Design of Biomass Gasifiers - Biomass Gasification, Pyrolysis and Torrefaction - page 250

TABLE 8.1 Comparison of Characteristics of Some Commercial Gasifiers

Fixed/
Moving Bed Parameters 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

650 C 1000 C 1260 C Gas exit temperature 450 800

.

Feedstock tolerance Low-rank coal Low-rank coal

and excellent
for biomass

Any coal including
caking but
unsuitable for
biomass

Oxidant requirements Low Moderate High

1090 C 1000 C 1990 C Reaction zone temperature 800

Steam requirement High Moderate Low

Nature of ash produced Dry Dry Slagging

Cold-gas efficiency 80% 89% 80%

Application Small

capacities

Medium-size
units

Large capacities

Problem areas Tar production

and utilization
of fines

Carbon
conversion

Raw-gas cooling

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.

.