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coal,fuel substance of plant origin, largely or almost entirely composed of carbon with varying amounts of mineral matter.
There is a complete series of carbonaceous fuels, which differ from each other in the relative amounts of moisture, volatile matter, and fixed carbon they contain. Of the carbonaceous fuels, those containing the largest amounts of fixed carbon and the smallest amounts of moisture and volatile matter are the most useful to humans. The lowest in carbon content, peat, is followed in ascending order by lignitelignite
or brown coal,
carbonaceous fuel intermediate between coal and peat, brown or yellowish in color and woody in texture. It contains more moisture than coal and tends to dry and crumble when exposed to the air; the flame is long and smoky and the heating power
..... Click the link for more information. , also called brown coal, and the various forms of coal—subbituminous coal or black lignite (a slightly higher grade than lignite), bituminous coal, semibituminous (a high-grade bituminous coal), semianthracite (a low-grade anthracite), and anthracite.
Lignite and subbituminous coal, because of the high percentage of moisture they contain, tend to crumble on exposure to the air. Bituminous coal, being more consolidated, does not crumble easily; it is a deep black in color, burns readily, and is used extensively as fuel in industries and on railroads and in making cokecoke,
substance obtained by the destructive distillation of bituminous coal. Coke bears the same relation to coal as does charcoal to wood. A hard, gray, massive, porous fuel, coke is the solid residue remaining after bituminous coal is heated to a high temperature out of
..... Click the link for more information. . Anthracite, which is nearly pure carbon, is very hard, black, and lustrous and is extensively used as a domestic fuel. Cannel coal, a dull, homogeneous variety of bituminous coal, is composed of pollen grains, spores, and other particles of plant origin. It ignites and burns easily, with a candlelike flame, but its fuel value is low.
The vegetable origin of coal is supported by the presence in coal of carbonized fibers, stems, leaves, and seeds of plants, which can be detected with the naked eye in the softer varieties and with the microscope in harder coal. Sometimes carbonized tree stumps have been found standing in layers of coal. The general interpretation of these facts is that coal originated in swamps similar to present-day peat bogs and in lagoons, probably partly from plants growing in the area and partly from plant material carried in by water and wind. From the thickness of coal seams, it is assumed that the coal swamps were located near sea level and were subject to repeated submergence, so that a great quantity of vegetable matter accumulated over a long period of time.
The initial processes of disintegration and decomposition of the organic matter were brought about by the action of bacteria and other microorganisms. Peat, the first product formed, is altered to form lignite and coal through metamorphism. The pressure of the accumulated layers of overlying sediments and rock upon the submerged plant matter forced out much of the water and caused some of the volatile substances to escape and the nonvolatile carbon material to form a more compact mass. The greater the stress exerted in the process of metamorphism, the higher was the grade of coal produced. Cannel coal was probably formed in ponds, rather than in lagoons or swamps, as it occurs in lenticular masses and is frequently found to contain fossil fish. Coal was formed chiefly in the Carboniferous periodCarboniferous period
, fifth period of the Paleozoic era of geologic time (see Geologic Timescale, table), from 350 to 290 million years ago. Historical Geology of the Period
..... Click the link for more information. of geologic time, but valuable deposits date also from the Permian, Triassic, Jurassic, Cretaceous, and Tertiary periods.
Coal is found in beds or seams interstratified with shales, clays, sandstones, or (rarely) limestones. It is usually underlaid by an underclay (a layer of clay containing roots of plants). The coal is removed by strip (surface) mining or underground mining methods (see coal miningcoal mining,
physical extraction of coal resources to yield coal; also, the business of exploring for, developing, mining, and transporting coal in any form. Strip mining is the process in which the overburden (earth and rock material overlying the coal) is removed to expose a
..... Click the link for more information. ).
