Effects of Measurement Materials and Oxygen Partial Pressure on the Viscosity of Synthetic Eastern and Western United States Coal Slags

Abstract

The viscosity of the molten ash (slag) resulting from the mineral constituents in carbon feedstock used in slagging gasifiers is critical for controlling the gasification process. The viscosity of two synthetic slags with compositions resembling the mineral impurities in average eastern and western coal feedstock was examined at temperatures from 1300–1500 °C using a rotating bob viscometer. A few combinations of atmospheres and experimental materials were investigated with respect to one another to determine slag viscosity. A CO/CO2 atmosphere (CO/CO2 = 1.8, corresponding to a P O2 = 10–8 atm) is required to sustain ferrous ions in FeO-containing slags, an environment that is oxidizing to most metals. Iron oxide in the slag prevents usage of Fe parts. In unpurified Ar, the Fe metal surface oxidizes. Using purified argon prevents iron measurement components from oxidation; however, the metallic surfaces act as nucleation sites for the reduction of the Fe oxide in the slag into metallic Fe. Dissolution of ceramic materials into the slag, including Al2O3 and ZrO2, occurs in both atmospheres. Therefore, evaluating slag properties in the laboratory is challenging. The measured viscosities of two synthetic slags in this study diverged depending upon material selection. This difference is likely attributable to container/spindle-slag interactions. Viscosity measurements of the eastern coal slag using all ceramic parts agreed best with FactSage prediction above 1350 °C, with an average activation energy of 271.2 kJ. For western coal slag, the dissolution of container/spindle materials was substantial during the measurement, with precipitation of crystalline phase noted. The experimental viscosity data of the western coal slag agreed best with Kalmanovitch prediction above 1350 °C. The activation energy changed dramatically for both data sets of western coal slag, likely indicating the Newtonian-to-non-Newtonian transition.

Correlation between the critical viscosity and ash fusion temperatures of coal gasifier ashes

“…The increase in measurement time is a potential source of error from composition drift. The high-density alumina spindles and crucibles used in this study are known to dissolve in molten coal ash slags over time [132,133]. Kim and Oh reported that the alumina content of coal ash slags measured with alumina spindles and crucibles increased by 2 to 5% compared to slags measured with platinum-lined alumina crucibles [134]; they noted that a 2.1% increase in alumina raised the liquidus temperature by 32°C for an anorthite primary phase field.…”

A Critical Review of Mineral Matter Related Issues during Gasification of Coal in Fixed, Fluidized, and Entrained Flow Gasifiers

“…However to avoid this issue, slag viscosity predictions are more preferred to slag viscosity measurements. Zhu et al showed that viscosity models can predict viscosity measured in the laboratory [91]. Slag viscosity can be predicted using models like Urbain, Feredey, Watt and Fereday, Kalmanovitch-Urbain and many others [92].…”

Slag Behavior in Gasifiers. Part I: Influence of Coal Properties and Gasification Conditions

“…For predicting the viscosity of homogeneous liquid slag, the models of Urbain, Fereday, and the silica percentage model are used, as well as modified versions of those models (such as Kalmanovitch-Urbain, Watt and Fereday, and S 2 ) [68,75,76]. The main challenge of all these models is their validity over a limited composition and temperature range, and their accuracy for different slag systems (e.g., Kalmanovitch model best predicts western coal slag above 1350 °C [77]). To predict the viscosity of the crystalline phase slags, Annen's or Einstein's models were used [68,76].…”