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).