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Thermodynamic evaluation of high-pressure coal gasification
Thernmdynamic evaluation of high-pressure coal gasification (Received 29 May 1963; in revised form 20 June 1963) RECENTLY SEVERAL publications have appeared [l, 2, 31 on the gasification of coal under pressure by the Lurgi process. These have concentrated on the purification and enrichment of the towns gas produced, the gasification process itself receiving less attention. It was felt desirable to calculate the thermodynamically expected gas analysis based on the process variables used in practice, and to compare these with the practical results. In this way an assessment could be made of the degree of attainment of equilibrium. It isnot the intention here to describe the plant itself, it being sufficient to regard the operation as one in which steam and oxygen under pressure are passed counter currently to a bed of suitably sized non-caking solid fuel (coal or briquettes). The fuel bed, described from the bottom upwards, consists of more or less well defined zones of ash, oxidation zone, reduction and hydrogenation zones and a distillation zone respectively. Any thermodynamic evaluation depends on a knowledge of pressure and temperature. Here the pressure is taken to be the mean operating pressure (the pressure drop across the bed is small in comparison with the mean pressure) and the temperature, somewhat less well defined, as that corresponding to the top of the reduction zones, where the gaseous equilibria apart from the distillation of volatile matter, are probably frozen. The steam referred to is that admitted with the oxygen at the bottom of the producer, any steam produced by the evaporation of moisture in the fuel being disregarded since it does not pass through the reaction zones. Higher pressures lead to higher rates of gasification. The space velocity of the gases up through the fuel bed is much less than that in low pressure producers, being arranged in practice to be approximately inversely proportional to the pressure. The much lower space velocity does not necessarily lead to a more complete attainment of equilibrium, because there is, proportionate to the pressure, more gas to react. Since the times for reduction of the initial pressure of reactants are respectively directly proportional to pressure for a zero order reaction, independent of pressure for a first order reaction and inversely proportional to pressure for a second order reaction, it is clear that increase of pressure would unambiguously increase the degree of attainment of equilibrium for the gasification process only if all the component reactions involved are of order greater than unity. Without detailed investigation (see [4] for a kinetic treatment of the gasification process at atmospheric pressure) no definite statement can be made as to the effect of pressure on the degree of attainment of equilibrium, other factors affecting it remaining constant. Methane, a desirable constituent of towns gas in view of its high calorific value, is expected to be formed more readily under high pressure. It is of interest to study its expected degree of formation under varying conditions.
No discussion of the enthalpy changes accompanying reaction is made, it being assumed that the temperatures used in the calculation can be attained by the reactions involved, supplemented by various heat transport phenomena. Of course implicit account is taken of the heats of reaction by considering the changes with temperature of the equilibrium constants. Method of Calculation The species expected at equilibrium are COz, CO, H20, He, CH4 and 02. Hydrocarbons other than methane will only be present in very small amounts, as can be verified from free energy data. Only two mass balance equations in the gaseous phase can be written: for hydrogen and oxygen. A mass balance in the gaseous phase for carbon can not be made: indeed it is one of the functions of the calculation to enable the maximum amount of carbon gasified to be found. For six species, four mass-action equations are required to solve the problem. Any four involving the above species and carbon will do, provided they are independent. The following equilibria were chosen, for which the equilibrium constants at varying temperatures are well known [S], and listed for three chosen temperatures in Table 1.
725
Reaction co +t02+co2 c +CO2~2CO COz+Ha-+CO+HzO C +2Ha +CH4
Equilibrium Constant (KP) Kl K2 K3 K4 Table 1.
