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The University of New South Wales Faculty of Science School of Materials Science and Engineering

High Temperature Phenomena Occurring during Reactions of Agricultural Wastes in Electric Arc Furnace Steelmaking: Interactions with Gas and Slag Phases

A Thesis in Materials Science and Engineering by Nur Farhana Diyana MOHD YUNOS

Submitted in Partial Fulfillment of the requirements for the Degree of DOCTOR OF PHILOSOPHY March 2012

PLEASE TYPE THE UNIVERSITY OF NEW SOUTH WALES Thesis/Dissertation Sheet Surname or Family name: BINTI MOHD YUNOS First name: NUR FARHANA DIYANA

Other name/s:

Abbreviation for degree as given in the University calendar: Ph.D School: MATERIALS SCIENCE AND ENGINEERING

Faculty: SCIENCE

Title: HIGH TEMPERATURE PHENOMENA OCCURING DURING REACTIONS OF AGRICULTURAL WASTES IN ELECTRIC ARC FURNACE STEELMAKING: INTERACTIONS WITH GAS AND SLAG PHASES

Abstract 350 words maximum: (PLEASE TYPE) Iron and steel making is an energy intensive industrial sector using mainly coal as the heat source and reduction agent. The industry gives rise to about 10 % of the anthropogenic CO2 emissions in the world. Due to the challenge for CO2 mitigation, interest for agricultural waste (palm and coconut shells) use as a renewable energy and carbon source as heating agent and reducing agent contributes to energy conservation and emission reduction, and can partially replace coal and coke. In the present study, the conventional material investigated was metallurgical coke which was blended with different proportions of palm and coconut shells as well as agricultural waste chars in order to reduce the waste in the landfill. Metallurgical coke, palm shell/coke blends and coconut shell/coke blends were combusted in a drop tube furnace (DTF) at 1200 °C under 20% O2 and 80% N2 gas mixture while palm char was devolatilized at 450 °C under N2 atmosphere. Subsequently, the residual materials were put in contact with an EAF iron oxide rich slags and their interfacial reactions and phenomena have been studied at 1550 °C in a horizontal tube furnace under inert atmosphere (1 L/min Ar) with off gases measured using an IR analyser. The initial devolatilization and the subsequent step of combustion of these samples are conducted in a Drop Tube Furnace (DTF) and in a Thermogravimetric Analyser (TGA), respectively, while the sessile drop approach was used to investigate the interfacial reactions taking place in the slag/carbon region. A Thermogravimetric Analyser coupled with Mass Spectrometer (TGA-MS) was also used to study the behavior of coke and agricultural wastes at high temperatures in order to understand the thermal behavior and gas products that evolved at high temperatures. The weight loss profiles, gas formation and products distribution were significantly different between the coke and agricultural waste samples. It was found that more gases were released from agricultural waste than from coke that participated in the subsequent carbon/slag reactions. In the gas phase reaction studies, the blends containing agricultural waste materials indicated higher combustion efficiencies compared to coke alone with an improved surface area resulted from volatile matter removal. The role of chemical structure and properties, as well as inorganic matter in agricultural waste blends also influenced the combustion performance. The rate of devolatilization appears to improve the coke/palm shell blends burnout as well as its foaming behavior when put in contact with an iron oxide rich slag. For carbon/slag interactions, experiments were conducted using the sessile drop technique (1550 °C) with off gases (CO, CO2) measured using an IR analyzer; the wetting behaviour was determined from contact angle measurements and estimation of slag foam volumes were calculated using specialized software. Off gas analyses following the carbon/slag interfacial reactions have been measured for all the carbonaceous materials and significantly different gas concentrations have been observed. The rates of total gas generation (CO+CO2) from palm char was comparable to those seen in coke; however the gases released from palm chars were extent over a longer period of time and allowed their entrapment in the slag matrix, enhancing the volume of the slag. A slower rate of FeO reduction is seen when coke reacted with the Electric Arc Furnace (EAF) slag, while the palm shell blends showed a faster reduction. Independent of the carbon material used as a substrate, the final stage of reaction reveals comparable contact angles due to similar extents of reduction and Fe deposition at the interface. The steady gas generation seen in palm char compared to coke allows the formation of a highly porous particle, promoting gasification and allowing more gases to be trapped in the slag phase. These results indicate that partial replacement of coke with palm shells is not only viable, but efficient leading to improved/sustained interactions with EAF slag. Optimization between the two phenomena, reduction and foaming is required for improved EAF process performance.

Declaration relating to disposition of project thesis/dissertation I hereby grant to the University of New South Wales or its agents the right to archive and to make available my thesis or dissertation in whole or in part in the University libraries in all forms of media, now or here after known, subject to the provisions of the Copyright Act 1968. I retain all property rights, such as patent rights. I also retain the right to use in future works (such as articles or books) all or part of this thesis or dissertation. I also authorise University Microfilms to use the 350 word abstract of my thesis in Dissertation Abstracts International (this is applicable to doctoral theses only).

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The University recognises that there may be exceptional circumstances requiring restrictions on copying or conditions on use. Requests for restriction for a period of up to 2 years must be made in writing. Requests for a longer period of restriction may be considered in exceptional circumstances and require the approval of the Dean of Graduate Research. FOR OFFICE USE ONLY

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THIS SHEET IS TO BE GLUED TO THE INSIDE FRONT COVER OF THE THESIS

ORIGINALITY STATEMENT I hereby declare that this submission is my own work and to the best of my knowledge it contains no materials previously published or written by another person, nor material which to a substantial extent has been accepted for the award of any other degree or diploma at UNSW or any other educational institution, except where due acknowledgement is made in the thesis. Any contribution made to the research by others, with whom I have worked at UNSW or elsewhere, is explicitly acknowledged in the thesis. I also declare that the intellectual content of this thesis is the product of my own work, except to the extent that assistance from others in the project’s design and conception or in style, presentation and linguistic expression is acknowledged. Signed …………………………………… Nur Farhana Diyana Mohd Yunos

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COPYRIGHT STATEMENT ‘I hereby grant the University of New South Wales or its agents the right to archive and to make available my thesis or dissertation in whole or part in the University libraries in all forms of media, now or here after known, subject to the provisions of the Copyright Act 1968. I retain all proprietary rights, such as patent rights. I also retain the right to use in future works (such as articles or books) all or part of this thesis or dissertation. I also authorize University Microfilms to use the 350 word abstract of my thesis in Dissertation Abstract International (this is applicable to doctoral theses only). I have either used no substantial portions of copyright material in my thesis or I have obtained permission to use copyright material; where permission has not been granted I have applied/will apply for a partial restriction of the digital copy of my thesis or dissertation.' Signed ……………………………………………........................ Date ……………………………………………...........................

AUTHENTICITY STATEMENT ‘I certify that the Library deposit digital copy is a direct equivalent of the final officially approved version of my thesis. No emendation of content has occurred and if there are any minor variations in formatting, they are the result of the conversion to digital format.’ Signed ……………………………………………....................... Date ……………………………………………...........................

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ACKNOWLEDGEMENTS During my doctoral research I have been accompanied and supported by many people. Now, I have the opportunity to express my gratitude to all of them. I would like to express my deepest sense of gratitude and appreciation to my supervisors Professor Veena Sahajwalla, Assoc Prof Rita Khanna and Dr Magdalena Zaharia for their unrelenting support and guidance throughout my research. My sincerest gratitude goes to them for their excellent guidance and for providing valuable advice, comments and suggestions at various points during the course of this research. Their expertise was also invaluable in interpreting the results and in providing suggestions on preparing papers and research documents. I also wish to acknowledge the financial support provided by the Ministry of Higher Education Malaysia and University of Malaysia Perlis. I am grateful to Mr Narendra Saha Chaudhury for his valuable assistance in setting up and maintaining the experimental apparatus. In addition, special thanks to staff and colleagues at the Centre for Sustainable Materials Research & Technology (SMaRT@UNSW), School of Materials Science and Engineering and Chemical Engineering for their help, comments and advices as well as for making my time enjoyable throughout the course of this project. Finally, I express my deepest gratitude to my husband, Muhammad Asri Idris and family for motivation, patience, sacrifices and forever love through the period. I also thank my lovely daughter Asfa Haziqa, who is my beautiful gift from God, for bearing with me during the course of my Ph.D.

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ABSTRACT Iron and steel making is an energy intensive industrial sector using mainly coal as the heat source and reduction agent. The industry gives rise to about 5 % of the anthropogenic CO2 emissions in the world. Due to the challenge for CO2 mitigation, interest for agricultural waste (palm and coconut shells) use as a renewable energy and carbon source as heating agent and reducing agent contributes to energy conservation and emission reduction, and can partially replace coal and coke. In the present study, the conventional material investigated was metallurgical coke which was blended with different proportions of palm and coconut shells as well as agricultural waste chars in order to reduce the waste in the landfill. Metallurgical coke, palm shell/coke blends and coconut shell/coke blends were combusted in a drop tube furnace (DTF) at 1200 °C under 20% O2 and 80% N2 gas mixture while palm char was devolatilized at 450 °C under N2 atmosphere. Subsequently, the residual materials were put in contact with an EAF iron oxide rich slags and their interfacial reactions and phenomena have been studied at 1550 °C in a horizontal tube furnace under inert atmosphere (l L/min Ar) with off gases measured using an IR analyser. The initial devolatilization and the subsequent step of combustion of these samples are conducted in a Drop Tube Furnace (DTF) and in a Thermogravimetric Analyser (TGA), respectively, while the sessile drop approach was used to investigate the interfacial reactions taking place in the slag/carbon region. A Thermogravimetric Analyser coupled with Mass Spectrometer (TGA-MS) was also used to study the behavior of coke and agricultural wastes at high temperatures in order to understand the thermal behavior and gas products that evolved at high temperatures. The weight loss profiles, gas formation and products distribution were significantly different between the coke and agricultural waste samples. It was found that more gases were released from agricultural waste than from coke that participated in the subsequent carbon/slag reactions. In the gas phase reaction studies, the blends containing agricultural waste materials indicated higher combustion efficiencies compared to coke alone with an improved v

surface area resulted from volatile matter removal. The role of chemical structure and properties, as well as inorganic matter in agricultural waste blends also influenced the combustion performance. The rate of devolatilization appears to improve the coke/palm shell blends burnout as well as its foaming behavior when put in contact with an iron oxide rich slag. For carbon/slag interactions, experiments were conducted using the sessile drop technique (1550 °C) with off gases (CO, CO2) measured using an IR analyzer; the wetting behaviour was determined from contact angle measurements and estimation of slag foam volumes were calculated using specialized software. Off gas analyses following the carbon/slag interfacial reactions have been measured for all the carbonaceous materials and significantly different gas concentrations have been observed. The rates of total gas generation (CO+CO2) from palm char was comparable to those seen in coke; however the gases released from palm char was extent over a longer period of time and allowed their entrapment in the slag matrix, enhancing the volume of the slag. A slower rate of FeO reduction is seen when coke reacted with the Electric Arc Furnace (EAF) slag, while the palm shell blends showed a faster reduction. Independent of the carbon material used as a substrate, the final stage of reaction reveals comparable contact angles due to similar extents of reduction and Fe deposition at the interface. The steady gas generation seen in palm char compared to coke allows the formation of a highly porous particle, promoting gasification and allowing more gases to be trapped in the slag phase. These results indicate that partial replacement of coke with palm shells is not only viable, but efficient leading to improved/sustained interactions with EAF slag. Optimization between the two phenomena, reduction and foaming is required for improved EAF process performance.

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Table of Contents ORIGINALITY STATEMENT ..................................................................................... ii COPYRIGHT STATEMENT .......................................................................................iii AUTHENTICITY STATEMENT ................................................................................iii ACKNOWLEDGEMENTS........................................................................................... iv ABSTRACT ..................................................................................................................... v Table of Contents ............................................................................................................ vii List of Figures ................................................................................................................ xiv List of Tables................................................................................................................ xxiv List of Publications ....................................................................................................xxviii CHAPTER 1 .................................................................................................................... 1 1

Introduction ............................................................................................................... 1 1.1

Background ....................................................................................................... 1

1.2

Research Objectives .......................................................................................... 5

CHAPTER 2 .................................................................................................................... 6 2

Literature Review ...................................................................................................... 6 2.1

Agricultural Waste as a Source of Renewable Materials .................................. 6

2.2

Environmental Impact ....................................................................................... 9

2.3

Recycling of Agricultural Wastes in Steelmaking .......................................... 10

2.4

Properties and Composition of Agricultural Waste and Metallurgical Coke . 11 2.4.1 Cellulose .................................................................................................. 13 vii

2.4.2 Hemicellulose .......................................................................................... 13 2.4.3 Lignin ...................................................................................................... 14 2.4.4 Inorganic Minerals................................................................................... 16 2.4.5 Metallurgical Coke .................................................................................. 20 2.4.6 Activated Carbon ..................................................................................... 20 2.5

Gas Phase Reactions ....................................................................................... 21 2.5.1 Pyrolysis .................................................................................................. 22 2.5.2 Heterogeneous Char Combustion ............................................................ 25 2.5.3 Kinetic Reactions Regimes...................................................................... 26 2.5.4 Inorganic Effects in Combustion ............................................................. 27 2.5.4.1 Effects Due to the Inorganic Ash Compounds ................................. 28 2.5.5 Determination of the Combustion Efficiency of Coke and its Blends with Agricultural Waste................................................................................... 32 2.5.5.1 Factors Affecting Gas Phase Reactions............................................ 32

2.6

High Temperature Carbon/Slag Interactions .................................................. 36 2.6.1 Experimental Techniques for Carbon/Slag Interactions.......................... 37 2.6.2 Previous Studies on Carbon/Slag Interaction .......................................... 43 2.6.3 Factors Affecting Carbon/Slag Interactions ............................................ 45 2.6.4 FeO Reduction in EAF Steelmaking Slags ............................................. 49 2.6.4.1 EAF Operating Conditions with Slags ............................................. 52 2.6.4.2 Previous Studies on FeO Reduction and Factors Affecting FeO Reduction by Solid Carbon .............................................................. 53 viii

2.7

Summary ......................................................................................................... 57

CHAPTER 3 .................................................................................................................. 59 3

Experimental ........................................................................................................... 59

Characterization of Experimental Materials ................................................................... 59 Gas Phase Reactions Studies........................................................................................... 60 3.1

Preparation of Carbonaceous Materials .......................................................... 60

3.2

Chemical Characterization of Specimens ....................................................... 66 3.2.1 Thermogravimetric Analysis with Mass Spectroscopy (TGA-MS) ........ 66 3.2.2 X-Ray Diffraction Analysis (XRD)......................................................... 66 3.2.3 Fourier Transform Infrared Spectroscopy (FTIR) ................................... 67 3.2.4 Nuclear Magnetic Resonance Spectroscopy (13C NMR) ........................ 68

3.3

Physical Characterization of Specimens ......................................................... 69 3.3.1 Surface Area Measurements .................................................................... 69 3.3.2 Scanning Electron Microscopy (SEM) .................................................... 69

Carbon/slag Interaction Studies ...................................................................................... 70 3.4

Sample Preparation ......................................................................................... 70 3.4.1 Interfacial Phenomena-Optical Microscopy and SEM ............................ 73 3.4.2 Interfacial Phenomena – Wetting Behaviour .......................................... 74

3.5

Experimental Apparatus .................................................................................. 76 3.5.1 Drop Tube Furnace (DTF) – Gas Phase Reaction Studies ...................... 76 3.5.2 LECO Carbon Analyser .......................................................................... 78 ix

3.5.3 Muffle Furnace – Ash Measurement ....................................................... 79 3.5.4 Thermogravimetric Analyzer (TGA) ...................................................... 80 3.5.5 Ash Tracer Method – Combustion Efficiency......................................... 83 3.5.6 Inorganic Tracer Method ......................................................................... 83 3.5.6.1 The mass burnout of coke/agricultural waste blends ....................... 86 3.5.7 Minimization of Error in Combustion Efficiency Determination ........... 87 3.5.8 Horizontal Tube Furnace – Carbon/slag Interactions .............................. 87 3.5.9 Infra-Red Analyzer (IR) – Off-Gas Generation ...................................... 90 3.5.10 X-Ray Fluoroscopy (XRF) – Slag Characterization ............................... 91 3.6

Reproducibility of Carbon/slag Interaction Experiments ............................... 92 3.6.1 Minimization of Error in Contact Angle Determination ......................... 92 3.6.2 Estimation of Error in Slag Foaming Behavior (Vt/V0) Measurements .. 93

CHAPTER 4 .................................................................................................................. 94 4

Combustion & Structural Transformations of Coke/ Palm Shell Blends: Results & Discussions.............................................................................................................. 94 4.1

Gas Phase Reactions of Metallurgical Coke and its Blends with Palm Shell . 94 4.1.1 Effect of High Temperature on the Behaviour of the Carbonaceous Material.................................................................................................... 95 4.1.2 Effect of Blending on Combustion ........................................................ 103 4.1.3 Physical Properties and Structural Transformations ............................. 111 4.1.3.1 Surface Area Measurements – BET Surface Area ......................... 111

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4.1.3.2 Structural Transformations - Scanning Electron Microscopy (SEM) ........................................................................................................ 115 4.1.4 The Role of Chemical Properties and Carbon Structures ...................... 118 4.1.4.1 Chemical Structures ....................................................................... 118 4.1.4.2 Chemical Properties – X-ray Diffractions ...................................... 120 4.1.4.3 Chemical Bonding – Fourier Transform Infrared Spectroscopy (FTIR) ............................................................................................. 131 4.1.4.4 Carbon Structures –Nuclear Magnetic Resonance (13C NMR Spectroscopy) ................................................................................. 137 4.1.4.5 Inorganic Minerals ......................................................................... 145 4.2

Summary ....................................................................................................... 148

CHAPTER 5 ................................................................................................................ 149 5

Combustion & Structural Transformations of Coke/ Coconut Shell Blends: Results & Discussion ......................................................................................................... 149 5.1

Gas Phase Reactions of Metallurgical Coke and its Blends with Coconut Shell ...................................................................................................................... 149 5.1.1 Effect of High Temperature on the Behaviour of the Carbonaceous Material.................................................................................................. 151 5.1.2 Effect on Blending on Combustion ....................................................... 156 5.1.3 Physical Properties and Structural Transformations ............................. 163 5.1.3.1 Surface Area Measurement – BET Surface Area ........................... 163 5.1.3.2 Structural Transformations - Scanning Electron Microscopy (SEM) ........................................................................................................ 166 xi

5.1.4 The Role of Chemical Properties and Carbon Structures ...................... 170 5.1.4.1 Chemical Structures ....................................................................... 170 5.1.4.2 Chemical Properties – X-ray Diffractions ...................................... 171 5.1.4.3 Chemical Bonding – Fourier Transform Infrared Spectroscopy (FTIR) ............................................................................................. 179 5.1.4.4 Carbon Structures –Nuclear Magnetic Resonance (13C NMR Spectroscopy) ................................................................................. 183 5.1.4.5 Inorganic Minerals ......................................................................... 186 5.2

Comparison of Coke/Palm Shells & Coke/Coconut Shells Blends in Gas Phase Reactions ............................................................................................ 189

5.3

Summary ....................................................................................................... 201

CHAPTER 6 ................................................................................................................ 203 6

Conclusions – Gas Phase Reactions Studies ......................................................... 203

CHAPTER 7 ................................................................................................................ 205 7

High Temperature Reactions: Carbon/Slag Interactions – Results and Discussion ............................................................................................................................... 205 7.1

Influence of Carbonaceous Material on Carbon/Slag Interactions ............... 208 7.1.1 Off-gas (CO, CO2) Generations ............................................................ 209 7.1.2 Contact Angle Measurements................................................................ 217 7.1.3 High Temperature, In-situ Observations ............................................... 219 7.1.4 Slag Foaming ......................................................................................... 225 7.1.5 Interfacial Phenomena - Optical and SEM Studies ............................... 234 xii

7.2

FeO Reduction .............................................................................................. 246 7.2.1 Estimation of the reaction rate constant ................................................ 252

7.3

Discussion on Slag Foaming of Carbonaceous Material .............................. 256 7.3.1 Influence of Gas Generation on Slag Foaming ..................................... 257 7.3.2 Influence of Volatiles from the Agricultural Wastes/Polymers on Slag Foaming ................................................................................................. 261 7.3.3 Influence of Mineral Matter from the Agricultural Wastes on Slag Foaming ................................................................................................. 264 7.3.4 Influence of Carbon Structures from the Agricultural Wastes on Slag Foaming ................................................................................................. 265 7.3.5 Influence of Entrapped Gas Bubbles in the Bulk Slag on Slag Foaming ............................................................................................................... 269

CHAPTER 8 ................................................................................................................ 273 8

Conclusions – Carbon/Slag Interactions Studies .................................................. 273

CHAPTER 9 ................................................................................................................ 276 9

Summary and Conclusions.................................................................................... 276

CHAPTER 10 .............................................................................................................. 280 10

References ............................................................................................................. 280

APPENDICES ............................................................................................................. 305 APPENDIX A .............................................................................................................. 306 APPENDIX B .............................................................................................................. 313

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List of Figures Figure 2-1 Molecular structure of common fatty acid in palm oil plant ...................... 11 Figure 2-2 Polymers of cellulose (Graboski, M. and Bain, R., 1981) .......................... 13 Figure 2-3 Monomers of xylan (Graboski, M. and Bain, R., 1981) ............................. 14 Figure 2-4 A hypothetical structure of lignin (Blazej, A. and Kosik, M., 1993).......... 15 Figure 2-5 Process during coal/agricultural wastes pyrolysis, gasification and combustion .......................................................................................................... 21 Figure 2-6 Reaction mechanism of pyrolysis and steam gasification (dotted line represents the reaction with steam): (a) cellulose; (b) lignin .............................. 24 Figure 2-7 Proposed catalytic mechanism for the reaction of iron atoms in the char structure (Backreedy, R. I., Jones, J. M., Pourkashanian, M. et al., 2003) ......... 31 Figure 2-8 Proposed catalytic mechanism for the reaction of alkali metals (Huang, H. Y. and Yang, R. T., 1999) ................................................................................... 31 Figure 2-9 Representative example of the video screen during data processing. The coordinate for point's p1 and p2 are displayed in the data line just above the image, along with the computed values of volume and area of contact (arbitrary units) (Khanna, R., Spink, J. and Sahajwalla, V., 2007, Rahman, M., 2006) ..... 40 Figure 2-10 Schematic diagram of the novel processing software for volume and contact area measurements (Khanna, R., Mahfuzur, R., Richard, L. et al., 2007) ............................................................................................................................. 41 Figure 2-11 The influence of S and P2O5 on the surface tension of lime-silica slags containing 30% FeO at 1673K (Skupien, D. and Gaskell, D., 2000) ................. 47 Figure 2-12 The relationship between the foaming index and viscosity for CaO-SiO2FeO system (Kozakevitch, P. and Olette, M., 1971)........................................... 48

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Figure 2-13 Foam index as a function of slag basicity (Ito, K. and Fruehan, R., 1989) ............................................................................................................................. 49 Figure 3-1 Bar chart of the materials investigated in this study illustrating the relative concentrations of palm shells and MC content in the blends .............................. 61 Figure 3-2 Bar chart of the materials investigated in this study illustrating the relative concentrations of coconut shells and MC content in the blends ......................... 61 Figure 3-3 Bubble diameter measuring using Adobe® Photoshop 7.0 ........................ 74 Figure 3-4 Illustration of the contact angle measurement using ANGLE software ..... 75 Figure 3-5 Schematic diagram of DTF (DTF is an abbreviation of Drop Tube Furnace) ............................................................................................................................. 77 Figure 3-6 Front view of the Thermogravimetric Analyzer (TGA) with key features highlighted .......................................................................................................... 81 Figure 3-7 A schematic diagram of the change in mass of representative coke or agricultural waste particle during combustion .................................................... 85 Figure 3-8 Schematic Diagram of the Horizontal Tube Resistance Furnace used for Gas Entrapment Tests ......................................................................................... 88 Figure 4-1 (a) – (f) The mass loss of metallurgical coke with on-line MS analysis of the gas products during pyrolysis ........................................................................ 97 Figure 4-2 (a) – (f) The mass loss of palm shells with on-line MS analysis of the gas products during pyrolysis .................................................................................. 100 Figure 4-3 (a) – (f) The mass loss of palm char with on-line MS analysis of the gas products during pyrolysis .................................................................................. 102 Figure 4-4 Weight loss curves of MC and its blends with palm shells at temperature, 1200 ºC in N2 atmosphere ................................................................................. 105

xv

Figure 4-5 Comparison of total weight loss curve of MC and its blends with palm shells at temperature, 1200 ºC in N2 atmosphere .............................................. 105 Figure 4-6 Effect of blending MC with varying palm shell content in the blends on the combustion at 1200 °C in the presence of 80% N2; 20% O2 ............................ 107 Figure 4-7 Effect of volatile matter on (a) blend composition and on (b) the combustion performance of palm shell/MC blends .......................................... 108 Figure 4-8 Changes in micropore surface area for palm shell before and after combustion in DTF as a function of the palm shell content in the blends ........ 112 Figure 4-9 The changes in micropore surface area for palm shell wastes content in the blend .................................................................................................................. 113 Figure 4-10 Images of (a) raw MC and (b) MC after combustion in gas phase at 1200 ºC (80% N2; 20% O2) ........................................................................................ 114 Figure 4-11 Images of (a) raw palm shells and (b) palm shell after combustion in gas phase reactions at 1200 ºC (80% N2; 20% O2) ................................................. 115 Figure 4-12 SEM micrograph of (a) raw MC, (b) MC after combustion at 1200°C in DTF, (c) cross-sectional image of raw MC and (d) cross-section of MC char . 116 Figure 4-13 SEM micrograph of (a) raw palm shell, (b) palm shell after combustion at 1200°C in DTF, (c) cross-sectional image of raw palm shell and (d) crosssection of palm shell char.................................................................................. 117 Figure 4-14 Cellulose, hemicellulose and lignin contents of palm shell (Wan Daud, W. M. A. and Wan Ali, W. S., 2004)...................................................................... 119 Figure 4-15 XRD patterns of metallurgical coke before and after combustions (T=1200 ºC with 20% O2; 80% N2) .................................................................. 121 Figure 4-16 XRD patterns of palm shells before and after combustions (T=1200 ºC: 20% O2; 80% N2) .............................................................................................. 124 xvi

Figure 4-17 X-ray diffraction analysis of coke/palm shell blends; P1 before and after combustion temperature 1200 ºC in 20% O2 and 80% N2 atmosphere............. 126 Figure 4-18 X-ray diffraction analysis of coke/palm shell blends; P2 before and after combustion temperature 1200 ºC in 20% O2 and 80% N2 atmosphere............. 127 Figure 4-19 X-ray diffraction analysis of coke/palm shell blends; P3 before and after combustion temperature 1200 ºC in 20% O2 and 80% N2 atmosphere............. 128 Figure 4-20 FTIR spectra of metallurgical coke before and after combustion in DTF at temperature 1200 ºC in 20% O2 and 80% N2 atmosphere ................................ 132 Figure 4-21 FTIR spectra of palm shells before and after combustion in DTF DTF at temperature 1200 ºC in 20% O2 and 80% N2 atmosphere ................................ 135 Figure 4-22 CP/MAS 13C NMR spectra of (a) raw MC and (b) MC at 1200ºC in a drop tube furnace [80% N2, 20% O2] (* spinning sideband) .................................... 138 Figure 4-23 Spectrum of CP/MAS 13C NMR of (a) raw palm shells and (b) palm shells char at 1200ºC in a drop tube furnace [80% N2, 20% O2] (* spinning sideband) ........................................................................................................................... 142 Figure 4-24 Summarize of mechanism governing the breakdown of the structures before and after combustion in DTF at 1200 C (80% N2; 20% O2) for palm shells and MC.................................................................................................... 144 Figure 4-25 Fe2O3 component present in MC and MC /palm shell blends ash .......... 145 Figure 4-26 K2O and Na2O components present in the ash from MC and MC / palm shell blends ........................................................................................................ 146 Figure 5-1 (a) – (f) The mass loss of coconut shells with on-line MS analysis of the gas products during pyrolysis............................................................................ 153 Figure 5-2 (a) – (f) The mass loss of coconut char with on-line MS analysis of the gas products during pyrolysis .................................................................................. 155 xvii

Figure 5-3 Weight loss curves of MC and its blends with coconut shells at temperature, 1200 ºC; N2 atmosphere ............................................................... 157 Figure 5-4 Comparison of total weight loss curve (50 seconds) of MC and its blends with coconut shells at temperature, 1200 ºC; N2 atmosphere ........................... 157 Figure 5-5 Effect of blending MC with varying coconut shell content in the blends on the combustion at 1200 °C in the presence of 80% N2; 20 % O2 ..................... 158 Figure 5-6 Effect of volatile matter on (a) blend composition and on (b) the combustion performance of coconut shell/MC blends...................................... 160 Figure 5-7 Changes in the micropore surface area before and after combustion in DTF as a function of the coconut shell content in the blends .................................... 164 Figure 5-8 The changes in micropore surface area for coconut shell wastes content in the blend ............................................................................................................ 165 Figure 5-9 Images of (a) raw coconut shells and (b) coconut shell after combustion in gas phase reactions at 1200 ºC (80% N2; 20% O2) ........................................... 166 Figure 5-10 SEM micrographs of transverse section of (a) raw coconut shell, (b) coconut shell after combustion at 1200 °C in DTF and cross-sectional images of (c) raw coconut shell, (d) coconut shell char (e) and (f) edge section of coconut shell char ........................................................................................................... 167 Figure 5-11 (a) and (b) SEM micrograph of coconut shell after combustion in DTF showing fragmented particles ........................................................................... 169 Figure 5-12 Cellulose, hemicellulose and lignin contents of coconut shell (Wan Daud, W. M. A. and Wan Ali, W. S., 2004). ............................................................... 170 Figure 5-13 XRD patterns of coconut shells before and after combustions (T=1200 ºC: 20% O2; 80% N2) .............................................................................................. 172 Figure 5-14 X-ray diffraction analysis of coke/coconut shell blends; C1 before and after combustion temperature 1200 ºC in 20% O2 and 80% N2 atmosphere .... 175 xviii

