Physicochemical properties and hydrophobicity enhancement of water-washed durian peel by oxidative torrefaction at 250 °C

Durian peel presents disposal challenges but offers potential as a renewable biofuel feedstock. However, high ash content, hygroscopicity, and low energy density limit its direct application. This study examined the effects of oxidative torrefaction at 250 °C (0–21 vol% O₂) combined with water washing pre-treatment on the physicochemical and hydrophobic properties of durian peel. Fuel quality was evaluated by proximate and ultimate analyses, HHV, EMC, water contact angle, FTIR, and SEM. Water washing reduced ash content by ~80%. Increasing oxygen concentration promoted devolatilization and carbonisation, with carbon content rising above 55 wt% and HHV reaching 22.34 MJ/kg. The lowest EMC (7.25%) and hydrophobic stability were obtained for washed samples at 5 vol% O₂. SEM revealed porous carbon-rich structures, while FTIR confirmed the removal of hydrophilic groups. The combined method improved energy density, minimised ash, and significantly enhanced hydrophobicity, producing coal-like biofuel suitable for storage, transportation, and co-firing applications.