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.