Development of operating conditions to improve ...

(3)  experimental trials and evaluation of the subsequent data by appropriate models; (4) development of models of the EAF and of dedusting systems that can be used to control the process and to calculate the data that is not measurable; (5) development of operating procedures that allow for an increase in the chemical energy yield of the process and for control of the exhaust gas volume and dust load. The project proved that airtight operations can lead to a decrease of the electrical energy demand if an accurate control of the process is performed. This control is necessary to optimise the coal and oxygen injection, so as to compensate for the decrease of electrical energy with chemical energy. In general, it is necessary to link the optimum postcombustion ratio inside the furnace with the injection apparatuses, the vessel geometry and the type of steel produced. As a consequence, this ratio must be worked out on a case-by-case basis. The results obtained provide guidelines for defining the strategy for post-combustion control of the different types of EAF technology investigated.

Price (excluding VAT) in Luxembourg: EUR 25

EUR 22973

(2) development of operating procedures and of monitoring devices suitable to control an airtight EAF;

EUR 22973

PROJECT REPORT

(1) identification of the intervention on the furnace to increase air-tightening;

Development of operating conditions to improve chemical energy yield and performance of dedusting in airtight EAF

Consequently, the activities carried out in this research project were as follows:

EC

The increase of the air-tightening of the furnace is advantageous because, with the reduction of the amount of uncontrolled air that enters the furnace, it is possible to have better control of the CO post-combustion in some zones of the furnace that are useful for chemical energy recovery. Furthermore, air-tightening is also useful for reducing dangerous and polluting emissions.

KI-NA-22973-EN-S

The industrial goal of this project is to make the EAF process, improving the chemical energy yield, more efficient and more flexible. The way to obtain this result is through optimisation of the CO post-combustion ratio inside the furnace operated in airtight conditions.

Development of operating conditions to improve chemical energy yield and performance of dedusting in airtight EAF

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European Commission

technical steel research Development of operating conditions to improve chemical energy yield and performance of dedusting in airtight EAF U. Martini (1), B. Kleimt, S. Zisser (2), H. Pfeifer, M. Kirschen, V. Velikorodov (3), U. De Miranda (4), R. Kühn, J. Deng (5), J. Siig, H. J. Wahlers (6), (1) CSM: Centro Sviluppo Materiali SpA — Via di Castel Romano, 100, I-00128 Rome (2) BFI: Betriebsforschungsinstitut — Sohnstraße 65, Postfach 105145, D-40237 Düsseldorf (3) RWTH: Institut für Industrieofenbau und Wärmetechnik im Hüttenwesen — Kopernikusstrasse 16, D-52074 Aachen (4) ORI: ORI-Martin — Via Canovetti Cosimo, 13, I-25128 Brescia (5) GMH: Georgsmarienhütte GmbH — Neue Hüttenstraße 1, D-49124 Georgsmarienhütte (6) TKN: ThyssenKrupp Nirosta — Werk Bochum Alleestraße 165, D-44793 Bochum

Contract No 7210-PR/328 1 July 2002 to 30 June 2005

Final report

Directorate-General for Research

2007

EUR 22973 EN

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INDEX List of figures .............................................................................................................................................................................................. 4 List of tables ................................................................................................................................................................................................ 6 Long Abstract ............................................................................................................................................................................................. 7 Introduction .......................................................................................................................................................................................................... 7 Objectives of the project ...................................................................................................................................................................................... 7 Description of the work........................................................................................................................................................................................ 7 Introduction .............................................................................................................................................................................................. 11 Objectives .................................................................................................................................................................................................. 11 Structure of the work............................................................................................................................................................................... 14 Description of the tasks............................................................................................................................................................................ 15 1 Set up of airtight furnace operations .................................................................................................................................................. 17 1.1 Evaluation of Air tightening strategies ........................................................................................................................................................ 17 1.2. Designing and realisation of modification to close the furnace ................................................................................................................. 19 ORI Martin EAF ................................................................................................................................................................................................ 19 1.3. Development, testing and installation of measurement systems and techniques for airtight conditions................................................... 25 1.3.1 Description of the CSM optical sensors ............................................................................................................................................. 25 1.3.2 Laboratory testing and calibration of the CSM sensors ..................................................................................................................... 29 1.3.3 CSM optical sensors: test on pilot plant ............................................................................................................................................. 31 1.3.4 CSM optical sensors: testing on industrial plants............................................................................................................................... 35 2. Development of airtight operations.................................................................................................................................................... 41 2.1 Performance of plant trials under airtight conditions with measurement of process data .......................................................................... 41 2.1.1 ORI Martin EAF ................................................................................................................................................................................. 41 2.1.2 GMH EAF........................................................................................................................................................................................... 43 2.1.3 TKN EAF ............................................................................................................................................................................................ 45 2.2 Evaluation of process data for static and dynamic mass and energy balance calculations......................................................................... 52 2.2.1 Evaluations with the statistical BFI model for the electrical energy demand........................................................................... 52 2.2.2 Evaluations with the BFI dynamic process model ............................................................................................................................. 59 2.2.3 Evaluation of process data of TKN EAF Bochum ............................................................................................................................. 67 2.3. Identification, optimisation of operation parameters and plant trials for verification of operation improvement .................................... 68 2.3.1 ORI Martin Furnace ............................................................................................................................................................................ 69 2.3.2 GMH furnace ...................................................................................................................................................................................... 72 2.3.3 TKN Furnace....................................................................................................................................................................................... 77 3. On-line observation and control of airtight operations ................................................................................................................... 81 3.1. Differential on-line energy balance at the GMH furnace ........................................................................................................................... 81 3.2 On-line monitoring and control system for airtight operations ................................................................................................................... 84 4. Measurement at and simulation of dedusting plants ....................................................................................................................... 87 4.1 Measurement of volume flow, temperature and gas composition for dedusting units ............................................................................... 87 4.2 Measurement of EAF off gas for static energy balance .............................................................................................................................. 93 4.3 Development of mathematical models for dedusting plant units ................................................................................................................ 93 4.4 Simulation of dedusting plants..................................................................................................................................................................... 96 4.4.1 Simulation of dedusting plants based on cell models......................................................................................................................... 96 4.4.2 CFD simulation of dedusting plants ................................................................................................................................................. 104 5 Designing and testing of operating practices in experimental trials.............................................................................................. 109 5.1 High coal injection ..................................................................................................................................................................................... 109 5.2 Chemical energy optimisation ................................................................................................................................................................... 109 5.2.1 ORI Martin ........................................................................................................................................................................................ 109 5.2.2 GMH.................................................................................................................................................................................................. 110 5.2.3 TKN................................................................................................................................................................................................... 111 5.3 Control of exhaust gas [melting with/without O2]..................................................................................................................................... 113 5.4 Control of dedusting system ...................................................................................................................................................................... 113 5.5 Improve power input and energy yield ...................................................................................................................................................... 113 5.6 Avoid peak temperatures in the O2 lancing period.................................................................................................................................... 115 6 Evaluation of airtight EAF performances ........................................................................................................................................ 117 General conclusions ............................................................................................................................................................................... 125 Recommendations .................................................................................................................................................................................. 126 Exploitation and disseminations ........................................................................................................................................................... 126 7. References ........................................................................................................................................................................................... 129

