Discovering Coal

By: Sulagna Chattopadhyay
The plant inhabitants of prehistoric swamps serve as a source of coal. Peat, lignite, bituminous and anthracite are the major varieties of coal found.
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Unassuming it may be, the black lumps of coal at your barbecue party, but its strength is legendary. Coal has been one of the most widely used sources of energy either for domestic use, or a fuel for power generating plants or for that matter in railways and steamships. Before the humans knew that use of coal, wood and charcoal were used as fuels. The Graeco-Romans were the first people in history to use coal as a domestic fuel.

Gradually, its use spread to other countries in Europe where coal was used to keep homes warm in the cold weather. However, it was from the late 18th century onwards that coal began to be used in the steam engines and became the cornerstone of the industrial revolution in Britain. Thus began the commercial mining of coal in different countries. Today, apart from being an important source of energy, coal is used in the production of materials like tar, pitch (this is a solid volatile substance obtained from the distillation of coal in the total absence of air, not to be confused with coking coal), ammonia, fertiliser, drugs and also in the production of dyes.

Forests of Carbon

Large, heavy creatures roamed the earth millions of years ago. Dinosaurs were a reality in the carboniferous age. Move your mental eye away from the dinosaurs in the movie Jurassic Park, and focus on the lush trees and thick vegetation that provided the backdrop of heightened action. That lush vegetation is coal today. In the geological time scale Jurassic and Triassic are part of the carboniferous age.

Although scientists have unanimously admitted that coal is a product of plant origin, its region of production is open to debate. Some argue that coal originated in a sea full of algae or in lakes. Others argue that the vast quantities of wood fell into water bodies and produced coal in situ. A few think that great forests or woods were caught in a huge drift, pushed by a great flow of water, eventually producing coal.

It has been estimated that a 30 cm thick layer of bituminous coal required the deposition of plant remains for 125-150 million years while an anthracite layer of the same thickness required nearly 175-200 million years of deposition. For transformation of coal from plant remains, temperate to tropical climate and moderate to heavy rainfall was required. Transformation of coal began with biochemical erosion and ended with chemical conversion in presence of high temperature and pressure exerted by the overlying sediments. In fact almost all the delicate parts of a plant are preserved and ultimately transformed into coal. Over time, layer upon layer was laid down, giving rise to the sedimentary process. Followed by intense heat, and compression, earth movements and contortions, the vegetation was compacted and the carbon in it turned to coal.

Coal Forming Forest Plants

Leaves, stems, spores, tree trunks, branches, plant roots, resins, charred wood from swamp fires, other organic and mineral (inorganic) matter were deposited within the ancient swamp basin. Accumulation of mud, silt and other sedimentary rock forming substances with these materials, constitute a coal bed. Coal, therefore occurs in a series of layers called ‘seam’ which are separated by layers of other rocks. A stratified scale can establish the age of coal deposits and their position in allied beds. Such specific information can establish the suitability of coal for energy production, chemical separation and manufacture of steel.

The plant inhabitants of Palaeozoic-Mesozoic (and to some extent Cenozoic) swamps serve as a source of coal. An enormous number of fossil evidences recovered so far prove the presence of world’s first great forests in the permo-carboniferous period. This forest association, comprising a variety of plant groups such as lycopsids, horsetails, ferns, pteridosperms, coniferophytes, etc., had faced mass extinction at the end of Permian (about 250 million years ago). The complete burial and transformation of these plants resulted in the Carboniferous coal (estimated age 280-360 million years), which today constitutes the majority of the world’s coal deposits.

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Besides coal of Permo-Carboniferous origin, formation also took place in three other geologic periods. Thus we have Triassic coal (estimated age 205-245 million years), Cretaceous coal (estimated age 70-140 million years) and Tertiary coal (estimated age 2-70 million years). Plant groups like ferns, cycadophytes, ginkgos, coniferophytes and few primitive angiospermous members shared the status of Mesozoic (both for Triassic and Cretaceous) coal formers. Tertiary coal, the youngest among the world’s coal deposits, evolved from perfect preservation of certain deciduous angiospermous genera, ginkgos and some conifers.

