Continuous Pyrolysis of Sewage Sludge in a Screw-Feeding Reactor: Products Characterization and Ecological Risk Assessment of Heavy Metals

Pyrolysis of dried sewage sludge is regarded as an effective treatment method as well as a promising technology for energy/fuel production. In this work, thermochemical conversion of dried sewage sludge in a continuous reactor at different pyrolysis temperatures (400-800 °C) and solid residence times (6-46 min) was conducted. The pyrolysis products obtained using different pyrolysis conditions were extensively investigated. It is indicated that high pyrolysis temperature (>700 °C) and long solid residence time (>23 min) could enhance secondary reactions and decrease the yield of bio-oil. The maximum yield of bio-oil of 16.69% was achieved at reaction temperature of 700 °C and solid residence time of 23 min. The FTIR and GC-MS analyses of the bio-oil obtained at optimum condition indicated that it contained large amounts of phenols and esters. H₂ and CO₂ were the main components of pyrolysis gas, with a total amount that exceeded 52.18%. The characteristics of the char, including elemental composition, functional groups, and combustion property were investigated by XRF, FTIR, and TG. FTIR analysis of the char showed that the main functional groups are M-X, C-O, and C-H. The volatile content in the char decreased with an increase of pyrolysis temperature, while it increased with an increase of solid residence time. Heavy metals distribution in pyrolysis products of dried sewage sludge were investigated by inductively coupled plasma atomic emission spectroscopy. The results showed that most of the heavy metals remained in char. In addition, the potential ecological risk assessment of heavy metals was assessed.