Origin and Formation of Coal

Coal is a compact stratified mass of mummified plants which have in part suffered decay to
varying degrees of completeness and free from all save a very low percentage of other matter (given by
Stopes and Wheeler).

Coal is a 'rock', being composed of a number of different constituents in varying

ratios. It is sometimes called "mineral fuel" or "mineralised plants" or "carbonised vegetable matter".

Geological evidence provides ample justification for the belief that coal has been formed from
terrestrial plant remains. The process of conversion of woody, peaty residues to coal is termed
coalification. It is in part biochemical and in part metamorphic. The biochemical stage is essentially part
of the conversion of vegetable matter into peat, during which microorganisms are active in reconstituting
the organic matter. The biochemical stage is eventually brought to an end when conditions become
unsuitable for bacterial activity.

A dynamo chemical process follows when the vegetable matter is covered with a considerable
thickness of sediments, and is thus subjected to pressure and heat. Spring has shown by experiment that
peat, subjected to a pressure of 6000 atmospheres, is converted to a coal-like substance. Under these
circumstances there is a progressive elimination of water and other gases, with a storing up of carbon. The
series peat, lignite, bituminous coal, anthracite may represent successive stages of this process.

It has been found from extensive studies that coal may be formed from many types of plants.
There are practically no botanical restrictions on the formation of coal. Any kind of plant residue that can
be geologically conserved may form coal (Schopf, 1973). Thus the climatic conditions that favoured coal
accumulation were mild temperate to subtropical, with moderate to heavy rainfall well distributed
throughout the year.

Coal is found at varying depths below the surface in association with strata consisting largely of
alternating shales and sandstones, with occasional beds of limestone in some areas. Such strata were
evidently laid down in close proximity to land.

Coal seams vary in thickness from a few inches to as much as 300ft. (exceptionally)
but the average is about3-4ft and aggregate thickness of seams in any one locality rarely
exceeds 100ft. On the other hand, the total thickness of coal-bearing strata (termed the coal-
measures) sometimes reaches 10,000 ft. Deposition of such a thickness of sedimentary strata implies
gradual subsidence in the area of deposition over millions of years, with occasional pauses whilst
coal seams themselves were being formed.

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Theories of Coal Formation

In Situ Theory

1. Present day in-situ accumulation of vast amount of plant materials in the swamps, e.g., Peat beds
in Dismal Swamps (U.S.A.).
2. Fossilized tree-trunks are found standing erect with their roots firmly fixed in the under clays,
which is supposed to represent the original soils.
3. Under clays or seat earth are found to be poor in alkalis, lime and oxide. It is suggested that these
materials have been extracted and utilized by plants that grew on them.
4. Relative purity of coal seams, suggesting unmixed with foreign materials.
5. Absence of aquatic fossils in coal seams.
6. Lenses of canal coal are often found in bituminous coal seams.
7. Less possibility of enormous amount of plant material accumulation by drift theory, without
being mixed with considerable amount of inorganic sediments

Drift Theory

1. Present day downstream transportation of large quantities of timber and tree trunks by the rivers,
e.g., Peat beds in deltas (Ganga, Mississippi)
2. Tree-trunks are inclined or in a prostrate position.
3. Seat earth or under clays are very often absent and coal seams lie directly on sandstone, shale or
conglomerates.
4. High proportion of inorganic materials with coal seams (ash - Gondwana coal -2 0%)
5. Marine fossils present at the top or bottom of coal seams, e.g. Makum coal field.
6. The rocks associated with coal seams are sedimentary. Many seams are stratified, with
partings of shale, clay or sandstone.
7. According to drift theory, it is believed that the transported decayed vegetation would reach its
resting-place in a more or less condensed state, so that it was possible to have thick seams of
coal.

It is now accepted that coal seams of some regions like Western Europe, had formed in situ,
whereas those of other regions like India had been formed by drift.

