Introduction and Background

Coal, essentially fossilized plant material, has been used as an energy source for centuries. As early plants decomposed in the absence of oxygen (for example, at the bottom of deep lakes), oxygen and hydrogen atoms broke off the long organic molecules in the plant material, leaving mostly atoms of carbon, with some other impurities. Formed as long as ago as 300 million years and more, during the Carboniferous Period (named for obvious reasons), coal's main useful component is carbon.

Coal comes in various forms. Older coal is harder, higher in energy content and has a higher proportion of carbon. Anthracite is the hardest, purest version, with a carbon content of about 90 percent. It is also rarer, found in the United States almost exclusively in eastern Pennsylvania. Its energy content is about 25 million Btus per short ton, 67 percent higher than lignite, the softest...