• • CO2 torrefaction at 300 °C raised the softening temperature (ST) of walnut shell ash by 251 °C and vinegar residue ash by 117 °C compared to N2 torrefaction, directly reducing slagging propensity in boilers.
• • CO2 atmosphere increased the HCl-soluble and insoluble AAEMs fractions, as quantified by chemical fractionation, indicating a shift toward more stable, high-melting-point mineral forms.
• • XRD analysis confirmed a higher relative content of alkaline earth aluminosilicates in ash after CO2 torrefaction, which are known to elevate ash fusion temperatures.
• • The weight loss and ash yield of biomass were higher under CO2 than N2 torrefaction, suggesting that CO2 actively participates in devolatilization and mineral transformation, enhancing fuel upgrading.