Kurose et al.(2001) analyzed experimentally the ...... [13] Choi Y.C., X.Y. Li, T.J. Park, J.H. Jim and J.G. Lee (2001) Numerical study on the coal gasification ...
NUMERICAL AND EXPERIMENTAL STUDIES ON LPG-ASSISTED GASIFICATION OF PULVERIZED HIGH-ASH INDIAN COAL A PROJECT REPORT Submitted in the partial fulfillment of the requirements for the award of the degree of
MASTER OF TECH OLOGY in Mechanical Engineering by P. SE THIL KUMAR (ME08M029) Under the guidance of Prof. T. SU DARARAJA A D Prof. U.S.P SHET
THERMODYNAMICS AND COMBUSTION ENGINEERING LABORATORY DEPARTME T OF MECHA ICAL E GI EERI G I DIA I STITUTE OF TECH OLOGY MADRAS CHE AI - 600 036, I DIA
MAY-2010
This work is dedicated to My parents, Brother and Sister.
ACKOWLEDGEMET I owe my deepest gratitude to Prof. T. Sundararajan for his guidance. His flawless ideas never failed and always made my research easier.
I am greatly indebted to
Prof. U.S.P Shet for his valuable suggestions during my research period. He was the one who thought me how a true experimentalist should be. It was an honor to work with both of them. My sincere thanks to Prof. S. P. Venkatesan, Head of the Department of Mechanical Engineering who laid the foundation of measurement techniques in me.
I also like to
thank Prof. K. Srinivasan , ( H O L -TDCE) for his support and ideas. I wish to express my profound gratitude to Prof. K. Ramamurthy, Prof. V. Babu, Dr. V. Raghavan and Dr. Ravikiran Sangras for all the useful suggestions and support I gathered during the course of my study at TDCE Laboratory. I am very much thankful to Sandeep, Revanth, Gopal , Siva, Giri sir, Suresh kumar, Ganesh, Arun, Vaibhav for their wonderful friend ship and support. I am grateful to Messrs. Diwakar , Kandaswamy,
Balaji, Jogendra, arayanan, Boopathy,
Sivakumar , Panendra , Srinivasan for providing a wonderful atmosphere in TDCL. My special and sincere thanks to Mr. A.Ajilkumar suggested
for his wonderful ideas
during my research. Without him I wouldn’t
he
have performed any
experimentation studies. Last, but not the least, I like to thank my Mother, Father, Brother and my Sister Shamu, for their prolonged patience and support.
P .SETHIL KUMAR
i
ABSTRACT
KEYWORDS : Coal gasification; Entrained flow gasifier; Numerical modeling; Pilot assisted gasifier; Devolatilisation models; Air ratio; Steam ratio Carbon conversion; Higher Heating Value; Cold gas efficiency.
Coal is the most abundant fossil fuel that will replace all petroleum products in the future. Currently India is the third largest producer of coal contributing 7.4% of total world’s coal production. Coal accounts for 55% of India's energy needs of which 75% is consumed by the power sector. The current way of extracting power from coal is highly inefficient and causes a threat to environment
by emitting
harmful pollutants .
Gasification of coal is a classical method that has played a major role in the clean combustion of coal. An advanced version known as the Integrated Gasification Combined Cycle (IGCC) is one of the new invention that has attracted several industries using coal as their primary input. Gasification plays a major role in this methodology and needs more attentions. Gasification reactions consist of both homogeneous and heterogeneous steps. These reactions may be exothermic or endothermic. Unfortunately during gasification endothermic reactions overweigh exothermic reactions creating an energy depravation. In order to continue the reaction a constant energy source supplying the necessary energy for the endothermic reactions is required. Wall heating, initial preheating of inlet air and pilot burners are a few of these methods employed in industries to provide the energy required for sustaining the gasification reactions. It has been noted that not much literature is available specifically, for
analyzing
the effect of pilot fuel on the
gasification characteristics of pulverized high ash Indian sub bituminous coal. In present study, the effect of pilot fuel (LPG) and the input parameters such as air and steam ratios on atmospheric pressure gasification are investigated through numerical simulations and the predictions are validated with experimental results. Numerical modeling is proved to be one of the major tools for generating the necessary details of the gasification characteristics even before the prototype of gasifier is erected.
