Coal

Coal Coal is a combustible compact black or brownish black sedimentary rock usually occurring in rock strata in layers or veins called coal beds or coal seams. It is formed from vegetation, which has been consolidated between other rock strata and altered by the combined effects of pressure and heat over millions of years to form coal seams. The harder forms can be regarded as metamorphic rock because of its exposure to elevated temperature and pressure. The quality of each coal deposit is determined by temperature and pressure and by the length of time in formation, which is referred as its ‘organic maturity’. The degree of change undergone by a coal as it matures from peat to anthracite is known as coalification. Coalification has an important bearing on the physical and chemical properties of coal and is referred to as the ‘rank’ of the coal. Ranking is determined by the degree of transformation of the original plant material to carbon. The ranks of coals, from those with the least carbon to those with the most carbon, are lignite, sub-bituminous, bituminous and anthracite. Low rank coals are typically softer, friable materials with a dull and earthy appearance. Higher rank coals are generally harder and stronger and often have a black and vitreous luster. Coal is composed primarily of carbon along with varying amounts of other elements mainly hydrogen, oxygen, nitrogen and sulphur. High-rank coals are high in carbon and therefore heat value, but low in hydrogen and oxygen. Low-rank coals are low in carbon but high in hydrogen and oxygen content. The relative amount of moisture, volatile matter, and fixed carbon content varies from one to the other end of the coalification series. The moisture and volatile matter decrease with enhancement of rank while carbon content increases i.e., carbon content is lowest in peat and highest in anthracite. The quality of...