Biomedical Engineering Reference
In-Depth Information
Gasification technologies
Entrained flow
￿ Koppers-Totzek
gasifier
Fluidized bed
￿ Winkler process
￿ KBR transport gasifier
￿ Twin-reactor gasifier
￿ EBARA gasifier
￿ GTI membrane gasifier
Moving bed
￿ Seimens SFG
gasifier
￿ E-gas gasifier
￿ MHI gasifier
￿ EAGLE gasifier
￿ Lurgi dry-bottom
gasifier
￿ BGL slagging gasifier
￿ Rotating fluidized-bed
gasifiers
￿ Internal circulating gasifier
￿ Foster wheeler CFB
gasifier
Downdraft
Updraft
Crossdraft
Coaxial
downflow
Opposed jet
Bubbling
Circulating
Twin bed
FIGURE 8.1 Gasification technologies and their commercial suppliers.
Fluid bed
Updraft
Entrained flow
Downdraft
10 kW
100 kW
1 MW
10 MW
100 MW
1000 MW
Thermal input
FIGURE 8.2 Range of applicability for biomass gasifier types.
8.2 FIXED-BED/MOVING-BED GASIFIERS
In entrained-flow and fluidized-bed gasifiers, the gasifying medium conveys
the fuel particles through the reactor, but in a fixed-bed (also known as
moving-bed) gasifier, the fuel is supported on a grate (hence its name). This
type is also called moving bed because the fuel moves down in the gasifier
as a plug. Fixed-bed gasifiers can be built inexpensively in small sizes,
which is one of their major attractions. For this reason, large numbers of
small-scale moving-bed biomass gasifiers are in use around the world.
Both mixing and heat transfer within the moving (fixed) bed are rather
poor, which makes it difficult to achieve uniform distribution of fuel, tem-
perature, and gas composition across the cross-section of the gasifier. Thus,
fuels that are prone to agglomeration can potentially form agglomerates dur-
ing gasification. This is why fixed-bed gasifiers are not very effective for
biomass fuels or coal with a high caking index in large-capacity units.
There are three main types of fixed- or moving-bed gasifier: (i) updraft,
(ii) downdraft, and (iii) crossdraft. Table 8.2 compares their characteristics.
 
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