Flow properties of low-rank coal ash slags: Implications for slagging gasification

For predicting satisfactory operation in slagging fixed-bed gasification, a slag should have a viscosity of < 10 Pa s at 1300 °C when measured in a reducing atmosphere. Various phenomena related to slag flow observed qualitatively in pilot plant testing can be explained or predicted from results obtained in the laboratory. Hysteresis between cooling cycle and heating cycle viscosities requires that slag must be reheated well above its initial temperature to resume slag tapping that was interrupted by a temperature fluctuation. A transition from a reducing to an oxidizing atmosphere in the gasifier hearth, such as may be caused by fuel bed bridging, will raise slag viscosity enough to impair or stop flow even in the absence of hearth cooling. Formation of metallic iron by partial reduction of the slag will raise viscosity, but for low-silica slags which give good slagging operation in the gasifier this effect is not usually serious.