Influence of nozzle height to width ratio on ignition and NOx 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 NOx 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 hf/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 hf/b = 2.32. The smaller hf/b delayed ignition and caused the flame to concentrate excessively on the axis of the furnace, while the larger hf/b easily caused the deflection of the pulverized coal flame, and a high-temperature flame zone emerged close to the furnace wall. NOx 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 hf/b varied from 1.65 to 3.22. Furthermore, the ratio (denoted by SR/SC) 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 SR/SC rose monotonously, but its restraining effects on NOx formation decreased as hf/b increased. The results suggested that in a test furnace, regulating the nozzle hf/b conditions sharply reduces NOx emissions and improves the combustion efficiency of SC blends possessing an appropriate jet rigidity.