Evidences from Indian coal: -The repetition of a regular sequence of sandstones, shale, calcareous
sands and coal seams in the coal bearing strata suggesting continuous deposition in sedimentary basins,
the great thickness of the coal seams, the general absence of seated earth or under clays, the high
proportion of impregnated matter in coal, the absence of any erect tree trunks found in the base beneath
the coal seams and the presence of terrestrial fossils are some of the points suggesting a drift origin for
Indian coals.

Mode of occurrence of coal

I) Horizontal and low dipping platforms,
II) Confined to major folded structures of simple composition, sometimes slightly
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 dislocated, and
III) Associated with complicated folded structures, dislocated with sharp changes in attitude.

Irregularities in Coal Seam

• Washouts or barren areas in seam (coal seam is missing over a more or less extensive area).
• Thickening of coal seams.
• Splitting of seams, either regionally or locally.
• Faults or dislocations of the strata, ranging from inches to thousands of yards. Faults may be
normal, or reversed, or (more rarely) transcurrent.
• Igneous intrusions, consisting of rocks consolidated from the molten state and forming either
dykes or sills.

Physical Properties of Coal

Type Colour Lusture Fracture Cleat Sp. Gr. Hardness

Lignite Brown Dull Uneven Absent 1.1 to1.5 1

Bituminous Black Vitreous Uneven Present 1.2 to1.3 2

Anthracite Black Metallic Conchoidal Absent 1.3 to 1.45 3

Chemical Constituents of Coal

Chemically, coal consists of a mixture of complex organic compounds along with small amounts
of inorganic mineral and moisture. The elementary composition of coal is very simple, carbon, hydrogen
and oxygen being the principal constituents, along with small amounts of nitrogen and sulphur.

The proportion of these elements is found to progressively increase or decrease with the advance
of coalification process starting from vegetal debris. There is a steady increase of carbon and a
corresponding decrease in oxygen content from wood to anthracite, hydrogen also decreases but very

The chemical composition of coal is determined either by the "Proximate Analyses" or by the
“Ultimate Analyses". The Proximate Analyses consists in determining the moisture content, volatile
matter content, ash, fixed carbon and calorific value. The Ultimate Analysis determines the relative
percentage of different elements present in the coal like hydrogen, oxygen, nitrogen, sulfur and carbon.

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 Sulphur is an injurious impurity commonly present in most coals in the form of marcasite or
pyrite. Ash is the residual of noncombustible matter in coal that comes from included silt, clay, silica, or
other substance.

Types of coal Carbon Hydrogen Nitrogen Oxygen

Anthracite 93.5 2.80 1.00 2.70
Bituminous 84.20 5.60 1.50 8.70
Lignite 73.00 5.20 1.30 20.50
Peat 55.40 6.30 1.70 36.60
Wood 49.70 6.20 0.90 43.20

Major stages of the development from peat to meta-anthracite

Chemical Analysis of Coal

The chemical analysis of coal comprises of “proximate” and “ultimate” analysis. The American
method of proximate analysis is faster and cheaper than the European method of ultimate analysis. The
method of analysis depends on certain set of physical and chemical parameters of coal.

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 The proximate analysis consists of four items: fixed carbon, volatile matter, moisture and ash, all
on a weight percent basis.

The ultimate analysis provides an element-by-element composition of the coal's organic fraction,
namely: carbon, hydrogen, oxygen and sulfur, all on a weight percent basis.

