Two-stage liquefaction of sewage sludge in methanol-water mixed solvents with low-medium temperature

[Display omitted] •Low-medium temperature continuous HTL of sludge in methanol-water co-solvent.•Prolonging the reaction time at low temperature can increase the yield of bio-oil.•Two-stage liquefaction reduces energy consumption and increases energy recovery.•Two-stage HTL can promote the accumulation of organic matter from aqueous to bio-oil.•Migration and transformation of nitrogen in two-stage HTL process. Hydrothermal liquefaction could be a promising technology to convert sewage sludge into bio-oil for energy recovery. In this study, in order to obtain the maximum yield of bio-oil and improve the energy recovery of sludge, the influence of temperature and residence time on the multi-temperature continuously liquefaction of sludge in methanol-water mixed solvents was investigated. The results showed that the two-stage liquefaction significantly affected the yield, elemental composition, higher heating value, energy recovery and chemical composition of bio-oil. By optimizing the reaction conditions of hydrothermal liquefaction, the maximum utilization of sludge resources was realized. The maximum yield (42.2 %) and ester content (41.8 %) of bio-oil were obtained when the reaction conditions were 260 °C, 40 min for the first stage and 340 °C, 40 min for the second stage. The migration and transformation of nitrogen indicated that nitrogen elements were mainly concentrated in the liquid product and the N-heterocyclic compounds are the main forms of nitrogen in bio-oils.