Impacts of coal particle size and secondary air ratio on NO and N2O emissions in a 10 kW circulating fluidized bed: Experimental and numerical simulation

• Experiments combining CPFD simulation to investigate a 10 kW CFBC. • Fine coal particles (0–0.3 mm) yield the highest N 2 O due to rapid devolatilization. • Medium-sized particles (0–0.6 mm) achieve the lowest N 2 O levels. • Moderate secondary-to-primary air ratio of 20–30% minimizes total NO and N 2 O emissions. • Coordinated control provides guidance for boiler ultra-low-load operation. Circulating fluidized bed combustion (CFBC) is a clean coal technology renowned for its fuel flexibility, high efficiency, and low emissions. However, under ultra-low boiler loads, reduced furnace temperatures weaken combustion stability and selective non-catalytic reduction (SNCR) performance, leading to elevated NO and N 2 O emissions. To address the critical knowledge gap regarding N 2 O and NO formation and emission mechanisms under these extreme conditions, this study combines experimental measurements with computational particle fluid dynamics (CPFD) based numerical simulations to explicitly link particle-size-dependent devolatilization kinetics to N 2 O and NO evolution. Results indicate that while coal particle size exerts a modest influence on average NO emissions, it significantly impacts N 2 O formation. Specifically, fine particles (0–0.3 mm) produce the highest N 2 O emissions due to rapid devolatilization and intense volatile-N release, whereas medium-sized particles (0–0.6 mm) yield the lowest N 2 O levels. In contrast, coarse particles (0–0.9 mm) exhibit approximately 15% lower NO but moderate N 2 O levels, attributed to enhanced heterogeneous reduction on char surfaces. Furthermore, increasing the secondary-to-primary air ratio redistributes oxygen and shifts the combustion zone toward the riser wall. Both experiments and simulations identify an optimal ratio range of 20–30%, in which the 30% condition minimizes NO and reduces combined emissions by approximately 17%. These findings provide novel insights into load-flexible CFBC boiler operation, demonstrating that the coordinated control of coal particle size and secondary-to-primary air staging can achieve lower NO and N 2 O emissions and more stable combustion.