Basis of analytical data

Before proceeding to the analysis of the coal, it is important to understand how the moisture,
ash, volatile matter and fixed carbon relate to one another and the basis on which analytical data are
presented. Coal analyses may be reported as follows:

1. As received’ basis (a.r.), also ‘as sampled’. The data are expressed as percentages of the coal
including the total moisture content, i.e. including both the surface and the air-dried moisture
content of the coal.
2. Air-dried’ basis (a.d.b.). The data are expressed as percentages of the air-dried coal; this includes
the air-dried moisture but not the surface moisture of the coal.
3. Dry’ basis (dry). The data are expressed as percentages of the coal after all the moisture has been
removed.
4. Dry ash-free’ basis (d.a.f.). The coal is considered to consist of volatile matter and fixed carbon
on the basis of recalculation with moisture and ash removed. It should be noted that this does not
allow for the volatile matter derived from minerals present in the air-dried coal. This basis is used
as the easiest way to compare organic fractions of coals.
5. Dry, mineral matter-free’ basis (d.m.m.f.). Here it is necessary that the total amount of
mineral matter rather than ash is determined, so that the volatile matter content in the mineral
matter can be removed.

Proximate analysis –

a) Moisture - It is an important property of coal, as all coals are mined wet. Groundwater and other
extraneous moisture are known as adventitious moisture and are readily evaporated. Moisture
held within the coal itself is known as inherent moisture and is analyzed quantitatively. Moisture
may occur in four possible forms within coal:
 Surface moisture: water held on the surface of coal particles or macerals
 Hydroscopic moisture: water held by capillary action within the micro-fractures of the coal
 Decomposition moisture: water held within the coal's decomposed organic compounds
 Mineral moisture: water which comprises part of the crystal structure of hydrous silicates such as
clays
Determination of moisture content - Loss in weight of coal caused by heating of coal sample for one
hour at 105˚C is the moisture content of coal. A known amount of finely powdered coal sample is kept in
a silica crucible and heated in a muffle furnace at 105-110˚C for one hour. There after the crucible is
taken out, cooled in a dessicator and weighed. The percentage of moisture is given by
% moisture in coal = {loss in wt. of coal * 100}/ wt. of coal initially taken

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 b) Volatile Matter: Volatile matter in coal refers to the components of coal, except for moisture,
which are liberated at high temperature in the absence of air. This is usually a mixture of short
and long chain hydrocarbons, aromatic hydrocarbons and some sulfur.

Determination of Volatile Matter - It is the loss in weight of moisture free powdered coal when heated
in a crucible fitted with cover in a muffle furnace at 925˚C for 7 minutes.

% volatile matter = {loss in wt. of moisture free coal * 100}/ wt. of moisture free coal taken

c) Ash: Ash content of coal is the non-combustible residue left after coal is burnt. It represents the
bulk mineral matter after carbon, oxygen, sulfur and water (including from clays) has been driven
off during combustion.

Mineral Matter: There are two types: (1) Inherent (2) Extraneous. Inherent comes from inorganic
constituent of plant materials, but the amount is less. Extraneous comes from the amount that gets
associated with substances during its conversion process. Relation between mineral matter and
ash is given below:

M.M. = 1.08Ash + 0.555

For Indian coals: M.M. = 1.1Ash

Determination of Ash - It is the weight of residue obtained after burning a weighed quantity of coal in
an open crucible (i.e. in presence of air) at 750˚C in a muffle furnace till a constant weighed is achieved.

% ash in coal = {wt. of residue ash formed * 100} / wt. of coal initially taken

d) Fixed Carbon: The fixed carbon content of the coal is the carbon found in the material which is
left after volatile materials are driven off. This differs from the ultimate carbon content of the coal
because some carbon is lost in hydrocarbons with the volatiles. Fixed carbon is used as an
estimate of the amount of coke that will be yielded from a sample of coal. Fixed carbon is
determined by removing the mass of volatiles determined by the volatility test, above, from the
original mass of the coal sample.

Determination of Fixed Carbon - It is determined indirectly by deducting the sum total of moisture,
Volatile matter, and ash percentage from 100.

% fixed carbon in coal = 100-(% moisture+% ash+% volatile matter)

Some examples of proximate and ultimate analyses are given in the table below:

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Classification of Coal

The coals are classified basically under ASTM classification (Based on Proximate Analysis) and
Seyler’s classification (Based on Ultimate Analysis). But many countries have their own classification on
the basis of above basic classification depending on different properties, nature, economic uses of coal.