The chief coal fields of the United States are the Appalachian (from N Pennsylvania into Alabama), the Eastern Interior (Illinois, Kentucky, and Indiana), the Northern Interior (Michigan), the Western Interior (Iowa, Kansas, Missouri, Oklahoma, and Arkansas), the Rocky Mountain (Colorado, Wyoming, Utah, New Mexico, Montana, and North Dakota), the Pacific (Washington), and the Gulf Coast (Texas, Arkansas, and Louisiana). In Europe the chief coal-producing countries are Germany, Russia, Ukraine, and Poland. There are valuable coal fields in China, India, Indonesia, Australia, South Africa, and Korea but only a few in South America, mainly in Colombia.
a solid, combustible mineral of sedimentary origin composed of organic matter, mineral solids, and moisture. The organic matter is the product of the conversion of higher and lower plants and planktonic microorganisms. By convention, the content of mineral solids in coal must be no greater than 50 percent.
Coals occur in the earth’s crust in beds and in sheetlike and lenticular deposits and may have an earthy, massive, layered, or granular texture. They range in color from brown to black.
General information. Coal is one of the most important raw materials for the production of energy and provides 30–35 percent of the world’s fuel. Between 1950 and 1974 world coal production increased by 70 percent and passed the level of 3 billion tons per year.
Coal makes up 87.5 percent of the world’s estimated reserves of fossil fuels, which amount to 12.8 trillion tons of standard fuel (seeSTANDARD FUEL). The USSR has the largest coal reserves. As of 1968, the known and estimated Soviet reserves meeting current requirements for quality and thickness of workable beds totaled 5.7 trillion tons. In terms of standard fuel, the figure was 4.6 trillion tons.
The principal industrial uses of coal are the production of electric power, the manufacture of metallurgical coke, the obtaining of heat and power in various other industrial processes, and the manufacture of up to 300 different kinds of products through chemical processing of coal. Coal is increasingly used to obtain high-carbon coal-graphite construction materials, waxes, plastics, synthetic liquid and gas high-energy fuels, aromatic products (through hydrogenation), and humic acids with a high nitrogen content for fertilizers. Germanium and gallium are extracted from coal (seeDISPERSED-ELEMENT ORE). Other promising uses include the extraction of sulfur from coal and the use of high-alumina ash and waste material from upgrading to produce aluminum, to provide ceramic and refractory raw materials, and to carry out treatment of industrial waste waters.
The possible uses of coals in industry depend on the coals’ composition and properties. As a result of differences in the source materials and in the conditions under which the source materials were transformed, the composition and properties of coals exhibit great diversity.
Origin, composition, and properties. Coals are divided into three genetic types according to the composition of the primary component, which is organic matter: humic coals, also known as humolites or humulites; sapropelic coals, or sapropelites; and coals of the saprohumolith and humosapropelic series (in the present article such coals are referred to simply as saprohumolitic coals).
The humic coals are the most common type. The source material for these coals consisted of remains of higher terrestrial plants. The remains were deposited mostly in swamps along the low-lying shores of seas, bays, lagoons, and freshwater lake and river basins; that is, a process of autochthonous accumulation occurred. Allochthonous accumulation, wherein plant material and products of its transformation were washed from adjacent land areas and deposited in stagnant bodies of water, was more limited. As a result of biochemical decomposition, the accumulated plant material was turned into peat; important factors in this process were the extent of inundation and the chemical composition of the aqueous medium. Anaerobic conditions in the aqueous medium led to gelification of the organic material and thus provided the basis for formation of the lustrous vitrinite, or gelenitic, coals. On the other hand, aerobic conditions and the existence of an oxidizing medium promoted the fusinization of tissues, that is, the formation of the fibrous and sooty fusinite coals. Eluviation, the leaching of the products of oxidation of lignocellulosic tissues by flowing water, was accompanied by enrichment of the organic mass with the remains of the most resistant parts of plants, for example, spore cases, cuticles, resin bodies, and corky bark tissue. These parts of plants are characteristic of the dull liptinite coals. Coals composed almost entirely of resistant tissue elements (plant remains that have preserved their structure and outlines) are assigned to a special group called liptobioliths (seeCAUSTOBIOLITHS).
The sapropelic coals are products of the transformation of lower plants and planktonic microorganisms that accumulate in the sapropel of lakes and marine lagoons. At equivalent stages of transformation of organic matter, the sapropelic coals differ from the humic coals by a higher yield of volatile matter (60–80 percent) and a higher hydrogen content (8–12 percent).
