Insight into characteristics and mechanism of CaO-enhanced municipal sewage sludge decoupled combustion for pollutant emissions control

Abstract

Using organic-rich sludge as an alternative to coal has emerged as a critical area in the utilization of renewable solid waste fuels to address the growing energy crisis. To tackle the issue of high nitrogen oxides emissions during sludge combustion, this study presents the first fundamental investigations into the decoupled combustion (DC) of municipal sewage sludge (MSS), focusing on the in-situ reduction of nitrogen oxides generated by MSS pyrolysis char combustion through its pyrolysis volatiles and examining the role of CaO in the DC process. Systematic experiments are conducted to demonstrate the feasibility of the DC process, explore the effects of CaO on DC efficacy, investigate the mechanisms of DC with and without CaO, and reveal the migration and transformation pathways of nitrogen. Results indicate that the DC process functions as a “Self-Thermal De-NO_x” process, primarily utilizing pyrolysis-derived NH₃ to reduce NO from char combustion while inhibiting N₂O formation. Compared to conventional combustion (CC), DC reduces emissions by up to 49.37 % at 950 °C. When CaO is used as the bed material, it mainly mitigates the detrimental impacts of HCN and combustible gases such as CH₄, H₂, and CO on the selective reduction of NO by NH₃ through inhibiting HCN formation, reducing the total production of combustible gases and promoting NH₃ production, thereby enhancing the nitrogen reduction efficacy of the DC process. Compared to CC, DC with CaO (DC-CaO) reduces emissions by up to 64.01 % at 850 °C. This study provides key theoretical insights for the efficient and environmentally friendly utilization of renewable solid waste as an energy source.