THE QUALITY OF WESTERN CANADIAN COKING COAL

DAVID E. PEARSON
British Columbia Ministry of Energy, Mines and Petroleum Resources

COAL MINING AND TECHNOLOGY

Source of information: http://coalpetrography.com/research/quality.htm


Abstract


    Western Canadian coking coals are located in the Rocky Mountains and foothills of British Columbia and Alberta. They are typical of non-marine coals, characterized by total sulphur contents of less than one per cent and with ash contents of 10-30 weight per cent. This mineral matter is composed principally of kaolinite and quartz.

    The petrographic compositions of most of the coking coals which constitute the 2.065 billion tonnes of British Columbia's measured reserves, and the compositions of the metallurgical coal currently exported from Alberta, show them to be rich in the inertinite macerals, macrinite, fusinite and semifusinite. When the mean maximum reflectance of vitrinite in oil (Romax) is used as the coal rank parameter, this high inertinite content is reflected in the lower volatile-matter yields, but the relatively inert character is also displayed by lower free swelling indices (FSI), lower maximum dilatations, lower maximum fluidities and lower coke strengths than other coals of the same rank, but richer in the reactive macerals, vitrinite, exinite and semifusinite. Despite these reduced values in the so-called caking tests, inertinite-rich coals produce strong coke.

    A system of classification of coking coals is introduced which is based on rank, as indicated by Romax, and either petrographic composition, maximum dilatation, maximum fluidity or, to a lesser extent, FSI and volatile-matter yield. Six principal coal groups can be distinguished among coking coals using this versatile classification. Inertinite-rich coals are assigned to the Keystone Group (G1), Balmer Group (G3) or Moura Group (G4), each group being named for a coal typical of that population of coals. Reactive-rich coals are assigned to the Pittston Group (G2), Kellerman Group (G5) or Big Ben Group (G6). A multiple-regression analysis of coal quality versus price shows how the value of coking coal in the Japanese market appears to be related to reflectance and the free swelling index, and confirms the economic viability of the classification.

    British Columbia's measured reserves of coking coal contain representatives of four of these groups; Balmer Group (73%), Moura Group (16%), Pittston Group (3.4%) and Kellerman Group (7.6%).

Introduction


    Western Canadian coking coals are located in coalfields in the Rocky Mountains and foothills of British Columbia and Alberta in two major stratigraphic zones, both of lower Cretaceous age. These coalfields are the Peace River coalfield and the East Kootenay coalfield. In 1971, the size of this coal resource was estimated to be 86.1 billion short tons, of which 57.4 billion short tons was in British Columbia (Latour, 1972). The figures for British Columbia were revised in 1976 when the coking coal resource was estimated to be 30.7 billion short tons (Gilchrist, 1976). Although the size of the resource is reasonably known and measured reserves of over 2.065 billion tonnes are reported by companies, little has been written about the quality of the coal. The purpose of this paper is to (i) establish the quality of coking coals in British Columbia and Alberta in relation to others that enter the international coking coal market, (ii) demonstrate how the price of coking coal is dependent on quality, (iii) examine the role of inertinite-rich coals in blends and (iv) assess the composition of British Columbia's measured reserves.

Composition and Rank of Coals


    The natural constituents of coal can be divided into two groups: (i) the organic fraction, which can be further subdivided into microscopically identifiable macerals; and (ii) the inorganic fraction, which is commonly identified as ash subsequent to combustion, but which may be isolated in the form of mineral matter by low-temperature ashing (LTA). The organic fraction can be further subdivided on the basis of its rank or maturity.

Maceral Composition of Coals


    Coal is composed of microscopically recognizable constituents, called macerals, which differ from one another in form and reflectance. Macerals are analogous to minerals in inorganic rocks. Three principal maceral groups are identified and these are, in increasing order of carbon content, exinite, vitrinite and inertinite . In a single coal, vitrinite, which is usually the commonest maceral, has a higher reflectance than the associated exinite, but a lower reflectance than inertinite.

    There is, therefore, a correlation between carbon content and reflectance and this is used to precisely determine rank. Petrographers in Canada and many other countries use the mean maximum reflectance of vitrinite in oil (Romax), at 546 µ, as the level of organic maturity, or rank, of a coal sample.

