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Equilibrium calculations in coal gasification
Int. J. Hydrogen Energy, Vol. 15. No. 2. pp. 125-131, 1990. Printed in Great Britain.
0360-3199/90$3.00+ 0.00 Pergamon Press plc. '.~5~1990ImernationalAssociationfor Hydrogen Energy.
EQUILIBRIUM CALCULATIONS IN COAL GASIFICATION G. KOVACIK, M. O~UZT6RELI*, A. CHAMBERSand B. OZ0M Alberta Research Council, Devon, Alberta T0C 1E0, Canada (Received for publication 2 August 1989) Abstract--Coal gasification is commercially important due to its application m ammonia, hydrogen, methanol and synthetic fuel production and in integrated gasification combined cycle (1GCC) power plants. Accurate process evaluations are required for the development of new generation gasification technologies. Perhaps the most crucial component of this evaluation is the accurate prediction of product gas quality for varying feed streams and operating conditions. Operating conditions such as reactor hydrodynamics, temperature and pressure are variables which differentiate the gasifiers that are being offered for commercial applications (fixed bed, fluid bed, entrained flow, dry or slurry feed, air or oxygen blown). The present work demonstrates that for reactive coal feedstocks and for gasifers operating at relatively high temperature and pressure (entrained flow and fluidized beds), product gas compositions can be estimated, regardless of gasifier type, using chemical equilibrium principles.
NOMENCLATURE Aj ak,j b~ g n P R T
n u m b e r of moles o f j t h specie stoichiometric coefficient of k th atom i n j t h specie total n u m b e r of kth atom Gibbs free energy per mole of mixture, J mol n u m b e r of moles pressure, N m 2 universal gas constant, 8.3143 J m o 1 - 1 K -1 temperature, K
Greek letters ~,.~ stoichiometric coefficient of j t h species in ith reaction 2, Lagrangian multipliers q~ multipliers # chemical potential, J mol- 1 variational operator Subscripts i indice for chemical reactions j indice for chemical species k indice for atoms INTRODUCTION Coal gasification involves the simultaneous processes of thermal pyrolysis, homogeneous gas phase and heterogeneous gas-solid reactions between the feed coal and the feed and product gases. The product gas is typically composed of H 2, CO, CH4, N 2 and trace amounts of hydrocarbons, and its composition is a function of gasification reactor operating temperature and pressure,
*Petro Simulator Systems Inc., Edmonton, Alberta, Canada.
as well as feed coal and feed gas compositions. If air is used as the main reactant to gasify coal, the product gas is described as low BTU gas because it contains N 2 as a major component. I f O 2 is used to gasify coal the product gas is termed intermediate BTU gas (synthesis gas) which contains only traces of N 2. Intermediate BTU gas can be used either as an energy source or as a synthesis gas for the production of chemical and synthetic liquid and gaseous fuels. Synthetic natural gas contains over 90 percent by volume CH4, which is produced by further processing of intermediate BTU gas. Coal gasification processes are being utilized to produce synthetic natural gas and fuel, ammonia, feedstock for the chemical industry and electricity through IGCC power generation. Extensive research and development of gasification technology is proceeding in the United States, Europe and Japan. The advantages of IGCC for power have utility companies interested in the recent developments of the gasification technology. Several gasification technologies are in the demonstration phase. These technologies utilize fixed bed, moving bed, fluidized bed or entrained bed reactors operating at a variety of pressures, temperatures and residence times. Each reactor has its own advantages, disadvantages and economics for the gasification of a particular coal. Prior to selection of the optimum technology for a particular coal some engineering evaluations must be performed. One of the major criteria needed is an accurate estimate of the gasifier product gas composition. This is necessary because the gas composition would effect the required downstream treatment of the product gas and its ultimate use. Product gas composition is one of the major variables affecting the evaluation and selection of any coal gasification process. Product gas composition depends on coal type, feed gas composition, feed coal to feed gas 125
126
G. KOVACIK et al.
ratio, and the type of gasification reactor being used. Variables which affect the reactor performance include the operating temperature and pressure, feed gas composition and temperature, and the reactor hydrodynamics. Reactor temperature and pressure are important for the thermodynamics (chemical equilibrium) and the dynamics (reaction kinetics) of the process while the reactor hydrodynamics are important for the practical application to ensure proper mixing, gas-solid contact, recycle ratio, gas and solid residence times etc. Research on the effect of these factors has resulted in changes in gasification technology with the development of several new, commercially applicable reactors in the last one or two decades. This study demonstrates that product gas composition can be predicted under certain conditions, especially under the operating conditions of new generation gasifiers, provided that chemical kinetics is not a controlling factor, and the operating temperature for the gasifier can be defined. F O R M U L A T I O N OF THE PROBLEM Coal gasification can be considered as rapid thermal pyrolysis of coal coupled with parallel and consecutive homogeneous gas phase and heterogeneous gas-solid reactions. Thousands of chemical reactions may occur among thousands of chemical species in coal gasification. However, from the reactor engineering point of view, char, CO2, CO, 02, H20, CH4, H2, H2S and SO 2 are the main species which can be identified by conventional analytical techniques. Many chemical reactions may occur among these species, from which a set of independent reactions must be chosen for reactor engineering analysis. If s and r denote the number of species and number of reactions involved in the gasification process, respectively, the following relation can be written for the stoichiometry of r reactions ~ ~i.jAj = 0
( i = 1,2 . . . . r)
(1)
j=l
where ~ti.jis the stoichiometric coefficient of j-th specie in i-th reaction and Aj is thej-th specie. If r reactions are
independent, only the trivial set of multiples t/i = 0; i = 1, 2 . . , r can satisfy the following relation [1]: r/,~i,s = 0
( j = 1, 2 . . , s).
