On a premixed NH3/O2 jet flame in hot coflow of gaseous H2O versus N2

This numerical study comparatively investigates a premixed NH3/O2 jet flame in hot coflow (JHC) of H2O (steam) versus that of N2. To differentiate physical and chemical effects of coflowing either H2O or N2, FH2O and FN2 are introduced as fictitious gases which have the same physical properties as their real counterparts but do not participate in any chemical reaction. All present JHC flames are simulated under MILD (moderate or intense low-oxygen dilution) condition at the coflow temperature of TC = 1500 K and the equivalence ratio of Phi J = 0.4 to 1.6. Specifically, the premixed ammonia flames are characterized by the mean temperatures, key radical concentrations and thermal efficiency, as well as their NOx formation mechanisms and emission features. It is found that the use of H2O versus N2 for dilution significantly reduces NOx emissions from the ammonia MILD combustion, albeit at the cost of reduced combustion efficiency. Interestingly, the H2O dilution affects combustion both physically and chemically, in contrast to the N2 dilution primarily impacting the process physically. Notably, the change to H2O dilution can lower peak temperatures and thus promote a more uniform temperature distribution. Furthermore, the shift from N2 to H2O for dilution influences NO emissions, primarily tied to variations in the HNO route, notably reaction R180, while boosting the DeNOx mechanism. This shift also alters the production rates of key radicals, e.g., H and O being generated more slowly with an intricate effect on OH production, depending on Phi J.