Coal is not always Black

Although chemically coal contains carbon, which not only gives it its colour but also determines its heating capacity, hydrogen, nitrogen, and substances like sand, mud, gravel dilute and deplete its darkness. Diluters of coal’s dark beauty such as sand gravel, mud and moisture, lower the heating capacity too and render certain deposits useless. A classification based on heating capacity and impurities is given below:

Peat: It is the first stage in the formation of coal and contains high amount of moisture but very low carbon. In a normal fossilization process, coal formation begins when vegetation is buried and partly decomposed to form peat. Peat is a fibrous, pale to dark brown, somewhat porous and a light-weight intermediate. It looks like charcoal and is also known as bog coal. Because of the low carbon content (i.e. 25 to 30 per cent), peat has a low heat value, and therefore it is not generally used as an industrial fuel. Its calorific value is only 75 therms per tonne as compared to bituminous, which is approximately 275 therms.

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Fig. 2: Shaft Mining

Lignite: Also called brown coal, as it is usually brownish in colour and crumbly in texture. Lignite or soft brown coal is formed from peat after the evaporation of aqueous parts and other gases. Lignite is mainly used for preparation of petrol and natural gases and warming-pans. Lignite has a high moisture content and emits a lot of vapour and smoke when burned. It is soft in disposition, has little heat value (because its carbon content is only 35 to 40 per cent and the calorific value is even less than 24 megajoules/kg) and easily cracks and crumbles when exposed to air. It is generally used only as a supplement to bituminous or anthracite.

Bituminous: It is the most widely used coal type in the world. By the increasing pressure and temperature of the overlying sediment, lignite transforms into better quality bituminous. Comparatively hard bituminous coal is powdery in texture and with characteristic stratification. It generates less amount of smoke during burning and is mainly used in industries, thermal power plants, households, steam locomotives and in gas production. Its carbon content is very high, about 80 per cent, giving it a black appearance and a hard texture. Its high carbon content also renders a high heat value of 26.7 megajoules/kg. In addition, low emission of smoke and minimal deposits makes bituminous coal the most popular industrial choice. It is this coal when heated in a special oven produces coke, which is an essential raw material for the iron and steel industry.

Another type, i.e., sub bituminous is a coal whose properties range from those of lignite to those of bituminous and are used primarily as fuel for steam-electric power generation. It may look dull, dark brown to black, soft and crumbly.

Anthracite: It is the hardest and the best type of coal with a shiny black appearance. With a carbon content above 90-95 per cent and little impurities anthracite’s heating capacity is higher, burns longer and leaves little residue and smoke. Yet anthracite, though the best, is not popular. Firstly, because the deposits are scarce, and secondly, because it’s the hardest coal mining is not only difficult but expensive too. Thirdly, anthracite has a high ignition temperature, and takes a long time to kindle. Thus, anthracite constitutes only about 5 per cent of the total coal production of the world and, is not commonly used in industry and transport. However, it may be used for domestic heating, bakeries and for boilers.

Extracting Coal

Man has discovered many ways to access this useful resource. Some of the methods are listed below:

Stripping or opencast mining: Opencast mining or stripping is well suited for areas where coal seam lies at or near the surface not more than 60 m deep. The seam should be more or less horizontal with the prerequisite for a thin and soft overlying strata, very much like a crumbly black current pie.

This is the easiest way to mine, as it hardly disturbs the underlying strata, although large tracts of agricultural land may be devastated. Modern conservation techniques can now restore former areas of opencast mining, examples of which may be seen in the Appalachians and Australia.

Hill slope boring: Giant sized augers (boring instruments) are used to dig out coal on hill slopes. These augers can reach as far as 105 m below the ground.

Underground mining: Also known as drift or adit mining where a tunnel is cut into the coal bearing stratum. This type of mining operation is undertaken in hilly areas, where there is a slightly inclined or horizontal coal seam with a thick overburden.

Slope mining: This is practiced in areas with steeply tilted coal seams or where coal is below a thick overburden. An inclined tunnel known as slope is constructed and a conveyor belt of a cable car is used for bringing out coal through the tunnel.

Shaft mining: This method of mining is used for reaching deep-seated seams (305 m to 1500 m below the surface). Vertical shafts are sunk to reach the coal bed, and a network of galleries is dug underground. Lifts or box like cages are used to access the mine. Often explosives are set in the coal face for loosening coal into lumps.

Shaft mining is the most expensive of the mining methods because of the overhead costs of ventilation, lighting, water supply and underground haulage. The safety in the mines has to be ensured by providing proper ventilation so that fire accidents don’t occur. This also helps reducing health hazards for miners. Miners also need to be aided with better geological information pertaining to the seasonal movement of water tables in order to equip them against the danger posed by the crushing inflow of water. Also efficient pumping stations to pump the water out of the mines need to be placed. The mining company must take precautions against such possibilities as gas poisoning, explosion, floods and the collapse of tunnel roofs.

 

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