Classification and formation of coal

The term coal is generally applied to a sedimentary formation of highly carbonaceous character that is derived from vegetable matter involving set of process such as burial, compaction and biochemical transformation.


A number of classifications for coals are available of these the one most widely adopted is based on the rank of coal that defines degree of transformation of wood into coal through the natural processes of deposition, compaction and biochemical changes.


It is essentially a partly changed vegetable mater in the first stage of transformation to coal. The vegetable structure is easily visible and the evidence of its being in the processes of transformation. to coal. Peat is generally composed of remains of moss like plants but occasionally may contain reeds and partially altered portions of trees of higher order.

Types:  Two types of peat are commonly recognized:

i)                   Bog Peat, which is evolved out of lower type of vegetation, like mosses.

ii)                Mountain Peat that is decomposed and partially altered form of higher type’s of trees.


Peat is a low value fuel in its application .It finds uses where available in abundance as i) Domestic fuel ii) gas purifier iii) For steam raising.


It is a variously coloured variety of coal of lowest rank. In lignite transformation of vegetable matter to coal like material is almost complete. Fibrous texture is also shown by some lignite’s.

Composition: Typical lignite has following composition:

Fixed Carbon : 50 percent; oxygen 20-25 percent Hydrogen : 05 perecnt;nitrogen 02-05 percent Sulphur: 01-02 percent.

Uses: These are used as domestic fuels and also in industry for distillation and gasification. This variety of coal has also been used in steam locomotives and for producing gas.


It is also known as the common coal,someties as coking coal and is ,in fact, the most common and important variety commercial coals. These are commonly black in colour, compact in structure breaking into almost cubical fragments when struck with hammer. They have a black streak. Bituminous coals burn freely leaving only a small mineral residue.

Types and composition:

The common bituminous coal is sometimes distinguished into three different types on the basis of its carbon content: sub-bituminous, bituminous and semi-bituminous coals.


It is a coal highest rank in which original organic source has been completely transformed into carbonaceous substance. It is very hard, jet black in colour, compact in structure and showing an almost metallic luster.


Anthracite is a favourite domestic fuel where available. It is also used for steam raising and other heating purposes. However because of its very low volatile matter content it is not suitable for making coke.

Formationof coal

There is absolutely no doubt regarding the nature of the source material form which coal is derived it is certainly always vegetable matter of one type or another. The two types of sources yielded vegetable material for the formation of the coal.

i) The higher vegetation:

It including herbs, shrubs and trees, growing on the plains, plateaus, sub-mountaineous and areas and characterized with wood tissue rich in cellulose and lignin and protein this type of source has been named as the humic sediment.

ii)The lower vegetation:

It comprising chiefly plank tonic algae, as soften found at the bottom of lakes and seas, submerged under water. This source has been named as sapropelic sediments.

On the basis of place of accumulation of the source material, the environment could be distinguished as:

Geosynclinal type:

When accumulation took place in great sea basins characterized with considerable depth.

Intermediate type:

When deposition of source material took place along the sea shores at shallower depths.

Platform type:

Such as lagoons, lake basins and marshes and swamps.

In these littoral environments decomposition of cellulose from humic organic matter is an easily accomplished process. IN other environment also the process may take the route of transport of source material its accumulation, compaction, dehyfdration and polymerisation to bituminous coal to anthracite are all environment dependent. These are completed in an better manner in geosynclinal situations that is under great depths in the presence of heat and pressure and seldom reach completion in littoral conditions.

Summarisingly the transformation of organic source material into coal takes place due to:

i)       Bio  chemical  decomposition  which  is  achieved  by  certain  type  of  bacteria

and involves breakdown of organic matter of wood into coal constituents.

ii)      Dynamo chemical transformation which involves alteration of original coal structure into more compact, metamorphosed varieties chiefly under the influence of temperature and pressure factors.