Numerical investigations along with the detailed combustion process analysis
can be
used to improve the gasifier performance. A gasifier operating with different mass flow rates of pilot fuel are compared numerically with a gasifier operating with constant wall heating. For this analysis a 16 kW capacity gasifier is considered. A steady state
axi-symmetric model is developed and analyzed using FLUENT 6.3
software. The model consists of devolatilisation , combustion of volatiles, char combustion and gasification and other gas phase reactions. The continuous (gas) phase conservation equations are solved in an Eulerian frame and those of the particle phase are solved in a Lagrangian frame, with coupling between the two phases carried out through interactive source terms. The dispersion of the particles due to turbulence is predicted using a stochastic tracking model, in conjunction with the gas phase turbulent models (mainly the k-ε model). The effect of ash in the coal is assessed by suitably modifying the preexponential factor and diffusion rate constant in char reaction steps. The effects of LPG (pilot fuel )on the gasification characteristics, specifically the gasifier temperature, CO, H2, CO2 and H2O mole fractions, are assessed. The simulation for this case is done with four LPG mass flow rates corresponding to 5%, 10%, 15% and 20% of total gasifier power. The experiments were conducted in a 1m length , 72mm diameter horizontal, tubular gasifier with LPG as pilot fuel, operating at atmospheric pressure. Several runs of experiment with different LPG flow rates and air ratio (the ratio of actual flow rate of air to stoichiometric air) corresponding to 0.30 were conducted. During the runs, no steam was allowed to enter the gasifier. The experimental values obtained are thus compared with numerically simulated results. It was found that the mole fraction of CO is decreasing with the increasing mass flow rates of LPG. But H2 mole fraction increased with LPG mass flow rate. The sensitivity of different devolatilisation models
on gasification was found and
compared with the experimental data. The 2-Step Kobayashi model showed a good agreement with experimental data. The effects of air ratio on gasification are determined for pilot assisted gasification as well as the no pilot case numerically. In the pilot case, the CO mole fraction at the exit of
iii
the gasifier is reduced and it becomes almost nil for the air ratio of 0.45. The H2 mole fraction increases for the air ratios of 0.30, 0.35, 0.40 and slightly decreases with air ratio of 0.45, similar observations were seen in the no pilot case. The effect of steam on H2 and CO was investigated in the no pilot case. It was found that H2 mole fraction increased more rapidly but for higher steam flow rate it reduces due to flame quenching and incomplete combustion. The effect of air ratio and steam ratio on parameters like carbon conversion, higher heating value of product gas and Cold gas efficiency were found out. It was found that the carbon conversion increases with air ratio, but the higher heating value of the product gas decreases along with the cold gas efficiency. At higher steam ratios the carbon conversion increases along with the cold gas efficiency. But the changes are very small indicating steam has only a minor influence on these parameters. .