Figure 5-15 X-ray diffraction analysis of coke/coconut shell blends; C2 before and after combustion temperature 1200 ºC in 20% O2 and 80% N2 atmosphere .... 176 Figure 5-16 X-ray diffraction analysis of coke/coconut shell blends; C3 before and after combustion temperature 1200 ºC in 20% O2 and 80% N2 atmosphere .... 177 Figure 5-17 FTIR spectra of coconut shell before and after combustion in DTF ...... 180 Figure 5-18 Spectrum of CP/MAS 13C NMR of (a) raw coconut shell and (b) coconut shell char at 1200ºC in a drop tube furnace [80% N2, 20% O2] (* spinning sideband) ........................................................................................................... 185 Figure 5-19 Fe2O3 component present in the ash from MC and MC /coconut shell blends ................................................................................................................ 187 Figure 5-20 K2O and Na2O components present in the ash from MC and MC - coconut shell blends ........................................................................................................ 188 Figure 5-21 (a) Comparison of residual mass (%) in TGA with time for 100% MC and its mixtures with palm shells (b) variation of rate constant of the palm shell blends with volatile matter at 1200ºC in N2 atmosphere .................................. 191 Figure 5-22 (a) Comparison of residual mass (%) in TGA with time for 100% MC and its mixtures with coconut shells (b) variation of rate constant of the coconut shell blends with volatile matter at 1200ºC in N2 atmosphere .................................. 192 Figure 5-23 (a) and (c) Comparison of residual mass (%) in TGA with time for 100% MC and its mixtures with palm and coconut shells (b) and (d) variation of rate constant of the agricultural blends with volatile matter at 1200ºC in N2 atmosphere ........................................................................................................ 195 Figure 5-24 Combustion performances of palm shell-coke and coconut shell-coke blends (Blend 3* = Blend P3 and Blend C3) .................................................... 197 Figure 5-25 The changes in micropore surface area for agricultural wastes content in the blend ............................................................................................................ 199 xix

Figure 5-26 SEM micrographs of (a) MC, (b) palm shells and (c) coconut shells collected after reaction in the DTF at 1200°C in atmosphere of 20% O2 and 80% N2, polished section of the residual particle x1000 ........................................... 200 Figure 7-1 Schematic diagram illustrating the effect of carbonaceous materials rate of devolatilization on slag foaming ....................................................................... 206 Figure 7-2 (I) Metallurgical coke/slag and (II) Palm char/slag with (a) generated gas concentrations (ppm) in terms of CO and CO2 gases and (b) the cumulative volume of gases (mol) of CO and CO2 ............................................................. 210 Figure 7-3 Total number of moles (a) carbon and (b) oxygen removed from the metallurgical coke (MC) and palm char (PC) substrate in contact with a slag at temperature 1550 ºC .......................................................................................... 212 Figure 7-4 Generated gas concentrations, CO+CO2 (ppm) as a function of time and carbon based materials (a) metallurgical coke, (b) P1 blends, (c) P2 blends and (d) P3 blends ..................................................................................................... 214 Figure 7-5 Total cumulative number of moles of gas generated (CO+CO2) as a function of time and carbonaceous material used ............................................. 216 Figure 7-6 Variation of contact angle with time for carbonaceous substrate (a) palm char (PC), (b) P1 blends, (c) P2 blends and (d) P3 blends in contact with slag at 1550ºC ............................................................................................................... 218 Figure 7-7 High temperature photographs of slag droplets in contact with (a) 100% MC and (b) 100% Palm char at 1550 ºC as a function of time ......................... 221 Figure 7-8 High temperature photographs of slag droplets in contact with (a) 100% MC and (c) P1 blends at 1550 ºC as a function of time .................................... 222 Figure 7-9 High temperature photographs of slag droplets in contact with (a) 100% MC and (d) P2 blends at 1550 ºC as a function of time .................................... 223

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Figure 7-10 High temperature photographs of slag droplets in contact with (a) 100% MC and (e) P3 blends at 1550 ºC as a function of time .................................... 224 Figure 7-11 (a) Instantaneous gases (CO+CO2), ppm, generated and (b) Volume ratio of 100% Metallurgical coke (MC) and 100% Palm char (PC) with slag with respect to reaction time ..................................................................................... 226 Figure 7-12 (a) Instantaneous gases (CO+CO2), ppm, generated and (b) Volume ratio of 100% Metallurgical coke (MC) and P1 blends with slag with respect to reaction time ...................................................................................................... 229 Figure 7-13 (a) Instantaneous gases (CO+CO2), ppm, generated and (b) Volume ratio of 100% Metallurgical coke (MC) and P2 blends with slag with respect to reaction time ...................................................................................................... 230 Figure 7-14 (a) Instantaneous gases (CO+CO2), ppm, generated and (b) Volume ratio of 100% Metallurgical coke (MC) and P3 blends with slag with respect to reaction time ...................................................................................................... 231 Figure 7-15 Instantaneous gases (CO+CO2), ppm generated from 100% metallurgical coke (MC), 100% Palm char (PC), P1 blends, P2 blends and P3 blends reacting with slag as a function of time .......................................................................... 233 Figure 7-16 Optical microscopy images of (a) Slag/100% metallurgical coke (MC) and (b) Slag/Palm char (PC) as a function of time .................................................. 236 Figure 7-17 Optical microscopy images of (a) Slag/100% metallurgical coke (MC) and (c) Slag/P1 blends as a function of time............................................................ 237 Figure 7-18 Optical microscopy images of (a) Slag/100% metallurgical coke (MC) and (d) Slag/P2 blends as a function of time ........................................................... 238 Figure 7-19 Optical microscopy images of (a) Slag/100% metallurgical coke (MC) and (e) Slag/P3 blends as a function of time............................................................ 239

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Figure 7-20 (a) Optical microscopy of MC/slag, (b) SEM, mapping on the inner region of quenched MC/slag and (c) EDS spectra of quenched metallurgical coke/slag assembly at 1550 °C after 15 min of contact .................................................... 242 Figure 7-21 (a) Optical microscopy of Palm char/slag, (b) SEM, mapping on the inner region of quenched Palm char/slag and (c) EDS spectra of quenched Palm char/slag assembly at 1550 °C after 15 min of contact ..................................... 243 Figure 7-22 (a) Optical microscopy of P1 blend/slag, (b) SEM, mapping on the inner region of quenched P1 blend/slag and (c) EDS spectra of quenched P1 blend/slag assembly at 1550 °C after 15 min of contact ................................... 245 Figure 7-23 Total removed oxygen, moles as a function of time and carbonaceous material used ..................................................................................................... 249 Figure 7-24 FeO concentration in the slag with proceeding reaction ......................... 250 Figure 7-25 Reaction rate constant as a function of carbon material used ................. 253 Figure 7-26 (a) Volume ratio of 100% Metallurgical coke (MC) and 100% Palm char (PC) and P3 blends with slag and (b) Gases (CO+CO2), ppm, generated with respect to reaction time ..................................................................................... 258 Figure 7-27 High temperature images of slag droplet in contact with (a) metallurgical coke (MC), (b) 100% palm char (PC) and P3 blends at 1550 ºC as a function of time.................................................................................................................... 259 Figure 7-28 High temperature images of slag droplet in contact with (a) rubber/coke blend (R3) (Zaharia, M., 2010) and (b) palm shell/coke blend (P3) at 1550 ºC as a function of time .............................................................................................. 267 Figure 7-29 Reaction rate constant as a function of carbon material used ................. 268 Figure 7-30 Optical microscopy images of (a) Slag/HDPE blend (HDPE 3), (b) Slag/rubber blend (R3), (c) Slag/PET blend (PET 3) (Kongkarat, S., 2011,

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Rahman, M. M., 2010, Zaharia, M., 2010), (d) Slag/palm shell (P3) blend as a function of time and (e) Slag/100% palm char (PC) ......................................... 270 Figure 10-1 A schematic diagram of the change in mass of representative coke or agricultural waste particle during combustion .................................................. 310

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List of Tables Table 2-1 Cellulose, holocellulose and lignin contents of palm shell and coconut shell (Wan Daud, W. M. A. and Wan Ali, W. S., 2004) ............................................. 16 Table 2-2 Inorganic minerals in agricultural waste by XRF analysis – literature review ............................................................................................................................. 17 Table 2-3 Ultimate analysis of palm shell wastes material – literature review .............. 18 Table 2-4 Proximate analysis of palm shell wastes material – literature review ........... 18 Table 2-5 Ultimate analysis of coconut shell wastes material – literature review ......... 19 Table 2-6 Proximate analysis of coconut shell wastes material – literature review....... 19 Table 2-7 Data for the SSM Trial Heats: Comparison between Metallurgical Coke and Rubber Tyre/Coke Mixture (Sahajwalla, Zaharia et al. 2009) ............................ 52 Table 3-1 Summary of chemical analysis of palm shells, palm shells char (PC), metallurgical coke (MC) and palm shell blends prior to high temperature reactions .............................................................................................................. 63 Table 3-2 Summary of chemical analysis of coconut shell, coconut shells char (CC), metallurgical coke (MC) and coconut shell blends prior to high temperature reactions .............................................................................................................. 63 Table 3-3 Ash analysis for metallurgical coke, agricultural wastes and chars ............... 64 Table 3-4 Ash analysis for coke/agricultural waste blends ............................................ 65 Table 3-5 Experimental conditions for X-Ray diffraction studies ................................. 67 Table 3-6 Experimental conditions for 13C NMR spectroscopy .................................... 68 Table 3-7 Summary of chemical analysis of metallurgical coke and agricultural wastes char after high temperature reactions .................................................................. 71

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Table 3-8 Composition of slag ....................................................................................... 72 Table 3-9 Operating conditions of DTF for combustion studies.................................... 78 Table 3-10 Operating conditions of carbon and sulphur measurements ........................ 79 Table 3-11 Experimental conditions for ashing studies ................................................. 79 Table 3-12 Operating condition of TGA for combustion and devolatilization studies under continuous conditions ............................................................................... 82 Table 3-13 The amounts of major inorganic elements (based on their oxides) present in the ash (XRF) of the coke and agricultural waste materials at 1200 °C ............. 84 Table 3-14 The experimental conditions for the carbon/slag interactions ..................... 88 Table 3-15 The experimental conditions for reduction studies ...................................... 90 Table 4-1 XRF analysis showing the amounts of major inorganic elements (based on their oxides) present in the ash of the coke, palm shell blends and palm shells following treatment at 1200 °C under 20% oxygen and 80% nitrogen atsmosphere ....................................................................................................... 110 Table 4-2 XRD peaks characteristic of raw MC and MC after combustion in DTF 1200 ºC with atmosphere (80% N2; 20% O2) ............................................................ 122 Table 4-3 XRD peaks characteristic of raw palm shells and palm shells after combustion in DTF 1200 ºC with atmosphere (80% N2; 20% O2) ................... 125 Table 4-4 XRD peaks characteristic for all samples P1, P2 and P3 blends before and after combustion in DTF 1200 ºC with atmosphere (80% N2; 20% O2) ........... 130 Table 4-5 Characterization of FTIR peak profiles of raw MC and MC after combustion at 1200 ºC at atmosphere 80% N2; 20% O2 ...................................................... 133 Table 4-6 Characterization of FTIR peak profiles of raw palm shells and palm shells after combustion at 1200 ºC at atmosphere 80% N2; 20% O2 .......................... 136 xxv

Table 4-7 Signal assignments for CP/MAS 13C NMR of MC (Erdenetsogt, B.-O., Lee, I., Lee, S. K. et al., 2010) .................................................................................. 139 Table 4-8 Resonance assignments for CP/MAS 13C NMR spectra of agricultural wastes (Bardet, M., Hediger, S., Gerbaud, G. et al., 2007, Link, S., Arvelakis, S., Spliethoff, H. et al., 2008) ................................................................................. 143 Table 5-1 XRF analysis showing the amounts of major inorganic elements (based on their oxides) present in the ash of the coke, coconut shell blends and coconut shells following treatment at 1200 °C under 20% oxygen and 80% nitrogen atsmosphere ....................................................................................................... 162 Table 5-2 XRD peaks characteristic of raw coconut shells and coconut shells after combustion in DTF 1200 ºC with atmosphere (80% N2; 20% O2) ................... 173 Table 5-3 XRD peaks characteristic for all samples C1, C2 and C3 blends before and after combustion in DTF 1200 ºC with atmosphere (80% N2; 20% O2) ........... 178 Table 5-4 Characterization of FTIR peak profiles of raw coconut shells and coconut shells after combustion at 1200 ºC at atmosphere (80% N2; 20% O2).............. 182 Table 5-5 Type of molecular bonds and bond energies required to break the fuels considered in this study (Blazej, A. and Kosik, M., 1993, Smith, L. H., and, S. L. D. and Fletcher, 1994)....................................................................................... 196 Table 7-1 Minimum bubble diameter in slag droplets in contact with metallurgical coke, palm char and palm shell blends.............................................................. 240 Table 7-2 Maximum bubble diameter in slag droplets in contact with metallurgical coke, palm char and palm shell blends.............................................................. 241 Table 7-3 Reaction rate constant (moles/cm2 s) for 100% metallurgical coke, 100% palm char and palm shell/coke blends .............................................................. 254 Table 7-4 Chemical composition of coke, palm char, rubber and PET ....................... 256

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Table 7-5 Comparison of the maximum rate of FeO reduction by different carbonaceous materials obtained from literature .............................................. 262 Table 7-6 Type of crystallization of polymers and molecular bonds in the carbonaceous materials (Antal, M. J., Jr. and Varhegyi, G., 1995, Orfão, J. J. M., Antunes, F. J. A. and Figueiredo, J. L., 1999, Sharma, R. K., Wooten, J. B., Baliga, V. L. et al., 2004) ................................................................................................................. 266 Table 7-7 The range of diameters of small gas bubbles entrapped in the slag droplet for coke and its blends with polymer between 2 – 10 minutes of reaction............. 271 Table 10-1 Ash analysis of metallurgical coke ............................................................ 312

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List of Publications Publications: 1.)

M. Yunos, N. F, Zaharia M, Idris M. A., Nath D., Khanna R., Sahajwalla V. (2012), “Recycling Agricultural Waste from Palm Shells during Electric Arc Furnace Steelmaking”, Energy and Fuels, Vol. 26, (1), pp. 278-286.

2.)

M. Yunos, N. F., Zaharia M., Ahmad K. R., Nath D., Iwase M., Sahajwalla V. (2011), "Structural Transformation of Agricultural Waste/Coke Blends and Their Implications during High Temperature Processes", ISIJ International, Vol. 51, (7), pp. 1185–1193.

3.)

M. Yunos, N. F., Ahmad K. R., Zaharia M., Sahajwalla V. (2011), "Combustion of Agricultural Waste and Coke Blends during High Temperature Processes: The Effect of Physical, Chemical and Surface Properties", Journal of the Japanese Society for Experimental Mechanics, Vol. 11, (Special Issue), pp. 261266.

Conference: 1.)

M. Yunos, N. F, Ahmad K. R, Zaharia M, Sahajwalla V (2011), "Agricultural Wastes-A Resource for EAF Steelmaking", AISTech 2011 - The Iron & Steel Technology Conference and Exposition, Indianapolis, US, pp. 877-875.

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Chapter 1: Introduction

CHAPTER 1 1 Introduction

1.1

Background

Nowadays the environmental benefits of wood or other forms of biomass/agricultural wastes associated with the reduction of CO2 in the atmosphere is attracting wide attention due to its CO2-neutrality, its contribution to the preservation of natural resources and its flexibility in the production of solid, liquid and gaseous fuels. Biomass/agricultural wastes can be used in the production of chemicals and liquid fuels, charcoal for use in metallurgy, carbon adsorbents from wood and biomass/agricultural wastes and for co-firing with coal/coke in steelmaking. Steelmaking is an energy intensive industrial sector and contributes significantly to the anthropogenic CO2 emissions in the world. With an estimated value of 20 – 35% of the annual industrial energy consumption and is one of the largest CO2 emitter accounting for approximately 5% of the total CO2 emission (Ooi, T. C., Aries, E., Ewan, B. C. R. et al., 2008, Yanjia, W. and Chandler, W., 2010). Since many of the unit processes in the chain from ore to steel have already evolved to a mature state, and further optimisation of their operation to reduce the emissions is difficult, global concern about increasing greenhouse gas concentrations in the atmosphere drives researchers to look for other solutions, such as capturing and storing the arising CO2 emissions,(Ariyama, T. and Sato, M., 2006, Danloy, G., Berthelemot, A. and Grant, M., 2009, Huijgen, W. J. J . and Comans, R. N. J., 2005 ) or replacing the fossil hydrocarbons (coal, oil and natural gas) by agricultural waste materials. Two types of agricultural waste materials used for this study are palm shell and coconut shell.

1


The combustion of carbon based materials releases chemical energy. However, their effective usage as energy depends on the kinetics of their reaction with oxygen and the extent to which they are converted to CO gas (the reaction that liberates energy). Malaysia is the second largest palm oil (Elaeis guineensis) producing country in the world, with 30 million tonnes, out of which 8.2 million tonnes are discarded as palm waste, consisting of empty fruit bunches, fibres and shells. Agricultural waste materials derived from palm shells are among the main renewable waste source in Malaysia. Approximately 2.01 million tonnes of palm shells were generated in 2010 alone, and a steady 5% increase has been seen in the last few years (Azri, S. M., 2008, Chor, L. Y., 2010, Mae, K., Hasegawa, I., Sakai, N. et al., 2000). Coconut shell (Cocos nucifera) consists of a hard and thick bony endocarp material, which presents serious disposal problems in the local environments (Chung, J. K ., 1997, Guo, S., Peng, J., Li, W. et al., 2009). Recycling this unused resource would add to its economic value, helping reduce the cost of waste disposal and most importantly, providing an inexpensive alternative to conventional coke (Heschel, W. and Klose, E., 1995, Su, W., Zhou, L. and Zhou, Y., 2006). Agricultural wastes having high volatile matter content, may find their possible utilization in combustion with low volatile coke. As compared to coke, agricultural wastes also contain high oxygen and easy release of volatile matter in a combustor. All these characteristics of agricultural wastes have been found to have large influence on the burn out time of blends of coke and agricultural wastes. Moreover, agricultural waste chars were found to have porous and highly disordered carbon structure and belong to the class of most reactive carbon materials. The porosity within the chars causes more accessibility of the reactive gas to active sites resulting in the very good combustion reactivity. Agricultural waste materials are a renewable source of carbon and a possible replacement for coal/coke used in high temperature processes. The aims of this high temperature study involving blending palm shell/coconut shell wastes with coke with application in high temperature processes include developing an understanding of the following high temperature phenomena: 2


1.

Gas phase reactions: pyrolysis and its subsequent step combustion; of high importance and novelty is the influence of palm shell and coconut shell wastes blend with coke on the kinetics of devolatilization, gas formations, crystallinity and associated structural transformations as a result of high temperature gas phase reactions. The intra and intermolecular bonding present in the structure of the raw materials are expected to play a significant role in controlling gas phase reactions which can influence the structure of the resulting carbonaceous particle and also subsequent carbon/ slag reactions.

2.

Carbon/slag interactions: the generated carbonaceous residue left behind after the combustion reaction can replace some of the coke in promoting FeO reduction reactions and foaming slag. The influence of gas generation, volatile matter and mineral matter from agricultural wastes are expected for controlling in carbon/slag interactions.

Slag foaming is a feature of most conventional Electric Arc Furnace steelmaking (EAF) processes. In the modern EAF, maintaining a stable foamed slag to submerge the arc is critical for high productivity operation. In the slag foaming process, carbon is injected into the slag, reacts with FeO in the slag producing CO, which foams the slag. Slag foams have been investigated with smaller bubbles than those used in the previous studies (Hara, S. and Ogino, K., 1992, Zhang, Y., 1995). The bubbles were generated by argon gas injection with the nozzle of multiple small orifices and by the slag/metal interfacial reaction of FeO in the slag with carbon in the liquid iron. Slag foaming is influenced by two main factors: 1) rate of gas evolution by the reduction reactions and 2) the stability of the foamy layer in the melt. The foaming stability depends on the high viscosity, low surface tension and increased suspension of second phase particle (Hara, S., 1990, Nexhip, C., Shouyi, S. and Jahanshahi, S., 2004, Pathak, D. C., 1997). Besides the formation of gases in the slag layer, a very important reaction, when carbon is put in contact with the slag, is the reduction of iron oxide (Nagasaka, T., Hino, M. and Ban-Ya, S., 2000). 3


The type of carbonaceous material and its properties play a critical role in governing its reaction with gas and slag phases. Previous studies have focused attention on slag properties and their influence on foaming, e.g. the foam index incorporates slag properties (Ito, K. and Fruehan, R., 1989). Our groups investigated the influence of carbonaceous materials on high temperature reactions in EAF steelmaking (Rahman, M., 2006, Rahman, M., Khanna, R., Sahajwalla, V. et al., 2009, Sahajwalla, 2009, Zaharia, M., Sahajwalla, V., Khanna, R. et al., 2009), while Corbari and Fruehan (Corbari, R., Matsuura, H., Halder, S. et al., 2009) measured the rate of carbon/slag reaction using five different types of carbonaceous materials. However, the significant influences of agricultural wastes are not yet fully understood, such as: volatile matter, mineral matter, carbon structures and gas formations. This study investigates the partial replacement of coke with waste materials such as palm shells and coconut shells and their influence upon gas phase reaction and slag/carbon interactions, including the influence of volatiles, gas formations, carbon structures, and mineral matter. The fundamentals of these reactions are key to understanding combustion efficiency and carbon/slag interactions.

4


1.2

Research Objectives

The present study investigates the influence of metallurgical coke and its blends with agricultural waste materials (palm and coconut shells) on gas phase reactions, carbon/slag interactions and associated gas generation. The objectives of this research are listed below: •

To study the gas phase reactions of different blends of agricultural wastes/coke and establish the influence of volatiles, gas formations, carbon structure, crystallnity, mineral matter and their release on structural transformations and gas phase reactions.

•

To establish the properties of the residual char and develop fundamental understanding of the interdependence between properties and reactions with gas and slag phases.

•

To study carbon/slag interactions in terms of reduction reactions and subsequent gas entrapment at the carbon/slag interface. Specifically, the aims are: 

Study the gas generation from FeO reduction as a result of carbon/slag interaction with the subsequent gas entrapment which ultimately influences relative slag volumes obtained.



Study gas entrapment in the EAF slag using quantitative technique involving slag volume measurements with respect to time and also through qualitative understanding of gas bubbles trapped within the slag.



Study the influence of gas generation, volatiles matter, mineral matter and entrapped gas bubbles in the bulk slag on slag foaming from agricultural wastes.

5

Chapter 2: Literature Review

CHAPTER 2 2 Literature Review

2.1

Agricultural Waste as a Source of Renewable Materials

Agricultural waste is an important contributor to the world economy. Today, various forms of agricultural wastes energy are consumed all over the world. Agricultural waste provides a clean, renewable material source that could dramatically improve the environment, economy and energy security. In developing countries (e.g. Malaysia), the use of agricultural waste is of high interest, since these countries have economy largely based on agriculture and forestry. The use of these materials will depend on the state of the art of safe economic technologies which are able to transform them into manageable products (Sensöz, S., Demiral, I. and Ferdi Gerçel, H., 2006). The use of agricultural waste as carbon/energy source is of interest due to the following benefits: (i)

Agricultural waste is a renewable, potentially sustainable and relatively environmentally benign source of energy.

(ii)

A huge array of diverse materials, frequently stereo chemically defined are available from the agricultural waste giving the user many new structural features to exploit (Bozell, J. J., 2008, Demirbas, A., 2006).

(iii) Increased use of agricultural waste would extend the lifetime of diminishing crude oil supplies. (iv) Agricultural waste fuels have negligible sulfur content and therefore, do not contribute to sulfur dioxide emissions that cause acid rain.

6


(v)

The combustion of agricultural waste produces less ash than coal combustion and the ash produced can be used as a soil additive in fields, etc.

(vi) The combustion of agricultural waste is an effective use of waste products that reduces the significant problem of waste disposal, particularly in municipal areas. (vii) Agricultural waste provides a clean, renewable energy source that could improve our environment, economy and energy securities (Annamalai, K., Priyadarsan, S., Arumugam, S. et al., 2007, Bozell, J. J., 2008) and (viii) Use of agricultural waste could be a way to prevent more carbon dioxide production in the atmosphere as it does not increase the atmospheric carbon dioxide level. Agricultural wastes are known to grow in a sustained way through the fixation and release of CO2, mitigating global warming problems. In fact, the amount of CO2 produced during the combustion of the fuels is the same amount absorbed during grown of the plants. This is particularly the case of energy crops and agricultural residues (Munir, S., Daood, S. S., Nimmo, W. et al., 2009). At present the palm oil industry generates the most agricultural waste from the oil extraction process such as the mesocarp, fibre, shell, empty fruit bunch (EFB) and palm oil mill effluent (POME). About 2.01 million tons of palm oil wastes are generated every year in Malaysian alone, and this keep increasing at 5% annually (Yang et al., 2006). Coconut shells have little or no economic value and their disposal is costly, also cause environmental problems. It is used as a reference for shell type materials in assessing the viability of other materials. Therefore, by subjecting palm shell and coconut shell to identical experimental conditions, the structure of the carbonaceous material can be compared. Agricultural wastes may vary in its physical and chemical properties due to its diverse origin and species. However, agricultural waste is structurally composed of cellulose, hemicellulose, lignin, extractives and inorganics. From the chemistry point of view, 7


agricultural waste is composed of series of long chain hydrocarbons with functional groups such as hydroxyls and carboxyl. Furthermore, it can be defined as a hydrocarbon, which consists mainly of carbon, hydrogen, oxygen and nitrogen. Some types of agricultural waste present significant proportions of inorganic species. The concentration of ashes generated for this inorganic goes 1% in softwoods until 15% in herbaceous biomass and agricultural wastes (Yaman, S., 2004). Agricultural waste is the fourth largest energy source in the world after coal, petroleum and natural gas, providing about 14% of the world’s primary energy consumption (Annamalai, K., Priyadarsan, S., Arumugam, S. et al., 2007). Agricultural waste is used to meet a variety of energy needs, including generating electricity, fuelling vehicles and providing process heat for industries (Bridgwater, A. V., 1999, Bridgwater, A. V., Meier, D. and Radlein, D., 1999). It is the only renewable source of carbon that can be converted into convenient solid, liquid and gaseous fuels through different conversion processes (Özbay, N., Pütün, A. E., Uzun, B. B. et al., 2001).

8


2.2

Environmental Impact

Atmospheric gases such as carbon dioxide, nitrous oxide and methane can regulate temperature of the earth. These greenhouse gases particularly CO2 allow energy from the sun to penetrate to the earth, but trap the heat radiated from the earth’s surface. Researchers, scientists and others are concerned about those gases being emitted to atmosphere by human activities which will increase the global warming at a rate extraordinary in human history. The CO2 emission from the usage of fossil fuels that provide about 85% of the total world demand for primary energy, cause an increase of the CO2 concentration in the atmosphere (Zanzi, R., Sjöström, K. and Björnbom, E., 1996). Emission of mainly sulphur dioxide, nitrous oxide and hydrochloric gases to atmosphere can cause acid rain. Sulphur oxides and nitrogen oxides can be transformed in the atmosphere to H2SO4 and HNO3. Sulphur oxides are produced in combustion of sulphur–bearing fuels such as petroleum and coal. Sulphur oxides emission from the utilization of biomass/agricultural waste is negligible because they contain minimal sulphur. Another acidic gaseous pollutant is hydrochloric acid (HCl) gas, produced from chlorine and mainly associated with combustion of municipal wastes. HCl also plays an important role for dioxin formation during combustion. Special attention is being paid to the nitrogen oxides emission from combustion of nitrogen such as biomass/agricultural waste, coal, peat or municipal waste. The nitrogen oxides emission from combustion of nitrogen comes from two sources, thermal nitrogen oxides and fuel nitrogen oxides. The formed from the nitrogen in the combustion air and its formation is more or less dependent on the temperature and pressure in the combustor. The latter comes from the oxidation of nitrogen in the fuel and is not particularly temperature sensitive. All the oxides of nitrogen also enhance the greenhouse effect.

9


During gasification, the nitrogen forms ammonia (NH3). Some hydrogen cyanide (HCN) and nitrogen monoxide (NO) may also be formed. During combustion of the gases, ammonia and cyanides undergo oxidation to nitrogen oxides the pyrolysis in the initial step in both gasification and combustion (Zanzi, R., Sjöström, K. and Björnbom, E., 2002).

2.3

Recycling of Agricultural Wastes in Steelmaking

Palm shell is a waste product of the palm oil industry. Currently, the material has no specific technical uses and it creates a huge disposal problem (Hussain, A., Ani, F. N. and Darus, A. N., 2006). It is an added advantage to the oil palm industry if the excess shell can be turned into useful and valuable products. It was estimated that about 1120 kg of palm shells are produced per hectare of oil palm planted area (Azri, S. M., 2008). The proximate analysis data of fixed carbon and ash content in palm shell shows that it is a suitable raw material for the steelmaking (R.C. Bansal, J.-B. D., F. Stoeckli, 1988). Coconut shell is chosen for comparison, since it is commercially used as a precursor for metallurgical industries. It is used as a reference for shell type materials in assessing the viability of other materials. Therefore, by subjecting palm shell and coconut shell to identical experimental conditions, the structure of the carbonaceous material in term of structured can be compared. In previous study, charcoal from hardwood species in small blast furnaces is being employed for iron production in Brazilian steel industry (Emmerich, F. G. and Luengo, C. A., 1994). The use of wood char in ironmaking has been extensively reviewed by Gupta, Burgess and Dell’Amico et al. (Burgess, J., 2004, Gupta, R. C., 2003, M. Dell’Amico, P. Fung, R. L. and O’Connor, J. M . a. M ., 2004). High resistivity of charcoal resulted in more efficient operation with respect to energy and electrode consumption. Low ash content charcoal (0.4 %) is derived from eucalyptus type wood, while timber waste (such as saw dust, chips and cuttings) is used to 10


improved gas permeability of the burden and prevent charge crusting and decreases electrical conductivity (Spratt, D. M. and Brosnan, J. G ., 1990). Babich et al. (Babich, A., Senk, D. and Fernandez, M., 2010) found that the combustion efficiency of all the tested charcoals is better or comparable with conventional coals. The use of palm shell charcoal for the production of good quality steel was considered by Emmerich et al. (Emmerich, F. G. and Luengo, C. A., 1996). Recently, carbon iron ore composite consisting of biomass char coated with submicron iron oxide powder and iron ore fines were proposed to improve the reduction rate in the blast furnace (Watanabe, K., Ueda, S., Inoue, R. et al., 2010).

