the coking properties of coal at elevated pressures. - Argonne ...

step, reaction (I) is suggested for both reactions, with the subsequent steps being different for
C02 and H20 as the gaseous oxidant.
C - C02 REACTION
C - H20 REACTION
M2C03 + 2C = 2M + 3CO
2M + co2 = M20 + co
M20 + C02 = M2C03
c + co2 = 2co
M2C03 + 2C = 2M + 3CO
2M + 2H20 = 2MOH + H2
2MOH + CO = M2C03 + H2
C + H 20 = CO + H2
Table n: Carbon Gasification Catalyzed by Na2C03 or K2C03
Although reaction (I) possesses a positive free energy change at temperatures in the 600-
1000°C range, at low partial pressures of CO the reaction can proceed rapidly, as shown by the
TGA data in Figure IS. Figure IS shows the equilibrium stability regions of K C03 and K(P, for
reaction (l), calculated from free energy data, as functions of temperaturg and the partial
pressures (in atmospheres) of K(g) and CO. The sloping lines at each temperature separate the
region of stability of %C03 (upper left) from that of K(g) (lower right). An overall gasification
rate of 5 x IO- min- (about the maximu3attained in this study at 9OO0C) would give an
ambient partial pressure of CO of about 10 atm. above the gasifying char sample. At this
value of Pco, and with a similar value of PK, Figure 15 indicates that reaction (1) would be
thermodynamically possible for all temperatures above about 800°C. At a temperature of
6OO0C the measured gasification rate, and the ambient value of P are about two orders of
magnitude ess than at 9OO0C and Figure 15 shows that reaction (IF?: again possible for P
?l
Pco = IO- atm. The steady state values of P reaction will in fact be much less than $ =
because of the occurrence of reactions (2) and [f& DiFect evidence that reaction (3) does oc%?
has recently been obtained by Wood et al., using high temperature Knudsen cell mass
spectrometry. On heating K C03 and carbon together at temperatures of 500°C and above, the
evolution of K vapor and 20 could be measured. Thus, reaction (I), which is inhibited by
increasing amounts of CO in the gas phase, is probably the rate determining step in the case of
the gasification reactions catalyzed by Na2C0 and K CO . Reactions (2) and (3) and also (4)
and (5) have large negative free energy values a? gasifi&tiofi temperatures and are thus favored
thermodynamically, although little is known about their kinetics.
A somewhat different pathway probably operates in the case of the reactions catalyzed by
Li CO as Li 0 is more stable than the oxides of Na and K, also Li CO is more easily
hy&olyzed
3
to Aydroxide than the carbonates of Na and K. A possible sequegce Jf reactions that
might be involved in the catalysis process in the case of Li CO is shown in Table Ill below.
Evidence for the occurrence of these individual reactions is &ill,%owever, indirect and future
efforts will be directed to exploring the catalytic process by the aid of isotopic tracers and in
determining the relative rates of the elementary steps involved in the catalyzed gasification
process.
79
(2)
(3)
(1)
(4)
(5)