In practice, a stoichiometrically independent reaction set can be found from the Gauss elimination of the matrix ~i.j of equation (1). Reactions corresponding to non-zero diagonal elements form the set of stoichiometrically independent chemical reactions. In the present study, the reactions listed in Table 1 were found to be independent. Prediction of product gas composition is almost impossible using available information about the kinetics of coal gasification reactions. This is due to the fact that a prediction which is based on the kinetics of the process requires the accurate knowledge of a large number of dynamic parameters. Measurements to evaluate these parameters are always influenced by the hydrodynamics of the reacting system [2]. Thus, product gas composition estimation using kinetic models becomes exceedingly difficult. In many practical applications the reactions in Table 1 occur under conditions where chemical kinetics are not the controlling mechanism. This leads to another approach to predict product gas composition using the principles of chemical equilibrium. If the gasification reactions occur fast enough, it may be assumed that the reaction products are in chemical equilibrium. Realistically, a coal gasification system should reach chemical equilibrium if the operating temperature and pressure of the system are high, i.e. the gasification reaction rates are high. In such cases, product gas composition could be calculated using chemical equilibrium principles for multicomponent, multireaction systems, provided that the feed compositions and the reactor operating conditions are known. The problems in applying this technique are: (i) at what operating temperature and pressure can one assume that the gasification process occurs at chemical equilibrium, (ii) what percentage of the feed coal is gasified in the reactor, (iii) what operating temperature should be used for the equilibrium calculations?
Table 1. Chemical reactions and associated reaction enthalpies in a coal gasification process (0. I MPa, 298 K) Reaction 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.
Moist coal Dry coal C (s) + 02 (g) C (s) + CO2(g) CO (g) + H20 (g) CO (g) + 3 H2(g) CH 4(g) + 2 02 (g) H2S (g) + 1.5 02 (g) SO2(g) + 0.5 02 (g) SO3(g) + H a (g)
(2)
i-1
Dry coal + H20 Char + Vol. + Tar CO2(g) 2 CO (g) CO2(g) + H2(g) CH4(g) + H20 (g) CO2(g) + 2 H20 (g) SO2(g) + H20 (g) SO3(g) SO2(g) + H 2 0 (g)
AH (kJ mol- l) 44.05 (endothermic) - 393.78 172.58 - 41.19 - 206.29 - 802.97 - 519.04 - 98.18 -- 143.82
EQUILIBRIUM CALCULATIONS IN COAL GASIFICATION Chemical equilibrium calculations can be performed for a gasifier if the coal conversion and the reactor pressure, P, are known, and an average reactor temperature, T, can be defined. An average reactor temperature is diËficult to define, due to temperature gradients that exist within the reactor. For practical purposes a value for the reactor temperature can be assigned. It is known that entrained flow gasifiers generally operate at temperatures above the ash melting temperature and fluidized bed gasifiers generally operate at temperatures below the ash melting temperature [3, 4]. The operating temperature for a particular gasifier using a particular coal can be estimated from the ash fusion analysis of the coal. Chemical equilibrium composition can be calculated by minimization of Gibbs free energy, if the thermodynamic state of the system is given by the temperature and pressure. For a system composed o f j ( j = 1, 2 . . . . s) chemical species, among which i(i = 1, 2, . . , r) chemical reactions can occur, equilibrium composition can be calculated by minimization of Gibbs free energy g
= L l(~n,
(3)
/=l
where l~i is the molar Gibbs free energy,
and nj is the number of moles of the j t h species. In minimizing Gibbs free energy, g, atomic balances must also be satisfied. If k (k = 1, 2 . . . . l) are the atoms that exist in the reacting system with atomic stoichiometric coeffÉcients akj (the number of kth atoms in one mole of species./), then the atomic balances can be expressed as L akjnj
b ~'=0
--
k
( k = l , 2 , . . , 1).
(5)
from equation (5)). Equilibrium composition can be calculated if the thermodynamic data of the chemical species are known [5, 6]. In the present study equilibrium calculations were performed using the Facility for the Analysis of Chemical Thermodynamics (F*A*C*T)t developed at the University of Montreal. Product gas compositions calculated from chemical equilibrium were mainly dominated by nine molecular species, CO, H 2, CO2, H20, CH4, H2S, COS, NH 3 and N 2. RESULTS A N D DISCUSSION Equilibrium calculations of gasification processes can be performed for known coal and gas feed rates, their respective elemental compositions, and for known gasifier pressure and temperature. In practice, however, large concentration and temperature gradients may exist in the gasifier, which are factors effecting gasifier performance and design. Equilibrium calculations can be performed assuming the gasification process is adiabatic, isothermal or between the two extremes. Data including feed and product compositions, were available for a number of different gasification processes. These data, generally, did not include gasifier temperatures and energy balance information. Equilibrium calculations (a)
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Temperature (K) Fig. 1. Equilibrium predictions of product gas composition for the gasification of Pittsburgh No. 8 coal using TEXACO process. Best fit temperature is 1288 K.
128
G. KOVACIK et al.
Table 2. TEXACO Coal gasification process data and equilibrium predictions (experimental data from TEXACO coal gasification process, company brochure, TEXACO Development Corporation, 2000 Westchester, White Plains, NY 10650 U.S.A.) Cases studied
1
2
3
4
5
Coal Feed type Dry analysis (wt%) C H N S O Ash Product gas composition (Dry, mol%) CO H2 CO 2 H20 CH 4 N2
H2S + COS Best fit temp. (K) Predicted gas composition (Dry, mol%) CO H2 CO 2 H20 CH 4 N2 H2S+COS
Pittsburgh No. 8
French
Utah
6
7
Petroleum coke W. German
S. African Delayed
Fluid
8
9
Coal liquef, residue Molten
Slurry
74.16 5.15 1.18 3.27 6.70 9.54
78.08 5.26 0.85 0.47 8.23 7.11
68.21 4.78 1.22 0.37 15.69 9.73
73.93 4.65 1.50 1.08 5.85 13.01
65.60 3.51 1.53 0.87 7.79 20.70
88.50 3.90 1.50 5.50 0.I0 0.50
85.98 2.00 0.98 8.31 2.27 0.46
68.39 4.75 0.98 1.87 2.21 21.80
68.39 4.75 0.98 1.87 2.21 21.80
39.95 30.78 11.43 16.43 0.04 0.49 0.88
37.36 29.26 13.30 19.43 0.16 0.37 0.12
30.88 26.71 15.91 25.67 0.22 0.50 0.11
39.46 29.33 12.59 17.47 0.25 0.60 0.30
36.53 26.01 15.67 20.82 0.02 0.68 0.27
46.20 28.69 10.68 12.37 0.17 0.55 1.34
47.14 24.33 13.16 12.67 0.09 0.42 2.19
46.31 35.54 6.41 10.46 0.27 0.45 0.56
33.48 28.56 13.09 23.72 0.23 0.42 0.50
1288.2
1290.5
1235.7
1260.6
1337.0
1317.7
1346.5
1286.1
40.95 29.50 11.58 15.54 1.09 0.37 0.88
37.82 28.63 13.45 19.03 0.72 0.24 0.11
31.54 27.05 16.61 23.25 1.07 0.38 0.10
37.37 25.79 15.99 19.19 0.84 0.54 0.27
46.39 27.54 10.65 13.12 0.55 0.42 1.34
47.16 23.31 13.39 13.17 0.46 0.30 2.22
47.61 34.35 6.29 9.61 1.22 0.34 0.56
34.09 29.31 13.57 21.57 0.67 0.30 0.49
were therefore performed assuming isothermal gasifier operation. In this study, calculated product gas compositions using chemical equilibrium principles were compared with the experimental data available for the Texaco, P R E N F L O and K R W gasification processes. Equilibrium calculations were performed using the reported coal conversion data to adjust elemental carbon input to the gasifier, while H, O, N and S content of the coal were assumed to react to 100% conversion. This adjustment of carbon feed data is reasonable, since unconverted char from gasification processes would be mainly composed o f carbon and ash. Product gas compositions (CO, H 2, CO2, H 2 0 ) derived from the equilibrium calculations were compared with those obtained experimentally. The best fit temperature which predicted product gas composition using chemical equilibrium was obtained by minimizing the combined least squares errors between calculated and measured CO, H : , CO2 and H 2 0 compositions. The sulfur species content of the product gas (H 2 S and COS), calculated at this temperature, was in good agreement
*Texaco Coal Gasification Process, Company Brochure, Texaco Development Corporation, 2000 Westchester Avenue, White Plains, NY 10650, U.S.A.