The saprohumolitic coals are a transitional group of coals and result from the transformation of higher and lower plants. Sapropelic and saprohumolitic coals usually occur in layers and lenses in beds of humic coals. High-ash varieties of sapropelic coals are called combustible shales; they often form independent coalfields or coal deposits, such as the Baltic shale basin.
The inorganic solids either appear in a finely dispersed state in the organic groundmass or take the form of thin interlayers and lenses or of crystals or concretions. The sources of the inorganic matter in coals may have been inorganic components of the coal-forming plants, terrigenous material brought into peat-formation areas by water and wind, or new minerals that precipitated from waters circulating in the peat swamps. The inorganic constituents may be quartz, clay minerals (primarily kaolinites), feldspars, pyrite, marcasite, carbonates, and other compounds containing silicon, aluminum, iron, calcium, magnesium, potassium, sodium, titanium, and rare and trace elements (such as uranium, germanium, gallium, and vanadium). There is broad variation in the content of the inorganic matter; most of it becomes ash when the coal is burned.
The direction and intensity of oxidizing and reducing microbiological processes were determined by differences in the source material, the extent of inundation of the peat swamps, the chemical composition of the medium, and the depositional environment of sedimentation and peat accumulation. Such differences created, in the peat stage, the basis for the formation of different genetic types of coal (seeCOAL PETROLOGY). Peat formation and peat accumulation ended with the peat swamp becoming covered by sediments that subsequently formed the roof of the coal bed. The peat was transformed into brown coal as a result of diagenetic and biochemical reducing processes that occurred at relatively low temperatures and pressure; the diagenetic processes included compaction, the dehydration of sediments, and the evolution of gas. Coals containing slightly decomposed wood remains cemented by earthy coal are called lignites.
The brown coals are widely distributed. Brown coals and lignites account for 42 percent of world coal reserves. Because many beds of brown coals and lignites lie close to the surface and are of great thickness, extensive use is made of surface mining. The economic and technological advantages of surface mining largely compensate for the relatively low quality of the coals.
The prolonged action of elevated temperatures and pressure transform brown coals into hard coals and hard coals into anthracites. (In the Soviet classification system, which is used in the present article, the term “hard coal” [kamennyi ugol’] is applied to coals of rank higher than that of brown coal; sometimes, but not always, the anthracites are regarded as forming a separate group. It should be noted that in the USA “hard coal” is used as a synonym for “anthracite” and contrasts with the term “soft coal” [bituminous coal].)
The term “coalification” is applied to the irreversible process of gradual change in the chemical composition (primarily in the direction of carbonization) and the physical and technological properties of the organic matter in the transformations from peat to anthracite. In the stages of the transformation of brown coals to hard coals and hard coals to anthracites, coalification due to processes occurring in the earth’s crust is called coal metamorphism. Three basic types of metamorphism are distinguished: regional, thermal, and contact. The term “regional metamorphism” is used with respect to coals buried deep in the crust and covers changes due to heat from the interior and to the pressure of the overlying rock mass. Thermal metamorphism is due to the heat given off by magmatic bodies that overlie or penetrate the coal-bearing strata or that penetrate the underlying deposits. Contact metamorphism is due to the action of the heat of igneous rocks that are intruded into coal beds or that cut across them. It is possible that a fourth type of metamorphism—dynamic metamorphism—occurs in the coalification process. This type results from an increase in temperature in regions affected by tectonic (compressive and shearing) forces. The role of dynamic metamorphism, however, remains problematic.
The molecular restructuring of organic matter in metamorphism is accompanied by a consistent increase in the relative content of carbon and by a decrease in the oxygen content and the yield of volatile matter. A number of characteristics of coals exhibit definite regularities of variation in the process of coalification, the regularities being of particular significance in the middle stages of the process. Such characteristics include hydrogen content, heat value, hardness, density, friability, and various physical properties, for example, optical and electrical properties (Figure 1). In order to determine these stages, or ranks, use may be made of the yield of volatile matter Vc, the carbon content, the microhardness, or other characteristics of the chemical composition and physical properties of the coals. The most effective method of determining the stage of coalification is based on the reflectance R of vitrinite.
