Page 286 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

262 Biomass Gasification, Pyrolysis and Torrefaction

Product
gas
Gasification
reactor

Fast bed
Air
Return leg
Waste &
biomass 900 C

Loopseal
Preheated air
Steam

Bottom ash cooling screw
Bottom ash

FIGURE 8.10 Circulating fluidized bed gasifier.

as fast-fluidized bed. Depending on the fuel and the application, the riser
operates at a temperature of 800 1000 C.

The hot gas from the gasifier passes through a cyclone, which separates
most of the solid particles associated with it, and the loop seal returns the
particles to the bottom of the gasifier. Foster Wheeler developed a CFB gas-
ifier where an air preheater is located in the standpipe below the cyclone to
raise the temperature of the gasification air and indirectly raise the gasifier
temperature (Figure 8.10).
Many commercial gasifiers of this type have been installed in different
countries. One of the biggest is a 140 MW CFB gasifier attached to a
560 MW e pulverized coal (PC) fired unit at Vaasa, Finland, for biomass
cofiring. It provides a cheap supplementary fuel by gasifying wood, peat and
straw replacing upto 40% coal. Several manufacturers around the world have
developed versions of the CFB gasifier that work on the same principle and
vary only in engineering details.

8.3.2.1 Transport Gasifier
This type of gasifier has the characteristics of both entrained-flow and
fluidized-bed reactors. The hydrodynamics of a transport gasifier is similar
to that of a fluid catalytic cracking reactor. A transport gasifier operates at