1279.3 40.15 28.62 12.80 16.59 1.07 0.47 0.30
with the experimental data. Texaco gasification product gas data were reported on a wet basis, and best fit temperatures were obtained directly by comparison with calculated equilibrium compositions. Both P R E N F L O and K R W product gas data, however, were reported on a dry basis and calculated equilibrium concentrations were renormalized with H : O removed before comparisons were made on a dry basis. The absence of H 2 0 data and sparsely documented operating conditions in some cases, introduced additional degrees of freedom in obtaining the best fit temperatures. In these cases, the errors between calculated equilibrium and measured product gas compositions were much larger. The Texaco Coal Gasification Process* utilizes an entrained flow reactor operating at 4 M P a pressure and over a temperature range of 1300 to 2300 K. Product gas compositions were calculated at 4 M P a pressure and varying gasification temperatures for 9 different feedstocks, including coals, petroleum cokes and coal liquefaction residues. F o r each feedstock the temperature that provides the best fit to the experimental data was calculated as described above. Figure l(a) illustrates a typical result for the gasification of Pittsburgh No. 8 coal. The temperature at which the calculated and experimental data were in the closest agreement was found to be 1288 K. The calculated values of the sulfur species
EQUILIBRIUM CALCULATIONS IN COAL GASIFICATION Table 3. PRENFLO Coal gasification process data and equilibrium predictions (experimental data from PRENFLO pressurized entrained-flow gasification, company brochure. Krupp Koopers GmbH, Altendolfer Strasse 120, 4300 Essen 1, Federal Republic of Germany) Cases studied
1
2
Gottelborn coal Dry analysis (wt%) C H N S O Ash
75.7 4.9 1.4 0.6 10.1 7.3
76.2 4.7 1.2 1.0 8.0 8.8
Product gas composition (Dry, mol%) CO H, CO 2 CH 4 N, H2S + COS
65.30 31.60 2.00 0.00 0.90 0.20
65.90 26.53 1.97 0.00 5.34 0.26
Best fit temp. (K) Predicted gas composition (Dry, mol%) CO H, C() 2 Clt 4 N, H~S 4- COS
1600.0 65.88 29.79 3.53 0.04 0.55 0.20
1500.0 68.60 23.30 2.37 0.10 5.27 0.35
oxygen and a steam-oxygen mixture. The results are summarized in Table 3. In the first case, feed rates were well documented, and simulations were readily performed. In the second case, however, feed parameters were not fully documented, and iterative calculations were performed to estimate the oxygen feed and nitrogen purge rates. In estimating the product composition, the nitrogen balance was first satisfied, since it is an inert element. The predicted and experimentally obtained product gas compositions are presented in Figs 2 and 3. Equilibrium temperatures for case one and two were estimated at 1600 and 1500 K, respectively, from the best fit of the equilibrium calculations to the reported data, on a dry basis. Again, the sulfur species concentrations were calculated using the best fit temperature. For case one, the calculated and reported sulfur species concentrations were in good agreement. The error between predicted and reported values of (H 2S and COS) in case two may be due to the uncertainties in feed conditions. These equilibrium temperatures are comparable to the stated product gas temperatures between 1620 and 1870K. Equilibrium composition was relatively insensitive to temperature above 1300 K (Figs 2 and 3). The gasification of Highvale coal (Alberta, Canada) using the K R W fluidized bed process was analysed for the two setpoints listed in Table 4 [7]. Coal, along with (a)
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in the product gas using this best fit temperature were also nearly identical to the experimental data, as seen in Fig. l(b). This procedure was performed for the 9 different feedstocks and the results are summarized in Table 2. The best fit temperatures varied from 1236 to 1347 K. It is interesting to note that lower temperatures were obtained for lower rank coals, i.e. 1237 K for the U t a h coal, while higher temperatures were calculated for cokes, i.e. 1337 K for the delayed petroleum coke. These differences may be explained by the differences in gasification reactivities of the feedstocks. This result was expected since bituminous coal is known to be more reactive than petroleum coke, therefore, allowing operation of the gasifier at lower temperatures while maintaining a high carbon conversion. In the P R E N F L O t gasification process, coal is gasified with oxygen or steam/oxygen mixtures at a temperature above the coal ash melting temperature and at a pressure of approximately 3 MPa in an entrained flow reactor, Coal ash is removed from the reactor as a molten slag. The raw product gas leaves the gasifier at a temperature between 1620 and 1870 K. Two cases were analysed for the gasification of pulverized Gottenborn coal using
129
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Temperature tPRENFLO Pressurized Entrained-flow Gasification, Company Brochure, Krupp Koopers GmbH, Altendolfer Strasse 120, 4300 Essen 1, F.R.G.
...... t800
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1900
(K)
Fig. 2. Equilibrium predictions of product gas composition for the gasification of Gottenborn coal using PRENFLO process, case I. Best fit temperature is 1600 K.
