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.

Fluidized bed combustion. Fuel is lifted by a stream of air from underneath the bed. Fuel efficiency is good and sulfur dioxide and nitrogen oxide emissions are lower than with conventional boilers. (McGraw-Hill Inc. Reproduced by permission.)

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...