Under the term “coal” there is a number of different fossil fuels. Figure bellow shows the international division of coal created by International Coal Classification of the UN Economic Commission for Europe (ECE), depending on moisture content, coal rank and calorific value. Low-rank coal is the one with lower carbon contents and higher content of hydrogen and oxygen, while high-rank coal has higher carbon content (and therefore higher calorific value) with lower content of hydrogen and oxygen (lower moisture).
Lignite is used in electricity production, same goes for sub-bituminous coal, which is also used in cement production and other related industries. Steam coal (or thermal coal) has similar application, but is more suitable for transport and has better quality. Coking coal is used to produce coke, which later is fed to the furnaces that make iron and therefore steel. Coking coal has not only higher calorific value from steam coal, but a certain physical features that are suitable to transform itself to hard but porous coke. The last type – anthracite – is used mainly by households and some industries (i.a. to produce smokeless fuel).
Polish coal classification is based on coal types, depending on few parameters: volatile matter content, Roga Index, calorific value (daf), dilatation and free swelling index. First two are important for all coal types, calorific value only for coal used in energy production, the last two parameters is relevant only to coking coal. Types are labeled by two-digits symbol, with third digit characterizing some additional parameters (e.g. “flaming coal” is labeled 31; 31.1 for coal of CV 31,000 kJ/kg and lower, 31.2 for coal of CV higher than 31,000 kJ/kg).
As mentioned at the beginning, each market or country has its own classification. Some countries (i.a. Australia, France, Japan, Korea, Mexico and US) classify sub-bituminous coal to energy coal, while all the others as energy coal considers only anthracite and other bituminous coal. Table bellow shows different naming for certain types of coal in various markets (types are listed from lowest volatile matter content to highest).
Coal is also divided by sizes. In general, international industrial consumers such as energy or heating sector use basically only coal of grain size 0-50 mm. In Poland, coal of that size accounts for around 87,5% of total production (more specifically, MI: 0-31.5mm and MII: 0-20mm). Coal of grain size above 50 mm is usually used by small customers on the retail market for household heating purposes. Because the naming for different coal sizes is also not unified, international traders stick with sizes expressed in millimeters. Coal grain size influences the burning process, mainly the time needed for the full burnout of certain volume of the fuel.