Environmental Engineering Reference
In-Depth Information
composition and tar formation. Usually, when investigating a material as an in-bed
catalyst for tar elimination and/or upgrading of the gas composition, it is used as
an additive compound instead of a bed material.
In situ
use of catalysts in the gasifier has been studied widely, and additives
like dolomites (Corella et al., 1991; Delgado et al., 1996; Gil et al., 1999; Ising,
2002; Olivares et al., 1997), magnesites (Corella et al., 1991; Delgado et al., 1996;
Siedlecki et al., 2009), limestones (Corella et al., 1991; Delgado et al., 1996), olivines
(Aznar et al., 2007; Corella et al., 2004b; Devi et al., 2005a, 2005b), and Ni-based
catalysts (Pfeifer et al., 2004) have found application. Table 10.9 refers to the
natural rock materials as having relatively good activity. Generally, they are cheap,
but show less favorable attrition behavior, and therefore, they impose higher capacity
requirements to remove particles. Successful demonstration of this
in situ
measure has
been shown using magnesite as bed material in the 18 MW
th
CFB demonstration
gasifier of Värnamo (Ståhl et al., 2004).
In situ
destruction seriously reduces the
cleaning needed in downstream processing of product gas. In addition, catalytically
active materials applied in the gasifier can also promote char gasification, modify the
major product gas concentrations and decrease tar levels, and prevent agglomeration
in the gasifier.
10.5.3.2 Secondary Tar Reduction Methods
Secondary tar reduction techniques
are accomplished using reactor units that are placed downstream the gasifier.
Research and implementation have mainly focused on fixed bed reactors, although
(circulating) fluidized beds have also been selected. An example of commercial
use was technology offered by the (former) Swedish company TPS, using dolomite
in a second CFB reactor (Rensfelt, 1997). A disadvantage is that such reactors show
attrition issues as the mineral rock materials are soft. In order to comply with the
rigorous requirements concerning tar concentration levels, combining primary
measures and downstream reduction may be effective in view of keeping catalyst life-
times acceptable. Dayton (2002), Abu El-Rub et al. (2004), and Yung et al. (2009)
have presented excellent reviews on tar reduction by secondary measures. El-Rub
has performed extensive research toward the use of biomass-derived char to
reduce tar levels (Abu El-Rub et al., 2008). This material is active, cheap, and made
in situ
but shows variable quality and is consumed in the reactions, so it must be
supplied continually.
Novel materials concern monoliths with impregnated metal catalysts (often
Ni based) in relatively dusty syngas flows. Corella et al. (2004a) used them for
product gas obtained from biomass gasification, but they concluded that the
activity was only moderate and to be improved. They also indicate that upstream
tar concentrations need to be reduced to approximately 2 g
m
n
−3
. Another issue
indicated is the presence of sticky ash due to alkali eutectics. The monolith concept
is applied to reduce tar in the CHP demonstration project in Skive (Denmark).
Recently, in China, a new catalyst was tested (60 h run) with real gas from a fluidized
bed biomass gasifier (scale, 150
h
−1
) impregnated in a monolith that shows
promising properties concerning tar conversion. Coking and sintering of the catalyst
−
300 kg
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