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Coal gasification by microwave plasma in water vapour
Short
Coal
Djamal
gasification
Djebabra,
Communications
by microwave
Odile
Dessaux
and
plasma
Pierre
in water
vapour
Goudmand
Laboratoire de PhysicoChimie de I’Energstique et des Plasmas, Universitb de Lille, Flandres-Artois 59655 Villeneuve d*Ascq cedex, France (Received 9 July 1990; revised 24 April 1991)
des Sciences
et Techniques
The influence of several parameters on the H, and CO yields from gasification of a coal (Blanzy) by microwave plasma in water vapour, in a static system, is discussed. The yields of H, and CO significantly increase for low coal weights placed in the discharge illustrating that the gasification occurs on the surface of the coal layer. The time necessary for complete gasification and the gasification yields are measured for several initial weights of coal. The relative yields of H, and CO depend on the initial water vapour pressure and are independent of coal granulometry. The reactivity of a coal which was pre-reacted in the discharge is similar to that of an original coal, so a possible recycling of the solid can be envisaged. (Keywords: coal; gasification;
microwave techniques)
A microwave plasma in water vapour creates radicals or ions in thermodynamical non-equilibrium, which react with coal so that the activation energy of C + H,O + H, + CO, is the reaction: greatly lowered. In a static system, previous studies’,’ were essentially concerned with the effect of the type of coal on the gasification yield in the presence of different plasma gases (Ar+H,O, Ar). The main reaction products were H,, CO, CO,, CH, and C,H, with H, and CO being the most important. The highest yields were obtained for lignite and kerogen (33.5 and 31.8 wt%, respectively). In the present work, only the H, and CO yields are studied.
STUDY CONDITIONS AND PARAMETERS ILLUSTRATING THE EXPERIMENTAL RESULTS The conditions which affect the H, and CO yields are: weight of coal which is placed in the discharge zone, m,; discharge time, r; initial water vapour pressure, P,; granulometry, d; incident microwave power, w; the gas surrounding the coal in the discharge, and the type of carbonaceous material used. In this study, we have used the coal Blanzy, the analysis of which is given in Table I. The incident microwave power is kept constant (100 W). Three parameters illustrate the evolution from the gasification reaction:
001&2361/91/121473~3 0 1991 Butterworth-Heinemann
Ltd.
RESULTS EfSecr of initial weight of coal
the number of H, and CO moles: nH2 and nco; the pressure, P’=P,,+P,-o which is proportional to n = nH2+ nc,; the ratio P’/n must satisfy the equation for an ideal gas; the loss in weight during the reaction process, m.
EXPERIMENTAL A known weight of coal, m,, is placed in a quartz tube so that all the coal powder is in the discharge zone. In a vacuum (~0.10 hPa), a water vapour pressure, P,, is introduced in the reactor. The discharge was then produced over z min. After the discharge, the pressure in the reactor is P. The reaction products are then trapped at liquid nitrogen temperature; the measured pressure is P’. Helium is added to the products which are not condensed at liquid nitrogen temperature (HZ and CO) up to a pressure of 1200 hPa. The H, and CO are quantitatively analysed by gas chromatography (g.c.)3 with a molecular sieve column. The discharge tube which contains the coal is weighed before and after reaction to determine the loss in weight of coal during the reaction, m.
The experimental results are given as the arithmetic mean of results of three to five experiments4. To study the effect of one parameter, the values of all other parameters are kept constant: 5 = 6 min, P, = 21.33 hPa and 0.063
Table 1
Analyses of coal Blanzy-original
The effect of m, on H, and CO yields and on the loss in weight m is studied for m, values between 1 mg and 120 mg. The results are given in Table 2. The consistent relatively low maximal value of H, and CO yields which is observed for m, > 80 mg can be explained in two ways: the products of the reaction are not eliminated; and by the depletion of reactive species coming from the discharge in the outer layer of the coal deposition.
