Liquid-nitrogen milling: a catalyst-like pretreatment for energy conversion affecting olive stone slow pyrolysis kinetics

• Cryomilling induces brittle fracture yielding uniform particles (d 50 26 µm). • Lignin activation energy is reduced up to 4-fold compared to untreated stone. • Stable entropy (ΔS) profile proves structural homogenization of the biomass. • Liquid-nitrogen milling acts as a catalyst-like physical pretreatment. The recalcitrant structure of olive stone (OS) hinders its valorization, which is essential for renewable energy production. This study evaluates the impact of cryogenic versus room-temperature ball milling on OS pyrolysis kinetics using thermogravimetric analysis (TGA) and isoconversional methods. Cryomilling yielded the most homogeneous distribution and significant particle size reduction (down to 26 µm). Notably, this pretreatment substantially altered the pyrolysis process: the fragmentation of cellulose and lignin resulted in their decomposition at lower temperatures (thermal shift: cellulose ∼ 4 °C, lignin ∼ 20 °C), overlapping with that of hemicellulose (mass conversion shift ∼ 12–14 %). Furthermore, it notably reduced the overall activation energy, by up to fourfold for the lignin decomposition stage compared to untreated OS (from 911 kJ mol −1 for OS mm to 217 kJ mol −1 for OST cryo ). These findings demonstrate that cryogenic milling alters the apparent reaction pathway and structural features, significantly enhancing pyrolytic efficiency.