1000°K Ki K2 K3 K4
1.58 x 1O1” 1.90 7.29 x 10-l 9.83 x 1O-2
1200°K 5.52 x 107 5.71 x 101 144 1.61 x 10-e
1400°K 9.81 x lo5 6.29 x lo2 2.27 4.33 x 10-3
From the values of Ki it is clear that the concentration of oxygen at equilibrium will be extremely small, this agreeing of course with the known kinetic fact that oxygen virtually disappears in the fuel bed within a few solid particle diameters. The first reaction can therefore be ignored (leaving five species to be calculated from two mass balance equations and three mass action equations.) With mole number as variables the equations were solved for a variety of temperatures, pressures and steam/oxygen ratios. The results obtained are shown in Table 2. The equations were programmed for solution on a digital computer, using a method previously described [6]. It is known that for this particular ‘method the choice of components is
D. B.
SCULLY
important. Generally the method worked using carbon monoxide and hydrogen as components. In the few cases in which it failed with this choice, it succeeded with carbon dioxide and water as components. All twenty-eight cases were solved in approximately 1 min on the Atlas digital computer at Manchester University.
11
21 (%) Moles
31 (%) Moles
16.5 19.6 30.0 26.0 7.9
0.81 0.96 1.47 l-27 0.39
13.9 17.1 31.7 28.4 8.9
0.66 0.81 1.50 1.34 0.42
3.84 49.2 2.37 30.7 040 8.9 0.36 8.0 O-14 3.2
3.28 2.04 0.59 O-53 O-21
45.7 28.7 11.3 10.2 4.1
2.91 1.83 0.72 0.65 0.26
4.96 2.70 0.10 0.13 0.09
60.2 34.2 1.8 2.3 1.6
4.56 2.59 0.14 0.17 0.12
1(%) Moles
co Ha co2
Hz0 CH4 T = 1200°K co Ha coa Ha0 CH4
61.5 37.0 0.7 0.6 0.2
4.85 54.1 2.92 33.3 5.6 o-05 5.0 0.05 o-02 2-O
T = 1400°K co HZ CO2 :g
62.4 37.4 0-l 0.1 0.1
4.98 298 0.01 0.01 0.01
61.6 36.2 o-7 0.9 0.6
4.83 2.83 0.05 0.07 0.05
60.9 35.1 1.2 1.6 1.1
Moles steam = 4 Moles oxygen = 1 T = 1000°K Pressure (Atm) Species
11
1-
(%) Moles
21
(%) Moles / (~1 Moles I
co %a
44-o 10.2 38.0 6.4 1.4
3.73 31.6 0.86 3.22 27.2 19.9 0.54 13.3 8.0 0.12 T=
co Ha coa Hz0 CH4
59.1 39.5 0.6 0.6 0.3
5.82 52.0 3.89 35.5 5.2 0.06 5.1 0.06 o-03 2.2
2.24 125.8 1.43 22.6 1.41 24-O 0.94 17-l 0.57 10.5
(“/.) Moles
_-
1.71 22.7 144 1.49 19.8 1.26 1.58 26.3 1.67 1.13 19.2 1.22 069 12-O 0.76
1200°K 4.62 47.3 3.15 32.6 0.46 8.2 0.45 8.2 0.20 3.6
T=
31
-
43.9 30.5 ‘0.5 f 0.5 4.6
3.50 2.43 0.83 0.83 0.37
5.62 57.8 360 36.4 O-11 l-6 0.16 2.4 0.12 1.8
5.47 344 0.16 0.22 0.17
3.95 2.72 O-69 0.68 0.30
14 3°K
I
co H2 co2 Ha0 CH4
59.9 39.9 0.1 0.1 0.1
5.98 3.98 0.01 0.01 o-01
59.2 38.6 0.6 o-9 0.7
5.79 58.5 3.77 37.4 1.1 0.06 l-6 0.09 1.3 o-07
T= Pressure co HZ CGa Ha0 CH4
Table 2. Results of Equilibrium Calculation. Moles steam = 3 Moles oxygen = 1 T = 1000°K. Pressure (Atm) 1 Species (%) Moles
Moles steam = 5 Moles oxygen = 1
26.0 23.3 22.4 17-l 11.2
1000°K
T=
21
31
2.07 23.0 1.86 20.5 1.79 24.6 1.36 19.0 0.89 12.8
1.76 46.1 1.57 33.9 1.89 7.8 1.46 8.3 O-98 3.9
1200°K
21 4.61 42.8 3.39 31.7 0.78 9.9 0.83 10.6 0.39 5-O
31 4.09 3.03 0.95 1.01 0.48
Attainment of equilibrium with low pressure producers Before comparing the results of the above calculations with typical plant analyses, it is desirable to consider whether equilibrium is established in conventional producer gas practice, wherein data is more extensive than for high pressure gasifiers. Two cases are dealt with, the gas analyses being taken from the book, Efficient Use of Fuel [7,8]. In the first example the actual percentage of steam in the exit gas is given, whereas in the second example this has to be calculated from the initial gas composition. (a) At the top of a coke-fired producer, operated at atmospheric pressure and blown with a steam/air mixture, a gas analysis (by volume) was obtained as follows: CO2 4*7x, CO 27.0 %, Ha 8.5 %, Ha0 2.7 %, remainder Na. Assuming that the gases are in equilibrium, tables [5] giving the variation with temperature of KP where:
