COAL

solid, dark-coloured, carbon-rich material that occurs in stratified, sedimentary deposits. It is one of the most important of the primary fossil fuels. A brief treatment of coal follows. For full treatment, see Fuels, Fossil: Coal; Industries, Extraction and Processing: Fossil fuels. Coals are found in many parts of the world. They occur in stratified deposits both near the Earth's surface and at various depths. They include a rather broad range of substances, all owing their origin to the partial decomposition and chemical conversion of immense masses of organic matter. This process is known as coalification. Two major eras of coal formation are known in geologic history. The older includes the Carboniferous and Permian periods (from about 350,000,000 to 250,000,000 years ago). Much of the bituminous coal of eastern North America and Europe is Carboniferous in age. Most coals in Siberia, eastern Asia, and Australia are of Permian origin. The younger era began in the Cretaceous Period (about 135,000,000 years ago) and culminated during the Tertiary Period (about 65,000,000 to 2,500,000 years ago). From this era came nearly all of the world's lignites and subbituminous (brown) coals. The plant matter from which coals are derived has left few recognizable traces. However, the rocks directly above and below coal deposits bear fossil remains of plants belonging to the fern, horsetail, club moss, and gymnosperm groups. The first stage of coalification is the formation of peat. Ancient swamps covered huge areas of flat land, and the climate was mild and rainy. Vast expanses of stagnant water must have covered the land much of the time. These conditions favoured the transformation of vegetable debris into peat. In time the land subsided, the seas covered the land, and sediments were laid down on top of the submerged peat bogs. According to the most widely accepted theory of coal formation, the increase in temperature that accompanies depth of burial is primarily responsible for the metamorphosis apparent in the chain that begins with peat and ends with anthracite (the hardest coal and the one representing the highest degree of coalification). In addition, conditions such as the length of time and increase in pressure due to depth of burial seem to affect the properties of the coal formed. Coals vary in density, porosity, hardness, and reflectivity (i.e., the degree to which a coal reflects light). In colour, coals tend to be dark, usually black. All coals contain some inorganic matter, usually clays, sulfides, and chlorides. Trace elements found in coal include mercury, titanium, and manganese. Coals can be classified in various ways. The most widely used classification schemes are based on the degree to which coals have undergone coalification. Such varying degrees of coalification are called coal ranks, the major ones being lignite, subbituminous, bituminous, and anthracite. Because the amount of fixed carbon contained in a coal increases with rank (from lignite to anthracite) while the amount of its volatile matter released upon heating decreases, many of the properties of coal are at least in part rank dependent (e.g., the amount of gaseous products released by a coal upon heating or its suitability for producing metallurgical coke). Coal is also classified into rock types on the basis of petrological components called macerals. This classification is essentially genetic in nature because it deals with the materials from which coal was formed and the various coalification processes involved. In this system coal is divided into four principal types: vitrain, clarain, durain, and fusain. Another system of classification, developed primarily with commercial considerations in mind, groups coal into grades according to quality, particularly with reference to the amount of impurities present (e.g., low-sulfur). Coal has long been used for power generation, for the production of metallurgical coke, and as a source of various aromatic compounds employed as intermediates in synthesizing dyes, solvents, and drugs. It also has been converted into gases for use as fuel. The first such coal gas was manufactured in England in the late 18th century by destructive distillation. This type of gas was widely used for street lighting and home illumination until gaslight was displaced by electricity, and for heating and gas appliances until it was displaced by natural gas. The increasing cost of natural gas in the late 20th century, however, has led to the exploration of both new and old methods for producing gas from coal, among them a process developed in the 1870s in which coal is pulverized and mixed with air and steam at high temperatures. The continuing concern for finding alternative energy sources also has revived interest in the conversion of coal into liquid fuels similar to crude oil. A method that has been extensively evaluated involves pyrolysis and hydrogenationi.e., causing coal to react with hydrogen at high pressures, usually in the presence of a catalyst. Coal hydrogenation was extensively used in Germany during World War II, but the process has remained commercially unattractive because its cost is significantly higher than that of producing gasoline from petroleum. Since the late 1970s, various other technologies for economical liquefaction of coal have been investigated, particularly in oil-dependent countries possessing extensive coal reserves. solid, usually brown or black, carbon-rich material that most often occurs in stratified, sedimentary deposits. It is one of the most important of the primary fossil fuels. Coal contains more than 50 percent by weight (or 70 percent by volume) carbonaceous matter produced by the compaction and induration of altered plant remainsnamely, peat deposits. Different varieties of coal arise because of differences in the kinds of plant material (coal type), degree of coalification (coal rank), and range of impurities (coal grade). Although most coals occur in stratified, sedimentary deposits, the deposits may later be subjected to elevated temperatures and pressures caused by igneous intrusions or deformation during orogenesis (i.e., processes of mountain building), resulting in the development of anthracite and even graphite. Although the concentration of carbon in the Earth's crust does not exceed 0.1 percent by weight, it is indispensable to life and constitutes humankind's main source of energy. Additional reading Howard N. Eavenson, Coal Through the Ages, 2nd ed., rev. (1942), provides information about the early history of coal mining and coal utilization. Marie C. Stopes and R.V. Wheeler, Monograph on the Constitution of Coal (1918), is a classic work on the organic composition of coals. Information on this and other subjects and numerous references to earlier literature may be found in D.W. van Krevelen, Coal: Typology, Physics, Chemistry, Constitution, 3rd, completely rev. ed. (1993); E. Stach et al., Stach's Textbook of Coal Petrology, 3rd rev. and enlarged ed. (1982; originally published in German, 1935); and Duncan Murchison and T. Stanley Westoll (eds.), Coal and Coal-Bearing Strata (1968). Robert A. Meyers (ed.), Coal Structure (1982), examines the nature and origin of coal structure and porosity. Simon Walker, Major Coalfields of the World (1993), provides information about coal resources worldwide. Methods for estimating world coal reserves, the present status of coal reserves, and the prospects for future development of coal resources are examined in Gnter B. Fettweis, World Coal Resources: Methods of Assessment and Resources (1979; originally published in German, 1976); and Carroll L. Wilson, CoalBridge to the Future: A Report of the World Coal Study (1980). Douglas C. Peters (ed.), Geology in Coal Resource Utilization (1991), provides information concerning coal resources, reserve estimation, coal utilization, and the environment. Otto C. Kopp