Investigation of NO Production and Flame Structures in Ammonia-Hydrogen Flames

Ammonia/hydrogen fuel blends have recently emerged as a promising solution to the de-carbonization of the energy and transport sectors. However, concerns over performance and, more importantly, NOx emissions have impeded their progress so far. Before effective NOx mitigation strategies can be developed, the fundamental chemical mechanisms involved in NOx production in NH₃/H₂ flames must be well understood. Although NOx formation in hydrocarbons and the oxidative processes involved in NH3 combustion have been well studied, there is a significant lack of such information for NH₃/H₂ flames. Key insights on NO formation mechanisms and flame structures for NH₃/H₂ mixtures are required to develop and improve chemical kinetic models. In this work, laminar Bunsen NH₃/H₂ flames with 65/35 NH₃/H₂ volume fraction were tested at two equivalence ratios (rich and lean). For all test conditions, the adiabatic flame temperature was kept constant. Measurements of simultaneous OH/NO-PLIF and OH-PLIF/chemiluminescence (of NH*, NH₂* and OH*) were conducted and compared to computational results of four reaction mechanisms (available in literature) applicable to NH₃/H₂ flames. The maximum OH-PLIF signal gradient was used as a spatial reference point for each simultaneous measurement and one-dimensional line profiles were determined for each species of interest. The simultaneous OH/NO-PLIF images show that the NO signal intensity in the NH₃/H₂ flames were up to 100 times more than a CH₄ reference flame. OH* and NH* chemiluminescence results showed a good spatial correlation with the maximum OH-PLIF signal gradient for both test conditions. NH* also showed a positive correlation with the computed HRR values from lean to rich, indicating it is a promising candidate for direct HRR measurement but warrants further investigation over a wide range of equivalence and mixture ratios. The results also indicate that all the mechanisms underestimate the profile widths of the measured species. Similarly, the experimental results showed a much higher relative increase in NH₂ production from lean to rich compared to the computed profiles. The data analysis approach employed in this paper, based on simultaneous measurements, could be further used for optimizing chemical kinetics mechanisms for NH₃/H₂ flames.