Synergistic retention of heavy metals and in-situ reduction of NO and SO2 by co-combustion of sewage sludge and coal gangue: A promising approach for contaminant management and emission reduction

Abstract

This study aims to evaluate the influence of co-firing municipal sewage sludge with coal gangue at 800–1000 °C on the emissions of pollutants such as NO, SO2 and heavy metals (HMs). The results revealed that co-combustion resulted in smoother combustion (e.g., lower ignition and burnout temperatures, and a higher comprehensive combustion index) as well as a reduction in activation energy by approximately 30 kJ/mol when the sewage sludge ratio was above 60%. Moreover, the retention of HMs in the ashes during co-combustion was significantly improved through reactions between the dominant minerals (especially quartz (SiO2), berlinite (AlPO4), hedenbergite (CaFeSi2O6), magnetite (Fe3O4) and gehlenite (Ca2Al2SiO7) with HMs species presented in the wastes, leading to an increase in HMs content by 1 to 3 times. Additionally, in-situ desulfurization and denitration were observed during co-combustion at above 900 °C and below 850 °C, respectively. It is likely that berlinite has a direct impact on NO emissions, while gehlenite, whitlockite (Ca2.86Mg0.14(PO4)2) and diopside (CaMgSi2O8) present in the ashes are associated with the evolution of SO2 during co-combustion. These findings suggest that co-combustion of multiple wastes is a feasible approach for their resource utilization while reducing the environmental impacts associated with solid waste disposal.