Page 284 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

260 Biomass Gasification, Pyrolysis and Torrefaction

Gas outlet
Gas take-off

Dust separator

Coal
bunker

Return pipe for
accycling fuel

Gasifier
(5½-ft deep fluidized bed)
Screw feed
Traveling grate
Air or oxygen tuyere
Steam
Ash pit
FIGURE 8.8 A sketch of the original Winkler bubbling fluidized-bed gasifier.

materials associated with it are drained easily from the bottom of the bed.
The bed temperature is normally kept below 980 C for coal and below

900 C for biomass to avoid ash fusion and consequent agglomeration.
The gasifying medium may be supplied in two stages. The first-stage sup-
ply is adequate to maintain the fluidized bed at the desired temperature; the
second-stage supply, added above the bed, converts entrained unreacted char
particles and hydrocarbons into useful gas.
High-temperature Winkler (HTW) gasification is an example of high-
temperature, high-pressure bubbling fluidized-bed gasification for coal and
lignite. Developed by Rheinbraun AG of Germany, the process employs
a pressurized fluidized bed operating below the ash-melting point. To
improve carbon conversion efficiency, small char particles in the raw gas are
separated by a cyclone and returned to the bottom of the main reactor
(Figure 8.9).
The gasifying medium (steam and oxygen) is introduced into the fluid-
ized bed at different levels as well as above it. The bed is maintained at a

pressure of 10 bar while its temperature is maintained at about 800 Cto
avoid ash fusion. The overbed supply of the gasifying medium raises the
local temperature to about 1000 C to minimize production of methane and

other hydrocarbons.
The HTW process produces a better-quality gas compared with the gas
that is produced by traditional low-temperature fluidized beds. Though origi-
nally developed for coal, it is suitable for lignite and other reactive fuels like
biomass and treated municipal solid waste (MSW).