Sub–supercritical liquefaction of municipal wet sewage sludge to produce bio-oil: Effect of different organic–water mixed solvents

Abstract The effect of different mixed solvents (methanol–water (MeOH–H 2 O), n -hexane–water (HEX–H 2 O)) on liquefaction of municipal wet sewage sludge (WSS) was investigated in this study. The highest bio-oil yield of 46.5 wt% was achieved in MeOH–H 2 O, of which ester content was 38.1% and methyl ester comprised 37.4%. The lowest solid residue yield of 10.3 wt% was achieved in HEX–H 2 O. The highest fatty acid compounds content was 54.9% of bio-oil obtained in HEX–H 2 O, of which were in saturated state. The highest calorific value and the lowest oxygen content of bio-oil were achieved in HEX–H 2 O: 36.45 ± 0.38 MJ/kg and 7.65 ± 0.26 wt%, respectively. The thermal stability of solid residue was enhanced in HEX–H 2 O. The formation of esters was promoted in MeOH–H 2 O, whereas aliphatic compounds could be efficiently extracted in HEX–H 2 O.