Coal is a kind of combustible sedimentary rock which is formed by the natural process of various kinds of deteriorated plants and accumulated in basins for over hundreds of millions. Upon changes of the crust such as earthquake, eruption of volcano, or more soil deposition, the pressure increased and the temperature also rose.

The system of the American Society for Testing and Materials, ASTM, classifies the coal into 4 classes based on the most amount of carbon to the least amount of carbon, namely, Anthracite, Bituminous coal, Sub-bituminous coal, and Lignite. Those dead plants were converted into various kinds of coals.

General quality of coal is based on the carbon content in the coal. The rank is categorized by varying coal rock types. The high rank coal contains significant carbon content, gives the most amount of heat with less hydrogen and oxygen content. The low rank coal contains less carbon content but significant oxygen and oxygen content.

Types of Coal

The qualities of each type of coal:

Coal Heat Humidity Ash Sulphur
Anthracite High Low Low Low
Bituminous High Low Low Low
Sub-Bituminous Medium -High Medium Medium Medium
Lignite Low- Medium High High Low-High