Sustainable valorisation of sewage sludge via carbon dioxide-assisted pyrolysis

The escalating volume of sewage sludge (SS) generated poses challenges in disposal, given its potential harm to the environment and human health. This study explored sustainable solutions for SS management with a focus on energy recovery. Employing CO₂-assisted pyrolysis, we converted SS into flammable gases (H₂ and CO; syngas). Single-stage pyrolysis of SS in a CO₂ conditions demonstrated that CO₂ enhances flammable gas production (especially CO) through gas phase reactions (GPRs) with volatile matter (VM) at temperatures ≥520 °C. Specifically, the CO₂ partially oxidized the VM released from SS and concurrently underwent reduction into CO. To enhance the syngas production at temperatures ≤520 °C, multi-stage pyrolysis setup with additional heat energy and a Ni/Al₂O₃ catalyst were utilized. These configurations significantly increased flammable gas production, particularly CO, at temperatures ≤520 °C. Indeed, the flammable gas yield in the catalytic pyrolysis of SS increased from 200.3 mmol under N₂ conditions to 219.2 mmol under CO₂ conditions, representing a 4.4-fold increase compared to single-stage pyrolysis under CO₂ conditions (50.0 mmol). By integrating a water-gas-shift reaction, the flammable gases produced from CO₂-assisted catalytic pyrolysis were expected to have the potential to generate revenue of US$4.04 billion. These findings highlight the effectiveness of employing CO₂ in SS pyrolysis as a sustainable and effective approach for treating and valorising SS into valuable energy resources.