The ASTM classification: -

The coals are classified mainly as Anthracitic, Bituminous, Sub-Bituminous and Lignitic on the
basis of fixed carbon, volatile matter and calorific value.

A] Anthracitic: - This classification comprises nonagglomerating coal, with the following limits of fixed
carbon or B.T.U. (British Thermal Unit) mineral-matter-free basis for the listed groups:
(1) Meta-anthracite- Dry fixed carbon, 98% or more.
(2) Anthracite- Dry fixed carbon. 92% to 97%.
(3) Semi-anthracite- Dry fixed carbon, 86% to 91 %.

B] Bituminous: - This classification comprises either agglomerating or nonweathering coal with the
following properties for the listed groups:
(1) Low volatile- Dry fixed carbon, 78% to 85%.
(2) Medium volatile- Dry fixed carbon, 69% to 79%.
(3) High volatile A- Dry fixed carbon, less than 69%; and moist B.T.U. 14000 or more.
(4) High volatile B- Moist B.T.U. 13,000 to 13,999.
(5) High volatile C- Moist B.T.U. 11,000 to 12,999.

C] Sub-Bituminous: - This classification comprises both weathering and nonagglomerating coal with the
following other properties:

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 (1) Group A- Moist B.T.U. 11,000 to 12,999.
(2) Group B- Moist B.T.U. 9,500 to 10,999.
(3) Group C- Moist B.T.U. 8,300 to 9,499.

D] Lignite: - This classification comprises two groups:

(1) Lignite - Consolidated with moist B.T.U. less than 8,300.
(2) Brown coal - Unconsolidated with moist B.T.U. less than 8,300.

Seyler's Classification: -

Seyler classified the coals as Group on the basis of hydrogen content and Class on the basis of
carbon content.
Groups Hydrogen content (%)
Anthracitic under 4
Carbonaceous 4-4.5
Semi-Bituminous 0.5-5.0
Bituminous 5.0-5.8
Per-Bituminous over 5.8
Class Carbon Content (%)
Anthracitic over 93.3
Carbonaceous 93.3-91.2

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 Meta-Bituminous 91.2-89.0
Ortho-Bituminous 89.0-87.0
Para-Bituminous 87.0-84.0
Meta-Lignitous 84.0-80.0
Ortho-Lignitous 80.0-75.0.

Megascopic Constituents (Lithotypes) in Coal

1. Vitrain :
Black, glassy and vitreous
The most striking component of bituminous coals.
Occurs as thin band, commonly of <6-8mm in thickness
Usually very closely jointed
More brittle than other megascopic coal constituent
Often breaks with a conchoidal fracture

2. Clarain
Bright to semibright bands of finely laminated coal
Silky lusture
Commonly contains fine vitrain bands alternate with dull groundmass

3. Durain
Occurs as grey to black bands with a dull to slightly greasy lusture
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 Relatively hard compare than other lithotypes
Breaks into large blocky fragment
Sometimes may be confused with impure coal and carbonaceous shale

4. Fusain
Latin word Fusus means spindle
Fusain means charcoal
Soft friable material
Easily disintegrates in black fibrous powder
Occurs as thin lenses

Microscopic Constituents (Thin Section) in Coal

1. Anthraxylon
Orange red in colour, nonopaque, main constituent
The properties of anthraxylon bands of megascopic are identical with that of vitrain
Generally shows some of the original plant structures
Anisotropic in low rank coals

2. Translucent attritus
Yellowish in colour, nonopaque, associated debris
Consists of the complex residual organic degradation matter (<14μ)

3. Opaque attritus
Opaque
Consists of Fusain, <30μ wide; Amorphous matter; discrete granules

4. Fusain
Opaque

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Geological Distribution of Coalfields of India

In India the total reserve of coal is about 257.38 billion tones, which is only 6% of world reserve.
The leading producers of coal in India are Bihar, West Bengal, Madhya Pradesh, Andhra Pradesh,
Maharashtra, Orrisa, Assam etc.

