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Gasification of graphite in carbon dioxide and water vapor—the catalytic effects of alkali metal salts
WX!-6223/82/010059$03.@l~0 0 1982 Pergamon Press Ltd
Carbon Vol. 20, No. I. pp. 59-66, 1982 Printed in Great Britain.
GASIFICATION OF GRAPHITE IN CARBON DIOXIDE AND WATER VAPOR-THE CATALYTIC EFFECTS OF ALKALI METAL SALTS D. W. MCKEE Corporate Research and Development, General Electric Company, Schenectady, NY 12301,U.S.A. (Received 24 August 1981) Abstract-The catalytic effects of a series of alkali metal salts in promoting the gasification of a graphite powder by carbon dioxide and water vapor have been studied by thermogravimetry between 700 and 1100°C.Lithium salts, specifically the carbonate and hydroxide, were the most active catalysts for both reactions. Cyclic processes which may account for the observed catalytic effects were evaluated from the standpoint of thermodynamic feasibility.
were Linde Instrument Grade carbon dioxide, Linde Ultra High Purity Grade helium and mixtures of carbon dioxide with 10, 20 and 5Ovol.% CO and helium with 5 and 2Ovol.% CO. All gases were dried by passage through a Drierite desiccant tower before use. Measurements of the kinetics of gasification of the doped graphite samples in both CO, (1 atm, 0.1 MPa) and in water vapor (23 mm, 3.1 kPa HZ0 in a helium stream) were carried out in the Mettler Thermoanalyzer-2 automatically recording balance previously described [7]. Usually the balance was operated in the isothermal mode with weight changes being recorded as a function of time with the furnace set at a series of temperatures in the range 700-1100°C. The experiments in CO2 were carried out with a gas flow rate of 400 ml min-‘, whereas the experiments with water vapor were performed by saturating a 200 ml mitt-’ helium stream with water vapor at 25°C by passage through a fritted disk bubbler containing distilled water. At each temperature setting, gasification was continued for at least 15 min to ensure that steady state kinetics were attained. However, in all cases, the total weight loss of the sample was kept below lo%, to minimize the effects of changing surface area and catalyst concentration during each experiment. Some thermogravimetric measurements were made by heating salt-graphite mixtures in the thermobalance in various gaseous atmospheres (He, CO*, H20 vapor), using a linearly increasing temperature rate of 10°Cmin-‘. Thermodynamic calculations were carried out using free energy data obtained from the JANAF tables [8].
1.INTRODUCTION The reactions of carbon with carbon dioxide and steam are of great importance in the coal gasifier as these
reactions often determine the overall kinetics of the process for converting coal to syngas. For over 100 years it has been known that salts of the alkali and alkaline earth metals are active catalysts for these reactionsHI. However, in spite of intense effort in the past few years, there is still no generally accepted explanation for this catalytic effect. Currently there are a number of processes under development which utilize the catalytic properties of the Group IA alkali metal carbonates to improve the reactivity of coal in the gasifier. Typical is the Exxon process [2] in which lO-15% potassium carbonate is added to the coal to catalyze the reaction with steam at about 700°C. Recent studies of the catalyzed gasification reactions of graphite [3,4] and coal char [5] have suggested that the observed effects of salt additives can be interpreted in terms of sequences of elementary reactions which involve interactions of the catalyst particles with the carbonaceous substrate and the ambient gaseous environment. A tentative mechanism for the catalytic effect of the alkali metal salts has been proposed by the author [6]. This paper describes the results of further investigation into the behavior of the salts of the alkali metals as catalysts for the gasification of pure graphite by carbon dioxide and water vapor. Possible mechanisms for the observed catalytic effects are discussed from a thermodynamic viewpoint. 2.EXPERIMENTAL
3. RESULTS
The substrate used in the catalyzed gasification experiments was spectroscopic grade graphite powder (Ultra Carbon Corp., Type UCP-2, -325 mesh) containing less than Sppm Si and Mg impurities. The salts investigated as catalysts included Fisher ACS Certified or Baker Analyzed Reagent Grade Li2C03, Na,CO,, KXO,, LiOH, KzS04, KNOs, K&OS, KC1 and K3P04. Salt samples were finely ground in an agate mortar and weighed amounts were then intimately mixed with the graphite powder in a Fisher Minimill to give an initial concentration of 5 wt.% of each additive. The gases used
Gasificationrates in 1 atm (0.1 MPa) CO* of 200 mg samples of graphite powder doped with 5 wt.% of the alkali metal carbonates are shown in Table 1 for three typical temperatures. Although the Na and K sajts were closely similar in behavior, L&CO3 was consistently more active. Five per cent of this latter salt increased the gasification rate of the graphite by a factor of about lo4 at 900°C. Figures 1 and 2 show Arrhenius plots (rates vs l/Z’) for the graphite-CO1 reaction for samples of pure graphite 59
60
D. W. MCKEE
1
Table 1. Alkali metal carbonates as catalysts in the graphite-CO* reactions
33
Salt (5%)
Gasification
Rate mdmin
(200 mp.
_8@~
cot
samples) e”c
__
0.00018
Li2CO
0.0062
0.130
1.40
K2C03
3
O.M20
0.033
0.55
Na2C03
0.0018
0.025
0.32
-1
and graphite doped with 5% Li2C03 (Fig. 1) and 5% K&O3 (Fig. 2). The effects of CO added to the COZ gas stream at several different concentrations are also shown. In addition to enormously increasing the rate of the reaction, the effect of the added salt was to decrease the apparent activation energy from 412 kJ/mol for pure graphite to 239 kJ/mol for graphite-5% L&CO3 and to 293 kJ/mol for graphite-5% K2C03. CO had an inhibiting effect both on the catalyzed and on the uncatalyzed reaction, the retardation being most marked at the lower temperatures. Thus 20% CO in the gas stream decreased the rate of reaction on pure graphite at 1000°C by a factor of about 10 and increased the apparent activation energy from 412 to 452 kJ/mol. The effects of CO on the salt-catalyzed reaction was only slight at 900°C and above in the case of L&CO, but was more pronounced at lower temperatures. 2O%CO added to the gas phase decreased the K2COj-catalyzed reaction rate by a factor
I 1000
000 *c 900 2OOmqGRAPHITE IGI- 5%h2CO3 (A) CO2
IO
t ‘,
70
1.5
90
85 IIT’K
90
95
IO<
X IO’
Fig. 2. Effects of addition of 5% K&O, on the gasification rates of graphite (G) in CO* and COs-CO mixtures. Weight loss rates vs l/T K in 1atm (0.1 MPa) flowing gas at 400ml min-‘.
of about five at 900°C. In the case of the Li&Oscatalyzed reaction, the effect of added CO was to increase the apparent activation energy, whereas with the K&O3 catalyst, the pre-exponential factor was most affected. The reason for this difference is not clear. The relative catalytic activities of a series of potassium salts (added at 5% concentration) on the graphite-CO* reaction are compared in Table 2 at three typical temperatures. A strong anion effect was found with salts of oxyacids, such as carbonate, sulfate, and nitrate, being more effective as catalysts than silicates or halides. When graphite powder and K&O3 are heated together in an inert atmosphere, a solid state reaction occurs at elevated temperatures with the formation of potassium vapor and CO[3], as a result of the reaction,
I
I
ir i
010
E
“c 2 z
0
s E
K&O3 + 2C = 2K + 3C0.
