Environmental Engineering Reference
In-Depth Information
Gas
Biomass
Tar + water
CO, CO
2
, CH
4
, H
2
Char + water
FIGURE 10.11
Global mechanism for biomass conversion under hydrothermal conditions.
10.4.3 Chemistry and Thermodynamics
Figure 10.11 shows a global mechanism for biomass conversion under hydrothermal
conditions. Biomass gasification in SCW is supposed to be the result of thermal
decomposition reactions, followed by homogeneous reactions (water
-
gas shift and
the methanization) of the resulting gases.
In general, the overall reaction equation (for glucose as model compound) can be
written as
uC
6
H
12
O
6
+vH
2
O
!
wCO
2
+ xCH
4
+ yCO+ zH
2
ð
RX
:
10
:
14
Þ
The desired reaction of complete reforming to hydrogen is
C
6
H
12
O
6
+6H
2
O
!
6CO
2
+ 12H
2
ð
RX
:
10
:
15
Þ
Maximal methane yields are obtained by
C
6
H
12
O
6
!
3CH
4
+ 3CO
2
ð
RX
:
10
:
16
Þ
Figure 10.12 presents results of thermodynamic calculations for supercritical gasifica-
tion of C
6
H
12
O
6
, which have been obtained with a model based on Gibbs free energy
minimization (see Chapter 5). Such calculations have a limited quantitative value in
case the reactions involved are too slow to reach equilibrium, but they may be useful
in predicting trends and the results desired upon application of an appropriate catalyst.
Complete gasification of the organic feedstock is thermodynamically possible for
both the proposed low- and high-temperature gasification processes. Actually, dry
matter concentrations of up to 50 wt% do not have thermodynamic or stoichiometric
limitations (see Figure 10.12a) regarding the conversion. Thermodynamic equilib-
rium calculations predict that high-temperature gasification produces a hydrogen-rich
gas (at least for dry matter contents of less than 10 wt%), while at low temperature a
methane-rich gas results (see Figure 10.12b and c). This shift in the product distribu-
tion is also observed experimentally.
For low-temperature gasification, the content of dry matter in the feed does not
influence the product distribution to a large extent; beyond 5 wt%, the yields are
almost unaffected (see Figure 10.12b). In contrast, at higher temperature, there is a con-
tinuous varying product distribution ranging from nearly pure hydrogen for very low
weight percentages of dry matter to a mixture of ca. 50 mol% hydrogen and 50 mol
% methane for high organic fractions in the feed. Once above 15MPa, the operating
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