Page 229 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

206 Biomass Gasification, Pyrolysis and Torrefaction

The rate of the char gasification reaction in CO 2 is insignificant below
1000 K.

7.3.3.3 Water Gas Reaction
The gasification of char in steam, known as the water gas reaction, is per-
haps the most important gasification reaction.

C 1 H 2 O2CO 1 H 2 ðR2 in Table 7:2Þ (7.10)
The first step involves the dissociation of H 2 O on a free active site of car-
bon (C fas ), releasing hydrogen and forming a surface oxide complex of car-
bon C(O). In the second and third steps, the surface oxide complex produces
a new free active site and a molecule of CO.
k w1
Step 1: C fas 1 H 2 O ! CðOÞ 1 H 2 (7.11)

k w2
Step 2: CðOÞ 1 H 2 ! C fas 1 H 2 O (7.12)
k w3
Step 3: CðOÞ ! CO (7.13)
Some models (Blasi, 2009) also include the possibility of hydrogen inhi-
bition by C(H) or C(H) 2 complexes as below:
C fas 1 H 2 2CðHÞ 2 (7.14)

C fas 1 0:5H 2 2CðHÞ (7.15)
The presence of hydrogen has a strong inhibiting effect on the char gasi-
fication rate in H 2 O. For example, 30% hydrogen in the gasification atmo-
sphere can reduce the gasification rate by a factor as high as 15 (Barrio
et al., 2001). So an effective means of accelerating the water gas reaction is
continuous removal of hydrogen from the reaction site.

7.3.3.4 Shift Reaction
Unlike the above reactions, shift reaction takes place between steam and an
intermediate product of the gasification reaction. The other difference of this
important reaction is that it is a gas-phase reaction. This reaction increases
the hydrogen content of the gasification product at the expense of carbon
monoxide. Some literature (Klass, 1998, p. 277) refers this reaction also as
“water gas shift reaction,” though it is much different from the water gas
reaction (R2).
CO 1 H 2 O2CO 2 1 H 2 241:2kJ=mol ðreaction R9 in Table 5:2Þ (7.16)
This is a prestep in syngas production in the downstream of a gasifier,
where the ratio of hydrogen and carbon monoxide in the product gas is
critical.