Valorization of sewage sludge via a pyrolytic platform using carbon dioxide as a reactive gas medium

This study demonstrates that CO₂ enhances the production of a syngas component and also hinders the formation of benzene derivatives and polycyclic aromatic hydrocarbons (PAHs) during the pyrolysis of sewage sludge. A thermogravimetric analysis and two lab-scale pyrolysis setups (single-stage and two-stage) were used to understand the role of CO₂ in the pyrolysis of sewage sludge. The use of CO₂ enhanced the thermal cracking of volatile species formed during the pyrolysis, resulting in the enhanced production of carbon monoxide (CO) at temperatures higher than 550 °C. In addition, less tar (e.g., 20% in N₂; 17% in CO₂) and more gas product (39% in N₂; 44% in CO₂) were formed in the presence of CO₂ after the pyrolysis of sewage sludge, indicating that the use of CO₂ shifts pyrolytic carbon distribution from tar to pyrolysis gas. In addition to the decrease in tar, the PAH content in the tar was also decreased, meaning that CO₂ suppresses the formation of benzene derivatives and PAHs during pyrolysis. Solid residues formed following the pyrolysis (in the presence of N₂ and CO₂) were also characterized, which showed that Ca, K, and Mg are effectively immobilized in the solid product. These residues can potentially be used for soil amendment. This study suggests that utilizing CO₂ increases the thermal efficiency of sewage sludge pyrolysis and suppresses the formation of harmful chemical species such as PAHs. A pyrolysis process operated using CO₂ would be an effective treatment method for byproducts of municipal and industrial wastewater treatment processing (e.g., sewage sludge).