Coal, one of mankind's primary \Tenergy sources\t, is a rock derived from vegetable matter through the process of metamorphism, which requires that heat and pressure act over long periods on this matter, altering both its chemical and physical characteristics. Because the final product may have a variety of chemical compositions, coal is classified as a rock rather than as a mineral. It is further classified according to grade, or purity, defined as the percentage of carbon content; according to type, depending on variations in the original plant composition; and according to rank, depending on the degree of metamorphism. Of these three criteria, purity (grade) and degree of metamorphism (rank) are most important in practice. FORMATION AND DISTRIBUTION The initial stage of coal formation is \Tpeat\t, decomposed organic matter, itself not regarded as a variety of coal. All types of peat easily retain water, and in saturated natural deposits include more than 75 percent moisture. The origin of coal is directly related to the dehydration and alteration of the plant parts that are initially conserved in peat deposits. The most critical stage in geologic preservation of peat and its alteration to coal depends on the deposition of sedimentary cover, or overburden, above the peat deposit. Only in appropriate geologic settings can a protective layer of sediment be deposited. The overburden serves by its weight to press out and reduce the moisture from the deposit, a chief criterion distinguishing peat from the lowest-rank lignitic coal. Peat deposits that do not receive a sedimentary cover are microbially degraded and oxidized, and eventually contribute a residue to \Thumus\t in the soil. Consequently the geologic history of a coal bed is initiated largely by its geologic setting, and the rank of the coal depends largely on its subsequent geologic history. Low rank coal closely related to peat but with less moisture is classed as \Tlignite\t, or brown coal. Coal with still less moisture is called bituminous, and the driest and most compressed coal is anthracite. Lignite also has the lowest carbon content and an anthracite the highest, so lignite is low grade and anthracite high grade. Bituminous coal is used in industry and in the production of \Tcoal tar\t and \Tcoke\t through destructive distillation, while the hard lustrous anthracite is used as a domestic fuel. Cannel coal is a highly volatile variety of bituminous coal that burns with a bright flame. Conditions that led to coal formation have existed from Devonian times, 390 million years ago, when the first peat deposits were formed from terrestrial vegetation. Older coal deposits that date from eras even before the advent of land plants must have been formed underwater from the organic remains of algae. Although coal from terrestrial plants of Devonian age is noncommercial and something of a curiosity, commercial coal deposits occur in rocks of all the later geologic periods. Coal of Permian age (280 to 230 million years old) and Triassic age (230 to 190 million years old) is found in Antarctica. Tertiary age coal (65 million to 2 million years old) is mined in Spitzbergen, and much of the low sulfur coal now being exploited in the western United States is also from the older Tertiary Period. Two beds of very low rank brown coal exceeding 300 m (1,000 ft) in thickness occur in the Tertiary deposits of Victoria in Australia. Much of the higher rank coal of the western United States occurs in Cretaceous deposits, 140 million to 65 million years old. Extensive coal deposits of Jurassic age (190 to 140 million years old) occur in the Angara Basin in Siberia. Triassic coal seems to be less extensive, but it occurs in the Deep River coal field in North Carolina, with a lesser deposit in the Richmond Basin of Virginia. The Kuznetsk Basin in Central Siberia and the Fushun Basin in China and Manchuria are noted for deposits of Permian age coal. Coal of Australia, India, and southern Africa is chiefly of Permian age. The most extensive and most important deposits of coal, however, are in the Appalachian Trough and the interior basins of North America; in Nova Scotia; in Great Britain; in the Belgian, Dutch, and Ruhr deposits in Germany; in Silesia; and in the great Donetz field of southern Russia. All of these latter deposits were formed in the Carboniferous Period, 340 to 280 million years ago. Coal is thus widely distributed and generally available in ample supply. The United States has approximately 31 percent of the known recoverable coal reserves of the world. The \TUSSR\t produces more coal than the United States, however; it has 23 percent of the known recoverable coal reserves. Europe has 13 percent and China 22 percent. At an annual production rate of about 3.5 billion metric tons (3.8 billion U.S. tons) worldwide, serious depletion of resources will take several hundred years. However, coal resources are not inexhaustible; in many areas the best and most accessible coal has already been depleted. UNDERGROUND MINING Little is known of the earliest methods for obtaining large quantities of coal. Coal was used in China in 1100 BC, in Wales during the Bronze Age, and as a home heating fuel in England during the late Middle Ages. In the United States, the discovery of coal was first recorded by French explorers on the Illinois River. Its actual use in the United States was first reported in Virginia in 1702, and Virginia recorded (1730) the first commercial mining in the country, near Richmond. Although coal was at first gathered from outcroppings, all large-scale early mining was done by underground methods, since none of the large equipment required for surface