130
G. KOVACIK et al.
limestone, was pneumatically transported by air into the gasifier operating at a pressure of 1.5 MPa. The coal was gasified by air and steam with average bed temperatures of 1174 and 1194 K for setpoints 1 and 2, respectively. Limestone and ash were continuously withdrawn from the bed to maintain a constant bed height. A portion of the product gas was recycled to the bed for temperature control and to maintain fluidization conditions. Product gas compositions were calculated using F*A*C*T, with the molar feeds based on 100 moles of feed coal as listed in Table 4. The carbon in the feed coal was adjusted by the conversion factor (% carbon conversion), to account for the unreacted carbon. Wet equilibrium compositions were calculated and adjusted for a dry basis. Dry nitrogen concentrations of about 64.5% for setpoint 1 and 63.5% for setpoint 2 were calculated using the equilibrium model. Measured nitrogen values of 62.5% were reported on a dry basis in Table 4. Discrepancies between measured and calculated values may be due to an overestimated equilibrium moisture content before adjusting the product gas composition to a dry basis, or due to experimental errors. Figure 4(a) and (b) shows the dry equilibrium product gas compositions for CO, H 2 and CO2 between 1100 and 1300 K, with best fit temperatures at 1210 and 1230K for setpoints 1 and 2, respectively. These best fit temperatures both agree well
Table 4. KRW Coal gasification process data and equilibrium predictions (Experimental data from TransAlta, 1988). Highvale coal Analysis (wt%) C H N O S Ash Moisture Average bed temp. (K) Product gas composition (Dry, mol%) CO H2 CO2 CH 4
N2 H2S + COS
Best fit temp. (K) Predicted gas composition (Dry, mol%) CO Ha CO2 CH 4
(a)
N2 H2S + COS i
0.8
i
i
50.42 3.34 1.22 11.18 0.17 11.91 21.76
50.42 3.34 1.22 11.18 0.17 11.91 21.76
Setpoint 1 1174
Setpoint 2 1194
9.5 13.1 13.5 1.3 62.5 0.05
10.8 12.7 12.9 1.1 62.5 0.06
1210.2 9.25 13.90 12.40 0.004 64.37 0.033
1230 10.66 13.59 11.90 0.004 63.81 0.31
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t700
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1800
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Fig. 3. Equilibrium predictions of product gas composition for the gasification of Gottenborn coal using PRENFLO process, case 2. Best fit temperature is 1500K.
with the corresponding measured average bed temperatures of 1174 and 1194 K, respectively. In both cases, the best fit equilibrium temperature was about 35 K above the experimentally measured value. Product gas composition calculations have been reported elsewhere for a bench scale spouted bed coal gasifier [4]. In these calculations the gasification system was assumed to be composed of H2, CO, CO2, CH4, H2 O, and N2 molecular species. Equilibrium calculations were performed using published chemical equilibrium constants for the water-gas shift and methanation reactions (5th and 6~ reactions in Table 1) and the elemental balances of H, O, C and N. Watkinson et al. [4] observed that, for an unreactive coal, experimentally measured product gas compositions were predicted accurately using chemical equlibrium calculations when reactor temperatures were above 1300K. Below this temperature large differences between experimental and calculated product gas compositions were observed. For a more reactive coal, however, Watkinson et al. [4] found good agreement between observed and calculated values at temperatures as low as 1050 K. This trend was also observed here when F*A*C*T was applied to the data presented by Watkinson et al. [4]. CONCLUSIONS In this study a commercially available chemical equilibrium software package (F*A*C*T) was used to
EQUILIBRIUM CALCULATIONS IN COAL GASIFICATION
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fluidized bed (KRW). F r o m this work, it was demonstrated that gasifier product gas compositions can be calculated using the F * A * C * T chemical equilibrium software for a wide range of coals and gasifiers. The coal compositions, extent of conversion and reactor operating conditions are required as input information for the calculations. For gasifier temperatures above 1300 K the calculated product gas composition, assuming chemical equilibrium, was found to be insensitive to reactor temperature. In many cases, therefore, large scale gasification experiments would not be required to determine product gas compositions.
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Acknowledgements--This work was supported by the Alberta/ Canada Energy Resources Research Fund administered by the Alberta Office of Coal Research and Technology, Alberta Energy and by the Alberta Research Council.
1-12
REFERENCES
0.00 itO0
~200
Temperature
t300
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Fig. 4. Equilibrium predictions of product gas composition for the gasification of Highvale coal using KRW process, setpoint 1, and setpoint 2. Best fit temperatures are 1210 and 1230 K.
calculate product gas compositions for a wide range of feedstocks in various gasification technologies. The results of the equilibrium calculations were compared with published data for the gasification of various feedstocks, ranging from sub-bituminous coal to petroleum coke, in various gasification reactors, including dry feed, oxygen blown entrained flow ( P R E N F L O ) , slurry feed, oxygen blown entrained flow ( T E X A C O ) and air blown
1. R. Aris, Elementary Chemical Reactor Analysis, pp. 8 28. Prentice Hall, Englewood Cliffs, N.J., U.S.A. (1969). 2. H. Hofmann, Kinetic data analysis and parameter estimation in M. I. deLasa (ed.), Chemical Reactor Design and Technology, pp. 69-105, NATO ASI No. 110 (1986). 3. L. K. Mudge, G. F. Schiefelbein C. T Li and R. H. Moore, The Gasification of Coal, a Battelle Energy Program Report, Battelle Pacific Northwest Laboralories, Richland. Washington, U.S.A. (1974). 4. A. P. Watkinson, G. Cheng and C. J. Lira, Oxygen- steam gasification of coals in a spouted bed, Can. J. Chem:. Engng 65, 791 798 (1987). 5. F. J. Zeleznik and S. Gordon, Calculation of complex chemical equilibria, Ind. Engng ('hem. 60, 27 57 (1968). 6. S. Gordon and B. J. McBride, Computer Program for Calculation of Chemical Equilibrium Compositions, Rocket Performance, Incident and Reflected Shocks, and Chapman Jouguet Detonations, NASA, SP-273, National Aeronautics and Space Administration, Washington, D.C. (1976). 7. TransAlta Utilities Corporation, Fluidized Bed Gasification of Highvtde Coal, DSS File No. 06SQ.23440-7-9134 (1988).
0360-3199/90$3.00+ 0.00 Pergamon Press plc. '.~5~1990ImernationalAssociationfor Hydrogen Energy.
