HTL of sugarcane bagasse and soybean oil in water–ethanol system: Experimental assessment of carbon and energy recovery

Hydrothermal liquefaction (HTL) is a promising thermochemical route for converting wet biomass into energy-dense liquid fuels. The use of mixed feedstocks and co-solvent systems may further improve process performance and product quality. In this study, the effects of temperature, holding time, and feedstock composition on the HTL of sugarcane bagasse were investigated in a water–ethanol medium using a batch reactor. Two systems were compared: a ternary system (TS), composed of sugarcane bagasse, water, and ethanol, and a quaternary system (QS), containing soybean oil as an additional lipid-rich co-feedstock. Experiments were performed at 260–320 °C and holding times of 0–90 min. Biocrude yield, elemental composition, higher heating value (HHV), carbon recovery (CR), and energy recovery (ER) were determined. Soybean oil substantially increased biocrude yield from approximately 32–42 wt% in the TS to 61–71 wt% in the QS and increased the average HHV from approximately 29 to 36 MJ kg⁻ 1 . Carbon and energy recoveries were also markedly improved. Van Krevelen analysis showed that QS biocrudes had lower oxygen content, higher H/C ratios, and improved fuel quality. Response-surface regression models revealed that interaction and curvature effects of temperature and holding time were more relevant than their individual linear effects, indicating the existence of optimum operating regions. Combined TS + QS modeling using a dummy variable showed that soybean oil mainly shifted the magnitude of the responses while preserving the overall response-surface topology. This interpretation was supported by an additivity assessment, with average synergy indices of − 4.1 % for biocrude yield and − 3.5 % for energy recovery, indicating predominantly additive behavior. The HHV synergy index averaged + 0.55 %, indicating that the higher energy density of the quaternary biocrudes was largely consistent with the additive contribution of soybean oil, rather than reflecting a systematic positive synergistic effect. Overall, soybean oil acts primarily as an additional source of carbon and energy while simultaneously enhancing biocrude quality, providing a practical strategy for improving HTL performance of lignocellulosic feedstocks.