2.4

Properties and Composition of Agricultural Waste and Metallurgical Coke

Agricultural waste contains as main elements of C, H and O. The molecular structure of palm shell based on one C atom can be written as CHxOy (CH1.61O0.51) (Lee, D. H., Yang, H., Yan, R. et al., 2007). Palm shell is a waste derived from palm oil which contains as main components saturated and unsaturated aliphatic carboxylic acids with carbon chain length in the range of C6 up to C24 such as the palmitic acid, (CH3 – (CH2)14 – COOH) (Figure 2-1).

Figure 2-1 Molecular structure of common fatty acid in palm oil plant

The chemical composition of agricultural waste is very different from that of coal, coke, etc. The presence of large amounts of oxygen in plant carbohydrate polymers means the pyrolytic chemistry differs sharply from these other fossil fuels (coal). Plant agricultural wastes are essentially a composite material constructed from oxygen-containing organic polymers. The major structural chemical components with high molar masses are 11


carbohydrate polymers and oligomers and lignin. The organic and inorganic minerals are also present in agricultural waste. The major constituents consist of cellulose (a polymer glucosan), hemicelluloses (xylan/polyose), lignin, organic extractives, and inorganic minerals.

12


2.4.1

Cellulose

Cellulose, which is shown in Figure 2-2, approximately constitutes 40% (w/w) of the mass of agricultural waste materials. Its structure is composed of d-glucose (6 carbon sugar) units (C6H10O5) bound together by ether-type linkages (glycosidic bonds). The weakest bond in the chain is the C-O glycosidic bond. The main functional group present in this compound is the hydroxyl group.

Figure 2-2 Polymers of cellulose (Graboski, M. and Bain, R., 1981)

2.4.2

Hemicellulose

Xylan, the most abundant form of hemicellulose, consists of various forms of monomers of d-xylose (5 carbon sugar units: C5H8O4). These monomers are linked together by ether-type linkages similar to those of cellulose (Antal, M. J., Jr . and Varhegyi, G., 1995, Graboski, M. and Bain, R., 1981, Shafizadeh, F., 1982). Xylan contains mainly hydroxyl (-OH), carboxyl (-COOH) and methoxyl (CH3O-) functional groups. Figure 2-3 depicts some important monomers of xylan.

13


Figure 2-3 Monomers of xylan (Graboski, M. and Bain, R., 1981)

2.4.3

Lignin

Lignin, the non-carbohydrate component of the cell wall, is composed of several monomer units, all of which possess phenylpropane-based structure. Lignin exists as an amorphous form around the cellulose fibres, and cements these fibres together. Lignin is connected to cellulose directly by ether bonds. A hypothetical structure of lignin is illustrated in Figure 2-4.

14


Figure 2-4 A hypothetical structure of lignin (Blazej, A. and Kosik, M., 1993).

From a structural point of view, lignin occupies the strongest structure due to the strength of methylene units (-C2H2-) and π bonds that constitute the aromatic ring (Antal, M. J., Jr . and Varhegyi, G., 1995, Graboski, M. and Bain, R., 1981, Shafizadeh, F., 1982). When agricultural waste becomes degraded under high temperature, lignin is considered to be most resistant to biochemical degradation and presumed to be largely intact. Lignin, hemicellulose and lignin are deemed to constitute the main chemical structure of agricultural waste materials. Therefore, the macromolecular structure of agricultural waste can be perceived to consist mainly of polymers of aliphatic (from cellulose and hemicellulose) and aromatic molecular (from lignin) incorporated with oxygencontaining functional groups.

15


Table 2-1 shows the cellulose and lignin content in the agricultural samples employed in this study. The term of holocellulose represents the total cellulose, which is composed of cellulose and hemicelluloses.

Table 2-1 Cellulose, holocellulose and lignin contents of palm shell and coconut shell (Wan Daud, W. M. A. and Wan Ali, W. S., 2004) Compositions (%) Cellulose Holocellulose Lignin

2.4.4

Palm Shell 29.7 47.7 53.4

Coconut shell 19.8 68.7 30.1

Inorganic Minerals

Tables 2-2 to 2-6 summarize the inorganic matter and chemical composition present in agricultural waste materials that used for this study as well as the results from previous literature.

16


Table 2-2 Inorganic minerals in agricultural waste by XRF analysis – literature review Ash analysis by XRF (%) Author

(Afrane, G. and (Ghani, W. M. A. W., Achaw, O.-W., Firdaus, M. S. and 2008) Loung, C. J., 2008)

Present study

SiO2

20.65

41.73

Coconut Shell 16.10

Fe2O3

31.5

17.78

46.80

20.50

Al2O3

1.1

8.46

1.30

8.90

TiO2

0.07

5.39

0.09

0.30

P2O5

1.45

2.57

1.90

4.10

Mn3O4

1.32

1.27

0.40

0.20

CaO

12.8

10.27

3.50

2.20

MgO

2.95

1.01

2.40

2.90

Na2O

10.6

4.38

7.90

1.10

K2O

16.5

5.51

16.90

4.00

SO3

1.2

1.02

2.70

1.10

Components

Coconut Shell

Palm Shell

Palm Shell 55.70

17


Table 2-3 Ultimate analysis of palm shell wastes material – literature review Author Present study (Hussain, A., 2006) (Mahlia, T. M. I., Abdulmuin, M. Z., Alamsyah, T. M. I. et al., 2001) (Emmerich, F. G. and Luengo, C. A., 1996) (Harimi, M., Megat Ahmad, M. M. H., Sapuan, S. M. et al., 2005) (Sumathi, S., Bhatia, S., Lee, K. T. et al., 2009)

Carbon (wt. %) 60.8

Hydrogen (wt. %) 5.74

Nitrogen (wt. %) 0.4

Sulphur (wt. %) 0.04

Oxygen (wt. %) 32.8

47.6

6.2

0.7

0

43.3

55.4

6.3

0.6

0.2

37.3

54.6

6.0

0.3

0.0

39.0

43.8

5.2

0.5

0.17

49.1

50.1

6.8

1.88

0.0

41.1

Table 2-4 Proximate analysis of palm shell wastes material – literature review Volatile matter (wt. %)

Fixed Carbon (wt. %)

Moisture (wt. %)

Ash (wt. %)

Present study

71.3

18.1

6.6

4.0

(Hussain, A., 2006)

77.4

19.3

6.8

3.3

77.6

19.8

6.0

2.6

65.8

28.3

4.8

0.9

73.6

18.2

18.2

0.9

72.4

18.4

7.9

1.1

Author

(Guo, J., Luo, Y., Lua, A. C. et al., 2007) (Lua, A. C., Lau, F. Y. and Guo, J., 2006) (Sumathi, S., Bhatia, S., Lee, K. T. et al., 2009) (Wan Daud, W. M. A. and Wan Ali, W. S., 2004)

18


Table 2-5 Ultimate analysis of coconut shell wastes material – literature review Carbon (wt. %) 58.60

Hydrogen (wt. %) 5.70

Nitrogen (wt. %) 0.20

Sulphur (wt. %) 0.03

Oxygen (wt. %) 35.40

(Bandyopadhyay, S., Chowdhury, R. and Biswas, G. K., 1999)

51.00

6.00

0.40

0.01

41.80

(Wan Daud, W. M. A. and Wan Ali, W. S., 2004)

48.63

6.51

0.10

0.08

44.60

(Tsamba, A. J., Yang, W. and Blasiak, W., 2006)

53.90

5.70

0.10

0.02

39.40

(Mohd Din, A. T., Hameed, B. H. and Ahmad, A. L., 2009)

48.00

7.60

0.10

0.55

43.50

Author Present study

Table 2-6 Proximate analysis of coconut shell wastes material – literature review Volatile matter (wt. %) 77.80

Fixed Carbon (wt. %) 15.10

69.11

(Wan Daud, W. M. A. and Wan Ali, W. S., 2004)

Moisture (wt. %)

Ash (wt. %)

6.00

1.10

22.45

6.00

2.44

73.09

18.60

8.21

0.10

(Tsamba, A. J., Yang, W. and Blasiak, W., 2006)

74.90

18.40

6.00

0.70

(Mohd Din, A. T., Hameed, B. H. and Ahmad, A. L., 2009)

72.29

21.02

6.11

0.58

Author Present study (Bandyopadhyay, S., Chowdhury, R. and Biswas, G. K., 1999)

19


2.4.5

Metallurgical Coke

Metallurgical coke is a macro porous carbon material of high strength produced by carbonization of coals of specific rank or of coal blends at temperatures up to 1400 K (Díez, M. A., Alvarez, R. and Barriocanal, C., 2002). The metallurgical coke consist a good strength and a fixed carbon content of 71 - 90 wt%, with volatiles ranging between 1 - 5 wt%, and 0.2 - 1.5 wt% sulphur content. Moisture content is a direct consequence of the coke-quenching process with some dependence on size. Coke is primarily used as a fuel and reductant in electric arc furnaces (Zaharia, M., 2010); however, it also functions as a support for other raw materials in the blast furnace. This material has been widely used in the applications for electric furnaces in production of steel, ferro alloys, graphite and refractories. Coke is also added to steel ladles to re-carburize the melt after discharge from the steelmaking furnace.

2.4.6

Activated Carbon

Activated carbon can be prepared from a large number of materials. These materials are usually high in carbon and volatile contents but low in inorganic contents. Some of the most common precursors for activated carbons are coal, palm shell, coconut shell, lignite and wood. For materials like coal, studies have shown that increasing the temperature initially increased the surface area and porosity and it reached its maximum between 700 – 800 °C (Wan Daud, W. M . A. and Wan Ali, W. S ., 2004). Similar trends was found in lignin, where the effect of high temperature on char porosity and surface area. For palm shells and coconut shells, the surface area and porosity showed an increase with increase temperature. (Lua, A. C. et al., 2006).

20


2.5

Gas Phase Reactions

Introduction The processes during heating and combustion of coal/coke are illustrated in Figure 2-5, and they are similar for biomass/agricultural waste except for high volatile matter. The process of release of gases from solid fuels in the absence of oxygen is called pyrolysis, while the combined process of pyrolysis and partial oxidation of fuel in the presence of oxygen is known as gasification. If all combustible gases and solid carbon are oxidized to CO2 and H2O, the process is known as combustion.

Homoge nous reactions

Moisture Vapo ur

Volatile Matter

Off gas

Coal/coke Biomass/ agricultural waste

Char & Ash

(Pyrolysis, coal/coke 400 °C, Biomass/ Agricultural waste 200 °C)

Heterogeneous char reactions (Char combustion)

Ash

Figure 2-5 Process during coal/agricultural wastes pyrolysis, gasification and combustion

21


2.5.1

Pyrolysis

Pyrolysis offers a simple way of processing all of the complex polymeric structure found in agricultural waste lignocellulosic residue materials. It can offer high yields of liquid product that retains the elements needed for chemical analysis and products. Pyrolysis is the thermal decomposition process under inert atmosphere through which a biomass/agricultural waste decomposes to various products like solid char, liquid tar and gases, some of which have high calorific values. The operating conditions and the agricultural waste pyrolysis units can maximize the yields of tars and condensable products or char (Antal, M. J., Mochidzuki, K. and Paredes, L. S., 2003, Bridgwater, A. V., 1999, Bridgwater, A. V., 2003). In the former case, the conversion process is indicated as fast pyrolysis (Bridgwater, A. V., 2003) and, after cooling and condensation, bio-oil is obtained. Fast pyrolysis requires high heating and heat transfer rates in the reaction zone, a primary conversion temperature of about 800 K, and short residence time of products in the vapour phase (below 2 s at about 700 K) with a rapid cooling of the vapour-phase products to limit the extent of secondary reactions. Conversely, the conventional or slow pyrolysis, process produces comparable yields of char, gas, and tar species. It is important to describe the evolution of gas and tar volatile products as a function of the process conditions. Yields of CO and H2 are the desired products of the gasification process and are favoured by these secondary pyrolysis reactions of volatile species. Most of the pyrolysis characteristics reported in literature are for woody materials. A few attempts have been made at correlating the pyrolysis characteristics of biomass with those of its constituents, viz. cellulose, hemicellulose, lignin and extractives (Antal, M. J., Jr. and Varhegyi, G., 1995). Pyrolysis of palm shell waste resulted in biomass chemicals such as methyl derivatives, acetic acid and most certainly phenols derivatives (Islam, M. N., Zailani, R. and Ani, F. N., 1999). Moreover, Yaman (Yaman, S., 2004) presents about two hundreds articles in biomass research studies and none of them is a coconut shells or palm shells. It can conclude that there are a limited number of researchers that have dedicated their investigation to these agricultural wastes. 22


Raveendran et al. (Raveendran, K., Ganesh, A. and Khilar, K. C., 1995) studied about 13 different types of biomass samples in which coconut shell is included. However, not too much attention was give particularly to this material. Thus, in this study, palm shell and coconut shell pyrolysis are investigated using thermogravimetry with the endeavor of characterizing their thermal decomposition process, particularly, pyrolysis profiles and gas revolutions. Figure 2-6 present summaries of reaction mechanisms of pyrolysis/steam gasification of cellulose and lignin, respectively by TGA-MS in the literature. Lignin was decomposed in the temperature range of 550-773 K, yielding 60 wt % of nascent char. Evolution of CO2 peaked at 673 K. A significant increase in H2 evolution was observed above 773 K; it reached a peak at 873 K. However, no pronounced evolution of CO and CO2 was observed in this temperature range. These results imply that aromatization and carbonization of the lignin-nascent char proceed to yield H2 and char. Evolution of CO2 and CO exhibited a weak peak at 973 K in accordance with cellulose pyrolysis. Above 823 K, H2 evolution increased drastically. Subsequently, a steep rise in CO2 and CO evolution was observed. These suggest that the decomposition of lignocellulosic structure from agricultural waste/biomass will released more gases at high temperature to participate for subsequent carbon/slag interactions in this present study.

23


Figure 2-6 Reaction mechanism of pyrolysis and steam gasification (dotted line represents the reaction with steam): (a) cellulose; (b) lignin

24


2.5.2

Heterogeneous Char Combustion

At high temperatures, solid fuel particles evolve the volatiles that burn in diffusion envelope flames. For most solid fuels, a heterogeneous char combustion phase follows. Heterogeneous char reaction is one of the main controlling steps in the combustion process of carbonaceous materials which affects the heat released in the combustion system. Laurendeau (Laurendeau, N. M., 1978) and later on Smith (Smith, I. W., 1982) and Smith and Smooth (Smith, L. H., and, S. L. D. and Fletcher, 1994) have focused on the kinetic measurements of various carbonaceous materials. However, due to the complexity of these processes, none of the previously developed methods are completely reliable without further experimental measurements. 1.

Two stages are encountered in the combustion of a carbon based material (Figure 2-5): Sample pyrolysis during the volatile matter is removed, forming a solid residue with certain physical and chemical structure, i.e. char Carbon based material → Char + Volatiles

2.

A heterogeneous reaction of the residual char or char combustion C + O2 = CO2 C + ½ O2 = CO

Eq. 2-1 Eq. 2-2

The pyrolysis reaction is much faster relative to the heterogeneous char combustion, which spreads over a longer period of time and is not the rate limiting step for the whole combustion process. Nevertheless, it determines the amount and the structure of char generated.

25


2.5.3

Kinetic Reactions Regimes

Combustion occurs, as a heterogeneous gas solid reaction generally does, in three different regimes controlled by the interplay of transport processes and chemical reactions (Walker, P. L. J., S helef, M. and Anderson, R. A., 1968). Three different situations may occur that are commonly referred to as regime I (or zone I), regime II, and regime III. In regime I, the overall burning rate is controlled by the chemical heterogeneous reaction between O2 and the carbon of the char particle. At low temperature zone, O2 will fully penetrate the porous char so that reactivity increases with total internal surface area. The chemical kinetics is extremely low, therefore the O2 diffuses into the interior of the porous char faster than it is consumed, providing high diffusion rate. The char burns across the whole particle; the particle size (dp) remains constant with burn-off, while the particle density (σp) decreases. The reaction rate of the particle, ρm (the reaction rate per unit remaining carbon mass), can be expresses as:

ρ m = ki ⋅ Ai ⋅ POm 2

Eq. 2-3

Where: ki, and Ai are reactivity constant and specific total surface area of the residue, respectively, while POm2 represent the oxygen partial pressure as a function of m reaction order. In regime II (intermediate temperature), the rate is determined by the chemical reaction as well as by internal diffusion of O2 in the char pores. The reactivity of the char will depend mainly on internal burning and the rate gaseous diffusion through the char wall thickness, total porosity and pore size distribution of the char. During oxidation, large pores increase in size and the material between the large pores tends to shrink. Although the small pores within the granules are associated with a larger surface area for reaction, O2 penetration into these pores is far less extensive than into the large pores. This limiting condition is referred to as regime II. Smith and Hurt (Smith, L. H., and, S. L. D. and Fletcher, 1994) derived the reaction rate and burnout time for a char particle in zone II, such that:

26


[

]

ρ a = 2 ⋅ 2σ p ⋅ ki ⋅ Ai ⋅ De ⋅ POm +1 ⋅ (1 − x )m +1 / (m + 1) 2

0.5

Eq. 2-4

Where: De is effective pore diffusivity and x is the actual reaction rate to the maximum possible reaction rate. As the temperature is further increased, reactivity may be strongly controlled by diffusion of O2 through the boundary layer to the particle surface. The rate of diffusion of the reactant gas toward a particle is determined by its external dimensions, so reactivity may be influenced by particle size of the carbonaceous material. Finally, in regime III, external diffusion of O2 from the bulk phase to the particle surface controls the burning rate. The chemical reactivity of the char will not influence the reaction rate. Nevertheless, for the Ist and IInd regime, where the char heterogeneous reaction is controlled by either the chemical reaction of the combination of char chemical reaction and pore diffusion, the char chemical structure and physical structure may have important implications for char reactivity. Many combustion studies indicate that, by using a reactor that entrains the carbon particles in the gas stream at relatively high temperatures, leads to a reaction zone (kinetic regime II) or in the transition region between zone I and II (Hurt, R. H., 1998, Shim, H.-S., Hurt, R. H. and Yang, N. Y. C., 2000, Smith, L. H., and, S. L. D. and Fletcher, 1994).

2.5.4

Inorganic Effects in Combustion

The combustion or co-combustion of coal/coke and biomass/agricultural waste involves devolatilisation of the solid fuel particle and the gas phase combustion of the volatiles, followed by the burning of the resultant char in an atmosphere, the temperature and oxygen concentration of which are large controlled by the combustion of the volatiles. The reactivities of the coal char vary from one char to another because of differences in the char surface area which determines both the available surface area and the number of active sites, and differences in the content and nature of the inorganics that can act as 27


a catalyst or inhibitor. In the case of agricultural waste, the carbon structures can vary particularly due to the incorporation of O-atoms which has a disruptive effect on the char structure. As the original char particle burns the char structure changes and ultimately becomes annealed. This reduces the reactivity of the remaining char that can result in a greater tendency to form unburned carbon. In addition, the ash content increases and this can have an inhibiting effect on the char reactivity. As the temperature reduces towards the end of the combustion chamber the reaction regime changes from zone II to I. At these reduced reaction temperatures catalytic effects due to metals in the ash can become more influential, and the presence or indeed absence of catalytic reactions can play a significant role. The effects of ash inorganic species are still uncertain. Species such as iron and calcium are naturally present in coal often with no effect on the combustion reaction and yet metals are sometimes added as a combustion improver, and the controlling factors are not well understood. Agricultural wastes chars are generally more disordered compared to coal chars and have significant oxygen content. These, and the easy interaction with metal catalysts change the reactivity of the biomass char very significantly (Backreedy, R. I., Jones, J. M., Pourkashanian, M. et al., 2003).

2.5.4.1 Effects Due to the Inorganic Ash Compounds There are two major features that are important in coke and agricultural waste char combustion and these are the formation of ash and catalyst effects.

•

Ash formation

Ash is formed during combustion and the fate of this ash is controlled by the temperature in the furnace and the ash composition, i.e. whether it is molten or not. Several approaches have been taken to model this effect and this can be considered to 28


have one of two roles. In the first, the ash is considered to form a protective layer (Hampartsoumian, E., Murdoch, P. L., Pourkashanian, M. et al., 1993, Zolin, A., Jensen, A., Jensen, P. A. et al., 2001) around the burning char particle and as reaction proceeds this inhibits combustion. The second approach is to consider that the ash is thrown off the char particle leaving only ash within the surface layer but which still has a blocking effect, that is, only a fraction of the surface is exposed. Application is therefore still difficult because of the uncertainties in applying these effects which can cause a disproportionate amount of inhibition for small char particles.

•

Effects of catalysts

Catalysts may influence both devolatilisation and char burnout, but the former is only of significance in the devolatilisation of biomass (Chen, Y., Charpenay, S., Jensen, A. et al., 1998). It is well known that metals will catalyse the oxidation of carbon at temperatures of about 500 ºC. However, there is some difficulty in determining the influence of catalysts at furnace conditions. Metals may occur in coal combustion either distributed within the char matrix or as separate clusters of metal oxides or in the case of volatile metals as condensed vapour. Three classes of catalysis may be identified as follows:



Class 1. Distributed metals:

Only a small part of the mineral matter is present as organometallic-type compounds and can result in a distributed form of metals along the reaction interface. Few studies have been made of such but the evidence from studies (Jones, J. M ., Agnew, J., Kennedy, J. et al., 1997) involving Fe and V show that the effect is to not to change the activation energy of char oxidation but to change the pre-exponential factor, that is there is an acceleration of the reaction. Backreedy et al. (Backreedy, R. I., Jones, J. M ., Pourkashanian, M. et al., 2002) have put forward a mechanism for this process which is based on the formation of a C–metal bond as shown in Figure 2-7 that outlines the 29


oxidation mechanism. The effect is twofold, a weakening effect on the adjacent C–C bond and a role as an oxygen donor. In the case of alkali metals a different mechanism is likely to hold. In this mechanism (Jones, J. M., Darvell, L. I., Pourkashanian, M. et al., 2005) a C–O–M bond is formed (where M can be Na, K or Ca) as shown in Figure 2-8 and this could be particularly important in the case of oxygenated biomass compounds. The effect here is the weakening of the adjacent C–C bond and hence catalysing the reaction.

 Class 2. Clusters of metal atoms formed by condensation: This class of catalysts effectively operates by being released into the gas phase during devolatilisation, and typical metals would be potassium and sodium. The metal can deposit in two ways. The first is in the form of clusters and the second is via attachment through a C–O– bond that can also act as the nucleus for a cluster. There is evidence that in the case of clusters the activation energy for reaction is lower (Baker, R. T. K., Dumesic, J. A . and Chludzinski, J. J., 1986 ) and this mechanism is the preferential reaction at low temperatures.

 Class 3. Ash clusters: The presence of ash clusters does not seem to have much influence on the reaction rate because the degree of contact with carbon at the interface is small. The effects of these catalysts are subsumed in the values of alpha here but it would be desirable to separate the influence of the terms into pure char combustion and the catalytic effect.

30


Figure 2-7 Proposed catalytic mechanism for the reaction of iron atoms in the char structure (Backreedy, R. I., Jones, J. M., Pourkashanian, M. et al., 2003)

Figure 2-8 Proposed catalytic mechanism for the reaction of alkali metals (Huang, H. Y. and Yang, R. T., 1999)

31


2.5.5

Determination of the Combustion Efficiency of Coke and its Blends with Agricultural Waste

2.5.5.1 Factors Affecting Gas Phase Reactions The processes involved in gas phase reactions, such as pyrolysis and combustion are very complex. Depending on the furnace type, carbonaceous material composition, particle size, the relative rates of heating, decomposition and oxygen transfer, the gas phase reactions may occur in separate stages or simultaneously.  Furnace type During high temperature gas phase reactions, physical and chemical changes appear. Research on combustion performance of different carbon based material has been reported widely in the literature. Drop tube furnace (DTF), which is an entrained flow reactor, thermogravimetric analyser (TGA) and fixed bed reactor (FB), are the type of furnaces, used in gas phase reaction studies. At low temperature, where the reaction occurs in the kinetic regime I, TGA and FB are the most commonly used reactors. At high temperature, where the reaction occurs in the kinetic controlled regime II and III, the heterogeneous char reactions can finish in a very short time. Neither TGA nor FB has the ability of withdrawing the particle so fast, thus the flow particle technique, in which a stream of carbonaceous particle is entrained into a gas atmosphere is more suitable for this purpose. The drop tube furnace has been widely considered by many authors (El-Samed, A. K. A., Hampartsoumian, E., Farag, T. M. et al., 1990, Kim, B. C. and Gupta, S., 2007, Liddy, J. P ., Newey, D. C. and Wilson, T., 1987, Wells, W. F ., Kramer, S. K., Smoot, L. D. et al., 1985).

32


 Heating rate Whilst the combustion process occurs very rapidly, of great importance is the heating rate of the carbonaceous particle. Such a parameter determines the nature of the devolatilization products, including the resulting char. Higher heating rates result in greater amount of volatiles released and thus more changes in the porous structure and the sizes of the remaining char particles (Kimber, G. M. and Gray, M. D., 1967).

 Residence Time and Temperature Residence time is a very important parameter in char preparation, since char structure is very sensitive to the residence time, especially in the earlier stages of pyrolysis. When the flow particle technique is employed, such as DTF, the residence time can be adjusted by changing the position of the collecting probe. Preliminary measurements have been performed previously (Solomon, P. R., Serio, M. A., Carangelo, R. M. et al., 1986) by recording the signals from transistors installed at both ends of the working tube. Fletcher et al. (Fletcher, T. H., Kerstein, A. R., Pugmire, R. J. et al., 1990) and Pugmire et al. (Van Niekerk, D., Pugmire, R. J., S olum, M. S. et al., 2008) have investigated the structural evolution of chars of two American coals, considering their residence time. It was discovered that a higher residence time leads to a more ordered graphitic like carbonaceous structure. During combustion, chars obtained at higher temperatures give carbonaceous matrices of higher surface area. This can be related to the fact that chars obtained at higher temperature have almost totally expelled their total volatile matter, thus forming matrice with larger surface areas which are hence more reactive. Van Niekerk et al. (Van Niekerk, D., Pugmire, R. J., S olum, M. S. et al., 2008) considered a few American coals and applied different temperatures and residence times. They discovered using NMR technique, an increase in the number of carbons per aromatic cluster with increasing temperature and residence times. Arrenillas et al. 33


(Arenillas, A., Rubiera, F., Arias, B. et al., 2004) studied the relationship between structure and reactivity of different carbonaceous materials as a function of temperature. During the first stages of combustion, as a result of higher temperature, the crystallites or the so-called turbo static structure developed gradually. An increase in the heat treatment temperature causes a substantial decrease in BET surface area accompanied by a loss in active sites. This study agreed well with the work done by Liming Lu et al. (Lu, L., Sahajwalla, V. and Harris, D., 2000), which observed an increase in the size of the graphitic planes with increasing treatment temperature. Hecker & McDonald et al. (McDonald, K. M., Hyde, W. D. and Hecker, W. C., 1992) prepared in a reactive methane flame at a maximum particle temperature of 1700 K, a series of chars derived from Diets sub-bituminous coal. An increase in CO2 and N2 surface area with increasing residence time was seen.

 Effect of the carbonaceous material composition Every carbonaceous material is characterized and ranked on the basis of its chemical composition, such as volatile matter, ash and carbon content. The carbon content determines the calorific value of the fuel as well as the temperature of the flame. Generally speaking, the gross properties increase with increasing carbon content. The volatile matter content determines the flame stability. It was established that with decreasing volatile matter, the ignition of the carbonaceous materials delayed and the combustion is slow. As the residence time in DTF is of the order of 1 to 2 s, it may be expected that the carbon based material with poor combustion performance will pass through the furnace in a partly unburned state. Volatile present in the composition of the carbonaceous material leads to the formation of pores in the residue when subjected to a high temperature process. Depending on the total amount of volatiles in the carbonaceous matrix a certain number of pores of different dimensions are formed. Depending on their sizes the pores within a carbonaceous particle may be classified into three categories; namely, micropore (50 nm) (Everett, D. H. and Haynes, J. M., 1972). A pore tree model was initially proposed by Simons and Finson (Simons, G. A. and Finson, M. L., 1979) which tries to explain the change in pore distribution by considering phenomena such as pore combination and pore growing due to chemical reaction. With increasing number and distribution of pores, its surface area increases, allowing the oxygen to penetrate the carbonaceous matrix and deplete it. The surface area developed by micro and mesopores is the one that accounts for increased combustion performance. However, a strong evolution of volatile has an endothermic effect and might deplete the dense phase of oxygen and cause hot spots (Ogada, T. and Werther, J., 1996) and possible particle fragmentation.

35


2.6

High Temperature Carbon/Slag Interactions

Introduction The interfacial phenomena between the slag and carbon are expected to play a major role in slag foaming during EAF steelmaking because they could dictate the kinetics of reduction reactions. EAF steelmaking slags mainly contain iron oxide, silica, alumina, lime, manganese and magnesia. In the temperature range of interest, 1500 °C to 1700 °C, iron oxide, manganese and silica are the main reducible oxides present in the slag. In addition, the ash impurities present in the char also contain iron oxide and silica to varying degrees and could participate in reduction reactions. Slag foaming is achieved by injecting oxygen and carbonaceous materials (carbon/slag interactions) into the molten metal bath and the slag respectively. Some oxygen is required for oxidizing impurities and to reduce the carbon content while the simultaneous injection of carbonaceous materials like inject coke, petroleum coke, natural graphite, synthetic graphite, char etc (slag foaming injectant – different carbonaceous materials) and oxygen into the slag layer produces the most effective foamy slag. By understand the carbon/slag interactions is of great importance due to their extensive application in a number of metallurgical processes, such as steelmaking, blast furnace ironmaking and direct reduction ironmaking processes (Hara, S. and Ogino, K., 1992, Jiang, R. and Fruehan, R., 1991, Katayama, H., 1992, Ogawa, Y., 1992). It is widely accepted by the industries involved in high temperature materials processing, that physical property data are extremely valuable in designing new processes and improving process control and product quality. A key carbon/slag interaction is slag foaming which involves the entrapment of gas bubbles in the slag layer when carbon based materials interact with O2 in the steel melt. This gas foam the slag from its normal thickness to a higher volume depending on different parameters such as: carbon material employed, amount of oxygen injected,

36


FeO content in the slag, temperature, etc. CO gas bubbles are generated according to the reaction between solid carbon and the FeO present in the slag: FeO + C = Fe + CO

Eq. 2-5

This reaction is endothermic and can lower the temperature of the slag; meanwhile, the carbon entering the bath will react with oxygen as follows: C + O = CO

Eq. 2-6

The CO produced will foam the slag and react in part with the available FeO, resulting in the reaction: CO + FeO = Fe + CO2

Eq. 2-7

Benefits of the slag foaming practice have been widely reported in the literature (Bisio, G., Rubatto, G. and Martini, R., 2000, Wang J and B, Z., 2007). Mostly, adequate slag foaming occurs at the beginning of refining but decreases towards the end of the heat. From a metallurgical point of view, slag foaming is a quite complicated phenomena and it can be difficult to establish as well as control. Operators commonly estimate the foaming behavior during operation by visually inspecting the slag and by listening to the sound of the EAF process. In practice, the steelmaker' aim is to control the foaming behavior such that, the right amount of foam is created at the right time and is maintained for the desired amount of time (Pretorius, E. B. and Carlisle, R. C., 1999).