and pre-reacted Proximate analysis (wt%) of original coal
Elemental analysis (%)
C
H N 0 (by diff.) S Ash
Original coal
Pre-reacted coal
79.2 3.3
78.6 3.5
1.4 4.0 1.4 9.0
1.6 3.5 1.6 11.2
Moisture 1.91 Volatile matter (dry basis) 9.12 Ash (dry basis) 11.25 Fixed C (dry basis) 79.63
FUEL,
1991,
Vol 70, December
1473
Short Communications E#ect qf discharge time The effect of t is studied for mO= 1, 5, 10 and 60 mg. Among the parameters described, the total pressure P can be continuously measured. It is the sum of the partial pressures of all gaseous products and of the pressure of nondecomposed water vapour. A plot of P as a function of t (Figure I) allows the determination of the time T* from which the gasification can be considered as
T (min)
Figure 1 Plot of P versus z. Values of M, (mg): A, 1; B, 5; C, 10; D, 60
Table 2
Effect of initial weight of coal m,
1 5 10 30 60 80 100 120
Table 3
mo (mg)
complete. It is defined as the time from which the variation of P, during 10 min, is equal to 1.33 hPa. The study of the influence of the discharge time, for T values
hI (x 105) (mol)
nco
(x 105) (mol)
P’ (hPa)
(W
4.3 6.1 6.5 13.1 14.8 16.6 16.9 16.5
2.6 4.7 5.5 11.1 13.2 14.6 15.0 15.2
13 18 22 37 42 47 49 49
0.39 0.57
m
I .o 1.83 2.5 3.4 3.3 4.0
Effect of discharge time T
nco
nI12
r (min)
(x IO') (mol)
(x 105) (mol)
m
p’ (hPa)
(w)
1
6 10 15
4.3 6.9 7.4
2.6 4.4 4.6
13 18 20
0.39 0.56 0.65
5
3 6 10 20 30
3.5 6.1 8.0 12.4 13.1
3.3 4.7 5.4 9.4 10.5
12 18 21 35 37
0.54 0.57 0.83 1.2 1.4
10
2 6 20 40
5.1 6.5 14.2 16.9
3.8 5.5 12.2 14.4
15 22 42 52
0.71 1.o
6 17 30 50
14.8 19.9 25.8 28.9
13.2 18.3 23.5 26.2
42 60 74 85
2.5 3.35 3.8 4.7
60
Table 4
1.95 2.5
Etfect of initial pressure of water vapour P,
2Pa)
(x 105) (mol)
( x 10’) (mol)
P’ (hPa)
WI (m8)
6.67 13.33 21.33
3.6 4.9 6.1
3.8 4.5 4.7
11.5 15.5 18.0
0.44 0.51 0.57
nco
4,
1474
FUEL,
1991,
Vol
70,
December
values of m,, so the dissociation rate of the water vapour and H, concentration increases. Effect of the values qf the initial water vapour pressure and qf granulometry The effect of the P, value is studied for three P, values: 6.67, 13.33 and 21.33 hPa, with the last value being the highest under our experimental conditions. The value of m, is kept constant at 5 mg. Tab/e 4 shows that nH2, n,, (to a smaller extent) and m increase with P,. This observation can be explained by an increase in the H and OH concentrations in the discharge zone. Four ranges of granulometry have been used: d(mm) < 0.063, 0.063 < d(mm)<0.100,0.250
Short Communications previously reacted (p.r.) or which has not previously reacted (n.p.r.) in the discharge, are compared. The m, value is 52 mg. After reaction, a manual separation of the ashes is carried out. The coal which is recovered (37.50 mg) reacts in the discharge for 6 min. The values of PL and %. are, respectively, 38 + 1 hPa and 2.9f0.2 mg. These values are compared with those (Ph,+r, = 39 + 2 hPa and mn.p.r. = 3.6kO.3 mg) obtained from 37.5 mg of coal which was not previously reacted. There is no considerable decrease in the Hz and CO yields of the pre-reacted coal. The analysis of the coal shows (Table
1) that the composition of this coal is not modified in comparison with that of the coal which was not previously reacted. This result confirms the hypothesis of the gasification of the surface of the coal layer. Two observations provide evidence that H, coming from the reaction coal + water vapour plasma essentially originates from the decomposition of the water vapour: the first is that the amount of H, is the same in the recuperated coal as in the coal which has not reacted; and the second is by comparison of the values of H, yield of coal gasification by plasma of water vapour and the initial H, content of the coal.
REFERENCES
1 Vastola, F. J., Walker, P. L. and Wightman, 2
J. P. Carbon 1963,1, 11; Chrm. Eng. 1962. 44, 2 Fu, Y. C. and Blaustein, B. D. Chem. Ind. 1967, 1257; Fuel 1968, 47, 463; Ind. Eng. Chem. Proc. Design Dewlop.
