Environmental Engineering Reference
In-Depth Information
are prevented on the material, which consists of a NiO
-Al
2
O
3
/cordierite
monolithic catalyst with a pore channel size of 7 by 7 mm. Moreover, such a monolith
unit can function properly even under high dust load conditions (
-
MgO/
γ
m
n
−3
) (Wang
330 g
et al., 2011).
Particle gas cleanup can be combined with tar conversion, which is a novel
development that holds good promise for process integration and intensification
(Nacken et al., 2009).
10.5.4 Sulfur Species Cleanup
During different gasification processes, depending on the fuel-bound sulfur content
of the fuel source, sulfur species are formed as part of the product gas. Most biomass
types contain little to no sulfur (0
2.0 wt%). Biomass-derived feedstock such as
municipal solid waste or sewage sludge, though, contains higher sulfur levels.
The species formed during gasification are usually H
2
S (major compound), COS,
CS
2
, and organic compounds like thiophenes and mercaptans. Up to 93
−
96% of
the sulfur appears as H
2
S in a typical gasification process (Higman and Van
der Burgt, 2003). The demonstration-scale woody biomass gasifier in Güssing
(Austria) shows sulfur compound levels of
-
30 ppm organic sul-
fur (e.g., thiophene) (Weber et al., 2010). For emission control, not the strictest reduc-
tion measures are needed (yet), but rather these measures are needed for protection of
downstream catalysts as they can be poisoned or materials can be corroded by traces
of such compounds.
The overall thermal efficiency of a biomass gasification-based energy conversion
system can be kept relatively high when hot gas desulfurization is applied, due to the
fact that product gas cooling is not needed. Sulfur cleanup can be accomplished
by using either primary methods (in the gasifier) or downstream capture, or a
combination.
150 ppmH
2
Sand
10.5.4.1 Primary Sulfur Species Concentration Reduction Methods
Like for tar
species reduction, natural rock materials, limestone and dolomite, from various
sources are among the most commonly used materials for sulfur capture in fluidized
bed gasification applications. An extensive review has been presented by Meng et al.
(2010). Natural rock materials have a widespread availability and are comparatively
cheap. They show different compositions depending on geographic sourcing, which
has significant effects on the final quality of the gas. Again, problems of attrition and
incomplete conversion below the calcining temperatures have been shown for lime-
stone. In dolomite, the higher Mg/Ca ratio causes an even higher attrition, but dolo-
mite generally shows an increased reaction rate and reduced agglomeration behavior.
Nowadays, also other Ca-based materials are being investigated that show both
increased strength and absorption capacity (Akiti et al., 2002).
10.5.4.2 Downstream Sulfur Capture
Downstream of the gasifier, many different
metal oxide materials can be utilized for high-temperature gas cleaning, with each hav-
ing their own limitations and advantages. Key performance criteria for downstream
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