Biomedical Engineering Reference
In-Depth Information
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).
k
w
1
P
H
2
O
s
2
1
r
w
5
(7.46)
1
1
ð
k
w
1
=
k
w
3
Þ
P
H
2
O
1
ð
k
w
2
=
k
w
3
Þ
P
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
10
7
exp
bar
2
1
s
2
1
:
ð
2
=
Þ
k
w
1
5
2
0
3
199
RT
10
6
exp
bar
2
1
s
2
1
k
w
2
5
1
:
8
ð
2
146
=
RT
Þ
3
bar
2
1
s
2
1
Most kinetic analysis, however, uses a simpler nth-order expression for
the reaction rate:
10
7
exp
k
w
3
5
8
:
4
ð
2
225
=
RT
Þ
3
E
RT
P
H
2
O
s
2
1
A
w
e
2
r
w
5
(7.47)
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:
C
2H
2
3
CH
4
(7.48)
1
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
A
10
2
3
/s and E
5
4.189
3
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.
Search WWH ::
Custom Search