2.6.1

Experimental Techniques for Carbon/Slag Interactions

Previously, a number of different experimental techniques have been used to study the entrapment of gases resulting from carbon/slag interactions. Ito and Fruehan (Ito, K. and Fruehan, R., 1989) used a tall alumina crucible where a foamy slag was placed and the surface position of the slag was detected with a stainless steel electric probe while Kitamura and Okohira (Kitamura, S. and Okohira, K., 1992) measured the slag 37


height by the slag adhesion length on a steel rod inserted in the crucible at 60 sec intervals. Kapilashrami (Kapilashrami, A. and Görnerup, M., 2006) used X-ray radiographic images and measured the foam height visually. Khanna et al. (Khanna, R., Spink, J. and Sahajwalla, V., 2007, Rahman, M., 2006) used the sessile drop arrangement and a novel video processing software to measure the volume changes in the foamy slag. Dynamic changes in volume and contact area of the foamy slag were determined from the captured images with the help of especially designed software detailed subsequently. A schematic diagram illustrates every step involved in the novel technique, developed with the aim to quantify the slag volume changes during carbon/slag interactions (see Figure 2-9). Detailed image analysis techniques are available in the literature for the accurate determination of contact angles and surface tension from the shape of sessile drops; Maze and Burnet (Maze, C. and Burnet, G., 1971) have developed a numerical algorithm using a nonlinear regression analysis for computing droplet profile from a number of arbitrarily selected coordinates on the experimentally measured sessile drop. However, in a reactive system, a detailed mathematical drop-shape analysis will be extremely difficult due to continuous/sporadic evolution of gases and dynamic changes in the droplet shape and size. As the droplet is not able to reach an equilibrium configuration, fitting a mathematically computed profile to the observed droplet through detailed image analysis is not likely to yield valuable/ reliable information. As the primary focus of this work was on a quantitative estimation of rapidly changing droplet volume and slag foaming, the slag droplet on the substrate was assumed to have a truncated spherical shape. A very large number (>1000) of slag droplets were analyzed to determine slag foaming a a function of time. Minor deviations from the spherical outline were not explicitly taken into account and were neglected (estimated error - 1000 ºC) attributed to the hemicellulose and lignin structure present in coconuts shells. H2 and CH4 might be attributed to the higher content of aromatic ring and O–CH3 functional groups in the origin lignin sample, as the H2 from organics pyrolysis mainly came from the cracking and deformation of C=C and C–H while CH4 was mainly brought by the cracking of methoxyl. Cellulose resulted in the highest CO release, due to the higher carbonyl content in it while hemicellulose produced high CO2 because of

151

Chapter 5: Combustion & Structural Transformations of Coke/Coconut Shell Blends

the higher carboxyl content (Haiping, Y., Yan, R., Chen, H. et al., 2007). The release of C3H4 and C2H6 was generally very low (Figure 5-1 (c) and (f)).

152


Coconut Shells 100

3.5

Hydrogen

(d)

(a) 3.0

Mass (%)

-11

Ion Current, x 10 (A)

80 60 40 20

H2 2.5 2.0 1.5

0 0

200

400

600

1.0

800 1000 1200 1400

0

200

400

600

800

1000 1200 1400

Temp (ºC)

Temp. (°C) 1.6

2.5

-10

-11


CO/ CO2 Ion Current, x 10 (A)

(b)

3.0

2.0 1.5 CO 1.0 0.5

CO2

200

400

600

800

Wate r

1.2 1.0 0.8 H2 O

0.6 0.4

0.0

0

(e)

1.4

0.2

1000 1200 1400

0

200

400

(c)

1.0

-10


Hydrocarbons

-11


1.2

8.0 6.0 CH4

4.0 C2 H6

2.0

600

800

1000 1200 1400

Temp (ºC)

Temp (ºC)

CH3

0.0

(f)

2.3

Hydrocarbons

2.2 2.1 2.0 1.9

C3 H4

1.8 1.7

0

200

400

600

800 1000 1200 1400

0

200

Temp (ºC)

400

600

800

1000 1200 1400

Temp (ºC)

Figure 5-1 (a) – (f) The mass loss of coconut shells with on-line MS analysis of the gas products during pyrolysis

153


The pyrolysis characteristics for coconut char is shown in Figure 5-2 (a) – (f). The decomposition took place slowly under the whole temperature range from ambient to 1450 ºC. Two peaks of CO release (450 and 600 ºC) were shown at lower temperature (600 ºC) (Figure 5-2 (b)). The release of CO was attributed of carbonyl (C–O–C) and carboxyl (C=O) at low temperature and the second was attributed to the thermal cracking of tar (Yang, H., Yan, R., Chen, H., Zheng, C. et al., 2006). CO2 was released at 200 ºC and 1000 ºC. CO2 release was mainly caused by the primary pyrolysis, while secondary pyrolysis was the main source for release of CO and CH4. H2 gas was released at a higher temperature (600 ºC) (Figure 5-2 (d)).

154


Coconut Char 100

4.5

Hydrogen

(d)

(a) 4.0

Ion Current, x 10

Mass (%)

-11

(A)

80 60 40 20

3.5 H2 3.0 2.5

0

2.0

0

200

400

600

0

800 1000 1200 1400

200

400

(b)

(A) -10

Ion Current, x 10

(A) -12

Ion Current, x 10

4.0 CO 3.0 2.0 CO2

(e)

1.3

Wate r

1.2 1.1 1.0 H2 O

0.9 0.8

0.0

0

200

400

600

0.7

800 1000 1200 1400

0

200

400

(c)

Hydrocarbons CH4

4.0 3.0 2.0

CH3

1.0

C2 H6

(f)

2.0 -8

5.0


(A)

6.0

600

800

1000 1200 1400

Temp (ºC)

Temp (ºC)

-12

1000 1200 1400

1.4

CO/ CO2

1.0

Ion Current, x 10

800

Temp (ºC)

Temp. (°C)

5.0

600

Hydrocarbons

1.8 1.7 1.6 1.5 C3 H4

1.4 1.3

0.0

1.2

0

200

400

600

800 1000 1200 1400

0

200

400

600

800

1000 1200 1400

Temp (ºC)

Temp (ºC)

Figure 5-2 (a) – (f) The mass loss of coconut char with on-line MS analysis of the gas products during pyrolysis

155


5.1.2

Effect on Blending on Combustion

The thermal behavior of coke and its blends with coconut shell blends during pyrolysis in N2 atmosphere, under isothermal conditions was studied and is shown in Figure 5-3. The pyrolysis reaction is much faster relative to heterogeneous char combustion, which is spread over a longer period of time. The total weight loss of MC and its blends with coconut shell during pyrolysis occurring in less than 50 seconds. Under thermal effect, the maximum weight loss observed in the TGA test was in order as proximate analysis, (being 2.1 % MC, 8.4 % for C1 blends , 10.5 % total weight loss for C2 blends and 11.1 % weight loss for C3 blends as can be seen in Figure 5-3. Residual mass decreased with increasing coconut shell content in the blend. This trend is due to the high volatile content of the coconut shells compared to coke alone. The difference is attributed to the differences in the strength of the molecular structure of the material. The lignocellulosic bond strength from coconut shells are relatively weaker and less heat resistance compared to coke structure (Sadhukhan, A. K., Gupta, P. and Saha, R. K., 2009, Smith, L. H., and, S. L. D. and Fletcher, 1994, Ulloa, C. A., Gordon, A. L. and García, X. A., 2009, Vuthaluru, H. B., 2004). Hence, the mass loss is lower (higher residual mass) compared to the pyrolysis of blends containing coconut shell proportions. Nitrogen atmosphere and high temperatures leads to weight loss derived mainly from volatile release, whereas combustion conditions leads to oxidation of volatile matter and residual carbon as shown in Figures 5-3 and 5-4. Different behaviors are exhibited by coke and their corresponding coconut shell blends, with larger fractions of coconut shell released as volatiles during the combustion process. This high volatile yield was seen to occur over a relatively short time and is believed to influence the time required for complete combustion when compared to coke alone (Yoshizawa, N., Maruyama, K., Yamada, Y. et al., 2000).

156


100.0 MC

Residual Mass, %

95.0 90.0

C1

85.0

C2 C3

80.0 75.0 70.0

0

50

100 Time, sec

150

200

Figure 5-3 Weight loss curves of MC and its blends with coconut shells at temperature, 1200 ºC; N2 atmosphere

Total Weight Loss, %

25.0 20.0 15.0 C3 C2

10.0 C1 5.0 0.0

MC Coconut shells content in the blend

Figure 5-4 Comparison of total weight loss curve (50 seconds) of MC and its blends with coconut shells at temperature, 1200 ºC; N2 atmosphere

157


In order to quantify the combustion performances, the burnout of the samples has been calculated on the basis of ash conversion. Figure 5-5 exhibits the extent of burnout in the drop tube furnace of the coke and its blends in different proportions with coconut shells at a fixed temperature of 1200 ºC and 80% N2; 20% O2 concentration. Coke burnout values were 10 % while 100 % coconut shells showed burnout values of 15.1 %. It can be seen that coconut shell blends (C1 to C3 blends) showed higher burnout values compared to the parent coke. A small increase is observed with increasing coconut shell content in the blend as expected on the basis of their volatile matter content (C1 = 20.7 %; C2 = 21.1 % and C3 = 22.2 %).

30.0

Burnout, %

25.0 20.0 C1

C2

C3

15.0 Raw coconut shells 10.0 MC 5.0 0.0 Coconut shells content in the blend

Figure 5-5 Effect of blending MC with varying coconut shell content in the blends on the combustion at 1200 °C in the presence of 80% N2; 20 % O2

158


The amount of volatile matter present in the carbonaceous material blends are known to influence the burnout values to a certain extent (Sahajwalla, V., Zaharia, M., Kongkarat, S. et al., 2009). Proximate analyses of the coconut shell/coke blends indicate an increase in VM with increasing coconut shell content in the blend (Figure 56 (a)). An almost linear increase in burn-out is observed with increasing VM for the three blends (Figure 5-6 (b)). Higher combustion efficiency of coconut shell/coke blends is expected to be significantly influenced by the quantity and nature of volatiles released during gas phase reactions. However, for 100% coconut shells with higher VM showed a low value of burnout. This is due to low of fixed carbon content in coconut shells. Thus, by blending with MC which is high fixed carbon content, the blending is expected to improve the burnout.

159


100.0 Volatile Matter, %

(a) 77.8%

80.0 60.0 40.0 27.9% 19.4%

20.0

12.2%

6.1% 0.0

C1

MC

C2

C3 Coconut Shell

30.0 (b) 25.0 Burnout, %

C1

C2

C3

20.0 15.0 Coconut shells 10.0 MC 5.0 0.0 0

20

40 60 80 Volatile Matter, %

100

Figure 5-6 Effect of volatile matter on (a) blend composition and on (b) the combustion performance of coconut shell/MC blends

160


The changes in the original structure of the matrix of the coconut shells material is the result of the cross-linking of the reactive points brought about by the disruption of the original polymeric structure of the precursor material (i.e. the cellulosic and lignin units of the raw material) and the subsequent reconstitution of a new matrix structure during gas phase reactions that improve the combustion performance. These will be discussed in next section 5.1.3.2. Moreover, agricultural wastes (coconut shells) differ from coke in many important ways, including the organic, inorganic, energy content, and physical properties. Relative to coke, agricultural waste materials generally has less carbon, more oxygen, more silica, sodium, phosphorus and potassium, lower heating value, higher moisture content, and lower density (Demirbas, A., 2006). The influence of inorganic matter will be discussed in sub-section 5.1.4. Table 5-1 showed the amounts of the inorganic species present in the fully-combusted MC/agricultural waste blends obtained from the measurement. Table 5-1 shows that the amount of oxides still exist in coconut shells after combustion with approximately 2 %. This method is used in order to quantify the accurate values of mass burn-off (ash tracer method) during the combustion of MC/agricultural waste blends.

161


Table 5-1 XRF analysis showing the amounts of major inorganic elements (based on their oxides) present in the ash of the coke, coconut shell blends and coconut shells following treatment at 1200 °C under 20% oxygen and 80% nitrogen atsmosphere

Amount of SiO2 (%w/w of raw fuel)

Amount of Al2O3 (%w/w of raw fuel)

Amount of TiO2 (%w/w of raw fuel)

Metallurgical Coke

2.00

0.00

2.09

Coconut shell

1.43

1.33

1.21

C1

2.02

2.02

2.04

C2

1.96

1.95

1.99

C3

1.93

2.06

2.06

Material

162


5.1.3

Physical Properties and Structural Transformations

It has been established that at elevated temperatures and low residence times, as in the case of coal combustion, the reaction is likely to be controlled by pore diffusion and chemical reactions on pore surfaces. In such case, the physical parameters of the reacted materials play a very important role. Therefore, in the present work, the surface accessibility of coke and its blends with coconut shell was investigated in detail and was assessed quantitatively by BET surface area measurement and qualitatively by SEM photomicrographs of the char particles collected after oxidation reaction in the drop tube furnace.

5.1.3.1 Surface Area Measurement – BET Surface Area The BET surface area was measured for MC and its mixtures in different proportions with coconut shells; N2 was pushed in the micropore channels. Figure 5-7 illustrates the BET surface area of the raw agricultural waste materials containing coconut shell samples which showed an increase in the surface area after combustion in DTF. The BET surface area results demonstrated minor changes in the BET surface area for coke, from 31.6 to 34.9 m2/g, while a high surface area had developed for coconut shell blends after the reaction in DTF i.e., from 26.3 to 36.7 m2/g for C1 blend, from 13.9 to 25.7 m2/g for C2 blend, and from 7.4 to 17.2 m2/g for C3 blend in Figure 5-7. In the case of coconut shell blends, the initial increase in combustion efficiency is supported by the increase in surface area (high surface area = 36.7 m2 in Figure 5-7), followed by a small increase in combustion efficiency (Figure 5-5).

163


50.0

40.0

2

BET Surface Area (m /g)

Before DTF After DTF

30.0

20.0

10.0

0.0 Coconut shells MC C1 C2 C3 Coconut shell content in the blends (%)

Figure 5-7 Changes in the micropore surface area before and after combustion in DTF as a function of the coconut shell content in the blends

The changes in surface area before and after combustion have been estimated for coke and its blends in different proportions with coconut shell and an index of surface area change has been defined as ΔSA (%) in Figure 5-8. In the case of coconut shell blends, the initial increase in the combustion efficiency is supported by the first ΔSA which is rises from C1 blends = 28.4 % to C2 blends = 45.7 %, however was seen to decrease afterwards for C3 in the blends. The drop in ΔSA might be due to particle fragmentation (coconut shell sample = 23.6 %). Further SEM studies were undertaken to investigate this aspect (Figure 5- 11).

164


100

Δ Surface Area (%)

80

60 C3 40

C2 C1

20

Coconut shells 0

MC Coconut shells content in the blend (%)

Figure 5-8 The changes in micropore surface area for coconut shell wastes content in the blend

Surface area modification during combustion is known to influence crack formation (Mitchell, R. E. and Akanetuk, A. E. J., 1996). It showed that a sharp change in the micropore surface area promotes a severe cracking and further SEM imaging is considered to demonstrate the behavior of coconut shell/ coke blends. From Figure 5-9 (a), the raw shells (brown color which is the fiber from the shell of the coconuts. After combustion in gas phase reactions, the physical structure turned into dark black color with porous structures. The ash particles (light grey particle) can be seen in the coconut shell chars (Figure 5-9 (b)).

165


(a)

(b)

0.05 mm

0.05 mm Raw Coconut Shells

Coconut Shells after combustion in DTF

Figure 5-9 Images of (a) raw coconut shells and (b) coconut shell after combustion in gas phase reactions at 1200 ºC (80% N2; 20% O2)

5.1.3.2 Structural Transformations - Scanning Electron Microscopy (SEM) The physical properties and structural transformations of raw coke and coke after combustion by SEM have been presented in Chapter 4. SEM micrographs of coconut shell and its residual mass after combustion are presented in Figure 5-10 (a) – (f). As it can be seen, the raw coconut shell and its cross-section clearly show pore structures which are made of cylinder-like tubes (Figure 5-10 (a)) composed of layers of several flat sheets. The cross section of the raw coconut shell gives more detail on the morphology of the cell like tubes which are seen to lie almost distinct from each other. After combustion the individual sheets of the tube walls have fused together and are no longer distinguishable from each other (Figure 5-10 (d) and (f)). Moreover, the walls of the individual tubes have also undergone a similar transformation. These have fused together at their point of contact with the result that a single solid matrix has been formed with interspersed pores. A similar observation on the pore structure and transformation following combustion was made by Achaw et.al. (Achaw, O. W. an d Afrane, G., 2008) implying that the cell structure was lost after devolatilization. 166


Figure 5-10 SEM micrographs of transverse section of (a) raw coconut shell, (b) coconut shell after combustion at 1200 °C in DTF and cross-sectional images of (c) raw coconut shell, (d) coconut shell char (e) and (f) edge section of coconut shell char

167


The micrographs of 100% coconut shell following combustion are taken to support the surface area measurements (Figure 5-11). As it can be seen from Figure 5-11 (a) and (b) particle fragmentation is clearly present and a small fragment has detached from the particle (Figure 5-11 (b)). A narrow fault can be seen on the particle surface (Figure 511 (a)) indicating that it is likely to form another fragment during further reactions. It is suggested that the fault in the shell of the particle results from the connection of neighbouring macropores on the particle surface that connect the external environment to the internal void. Moreover, the irregular thickness of the coconut shell particles provides some weak parts, which may be consumed rapidly, contributing to the formation of the surface pores and particle fragmentation when compared to palm shell. The association of these physical changes to reaction would influence the combustion performance as well as other factors; e.g. VM, ash/catalytic effects and chemical structures.

168


Figure 5-11 (a) and (b) SEM micrograph of coconut shell after combustion in DTF showing fragmented particles

The char particles resulting from the coconut shells seem to contain a greater number of large pores when compared to those in coke-char particles, which may be attributed to the evolution of volatile species from the interior of the particle during the combustion process.

169


5.1.4

The Role of Chemical Properties and Carbon Structures

5.1.4.1 Chemical Structures Lignocellulosic of agricultural waste basic components are hemicellulose, cellulose and lignin. Previous study (Haiping, Y., Yan, R., Chen, H. et al., 2007), confirmed that lignin starts decomposing at low temperatures (160 – 170 °C) and continues to decompose at a low rate until approx. 900 °C. Hemicellulose is the second component to start decomposing, followed by cellulose, in a narrow temperature interval from about 200 to 400 °C. This is the interval in which the main decomposition takes place and accounts for the greatest decomposition in the biomass pyrolysis process consisting of degradation reactions (Fisher, T., Hajaligol, M., Waymack, B. et al., 2002, Hosoya, T., Kawamoto, H. and Saka, S., 2007). Figure 5-12, shows the content of cellulose was 19.8 %, hemicellulose equally 50.1 % and lignin was 30.1 %. The high content of cellulose and hemicellulose will enhance the combustions performance compared with coke which contains aromatic carbon.

Allotropes of Carbon

MC Coconut shell

Cellulose 19.8

Hemicellulose 50.1

Lignin 30.1

Figure 5-12 Cellulose, hemicellulose and lignin contents of coconut shell (Wan Daud, W. M. A. and Wan Ali, W. S., 2004).

170


5.1.4.2 Chemical Properties – X-ray Diffractions Structural modifications of coconut shell and its blends with MC have been characterized by X-ray diffraction experiments in Figure 5-13 and Table 5-2 illustrates the resulting x-ray spectra. Based on visual inspections of Figure 4-15 (Chapter 4), it could be suggested that coke peaks correspond to a type of intermediate structure between graphitic and amorphous carbon with a wide range of carbon lattice spacing. The raw coconut shells show characteristic peaks of amorphous carbonaceous constituents in their structural matrices. A peak designated as μ band, was observed at 2θ = 15.8º, which may be attributed to the packing of the structure such as aliphatic side chains or condensed saturated rings which was cellulose. Moreover, the peaks detected the basic compounds in coconut shells was oxides elements such as sodium, aluminum and silicate. It can be observed from Figure 5-13 that the amorphous carbonaceous constituents in the raw coconut shells undergo massive conversion during devolatilisation as indicated by the disappearance of the hump and the presence of a plateau section in the diffraction spectrum between 10° - 30°. The long carbon ring which is expected to be lignin was assigned at 2θ ~ 44.6). The overall feature of the diffraction spectrum of the coconut shells after combustion resembles by previous reported by (Wornat, M. J., Hurt, R. H., Yang, N. Y. C. et al., 1995) at similar combustion temperatures and mass burnout.

171


10000

(Cn(H O)n)

Raw Coconut Shells

2

8000

2

C (Cn(H O)n)

2

4000

Na Al Si O SiO 6, 4 4 17

Cellulose

6000

Intensity (counts)

2000

0

50000 Coconut Shells after DTF

SiO

2

2

2

2

C (Cn(H O)n)

C

SiO

10000

Na Al Si O SiO6, 4 4 17

20000

2

2

30000

(Cn(H O)n)

SiO 2 (Cn(H O)n)

40000

0 10

20

30 Scattering angle (2θ)

40

50

Figure 5-13 XRD patterns of coconut shells before and after combustions (T=1200 ºC: 20% O2; 80% N2)

172


Table 5-2 XRD peaks characteristic of raw coconut shells and coconut shells after combustion in DTF 1200 ºC with atmosphere (80% N2; 20% O2)

Sample

Raw coconut shells

Coconut shells after DTF

Peak Position (2θ)

dspacing (Å)

h

k

l

15.8

5.61

-

-

-

Cellulose [C6(H2O)5)]n

26.6 34.7 44.6 50.1 21.9 26.5 34.7 36.2 44.6 50.1

3.36 2.58 2.03 1.82 4.05 3.36 2.58 2.48 2.03 1.82

0 1 1 1 0 1 0 1

0 3 1 0 0 3 2 1

2 2 3 0 2 2 1 2

Graphite (C) Anorthite (Ca, Na)(Si, Al)O Carbon (Cn(H2O)n) Quartz (SiO2) Quartz (SiO2) Graphite (C) Anorthite (Ca, Na)(Si, Al)O Quartz (SiO2) Carbon (Cn(H2O)n) Quartz (SiO2)

Crystal Size

Possible Compound (chemical formula)

173


Figures 5-14 to 5-16 show that, with increasing combustion temperature, the (002) peak position shifts from 24.4º to 26.5º, closer to that of graphite (26.6º) and all the coconut shells/coke blends peaks become more sharper. In general, diffuse and broad bands in XRD patterns represent the existence of short range order in the carbon structure, while sharp and narrow peaks correspond to highly crystalline region with high degree of long-range order. A summary of peak characteristics are presented in Table 5-3. The only noticeable peak emerging from coconut shell/coke blends (C1, C2 and C3 blends sample), its diffraction spectrum situated at 2θ = 21.9º (100), 43.1º (103) and 50.1º (112) strongly suggest the existence of the compounds of SiO2 (quartz), the most abundant oxide in the ash. Highest carbon peak was found at 2θ = º26.5 (002). At peakθ2= 29º and 44.6º corresponds to carbon ring of compound in the coconut shell blends. After combustion peak characteristics of cellulose (2θ = 29.4º) was seen to diminish with high temperature of reactions.

174


50000 Raw_C1 blends

C

40000

4 2

2

C H NO

SiO

2

12

11

C

2

C SiO

C ; SiO

2

3

CaCO

10000 Intensity (counts)

4

C

20000

SiO 2 FeAl O

Cellulose

30000

0 50000 C1 blends after DTF 40000

0

20


40

SiO 2 SiO

2

4 2

C

2

2

C SiO

C

6

10

C ; SiO SiO 2

13 2

CaCO 3 Al Si O

10000

C

SiO

2

2

FeAl O

4

20000

FeAl O ,SiO

2

C

30000

50

Figure 5-14 X-ray diffraction analysis of coke/coconut shell blends; C1 before and after combustion temperature 1200 ºC in 20% O2 and 80% N2 atmosphere

175


50000 Raw_C2 blends C

40000

4

4

SiO

2

2

C ; SiO 2 SiO 2 C

2

SiO

Intensity (counts)

10000

FeAl O ,SiO

C

2

2

CaCO 3 C

20000

SiO 2 FeAl O

Cellulose

30000

0

50000 C2 blends after DTF

C

40000

2

SiO 2 SiO

2

4

FeAl O ,SiO

C

2

C ; SiO SiO 2

2

SiO

10000

C

6

C

2

CaCO 3 Al Si O

13

2

C

SiO

2

20000

FeAl O

4

2

30000

0 10

20


40

50


176


50000 Raw_C3 blends 40000

4

10

3

30

4

2

2

40

SiO

C SiO

2

CaCO

20

C ; SiO 2 SiO 2 C

Intensity (counts)

10000

0

FeAl O ,SiO

2

2

C

20000

SiO 2 FeAl O

Cellulose

C

30000

50

50000 C3 blends after DTF

C

40000

SiO 2 SiO

2

2

4

FeAl O ,SiO

C

2

C ; SiO SiO 2

2

C

6

10000

C SiO

2

CaCO 3 Al Si O

13

2

C

SiO

2

20000

FeAl O

4

2

30000

0 10

20


40

50


177


Table 5-3 XRD peaks characteristic for all samples C1, C2 and C3 blends before and after combustion in DTF 1200 ºC with atmosphere (80% N2; 20% O2)

Sample

Raw C1, C2, C3 blends

C1, C2, C3 blends after DTF

Peak Position (2θ) 12.8

dspacing (Å) 5.61

h

k

l

-

-

-

Cellulose [C6(H2O)5)]n

21.9

4.05

1

0

0

Quartz (SiO2)

29.3

3.04

0

0

2

Calcite (CaCO3)

44.6

2.03

-

-

-

Carbon (Cn(H2O)n)

50.1

1.82

1

1

3

Quartz (SiO2)

26.5

3.36

0

0

2

Graphite (C), Quartz (SiO2)

29.3

3.04

0

0

2

Calcite (CaCO3)

33.1

2.71

0

2

2

Mullite (Al6Si2O13)

44.6

2.03

-

-

-

Carbon (Cn(H2O)n)

50.1

1.82

1

1

3

Quartz (SiO2)

Crystal Size

Possible Compound (chemical formula)

178


5.1.4.3 Chemical Bonding – Fourier Transform Infrared Spectroscopy (FTIR) The assignment of peaks in the FTIR spectra was carried out by comparison with literature data (Biagini, E., Barontini, F. and Tognotti, L., 2006, Shao, J., Yan, R., Chen, H. et al., 2007). Table 5-4 shows the regions of the spectra for the present work and FTIR spectra of the coconut shell before and after reaction in DTF are given in Figure 5-17. Bands corresponding to aromatic CH, OH and CO can be distinguished. The presence of the sharp peaks between 1640 and 1700 cm-1 could be attributed to C– O (carbonyl) stretching vibration indicative of ketones, phenols, carboxylic acids or aldehydes for raw sample, and represent C=C stretching vibrations indicative of alkenes and aromatics. The samples after combusted showed that the moisture was not completely removed due to a few seconds of reaction in DTF.

179


100 Raw Coconut Shells

95

CH stretching

80

2

75

Moisture

65 60 100

Coconut shells after DTF

3

C-O C-CH bending

3

C-CH bending

Aromatic (lignin)

80

O-H stretching N-H stretching

85

2

90

CH stretching

95

Moisture

% Transmittance

3

70

C-1 ring

85

N-H stretching aromatic rings (lignin) C-CH bending 3 C-H C-O C-C alkene C-CH bending

C=O

90

75 70 65 60 4000

3500

3000

2500 2000 Wavenumbers (cm-1)

1500

1000

500

Figure 5-17 FTIR spectra of coconut shell before and after combustion in DTF

180


The spectrum of the combusted coconut shell was seen to decrease after burning. The spectrum showed the absence of most bands and mainly an aromatic polymer of carbon atoms while the aliphatic rings were not strongly detected. Mok et al. (Mok, W. S. L., Antal, M. J., Szabo, P. et al., 2002) made similar observations from the FTIR analysis of cellulose chars prepared at temperatures of up to 450 °C. The major chemical changes were reported to be due to dehydration, carbonyl group formation and elimination, decomposition of aliphatic and formation of aromatic char units has been revealed. This study indicated that at least some steps (e.g. dehydrations, decomposition of aliphatic) in the chemical transformations occurring during devolatilization may be similar for different biomasses. The peak at around 1513 cm-1 could correspond to C-C stretching vibrations related to alkenes and aromatic components. Below 1500 cm-1, all bands are complex and have their origin in a variety of vibrational modes. C–H stretching and bending vibrations between 1380 and 1465 cm-1 indicate the presence of alkane groups in pyrolysis oils derived from biomass (Tsai, W. T., Lee, M. K. and Chang, Y. M., 2006). Absorption bands possibly due to C–O vibrations from carbonyl components (i.e., alcohols, esters, carboxylic acids or ethers) seem to occur between 1300 and 900 cm-1.

