Measurements and simulations on effects of elevated pressure and strain rate on NO_x emissions in laminar premixed NH₃/CH₄/air and NH₃/H₂/air flames

Efforts to reduce NOx emissions by utilizing ammonia-fired combustors are made through various methods; this work investigates the possibility of combining the effects of pressure and stretch rate to reduce NOx emissions in laminar premixed flames. NOx emissions, including NO, NO2, N2O, NH3, and HCN, from NH3/CH4/air and NH3/H-2/air unstrained and strained laminar premixed flames were separately measured and simulated with a high-pressure heat flux burner and CHEMKIN software. The key points are that increasing the pressure can decrease most NOx emissions, including NO, N2O, and HCN. However, it slightly increases the unburnt ammonia emissions and largely increases the NO2 formation. Medium XNH3 conditions are the most favorable for reducing NO emissions by increasing pressure(.) High strain rates can reduce NO emissions, but only in stoichiometric and rich conditions; they reversely increase NO emissions for lean equivalence ratios before the NO peak equivalence ratio (phi). The reason is explained by kinetic and reaction pathway analyses, which showed that in high-strain flames, more fuel-N species participated in the reaction to form N-2 instead of NO for higher phi, while O and OH increased for lower phi, promoting NO formation. The combination of increasing pressure and strain rate can efficiently reduce NO emissions but ensure full ammonia oxidation for the medium ammonia content at phi = 0.9 similar to 1.1. This sheds light on the utilization of highly strained ammonia-blend flames in more practical high-pressure burners to reduce NOx emissions.