    Vitrinite is thought to be derived mainly from the original woody tissue of trees in peat swamps. In Pennsylvanian-age coals of Western Europe and the eastern United States, it often constitutes 60-80% of the macerals (ICCP Handbook 1971), whereas in Permian-age Gondwana coals of the southern hemisphere it rarely exceeds 80%, and in some cases comprises less than 50% of the total macerals (Chandra and Taylor, 1975).

    Exinite , which is derived from pollen, spores and leaf epidermis, is technologically important, because it enhances the fluidity of coal (Krevelen, 1961). Exinite contents of 1% are common in Gondwana coals of the southern hemisphere, but generally they are lower than in Pennsylvanian-age coals, which possess exinite contents of 5-20%. Both exinite and vitrinite are capable of yielding petroleurn-type hydrocarbons (Teichmüller and Teichmüller, 1975).

    The inertinite group of macerals derives its name from its more or less non-reactive character shown during the carbonization process. Whereas exinite and vitrinite melt, with an evolution of volatiles, inertinites generally remain intact. Inertinite is derived from fungal remains, charcoal and partly charred wood. Gondwana coals in general and many lower Cretaceous Kootenay Formation coals of British Columbia and Alberta specifically are characteristically enriched in inertinite macerals. For example, in Australia, Taylor and Cook (1962) found up to 85% inertinite macerals in a whole seam section; lower Kootenay coals usually comprise 20-30% inertinite (Cameron 1972, Pearson and Grieve 1978 and in prep.). Pennsylvanian-age coking coals generally possess 5-20% inertinite.

    The maceral content and volatile-matter yield of a coal may be considerably influenced by the post-depositional chemical environment to which the normally acid peat is subjected. In the case of a marine cover to a peat, the maceral proportions remain unaffected, but anaerobic bacteria flourish and promote advanced decomposition, together with reduction of seawater sulphate to sulphide. This leads to a per-hydrous coal, rich in sulphur, with a higher volatile yield than normal (Francis 1961, Teichmüller and Teichmüller 1975). By contrast, in those cases where peats are covered by fresh water, if coupled with periodic oxidizing conditions, volatile-matter yields are reduced, and the inertinite content may be increased at the expense of vitrinite and exinite.

    In the East Kootenay coalfield, all seams have roof strata and sulphur contents which indicate non-marine conditions, and in the Peace River coalfield only two instances are known where marine strata form the seam roof. Thus, the influence of the post-depositional chemical environment contributes to the inertinite-rich character and probably explains why measured reserves of Western Canadian coking coals, with few exceptions, are rich in inertinite macerals.

Mineral-Matter Composition of Coals


    The ash of a coal is the uncombustible oxide residue which remains after combustion. By contrast, mineral matter is the natural mineral assemblage of a coal that contains syngenetic, diagenetic and epigenetic species. Mineral matter is recorded during petrographic examination of coals, but is identified more accurately by X-ray diffraction analysis of low-temperature ash (LTA) obtained by radio-frequency plasma-ashing of coal (Rao and Gluskoter, 1973; Mitchell and Gluskoter, 1976; Pearson and Kwong, 1979).

    The post-depositional chemical environment of a peat swamp, as well as being an influence on the maceral compostion of a coal as noted above, is the major factor determining the amount and kind of mineral species present in coal. Although there is little doubt that some minerals present in coal are detrital, a large proportion are authigenic. Therefore an evaluation of mineral assemblages can suggest the physico-chemical conditions in the peat swamps. For example, where a peat swamp is known to have had a marine cover, the change in pH from acid to alkaline is accompanied by advanced bacterial action, the production of sulphide and a consequent nucleation of pyrite. Calcite and illite may also be formed. In the case of a fresh-water cover, the acid nature of the swamp is preserved, and the absence of seawater-salts limits total sulphur to that which is contained in plant and animal protein (Teichmüller and Teichmüller, 1975). The acid environment is, however, ideal for the nucleation of kaolinite. Thus, freshwater coals are characterized by kaolinite-quartz and low sulphur contents, and marine-influenced coals are characterized by high sulphur contents and mineral assemblages of pyrite-calcite-illite-quartz. Figure 4 shows LTA-diffractograms typical of early Cretaceous non-marine coals from British Columbia (Balmer, Fording and Denison's Belcourt property) and, for contrast, a marine-influenced Pennsylvanian-age coal (Herrin-Illinois No. 6).
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