Abstract: The coal chemical industry has expanded rapidly, producing a substantial amount of semi-coke. However, its large-scale, low-carbon utilization remains challenging. Semi-coke has low volatile content, making it hard to ignite and burn completely. Blending semi-coke with biomass, which exhibits superior combustion characteristics, is a promising approach to enhance combustion performance and increase burnout rates. Due to the high fixed carbon content of semi-coke, its combustion generates a significant amount of CO₂. Flameless oxy-fuel combustion can realize both high concentration CO₂ capture and significantly reduce NO emissions. However, research on the co-combustion of semi-coke and biomass in flameless oxy-fuel atmospheres is limited, and the mechanisms of nitrogen conversion are not well understood. To study fuel nitrogen conversion in semi-coke and biomass co-combustion under flameless oxy-fuel conditions, numerical simulation is carried out based on detailed model validation. The effects of biomass blending ratio on nitrogen conversion characteristics are thoroughly investigated, and the individual contributions of different NO types are analyzed. Results show that flameless oxy-fuel combustion maintains a high level of flue gas entrainment. As the biomass blending ratio increases, the high-temperature zone shifts forward, and the peak temperature rises. Additionally, the low-oxygen zone expands, volatile content increases, and CO concentration rises with a wider distribution. Compared to pure semi-coke combustion, pure biomass combustion increases the peak temperature by 105 K. When the biomass blending ratio is 25%, the total outlet NO concentration is significantly reduced. In contrast, when the biomass blending ratio is between 50% and 100%, there is no significant change in the total outlet NO concentration. The total NO concentration at the outlet is primarily determined by the generation of fuel NO and NO reburning, with fuel NO playing a dominant role. As the biomass blending ratio increases from 0 to 75%, the NO reburning amount rises from 5×10⁻⁶ to 35×10⁻⁶. When the biomass blending ratio is less than 50%, the formation of fuel NO continuously decreases, driven mainly by the reduction in semi-coke NO, while the increase in biomass NO has a limited impact. A biomass blending ratio of 25% is recommended, as it can effectively suppress NO generation. At this ratio, the total outlet NO concentration is reduced by 33% compared to pure semi-coke combustion, mainly due to a 30% decrease in the generation of fuel NO, while the impact of NO reburning is relatively minor.