Towards non-intrusive, quantitative N2O Raman measurements in ammonia flames

Understanding N2O formation and consumption in ammonia combustion is crucial in realizing the impact of ammonia as an alternative fuel to mitigate the impact of climate change. This study demonstrates the feasibility of using Raman spectroscopy for in-situ N2O measurements in ammonia flames. Raman spectra were acquired along a NH3/H2/N2-air flame in a laminar opposed jet burner, using a pulsed laser combined with a three-disk rotating shutter system to suppress the luminous flame background. This setup enabled the clear detection of the N2O Raman spectrum. Raman libraries of N2, O2, H2, NO, and N2O were fitted to the spectra using a newly developed fitting routine. This yielded qualitative N2O mole fractions along the flame that align closely with numerical simulations based on recently published chemical reaction models for ammonia oxidation, paving the way for future quantitative N2O measurements in ammonia flames. Since no prior libraries for temperature-dependent N2O Raman spectra were available, a methodology for its simulation is introduced. High-resolution N2O spectra were acquired between 295 and 1091K as validation data for the simulation. Despite minor deviations, the simulation effectively captures the spectral shape and temperature dependence of the Raman cross sections, enabling its use in the spectral fitting routine towards quantitative in-situ concentration measurements.