Coal

British coalfields in the nineteenth century.

A piece of bituminous coal

A piece of high-quality anthracite coal

Coal is a black rock, and a solid fossil fuel.

It is usually very hard. Coal is made of 65%–95% carbon. It also has hydrocarbons and some other compounds in it.

Coal is a metamorphic rock formed from peat by the pressure of rocks laid down later on top. The harder forms of coal, such as anthracite, have been changed by higher temperature as well as pressure.

Peat, and therefore coal, is formed from the remains of plants which lived millions of years ago in tropical wetlands, such as those of the late Carboniferous period (the Pennsylvanian). A similar substance made from wood by heating it in an airless space is called charcoal.

Coal can be burned for energy or heat. About two-thirds of the coal mined today is burned in power stations to make electricity. Coal is becoming less popular in new power plants as less expensive and less polluting technologies such as natural gas and hydroelectricity take over.

Coal can be dry-distilled (heated in high temperature in a place where there is no oxygen) to produce coke. Coke is even better fuel than coal, and coke can be used in smelting for reducing metals from ores.

Types

Coastal exposure of the Point Aconi Seam (bituminous coal; Pennsylvanian).

Under suitable conditions, plant material is transformed step by step into

1. Peat, which has industrial importance as a fuel in some regions, for example, Ireland and Finland. In its dehydrated form, peat is a highly effective absorbent for fuel and oil spills on land and water
2. Lignite (brown coal) is the lowest rank of coal and is used as fuel for electric power generation. Jet is a compact form of lignite that is sometimes polished and has long been used as an ornamental stone.
3. Sub-bituminous coal is used as fuel for steam-electric power generation. Also, it is a source of light aromatic hydrocarbons for the chemical synthesis industry.
4. Bituminous coal is a dense rock, black but sometimes dark brown. It used as fuel in steam-electric power generation, and for heat and power applications in manufacturing; also and to make coke
5. Steam coal was once widely used as a fuel for steam locomotives. In this specialized use it is sometimes known as sea-coal in the U.S. Small steam coal (dry small steam nuts or DSSN) was used as a fuel for domestic water heating
6. Anthracite is the highest rank: a harder, glossy, black coal used primarily for residential and commercial space heating.
7. Graphite is difficult to ignite and is not so commonly used as fuel: it is mostly used in pencils and, when powdered, as a lubricant.

Back in fashion

After 50 or 60 years of decline, coal is back in fashion.

"Despite stringent carbon emissions targets in Europe designed to slow global warming and massive investment in renewable energy in China, demand for this most ancient source of energy is greater than ever.

In fact, coal was the fastest growing form of energy in the world outside renewables last year, with production up 6% on 2010, twice the rate of increase of gas and more than four times that of oil". The world needs energy, and coal-fired power stations are the cheapest way to provide it. Population growth, especially in Asia, and the growth of the middle classes in China, are forces driving the need for more energy. To meet that need coal is required, because more favoured sources fall short. Here are the reasons:

1. Oil and gas are running out, and are getting much more expensive.
2. Few atomic power stations are being built.
3. Power from water turbines is only available in some places.
4. Power from wind farms has other problems, such as expense, unreliability of supply and pollution of the living environment.

Some of the world's largest economies are turning to coal. "Germany's decision to scrap all nuclear power and build more coal-fired power stations can only boost production further". On the other hand, coal burning is the most polluting form of energy. Though capturing the carbon produced is possible, it is seldom done.

Facts & figures

• Coal makes up about 40% of the world's carbon dioxide emissions from fuels
• Coal-fired power stations produce almost half the electricity produced in the US
• Coal, when burnt, gives off almost a third more carbon dioxide per unit of energy than oil, and 80% more than natural gas
• Coal provides about a quarter of the world's energy needs
• Almost 70% of world steel production depends on burning coal.