The plants which formed coal captured energy from the sun through photosynthesis to create the compounds that make up plant tissues. The most important element in the plant material is carbon, which gives coal most of its energy. Most of our coal was formed about 300 million years ago, when much of the earth was covered by steamy swamps. As plants and trees died, their remains sank to the bottom of the swampy areas burying layer upon layer and eventually forming a soggy, dense material called peat. Pressure caused by their weight squeezed water from the peat. Increasingly deeper burial and the heat associated with it gradually changed the material to coal.
Stages of Development.
1. Forest and Bog: Low land water accumulated above the trees and vegetation. It does not have any definite shape. It is a jelly like material. It is grayish in color. It does not have any structure because it is not solidified. All the structure that is present in the original plant are present in the bog mass.
2. Peat:It has not yet attained the hardness as coal, is friable in nature. It does not have any definite structure. Peat has not yet been affected by heat and pressure. It is purely sedimentary in nature. It is light, porous and fibrous substance light grayish brown to dark brown color
3. Lignite: It is fragile and breaks into powder on handling. It is dull brown to blackish brown in
color. It does not have any clear structure. Since it is very little affected by heat and pressure, it has not attained and design. Some much more matured lignite has within it the macerals. Some of the lower rank lignite possesses clear remains of plant structures. Since it is very near to low rank coal, its heat value is quite
substantial.
4. Bituminous: It has been formed due to slow application of heat and static pressure. It is pitch to dark black in color. The physical properties of bituminous coal are determined by the percentage
distribution of macerals groups, such as vitrain, clarin, durain and fusain. Bituminous coal covers a wide range of variety, from low volatile, low moisture to high volatile; high moisture. It is considered as all purpose coal because of its excellent heating quality. Calorific value ranges between 11000-15000 B.Th.U (6100-
8300 K.Cal/kg).
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