1969, 8, 257;
Fu, Y. C.. Blaustein, B. D. and Wender, I. Chem. Eng. Progr. Svmp. Ser. 1971, 67. 47 3
4
Djebabra, D. PhD Thesis University of Lille, 1990 Lacroix, Y. ‘Analyse chimique. InterprCtation des rksultats par le calcul statistique’, Masson, Paris, 1973
FUEL, 1991, Vol 70, December
1475
Coal
Djamal
gasification
Djebabra,
Communications
by microwave
Odile
Dessaux
and
plasma
Pierre
in water
vapour
Goudmand
Laboratoire de PhysicoChimie de I’Energstique et des Plasmas, Universitb de Lille, Flandres-Artois 59655 Villeneuve d*Ascq cedex, France (Received 9 July 1990; revised 24 April 1991)
des Sciences
et Techniques
The influence of several parameters on the H, and CO yields from gasification of a coal (Blanzy) by microwave plasma in water vapour, in a static system, is discussed. The yields of H, and CO significantly increase for low coal weights placed in the discharge illustrating that the gasification occurs on the surface of the coal layer. The time necessary for complete gasification and the gasification yields are measured for several initial weights of coal. The relative yields of H, and CO depend on the initial water vapour pressure and are independent of coal granulometry. The reactivity of a coal which was pre-reacted in the discharge is similar to that of an original coal, so a possible recycling of the solid can be envisaged. (Keywords: coal; gasification;
microwave techniques)
A microwave plasma in water vapour creates radicals or ions in thermodynamical non-equilibrium, which react with coal so that the activation energy of C + H,O + H, + CO, is the reaction: greatly lowered. In a static system, previous studies’,’ were essentially concerned with the effect of the type of coal on the gasification yield in the presence of different plasma gases (Ar+H,O, Ar). The main reaction products were H,, CO, CO,, CH, and C,H, with H, and CO being the most important. The highest yields were obtained for lignite and kerogen (33.5 and 31.8 wt%, respectively). In the present work, only the H, and CO yields are studied.
STUDY CONDITIONS AND PARAMETERS ILLUSTRATING THE EXPERIMENTAL RESULTS The conditions which affect the H, and CO yields are: weight of coal which is placed in the discharge zone, m,; discharge time, r; initial water vapour pressure, P,; granulometry, d; incident microwave power, w; the gas surrounding the coal in the discharge, and the type of carbonaceous material used. In this study, we have used the coal Blanzy, the analysis of which is given in Table I. The incident microwave power is kept constant (100 W). Three parameters illustrate the evolution from the gasification reaction:
001&2361/91/121473~3 0 1991 Butterworth-Heinemann
Ltd.
RESULTS EfSecr of initial weight of coal
the number of H, and CO moles: nH2 and nco; the pressure, P’=P,,+P,-o which is proportional to n = nH2+ nc,; the ratio P’/n must satisfy the equation for an ideal gas; the loss in weight during the reaction process, m.
EXPERIMENTAL A known weight of coal, m,, is placed in a quartz tube so that all the coal powder is in the discharge zone. In a vacuum (~0.10 hPa), a water vapour pressure, P,, is introduced in the reactor. The discharge was then produced over z min. After the discharge, the pressure in the reactor is P. The reaction products are then trapped at liquid nitrogen temperature; the measured pressure is P’. Helium is added to the products which are not condensed at liquid nitrogen temperature (HZ and CO) up to a pressure of 1200 hPa. The H, and CO are quantitatively analysed by gas chromatography (g.c.)3 with a molecular sieve column. The discharge tube which contains the coal is weighed before and after reaction to determine the loss in weight of coal during the reaction, m.
The experimental results are given as the arithmetic mean of results of three to five experiments4. To study the effect of one parameter, the values of all other parameters are kept constant: 5 = 6 min, P, = 21.33 hPa and 0.063
Table 1
Analyses of coal Blanzy-original
The effect of m, on H, and CO yields and on the loss in weight m is studied for m, values between 1 mg and 120 mg. The results are given in Table 2. The consistent relatively low maximal value of H, and CO yields which is observed for m, > 80 mg can be explained in two ways: the products of the reaction are not eliminated; and by the depletion of reactive species coming from the discharge in the outer layer of the coal deposition.
