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Coal is an organic sedimentary rock that is formed by the compaction and hardening of plant remains. It is typically black or brownish-black in color. It consists mostly of carbon, but includes other elements, primarily hydrogen, sulfur, oxygen, and nitrogen. Coal typically occurs in layers, called coal seams, interbedded within other sedimentary rock strata, such as shales and sandstones. Most coal seams are 0.5 to 3 meters thick, although some reach thicknesses of more than 30 meters. Coal seams can extend for hundreds, sometimes even thousands, of square kilometers.
Because coal is mostly composed of carbon, it is a rock that burns in air. The heat that is released by burning coal is used as the primary source of energy for the generation of electricity. About 93 percent of the coal consumed in the United States is used for producing electric power. Power plants that generate electricity use the heat from burning coal to boil water and make steam. The release of steam is then used to turn turbines that drive electric generators. Across the United States, there are more than 600 coal-fueled power plants. Together, these plants create almost half of all the electricity produced in the United States.
Coal has many industrial uses including the production of metal, cement and paper. In addition, coal-derived compounds are used in making plastics, roofing, linoleum, synthetic fibers, insecticides, fertilizers, paint products, and medicines.
Most coal was formed from the lush growth of trees and plants in coastal freshwater swamps. Such swamps are now rare, but were widespread at various times in the geologic past. For example, about 300 million years ago, average global temperatures were similar to those of today. At that time, the continent of North America was located along the equator. The hot and humid climate at that location supported the growth of huge freshwater swamps, especially along river floodplains and deltas. These swamps extended hundreds of square kilometers and were full of ferns and large trees. As plants and trees died, their remains sank to the bottom of the swampy areas and more plants and trees grew on top of them. In this way, thick deposits of almost pure plant debris built up, eventually forming a soggy, dense material called peat.
Over long periods of time, sand, clay, and other mineral matter carried by nearby rivers buried the peat. Upon burial under several kilometers of sediment, the pressure caused by the weight of the material above the peat caused water to be squeezed out. Also, temperature generally increases with depth within Earth's crust (between 25-30° C per km of depth). Increasingly deeper burial and the heat associated with it caused some of the plant matter to be altered chemically. As the material underwent change, gases such as carbon dioxide (CO2) and methane (CH4) were produced. Pressure from burial forced these gases out of the deposit. Under continual compression, volatile gases were lost and the deposit was gradually compacted to form solid coal.
Coal can be classified according to the amount of carbon it contains and on the amount of heat energy it can produce. When peat continues to be compressed and heated, it becomes denser and the relative amount of carbon it contains increases. Over more time and pressure, peat forms a type of soft brown coal called lignite. Carbon concentrations in lignite are generally around 40 percent. Over more time, and under increasingly more pressure, lignite may become bituminous, or soft, coal. Bituminous coal forms at temperatures between 100° C to 200° C and contains up to 85 percent carbon. Under considerable heat and pressure, bituminous coal may become anthracite, or hard coal. Anthracite forms at temperatures between 200° C to 300° C and at depths of 8 to 10 kilometers. In anthracite, carbon concentrations reach 90 to 95 percent. The greater the percentage of carbon, the more energy released when the coal is burned.
Coal deposits are always interbedded with other sedimentary rocks, mainly sandstones and shales. This occurs because the environments in which sediments are deposited change with time. For example, an area that was a coal swamp may become buried by sand or mud from a nearby river system. Eventually the coal swamp reestablishes itself. Upon burial, the plant material is converted to coal, and the sand and mud form sandstone and shale beds. In this way, there is an alternation of other sedimentary rock types with the coal deposit.
The United States has more coal than any other country, about 30 percent of the world's known reserves. It is estimated that the United States has over 260 billion tons of coal that can be recovered and used. This is enough to last more than 200 years, based on the rate at which coal use used today. The amount of coal produced in the United States in 2010 was over one billion tons. Coal is mined in 26 states with almost 41 percent of all the coal produced in the United States coming from Wyoming (443 million tons in 2010). The next highest coal-producing states are West Virginia (135 million tons), Kentucky (105 million tons), Pennsylvania (59 million tons), and Montana (45 million tons).
