The role of coal petrography in understanding the properties of South African coal

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The variations in physical and chemical properties of coal can only be explained in terms of its petrographical composition and its rank. By means of a TREND computer programme quantitative relationships (correlation coefficients > 0.97) were established between the contents of total carbon and hydrogen (daf) of South African humic coals and their volatile-matter and oxygen contents, their calorific value (all daf) and their mean maximum reflectance of vitrinite ( R max). The inherent moisture content (ash-free), the vitrinite content (mineral-matter-free), and the Hardgrove and free-swelling indices yielded only semiquantitative relationships (correlation coefficients between 0.80 and 0.93 and high standard deviations) when expressed in terms of total carbon and hydrogen contents (daf). These interrelationships are shown graphically by means of a Seyler diagram modified for South African coal. The relatively poor correlation in the case of vitrinite content is due to the presence of fairly large amounts of reactive semifusinite in most South African coals, and for this reason it is necessary to distinguish between normal, perhydrous and subhydrous coals, depending on the relative difference between determined and inferred vitrinite contents. Due to the high content of low reflecting inertinite the modal reflectance of vitrinite is also regarded to a better rank parameter than the mean reflectance, as the former is not affected by sporadic low or high values. From the modified Seyler diagram it is also possible to devise a grade/type/rank classification for South African coals, where grade is given by the ash content (moisture-free basis). This classification complements Alpern's universal classification of solid fossil fuels. The Seyler diagram can also be used to delineate fields of the most useful application of South African coals, based on the relevant coal properties.