Solvent extraction of South African coal using a low volatile, coal-derived solvent

Abstract

Coal is an important fuel for countries with large coal reserves such as South Africa since it is expected that oil and natural gas prices will continue to rise. The Waterberg coalfield contains 40% of South Africa's remaining coal reserves, but due to the lack of a good infrastructure (water and railway) there are currently only a few operations in the Waterberg. The utilization of remote coal reserves, such as the Waterberg coalfield, is difficult and requires an investigation of possible coal conversion processes. The extraction of high-ash bituminous coal, using an organic solvent, had been studied by coal researchers before as a possible method to convert coal into valuable products. The purpose of the solvent is to extract organic material from the coal while the inorganic mineral matter (ash) is left behind. Advantages of the solvent extraction process include mild operating conditions compared to an indirect liquefaction process, low C02 release and coal as major utility since the solvent can be recycled. Therefore, the aim of this study was to investigate the solvent extraction of South African coal using a coal-derived solvent. Batch extraction experiments were carried out for two South African coals - one a vitrinite-rich coal (Waterberg) and the other an inertinite-rich coal (Brandspruit). A vitrinite-rich American coal (lllinois#6) was used as benchmark. Residue oil was the selected coal-derived industrial solvent used for the extractions. High extraction yields (d.a.f) were obtained for Waterberg (29-63%) and lllinois#6 coal (55-74%), while only limited success was achieved with Brandspruit coal (maximum 17%). This shows that solvent extraction, using residue oil as solvent, is a possible coal conversion process to convert vitrinite-rich South African coal into valuable products. For the operating conditions investigated in this study it was found that 370^ is the optimum extraction temperature and 5:1 the optimum solvent to coal ratio, while the particle size did not have a significant influence on the extraction yield. Continuous extraction experiments were carried out for the same two South African coals used in the batch extraction experiments and also with residue oil as solvent. Extraction temperatures and residence times were limited by the experimental setup. Negative extraction yields were obtained that could be explained by the effect of coal particles adsorbing compounds in the residue oil during the filtration step, which was performed at room temperature. To compensate for the adsorption process, the extraction yields were calculated with the extraction data at the lowest temperature as basis instead of the feed coal data. Extraction yields (d.a.f) of 20-52% were then obtained for the Waterberg coal and 3-12% for Brandspruit coal, with the highest extraction yield observed at 340°C and residence time of 12 min. In general, an increase in extraction yield with increase in temperature as well as with increase in residence time was observed. The shrinking-core model was used to describe the solvent extraction process and provided a good fit for the experimental data. Activation energies of 324 kJ/mol for Waterberg coal and 134 kJ/mol for Brandspruit coal were determined. Finally, it was found that most of the results of the continuous extraction experiments were in line with the results of the batch extraction experiments. In conclusion, the objectives of this investigation were met and form a good basis for further extraction research. In general it can be recommended to expand the conditions of this investigation to check the accuracy of the conclusions made. The most important areas for future research in developing a commercial-scale process include separation of residue coal from the extract, recycling of the solvent, and hydrogenation studies on the liquid extract product.