Abstract: The large-scale development of the coal chemical industry produces a substantial amount of semi-coke. This byproduct is characterized by low volatile matter content and a high proportion of fixed carbon, leading to difficulties in ignition and high carbon emissions, posing challenges for its large-scale, clean, and low-carbon utilization. Blending semi-coke with pulverized coal can improve its combustion characteristics; combining this with oxy-fuel combustion enables high-concentration CO2 capture, and further incorporating Moderate or Intense Low oxygen Dilution (MILD) combustion enhances denitration performance. In this study, experiments on fuel-nitrogen conversion during combustion of semi-coke and pulverized coal blends under MILD oxy-fuel conditions were conducted using a volatile-char decoupled combustion experimental platform. The effects of combustion temperature (1273–1773 K), oxygen concentration (3%–30%), pulverized coal blending ratio (0–100%), and CO2 concentration (0–60%) on NO generation were investigated under both decoupled and coupled combustion modes. The results show that with increasing temperature, NO release first decreases and then increases, reaching a minimum at 1673 K. In a CO2-rich atmosphere, gasification reactions promote early release of char-bound nitrogen; when the temperature exceeds 1473 K, the amount of NO derived from char can be reduced to nearly zero, and the char-CO2 gasification reaction is largely unaffected by oxygen concentration. Co-firing semi-coke with pulverized coal synergistically reduces NO emissions, with the optimal NOx reduction effect observed at a pulverized coal blending ratio of 50%, under which NO emissions decrease by 51.7% compared to pure semi-coke combustion under coupled combustion conditions. Increasing CO2 concentration suppresses fuel-NO formation, mainly due to the production of CO (which acts as a reducing agent) from gasification and the scavenging of H radicals by CO2. Comparative analysis indicates the nitrogen redistribution effect induced by the CO2 gasification reaction in decoupled combustion is overall stronger, compared with the NO reduction effect of char in coupled combustion. This study provides theoretical support for the development of high-precision numerical models of fuel-NO and low-nitrogen combustion regulation for semi-coke and pulverized coal blends under air and oxy-fuel atmospheres.