mining had been developed. The coal was extracted by hand, using picks and bars to remove it from the solid bed. Once extracted, it was shoveled into baskets, boxes, or wheelbarrows and taken outside. Later, mine cars were developed that were drawn over planks covered with iron straps and, eventually, rails. Motor power was supplied by humans, dogs, ponies, mules, and horses. In the late 18th century black powder explosives were introduced to blast the coal. Holes for the explosive were drilled by hand. Undercutting, a procedure in which the coal is cut out from the base of the coal seam to provide a free face for the explosive to break the coal, was also done by hand. Basic machinery developments during the late 1700s and the 1800s, notably the locomotive for transport and the pump for drainage purposes, greatly aided underground coal mining. Following World War I, oil and gas began to displace coal in the small industry and home heating markets because of their convenience. The introduction of diesel locomotives following World War II further reduced the traditional coal markets; the requirements of electric power generation partially offset the loss of these markets. The market loss created a demand for increased mechanization to increase productivity, so that the competition of oil and gas could be met. In modern mining, access to underground mines is gained by three primary methods. In the drift mine method, the seam of coal is exposed to the surface on the side of a hill or mountain, and the mine opening is made directly into the coal seam. This is generally the easiest and least expensive way to open an underground coal mine. In the slope mine method, an inclined opening through rock strata is used to gain access to the coal seam. If the coal seam itself is inclined, the slope may follow the seam. Slope mine access is usually used where less overburden is present. In the mine shaft method, the coal seam is reached by a vertical opening from the surface. Combinations of access methods may be used, depending on conditions of the coal seams. Once access is gained to the coal bed, three primary mining systems are used in the United States. Classified according to the equipment used, they are the conventional, continuous, and longwall methods. Conventional Mining In conventional mining the coal is first cut with an undercutter, a large chain saw on wheels. Holes are then drilled in the coal, using mobile or hand-held electric or hydraulic drills. The holes are loaded with explosives or other breaking methods that dislodge the coal from the seam. A loading machine places the coal into a specially designed shuttle car, which transports it either to a conveyor belt or to mine cars for transportation to the surface. Roof support in the mine is now necessary; it can be provided by wooden timbers, steel beams set on posts, or most frequently by roof bolts, which are steel rods anchored into holes drilled in the roof, laminating the overlying rock strata. This is similar to laminated wooden beams that greatly strengthen individual thin sections of wood. Continuous Mining The continuous mining system uses a single machine, called a continuous miner, that replaces the undercutter, drill, explosives, and loading machine used in the conventional mining system. This machine breaks the coal and loads it into shuttle cars for transportation to either conveyor belts or mine cars. The three types of continuous miners are the boring, ripper, and milling machines. The boring machine cuts or breaks the coal with arms that rotate flat against the coal face. This machine produces an arched opening in the working place and provides advantages in roof support, but the height and width of the opening created is restricted. The ripper miner cuts the coal with a series of chains operating vertically on the coal face. This machine, similar to an undercutter, can load the coal that it cuts or breaks. The ripper miner is more flexible in the height and width of the mine opening it can create. The milling or drum miner cuts the coal with bits mounted on heads or drums rotating vertically parallel to the coal face. It is widely used in the industry today. Longwall Mining In the longwall mining system, large blocks of coal, 100 to 200 m (300 to 600 ft) in width are extracted that had been exposed previously by other mining systems. These blocks are completely extracted by continuous operations, using self-advancing hydraulic jacks to provide roof support during the mining operations. These jacks (chocks) advance as the coal is mined, thus allowing the roof behind the jacks to collapse. Longwall mining machines cut the coal parallel to the coal face. The broken coal falls into a chain conveyor that removes it to a conveyor belt. The two general types of longwall mining machines are plows (planers) and shearers. The plow is a blade arrangement pulled across the face by a heavy chain; it is similar to the plow used by farmers. This machine, which cuts to a depth of about 8 to 15 cm (3 to 6 in), also forces the broken coal into the chain conveyor. The shearer is a rotating drum with either a single or a double drum; it is similar to a drum miner. The shearer can cut to a depth of 69 cm (27 in). Developing the Mine Using the above-mentioned techniques, mine development follows a "room and pillar" plan. Rooms are the openings from which coal is extracted, and pillars are the blocks of coal that are left for roof support. Sets of rooms are connected with cross cuts (openings driven at an angle to rooms); pillars are left between rooms and cross cuts. To open a coal mine, development mining is done. When the mine has been developed, pillars