Environmental Engineering Reference
In-Depth Information
10.4.4 Reported Results for Low-Temperature and High-Temperature
Gasification: Catalyst Selection and Development
To achieve complete conversion of the feed into gases, a catalyst is always required in
a hydrothermal gasification process. Moreover, that catalyst plays an important role in
steering the product distribution. In the following part, gasification in hot compressed
water is discussed for low (250
400
C) and high (>550
C) temperature separately.
−
10.4.4.1 Low-Temperature Gasification
In an extensive research program that
started in the 1980s, the Pacific Northwest National Laboratory (PNNL, United
States) developed a catalytic process for the destruction of organic waste at approx-
imately 350
C while producing a methane-rich gas. Tests were carried out at labora-
tory and pilot scale focusing on both catalyst and process development. Ruthenium on
rutile titania, ruthenium on carbon, and stabilized nickel catalysts showed the highest
activity and the best stability. With these catalysts, nearly 100% gasification of model
components (1
10 wt% in water) was achieved, while without catalyst the extent of
gasification is very limited at this temperature. The gas produced consisted of nearly
only CH
4
and CO
2
, as dictated by the overall thermodynamic equilibrium. The cat-
alytic process was carried out in a series of fixed bed reactors. When using feedstock
materials with the tendency to produce char/coke, a continuous stirred tank reactor
(CSTR) was required before the fixed bed to soften the feed and to prevent the buildup
of solids. Pilot plant runs using complex feeds such as potato waste and manure were
carried out. The required liquid hourly space velocity (LHSV) was in the range of
1.5
-
h
−1
. For a waste disposal process, these LHSVs are accept-
able, but for the production of gaseous energy carriers from biomass, the activity is too
low. Researchers at PSI (Peterson et al., 2008) reported high extents of gasification
and equilibrium methane yields for concentrated (up to 30 wt%) wood sawdust
slurries using Raney nickel as catalyst at 400
C. For complete gasification, 90 min
reaction time was required in their batch reactor.
How the catalysts enhance the extent of gasification at these low temperatures has
not been completely clarified. Either they accelerate the rate of the gasification reac-
tion relative to the rate of polycondensation/polymerization reactions, or they are able
to gasify the formed polymers, or a combination of both. Obviously, these catalysts
catalyze all gas-phase component reactions because a good agreement was found
between the observed gas composition and the gas composition dictated by thermo-
dynamic equilibrium. Reported problems with respect to the catalysts are poisoning
by trace components such as sulfur, magnesium, and calcium and the growth of the
active metal crystals during operation (sintering). A general problem of the near-
supercritical and supercritical region is that it enhances leaching of the catalytic active
phases and degeneration of the support. Hot compressed water is a good solvent for
most organic chemicals and thus especially useful to keep coke precursors dissolved.
Further, if coke is formed on the surface of the catalyst, the high H
2
O concentration
helps in keeping it clean via gasification. In accordance with that, it was found that
coke formation on the catalyst surface is only a minor problem.
3.5 m
n
3
feed
m
cat
−3
-
Search WWH ::
Custom Search