Coal is a combustible sedimentary rock composed mostly of carbon and hydrocarbons. It is a non-renewable resource. It is black or brownish-black in colour.
The energy in coal comes from the energy stored by plants that lived hundreds of millions of years ago in swampy forests.
Depending on its carbon content, it can be divided into -
Lignite – This is a brownish-black coal with high moisture and ash content, which has the lowest heating value of the four types of coal. It is considered an “immature” coal that is still soft. Its carbon content is 25-35%.It is used for generating electricity.
Subbituminous coal – This is a dull black coal with a higher heating value than lignite, and is used principally for electricity and heating.
Bituminous coal – It has a carbon content of around 60- 80% and is mainly used for power generation and for manufacturing another fuel called coke.
It is the most commonly used type of coal for electric power generation. It is a dark, hard coal that has a higher heating value than lignite and subbituminous coal, but a lower heating value than anthracite.
Anthracite – This is also known as "hard coal" that was formed from bituminous coal under increased pressures in rock strata during the creation of mountain ranges. It is very hard and shiny. This type of coal is the most compact and therefore, has the highest energy content of the four levels of coal. Its carbon content is 92-98%. It is used for heating and generating electricity.
Formation of Coal:
Trees and other plants that grew in swampy areas 300 to 400 million years ago died and their remains got buried in the swampy soil. Over time, these remains were covered with layers of sediments such as mud and sand.
In low-oxygen conditions, the buried plant material formed a dark brown material called peat.
Gradually more and more sediments got deposited over peat. Peat was compressed between layers of sediments and formed lignite.
Further compression resulted in the formation of bituminous coal.
Even further compression resulted in the formation of anthracite.
Coal is processed further to obtain useful materials such as coke, coal tar, and coal gas.
Coke - It is a hard, dry fuel produced by heating bituminous coal to a very high temperature in the absence of air. Coke has high carbon content and is used as a fuel and in steel manufacture.
Coal tar - It is a thick, black, opaque fluid obtained as a by-product of the process of manufacturing coke. It is used in anti-dandruff shampoos, ointments, soaps, perfumes, etc.
Coal gas - It is a gaseous fuel obtained as a by-product of the process of manufacturing coke. Before the development of natural gas supplies, it was used as domestic and lighting fuel.
