The influence of the roof and floor geological structures on the ash composition produced from coal at UCG temperatures

Abstract

The mineralogy of the ash and slag formed at typical UCG temperatures were investigated using a bituminous coal from the Theunissen UCG site in the Free State province of South Africa. The ash and slag samples were produced from the coal, and the surrounding roof and floor geological structures at 1000, 1100, 1200 and 1300°C. XRD results show an increase in crystalline phases, with a decrease in the amorphous content as the temperature increases, with mullite and quartz found to be the dominant minerals in the crystalline phase. FTIR spectroscopy results reveal the appearance of peaks related to the crystalline phase of mullite with increasing temperature. SEM results show the formation of spherical particles, with the appearance of cenospheres, as the temperature increases. The samples produced at 1000°C had significantly lower surface area and porosity values than the blended coal sample, but these values remain similar for the ash samples produced between 1000°C and 1300°C, where only slight decreases are observed with an increase in temperature. Results indicate that the higher the temperature in a UCG cavity, the less leaching of inorganic species should occur