Maceral effects in the determination of proximate volatiles in coals

The proximate and ultimate analyses of coals are commonly regarded as sufficient for a satisfactory description of their chemical composition. This is no doubt true if coal is considered as a whole, but it has to be borne in mind that coals are actually composed of fairly different materials (macerals), and that the corresponding chemical characterization data are average values of the individual compositions of the macerals present in the coal. As regards the determination of coal maturity, the volatile matter content is widely used as a rank parameter, although this characteristic varies not only with coalification but also with maceral composition. A full description of the chemistry of coals, for the prediction of their behavior during industrial processing, requires that the characteristics of their maceral components is also known. This paper is an attempt to evaluate the relative contributions of vitrinite, liptinite, and inertinite to the volatile matter content of coals, over the entire coalification scale. Proximate and petrographic analyses were carried out on 39 coals, and data from 992 more coals and 83 coal fractions were obtained from the literature and coal databases. A best-fit strategy was run with these data, so that the volatile matter contents of the three maceral groups and their variations with rank were determined. The results showed qualitative trends similar to those reported in the literature, but a substantial improvement in their predicting ability was achieved, and the ranges of applicability were expanded to the entire coalification scale.