Valorization of Sargassum via Hydrothermal Co-Liquefaction with Food Waste: Effects of Reaction Temperature and Feedstock Ratio on Biocrude Yield and Fuel Properties

The massive invasion of Sargassum (SG) in coastal regions has emerged as a growing environmental and economic challenge, driving interest in sustainable valorization strategies. Although hydrothermal liquefaction (HTL) has demonstrated potential for valorizing SG into biocrude, its high ash and low lipid content limit conversion efficiency. In this context, hydrothermal co-liquefaction (CoHTL) offers a promising route by leveraging synergistic interactions with complementary feedstocks. This study investigates the effects of HTL temperature (275–350 °C) and feedstock ratio (5–15 wt% SFW) on biocrude production during the CoHTL of SG with simulated food waste (SFW). CoHTL with 5 wt% SFW at 300 °C produced the maximum biocrude yield of 45.6 ± 0.8 wt%, which was 27.6 wt% higher than that obtained from the individual HTL of SG, indicating a significant positive synergistic effect. However, this synergy decreased with increasing temperature and SFW fraction, with temperature exerting a more pronounced influence than feedstock ratio. CoHTL also produced biocrude with higher carbon and energy contents than HTL of SG, reaching up to 78.7 ± 0.3% and 37.2 ± 0.1 MJ/kg, respectively. The boiling point distribution showed a dominance of lighter volatile compounds in the 125–340 °C range, although this fraction decreased slightly after CoHTL. In addition, a slight increase in nitrogen content was observed in the CoHTL biocrude, indicating a trade-off associated with the process. Overall, CoHTL with SFW is an effective strategy for improving biocrude yield and energy recovery from SG, offering an enhanced pathway for its valorization.