Abstract

MOKHAHLANE, L.S.; GOMO, M.  and  VERMEULEN, D.. Acid-base accounting of unburned coal from underground coal gasification at Majuba pilot plant. J. S. Afr. Inst. Min. Metall. [online]. 2018, vol.118, n.10, pp.1041-1046. ISSN 2411-9717.  http://dx.doi.org/10.17159/2411-9717/2018/v118n10a4.

Underground coal gasification (UCG) is an unconventional mining method that converts coal in situ into a fuel gas that can be used for electricity generation. Residue products from UCG have the potential to leach into surrounding groundwaters. The geochemistry and leaching dynamics of these products are explored in this study. The products include char, ash, and the heat-affected zone in the surrounding rocks. Core samples from the pilot plant at Majuba are the first ever to be recovered from a UCG cavity in Africa, and they give key insights into the geochemistry of the gasification zone. Mineralogical and chemical analyses were performed on the samples, and acid-base accounting (ABA) was used to predict the acid-producing capacity of the gasification zone, particularly for char samples. Some of the char contained pyrite, although not all samples were acid-producing as determined by the ABA analysis. The ABA results showed that some of the unburned coal has moderate levels of sulphur, which could be the driving medium for acidic conditions. The ABA analysis indicated that water in contact with the gasification zone would eventually have a pH lower than 7, which could lead to acid rock drainage. These results form part of a preliminary investigation into the geochemistry of the reaction zone, post gasification.

Keywords : underground coal gasification; residue products; geochemistry; acid-base accounting.

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