Insights into the Synergistic Effect and Inhibition Mechanism of Composite Conditioner on Sulfur-Containing Gases during Sewage Sludge Pyrolysis

Sewage sludge odorous gas release is a key barrier to resource utilization, and conditioners can mitigate the release of sulfur-containing gases. The gas release characteristics and sulfur compound distribution in pyrolysis products under both single and composite conditioning strategies of CaO, Fe₂O₃, and FeCl₃ were investigated. This study focused on the inhibition mechanisms of these conditioners on sulfur-containing gas emissions and compared the theoretical and experimental sulfur content in the products to evaluate the potential synergistic effects of the composite conditioners. The findings indicated that at 650 °C, CaO, Fe₂O₃, and FeCl₃ inhibited H₂S release by 35.8%, 23.2%, and 9.1%, respectively. Notably, the composite of CaO with FeCl₃ at temperatures ranging from 350 to 450 °C and the combination of Fe₂O₃ with FeCl₃ at 650 °C were found to exert synergistic suppression on H₂S emissions. The strongly alkaline CaO inhibited the metathesis reaction between HCl, a decomposition product of FeCl₃, and the sulfur-containing compounds within the sewage sludge, thereby exerting a synergistic suppression on the emission of H₂S. Conversely, at temperatures exceeding 550 °C, the formation of Ca-Fe compounds, such as FeCa₂O₄, appeared to diminish the sulfur-fixing capacity of the conditioners, resulting in increased H₂S emissions. For instance, the combination of CaO and FeCl₃ at 450 °C was found to synergistically reduce H₂S emissions by 56.3%, while the combination of CaO and Fe₂O₃ at 650 °C synergistically enhances the release of H₂S by 23.6%. The insights gained from this study are instrumental in optimizing the pyrolysis of sewage sludge, aiming to minimize its environmental footprint and enhance the efficiency of resource recovery.