EQUILIBRIUM CALCULATIONS IN COAL GASIFICATION G. KOVACIK, M. O~UZT6RELI*, A. CHAMBERSand B. OZ0M Alberta Research Council, Devon, Alberta T0C 1E0, Canada (Received for publication 2 August 1989) Abstract--Coal gasification is commercially important due to its application m ammonia, hydrogen, methanol and synthetic fuel production and in integrated gasification combined cycle (1GCC) power plants. Accurate process evaluations are required for the development of new generation gasification technologies. Perhaps the most crucial component of this evaluation is the accurate prediction of product gas quality for varying feed streams and operating conditions. Operating conditions such as reactor hydrodynamics, temperature and pressure are variables which differentiate the gasifiers that are being offered for commercial applications (fixed bed, fluid bed, entrained flow, dry or slurry feed, air or oxygen blown). The present work demonstrates that for reactive coal feedstocks and for gasifers operating at relatively high temperature and pressure (entrained flow and fluidized beds), product gas compositions can be estimated, regardless of gasifier type, using chemical equilibrium principles.
NOMENCLATURE Aj ak,j b~ g n P R T
n u m b e r of moles o f j t h specie stoichiometric coefficient of k th atom i n j t h specie total n u m b e r of kth atom Gibbs free energy per mole of mixture, J mol n u m b e r of moles pressure, N m 2 universal gas constant, 8.3143 J m o 1 - 1 K -1 temperature, K
Greek letters ~,.~ stoichiometric coefficient of j t h species in ith reaction 2, Lagrangian multipliers q~ multipliers # chemical potential, J mol- 1 variational operator Subscripts i indice for chemical reactions j indice for chemical species k indice for atoms INTRODUCTION Coal gasification involves the simultaneous processes of thermal pyrolysis, homogeneous gas phase and heterogeneous gas-solid reactions between the feed coal and the feed and product gases. The product gas is typically composed of H 2, CO, CH4, N 2 and trace amounts of hydrocarbons, and its composition is a function of gasification reactor operating temperature and pressure,
*Petro Simulator Systems Inc., Edmonton, Alberta, Canada.
as well as feed coal and feed gas compositions. If air is used as the main reactant to gasify coal, the product gas is described as low BTU gas because it contains N 2 as a major component. I f O 2 is used to gasify coal the product gas is termed intermediate BTU gas (synthesis gas) which contains only traces of N 2. Intermediate BTU gas can be used either as an energy source or as a synthesis gas for the production of chemical and synthetic liquid and gaseous fuels. Synthetic natural gas contains over 90 percent by volume CH4, which is produced by further processing of intermediate BTU gas. Coal gasification processes are being utilized to produce synthetic natural gas and fuel, ammonia, feedstock for the chemical industry and electricity through IGCC power generation. Extensive research and development of gasification technology is proceeding in the United States, Europe and Japan. The advantages of IGCC for power have utility companies interested in the recent developments of the gasification technology. Several gasification technologies are in the demonstration phase. These technologies utilize fixed bed, moving bed, fluidized bed or entrained bed reactors operating at a variety of pressures, temperatures and residence times. Each reactor has its own advantages, disadvantages and economics for the gasification of a particular coal. Prior to selection of the optimum technology for a particular coal some engineering evaluations must be performed. One of the major criteria needed is an accurate estimate of the gasifier product gas composition. This is necessary because the gas composition would effect the required downstream treatment of the product gas and its ultimate use. Product gas composition is one of the major variables affecting the evaluation and selection of any coal gasification process. Product gas composition depends on coal type, feed gas composition, feed coal to feed gas 125
126
G. KOVACIK et al.
ratio, and the type of gasification reactor being used. Variables which affect the reactor performance include the operating temperature and pressure, feed gas composition and temperature, and the reactor hydrodynamics. Reactor temperature and pressure are important for the thermodynamics (chemical equilibrium) and the dynamics (reaction kinetics) of the process while the reactor hydrodynamics are important for the practical application to ensure proper mixing, gas-solid contact, recycle ratio, gas and solid residence times etc. Research on the effect of these factors has resulted in changes in gasification technology with the development of several new, commercially applicable reactors in the last one or two decades. This study demonstrates that product gas composition can be predicted under certain conditions, especially under the operating conditions of new generation gasifiers, provided that chemical kinetics is not a controlling factor, and the operating temperature for the gasifier can be defined. F O R M U L A T I O N OF THE PROBLEM Coal gasification can be considered as rapid thermal pyrolysis of coal coupled with parallel and consecutive homogeneous gas phase and heterogeneous gas-solid reactions. Thousands of chemical reactions may occur among thousands of chemical species in coal gasification. However, from the reactor engineering point of view, char, CO2, CO, 02, H20, CH4, H2, H2S and SO 2 are the main species which can be identified by conventional analytical techniques. Many chemical reactions may occur among these species, from which a set of independent reactions must be chosen for reactor engineering analysis. If s and r denote the number of species and number of reactions involved in the gasification process, respectively, the following relation can be written for the stoichiometry of r reactions ~ ~i.jAj = 0
( i = 1,2 . . . . r)
(1)
j=l
where ~ti.jis the stoichiometric coefficient of j-th specie in i-th reaction and Aj is thej-th specie. If r reactions are
independent, only the trivial set of multiples t/i = 0; i = 1, 2 . . , r can satisfy the following relation [1]: r/,~i,s = 0
( j = 1, 2 . . , s).
In practice, a stoichiometrically independent reaction set can be found from the Gauss elimination of the matrix ~i.j of equation (1). Reactions corresponding to non-zero diagonal elements form the set of stoichiometrically independent chemical reactions. In the present study, the reactions listed in Table 1 were found to be independent. Prediction of product gas composition is almost impossible using available information about the kinetics of coal gasification reactions. This is due to the fact that a prediction which is based on the kinetics of the process requires the accurate knowledge of a large number of dynamic parameters. Measurements to evaluate these parameters are always influenced by the hydrodynamics of the reacting system [2]. Thus, product gas composition estimation using kinetic models becomes exceedingly difficult. In many practical applications the reactions in Table 1 occur under conditions where chemical kinetics are not the controlling mechanism. This leads to another approach to predict product gas composition using the principles of chemical equilibrium. If the gasification reactions occur fast enough, it may be assumed that the reaction products are in chemical equilibrium. Realistically, a coal gasification system should reach chemical equilibrium if the operating temperature and pressure of the system are high, i.e. the gasification reaction rates are high. In such cases, product gas composition could be calculated using chemical equilibrium principles for multicomponent, multireaction systems, provided that the feed compositions and the reactor operating conditions are known. The problems in applying this technique are: (i) at what operating temperature and pressure can one assume that the gasification process occurs at chemical equilibrium, (ii) what percentage of the feed coal is gasified in the reactor, (iii) what operating temperature should be used for the equilibrium calculations?
