Co-Combustion of Sewage Sludge Char with Zhundong Coal: Ash Fusion Characteristics and NO/SO 2 Emission Behaviors

The co-combustion characteristics of sewage sludge char (SSC) and Zhundong coal (ZC) at various blending ratios (20–80% ZC) were investigated in this study. The ash fusion characteristics, mineral transformations, and microstructure properties were analyzed using an ash fusion test, XRD, and SEM-EDS, respectively. The NO and SO2 emission behaviors were measured in a tubular furnace. The results indicated that the ash fusion temperature of SSC showed a non-monotonic response to an increasing ZC proportion, reaching a minimum at 40% ZC and a maximum at 80% ZC. The ash microstructure properties of the blends transformed from a dense block-like structure to a loose and porous structure. The lowest NO emission amount was observed at 60% ZC, with a value of 0.76 mg·g−1, whereas the maximum discrepancy between the calculated and experimental results was identified at 40% ZC (0.58 mg·g−1). In contrast, the SO2 emission amount increased with ZC proportion and peaked at 3.11 mg·g−1 for 80% ZC, with a maximum negative deviation of −1.07 mg·g−1.The co-combustion of SSC with ZC suppressed NO but promoted SO2 release. An optimal blending ratio was needed to balance pollutant control and ash fusion performance. These findings provide guidance for the clean and efficient co-combustion of SSC and ZC.