Viscosity of Spinel Primary Phase Field Slags from Australian Brown Coals

The achievement of a steady rate of slag tapping in entrained-flow gasifiers relies on their continuous, smooth operation. In turn, this requires the viscosity of coal ash slags to be within 5-25 Pa.s at operating temperatures. However, slagging behavior, including viscosity, strongly depends on coal ash mineral and chemical composition. These vary significantly in Australian brown coal ashes, making continuous gasifier operation difficult. In our previous work, we identified brown coal ash compositions that satisfy entrained-flow gasification conditions and were attributed to specific primary phase fields of the corresponding slag compositions. In this work, we experimentally determined the viscosity and phase composition of slag samples prepared from the spinel primary phase field. On the basis of these results, we evaluated different viscosity models to identify those that accurately described the viscosity of the liquid phase. The models were then applied to describe the viscosity of two-phase slags with spinel crystals and evaluate effective slag viscosity. Lastly, we developed a model to describe the kinetics of spinel growth during slag isothermal crystallization and linked the model to the effective viscosity of the investigated slags.