iv
TABLE OF CO TE TS
Page ACKNOWLEDGEMENTS…………………………………………………................i ABSTRACT…………………………………………………………………..............ii LIST OF TABLES……………………………………………………………...........viii LIST OF FIGURES……………………………………………………………..........ix ABBREVIATIONS…………………………………………………………………xiii NOMENCLATURE…………………………………………………………….......xiv
CHAPTER 1
I TRODUCTIO
1.1
Coal and its Importance….……………………………………………………...1
1.2
Integrated Combined Gasification Cycle ( I.G.C.C )……………………………2
1.3
Coal Gasification…………………………………………………………….......2
1.4
Types of Gasifier…………………………………………………………..……..3 1.4.1
Fixed Bed Gasifier ……………………………………………….............4
1.4.2
Fluidized Bed Gasifiers………………………………………...…………4
1.4.3
Entrained Bed Gasifier…………………………..…………………..........6
1.5
Overview of Gasification Processes……………………………............................6
1.6
Numerical Simulation of Gasification…………………………………………….7
1.7
Organization of Thesis…………………………..………………………………...7
CHAPTER 2 2.1
LITERATURE REVIEW
Introduction………………………………………………………………………..9
v
2.2
Coal Gasification Aspects ………………………………………………………...9
2.3
High Ash Coal and its Utility to Coal Gasification ………………..……………10
2.4
Entrained Flow Air Gasification…………………….…………………………...11
2.5
Objectives of Present Studies………………………………………………….....14
CHAPTER 3
UMERICAL METHODOLOGY
3.1
Introduction………………………………………………………………………15
3.2
Gas Phase Conservation Equations …………..………………………….............16
3.3
Discrete Phase Model…………………………………….……………………...18
3.4
Particle Size and Distribution….……………………………...………...…….....21
3.5
Chemistry Modeling……………………………………………………..............24 3.5.1
Devolatilization Models……….…………................................................24
3.5.2
Heterogeneous Reactions………………………………………………..32
3.5.3
Homogenous Reactions…………………………...……………………..33
3.6
Boundary Conditions…………………………….................................................35
3.7
Numerical Schema……………………………………………………………….36
3.8
Grid Independence Study………………………………………………………...37
CHAPTER 4
EXPERIME TAL METHODOLOGY
4.1
Introduction………………………………………………………………………39
4.2
Coal Preparation and Analysis…………………...................................................39
4.3
Experimental Setup………………………………................................................40
4.4
Radiation Correction for Thermocouple…………………………………………45
vi
4.5
Uncertainties in Measurements ………………………………………………….45
4.6
Experimental Procedure…………...……………………………………………..46
4.7
Closure…………...………………………………………………………………47
CHAPTER 5
5.1
UMERICAL A D EXPERIME TAL STUDY O
ATMOSPHERIC PRESSURE E TRAI ED FLOW GASIFIER WITH A D WITHOUT PILOT Introduction……………………………………………………………..………..48
5.2
Experimental Conditions …………………………..……………………............48
5.3
Numerical Methodology…………………………………………………………49
5.4
Comparison of Numerical and Experimental Results….………….……………..51
CHAPTER 6 6.1
UMERICAL STUDY O HIGH ASH COAL GASIFICATIO CHARACTERISTICS Introduction………………………………………………………………………62
6.2
Comparison of Different Devolatilization Models………………………………63
6.3
Effect of Air Ratio and Steam Ratio on Gasification Characteristics....................69 6.3.1
Numerical Study of Gasification with Pilot Burner……………………...69
6.3.2
Numerical Study of Gasification Without Pilot Burner………………….75
CHAPTER 7
CO CLUSIO A D SCOPE OF FUTURE WORK
7.1
Summary…………................................................................................................84
7.2
Major Conclusions Drawn from the Present Study…….………..…....................84
7.3
Scope for Future Studies…………………………………....................................86
REFERE CES………………………………………………………………………….88
vii
LIST OF TABLES Table o.
Title
Page o.
3.1
Particle size distribution data……………………………………...... 22
3.2
Particle size data in Rossin-Rammler form…………………….......
3.3
Size input parameters……………………………………………...... 24
3.4
Parameters for CPD modeling…………………………………........
3.5
Proximate and ultimate analysis of coal…………………….............
23
31 34
3.6
Kinetic constants used in numerical simulations………………….... 35
3.7
Grid temperature variation………………………………………...... 37
4.1
Proximate and Ultimate analyses of coal in present experiments…………………………………………………………. 40
5.1
Mass flow rates of LPG and air in pilot burner in terms of total gasifier power……………………………………………………….
49
5.2
Input parameters for numerical and experimental analysis…….......................................................................................
51
6.1
The variation of carbon conversion and cold gas efficiency with AR……............................................................................................... 80
6.2
Effect of Steam Ratio (SR) on Carbon Conversion and Cold Gas Efficiency............................................................................................ 83
viii
LIST OF FIGURES Figure o.
Title
Page o.
1.1 Types of gasifier (a) Fixed bed gasifier ,(b) Fluidized bed gasifier (c)Entrained bed gasifier (Source : www.ccsd.biz)....................................
