Coal is the most abundantly found, non-renewable fossil fuel that is combusted and used to generate electricity. Most of the coal deposits that we use were formed around\( 300 million years ago\). During this time, much of the earth was covered with steamy swamps. When the plants and trees are dead, their remains were buried underneath the swamps. These organic remains are converted into peat under certain conditions. The peat then transforms into coal under the right conditions. This process of conversion is called carbonization.
Carbonization takes place under incredible heat and pressure. About 3 meters of layered vegetation eventually compresses into a third of a meter of coal! 
Despite the dangerous mining techniques and combustion, which is hazardous to both miners and the environment, coal remains the largest source of energy for generating electricity in the world.
It is used as both domestic and industrial fuel, particularly in iron and steel, steam engines to generate electricity. Electricity produced by burning coal is called Thermal Power.
Thermal power
Coal is classified into four types based on carbon content. They are:
1. Anthracite
2. Bituminous
3. Lignite
4. Peat.

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China is the largest coal producer of coal in the world. Beside this, India, USA, Australia, Indonesia and Russia also produce more coal.
The coal-producing areas of India are Raniganj in West Bengal, Neyveli in Tamil Nadu, Jharia, Dhanbad, and Bokaro in Jharkhand.