Characterization of biocrude oil obtained from the catalytic hydrothermal liquefaction of municipal sewage sludge with a Ni-based hydrochar

The hydrothermal liquefaction (HTL) technology for municipal sewage sludge (referred to as sludge) treatment presents an effective method to improve organic matter content in sludges. However, challenges remain, particularly regarding the optimization of biocrude oil yield and by-product management during the process. In this paper, a catalyst was prepared by loading NiO with hydrochar (HC), a product of calcined HTL solid-phase product of sludge, as a carrier, and the NiO/HC catalyst was used in the hydrothermal liquefaction process of sludge. The performance of the catalyst was evaluated. The catalytic pattern of NiO/HC catalysts for hydrothermal liquefaction of sludge was systematically investigated by varying the key reaction parameters such as hydrothermal liquefaction reaction temperatures, times, and catalyst loadings. During the calcination process of the solid-phase product, some of the crystals grow along a specific direction, forming a fibrous structure with rough surface pores, which increases the contact area with the reactants, so the loaded NiO can increase the active sites on the surface. The NiO particles are uniformly distributed on the HC surface and show certain aggregation characteristics. Biocrude oil overwhelmingly increased with reaction temperature and reaction time increasing, and increased with catalyst loading decreasing. The highest yield of biocrude oil was obtained at 350 °C, 60 min and 5 wt% catalyst, which reached up to 17.93 wt%. The highest quality of biocrude oil was obtained at 325 °C, 20 min, and 10 wt% catalyst, at which time the higher heating value (HHV) reached 39.40 MJ kg⁻¹ and the recovery rate (ER) reached 50.91 %. Compared with conventional catalysts, NiO/HC catalysts exhibit better stability and reusability. By adjusting the reaction parameters, the yield and quality of biocrude oil can be further improved. It provides theoretical support for the resource utilization of sludge.