Torrefaction of corn stover prior to co-gasification with coal not only improves fuel properties but also significantly alters the speciation of inorganic elements, affecting ash slagging and melting behavior. This study clarifies how torrefaction mitigates slagging risk through a comparative investigation of ash evolution in raw and torrefied corn stover-coal blends. In the raw corn stover-coal system, a critical slagging risk emerged at a 30 wt.% biomass ratio due to the formation of metastable leucite. Torrefaction converted water-soluble alkali and alkaline earth metals (AAEMs) into more stable organic or residual forms, enhancing their retention during ash formation and redirecting high-temperature mineral evolution. This shift suppressed metastable leucite formation and promoted thermally stable silicates, thereby delaying initial melt formation. Importantly, torrefaction markedly narrowed the ash melting interval (ΔT): while ΔT for raw blends remained >150 °C across all ratios, for torrefied blends it sharply decreased with increasing biomass proportion, reaching an optimal 44 °C at 50 wt.% blending. This sharp melting transition eliminated the critical slagging point observed in raw blends and promoted homogeneous slag formation. Consequently, the deformation temperature (DT) of torrefied corn stover-coal ash increased by up to 140 °C compared to raw blends, effectively mitigating slagging risk at the previously critical 30 wt.% ratio. This work provides a theoretical basis for designing corn stover-coal blends with favorable slagging characteristics via torrefaction pretreatment.
 

Torrefaction; AAEMs speciation; Slagging; Melting; Inorganics evolution