Mechanism investigation of model protein during two-stage hydrothermal liquefaction: Effects of operated conditions in each stage

Two-stage hydrothermal liquefaction (2-stage HTL) has raised attention in treating protein-rich biomass recently. This research is intended to fill the research gap in mechanism investigation about protein conversion during 2-stage HTL. The responses of the model protein, bovine serum albumin, to direct HTL (300 °C, 30 min) and 2-stage HTL with different conditions (150–250 °C, 10–30 min for low-temperature stage, and 250–350 °C, 10–30 min for high-temperature stage) were investigated and compared in terms of products yields, properties and chemical composition of bio-oil. The underlying reaction pathways of protein were also proposed. Results revealed that only proper temperature and longer residence time for low-temperature stage of 2-stage HTL were beneficial to obtain higher bio-oil yield and less solid residue but with higher contents of N in bio-oil, in comparison to that from direct HTL. These were mainly contributed by formation of diketopiperazines and cyclization of amine/amide. The maximum bio-oil yield (20.7 %) was obtained at 2-stage HTL of 200 °C for 30 min followed by 300 °C for 20 min. While higher temperature and holding residence time for high-temperature stage could decrease the N contents by deamination of amine and decomposition of diketopiperazines. This work will facilitate to understand the whole process and provide reference for mitigating N-related issues in 2-stage HTL of protein-rich biomass. © 2024 Elsevier Ltd