Note to Figure 1. Variation in a number of quantities characterizing coals as the degree of coalification increases (after I. V. Eremin and E. M. Pakh): (Ww) working moisture, () heat value, (C) carbon content, (Vc) yield of volatile matter, (R) reflectance of vitrinite
In the middle stages of metamorphism hard coals acquire caking properties, that is, the ability of the gelified and lipid components of the organic matter, when the coals are heated under certain conditions, to change to the plastic state and form the porous solid material called coke. The reserves of coals with a high caking index make up 10–15 percent of the total hard coal reserves. This fact is due to the higher intensity of transformation of organic matter in the middle stages of metamorphism. Caking coals develop at temperatures ranging from about 130° to 160°-180°C; by contrast, the total range of temperatures responsible for metamorphism lies between 70°-90°C for long-flame coals and 300°-350°C for anthracites. The highest-grade caking coals were formed in coalfields that experienced regional metamorphism, the coal-bearing strata being deeply buried. In thermal and contact metamorphism, the sharp change in temperatures and the low pressure caused the transformation of organic matter to occur unevenly, and the quality of the resulting coals is not persistent with respect to technological properties. In addition to metamorphism, the rocks of coal-bearing formations underwent cata-genetic transformations (seeCATAGENESIS).
In zones of aeration and the action of subterranean waters near the earth’s surface, coals are subjected to oxidation. With respect to its effect on the chemical composition and physical properties of coals, oxidation works in a direction opposite to that of metamorphism. The coals lose their strength properties and may finally be converted into sooty matter; in addition, the coals lose their caking properties. The relative oxygen content of the coals increases, the carbon content decreases, the moisture and ash content increase, and the heat value drops sharply. The depth to which coal oxidation is effective varies from 0 to 100 m, depending on present-day and ancient relief, the position of the ground-water level, climatic conditions, material composition, and the degree of metamorphism.
Differences in material composition and the degree of metamorphism have resulted in substantial differentiation in the technological properties of coals. To provide a basis for their rational use in industry, coals in the USSR are classified into rank varieties (marki) and technological groups. This classification makes use of parameters that characterize the behavior of the coals under the action of heat (see Table 1). For a working mass of ash-free coal, a gross, or high, heat value of 5,700 kilocalories per kg (kcal/kg), or 23.86 megajoules (MJ), is taken as the point of division between brown and hard coals.
In the case of coals used for the production of heat and power, the principal index is the net, or low, heat value Qn. Upon conversion to working fuel, the ranges of the net heat value are 2,000–5,000 kcal/kg (8.372–20.930 MJ) for brown coals, 4,100–6,900 kcal/kg (17.162–28.893 MJ) for hard coals, and 5,700–6,400 kcal/kg (23.86–26.79 MJ) for anthracites. The low net heat values of brown coals are due to the low degree of coalification of the organic matter, the slight compaction of the material, and, consequently, the high natural moisture content of such coals, which ranges from 15 to 58 percent. Brown coals are classified by content of working moisture Ww into three technological groups: Bl with Ww > 40 percent, B2 with Ww = 30–40 percent, and B3 with Ww < 30 percent.
The industrial classification of hard coals into rank varieties is based on two parameters that characterize the results of high-temperature dry distillation, or coking, of the coals: (1) the yield of volatile matter that forms upon decomposition of the organic matter and, to some extent, the inorganic material (sulfides, carbonates, and hydrated minerals) and (2) the caking index of the ash-free combustible residue, the coke. As the degree of coalification increases, the yield of volatile matter by weight (Vc) from coals decreases steadily from 45 to 8 percent for hard coals and from 8 to 2 percent for anthracites.