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List of figures Figure

Figure 1: typical EAF off-gas composition before the start of the project Figure 2: EAF off-gas composition measured at GMH EAF Figure 3: ORI Martin EAF - Schematic drawing of the arrangement for heats in airtight conditions (the additional lance for pulverised coal is not shown) Figure 4: ORI Martin EAF, schematic drawing of the furnace plus a part of the tunnel of the Conveyor system Figure 5: Location of burners and injectors at the GMH DC EAF for the first trial campaign Figure 6: Location of burners and injectors at the GMH DC EAF for the 2nd and 3rd trial campaigns Figure 7: Location of burners and injectors at the GMH DC EAF for the 4th trial campaign and the 5th trial campaign Figure 8: Location of burners and injectors at the GMH DC furnace after revamping (since August 2005) Figure 9: TKN EAF: openings for air infiltration into the EAF vessel and dedusting system, schematically Figure 10: CO and CO2 infrared absorption spectra[49] Figure 11: LEA: overall scheme Figure 12: LEA: scheme of the optical path Figure 13: LEA: overall view of the device (without the cover) Figure 14: schematic drawing of the device Figure 15: TCP: photos of the device Figure 16: device used for LEA calibration Figure 17: Laboratory assessment for LEA calibration Figure 18: LEA calibration with N2/CO2 mixtures Figure 19: LEA calibration with N2/CO mixtures Figure 20: images obtained from TCP during the laboratory calibration at 1500°C Figure 21: View of CSM EAF pilot plant Figure 22: LEA: Schematic drawing of the experimental assessment on EAF pilot plant Figure 23: CO concentration into EAF pilot plant (in red) compared with CO concentration measured at the stack Figure 24: View of combustion chamber apparatus used for TCP testing Figure 25: Assessment of the combustion chamber for TCP testing Figure 26: Testing of TCP with combustion chamber Figure 27: TCP testing with CSM combustion chamber: variation of flame temperature as a consequence of the injection of pulverised coal Figure 28: LEA output during a phase of a heat at ORI Martin EAF Figure 29: LEA – ratio signals during the phase of a heat shown in Figure 28 Figure 30: Set of the system to install the optical sensors to ORI Martin EAF Figure 31: CO, CO2 and O2 concentration in the EAF off-gas measured with Mass Spectrometer during a representative heat of the year 2004 Figure 32: Comparison between the signal of LEA and the ratio CO/CO2 from MS measurements Figure 33: Comparison between the signal of LEA after smoothing and the ratio CO/CO2 from MS measurements Figure 34: Images obtained by TCP device monitoring off-gas temperature Figure 35: EAF off-gas measurement by TCP device Figure 36: Sampling point of the EAF off-gas Figure 37: EAF gas composition measured with MS in the points A and B during a phase of a heat Figure 38: Influence of EAF operative conditions (pressure, slag door) on the off-gas flow rate Figure 39: Off-gas analysis at the GMH furnace Figure 40: Measured volume flow at the DEC flap and in the by-pass with DEC (Nov. 08, 2003) Figure 41: Measured volume flow at the DEC flap and in the by-pass without DEC (Nov. 27, 2003) Figure 42: Measured off-gas enthalpies at DEC flap vs. electrical energy input (2003) Figure 43: Off-gas and heat data with and without Direct Exhaust Control (May 2003) Figure 44: Operating DEC (8. Nov. 2003) and failure of DEC (21. Nov. 2003) Figure 45: Secondary off-gas enthalpy flow rate (point 3) vs. furnace pressure, with DEC (8. Nov. 2003) Figure 46: Secondary off-gas enthalpy flow rate (point 3) vs. furnace pressure, without DEC (28. Nov. 2003) Figure 47: Calculated versus actual electrical energy consumption for different trial campaigns at the GMH DC furnace Figure 48: Calculated versus actual electrical energy consumption for GMH heats from standard and nearly airtight operation Figure 49: Calculated versus actual electrical energy consumption for ORI Martin AC Consteel furnace Figure 50: Modelling error of electrical energy demand versus specific oxygen consumption for Ori Martin Consteel furnace Figure 51: Modelling error of electrical energy demand versus specific coal injection for Ori Martin Consteel furnace Figure 52: Modelling error of electrical energy demand versus specific oxygen / coal injection ratio for Ori Martin Consteel furnace Figure 53: Structure of the dynamic process model with input data available at the GMH DC-Electric Arc

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Furnace Figure 54: Electrical energy input, burner and oxygen injection rates of a GMH example heat from the trial campaign with high oxygen input Figure 55: Cooling water and off-gas data of a GMH example heat from the trial campaign with high oxygen input Figure 56: Post-combustion and exhaust gas system data of a GMH example heat from the trial campaign with high oxygen input Figure 57: Energy input and loss rates as well as melt temperature evolution calculated for a GMH example heat from the trial campaign with high oxygen input Figure 58: Low coal addition: Electrical Energy Demand as a function of Oxygen injection Figure 59: High coal addition: Electrical Energy Demand as a function of Oxygen injection Figure 60: Campaigns of 2004 with low coal: electrical energy demand vs. CO post Figure 61: Campaigns of 2004 with high coal: electrical energy demand vs. CO post Figure 62: Campaign 1-2005 with high coal: electrical energy demand vs. CO post Figure 63: Campaign 2-2005 with high coal: electrical energy demand vs. CO post combustion ratio Figure 64: Electrical energy consumption and power-on time versus Figure 65: Modelling error of electrical energy demand versus post-combustion ratio for GMH DC furnace Figure 66: Amounts of injected oxygen and carbon versus Figure 67: Off-gas and leakage air volume versus injected oxygen Figure 68: Relation of off-gas and leakage air amount versus CO post-combustion ratio Figure 69: Modelling error of electrical energy demand versus injected oxygen for the GMH DC furnace Figure 70: Average off-gas and leakage air flow rate depending on percentage of operation time with closed furnace door Figure 71: Leakage air flow rate depending on furnace pressure for different fur-nace door positions Figure 72: Decreased off-gas extraction increases risk of electrical breakdown [33] Figure 73: Measured specific off-gas enthalpy versus specific electrical energy Figure 74: Off-gas composition for a GMH example heat Figure 75: Off-gas flow rate and temperature for a GMH example heat Figure 76: Differential energy balance for a GMH example heat (Part I) Figure 77: Differential energy balance for a GMH example heat (Part II) Figure 78: On-line Monitor of energy balance and post-combustion ratio Figure 79: Correlation between post combustion ratio and energy utilization Figure 80: Schematic drawing of the modelled system constituted by ORI Martin EAF plus Conveyor Figure 81: Sketch and off-gas monitoring points of the dedusting system (145t-EAF) Figure 82: Equipment for off-gas measurements Figure 83: Measurement point 1 at the gap between EAF elbow and hot gas duct (left: 2002, point 1, right: Nov. 2003, point 1*) Figure 84: off -gas composition at point 1 (Mai 2003 Figure 85: off-gas composition at point 2 (Mai 2003) Figure 86: Measured CO2 gas concentrations at point 2 (Nov. 08, 2003) Figure 87: Measured CO2 concentration at point 1* and at point 2 (Nov. 27, 2003) Figure 88: Measured primary and secondary gas volume flows (points 2 and 3, one day) Figure 89: Measured off-gas temperatures of primary off-gas duct (point 2 and before filter) and secondary dedusting (point 3, one day) Figure 90: Measured off-gas temperatures of primary off-gas duct (point 2 and before filter) and secondary dedusting (point 3, one heat) Figure 91: Measured off-gas temperatures of primary off-gas duct (point 2 and before filter) and secondary dedusting (point 3, one heat) Figure 92: Calculated enthalpy flow rate of primary dedusting system at the EAF elbow (one heat) Figure 93: Calculated enthalpy flow rate of secondary dedusting: line 1, line 2 and bypass (one heat, DEC flap position) Figure 94: Off-gas temperature (point 3) vs. furnace pressure, with DEC (Nov. 8, 2003) Figure 95: Off-gas temperature (point 3) vs. furnace pressure, without DEC (Nov. 28, 2003) Figure 96: Modular layout of the simulation program Figure 97: Element of water cooled duct with heat transfer Figure 98: Measured off-gas mass flows of primary duct and bypass (one heat) Figure 99: Computed and measured off-gas temperatures at point 2 (different heats) Figure 100: Computed and measured off-gas temperatures after mixing chamber (different heats) Figure 101: Calculated off-gas temperatures as function of air to EAF off-gas ratio Figure 102: Sketch of the pressurized water cooling system of the Bochum EAF [27] Figure 103: Calculated heat flow density and heat transfer coefficient, αγ, in the primary dedusting duct, at 1.5 m from the duct entrance (point 1*) Figure 104: Calculated heat flow density and heat transfer coefficients αγ and ακ over total length of the water cooled duct (during oxygen injection, time mark A in Figure 103) Figure 105: Calculated specific heat flow from gas stream to hot water cooling system Figure 106: Measured water volume flow rates in the EAF wall cooling panels Figure 107: Sketch of EAF and of primary dedusting system

