The effect of solids and phase compositions on viscosity behaviour and TCV of slags from Australian bituminous coals

The flow behaviour of coal mineral matter at high temperatures is an important parameter for coal use in entrained flow gasification technologies. Slag flow behaviour is characterised by the temperature dependence of slag viscosity, including the temperature of critical viscosity (TCV), which is strongly dependent on slag composition. Moreover, at temperatures below the liquidus temperature this compositional dependence is complicated by the precipitation of solids, and the resultant change in chemical composition of the liquid phase. In the present study, the viscosity behaviour of selected slag compositions is investigated in terms of the dynamics of solid precipitation, compositional changes, and the morphology of crystallised solids. Relationships between slag microstructure, phase composition and viscosity behaviour are described on the basis of viscosity measurements, thermodynamic calculations, and slag quenching experiments. It is shown that the viscosity–temperature behaviour of specific slag compositions was dependent on the amount and size of the solids in the slag, silica/alumina ratio (S/A) and compositional changes of the liquid phase due to solid phase precipitation.