Evidence of coal conversion in the Majuba underground coal gasification pilot plant, South Africa

Abstract

Underground coal gasification (UCG) is a coal conversion method that permits coal resources to be exploited in situ using high-temperature conversion reactions. An understanding of the chemical, mineralogical, and petrographic properties of the coal, and resultant unburned carbon, degasified coal, and ash (minerals) in the UCG gasification zone, is fundamental to determining conversion rates, gas composition, and environmental and groundwater risk assessments. This study aims to provide a mineralogical and petrographic characterization of UCG residues obtained from the Eskom Majuba UCG pilot plant site in South Africa. Samples were selected from a verification borehole (VH3) drilled at the site following the pilot trial. The Permian-age Karoo Basin Majuba Gus Seam coal is medium rank C bituminous, inertinite-rich, with variable ash content. All the coal samples extracted from the borehole show evidence of heating and conversion. The volatile matter content is very low, and the mean random vitrinite reflectance values are above 5 (%RoVmr). It is possible to use vitrinite reflectance data to estimate the probable temperatures achieved in the UCG georeactor. The UCG samples in this study were exposed to maximum temperatures of around 1300°C. There is a very slight temperature gradient through the seam, indicative of fairly even heat distribution and release of volatile gases. Cracks within the degasified coals were filled with molten glassy material. Most of the iron sulfide mineralization is the degasified coal samples was transformed to pyrrhotite. The gasified samples show lower levels of sulfur as compared to unheated coal from this seam.