Kpzp% PC02 show that the temperature is about 1280°K. For the reaction CO + Ha0 = COa + Ha the gas analysis corresponds to a temperature of 1295°K. These temperatures are almost equal, showing that the assumption that equilibrium is attained is reasonable. (b) A coke-fed mechanical producer, supplied with a steam/ air mixture of blast steam saturated temperature (B.S.S.T.) 60°C eave a teas of drv analvsis as follows: COa 10%. CO 20.8 %, Ha 8-8x, Ne 80.3 %.-Taking the figures to represent moles and assuming equilibrium, the CO and COa analyses correspond to a temperature of 1200°K. From the water gas shift reaction a second temperature can beobtained, provided the amount of water vapour, not given in the analysis, is found. This is readily done, using the B.S.S.T., the moles of nitrogen and the experimental fact that the steam decomposition is 47 per cent. The equilibrium temperature works out to be 1400°K. Comparison with the temperature of 1200°K shows that equilibrium has not been attained. The reason is that excess steam has resulted in a lowering of the fuel bed temperature, the carbon-steam reaction being endothermic. In the first example presumably the B.S.S.T. was lower, resulting in a higher fuel bed temperature with consequent higher reactivity and greater approach to equilibrium. It is apparent that thermodynamic data are useful in these cases and would be expected to be revealing in the application to high pressure gasifiers. Discussion of theoretical results The results of the calculation at 1000°K show that there is a largeincrease in percentage of methane when the operating
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Thermodynamic evaluation of high-presure coal gasitication pressure is increased from atmospheric pressure to 30 atm gauge. As against this, temperature has a very adverse effect of methane production, thus offsetting the advantage of high pressure, there being a decrease in methane production of more than 50 per cent, at a pressure of 30 atm gauge if the temperature is raised from 1000 to 1200°K. The changes in the ratio of carbon monoxide to carbon dioxide with temperature and pressure are interesting. For any given tem‘perature this ratio decreases quite considerably with increase of pressure over the range considered. With increase of temperature there is an even more striking increase of this ratio. The effect of increase in pressure on the amount of undecomposed steam is noteworthy. For a steam to oxygen molar ratio of three and a temperature of 1000°K at atmospheric pressure 14 per cent of the steam remains undecomposed, whereas at 30 atm gauge 45 % is undecomposed. With regard to the production of hydrogen, increasing pressure decreases the amount whereas increasing temperature increases it. Comparison with plant analyses The analyses for unpuritled gas for two Lurgi generators have been given recently 121, for the Morwell plant in Australia and the Westfield plant in Scotland. These together with the computed thermodynamic result are reproduced in Table 3. Table 3. Plant and computed gasi$cation analyses (%)
Morwell 30 atm 35 :: H8 CH4 Ns CnHm
:: 13 2 0
Westfield 30 atm 27.7 21.1 39.2 10.1 1.0 0.9
Calculated 24 atm 1000°K 28.5 30.0 26.9 14.6 0 0
Calculated, corrected for distilled Hs 26.1 27-5 33.0 13.4 0 0
The higher ratio of CO&O at the Morwell plant and also the higher percentage of methane show the effect of increasing pressure. The third column of figures in Table 3 gives the calculated results, omitting the calculated amount of water (which is not given in the dry gas plant analyses) at 24 atm, 1000°K and a steam/oxygen molar ratio of 5. To make a fair comparison of the plant analysis with the calculated results for the hydrogen, it is necessary to increase the calculated hydrogen to allow for its additional production by destructive distillation of solid fuel at the top of the bed.