181


Table 5-4 Characterization of FTIR peak profiles of raw coconut shells and coconut shells after combustion at 1200 ºC at atmosphere (80% N2; 20% O2) Peak location range (cm-1) 3600-3400 3000-2850 1738-1700 1680-1650 1655-1635 1650-1580 1597-1513 1465-1440 1375-1350 1250-1200 1175-1120 1086-1030 900-600

Possible Functional Groups O-H stretching C-H asymmetric and symmetric stretching in methyl and methylene group C=O stretching in acetyl and uronic ester groups or in carboxylic group of ferulic and coumaric acids C=C, symmetric aromatic O-Hydroxyl diaryl ketones N-H bending in primary amine Aromatic rings (lignin) CH3 (aliphatic) C-H rocking in alkanes or C-H stretching in methyl and phenolic alcohol Si-CH2 stretching in alkane or C-C plus C-O plus C=O stretching C-C alkenes C-O deformation in secondary alcohol and aliphatic ether or aromatic; C-H in plan deformation plus C-O deformation in primary alcohol C-1 groups frequency

Raw Coconut Shells 3436

Coconut Shells after DTF 3442

2920

2955

1736

-

1638 1560 1513

1637 1560 1513

1461

1458

1371

1365

1244

-

1166

-

1048

-

669

-

182


5.1.4.4 Carbon Structures –Nuclear Magnetic Resonance (13C NMR Spectroscopy) A large number of structural studies based on solid-state

13

C NMR spectroscopy of

crystalline and amorphous cellulose, cellulose polymorphs, and model compounds have been reported in literature (Alesiani, M., Proietti, F., Capuani, S. et al., 2005, Sekiguchi, Y., Frye, J. S . and Shafizadeh, F., 1983, Snape, C. E., Axelson, D. E., Botto, R. E. et al., 1989). Solid-state NMR proved to be especially useful to characterize the process of degradation of wood/ biomass/ agricultural waste, revealing significant differences in the chemical structure. Solid-state CP/MAS

13

C NMR

spectroscopy is a powerful tool for investigation of cellulose structure. If it is used in conjunction with spectral fitting, it is possible to study and monitor changes in both cell wall structure and composition determined by chemical, mechanical, and thermal factors. The carbon structure of coke had been discussed in Chapter 4 (Figure 4-22). Raw cokes mainly consist of aliphatic and aromatic carbon structures. However, coconut shells showed variety of carbon resonances. The raw coconut shells shows a peak at 22 ppm assigned to the acetyl methyl groups of hemicelluloses in Figure 5-18 (a) and resonance assignment of peak was presented in Table 4-8. A peak at 56 ppm is attributed to the methoxy groups of lignin. The peak at 65 ppm is aliphatic C-6 carbons of crystalline cellulose (Link, S., Arvelakis, S., Spliethoff, H. et al., 2008). Moreover, at peak at 72 and 75 ppm, the coconut shells have the highest peak, corresponding to the C-2, C-3, and C-5 carbons of cellulose. C-4 amorphous and crystalline cellulose at 84 and 89 ppm present in the

raw coconut shells, respectively, while at 105 ppm a sharp peak

indicating the presence of anomeric carbon of sugars (aldehyde and ketones). Lignin syringyl C-1, syringyl C-5, and quaiacyl C-2 are also detected in the raw coconut samples exhibiting a broad line in the region between 134 and 137 ppm. The signals at 153 ppm correspond to syringyl C-3 and syringyl C-5. In the case of coconut shell, the signal at 162 ppm corresponds to 4-hydorxyphenyl C-4. A peak of carbonyl groups (COOH) of hemicelluloses seen at 173 ppm which are well known to have a high reactivity (Scholze, B., Hanser, C. and Meier, D., 2001). 183


The NMR spectra of the residual chars derived from combustion of coconut shell wastes are presented in Figure 5-18 (b). As can be seen in this figure, only one peak is detected at 124-126 ppm which is assigned to aromatic lignin (124 ppm-issuing from polyaromatic hydrocarbons). In can be concluded that aromaticity of the chars increased due to the loss of the less stable aliphatic groups. The broad resonances in Figure 5-18 (b) demonstrate the amorphous nature of the chars due to the presence of free radicals and the complex structure of this material. The aromatic component of lignin is very resistant to thermal degradation, and the resulting char is highly refractory. The lignocellulosic structure was lost and transformed to polycyclic material with a preponderance of aromatic structures as the temperature of treatment increases. All cellulose and lingo-cellulosic materials under thermal treatment with final temperatures between 800 and 1000°C were seen to undergo structural transformation, resulting in a more ordered structure (Bardet, M., Hediger, S., Gerbaud, G. et al., 2007). As in the present study, residues reaches end temperature of 1200ºC, such a transformation is expected for the employed agricultural waste materials.

184


(a)

Raw coconut shells Cellu lose C-2, C-3, C-5

Aromatic C-1 carbon of sugars (cellulose) O

Acetyl carbonyl groups of hemicellulose O

C-6 carbon crystalline cellu lose

-C-O Oxygen substituted (-OCH and –OH) aromat ic carbons

C-4 carbon CH3

-C-O Ketones O

O

Methoxy group Lignin :- OCH3

am Lignin

CH2 in saturated aliphatic chain

cr

-C-OCH3

R-C-R

Lignin aromatic 200

Acetyl methyl groups of hemicellu lose CH3

150

Aliphatic 100

50

(b)

[ppm]

0

Coconut shells at 1200ºC Aromatic-C

*

*

aromatic 200

150

100

50

0

[ppm]

Figure 5-18 Spectrum of CP/MAS 13C NMR of (a) raw coconut shell and (b) coconut shell char at 1200ºC in a drop tube furnace [80% N2, 20% O2] (* spinning sideband)

185


5.1.4.5 Inorganic Minerals Minerals, such as iron and alkali metals in agricultural wastes contribute to the mineral matter in the blends and influence combustion. Mineral phases in coke reactivity were investigated Grigore et al. (Grigore, M., Sakurovs, R., French, D. et al., 2006) identified Australian coke containing more than 7 wt.% of iron oxide in the ash having catalytic effects. From Figure 5-19, coke had lower content of Fe2O3 in the ash (5.7 wt. %) than coconut shells which was 46.8 wt. % of Fe2O3 in the ash. This values are higher than the Fe2O3 content in coke, thus could influence the combustion performance of the blends. Increasing the proportion of coconut shells in the blend, the iron oxide content also will increase. Backreedy et.al. (Backreedy, R. I., Jones, J. M., Pourkashanian, M. et al., 2002) studied the mechanism on the formation of a C - metal bond during devolatilisation. The alkali metals, C – O – M bond (where M can be Fe, Na, K or Ca) was seen to be formed which effected the C – C bond and hence catalysed the gas phase reaction (R Backreedy, J. M. J., 2002). Yamashita et. al (Yamashita, H. and Tomita, A., 1993) found that during devolatilization, the chemical form of iron species changed stepwise with high temperature. Thus, a rapid increase of the burnout is expected.

186


50 Coconut shells

2 3

Fe O in Ash (%)

40

30

20

10

0

MC C1

C2 C3

Coconut shell content in the blends (%)

Figure 5-19 Fe2O3 component present in the ash from MC and MC /coconut shell blends

Figure 5-20 showed the presence of (Na2O, K2O) in coke ash is less than 1 wt.%. However, coconut shell has higher content of Na2O = 7.9 wt.% and K2O = 16.9 wt.%. By blending coke with agricultural wastes, the presence of alkali (Na2O, K2O) in the agricultural wastes ash (Figure 5-20) might catalytically enhance the combustion performance of the blends (Carlos, A. C., Hooshang, P. and Christian, R., 2001). From previous study, alkali metals such as potassium

and sodium in coke are

associated with aluminosilicate in an unexchangeable ion form and they are believed to be catalytically inactive (Lang, R. J. and Neavel, R. C., 1982).

187


20 K2O: MC/coconut shell blends Na2O: MC/coconut shell blends

Na2O in Ash (%)

10

2

K O in Ash (%)

15

5

0 MC C1 C2 C3 Coconut shells Coconut shell content in the blends (%)

Figure 5-20 K2O and Na2O components present in the ash from MC and MC - coconut shell blends

The inorganic elements are located at the edge sites of the carbon layer and at the vacancies between the carbon layers in the graphitic-like carbon building blocks of coke/agricultural blends also known as carbon active sites (Laurendeau, N. M., 1978, Mims, C. A., Chludzinski, J. J., P abst, J. K. et al., 1984, Walker Jr, P. L., Taylor, R. L. and Ranish, J. M ., 1991). Their location allows them to catalytically enhance oxygen reactivity of neighbouring carbon atoms in the carbon matrix (MacPhee, J. A., Charland, J. P. and Giroux, L., 2006, Walker Jr, P. L., Taylor, R. L. and Ranish, J. M., 1991). Rate of pyrolysis are reported in the next section 5.2, while a thorough comparison between palm shell/coke and coconuts shell/coke blends is clearly explained along with the differences in their combustion and structural transformations.

188


5.2

Comparison of Coke/Palm Shells & Coke/Coconut Shells Blends in Gas Phase Reactions

This section reports comparison of coke/palm shell and coke/coconut shell blends in gas phase reactions. As the combustion reaction involves an initial thermal decomposition, followed by pyrolysed char combustion, the weight loss of the carbon based materials was studied in 100 % N2 and in a gas mixture consisting of 20% O2 and 80% N2, respectively. The weight loss curve is plotted as a function of reaction time in Figure 5-21 (a) and Figure 5-22 (a). The total weight loss of MC and it blends with palm shell and coconut shell occurred in less than 100 seconds. Such trend supports a lack of synergy between the two fuels, which is in good accordance with the previously published literatures (Meesri, C. and Moghtaderi, B., 2002, Vuthaluru, H. B., 2004). The extent of volatile release can be quantitatively compared in terms of devolatilization rates (Kim, B.-c., Gupta, S., Lee, S.-h. et al., 2008), which is calculated from the weight loss data by considering the first-order kinetics (Eq. 5.1): - dW / dt = K (W0 –W∞)

Eq. 5-1

Where: W,W∞, and dW / dt are the initial mass of the sample, the final mass of the solid residue, and the rate of the mass change in a time range of 20s, and K is the rate constant, in s-1, respectively. Correlating the rate constant of the tested samples with the actual amount of volatiles present in the initial blends Figure 5-21 (b) and Figure 5-22 (b), an increase in devolatilization rates is seen with an increase in agricultural content in the blends. Faster rates of devolatilization were seen for P3 blends compared to coke/coconut blends, while pure coke, as expected, shows the slowest rate. Figure 5-21 (b) showed an increased in rates of devolatilization from C1 to C2 blends which then decreased for the C3 sample. The difference in lignocellulosic structure of both samples agricultural waste materials is expected to influence the rates. 189


On the other hand, palm shell blends showed a continuous increase with increasing palm shell proportion in the blend. Previous studies on the pyrolysis of agricultural waste (Antal, M. J., Jr. and Varhegyi, G., 1995), report that lignin gives a higher char yield than cellulose or hemicellulose. A relatively greater weight loss of palm shell (and lower residual mass) could be attributed to its higher cellulose content (palm shells cellulose = 29.7 %; coconut shells cellulose = 19.8%), which is less stable compared to lignin (Figure 4-14). These results suggest that cellulose content in the agricultural wastes may enhance the combustion characteristics and decomposition of lignin since the cellulose compounds have a structure of branching chain of polysaccharides and no aromatic compounds, which are easily volatilized. Consequently, the agricultural wastes will burn at the flowing steps; first, the cellulose components in the agricultural wastes are volatilized, so that the porosity in the char particles of agricultural wastes increases and that oxygen easily diffuses into the char particles. Next, the lignin components in the agricultural wastes can also react with oxygen diffused even if the reactivity of lignin itself is low. In other words, this discussion suggests that the char morphology will be one of the important indices to evaluate the agricultural wastes reactivity during combustion.

190


100.0 MC 95.0

Residual Mass, %

P1 90.0 P2

85.0

P3

80.0 75.0 (a) 70.0 0

50

100

150

200

Time, sec

10

Rate constant (K, s-1x 10-3)

(b) 8

P3

6

4 P2 2

P1 MC

0 0

5

10

15 20 25 30 Volatile matter (%)

35

40

Figure 5-21 (a) Comparison of residual mass (%) in TGA with time for 100% MC and its mixtures with palm shells (b) variation of rate constant of the palm shell blends with volatile matter at 1200ºC in N2 atmosphere

191


100.0 MC

Residual Mass, %

95.0 90.0

C1

85.0

C2 C3

80.0 75.0 (a) 70.0

0

50

100

150

200

Time, sec

10


(b) 8

6

4 C2 2

C3

C1 MC

0 0

5

10

15 20 25 30 Volatile matter (%)

35

40

Figure 5-22 (a) Comparison of residual mass (%) in TGA with time for 100% MC and its mixtures with coconut shells (b) variation of rate constant of the coconut shell blends with volatile matter at 1200ºC in N2 atmosphere

192


Thermogravimetric analysis has been performed for MC and its corresponding palm shell and coconut shell blends as compared with previous study (rubber blends) (Zaharia, M., 2010). The weight loss curve is plotted as a function of reaction time in Figure 5-23 (a) and (c). The curve for each agricultural blend was found along with the curves of the reference material (MC). Such trend supports a lack of synergy between the two fuels, which is in good accordance with the previously published literatures (Zaharia, M., 2010). The devolatilization of rubber/coke was chosen for comparison from a previous study (Sahajwalla, V., Zaharia, M., Kongkarat, S. et al., 2009, Zaharia, M., 2010) due to the volatile matter content in the polymeric material. Rubber was chosen since it’s containing high volatile that similar to agricultural wastes (~ 7080 %). Correlating the rate constant of the tested samples with the actual amount of volatiles present in the initial blends (Figure 5-23 (b) and (d)), an increase in devolatilization rates with an increasing agricultural content in the blends is seen. The rate constant of palm shell blends (P3 blends) is similar to the rate constant of rubber blend (R3) which is same in proportion. However, lower rates of devolatilization were seen for coconut shell blends compared to pure coke, which is expected, shows the slowest rate. Coconuts shells blend showed a lower rate due to its structure transformation (particle fragmentation) during devolatilization compared to palm shells blend. This phenomenon occurred very fast at high temperature where the devolatilization rates would decreased. Different behaviors are shown by coke and its blends with agricultural wastes, with larger fractions from the blends being released as volatiles during the combustion process. This high amount of volatile was seen to occur over a relatively short time and is believed to influence the time required for complete combustion compared to coke. The estimated weight loss rate for P3 blend is the fastest, while 100% coke showed the lowest decomposition rate. On the other hand, palm shell blends show a continuous increase with increasing palm shell proportion in the blend. The greater weight loss of palm shell (and lower residual mass) could be attributed to its higher cellulose content, which is less stable compared 193


to lignin. Therefore, cellulose might have a more dominant influence on devolatilization compared to lignin. Previous studies on the pyrolysis of agricultural waste (Antal, M. J., Jr. and Varhegyi, G., 1995), agrees that lignin gives a higher char yield than cellulose or hemicellulose. However, in the case of rubber, the long hydrocarbon structure with higher volatile matter would influence the rate of devolatilization compared to agricultural waste materials. Moreover, such a behavior is expected considering the high moisture content in the agricultural wastes retarding the VM released. However, the presence of potassium, iron oxide and sodium present in agricultural waste, is known to be a strong catalyst in gas phase reactions (Lang, R. J. and Neavel, R. C., 1982, Miura, K., Hashimoto, K. and Silveston, P. L., 1989, Yunos, N. F. M., Zaharia, M., Ahmad, K. R. et al., 2011), promoting a high devolatilization rate compared to coke alone.

194


MC P1

90.0 P2 P3

80.0

R3

70.0 60.0

100.0

Residual Mass, %

Residual Mass, %

100.0

MC

90.0

C1

80.0

C2 C3 R3

70.0 60.0

(a) 50.0

0

(c) 50

100 Time, sec

150

200

50.0

R3

8

P3

6 4 P2

2

100 Time, sec

150

200

P1

(d) R3

8 6 4 C2

0

5

C3

C1

2 MC

MC

0

50

10

(b)



10

0

10 15 20 25 30 Volatile matter (%)

35

40

0

0

5

10 15 20 25 30 Volatile matter (%)

35

40

Figure 5-23 (a) and (c) Comparison of residual mass (%) in TGA with time for 100% MC and its mixtures with palm and coconut shells (b) and (d) variation of rate constant of the agricultural blends with volatile matter at 1200ºC in N2 atmosphere Where: [MC =100% coke; P3 = palm shell blends; C3 = coconut shell blends; R3 = rubber blends].

195


Furthermore, the difference is also attributed to the difference in the strength of the molecular structure of the fuels. The polymers cellulose, hemicellulose and lignin are linked together with relatively weak ether bonds are less resistant to the heat at low temperatures (Blazej, A. and Kosik, M., 1993). In contrast, the coke structure, which mostly comprises dense polycyclic aromatic hydrocarbons, are more resistant to the heat (Blazej, A. and Kosik, M., 1993, Smith, L. H., and, S. L. D. and Fletcher, 1994). The coke considered for the present study was expected to have a high aromatic content. As a consequence, a small amount of fragmented polycyclic aromatic compounds would be expected to result from the immobile phase. Hence the mass loss was lower (high residual mass) than from pyrolysis of blends involving agricultural waste samples. The lower amount of volatiles, the slowest rate of devolatilization and the highly aromatic structure present in coke leads to lower mass loss and little change in structure. Table 5-5 summarizes the intramolecular bonds present in the samples used in the present study along with the corresponding energies required to break these bonds.

Table 5-5 Type of molecular bonds and bond energies required to break the fuels considered in this study (Blazej, A. and Kosik, M., 1993, Smith, L. H., and, S. L. D. and Fletcher, 1994) Samples Metallurgical coke, MC Cellulose, hemicellulose and lignin (agricultural waste/woody/ biomass)

Bonds

Bond Energies (kJ/mol)

C=C

>1000

R-O-R

~ 380-420

196


Quantitative determination of the combustion performance of coke and its corresponding agricultural waste blends indicated a direct correlation showing an increase in burnout with increasing agricultural waste proportions in the blend. Higher burnout values recorded by the palm shells blend over the coconut shells blend were observed. (Figure 5-24).

30.0

Burnout, %

25.0 20.0 15.0 10.0 5.0

Palm shells Coconut shells

0.0 MC Blend 1 Blend 2 Blend 3 Palm & Coconut shells content in the blend

Figure 5-24 Combustion performances of palm shell-coke and coconut shell-coke blends (Blend 3* = Blend P3 and Blend C3)

Looking at the decomposition pattern of palm shells and coconut shells, it may be assumed that the slightly higher burnout of the palm shells over the coconut shells may be due to the direct conversion of lignocellulosic structure, changes in surface area and inorganic material that is present in the agricultural waste materials.

197


The development of porosity in the residual chars seems to depend on the amount of volatile matter removed and the subsequent structural changes of the residual carbon. Agricultural waste materials as a raw have high levels of volatiles. When used in conjunction with coke matrix, structural and physical changes might occur because of the volatiles released following combustion. Figure 5-25 shows that the change is more significant in the case of palm blends compared to coconut blends, while coke develops the least change. This will support the observations that for palm shell the combustion show an increase (continuous) (see thermogravimetric analysis in Figure 5-22). In the case of coconut shell, the initial increase in combustion efficiency is supported by the first ΔSA which is appreciable and rises. However the combustion was decreased afterward for C3 blends due to particle fragmentation compared to palm shell samples discussed earlier. The changes in the original structure of the matrix of the raw material may be attributed to the cross-linking of the reactive points of the cylinders brought about by the disruption of the original polymeric structure of the precursor material (i.e. the cellulosic and lignin units of the raw material) and the subsequent reconstitution of a new matrix structure during the devolatilization as observed by Byrne and Marsh (Marsh, H. and Reinoso, F. R., 2006).

198


100 Palm shells

Δ Surface Area (%)

80

P2

P3

60 P1 C3

40 C2 C1

20

0

Coconut shells

MC Agricultural wastes content in the blend (%)

Figure 5-25 The changes in micropore surface area for agricultural wastes content in the blend

Scanning electron microscopy was used for a quantitative determination of the residual char morphology. Figures 5-26 (a) – (c) show the structure of 3 samples blended with coke develop a high porous structure. For palm shells sample, the pore structures are opening up while coconut shells showed that cylinders were still present, even though the interiors are clearly being consumed. Thus, after the initial consumption of the interiors, the cylinders are not providing any further opportunities for increase, as it is difficult to find too many “edges”, and the surface area itself keeps decreasing, compared to palm shells (Figure 5-26 (b)) where we clearly saw the entire structure uniformly opening up and creating significant changes in surface area (Figure 4-13 in Chapter 4).

199


(a)

(c)

(b)

Met. coke

Palm shells

Coconut shells

Figure 5-26 SEM micrographs of (a) MC, (b) palm shells and (c) coconut shells collected after reaction in the DTF at 1200°C in atmosphere of 20% O2 and 80% N2, polished section of the residual particle x1000

A good agreement between the surface area (porosity) and morphological changes of the agricultural/coke blends with combustion performance and devolatilization is established in this study. Similar correlations between physical parameters of the carbonaceous materials and their derived burnout were observed in the literature (Sahajwalla, V., Zaharia, M., Kongkarat, S. et al., 2009, Zaharia, M., Sahajwalla, V., Kim, B.-C. et al., 2009). These researchers found that the changes in morphology would enhance the combustion performance at similar conditions.

200


5.3

Summary

Investigation of comparative combustion characteristics and performances of coke and its blends in different proportions with palm and coconut shells have been investigated. The effect of addition of agricultural waste on the combustion behavior of its blends with metallurgical coke are summarize below: 1)

Fundamental pyrolysis and combustion behaviors for two types of agricultural wastes were tested by a TGA-MS. At temperatures above 1000 ºC, the gas products evolving from coconut and palm shells pyrolysis measured by MS showed CO, CO2 and H2 were released.

2)

Increased combustion efficiency is observed with increasing agricultural wastes proportion in the blend.

3)

The effect on the structure of the particles, such as surface accessibility developed following gas phase reactions in the DTF also influences the combustion efficiency of coke/palm shell and coke/coconut shell blends.

4)

Good correlations were found between the extent of mass burnout of the blended residues and that of their pore, therefore supporting the dominant influence of the carbon surface accessibility (porosity) and blend components. The SEM photomicrographs illustrated a good agreement with the trends observed in measured surface area in the carbonaceous matrix.

5)

Minerals, such as iron and alkali metals in agricultural waste contribute to the mineral matter in the blends and influence combustion performances.

6)

Combustion efficiency depends on the amount of mineral matter coupled with the rate of release of VM products and also with the effect on the structure of the particle formed. The findings clearly indicate that blending coke with recycled agricultural waste materials would enhance overall combustion efficiency.

201


7)

Palm shell blends developed higher devolatilization rates when compared to coconut shell blends where the lower devolatilization rates appear to affect the structure. The difference in lignocellulosic structure of the materials is strongly connected to their kinetic behavior.

202

Chapter 6: Conclusions – Gas Phase Reactions

CHAPTER 6 6 Conclusions – Gas Phase Reactions Studies

The combustion characteristics and performances of coke and agricultural waste blends in different proportion (palm and coconut shells) have been investigated. The aim of this study is to understand the fundamentals of gas phase reactions and structural transformations. TGA (N2 atmosphere) and DTF (20% O2 and 80% N2 gas mixture) are used for gas phase reaction purposes. The thermal decomposition (devolatilisation) and char oxidation of MC and agricultural waste materials were studied. Residual mass was found to decrease with increasing the agricultural wastes content in the blend. This behavior might be due to the high volatile content of agricultural waste materials when compared to coke alone. The intensity of volatile release was quantitatively compared in terms of devolatilization rates. Coconut shell blends show a steady reaction rate with increasing concentration of coconut in the blend, while palm shell blends show an increase as the proportion of the palm shell in the blend rises. At temperatures above 1000 ºC, the gas products evolving from palm and coconut shells pyrolysis measured by TGA-MS showed CO, CO2 and H2 as the main gases. These were attributed to the lignocellulosic structure present in the in agricultural waste allowing a continuous gas release for participation in the subsequent carbon/slag interactions. The combustion performance of coke and its blends with agricultural wastes was quantified. A higher burnout was estimated for the palm/coconut blends when compared to coke alone. 203

Chapter 6: Conclusions – Gas Phase Reactions

Surface area measurements and qualitative morphological characterization of the agricultural/coke blends through SEM were conducted. The changes in surface area, before and after gas phase reactions, were estimated and palm shell blends showed a larger increase in surface area compared to coconut shell blends, while the changes occurring in the coke particles are marginal. Particle fragmentation is believed to hinder pore development in coconut shell while an agreement with the trends observed in the rate of devolatilization is consistent and support with an increase in combustion performance of palm shell blends. A highly amorphous structure characterized the palm and coconut shell wastes with broad diffuse spectra and a significant amount of highly disordered material. The changes in surface area are more significant when the proportion of agricultural waste materials increases. SEM micrograph supported the BET measurement showing enlarged pores develop in the agricultural waste materials. Palm shell’s cell structures were seen to open up to a significant extent and structural changes were observed accompanied by higher surface area. In contrast, the structural transformations that occur in the coconut shell blends retain the cylindrical cell structures, which show a lower surface area compared to palm shell blends. Minerals, such as iron and alkali metals in agricultural waste also contribute to the mineral matter in the blends and influence combustion. The combined modification of pore and cell structure of agricultural waste materials (palm and coconut shells) are shown to contribute to their structural transformations, which results in improvement in combustion efficiency in high temperature processes.

204

Chapter 7: Discussions on Carbon/Slag Interactions

CHAPTER 7 7 High Temperature Reactions: Carbon/Slag Interactions – Results and Discussion

The sessile drop method was used to investigate carbon/slag interactions at 1550 ºC in a horizontal tube furnace under inert atmosphere (1 L/min Ar, 99.99% purity). These studies include the off-gas generations, interfacial phenomena and slag foaming behavior of EAF slags. The carbonaceous materials used were metallurgical coke, palm and coconut shells char and the blends with different proportions together with an EAF rich iron oxides slag. The agricultural waste/coke blends were initially subjected to rapid gas reaction in a drop tube furnace (DTF) in an atmosphere of (20% O2 and 80% N2) gas mixture at temperature of 1200 ºC while agricultural wastes were devolatilized at 450 ºC, as explained in Chapter 3. Coke was used without prior treatment due to low amount of volatile present in its matrix. Later on, these carbonaceous materials were subjected to carbon/slag reactions. These carbon/slag interactions were monitored continuously throughout the experimental run by video recording of reaction assembly as well as gas generation. When a carbonaceous material is injected in the EAF furnace it experiences an initial devolatilization, followed by combustion and later on the residues interact with the available slag. The iron oxide rich EAF slag reacts with the available carbon as a result metallic iron is produced. The reaction between carbon and FeO in the slag leads to the formation of CO and CO2 gases which are entrapped in the slag phase and subsequently released. The characteristics of the carbonaceous materials in the residual chars obtained after devolatilization and subsequent combustion reaction will influence carbon/slag 205


interactions. Thermogravimetric analysis (TGA) has been performed on the raw materials and the devolatilization rates of coke and coke/agricultural waste blends were estimated. The rates of devolatilization of the carbonaceous materials studied in this present work will affect the gases released, influencing the carbon/slag interactions. Medium rates of volatile matter evolution will lead to steady gas releases which might be available during subsequent contact with the EAF slag allowing the entrapment of gases over a longer period of time, while higher rates of gas generations might lead to poor gas entrapment allowing a fast passage through the slag (Figure 7-1).

Carbonaceous material Rate of devolatilization

high

Fast gas release

Poor foaming

medium

Steady gas release

Stable foaming

low

Low gas release

Poor foaming

Figure 7-1 Schematic diagram illustrating the effect of carbonaceous materials rate of devolatilization on slag foaming

The initial rapid gas phase reaction of coke/palm shell blends were achieved during passage through DTF. This resulted in volatiles still being present in the blends as unburned hydrocarbons. The structure of the resulting carbon also plays a very important role in the subsequent carbon/slag interactions. An ordered graphitic structure has a low degree of reactivity; while a more disordered material contains active sites \ and volatiles (hydrocarbons) are very reactive. The reactions occurring at the slag/carbon interface (1550 ºC) are expected to be affected by the presence of an increased level of hydrocarbons. These hydrocarbons 206


(CnHm) are expected to further decompose into carbon and hydrogen in Eq. 2-11 (Dankwah, J. R., Koshy, P., Saha-Chaudhury, N. M. et al., 2011). The hydrocarbon could also act as a sink for CO2 gas to produce CO and H2 in Eq. 2-12. When put in contact with an EAF rich iron oxide slag, the presence of FeO leads to reduction reaction depending on the reducing agents including C, CO and H2 released at high temperature. The reduction of FeO by hydrocarbons leads to the formation of CO, CO2 and H2O. These gases can later be found at slag/metal interface allowing the entire bath to become foamy. A fluctuation in volume is dependent on the rate of gas evolution, the size of gas bubble as well as physical and chemical properties of the slag. Moreover, CO2 can be produced from reaction with H2O based on auxiliary reaction and H2O with C to produce CO from cracking the hydrocarbon (Dankwah, J. R., Koshy, P., Saha-Chaudhury, N. M. et al., 2011, Demirbas, A., 2002). The reactivity of carbon is also related to the porosity of structure and more porous structure will result in a larger exposed area per unit mass and this will further enhance gasification of carbon. The gases released from the carbonaceous material will assist with carbon/slag reactions including FeO reduction. The interactions between carbons based materials with EAF rich slags at high temperature leading gas generation. Part of the gases generated remains trapped in the slag for a certain period of time depending on the process parameters and slag properties. Interfacial reactions between solid carbon and the slag phase determine their degree of interaction.

207


7.1

Influence of Carbonaceous Material on Carbon/Slag Interactions

For a better understanding of carbon/slag interactions, off-gas measurement enable determination of the amounts of CO and CO2 formed during carbon/slag interactions and FeO reduction as a function of time. In the present study, palm shell sample is chosen for comparison with conventional material (coke) due to its capability to maintain the slag volume and easy to quantify the sizes. While, coconut shell reacted very fast with slag that makes hard to quantify the slag volume ratio and contact angle measurement because it kept sinking into substrate. The gas entrapment phenomena has been quantified using sessile drop arrangement and the novel processing software (Khanna, R., Mahfuzur, R., Richard, L. et al., 2007) for estimation of slag volume ratios as a function of time and quantitative measurement of the reduced metal formation and gas behavior by using optical microscopy and SEM. These carbonaceous materials will react with an EAF rich iron oxide slag, containing 34.9 % iron oxide. The complete slag composition is presented in detailed in Table 3-8. When the samples assembly attained the desired temperature, changes in the slag volume were observed. Fluctuations were clearly visible and were attributed to subsequent formation and gas release. Depending on the chemical nature and composition of the carbonaceous material different volume ratio trends have been observed through in-situ high temperature determination of slag volumes as a function of time are presented in the following sub-sections.

