Effects of flue gas recirculation on self-excited combustion instability and NOx emission of a premixed flame

Combustion instability has been a common problem accompanied with the low NOx emission technologies for the industrial premixed burner. The effects of flue gas recirculation ratio (FR) on the characteristics of combustion instability and NOx emissions of a premixed flame in a 350 kW industrial boiler are investigated by experimental tests and numerical simulations. The cavity acoustic mode of the whole chamber is computed by a Helmholtz solver combined with CFD simulations. As the FR increased from 0 to 20%, the NOx emission is reduced by around 85% and self-excited combustion instability is trigged when the FR exceeds 10%. Four modes -combustion noise, limited cycle, intermittence and breathing -of combustion instability with the different FRs are recorded in the experiment. Two dominant pressure oscillation frequencies ranging from 21 to 25 Hz and from 1 to 2 Hz are observed when the FR is 10-20%. The measured pressure oscillation modes are compared with the acoustic simulation results. The results show that these two dominant frequencies are consistent with the first and second acoustic modes of the whole boiler cavity. The difference between the dominant oscillation frequency and the cavity acoustic eigenfrequency may be caused by the time delay of the response of the flame. The high amplitude pressure oscillations featured as ultra-low frequency (1-2 Hz) are caused by the periodic extinction and ignition of the flame under a high FR (> 18%).