Table 1. Chemical reactions and associated reaction enthalpies in a coal gasification process (0. I MPa, 298 K) Reaction 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.
Moist coal Dry coal C (s) + 02 (g) C (s) + CO2(g) CO (g) + H20 (g) CO (g) + 3 H2(g) CH 4(g) + 2 02 (g) H2S (g) + 1.5 02 (g) SO2(g) + 0.5 02 (g) SO3(g) + H a (g)
(2)
i-1
Dry coal + H20 Char + Vol. + Tar CO2(g) 2 CO (g) CO2(g) + H2(g) CH4(g) + H20 (g) CO2(g) + 2 H20 (g) SO2(g) + H20 (g) SO3(g) SO2(g) + H 2 0 (g)
AH (kJ mol- l) 44.05 (endothermic) - 393.78 172.58 - 41.19 - 206.29 - 802.97 - 519.04 - 98.18 -- 143.82
EQUILIBRIUM CALCULATIONS IN COAL GASIFICATION Chemical equilibrium calculations can be performed for a gasifier if the coal conversion and the reactor pressure, P, are known, and an average reactor temperature, T, can be defined. An average reactor temperature is diËficult to define, due to temperature gradients that exist within the reactor. For practical purposes a value for the reactor temperature can be assigned. It is known that entrained flow gasifiers generally operate at temperatures above the ash melting temperature and fluidized bed gasifiers generally operate at temperatures below the ash melting temperature [3, 4]. The operating temperature for a particular gasifier using a particular coal can be estimated from the ash fusion analysis of the coal. Chemical equilibrium composition can be calculated by minimization of Gibbs free energy, if the thermodynamic state of the system is given by the temperature and pressure. For a system composed o f j ( j = 1, 2 . . . . s) chemical species, among which i(i = 1, 2, . . , r) chemical reactions can occur, equilibrium composition can be calculated by minimization of Gibbs free energy g
= L l(~n,
(3)
/=l
where l~i is the molar Gibbs free energy,
and nj is the number of moles of the j t h species. In minimizing Gibbs free energy, g, atomic balances must also be satisfied. If k (k = 1, 2 . . . . l) are the atoms that exist in the reacting system with atomic stoichiometric coeffÉcients akj (the number of kth atoms in one mole of species./), then the atomic balances can be expressed as L akjnj
b ~'=0
--
k
( k = l , 2 , . . , 1).
(5)
from equation (5)). Equilibrium composition can be calculated if the thermodynamic data of the chemical species are known [5, 6]. In the present study equilibrium calculations were performed using the Facility for the Analysis of Chemical Thermodynamics (F*A*C*T)t developed at the University of Montreal. Product gas compositions calculated from chemical equilibrium were mainly dominated by nine molecular species, CO, H 2, CO2, H20, CH4, H2S, COS, NH 3 and N 2. RESULTS A N D DISCUSSION Equilibrium calculations of gasification processes can be performed for known coal and gas feed rates, their respective elemental compositions, and for known gasifier pressure and temperature. In practice, however, large concentration and temperature gradients may exist in the gasifier, which are factors effecting gasifier performance and design. Equilibrium calculations can be performed assuming the gasification process is adiabatic, isothermal or between the two extremes. Data including feed and product compositions, were available for a number of different gasification processes. These data, generally, did not include gasifier temperatures and energy balance information. Equilibrium calculations (a)
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U
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Temperature (K) Fig. 1. Equilibrium predictions of product gas composition for the gasification of Pittsburgh No. 8 coal using TEXACO process. Best fit temperature is 1288 K.
128
G. KOVACIK et al.
Table 2. TEXACO Coal gasification process data and equilibrium predictions (experimental data from TEXACO coal gasification process, company brochure, TEXACO Development Corporation, 2000 Westchester, White Plains, NY 10650 U.S.A.) Cases studied
1
2
3
4
5
Coal Feed type Dry analysis (wt%) C H N S O Ash Product gas composition (Dry, mol%) CO H2 CO 2 H20 CH 4 N2
H2S + COS Best fit temp. (K) Predicted gas composition (Dry, mol%) CO H2 CO 2 H20 CH 4 N2 H2S+COS
Pittsburgh No. 8
French
Utah
6
7
Petroleum coke W. German
S. African Delayed
Fluid
8
9
Coal liquef, residue Molten
Slurry
74.16 5.15 1.18 3.27 6.70 9.54
78.08 5.26 0.85 0.47 8.23 7.11
68.21 4.78 1.22 0.37 15.69 9.73
73.93 4.65 1.50 1.08 5.85 13.01
65.60 3.51 1.53 0.87 7.79 20.70
88.50 3.90 1.50 5.50 0.I0 0.50
85.98 2.00 0.98 8.31 2.27 0.46
68.39 4.75 0.98 1.87 2.21 21.80
68.39 4.75 0.98 1.87 2.21 21.80
39.95 30.78 11.43 16.43 0.04 0.49 0.88
37.36 29.26 13.30 19.43 0.16 0.37 0.12
30.88 26.71 15.91 25.67 0.22 0.50 0.11
39.46 29.33 12.59 17.47 0.25 0.60 0.30
36.53 26.01 15.67 20.82 0.02 0.68 0.27
46.20 28.69 10.68 12.37 0.17 0.55 1.34
47.14 24.33 13.16 12.67 0.09 0.42 2.19
46.31 35.54 6.41 10.46 0.27 0.45 0.56
33.48 28.56 13.09 23.72 0.23 0.42 0.50
1288.2
1290.5
1235.7
1260.6
1337.0
1317.7
1346.5
1286.1
40.95 29.50 11.58 15.54 1.09 0.37 0.88
37.82 28.63 13.45 19.03 0.72 0.24 0.11
31.54 27.05 16.61 23.25 1.07 0.38 0.10
37.37 25.79 15.99 19.19 0.84 0.54 0.27
46.39 27.54 10.65 13.12 0.55 0.42 1.34
47.16 23.31 13.39 13.17 0.46 0.30 2.22
47.61 34.35 6.29 9.61 1.22 0.34 0.56
34.09 29.31 13.57 21.57 0.67 0.30 0.49
were therefore performed assuming isothermal gasifier operation. In this study, calculated product gas compositions using chemical equilibrium principles were compared with the experimental data available for the Texaco, P R E N F L O and K R W gasification processes. Equilibrium calculations were performed using the reported coal conversion data to adjust elemental carbon input to the gasifier, while H, O, N and S content of the coal were assumed to react to 100% conversion. This adjustment of carbon feed data is reasonable, since unconverted char from gasification processes would be mainly composed o f carbon and ash. Product gas compositions (CO, H 2, CO2, H 2 0 ) derived from the equilibrium calculations were compared with those obtained experimentally. The best fit temperature which predicted product gas composition using chemical equilibrium was obtained by minimizing the combined least squares errors between calculated and measured CO, H : , CO2 and H 2 0 compositions. The sulfur species content of the product gas (H 2 S and COS), calculated at this temperature, was in good agreement
*Texaco Coal Gasification Process, Company Brochure, Texaco Development Corporation, 2000 Westchester Avenue, White Plains, NY 10650, U.S.A.
