Multistage Reaction Characteristics and Ash Mineral Evolution in Coal–Biomass Co-Combustion Process

This study investigates the combustion characteristics and ash behavior of coal–biomass co-combustion using Zhujixi coal and corn straw in a fixed-bed system. The research analyzes combustion stage division, gas release patterns, and mineral evolution of ash under varying blending ratios. Results indicate that biomass addition modifies the dynamic features of the combustion process by advancing the CO2 release peak; extending the release of CO, CH4, and H2; and enhancing the completeness of char oxidation. At moderate blending levels (20–60%), oxygen utilization is significantly improved and combustion stability is strengthened. Ash fusion temperatures exhibit a consistent decline with increasing biomass proportion due to the formation of low-melting eutectic phases such as KAlSiO4 and K, Ca-based phosphates. Mineralogical analysis further reveals that coal ash components promote the immobilization of alkali metals, thereby suppressing potassium volatilization. A blending ratio of 40% demonstrates the most favorable balance between burnout performance, oxygen efficiency, and alkali fixation, surpassing both pure coal and high-ratio biomass conditions. This optimized ratio not only improves energy conversion efficiency but also reduces slagging and corrosion risks, offering practical guidance for cleaner coal power transformation, stable boiler operation, and long-term reduction of carbon and pollutant emissions.