In the USSR the caking index of coals is determined with laboratory equipment through the use of the plastometric method proposed in 1932 by the Soviet scientists L. M. Sapozhnikov and L. P. Bazilevich. The determination is based on the thickness y of the plastic layer formed upon heating, with the shrinkage x being taken into account; both y and x are expressed in millimeters. Hard coals of the middle stages of coalification with a plastic layer 10–35 mm thick (rank varieties C and F) have the greatest caking properties. The caking index of coals decreases as the degree of metamorphism increases or decreases. Varieties LF and L are characterized by a slightly caked powder-like nonvolatile residue. Table 1 gives the basic indexes of coal quality for different stages of coalification as classified into rank varieties in the USSR.
|Table 1. Basic indexes of the quality of different rank varieties of coal|
|Rank variety||Symbol||Yield of volatile matter Vc* (%)||Carbon content Cc* (%)||Heat value Qc* (kcal/kg)||Reflectance of vitrinite in oil immersion R° (%)|
|* Average values for coals consisting primarily of vitrinite|
|Brown. . . . . . . . .||B||41 or more||76 or less||6,900–7,500||0.30–0.49|
|Long flame. . . . . . . . .||LF||39 or more||76||7,500–8,000||0.50–0.64|
|Gas. . . . . . . . .||G||36||83||7,900–8,600||0.65–0.84|
|Fat. . . . . . . . .||F||30||86||8,300–8,700||0.85–1.14|
|Lean caking. . . . . . . . .||LCa||15||89||8,450–8,780||1.75–2.04|
|Lean. . . . . . . . .||L||12||90||7,300–8,750||2.05–2.49|
|Anthracite. . . . . . . . .||A||less than 8||91 or more||8,100–8,750||2.50–6.00|
In addition to those shown in the table, intermediate varieties are distinguished in some coalfields: gas-fat coals (GF), coking-fat coals (CF), coking-two coals (C2), and weak-caking coals (WCa). The coal varieties G, GF, F, CF, C, and LCa are subdivided into technological groups by caking properties. To indicate the technological group, a number is added to the letter designation of the coal variety. The number gives the minimum thickness y of the plastic layer in the coal; examples are G6, G17, and CF14. The values of the classification indexes Vc and y for coals of specific basins are set by all-Union state standards. Metallurgical coke is produced from a mixture of different coals; coals with good caking properties make up the primary component of the blend.
The division of coals into brown coals, hard coals, and anthracites is used in most European countries; in some countries the lignites are also regarded as a separate class. The international classification system for hard coals adopted in 1956 by the Economic Commission for Europe of the United Nations is based on the yield of volatile matter Vc. For coals with Vc < 33 percent, the parameters used are caking and coking properties; for coals with Vc > 33 percent, the gross heat value on the moist ash-free basis is used. Coal varieties are indicated by a three-number code in which the first digit indicates the class of the coal (according to Vc or ), the second shows the group (according to the caking property as determined by the Roga method or the swelling index in the crucible), and the third gives the subgroup (according to the coking property as determined by the Audibert-Arnu or Gray-King methods). The USA and a few other countries divide coals into the following classes: lignite, subbituminous coal, bituminous coal, and anthracite. The heat value on the ash-free basis is used as the classification parameter for lignites, for subbituminous coals, and for bituminous coals with Vc > 31 percent. For bituminous coals with Vc < 31 percent and for anthracites, the yield of volatile matter and the content of fixed carbon are used.
The classification of coals into rank varieties reflects a number of definite technological properties of the different coals and provides a basic standard for the industrial use of coals. Supplementary technical requirements are established for particular uses. Because the heat value of coals and the economic indexes of the use of coals drop sharply as the content of inert material (ash and moisture) increases, it is advantageous to briquette coals with high natural moisture and to subject high-ash coals to preliminary upgrading (seeCONCENTRATION OF MINERALS). The maximum ash content of coals used in layer combustion should not exceed 20–37 percent; for dust combustion the figure is 45 percent.
For coking, low-ash (upgraded) caking hard coals are used, with limits being placed on the sulfur and phosphorus content. Standard values are established for such quality parameters as the caking index, sulfur content, ash content, lump size, heat-shock resistance, resin content, and bitumen content for various purposes, including low-temperature carbonization, gasification, and the production of liquid fuel and waxes.