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Figure 108: Coefficients c, l and f for one heat without DEC (CID 793063) Figure 109: Coefficients c, l and f for one heat with DEC (CID 793307) Figure 110: Schematic representation of the modeled EAF elbow and primary gas duct Figure 111: Calculated gas flow pattern for Y = 0 m Figure 112: Calculated CO and CO2 concentrations in the primary dedusting system Figure 113: Calculated gas temperature distribution [K] in the primary dedusting system Figure 114: Electrical Energy Demand vs. Post Cosmbustion ratio of CO at ORI Martin EAF. Figure 115: Off-gas enthalpy vs. electrical energy input for two heats of GMH and TKN in 2003 Figure 116: Specific electric energy as function of specific oxygen input Figure 117: Specific off-gas enthalpy at point 2 as function of specific oxygen input Figure 118: Modelling error of electrical energy demand versus percentage of operation time with closed furnace door for the new GMH furnace Figure 119: Measured off-gas composition at the EAF elbow indicating lower volume flow rate of nitrogen into the EAF and increasing air tightness of the EAF Figure 120: Decrease of specific oxygen input, specific input of slag formers and specific electric energy demand in 2004 to 2005 Figure 121: Decrease of mean off-gas temperature after hot gas duct with decreased oxygen input Figure 122: standard case Figure 123: elimination of the gaps (slag door, roof/vessel) Figure 124: elimination of the gaps – increasing of the use of chemical energy Figure 125: elimination of the gaps plus pressure raising – increasing of the use of chemical energy Figure 126: elimination of the gaps plus pressure raising – increasing of the use of chemical energy (PCR=0.45) Figure 127: Hypothesized case of elimination of air entrance Figure 128: Sankey diagram for GMH example heat (old furnace configuration) with high oxygen input Figure 129: Sankey diagram for GMH example heat (old furnace configuration) under nearly airtight operation

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List of tables Table

Table 1: Modifications of EAF operation and dedusting system Table 2: Basis sets of operating conditions for ORI Martin test trials Table 3: Trial campaigns at GMH with data for evaluation with BFI models Table 4: Mean values of measured specific off-gas mass and enthalpy Table 5: Mean values of process parameters and measured specific off-gas mass and enthalpy Table 6: Measured air volume flow rates as function of uncovered surface of intake Table 7: BFI Formula for electrical energy demand of arc furnaces Table 8: Average values of input variables for energy demand calculation of the GMH DC-EAF with different operating conditions Table 9: Average values of input variables for energy demand calculation of ORI Martin heats with different operating conditions Table 10: Summary of the experimental campaigns carried out with different amount of C addition Table 11: Analysis of energetic efficiency of oxygen input at GMH furnace

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Long Abstract Introduction There is a general trend toward an increase of the steel production based on Electric Arc Furnace (EAF) route. This represent an important technological challenge to improve efficiency and flexibility of the EAf process. The present project was focused on the development and application of process concept and plant solution to increase the efficiency and flexibility of the EAF process. The industrial goal is to improve Chemical Energy Yield and Performance of Dedusting in Airtight EAF. The project concentrates on the special requirements of the airtight EAF operation, which is currently planned to be introduced in many electric steelmaking plants. Objectives of the project Airtight furnace design and operation restricts the easy accessibility to the furnace through the slag door for injection of oxygen, carbon and other materials, for removal of the excess furnace slag and for measurement of the steel and slag properties. The technical objective of the project is the development of operating procedures suitable to control airtight EAFs in order to increase productivity and steel quality, saving electrical energy and reducing dangerous and polluting emissions. To obtain these goals, three main technical objectives have been defined:

•

Application of new measurement techniques for airtight operation This permits to observe the airtight furnace operation and to provide the basis for on-line process control

•

Development of operation procedures to increase the use of chemical energy in EAF This permits to directly reduce electric energy consumption and to increase the ability to control scrap pre-heating with a global saving of energy

•

Definition of operating conditions and plant configurations to obtain a controlled amount of exhaust gas, dust production and polluting emissions This permits to obtain a high productivity with low total specific energy consumption and reducing at lowest values dust load and polluting emissions

Key factors in the project are: 1. Use of Aitight furnaces This is obtained managing the EAF process and the furnace so to reduce the income of uncontrolled air 2. Use of high amounts of carbonaceous materials (as pig iron and coal – lump and powder) and oxygen On-line measurements and models must be used to define the appropriate conditions to control the gas produced in the furnace Description of the work The work has been structured according to the following tasks. Task 1. Set up of airtight furnace operations Aim of the task is the definition of plant configuration and necessary instrument to run airtight operation. Measurements of EAF off-gas, the evaluation of EAF operation practices and of EAF configurations allowed the definition of the modifications to the furnace and to the operating practices to perform the first trials. The leakages in the vessel, the slag door position (open/close) and the off-gas extraction have been characterised as key-factors to be controlled. Injection devices and/or modifications of the operating procedure have been defined to carry out heats with improved air-tightening of the furnace. New devices (optical sensors) to monitor the EAF have been developed and tested at laboratory, pilot plant and industrial level.

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Task 2. Development of airtight operation Aim of the task is the definition of the best operating procedures for an airtight EAF. Different series of plant trials have been carried out varying the operating procedures (i.e. oxygen injection, coal injection) and the furnace sealing (i.e.: percentage of time with closed slag door, percentage of sealing of the gaps, regulation of off-gas extraction). Fundamental process data have been monitored and evaluated by means of mathematical models. In this way it has been possible to establish relationships between the electrical energy demand of the process and the different plant configurations (which determine air tightness level) and operating conditions (specially coal feeding rate and oxygen flow rate). This activity has permitted to define the control strategy of airtight operations.

Task 3. On-line observation and control of airtight operations Aim of the task is the control of a furnace in airtight conditions. To do this, the appropriate instruments to monitor revealing process parameters have been identified and implemented.

Different models have been developed and implemented on computer software in order to have an instrument to be coupled with the monitoring systems that allows controlling the furnace. The models are an on-line energy balance model which provide information on the energetic behaviour of a heat during the treatment and a mass/energy balance model which allows the regulation of the oxygen injection so to have a certain post combustion ratio. Both these models and the monitoring systems were used during the experimental trials carried out in this project. Task 4. Measurement at and simulation of dedusting plants

An important aspect of EAF process is to control amount of exhaust gas and to reduce dust production and polluting emissions. The aim of the task is the control of process parameters of dedusting plant in airtight conditions. This has been carried out with physical measurements of EAF off-gas flow rate, composition and temperature with purposely devoted sensors in different zones of dedusting plant. Due the difficulty or the impossibility of the measure of some parameters of dedusting system (i.e.: volume flow rate behind elbow, gas temperature at the post combustion zone), a cell model has been purposely developed to calculate these parameters. Furthermore, fluid-dynamic simulations of the primary dedusting system allowed the calculation of the temperature and of the chemical species distribution in this zone. Task 5. Designing and testing of operating practices in experimental trials

Aim of the task is the testing of operating practices allowing the improvement of the chemical energy yield of the process. The increase of coal addition, the control of chemical energy utilisation and of exhaust gas volume and dust load have been verified as well as the improvement of power input and energy yield of the furnaces involved in the project. Task 6. Evaluation of airtight EAF performance