208


7.1.1

Off-gas (CO, CO2) Generations

Off-gas generation (CO and CO2) during the high temperature reactions at 1550 ºC between the carbonaceous substrate and the slag (predominantly reduction of iron oxide by carbon) was determined by using IR analyser results as shown in Figure 7-2. Figures 7-2 (I and II (a)) respectively provide volume concentrations (ppm) in terms of CO and CO2 gases resulting from the interactions of the EAF slag with coke and palm char samples. Metallurgical coke showed significant level of carbon/slag interaction with the CO gas concentration emitted reaching 28000 ppm after 100 s of contact (Figure 7-2 (I, a)). The CO gas concentration from palm shells char/slag showed a sharp rise, attaining a value of 23000 ppm in less than 100 seconds, increased slowly and the reaction slowing down after 300 s (Figure 7-2 (II, a)). The CO concentration decreased gradually which attributed to the complex structure found in the palm shells breaking down at a slower pace. The CO2 volume was seen to be significantly lower (~200 ppm) compared to coke (13000 ppm) and this might be due to the hydrogen content present in the agricultural wastes char (~ 6 %) which acts as a CO2 sink (Dankwah, J. R., Koshy, P., Saha-Chaudhury, N. M. et al., 2011). Figure 7-2 (I and II (b)) shows the cumulative volumes of gases emitted (CO, CO2) from a metallurgical coke/slag as well as from the palm char/slag. In general, there is a time delay observed of about 40 – 60 seconds involved before the Infrared analyser starts recording the CO and CO2 data. The product gases take this time to travel from the reaction site to the Infrared analyser. This was initially ascertained by passing Argon gas through the furnace tube and the observing time needed for the IR to respond. The off-gas results for metallurgical coke were significantly different from palm char for both CO and CO2 levels. After 100 s reaction at 1550 °C, the cumulative volume of CO gas emitted from the metallurgical coke was seen to increase up to 0.5 x 10-4 mol with CO2 levels = 0.25 x 10-4 mol. On the other hand, CO gas emitted from palm char was nearly 3 times higher (1.15 x 10-4 mol) than the corresponding result from coke. The total amount of CO2 evolved gases from palm char was lower than coke indicated that agricultural waste has potential to reduce the CO2 emissions. These results were in 209


good agreement with TGA-MS analysis where more CO and CO2 were released at high temperatures (Figure 4-3). A higher content of oxygen present in the agricultural waste was also an important factor affecting the high temperatures reactions of agricultural wastes char with EAF steelmaking slags. Palm char contain high oxygen level where the gas will release as CO and CO2 during the interaction with slags.

(I) Metallurgical coke/ slag

25000

CO

20000 CO

15000

2

10000 5000 0 0

200

400

600

800

(b)

1.2 1.0 x 10-4 (mol)

30000

1.4 (a)

Cumulative volume of gases,

Gas generated (CO/CO2), ppm

35000

0.8 0.6

CO

0.4 CO

2

0.2 0.0

1000

0

200

Time, s

400 600 Time, s

800

1000

(II) Palm char/ slag

CO

20000 15000 10000 5000 0 0

CO

2

200

400

600

800

1000

CO

1.0 x 10 (mol)

25000

(b)

1.2

-4

30000

1.4 (a)

Cumulative volume of gases,


35000

0.8 0.6 0.4 0.2 0.0

CO

2

0

Time, s

200

400 600 Time, s

800

1000

Figure 7-2 (I) Metallurgical coke/slag and (II) Palm char/slag with (a) generated gas concentrations (ppm) in terms of CO and CO2 gases and (b) the cumulative volume of gases (mol) of CO and CO2 210


A number of reduction reactions involving oxides in EAF slags, ash impurities, and oxidation of carbon are expected to take place in the interfacial reactions. In Figure 7-3, the number of moles of carbon and oxygen removed during metallurgical coke/slag and palm char/slag interactions have been plotted. Extensive gas generation was observed from palm char in contact with slag which is predominantly CO. Metallurgical coke showed slightly lower levels of gas generation. Moles of oxygen and carbon removed increased as a function of temperature indicating enhanced reactivity and iron oxide reduction.

211


-5

Carbon removed x10 ,moles

1.5

(a)

1.0

0.5

0.0 0 1.5

200

400 600 Time, s

800

(b)

1000

MC PC

-5

Oxygen removed x10 ,moles

MC PC

1.0

0.5

0.0 0

200

400 600 Time, s

800

1000

Figure 7-3 Total number of moles (a) carbon and (b) oxygen removed from the metallurgical coke (MC) and palm char (PC) substrate in contact with a slag at temperature 1550 ºC

212


Figures 7-4 (a) – (d) respectively shows the gas (CO+CO2) generation for palm char, metallurgical coke and its blends with palm shells during interactions with slag at 1550 °C. From the total gas (CO+CO2) generated graph, it can be observed that, with increasing palm shells concentration in the blends, the total gas generation was decreased. The might be due to the lower of CO2 generated from palm shell blends where the CO2 generated showed a lower level than coke. Metallurgical coke showed a significant level of carbon/slag interaction with the CO gas emitted reaching 28000 ppm after 100 s of contact. CO gas from palm shells/coke blends was higher than that from metallurgical coke substrates. In the initial stages of interaction, the highest amounts of total cumulative of moles generated (CO+CO2) gases were emitted by P1 blends (1.2 x 10-5 moles) in Figure 7-5 (b). The CO2 gas was seen decreasing with increasing the palm shell concentration in the blends indicating less of CO2 emissions (Figure 7-5 (b) – (d).

213

50000 (a) MC




40000 30000

CO

20000

CO

2

10000 0 0

200

400

600

800

50000 (b) P1 blends

30000 CO

20000

2

10000 0 0

1000

CO

40000

200

400

50000 (c) P2 blends CO

40000 30000 20000

CO

2

10000 0 0

200

400

600

800

1000

800

1000

Time, s



Time, s

600

800

1000

Time, s

50000 (d) P3 blends 40000

CO

30000 20000

CO

2

10000 0 0

200

400

600

Time, s

Figure 7-4 Generated gas concentrations, CO+CO2 (ppm) as a function of time and carbon based materials (a) metallurgical coke, (b) P1 blends, (c) P2 blends and (d) P3 blends

214


Figure 7-5 (a) – (d) shows the total amount of removed gasses produced from metallurgical coke and palm shell blends when put in contact with an EAF slag. In general, there is a time delay observed about 40 to 60 seconds involved before the Infrared analyser starts recording the CO and CO2 gases. The product gases take this time to travel from the reaction site to the Infrared analyser. This was initially ascertained by passing argon through the furnace tube and observing the time needed for the IR to respond. As a result, all the graphs show the reduction starting after a delay of 40 to 60 seconds. No significant differences were observed in the time of initiation of the evolution of gases with the variation of carbonaceous material or slag employed in the present study. The variations in amounts of removed oxygen, i.e. the amount of CO and CO2 show a decrease with increasing palm shell content in the blends. By direct observations of the graphs in Figure 7-5 , it can be observed that the lowest amount gas emissions are obtained from the raw coke, increasing when palm shells were partially replaced the carbonaceous substrate as in P1 blends (Figure 7-5 (b)).

215


1.6 (a)

100% MC 100% PC

1.2 0.8

0.4 0.0

0

200

400 600 Time, s

800

Total (CO+CO2) removed x 10-5 , moles


1.6

0.8

0.4

0

200

400 600 Time, s

800

1000

1.6 (c)

100% MC P2 blends

1.2

0.8 0.4

0

200

400 600 Time, s

800

1000



100% MC P1 blends

1.2

0.0

1000

1.6

0.0

(b)

(d)

100% MC P3 blends

1.2

0.8 0.4 0.0

0

200

400 600 Time, s

800

1000

Figure 7-5 Total cumulative number of moles of gas generated (CO+CO2) as a function of time and carbonaceous material used

216


7.1.2

Contact Angle Measurements

The slag/carbon interactions of metallurgical coke, palm char and palm shells/coke blends were measured at 1550 °C with EAF slags are presented in Figures 7-6 (a) to (d) in terms of contact angles. For metallurgical coke, the initial contact angle was 107° and then it increased up to 124° at 150 s. Then the contact angle decreased to 102° at 1000 s, indicating a minor improvement in the wetting behavior with time (Figure 7-6 (a)). For palm shell/coke blends, the wettability was seen to improve slightly with decreasing levels of concentrations of palm shells. The slag droplet showed high fluctuating of contact angles during the experimental run. For Blend P1 showed dynamic non-wetting behavior with the contact angle was higher than the corresponding values for metallurgical coke (Figure 7-6 (b)). The slags also showed a poor wettability on Blend P2 where high contact angles rapidly increased to 122° in 10 min (Figure 7-6 (c)). While the contact angles for Blend P3 was 118° for 100 s and decreased to 100° at 10 min (Figure 7-6 (d)). These fluctuations are believed to have been caused by gases generated during carbon/slag interactions, their trapping and subsequent release from the slag droplet. Based on thermodynamic considerations, the wetting line in a wetting carbon/slag system will move only if there is a net decrease in the free energy at the carbon/slag interface. This decrease in the free energy during wetting is a result of interfacial reaction and lowering of interfacial tension. According to Pask (Pask, J., 1987 ), the solid carbon substrate is an active participant in the reaction and the free energy changes during the reactions contribute to reduction in the interfacial energy. However, the initial contact angle being high, generally >100º, the decrease in the contact angle is to be expected where the volume expansion will control the measurements indicating high gas entrapments. Concluding from the measured values of the contact angles, there were observed a positive influence of FeO reduction reaction on improving wettability in palm char with higher amount of iron oxide in the ash, which is influenced by composition of carbon content, slag composition (FeO content) and temperature. 217


140

140 (a)

(b) 130 Contact angle (°)

Contact angle (°)

130 120 110 100 90 80

200

400 600 Time, s

800

110 100 90

MC PC 0

120

80 0

1000

140

400 600 Time, s

800

1000

(d)

130

130 Contact angle (°)

Contact angle (°)

200

140 (c)

120 110 100 90 80 0

MC P1

MC P2 200

400 600 Time, s

800

1000

120 110 100 90 80 0

MC P3 200

400 600 Time, s

800

1000

Figure 7-6 Variation of contact angle with time for carbonaceous substrate (a) palm char (PC), (b) P1 blends, (c) P2 blends and (d) P3 blends in contact with slag at 1550ºC

218


7.1.3

High Temperature, In-situ Observations

The sessile drop method was used to investigate the gas entrapment in metallurgical slags as a result of carbon/slag interactions. The changes during carbon/slag interactions were monitored continuously throughout the experimental run by video recording and monitoring gas data. In order to qualitatively demonstrate the gas entrapment phenomenon a few representative dynamic wetting images of the slag droplet in contact with palm char, metallurgical coke and its blends with palm shell are shown in Figures 7-7 to 7-7 (a) – (e). Gas bubbles are predominantly visible in the video recording and are generated throughout the interface just after melting. In fact, the generated gas spreads into the slag droplet increasing its volume. The volume decreases when the gases are released. The entire sequence of events is presented as a function of time and t=0 represents the initial melting stage. The size of the slag droplet in contact with metallurgical coke (Figure 7-7 (a)) did not show a significant variation with time during the initial stages. After 10 minutes of reaction, the slag droplet fluctuated to a small extent in volume and maintained a reasonably spherical shape. The fluctuations in droplet size are associated with the generation and subsequent release of gases. In the case of palm shell char, the volume increase and associated were observed fluctuations throughout the experiment, indicated a sustained improvement in slag volume (Figure 7-8 (b)). The EAF slag reacting with P1 blends increased in volume and attained a spherical shape. Fluctuations appeared throughout the experiment, showing an improvement in volume when compared to coke alone (Figure 7-9 (c)). When the slag reacted with P2 blends (Figure 7-10 (d)) an initial droplet of high volume was seen, followed by a fast released of gases and immediate decrease in droplet. The fluctuations continue up to 15 min of contact. A different behavior was seen when the EAF slag reacted with the carbonaceous substrate, P3 blends. An increase in volume was clearly visible due to a continuous/sporadic evolution of CO gas bubbles within the slag droplet. Fluctuations appeared throughout the experiment, showed an improvement in volume compared to 219


coke alone. (Figure 7-10 (e)). Images were captured for every 5 s during the initial stages of contact and every 30 s during later periods.

220


Metallurgical Coke (MC) (a)

t = 120 sec

t = 60 sec

t = 600 sec

t = 900 sec

Palm char (PC) (b)

t = 60 sec

t = 600 sec

t = 120 sec

t = 900 sec

Figure 7-7 High temperature photographs of slag droplets in contact with (a) 100% MC and (b) 100% Palm char at 1550 ºC as a function of time

221



t = 120 sec

t = 60 sec

t = 600 sec

t = 900 sec

P1 blends (c)

t = 60 sec

t = 600 sec

t = 120 sec

t = 900 sec

Figure 7-8 High temperature photographs of slag droplets in contact with (a) 100% MC and (c) P1 blends at 1550 ºC as a function of time

222



t = 120 sec

t = 60 sec

t = 600 sec

t = 900 sec

P2 blends (d)

t = 60 sec

t = 600 sec

t = 120 sec

t = 900 sec

Figure 7-9 High temperature photographs of slag droplets in contact with (a) 100% MC and (d) P2 blends at 1550 ºC as a function of time

223



t = 120 sec

t = 60 sec

t = 600 sec

t = 900 sec

P3 blends (e)

t = 60 sec

t = 600 sec

t = 120 sec

t = 900 sec

Figure 7-10 High temperature photographs of slag droplets in contact with (a) 100% MC and (e) P3 blends at 1550 ºC as a function of time

224


7.1.4

Slag Foaming

Carbon/slag interactions were performed in high temperature furnaces with monitoring facilities to observe and quantify during the reactions. All experimental runs were recorded up to 30 minutes, but due to insignificant changes in the slag volume after 1000 seconds no more data points are presented. Gas hold-up in the slag droplet was measured in terms of Vt/V0, where Vt is the volume of slag droplet at time, t and V0 is the initial slag volume (Figure 7-11). At the initial slag melting stage is represented by t = 0, where the volume ratio is 1. Changes are observed corresponding to CO and CO2 generation and entrapment, as an increased droplet volume. After a certain period of time, gas release has reduced considerably and Vt /V0 stabilizes. Figure 7-11 (a) presents the volumes of CO and CO2 generated at any time, t for 100% palm char and 100% coke, monitored by the IR gas analyzer. The instantaneous gases (CO+CO2) generated peaked around 39000 ppm for coke and the evolution of CO and CO2 gases released from palm char slowly decreased with time (Figure 7-11 (a)). These results indicate that the high levels of gas generation from coke/slag released than palm char/slag resulting in faster gas escaping after 300 seconds of reactions. From Figure 7-11 (b), the palm shells char has a significant variation in volume ratios, with the volume ratio fluctuating between 1.0 and 1.3 up to 100 s and between 1.0 and 1.2 after 200 s. This indicates that the decrease in slag volume took longer when palm shell char was used, suggesting significant levels of gas entrapment and subsequent release during carbon/slag interactions. Metallurgical coke has the volume ratio in the range of 0.8 to 1.0. After 600 s the volume ratio decreased to 0.7 indicating a lower extent of gas entrapment by the slag. The results suggest the potential of palm shells char injection, to give immediate and observable differences to slag foaming, increasing foam volume.

225


50000 (a)

100% MC 100% PC

30000

2

(CO+CO ), ppm

40000

20000 10000 0 0

200

400 600 Time, s

800

1000

2.0 (b)

100% MC 100% PC

1.6

t

Volume ratio, V /V

0

1.8

1.4 1.2 1.0 0.8 0.6 0

200

400 600 Time, s

800

1000

Figure 7-11 (a) Instantaneous gases (CO+CO2), ppm, generated and (b) Volume ratio of 100% Metallurgical coke (MC) and 100% Palm char (PC) with slag with respect to reaction time

226


Figure 7-12 (b) reveals the volume ratio Vt /V0 of the slag reacting with metallurgical coke and P1 blends. The slag volume ratios for P1 blends showed significantly different trends with much higher level of gases in the slag volume in the first 100 seconds of reaction, sustained for 400 seconds and continue fluctuated after 600 seconds (1.8) where the gas generated from P1 blends showed high gas released, 62000 ppm (Figure 7-12 (a)). This indicates that the decrease in slag volume took much longer than coke suggesting significant levels of gas entrapped which was subsequently released. P2 blends showed a volume ratio comparable to P1 blends (Figure 7-13 (b)). The volume ratio showed 1.4 for first 100 seconds due to the gas trapped within the slag, dropping down to a value of 0.8 after 300 seconds. After 700 seconds, the fluctuation resulted in high volume ratio 1.7 and dropping down afterwards with the release gases and consumption of carbon. It was seen that the gas generated from P2 blends are lower than P1 blends (Figure 7-13 (a); 58000 ppm). P3 blends showed significantly lower levels of gas entrapment compared to other blends. The volume ratio (Vt /V0) showed large fluctuations, with the drop growing up to 1.9 due to the gas trapped within the slag droplet and then dropping down to a much smaller size with the release of the gas. This trend continued several times during the first 200 s of contact; the volume ratio eventually settled down to 1.1 after 1000 s (Figure 7-14 (b)). For palm shell blends, slag foaming behavior was found to be similar for all cases where after 600 seconds of reaction, the volume sizes increased (Figures 7-12 and 7-14). This might be due to the presence of small bubbles that hold up all through the period of time reactions. P1 blends showed the best gas entrapment and foaming behavior compare to coke and other blends. When more proportion of palm shell in the blends (P2 and P3 blends), the volatile matter was increase compared to P1 blends. Volatile matter was expected to loss during experimental where the substrate needs to stay in the cool zone before purge into hot zone. Even in the cool zone, the temperature is reached 1450 °C. At this temperature, palm shells blend was burn before the exactly time to interact with slag at

227


1550 °C. Thus, the gas released from the carbon/slag interactions for P2 and P3 blends was expected to be low than P1 blends. A stable slag volume is strongly related to an optimum rate of gas generation where 100% palm char shows a steady behavior compared to the three blends. Higher gas generation leads to higher flow rate of gas with increase velocities. When the rate of gas generation is too high, the slag is unable to hold the gases and the foaming phenomenon is affected.

228


70000 (a)

100% MC P1 blends

50000 40000

2

(CO+CO ), ppm

60000

30000 20000 10000 0 0

200

400 600 Time, s

800

1000

2.0 100% MC P1 blends

(b) 1.6

t

Volume ratio, V /V

0

1.8

1.4 1.2 1.0 0.8 0.6 0

200

400 600 Time, s

800

1000

Figure 7-12 (a) Instantaneous gases (CO+CO2), ppm, generated and (b) Volume ratio of 100% Metallurgical coke (MC) and P1 blends with slag with respect to reaction time

229


70000 (a)

100% MC P2 blends

50000 40000

2

(CO+CO ), ppm

60000

30000 20000 10000 0 0

200

400 600 Time, s

800

1000

2.0 100% MC P2 blends

(b) 1.6

t

Volume ratio, V /V

0

1.8

1.4 1.2 1.0 0.8 0.6 0

200

400 600 Time, s

800

1000


230


70000 (a)

100% MC P3 blends

50000 40000

2

(CO+CO ), ppm

60000

30000 20000 10000 0 0

200

400 600 Time, s

800

1000

100% MC P3 blends

2.0 (b) 1.6

t

Volume ratio, V /V

0

1.8

1.4 1.2 1.0 0.8 0.6 0

200

400 600 Time, s

800

1000


231


Figure 7-15 has been plotted for a quick visual comparison of the instantaneous gases (CO+CO2), ppm generated when EAF slag put in contact with coke, palm char, P1 blends, P2 blends and P3 blends. The trend observed in the gas generation data for all samples are expected based on the amount of residual gases available due to palm shells addition and increased surface area following initial gas phase reactions. While the amount of gases generated for coke peaked at a value around 39000 ppm, P1 blends attained 62000 ppm followed by P2 (59000 ppm) and P3 blends (52000 ppm). The lower gas generated was seen with increasing the palm shell in the blends due to lower CO2 gas generations. This might be due to the slower reactivity of CO2 with coke where higher levels of CO2 are left behind which does not help with gas generation of CO. Whereas for 100% palm char, the CO2 reactivity was greater as FeO reduction continues further generated CO2 and converted to CO. The gas was observed over an extended period time (Figure 4-3). Thus, it can conclude that 100% palm char released the gas steady and continuously; this expected due to slowly breaking of the lignocellulosic structure for participation in the subsequent carbon/slag interactions.

232


70000 100% MC 100% PC P1 blends P2 blends P3 blends

50000 40000

2

(CO+CO ), ppm

60000

30000 20000 10000 0 0

200

400 600 Time, s

800

1000

Figure 7-15 Instantaneous gases (CO+CO2), ppm generated from 100% metallurgical coke (MC), 100% Palm char (PC), P1 blends, P2 blends and P3 blends reacting with slag as a function of time

233


7.1.5

Interfacial Phenomena - Optical and SEM Studies

The sample was pushed in the hot zone and the counter was started after the slag droplet showed the first signs of melting. Throughout the experiment the reaction time was closely monitored. The slag droplet resting on the carbonaceous substrate was quenched by withdrawing the supporting tray into the cold zone of the furnace after fixed periods of time, being: 60, 120, 600 and 900 seconds. The optical images exhibited different characteristics depending on the carbon based material used as substrate. Optical micrographs of the quenched samples during reactions at different times were investigated to develop an understanding of the role of the gas entrapment on slag foaming (Figure 7-16 (a) and (b)). Extensive reduction of iron oxide (Figure 7-16 (a)) was observed after 1 min of contact when the EAF slag was put in contact with coke. A large number of molten iron droplets (shiny round particles) were also observed dispersed throughout the slag matrix sitting either at the carbon/slag or gas/slag interface. Gas bubbles trapped within the slag droplet were generally quite small in size. After 10 min of contact, the reduced iron in slag droplet was seen to be precipitated on the substrate. A close inspection of the cross section of the slag/palm char sample revealed a different behavior. From Figure 7-16 (b), we observe a large numbers of small diameter gas bubbles were dispersed throughout the entire slag volume. Thus, overall slag volume is dictated by large number of small bubbles. With increasing time while the large gas bubbles were still present, their numbers were much reduced. The evolution of gases was seen clearly and was due to the reaction of carbon with the FeO in the slag droplet, resulting in the release of CO and CO2 gases. The subsequent entrapment and released these gases contribute to volume fluctuations of the slag droplets; Fe was seen deposited at the slag/carbon interface. At the latter stages of the reaction, as the reduction was approaching completion where the molten metal resting at the interface was appeared as a bright round shiny droplet. Significantly different behavior (Figure 7-17 (c)) was observed when the same EAF slag was put in contact with the residue resulting from P1 blends. From the figure, we 234


observe large bubble and small of gas bubbles dispersed throughout the entire slag volume. Metallic iron is observed at the slag/gas interface and the overall slag volume is dictated by the large number of small bubbles. With elapsing time, small gas bubbles are still present, although not as numerous as the initial stages of reaction. However, these small bubbles contribute to forming a large bubble as seen after 15 minutes of reaction. Metallographic examination of the slag/P2 blends assembly shows in the first minute of reaction small and large gas bubbles in the cross sections (Figure 7-18 (d)). Small shiny particle are dispersed throughout the slag area representing the reduced Fe. Metallic Fe is seen also in different locations such as the slag/gas and the carbon/slag interface. After 15 minutes of reaction, the optical images illustrate the reduced iron is deposited at the carbon/slag interface. Examination of slag/P3 blends assembly, indicated after 60 seconds of reaction, a large gaseous region trapped between the substrate and the slag droplet along with several much smaller gas bubbles (Figure 7-19 (e)). With increasing reaction time, gases still present in the droplet. With the reduction of iron oxide in slag nearing completion, the molten iron droplets tended to increase in size and precipitate on the substrate. It can concluded that 100% palm char showed better foaming where the presence of smaller bubbles all through the time period would sustain the foaming when compared to palm shell blends (Figure 7-11 (b.)). The iron reduction rate was expected to be lower for palm char due to a steady released of gases (Figure 7-24).

235


Metallurgical coke (MC) (b)

(a) Gas Fe

500 µm

500500 µmµm t=120 sec

t=60 sec

Slag

Fe

500 µm

Gas

500 µm 500 µm

t=600 sec

t=900 sec

Palm char (PC) (b)

500 µm

500 µm

t=120 sec

t=60 sec

Gas

Slag

500 µm

500 µm t=600 sec

Fe

t=900 sec

Figure 7-16 Optical microscopy images of (a) Slag/100% metallurgical coke (MC) and (b) Slag/Palm char (PC) as a function of time

236



(a) Gas Fe

500 µm

500500 µmµm t=120 sec

t=60 sec

Slag

Gas

Fe

500 µm

500 µm 500 µm

t=600 sec

t=900 sec

P1 blends (c)

500 µm

500 µm t=60 sec

t=120 sec Slag

Fe

500 µm

Gas

500 µm t=600 sec

t=900 sec

Figure 7-17 Optical microscopy images of (a) Slag/100% metallurgical coke (MC) and (c) Slag/P1 blends as a function of time

237



(a) Gas Fe

500 µm

500500 µmµm t=120 sec

t=60 sec

Slag

Gas

Fe

500 µm

500 µm 500 µm

t=600 sec

t=900 sec

P2 blends (d)

500 µm

500 µm t=60 sec

t=120 sec

Fe

500 µm

500 µm t=600 sec

Gas

Slag

t=900 sec

Figure 7-18 Optical microscopy images of (a) Slag/100% metallurgical coke (MC) and (d) Slag/P2 blends as a function of time

238



(a) Gas Fe

500 µm

500500 µmµm t=120 sec

t=60 sec

Slag

Gas

Fe

500 µm

500 µm 500 µm

t=600 sec

t=900 sec

P3 blends (e)

500 µm

500 µm t=60 sec

t=120 sec

Gas Slag

500 µm

Fe

500 µm t=600 sec

t=900 sec

Figure 7-19 Optical microscopy images of (a) Slag/100% metallurgical coke (MC) and (e) Slag/P3 blends as a function of time

239


In Tables 7-1 and 7-2, the measured bubble diameters from the optical images for various samples at different times are reported. The quantitative estimation of these diameters was carried out using the magnification scale marked on optical microscopy images by Adobe® Photoshop 7.0 software. The minimum bubble diameters observed for palm char were generally smaller than the corresponding values measured for metallurgical coke. Due to their better retention, these smaller bubbles are expected to lead to improved slag foaming. Maximum gas bubble diameters with P3 blends were generally higher than the corresponding values for metallurgical coke. Palm shell/coke blends did not show any well defined trends as a function of palm shells concentration. On the other hand, minimum bubble diameters observed for palm shell/coke blends were generally smaller than the corresponding values measured for metallurgical coke. These smaller bubbles in the blends could contribute to improved slag foaming observed for the blends, due to their better retention, which leads to larger slag volumes.

Table 7-1 Minimum bubble diameter in slag droplets in contact with metallurgical coke, palm char and palm shell blends Minimum Bubble Diameter (µm) Sample

1 min

2 mins

10 mins

15 mins

Met. Coke (MC)

111.2

111.0

83.3

76.8

Palm char (PC)

55.6

55.8

55.5

38.5

P1 blends

76.9

75.4

39.7

34.2

P2 blends

76.8

76.9

38.4

35.5

P3 blends

40.4

76.9

40.5

38.5

240


Table 7-2 Maximum bubble diameter in slag droplets in contact with metallurgical coke, palm char and palm shell blends Maximum Bubble Diameter (µm) Sample

1 min

2 mins

10 mins

15 mins

Met. Coke (MC)

498.5

250.5

194.6

187.5

Palm char (PC)

388.9

400.5

277.8

230.8

P1 blends

384.6

173.1

153.8

326.9

P2 blends

491.5

230.1

192.3

115.4

P3 blends

576.9

384.6

423.1

230.8

These results are consistent with previous research where the minimum size gas bubble diameters in rubber-coke blends/slag leads to better foaming (Rahman, M. M., 2010, Zaharia, M., 2010). Teasdale and Hayes (Teasdale, S. L. and Hayes, P. C., 2005) proposed a gas ferrying mechanism, wherein CO gas produced in the initial reaction between carbon and slag gets transported through the gas phase and reacts with FeO in slag to produce CO2 and metallic iron. This CO2 is ferried back to carbon, where gasification of CO2 to CO takes place via the Boudouard reaction. Gas entrapment in slags as a result of carbon/slag interactions is a dynamic phenomenon depending on a number of physical and chemical factors. In addition, ash oxides present in the substrate can also participate in reduction reaction and associated gas generation. These could also account for excess CO and CO2 generated (Rahman, M., Khanna, R., Sahajwalla, V. et al., 2009). A SEM/EDS with mapping technique, on the slag droplet after 15 min of reaction with carbonaceous material is shown in Figures 7-20 to 7-22. Predominant components in this region were CaO, MgO, Al2O3 and SiO2 with levels of molten iron and sulphur being observed (Figure 7-20 (c)). EDS spectra for metallurgical coke/slag (Figure 7-20 (b)) showed local regions of molten iron superimposed on other slag oxides. Red regions was found in round shapes which represent the reduced metal oxides (FeO). The 241


level of sulphur was higher than palm char/slag (Figure 7-21 (b)). Sulphur plays an important role in suppressing slag foaming which tends to increase the size of CO bubbles and affect stability (Kapilashrami, A. and Görnerup, M., 2006, Morales, R. D. and Rodríguez-Hernández, H., 2003). It was in good agreement where the size of bubbles increases from MC (Table 7-1) due to the influence of high sulphur.

(a)

O 500 µm

S

Fe

(b)

(c) Ca

MC after 15 min reactions

10 µm

Mg

Figure 7-20 (a) Optical microscopy of MC/slag, (b) SEM, mapping on the inner region of quenched MC/slag and (c) EDS spectra of quenched metallurgical coke/slag assembly

at 1550 °C after 15 min of contact

242


Figure 7-21 shows the SEM micrograph with mapping of palm char at 1550 ºC after reaction with slag. EDS analysis is conducted in three specific points to determine the variations of composition. The presence of oxygen was seen while the level of sulphur is not detected. Palm char showed a better slag foaming compared to coke and the influence could be due to lower sulphur and high content of phosphorus (P2O5) and iron oxide (Fe2O3) that present in the ash (Figure 7- 21 (b) and Table 3-1). This analysis is consistent with the results from the size of bubbles where palm char shows smaller bubble sizes compared to coke (Table 7-1). Gaskell and Skupien et. al had summarized the influence of sulphur and phosphorus on surface tension, and P2O5 lowered the surface tension thus allowing more stability of the gas bubble generations. (Skupien, D. and Gaskell, D., 2000).

