Page 283 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

Chapter | 8 Design of Biomass Gasifiers 259

Crossdraft gasifiers can be very light and small (,10 kW e ). Since layers
of fuel and ash insulate the walls from the high-temperature zone, the gas-
ifier vessel can be constructed of ordinary steel with refractory linings on the
nozzle and gas exit zone.
The crossdraft design is less suitable for high-ash or high-tar fuels, but it
can handle high-moisture fuels if the top is open so that the moisture can
escape. Particle size should be controlled, as unscreened fuel runs the risk of
bridging and channeling. Crossdraft gasifiers work better with charcoal or
pyrolyzed fuels. For unpyrolyzed fuels, the height of the air nozzle above the
grate becomes critical (Reed and Das, 1988, p. 32).

8.3 FLUIDIZED-BED GASIFIERS
Fluidized-bed gasifiers are noted for their excellent mixing and temperature
uniformity. A fluidized bed is made of granular solids called bed materials,
which are kept in a semi-suspended condition (fluidized state) by the passage
of the gasifying medium through them at the appropriate velocities. The
excellent gas solid mixing and the large thermal inertia of the bed make
this type of gasifier relatively insensitive to the fuel’s quality (Basu, 2006).
Along with this, the temperature uniformity greatly reduces the risk of fuel
agglomeration.
The fluidized-bed design has proved to be particularly advantageous for
gasification of biomass. Its tar production lies between that for updraft
3 3
(B50 g/nm ) and downdraft gasifiers (B1 g/nm ), with an average value of
3
around 10 g/nm (Milne et al., 1998, p. 14). There are two principal
fluidized-bed types: bubbling and circulating.

8.3.1 Bubbling Fluidized-Bed Gasifier
The bubbling fluidized-bed gasifier, developed by Fritz Winkler in 1921, is
perhaps the oldest commercial application of fluidized beds; it has been in
commercial use for many years for the gasification of coal (Figure 8.8). For
biomass gasification, it is one of the most popular options. A fairly large num-
ber of bubbling fluidized-bed gasifiers of varying designs have been devel-
oped and are in operation (Lim and Alimuddin, 2008; Narva ´ez et al., 1996).
Because they are particularly suitable for medium-size units (,25 MW th ),
many biomass gasifiers operate on the bubbling fluidized-bed regime.
Depending on operating conditions, bubbling-bed gasifiers can be grouped as
low-temperature and high-temperature types. They can also operate at atmo-
spheric or elevated pressures.
In the most common type of fluidized bed, biomass crushed to less than
10 mm is fed into a bed of hot materials. These bed materials are fluidized
with steam, air, or oxygen, or their combination, depending on the choice of
gasification medium. The ash generated from either the fuel or the inorganic