Hydrotreating the Low-Boiling-Point Fraction of Biocrude in Hydrogen Donor Solvents for Production of Trace-Sulfur Liquid Fuel

The low-boiling-point distillate (LBD) via vacuum distillation of biocrude produced from hydrothermal liquefaction of soybean straw was hydrotreated to produce trace-sulfur liquid fuel. The effects of five hydrogen donor solvents (HDSs), including cyclohexene, cyclohexane, decahydronaphthalene, tetrahydronaphthalene, and Indane, on heteroatom removal efficiency were examined at 350 °C for 2 h with 6 MPa H₂ added and 5 wt %_(Pt/C)/feed. The LBD to HDS mass ratio was 1:1. HDSs not only could reduce the production of solid and gas and increase the yield of upgraded oil but also could favor denitrogenation, desulfurization, and deoxygenation of the upgraded oil. Among the HDSs examined, decahydronaphthalene showed the best performance for denitrogenation, and tetrahydronaphthalene was the most suitable HDS for deoxygenation and desulfurization. By employing a decahydronaphthalene and tetrahydronaphthalene mixture (w/w, 1:1) as the reaction medium, the effects of temperature (300–450 °C), time (1–6 h), H₂ pressure (0.1–10 MPa), and Pt/C loading (0–20 wt %) on the product yields and quality of the upgraded oil produced from hydrotreating the LBD were examined. The upgraded oil was the dominant product fraction under all tested reaction conditions and varied between 76.7 and 87.3 wt %. The HDS mainly acted as a hydrogen transfer agent in the LBD hydrotreatment process, during which the HDS provided the hydrogen for the hydrogenation reaction, and this consumed hydrogen was resaturated by the external hydrogen source. A more positive synergistic effect was observed for the removal of N, O, and S when using the decahydronaphthalene and tetrahydronaphthalene mixture than when using decahydronaphthalene or tetrahydronaphthalene alone. N was the most difficult heteroatom to remove, followed by O and S. Catalyst loading was the most influential factor affecting the N, O, and S removal efficiencies. Under optimal reaction conditions, 93% of N, 95% of O, and 99% of S in the LBD and HDS blend were removed, which corresponded to contents of 0.05 wt %, 0.42 wt %, and 21 ppm in the upgraded oil, respectively. The equilibrium restrictions on denitrogenation, deoxygenation, and desulfurization were essential factors affecting the removal efficiencies of heteroatoms.