Coal is a blackish, complex mixture of compounds of carbon, hydrogen and oxygen. It also consist small amounts of nitrogen and sulphur compounds. Coal was formed by the decomposition of dead plants buried under the earth’s crust millions of years ago. Conversion of wood into coal is called carbonization.it is a very slow bio-chemical process. It takes place in the absence of air under high pressure and high temperature over a long period of time (about 500 million of years).

Wood contains about 40% of carbon, so depending upon the extent of carbonization process; four types of coal are found. These types differ with respect to carbon content, volatile matter and moisture.

Types of coal and carbon contents it contain and its use:

Peat: It contains 60% carbon contents. It is inferior quality of coal used in Kiln.
Lignite: It contains 70% carbon contents. It is soft coal used in thermal power stations.
Bituminous: It contains 80% carbon contents. It is common variety of coal used as household coal.
Anthracite: it contains 90% carbon contents. It is superior quality hard coal that is used in industry.

Coal has become a major source of organic compounds, because of destructive distillation. The strong heating of coal in the absence of air is called destructive distillation.