001
%
0 001
I
00001 IIT
‘K
X
IO’
Fig. 1. Effects of additionof 5% LizCOJon the gasification rates of graphite (G) in CO2 and CO*-CO mixtures. Weight loss rates vs I/ 7’K in 1 atm (0.1 MPa) flowing gas at 400 ml min-‘.
This reaction may play an important role in the K&O,catalyzed gasification reaction with COZ or other oxidants. It was of interest, therefore, to compare the rate of this reaction with that of the salt-catalyzed graphiteCOZ reaction over the same temperature range. Figure 3 shows, as functions of l/T, (A) the rate of weight loss from a 1: 1 graphite-K&OX mixture on heating in pure dry helium at a series of temperatures between 700 and 95O”C,(B) the rate of weight loss as a result of vaporization of the salt in the helium steam, and (C) the gasification rate of a graphite-5% K*CO:, sample in CO*. If correction is made for the loss of potassium vapor in curve (A), the rate of weight loss due to CO formation
61
Gasificationof graphite in carbon dioxide and water vapor
Table 2. Potassium salts as catalysts for the graphite-CO2 the two series of reactions. The relative activities of a reaction series of potassium salts (added at 5% concentration) for the catalyzed gasification of graphite powder in water Gasification Rate mllmin (200 mn samples), Salt (5%) vapor (23 mm, 3.1 kPa) are shown in Table 3 at three y&c gas -700% typical temperatures. The most active salts were again the nitrate, sulfate and carbonate with the phosphate and 0.0018 __ silicate being quite inert. KC1 was noticeably more active 0.55 0.033 K2C03 0.0012 in this reaction than with COz, possible because a small amount of KOH was formed by hydrolysis in the 0.28 0.025 KZSOB 0.0016 presence of water vapor. The identity of the active 0.25 0.018 KN03 0.009 species responsible for the catalytic activity was prob0.082 0.0065 K2Si03 0.0003 ably different for the two gasification reactions. Thus, Table 4 summarizes rate data obtained in water vapor 0.032 0.0006 KC1 __ and in CO* when either 5%Li&03 or LiOH was added to the graphite initially. Even though gasification rates were consistently higher in water vapor than in CO*, from the salt-graphite reaction between 700 and 950°C both salts behaved similarly in the two gaseous environments. As discussed below, it is likely that LiOH was consistent with that of the salt-catalyzed graphitewas converted to L&COS in the presence of CO?, and COZ reaction, curve (C), even though the amount of salt LiXOS was hydrolyzed to LiOH in the presence of HzO. present was quite different in the two cases. Because of The behavior of K&O3 and the effect of added CO on the low surface area of the graphite, catalyzed the rates of the catalyzed graphite-H20 reaction are gasification rates were insensitive to salt concentration illustrated by the Arrhenius plots shown in Fig. 4. As when the latter exceeded about 2 wt.%. Thus, the gasification rate of a 1: 1 graphite-KzCOj mixture in COz before, addition of CO to the gas stream resulted in a marked decrease in overall rate for both the catalyzed was almost identical to that of the ~aphite-5%K~CO~ and the uncatalyzed reactions. Thus at 8OO”C,rates sample shown in Fig. 3 (curve C). The alkali salts which are efl’ective catalysts for the were decreased by about an order of magnitude when 20% CO was added to the gas. graphite-CO2 reaction were also very active catalysts for Thermogravimetric curves (weight changes vs temthe graphite-H,0 reaction, although there were differences in the behavior of the individual catalysts in perature) for pure L&CO3 and graphite-L&CO3 mixtures on heating at a rate of 10°Cmin-’ in various gaseous atmospheres are shown in Fig. 5. A variety of reactions were possible depending on the composition of the solid and gas phases. Thus, pure Li2C03, on heating in pure dry helium, lost weight appreciably above the melting point (723”C), as a result of both vaporization and dissociation, L&CO, = L&O t COz. At 8OO”C,the equilibrium dissociation pressure of molten L&CO, is approximately 110mm CO,[9]. This decomposition was suppressed in the presence of 1atm CO? and the small weight loss observed in this case was presumably due to vaporization of the salt. The weight loss Table 3. Potassium salts as catalysts for the graphite-H,0 reaction Salt (5%)
Gasification 700°C --
-_
Fig. 3. Comparison of the weight loss rates vs l/T K for (A) 200mg of a 1: 1 graphite-K,C03 mixture heated in helium, (8) 100mg K&O3 heated in helium, and (C) 200mg of a graphite-5% K&O3 mixture heated in COz.
-_
Rate
mdmin
F@c
0.0014
(200
rn~ sampled
?oO*g
0.011
KNOj
0.010
0.090
0.65
Kz%
0.0057
0.058
cl.44
KZC03
0.0070
0.070
0.55
KC1
0.0013
0.022
0.23
K3PO4
0.0004
0.0070
0.070
0.0020
0.015
K*S103
-_
D. W. MCKEE Table 4. Lithium salts as catalysts in graphite-CO* and graphite-H*0 reactions Gasification Rate ma/min (200 mg. samrdes)
Salt 13%)
H20 (23 mm, 3.1 kPd
CO2 fl atm. O.lMPa) #Oys
z!%
800°C _-
mos
30s
Li2C03
0.041
0.47
1.0
0.0060
0.13
1.52
LiOH
0.047
0.51
I.5
0.0045
0.10
1.42
of the graphite-L&CO3 mixture in COZ, which became rapid above 9OO”C,was due almost entirely to the saltcatalyzed gasification of the graphite. However, the graphite-Li&!O, mixture on heating in helium lost weight rapidly because of salt dissociation, vapori~tion, and the occurrence of the reactions, LXO, C 2C = 2Li t 3C0 and Li,C!OS+ C = Liz0 t 2C0. L&CO3 was appreciably hydrolyzed at elevated temperatures,
combined effects of this hydrolysis, vaporization of the salts, and catalyzed gasification of the graphite. The thermodynamics of these reactions are discussed below. Thermogravimetric curves are shown in Fig. 6 for pure LiOH (IOmg), pure graphite (200 mg) and a graphite (200 mg)-5% LiOH (10 mg) mixture on heating at a rate of lO”/min in pure dry helium. The LiOH began to lose weight at 4OO”C, reaching a plateau at MO-750°C. Complete decomposition of the remaining hydroxide to oxide occurred between 800 and 850°C and at higher temperatures there was no further change in weight. The expected weight losses for several possible reactions are indicated by the horizontal lines on the vertical axis of Fig. 6. The measured weight loss agreed well with that expected for the dehydration reaction, 2 LiOH = L&O t Hz0
LiZCOst 2Hz0 = 2LiOH t CO*, and the graphite-L&CO3 mixture lost weight rapidly above 800°C when heated in water vapor due to the
Id00
y&g
(Calcd. wt. loss - 3.8 mg).
