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To investigate the thermal behaviors and NO x emission properties during combustion of sewage sludge (SS), sawdust (SD), and their blends (SS5SD5, SS3SD7, and SS1SD9 with SD proportions of 50, 70, and 90 wt %, respectively), tests were conducted using thermogravimetry-mass spectrometry (TG-MS), Fourier transform infrared spectroscopy (FTIR), and a tube furnace in this study. Results indicated that hydrogen in the fuel was mainly released during volatile combustion, and carbon conversion proceeded during the whole combustion process. With the SD proportion increasing, samples exhibited better combustion characteristics. Compared to SD, SS emitted more NO x due to its higher nitrogen content but showed lower conversion ratios from fuel nitrogen to NO x , and the NO x yields decreased significantly with the increase in SD proportion. NO x emissions of higher volatile samples were more sensitive to temperature, and NO x yields of SD and SS1SD9 continued to decrease from 800 to 1000 °C, whereas NO x yields of SS, SS5SD5, and SS3SD7 changed slightly from 800 to 900 °C and decreased significantly from 900 to 1000 °C. Synergistic effects of cocombustion on NO x emission varied with the blending ratio and temperature. SS5SD5 and SS3SD7 always presented a positive NO x reduction performance, and SS1SD9 exhibited opposite NO x reduction effects at different temperatures. Synthetically considering the SS disposal capacity, combustion characteristic, and NO x yield, an SS proportion of around 30% in blends is more recommended in practical applications., Competing Interests: The authors declare no competing financial interest., (© 2022 The Authors. Published by American Chemical Society.)