Thermochemical behaviour of sewage sludge during its slow pyrolysis

Processing of sewage sludge waste is a major concern in modern societies, and pyrolysis is one of the technological options that can convert sewage sludge into useful energy and biofuels. The aim of this study was to investigate the thermochemical behaviour of sewage sludge during its slow pyrolysis from room temperature to 1000°C at a heating rate of 10°C/min. Thermogravimetric analysis showed that the pyrolysis process of sewage sludge consisted of three main steps in the respective temperature zones of <180°C, 180–600°C, and >600°C. The most intense mass loss rate occurred between 210 and 550°C with a maximum peak value of 0.21wt%/°C at 328°C. Computer aided thermal analysis was applied to determine the specific heat and the reaction heats during pyrolysis of the sewage sludge. Three main reaction peaks were identified. Gas chromatography results indicated that CO2 and CO were the primary gas species evolved in the low (<500°C) and high (500–1000°C) temperature ranges, respectively. The yields of solid, liquid and gas products at 500 °C were 55.7, 35.6, and 8.7 wt%, respectively. Understanding of the thermochemical behaviour of sewage sludge will facilitate more elaborate control of sewage sludge pyrolysis to produce energy and biofuels.