that were left during the development may be mined. This is known as retreat (second) mining. Often two or more seams must be mined; if the upper seam is mined first, the lower seams will not be affected. SURFACE MINING Surface mining, commonly known as strip mining, began in about 1910 when steam shovels began to be used. Today 60% of all coal is mined by this method. Strip mining is divided into three general classifications: area, contour, and open pit mining. Area mining prevails in the West and Midwest where the minable coal seams are relatively flat under either a level or gently rolling surface. The depth from the surface to the coal remains relatively constant. With these conditions, the property can be mined using either a dragline or a shovel to excavate the overburden in a series of parallel openings. The topsoil is removed first and stockpiled for later reclamation. The overburden from each opening is deposited into the previous opening after the coal has been extracted. Peaks created by this deposition must be leveled and covered with topsoil for reclamation. Contour mining is used in hilly and mountainous terrain, and is found in the eastern United States where the coal outcrops on the side of a hill. The mining begins on the outcrops and follows the outcrop along its contour. The topsoil is first removed and stockpiled for later reclamation; the overburden is then removed by shovel, dragline, scraper, trucks, front-end loaders, or bulldozers. This material is saved and eventually placed into another excavation. Succeeding excavation material is placed into the preceding excavation, leveled, and topsoil distributed over the area. This is known as the carry back method. Other contour methods are mountaintop removal and valley fill, which are valuable in creating flat lands in hilly or mountainous areas. The topsoil is removed and stockpiled, and the overburden transported to a valley and used as fill. Then the coal is extracted and the land left in a level condition. The topsoil is then redistributed. Open pit mining is similar to quarrying. In the western states, coal seams vary from 12 to 30 m (40 to 100 ft) in thickness, with thin overburden. Because the coal seam is so thick, it is impossible to completely backfill the mined-out area with overburden. The overburden may be transported directly to mined-out areas by scrapers or trucks. Auger mining is used when the economic limits of normal strip mining have been reached. At this point the coal seam is still exposed in the "highwall." To recover the coal, several methods might be used. If a sufficient tonnage of coal remains, a conventional underground mine could be used. Lesser tonnages might dictate either a "punch" underground mine or augering. The auger is a a large bit and brace that bites into the coal while the spiral of the bits removes the coal. Coal can be mined by this method up to depths of 60 to 90 m (200 to 300 ft); the auger has sections that are added to obtain these depths. When the depth has been reached the auger flights are reversed and removed and the machine is moved to another solid face of coal. Robert T. Reeder COAL-MINING SAFETY Underground coal mining is the most hazardous industrial occupation in the United States. The principal hazard is explosion, caused by the presence in the mines of coal dust and methane gas, both produced in the mining process. The technology exists to prevent dangerous concentrations of both substances, but it is not always used effectively. Coal dust is also the cause of the disease \Tblack lung\t. The passage of the Federal Mine Health and Safety Act (1969) established the Mine Safety and Health Administration (MSHA), which laid down minimum mine safety and health standards, mandated a schedule of frequent mine inspections, and set rigorous civil and criminal penalties for mine operators and miners who violated the standards. The act also provided compensation for miners suffering from black-lung disease. Following implementation of the act, the mine fatality rate dropped (from 0.24 to 0.09 per 200,000 work hours) and remained at a low level through 1980. Contributing to the increased fatality rate since that year have been a number of major mine disasters--four in 1981 alone--which indicate that despite the precautions in the mine safety laws, or because of lax administration, mining still creates unacceptably high levels of hazard. The Reagan administration cut funding for MSHA and reduced the number of mine inspectors, stressing voluntary compliance with mine safety laws rather than punitive measures such as fines or imprisonment. In 1982, however, President Reagan lifted a hiring freeze that had been imposed at MSHA, and added $2 million to the agency's budget, in order to avert a protest strike threatened by the \TUnited Mine Workers\t. To support the financially troubled Black Lung Disability Trust Fund, which pays benefits to minors who are disabled by the disease, Congress had doubled the tax on coal paid by coal producers (1981), and at the same time, had tightened benefit requirements for miners claiming disability. In 1986, with the Fund operating at the deficit of $2.5 million, the tax on coal was raised once again. ENVIRONMENTAL CONSIDERATIONS Both the mining of coal and the use of coal for fuel are processes that have the potential for causing environmental damage. The problems associated with coal mining include the destruction of land when it is strip mined and the various surface effects of underground mining: slag heaps, long-burning mine fires, and the occasional undermining and collapse of the surface into abandoned mines. The environmental effects of coal burning are less