The graphite-5% LiOH mixture showed similar behavior at low temperatures, however, above 750°C weight losses became pronounced because of the occurrence of the reactions,
8;rO’C
3bo ---
GRAPHITE,
200~19
-
GRAPHITE-
S%K&O+
2LiOH + C = 2Li t CO + HZ0 Calcd. wt. loss - 9.6 mg)
200mg
(A) P~~o=O.03 &I)
P”,o’oo3
Pc#y 0.05
(Cl
P “to = 0.03
PC** 020
(PRESSURES
-
IN ATM *O.IYPo
2LiOH t C = L&O f CO -t Hz (Calcd. wt. loss - 6.3 mg) or Liz0 + C = 2Li t CO.
f
20
s g ” Y
I5
-’\CCl ‘V8l I 75
I 8.5
I 80 IIT’K
I 90
I 95
IO 5
I
0 600
100
X 104
Fig. 4. Effects of addition of 5% K&OS on the gasification rates of graphite (G) in Hz0 vapor and H&CO mixtures. Weight loss rates vs 1ITK.
700
Boa TEYPERATURE
900
1000
‘C
5. Thermo~a~e~c weight loss vs temperature curves for LizCOs (~~rng) and 200mg of a ~aphit&Li*C~ 1: 1 mixture in helium, COz, and water vapor. Heating rate = 10°Cmin-‘.
Fig.
Gasification of graphite in carbon dioxide and water vapor
I
4
I
tAW.9.6mp)
1
PLiOH*C=Li~O+CO*ti~ DRY HELIIJY AT/At a IO*
r5 w4
s
2LiOtl:
400ml min-’
min+
Liz0
5 03 z !a
-i2 0
200
800
400 TEtE7RAT”RE
1000
‘C
Fii. 6. Thermo~avime~ic weight loss vs temperature curves for LiOH (IOmg) and 200mg of a ~aph~te-5%LiOH mixture in helium. Heating rate = 10°Cmin-‘.
In a helium stream with low partial pressures of CO, Hz and H20, both these reactions would be expected to occur at elevated temperat~es, as discussed below. 4. DI!3ClJSSION A plausible mechanism for the KtCOp-catalyzed CCO2 gasification reaction is the sequence, K&O3 + 2C = 2K + 3C0
(1)
2K+COz=K,O+CO
(2)
KzO + CO* = K&O3
(3)
Figure 7 shows the equilib~um stability regions of K*CO, and K(g), calculated assuming the occurrence of reaction (l), as functions of PK, PC0 and temperature. The diagram was derived using the free energy relation, AG = AG”+ RT In P&P5*. LOG -8
An overall gasification rate of 1 mg mitt-’ (about the maximum attained in this study), would give an ambient partial pressure of CO of about 10e3atm (O,l kPa). At this value of PC0 and with a similar value of Pk, Fig. 7 indicates that reaction (1) would be thermodynamically possible for all temperatures above about 700°C. This reaction, while is probably the rate determining step in the overall gasification process in the case of the K2C03-(Fig. 3) and the Na~CO~-ca~yzed C-CO, reaction, is inhibited by increasing amounts of CO in the gas phase (Fig. 2). Reaction (1) is less favorable in the case of Li&O, as indicated in Fig. 8, which shows the calculated equilibrium regions of Li,CO, [ 1] and Li [ 1] in the presence of carbon, as a function of temperature and PC0 fatm). At 7OO”C,the partial pressure of CO in the system would have to be below 10-‘atm (lo-*Pa) to permit the formation of Li metal. However, this reaction may occur to some extent even with L&CO3above about 900°C. Reactions (2) and (3) above are very favorable for Na and K, the reactions having large negative free energies at gasification temperatures. Hence (2) and (3) probably takes place to reform the carbonate phase subsequent to the formation of alkali metal vapor by reaction (1). In the case of the Li&O, catalyst, reaction with the graphite probably proceeds only as far as the formation of monoxide, M&O, t C = MzO + 2C0 MzO t COz = M&O,.
&A) (3)
.
c+co,=2co
-10
63
Equilibrium stability regions of MzO and MzCOrr, assuming the occurrence of these two reactions for the three alkali metals, are shown in Fig. 9, as functions of temperature, PC0 and Pco2 (in atm). These curves were calculated assuming unit activity of the various phases and mutual solubility between oxide and carbonate would broaden the stability boundaries. The solid curves
PK
-6
-4
-2
0
0
-6
P 3
-8 K2CO3
t-
+ 26*2K
+ SC0
-l -10
Fig. 7. Equ~ibrium stability regions of K and K.&OS for the reaction, K&O3 t 2C = 2K + 3C0, as functions of Pco. PK (in atm), and temperature.
Fig.8. Equ~ib~~ stability regions of Li (1)and U&OS(1)for the reaction, L&CO3 + 2C= 2Li + 3C0, as functions of PC0(in atm) and temperature.
D. W.
MCKEE
reaction sequence, rather than by the physical state of the catalyst particles. Other group IA metal salts, such as nitrates and sulfates, which decompose to form alkali monoxides on heating, are also capable of participating in cyclic processes as listed above and are active catalysts for the C-CO, reaction, whereas more stable salts such as halides, silicates and phosphates are inactive (Table 2). In the case of the C-H20 reaction, a plausible sequence of steps which might be involved in the presence of an alkali carbonate catalyst is, M2COF+ 2c = 2M + 3co
(1)
2M + 2H20 = 2MOH + Hz
(4)
2MOH t CO = M&OS t Hz
(5)
C+H,O=COtH)
/
/
Fig. 9. Equilibrium stability regions of MzO and M&O3 corresponding to the reactions, M&O3 t C = M20 + 2C0 (-) and M2C03= MzOt CO,(-----) for Li, Na, and K species, as functions of Pco. Pco2 (in atm), and temperature.