In the United States, coal production is spread across three coal-producing regions. The Appalachian Coal Region contains much of the nation's bituminous coal and almost all of its anthracite. Coal is also abundant in the Interior Coal Region, with Texas being the largest producer. Coal in the Western Coal Region, from North Dakota to New Mexico, is mostly lignite and bituminous. This area includes Wyoming and the Powder River Basin. The Western Coal Region contains the largest coal mines in the world.
Coal found near the Earth's surface, generally at depths less than 50 meters, can be uncovered and removed by giant machines using a method called surface mining. Surface mining techniques account for 60 percent of coal produced in the United States. In surface mining, topsoil is removed first and set aside to be used later in reclaiming the land. Then, explosives are used to break up any rock above the coal seam. Following this, heavy machines, such as draglines, wheel excavators, and large shovels, remove layers of rock to expose the coal in an open mine. The coal is then drilled, fragmented, and loaded into large trucks and removed for processing. Once mining in area is finished, the layers of rock are returned to the site, the topsoil is replaced, and the area is replanted to reduce the environmental impact.
Underground mining methods are used in places where coal seams lie deep below the surface, usually at depths greater than 50 meters. Coal is removed from the ground through networks of tunnels, passages, and openings that are connected to the surface. If the coal appears at the surface of a hillside, a drift mine can be driven horizontally into the coal. Sometimes, a slope mine is constructed with the mine shaft slanting down from the surface to the coal. The most common type of mine is the shaft mine. This involves cutting vertical shafts to the coal. Underground mining operations rely on heavy tunneling equipment to extract the coal. For example, longwall mining machines have massive shearers that cut coal from a wall face. The coal then falls onto a conveyor belt for removal to the surface.
Underground mining operations pose several hazards. One hazard is the collapse of the mine as supporting coal is removed. To prevent this from happening, large hydraulic systems are put in place to support the floor, walls, and ceilings. Another hazard is flammable gas and coal dust. As coal is cut, methane trapped in the coal is released and dust particles are created. This hazard is reduced by ventilating and spraying the air with water.
After being mined, coal is prepared for easier transportation and use. Usually, this is done at a facility located near the mining site. The coal may be crushed and non-coal materials, such as rock and dirt, are removed.
After coal is processed, it is transported from a mine site to a vendor or user, such as a coal-fueled power plant. The way that coal is transported depends on distance. Generally, coal is transported over very short distances by conveyor belt and longer distances by truck. Trains and barges are much more economical for even longer distances. For example, Powder River Basin coal is transported by train to power plants as far away as Georgia. Most major power stations have their own railways. Alternatively, coal can be mixed with water to form a coal slurry that is transported through a pipeline. Transportation costs account for a large share of the price of the coal that is delivered.
Coal production and use can affect the environment in various ways. For example, mining coal involves the removal of at least some rock that surrounds the coal. This rock is stored in mine openings or nearby the mine until it is replaced. In many cases, this rock contains the mineral pyrite (FeS2). When pyrite comes in contact with water and air, it can form sulfuric acid. As water drains from the mine site, the acid flows into streams and rivers, impacting fish, plants, and aquatic animals. It can also percolate down into the ground, leaching heavy metals from rock and depositing them in the groundwater system.
The burning of coal is also cause for environmental concern. Coal-fueled power plants emit sulfur dioxide (SO2) and nitrous oxides (NOx). These oxides react with water vapor in the air to form tiny droplets of sulfuric and nitric acid. Rain that includes these droplets is acidic and causes damage to vegetation and creates acidic streams and lakes that are harder for fish to live in. Coal-fired power plants also emit carbon dioxide, a greenhouse gas that some believe is changing the energy budget of the Earth and causing global climate to warm.
Efforts are being made to control the gases emitted from coal-fueled power plants. For example, scrubbers are currently used in most coal plants to remove some of the most harmful pollutants, especially sulfur dioxide. This technology involves spraying the gases resulting from burning coal with limestone powder (either mixed with water or dry). Sulfur dioxide reacts with the limestone and is then filtered out.