Thermodynamic Analysis of Alkali Metal Partitioning and Kaolin-Induced Phase Evolution During Wheat Straw Gasification

Alkali metal immobilization mediated by kaolin during wheat straw gasification was investigated using thermodynamic equilibrium calculations. Kaolin promoted the formation of stable K- and Na-bearing aluminosilicates, including feldspar, leucite, and nepheline. The controlling mechanism shifted from mineral-phase reactions below 1100 °C to a molten slag structure at higher temperatures. Increasing kaolin addition reduced the slag structure parameter R from 0.43 to 0.09, indicating enhanced network polymerization. The calculated evolution of Al-containing network units was consistent with enhanced K+/Na+ charge compensation around tetrahedrally coordinated Al, suggesting a possible structural origin for the increased thermodynamic stability of alkali metals. Gibbs free energy calculations demonstrated the improved thermodynamic stability of K and Na in the slag phase. At a kaolin addition of 5 wt.%, the K release fraction decreased by at least 29.36 percentage points relative to wheat-straw ash even at high temperatures over 1400–1600 °C. The results clarified the thermodynamic relationships among ash composition, phase evolution, and alkali partitioning during kaolin-assisted gasification.