For decades, fluidized-bed reactors have been used in
noncombustion reactions in which the thorough mixing and intimate contact of
the reactants in a fluidized bed result in high product yield with improved
economy of time and energy.
Although conventional methods of burning coal can also
generate energy with very high efficiency, fluidized-bed combustion can burn
coal efficiently at a temperature low enough to avoid many of the problems of
conventional combustion.
The outstanding advantage of fluidized-bed combustion (FBC)
is its ability to burn high-sulfur coal in an environmentally acceptable manner
without the use of flue-gas scrubbers. A secondary benefit is the formation of
lower levels of nitrogen oxides compared to other combustion methods.
Crushed fuel and sorbent are fed mechanically or
pneumatically to the lower portion of the combustor.
Primary air is supplied to the bottom of the combustor
through an air distributor, with secondary air fed through one or more
elevations of air ports in the lower combustor.
Combustion takes place throughout the combustor, which is
filled with bed material. Flue gas and entrained solids leave the combustor and
enter one or more cyclones where the solids are separated and fall to a seal
pot.
From the seal pot, the solids are recycled to the combustor.
Optionally, some solids may be diverted through a plug valve to an external
fluidizedbed heat exchanger (FBHE) and back to the combustor.
In the FBHE, tube bundles absorb heat from the fluidized
solids. Bed temperature in the combustor is essentially uniform and is
maintained at an optimum level for sulfur capture and combustion efficiency by
heat absorption in the walls of the combustor and in the FBHE (if used).
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