5
3.1 Cumulative size distribution of coal particles.…………………………...
23
3.2 Chemical reaction schema for modeling of bridge cleavage……………
27
3.3
Axi-symmetric view of the gasifier with boundary conditions…………..
36
3.4 Axial variation of temperature for three different grid sizes……………..
37
4.1 Schematic representation of the experimental set-up…………………….
41
4.2 Photographic view of the gasifier unit…………………………………..
42
4.3 Sectional view of inlet ports……………………………………………..
42
4.4 Exhaust system for product gas dilution…………………………………
44
5.1 Modified geometry for modeling of no-pilot case……………………….
50
5.2 Temperature contours for (a) Pilot power equivalent to 5% of total gasifier power (TGP) (b) Pilot power equivalent to 10% of TGP (c) Pilot power equivalent to 15% of TGP (d) Pilot power equivalent to 20% of TGP (e) No pilot case………………………………………………………. 53 5.3 Contours showing CO mole fraction for (a) Pilot power equivalent to 5% of total gasifier power (TGP) (b) Pilot power equivalent to 10% of TGP (c) Pilot power equivalent to 15% of TGP (d) Pilot power equivalent to 20% of TGP (e) No pilot case………………………………………………. 54
ix
5.4 Contours showing O2 mole fraction for (a) Pilot power equivalent to 5% of total gasifier power (TGP) (b) Pilot power equivalent to 10% of TGP (c) Pilot power equivalent to 15% of TGP (d) Pilot power equivalent to 20% of TGP (e) No pilot case……………………………........................................... 55 5.5 Contours showing CO2 mole fraction for (a) Pilot power equivalent to 5% of total gasifier power (TGP) (b) Pilot power equivalent to 10% of TGP (c) Pilot power equivalent to 15% of TGP (d) Pilot power equivalent to 20% of TGP (e) No pilot case………………………………………………………... 56
5.6
Contours showing H2 mole fraction for (a) Pilot power equivalent to 5% of total gasifier power (TGP) (b) Pilot power equivalent to 10% of TGP (c) Pilot power equivalent to 15% of TGP (d) Pilot power equivalent to 20% of TGP (e) No pilot case……………………………........................................... 57
5.7 Contours showing H2O mole fraction for (a) Pilot power equivalent to 5% of total gasifier power (TGP) (b) Pilot power equivalent to 10% of TGP (c) Pilot power equivalent to 15% of TGP (d) Pilot power equivalent to 20% of 58 TGP (e) No pilot case……………………………………………………… 5.8 Contours showing volatiles mole fraction for (a) Pilot power equivalent to 5% of total gasifier power (TGP) (b) Pilot power equivalent to 10% of TGP (c) Pilot power equivalent to 15% of TGP (d) Pilot power equivalent to 20% of TGP (e) No pilot case………………........................................................... 59
5.9 Comparison of numerical and experimental results of axial temperature plots……………………………………………………………………........... 60 5.10 Experimental and theoretical comparison of the mole fractions of CO, CO2 & H2………………………………………………………………………….. 61 6.1 Axial variations of temperature for different devolatilisation models and comparision with experimental data…………………………………….........
64
6.2 Variations of CO2, CO and H2 mole fractions for different devolatilisation models and comparison with experimental results………………………….. 64
6.3
Axial variation of volatile mole fraction for different devolatilisation model…………………………………………………………………………. 65
x
6.4
Contour plots showing the effects of different devolatilisation models on temperature (a) 2-step Kobayashi Model (b)Constant rate Model (c) C P D Model (d) Finite rate Model……………….. 66
6.5
Contour plots showing the effects of different devolatilisation models on CO mole fraction (a) 2-step Kobayashi Model (b)Constant rate Model (c) C P D Model (d) Finite rate Model….. 67
6.6
Contour plots showing the effects of different devolatilisation models on CO2 mole fraction (a) 2-step Kobayashi Model (b)Constant rate Model (c) C P D Model (d) Finite rate Model….. 67
6.7
Contour plots showing the effects of different devolatilisation models on Volatile mole fraction (a) 2-step Kobayashi Model (b)Constant rate Model (c) C P D Model (d) Finite rate Model….. 68
6.8
Contour plots showing the effects of different devolatilisation models on H2 mole fraction (a) 2-step Kobayashi Model (b)Constant rate Model (c) C P D Model (d) Finite rate Model….. 68
6.9
Axial variation of temperature for different AR…………………...
