Viscosity and crystallisation behaviour of coal ash slag from the primary phase of anorthite

Abstract To achieve a stable slag-tapping process in entrained-flow coal gasifiers, the acceptable viscosity of coal ash slag is within 5–25 Pa·s in the temperature range 1200–1500 °C and the tapping temperature should be above the temperature of critical viscosity (Tcv) to avoid massive crystallisation. In general, coal slag from anorthite primary phase field can meet these requirements. In this work, we determined the viscosity, phase composition and crystallisation behaviour of synthetic coal slag from anorthite primary phase field using a high-temperature viscometer, thermodynamic modelling and microstructural analysis. On the basis of our results, we identified the optimal model for predicting the viscosity of fully liquid slag, and proposed a new empirical equation to predict Tcv. This has enabled us to develop a viscosity model for partially crystalline slag with the considered anorthite aspect ratio. Finally, we evaluated the isothermal crystallisation kinetics of anorthite using a modified Kolmogrorov–Johnson–Mehl–Avrami equation, and determined the effects of liquid viscosity, degree of supercooling, theoretical solid content, iron oxidation and alkali oxides on these kinetics. Based on the above results, we present here a comprehensive method for predicting the viscosity and crystallisation behaviour of slag from the anorthite primary phase field.