Origin of coal

There are many stages and concepts regarding the origin of

coal; these includes –

1. Accumulation of vegetable matter: the presence of

various plant structures as seen under the mcroscope

has proved beyond doubt that coals are fomed from

plant remians. It is now generally admitted that coal is

formed from the remains of plants that once flourished

in some areas of the earth where climate was facourable

for the luxurient growth of flora. The several kinds of

coal represent different degrees in the chemical

decomposition and physical alteration of the original

plant material. The change in chemical composition

during the various stages in the formation of coal from

the parent organic debris are shown in table below:

Carbo

n %

Hydroge

n %

Oxyge

n %

Nitroge

n %

Calorific

value

(Kilo

Joule/Kg

)

Wood 50.0 6.0 43.0 1.0 14,400-

17,400

Peat 57.0 6.0 35.3 1.7 13,800-

20,000

Lignite 65.0 5.2 28.3 1.5 23,000-

29,000

Bituminou

s

84.0 5.2 9.3 1.5 29,000-

35,000

Anthracite 93.5 2.8 2.8 0.9 35,000-

35,400

Table: chemical change from wood to anthracite

2. Accumulation of plant debris: there are existing

theories on the accumulation of vegetable matter. Once

is ‘insitu’ theory and the other is ‘drift’ theory:

a. Insitu theory: it is considered that old forests or

marshes due to disturbaces on the surface of the

earth were buried at the place of their growht

under a cover of sediments. In course of time

due to overburden pressure and temperature it

was transformed into coal. Upright fossil tree

trunks or fossill roots found in coal seams

suggests growht in the original position of the

forest.

b. Drift theory: according to this theory vegetable

matters are shifted from the place of their origin

by running water or glacier and accumulate in

deep lake, estuary, river valleys and afterwards

are covered with sediments like sands or clays.

Seat earths are often absent in the coal seams

produced by drifting of plant material and seams

lie directly on sandstone, conglomerate or shale,

upright tree trunks are absent. Because of the

drifted nature of accumulation of vegetable

matter the resultant coal seam shows wider

variation in composition, particularly a high

amount of detritus material such as sand or clay

in the coal seams. As a result the ash contents

are high in coals formed due to accumulation of

drifted vegetable mateial.

3. Transformation of vegetable matter into coal: is a very

complex process, which occurs in two distinct stages –

the biochemical and the geochemical (dynamochenical)

stages, which is designated as coalification process.

a. Biochemical stage: during the initial stages of

biochemical phase the decomposition and

degradation of plant material take place dur to

atmospheric oxidation by fungi and also by

aerobic bacteria. The plant debris are coverted

into the precursors of the coal micro-

constituents and and the main controlling factors

for such biochemmical trnasformation are

hydrogen-ion concentration and redox potential

of the medium. If decompositon and

degradation of plant material by atmosphereic

oxidation by bacteria and by bacterua and fungi

are allowed to continue unhindered, then

eventually bothing will be left.

b. Coalification: the progress change in the

composition of organic materail in the formation

of coal is called coalification. As it occurs the

coal is said to increase in maturity or rank. Coal

is essentially formed form plant substances.

Chemically, therefore, two substance-cellulose

and lignin, htat predominate in plant structire

and make up the entire composition of wood,

contribute significantly in the formation of coal.

Both cellulose and lignin are complex, high

molecular weight compounds made essentially

of carbon, hydrogen and oxygen. Mechanistic

theories: various workers have contributed to

explain the chemical changes during

coalification. Regnault (1905) based on analysis

of mine gases postulated the following

mechanism:

(C6H10O5)4 ® C9H6O + 7CH4 + 8CO2 + 3H2O...Eq (1)

Cellulose Bitumnous Coal

Parr (1906), explain the mechanism of conversion to

lignite and bituminous coal by the following equation

(C6H10O5)5 ® C20H22O4 + 3CH4 + 6CO2 + CO +8H2O

…………………………………………………….Eq (2)

Cellulose

(C6H10O5)5 ® C20H22O3 + 5CH4 + 8CO2 + CO +10H2O

……………………………………………………...Eq (3)

Cellulose

c. Cellulose and Lignin theory: Bone (1918)

explained the physical changes that occurred

during heating of coals. According to him, coal

contains about 7% cellulose about 25% lignin.

These substances have different characteristics

properties and he utilised these difference in

properties of cellulose and lignin to explain the

variation in swelling and coking properteis with

rank of coal. According to Bone plant materials

underwent geochemical changes due to the

weight of the overlying burden pressure and

temperature due to depth. The waxy resinous

material in the plant first changed into lignin in

lignited and further to bitumen in bituminous

coals. The bitumen retained the softening

characteristics. On heating this melted and

mixed into the fibrous cellulose structure and

one cooling formed a solid coherent mass.

