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 CO ₂ -assisted pyrolysis, we converted SS into flammable gases (H ₂ and CO; syngas). Single-stage pyrolysis of SS in a CO ₂ conditions demonstrated that CO ₂ enhances flammable gas production (especially CO) through gas phase reactions (GPRs) with volatile matter (VM) at temperatures ≥520 °C. Specifically, the CO ₂ 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/Al ₂ O ₃ 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 N ₂ conditions to 219.2 mmol under CO ₂ conditions, representing a 4.4-fold increase compared to single-stage pyrolysis under CO ₂ conditions (50.0 mmol). By integrating a water-gas-shift reaction, the flammable gases produced from CO ₂ -assisted catalytic pyrolysis were expected to have the potential to generate revenue of US$4.04 billion. These findings highlight the effectiveness of employing CO ₂ in SS pyrolysis as a sustainable and effective approach for treating and valorising SS into valuable energy resources.
Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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