Shrinking-melting mechanism of biomass ashes induced by phase transformation of alkali metal minerals for entrained flow gasification

Biomass ashes with high contents of alkali metals and chlorine exhibit special physical and chemical properties, compared to coal ash, which is key to the stable operation of entrained flow gasification. The current work investigated the melting behaviors of biomass ashes, including rice straw ash (RSA), corn stover ash (CSA), poplar shavings ash (PSA), and bamboo powder ash (BPA). In-situ experiments via high-temperature micro hot stage, thermogravimetric analysis-differential scanning calorimetry (TG-DSC) with analytical apparatuses were used to characterize the melting properties. Results revealed the weight loss of biomass ash during the heating process primarily attributed to the alkali metal release (e.g., KCl and NaCl) and mineral phase transformations. Temperature, heating rate, and ash mineral component were considered as factors affecting the melting behavior. CSA exhibited the strongest shrinkage ratio (∼90 %), while PSA exhibited a 60 % shrinkage ratio. Transformations of sylvite, leucite, and quartz were proved to drive the shrinkage behavior. The ash shrinkage was the combined results of substance release and sintering with mineral and phase transformations, with two stages defined to characterize the changes. These findings offered mechanistic insights into the biomass ash melting and shrinkage behaviors, providing a potential foundation for optimizing slag discharge in entrained-flow gasifiers.