Gasification Theory - Biomass Gasification, Pyrolysis and Torrefaction - page 216

Though the bed solids are well mixed, the fluidizing gas remains gen-
erally in plug-flow mode, entering from the bottom and leaving from the
top. Upon entering the bottom of the bed, the oxygen goes into fast exo-
thermic reactions (R4, R5, and R8 in Table 7.2 ) with char mixed with
bed materials. The bed materials immediately disperse the heat released
by these reactions to the entire fluidized bed. The amount of heat released
near the bottom grid depends on the oxygen content of the fluidizing gas
and the amount of char that comes in contact with it. The local tempera-
ture in this region depends on how vigorously the bed solids disperse heat
from the combustion zone.
Subsequent gasification reactions take place further up as the gas rises.
The bubbles of the fluidized bed can serve as the primary conduit to the top.
They are relatively solids free. While they help in mixing, the bubbles can
also allow gas to bypass the solids without participating in the gasification
reactions. The pyrolysis products coming in contact with the hot solids break
down into noncondensable gases. If they escape the bed and rise into the
cooler freeboard, tar and char are formed.
A bubbling fluidized bed cannot achieve complete char conversion
because of the back-mixing of solids. The high degree of solid mixing helps
a bubbling fluidized-bed gasifier achieve temperature uniformity, but owing
to the intimate mixing of fully gasified and partially gasified fuel particles,
any solids leaving the bed contain some partially gasified char. Char particles
entrained from a bubbling bed can also contribute to the loss in a gasifier.
The other important problem with fluidized-bed gasifiers is the slow diffu-
sion of oxygen from the bubbles to the emulsion phase. This encourages the
combustion reaction in the bubble phase, which decreases gasification
efficiency.
In a circulating fluidized bed (CFB), solids circulate around a loop that is
characterized by intense mixing and longer solid residence time within its
solid circulation loop. The absence of any bubbles avoids the gas-bypassing
problem of bubbling fluidized beds.
Fluidized-bed gasifiers typically operate in the temperature range of
800

1000 C to avoid ash agglomeration. This is satisfactory for reactive
fuels such as biomass, municipal solid waste (MSW), and lignite. Since
fluidized-bed gasifiers operate at relatively low temperatures, most high-ash
fuels, depending on ash chemistry, can be gasified without the problem of
ash sintering and agglomeration.
Owing to the large thermal inertia and vigorous mixing in fluidized-
bed gasifiers, a wider range of fuels or a mixture of them can be
gasified. This feature is especially attractive for biomass fuels, such as
agricultural residues and wood, that may be available for gasification at
different times of the year. For these reasons, many developmental activi-
ties on large-scale biomass gasification are focused on fluidized-bed
technologies.