Efficient renewable fuel production from sewage sludge using a supercritical fluid route

•Non-catalytic, non H2 sewage sludge liquefaction was held in supercritical fluids.•High bio-oil yields of 87.8 wt% and HHV of 34.6 MJ kg−1 were achieved.•The presence of water increases the conversion up to 96% and HHV up to 37.3 MJ kg−1.•Ester and nitrogenated compounds were the major compounds in the bio-oil.

Conversion of sewage sludge into fuel was investigated using a catalyst- and external hydrogen-free supercritical fluid route. When dried sewage sludge was treated in supercritical ethanol, an extremely high bio-oil yield of 87.8 wt% and a remarkable higher heating value (HHV) of 34.6 MJ kg−1 were obtained. The possibility of using non-dried sewage sludge as a fuel source was tested using various alcohol-water mixtures; the presence of water resulted in an almost complete (96%) conversion and high HHVs of 36.8 and 37.3 MJ kg−1 for methanol-water (3:7, v/v) and ethanol-water (3:7, v/v) mixtures, respectively. The main chemical compounds in the bio-oils were found to be esters and nitrogenated species. Plausible reaction mechanisms for sewage sludge conversion in each supercritical fluid are discussed.

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