The behaviour of potassium and sodium species during the thermal treatment of a demineralized Highveld coal

Abstract

A series of experiments was conducted to investigate the potential influence of pre- and post adding of catalysts to a demineralized coal char. The catalysts were chosen according to yield better catalytic activity and be inexpensive. CO2 gasification was conducted on the samples in a temperature range of 500 °C to 900 °C. The coal chosen was a high-inertinite, high-ash, Highveld bituminous coal. The catalysts chosen were sodium carbonate, potassium carbonate, and a mixture of the two catalysts. Different methods were used to investigate the factors influencing the reactivity of the demineralized coal char, and the extent of the influence from the catalysts. Proximate analysis, ultimate analysis and ash yields were conducted on the starting material to determine the change the demineralization had on the coal. Ash fusion temperatures of the samples were also obtained. The results indicated that demineralization lowered the ash content, as well as the ash fusion temperatures, but the ultimate analysis showed consistency in both sets of samples. Mass losses obtained during the thermal treatment experiments under CO2 atmosphere showed an increase in mass loss in the order of samples without addition of catalysts to the smallest amount of addition. Potassium carbonate showed the largest increase in mass loss during CO2 thermal treatment, together with the mixture of the two catalysts. Samples with pre-added catalysts also had a larger mass loss than samples with post-added catalysts. According to the XRD and QEMSCAN results, some potassium species are retained in the ash, which is confirmed by XRF results. The XRF results also showed that the amount of alkali species retained is quite large.