Basic patterns for coal accumulation. Coal formation is a regional geologic process. The occurrence and renewal of the process requires a favorable combination of tectonic, climatic, geo-morphological, phytocoenologic, and other factors. The major periods of coal formation were associated with periods of slow oscillatory movements of the earth’s crust against a background of general, prolonged submergence of large areas and regions. The emergence of land vegetation in the Lower Paleozoic and the evolution of the vegetation in subsequent earth history were of great importance for coal formation. Humic coals are found in sedimentary beds beginning with the Silurian, and industrially significant coal accumulations are found beginning with the Devonian. The hygrophilous fernlike plants that developed in the middle of the Paleozoic limited the distribution of coal accumulation regions to coastal plains or plains that gradually lost their connection with the sea; the resulting coal formations are said to be paralic. With the subsequent evolution of plant forms and their dispersal on land, the regions of coal formation shifted to the interior of the continents, and the development of limnic coal formations become predominant.
Russian and Soviet geologists have made a substantial contribution to knowledge of the processes of coal formation, the patterns of spatial distribution of coal reserves, and other problems of coal geology. The first specialists in the geology of coalfields were L. I. Lutugin and his students, including V. I. Iavorskii, P. I. Stepanov, and A. A. Gapeev. In addition, important work was done by M. A. Usov, Iu. A. Zhemchuzhnikov, I. I. Gorskii, G. A. Ivanov, M. M. Prigorovskii, A. K. Matveev, and G. F. Krasheninnikov. The development of coal geology in other countries is associated with the names of, for example, the Germans H. Potonié, K. Naumann, M. Teichmüller, R. Teichmüller, and E. Stach, the Britons M. Stopes, K. Marshall, and W. Francis, the Americans R. Thiessen and D. White, the Dutchman D. von Krevelen, and the Czech V. Havlena.
An analysis of the stratigraphic and paleogeographic distribution of coal bodies in the earth was the basis of the theory of belts and major zones of coal formation developed by P. I. Stepanov in 1937. He established a definite pattern in the location of coal regions and coalfields of the same age: they are arranged in latitudinal or submeridional belts associated with areas of the earth’s surface where paleoclimatic and geotectonic conditions favored the accumulation of coal. Making use of the stratigraphic distribution of world coal reserves, P. I. Stepanov identified two peaks of coal formation: in the Upper Carboniferous and Permian and in the Paleogene and Neogene. In addition, he suggested that a third peak may have existed in the Jurassic and Lower Cretaceous. Later investigators confirmed these patterns. The stratigraphic distribution of world coal reserves, which in 1970 were estimated to total 14 trillion tons, is shown in Figure 2. The principal coal reserves in the USSR are concentrated in Permian (48.5 percent) and Jurassic-Cretaceous (39 percent) basins.
Coal formation is a regional geologic process that occurred on all continents (Figure 3). Regions of continuous coal-bearing formations vary in area from a few square kilometers to hundreds of thousands of square kilometers, and thicknesses range from tens of meters to 20 km. The number of coal beds in the formations ranges from a few to several hundred. According to current notions, all the basic features of coal-bearing formations are determined by the character and intensity of the oscillatory movements of the earth’s crust, in close association with the history of structural development and with paleogeography. These features include the thickness of the formation, the spatial variation in composition and structure, the relationship with the enclosing rocks, the degree of metamorphism, tectonics, and quantitative and qualitative characteristics of coal-bearing capacity. Thus, coal-bearing formations associated with foredeeps and with large inherited and superimposed basins on a folded base (seeTECTONIC TROUGH) are characterized by a large thickness, by a zonation of tectonic construction (from strongly dislocated structures on the boundary with orogenic regions to less disturbed structures in the central part of the basin and parts near platforms), by a multiplicity of beds, by horizontal and vertical zonation in the manifestation of regional coal metamorphism, and by a broad range of rank varieties (from brown coals to anthracites). In the USSR, the coalfields providing raw material for the by-product coke industry—that is, the Donets, Kuznetsk, Karaganda, and Pechora fields—involve formations of this kind.
Large-scale coal formation processes are confined to platform regions. Thick coal beds often accumulated in the coal-bearing formations associated with postorogenic (Cheliabinsk and Turgai), inherited, and superimposed (Kansk-Achinsk, Maikiuben’, and Southern Ural) basins. Thin coal-bearing formations with low coal-bearing capacity are associated with platform syneclises (Moscow and Irkutsk). The degree of coalification of the coals of platform formations is low; brown coals and hard coals of varieties LF and G predominate. Extensive coal formation did not occur in orogenic regions; coal is, however, found in local areas where conditions were favorable for continental sediment accumulation. Because of their complex tectonic structure, such deposits are of limited industrial importance.
