Investigations on the Effects of Flue Gas Recirculation on the Combustion Instability of a Premixed Flame for the Low Nox Emissions

Abstract

The combustion instability has been a common problem accompany with the low NOx emission technologies in industrial premixed burner. The effects of flue gas recirculation ratio (FR) on the characteristics of combustion instability and NOx emissions of an industrial scale premixed burner were investigated with experiments and numerical simulation. The acoustic mode of the whole chamber was computed with a Helmholtz solver combined with CFD simulation. The NOx emissions significantly decrease with the increasing of the FR. As FR increases from 0% to 20% the NOx emission reduces about 85%. Four modes of pressure oscillations in the combustion chamber with different FRs were found in the experiment. The pressure oscillation exhibits combustion noise mode at low FRs (<10%) in which conditions the pressure oscillation amplitude is low and there is no distinct frequency. Two dominant pressure oscillation frequency ranging from 21Hz to 25Hz and ranging from 1Hz to 2Hz are observed under relatively high FRs. The first acoustic mode frequency is 5.3Hz~5.7Hz and the second acoustic mode frequency is 23.1Hz~24.1Hz which are close to the pressure oscillation frequencies observed in the experiment. The ultra-low frequency and high amplitude pressure oscillations are attributed the periodic overall extinguish and ignition of the flame under high FRs. The difference between the pressure oscillation frequencies and the acoustic cavity mode frequencies may be attributed to the interaction of the response of the flame and the cavity acoustic mode which can lead to mode transition.