in Fig. 9 indicate that reduction of carbonate to oxide by reaction (1A) is favorable for Li over a wide range of PC0 and temperature, but less favorable for Na and K. On the other hand, the dashed curves show that reaction (3) is likely with PcoZ = 1 atm (0.1 MPa) for all three alkali metals. Thus, a cyclic process based on these two reactions is possible for the Li catalyst in the presence of 1 atmCO2 and a low partial pressure of CO, the rate determining step probably being reaction (1A). This reaction and therefore the overall gasification rate will be inhibited by increasing concentrations of CO in the gas, as found experimentally (Fig. 1). It is also possible that the physical state of the catalyst may influence the relative catalytic activity of the alkali carbonates. Thus, at gasification temperatures, Li2COs (m.p. 72O“C)will be in the liquid state and will tend to spread over the graphite surface, facilitating reaction between the salt phase and the graphite substrate. On the other hand, particles of KzC03 (m.p. 910°C) would remain in the solid state and reaction would be limited to regions of contact between the two solid phases. However, on this basis, NazC03 (m.p. 815°C) would be expected to be more active than K&O, at, for example, 900°C whereas in fact (Table 1) the reverse is the case. Reaction (1) above is somewhat more favorable for K&O:, than for Na2C0, and hence it seems more likely that the relative activities of these salts are determined by the ease of occurrence of the elementary steps in the
As discussed above, reaction (1) is thermodynamicalIy favorable for Na and K and is probably the rate determining step for gasification catalyzed by these salts. The catalyzed reactions are inhibited by increasing concentrations of CO in the gas (Fig. 4), as expected. Both reactions (4) and (5) have large negative free energies and are likely to occur readily at temperatures in the gasification range. In the case of Li and to a lesser extent with the other alkali metals, hydrolysis of the carbonate to hydroxide is likely in the presence of water vapor. A possible sequence of reaction steps is then, MTCOXt HZ0 = 2MOH t CO*
(6)
2MOHtC=M,OtCO+H,
(7)
M20 t Hz0 = 2MOH.
03)
As indicated by the equilibrium stability dia~am, Fig. 10, for reaction (6) in the case of Li, in I atm (0.1 MPa) CO,,
LOGPH,O 0 -II
-10 < 'LqCO3
t
It20 :ZLiOH
+ CO2
1300K /!
LiOH
Fig. 10. Equilibriumstabilityregionsof LiOH and Li2C03for the reaction, Li$O, t HZ0= 2LiOH+ CO*, as functions of PHZO, Pco, (in ah-n)and temperature.
65
Gasification of graphite in carbon dioxide and water vapor
the stable species expected will be carbonate, whereas with low values of Pco2 and PHZO= 23 mm (3.1 kPa) as in the present series of experiments, the dominant species will be LiOH. These expections were confirmed by the results shown in Table 4 which indicate that the Li&O, additive behaved like LiOH in the presence of water vapor. The equilibrium stability diagram in Fig. 11 indicates further that LiOH can be reduced to metal at 1100K only at low values of PC0 and PH20, whereas as shown in Fig. 12, reduction to Lip0 via reaction (7) is possible over a wide range of temperature, PC0 and PHI. In a related study[lO] of the graphite-H*0 reaction, catalyzed by C”-labelled BaC03, Yates and the author found that both C’* and Cl3 gaseous species (CO and CO*) were evolved during gasification of the graphite, indicating that chemical participation of the salts in the catalyzed gasification reaction was indeed taking place. There was also evidence that the graphite promoted the decomposition of the salt, with evolution of C1302, at temperatures as low as 600°C. In the present thermogravimetric study, the reaction between K2C03 and graphite was detectable only at temperatures above above 750°C (Fig. 3). However, using temperature programmed desorption, Freriks et al. [l l] have recently found that carbons derived from polyfurfuryl alcohol and impregnated with K&O, evolve CO2 at temperatures around 700°C. Also, Mims and Pabst have reported that coal char catalyzes the decomposition of K2Ci40,, with the formation of C’402, at temperatures as low as 500°C[12]. Caution should be used in interpreting these latter results, however, as coal contains a number of mineral constituents, specifically clays, and quartz, which can react with K,CO, to yield CO2 at low temperatures. These isotopic tracer experiments should therefore be repeated with a pure carbon substrate. However, there is a definite suggestion that alkali carbonates can decompose to oxide and CO, at quite low temperatures in the presence of carbon. This process, if
LOG PHzO 23mm (31 kPa)
LOG P,,
-6
-
8
-8
ZLIOH + C: LI?O + CO + Hz ---IO
I
I
I
I
I
Fig. 12. Equilibrium stability regions of L&O and LiOH for the reaction, 2LiOH + C = Liz0 t CO t Hz, as functions of PHZ,Pco (in atm), and temperature.
it occurs, would be expected to be followed by the formation of alkali metal, as the reaction KzO+C=2K+CO has a negative free energy change above 800°C. A cyclic sequence of reactions could then be established during the catalyzed gasification reaction, as outlined above. It has also been proposed [ 131,that alkali metal formed by reaction of the K&O, with the graphite might form an intercalation compound and thereby enhance the reactivity of the carbon substrate. However, alkali metal intercalation compounds are stable only at fairly low temperatures, and there is at present no direct evidence that they exist under gasification conditions. Also, sodium does not readily form intercalation compounds with graphite even at low temperatures, whereas sodium salts are quite active catalysts for graphite gasification. It is possible that the ready migration of alkali metal between the basal planes of the graphite at elevated temperatures may help to promote the dispersion of the active catalyst throughout the carbon matrix [ 141. 5.CONCLUSIONS
The experimental observation that oxysalts of the alkali metals are effective catalysts in the reaction of graphite with carbon dioxide and water vapor is explicable on the basis of the participation of the catalyst in a cyclic series of elementary reactions. A likely sequence of steps involves reaction of the salt with the graphite to form alkali metal or oxide, followed by reaction of this active intermediate species with the constituents of the gas phase. The details of the catalytic process depend on the temperature, the salts present, and the nature of the oxidizing gas. ELiOH(fl+
C(s) = 2LiIP)
+ COlgl+
Ii 20(g)
Fig. 11. Equilibrium stability regions of Li and LiOH at 1100and 1300K for the reaction, 2 LiOH + C = 2Li + CO + Hz, as functions of Pco and PH20(in atm). Car Vol. 20. No. I-E
REFERENCES
1. T. du Motay, Br. Pat. 2548(1867). 2. J. E. Gallagher, Jr. and C. A. Euker, Jr., Energy Rex 4, 137 (1980). 3. D. W. McKee and D. Chatterji, Carbon 16, 53 (1978).
66
D. W. MCKEE
4. D. W. McKee, Fuel 59, 308 (1980). 5. M. J. Veraa and A. T. Bell, Fuel 57, 194(1978). 6. D: W. McKee, Chemistry and Physics of Carbon (Edited by P. L. Walker, Jr. and P. A. Thrower). Vol. 16. D. 1. Marcel Dekker, New York (1981). ” _ 7. D. W. McKee and D. Chatterji, Carbon 13,381 (1975). 8. JANAF Thermochemical Tables, 2nd Edn, NSRDS-NBS 37 (1971). 9. G. J. Janz, Molten Salt Handbook, p. 77. Academic Press, New York (1967).
10. D. W. McKee and J. T. Yates, Jr., J. Catalysis, 71, 308 (1981). 11. I. L. C. Freriks, H. M. H. van Wechem, J. C. M. Stuiver and R. Bouwman, Fuel 60,463 (1981). 12. C. A. Mims and J. K. Pabst,‘Predrints ACS Div., Fuel. Chem. 25(3), 263 (1980). 13. W.-Y. Wen, Catal. Reu. Sci. Engng 22(l), l(l980). 14. H. Marsh and A. Wilkinson, Paper CR-26, presented at 15th Biennial Carbon Conf. Philadelphia, PA, June 1981.
