A comprehensive parametric study on NO and N2O formation in ammonia-methane cofired premixed flames: Spatially resolved measurements and kinetic analysis

Understanding the mechanism of NOx formation and destruction is a prerequisite for the development of effective NOx mitigation techniques in ammonia flames. Laboratory-scale laminar ammonia (NH₃) flames are well suited for such fundamental kinetic studies as the complex interaction between chemistry and fluid flow can be largely decoupled. However, quantitative and spatially resolved NO/N₂O concentration data in canonical laminar ammonia flames are surprisingly scarce. Such data is, on the other hand, crucial for developing and validating kinetic models for NH₃ combustion. In this regard, we developed a novel NO/N₂O measurement method combining microprobe sampling and calibration-free mid-infrared laser absorption spectroscopy and realized spatially-resolved detection with high accuracy and large dynamic ratio. The fidelity of the method has been rigorously tested before being applied to perform a comprehensive parametric study on NO and N₂O formation in NH₃-CH₄ co-fired burner-stabilized premixed flames. The effects of NH₃ fuel ratio, equivalence ratio and flame temperature on NO/N₂O formation have been experimentally determined. Corresponding numerical modelling was also performed using literature-based mechanisms to provide kinetic insights into the experimental observations. It is found that existing mechanisms generally have satisfactory predictions in fuel-lean flames; however, under fuel-rich conditions, these mechanisms overpredict NO formation but underestimate its destruction, leading to significant over-prediction. Such discrepancies were further investigated through sensitivity and reaction pathway analysis. The present study not only provides extensive spatially-resolved NO/N₂O data in ammonia flames that are urgently needed for the development and validation of NH₃ combustion mechanisms, the comparison between neat CH₄ and NH₃-cofired flames also points to the fact that traditional NOx mitigation techniques that are popular in combustion of hydrocarbon fuels may not be appropriate in NH₃ flames. Perhaps most interestingly, the present experimental results show an oxygen-enriched oxidizer can in some cases reduce NO emission from NH₃ flames.