Effect of exhaust tube vortex on NOx reduction and combustion characteristics in a swirl-stabilized pulverized coal flame

•In-furnace temperature is measured and 2-D contours are achieved.•Behaviors of the burner aerodynamics and swirl intensity are investigated.•Effectiveness of the ETV is evaluated by detailed measurements.•The synergistic effect of the ETV and air staging is defined and evaluated. Experimental investigations were conducted on the effect of the exhaust tube vortex (ETV) and the synergistic effect of the ETV and air staging on nitrogen oxide (NO) concentration and carbon monoxide (CO) concentration, as well as carbon burnout in a swirl-stabilized pulverized coal-fired furnace. To elucidate the aforementioned effects, four different swirl combinations were prepared derived from co-swirl and counter-swirl through five staging air injection positions. The results show that the ETV contributes to unburned carbon (UBC) combustion through its hollow-tube structure in the co-swirling flame. In addition, the temperature is distinguishable in the case of the 60°–60° co-swirl combination (Sco, 1.37/1.50), and 60°–60° counter-swirl combination (Sct, 1.37/1.50) because of the shape of the inner recirculation zone (IRZ) and the occurrence of the ETV. Although the ETV effect follows the swirl intensity, the 50°–50° co-swirl combination (Sco, 0.95/1.03) is the most favorable for the ETV effect under the unstaged condition, considering the behavior of the burner aerodynamics that contains the IRZ and the flame propagation dependent on the swirl vane angle. The result without an ETV is reflected in Sct, 1.37/1.50, meaning incomplete combustion of the coal during the combustion procedure. In terms of the synergistic effect, NO and CO reduction rates, as well as carbon burnout are needed to evaluate the best overall performance. According to evaluation, Sco, 1.37/1.50 at staging level 3 (SL3) has the highest synergistic effect because the effect of air staging is maximized in SL3 given the longest particle residence time in the re-burning zone.