Most of the coal deposits are about 300 million years old. The recent coal deposits of the Tertiary Age are generally of inferior quality. Depending upon the percentage of carbon present, the coal can be grouped in four types, such as peat, lignite, bituminous and anthracite.
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1. Peat:
It represents the first stage of coal formation, i.e. from wood to coal, today, peat is being formed at many places. It has a high percentage of moisture and volatile matter. The carbon content in peat is less than 40 per cent. It burns like wood and gives more smoke and less heat. It leaves a large amount of ash after burning. Its low heating capacity reduces its value as an industrial fuel.
2. Lignite:
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It is generally regarded as the next stage of coal formation after peat. It is also known as the brown coal. Lignite is soft, but more compact than peat. The carbon content varies from 40 per cent to 60 per cent. Lignite has large percentage of moisture and less amount of combustible matter.
The increasing demand for coal has enhanced its use in thermal power stations and in some industries. In India, lignite is mostly found in Rajasthan, Tamil Nadu, Assam and Jammu and Kashmir states.
3. Bituminous:
It is the hard and compact variety of coal. The carbon content varies from about 60 per cent to 80 per cent. Almost 80 per cent of the world’s total output of coal is of the bituminous type. The moisture and the volatile contents are also less. This type of coal is also known bunker coal to the sailors.
It is widely used in the making of coke and gas. Coke is mainly used in the iron and steel industry for smelting iron ore in blast furnaces. Bituminous coal is found in Jharkhand, Orissa, West-Bengal, Madhya Pradesh and Chhattisgarh.
4. Anthracite:
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It is the hardest and the best quality of coal. The carbon content varies from about 80 per cent to 90 per cent. Anthracite, practically, has no volatile matter. It does not ignite easily, but once lighted, it has the highest heating capacity.
It burns for a long time and leaves very little ash behind. Only about 5 per cent of the world’s total coal is anthracite. In India this type of coal is found only in Jammu and Kashmir State and that too in very small quantity.