Coal combustion is a process that involves the burning of certain types of sedimentary or metamorphic rocks to generate heat energy. The primary use for coal combustion is to fire boilers in power plants, though it has also been burned for other purposes such as heating and a source of locomotive power. In countries such as the United States, only about 10% of the total coal consumed each year is used for purposes other than generating power. Billions of metric tons of coal are combusted each year, accounting for about half of all electricity generated throughout the world.
There are several different kinds of coal and coal precursors, all of which are formed when certain biological materials are exposed to very high pressures over long periods of time. Most of these materials came be used for fuel, from peat to anthracite coal. High concentrations of carbon render coal highly combustible and allow it to burn longer than materials such as wood. Sub-bituminous and bituminous coals are classified as sedimentary rocks and contain more impurities than the harder, metamorphic anthracite. All three are commonly used to fuel coal-fired power generators.
When coal is combusted as part of a power generation process, it is typically first pulverized into a fine dust. The coal dust is then ignited within a furnace that is attached to a boiler. Coal combustion can result in a great deal of heat, which causes water in the boiler to turn into steam. The steam may then be used to activate turbines that are capable of generating electricity.
One of the main coal combustion byproducts is ash, which may take the form of bottom or fly ash. Many of the impurities present in coal can escape from a power plant in the form of fly ash if proper measures are not taken, while others are trapped in the bottom ash. Both varieties of ashes can be reclaimed for use in cement or separated out into the base impurities such as aluminum and iron, though uranium and other fissionable materials are sometimes found as well. One way to cut down on the amount of ash generated is to combust a slurry of water and coal rather than airborne dust.
In addition to power generation, coal combustion is also used for various other industrial applications. One common usage for coal is to turn it into coke, which is a substance that is very rich in carbon. The coking process involves subjecting certain types of coal to very high temperatures without providing sufficient oxygen for full combustion to take place. Coke has several uses, though it is essential to the operation of blast furnaces in the production of steel.