Design of Biomass Gasifiers - Biomass Gasification, Pyrolysis and Torrefaction - page 261

Syngas
cooler

HTW
gasifier

Feed

Freeboard

Lock
hopper

Fluid bed

Feed
screw

Air/oxygen/
steam

Cooling screw

Lock
hopper

FIGURE 8.9 HTW bubbling fluidized-bed gasifier.

8.3.2 Circulating Fluidized-Bed Gasifier
A circulating fluidized-bed (CFB) gasifier has a special appeal for biomass
gasification because of the long gas residence time it provides. It is espe-
cially suitable for fuels with high volatiles. A CFB reactor typically com-
prises a riser, a cyclone, and a solid recycle device ( Figure 8.10 ). The riser
serves as the gasifier reactor.
Although the HTW process ( Figure 8.9 ) appears similar to a CFB, it is
only a bubbling bed with limited solid recycle. The circulating and bubbling
fluidized beds are significantly different in their hydrodynamic behavior. In
a CFB, the solids are dispersed all over the tall riser, allowing a long resi-
dence time for the gas as well as for the fine particles. The fluidization
velocity in a CFB is much higher (3.5

5.5 m/s) than that in a bubbling bed

(0.5 1.0 m/s). Also, there is large-scale migration of solids out of the CFB

riser. These are captured and continuously returned to the riser's base. The
recycle rate of the solids and the fluidization velocity in the riser are suffi-
ciently high to maintain the riser in a special hydrodynamic condition known