(a)

O

S

Fe

(b)

500 µm

(c) Ca

Palm char after 15 min reactions

30 µm

Mg

Figure 7-21 (a) Optical microscopy of Palm char/slag, (b) SEM, mapping on the inner region of quenched Palm char/slag and (c) EDS spectra of quenched Palm char/slag

assembly at 1550 °C after 15 min of contact

243


Further, the blends of palm shell and coke in contact with slag is presented in Figure 722. The SEM/EDS with mapping technique shows P1 blends in contact with slag after 15 minutes of reaction. The micrograph indicated that iron is deposited as a result of carbon/slag interactions releasing gas bubbles which lead to fluctuations in the liquid slag droplet. It is assumed that the produced metallic iron has significant levels as seen in EDS spectra. The other selected images show the slag contains CaO, SiO2, MgO and Al2O3. With increasing palm shells content in the carbonaceous blend the gas generation increase which is important for foaming. This can be explained on the basis of increased volatile matter in the palm shells and rates of FeO reduction which leads to greater levels of gas release inside the slag thus will lead to increase the bubble size. The present studies show that an improvement in slag foaming is governed by gas generation, its entrapment and subsequent release. These phenomena are controlled by both properties of carbonaceous materials and slag properties. Carbonaceous materials have a significant influence on the kinetics of reactions that generate gases and also contribute to changes in slag compositions during/slag carbon interactions.

244


(a)

O

S

Fe

(b)

500 µm

(c) Ca

P1 after 15 min reactions

10 µm

Mg

Figure 7-22 (a) Optical microscopy of P1 blend/slag, (b) SEM, mapping on the inner region of quenched P1 blend/slag and (c) EDS spectra of quenched P1 blend/slag

assembly at 1550 °C after 15 min of contact

245


7.2

FeO Reduction

In the direct reduction, carbon is presumed to react directly with iron oxide (Fe2O3), which is very rapid then converted to FeO, followed by further reduction of FeO to Fe, producing carbon monoxide with carbon dioxide. The reduction of FeO to Fe will takes place at a slower step (Rao, Y., 1971). The direct reduction may be visualized as beginning at the points of contact between iron oxide and carbon particles; and oxygen is removed from the solid in the form of CO and CO2. The assumption can be made into 2 steps; where a slag containing FeO and Fe2O3 can be reduced by carbon (Equation 211 and Equation 2-12). As it assumed that the reduction of Fe2O3 to FeO occurs quite rapidly it has not been considered for calculation purposes. Based on previous studies (Fruehan, R., 1977) which indicated that in the first stage of reduction, Fe2O3 to FeO, the reaction product was essentially all CO2, the possibility of carbon gasification reaction in controlling the reduction rate is expected. The off-gas analysis resulting from iron oxide rich slag in contact with various carbonaceous materials in the present study allowed the quantification of the reduction reaction. Carbon based materials such as coke, coal chars and graphite are commonly used to remove oxygen from the metallic oxide component. The gas generated from above mentioned reaction will form bubbles at the carbon/slag interface leading to the formation of a gas film. The maximum amount of oxygen that can be removed from both the slag and the carbonaceous substrate is calculated in terms of number of moles of oxygen. The oxygen content in the evolved gas by the reduction reaction can be accounted for the reduction of FeO in the slag. According to the mass balance for oxygen, the following relationships can be derived:

FeO + xC = Fe + (2 x − 1)CO + (1 − x)CO2 − n FeO =n CO +2.nCO

−

d (nFeO) = − A × n FeO dt

Eq. 7-1 Eq. 7-2 246


where: n and A are respectively the number of moles (mole-1), and A is the reaction area (cm2). The data obtained from IR analyzer on the reduction reactions during carbon/slag interactions are presented in Figure 7-23. The off-gas analyzer monitored the amount of CO and CO2 gases evolved during the carbon/slag interactions, ppm values. The volume flow rates of CO and CO2 gases were calculated considering that 1 liter/min of Argon was purging the furnace environment. Based on standard temperature and pressure (STP) conditions one mole of a standard gas occupies 22.4 liters’ of volume. From Equation 7-2 the time dependent reduction involves the removal of one mole of oxygen during the reduction of one mole of FeO. Thus, each of moles of CO detected corresponds to a mole of oxygen removed from the FeO present in the slag, while each mole of CO2 corresponds to two moles of oxygen. A further conversion into equivalent number of moles of oxygen removed have been considered and plotted as a function of time (Figure 7-23). From Figure 7-23 (a), palm char samples showed a higher number of moles of oxygen were removed from the slag when compared to the amounts of moles of oxygen from the interaction with metallurgical coke however with a slower rate. When palm shells were added with coke (P1 blends), the oxygen content estimated from the evolved gases increased further when compared to coke/slag assembly (Figure 7-23 (b)). However, when palm shells accounted for higher proportions in the carbonaceous blends, as P2 and P3 blends; the amounts of oxygen removed were seen decreased. This might be attributed due to lower of CO2 released from agricultural waste materials. P1 blends/slag led to the highest amount of removed oxygen accounting for the highest amount of oxygen removed when compared to all carbonaceous substrates. It was expected because after combustion in DTF, the residual char derived from the palm shell blends conserved unburned volatiles as Table 3-7 shows. Moreover, the role of solid coke was expected to gasify the CO2 and generated CO, thus would release more gases compared to other blends where more palm shells rather than coke. Modifications in the carbonaceous structure of palm shell/coke blends appeared during the initial gas phase reactions (Yunos, N. F. M., Zaharia, M., Ahmad, K. R. et al., 247


2011) which improved the generation of reducing gas and thus increased the quantity of removed oxygen. The rate of gas generation is enhanced when palm shell/coke blends are compared to coke alone. Another factor that could influence the total of moles removed could be the gases generated from breaking of lignocellulosic structure from palm shells. In Chapter 4 showed that the breaking of lignocellulosic structure at high temperature will release a considerable amount of CO and CO2 gases that will participate in carbon/slag interactions.

248


0.0005

0.0003 0.0002 0.0001 0.0000 0

200

400 600 Time, sec

0.0005

0.0003 0.0002 0.0001

200

400 600 Time, sec

800

1000

0.0003 0.0002 0.0001

200

400 600 Time, sec

0.0005

800

1000

100% MC P3 blends

(d)

Total removed O, moles

0.0004

0.0004

0.0000 0

1000

100% MC P2 blends

(c)


800

100% MC P1 blends

(b)



0.0004

0.0000 0

0.0005

100% MC 100% PC

(a)

0.0004 0.0003 0.0002 0.0001 0.0000 0

200

400 600 Time, sec

800

1000

Figure 7-23 Total removed oxygen, moles as a function of time and carbonaceous material used

The change in FeO content in the slag was calculated using the amount of oxygen evolved of the blends and the initial FeO content in the slag (see Appendix B). The FeO concentration in the slag with respect with to time is estimated and further represented in Figure 7-24. Three distinct stages are observed: I. An incubation period attributes to a delay due to the time required for the transport of the gas and the response from the IR analyzer. The remaining time period before the initiation of reaction is possibly due to the nucleation and growth of the generated gas bubbles. 249


II. A steady state period. III. A degradation period characterized by local deficiency of FeO at the slag/gas interface. Because alterations in experimental conditions change the reaction rates of region (I) and (III), the reaction-rate constant is estimated from the slope region (II), where the concentration of FeO changes linearly with time.

FeO concentration, %

40

Temp: 1550 ºC Stage I Stage II

30

100% MC 100% PC P1 blends P2 blends P3 blends

20 Stage III

10 0

0

50 100 150 200 250 300 350 Time, sec

Figure 7-24 FeO concentration in the slag with proceeding reaction

250


An initial slow reduction section followed by a linear rapid reduction and finally a slow degradation region characterised by local deficiency of FeO. The pure palm char showed the largest duration of the initial slow region followed by the pure coke; the length of the initial slow region appeared to decrease with a decreased in the level of palm shells blended with the coke (Figure 7-24). The reduction of the FeO present in the slag took place in about 300 seconds, and FeO in the slag was reduced completely for coke sample. When P1 blends was the carbonacous substrate, the FeO concentration in the slag decreased faster being consumed in about 160 seconds. Increasing the palm shell in the blends as in P3 blends, a lower reduction of the available FeO in the slag was observed. However, all the carbonaceous blends showed faster FeO reduction when compared to coke alone. On the other hand, 100% palm char showed a lower rate of FeO reduction and taking a longer period of time to be completely reduced in about 360 seconds. From Figure 7-24 the time required for complete reduction of the FeO from the slag decreased with decreasing amount of palm shell blended in coke. The role of solid coke was expected to gasify the CO2 and generated CO, thus would release more gases compared to other blends where more palm shells rather than coke. The relatively lower times of reduction recorded for the blends (50 to 60 seconds) compared to pure coke may be attributed to the presence of a hydrogen environment provided by the agricultural waste materials. Higher hydrogen contents promote fluidity at high temperatures and gas formation (Ono-Nakazato, H., Yonezawa, T. and Usui, T., 2003, Sohn, I. and Fruehan, R., 2005). Hydrogen gas, apart from being a faster reducing agent than both carbon monoxide and solid carbon also enhances the rate of reduction of iron oxides by carbon monoxide (Ono-Nakazato, H., Yonezawa, T. and Usui, T., 2003) when it is added to a reduction system containing the latter. Optical and SEM studies are cnsidered to support the calculations based on gas generation data and are presented in section 7.1.5.

251


7.2.1

Estimation of the reaction rate constant

The rate of reduction is equivalent to the rate of CO and CO2 gases produced from the system. Iron oxide is the main oxide getting reduced as a result of the interaction of slag with various carbonaceous materials under investigations. Based on these gases, the variation of the percentage of FeO remianing in the system is plotted as a function of time. The slope of the graph is calculated to determine the reaction rate (R) by considering linear portion. A first order kinetic reduction is asummed and a similar aprroach has been reported in the literature (Bafghi, M., 1993, Mehta, A. S. and Sahajwalla, V., 2000, Sarma, B., Cramb, A. and Fruehan, R., 1996, Story, S., Sarma, B., Fruehan, R. et al., 1998).

R FeO = ( S FeO / 100) * ρ s / MWFeO

Eq. 7-3

where: RFeO is the reaction rate of FeO (moles/cm3 s), SFeO is the slope of the graph, % FeO/s, ρs is the density of the slag (g/cm3) and MWFeO is the molecular weight of FeO (g/mole). On the basis of the reaction rate, R, calculated using Eq. 7-4, the apparent reaction rate constant is calculated as follows:

RFeO = K 0 ( A / Vslag )

Eq. 7-4

Combining Eq. 7-7 and 7-8 the reaction rate constant is given by Eq. 7-5:

K0 =

S FeO ρ slag A . . MWFeO 100 Vslag

Eq. 7-5

where: K is the reaction rate constant in moles/cm2 s, A is the interfacial area of contact in cm2 and Vslag is the slag volume in cm3. The reaction rate, expressed in mol.cm-2.s-1 is the slope of the points devided by the interfacial area. The rates are deduced from CO and CO2 gas volumes assuming the oxygen is removed as CO and CO2. From Figure 7-25, it was noted that previously in 252


section 7.1.1, that the CO gas generation (moles) decreases proportionally to palm shell content in the blends, recording the highest value when the blend contained the lowest proportion as in P1 blends. Using Eq. 7-4 to 7-5 and the amount of gas generated from reaction of slag/coke and slag/coke-palm shell blends, the rates of reduction were calculated and are presented in Figure 7-25 as a function of carbon materials used as a substrate.

6.0 5.0 4.0

P3 blends

P2 blends

1.0

P1 blends

2.0

Palm char

3.0 Met. coke

2

-5

Rate constant, moles/cm s (x10 )

7.0

0.0

Figure 7-25 Reaction rate constant as a function of carbon material used

The rate of reaction of FeO (Table 7-3) decreased with the proportion of palm shells blended with the coke, with the blends recording much higher values than for pure coke or pure palm char. For 100% metallurgical coke, a value of 2.16 x 10-5 moles/cm2 s was calculated as the maximum rate, while a lower value of 0.937 x 10-5 moles/cm2 s was calculated for the 100% palm char. The corresponding rates for the blends were 6.23 x 10-5 moles/cm2 s, 4.37 x 10-5 moles/cm2 s and 3.10 x 10-5 moles/cm2 s for blends P1, P2 and P3, respectively. 253


Table 7-3 Reaction rate constant (moles/cm2 s) for 100% metallurgical coke, 100% palm char and palm shell/coke blends Substrate

Reaction rate, K (moles/cm2 s)

Met. coke

2.16283 x 10-5

Palm char

0.93721 x 10-5

P1 blends

6.22927 x 10-5

P2 blends

4.37068 x 10-5

P3 blends

3.09583 x 10-5

Slower rates of gas generation from 100% palm char made it easier for slag to trap gases. A relatively slower rate of gas generation could have a lower impact on the modification of slag composition and would allow for gas bubbles to be trapped in the slag sample instead of rapidly escaping from the slag (Yunos, N. F. M., Zaharia, M., Ahmad, K. R. et al., 2011). These resulted in the better slag foaming in the case of the palm char compared to coke. The reason for such a behavior might be the oxygen and oxides in the ash of palm char which reacted as surface active. Oxygen present will decrease the surface tension in iron melt and the interfacial tension at the slag/metal boundary. Surface active elements in liquid have shown to retard the rate of interfacial reactions (Richardson, F. D., 1974). Oxides such as P2O5 and SiO2, which are surface active in slags, have shown to reduce the rate of reduction of iron oxide in slags. Palm char sample contains high P2O5 and SiO2 in the ash might be one of the factors to lower the rate of FeO reduction. The other reason of lower rate of reduction might be due to the low of fixed carbon content (55.6 wt. %) where more carbon is needed to react with slag in producing more gases. In a study performed by Fruehan (Fruehan, R., 1977) on the rate of reduction of iron oxide by different carbon based materials such as coconut charcoal, coal char and metallurgical coke in an inert atmosphere (T = 900 ºC and T = 1200 ºC), a strong dependence of the reduction rate on the type and the amount of carbon used was seen. 254


Thus, palm shell blends showed better reduction compared to coke and palm char due to the carbon in coke which could gasify CO2 to CO gas during the interactions. Such findings support an overall rate of reduction controlled by the rate of oxidation of carbon. At high temperatures, oxidation of carbon and partially mass transfer of FeO was reported to control the overall reaction.

255


7.3

Discussion on Slag Foaming of Carbonaceous Material

Interfacial phenomena occurring between polymer/coke blends with steelmaking slags were investigated at 1550 ºC (Rahman, M., 2006, Zaharia, M., Sahajwalla, V., Kim, B.-C. et al., 2009). This section presents in-depth discussion on the effect of palm shell blends and char, on the slag/carbon interactions, along with the effect of rubber and polyethylene (PET), which are polymers containing high levels of volatiles. The substitution of waste polymers by blending with metallurgical coke was found to modify blends characteristics; significant differences have been observed in the carbon/slag interactions of polymer/coke blends in previous studies (Zaharia, M. 2010 and Kongkarat, S. 2011). Metallurgical coke, rubber/coke blends (R3), polymers, namely Polyethylene Terephthalate (PET)/coke blends, palm shell/coke blends (P3) and 100% palm char have been chosen for comparison in the slag foaming behavior due to distinct in chemical structure. Rubber contains high carbon, sulfur and hydrogen while PET contains carbon, hydrogen and high oxygen. The discussions on volatiles produced from rubber and PET are presented to compare them with palm shells which contains volatiles and oxygen. Table 7-4 shows the chemical composition of rubber, PET, palm char and metallurgical coke. Table 7-4 Chemical composition of coke, palm char, rubber and PET Materials

VM

C

H

O

N

S

Ash

Met. coke

6.1

80.4

1.13

-

1.24

0.36

17.2

Palm char

33.6

76.5

5.73

17.5

0.58

0.05

5.6

P3 blend

25.0

71.8

5.8

21.5

0.91

0.04

13.5

*Rubber, R3

20.9

84.7

7.6

-

-

2.0

5.7

**PET

23.1

62.5

4.2

33.3

-

-

-

*(Zaharia, M., 2010) **(Kongkarat, S., 2011) 256


7.3.1

Influence of Gas Generation on Slag Foaming

The rate of gas generation has been reported to have a significant effect on the slag foaming behavior and this influence was found to depend on the basic characteristics of carbonaceous materials. Rahman et. al. (Rahman, M. M., 2010, Sahajwalla, V., Rahman, M., Khanna, R. et al., 2009) investigated interactions between an EAF slag containing high levels of FeO with metallurgical coke and natural graphite at 1550 ºC using the sessile drop technique. These authors found that the gas generation of both CO and CO2 from the slag/coke assembly was significantly higher than that of slag/graphite, also indicating an extensive/rapid FeO reduction by metallurgical coke. Even though the rate of gas generation for coke was very high, poor gas entrapment was observed within molten slag. Rapid gas generation can lead to the escape of gas bubbles from the slag sample. It was mentioned that high levels of gas generation resulted in a strong likelihood of convective transport of reactants and products across the slag/coke interface with oxides in coke ash partially dissolving in molten slag and modifying slag composition. The carbonaceous materials (coke, 100% palm char and P3 blends) under investigation showed significant differences in gas generation and entrapment and their foaming behavior (Figure 7-26). Metallurgical coke showed lowest level of gas generation as well as lowest slag foaming which was not sustained over extended period. High levels of off-gases (CO, CO2) were emitted from the slag/metallurgical coke substrate; CO2 levels were relatively much higher than 100% palm char. 100% palm char showed a steady gas released and continuously fluctuating for a longer time. On the other hand, P3 blends showed increased gas generation compared to coke and fluctuating behavior with droplets increasing in size and then releasing gas. A high volume ratio greater than 1 was maintained for an extended period.

257


100% MC 100% PC P3 blends

2.0 (a) 1.6

t

Volume ratio, V /V

0

1.8

1.4 1.2 1.0 0.8 0.6 0

200

400 600 Time, s

800

1000

70000 (b)

100% MC 100% PC P3 blends

50000 40000

2

(CO+CO ), ppm

60000

30000 20000 10000 0 0

200

400 600 Time, s

800

1000

Figure 7-26 (a) Volume ratio of 100% Metallurgical coke (MC) and 100% Palm char (PC) and P3 blends with slag and (b) Gases (CO+CO2), ppm, generated with respect to reaction time

258


Just from visual observation these carbonaceous materials (palm shells) influenced the foaming behavior. High temperature images captured during specific times of reaction showed an increase in volume when carbonaceous material consisted palm shells, while coke show similar slag droplet (Figure 7-27 (a) – (c)). Volume ratio measurements and optical imaging are also considered to determine the gas entrapment and the size of

(c) P3 blends

(b) 100% Palm char

(a) 100% MC

bubbles present in the slag after interactions with coke, 100% palm char and P3 blends.

t = 60 sec

t = 120 sec

t = 600 sec

t = 900 sec

Figure 7-27 High temperature images of slag droplet in contact with (a) metallurgical coke (MC), (b) 100% palm char (PC) and P3 blends at 1550 ºC as a function of time

259


Slag composition can be affected in two different ways: reduction reactions and possible transfer of ash oxides from coke into molten slag. The extent of FeO reduction as a function of time can in turn lead to the differences in physical properties of the molten slag, such as viscosity, surface tension and density, and this would affect slag foaming (Paramguru, RK 1997). In the present study, palm char showed a relatively slow rate of CO and CO2 generation compared to the case of coke; and a greater level of gas entrapment was observed in the slag sample. A higher rate of gas generation in the case of coke may lead to local turbulence at the interface and transfer ash oxides into the molten slag and modify slag composition. The modification of slag composition may lead to the changes in the physical properties of slag, and thus impact the ability of slag to hold the gas (Rahman et al. 2009). Conversely, a relatively slower rate of gas generation in the case of palm char could cause the gas bubbles to be trapped in the slag sample instead of rapidly escaping from the slag. In the other case, palm shell blends showed higher rate of FeO reduction compared to coke (Table 7-3). While in the case of rubber (Zaharia, M., 2010) their interaction with slag showed a high rate of gas generation which resulted a higher FeO reduction rate compared to coke. These blends (rubber/palm shells blend) showed similar behaviour due to volatile matter content, where palm shell/coke has 25 wt. % and rubber/coke blend has 20.9 wt. % (Table 7-3). Optimal rates of gas generations are beneficial for both improved foaming and FeO reduction. The higher rates of gas generation for blends compared to coke are important for optimisation. For 100% palm char, the FeO reduction rate is about half that of coke which indicates that the blends lead to optimum outcomes not the 100% palm char. These showed palm char is better in slag foaming compared to coke.

260


7.3.2

Influence of Volatiles from the Agricultural Wastes/Polymers on Slag Foaming

The foaming of slag is predominantly caused by the retention of CO and CO2 generated from the reactions between slag and solid or solute carbon. The palm chars are expected to improve the carbon/slag interactions due to the availability of gases such as CO and CO2 and other hydrocarbons above 900 ºC. When palm shells reacted with an iron oxide rich EAF slag, the presence of FeO leads to a reduction reaction depending on the reducing agents including C, CO and H2 released at high temperature (Chapter 4). This result is attributed to the presence of hydrogen that promotes gas formation. H2 peaks were also detected in the last stages of thermal decomposition (Figure 4-1 in Chapter 4). H2 comes from the condensation of aromatic structures (lignin) or the decomposition of heterocyclic compounds (cellulose/ hemicellulose), processes that occur at high temperatures (Haiping, Y., Yan, R., Chen, H. et al., 2007, Hasegawa, I., Tabata, K., Okuma, O. et al., 2004, van Heek, K. H. and Hodek, W., 1994 ). Hydrogen gas, apart from being a faster reducing agent than both carbon monoxide and solid carbon, also enhances the rate of FeO reduction (Fruehan, R., 1977). It can be seen that from equations: FeO + H2 (g) = Fe + H2O

Eq. 7-1

FeO + CO (g) = Fe + CO2 (g)

Eq. 7-2

The reduction of FeO by H2 and CO will produce CO2 and H2O. Moreover, CO2 produced by Eq. (7-11) will react with any available carbon at high temperatures, thus the key advantage palm shell comes from its increased hydrogen content, in comparison to metallurgical coke. Interactions between FeO containing slag with plastics/coke and rubber/coke blends at 1550 ºC using the sessile drop technique were respectively investigated by (Kongkarat, S., 2011, Rahman, M. M., 2010, Zaharia, M., 2010). Using a similar proportion of agricultural waste/coke blend used in the present study, (Rahman, M. M., 2010) reported that the maximum FeO reduction rate for metallurgical coke was 1.52 x 10-5 261


mole.cm-2.s-1, while the reaction rate was observed to increase when a HDPE/Coke blend was used with a maximum value of 1.91 x 10-5 mole.cm-2.s-1. Similarly, (Zaharia, M., 2010) reported that the maximum rate of FeO reduction for metallurgical coke was 1.89 x 10-5 mole.cm-2.s-1, while the reaction rate was also found to increase when a rubber tyre/coke blend was used with a maximum value of 2.5 x 10-5 mole.cm-2.s-1. Recently, the FeO reaction using PET/Coke was also conducted at 1550 ºC by (Kongkarat, S., 2011), where the reaction rate for PET/coke blend was 1.5 x 10-5 mole.cm-2.s-1 and coke was 1.74 x 10-5 mole.cm-2.s-1. The reaction rate for coke, palm shell/coke and 100% palm char in the present study were compared with other values reported in literature and is shown in Table 7-5. It was found that rubber showed a higher rate of FeO reduction than that of coke. However, the results observed in the present study indicate an opposite trend for 100% palm char, where the rate of FeO reduction was found to be lower than that for coke. The lower rate of palm char might be due to low fixed carbon; while in the case of rubber and palm shell blends, they showed similar high rate of FeO reduction with high fixed carbon content.

Table 7-5 Comparison of the maximum rate of FeO reduction by different carbonaceous materials obtained from literature Materials

Reaction Rate

Researcher

Coke

1.89 x 10-5 mole.cm-2.s-1

(Zaharia, M., 2010)

Rubber/Coke

2.5 x 10-5 mole.cm-2.s-1

(Zaharia, M., 2010)

Coke

2.2 x 10-5 mole.cm-2.s-1

Present study

Palm char

0.94 x 10-5 mole.cm-2.s-1

Present study

Palm shell/Coke

3.1 x 10-5 mole.cm-2.s-1

Present study

262


The differences between the values of reaction rates in the case of metallurgical coke reported in the present study and from the previous studies (Kongkarat, S., 2011, Zaharia, M., 2010), are possibly due to the difference in the properties of slags and cokes used. In the case of polymer/coke blends and palm shell/coke blends, the polymer and lignocellulosic characteristics were found to have a significant effect on their interactions with slag. As shown in Table 7-4, e.g. rubber tyre which contains high levels of carbon and hydrogen where higher carbon and hydrogen is in the form of volatile matter (CH4 and H2), can aid and supplement gas generation and FeO reduction when reacted with molten slag. In the case of 100% palm char, the oxygen content that present could also oxidize Fe product to generate FeO. That is why 100% palm char shows an extended time period which the reaction appears to take place. The influence of oxygen which may reoxidize some of the reduced Fe in the slag, then reform FeO into the bulk slag (Kongkarat, S., 2011). These results show that FeO in the slag was not only reduced by solid carbon, but also by volatiles in the 100% palm char when compared to rubber/coke blends where rubber does not contain any oxygen. These additional reactions can contribute to the improvement in slag foaming behaviour in the case of palm shell/coke blends compared to coke. Ozawa et. al (Ozawa, M. and Kitagawa, S., 1986) investigated the reduction of FeO in molten slags by solid carbon in electric arc furnace steelmaking and pointed out that factors like quality of solid carbon injected, boundary area of reaction, wherein solid carbon reacts with FeO and nature of slag could influence the reduction rate of FeO in slag. They reported that, the volatile matter in solid carbon has a greater influence on the reducing reactions. The reduction of carbon materials with high volatile was controlled by the chemical reactions, while in the carbon with lower volatile matter; the reduction was controlled by the transport of FeO in slag.

263


7.3.3

Influence of Mineral Matter from the Agricultural Wastes on Slag Foaming

Due to high levels of gas generation, there is therefore a strong likelihood of gas escaping quickly due to greater velocity. Due to extensive reduction of iron oxide, its levels in slag will be lowered. According to slag phase diagrams (Slag Atlas, 1995), a lower iron oxide content will increase the melting point of slag; which will in turn reduce its liquid fraction and enhance the viscosity of the slag. Surface active elements, such as oxygen and sulphur can affect the surface tension (γ) of the liquid metal (Jimbo, I. et al., 1993). A previous study on surface tension and interfacial tensions shows that the surface tension of liquid iron decreases with increasing oxygen and sulphur content. Oxygen is expected to come from ash of palm char and mineral matter (oxides) in the coke. The influence of oxygen which may reoxidize some of the reduced Fe in the slag, then reform FeO into the bulk slag. In the present study, the FeO level should decreased ever time, thus the rate will be lower as we see in the case for 100% palm char. Other interactions such as mineral matter (SiO2) from metallurgical coke affect the surface tension and making it easier for gases to escape (Hara, S. and Ogino, K., 1986, Rahman, M. M., 2010). Silica was a mineral matter (57.4 wt. %) present in coke ash (17.2 wt. %) (Table 3-3 and Table 7-4), and its diffusion from the substrate into slag will lead to a lowering of surface tension of the slag. Alumina was another significant mineral matter (26.5 wt. %) present in coke ash. According to atomistic simulations of (Khanna, R. and Sahajwalla, V., 2005), poor wetting between alumina and molten iron results in a strong tendency between alumina and liquid iron to be mutually exclusive from their immediate neighborhood. A similar finding was observed from (Mehta, A. S. and Sahajwalla, V., 2000). In the present study, the interactions of palm char (5.6 % ash) and metallurgical coke (17.2 % ash) with EAF slag showed major differences in gas generation, carbon/slag interactions and rate of iron oxide reduction (Table 7-3). Metallurgical coke showed rapid iron oxide reduction, very high rates of gas generation but poor foaming behavior. Palm char on the other hand showed excellent slag volumes but slower reduction of iron oxide. Slower rates of gas generation made it easier for slag to trap gases. The reason 264


for such a behavior might be due to the components in the mineral matter (ash) of palm char which are surface active (P2O5 and SiO2). Surface active components in liquid have shown to retard the rate of interfacial reactions (Richardson, F. D., 1974). Components such as P2O5 and SiO2, which are surface active in slags, have shown to reduce the rate of reduction of iron oxide in slags. Palm char sample contains P2O5 and SiO2 in the ash which might be one of the factors for lower rate of FeO reduction (Table 3-3). The influence of P2O5 and sulphur were studied and the previous authors observed that the addition of sulphur increased surface tension while more phosphorus oxide caused a decrease in surface tension, thus P2O5 was considered to be surface active (Figure 211). Figure 7-28 shows the influence of S and P2O5 addition on the surface tension of 30% FeO containing slags. Similar behavior has been observed in previous work Kozakevitch, Elliot, Bhattacharyya and Gaskell. (Bhattacharyya, P. and Gaskell, D., 1996, Elliott, J. F., 1988, Kozakevitch, P. and Olette, M., 1971).

