Page 249 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

Chapter | 7 Gasification Theory 225

7.4.1.5 Water Gas Reaction
Referring to the water gas reaction, the kinetic rate, r w , may also be written
in Langmuir Hinshelwood form to consider the inhibiting effect of hydro-
gen and other complexes (Blasi, 2009).
P H 2 O 21
k w 1
r w 5 s (7.46)
1 1 ðk w 1 =k w 3 ÞP H 2 O 1 ðk w 2 =k w 3 ÞP H 2
where P i is the partial pressure of gas i in bars.
Typical rate constants according to Barrio et al. (2001) for beech wood are
7
s
5 2:0 3 10 expð2199=RTÞ bar 21 21
k w 1
6
s
5 1:8 3 10 expð2146=RTÞ bar 21 21
k w 2
7
s
5 8:4 3 10 expð2225=RTÞ bar 21 21
k w 3
Most kinetic analysis, however, uses a simpler nth-order expression for
the reaction rate:
E
r w 5 A w e 2 RT P n s 21 (7.47)
H 2 O
Typical values for the activation energy, E, for steam gasification of char
for some biomass types are given in Table 7.8.
7.4.1.6 Hydrogasification Reaction
The hydrogasification reaction is as follows:
(7.48)
C 1 2H 2 3CH 4
With freshly devolatilized char, this reaction progresses rapidly, but
graphitization of carbon soon causes the rate to drop to a low value. The
reaction involves volume increase and so pressure has a positive influence
on it. High pressure and rapid heating help this reaction. Wang and
Kinoshita (1993) measured the rate of this reaction and obtained values of
23
A 5 4.189 3 10 /s and E 5 19.21 kJ/mol.

7.4.1.7 Steam Reforming of Hydrocarbon
For production of syngas (CO, H 2 ) direct reforming of hydrocarbon is an
option. Here, a mixture of hydrocarbon and steam is passed over a nickel-
based catalyst at 700 900 C. The final composition of the product gas

depends on the following factors (Littlewood, 1977):
H/C ratio of the feed
Steam/carbon ratio
Reaction temperature
Operating pressure.