There are many classifications of coal, and they are generally classified based on the following properties of coal: ① ash content; ② own structure: anthracite, bituminous, sub-bituminous and lignite; ③ calorific value; ④ sulfur content: high or low; ⑤ coke grade; ⑥ adhesion.
Henri-Victor Regnault carried out an industrial analysis of the coal to determine the percentage of moisture, volatile matter (gas volatilized when the coal is heated), fixed carbon (solids remaining in the coal after volatilization and analysis), ash (impurities in coal such as iron, silicon, aluminum, and other unburned substances), and then classified the coal. Its industrial analysis is based on a “received basis”.
Clarence Seyler used elemental analysis to determine the chemical constituents of coal: carbon, hydrogen, oxygen and nitrogen, but not sulfur and ash. British scientist Marie Stopes classified coal according to its microscopic composition: bright coal (ordinary bright coal), mirror coal (glossy black coal), dark coal (dull rough coal), and black coal (also known as silk charcoal, natural charcoal, which is a soft powdered coal). Because moisture, minerals, or ash are not the flammable qualities of coal, coal composition analysis can be based on several criteria: as-received, air-dried, dry, dry-ash-free, and dry-mineral-free. The most commonly used benchmark in various classification schemes is proposed by Ward (1984):
(1) Received base: contains fixed carbon, volatile matter (organic and mineral), ash, moisture (internal water and surface water);
(2) Air drying base: fixed carbon, volatile matter, ash and internal water;
(3) Drying base: fixed carbon, volatile matter and ash;
(4) Dry ashless base: fixed carbon and volatile matter;
(5) Dry mineral-free base: fixed carbon and volatile organic compounds; also known as clean coal (pure coal).
Coal classification guidelines written by the American Standards Association (ASA) and the American Society for Testing and Materials (ASTM) are used in commercial classification systems as well as in scientific research. The ASTM system (ASTM D388) classifies coal into the four grades mentioned above, each grade is subdivided into several sub-classes. High rank coal has high content of fixed carbon (calorific value) and volatile matter (dry mineral-free base), and low content of hydrogen and oxygen elements; low rank coals are low in carbon but high in hydrogen and oxygen and can be subdivided according to their calorific value (dry mineral-free basis).
Anthracite, as the highest rank coal, burns with a small flame and produces less smoke. Its fixed carbon content is the highest among all coals, reaching 86% to 98%, and its energy is relatively high. Its calorific value is 13500~15600Btu9/1b9 (14.2~16.5MJ/lb), and its moisture and volatile content are relatively low. While anthracite can be used in power plants to generate electricity and in the steel industry, it is primarily used for domestic heating. Anthracite is a scarce coal. Bituminous coal is a second-order soft coal that produces smoke and fly ash when burned. Its fixed carbon content is 46%~86%, and its calorific value is high, between 10500~15000Btu/lb. Bituminous coal is the most abundant and economically minable coal in the world and the main fuel for steam turbine-powered power plants, some of which can be converted into coke for the steel industry. Sub-bituminous coal is the third rank coal with a fixed carbon content of 35%~45%, a higher moisture content than bituminous coal, and a calorific value of 8300~13000Btu/lb. Although subscale coal has a low calorific value, it is widely used in power plants because of its low sulfur content and low environmental pollution. Lignite is also a soft coal with a fixed carbon content of 25%~35%, and its calorific value is the lowest at 4800~8300 Btu/lb. Brown medium is mainly used for power generation. With the increase of coal rank, the moisture and volatile content of coal decreased, and the fixed carbon content and calorific value increased.
Figure 1 gives a more detailed classification of the various coals in the US, UK and Germany (data from EURACOAL website)
In addition to being used for heat and power generation, coal is an important raw material for manufacturing. Hydrocarbon gases and coal tar from coal retorting (carbonization) can be used to synthesize drugs, dyes, plastics, solvents, and many other organic chemicals. Hydrogenation and liquefaction of coal under high pressure, and indirect liquefaction using Fischer-Tropsch synthesis technology are effective and feasible methods for producing clean liquid dyes and lubricants.