Effect of cooling rate and alumina dissolution on the determination of temperature of critical viscosity of molten slag

In slagging gasifiers with crystalline slag, the fluctuation in temperature near the temperature of critical viscosity (Tcv) can rapidly increase the molten slag viscosity and cause an accumulation of slag inside the gasifier. In order to prevent operational problems caused by the slag, one therefore needs to obtain an accurate measurement of Tcv. The effects of cooling profile and the extent of alumina dissolution on the experimentally determined Tcv were investigated. A synthetic slag was prepared using a composition of Alaskan Usibelli ash, which exhibited crystalline slag behavior, and Tcv was determined under constant cooling and equilibrium conditions. The Tcv measured under 2°C/min of constant cooling was 33°C lower than the Tcv under equilibrium conditions. The effect of alumina dissolution was determined using a platinum (Pt) lined crucible. At a 2°C/min cooling rate, the Tcv measured using a bare alumina crucible was 23°C higher than that from the Pt-lined crucible, offsetting the difference caused by constant cooling. In order to determine a safe gasification temperature range, however, the measurement of Tcv under constant cooling conditions should be avoided.