Contrasting morphology and oxidation kinetics in forward and reverse smolder-derived chars from pine needles

Smoldering is categorized into forward and reverse modes, propagating with or against the wind orientation. The inherent structural heterogeneity and complex smoldering dynamics of forest residues have left their bioenergy potential underexplored under contrasting airflows. This study combined multi-scale characterization ( μ -CT, SEM, Micro-FTIR, Raman spectroscopy, and coupled TG-FTIR-MS) to reveal the distinct physicochemical properties of pine needles (PN) and its smolder-derived chars. The results revealed that PN features a multiscale pore structure across its inter- and intra-particle regions, as characterized by μ -CT and SEM, respectively. The char produced by forward smoldering (FSC) exhibited denser and thicker-walled pores (5–20 μm), whereas the char produced by reverse smoldering (RSC) retained loose and thinner-walled pores (>20 μm). The thermal stability of RSC was reduced due to its less-ordered carbon structure, as evidenced by the Micro-FTIR and Raman analysis. At 200–400 °C, RSC exhibited higher activation energy than FSC (135.98 vs. 92.10 kJ/mol), indicating its greater resistance to initial oxidation. However, RSC's activation energy became lower (101.89 vs. 113.66 kJ/mol) at 400–500 °C, reflecting the enhanced reactivity of the secondary char oxidation. These findings pave the way for tailoring smoldering conditions (e.g., wind orientation and temperature) to convert forest residues into chars with desired reactivity for bioenergy applications. • The char produced by forward smoldering retains larger pores and thinner walls. • Reverse-smoldering generated char shows higher thermal stability at 200–400 °C due to its more ordered carbon structure. • Reverse-smoldering generated char has lower activation energy at 400–500 °C due to its more porous structure. • Opposite reactivity of chars produced by forward and reverse smoldering demands flow orientation control for bioenergy.