Effects of the outer secondary air cone length on the combustion characteristics and NOx emissions of the swirl burner in a 0.5 MW pilot-scale facility during air-staged combustion

• Effect of outer secondary air cone length on burner performance was determined.
• The size of central recirculation zone increases with increasing the length.
• NOx emissions and unburned carbon in fly ash decrease with increasing the length.
• Considering NOx emissions and burnout rate, a length of 49 mm is advisable.

Cold air experiments using a centrally fuel-rich pulverized-coal swirl burner model were performed within a laboratory-scale facility to study the effects of the outer secondary air cone length on the aerodynamic fields. The divergent angle of the air jet and the size of the central recirculation zone were found to increase with increasing outer secondary air cone lengths. The 0.5 MW pilot-scale facility was also used to optimize the length of the outer secondary air cone by measuring the flue gas temperature, and O2, CO and NOx concentrations, at different cone lengths. As the cone length was varied from 0 to 49 mm, the NOx emissions rapidly fell from 778 to 458 mg/m3 (O2 6%), while the unburned carbon content in the fly ash decreased from 5.83% to 3.51%, and CO concentration decreased from 1355 mg/m3 (O2 6%) to 99 mg/m3 (O2 6%). Based on these results, the optimum length of the outer secondary air cone was determined to be 49 mm.