Viscous deformation as a measure of heat work during coal ash fusibility testing

Abstract

Coal ash fusibility is important in evaluating coal quality and managing coal ash in boilers and gasifiers. Ash fusibility test (AFT) data constitute the basis for many ash and slag management models. Coal ash is a mixture of inorganic compounds derived from sedimentary minerals. As is the case for the bending of pyrometric cones used to control the firing of traditional ceramics, the deformation of coal ash cones is a measure of heat work and not temperature. Heat work, the combined effect of time and temperature, is measured by cone deformation that results from the integration of viscous strain over time. The effective viscosity of 4726 coal ash samples from the U.S. Geological Survey Coal Quality (COALQUAL) database were evaluated at their AFT critical temperatures and found to be lower than the working range of glass, which is consistent with the heat work interpretation of AFT critical temperatures. Thermophysical model data also showed strong positive linear correlations between AFT critical temperatures and the liquidus temperature, as was also the case for pyrometric cones, but not with their solidus temperatures. The finding provides a thermophysical basis for AFT models based on composition while also explaining the spurious correlation between the critical viscosity temperature (TCV) in coal ash slags and AFT critical temperatures. Silicate liquid immiscibility and phase inversion between iron-rich and iron-poor phases results in a sharp change in the effective viscosity of the slag emulsion. High effective viscosities magnify the effect of heating rate variability on AFT critical temperature measurements.