A Numerical Investigation on Effects of Hydrogen Enrichment and Turbulence on NO Formation Pathways in Premixed Ammonia/Air Flames

NOx emission reduction is one of the major challenges when using ammonia/hydrogen blends as an alternative fuel for carbon-free combustion. In this study, chemical reaction pathways of NO formation in planar premixed flames of stoichiometric ammonia/hydrogen/air at atmospheric pressure and reactants temperature of 298 K are investigated under laminar and decaying turbulent conditions using quasi-DNS with detailed chemistry and the mixture-averaged transport model. The sweep parameter, α, is the volumetric ratio of hydrogen to the ammonia/hydrogen mixture. Here, for unstrained laminar conditions, α = 0, 0.4, and 0.6 and for turbulent condition, α = 0.4 within the thin reaction zones region (Ka ≈ 34.2) are studied. Under 1D laminar conditions, increasing α enhances NO formation drastically, around 3 times when comparing α = 0 and 0.6. At different α, despite the high activity of the reactions which belong to the Zeldovich pathway, the contribution of this pathway is insignificant in NO formation. However, HNO and N2O pathways have the most significant roles. For instance, at α = 0.4, R85 (NH+NO ⇔ N2O+H) in the N2O pathway and R144 (HNO+H ⇔ NO+H2) in the HNO pathway have major roles in NO formation. Higher NO formation at larger α is found to be due to the increased H and O radicals within the reaction zone as well as the increased reaction rates because of the higher flame temperature. Under the 3D turbulent condition (α = 0.4), it is observed that turbulence does not switch the pathways trends across the flame brush compared to the laminar condition. In the reaction zone, however, it is observed that NO formation is lower (higher) in the regions convex (concave) toward the reactants. The underlying reason for this effect is the preferential diffusion of H2 to the regions of the flame front convex toward the reactants, which consumes NO and also depletes O and OH radicals, which are necessary for NO formation.