visible but perhaps more serious. They include the dispersal in the air of particulate matter from coal smoke, the venting of sulfur compounds into the atmosphere with the subsequent acidification of atmospheric moisture (see \Tacid rain\t), the pollution of water used in coal fueling operations, and the pollution of land used to store coal residues, such as ash and the sludge from smokestacks. In 1977 the U.S. Congress passed the Surface Mining Control and Reclamation Act, which was intended primarily to protect those coal-bearing lands in the Western portion of the country which were slated for strip mining, although it included provisions for reclaiming some of the already damaged 525,000 ha (1.3 million acres) of strip-mined land in the Eastern coalfields. The act set standards for the environmental protection of strip-mined lands, which were to be enforced jointly by the states and the federal Office of Surface Mining (OSM). In the years since the passage of the act, however, relatively few strip-mined sites have been reclaimed. Strip miners have exploited loopholes in the law; states have been lax in enforcing reclamation requirements; and, although the OSM has assessed mining companies for millions of dollars in violation penalties, almost no fines have been actually collected. Under the 1970 Clean Air Act and the 1972 Water Pollution Control Act, coal-fueled electric generating plants and industries, such as steel, that use coal as fuel were required to begin installing smokestack scrubbers--devices that are designed to remove particulates and sulfur compounds from flue gases (see \Tsmoke\t). Because of their high cost to industry, the Reagan administration proposed easing such pollution-control requirements. COAL GASIFICATION Coal gasification is a process for converting coal to combustible gases that can be used as fuels or as raw materials for the manufacture of chemicals and fertilizers. (Coal liquefaction uses similar processes to produce liquid oil products as well as fuel gases.) Simple gasification processes have been in use since the early 19th century, and until the 1940s--when natural gas first became widely available--almost all European and U.S. fuel gas was produced from coal. Interest in coal gasification has recently been renewed, however, because natural gas reserves may begin to diminish in the near future. In 1980 the U.S. Congress authorized a $20-billion fund to be used by the energy industry to develop synthetic fuels, including coal gases and oils. Under the Reagan administration, much of this "synfuels" funding was suspended, although a few federally subsidized projects were continued and several unsubsidized research plants have been built by industry. In order to convert coal into gas, the heavy coal hydrocarbon molecules must be "cracked," or converted into lighter molecules in high temperature, high pressure processes involving the reaction of coal with water and oxygen. The sulfur and nitrogen that were present in the solid coal are removed as hydrogen sulfide and ammonia. Carbon monoxide and hydrogen are released to form synthesis gas, which can be burned as a low-thermal-content industrial fuel, or used as a feedstock for the production of fertilizers and other \Tpetrochemicals\t. Synthesis gas may be further processed to produce \Tmethane\t, a high-thermal-content gas that is a substitute for natural gas and can be transported through pipelines. The first commercial power plant using a nonpolluting technique for producing gas from high-sulfur coal began a 5-year test program in early 1986. The process in this experimental plant uses pulverized coal mixed with water and oxygen. This slurry is partially burnt in a gasifier, under very high temperature and pressure. High-pressure steam produced in the gasifier is drawn off and used to drive a steam turbine. The hot gas is cooled and its heat captured by water, producing more steam. Sulfur and nitrogen compounds are removed from the cooled gas, which then drives a gas turbine. Advanced gasification techniques for the large-scale production of fuel are still in the experimental stage, but they offer the potential for the pollution-free use of all grades of coal, including the high-sulfur grades that are now unsuitable for use because of the pollution they produce. COAL PRODUCTION In the decade 1950-60 the use of oil for energy began to soar in the United States, and U.S. coal production fell from 508 to 394 metric tons (560 to 434 U.S. tons). Production crept up in the 1960s, with most of the additional coal being used to fuel steam-powered electric generating plants (see \Tpower, generation and transmission of\t). The 1973 Arab oil embargo and the subsequent steep increases in oil prices, however, made the cost of coal competitive with that of oil for the first time since World War II. The United States produced about 834 million metric tons (920 million U.S. tons) of coal annually in the late 1980s. Eighty-five percent of this total production was used for electricity generation, and about 9 percent was exported, principally to Europe and Japan. Bibliography: Ackerman, Bruce A., and Hassler, William T., Clean Coal--Dirty Air (1981); Dix, K., What's a Coal Miner to Do? The Mechanization of Coal Mining (1989); Engineering and Economics Research, Inc., Staff, Emerging Clean Coal Technologies (1987); Gordon, R. L., World Coal: Economics, Policies, Prospects (1987); Hessley, R. K., Coal Science (1986); Johnson, C., and Hildebrant, E., Still Stripping the Law on Coal (1984); Merritt, R. D., Coal Exploration, Mine Planning and Development (1986); Seltzer, C., Fire in the Hole (1985); Wiener, D. P., Reclaiming the West (1980).