Water leaching of inorganic species from coal ash and slag generated at typical Underground Coal Gasification (UCG) temperatures

MSc (Chemistry), North-West University, Potchefstroom Campus, 2017

Underground coal gasification (UCG) is a better and more environmentally friendly way of gasifiying coal, using coal seams that are not economically viable to mine or coal seams that are deep underground. This technology has a lot of advantages in comparison with conventional or surface gasification. Recently some companies in South Africa have started to explore the practise of UCG technology for the gasification of coal. One such plant is currently being erected in the Theunissen area of the Free State Province of South Africa. With the advancement of this technology in South Africa, there is a need to study the mineralogy and water leaching of the ash and slag formed at typical temperatures of the UCG process. This study presents results obtained using a bituminous coal from the Theunissen underground coal gasification (UCG) site in the Free State Province, South Africa and results regarding the subsequent leaching from the coal and ash samples obtained after a heating process at temperatures expected during UCG. The coal sample was blended to contain 15% of the roof and 5% of the floor samples from the Theunissen UCG site in order to mimic possible mineral compositions during a UCG process. The ash and slag samples were prepared in air at 1000, 1100, 1200 and 1300oC using a furnace, and the mineralogy of the produced ash samples was characterised using various analytical methods such as XRD, XRF, FTIR, surface area (CO2, and N2), and SEM-EDX. Results from the XRD experiments show an increase in the crystalline phase with a decrease in the amorphous phase as the temperature of gasification increases, with mullite and quartz found to be the dominated minerals in the crystalline phase. FTIR spectroscopy results reveal the disappearance of peaks associated with certain functional groups of the carbon matrix as the temperature of gasification increases, with the appearance of peaks related to the crystalline phase of mullite. SEM results show the formation of cenospheres in the ash samples and slag as the temperature of the ashing rises. The surface area results show a reduction in surface area and porosity as the temperature of the ashing increases, with the slag at 1300oC having the lowest surface area and porosity. Leaching experiments were conducted on the coal and the ash samples (1000oC, 1100oC and 1300oC) using two lixiviants: groundwater (GW) from the Theunissen (UCG) site in the Free State Province, South Africa and deionised water (DW). A batch leaching method and a column leaching method was used. Some parameters were varied during the leaching tests, such as liquid to solid ratio, leaching temperature and the effect of ashing temperature on the leachability of the inorganic compounds in the ash samples. A comparison between the batch and the column leaching method indicate that more leachants were obtained using the column leaching method. The leachates obtained during the leaching study were analysed using ICP-OES, ICP-MS and IC. From the leaching results, it was found that Ca species and SO42- ions were the most leached species and ions during the leaching tests, with a minor release of K, Mg, Al, Fe, Si, F-, NO2-, NO3- for both leaching methods. The Cl- and Na already in the groundwater contributed to relatively high values for these species in the leachates when the groundwater was used as lixiviant. The concentrations of the leachants increase slightly when the leaching temperature was increased from room temperature to 50oC, during the batch leaching tests. The species leached from the coal and ash samples were correlated with amounts in the original coal and ash samples. It was found that species in the leached samples were less when compared with the initial concentration of the species before leaching test. As expected fewer species leached out of the ash that was formed at 1300°C as a result of the slag being formed and the inorganic species being caught up in the melted mass. More leachants (Ca and SO42-) were observed using the column leaching method. The effect of ashing temperature was also investigated. It was found that the ash produced at 1300oC (fused form) produced lower concentrations of elements and ions when compared with the ash produced at 1000 and 1100oC. The amounts of the trace and heavy metals in the leachates were compared with the minimum standard requirements of the Department of Water Affairs and Forestry (DWAF) of South Africa and the Environmental Protection Agency (EPA) of USA. It was found that the concentration of the trace and heavy element were below limits as recommended by DWAF and EPA, except for Cd and Al, where slightly higher amounts were observed

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