Carbon Vol. 20, No. I. pp. 59-66, 1982 Printed in Great Britain.
GASIFICATION OF GRAPHITE IN CARBON DIOXIDE AND WATER VAPOR-THE CATALYTIC EFFECTS OF ALKALI METAL SALTS D. W. MCKEE Corporate Research and Development, General Electric Company, Schenectady, NY 12301,U.S.A. (Received 24 August 1981) Abstract-The catalytic effects of a series of alkali metal salts in promoting the gasification of a graphite powder by carbon dioxide and water vapor have been studied by thermogravimetry between 700 and 1100°C.Lithium salts, specifically the carbonate and hydroxide, were the most active catalysts for both reactions. Cyclic processes which may account for the observed catalytic effects were evaluated from the standpoint of thermodynamic feasibility.
were Linde Instrument Grade carbon dioxide, Linde Ultra High Purity Grade helium and mixtures of carbon dioxide with 10, 20 and 5Ovol.% CO and helium with 5 and 2Ovol.% CO. All gases were dried by passage through a Drierite desiccant tower before use. Measurements of the kinetics of gasification of the doped graphite samples in both CO, (1 atm, 0.1 MPa) and in water vapor (23 mm, 3.1 kPa HZ0 in a helium stream) were carried out in the Mettler Thermoanalyzer-2 automatically recording balance previously described [7]. Usually the balance was operated in the isothermal mode with weight changes being recorded as a function of time with the furnace set at a series of temperatures in the range 700-1100°C. The experiments in CO2 were carried out with a gas flow rate of 400 ml min-‘, whereas the experiments with water vapor were performed by saturating a 200 ml mitt-’ helium stream with water vapor at 25°C by passage through a fritted disk bubbler containing distilled water. At each temperature setting, gasification was continued for at least 15 min to ensure that steady state kinetics were attained. However, in all cases, the total weight loss of the sample was kept below lo%, to minimize the effects of changing surface area and catalyst concentration during each experiment. Some thermogravimetric measurements were made by heating salt-graphite mixtures in the thermobalance in various gaseous atmospheres (He, CO*, H20 vapor), using a linearly increasing temperature rate of 10°Cmin-‘. Thermodynamic calculations were carried out using free energy data obtained from the JANAF tables [8].
1.INTRODUCTION The reactions of carbon with carbon dioxide and steam are of great importance in the coal gasifier as these
reactions often determine the overall kinetics of the process for converting coal to syngas. For over 100 years it has been known that salts of the alkali and alkaline earth metals are active catalysts for these reactionsHI. However, in spite of intense effort in the past few years, there is still no generally accepted explanation for this catalytic effect. Currently there are a number of processes under development which utilize the catalytic properties of the Group IA alkali metal carbonates to improve the reactivity of coal in the gasifier. Typical is the Exxon process [2] in which lO-15% potassium carbonate is added to the coal to catalyze the reaction with steam at about 700°C. Recent studies of the catalyzed gasification reactions of graphite [3,4] and coal char [5] have suggested that the observed effects of salt additives can be interpreted in terms of sequences of elementary reactions which involve interactions of the catalyst particles with the carbonaceous substrate and the ambient gaseous environment. A tentative mechanism for the catalytic effect of the alkali metal salts has been proposed by the author [6]. This paper describes the results of further investigation into the behavior of the salts of the alkali metals as catalysts for the gasification of pure graphite by carbon dioxide and water vapor. Possible mechanisms for the observed catalytic effects are discussed from a thermodynamic viewpoint. 2.EXPERIMENTAL
3. RESULTS
The substrate used in the catalyzed gasification experiments was spectroscopic grade graphite powder (Ultra Carbon Corp., Type UCP-2, -325 mesh) containing less than Sppm Si and Mg impurities. The salts investigated as catalysts included Fisher ACS Certified or Baker Analyzed Reagent Grade Li2C03, Na,CO,, KXO,, LiOH, KzS04, KNOs, K&OS, KC1 and K3P04. Salt samples were finely ground in an agate mortar and weighed amounts were then intimately mixed with the graphite powder in a Fisher Minimill to give an initial concentration of 5 wt.% of each additive. The gases used
Gasificationrates in 1 atm (0.1 MPa) CO* of 200 mg samples of graphite powder doped with 5 wt.% of the alkali metal carbonates are shown in Table 1 for three typical temperatures. Although the Na and K sajts were closely similar in behavior, L&CO3 was consistently more active. Five per cent of this latter salt increased the gasification rate of the graphite by a factor of about lo4 at 900°C. Figures 1 and 2 show Arrhenius plots (rates vs l/Z’) for the graphite-CO1 reaction for samples of pure graphite 59
60
D. W. MCKEE
1
Table 1. Alkali metal carbonates as catalysts in the graphite-CO* reactions
33
Salt (5%)
Gasification
Rate mdmin
(200 mp.
_8@~
cot
samples) e”c
__
0.00018
Li2CO
0.0062
0.130
1.40
K2C03
3
O.M20
0.033
0.55
Na2C03
0.0018
0.025
0.32
-1
and graphite doped with 5% Li2C03 (Fig. 1) and 5% K&O3 (Fig. 2). The effects of CO added to the COZ gas stream at several different concentrations are also shown. In addition to enormously increasing the rate of the reaction, the effect of the added salt was to decrease the apparent activation energy from 412 kJ/mol for pure graphite to 239 kJ/mol for graphite-5% L&CO3 and to 293 kJ/mol for graphite-5% K2C03. CO had an inhibiting effect both on the catalyzed and on the uncatalyzed reaction, the retardation being most marked at the lower temperatures. Thus 20% CO in the gas stream decreased the rate of reaction on pure graphite at 1000°C by a factor of about 10 and increased the apparent activation energy from 412 to 452 kJ/mol. The effects of CO on the salt-catalyzed reaction was only slight at 900°C and above in the case of L&CO, but was more pronounced at lower temperatures. 2O%CO added to the gas phase decreased the K2COj-catalyzed reaction rate by a factor
I 1000
000 *c 900 2OOmqGRAPHITE IGI- 5%h2CO3 (A) CO2
IO
t ‘,
70
1.5
90
85 IIT’K
90
95
IO<
X IO’
Fig. 2. Effects of addition of 5% K&O, on the gasification rates of graphite (G) in CO* and COs-CO mixtures. Weight loss rates vs l/T K in 1atm (0.1 MPa) flowing gas at 400ml min-‘.
of about five at 900°C. In the case of the Li&Oscatalyzed reaction, the effect of added CO was to increase the apparent activation energy, whereas with the K&O3 catalyst, the pre-exponential factor was most affected. The reason for this difference is not clear. The relative catalytic activities of a series of potassium salts (added at 5% concentration) on the graphite-CO* reaction are compared in Table 2 at three typical temperatures. A strong anion effect was found with salts of oxyacids, such as carbonate, sulfate, and nitrate, being more effective as catalysts than silicates or halides. When graphite powder and K&O3 are heated together in an inert atmosphere, a solid state reaction occurs at elevated temperatures with the formation of potassium vapor and CO[3], as a result of the reaction,
I
I
ir i
010
E
“c 2 z
0
s E
K&O3 + 2C = 2K + 3C0.
