What Is Coal?

Coal is a combustible, carbon-rich sedimentary rock formed over hundreds of millions of years from the compaction and chemical alteration of ancient plant matter buried under layers of sediment. It ranks as one of the most abundant fossil fuels by energy content and has been the primary fuel for electricity generation worldwide since the early twentieth century. Coal's high energy density, typically around 24 megajoules per kilogram for bituminous grades, made it the fuel of the Industrial Revolution and continues to supply a substantial fraction of global electric power, though its combustion produces carbon dioxide, sulfur dioxide, and particulate emissions that drive environmental and health concerns.

The study of coal draws on geology, chemistry, combustion engineering, and environmental science. In electrical and power engineering, coal is analyzed for its thermodynamic properties, combustion characteristics, and the design of pulverized-coal boilers, flue gas treatment systems, and integrated gasification combined cycle plants.

Coal Types and Ranks

Coal is classified into four principal ranks based on carbon content, energy content, and the degree of geological transformation from the original plant material. Lignite, the lowest rank, contains 25 to 35 percent carbon and retains high moisture, yielding the lowest heating value among the ranks. Subbituminous coal, at 35 to 45 percent carbon, represents a transitional stage and is mined in large volumes in Wyoming's Powder River Basin. Bituminous coal, with 45 to 86 percent carbon, is the most abundant rank in U.S. production and serves both electricity generation and metallurgical applications in steelmaking. Anthracite, the highest rank at 86 to 97 percent carbon, has the greatest heating value but represents less than one percent of global production. The U.S. Energy Information Administration's coal overview provides a detailed breakdown of rank characteristics, reserve distributions, and consumption trends.

Coal Mining

Coal is extracted by two primary methods determined by the depth and configuration of the seam. Surface mining, which includes strip mining and open-pit methods, removes overburden to expose shallow seams and accounts for the majority of U.S. coal production by tonnage. Underground mining, using longwall or room-and-pillar methods, recovers deeper seams that are impractical to reach from the surface. Longwall mining uses a shearer that traverses a long working face while hydraulic shields support the roof, achieving high extraction rates in thick seams. The extracted coal is crushed, sized, and cleaned to remove ash and sulfur-bearing minerals before transport by rail, barge, or conveyor to power plants or export terminals. Safety engineering in underground coal mines addresses methane accumulation, roof stability, and dust explosion prevention, topics addressed in standards from the U.S. Mine Safety and Health Administration. The IEA coal energy system overview tracks global production, trade, and consumption patterns.

Coal in Power Generation and Industrial Use

In coal-fired power plants, pulverized coal is blown into a combustion chamber where it burns at temperatures exceeding 1400 degrees Celsius, heating water in boilers to produce high-pressure steam that drives turbine generators. Flue gas desulfurization systems remove sulfur dioxide by reacting flue gas with a limestone slurry, while electrostatic precipitators and fabric filters capture fly ash. In integrated gasification combined cycle (IGCC) plants, coal is partially oxidized with steam and oxygen to produce a synthetic gas that fuels a gas turbine, with the hot exhaust then driving a steam turbine, improving overall efficiency and reducing criteria pollutant emissions. Coal also supplies the metallurgical coke used in blast furnaces to reduce iron ore, and coal-derived products including tars and light oils serve as feedstocks for chemical manufacturing.

Applications

Coal has applications across several industrial and energy sectors, including:

• Electricity generation in steam-cycle power plants across Asia, Europe, and North America
• Metallurgical coke production for iron and steel manufacturing via blast furnace reduction
• Synthesis gas production through gasification for chemical feedstocks and synthetic fuels
• Activated carbon manufacturing for water treatment and industrial filtration
• Carbon fiber and electrode precursors derived from high-rank coal and coal tar pitch