69
6.10
Axial variation of CO2 mole fraction for different Air Ratios (AR)...
70
6.11
Axial variation of CO mole fraction for different Air Ratios (AR)..
71
6.12
Axial variation of H2O mole fraction for different Air Ratios (AR)... 71
6.13
Axial variation of H2 mole fraction for different Air Ratios (AR)….
6.14
Temperature contours for (a)AR=0.30, (b)AR=0.35, (c)AR=0.40 and (d)AR=0.45.................................................................................. 72
6.15
CO mole fraction contours for (a)AR=0.30, (b)AR=0.35, (c)AR=0.40 and (d)AR=0.45............................................................. 73
6.16
CO2 mole fraction contours for (a)AR=0.30, (b)AR=0.35, (c)AR=0.40 and (d)AR=0.45.............................................................. 74
xi
72
6.17 H2 mole fraction contours for (a)AR=0.30, (b)AR=0.35, (c)AR=0.40 and (d)AR=0.45........................................................ 74 6.18 H2O mole fraction contours for (a)AR=0.30, (b)AR=0.35, (c)AR=0.40 and (d)AR=0.45........................................................ 75 6.19 Radial plots (no pilot case) of CO2 for (a) AR=0.30, (b)AR=0.35, (c)AR=0.40 and (d)AR=0.45 …………………….. 76 6.20 Radial plots (no pilot case) of CO for (a) AR=0.30, (b)AR=0.35, (c)AR=0.40 and (d)AR=0.45…………………….. 77
6.21
Axial variation of H2 for different Air Ratio (AR)……………...
78
6.22
Axial variation of CO for different Air Ratio (AR)……………
78
6.23 Temperature contours (no pilot) for (a)AR=0.30, (b)AR=0.35, (c)AR=0.40 and (d)AR=0.45.......................................................
79
6.24 H2O mole fraction contours (no pilot) for (a)AR=0.30, (b)AR=0.35, (c)AR=0.40 and (d)AR=0.45................................... 79 6.25 Carbon conversion and Cold Gas Efficiency for different Air Ratios (AR)……………………………………………………...
80
6.26 Temperature contours for no pilot case , AR=0.30 , (a) SR=0, (b) SR=0.1, (c) SR=0.2 and (d) SR=0.3........................................ 81 6.27 H2 mole fraction contours for no pilot case , AR=0.30 , (a) SR=0, (b) SR=0.1, (c) SR=0.2 and (d) SR=0.3...........................
82
6.28
Axial variation of H2 mole fraction for different SR…………...
82
6.29
Carbon conversion and Cold Gas Efficiency for different Steam Ratios (SR)……………………………………………………..