7.3.4

Influence of Carbon Structures from the Agricultural Wastes on Slag Foaming

The other factor that influence a better foaming from agricultural waste is the interactions of lignocellulosic structure (cellulose, hemicellulose and lignin) in palm shells to break at high temperatures could also influence for low FeO reduction rate (Table 7-6). Hydrogen bonding, intermolecular and intramolecular, is recognized as one important linkage between cellulose and hemicellulose (Henriksson, Å. and Gatenholm, P., 2001, Schmidt, M., Gierlinger, N., Schade, U. et al., 2006) while hydrogen bonding in lignin was found to exist chemically linked to polysaccharides (Miyoshi, K., Uezu, K., Sakurai, K. et al., 2006). It was found that the hydrogen bonding in lignin is more stable at high temperatures (Zhang, X., Yang, W. an d Blasiak, W., 2011) which is attributed to the high degree of branching and formation of highly condensed aromatic. Thus, the gas generated from palm shell char was released at slower rate due to the complex structure of lignin. 265


Table 7-6 Type of crystallization of polymers and molecular bonds in the carbonaceous materials (Antal, M. J., Jr. and Varhegyi, G., 1995, Orfão, J. J. M ., Antunes, F. J. A. and Figueiredo, J. L ., 1999, Sharma, R. K., Wooten, J. B ., Baliga, V. L. et al., 2004) Samples

Crystallization Structure

Bond

amorphous (turbostatic)

C=C

Lignin

amorphous

(-C2H2-), π bonds aromatic

Cellulose

crystalline

[(C6H10O5)n]

Hemicellulose

amorphous

[(C5H8O4)n]

Rubber (SBR)

amorphous

C-C within aromatic ring, C-S

Met. coke

Rubber is characterized by a three dimensional framework following the vulcanizing process when sulfur was introduced and C-S and S-S bonds were formed (Kameda, T. and Asakura, T., 2003). When rubber content was introduced in the carbonaceous blend, the gas generation increases compared to coke which is important for foaming. This can be explained on the basis of increased volatile matter in the rubber and higher rates of FeO reduction which leads to greater levels of gas release inside the slag. This can be understood on the basis of influence of sulfur on the surface tension of slags. Gaskel and Skupiel (Skupien, D. and Gaskell, D., 2000) measured the surface tensions in CaO-FeO-SiO2 system with a view to provide a better understanding of the phenomenon of slag foaming and considered the influence of S on steelmaking slags. Each slag contained 30 wt. pct FeO with the balance varying from CaO/SiO2 (wt. pct) = 0.43 to CaO/SiO2 (wt. pct) = 1.5. Sulfur was introduced as CaS and the surface tension of melts in the range of 0 to 3 wt pct were measured at 1400 ºC. Increasing the surface tension (effects of sulfur) leads to an increase in the interfacial energy and thus will lead to increasing the bubble size (Table 7-7). Therefore, as sulfur influences size of bubbles, eventually an increase in minimum bubble size decreases the 266


stability of foam. This suggested that when there is greater proportion of rubber in the system the stability of foam will decrease. Palm shell/coke blends sample with low sulfur content, was expected to improve the foam stability compared to rubber/coke blend samples. Figure 7-28 showed the comparison of high temperature images captured during specific times of reaction showed an increased in volume of palm shell/coke blends (P3) and rubber/coke blends (R3).

R3 blend

(a)

P3 blend

(b)

t = 60 sec

t = 120 sec

t = 240 sec

Figure 7-28 High temperature images of slag droplet in contact with (a) rubber/coke blend (R3) (Zaharia, M., 2010) and (b) palm shell/coke blend (P3) at 1550 ºC as a function of time

A series of cyclic reaction (Donskoi, E. and McElwain, D., 2003, Shi, J. Y., Donskoi, E., McElwain, D. L. S. et al., 2008) is set up as the carbon monoxide and hydrogen (products of the carbon gasification) in turn partially react with the iron oxide and further reactions continue. These cyclic reactions persist until all the iron oxide has been reduced to metallic iron, provided there is enough carbon in the system (Shi, J. Y ., Donskoi, E., McElwain, D. L. S. et al., 2008) as is the case in the present study. It is 267


thus clear that the presence of hydrogen and some fixed carbon in a carbonaceous material are both essential to initiate and maintain these cyclic reactions to obtain a high rate of reduction. This clearly explains why the blends generally perform better than both pure coke and the pure agricultural waste. The rates of reduction were presented to show the blend is better than the parent materials (palm char and coke) in Figure 7-29 as a function of carbon materials used as a substrate; where 100% metallurgical coke, a value of 2.16 x 10-5 moles/cm2 s, 100% palm char showed value of 0.937 x 10-5

3.5 3.0

1.5 1.0 0.5 0.0

P3 blends

2.0

100% Palm char

2.5 100% Met. coke

2

-5

Rate constant, moles/cm s (x10 )

moles/cm2 s and P3 blend showed 3.10 x 10-5 moles/cm2 s.

Carbonaceous material

Figure 7-29 Reaction rate constant as a function of carbon material used

268


7.3.5

Influence of Entrapped Gas Bubbles in the Bulk Slag on Slag Foaming

The foaming of slag is a result of the gas bubbles being retrained in the bulk liquid slag, and is influenced by amount and size of the entrapped gas bubbles. ((Zhang, Y. and Fruehan, R., 1995)) studied the effects of gas bubble size generated by argon gas injected into liquid slag and by the slag/metal interfacial reactions between the solute carbon in liquid metal and FeO in the slag on slag foaming. The authors concluded that foams with very fine bubbles have spherical bubble cells and are very stable, while foams with larger bubbles are less stable. (Rahman, M. M., 2010) measured the diameters of entrapped gas bubbles in the quenched droplets after reaction with coke and HDPE/Coke blends. This author reported that the minimum gas bubble diameter of the quenched slag droplets after 2, 4 and 8 minutes of reaction with coke was 90, 92 and 92 µm, respectively, while it was 29, 36, 39 µm in the case of HDPE/Coke (Figure 7-31 (a)). (Kongkarat, S., 2011) reported PET/Coke showed the smallest gas bubbles (36-62 µm) (Figure 7-30 (c)). The small gas bubbles are generated from the reduction reactions of FeO and these bubbles could contribute to both sustaining and improving the slag foaming behavior where a similar trend had been reported by (Rahman, M. M., 2010) showing that the gas bubble sizes for polymer/coke blends are generally smaller than that for coke alone, which is expected to lead to a better slag foaming behavior. A close inspection of the cross section of the slag/carbonaceous sample revealed the small gas bubbles in Figure 7-30.

269

500 µm

500 µm

500 µm

500 µm

500 µm

500 µm

500 µm

500 µm

(e) 100% Palm char

(d) Palm shell/ coke (P3)

(c) PET/coke (PET 3)

(b) rubber/ coke (R3)

(a) HDPE/coke (HDPE 3)


t = 60 sec

t = 120 sec

t = 600 sec

t = 900 sec

Figure 7-30 Optical microscopy images of (a) Slag/HDPE blend (HDPE 3), (b) Slag/rubber blend (R3), (c) Slag/PET blend (PET 3) (Kongkarat, S., 2011, Rahman, M. M., 2010, Zaharia, M., 2010), (d) Slag/palm shell (P3) blend as a function of time and (e) Slag/100% palm char (PC)

In the present study, the bubble diameters of palm char (38-56 µm) were smaller than coke and rubber/coke (37-85 µm)(Zaharia, M., 2010) in Figure 7-30 (b) and (d). The 270


size distribution of entrapped gas bubbles also contributes to the differences in slag foaming behavior of the samples. The diameters of the gas bubbles entrapped in the slag droplet after reaction with coke and carbonaceous blends were classified into 2 ranges based on their sizes. Bubbles were classified as small if their size < 100 μm and large if their size >100 μm. The ranges of diameters of the small gas bubbles entrapped in the slag are compared with the results from previous study in Table 7-7.

Table 7-7 The range of diameters of small gas bubbles entrapped in the slag droplet for coke and its blends with polymer between 2 – 10 minutes of reaction Samples Range of Small Bubbles Diameter between 2-10 Minutes of Reaction (µm) Sample

Bubbles Diameter (µm)

Researcher

Met. Coke (MC)

77-111

Present study

Palm char (PC)

38-56

Present study

P3 blends

39-77

Present study

HDPE/Coke blend

29-39

(Rahman, M. M., 2010)

PET/Coke blend

36-62

(Kongkarat, S., 2011)

Rubber/Coke blend

37-85

(Zaharia, M., 2010)

It was observed that the range of small bubbles diameters in the case of palm shell char observed between 2-10 minutes of reaction were somewhat smaller than the corresponding values for coke (77-111 µm). The small gas bubbles are generated from the reduction reactions of FeO and these bubbles could contribute to both sustaining and improving the slag foaming behavior. This explains the observations that palm shell containing samples show better foaming than coke alone. These results also show a trend similar to that reported by (Kongkarat, S., 2011, Rahman, M. M., 2010) and (Zaharia, M., 2010) showing that the gas bubble sizes for polymer/coke blends and

271


rubber/coke blends are generally smaller than that for coke alone, which is expected to lead to a better slag foaming behavior for these blends compared to coke alone. Detailed results presented in this section have clearly brought out the significant differences observed in a number of aspects of carbon/slag interactions for a range of carbonaceous materials. This in-depth study has shown that differences in key characteristics of carbonaceous materials can have a significant influence and result in a wide variety of interfacial phenomena and slag interactions. These carbonaceous materials (palm shells) are essential for maintaining sustained slag foaming through gas generation and subsequent retention of trapped gas bubbles within the slag.

272

Chapter 8: Conclusions – Carbon/Slag Interactions

CHAPTER 8 8 Conclusions – Carbon/Slag Interactions Studies

The present study has focused on interactions between palm char and palm shell blends with coke in different proportion with an EAF slags. The gas generation and entrapment in the slag phase and their associated interfacial phenomena such as iron oxide reduction has been investigated. Significant carbon/slag interactions occurred when palm char was used compared to the conventional coke as the gases evolving from metallurgical coke showed lower concentrations of CO compared to palm char. CO gas from palm shell blends were higher than the corresponding gases resulting from the parent coke (Figure 7-4). The primary source of this gas is through reduction of iron oxide in slag. The higher amount of (CO+CO2) generated by the palm shell blends could be attributed to a certain extent to the volatiles still trapped in the carbonaceous mixtures, which are available after the initial combustion reaction in the DTF. The lignocellulosic structure present in the in palm shells allowing a steady gas release for participation in the subsequent carbon/slag interactions (Chapter 4). An increase in slag volume and fluctuations were observed when palm char reacted with the EAF slag, while coke showed a low degree of fluctuations and poor volume ratios. Different behaviors were seen when the palm shell blends were put in contact with the same EAF slag. The palm shell blends showed a slag droplet with a tendency to hold gas within, resulting in a significant increase in the volume (Figure 7-11 to 7-14). This resulted in volume ratio fluctuations due to gas hold-up and subsequent release. The rates of chemical reactions producing gas (i.e. reduction of iron oxide, carbon gasification) and their variation with the progress of reactions have been quantified. The 273


rates have been calculated considering the slope of the second stage of FeO concentration in the slag consisting of a steady state reaction region. Metallurgical coke showed rapid iron oxide reduction, very high rates of gas generation but poor foaming behavior. Palm char on the other hand showed excellent slag volumes but slow reduction of iron oxide. Slower rates of gas generation made it easier for slag to trap gases. As expected, the rate was much higher for palm shell containing blends as compared to the parent coke and palm char. Larger changes in surface area values also support much higher reactivity for palm shell blends as compared to coke (Figure 4-8). Optical microscopic investigations showed the presence of small diameter gas bubbles trapped within the slag after contact with palm char. This showed the behavior during initial stages of contact of parent coke, palm char and palm shell blends (Figure 7-16 to 7-19). There are significant differences in behavior in each of these cases, attributed to the chemistry of the carbon source (e.g. volatile matter, mineral matter and lignocellulosic structure). Reduction of the slag with palm char, coke and palm shell/coke at 1550 °C results in rapid formation of metallic iron droplets at the carbon/slag interface. The influence of gas generation, volatiles, mineral matter, carbon structures and entrapped gas bubbles from agricultural waste on slag foaming had discussed above where; coke and palm shell/coke blends showed large gas bubbles attached to the metallic droplets, providing evidence of significant gasification (volatile matter) of the carbon in the metal and subsequent reduction of the slag through the CO gas produced by this pathway. An increase in slag volume and fluctuations continuously were observed when 100% palm char reacted with the EAF slag due to the inorganic matter (P2O5) that are react as surface active in carbon/slag interactions. Oxygen in palm char are believed may reoxidize some of the reduced Fe in the slag, then reform FeO into the bulk slag that lowering the FeO reduction rate compared to other carbonaceous material (rubber, PET and rubber). The lignocellulosic structures (carbon structure) in palm char also influence the FeO reduction rate due to amorphous structure of lignin. The small gas bubbles are generated from the reduction reactions of FeO from 100% palm char, and these bubbles could contribute to both sustaining and improving the slag foaming 274


behavior. Very fine metallic particles are also found to be distributed throughout the bulk slag. The compositions of the slag droplets are uniform throughout; indicating that mass transfer in the slag has been enhanced by the movement of gas bubbles through the liquid phase. Our results indicate that both carbonaceous materials properties and their optimum interaction with slag control interfacial phenomena between carbon and slag, which are essential for maintaining optimum levels of foaming through retention of trapped gas bubbles within the slag and also achieving FeO reduction rates which could be better than that for coke in some cases.

275

Chapter 9: Summary & Conclusions

CHAPTER 9 9 Summary and Conclusions

Investigations of combustion followed by subsequent carbon/slag interactions of coke, agricultural waste and coke with its blends containing different proportions of palm and coconut shells have been undertaken in this study. This study has shown that blends of agricultural waste with metallurgical coke could be used to partially replace the conventional metallurgical coke used in EAF steelmaking for its fuel and carbon requirements as a result of enhanced gas phase reactions and carbon/slag interactions obtained when 100% palm char are used. The following conclusions can be made from this study. (1.) The burnout of coke/palm shell and coke/coconut shell blends were seen to be higher than the burnout of coke and raw agricultural waste alone, while an increase was seen with increasing agricultural wastes proportion in the blend. An important role is seen to be played by the rate at which volatiles escape the carbonaceous blend which affects the physical structure of the resulting char. With increasing waste content, a continuously increasing rate of devolatilization developed by palm shell blends leads to a gradual improvement in combustion performance, while a steady devolatilization rate results in an almost constant burnout of coconut shell blends from C1 to C3 blends. The subsequent changes in the structure of the carbonaceous chars supports the differences in combustion performances estimated for the two agricultural wastes. Palm shell’s cell structures were seen to open up to a significant extent and structural changes were observed accompanied by an increase in surface area, In contrast, the structural transformation that 276


occur in the coconut shell blends retain the cylindrical cell structures, which show an initial decrease in surface area. (2.) The gas products evolving from agricultural wastes pyrolysis measured by TGA-MS showed CO, CO2 and H2 as the main gases. These were attributed to the lignocellulosic structure present in the in agricultural waste allowing a steady gas release for participation in the subsequent carbon/slag interactions. (3.) The high temperature gas phase reaction affects the chemical structure and bonding of the resulting chars. The aliphatic structure present in palm and coconut shell is consumed during combustion, leaving behind a more ordered aromatic structure. Minerals, such as iron and alkali metals in agricultural waste also contribute to the mineral matter in the blends and influence combustion. (4.) Faster rate of devolatilization in carbonaceous materials will release volatile much earlier and as a result less volatile are available to sustain subsequent reactions in the slag phase. However, 100% palm char showed a steady gas release due to a oxygen content, inorganic matter (phosphorus in ash) and lignocellulosic structure that slowly breaks down at high temperature and contributes to maintaining foaming. (5.) This in-depth study has brought out significant differences in the slag/carbon interactions of metallurgical coke and palm char and palm shell blends in contact with iron oxide rich slags. The gases evolving from metallurgical coke showed lower concentrations of CO and CO2 in comparison to those from the coke/palm shell blends. This could be attributed to a certain extent to the volatiles still trapped in the carbonaceous mixtures, as well as to the associated enhanced reactivity of the carbonaceous materials.

277


(6.) Metallurgical coke showed poor wetting compared to palm char at the temperatures of investigation. Contact angles were generally higher than 100° even after 10 minutes. Wettability of coke/palm shell blends by EAF steelmaking slags was observed at 1500 °C. The first stage of reaction developed an increased initial contact angle explained on the basis of volume changes during the reduction reaction. The slag droplet entrapped CO gas and slag volume was seen to expand. Following the reduction of iron oxide present in the slag, iron is deposited at the interface as a result the interfacial tension is lowered and the contact angle decreases. Increased rate of reduction due to increased FeO content in the slag leads to a faster deposition of iron at the interface and thus better wetting. However, in the final stage of reaction, the FeO reduction has progressed to similar extents for all carbonaceous and ultimately comparable equilibrium contact angles were observed. (7.) The measured rate constants, K, obtained in this study are of the order of 105

(mol/cm2. s) and are comparable to the values reported in the previous

literature for coke. At 1550 °C the rate constants for FeO reduction reaction, expressed in mol/cm2. s, decreased with increased palm shells content in the carbonaceous blend, such that: a rate constant of 2.16 x 10-5 moles/cm2 s was obtained when coke interacted with the EAF slag, increasing to 6.23 x 10-5 moles/cm2 s for P1 blends, 4.37 x 10-5 moles/cm2 s for P2 blends and 3.10 x 10-5 moles/cm2 s for P3 blends. However, 100% palm char showed lower iron reduction rate with value 0.937 x 10-5 moles/cm2 s. Metallurgical coke showed rapid iron oxide reduction, very high rates of gas generation but poor foaming behaviour. Palm char on the other hand showed excellent slag volumes but slow reduction of iron oxide. Slower rates of gas generation made it easier for slag to trap gases. (8.) Slag foaming behavior was quantified by volume measurements with respect to time using the sessile drop technique. An improved foaming behavior was seen when the palm shells was present in the carbonaceous 278


blend, showing either a fluctuating or stable slag volume depending on the palm shell proportion in the blend. Improvement of slag foaming is governed by rates of gas generation, its entrapment and subsequent release. These phenomena are controlled by both properties; i. e. carbonaceous materials (volatile matter, mineral matter, carbon structure) and slag properties. Carbonaceous materials have a significant influence on kinetics of reduction that generate gases and also contribute to changes in slag composition during slag/carbon interactions. (9.) The qualitative understanding of gas bubbles trapped within the slag through optical images was found to be in good agreement with the volume measurements of the EAF slag in contact with coke and its corresponding palm shell blends. The present study has brought out the significant role played by agricultural waste materials and the influence of their fundamental characteristics on various aspects of combustion performance and carbon/slag interactions, including FeO reduction, gas generation and its entrapment within the slag as well as the associated enhancement of the slag volumes. Balance between reduction and foaming must be established for improving the furnace efficiency and decreasing its losses. An increased FeO content decreases viscosity and decreases slag density and therefore the ability of the slag to foam is hindered. On the other hand, higher FeO content generates an increased amount of gases from reduction reactions, leading to enhanced chemical reactions. Large volumes of CO and CO2 gases are produced and the slag shows an improved foaming. However, when the rates are too high the gases escape faster and lower levels of gas entrapment are observed leading to lower slag volumes. Thus, palm chars could be used to partially replace metallurgical coke which in metallurgical processes including EAF steelmaking due to their characteristics which result in improved slag foaming and enhance the combustion performance.

279

Chapter 10: References

CHAPTER 10 10

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304

Appendices

APPENDICES APPENDIX A ESTIMATION OF COMBUSTION EFFICIENCY Ash Tracer Method Inorganic Tracer Method APPENDIX B FeO REDUCTION AND RATE OF REACTION CALCULATION

305

Appendices

APPENDIX A ESTIMATION OF COMBUSTION EFFICIENCY Ash Tracer Method The combustion efficiency is calculated by the ash tracer method based on the chemical analysis as follows: ɳ = (1 – (A0.Ci)/Ai.C0)) x 100%

Eq. 10-1

where: A0 and Ai are ash content (%) before and after combustion, while C0 and Ci represent the carbon content (%) before and after combustion.

306

Appendices Carbon LECO Analysis Sample 1 Coke

2 3

Before DTF After DTF Before DTF After DTF Before DTF After DTF

Carbon Content (%) 1 2 77.8 77.6 72.3 74.2 77.7 76.8 74.9 73.6 76.6 77.8 75.6 75.1

Average Carbon Content (%) 77.70 73.25 77.25 74.25 77.20 75.35

307

Appendices ASH Analysis – Muffle Furnace

Sample

Crucible mass (g)

Mass of crucible + sample before (g) after (g)

Sample mass (g)

Ash content (g)

Ash content (%)

Before DTF

35.573

37.078

35.802

1.505

0.229

15.2159

After DTF

35.961

37.462

36.276

1.501

0.315

20.9860

Before DTF

36.968

38.473

37.218

1.505

0.25

16.6113

After DTF

35.366

36.867

35.663

1.501

0.297

19.7868

Before DTF

36.962

38.468

37.215

1.506

0.253

16.7995

After DTF

35.367

36.868

35.573

1.501

0.206

13.7242

1

Average ash (%)

Before: 16.2089

2

3

After: 18.1657

308

Appendices Combustion Efficiency (%) Sample Coke Palm shells P1 P2 P3 Coconut shells C1 C2 C3

Before DTF After DTF Before DTF After DTF Before DTF After DTF Before DTF After DTF Before DTF After DTF Before DTF After DTF Before DTF After DTF Before DTF After DTF Before DTF After DTF

Average Ash Content (%) 16.2089 18.1657 2.9979 3.3956 15.2799 18.0088 14.2131 17.1142 12.4145 15.1703 2.7741 3.3933 15.6179 20.0714 14.0611 18.9750 12.4778 17.2140

Average Carbon Content (%) 77.4 74.3 48.2 49.0 73.1 71.9 73.6 71.0 73.8 69.0 47.4 52.0 73.7 75.1 70.0 74.5 68.5 73.5

Combustion Efficiency (%) 14.3086 10.1244 16.5460 19.8855 23.4886 10.2567 20.6739 21.1332 22.2226

309

Appendices Inorganic Tracer Method Principles of the inorganic tracer method and the derivation of relevant mathematical equations are described below: A raw coke or agricultural wastes particle

m0 xa,0 x i ,0 Initial Stage (Stage 0)

A raw coke or agricultural wastes char particle

m1 xa,1 xi,1 Initial Stage (Stage I)

A raw coke or agricultural wastes ash particle

ma xi,a Initial Stage (Stage II)

Figure 10-1 A schematic diagram of the change in mass of representative coke or agricultural waste particle during combustion This method (Meesri, C., 2003) is based on the assumption that the mass of the high melting temperature inorganic materials, especially silica (Si), alumina (Al), and titanium (Ti) is conserved during combustion of coke. Using Si, Al or Ti as xi in the following equations represents the mass percentage of oxides, it can be demonstrated that: 310

Appendices (m0)(xi,0 ) = (m1)( xi,1) = (ma) (xi,a)

Eq. 10-2

where, according to Figure A-1, m0 is the mass of raw coke/agricultural wastes particle, xi,0 is the mass percentage of oxides (Si, Al or Ti), m1 is the mass of coke/agricultural wastes char particle, xi,1 is the mass percentages of oxides (Si, Al or Ti), ma is the mass of coke/agricultural wastes ash particle, xi,a is the average value of mass percentages of oxides (Si, Al or Ti). Therefore: (m1) = (m0)( xi,0)/ (xi,1)

Eq. 10-3

(ma) = (m0)( xi,0)/ (xi,a)

Eq. 10-4

The extent of mass burnout of solid materials can be expressed as (U; % w/w, daf): U = (m0 - m1)/( m0 - ma)

Eq. 10-5

Hence, Eq. A-6 may represent based on the extent of oxides (SiO2, Al2O3 or TiO2): xi , 0 xi ,1  ( xi ,1 − xi , 0 )  xi , a  (m − m )   = U= 0 1 = xi , 0  ( xi , a − xi , 0 )  xi ,1  ( m0 − m a ) 1− xi , a 1−

Eq. 10-6

311

Appendices The value of U based on the mass percentage of SiO2, Al2O3 or TiO2 can be obtained by repeating the procedure above (Mitchell, R. E. and Akanetuk, A. E. J., 1996). Accuracy of the results obtained from Eq. A-6 is estimated to be in the range of ± 0.9 % at the 95 % confidence level (based on the precision data of the XRF analysis of Si, Al and Ti; details see Table A-1). Table 10-1 Ash analysis of metallurgical coke Sample

Coke Before DTF, xi,0

Coke After DTF, xi,1

(xi,1 - xi,0)

(xi,a-xi,0)

xi,a

(xi,a/xi,1)

(xi,1 - xi,0)/(xi,axi,0)(xi,a/xi,1)

SiO2 Fe2O3 Al2O3 TiO2 P2O5 Mn3O4 CaO MgO Na2O K2O SO3

57.4 5.9 26.5 1.2 0.6 0.3 4.1 0.6 0.2 0.7 2.5

57.2 7.1 26.5 1.1 0.51 0.4 3.8 0.61 0.2 0.63 1.4

-0.2 1.2 0 -0.1 -0.09 0.1 -0.3 0.01 0 -0.07 -1.1

-0.1 0.6 0 -0.05 -0.045 0.05 -0.15 0.005 0 -0.035 -0.55

57.3 6.5 26.5 1.15 0.555 0.35 3.95 0.605 0.2 0.665 1.95

1.0017 0.9155 1.0000 1.0455 1.0882 0.8750 1.0395 0.9918 1.0000 1.0556 1.3929

2.0035 1.8310 0.0000 2.0909 2.1765 1.7500 2.0789 1.9836 0.0000 2.1111 2.7857

312

Appendices

APPENDIX B FeO REDUCTION AND RATE OF REACTION CALCULATION CONVERSION OF THE NUMBER OF MOLES OF OXIDES The Argon flow rate is 1 Litre/min or 1/60 Litre/sec throughout the experiment. The volume flow rate of CO and CO2 gases evolved during proceeding reaction can be calculated as follows: Volume flowrate of CO,

VCO = 1/60 * CO (V %) / 100 Litre/sec

Eq. 10-7

Volume flowrate of CO2,

VCO2 = 1/60 * CO2 (V %) /1 00 Litre/sec

Eq. 10-8

One mole of a standard gas occupies 22.4 litres of volume at standard temperature and pressure (STP). Mole flow rate of CO,

MCO = VCO / 22.4 Mol/sec

Eq. 10-9

Mole flow rate of CO2,

MCO2 = VCO2 22.4 Mol/sec

Eq. 10-10

313

Appendices The number of moles of CO evolved during the time (ti+1 - ti), MCOi; = MCO |i (ti+1 - ti)

Eq. 10-11

The number of moles of CO2 evolved during the time (ti+1 - ti), MCO2 = MCO2 |i (ti+1 - ti)

Eq. 10-12

The cumulative number of moles of CO evolved during the time (t), n

M CO , cum = ∑ M CO | i (t i +1 − t i ) i =0

Eq. 10-13

The cumulative number of moles of CO2 evolved during the time (t), n

M CO2 , cum = ∑ M CO2 | i (t i +1 − t i ) i =0

Eq. 10-14

314

Appendices ESTIMATION OF TOTAL Carbon REMOVED The total amount of removed C is calculated as a function of time and presented as cumulative values, assuming that at the studied temperature the generated gases consists of CO and CO2 i. e. Total C removed (moles) = (CO, moles) + (CO2, moles)

Eq. 10-15

The cumulative number of moles of total Carbon removed during the time (t), n

n

i =0

i =0

M C , cum = ∑ M CO | i (t i +1 − t i ) + ∑ M CO2 | i (t i +1 − t i )

Eq. 10-16

ESTIMATION OF TOTAL Oxygen REMOVED The time-dependent reduction involves the removal of one mo le of oxygen during the reduction of one mole of FeO. Thus, each mole of CO detected corresponds to a mole of oxygen removed from the FeO present in the slag, while each mole of CO2 comport with two moles of oxygen. Total oxygen removed (moles) = (CO, moles) + 2 * (CO2, moles)

Eq. 10-17 315

Appendices The cumulative number of moles of total Oxygen removed during the time (t), n

n

i =0

i =0

M O , cum = ∑ M CO | i (t i +1 − t i ) + 2x ∑ M CO2 | i (t i +1 − t i )

Eq. 10-18

ESTIMATION OF % FeO CONCENTRATION IN THE SLAG Eq. 10-19

% FeO reduction = (MO, cum / MFeO t=0) dt/ 100

M FeOt = 0 =

m FeOt = 0

µ FeO

, m FeOt = 0 =

t =0

% FeOt =0 x mslag 100

n

n

i =0

i =0

% FeO reduction = (∑ M CO | i (t i +1 − t i ) + 2 x ∑ M CO2 | i (t i +1 − t i ) / M FeOt = 0 ) x

% FeO concentration = % FeOt =0 /(1 −

%FeO reduction ) 100

Eq. 10-20

1 100

Eq. 10-21

Eq. 10-22

316

Appendices Time, sec

CO, (ppm)

CO2, (ppm)

VCO, (lit/sec)

VCO2, (lit/sec)

MCO, (mol/sec)

MCO2, (mol/sec)

MCO, cumm (mol/sec)

MCO2, cumm (mol/sec)

MO, cumm mol/sec

MC, cumm (mol/sec)

%FeO red.

% FeO con.

0

795.24

1862.13

1.325E-05

3.104E-05

5.349E-09

1.253E-08

0

0

0

0

0.000

32.4

10

803.07

1856.82

1.338E-05

3.095E-05

5.402E-09

1.249E-08

5.402E-09

1.249E-08

3.038E-08

1.789E-08

0.452

32.2534

20

783.78

1855.89

1.306E-05

3.093E-05

5.272E-09

1.248E-08

1.067E-08

2.497E-08

6.062E-08

3.565E-08

0.902

32.1078

30

775.48

1859.07

1.292E-05

3.098E-05

5.216E-09

1.251E-08

1.589E-08

3.748E-08

9.085E-08

5.337E-08

1.351

31.9623

40

2128.29

2396.72

3.547E-05

3.995E-05

1.432E-08

1.612E-08

3.021E-08

5.36E-08

1.374E-07

8.381E-08

2.086

31.7243

50

8150.57

3605.6

0.0001358

6.009E-05

5.483E-08

2.425E-08

8.503E-08

7.786E-08

2.407E-07

1.629E-07

3.874

31.1450

60

16347.9

4565.96

0.0002725

7.61E-05

1.1E-07

3.071E-08

1.95E-07

1.086E-07

4.121E-07

3.036E-07

6.969

30.1420

70

22382.8

5391.74

0.000373

8.986E-05

1.506E-07

3.627E-08

3.456E-07

1.448E-07

6.352E-07

4.904E-07

11.050

28.8199

ESTIMATION OF RATE COSTANT The rate of reduction is equivalent to the rate of CO and CO2 gasses produced from the system. Iron oxide is the main oxide getting reduced as a result of the interaction of slag I with various carbonaceous materials under investigations. Based on these gases, the variation of the percentage of FeO remaining in the system is plotted as a function of time. The slope of the graph is calculated to determine the reaction rate (R) by considering the linear portion. A first order kinetic reduction it is assumed:

RFeO = ( S FeO / 100) * ρ s / MWFeO

Eq. 10-23

317

Appendices Where: RFeO is the reaction rate of FeO in moles/cm3 s, SFeO is the s lope of the graph, % FeO/s, ρs is the density of the slag in g/cm3 and MWFeO is the molecular weight of FeO in g/moles. On the basis of the reaction rate, R calculated using Eq., the apparent reaction rate constant is calculated as follows:

RFeO = ( K O ( AVslag )

Eq. 10-24

Combining Eq. B-17 and Eq. B-18 the reaction rate constant is given by Eq. B-19:

K0 =

S FeO ρ slag A . . MWFeO 100 Vslag

Eq. 10-25

where: K is the reaction rate constant in moles/cm2 s, A is the interfacial area of contact in cm2 and Vslag is the slag volume in cm3.

318

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