How Is Coal Formed?

Coal is a rock formed from the remains of the living things (plants). Therefore it is called a biogenic sedimentary rock. The Sun makes plants grow. The energy of the Sun received by plants during the process of photosynthesis is preserved in plant in the form of chemicals like carbohydrate and proteins. If these plants are preserved under pressure for millions of years they form coal. When coal is burned today, the ancient energy is released as heat.

Forests grow well in swampy conditions. Millions of years ago, forests died, and were buried in swamps before the wood had time to decay. As the mud and sand of the swamps slowly turned to stone, the makeup of the wood changed. Wood contains mainly cellulose which is made up of carbon, Hydrogen and oxygen. During the long process of coal formation, the hydrogen and oxygen were removed, leaving a concentrated deposit of carbon. Wood contains about 50 per cent carbon, coal can contain up to 90 percent carbon. Most coal began to form about 350 million years ago, during the carboniferous period. The huge swampy forests that grew then are treasured today as the world’s main coal deposits.

Stages in Coal formation:

The forest trees eventually die covered with swamp material. This is compressed into a layer. Before coal is formed a fibrous material called ‘peat’ develops. It is sometimes used as fuel and also as rich plant food. Peat forms when buried plant loses oxygen and compressed into the fibrous material. Sediments continue to be deposited that compress the peat into rock.  More oxygen is removed from the peat, turning it into soft brown coal called lignite. Eventually the wood is compressed so much that it becomes the compact glossy coal called bituminous coal.

How coal is mined.

Coal is extracted from the ground for mining. In many countries coal is found hundreds of meters underground. An underground coal mine is also called a pit. To reach the coal, miners first bore holes into the ground down to the coal seam. Then they dig tunnels along the seam, removing the coal as they go. In many mines a number of seams are worked at different levels. The miners travel along these tunnels to the coal face by railway.

In modern coal mines, the coal is cut form the face by special machines called shearers. As the coal is cut, it is carried back to the shaft on a conveyor belt or in small truck that run on rails. The coal is taken up to the surface in the lift.

Coal Gas Form Coal:

In 1727, Stephen Hales, a parish priest in Middlesex noticed that coal was heated in a sealed vessel it yielded an ‘inflammable air’. In 1801, Philipose Lebon, a French engineer suggested that such a gas could be used as a domestic fuel and to provide light. He also suggested that the gas could be piped from room to room by tubes. The production and distribution of coal gas gradually increased in many countries of Europe. In the 19^th century, the development of gas light changed life to a great extent. It provided light to house and streets.

Gas Mantle:

Carl Auer, an Australian physicist invented the gas mantle in 1885. He demonstrated that if a small mantle of gauze impregnated with oxides of Thorium and Cerium was placed around the flame it would glow white and increased the output of light. It was the, mantle which led to the popularity of gas light in the 19^th and the early years of 20^th century.

When coal is heated to above 1200 K out of contact with air, it gives a number of products such as coal gas, ammonia, coal tar coke. By fractional distillation of coal tar a number of aromatic hydro carbons are obtained.