A assignment on coal petrology
COAL FORMING EPOCHS, ORIGIN AND MODE OF OCCURRENCE Manash Pratim Gogoi th
M.sc. 4 semester; Roll no -07, Deptt. Applied Geology; Dibugarh University
SUMMARY This assignment is written on the basis of geological facts which govern the deposition and formation of coal during the process of sedimentation. The coal is a rock composed of combustible matters derived from the partial decomposition of plants. We shall consider coal as a biochemically formed sedimentary rock, although some geologists prefer to think of it as a metamorphic rock because it passes through various stages. The process of formation of begins with an accumulation of plant remains in an swamp. This accumulation is known as peat. Time, coupled with the pressure produced by deep burial and sometimes by earth movement, gradually transforms the organic matter into coal. Coals of
Tertiary and Permo-Carboniferous origin are important in India. In India, there are many deposits of coal are found and are producing coal upto the demand required by the countries economic need. Introduction Coal is an organic carbonaceous solid fuel of plant origin. In remote geological times, and particularly in the Carboniferous period, between 345 and 280 million years ago, much of the world was covered with luxuriant vegetation growing in swamps. Many of these plants were types of ferns, some as large as trees. This vegetation died and became submerged under water, where it gradually decomposed. As decomposition took place, the vegetable matter lost oxygen and hydrogen atoms, leaving a deposit with a high percentage of carbon. In this way peat bogs were formed. As time passed, layers of sand and mud settled from the water over some of the peat deposits. The pressure of these overlying layers,
movements of the Earth's crust, and, sometimes, volcanic heat, acted to compress and harden the deposits, thus producing coal. Various types of coal are recognized, according to their fixed carbon content. Peat, the first stage in the formation of coal, has a low fixed carbon content and a high moisture content. The carbon content is greater in lignite, the lowest rank of coal. Bituminous coal has even more carbon and a correspondingly higher heating value. Anthracite coal has the highest carbon content and heating value. Coal may be transformed by further pressure and heat into graphite, which is almost pure carbon. Other components of coal are volatile hydrocarbons, sulphur and nitrogen, and the minerals that remain as ash when the coal is burned. Some products of coal combustion have detrimental effects on the environment. Burning coal produces carbon dioxide, among other byproducts. Some scientists believe that, owing to the widespread use of coal and other fossil fuels, the amount of carbon dioxide in the Earth's atmosphere could increase to such an extent that changes in the Earth's climate will occur (see Global Warming; Greenhouse Effect). Also, sulphur and nitrogen in the coal form oxides during combustion that can contribute to the formation of acid rain. Acid rain is the result of a series of complex reactions involving chemicals and compounds from many industrial, transport, and natural sources. Sulphur dioxide (SO2) emissions from new coal-fired facilities are now controlled in many countries. As a result, sulphur dioxide emissions have dropped in those countries even though coal use has increased. Bituminous Coal, or soft coal, variety of coal intermediate in constitution and properties between anthracite, or hard coal,
and lignite, or brown coal. It is the most common kind of coal, used mostly for producing coke, a substance used in the refining of iron ore. It burns with a yellow, smoky flame. Anthracite (Greek, anthrax,”coal”), hard coal with the highest fixed-carbon content and the lowest amount of volatile material of all types of coal. It contains approximately 87.1 per cent carbon, 9.3 per cent ash, and 3.6 per cent volatile matter. Anthracite is glossy black, with a crystal structure and conchoidal, or shell-like, fracture. It is used mostly as a fuel and as a source of industrial carbon. Although harder to ignite than other coals, anthracite releases a great deal of energy when burned and gives off little smoke and soot. Anthracite was formed primarily at the end of the Carboniferous Period by earth movements that generated heat and pressure which transformed the carbonaceous material then present in the Earth. The world's leading producers of anthracite are, in order, China, the countries of the former Soviet Union, North Korea, South Korea, Spain, and Germany. Lignite, variety of coal, also known as brown coal, intermediate in quality between peat and bituminous coal. Geologically, lignite is of comparatively recent origin, occurring in Cretaceous and Tertiary strata. It is usually brownish-black in colour and often shows a distinct fibrous or woody structure. Lignite is inferior in calorific value to ordinary coal because of its high (30-75 per cent) water content and low (60-68 per cent) carbon content; the high (52.5-62.5 per cent) content of volatile matter causes lignite to disintegrate rapidly on exposure to air. The heat value of lignite is 7,807 kJ (7,400 BTU) per lb.
