The co-combustion of sewage sludge (SS) with coal offers a promising strategy for waste valorization, yet its effects on pollutant emissions (NO, SO₂, heavy metals (HMs)) remain insufficiently elucidated. This work investigated the NO/SO₂ release patterns, HMs retention and extractability, and ash fusion behaviors during SS/coal co-combustion. Comprehensive analyses of flue gases and residual ashes reveal multiple synergistic benefits. Co-firing SS promotes early ignition and enhances overall combustion reactivity of coal through the rapid release of abundant volatiles that facilitate fixed-carbon combustion, although excessive SS blending reduces heat release due to its low calorific value and high ash content. Concurrent in-situ reductions of NO and SO₂ are achieved: NO suppression results from intensified O₂ consumption and elevated CO formation, which inhibit NH₃/HCN precursor oxidation while promoting NO reduction via CO or char active sites; SO₂ mitigation is linked to sulfur retention by Ca-bearing minerals forming stable sulfates. Co-combustion enhances retention of Cr, Cu, Pb, and Zn in ash matrices while reducing their extractability, associated with silicate, aluminosilicate, and phosphate-rich phases that encapsulate HMs. These refractory phases elevate ash flow temperatures and inhibit liquid slag formation. Overall, SS/coal co-combustion achieves reduced gaseous emissions, stabilized HMs in ashes, and enhanced ash fusibility.
 

Coal combustion; Sewage sludge disposal; NO and SO2 reduction; Heavy metal retention; Ash fusibility