Oat Hull Valorization: Integrated Biofuel and Hydrochar Production via Hydrothermal Processing

The increasing global energy demand and the urgency for sustainable biomass utilization have intensified interest in hydrothermal processing for biofuel and biochar production. This study explores the hydrothermal liquefaction (HTL) and hydrothermal carbonization (HTC) of oat hulls, a lignocellulosic agro-residue, for producing biocrude and hydrochar. HTL experiments were conducted in a high-pressure batch reactor, varying temperature, reaction time, and feed concentration (270–340 °C, 20–60 min, 5–15 wt %) to optimize biocrude yield and composition. The highest biocrude yield of 19.1 wt % was achieved at 287 °C, 20.4 min, and 5 wt % feed concentration, with a higher heating value (HHV) of 32.5 MJ/kg. Elemental analysis of the biocrude revealed 71.6 wt % carbon, 7.1 wt % hydrogen, and 20.3 wt % oxygen, making it a promising candidate for further upgrading. GC-MS analysis of optimized biocrude comprised a complex mixture of long-chain fatty acids and derivatives (oleic acid: 37.5%; n-hexadecanoic acid: 10.9%), amides (9-octadecenamide: 21.1%), phenolics, and oxygenated aromatics, confirming its suitability for catalytic upgrading into high-quality biofuels. FTIR analysis indicated the transformation of lignocellulosic groups and the emergence of carbonyl and aromatic structures. Reuse of HTL process water across five cycles significantly improved biocrude yield, peaking at 26 wt % during the third cycle due to the accumulation of hydrolyzed intermediates such as organic acids and furans. However, HPLC revealed an accumulation of acidic organics. Acetic acid reached 475.6 mg/L, and levulinic acid reached 44.6 mg/L. At low levels, these accumulated compounds promote hydrolysis and deoxygenation, whereas at higher acidity, they inhibit biocrude formation. Hydrochar derived from HTC of the process water exhibited 65.5 wt % carbon and an HHV of 26.3 MJ/kg, confirming its potential as a solid biofuel. ICP-OES analysis of this hydrochar revealed a nutrient-rich composition (calcium: 3.74 mg/g; potassium: 0.74 mg/g; phosphorus: 1.81 mg/g) compared to that of fresh water-derived hydrochar (calcium: 1.48 mg/g; potassium: 0.56 mg/g; phosphorus: 0.66 mg/g), indicating enhanced suitability as a soil amendment and carbon sequestration agent. This study establishes a closed-loop hydrothermal valorization pathway for oat hulls, promoting energy recovery and advancing the circular bioeconomy.