Tuning two-stage hydrothermal liquefaction of food waste for biocrude synthesis

Two-stage hydrothermal liquefaction (HTL) has gained increasing attention in recent years due to its ability to produce biocrude with reduced heteroatom content, facilitate the recovery of sugar-rich process liquids, and minimize solid residue formation. The physicochemical properties of solids generated during the first stage (HTL-1) are strongly influenced by HTL-1 temperature, which in turn affects downstream biocrude production. Accordingly, this study aims to evaluate how HTL-1 solids produced at different HTL-1 temperatures influence the composition and fuel properties of biocrude derived from food waste (FW) in a two-stage HTL process. The research systematically explores HTL-1 temperatures ranging from 140 to 200 °C in combination with second stage (HTL-2) temperatures ranging from 275 to 350 °C. The results indicate that the overall biocrude yield, higher heating value (HHV), boiling point distribution, and chemical composition of two-stage HTL biocrude derived from low-temperature HTL-1 solids are substantially different from those produced using high-temperature HTL-1 solids. The maximum biocrude yield (23.63 ± 0.17 wt%) was achieved using HTL-1 140 °C solids at HTL-2 300 °C, while HTL-1 200 °C solids produced the lowest yields (10.38–16.86 wt%) but also the lowest N/C ratios (0.022–0.036), and the highest HHV (40.46 MJ kg -1 ) when HTL-2 performed at 350 °C. Analysis of biocrude formation pathways from different HTL-1 solids revealed that dehydration reactions predominated during biocrude formation from lower-temperature HTL-1 solids. Biocrude derived from HTL-1 200 °C solids exhibited the highest hydrocarbon content. These findings demonstrate that tailoring HTL-1 temperature provides a viable strategy to vary biocrude yield and quality in two-stage HTL systems by controlling the chemical pathways and composition of the HTL-1 solids.