A reasonable allowance for this is determined as follows. It is known that in the distillation of bituminous coals carbonized for the manufacture of towns gas, approximately 20 lb moles of hydrogen are produced per ton of coal gasified (by distillation alone, and not including additional hydrogen formed by steaming the retorts). This means that the ratio of atomic carbon to hydrogen is about 4. It is reasonable to add + of the total atoms of carbon in the calculated results to the moles of hydrogen calculated. The calculated results corrected for distilled hydrogen are shown in the fourth column of Table 3. It has been assumed here that the distilled hydrogen plays no part in the chemical reactions. The different assumption, that distilled hydrogen does take part in the reactions, would alter the computed results and would entail an involved calculation. This would be so because the amount of carbon gasitied is an unknown (to be calculated) rather than a known parameter. The present assumption is justifiable on the grounds (a) of simplicity, (b) the distilled hydrogen does not pass through the whole length of the reactor: indeed probably only through about a third of it. The agreement with the Westfield analysis is very satisfactory. The plant hydrogen percentage is still higher than expected on an equilibrium basis and the methane lower. Now the amount of calculated hydrogen would increase if a higher temperature than 1000°K had been used, as can be seen from Table 2, but a higher temperature would increase the CO/COs ratio rather sharply. As it is. the calculated COalCO ratio for 1000°K is rather lower than the plant figures~ndicating that a temperature somewhat below lOC!@‘Kwouldhavebeen more appropriate to secure agreement with the plant analysis for the latter ratio. These remarks would then appear to warrant the conclusion that the methane forming reaction C+2Hs+CH4 has not gone completely to equilibrium. This is consonant with the history of hydrogen production in gasiflers generally. Hydrogen will only begin to make its appearance in the primary reduction zone some third way or so up the gasitier. Since also the temperature is a falling one as the gases proceed up the bed from this zone, it is not surprising that the methane forming reaction does not reach equilibrium. Even so the methane formation is within 20 per cent of that expected thermodynamically. Acknowledgement-The author is indebted to Professor F. MORTONto whom he owes his interest in this problem. D. B. SCXJLLY Department of Chemical Engineering Faculty of Technology University of Manchester
REFERENCES HUBBMANO., BrennstofiChem. 1959 40 65. [ii RICKE~TST. S., J. Inst. Fuel 1961 34 177. M. C., Chem. Engr. 1962 No. 164 ASS. 131 SOLBETT J. M. and GOODMAN H. R., Busct-m R. M. and ARMSTRONGW. R., Zndustr. Engng. Chem. 1953 45 1856. [41 BATCHELDER I51 SPTERSH., Technical Data on Fuel, 6th edition, p. 214. 1’51 SCULLYD. B., Chem. Engng. Sci. 1962 17 977. 171 Eficient use ofFuel, H.M.S.O. 1958, p. 108. P31 Eficient use of Fuel, H.M.S.O. 1958, p. 438.
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No discussion of the enthalpy changes accompanying reaction is made, it being assumed that the temperatures used in the calculation can be attained by the reactions involved, supplemented by various heat transport phenomena. Of course implicit account is taken of the heats of reaction by considering the changes with temperature of the equilibrium constants. Method of Calculation The species expected at equilibrium are COz, CO, H20, He, CH4 and 02. Hydrocarbons other than methane will only be present in very small amounts, as can be verified from free energy data. Only two mass balance equations in the gaseous phase can be written: for hydrogen and oxygen. A mass balance in the gaseous phase for carbon can not be made: indeed it is one of the functions of the calculation to enable the maximum amount of carbon gasified to be found. For six species, four mass-action equations are required to solve the problem. Any four involving the above species and carbon will do, provided they are independent. The following equilibria were chosen, for which the equilibrium constants at varying temperatures are well known [S], and listed for three chosen temperatures in Table 1.
