Coal is a fossil fuel extracted from the ground by mining. It is a readily combustible black or brownish-black rock. It is composed primarily of carbon and hydrocarbons, along with assorted other elements, including sulfur.

Including inherent moisture, coal consists of more than 50 percent by weight and more than 70 percent by volume of carbonaceous material. Coal is formed from plant remains that have been compacted, hardened, chemically altered, and metamorphosed by heat and pressure over geologic time. It is suspected that coal was formed from prehistoric plants that grew in swamp ecosystems. When such plants died, their biomass was deposited in anaerobic, aquatic environments where low oxygen levels prevented their oxidation (rotting and release of carbon dioxide). Successive generations of this type of plant growth and death formed deep deposits of unoxidized organic matter that were subsequently covered by sediments and compacted into carboniferous deposits such as peat or bituminous or anthracite coal. Evidence of the types of plants that contributed to carboniferous deposits can occasionally be found in the shale and sandstone sediments that overlie coal deposits. It is believed that most coal was formed during the carboniferous era (280 to 345 million years ago).

Coal is primarily used as a solid fuel to produce heat by burning, which produces carbon dioxide, a greenhouse gas, along with SO₂. This produces sulfuric acid, which is responsible for the formation of suphate aerosol and acid rain. In electricity generation, the heat is used to create steam, which then is used to power turbine generators. Approximately 40% of the Earth's current electricity production is powered by coal, and the total known deposits recoverable by current technologies are sufficient for at least 300 years' use. Modern coal power plants utilize a variety of techniques to limit the harmfulness of their waste products and improve the efficiency of burning, though these techniques are not widely implemented in some countries, as they add to the capital cost of the power plant.

Types of coal

Bituminous coal is a dense coal, usually black, sometimes dark brown, often with well-defined bands of bright and dull material, used primarily as fuel in steam-electric power generation, with substantial quantities also used for heat and power applications in manufacturing and to make coke. Bituminous coal is the most abundant coal in active U.S. mining regions. Its moisture content usually is less than 20 percent. The heat content of bituminous coal ranges from 21 to 30 million Btu/ton (24 to 35 MJ/kg) on a moist, mineral-matter-free basis. The heat content of bituminous coal consumed in the United States averages 24 million Btu/ton (28 MJ/kg), on the as-received basis (i.e., containing both inherent moisture and mineral matter).

Anthracite is the highest rank of coal; used primarily for residential and commercial space heating. It is hard, brittle, and black lustrous coal, often referred to as hard coal, containing a high percentage of fixed carbon and a low percentage of volatile matter. The moisture content of fresh-mined anthracite generally is less than 15 percent. The heat content of anthracite ranges from 22 to 28 million Btu/ton (26 to 33 MJ/kg) on a moist, mineral-matter-free basis. The heat content of anthracite coal consumed in the United States averages 25 million Btu/ton (29 MJ/kg), on the as-received basis (i.e., containing both inherent moisture and mineral matter). Note: Since the 1980s, anthracite refuse or mine waste has been used for steam electric power generation. This fuel typically has a heat content of 15 million Btu/ton (17 MJ/kg) or less.

Lignite is the lowest rank of coal, often referred to as brown coal, used almost exclusively as fuel for steam-electric power generation. It is brownish-black and has a high inherent moisture content, sometimes as high as 45 percent. The heat content of lignite ranges from 9 to 17 million Btu/ton (10 to 20 MJ/kg) on a moist, mineral-matter-free basis. The heat content of lignite consumed in the United States averages 13 million Btu/ton (15 MJ/kg), on the as-received basis (i.e., containing both inherent moisture and mineral matter).

Subbituminous coal is a coal whose properties range from those of lignite to those of bituminous coal and are used primarily as fuel for steam-electric power generation. It may be dull, dark brown to black, soft and crumbly at the lower end of the range, to bright, jet-black, hard, and relatively strong at the upper end. Subbituminous coal contains 20 to 30 percent inherent moisture by weight. The heat content of subbituminous coal ranges from 17 to 24 million Btu per ton on a moist, mineral-matter-free basis. The heat content of subbituminous coal consumed in the United States averages 17 to 18 million Btu/ton (20 to 21 MJ/kg), on the as-received basis (i.e., containing both inherent moisture and mineral matter).

Coke is a solid carbonaceous residue derived from low-ash, low-sulfur bituminous coal from which the volatile constituents are driven off by baking in an oven at temperatures as high as 2,000 °F (1,000 °C) so that the fixed carbon and residual ash are fused together. Coke is used as a fuel and as a reducing agent in smelting iron ore in a blast furnace. Coke from coal is grey, hard, and porous and has a heating value of 24.8 million Btu/ton (29 MJ/kg). Byproducts of this conversion of coal to coke include coal-tar, ammonia, light oils, and "coal-gas". (Coke can also be made from petroleum)