Estimating the Iron Valence Distribution of Synthetic Coal Ash Slags through Thermophysical Modeling

Entrained-flow gasifiers convert coal into synthesis gas, a valuable feedstock for synthetic fuels and chemicals, as well as play an important role in integrated gasification combined cycle plants with carbon capture. Molten slag is removed through tapping and can be blocked by highly viscous slag. This can force an unscheduled shutdown of a gasifier. Therefore, slag viscosity management is a key aspect of gasifier operation. The viscosity–temperature relationship of a slag is strongly influenced by its iron valence distribution. Synthetic coal ash used to study slag flow in a gasifier should replicate the iron valence distribution present during gasification. Thermophysical modeling provides an alternative to estimating the iron valence distribution in addition to wet chemical analysis and Mössbauer spectroscopy. Estimated iron valence distributions of 13 synthetic coal ash slags using oxygen activity as an adjustable parameter were used in examining the effect of MgO as a secondary fluxing agent during gasification. The change in flow behavior index measured as a function of shear rate, corresponding to a transition from Newtonian to non-Newtonian flow, was found to take place at slag temperatures less than or equal to the liquidus temperature. The difficulty of obtaining equilibrated synthetic slag with a suitable iron valence distribution may be minimized using FeO instead of Fe2O3 during sample preparation.