Effectsof Minerals on Sewage Sludge Smoldering Combustion:Kinetics, Propagation Behaviors, and Product Characteristics

Smoldering combustion technology offers a promising low-energy alternative for the large-scale reduction of sewage sludge. While optimal process conditions for smoldering combustion of sewage sludge have been explored, the influence of fuel properties, particularly minerals, remains unclear. This study investigates the effects of minerals on the kinetics, propagation behavior, and product characteristics of sewage sludge smoldering combustion. Raw sludge (RS) and demineralized sludge (DS) were prepared and analyzed by using thermogravimetric analysis (TGA) and laboratory-scale smoldering experiments. The TGA results demonstrated that inherent minerals in sludge, including active minerals (K, Na, Ca, Fe, Cu, and Mn) and inert minerals (Al, Si, and P), exhibited mixed effects on combustion. Active minerals were found to reduce the activation energy for volatilization and char oxidation, thereby enhancing the combustion process. In contrast, inert minerals inhibited combustion by covering active sites and reducing heat transfer. Smoldering experiments demonstrated that DS exhibited a higher smoldering temperature (807.95 °C) and a slower propagation rate (0.66 cm/min) compared to RS due to increased organic content and reduced permeability after demineralization. Gas analysis revealed that DS emitted lower NOx levels, attributed to the reduced formation of NH₃ and HCN precursors, as well as increased CO levels that facilitated NO reduction. This study highlights the dual role of minerals in sewage sludge smoldering combustion and offers valuable insights into optimizing sludge disposal processes to enhance energy recovery and minimize pollutant emissions.