Biomedical Engineering Reference
In-Depth Information
TABLE 7.8
Activation Energy, Preexponential Factor, and Reaction Order
for Char for the Water
Gas Reaction
Activation
Energy E
(kJ/mol)
Char
Origin
Preexponential
Factor A w (s 2 1 bar 2 1 )
Reaction
Order n (
2
) References
10 8 s 2 1 bar 2 n
Birch
237
2.62
0.57
Barrio et al.
(2001)
3
10 8 s 2 1 bar 2 n
Beech
211
0.171
0.51
Barrio et al.
(2001)
3
10 8 s 2 1 atm 2 n
Wood
198
0.123
3
0.75
Hemati and
Laguerie (1988)
Various
biomass
180
200
0.04
1.0
Blasi (2009)
The mixture of CO and H 2 produced can be subsequently synthesized
into required liquid fuels or chemical feedstock. The reactions may be
described as:
4m
2
n
4m
1
n
4m
2
n
C m H n 1
H 2 O
3
CH 4 1
CO 2
(7.49)
4
8
8
CH 4 1
H 2 O
3
CO
3H 2
(7.50)
1
CO
H 2 O
3
CO 2 1
H 2
(7.51)
1
The first reaction ( Eq. (7.49) ) is favorable at high pressure, as it involves
an increase in volume in the forward direction. The equilibrium constant of
the first reaction increases with temperature while that of the third reaction
( Eq. (7.51) ), which is also known as the shift reaction, decreases.
7.4.1.8 Kinetics of Gas-Phase Reactions
Several gas-phase reactions play an important role in gasification. Among
them, the shift reaction (R9), which converts carbon monoxide into hydro-
gen, is most important.
k for
R9:CO
1
H 2 O
!
CO 2 1
H 2 2
41
:
1kJ
=
mol
(7.52)
This reaction is mildly exothermic. Since there is no volume change, it is
relatively insensitive to changes in pressure.
The equilibrium yield of the shift reaction decreases slowly with tempera-
ture. For a favorable yield, the reaction should be conducted at low tempera-
ture, but then the reaction rate will be slow. For an optimum rate, we need
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