Production of bio-oil with low oxygen and nitrogen contents by combined hydrothermal pretreatment and pyrolysis of sewage sludge

In this study, sewage sludge (SS) is first hydrothermally pretreated at 160–250 °C, and subsequently pyrolyzed at 550 °C to produce bio-oil. The effects of the hydrothermal pretreatment (HTP) temperature on the chemical composition, fuel properties, and energy balance of the process are investigated to assess its feasibility. The results show that the characteristics of SS are improved during HTP, including better thermal stability, decreased N content, and enhanced aromaticity, which could influence subsequent pyrolysis. After HTP, the composition of bio-oil shows clear variation. The contents of undesirable O-containing and N-containing compounds, especially acids and amides, are significantly decreased, while the yields of aliphatic hydrocarbons and aromatics increase by 9.53% and 23.18%，respectively. The H/C ratio and higher heating value (HHV) of bio-oil in Sample 190–550 increase to 0.136 and 36.382 MJ/kg, respectively, which are similar to those of biodiesel. The suggested optimal HTP temperature is 190 °C for the HHV and energy recovery rate (44.23%). Meanwhile, the formation pathway of bio-oil with low N and O contents is explored.