Fluidized Bed Combustion

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Because the inert material is at burn temperature, any carbon-containing fuel which is introduced into the bed of tumbling solids will burn rapidly, passing heat back into the swirling mass surrounding it. Thus a wide range of fuels which have been considered unsuitable—because of either their low BTU content or their tendency to burn incompletely—can be successfully consumed in a fluid bed . . . including coal mine tailings (10 to 20% carbon), timber waste, agricultural byproducts, and even pelletized garbage!

Furthermore, heat produced in a fluid bed can be transferred to a heat exchanger much more rapidly than can that generated in a conventional furnace. Because the hot inert matter in the bed comes in direct contact with both the oxidizing carbon and the tubes of a heat exchanger, heat transfer is by conduction (directly from one medium to another) rather than by convection (from one medium to another by way of an intervening moving medium). Between 50 to 100 BTU per square foot per °F can be transferred by a fluid bed, while a conventional furnace moves only about 10 BTU per square foot per °F. Accordingly, a fluidized bed requires far less heat-exchanger area to extract a similar amount of heat.

REDUCED POLLUTION

An additional benefit of a fluidized bed's method of BTU transfer is that it allows the heat exchanger to be used to control the temperature of combustion. Since the coils are set directly into the flame area—instead of above it, as is the case in a standard furnace—the heat that is extracted reduces the burn temperature (the fire can actually be put out by this technique). Therefore, in the case of coal, the combustion temperature can be maintained at the ideal of 1550°F—as opposed to the 3000°F that's sometimes generated in a normal furnace—which eliminates the clinkers usually associated with firing low-quality fuel. The reduced flame temperature also drastically limits the heater's production of nitrous oxide—a key element in photochemical smog—which is produced in significant quantities only above 1800°F.

Finally, the chemical makeup of the inert bed material can actually reduce or eliminate the pollution problems associated with certain "dirty" fuels. For example, high-sulfur coal can be burned in a bed of limestone with virtually no emission of sulfur . . . a pollutant which, in the opinion of the Environmental Protection Agency, is a major cause of acid rain. In a fluid bed, the calcium in the limestone reacts with the sulfur in the coal to produce calcium sulfate (CaSO 4 ) . . . which can be used as fertilizer. And—while suitable "civilizers" for other problem fuels have not yet been established—there are probably a number of "cleansing" materials . . . which may someday play a major role in providing us with low-pollution energy.

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