Coal can be divided into lignite, bituminous coal, lean coal and anthracite according to its formation time and coalification degree.

Brown coal. Because of its low coalification degree, the dry ash-free volatile matter can reach more than 40%, and the temperature at which the volatile matter begins to precipitate is low, which is easy to catch fire. Its water absorption capacity is strong, and the water content can usually reach 20%. There are many internal and external impurities in lignite, so its calorific value is not high, generally less than 16750kJ/kgo. Lignite is soft, easy to weather, easy to spontaneous combustion, difficult to store, not suitable for long-distance transportation, and belongs to local low-quality coal.

Bituminous coal refers to coal with high carbon content and many volatiles, and the dry ash-free volatiles are between 10%-40%. Bituminous coal is easy to ignite and burn, with less ash and moisture and higher calorific value. For some bituminous coal with high ash content and high moisture content, the calorific value is very low. Usually, bituminous coal with low calorific value less than or equal to 15490kJ/kg is called inferior bituminous coal, which is not easy to ignite and burn. Bituminous coal is black, soft, shiny and smoky when burned. It is the most widely distributed and abundant coal in nature.

Lean coal refers to coal with a lower coalification degree than anthracite and a dry ash-free volatile content higher than 10%~20%. Compared with bituminous coal, lean coal is more difficult to ignite and burn, with short flame and poor coke separation, and its calorific value is between anthracite and ordinary bituminous coal.

Anthracite refers to the coal with the highest degree of coalification, which is gray-black, metallic, hard and not easy to grind, commonly known as white coal. Its volatile matter is very little, and the dry ash-free volatile matter is less than or equal to10%; High carbon content, the highest dry ash-free carbon content can reach 95%~98%. So it is quite difficult to catch fire and it is not easy to burn out. When coal burns, it is smokeless, with only a short blue-green flame, and the ash is powdery and non-sticky. Because of the high carbon content and few internal and external impurities, the calorific value is generally high, and most of the calorific value of the receiving base is between 20930- 25 120kJ/kg. However, due to its low hydrogen content, its calorific value is lower than that of some high-quality bituminous coal. It is stable during storage and will not spontaneously ignite.

The coal used in general life is "bituminous coal" and "anthracite"! ! !