Depending on the bonding formed between the

molten lignin and cellulose matter, the strength

or the afflutinating propterty of the coke

depends. The coke may be softer or harder than

the coal.

Mode of occurrences

Diessel (1992) has described the following types of laminites

very oftenly seen in the coal measures:

1. Sandstone laminite: this is type of sandstone which

contains laminae of variable sized arenites

2. Shale-laminated sandstone: this is a type of sandstone

which is composed of >60% sandstone and between 40

and 10% shale laminae. The term shale here include

both silt and clay fractions in approximately equal

propertions.

3. Shale/sandstone laminite: this type consists of both the

rock types in equal proportions in the form of laminae.

4. Sand-laminated shale: this type contains between 40

and 10% arenaceous laminae.

5. Clay-laminated siltstone: this association is

characterised by 10 to 40% clay laminae alternate with

at least 60% silt laminae.

6. Silt-laminated claystone: this consists of 10 and 40%

silt laminae alternate with atleast 60% clay laminae.

Coal-laminated shale or coaly shale: this contains 10 to 40%

coal laminae interstratified with shaly matter. Carbonaceous

shale has a similar coal shale ration but the tow components are

unseparable into diffferent laminae.

Coal forming epochs

If we put a glance in the Geological Time Scale, we

will find that in Devonian Period (Precambrian Era), the spore-

bearing terrestrial plants showup its existence. This is followed

by the Carboniferous and Permian Periods where most of the

vafourable regions of earth were fully covered by the

vegetative propagation. Conifer and Cycad plants flourished in

Triassic Period (Palaeozoic) and hence many places in the

world bear the coal of Jurassic and Triassic Origin. Flowering

Plants came into existence during the Period of Tertiary,

mainly Oligocene Epoch.

In India, coal deposits are of two distinct geological

ages, Permian and Tertiary. Here the following table indicates

the major geological time distribution of coal –

Coal fields Geologic Horizons Locations

Tertiary

Early Pleistocene

to Upper Pliocene

Miocene

Lignites in the

Kashmir Valley.

Oligocene to Upper

Eocene

Jaipur, Nazira,

Makum etc

Middle Eocene Palana, Rajasthan,

Kutch

Lower Eocene Darranggiri,

Rongrenggiri,

Cherrapunji,

Mawlong etc

Upper Gondwana

Upper Jurassic Chikiala, Kota,

Maharastra

Upper Permian Raniganj, Jharia,

Bokaro etc

Lower Gondwana Lower Permian Damodar Valley,

Mahanadi Valley

and other south

Indian coal fields.

Coal is found in nearly every region of the world, but deposits

of present commercial importance are confined to Europe,

Asia, Australia, and North America. Great Britain, which led

the world in coal production until the 20th century, has

deposits in southern Scotland, England, and Wales. In western

Europe, important coalfields are found throughout the Alsace

region of France, in Belgium, and in the Saar and Ruhr valleys

in Germany. Central European deposits include those of

Poland, the Czech Republic, and Hungary. The most extensive

and valuable coalfield in the former Soviet Union is that of the

Donets Basin between the Dnepr and Don rivers; large deposits

have also recently been exploited in the Kuznetsk Basin in

western Siberia. The coalfields of north-western China are

among the largest in the world. The coal reserves of the United

States are divided into six major regions, only three of which

are mined extensively. The most productive region is the

Appalachian field, which includes parts of Pennsylvania, West

Virginia, Kentucky, Tennessee, Ohio, and Alabama. In the

Midwest one large field covers most of Illinois and sections of

Indiana and Kentucky. A thick field extends from Iowa through

Missouri, Kansas, and Oklahoma. These three regions produce

most of the coal mined in the United States. There are large

deposits of lignite and subbituminous coal in North Dakota,

South Dakota, and Montana. Subbituminous and bituminous

coal deposits are scattered throughout Wyoming, Utah,

Colorado, Arizona, and New Mexico. The Pacific Coast and

Alaska have small reserves of bituminous coal. Almost all the

anthracite in the United States is in a small area around

Scranton and Wilkes-Barre, in Pennsylvania. The best

bituminous coal for coking purposes comes from the Middle

Atlantic states. Estimates of world coal reserves vary widely.

According to the World Energy Council, recoverable world

reserves of anthracite, bituminous, and subbituminous coal in

the late 1980s exceeded 1.2 trillion tonnes. Of this recoverable

coal, China held about 43 per cent, the United States 17 per

cent, the former Soviet Union 12 per cent, South Africa 5 per

cent, and Australia 4 per cent. The World Coal Institute has

estimated that, at 1998 levels of production, coal reserves are

likely to last about another 200 years.