Investigation of the Influence of CaO/Na2O on the Flow Behavior of Coal Ash Slag under Entrained-Flow Gasification Conditions.

The flow behavior of slag plays a critical role in maintaining operational stability during entrained-flow gasification. Based on the chemical composition of typical Zhundong high-alkali coal ash, a series of synthetic ashes with different CaO/Na2O mass ratios were prepared, and the influence of the CaO/Na2O ratio on the flow behavior of coal ash under entrained-flow gasification conditions was explored through FactSage thermodynamic calculations. The results revealed that as the CaO/Na2O ratio increased, the crystalline phase in the slag gradually transformed from nepheline (NaAlSiO4) to anorthite (CaAl2Si2O8), which significantly affected the ash fusibility and slag viscosity. Regarding the ash melting characteristics, the ash melting temperatures (AFTs) exhibited an upward trend with increasing CaO/Na2O ratios. Specifically, when the CaO/Na2O ratio was below 8/12, nepheline was the dominant mineral phase and the increase in AFTs was relatively gradual. However, as the CaO/Na2O ratio continued to rise, the mineral in the slag shifted from nepheline-dominated to anorthite-dominated, leading to a marked increase in the AFTs. Regarding the slag viscosity-temperature characteristics, below the liquidus temperature, due to the transformation of the dominant crystalline mineral from the low-temperature eutectic phase nepheline to anorthite, the viscosity displayed nonmonotonic behavior. Specifically, as the CaO/Na2O ratio increased, the viscosity first decreased and then increased. Above the liquidus temperature, the oxygen structural units in the slag underwent a significant transformation, primarily from the Na+-compensated Si-O-Al-(Na)-type BO structure to the Si-O-Ca-type NBO structure. This result led to a decrease in BO structures and an increase in NBO structures, thereby decreasing the melt polymerization degree and ultimately leading to a monotonic decrease in viscosity with an increasing CaO/Na2O ratio. These findings can provide a theoretical basis for optimizing the blending of Zhundong coals with varying alkali contents to maintain a stable slag flow in entrained-flow gasifiers.