and pre-reacted Proximate analysis (wt%) of original coal
Elemental analysis (%)
C
H N 0 (by diff.) S Ash
Original coal
Pre-reacted coal
79.2 3.3
78.6 3.5
1.4 4.0 1.4 9.0
1.6 3.5 1.6 11.2
Moisture 1.91 Volatile matter (dry basis) 9.12 Ash (dry basis) 11.25 Fixed C (dry basis) 79.63
FUEL,
1991,
Vol 70, December
1473
Short Communications E#ect qf discharge time The effect of t is studied for mO= 1, 5, 10 and 60 mg. Among the parameters described, the total pressure P can be continuously measured. It is the sum of the partial pressures of all gaseous products and of the pressure of nondecomposed water vapour. A plot of P as a function of t (Figure I) allows the determination of the time T* from which the gasification can be considered as
T (min)
Figure 1 Plot of P versus z. Values of M, (mg): A, 1; B, 5; C, 10; D, 60
Table 2
Effect of initial weight of coal m,
1 5 10 30 60 80 100 120
Table 3
mo (mg)
complete. It is defined as the time from which the variation of P, during 10 min, is equal to 1.33 hPa. The study of the influence of the discharge time, for T values
hI (x 105) (mol)
nco
(x 105) (mol)
P’ (hPa)
(W
4.3 6.1 6.5 13.1 14.8 16.6 16.9 16.5
2.6 4.7 5.5 11.1 13.2 14.6 15.0 15.2
13 18 22 37 42 47 49 49
0.39 0.57
m
I .o 1.83 2.5 3.4 3.3 4.0
Effect of discharge time T
nco
nI12
r (min)
(x IO') (mol)
(x 105) (mol)
m
p’ (hPa)
(w)
1
6 10 15
4.3 6.9 7.4
2.6 4.4 4.6
13 18 20
0.39 0.56 0.65
5
3 6 10 20 30
3.5 6.1 8.0 12.4 13.1
3.3 4.7 5.4 9.4 10.5
12 18 21 35 37
0.54 0.57 0.83 1.2 1.4
10
2 6 20 40
5.1 6.5 14.2 16.9
3.8 5.5 12.2 14.4
15 22 42 52
0.71 1.o
6 17 30 50
14.8 19.9 25.8 28.9
13.2 18.3 23.5 26.2
42 60 74 85
2.5 3.35 3.8 4.7
60
Table 4
1.95 2.5
Etfect of initial pressure of water vapour P,
2Pa)
(x 105) (mol)
( x 10’) (mol)
P’ (hPa)
WI (m8)
6.67 13.33 21.33
3.6 4.9 6.1
3.8 4.5 4.7
11.5 15.5 18.0
0.44 0.51 0.57
nco
4,
1474
FUEL,
1991,
Vol
70,
December
values of m,, so the dissociation rate of the water vapour and H, concentration increases. Effect of the values qf the initial water vapour pressure and qf granulometry The effect of the P, value is studied for three P, values: 6.67, 13.33 and 21.33 hPa, with the last value being the highest under our experimental conditions. The value of m, is kept constant at 5 mg. Tab/e 4 shows that nH2, n,, (to a smaller extent) and m increase with P,. This observation can be explained by an increase in the H and OH concentrations in the discharge zone. Four ranges of granulometry have been used: d(mm) < 0.063, 0.063 < d(mm)<0.100,0.250
Short Communications previously reacted (p.r.) or which has not previously reacted (n.p.r.) in the discharge, are compared. The m, value is 52 mg. After reaction, a manual separation of the ashes is carried out. The coal which is recovered (37.50 mg) reacts in the discharge for 6 min. The values of PL and %. are, respectively, 38 + 1 hPa and 2.9f0.2 mg. These values are compared with those (Ph,+r, = 39 + 2 hPa and mn.p.r. = 3.6kO.3 mg) obtained from 37.5 mg of coal which was not previously reacted. There is no considerable decrease in the Hz and CO yields of the pre-reacted coal. The analysis of the coal shows (Table
1) that the composition of this coal is not modified in comparison with that of the coal which was not previously reacted. This result confirms the hypothesis of the gasification of the surface of the coal layer. Two observations provide evidence that H, coming from the reaction coal + water vapour plasma essentially originates from the decomposition of the water vapour: the first is that the amount of H, is the same in the recuperated coal as in the coal which has not reacted; and the second is by comparison of the values of H, yield of coal gasification by plasma of water vapour and the initial H, content of the coal.
REFERENCES
1 Vastola, F. J., Walker, P. L. and Wightman, 2
J. P. Carbon 1963,1, 11; Chrm. Eng. 1962. 44, 2 Fu, Y. C. and Blaustein, B. D. Chem. Ind. 1967, 1257; Fuel 1968, 47, 463; Ind. Eng. Chem. Proc. Design Dewlop.
1969, 8, 257;
Fu, Y. C.. Blaustein, B. D. and Wender, I. Chem. Eng. Progr. Svmp. Ser. 1971, 67. 47 3
4
Djebabra, D. PhD Thesis University of Lille, 1990 Lacroix, Y. ‘Analyse chimique. InterprCtation des rksultats par le calcul statistique’, Masson, Paris, 1973
FUEL, 1991, Vol 70, December
1475