<p>Ash fusion and alkali-metal volatilization characteristics are the main limiting factors during biomass gasification. The influences of coal gangue (CG) on the ash fusion temperatures (AFTs) and alkali-metal volatilization behaviors of biomass (eggplant straw (ES) and pepper straw (PS)) were investigated. The mineral evolution in ash was explored through X-ray powder diffraction, and the alkali-metal volatilization rate and distribution in gas phase were investigated via thermodynamic equilibrium calculation conducted with FactSage software. CG addition significantly increased the AFTs of both ES and PS, with a greater effect on the AFT of ES. The increase in AFT was ascribed to the formations of refractory alkali-metal aluminosilicates, which resulted from the reactions between the Si–Al–based minerals in CG ash and the alkali-metal salts (carbonates (K₂Ca(CO₃)₂, Na₂CO₃), sulfates (K₂SO₄, Na₂SO₄), and chloride (KCl)) in biomass ash. CG addition effectively suppressed the volatilization of alkali-metal in elemental and hydroxide forms, which originated from the carbonates and sulfates. However, CG exhibited a limited effectiveness in mitigating the volatilization of alkali-metal chlorides due to the poor reactivity between the Si–Al–based minerals and the alkali-metal chlorides. Moreover, the comparison between the calculated and experimental results of alkali-metal volatilization rates validated the feasibility of utilizing FactSage software to model the alkali-metal volatilization behaviors of biomass ash.</p>