Influence of nozzle height to width ratio on ignition and NO_x emission characteristics of semicoke/bituminous coal blends in a 300 kW pulverized coal-fired furnace

Abstract

To improve the ignition behavior and to reduce the high NO_x emissions of blended pulverized fuels (PF) of semicoke (SC), large-scale experiments were conducted in a 300 kW fired furnace at various nozzle settings, i.e., ratios (denoted by h_f/b) of the height of the rectangular burner nozzle to its width of 1.65, 2.32, and 3.22. The combustion tests indicate that the flame stability, ignition performance, and fuel burnout ratio were significantly improved at a nozzle setting of h_f/b = 2.32. The smaller h_f/b delayed ignition and caused the flame to concentrate excessively on the axis of the furnace, while the larger h_f/b easily caused the deflection of the pulverized coal flame, and a high-temperature flame zone emerged close to the furnace wall. NO_x emissions at the outlet of the primary zone decreased from 447 to 354 mg/m³ (O₂ = 6%), and the ignition distance decreased from 420 to 246 mm when the h_f/b varied from 1.65 to 3.22. Furthermore, the ratio (denoted by S_R/S_C) of the strong reduction zone area to the combustion reaction zone area was defined experimentally by the CO concentration to evaluate the reduction zone. The S_R/S_C rose monotonously, but its restraining effects on NO_x formation decreased as h_f/b increased. The results suggested that in a test furnace, regulating the nozzle h_f/b conditions sharply reduces NO_x emissions and improves the combustion efficiency of SC blends possessing an appropriate jet rigidity.

Graphical abstract

Keywords

rectangular jet burner / nozzle height to width ratio / ignition characteristics / pyrolyzed semicoke (SC) and bituminous blend / NO_x formation

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Acknowledgments

This work was supported by the National Key Research and Development Program of China (No. 2017YFB0602002) and National Natural Science Foundation of China (Grant No. 51536002).

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