Aim of the task is the evaluation of airtight EAF performances on the basis of the results of the preceding tasks. In general, it has been found the airtight conditions lead to a decrease of the electrical energy demand but it is necessary to provide oxygen because of the decrease of the air leakage. Additional decreases of the electrical energy demand can be achieved with a controlled CO post combustion ratio. This can be done by appropriate coal and oxygen injection rates but depends also to the design of the furnace and of the injection systems. Furthermore, it depends also to the kind of process that is related with the type of steel produced. Where has been investigated, the variation of oxygen injection has no correlation with the off-gas enthalpy at the primary dedusting system. As a consequence, the operations of dedusting system remain unchanged with improved airtight conditions. Conclusions A series of actions are required to improve the air-tightening of an EAF. Physical actions on the furnace (filling of the gaps) and on the injection devices (oxygen and carbon injectors type and positioning). Actions with the aim of the development of new operating procedures to carry out heats in airtight conditions (slag door closed, off-gas extraction decreased). To do this is necessary to have instruments (sensors, software) to monitor the process parameters, to calculate the not measurable parameters, to calculate the influence of these parameters on the energetic balance of the process and to control the

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process. The results proved that benefits in terms of decrease of electrical energy demand are achievable from airtight operations but it is necessary an accurate control of the process. This control is related to the type of furnace (furnace geometry, feeding system, positioning and type of injector systems) and of the steel grade. As a consequence, it is necessary the identification of the optimal strategy, and consequently the best operating procedures, for each kind of EAF technology. This has been made with the three different EAF considered in this project demonstrating the validity of the method. To summarise, all the performed actions permitted to: performing heats with airtight EAF evaluate the performance of the airtight EAF 1. optimising the chemical energy yield on the basis of the type of furnace and of the process carried out 2. to set the basis for the type of approach to be followed if an EAF must be transformed in an airtight EAF 3. to establish guidelines for the optimal control of an airtight EAF 4. to quantify the real benefits achievable in terms of the reduction of electrical energy demand as a function of the increase of the coal and oxygen injection.

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Introduction During the last decades, the Electric Arc Furnace (EAF) process achieved a very high production capacity due to high energy transfer density by use of high power transformers, oxygen injectors, gas burners etc [23, 24, 50, 52]. The significant improvement of the EAF performance is cause and effect of the notable increase of the amount of steel produced with this route, which is presently around 33% of the total world steel production. The trend is toward a further increase. The International Iron and Steel Institute (IISI) has carried out a study to describe the state of the art of Electric Arc Furnace (EAF) technology and its future trends, the IISI Delphi Survey. From the Delphi study an increasing use of electric steelmaking is forecast, with a prevision of 40% of liquid steel production in 2010 compared to 33% today. The study depicts a very positive future scenario for EAF and highlights the technology trends. Of major importance will be: • the reduction of electrical energy consumption • the increased use of carbon, carbonaceous materials and oxygen • the reduction of dust and polluting species • the implementation of flexible practices, permitting diversification of feeding and energy source Moreover the EAF based steelmaking route is seen, in the future, as a possible competitor for integrated steelmaking in the production of steel strip, but to achieve this position the EAF must be able to produce purer and more consistent steel products. To achieve this highly competitive position in the steel market a number of critical areas in which continuous improvement will be necessary have been already identified. Main examples are: • Process Efficiency - to seek improvement in productivity, quality, and energy efficiency. • Recycling - to increase steel recycling and recovery of iron units from plant solid wastes. • Environmental Engineering - to achieve further reductions in air and water emissions and generation of hazardous wastes, and to develop new processes to avoid pollution rather than control and treat it. Important technologies to achieve the above mentioned goals are: • scrap pre-heating systems and techniques • new injection systems for oxygen into the bath and for post-combustion purposes • new sensors and control systems • an air-tight EAF operation • quenching of off-gas after the post-combustion chamber in the fume collecting system • de-dusting and dust recycling systems. The present project was focused on the development and application of process concept and plant solution to increase the efficiency and flexibility of the EAF process. The industrial goal is to improve Chemical Energy Yield and Performance of Dedusting in Airtight EAF. The project concentrates on the special requirements of the airtight EAF operation, which is currently planned to be introduced in many electric steelmaking plants Objectives Airtight furnace design and operation restricts the easy accessibility to the furnace through the slag door for injection of oxygen, carbon and other materials, for removal of the excess furnace slag and for measurement of the steel and slag properties. The industrial goal of the project is the development of operating procedures suitable to control airtight EAFs with the final industrial objectives of increasing productivity and steel quality, saving electrical energy and reducing dangerous and polluting emissions.

11

To obtain these goals, three main technical objectives have been defined:

•

Application of new measurement techniques for airtight operation This permits to observe the airtight furnace operation and to provide the basis for on-line process control

•

Development of operation procedures to increase the use of chemical energy in EAF This permits to directly reduce electric energy consumption and to increase the ability to control scrap pre-heating with a global saving of energy

•

Definition of operating conditions and plant configurations to obtain a controlled amount of exhaust gas, dust production and polluting emissions This permits to obtain a high productivity with low total specific energy consumption and reducing at lowest values dust load and polluting emissions

Key factors in the project are: 1. Use of Aitight furnaces This is obtained managing the EAF process and the furnace so to reduce the income of uncontrolled air 2. Use of high amounts of carbonaceous materials (as pig iron and coal – lump and powder) and oxygen On-line measurements and models must be used to define the appropriate conditions to control the gas produced in the furnace Electric arc furnaces are directly exhausted for removing the dust and the gases from oxidation reactions. The exhaustion through the elbow on the furnace roof causes an ingress of leakage air through openings in the lower part of the vessel, mainly the slag door. Because of the chimney effect in the hot furnace, fumes may exit through openings in the upper part of the vessel, i.e. through the gaps around the electrodes and between the wall and the roof. Thus direct exhaustion is a balance between the exhaust gas flow rate, the leakage air input and the fumes output which has to be limited. From investigations at conventional furnaces it is known that about 75% of the gas exhausted through the elbow consists of leakage air [1,2]. Some oxygen from this air contributes to oxidation reactions in the furnace, but the high amount of nitrogen is a thermal ballast which has to be heated up to 1000 °C or more. In addition to its thermal energy content, the exhaust gas contains chemical energy by carbon monoxide and hydrogen which are completely post-combusted behind the elbow by air added at the entrance to the exhaust gas duct. At several furnaces post-combustion is shifted into the vessel by additional oxygen input. For its effect on the electrical energy consumption a factor of -2.8 kWh/m³ was found, corresponding to a heat transfer efficiency of about 40% [3,4]. Total energy losses by exhaust gas are about 150 kWh/t [2,5,6]. At some furnaces, values up to 300 kWh/t were found, depending on their operating practice [7]. It was also found that the exhaust gas energy losses and the electrical energy consumption are correlated with coefficients of up to 60%. Many efforts have been made to recover or to reduce the high energy content of the exhaust gas. Scrap preheating by exhaust gas may reduce the electrical energy consumption by about 50 kWh/t [1, 5, 9, 10]. This is rather low with respect to high investment and maintenance costs for scrap preheating. The reduction of the exhaust gas flow rate and thus its energy losses seems to be more promising. With the control system described in [11], the fume density above the furnace is measured by an optical device and is directly controlled by the gate valve in the exhaust gas duct. Furthermore the cross sections of the electrode gaps were reduced. According to the relations described above, the same limited output of fumes was thus achieved with lower exhaust gas flow rate and energy losses, resulting in 15 kWh/t lower electrical energy consumption. The even more effective way to lower the exhaust gas flow rate is to reduce the cross section for the input of leakage air which contributes most to the exhaust gas. This mainly means to close the slag door as far as possible and is known as Airtight Furnace operation [8-10]. The analysis of the exhaust gas composition at the elbow permits to control the oxidation process in the furnace and to improve the thermal efficiency of post-combustion by adapted oxygen input [9-12].

