Determination and statistical evaluation of the effect of minerals and mineral associations in specific dense medium fractions on ash fusion temperature

Abstract

During Sasol-Lurgi Fixed Bed Dry Bottom coal gasification, the mineral matter in coal undergoes various transformations. Heat induced transformation due to low ash fusion temperatures leads to agglomeration of the ash particles to sizes varying from tiny particles to lumps larger than 100mm. Channel burning and instability in the ash bed can occur as a result. It is therefore important to understand and anticipate the reactions of the mineral matter prior to feeding the coal into a gasifier. The principal aims of this thesis were to investigate the effect of minerals and mineral associations on the ash fusion temperature of coal. Ash fusion temperature was used as a measure of the expected behaviour of the ash bed during gasification. Representative samples of run-of-mine coal from three sources were density separated and then comprehensively characterised. The predominant basic oxides present were identified to be Ca, Mg and Fe due to the presence of calcite, dolomite and pyrite. The results showed highest linear correlations with Si02 and CaO and confirmed literature that low ash fusion temperatures may be attributed to increased basic oxide levels and low Si021A1203 ratios. The combined removal of Ca, Mg and Fe by chemical fractionation resulted in an increase in ash fusion temperature. Different combinations of minerals were removed by different leaching agents. Chemical fractionation selectively altered the mineral content of coal which in turn provided valuable information on mineral interactions. This was clearly illustrated in the models developed for the calculation of ash fusion temperature.