Particulars Coal reserve( in million tonnes)

Proved reserve 99,060.39 38.50% 99.06BT
Indicated reserve 1,20,177.39 46.70% 120.17BT
Inferred reserve 38,143.77 14.80% 38.143BT
Total coal 2,57,381.55 100% 257.38BT
reserve(upto 1200 m
depth)

In India, the greatest period of coal formation is the Permian. The coal bearing formation belongs
to Lower Gondwana Super Group and is collectively known as Damudas. The coal belongs to the lower
division of Damudas known as Barakars. Important lower Gondwana coal fields are- Jharia, Raniganj,
Karanpura etc.

The other important period of coal formation in India is the Tertiary period. Tertiary coals are
found in Andamans, Arunachal Pradesh, Assam, Himachal Pradesh, Jammu and Kashmir, Meghalaya and
Nagaland.

Geological Formation Localities

Early Pliocene Lignites in the Karewa formations of Kashmir valley.
to Upper
Pliocene
Miocene Lignites in the Cuddalore Formation of South Arcot, Tamilnadu
and Varkala and Quilon in Kerala.
Oligocene to Lignites in Barail Formation in Jaipur, Nazira, Namchi and Makum
Tertiary Upper Eocene coalfields of Upper Assam.
Coalfields Upper Eocene Coals in the Jaintia Formation of Cherrapunji, Mawlong and
Shillong in the Khasi and Jaintia hills; Garo and Mikir hills in
Assam.
Middle Eocene Lignites of Palana, Rajasthan; lignites of Kutch.
Lower Eocene Jammu coalfields – Kalakot, Metka, Mahogala, Dhanswal-
Sawalkot; coalfields of Western Assam; Darangiri, Rongrenggiri in
Garo hills; some fields of Khasi and Jaintia hills in Meghalaya.
Upper Upper Jurassic Chikiala and Kota in Godavari valley in the Kota Bed; Satpura

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 Gondwana region, Madhya Pradesh, in Jabalpur Formation; in Ghuneri in
Coalfields Kutch below the Umia Formation.
Upper Permian Raniganj, Jharia Bokaro and Karanpura coalfields of the Damodar
valley Bengal and Bihar.
Lower Lower Permian All lower Gondwana coalfields of the Peninsula including Damodar
Gondwana valley, Mahanadi-Brahmani valley, Pranhita-Godavari valley,
Coalfields Pench valley, Wardha valley, Son valley coalfields; coalfields of
Sub-Himalayan zone of Sikkim and Assam, Darjeeling district of
Bengal; Abor, Daphla and Aka hills of Assam.

Coal deposits of Northeastern region: -

The coal deposits of north eastern region of India are distributed in Arunachal Pradesh, Assam,
Nagaland and Meghalaya. The coal belts of NE are subdivided into two areas: 1) Coal fields of upper
Assam, Arunachal Pradesh, Nagaland and 2) Coal fields of Central and Lower Assam and Meghalaya.
The lower tertiary coal deposits in these areas belong to Barail and Jaintia Group. The coal bearing
horizons are sylhet limestone and cherra sandstone of Jaintia Group and Tikak Parbat and Boragolai
formation of Barail Group. The important coal fields in north eastern region are-

a) Assam: 1) Makum coal field

2) Dilli-Jeypore coal field

b) Arunachal Pradesh: 1) Namchik-Namphuk coal field

c) Meghalaya: 1) West darrangiri coal field

2) Bapung coal field
3) Siju coal field
4) Langrin coal field
5) Balphakram- Pendenguru coal field

d) Nagaland: 1) Borjan and tuensang coal field

2) Nazira coal field.

The Tertiary coal fields of north eastern region are characterized by low ash, high calorific value
and high sulphur content. The total reserve of coal of north eastern region upto a depth of 600 metres as
assessed by GSI is 945.03 million tones only, which is 0.37% of the national reserve.

*** ***

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