Effects of annular N2/O2 and CO2/O2 jets on combustion instabilities and NOx emissions in lean-premixed methane flames

Experiments on the effects of annular N-2/O-2 and CO2/O-2 jets on combustion instabilities and NOx emissions in lean-premixed methane flames were conducted. Two variables were investigated to validate the effectiveness of suppressing combustion instabilities and NOx emissions synchronously-the flow rate and the volume fraction of O-2. Results indicate that the amplitude-damped ratio of combustion instability can reach 79.53% under CO2/O-2 injection, sound pressure reduced from 30.3 Pa to 6 Pa. Damped ratio of NOx emissions achieve 45% under the case of CO2/O-2 injection, NOx emission reduced from 22.5 ppm to 12 ppm, but the N-2/O-2 injection case cannot significantly reduce NOx emissions. Mode shifting of flame heat release rate is observed during the process of annular injection, the amplitude of flame heat release rate increases to two times than that of the uncontrolled flames during the N-2/O-2 injection case, but the amplitude maintains a relatively stable level in the case of CO2/O-2 injection, under both N-2/O-2 and CO2/O-2 jets, oscillation frequency of flame heat release rate jumps from 265.5 Hz to approximately 125 Hz. The CO2/O-2 jets lead to a more uniform temperature field than the N-2/O-2 jets, thus contribute to lower NOx emissions. The application of CO2/O-2 injection improves combustion velocity and provides better mixing, the flame length becomes shorter and compact. This study realized the reduction of combustion instabilities and NOx emissions simultaneously, which could be conducive to the prevention of combustion instabilities or pollutant discharge in industrial gas turbine burners.