The role of fuel-rich clusters in flame stabilization and NO x emission reduction with precessing jet pulverized fuel flames

Abstract

Effects of controlling the local structure of turbulence in confined pulverized coal flames have been investigated using both reacting and nonreacting experiments. The reacting experiments, conducted in a 130-kW rotary kiln simulator, showed that flame ignition distance is reduced by more than a factor of 2 compared to a monochannel flame by use of a precessing jet of air to enlarge the scale of mixing in the emerging coal jet. NO x emissions are reduced from more than 200 ppm to 80 ppm (0% O 2 ) by using precessing jet burners. Qualitative cold-flow visualization studies demonstrated that precessing airflows break up the surrounding annular flow, simulating a coal transport flow, into much larger turbulent flow structures, causing increased residence times in the pre-ignition region. Particle-imaging techniques in a nonreacting flow show that, in the pre-ignition region, the motion of the smaller particles (<30 μ m) are affected by the large flow structures and form clusters with long residence times. Flame stability improvements are attributed to increased particle heating rates in the clusters by radiation due to increased absorptivity and increased residence time in the near burner zone. The NO x reduction can be explained by the generation of large-scale particle clusters that provide local instantaneous fuel-rich combustion environments. This work demonstrates that the production of large-scale clustering effects has the potential to provide an alternative means to simultaneously control NO x and improve flame stability relative to existing staging techniques, which control the mean stoichiometry of various flame zones.