82
xii
ABBREVIATIO S
AR
Air Ratio
CC
Carbon Conversion
CGE
Cold Gas Efficiency
CPD
Chemical Percolation Devolatilization
DPM
Discrete Phase Modeling
HHV
Higher Heating Value
IGCC
Integrated Gasification Combined Cycle
LPG
Liquefied Petroleum Gas
NMR
Nuclear Magnetic Resonator
NS
Navier-Stokes
PCI
Pulverised Coal Injection
Pc
Pulverised coal
Pf
Pulverized-fuel
PFB
Pressurized Fluidized Bed
RSM
Reynolds Stress Model
SNG
Substitute Natural Gas
SR TGP
Steam Ratio Total Gasifier Power
xiii
OMECLATURE English Symbols A
pre-exponential factor, s-1
Ap
surface area of the particle, m2
CD
drag coefficient
Cp
specific heat at constant pressure, J.kg-1.K-1
d
diameter of gasifier, m
dp
particle diameter, m
d
mean diameter of particle, m
D 0, r
diffusion coefficient for surface reaction
Dkm
binary diffusivity, m2.s-1
E
activation energy, J.kmol-1
FD
drag force, N
Hreac
enthalpy of char reaction, J.kg-1
K
thermal conductivity of gas phase, W.m-1.K-1
m
mass, kg
u
Nusselt number
p
pressure, Pa
Pr
Prandtl number
Pt
total pressure, MPa
r
radial coordinate, m
R
gas constant, J.kgmol-1.K-1
Re
Reynolds number
xiv
Rk
net rate of production of species k, kg.m-3.s-1
R k,r
kinetic rate of reaction, kg.m-2.s-1
SMk
source term due to exchange of momentum, kg.m-2.s-2
Sp
source term due to exchange of mass, kg.m-3.s-1
Sph
source term due to exchange of energy, W.m-3
S pYk
source term arising from evaporation from the particles, kg.m-3.s-1
t
time, s
T
gas phase temperature, K
Tp
particle temperature, K
u
velocity, m.s-1
z
axial distance, m
Y
gas phase mass fraction
Greek symbols θR α1, α2
radiation temperature, K stoichiometric coefficients of two competing rates
µ
viscosity, kg.s.m-1
εP
particle emissivity
ρ
density, kg.m-3
σ
Stefan-Boltzmann constant (=5.670 x 10-8 W.m-2.K-4)
ηr
effectiveness factor
xv
Subscripts i, j
indices of Cartesian vector components
k
index for species
p
particle phase
r
reaction
vol
volatile content in coal
xvi
CHAPTER 1 I TRODUCTIO 1.1
COAL A D ITS IMPORTA CE
“Coal is a compact stratified mass of metamorphosed plants which have in part suffered arrested decay to varying degree of completeness”- Stopes and Wheeler Coal is the most abundant fossil fuel that will replace all petroleum products in the forth coming future. Currently India is the third largest producer of coal contributing 7.4% of total world’s coal production. Coal accounts for 55% of India's energy needs of which 75% is consumed by the power sectors. In the foreseeable future, it is still expected to be utilized as a major source of fuel for power generation, but there is an increasing need for clean-coal power generation for higher efficiency. The worldwide consumption of coal is expected to increase in the upcoming decades (Annamalai and Ryan ,1993). Despite of high ash content in Indian coal, it is widely used in many industries for power generation and chemical processing. Coal is likely to remain a key energy source for India, for the next two decades as India has a significant amount of domestic coal reserve and a large installed-capacity for coal-based electricity production. The only question that remains in one’s mind is whether the current way of utilization of coal is efficient and clean. The environmental issues related to the disposal of the large quantity of fly ash produced during coal combustion remain unanswered. Currently in India, coal-fired power generation systems are largely based on the combustion of pulverized fuel because of the high surface area to volume ratio which enhances the heat and mass transfer between the coal and its surrounding air thereby resulting in higher burn- out efficiency. Since the stable and efficient combustion of pulverized coal particles strongly depends upon the physical and chemical properties of coal, the particle size and the operating conditions it is necessary to do research on such pulverized coal combustors and gasifiers.
1.2
I TEGRATED GASIFICATIO COMBI ED CYCLE ( I.G.C.C )
The current way of extracting power from coal is highly inefficient and causes a threat to environment due to the emissions. The emission of different pollutants, especially green house gases, may urge the environmental regulations to be a strong driver for new developments, in particular for decision makers that regulate the energy policies of states and regions. These developments include coal based electric power technologies, where the IGCC is an alternative to pulverized coal combustion systems as they obtain higher efficiencies and better environmental performance. In comparison with modern coal combustion technologies (Pulverized Coal Combustion (PCC), fluidized bed combustion (FBC), supercritical and ultra-supercritical technologies), IGCC systems are characterized by lower SOx and NOx emissions, comparable vapor organic carbon (VOC) emissions, 20% less CO2 emissions and use of 20–40% less water (Kings,1981). They operate at higher efficiencies, thus requiring less fuel and producing fewer emissions (Zheng and Furinsky, 2005). Commercially available IGCC power plant technologies produce substantially smaller volumes of solid wastes (approx. 1/2) than the new conventional coal plants (Shilling & Lee, 2003). Furthermore, IGCC solid wastes are less likely to cause environmental damage than fly ash from conventional coal plants because IGCC ash melts in the gasification process (Shilling & Lee, 2003).Gasifier is known to be the heart of IGCC and hence the investigation and analysis of the gasifiers, mainly pulverized coal gasifier is necessary.
