AAEM salt-dependent effects on pyrolysis behavior and product distribution of cellulose, xylan, and lignin

Alkali and alkaline-earth metals (AAEMs) are inherently present in biomass and exert pronounced influences during thermochemical conversion. However, the effects of different AAEM salt systems on the pyrolysis behavior and product distribution of individual biomass components remain insufficiently understood. In this work, the effects of representative AAEM salt systems based on different cations (K + , Na + , Ca 2+ , and Mg 2+ ) and potassium salts with different counter-anions (Cl - , CO 3 2- , HCO 3 - , SO 4 2- , PO 4 3- , and CH 3 COO - ) on the fast pyrolysis of cellulose, hemicellulose (xylan), and lignin were systematically investigated using thermogravimetric analysis coupled with detailed product characterization. The results demonstrate that AAEM salt composition strongly influences thermal decomposition behavior, product evolution, and carbon partitioning among char, bio-oil, and gas phases. Under chloride salt conditions, monovalent cations (K + and Na + ) generally promote depolymerization-related transformations and moderate formation of sugars or oxygenated compounds, whereas divalent cations (Ca 2+ and Mg 2+ ) induce distinct changes in ring-opening, dehydration, and furan-related product formation, likely associated with stronger interactions with oxygen-containing structures. Different potassium salt systems exhibit distinct product distributions, indicating that K + availability and counter-anion-associated chemistry, contribute to the observed pyrolysis behaviors. Phosphate- and sulfate-containing salts show stronger tendencies toward carbon retention, whereas acetate-, bicarbonate-, and carbonate-containing systems lead to different distributions of volatile oxygenates and hydrocarbons. In xylan pyrolysis, K 2 CO 3 - and KHCO 3 - -containing systems generate contrasting hydrocarbon and oxygenated product distributions, highlighting the influence of salt composition on intermediate transformation under intensified salt–biomass interactions. Overall, AAEM effects depend on metal species, counter-anion chemistry, biomass composition, and loading conditions. Lower AAEM concentrations favor stabilization of primary pyrolysis intermediates, whereas higher loadings promote secondary conversion processes that redistribute products among char, bio-oil, and gas phases. These findings provide mechanistic insights into the role of AAEM salts in biomass pyrolysis and offer guidance for tailoring product distributions toward renewable chemicals and carbon-rich materials.