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With airtight furnace operation, the air ingress could be reduced by a factor up to 3 and allowed a gain of 10 to 15 kWh/t in electrical energy consumption [8]. In [9] it is reported that by sealing the furnace the exhaust gas flow rate could be lowered by more than 50% and its energy losses by 45 kWh/t. A reduction of 50 kWh/t in electrical energy consumption is expected from airtight furnace operation in [10]. These values of energy saving are comparable to those from scrap preheating, but with much lower investment and maintenance costs. At the Institut für Industrieofenbau und Wärmetechnik im Hüttenwesen of the RWTH Aachen investigations based on off-line mass and energy balances of EAFs [6, 13-16] were performed to optimise the exhaust gas flow rate, to estimate and reduce the leakage air infiltration in the vessel of the EAF. With this knowledge of time dependent composition, flow rate and temperature of the direct exhausted off gas, the performance of the EAF dedusting system can be improved. Dedusting systems consist of a number of units for the transport, mixture, post combustion, cooling and cleaning of the EAF off gas. Depending on the type of layout some of the shown units are used. The increased productivity of the EAF’s, e. g. due to an increased amount of fuel-oxygen, are resulting in a higher specific off gas load to the dedusting system. The off gas composition, temperature und mass flow depends on the actual state of the melting process and the process parameters of the dedusting system. To achieve low total specific energy consumption with a high productivity the amount of exhaust gas must be minimised and the post combustion of CO in the EAF must be maximised. In the case of Airtight EAF, the volume flow of exhaust gas will be reduced (e. g. less amount of N2) and the CO off gas concentration may significantly differ from that of a conventional non Airtight EAF. In practise the amount of gas exhausted directly over the elbow is not well known. Measured volume flows in a dedusting system comprise normally the volume flow exhausted from the furnace via the elbow, the excess air flow in the gap between elbow and water cooled hot gas line and leakage air flows. The amount of excess air penetrating between elbow and water cooled hot gas line must be large enough for the complete post combustion of CO and H2. At high temperatures, O2 in the off gas and relative long retention times of the off gas in this area can produce additional amounts of NOx. The objectives of a dedusting system are e. g.

• • • •

drawing off of the exhaust primary and secondary exhaust gases from the furnace post combustion of CO, Volatile Organic Carbons (VOC, especially in the case of scrap preheating) and Dioxines and Furanes (D/F) cooling of the off gas or mixing with cold air up to appr. 120 °C (entry temperature baghouse) cleaning of the off gas in filter units

This target will be reached by the adequate match of units for a dedusting system. The layout of dedusting systems differ generally from the layout of systems for other thermal plants (e. g. power plants,...), because the volume flow, temperature and composition of the off gas change relatively fast during the melting process. In literature a number of authors are dealing with the emission control of arc furnace technology. The emission control of arc furnace technology, especially for EAF' s with scrap preheating, is investigated in [17]. The post combustion units are investigated in [18-19]. For the post-combustion of VOC a minimum temperature of 850 °C and a residence time of the gas particles of 1.5 s is necessary. In the case of scrap preheating the off gas must be heated up to fulfil this requirements. In the case of EAF' s with no scrap preheating the off gas temperature may be high enough to fulfil the post-combustion conditions in an optimal designed drop out box. Some dedusting plants for EAFs are only equipped with a water cooled hot gas line. In this case the post-combustion and cooling of the gas must be realised in this unit. The principle of such a system with steam cooling instead of cold water cooling is realised in the Bochum plant of TKN [20]. The application of computer analysis to electric arc furnace fume control is discussed in [21] but the models and the systematic is not given in detail. A complete simulation and the application to existing EAF dedusting systems is not available in the literature. Improvement of EAF performance by using airtight conditions, requires systems and techniques for online, real time, quick measures, not common in EAF process, and new control strategies based on the integration of electrical power control and subsidiary fuel control.

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Many works can be found on the problem of improving and develop measurement and sampling techniques. Optical measurements, based on laser techniques, can be used for both gas and slag analysis. Optical sensor based on mid-infrared tunable diode lasers (TDLs) to provide real-time information on both the temperature and composition of the gas produced in industrial furnaces are known from literature[26] and commercially available. This technique is a noninvasive, and permits on-line monitoring. It is, hence, particularly suitable for gas monitoring in airtight furnaces. Radar systems have been already experimented for the measurement of liquid level in steel bath in EAF [22]. Device to sample steel and/or slag and to perform measurements in the liquid bath are under study. For example CRM, with the financial grant of the ECSC, has developed, for the BOF operating at turndown, an automatic device for steel sampling and for temperature and oxygen measurements. BSE (Badische Stahl-Engineering GmbH) and More (Gemona, Italy) have installed in various EAF plants automatic devices for the steel sampling or/and for the measurement of temperature. Some of these devices operate without removing of the electrodes. A recent ECSC project [47] demonstrated that the reduction of the air ingress in EAF can lead to a reduction of the electrical energy demand of the process. Structure of the work The project work is based on the combination of advanced measurement technologies for the airtight furnace with the benefits of dynamic modelling of the EAF energy balance and the dedusting process. Both measures shall be applied for on-line process observation and a reliable control of the airtight furnace operation. The project is carried out by three research institutes with different working areas on the one side, and three Electric steelmaking plants with different furnace types and production programs on the other side. The research institutes are: Centro Sviluppo Materiali (CSM), Rome, Italy Betriebsforschungsinstitut (BFI), Düsseldorf, Germany Institut für Industrieofenbau und Wärmetechnik im Hüttenwesen (RWTH), Aachen, Germany The participating electric steelmaking plants are: ORI Martin (ORI), Brescia, Italy: AC EAF with Consteel® technology – tap weight 70 t Georgsmarienhütte (GMH), Georgsmarienhütte, Germany: DC EAF – tap weight 130 t Thyssen Krupp Nirosta (TKN), Bochum, Germany: UHP EAF – tap weight 145 t A schematic representation of the work structure and the tasks sharing among the partners is shown in the following schematic diagram:

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Description of the tasks The topics have been subdivided in 6 sections according to the following tasks: Task 1. Set up of airtight furnace operations The control of an airtight EAF requires:

Furnace modifications Elimination of the gaps in the vessel ad roof Elimination of slag door manipulators Installation of new injection systems Operating procedures to carry out heats with improved air-tightness Instruments/devices to monitor the airtight EAF To achieve these objectives, an analysis of the running conditions of the EAF involved in the project has been carried out. The leakages in the vessel, the slag door position (open/close) and the off-gas extraction have been characterised as key-factors to be controlled. Injection devices and/or modifications of the operating procedure have been defined to carry out heats with improved air-tightening of the furnace. New devices to monitor the EAF have been developed and tested. Task 2. Development of airtight operation The development of airtight operations requires:

Performance of plant trials under airtight conditions Monitoring of characteristic properties during plant trials Off-gas composition, flow rate and temperature Coal and oxygen injection Evaluation of measured process data with static and dynamic mass and energy balance calculations New plant trials for verification of operation improvement To achieve these objectives, different series of plan trials have been carried out on all the EAF involved in the project. Existing or new measuring systems have been implemented to monitor the characteristic properties of the process during the trials. The data of each series have been evaluated by purposely devoted models to obtain information on the energetic yield of the process with the

15

new operating conditions. This method allowed the improvement of the operating conditions that have been verified with further trials during all the project duration. Task 3. On-line observation and control of airtight operations The control of airtight operations requires:

On-line monitoring systems Mass/Energy balance models of the EAF To achieve these objectives, the on-line monitoring systems implemented on EAF have been coupled with two models implemented on computer software: 1. on-line energy balance model which provides information on the energetic behaviour of an EAF heat already during treatment 2. mass/energy balance model which allows the regulation of the oxygen injection so to obtain the optimal value to have the best chemical energy yield of the process Task 4. Measurement at and simulation of dedusting plants

