Coal is a solid stratified rock, composed mainly of hydrocarbons and capable of being used as a fuel to supply heat or light or both. It consists of carbon­ised remains of vegetation accumulated either in situ or transported by water and deposited. Practically all Indian coal seems to be of the latter type.

Chemically it consists essentially of carbon and hydrogen with subordinate amounts of oxygen and nitrogen. These are combined in very complex ways (Krishnan, 1982, p. 283). The quality of coal is determined by the ratio of carbon present in it. Owing to the pres­sure and heat the vegetation buried underground is transformed into peat (the inferior variety of coal slightly different from wood with carbon content less than 40%).

Under the increasing pressure and heat with the passage of time peat is converted into lignite (also called brown coal with carbon content ranging between 40 and 55%). When coal is buried deeply, moisture expelled, and the material sub­jected to increased temperatures, a firmer bitumi­nous coal results.

It is the most popular coal in commercial use with carbon content ranging be­tween 55 and 80 per cent. The name is derived after a liquid called bitumen released after heating. It is also called a soft coal. Further compression yields anthracite, the highest-quality hard coal with carbon content between 80 and 95 percent. Bituminous coal also yields coking coal (coke), gas coal and steam coal. Coking coal results from the heating of coal in the absence of oxygen, this burns off volatile gases. It is used in steel industry. Gas coal yields gas which is utilised for industrial and domestic pur­poses. Steam coal is the name given to the coal used to produce heat in thermal electrical power generat­ing plants.

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Occurrence

The coal found in India belongs to two geo­logical periods: the Gondwana and the Tertiary, the former accounting for overwhelmingly greater pro­portions of both the reserves and production.

Gondwana Coal

The Gondwana coal belongs to three geologi­cal formations, viz., Karharbaris (Upper Sakmarian), Barakars (Lower Permian) and the Ranging (Upper Permian) series. Of the 113 coalfields found in the country, 80 belong to the Lower Gondwana period.

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Andhra pradesh

These account for over 98 per cent of the total reserves and 99 per cent of the production of coal in India. These coal fields occupy more than 74 sepa­rate basins covering a total area of about 45,125 sq km. These basins varying in size from about 1 sq.km. To 1550 sq. km. are mainly confined to the Peninsular India.

These basins occur in the valleys of the rivers like Damodar (Jharkhand-West Bengal), Son (Madhya Pradesh-Bihar), Mahanadi (Chhattisgarh-Orissa), Godavari, Wardha (Maharashtra-Andhra Pradesh), Brahmani, lndravati, Narmada, Koel, Panch and Kanhan. These are small sedimentary basins running through the above stated river valleys.

Each contains a number of seams which vary in extent as well as in thickness. Some of the seams are only a few cm thick, while others attain a thickness of more than 20 m (Kargali seam in Bokaro coal-field is 28 m thick). In many coal fields seams are inter-bedded with shale and limestone, the former being ferruginous at some places. Gondwana coal mainly consists of three series: (a) Talchir, (b) Damuda, and (c) Panchet. The Damuda series in­cludes three groups of strata namely, Raniganj, Baren and Barakar which date back to the Permian period (250 million years ago).

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The Gondwana coal is laminated bituminous to sub-bituminous type with carbon between 55 and 60 per cent, high volatile matter and ash contents. The coal is both coking and non-coking. Coking coal used in metallurgical industries occurs in coalfields like Raniganj, Jharia, Bokaro, Ramgarh, Giridih and Karanpura. Non-coking coal is called gas or steam coal. Coal is free from moisture but contains ash (13 to 20%), sculpture and phosphorus in variable quantities. During the last 20 years a number of coal wateriest have been established to make it suitable.

Tertiary coal is found in the rocks of post- Oligocene times (15-60 million years) in the extra- Peninsular areas including Assam, Meghalaya, Arunachal Pradesh. Nagaland, West Bengal (Darjeeling district), Jammu and Kashmir, and south­ern states of Tamil Nadu, Kerala and Pondicherry. The coal is intensely crushed, friable, high sculpture percentage and low carbon content (40-55%). The thickness of coal seams varies from 6 m in Rajasthan to 23 m in Assam. The lignite coal of Tamil Nadu is utilised for generating thermal power.

The Jurassic-Cretaceous coal of the Mesozoic era is found in Kachchh (Gujarat) and Assam. The coal is of inferior variety with carbon percentage below 30 per cent.