Page 274 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

250 Biomass Gasification, Pyrolysis and Torrefaction

TABLE 8.1 Comparison of Characteristics of Some Commercial Gasifiers
Fixed/
Parameters Moving Bed Fluidized Bed Entrained Bed
Feed size ,51 mm ,6mm ,0.15 mm
Tolerance for fines Limited Good Excellent
Tolerance for coarse Very good Good Poor
Gas exit temperature 450 650 C 800 1000 C .1260 C

Feedstock tolerance Low-rank coal Low-rank coal Any coal including
and excellent caking but
for biomass unsuitable for
biomass
Oxidant requirements Low Moderate High
Reaction zone temperature 1090 C 800 1000 C 1990 C

Steam requirement High Moderate Low
Nature of ash produced Dry Dry Slagging
Cold-gas efficiency 80% 89% 80%
Application Small Medium-size Large capacities
capacities units
Problem areas Tar production Carbon Raw-gas cooling
and utilization conversion
of fines

Source: Data compiled from Basu (2006).

gasifiers are broadly divided into three principal types (Table 8.1): (i) fixed
or moving bed, (ii) fluidized bed, and (iii) entrained-flow bed. Each is further
subdivided into specific commercial types as shown in Figure 8.1.
One particular gasifier type is not necessarily suitable for the full range
of gasifier capacities. There is an appropriate range of application for each.
For example, the moving-bed (updraft and downdraft) type is used for
smaller units (,10 MW th ); the fluidized-bed type is more appropriate for
intermediate units (5 100 MW th ); entrained-flow reactors are used for large-
capacity units (.50 MW th ). Figure 8.2 developed with data from Maniatis
(2001) and Knoef (2005) shows the overlapped range of application for dif-
ferent types of gasifiers. Downdraft gasifiers are for the smallest size while
entrained-flow gasifiers are the largest size.