The control of process parameters of dedusting plants requires: Measurement of EAF off-gas flow rate, temperature and composition in different zones of the dedusting plant Calculation of the process parameters of the dedusting system that are difficult to measure Fluid-dynamic simulation of the dedusting plant to characterise the post-combustion at the primary dedusting system To achieve these objectives the following actions have been carried out: 1. Purposely developed instruments have been applied in different zones of the dedusting plant to measure the EAF off-gas composition, temperature and total flow rate 2. A cell model has been developed to calculate some not measurable parameters of the dedusting system as volume flow rate behind elbow or flap, gas temperature at the post combustion zone 3. A CFD simulation has been performed to obtain the distribution of temperature and of the chemical species in off-gas in the primary dedusting system Task 5. Designing and testing of operating practices in experimental trials

All the information obtained from tasks 1-4 allows the design of operating practices to: Increase coal injection Control the chemical energy utilisation Control exhaust gas volume and dust load Improve power input and energy yield To achieve these objectives experimental trials have been carried out with the support of the developed models of EAF plus dedusting systems and of the implemented monitoring systems. Task 6. Evaluation of airtight EAF performance

The results achieved on the basis of the tasks 1-5 permitted a global evaluation of the performance of airtight EAF considering the three different EAF technologies involved in this project. The derived general conclusions are reported in the Section 6 of this report (from page 117). Ricordarsi di aggiornare questo numero di pagina in funzione della nuova numerazione che verrà fatta.

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1 Set up of airtight furnace operations 1.1 Evaluation of Air tightening strategies At the start period of the project, a careful analysis of the running conditions of the three EAFs allowed to define the guidelines that have to be followed to move ahead air tightening conditions. This analysis has been carried out considering different aspects of the running conditions of the furnaces. The first aspect is the off-gas measurement at the elbow of the furnace. The second aspect is the evaluation of the oxygen and coal injection procedures. The third aspect is the evaluation of the EAF operation practice. Measurements of EAF off-gas At EAF Bochum, off-gas measurements at the elbow of the furnace showed low concentration of carbon monoxide and carbon dioxide during the first 30 minutes of stainless steel heats, but significant amounts of oxygen and nitrogen [23,24]. Hydrogen concentration was low during all periods of melting indicating high oxygen fugacity. Measured off-gas composition indicated high concentration of nitrogen due to high volume flow rate of air into the furnace. The low air tightness of the furnace in the year 2000 is explained with various leakages of the EAF vessel and due to constant off-gas extraction. At EAF ORI Martin, measurements of the off-gas composition at the entry of the tunnel showed a rather constant gas composition due to the regularity of the feeding materials with the Conveyor. The CO concentration remains in general higher than 20% and hydrogen concentration was lower than 5%. The nitrogen concentration in the off-gas is high during the whole heat duration. It indicates an high rate of uncontrolled air into the furnace. The low air tightness of the furnace is mainly due to the periods of slag door openings (to use an oxygen lance manipulator as additional oxygen source) and to the off-gas extraction system that maintain the furnace in low pressure conditions. Figure 1 shows in comparison the EAF off-gas composition of Bochum and ORI Martin for a typical heat

Figure 1: typical EAF off-gas composition before the start of the project At the EAF of GMH, an off-gas sample is taken directly from the furnace for analysis with a mass spectrometer regarding the components CO, CO2, O2, H2 and CH4. It can be assumed that the remaining part of the off-gas is mainly N2. CO and H2 show values between 40 and 50 %, indicating a low post combustion ratio. With the help of a tracer gas and a nitrogen balance, the off-gas and the leakage air flow rate are determined. The relatively low leakage air ingress, together with the low nitrogen content of the off-gas, indicates that the GMH furnace was already operated under nearly airtight conditions at

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the beginning of the project. Figure 2 shows the measured EAF off-gas composition at GMH EAF. Heat No. 642369

50

Off-gas H2

Off-gas analysis in %

40

Off-gas CO

30

20

Off-gas CO2 10

Off-gas O2 Off-gas CH4 0 0

10

20

30

time in min

40

50

60

70

Figure 2: EAF off-gas composition measured at GMH EAF Evaluation of oxygen and coal injection procedures At the GMH EAF, two scrap baskets are charged per heat. Fossil energies are added as charge coal with the scrap bucket and blowing coal via a door lance. For chemical energy input, the furnace was furthermore equipped with two natural gas/oxygen burners. Additional oxygen was supplied by two consumable door lances. The aim of GMH was to substitute these door lances as far as possible by shell injector systems, in order to allow an airtight operation of the furnace with a closed furnace door. At ORI Martin EAF, an O2 door lance manipulator is used in substitution to a wall mounted oxygen lance to melt the scrap charged with a bucket. By this lance is used to melt the scrap avoiding damages on the wall mounted lance due to anomalous contact with the charged scrap (electric discharge between lance and scrap). The aim is to perform heats with scrap fed only by Conveyor so to avoid the use of consumable oxygen lance trough the slag door. At Bochum EAF, the oxygen injection was at low capacity (between 8 m3/t and 10 m3/t) and as late as possible (see Figure 1_A the oxygen blowing period) to minimise the oxidation in stainless steel heats. Evaluation of EAF operation practices At TKN Bochum EAF in the year 2000, operation practise included melting of the first bucket with slightly elevated EAF roof in order to avoid breakdown of the electrode voltage to roof and wall panels of the UHP furnace. During that time maximum voltages (up to 1100 V) were applied from the beginning of melting. As a consequence, infiltration of air was high. Nominal capacity of the primary and secondary dedusting system is 165.000 m3STP/h (formerly 120.000 m3STP/h [25]) and 660.000 m3STP/h, respectively [23, 24]. The secondary dedusting system consists of two lines with 380.000 m3STP/h [25] and 260.000 m3STP/h capacity, respectively (see Figure 81). Nominal off-gas volume flow rates in the primary dedusting system and in line 1 of the secondary dedusting system were checked with measurements (chapter 4). Comparison of specific volume flow rates (759 m3STP/h and 4552 m3STP/h, respectively) with other steel plants do not indicate oversized dedusting system. Measured mean specific off-gas enthalpy at the elbow, 128 kWh/t, is moderate when compared to other furnaces[2], but mean specific off-gas mass is high: 293 kg/t [24]. Also, mean specific mass of air into the furnace is high: 267 kg/t [24]. In particular, volume flow rate of nitrogen decreases energy efficiency with a specific energy demand of 0.49 kWh/kgN2 when heated to 1500 °C (steel: 0.36 kWh/kgFe). As a consequence, a series of measures was defined in 2002 to 2004 to decrease extracted specific offgas mass and specific enthalpy, to decrease volume flow rate of nitrogen, to increase air-tightness of the furnace and to increase over-all energy efficiency.

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1.2. Designing and realisation of modification to close the furnace ORI Martin EAF At ORI Martin EAF the following actions have been carried out to move towards enhanced airtightening conditions: 1. Scrap addition is carried out by the Conveyor system (see Figure 3) Furthermore, being the variation of the charge composition and quantity cause of not so easily estimable uncertainty, the scrap used for a single heat has been chosen as homogeneous as possible. 2. Oxygen injection by a supersonic wall mounted lance (see Figure 3) the lance has a capacity of 2000÷4700 m3/h [stp]. The distance from the bath and the impact angle of the oxygen blown are maintained constant during the whole blowing period. In fact, the lance movement is regulated as a function of the total weight of the liquid phase into the furnace. 3. Carbon addition is made by the Conveyor as pig iron and lump coal and by a wall mounted lance as pulverised coal (see Figure 3) the Conveyor allows to feed the pig iron and coal (usually with size between 3 and 12 mm). Both these species are regularly distributed with the charge during the heat duration. The amount of pulverised coal injected by the wall mounted lance is of 1÷1.5 kg/t. 4. The slag door is maintained closed for over the 90% of the tap-to-tap time the consumable oxygen lance is not used, so the slag door can be maintained closed during the oxygen injection period. 5. The extraction of the off-gas is varied so to have different set points regulation of the furnace pressure 6. The gap between EAF roof and the vessel has been reduced 7. The gaps between EAF elbow and connecting car and between connecting car and the tunnel of the Conveyor has been reduced as more as possible so to avoid air entrance in the tunnel and a subsequent interference on the EAF off gas analysis (see Figure 4). This has been done at the end of 2003.

