51
Steam Gasification and Reforming Technologies
other gasifying agents. Two reaction mechanisms for carbon–steam reactions over
a wide range of gasification conditions are proposed [1,3]:
Mechanism A:
C + H O = (
)
(4.8)
C H O
f
2
2
A
C H O
( 2 ) A → CO + H 2
(4.9)
C f + H 2 = C(H 2 ) B
(4.10)
In this mechanism, C f designates unoccupied carbon sites, C(H 2 O) A and C(H 2 ) B
are adsorbed species of water and hydrogen, respectively. Equations 4.8 and 4.10
are reversible, whereas Equation 4.9 is irreversible. This mechanism is referred to
as inhibition by hydrogen adsorption because the adsorbed hydrogen prevents the
adsorption of steam molecules and thus prevents the gasification reaction [1,3]
Mechanism B:
C f + H 2 O = C(O) A + H 2
(4.11)
C O ) A → CO
(
(4.12)
Here gasification rate is affected by the competitive reaction of chemisorbed oxygen
with hydrogen, and this competition for sites limits the conversion of the adsorbed
oxygen to carbon monoxide. Thus, this mechanism is often called “inhibition by
oxygen exchange” [1,3].
Both of these mechanisms can express the gasification rate in the form as follows:
k p H 2
R =
1
(4.13)
k p H + k p H O
1+ 2
3
2
2
where:
R is the rate of gasification
p H 2 and p H O are partial pressures of hydrogen and water
2
k 1 is the kinetic constant
k 2 and k 3 are adsorption constants
and the expression fits the experimental data well. The rate expression can be derived,
assuming pseudo-steady states for adsorbed species.
The above discussion assumes pure carbon and steam as a gasifying agent. The
rate analysis gets more complex when coal and other gasifying agents are taken
into the considerations. Carbon in coal is distributed within the coal matrix and its
concentration depends on the rank of coal. The gasification of coal must therefore
consider transport processes within the coal matrix and accessibility of carbon within
the matrix. When other gasifying agents such as oxygen and hydrogen are added,
their roles on overall gasification reactions must also be considered. Alkali metal salts