1.3
COAL GASIFICATIO
Coal gasification has progressed from the traditional town mains gas processes common until the 1960s, but the chemical reactions involved are basically the same. The early processes drove off the volatile components in the coal and produced hydrogen by reaction of the coke with steam. Heat for the process, carbon monoxide and carbon dioxide were produced by reaction of the coke with steam, air or oxygen. Most modern gasification processes try to pyrolyse and gasify all the hydrogen and carbon in coal in completely automotive ways. Coal gasification is basically the combustion of coal with
2
insufficient oxygen to turn all the chemical energy into heat. Efficient combustion would turn all the carbon in the fuel to carbon dioxide and all the hydrogen compounds into water i.e. turn all the chemical energy of the coal into heat. But in gasification reactions due to insufficient oxygen, CO and H2 are evolved along with CO2. The aim of gasification is to turn as much as possible of the chemical energy in the fuel into chemical energy in gases. This requires provision of the right amount of oxygen and perhaps steam to balance heat release without excessive oxidation of hydrogen and carbon components. Apart from satisfying the domestic needs as the technology progressed, gasification has stepped to the next level playing a major role in petro-chemical industries.. The goal is to produce synthetic gas or substitute natural gas for the synthesis of gasoline, hydrogen and sulfur-free diesel fuel. In this regard, chemical feed stocks for methanol and ammonia production are also emphasized. For instance, high pressure gasification is used commercially for the production of gases to synthesize chemicals such as ammonia, although the feed material is heavy oil residue instead of coal. . The use of syngas produced from coal gasification reactions as a fuel for gas turbines has motivated further development of coal gasification technology.
1.4
TYPES OF GASIFIERS
Gasifiers are special types of combustors which operate with less air or oxygen. Their main objective is to produce CO and H2. Gasifiers can be classified based on oxidant feed, feedstock feeding direction and flow speed. Depending upon the direction of feedstock feeding, they are divided into (i) Co-current gasifiers in which the coal and oxidant move in the same direction. (ii) Counter-current Gasifiers in which the coal and the oxidant move in opposite directions. (iii) Updraft gasifiers in which the oxidant is supplied at the bottom and the syngas is extracted from the top of the gasifier. (iv) Downdraft Gasifiers, in which the oxidant is supplied at the top and syngas is extracted from the bottom.
3
On the basis of the oxidant feed, gasifiers are categorized into oxygen- blown and airblown gasifiers. Based on the flow speeds, gasifiers are classified as moving bed gasifier, fluidized bed gasifier and entrained flow gasifier.
1.4.1
Fixed Bed Gasifiers
Fixed beds (or moving beds as they are known in the United States) consist of beds of fuel through which the air or oxygen for gasification is blown. The solids move slowly towards the bottom of the gasifier, turning from coal to ash/slag on the way. In counter flow versions, the gas exits at the top of the gasifier. The technology has similarities to air rising through a domestic grate in a fireplace and ash falling through the grate. Fixed beds have limitations on the properties and the size range of fuel, sometimes requiring some fine coal to be briquetted to avoid gas channeling .The preferred particle size is 5-80mm. Internal heat exchange in counter flow versions of this gasifier gives a high proportion of chemical energy in the gas through the use of sensible heat in the product gas for devolatilisation and other endothermic reactions which increase the gas calorific value. This avoids the need for expensive high-temperature heat exchangers. The operating temperature ranges from 430°C to 1540°C. The by-products, tar and liquid volatile, flow down to the slagging area and decompose.
1.4.2
Fluidized Bed Gasifiers
Fluidized beds use coal ground to a few millimeters diameter (
