Biomedical Engineering Reference
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CFB regime. Solids, captured in the gas
solid separator at the gasifier exit,
must be recycled back to the gasifier at a rate sufficiently high to create a
“fast-fluidized” bed condition in the riser. Additional details about this are
available in Basu (2006) or Kunii and Levenspiel (1991).
8.8.2.3 Gasifier Height
Since gasification involves only partial oxidation of the fuel, the heat released
inside a gasifier is only a fraction of the fuel's heating value, and part of it is
absorbed by the gasifier's endothermic reactions. Thus, it is undesirable to
extract any further heat from the main gasifier column. For this reason, the height
of a fluidized-bed gasifier is not determined by heat-transfer considerations as
for fluidized-bed boilers. Instead, solid residence times are major considerations.
For coal it could be about an hour (Probstein and Hicks, 2006, p. 154).
The total height of the gasifier is made up of the height of the fluidized
bed and that of the freeboard above it:
Total gasifier height
bubbling bed height
ð
depth
Þ 1
freeboard height
(8.31)
5
8.8.2.4 Fluidized-Bed Height
The bed height (or depth) of a bubbling fluidized-bed gasifier is an important
design parameter. Gas
solid gasification reactions are slower than combus-
tion reactions, so a bubbling-bed gasifier
is necessarily deeper
than a
bubbling-bed combustor, which is typically 1.0
1.5 m deep for units larger
than 1 m in diameter. Besides pilot plant data or design experience, there is
presently no simple means of deciding the bed depth. A deeper bed allows
longer gas residence time, but the depth should not be so great compared to
its diameter as to cause slugging. The selection of bed height depends on
economics. A higher bed height means a higher pressure drop and also a tal-
ler reactor. It also should provide a longer residence time for better carbon
conversion.
The gasification agent, CO 2 or H 2 O, entering the grid takes a finite time
to react with char particles to produce the gas. The bulk of the gasifying
agent travels up through the bubbles but very little reaction takes place in
the bubble phase. Rather, the reaction takes place mostly in the emulsion
phase. The extent to which oxygen or steam is converted into fuel gases thus
depends on the gas exchange rate between the bubble and emulsion phases
as well as on the char
gas reaction rate in the emulsion phase. This is best
computed through a kinetic model of the gasifier as described in
Section 7.6.2. An alternative is to use an approach based on residence time,
as described next.
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