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a solid combustible mineral of plant origin; a type of mineral coal with a higher carbon content and greater density than brown coal (lignite). Hard coal is a dense rock of black and sometimes gray-black color with a shiny mat or semi-mat surface. It contains 75– percent and more carbon, 1.5–.7 percent hydrogen, 1.5– percent oxygen, 0.5– percent sulfur, up to 1.5 percent nitrogen, and 45– percent volatile matter; the amount of moisture varies between 4 percent and 14 percent; ash constitutes usually between 2– percent and 45 percent. The highest heat of combustion figured for a moist ashless mass of hard coal is 23.8 megajoules per kg (5, 700 kilocalories per kg).
Hard coals are formed from products of the decay of the organic remains of higher plants; these products have undergone changes (metamorphism) under the pressure of surrounding rocks in the earth’s crust at comparatively high temperatures. With an increase in the degree of metamorphism in the combustible mass of hard coal the carbon content increases consistently and, at the same time, the amount of oxygen, hydrogen, and volatile matter decreases; there are also changes in the heat of combustion and clinkering capacity. The industrial classification of hard coals by ranks that has been adopted in the USSR is based on changes in these qualities as determined by the results of the thermal decomposition of coal (the discharge of volatile matter and the characteristics of the nonvolatile remainder). The ranks are long flame, or candle (LF); gas, or bottle (G); gasmetabituminous (GF); metabituminous, or fat (F), coking metabituminous (CF); coking (C); lean clinkering (LC); lean (L); weak clinkering (WC); subanthracite (S); and anthracite (A). Anthracite coals are sometimes singled out as a separate group. Coals of ranks G, F, C, LC, and, to some extent, LF and L are generally used for coking. Between coal of rank LF and coal of ranks L-A there is a gradual decrease in moisture in the working fuel from 14 percent in LF coal to 4.5–5.0 percent in ranks L-A; at the same time the oxygen content (in the combustible mass) decreases from 15 percent to 1.5 percent and the hydrogen content drops from percent 5.7 to 1.5 percent. The content of sulfur, nitrogen, and ash does not depend on the rank to which the particular coal belongs. The heat of combustion of the combustible mass of coal rises steadily from 32.4 megajoules (MJ) per kg, or 7,750 kilocalories (kcal) per kg, for rank LF to 36.2–36.6 MJ/kg (8, 650–8,750 kcal/kg) for rank C and then decreases to 35.4–33.5 MJ/kg (8,450–8,000 cal/kg) for ranks S and A.
Hard coals are also classified by the size of the pieces obtained during extraction. The categories are slab (S) for more than 100 mm, macrofragmental (M) for 50– mm, nut (N) for 26– mm, fine (F) for 13– mm, seed (S) for 6– mm, chip (C) for less than 6 mm, and run-of-mine (R), which is not limited by size. Combinations of letters, such as LM, are used to indicate the rank and size of pieces for the coals.
The classifications of hard coals in a number of western European countries are based on approximately the same principles as those used in the USSR. The United States has the most extensive classification of hard coals, which is based on the release of volatile matter and the heat of combustion. The coals are divided into subbituminous, with a high discharge of volatile matter (corresponding to Soviet ranks LF and G); bituminous with moderate discharge of volatile matter (corresponding Soviet ranks F and C); bituminous with a low discharge of volatile matter (LC and L); and anthracite coals, which are divided into semianthracites (some L and A) and anthracites proper and meta-anthracites (A). In addition, there is an international classification of hard coals based on the content of volatile matter, the clinkering capacity, the coking capacity, and the industrial properties of the coals.
The formation of coal typifies all geological systems since the Silurian and Devonian; coal is very common in the deposits of the Carboniferous, Permian, and Jurassic systems. Coal occurs in the form of seams of varying thickness (from fractions of a meter to several dozen meters and more). The depth at which coals occur may vary; in some cases it emerges on the surface and in others it may occur as deep as 2, 000–, 500 m and lower. With the present level of mining technology, coal may be extracted by the open-pit method at depths to 350 m. The total geological reserves of hard coal in the USSR come to about 4, 700 billion tons (1968 estimate), which breaks down by rank (in billions of tons) to 1, 719 LF, 331 LF-G, 475 G, 69.4 GF, 156 F, 21.5 CF, 105 C, 88.2 LC, 634 WC, 205 L, 540 L-A, and 139 S and A. The largest reserves of hard coal in the USSR are found in the Tun-guska Basin. The largest developed coal basins in the USSR are the Donets, Kuznetsk, Pechora, and Karaganda basins. Major basins in other countries include the Appalachian and Pennsylvania Anthracite in the USA; Upper Silesia in Poland and its continuation into Czechoslovakia, the Ostrava-Karvina; the Ruhr in the Federal Republic of Germany; the Greater Hwang Ho Basin in China; South Wales in Great Britain; Valenciennes in France; and Brabant in Belgium. Coal has many uses: as home-heating and power fuel, as raw material for the metal and chemical industries, and for the extraction from the coal itself of rare and dispersed elements.
REFERENCESGapeev, A. A. Tverdye goriuchie iskopaemye. Moscow, 1949.
Zhemchuzhnikov, Iu. A., and A. I. Ginzburg. Osnovy petrologii uglei. Moscow, 1960.
Energeticheshoe toplivo SSSR: Spravochnik. Edited by T. A. Zikeev.Moscow, 1968.
A. K. MATVEEV