Multifactorial impacts on ash behaviors during oxy-fuel co-combustion of semi-coke and biomass

• Ash behaviors between silica-rich (RH) and alkali-rich (PW) biomass during oxy-fuel co-combustion with semi-coke were mechanistically studied. • Silica-rich RH ash inherently resists slagging/fouling, with SC blending further reducing corrosion/abrasion via mullite crystallization. • Alkali-rich PW ash exhibits severe slagging/fouling/corrosion from low-melting silicates, specifically mitigated beyond 50 % SC blending. • O 2 concentration, rather than CO 2 , dictates the ash evolution. Moreover, PW-SC ash indicates stronger dependence on the combustion atmosphere. Oxy-fuel co-combustion of semi-coke (SC) and biomass represents a promising strategy for reducing carbon emissions and utilizing solid industrial waste. However, the ash behavior mechanisms underlying this process remain inadequately understood. This study systematically investigates the multifactorial impacts on ash behaviors during oxy-fuel co-combustion of SC with two distinct types of biomass — rice husk (RH) and pine wood (PW). Through fixed-bed combustion experiments under varied atmospheres (O 2 /N 2 , O 2 /CO 2 ) and temperatures (900–1350 °C), coupled with XRF/XRD/AFT/SEM-EDS analysis, the slagging, fouling, corrosion, and abrasion propensities of ashes were evaluated using a suite of established indices. The results revealed markedly divergent ash behaviors of RH-SC and PW-SC blends. The silica-rich RH ash exhibited inherent slagging and fouling resistance. SC blending slightly enhanced these risks but effectively reduced the abrasion and corrosion propensities through mullite crystallization. PW ash exhibited high slagging, fouling, and corrosion propensities due to its abundant low-melting alkali silicate phases, while these propensities were substantially mitigated by SC blending — with a marked reduction when SC ratios exceeded 50 %. Transitioning from 20 % O 2 /80 % N 2 to 20 % O 2 /80 % CO 2 atmosphere had limited influence on ash behaviors. However, increasing O 2 concentration in O 2 /CO 2 atmosphere intensified slagging and fouling due to CaSO 4 -mediated eutectic formation. Oxy-fuel combustion decreased the abrasion propensity through mullite crystallization for RH-SC ash or increasing soft-phase formation for PW-SC ash. Elevated temperatures exacerbated slagging and fouling by enhancing ash fusion but improved abrasion resistance via crystallization of harder phases. Critically, sub-1200 °C operation with biomass-specific SC blending ratios is recommended to optimally mitigate ash-related risks. The findings provide fundamental insights into optimizing oxy-fuel co-combustion processes for sustainable energy utilization.