Fluidized Bed Combustion

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Fluidized bed combustion

Of all fossil fuels , coal exists in the largest amount. In fact, the world's coal resources appear to be sufficient to meet our energy needs for many hundreds of years. One aim of energy technology, therefore, is to find more efficient ways to make use of these coal reserves. One efficiency procedure that has been under investigation for at least two decades is known as fluidized bed combustion .

In a fluidized bed boiler, granulated coal and limestone are fed simultaneously onto a moving grate. A stream of air from below the grate lifts coal particles so that they are actually suspended in air when they begin to burn. The flow of air, small size of the coal particles, and exposure of the particles on all sides contribute to an increased rate of combustion. Heat produced by burning the coal is then used to boil water, run a turbine, and drive a generator, as in a conventional power plant.

The fluidized bed process has much to recommend it from an environmental standpoint. Sulfur and nitrogen oxides react with limestone added to the boiler along with the coal. The product of this reaction, primarily calcium sulfite and calcium sulfate, can be removed from the bottom of the boiler. However, disposing of large quantities of this waste product represents one of the drawbacks of the fluidized bed system.

Fly ash is also reduced in the fluidized bed process. As coal burns in the boiler, particles of fly ash tend to adhere to each other, forming larger particles that eventually settle out at the bottom of the boiler. Conventional methods of removal in the stack, such as electrostatic precipitation , can further increase efficiency with which particles are removed.

Incomplete combustion of coal is common in the fluidized bed process. However, carbon monoxide and hydrogen sulfide formed in this way are further oxidized in the space above the moving grate. The products of this further oxidation are then removed by the limestone (which reacts with sulfur dioxide ) or allowed to escape harmlessly in to the air (in the case of the carbon dioxide ). After being used as a scavenger in the process, the limestone, calcium sulfite, and calcium sulfate can be treated to release sulfur dioxide and regenerate the original limestone. The limestone can then be re-used and the sulfur dioxide employed to make sulfuric acid .

A further advantage of the fluidized bed process is that it operates at a lower temperature than does a conventional power plant. Thus, the temperature of cooling water ejected from the plant is lower, and the amount of thermal pollution of nearby waterways correspondingly lessened.

Writers in the 1970s expressed high hopes for the future of fluidized bed combustion systems, but the cost of such systems is still at least double that of a conventional plant. They are also only marginally more efficient than a conventional plant. Still, their environmental assets are obvious. They should reduce the amount of sulfur dioxide emitted by up to 90% and the amount of nitrogen oxides by more than 60%.

[David E. Newton ]