Ø These may broadly be defined as metamorphosed sedimentary rocks of carbonaceous character in which the raw material has mostly been supplied by plants of various groups.
Ø The original raw material passes through many biomechanical and biochemical
processes before it becomes a coal in technical terms;
Ø In most cases coals represent carbonized wood.
Ø The process of coal formation involves a series of stages similar to formation of sedimentary rocks such as wastage of forests and transport of the wood material through different natural
agencies to places of deposition, accumulation of the material in huge formations.
Ø Its burial under clays and other matter and its compaction and consolidation under superimposed load.
Ø Biochemical transformation of the organic matter so accumulated starts and is completed under the influence of aerobic and anaerobic bacteria available at the place of deposition.
Ø The degree of carbonification depends to a great extent on the time and type of environment in which the above processes have operated on the source material giving rise to different varieties
Ø It is the lowest grade coal that consists of only slightly altered vegetable matter. It may not be even considered as a coal. It has very low calorific value, high percentage of moisture and is rich in volatile matter.
Ø It is also known as brown coal and forms the poorest grade of coal with calorific value ranging between 6300-8300 B.th.V.
Ø It is compact and massive in structure with an upper specific gravity of 1.5 and hardness of 2.5 on
Mohs' Scale of Hardness.
Ø Some varieties of lignite may still show to a good extent the traces of original vegetable structure.
Ø These form a broad group of common coals having essential properties varying within wide limits.
Ø The fixed carbon ranges between 69-78 per cent and the calorific value between 9,500 B.th.V to
Ø Their common character is that they contain enough volatile matter, which makes them quite soft on heating, and they start agglomerating.
Some of bituminous coals may contain volatile matter to such a high extent as 30 per cent of their bulk.
Ø It is considered the highest-grade coal with fixed carbon ranging between 92-98 per cent.
Ø It has highest calorific value in coals and burns almost without any smoke, as the
volatile matter is negligible.
Ø Coals of different varieties are found to occur almost in all countries of the world, though in varying proportions.
Ø Coals form all-purpose fuels, some varieties being more suitable for specific industrial uses.