Figure 3: ORI Martin EAF - Schematic drawing of the arrangement for heats in airtight conditions (the additional lance for pulverised coal is not shown)

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Figure 4: ORI Martin EAF, schematic drawing of the furnace plus a part of the tunnel of the Conveyor system Different sets of operating conditions, in terms of scrap feeding and oxygen blowing have been experimented with different coal rates. On the basis of these conditions, a series of experimental campaigns have been carried out from the second half of 2003 up to the first half of 2005. In a preliminary phase, it has been evaluated the effect of the new running conditions of the furnace on the amount of uncontrolled air that enters the furnace. The attention was then focused on the state of the slag door and on the furnace pressure set point regulation. The quantification of the effect of the above mentioned parameters on the amount of uncontrolled air that enters the furnace has been carried out by direct measurements of the EAF off-gas flow rate under the different conditions followed by the application of a mathematical model of the ORI Martin EAF. To perform these measurements a mass spectrometer (MS) was used to analyse the EAF off-gas sampled at the exit of the furnace. These analyses allow to monitor the gas composition and to calculate the gas total flow rate by means of a “tracer gas” method. By means of these activities, it has been possible to establish the benefits of the enhanced air-tightening of the furnace on the chemical energy recovery deriving from the CO post combustion ratio. GMH DC EAF At GMH EAF, different furnace configurations have been experimented during the project. At the beginning of the project, the configuration of burners and injectors for oxygen and carbon at the GMH DC furnace was as follows: Burners (O2 / CH4): Oxygen injection: Carbon injectors: Dust injection:

2 with 3.5 MW each 2 Oxygen door lance manipulators 1 supersonic RCB Fuchs type shell injection system 1 supersonic JetBOx (PTI) shell injection system 1 Carbon door lance manipulator 2 shell injection systems via door lance

Carbon injection via the door lance manipulator had to be maintained because the availability of the carbon injection through RCB burner and JetBOx was not reliable enough. The configuration of burners and injectors at the beginning of the project is shown in Figure 5. The data of trial heats performed under this configuration were used as reference for evaluation of improvements of the chemical energy yield and airtight operation achieved during the project by modified injector configurations. With the initial injector configuration, a severe formation of skull occurred at the injector positions. Furthermore the durability of the RCB Fuchs type burner was not sufficient in continuous operation, as problems with the cooling water system of the burner box occurred. As the JetBOx showed better results, GMH decided to substitute the RCB burner by a second JetBOx during the third project semester. Furthermore for the injection of carbon and dust also shell injectors were installed within the JetBOx elements. The door lance manipulator remained for oxygen injection, mainly in the last part of

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the meltdown period to clear the area of the furnace door for sampling. The first trials with airtight operation were accompanied with problems in creating and maintaining a foamy slag. Therefore the positions of the carbon and the dust injection were exchanged, which lead to a significantly improved foamy slag creation. For the first trials with this new injector configuration, the lance manipulator was still used for oxygen and injection during the whole treatment. Nevertheless it was possible to close the furnace door almost completely during lance operation, leaving only a small gap for introduction of the lance tip. The post combustion injectors inside the furnace were moved to a deeper operation position at the same height as the gas burners, to improve the efficiency of post combustion. During the year 2004, further trial campaigns with different types and configurations of wall-mounted injectors for oxygen and carbon were performed, partially under nearly airtight furnace conditions. The wall mounted JetBOx in panel number 11, which had been damaged during production, was substituted by an used RCB-Burner, which was temporarily in operation during the first semester of 2004. However, it was not yet possible to realise a completely airtight operation, mainly due to a still insufficient efficiency of coal injection via a wall-mounted injector. Thus the major part of coal injection was temporarily performed through the lance manipulator. The resulting furnace configuration for airtight furnace operation is shown in Figure 6 for the status of November 2003 (Airtight I operation) and for May 2004 (Airtight II operation).

Figure 5: Location of burners and injectors at the GMH DC EAF for the first trial campaign (Standard operation, March 2003

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Figure 6: Location of burners and injectors at the GMH DC EAF for the 2nd and 3rd trial campaigns Starting with September 2004 a trial period with a Pyrejet burner/injector type of Air Liquide, which is better protected versus mechanical damages, has been performed. Trials with and without dust injection via this injector were performed. The resulting configuration of burners and injectors is shown in Figure 7 on the left. In February 2005, a second line for carbon injection was installed in panel number 15. The injection position is near the off-gas extraction area, as can be seen in Figure 7 on the right. Thus carbon injection was now possible from two different positions in the furnace. This should improve the performance of slag foaming in the superheating phase, to increase electrical energy input into the liquid melt by reducing the radiation losses of the arc. A further aim was to allow to close the furnace door during the melting process for a longer time (airtight operating conditions), as with the second coal injection line on the long run it should be possible to substitute the coal injection via the door lance manipulator. Characteristic for both of these trial campaigns was that a high amount of oxygen was injected via the door lance and the wall-mounted injectors. Thus the furnace was operated with a slag door opened most of the treatment time according to the necessity to operate the door lance for carbon and oxygen injection. However, for these trial heats almost no post-combustion oxygen was injected via the subsonic ALARC injectors.

Figure 7: Location of burners and injectors at the GMH DC EAF for the 4th trial campaign and the 5th trial campaign

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During July 2005, a complete revamping of the GMH DC furnace was performed. A new furnace vessel with a higher shell was installed, still requiring the charging of two scrap baskets. Furthermore a modified burner and injector configuration with 3 oxygen and carbon injection points was implemented. Thus a more homogeneous distribution of chemical energy as well as oxygen and carbon injection is now possible, allowing a fully airtight operation of the furnace for most of the treatment time. The new configuration of burners and injectors after the revamping is shown in Figure 8.

Figure 8: Location of burners and injectors at the GMH DC furnace after revamping (since August 2005)

TKN Bochum EAF A series of measures were taken on both EAF and dedusting system. Operation conditions of dedusting plant: • Leaks at the forced draft coolers were sealed • Grids at the entry of the secondary dedusting system (canopy) were removed • Cycles of maintenance for the coolers were changed • O2 and CO is measured continuously at the point 2 in the primary dedusting system (Figure 9) • Gap between EAF elbow and hot gas line is reduced to 100 mm • Minimum position of the DEC flap and the by-pass flap was set to 15% • Air injector at the hot gas duct was shut down after measurement of air volume flow rate [chapter 2.1] • Trials to control off-gas extraction by furnace pressure measurements Operation conditions of EAF: Continuous operation with closed slag door during melting Gap between EAF roof and elbow is sealed Gap between EAF elbow and hot gas line is reduced to 100 mm Equipment for the measurement of the furnace atmosphere pressure was checked, revised and tested Material input: Use of most recent database of analysis data in order to increase homogeneity of the input material Consideration of actual material yield in computation of nominal material input Instant adaptation of input when remains are detected at the preceding heat Minimum occurrence of remain is aimed Tapping weight strictly between 70t and 80t per ladle Strict compliance of estimated levels of the filled bucket for austenitic and ferritic heats

