Environmental Engineering Reference
In-Depth Information
a molecular weight larger than benzene (Brown et al., 2009; Van Paasen and Kiel,
2004), comparable to a European standard
for quantifying tars
(CEN, 2006). Tar formation is a commonly encountered issue in lower-temperature
(e.g., fluidized bed) gasification processes (Corella et al., 2006; Kinoshita et al., 1994;
Milne et al., 1998; Van Paasen and Kiel, 2004). Although the main issues related to tar
are formed by condensation in the equipment downstream the gasifier, which operates
at lower temperatures (typically below 500
C), tar also significantly contributes to the
heating value of the product gas. Therefore, gas cleaning is needed with respect to this
class of contaminants.
—
CEN/TS 15439
—
10.3.4 Comparison of Large-Scale Gasification
Technologies for Dry Biomass
An overview of the characteristics of different dry biomass-based gasifier technolo-
gies for the larger-scale applications
so excluding the fixed and moving bed types,
as these are less relevant for large-scale syngas generation
—
is given in Table 10.5.
The major challenges for fluidized bed gasification are in gas cleaning for removal
of particles and tars in particular, as well as agglomeration prevention. The main chal-
lenges for EF gasifiers are biomass pretreatment to obtain finely sized fuel and slag-
ging/fouling of heat transfer surfaces.
—
10.3.5 Some Other Types of Gasifier Concepts
Apart from fixed/moving bed, EF, and fluidized beds, there are also comparatively
novel reactor designs under development. One example is the cyclone gasifier.
It combines reaction and separation, intensifying the latter. Fuel is fed via different
tangential inlets using steam. Design and evaluation have been performed for atmos-
pheric and pressurized operation. In order to realize the pressurized design, the
cyclone is surrounded by a pressure vessel. Produced gas leaves swirling at the top
of the reactor; the remaining solids are separated by centrifugal and gravity forces
(Syred et al., 2004).
Another relatively new concept is the vertical vortex gasifier, which has been
derived from the downdraft moving bed gasifier. In this reactor, preheated air is used
as an oxidizer, and it is fed tangentially at the top of the reactor, so that a swirl is gen-
erated. By the strong circulating flow toward the wall, another upflow in the center
part is generated; here, pyrolysis gases generated from the top of a charring bed
are sucked upward toward the entering airstream(s). Then these tar-containing vola-
tiles are converted at temperature of about 1100
C. The heat generated in the top part
of the reactor radiates to the top part of the pyrolyzing bed, which enhances the process
of fuel conversion.
More design alternatives, separating in varying ways the different subprocesses of
gasification, have been suggested and tested. The Blue Tower is one such a concept. It
is an indirect staged reforming process with a heat-carrying system to transfer heat
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