725
Reaction co +t02+co2 c +CO2~2CO COz+Ha-+CO+HzO C +2Ha +CH4
Equilibrium Constant (KP) Kl K2 K3 K4 Table 1.
1000°K Ki K2 K3 K4
1.58 x 1O1” 1.90 7.29 x 10-l 9.83 x 1O-2
1200°K 5.52 x 107 5.71 x 101 144 1.61 x 10-e
1400°K 9.81 x lo5 6.29 x lo2 2.27 4.33 x 10-3
From the values of Ki it is clear that the concentration of oxygen at equilibrium will be extremely small, this agreeing of course with the known kinetic fact that oxygen virtually disappears in the fuel bed within a few solid particle diameters. The first reaction can therefore be ignored (leaving five species to be calculated from two mass balance equations and three mass action equations.) With mole number as variables the equations were solved for a variety of temperatures, pressures and steam/oxygen ratios. The results obtained are shown in Table 2. The equations were programmed for solution on a digital computer, using a method previously described [6]. It is known that for this particular ‘method the choice of components is
D. B.
SCULLY
important. Generally the method worked using carbon monoxide and hydrogen as components. In the few cases in which it failed with this choice, it succeeded with carbon dioxide and water as components. All twenty-eight cases were solved in approximately 1 min on the Atlas digital computer at Manchester University.
11
21 (%) Moles
31 (%) Moles
16.5 19.6 30.0 26.0 7.9
0.81 0.96 1.47 l-27 0.39
13.9 17.1 31.7 28.4 8.9
0.66 0.81 1.50 1.34 0.42
3.84 49.2 2.37 30.7 040 8.9 0.36 8.0 O-14 3.2
3.28 2.04 0.59 O-53 O-21
45.7 28.7 11.3 10.2 4.1
2.91 1.83 0.72 0.65 0.26
4.96 2.70 0.10 0.13 0.09
60.2 34.2 1.8 2.3 1.6
4.56 2.59 0.14 0.17 0.12
1(%) Moles
co Ha co2
Hz0 CH4 T = 1200°K co Ha coa Ha0 CH4
61.5 37.0 0.7 0.6 0.2
4.85 54.1 2.92 33.3 5.6 o-05 5.0 0.05 o-02 2-O
T = 1400°K co HZ CO2 :g
62.4 37.4 0-l 0.1 0.1
4.98 298 0.01 0.01 0.01
61.6 36.2 o-7 0.9 0.6
4.83 2.83 0.05 0.07 0.05
60.9 35.1 1.2 1.6 1.1
Moles steam = 4 Moles oxygen = 1 T = 1000°K Pressure (Atm) Species
11
1-
(%) Moles
21
(%) Moles / (~1 Moles I
co %a
44-o 10.2 38.0 6.4 1.4
3.73 31.6 0.86 3.22 27.2 19.9 0.54 13.3 8.0 0.12 T=
co Ha coa Hz0 CH4
59.1 39.5 0.6 0.6 0.3
5.82 52.0 3.89 35.5 5.2 0.06 5.1 0.06 o-03 2.2
2.24 125.8 1.43 22.6 1.41 24-O 0.94 17-l 0.57 10.5
(“/.) Moles
_-
1.71 22.7 144 1.49 19.8 1.26 1.58 26.3 1.67 1.13 19.2 1.22 069 12-O 0.76
1200°K 4.62 47.3 3.15 32.6 0.46 8.2 0.45 8.2 0.20 3.6
T=
31
-
43.9 30.5 ‘0.5 f 0.5 4.6
3.50 2.43 0.83 0.83 0.37
5.62 57.8 360 36.4 O-11 l-6 0.16 2.4 0.12 1.8
5.47 344 0.16 0.22 0.17
3.95 2.72 O-69 0.68 0.30
14 3°K
I
co H2 co2 Ha0 CH4
59.9 39.9 0.1 0.1 0.1
5.98 3.98 0.01 0.01 o-01
59.2 38.6 0.6 o-9 0.7
5.79 58.5 3.77 37.4 1.1 0.06 l-6 0.09 1.3 o-07
T= Pressure co HZ CGa Ha0 CH4
Table 2. Results of Equilibrium Calculation. Moles steam = 3 Moles oxygen = 1 T = 1000°K. Pressure (Atm) 1 Species (%) Moles
Moles steam = 5 Moles oxygen = 1
26.0 23.3 22.4 17-l 11.2
1000°K
T=
21
31
2.07 23.0 1.86 20.5 1.79 24.6 1.36 19.0 0.89 12.8
1.76 46.1 1.57 33.9 1.89 7.8 1.46 8.3 O-98 3.9
1200°K
21 4.61 42.8 3.39 31.7 0.78 9.9 0.83 10.6 0.39 5-O
31 4.09 3.03 0.95 1.01 0.48
Attainment of equilibrium with low pressure producers Before comparing the results of the above calculations with typical plant analyses, it is desirable to consider whether equilibrium is established in conventional producer gas practice, wherein data is more extensive than for high pressure gasifiers. Two cases are dealt with, the gas analyses being taken from the book, Efficient Use of Fuel [7,8]. In the first example the actual percentage of steam in the exit gas is given, whereas in the second example this has to be calculated from the initial gas composition. (a) At the top of a coke-fired producer, operated at atmospheric pressure and blown with a steam/air mixture, a gas analysis (by volume) was obtained as follows: CO2 4*7x, CO 27.0 %, Ha 8.5 %, Ha0 2.7 %, remainder Na. Assuming that the gases are in equilibrium, tables [5] giving the variation with temperature of KP where:
Kpzp% PC02 show that the temperature is about 1280°K. For the reaction CO + Ha0 = COa + Ha the gas analysis corresponds to a temperature of 1295°K. These temperatures are almost equal, showing that the assumption that equilibrium is attained is reasonable. (b) A coke-fed mechanical producer, supplied with a steam/ air mixture of blast steam saturated temperature (B.S.S.T.) 60°C eave a teas of drv analvsis as follows: COa 10%. CO 20.8 %, Ha 8-8x, Ne 80.3 %.-Taking the figures to represent moles and assuming equilibrium, the CO and COa analyses correspond to a temperature of 1200°K. From the water gas shift reaction a second temperature can beobtained, provided the amount of water vapour, not given in the analysis, is found. This is readily done, using the B.S.S.T., the moles of nitrogen and the experimental fact that the steam decomposition is 47 per cent. The equilibrium temperature works out to be 1400°K. Comparison with the temperature of 1200°K shows that equilibrium has not been attained. The reason is that excess steam has resulted in a lowering of the fuel bed temperature, the carbon-steam reaction being endothermic. In the first example presumably the B.S.S.T. was lower, resulting in a higher fuel bed temperature with consequent higher reactivity and greater approach to equilibrium. It is apparent that thermodynamic data are useful in these cases and would be expected to be revealing in the application to high pressure gasifiers. Discussion of theoretical results The results of the calculation at 1000°K show that there is a largeincrease in percentage of methane when the operating
726
Thermodynamic evaluation of high-presure coal gasitication pressure is increased from atmospheric pressure to 30 atm gauge. As against this, temperature has a very adverse effect of methane production, thus offsetting the advantage of high pressure, there being a decrease in methane production of more than 50 per cent, at a pressure of 30 atm gauge if the temperature is raised from 1000 to 1200°K. The changes in the ratio of carbon monoxide to carbon dioxide with temperature and pressure are interesting. For any given tem‘perature this ratio decreases quite considerably with increase of pressure over the range considered. With increase of temperature there is an even more striking increase of this ratio. The effect of increase in pressure on the amount of undecomposed steam is noteworthy. For a steam to oxygen molar ratio of three and a temperature of 1000°K at atmospheric pressure 14 per cent of the steam remains undecomposed, whereas at 30 atm gauge 45 % is undecomposed. With regard to the production of hydrogen, increasing pressure decreases the amount whereas increasing temperature increases it. Comparison with plant analyses The analyses for unpuritled gas for two Lurgi generators have been given recently 121, for the Morwell plant in Australia and the Westfield plant in Scotland. These together with the computed thermodynamic result are reproduced in Table 3. Table 3. Plant and computed gasi$cation analyses (%)
Morwell 30 atm 35 :: H8 CH4 Ns CnHm
:: 13 2 0
Westfield 30 atm 27.7 21.1 39.2 10.1 1.0 0.9
Calculated 24 atm 1000°K 28.5 30.0 26.9 14.6 0 0
Calculated, corrected for distilled Hs 26.1 27-5 33.0 13.4 0 0
The higher ratio of CO&O at the Morwell plant and also the higher percentage of methane show the effect of increasing pressure. The third column of figures in Table 3 gives the calculated results, omitting the calculated amount of water (which is not given in the dry gas plant analyses) at 24 atm, 1000°K and a steam/oxygen molar ratio of 5. To make a fair comparison of the plant analysis with the calculated results for the hydrogen, it is necessary to increase the calculated hydrogen to allow for its additional production by destructive distillation of solid fuel at the top of the bed.
