Types of coal

The term coal embraces a range of materials. Within this range there are a number of distinct types of coal, each with different physical properties. These properties affect the suitability of the coal for power generation.

The hardest of coals is anthracite. This coal contains the highest percentage of carbon (up to 98%) and very little volatile matter. As a result, anthracite from many sources is slow burning and difficult to fire in a power station boiler unless it is mixed with another fuel. There are large reserves of anthracite around the world, particularly in Asia. In consequence, power plants are being built to burn this fuel alone.

Though anthracite is abundant, the largest group of coals are the bituminous coals. These coals contain significant amounts of volatile matter. When they are heated they form a sticky mass, from which their name is derived. Bituminous coals normally contain above 70% carbon. They burn easily, especially when ground or pulverised. This makes them ideal fuels for power stations. Bituminous coals are further characterised, depending on the amount of volatile matter they contain, as high-, medium- or low-volatile bituminous coals.

A third category, sub-bituminous coals, are black or black-brown. These coals contain between 15% and 30% water, even though they appear dry. They burn well, making them suitable as power plant fuels.

The last group of coals that are widely used in power stations are lignites. These are brown rather than black and have a moisture content of 30-45%. Lignites are formed from plants which were rich in resins and contain a significant amount of volatile material. The amount of water in lignite, and its consequent low carbon content, makes the fuel uneconomic to transport over any great distance. Lignite-fired power stations are usually found adjacent to the source of fuel.

A type of unconsolidated lignite, usually found close to the surface of the earth where it can be strip-mined, is sometimes called brown coal. (This name is common in Germany.) Brown coal has a moisture content of around 45%. Peat is also burned in power plants, though rarely.

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