Investigation of the Influence of CaO/Na ₂ O 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.