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 or moisture. When burned it produces little ash and relatively low levels of pollution. Its energy density is generally higher than other coals at 23 MJ/kg to 33 MJ/kg. Anthracite is typically slow-burning and often difficult to fire in a power station boiler unless it is mixed with another fuel. While its energy content makes it attractive as a power plant fuel, the difficulty with firing it and its cost does not, so it has traditionally been used for heating rather than industrial use. However, it is becoming more common as a power plant fuel as countries with large reserves, such as Russia and Ukraine, switch to anthracite to free natural gas for export.
While anthracite reserves are important, the most abundant of the 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 between 45% and 70% carbon. Moisture con- tent is between 5% and 10%. They burn easily, especially when ground or pulverized. This makes them ideal fuels for power stations. Bituminous coals are further characterized, depending on the amount of volatile matter they contain, as high-, medium-, or low-volatile bituminous coals. Some bituminous coals contain high levels of sulfur, which can be a handicap for power generation purposes.
A third category, called sub-bituminous coals or soft coals, are black or black-brown. These coals contain between 35% and 45% carbon and 15% to 30% water, even though they appear dry. They burn well, making them suitable as power plant fuels, and sulfur content is low.
The last group of coals that are widely used in power stations are lignites. These are brown rather than black and have a carbon content of 20–35%. Moisture content is 30–50%. Lignites are formed from plants that are rich in resins and contain a significant amount of volatile material. The amount of water in lignites, and their 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 around 45%. Peat is also burned in power plants, though rarely.