Understanding the chemical pathways of NO formation in low-pressure burner stabilized premixed lean-to-rich hydrogen flames

Hydrogen (H₂) combustion, a potential clean energy solution, is hindered by NO emission that affects human health and the environment. A comprehensive understanding of NO formation during H₂ combustion is necessary to mitigate its emission. The dynamic interplay between the different formation pathways makes the interpretation of NO sub-mechanism difficult. Lack of comprehensive experimental data further limits the comprehension of NO formation process, especially for the non-thermal NO formation pathways. In this study, quantitative NO and temperature measurements were performed using in-situ laser diagnostics in 6 low-pressure burner stabilized H₂/O₂/N₂ flames over a wide range of equivalence ratios (0.35–1.50) at 35 and 70 Torr (4.67, and 9.33 kPa). The maximal temperature in the flames remain below 1500 K, which minimizes the thermal NO pathway and allows for a focused study of non-thermal pathways of NO formation. However, the low temperature restricts NO formation imposing severe challenges on experimental measurements. Several precautions were taken to address these challenges and reduce the experimental uncertainty. The maximal NO mole fraction in the flames is between 0.09 and 0.71 ppm. The experimental re-evaluation of two flames similar to Harrington (Harrington et al., Proc. Combust. Inst., 26, 1996) shows disagreement with the original experimental data but consistent with the simulation predictions. This re-evaluated dataset can potentially replace the existing controversial Harrington measurements for validation of NNH pathway. The simulations of the flames using three recent chemical kinetic models predict NO in satisfactory agreement with the experiment, even for the flames similar to Harrington. The study suggests that NNH pathway dominates NO formation in low-pressure, low temperature H₂ combustion irrespective of the equivalence ratio. The large set of novel experimental datasets generated in this study can serve as future chemical kinetic model validation targets, especially for the NNH pathway of NO formation.