Effects of secondary air injection on the emissions and stability of two-stage NH3-CH4-air swirl flames

Two-stage rich-lean combustion has been identified as a suitable strategy to lower NOx emissions and increase the combustion efficiency for ammonia (NH3) fueled gas turbines. Success requires a fine control of the secondary stage properties, where air is injected to oxidize all of the remaining fuel. This study investigates the effects of the air flow rate and injection holes geometry of the secondary stage on the exhaust emissions and flame stability. Experiments were conducted with a lab-scale piloted burner inspired from that fitted in the AE-T100 micro gas turbine. A rich NH3-CH4-air primary flame with an equivalence ratio of ϕprimary = 1.1 and an NH3vol fraction of 0.8 was used. The secondary air flow rate (Qsec) was varied from 0 up to the maximum value that eventually led to flame instability or blowout. The size and number of secondary air injection holes were varied too. Consistent with earlier reports for non-piloted burners, it is found that the two-stage rich-lean strategy abated NO emissions by about one order of magnitude compared to single-stage lean combustion, while also mitigating N2O emissions. However, exhaust gases emissions measurements and Dynamic Mode Decomposition (DMD) of high-speed chemiluminescence imaging highlighted the existence of a threshold for the secondary air flow rate above which a low frequency, longitudinal pulsation of the primary flame occurs, and NO emissions increase significantly. This threshold was found to depend on the number and diameter of the air holes. Increasing the number of secondary air holes at a constant diameter allowed to retain flame stability for higher secondary air flow rates, without degrading NO and N2O emissions. Data showed that both the secondary air flow rate and the velocity of the jets are parameters controlling the onset of this instability; a finding useful for the design of future two-stage rich-lean NH3 burners.