Coal

Coal is a combustible rock, black or dark brown in colour. It consists mainly of carbon, along with smaller amounts of hydrogen, sulphur, oxygen and nitrogen. It forms from plant and tree matter accumulated in water-rich, oxygen-poor areas, which then underwent prolonged pressure and heat.

Coal is one of the world's most abundant fossil fuels. It is used to generate electricity, produce iron and cement, and in other industries. It is also a major source of carbon dioxide emissions, which has made it central to climate change debate.

Formation

When large amounts of plant matter dies in a waterlogged area, it cannot fully decay because oxygen is scarce. This produces a fibrous substance called peat. As successive layers of sediment and rock accumulate on top, pressure and heat remove water and gas from the plant matter, increasing the carbon content. This produces different grades: lignite, sub-bituminous, bituminous, and anthracite, which has the highest carbon content.

Mining

Two methods are used. When the coal seam is near the surface, it is extracted by removing the overlying material (surface mining). When it lies deep underground, mining proceeds through shafts and tunnels. Underground mining carries hazards including gas explosions, roof collapse and respiratory diseases affecting workers.

Uses

The largest proportion is burned in power stations: heat boils water, and the steam drives turbines. A processed form called coke—coal heated without air—is used in blast furnaces to extract iron from ore. There are also cement and chemical plants.

Environmental impact

Burning coal releases carbon dioxide, sulphur dioxide, nitrogen oxides and fine particles into the air. Sulphur can cause acid rain. Ash residue contains heavy metals. Many countries have adopted policies to reduce coal use and shift to other energy sources, though the transition varies by region.