NOx reduction in a pulverised-coal swirl burner with high-velocity jets

The mechanism of NO reduction in a pulverised-coal swirl burner with high-velocity jets was experimentally analysed. Variations in the number of jets, injection angle and driving pressure were investigated. Temperature and species concentrations were measured, both at the exhaust of the small-scale furnace and within the flame. The velocity field was measured in a cold-flow model by means of a three-dimensional pressure probe. A high-velocity jet generates a non-symmetrical, powerful recirculation that strongly interacts with the swirling flow. In-flame measurements of both temperature and species concentrations support the view that the jet is deviated by the swirling flow, adopting a helical path; the jet mixing is delayed, and because of its high entrainment rate it drags small particles off the primary stream, thereby creating a staging process that reduces NO formation by 35-50% with respect to the burner without jets, but has little effect on combustion efficiency.