1279.3 40.15 28.62 12.80 16.59 1.07 0.47 0.30
with the experimental data. Texaco gasification product gas data were reported on a wet basis, and best fit temperatures were obtained directly by comparison with calculated equilibrium compositions. Both P R E N F L O and K R W product gas data, however, were reported on a dry basis and calculated equilibrium concentrations were renormalized with H : O removed before comparisons were made on a dry basis. The absence of H 2 0 data and sparsely documented operating conditions in some cases, introduced additional degrees of freedom in obtaining the best fit temperatures. In these cases, the errors between calculated equilibrium and measured product gas compositions were much larger. The Texaco Coal Gasification Process* utilizes an entrained flow reactor operating at 4 M P a pressure and over a temperature range of 1300 to 2300 K. Product gas compositions were calculated at 4 M P a pressure and varying gasification temperatures for 9 different feedstocks, including coals, petroleum cokes and coal liquefaction residues. F o r each feedstock the temperature that provides the best fit to the experimental data was calculated as described above. Figure l(a) illustrates a typical result for the gasification of Pittsburgh No. 8 coal. The temperature at which the calculated and experimental data were in the closest agreement was found to be 1288 K. The calculated values of the sulfur species
EQUILIBRIUM CALCULATIONS IN COAL GASIFICATION Table 3. PRENFLO Coal gasification process data and equilibrium predictions (experimental data from PRENFLO pressurized entrained-flow gasification, company brochure. Krupp Koopers GmbH, Altendolfer Strasse 120, 4300 Essen 1, Federal Republic of Germany) Cases studied
1
2
Gottelborn coal Dry analysis (wt%) C H N S O Ash
75.7 4.9 1.4 0.6 10.1 7.3
76.2 4.7 1.2 1.0 8.0 8.8
Product gas composition (Dry, mol%) CO H, CO 2 CH 4 N, H2S + COS
65.30 31.60 2.00 0.00 0.90 0.20
65.90 26.53 1.97 0.00 5.34 0.26
Best fit temp. (K) Predicted gas composition (Dry, mol%) CO H, C() 2 Clt 4 N, H~S 4- COS
1600.0 65.88 29.79 3.53 0.04 0.55 0.20
1500.0 68.60 23.30 2.37 0.10 5.27 0.35
oxygen and a steam-oxygen mixture. The results are summarized in Table 3. In the first case, feed rates were well documented, and simulations were readily performed. In the second case, however, feed parameters were not fully documented, and iterative calculations were performed to estimate the oxygen feed and nitrogen purge rates. In estimating the product composition, the nitrogen balance was first satisfied, since it is an inert element. The predicted and experimentally obtained product gas compositions are presented in Figs 2 and 3. Equilibrium temperatures for case one and two were estimated at 1600 and 1500 K, respectively, from the best fit of the equilibrium calculations to the reported data, on a dry basis. Again, the sulfur species concentrations were calculated using the best fit temperature. For case one, the calculated and reported sulfur species concentrations were in good agreement. The error between predicted and reported values of (H 2S and COS) in case two may be due to the uncertainties in feed conditions. These equilibrium temperatures are comparable to the stated product gas temperatures between 1620 and 1870K. Equilibrium composition was relatively insensitive to temperature above 1300 K (Figs 2 and 3). The gasification of Highvale coal (Alberta, Canada) using the K R W fluidized bed process was analysed for the two setpoints listed in Table 4 [7]. Coal, along with (a)
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i
co
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in the product gas using this best fit temperature were also nearly identical to the experimental data, as seen in Fig. l(b). This procedure was performed for the 9 different feedstocks and the results are summarized in Table 2. The best fit temperatures varied from 1236 to 1347 K. It is interesting to note that lower temperatures were obtained for lower rank coals, i.e. 1237 K for the U t a h coal, while higher temperatures were calculated for cokes, i.e. 1337 K for the delayed petroleum coke. These differences may be explained by the differences in gasification reactivities of the feedstocks. This result was expected since bituminous coal is known to be more reactive than petroleum coke, therefore, allowing operation of the gasifier at lower temperatures while maintaining a high carbon conversion. In the P R E N F L O t gasification process, coal is gasified with oxygen or steam/oxygen mixtures at a temperature above the coal ash melting temperature and at a pressure of approximately 3 MPa in an entrained flow reactor, Coal ash is removed from the reactor as a molten slag. The raw product gas leaves the gasifier at a temperature between 1620 and 1870 K. Two cases were analysed for the gasification of pulverized Gottenborn coal using
129
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1 I
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0.0 I ....... 1400
t ....... 1500
f ....... 1800
rc-°zt 1700
Temperature (b)
0.0025
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t900
(K)
l
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~- 0,002o- ~ . . . . . . . . . . . . . . . . . . . . . . . . . .o ~o 0 . 0 0 1 5 ' f..
u. t. f0
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Temperature tPRENFLO Pressurized Entrained-flow Gasification, Company Brochure, Krupp Koopers GmbH, Altendolfer Strasse 120, 4300 Essen 1, F.R.G.
...... t800
I
t800
1900
(K)
Fig. 2. Equilibrium predictions of product gas composition for the gasification of Gottenborn coal using PRENFLO process, case I. Best fit temperature is 1600 K.
