Coal or ‘black diamond’ as it is popularly known, is composed primarily of carbon and hydrocarbons, along with an assortment of other elements including sulphur. Coal as we are aware of is the buried and partly decomposed vegetation of the lost world, which has changed through immense period of geological time into brown or black combustible rock. In this sense, coal is a fossil fuel and the energy thus generated from it is of biological origin – bioenergy.
The coal age
Coal was the first fuel exploited for commercial energy. Although, ancient people knew the use of coal, it was only since the 9th century that English houses begun using coal. By the 13th century, its increased use in industry became noticeable which paved the way to industrial revolution in England. The extensive use of coal in India, however started in the last quarter of the 18th century.
Coal can be used as a primary fuel, i.e. it can be directly burned to obtain energy and it can also be used as a secondary fuel, burned to generate electricity as well as to produce convenient liquid fuel. From the daily household uses to propelling a ship, almost all activities in our modern industrial society make partial or total use of coal generated energy.
Coal has many by-products from which a range of secondary products evolve, such as ammonia, benzene, coal-gas, coal-tar, fertilizer, medicines, naphtha, paints, phenol, plastic etc.
Moreover, a significant percentage of the world’s coal is transformed into ‘coke’. Coke is a grey, hard and porous carbonaceous residue derived from low-ash, low-sulphur bituminous coal from which the volatile constituents are driven off by baking temperature are as high as 1000 degree Celsius so that the fixed carbon and residual ash are fused together. Coke is used as a fuel and as a reducing agent in smelting iron ore in blast furnace.
Types of Coal
Coal has been classified according to the percentage of carbon present within. In normal fossilization process, coal formation begins when vegetation is buried and partly decomposed to form peat. Peat is a fibrous, pale to dark brown coloured, somewhat porous and light- weight intermediate which is the initial stage of any type of coal forming rocks. It comprises of about 35 per cent carbon, containing various materials like biogenetic gases, volatile substances and water. Peat generates enormous smoke during burning.
Lignite or soft brown coal is formed from peat after the evaporation of aqueous parts and other gases. This is the lowest quality coal, crumbly in texture and contains about 30 to 50 per cent carbon. Lignite is mainly used for the preparation of petrol and natural gases and in warming-pans.
By the increasing pressure and temperature of the overlying sediment, lignite coal transforms into better quality bituminous. It contains about 50-85 percent carbon. 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. Another type, i.e., sub bituminous coal is a coal whose properties range from those of lignite to those of bituminous coal and are used primarily as fuel for steam-electric power generation. It may be dull, dark brown to black, soft and crumbly.
Anthracite is the highest class of coals containing about 90-95 percent carbon. It is hard ,and black in appearance with shiny metal like surface. It takes more time to burn and while burning it emits a smokeless blue flame. Anthracite coal is used primarily as a space heater in residential and commercial places.
How was coal formed
Scientists have unanimously admitted that coal is a product of plant origin. The argument that arises here is when the swamp or the basin of birth is analysed, some argue that coal originated in a sea full of algae or in lakes whereas others argue that the vast quantities of wood having fallen into the water bodies produced it. There are a few who think that great forests or woods in a huge drift, pushed by a great flow of water produced coal.
Presently it is an established fact thatcoal begins as peat at the very first stage. Later other classes of coals are produced by the ever-increasing pressure and temperature of the overlying sediments. It has been estimated that a 30cm thick layer of bituminous coal requires disposition of plant remains for 125-150 years while an anthracite layer of the same thickness requires nearly 175-200 years for deposition. For transformation of coal from plant remains, temperature to tropical climate and moderate to heavy rainfall is required. Transformation of coal begins with biochemical erosion and ends with the stage of 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.
Coal forming forest plants
Leaves, stems, spores, tree trunks, branches, plant roots, resins, charred wood from swamp fires, other organic and mineral 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 ‘stems’ which are separated by layers of other rocks. A stratifies 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 swamps serve as a source of coal. An enormous amount of fossil evidences has been recovered so far which proved the presence of the world’s first forests in Permo-Carboniferous period. This forest association, comprising a variety of plant groups such as lycopsids, horsetails, ferns, pteridosperms, coniferophytes, etc., had faced the 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 worlds coal deposits.
The great Indian coal seams
India is rich in her coal wealth and possesses a significant place in the annual upraising of coal in the world. Coal is extracted from the ground either by underground mining, open pit mining or strip mining.
According to the Indian Minerals Yearbook’s 56th edition, India has produced 662792 thousand tonnes coal in the year of 2016-17. Some of India’s largest coal producing states and their produce for the financial year of 2016-2017 are listed below:
|Coal producing states||Produce in the year 2016-17(in thousand tonnes)|
|Jammu and Kashmir||10|
Lignite is obtained from the Cretaceous rocks of Assam and in the Tertiary rocks of Assam, Kashmir, Kerala, Rajasthan and Tamil Nadu. A traceable amount of lignite is also found in the Darjeeling and Jalpaiguri districts of West Bengal.
Coal and Environment
Coal is the most impure of fuels. It causes great amount pollution in the environment. Its impurities range trace quantities of many metals, including uranium, thorium, large quantity of aluminium, sulphur and iron.
Various carbon containing compounds, oxides and hydrocarbons, particularly carbon dioxide which are considered as the chief air pollutants are released during coal combustion causing heavy rise in the green house gases and then air pollution. Apart form the industrial and household practices, emissions from the thermal power plants represent the largest source of artificial carbon dioxide generation. According to most climatologists, this is a primary cause of the green house effect and global warming. Another pollutant from the thermal power plant emissions, is fly-ash. It remains as suspended particulate matter in air causes pollution.
Coal accounts for 60 percent of the country’s commercial power requirements. Besides being a prime source of industrial energy, it is also the raw material for countless coal-based products. However, large scale uses of coal throughout the world produces huge amount of pollutants which brings forth situations unsuitable for the healthy sustenance of the biosphere.