Morphology of coal beds and their modes of occurrence. In most coal-bearing formations the coal occurs in beds extending over vast areas between almost parallel layers of enclosing rocks. The thickness of the beds is small compared to the area of distribution. In the marine coastal and coastal basin (lagoon and delta) sediment accumulation settings typical of coal-bearing formations associated with transitional (from orogenic to platform) regions, the coal beds were formed over areas of hundreds of square kilometers. Individual beds range in thickness from a few centimeters to a few meters, with a relatively high persistence of morphological features. The intracontinental (lake, lake-swamp, and river) sedimentation setting typical of platform regions resulted in the formation of beds covering a more limited area; in many cases the beds have a lenticular shape. Many coal deposits in such regions are tens or, occasionally, even hundreds of m thick over considerable areas.
In the practical evaluation of coal deposits in the USSR, it is customary to classify coal beds by thickness as being very thin (less than 0.5 m), thin (0.5–1.3 m), medium-thick (1.3–3.5 m), thick (3.5–15 m), or very thick (more than 15 m); with respect to persistence of morphology and coal quality, coal beds are classified as persistent, relatively persistent, or not persistent.
The persistence of the morphology of coal beds, which is usually evaluated for areas of a few square kilometers, reflects primarily regional and local splitting, which is a result of discontinuous, differentiated subsidences of the basin floor; uneven removal of sandy and argillaceous material; and fluctuations of
the water level. Changes in bed thickness also result from un-evenness in the bottom of the peat swamp and from erosion by the network of gulleys and streams or by marine transgression, both during the process of accumulation and after burial of the peat swamps and coals. In many cases the persistence of coal beds has been disrupted through processes of karst formation in the deposits underlying the coal-bearing strata, through the burning out of beds as a result of oxidation of the coal by atmospheric air, through the action of tectonic movements that lead to pinching and swelling, and through the assimilation of the coal by igneous rocks injected into the coal-bearing strata.
The mode of occurrence of coal beds exhibits considerable diversity. Only in a few coalfields and coal deposits of the platform group can the beds be described as gently rolling, almost horizontal, and undisturbed. Most coal-bearing formations have been subjected to folding accompanied by faulting (Figure 4). In the exploration and working of deposits the mode of occurrence of coal beds is evaluated for local areas of large coalfields and deposits that have sufficient coal reserves to support the operation of an underground or surface mine. On the scale of a mining field, the chief structural forms include monoclines—that is, the limbs of gently sloping syneclises and anticlises of platforms. Other important forms are (1) the limbs and closures of large synclines and anticlines and (2) brachyfolds of limited size and forms combining different folded forms of lesser orders. The accompanying folding and superimposed faulting imparts a blocklike character to the coal beds; individual blocks may range from small and flaky forms to blocks that are several square kilometers in size.
With respect to the principles of industrial geological evaluation that are accepted in the USSR, coal deposits and coal-bearing areas are divided into three groups according to complexity of geological structure; the classification takes into account the persistence of the morphology of the coal beds, the quality of the coals, and the effects of tectonic phenomena. The first group is made up of deposits or sections of simple structure and is characterized by persistent thicknesses of the principal workable coal beds, consistent coal quality, and undisturbed or mildly disturbed bedding. The second group includes deposits or sections of complex structure; the thickness and structure of most of the coal beds vary, or the quality of the coal is inconsistent. This group also includes coal-bearing areas where the morphology of the principal beds is persistent but the beds have undergone complex folding or intensive faulting. The third group consists of deposits or sections of very complex structure; the beds have been intensively disturbed by folding and faulting, by the appearance of small blocklike forms, or by the development of a complex, varying morphology. This grouping scheme is used in planning geological exploration work, calculating coal reserves, and planning the construction of coal-mining extraction enterprises. (See also and MINING, UNDERGROUND.)
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