Coal is a combustible, usually black or brownish-black rock formed from plant remains altered by burial over geological time. It is a fossil fuel and a nonrenewable source of energy, because its formation takes millions of years. Coal consists predominantly of carbon-rich organic material mixed with minerals and moisture. Its principal uses include electricity generation, industrial heating, and the production of coke for ironmaking. Its composition, heating value, and suitability for particular applications vary substantially between deposits. (usgs.gov)
Formation and geological occurrence
Coal formation begins where dead plant material accumulates faster than it decomposes, commonly in waterlogged wetlands. Limited oxygen availability slows decomposition, allowing organic matter to build up as peat. Subsequent burial beneath sediments compacts this material and exposes it to increasing heat. The physical and chemical transformation of buried peat into coal is called coalification. (pubs.usgs.gov)
During coalification, organic matter loses moisture and becomes relatively depleted in oxygen and hydrogen, while its carbon concentration increases. Some carbon and hydrogen form methane. Both burial conditions and geological history influence the resulting coal, but increasing temperature is generally more important than increasing pressure in determining its degree of maturation. Consequently, deposits of similar geological age can have different ranks. (pubs.usgs.gov)
Coal occurs in layers called beds or seams within sedimentary sequences. Deposits are distributed worldwide, especially in regions that once supported extensive marshes and forests. Geological investigation establishes seam thickness, depth, continuity, and composition, providing the information needed to assess potentially recoverable deposits. (usgs.gov)
Ranks and properties
Coal rank describes its degree of coalification, rather than its suitability for every industrial purpose. The main ranks, in ascending order, are:
- Lignite: Often called brown coal, it is relatively immature, moisture-rich, and low in heating value.
- Subbituminous coal: More mature than lignite, with properties intermediate between lignite and bituminous coal.
- Bituminous coal: Widely used for power generation; selected varieties are suitable for coke production.
- Anthracite: The highest major rank, generally hard and rich in carbon, with relatively little volatile matter. (eia.gov)
Rank and quality are distinct. Quality depends on the intended use and includes heating value, moisture, ash yield, sulfur content, trace elements, and coking behavior. A coal suitable for a power station may be unsuitable for metallurgical coke. Laboratory results also depend on their reporting basis: measurements for an as-received sample are not directly interchangeable with those expressed on moisture-free or moisture-and-ash-free bases. (pubs.usgs.gov)
Mining, preparation, and transportation
The choice of mining method depends strongly on the depth and geometry of a seam. Surface mining removes the soil and rock above a deposit, collectively called overburden, to expose the coal. Underground mining reaches deeper seams through shafts and tunnels. Both methods use machinery to extract and move the material, but their excavation requirements and effects on the surrounding land differ. (eia.gov)
After extraction, coal may pass through a preparation plant that removes unwanted rock and other impurities. Cleaning improves the usable fuel’s heating value and reduces some mineral contaminants, although it does not eliminate every impurity. Coal is transported by conveyors, trucks, railways, barges, and ships. Transportation can represent a substantial portion of delivered cost, which helps explain why some power stations are located close to mines. (eia.gov)
Energy and industrial applications
In conventional coal-fired power generation, combustion produces heat that turns water into steam. The steam drives a steam turbine connected to an electric generator, producing electricity. Coal also supplies process heat in industries such as paper and cement production, and some industrial facilities generate both electricity and useful heat. (eia.gov)
Certain coals can be heated with air excluded to produce coke, a carbon-rich material used in ironmaking. Coal processing also provides chemical raw materials, while conversion processes can produce gaseous and liquid fuels. These applications depend on coal’s chemical and physical properties, not simply on the amount available. (pubs.usgs.gov)
Coal-fueled steam engines played an important role in the Industrial Revolution. Improvements in engine efficiency extended steam power beyond mines into factories and mills. James Watt’s separate condenser, patented on January 5, 1769, reduced fuel consumption and helped make this wider application practical. (blog.sciencemuseum.org.uk)
Environmental and occupational effects
Coal mining can alter landscapes, disturb streams, and generate polluted drainage. Underground workings may cause the overlying ground to subside. Methane released from coal deposits creates an explosion hazard in mines and contributes to greenhouse gas emissions when discharged into the atmosphere. Exposure to respirable coal-mine dust can cause coal workers’ pneumoconiosis, also called black lung disease. (eia.gov)
Burning coal releases carbon dioxide, contributing to climate change. Other emissions include sulfur dioxide, nitrogen oxides, particulate matter, and mercury. Combustion also produces fly ash and bottom ash; contaminants from inadequately contained ash can enter groundwater. (eia.gov)
Pollution-control equipment addresses different components of these emissions. Scrubbers reduce sulfur dioxide, while electrostatic precipitators and fabric filters capture particles. Greater generating efficiency reduces the coal burned per unit of useful output. Carbon capture and storage separates carbon dioxide for subsequent underground storage; it addresses a different problem from conventional controls for sulfur compounds and particles. (eia.gov)