Pretreatment sequence of torrefaction and leaching for high-alkali biomass valorization: Focus on elemental...

The pretreatment sequence of torrefaction and leaching poses a significant challenge in upgrading high-alkali biomass. This study investigates the impact of pretreatment sequence (torrefaction and leaching) on straw-derived char structure, physicochemical properties, and mineral form the dual organic and inorganic perspectives, elucidating its role in inorganic distribution and torrefaction reaction. Results demonstrate that the leaching-torrefaction pathway, due to the absence of endogenous AAEMs catalysts, suppresses deoxygenation and devolatilization. This leads to a higher final yield (55.8-68.5 %), oxygen content (35.6-42.5 %), and volatiles (62.4-68.6 %) at 300 °C. In contrast, the torrefaction-leaching pathway enhances volatiles release (12.9 %) by promoting intramolecular deoxygenation, condensation, and aromatization. This process facilitates the conversion of volatiles into fixed carbon, evidenced by a transformation of O-alkyl C (23.0 %) into aromatic C (5.3 %) and aliphatic C (6.3 %). Mechanistic analysis reveals that endogenous AAEMs catalyze the cleavage and ring-opening of cellulose/hemicellulose and breakdown of lignin ether bonds. This catalysis enhances the aromaticity of char at the expense of yield. Furthermore, the pretreatment sequence alters inorganic mineral form. Reactions with oxygen-containing functional groups promote the transformation of minerals from water-soluble and acid-soluble towards ion-exchangeable. This work proposes a strategic torrefaction-leaching pathway for high-alkali biomass valorization, offering a viable approach for its scalable alternative and sustainable application in industry.