A reasonable allowance for this is determined as follows. It is known that in the distillation of bituminous coals carbonized for the manufacture of towns gas, approximately 20 lb moles of hydrogen are produced per ton of coal gasified (by distillation alone, and not including additional hydrogen formed by steaming the retorts). This means that the ratio of atomic carbon to hydrogen is about 4. It is reasonable to add + of the total atoms of carbon in the calculated results to the moles of hydrogen calculated. The calculated results corrected for distilled hydrogen are shown in the fourth column of Table 3. It has been assumed here that the distilled hydrogen plays no part in the chemical reactions. The different assumption, that distilled hydrogen does take part in the reactions, would alter the computed results and would entail an involved calculation. This would be so because the amount of carbon gasitied is an unknown (to be calculated) rather than a known parameter. The present assumption is justifiable on the grounds (a) of simplicity, (b) the distilled hydrogen does not pass through the whole length of the reactor: indeed probably only through about a third of it. The agreement with the Westfield analysis is very satisfactory. The plant hydrogen percentage is still higher than expected on an equilibrium basis and the methane lower. Now the amount of calculated hydrogen would increase if a higher temperature than 1000°K had been used, as can be seen from Table 2, but a higher temperature would increase the CO/COs ratio rather sharply. As it is. the calculated COalCO ratio for 1000°K is rather lower than the plant figures~ndicating that a temperature somewhat below lOC!@‘Kwouldhavebeen more appropriate to secure agreement with the plant analysis for the latter ratio. These remarks would then appear to warrant the conclusion that the methane forming reaction C+2Hs+CH4 has not gone completely to equilibrium. This is consonant with the history of hydrogen production in gasiflers generally. Hydrogen will only begin to make its appearance in the primary reduction zone some third way or so up the gasitier. Since also the temperature is a falling one as the gases proceed up the bed from this zone, it is not surprising that the methane forming reaction does not reach equilibrium. Even so the methane formation is within 20 per cent of that expected thermodynamically. Acknowledgement-The author is indebted to Professor F. MORTONto whom he owes his interest in this problem. D. B. SCXJLLY Department of Chemical Engineering Faculty of Technology University of Manchester
REFERENCES HUBBMANO., BrennstofiChem. 1959 40 65. [ii RICKE~TST. S., J. Inst. Fuel 1961 34 177. M. C., Chem. Engr. 1962 No. 164 ASS. 131 SOLBETT J. M. and GOODMAN H. R., Busct-m R. M. and ARMSTRONGW. R., Zndustr. Engng. Chem. 1953 45 1856. [41 BATCHELDER I51 SPTERSH., Technical Data on Fuel, 6th edition, p. 214. 1’51 SCULLYD. B., Chem. Engng. Sci. 1962 17 977. 171 Eficient use ofFuel, H.M.S.O. 1958, p. 108. P31 Eficient use of Fuel, H.M.S.O. 1958, p. 438.
727