Effect of hydrothermal conditions of temperature-pressure decoupling on the production characteristics of N/S-free biocrude from dry-rendered animal carcasses

To enhance the value of diseased and dead livestock resources, this study investigated the hydrothermal conversion of dry-rendered animal carcasses into N/S-free biocrude. Hydrothermal treatments were conducted under varying holding temperatures (HT), retention times (RT), and working pressures (WP) using temperature-pressure decoupling. The recovery, elemental composition, functional groups, and chemical composition of biocrude, and degradation characteristics of solid residue (SR), were analyzed. The results showed that all biocrudes were S-free and rich in hydrocarbons. At 200 °C, 15 min, and 10 MPa, the biocrude recovery reached nearly 100 % (100.76 %), and the aqueous phase (AP) yield was maximized, enabling efficient component separation. As RT increased, the optimal condition for producing N/S-free biocrude was identified (200 °C, 45 min, and 10 MPa), where the biocrude recovery exceeded 100 % (107.79 %), with an N content of only 0.15 % and a HHV of 38.14 MJ/kg, demonstrating its suitability for biofuel refinement, while the degradation of SR was more complete. WP significantly influenced the HHV, increasing it substantially from 32.06 to 38.14 MJ/kg as WP increased from self-generated pressure to 10 MPa. Furthermore, the conversion characteristics of N-free and N-containing compounds, along with the comprehensive reaction mechanism, were examined. This study provides a viable pathway for the efficient utilization of diseased and dead livestock resources and producing low-N or N-free biocrude from high-protein/high-fat biomass.