Coal

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Coal is classified as a sedimentary rock. It is a common non-renewable fuel used mainly in the production of electricity. It is a fossil fuel because it forms from dead plant matter. The quality of coal depends on how it formed; as the organic matter is subjected to greater heat and pressure, the carbon content increases.

Properties:

Coal is a combustible rock mainly composed of carbon along with variable quantities of other elements, mostly hydrogen, sulphur, oxygen and nitrogen. Coal occurs as layers, called coal beds or coal seams, that are found between other sedimentary rocks. Coal is slightly denser than water but less dense than most of the rocks of the Earth’s crust. The density varies depending on how porous (full of holes) the coal is. Sometimes the pores contain a gas called methane.

Coal properties vary depending on how much carbon is in the coal (coal rank). Black coal is so called because of its colour; it varies from having a bright, shiny lustre to being very dull, and from being relatively hard to soft. The term ‘black coal’ is used to refer to anthracite, as well as bituminous and sub-bituminous coals. Brown coal is also called lignite; it contains less carbon and so has less chemical potential energy than black coal and has more moisture in it.

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Uses

Brown and black coal are mostly used as a source of fuel for power stations. Burning the coal produces heat which is used to make steam, so the the coal is sometimes called thermal or steam coal. The steam spins turbines to generate electricity.
Using brown coal for power generation is problematic because of the high water content. It crumbles easily on exposure to the air which reduces its value as a fuel and requires specialised storage. It has also been known to spontaneously combust. Since the 1970s the amount of natural gas used to generate electricity has increased and there has been a gradual decline in the use of coal for power. The amount of greenhouse gases generated by burning gas for energy is far less than the volume produced by burning coal.

Certain types of bituminous coal are used to make an essential raw material for making steel — coke. This type of coal is also called metallurgical (steel making) coal and it is processed to produce coke. Coke is a hard porous substance composed of about 90% carbon, it is vital for making steel which is used in millions of items such as cars, trucks, fire engines, refrigerators and bridges.

Coal and peat is still used for residential and commercial heating in some parts of the world (e.g. Ireland and Finland). In its dehydrated form, peat is a highly effective absorbent for fuel and oil spills on land and water. It is also used as a conditioner for soil to make it more able to retain and slowly release water.

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Coal is also an important source of light aromatic hydrocarbons for the chemical synthesis industry. If coal is heated the gases and residues produced can be used in manufacturing plastics, explosives, dyes, pitch, ammonia, medicines, aspirin, soap, disinfectant, detergents, nylons, cosmetics, shampoo, toothpaste, synthetic rubber, fertilizers, cement, bricks and tiles. It is an important ingredient used in filters for water and air purification and in kidney dialysis machines. In addition coal is used to make carbon fibre, a very strong, light material used in construction, mountain bikes and tennis rackets.

Jet, a compact form of brown coal, is sometimes polished and has been used as an ornamental stone. Graphite which is often associated with coal, is almost entirely made of carbon; it is mostly used in pencils and when powdered, as a lubricant.