Supercritical water pyrolysis of sewage sludge

Municipal sewage sludge (SS) from wastewater treatment plant containing high water content (>85 wt.%), lead to the difficulty of co-combustion with MSW or coal due to the high cost of drying. This study explores an alternative method by supercritical water (SCW) pyrolysis of sewage sludge (SS) in a high pressure reaction vessel. The effects of temperature and moisture content of SS on yield and composition of the products (bio-oil, bio char and non-condensable gas) were studied. A temperature of 385 °C and moisture content of 85 wt.% were found to be the optimum conditions for the maximum bio-oil production of 37.23 wt.%, with a higher heating value of 31.08 MJ/kg. In the optimum condition, the yields of aliphatic hydrocarbon and phenols were about 29.23 wt.% and 12.51 wt.%, respectively. The physical and chemical properties of bio-char were analyzed by using XRF and BET. Results of GC analyses of NCG showed that it has the maximum HHV of 13.39 MJ/m3 at 445 °C and moisture content of 85 wt.%. The reaction path from SS to bio-oil through SCW pyrolysis was given. Moreover, carbon balance was calculated for the optimal condition, and finding out that 64.27 wt.% of the carbon content was transferred from SS to bio-oil. Finally, this work demonstrates that the SCW pyrolysis is a promising disposal method for SS.