Influence of primary air cone length on combustion characteristics and NOx emissions of a swirl burner from a 0.5 MW pulverized coal-fired furnace with air staging

Highlights: • Effect of primary air cone length on burner performance was determined. • The size of central recirculation zone increases with decreasing the length. • Performance of NOx emissions and combustion increases with decreasing length. • A burner outlet structure was optimized. - Abstract: This paper presents experimental research on cold flow, combustion characteristics, and NO_x emissions of a swirl burner with air staging. The aim of the research is to investigate the influence of primary air cone length on the swirl burner performance. Cold flow air experiments on a centrally fuel-rich swirl burner with an over-fire air ratio of 25% were performed, and it was observed that when the dimensionless primary-air cone length (i.e., L_p/L_i, L_p and L_i respectively denoting the primary-air cone length in the cold-modeling experiments and inner secondary-air cone length) increases from 0 to 0.75, the diameter and length of the recirculation zone reduces from 0.91 d and 1.51 d to 0.68 d and 0.57 d, respectively. An investigation was conducted in a 0.5 MW laboratory furnace in order to optimize the primary-air cone length of a centrally fuel-rich swirl burner. The flame stability, performance of ignition, and burnout are significantly improved by decreasing the primary-air cone length. The NO_x emissions measured at the furnace exit with a cone length L_p/L_i = 0 decreased by 50.3% compared to those measured with a cone length L_p/L_i = 1.0. Based on the evaluation of flame stability, performance of the ignition, burnout, and low NO_x emissions, the optimum value of primary-air cone length was observed to be L_p/L_i = 0.