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

Abstract 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.