001
%
0 001
I
00001 IIT
‘K
X
IO’
Fig. 1. Effects of additionof 5% LizCOJon the gasification rates of graphite (G) in CO2 and CO*-CO mixtures. Weight loss rates vs I/ 7’K in 1 atm (0.1 MPa) flowing gas at 400 ml min-‘.
This reaction may play an important role in the K&O,catalyzed gasification reaction with COZ or other oxidants. It was of interest, therefore, to compare the rate of this reaction with that of the salt-catalyzed graphiteCOZ reaction over the same temperature range. Figure 3 shows, as functions of l/T, (A) the rate of weight loss from a 1: 1 graphite-K&OX mixture on heating in pure dry helium at a series of temperatures between 700 and 95O”C,(B) the rate of weight loss as a result of vaporization of the salt in the helium steam, and (C) the gasification rate of a graphite-5% K*CO:, sample in CO*. If correction is made for the loss of potassium vapor in curve (A), the rate of weight loss due to CO formation
61
Gasificationof graphite in carbon dioxide and water vapor
Table 2. Potassium salts as catalysts for the graphite-CO2 the two series of reactions. The relative activities of a reaction series of potassium salts (added at 5% concentration) for the catalyzed gasification of graphite powder in water Gasification Rate mllmin (200 mn samples), Salt (5%) vapor (23 mm, 3.1 kPa) are shown in Table 3 at three y&c gas -700% typical temperatures. The most active salts were again the nitrate, sulfate and carbonate with the phosphate and 0.0018 __ silicate being quite inert. KC1 was noticeably more active 0.55 0.033 K2C03 0.0012 in this reaction than with COz, possible because a small amount of KOH was formed by hydrolysis in the 0.28 0.025 KZSOB 0.0016 presence of water vapor. The identity of the active 0.25 0.018 KN03 0.009 species responsible for the catalytic activity was prob0.082 0.0065 K2Si03 0.0003 ably different for the two gasification reactions. Thus, Table 4 summarizes rate data obtained in water vapor 0.032 0.0006 KC1 __ and in CO* when either 5%Li&03 or LiOH was added to the graphite initially. Even though gasification rates were consistently higher in water vapor than in CO*, from the salt-graphite reaction between 700 and 950°C both salts behaved similarly in the two gaseous environments. As discussed below, it is likely that LiOH was consistent with that of the salt-catalyzed graphitewas converted to L&COS in the presence of CO?, and COZ reaction, curve (C), even though the amount of salt LiXOS was hydrolyzed to LiOH in the presence of HzO. present was quite different in the two cases. Because of The behavior of K&O3 and the effect of added CO on the low surface area of the graphite, catalyzed the rates of the catalyzed graphite-H20 reaction are gasification rates were insensitive to salt concentration illustrated by the Arrhenius plots shown in Fig. 4. As when the latter exceeded about 2 wt.%. Thus, the gasification rate of a 1: 1 graphite-KzCOj mixture in COz before, addition of CO to the gas stream resulted in a marked decrease in overall rate for both the catalyzed was almost identical to that of the ~aphite-5%K~CO~ and the uncatalyzed reactions. Thus at 8OO”C,rates sample shown in Fig. 3 (curve C). The alkali salts which are efl’ective catalysts for the were decreased by about an order of magnitude when 20% CO was added to the gas. graphite-CO2 reaction were also very active catalysts for Thermogravimetric curves (weight changes vs temthe graphite-H,0 reaction, although there were differences in the behavior of the individual catalysts in perature) for pure L&CO3 and graphite-L&CO3 mixtures on heating at a rate of 10°Cmin-’ in various gaseous atmospheres are shown in Fig. 5. A variety of reactions were possible depending on the composition of the solid and gas phases. Thus, pure Li2C03, on heating in pure dry helium, lost weight appreciably above the melting point (723”C), as a result of both vaporization and dissociation, L&CO, = L&O t COz. At 8OO”C,the equilibrium dissociation pressure of molten L&CO, is approximately 110mm CO,[9]. This decomposition was suppressed in the presence of 1atm CO? and the small weight loss observed in this case was presumably due to vaporization of the salt. The weight loss Table 3. Potassium salts as catalysts for the graphite-H,0 reaction Salt (5%)
Gasification 700°C --
-_
Fig. 3. Comparison of the weight loss rates vs l/T K for (A) 200mg of a 1: 1 graphite-K,C03 mixture heated in helium, (8) 100mg K&O3 heated in helium, and (C) 200mg of a graphite-5% K&O3 mixture heated in COz.
-_
Rate
mdmin
F@c
0.0014
(200
rn~ sampled
?oO*g
0.011
KNOj
0.010
0.090
0.65
Kz%
0.0057
0.058
cl.44
KZC03
0.0070
0.070
0.55
KC1
0.0013
0.022
0.23
K3PO4
0.0004
0.0070
0.070
0.0020
0.015
K*S103
-_
D. W. MCKEE Table 4. Lithium salts as catalysts in graphite-CO* and graphite-H*0 reactions Gasification Rate ma/min (200 mg. samrdes)
Salt 13%)
H20 (23 mm, 3.1 kPd
CO2 fl atm. O.lMPa) #Oys
z!%
800°C _-
mos
30s
Li2C03
0.041
0.47
1.0
0.0060
0.13
1.52
LiOH
0.047
0.51
I.5
0.0045
0.10
1.42
of the graphite-L&CO3 mixture in COZ, which became rapid above 9OO”C,was due almost entirely to the saltcatalyzed gasification of the graphite. However, the graphite-Li&!O, mixture on heating in helium lost weight rapidly because of salt dissociation, vapori~tion, and the occurrence of the reactions, LXO, C 2C = 2Li t 3C0 and Li,C!OS+ C = Liz0 t 2C0. L&CO3 was appreciably hydrolyzed at elevated temperatures,
combined effects of this hydrolysis, vaporization of the salts, and catalyzed gasification of the graphite. The thermodynamics of these reactions are discussed below. Thermogravimetric curves are shown in Fig. 6 for pure LiOH (IOmg), pure graphite (200 mg) and a graphite (200 mg)-5% LiOH (10 mg) mixture on heating at a rate of lO”/min in pure dry helium. The LiOH began to lose weight at 4OO”C, reaching a plateau at MO-750°C. Complete decomposition of the remaining hydroxide to oxide occurred between 800 and 850°C and at higher temperatures there was no further change in weight. The expected weight losses for several possible reactions are indicated by the horizontal lines on the vertical axis of Fig. 6. The measured weight loss agreed well with that expected for the dehydration reaction, 2 LiOH = L&O t Hz0
LiZCOst 2Hz0 = 2LiOH t CO*, and the graphite-L&CO3 mixture lost weight rapidly above 800°C when heated in water vapor due to the
Id00
y&g
(Calcd. wt. loss - 3.8 mg).
