Coal is a black or brown rock that can be ignited and burned to produce energy in the form of heat. Coal's chemical makeup is a complex mix of elements that include sulfur, carbon, oxygen, hydrogen and nitrogen, as well as small quantities of aluminum, zirconium and many other minerals.
There are several different types and ranks of coal. Coal is classified by degrees of hardness, moisture and heat content. Anthracite is hard coal, almost pure carbon, and containing the highest Btu content. (British thermal unit - a measure of the energy required to raise the temperature of one pound of water by one degree Fahrenheit.); Bituminous is soft coal, the most common type in the United States, used to generate electricity and to make coke for the steel industry; Lignite is the softest coal, has the highest moisture content and is used for generating electricity in certain parts of the country and for conversion into synthetic gas; Subbituminous has a heating value between bituminous and lignite, and has low-faced carbon and high percentages of volatile matter and moisture.
To determine the origins of coal, one has to look back a long way. Some 300 million years ago, much of the United States was a swamp covered with giant ferns, reeds and other plants. When the plants died, they fell under water, where the oxygen was not sufficient for them to decay completely. A result was the formation of peat.
As the sea deposited layers of other materials over the peat through the centuries, the peat was tightly compacted and dried. It then hardened, in the final step of a natural process, to become coal.
Because coal is composed of once living material, it is known as a "fossil fuel." In the burning of coal, its fossil energy - the energy stored in the ancient plant material - is released. Oil and natural gas are also fossil fuels.