Oxidative torrefaction of durian peel under controlled oxygen with an integrated evaluation of energy yield and hygroscopicity

• Oxidative torrefaction of durian peel was evaluated under controlled O 2 (0–21 vol%). • Increasing temperature and O 2 increased severity, raising HHV (max 35.21 MJ kg −1 , d.a.f.). • Oxygen addition reduced energy and carbon yields, indicating a quality–recovery trade-off. • Hygroscopicity showed a transition: mild O 2 lowered EMC, while severe O 2 increased EMC. • HPEY integrated energy yield with moisture penalty, favoring mild, low-O 2 conditions for storage Oxidative torrefaction can upgrade biomass when maintaining an inert atmosphere is difficult at scale, but oxygen can also change hygroscopicity and deliverable energy. Durian peel was torrefied at 220–300 °C for 30 min under controlled oxygen concentrations (0–21 vol%) to quantify the coupled effects on chemical properties, fuel upgrading and moisture resistance. Increasing temperature and oxygen concentration increased severity and reduced solid yield. HHV increased and reached 35.21 MJ kg −1 (d.a.f.) at 300 °C and 21% O 2 , confirming carbon enrichment and deoxygenation. Elemental ratios (H/C and O/C) decreased with severity, consistent with progressive dehydration and devolatilization. In contrast, energy yield and carbon yield decreased with oxygen addition, indicating a trade-off between energy content and energy recovery. Hygroscopicity showed a severity-dependent transition: at 220–260 °C, low oxygen levels lowered EMC and increased contact angle, whereas higher severity decreased contact angle and increased EMC, consistent with pore opening and ash enrichment. To integrate upgrading with storage performance, a hygroscopicity-penalized energy yield (HPEY) was proposed by combining energy yield with an EMC-based moisture penalty. HPEY declined with increasing temperature and O 2 , showing that severe conditions did not improve deliverable energy under humid storage. Mild, low-oxygen conditions best balanced upgrading and moisture resistance.