Regulatory Mechanism of AAEMs Migration and Transformation on Biomass Ash Fusion Characteristics under CO2 Torrefaction Atmosphere
This study investigates the migration and transformation behavior of alkali and alkaline earth metals (AAEMs) during CO2 and N2 torrefaction of walnut shell (WS) and vinegar residue (VR), and their subsequent impact on ash fusion characteristics. Chemical fractionation and ICP-MS quantified AAEMs speciation and content. CO2 torrefaction increased biomass weight loss and ash yield compared to N2, and significantly elevated ash fusion temperatures: deformation temperature (DT), softening temperature (ST), hemispherical temperature (HT), and flow temperature (FT). At 300 °C under CO2, WS ash ST increased by 251 °C and VR ash ST by 117 °C. Chemical fractionation revealed that CO2 atmosphere promoted increases in HCl-soluble and insoluble AAEMs fractions. XRD confirmed increased relative content of high-melting-point alkaline earth aluminosilicates. CO2 torrefaction effectively mitigates ash deposition and slagging issues in biomass thermal conversion. The findings provide a mechanistic basis for tuning torrefaction atmosphere to control AAEMs speciation and enhance ash fusibility, offering a viable strategy for improving biomass fuel quality and boiler operational stability.