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Black coal
a solid fossil fuel of sedimentary origin. Black coal has a high carbon content and higher density than brown coal. Coal is a dense black rock, sometimes greyish-black, with a shiny, semi-matt or matt surface. It contains 75-97% or more carbon, 1.5-5.7% hydrogen, 1.5-15% oxygen, 0.5-4% sulphur, up to 1.5% nitrogen, and 2-45% volatile matter. The moisture content of coal ranges from 4% to 14%, and ash usually from 2-4% to 45%. Its gross calorific value on a dry ash-free basis is at least 30.5-36.8 MJ/kg.
Black coal is formed from the decomposition of organic plant residues that undergo changes (metamorphism) under pressure from surrounding rocks at relatively high temperature. A higher degree of metamorphism in the combustible mass of black coal leads to a higher carbon content and a simultaneous reduction in the amount of oxygen, hydrogen and volatile substances. This also changes its calorific value, coking power and other properties.
Changes in the quality of coal as a result of thermal decomposition (volatile content, characteristics of non-volatile residue) form the basis for the industrial classification of coal under the Russian system as follows: long-flame coal (denoted by D in the Russian system); gas coal (G); gas-fat coal (GZh); fat coal (Zh); coking fat coal (KZh); coking coal (K); lean coking coal (OS); meagre coal (T); weakly coking coal (SS); semi-anthracite coal (PA); and anthracite (A). Anthracites are sometimes classified as a separate group. Grades G, Zh, K and OS, and sometimes D and T, are used for coking. Between grades D and T-A, the moisture content declines from 14% (D) to 4.5-5% (T-A); the oxygen content of the combustible matter declines from 15% to 1.5%; and the hydrogen content from 5.7% to 1.5%. Sulphur, nitrogen and ash content does not depend on the grade of coal. Calorific value rises accordingly, from 32.5 MJ/kg (7,750 kcal/kg) at grade D to 36.2-36.6 MJ/kg (8,650-8,750 kcal/kg) at grade K, and falls again to 33.5-35.4 MJ/kg (8,000-8,450 kcal/kg) at grades PA and A.
Black coal is also classified under the Russian system by fraction size on extraction as follows: plate (P), which is larger than 100 mm; large (K), 50-100 mm; nut (O), 26-50 mm; small (M), 13-25 mm; kernel (S), 6-13 mm; dust (Sh), less than 6 mm; and ordinary (R), no size limit. Coal can therefore be classified by grade and size by combinations of letters, such as DK, etc.
