Coal is the compressed remains of tropical and subtropical plants, especially those of the Carboniferous and Permian periods. Geologists estimate that it took about 10 feet of leaves, tree trunks, and other organic matter to produce a one-foot layer of coal. The first stage in the transformation of decaying plant material into coal is the development of peat. Following long intervals of time, peat is transformed into lignite (brown coal) and eventually into bituminous coal.
Changes in the world climatic patterns explain why coal occurs in all continents, even Antarctica. 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 analyzed in two main ways: the "ultimate analysis" determines the total percentages of the elements present (carbon, oxygen, sulfur, and nitrogen); and the "proximate analysis" gives an empirical estimate of the amounts of moisture, ash, volatile materials, and fixed carbon. Coals are classified, or ranked, according to their fixed-carbon content, which increases progressively as they are formed. In ascending rank, the main types are lignite, or brown coal, which weathers quickly, may ignite spontaneously, and has a low calorific value; subbituminous coal, mainly used in generating stations; bituminous coal, the commonest type, used in generating stations and the home, and often converted into coke; and anthracite, a lustrous coal which burns slowly and well, and is the preferred domestic fuel.
Coal was burned in Glamorgan, Wales, in the 2nd millennium BC, and was known in China and the Roman Empire around the time of Christ. Coal mining was practiced throughout Europe and known to native Americans by the 13th century BC. The first commercial coal mine in the United States was at Richmond, Virginia (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 now again increasing.
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