A assignment on coal petrology

COAL FORMING EPOCHS, ORIGIN AND MODE OF OCCURRENCE

Manash Pratim Gogoi

M.sc. 4th 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.