Environmental Engineering Reference
In-Depth Information
Researchers from the university of Wisconsin-Madison have reported interesting
catalysis around 230
C for the production of hydrogen-rich gas from small oxyge-
nated hydrocarbons. They were able to decrease the methane formation rate via
C
O bond cleavage and methanation (hydrogenation) while maintaining high rates
of C
-
gas shift reaction for hydrogen production. They
used a Pt catalyst and nickel catalyst promoted with tin. High hydrogen yields were
obtained for methanol, ethylene glycol, and glycerol. However, with sorbitol and glu-
cose as feedstock, already significant amounts of methane were being produced next
to hydrogen. In an embryonic stage, the methodology of decelerating methane-
producing reactions at catalytic sites while keeping a high rate of catalytic hydrogen
production seems promising to produce hydrogen-rich gas at conditions for which
overall chemical equilibrium dictates methane-rich gas, viz., at subcritical temperature
and at the combination of high temperature and high concentration of organics. In this
concept, it will be important to decrease homogeneous reactions to undesired by-
products (oil/char/CH
4
) and to increase the reaction rate. This is quite a challenge
for both catalyst and reactor design.
-
C bond cleavage and the water
-
10.4.4.2 High-Temperature Gasification
For high temperatures (>500
C), alka-
lis have been proposed as catalysts. However, recovery of alkalis from the process
may be a problem, because alkalis hardly dissolve in SCW. One of the pioneers in
this field, Antal et al. (2000), has reported that leading the effluent of their empty tube
reactor over a fixed bed of activated carbon derived from coconut increased the extent
of gasification from 0.7 to 1.0. Despite the successful use of this activated carbon as a
catalyst on laboratory scale, it may not be the catalyst finally selected for the process.
Two important reasons for this are: (i) the catalytic activity nor its decline is under-
stood, and (ii) the rate of charcoal gasification is slow but certainly not zero, leading to
consumption of the catalyst.
Researchers at the University of Twente used the Ru/TiO
2
catalyst of PNNL and
found complete gasification of glucose (1
17 wt% solutions) at 600
C and approxi-
mately 60 s residence time (Kersten et al., 2006). The produced gas was at chemical
equilibrium. The reaction is much faster at 600
C compared to 350
C, which is ben-
eficial for the size of the reactor. However, no information is yet available concerning
the stability of catalysts in the high-temperature-range SCW. Gas produced by gasi-
fication in hot compressed water typically has a (very) low CO content. Gasification in
hot compressed water produces either methane- or hydrogen-rich gas in combination
with CO
2
. This makes the product gas unsuitable as synthesis gas for FT synthesis.
The hydrogen can be used for hydrogenation of bioliquids (see Chapter 11).
-
10.4.5 Reactor and Process Design Aspects
1.
High Temperatures
. High temperatures of above 600
C are needed in the reac-
tor to allow significant noncatalytic thermal degradation of the biomass mole-
cules and subsequent cracking or gasification of the intermediate-sized
fragments to the small molecules H
2
, CO, CH
4
, and CO
2
. Obviously, by apply-
ing catalysis, the required operating temperature can be reduced. It is, however,
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