Coal

Coal, hard, black mineral, predominantly carbon, the compressed remains of tropical and subtropical plants, especially those of the carboniferous and Permian geological periods, burned as a fuel. With its by-products coke and coal tar, it is vital to many modern industries. Coal formation began when plant debris accumulated in swamps, partially decomposing and forming peat layers. A rise in sea level or land subsidence buried these layers below marine sediments, whose weight compressed the peat, transforming it under high-temperature conditions to coal; the greater the pressure, the harder the coal. Coals are classified according to their fixed-carbon content, which increases progressively as they are formed. Lignite, or brown coal, which weathers quickly, may ignite spontaneously, and has a low calorific value. Subbituminous coal is mainly used in generating stations; bituminous coal is the commonest type, used in generating stations and the home, and often converted into coke; anthracite is a lustrous coal that burns slowly and well and is the preferred domestic fuel.

Coal was burned in Glamorgen, Wales, in the 2nd millennium B.C. and was known in China and the Roman Empire around the time of Jesus. Coal mining was practiced throughout Europe and known to Native Americans by the 13th century. The first commercial coal mine in the United States was at Richmond, Va. (opened 1745), and anthracite was mined in Pennsylvania by 1790. The Industrial Revolution created a huge and increasing demand for coal. This slackened in the 20th century as coal faced competition from abundant oil and gas, but production is again increasing. Annual world output is about 3 billion tons, 500 million tons from the United States. World coal reserves are estimated conservatively at about 7 trillion tons, enough to meet demand for centuries at present consumption rates.

See also: Diamond.