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Constraining heavy input (>5t) to one piece per heat Power input: Constraining the EAF power input by strict application of the computed program Preferential use of a certain model (“model B”) for power input Definition of time to charge the 2nd bucket Detailed definition of few exceptions (input of coils, change of quality, remains from preceding heat) Oxygen input: Definition of maximum amount of oxygen, starting time and volume flow rate for each melting period, respectively Slag door strictly closed before oxygen injection Time and energy management: Use of previewed time management of material flow (Gantt diagram) Close co-operation with AOD converters after every heat in order to estimate interruptions and with the aim to interrupt the EAF process after tapping and charging, not during melting. During EAF interruption: charge EAF vessel and open the gap between EAF elbow and hot gas duct of the primary dedusting system. Definition of procedure for tapping into two ladles Definition of feasible range and of optimum of tapping temperature for ferritic and austenitic heats Slag treatment: Input of the amount of slag formers is convinced to the computed value in order to obtain low viscous slag with the estimated basicity of 1.2 Sealing of the EAF comprised closure of the gaps between EAF roof and EAF elbow, between EAF vessel and EAF roof, and installation and continuous use of the slag door. Subsequent sealing of the EAF was monitored by off-gas measurements during plant trial periods (Table 1 and Figure 9). Closure of the gaps was realized by re-installation of the slag door (gap no. 1), by refractory linings (gap no. 2) and by use of new undeformed EAF roof (gap no. 3). For normal EAF operation the DEC flap is opened to 100 % during melting and closed to 20% (15%) during charging and tapping in favor to the bypass duct of the secondary dedusting system. The by-pass flap is opened to 100 % during charging and tapping and closed to 50% during melting. Capacity of the primary dedusting system at point 2 is 110.000 m3STP/h during melting and 75.000 m3STP/h during tapping. With closure of the DEC and opening of the bypass flap during charging and tapping, the capacity of the canopy hood dedusting is therefore increased from 660.000 m3STP/h to 700.000 m3STP/h in order to provide maximum collection of emissions to the opened dog-house. During two plant trials the furnace pressure signal was used to control the DEC flap position in order to minimize the extracted off-gas volume flow rate at the EAF elbow. During the last plant trial the by-pass flap is opened to 100 % permanently in order to reduce off-gas volume flow rate at the EAF elbow.

Figure 9: TKN EAF: openings for air infiltration into the EAF vessel and dedusting system, schematically

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Table 1: Modifications of EAF operation and dedusting system Campaign in

Gap Gap Gap between between between EAF elbow EAF elbow EAF roof and hot gas and EAF and EAF duct closed roof vessel during closed closed oxygen (with (new roof) lancing refractory linings) (1) (3) (2)

X -a -a X -b -b X X X X X

X X X X X X X X

X X X X X X X X

Slag door installed and closed before oxygen lancing

(4)

X X X X X X X

DEC flap controlled with measured EAF atmospher e pressure

X X -

1.3. Development, testing and installation of measurement systems and techniques for airtight conditions CSM developed two optical sensors that can be used to monitor revealing variables linked to the furnace. The attention was focused in particular on the measurement of the composition and temperature of the gaseous phase inside the furnace and at the elbow (off-gas) using non-invasive devices. The sensors can be connected with the inner part of the furnace via optical windows. More particularly, the devices developed have been the following: • optical sensor to measure the gas composition in terms of CO and CO2; the concentration of CO and CO2 in the furnace gas as well as the ratio between CO and CO2 is symptomatic for the control of the coal combustion and of CO post-combustion inside the furnace. For this reason, a device that can do it is appropriate to be coupled with an on-line control system of the furnace • optical sensor to measure the temperature of a phase radiating in a specific range. the radiating phase could be a body at elevated temperature like the refractory wall of the furnace, the liquid surface or the particles dispersed into the gaseous phase. For this reason, the device can be used to monitor either the gas temperature or the temperature of the bath (for example). As a consequence, the use of this device coupled with the preceding one is ideal to complete the dynamic control system of the furnace. In what follows, the two sensors have been named with two different acronyms: LEA is the CO/CO2 sensor and TCP is the sensor to monitor the temperature.

1.3.1 Description of the CSM optical sensors LEA sensor: measurement of CO and CO2 concentration The device is able measure both CO and CO2 and it is based on a light absorption technique. More particularly, the light emitted from a source passes through the gaseous phase to measure and then is detected from the device, so a current proportional to the light intensity is recorded. The presence in the

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gaseous phase of CO and/or CO2 produces a light absorption in specific range of wavelength which is related to the chemical species, so the light intensity decrease (in this range) and this decrease can be directly related to the concentration of the specie in the gaseous phase. In Figure 10, typical absorption spectra of CO and CO2 in the infrared region have been reported. The light coming from the source and went through the gaseous phase enters the device and is split in two optical path. The splitting is obtained by means of a chopper which is a rotating disk with transparent and opaque windows; the split light is transmitted to some interferential filters. The filters that have been used are centred at 4.61 µm, 4.26 µm and 4.40 µm respectively. The wavelength of 4.40 µm is used as signal background. The use of a specific filter for CO or CO2 (4.61 µm or 4.26 µm, respectively) in one of the two optical path and of the 4.40 µm filter on the other, allows the measurement of CO or CO2 concentration. The use of the filter centred at 4.26 µm in one of the two optical paths and of the filter centred at 4.61 µm in the other, allows the measurements of the ratio CO/CO2. The design with two different photo detectors simplify the problems connected with optical alignments of the second section of the optical instruments after the light chopping: the use of two separated photodetectors each one for the detection of a single wavelength allows the use of a single chopper blade. Figure 11 shows a schematic drawing of the whole device.

Figure 10: CO and CO2 infrared absorption spectra[49]

Figure 11: LEA: overall scheme Optical calculations produced the adoption of two lenses that focuses the source light on the splitting

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apparatus and of two other lenses for focusing the light coming from the filters on the detectors. The focal lengths of these lenses are 100 and 50 mm for the two lenses focusing the light on the chopper and 40 mm for the two lenses focusing the light on the detectors. The choice of the focal lengths of the first two lenses is related to the distance of the source light from the device. The material chooses for lenses is the CaF2. The measures of light intensity have been made by interferential filters that can use 1% or 4% bandwidth. The 4% bandwidth allows measuring greater concentrations with lower accuracy than the 1% bandwidth. The measure of the transmitted light intensity has been made by PbSe detectors. Figure 12 shows the simplified scheme of the optical path into the LEA. Figure 13 shows an overall view of the LEA device without its cover.

Figure 12: LEA: scheme of the optical path

Figure 13: LEA: overall view of the device (without the cover) The LEA is interfaced with a PC by means of multifunction I/O board National Instrument PCI 6023E. The software used is “Lab View”. This allows to monitor the signals of the two sensors and to collect them. TCP sensor: temperature measurement This device is based on the measure of the radiant energy of a continuum at two different wavelength. The simultaneous measurement of the radiant energy at two different wavelengths allows monitoring very rapid temperature variation. The instrument is composed of two monochromatic CCD video camera (JAI CV-M10RS-C-1/2”) with synchronised shutter speed (1/10,000). A simple optic, composed by two lens and a diaphragm, projects

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the light on a beam splitter. Then, the light is filtered by two different interferential filters and captured by the CCD video cameras. This optical instrument generates (25 times per second) two bi-dimensional maps of the same spot at the same time, filtered on two different wavelength. If the object is considered like a black body, the ratio of the maps signal is proportional to the temperature of the object. Figure 14 shows a schematic drawing of the apparatus. Figure 15 shows an overall view of the device without the cover (A) and a front view (B), respectively.

Figure 14: schematic drawing of the device

Figure 15: TCP: photos of the device The device allows performing a “Ratio Pyrometry”. According to the theories of the radiation of thermal energy from bodies above absolute zero, the radiance N at a wavelength λ of a real body is given by the following equation (Planck relation):

ε λ ⋅ C1

Nλ =

λ ⋅ e 5

C2 λ ⋅T

−1

where: ελ is the emissivity that depends on the wavelength and on the body temperature C1 and C2 are the Planck constants

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The two colors pyrometer method uses an approximation of the Planck relation, called Wien [45,46] relation, that gives a deviation less than 1% from the Planck relation if λT