130
G. KOVACIK et al.
limestone, was pneumatically transported by air into the gasifier operating at a pressure of 1.5 MPa. The coal was gasified by air and steam with average bed temperatures of 1174 and 1194 K for setpoints 1 and 2, respectively. Limestone and ash were continuously withdrawn from the bed to maintain a constant bed height. A portion of the product gas was recycled to the bed for temperature control and to maintain fluidization conditions. Product gas compositions were calculated using F*A*C*T, with the molar feeds based on 100 moles of feed coal as listed in Table 4. The carbon in the feed coal was adjusted by the conversion factor (% carbon conversion), to account for the unreacted carbon. Wet equilibrium compositions were calculated and adjusted for a dry basis. Dry nitrogen concentrations of about 64.5% for setpoint 1 and 63.5% for setpoint 2 were calculated using the equilibrium model. Measured nitrogen values of 62.5% were reported on a dry basis in Table 4. Discrepancies between measured and calculated values may be due to an overestimated equilibrium moisture content before adjusting the product gas composition to a dry basis, or due to experimental errors. Figure 4(a) and (b) shows the dry equilibrium product gas compositions for CO, H 2 and CO2 between 1100 and 1300 K, with best fit temperatures at 1210 and 1230K for setpoints 1 and 2, respectively. These best fit temperatures both agree well
Table 4. KRW Coal gasification process data and equilibrium predictions (Experimental data from TransAlta, 1988). Highvale coal Analysis (wt%) C H N O S Ash Moisture Average bed temp. (K) Product gas composition (Dry, mol%) CO H2 CO2 CH 4
N2 H2S + COS
Best fit temp. (K) Predicted gas composition (Dry, mol%) CO Ha CO2 CH 4
(a)
N2 H2S + COS i
0.8
i
i
50.42 3.34 1.22 11.18 0.17 11.91 21.76
50.42 3.34 1.22 11.18 0.17 11.91 21.76
Setpoint 1 1174
Setpoint 2 1194
9.5 13.1 13.5 1.3 62.5 0.05
10.8 12.7 12.9 1.1 62.5 0.06
1210.2 9.25 13.90 12.40 0.004 64.37 0.033
1230 10.66 13.59 11.90 0.004 63.81 0.31
i
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Temperature (b) o . o o s
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,
1
1 t600
t700
t800
(K)
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0.002"
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I
t300
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A. . . . . . . . . . . . . . . . .
t500
I
1600
Temperature
I
t700
1800
(K)
Fig. 3. Equilibrium predictions of product gas composition for the gasification of Gottenborn coal using PRENFLO process, case 2. Best fit temperature is 1500K.
with the corresponding measured average bed temperatures of 1174 and 1194 K, respectively. In both cases, the best fit equilibrium temperature was about 35 K above the experimentally measured value. Product gas composition calculations have been reported elsewhere for a bench scale spouted bed coal gasifier [4]. In these calculations the gasification system was assumed to be composed of H2, CO, CO2, CH4, H2 O, and N2 molecular species. Equilibrium calculations were performed using published chemical equilibrium constants for the water-gas shift and methanation reactions (5th and 6~ reactions in Table 1) and the elemental balances of H, O, C and N. Watkinson et al. [4] observed that, for an unreactive coal, experimentally measured product gas compositions were predicted accurately using chemical equlibrium calculations when reactor temperatures were above 1300K. Below this temperature large differences between experimental and calculated product gas compositions were observed. For a more reactive coal, however, Watkinson et al. [4] found good agreement between observed and calculated values at temperatures as low as 1050 K. This trend was also observed here when F*A*C*T was applied to the data presented by Watkinson et al. [4]. CONCLUSIONS In this study a commercially available chemical equilibrium software package (F*A*C*T) was used to
EQUILIBRIUM CALCULATIONS IN COAL GASIFICATION
'i
(a) 0.20 r" 0 -r-I
fluidized bed (KRW). F r o m this work, it was demonstrated that gasifier product gas compositions can be calculated using the F * A * C * T chemical equilibrium software for a wide range of coals and gasifiers. The coal compositions, extent of conversion and reactor operating conditions are required as input information for the calculations. For gasifier temperatures above 1300 K the calculated product gas composition, assuming chemical equilibrium, was found to be insensitive to reactor temperature. In many cases, therefore, large scale gasification experiments would not be required to determine product gas compositions.
i O . 15"
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Acknowledgements--This work was supported by the Alberta/ Canada Energy Resources Research Fund administered by the Alberta Office of Coal Research and Technology, Alberta Energy and by the Alberta Research Council.
1-12
REFERENCES
0.00 itO0
~200
Temperature
t300
(K)
Fig. 4. Equilibrium predictions of product gas composition for the gasification of Highvale coal using KRW process, setpoint 1, and setpoint 2. Best fit temperatures are 1210 and 1230 K.
calculate product gas compositions for a wide range of feedstocks in various gasification technologies. The results of the equilibrium calculations were compared with published data for the gasification of various feedstocks, ranging from sub-bituminous coal to petroleum coke, in various gasification reactors, including dry feed, oxygen blown entrained flow ( P R E N F L O ) , slurry feed, oxygen blown entrained flow ( T E X A C O ) and air blown
1. R. Aris, Elementary Chemical Reactor Analysis, pp. 8 28. Prentice Hall, Englewood Cliffs, N.J., U.S.A. (1969). 2. H. Hofmann, Kinetic data analysis and parameter estimation in M. I. deLasa (ed.), Chemical Reactor Design and Technology, pp. 69-105, NATO ASI No. 110 (1986). 3. L. K. Mudge, G. F. Schiefelbein C. T Li and R. H. Moore, The Gasification of Coal, a Battelle Energy Program Report, Battelle Pacific Northwest Laboralories, Richland. Washington, U.S.A. (1974). 4. A. P. Watkinson, G. Cheng and C. J. Lira, Oxygen- steam gasification of coals in a spouted bed, Can. J. Chem:. Engng 65, 791 798 (1987). 5. F. J. Zeleznik and S. Gordon, Calculation of complex chemical equilibria, Ind. Engng ('hem. 60, 27 57 (1968). 6. S. Gordon and B. J. McBride, Computer Program for Calculation of Chemical Equilibrium Compositions, Rocket Performance, Incident and Reflected Shocks, and Chapman Jouguet Detonations, NASA, SP-273, National Aeronautics and Space Administration, Washington, D.C. (1976). 7. TransAlta Utilities Corporation, Fluidized Bed Gasification of Highvtde Coal, DSS File No. 06SQ.23440-7-9134 (1988).