Influence of primary air cone geometry on near-field flow, fuel-air mixing and NOx performance of a centrally fuel-rich swirl burner with 600 MWe boiler validation

Under deep air-staged combustion, the secondary air flow rate of swirl burners is greatly reduced, making outlet recirculation structure, gas-solid mixing and early ignition behavior critical for stable combustion and low NOx emissions. This work examines the effects of primary air cone geometry on aerodynamic fields, fuel-air mixing and gas-solid flow in a centrally fuel-rich swirl burner for a 600 MWe wall-fired boiler. A cold model is employed for single-phase flow visualization, temperature-tracer mixing and three-dimensional phase-Doppler anemometry (PDA), validated on a 600 MWe industrial unit. As the primary air cone length decreases from 49 mm to 0 mm, the central recirculation zone (CRZ) length increases from 0.57d to 1.51d and its maximum diameter from 0.68d to 0.91d, while the axial onset of recirculation shifts ~70 mm toward the burner outlet. Shorter cones intensify the CRZ, promote earlier and stronger primary-secondary air mixing, increase radial and tangential mixing velocities, and drive fine particles outward, whereas longer cones preserve axial momentum, weaken the CRZ and maintain a strongly fuel-rich core near the centerline. PDA results further indicate that particle back-flow, concentration stratification and size segregation are highly sensitive to cone geometry. Industrial tests show that moving the primary-air outlet 0.1 m away from the burner wall shifts ignition downstream, reduces burner-throat gas temperatures (e.g. from 747°C and 823°C to 451°C and 714°C at two representative burners), and lowers NOx emissions from 344 to 313 mg/m³ at 6% O₂; compared with the pre-retrofit level of 663 mg/m³, this corresponds to an overall reduction of 52.8%. These findings demonstrate that optimizing primary air cone length and outlet position effectively coordinates ignition stability and ultra-low NOx emissions, providing practical design guidance for large wall-fired boilers under deep air-staged combustion and flexible load conditions.