The graphite-5% LiOH mixture showed similar behavior at low temperatures, however, above 750°C weight losses became pronounced because of the occurrence of the reactions,
8;rO’C
3bo ---
GRAPHITE,
200~19
-
GRAPHITE-
S%K&O+
2LiOH + C = 2Li t CO + HZ0 Calcd. wt. loss - 9.6 mg)
200mg
(A) P~~o=O.03 &I)
P”,o’oo3
Pc#y 0.05
(Cl
P “to = 0.03
PC** 020
(PRESSURES
-
IN ATM *O.IYPo
2LiOH t C = L&O f CO -t Hz (Calcd. wt. loss - 6.3 mg) or Liz0 + C = 2Li t CO.
f
20
s g ” Y
I5
-’\CCl ‘V8l I 75
I 8.5
I 80 IIT’K
I 90
I 95
IO 5
I
0 600
100
X 104
Fig. 4. Effects of addition of 5% K&OS on the gasification rates of graphite (G) in Hz0 vapor and H&CO mixtures. Weight loss rates vs 1ITK.
700
Boa TEYPERATURE
900
1000
‘C
5. Thermo~a~e~c weight loss vs temperature curves for LizCOs (~~rng) and 200mg of a ~aphit&Li*C~ 1: 1 mixture in helium, COz, and water vapor. Heating rate = 10°Cmin-‘.
Fig.
Gasification of graphite in carbon dioxide and water vapor
I
4
I
tAW.9.6mp)
1
PLiOH*C=Li~O+CO*ti~ DRY HELIIJY AT/At a IO*
r5 w4
s
2LiOtl:
400ml min-’
min+
Liz0
5 03 z !a
-i2 0
200
800
400 TEtE7RAT”RE
1000
‘C
Fii. 6. Thermo~avime~ic weight loss vs temperature curves for LiOH (IOmg) and 200mg of a ~aph~te-5%LiOH mixture in helium. Heating rate = 10°Cmin-‘.
In a helium stream with low partial pressures of CO, Hz and H20, both these reactions would be expected to occur at elevated temperat~es, as discussed below. 4. DI!3ClJSSION A plausible mechanism for the KtCOp-catalyzed CCO2 gasification reaction is the sequence, K&O3 + 2C = 2K + 3C0
(1)
2K+COz=K,O+CO
(2)
KzO + CO* = K&O3
(3)
Figure 7 shows the equilib~um stability regions of K*CO, and K(g), calculated assuming the occurrence of reaction (l), as functions of PK, PC0 and temperature. The diagram was derived using the free energy relation, AG = AG”+ RT In P&P5*. LOG -8
An overall gasification rate of 1 mg mitt-’ (about the maximum attained in this study), would give an ambient partial pressure of CO of about 10e3atm (O,l kPa). At this value of PC0 and with a similar value of Pk, Fig. 7 indicates that reaction (1) would be thermodynamically possible for all temperatures above about 700°C. This reaction, while is probably the rate determining step in the overall gasification process in the case of the K2C03-(Fig. 3) and the Na~CO~-ca~yzed C-CO, reaction, is inhibited by increasing amounts of CO in the gas phase (Fig. 2). Reaction (1) is less favorable in the case of Li&O, as indicated in Fig. 8, which shows the calculated equilibrium regions of Li,CO, [ 1] and Li [ 1] in the presence of carbon, as a function of temperature and PC0 fatm). At 7OO”C,the partial pressure of CO in the system would have to be below 10-‘atm (lo-*Pa) to permit the formation of Li metal. However, this reaction may occur to some extent even with L&CO3above about 900°C. Reactions (2) and (3) above are very favorable for Na and K, the reactions having large negative free energies at gasification temperatures. Hence (2) and (3) probably takes place to reform the carbonate phase subsequent to the formation of alkali metal vapor by reaction (1). In the case of the Li&O, catalyst, reaction with the graphite probably proceeds only as far as the formation of monoxide, M&O, t C = MzO + 2C0 MzO t COz = M&O,.
&A) (3)
.
c+co,=2co
-10
63
Equilibrium stability regions of MzO and MzCOrr, assuming the occurrence of these two reactions for the three alkali metals, are shown in Fig. 9, as functions of temperature, PC0 and Pco2 (in atm). These curves were calculated assuming unit activity of the various phases and mutual solubility between oxide and carbonate would broaden the stability boundaries. The solid curves
PK
-6
-4
-2
0
0
-6
P 3
-8 K2CO3
t-
+ 26*2K
+ SC0
-l -10
Fig. 7. Equ~ibrium stability regions of K and K.&OS for the reaction, K&O3 t 2C = 2K + 3C0, as functions of Pco. PK (in atm), and temperature.
Fig.8. Equ~ib~~ stability regions of Li (1)and U&OS(1)for the reaction, L&CO3 + 2C= 2Li + 3C0, as functions of PC0(in atm) and temperature.
D. W.
MCKEE
reaction sequence, rather than by the physical state of the catalyst particles. Other group IA metal salts, such as nitrates and sulfates, which decompose to form alkali monoxides on heating, are also capable of participating in cyclic processes as listed above and are active catalysts for the C-CO, reaction, whereas more stable salts such as halides, silicates and phosphates are inactive (Table 2). In the case of the C-H20 reaction, a plausible sequence of steps which might be involved in the presence of an alkali carbonate catalyst is, M2COF+ 2c = 2M + 3co
(1)
2M + 2H20 = 2MOH + Hz
(4)
2MOH t CO = M&OS t Hz
(5)
C+H,O=COtH)
/
/
Fig. 9. Equilibrium stability regions of MzO and M&O3 corresponding to the reactions, M&O3 t C = M20 + 2C0 (-) and M2C03= MzOt CO,(-----) for Li, Na, and K species, as functions of Pco. Pco2 (in atm), and temperature.
in Fig. 9 indicate that reduction of carbonate to oxide by reaction (1A) is favorable for Li over a wide range of PC0 and temperature, but less favorable for Na and K. On the other hand, the dashed curves show that reaction (3) is likely with PcoZ = 1 atm (0.1 MPa) for all three alkali metals. Thus, a cyclic process based on these two reactions is possible for the Li catalyst in the presence of 1 atmCO2 and a low partial pressure of CO, the rate determining step probably being reaction (1A). This reaction and therefore the overall gasification rate will be inhibited by increasing concentrations of CO in the gas, as found experimentally (Fig. 1). It is also possible that the physical state of the catalyst may influence the relative catalytic activity of the alkali carbonates. Thus, at gasification temperatures, Li2COs (m.p. 72O“C)will be in the liquid state and will tend to spread over the graphite surface, facilitating reaction between the salt phase and the graphite substrate. On the other hand, particles of KzC03 (m.p. 910°C) would remain in the solid state and reaction would be limited to regions of contact between the two solid phases. However, on this basis, NazC03 (m.p. 815°C) would be expected to be more active than K&O, at, for example, 900°C whereas in fact (Table 1) the reverse is the case. Reaction (1) above is somewhat more favorable for K&O:, than for Na2C0, and hence it seems more likely that the relative activities of these salts are determined by the ease of occurrence of the elementary steps in the
As discussed above, reaction (1) is thermodynamicalIy favorable for Na and K and is probably the rate determining step for gasification catalyzed by these salts. The catalyzed reactions are inhibited by increasing concentrations of CO in the gas (Fig. 4), as expected. Both reactions (4) and (5) have large negative free energies and are likely to occur readily at temperatures in the gasification range. In the case of Li and to a lesser extent with the other alkali metals, hydrolysis of the carbonate to hydroxide is likely in the presence of water vapor. A possible sequence of reaction steps is then, MTCOXt HZ0 = 2MOH t CO*
(6)
2MOHtC=M,OtCO+H,
(7)
M20 t Hz0 = 2MOH.
03)
As indicated by the equilibrium stability dia~am, Fig. 10, for reaction (6) in the case of Li, in I atm (0.1 MPa) CO,,
LOGPH,O 0 -II
-10 < 'LqCO3
t
It20 :ZLiOH
+ CO2
1300K /!
LiOH
Fig. 10. Equilibriumstabilityregionsof LiOH and Li2C03for the reaction, Li$O, t HZ0= 2LiOH+ CO*, as functions of PHZO, Pco, (in ah-n)and temperature.
65
Gasification of graphite in carbon dioxide and water vapor
the stable species expected will be carbonate, whereas with low values of Pco2 and PHZO= 23 mm (3.1 kPa) as in the present series of experiments, the dominant species will be LiOH. These expections were confirmed by the results shown in Table 4 which indicate that the Li&O, additive behaved like LiOH in the presence of water vapor. The equilibrium stability diagram in Fig. 11 indicates further that LiOH can be reduced to metal at 1100K only at low values of PC0 and PH20, whereas as shown in Fig. 12, reduction to Lip0 via reaction (7) is possible over a wide range of temperature, PC0 and PHI. In a related study[lO] of the graphite-H*0 reaction, catalyzed by C”-labelled BaC03, Yates and the author found that both C’* and Cl3 gaseous species (CO and CO*) were evolved during gasification of the graphite, indicating that chemical participation of the salts in the catalyzed gasification reaction was indeed taking place. There was also evidence that the graphite promoted the decomposition of the salt, with evolution of C1302, at temperatures as low as 600°C. In the present thermogravimetric study, the reaction between K2C03 and graphite was detectable only at temperatures above above 750°C (Fig. 3). However, using temperature programmed desorption, Freriks et al. [l l] have recently found that carbons derived from polyfurfuryl alcohol and impregnated with K&O, evolve CO2 at temperatures around 700°C. Also, Mims and Pabst have reported that coal char catalyzes the decomposition of K2Ci40,, with the formation of C’402, at temperatures as low as 500°C[12]. Caution should be used in interpreting these latter results, however, as coal contains a number of mineral constituents, specifically clays, and quartz, which can react with K,CO, to yield CO2 at low temperatures. These isotopic tracer experiments should therefore be repeated with a pure carbon substrate. However, there is a definite suggestion that alkali carbonates can decompose to oxide and CO, at quite low temperatures in the presence of carbon. This process, if
LOG PHzO 23mm (31 kPa)
LOG P,,
-6
-
8
-8
ZLIOH + C: LI?O + CO + Hz ---IO
I
I
I
I
I
Fig. 12. Equilibrium stability regions of L&O and LiOH for the reaction, 2LiOH + C = Liz0 t CO t Hz, as functions of PHZ,Pco (in atm), and temperature.
it occurs, would be expected to be followed by the formation of alkali metal, as the reaction KzO+C=2K+CO has a negative free energy change above 800°C. A cyclic sequence of reactions could then be established during the catalyzed gasification reaction, as outlined above. It has also been proposed [ 131,that alkali metal formed by reaction of the K&O, with the graphite might form an intercalation compound and thereby enhance the reactivity of the carbon substrate. However, alkali metal intercalation compounds are stable only at fairly low temperatures, and there is at present no direct evidence that they exist under gasification conditions. Also, sodium does not readily form intercalation compounds with graphite even at low temperatures, whereas sodium salts are quite active catalysts for graphite gasification. It is possible that the ready migration of alkali metal between the basal planes of the graphite at elevated temperatures may help to promote the dispersion of the active catalyst throughout the carbon matrix [ 141. 5.CONCLUSIONS
The experimental observation that oxysalts of the alkali metals are effective catalysts in the reaction of graphite with carbon dioxide and water vapor is explicable on the basis of the participation of the catalyst in a cyclic series of elementary reactions. A likely sequence of steps involves reaction of the salt with the graphite to form alkali metal or oxide, followed by reaction of this active intermediate species with the constituents of the gas phase. The details of the catalytic process depend on the temperature, the salts present, and the nature of the oxidizing gas. ELiOH(fl+
C(s) = 2LiIP)
+ COlgl+
Ii 20(g)
Fig. 11. Equilibrium stability regions of Li and LiOH at 1100and 1300K for the reaction, 2 LiOH + C = 2Li + CO + Hz, as functions of Pco and PH20(in atm). Car Vol. 20. No. I-E
REFERENCES
1. T. du Motay, Br. Pat. 2548(1867). 2. J. E. Gallagher, Jr. and C. A. Euker, Jr., Energy Rex 4, 137 (1980). 3. D. W. McKee and D. Chatterji, Carbon 16, 53 (1978).
66
D. W. MCKEE
4. D. W. McKee, Fuel 59, 308 (1980). 5. M. J. Veraa and A. T. Bell, Fuel 57, 194(1978). 6. D: W. McKee, Chemistry and Physics of Carbon (Edited by P. L. Walker, Jr. and P. A. Thrower). Vol. 16. D. 1. Marcel Dekker, New York (1981). ” _ 7. D. W. McKee and D. Chatterji, Carbon 13,381 (1975). 8. JANAF Thermochemical Tables, 2nd Edn, NSRDS-NBS 37 (1971). 9. G. J. Janz, Molten Salt Handbook, p. 77. Academic Press, New York (1967).
10. D. W. McKee and J. T. Yates, Jr., J. Catalysis, 71, 308 (1981). 11. I. L. C. Freriks, H. M. H. van Wechem, J. C. M. Stuiver and R. Bouwman, Fuel 60,463 (1981). 12. C. A. Mims and J. K. Pabst,‘Predrints ACS Div., Fuel. Chem. 25(3), 263 (1980). 13. W.-Y. Wen, Catal. Reu. Sci. Engng 22(l), l(l980). 14. H. Marsh and A. Wilkinson, Paper CR-26, presented at 15th Biennial Carbon Conf. Philadelphia, PA, June 1981.