NO emissions from H2/CH4–air mixtures with high H2 content: a parametric study using laminar premixed flames

Decarbonisation efforts for the combustion of hydrogen/methane-blends target a volume fraction of hydrogen, α , of at least 70 % but nitric oxide (NO) may increase with the amount of hydrogen in the mixture. The effects of mixture temperature, T u , pressure, p , equivalence ratio, Φ , and α on the absolute levels of NO and the relative contribution of the NO formation pathways are studied using one-dimensional laminar premixed flames for T u ∈ [ 300 , 500 , 700 K ] , p ∈ [ 1 , 5 , 10 , 30 bar ] , Φ ∈ [ 0.5 , 0.75 , 1.0 , 1.25 , 1.5 ] and α ∈ [ 0.7 , 0.85 , 1.0 ] . A detailed pathway flux analysis is employed for this purpose. The pathway contributions are similar for Φ = 0.75 to 1.25 irrespective of α as the thermal route is dominant, but its relative contribution is reduced for fuel-rich Φ = 1.5 and especially fuel-lean Φ = 0.5 conditions. The prompt pathway is dominant for Φ = 1.5 and α < 1.0 and it is replaced by thermal, NNH and N 2 O contributions for α = 1.0 depending on operating conditions. For Φ = 0.5 , the N 2 O contribution dominates for α < 1.0 but the NNH contribution can become equally important for α = 1.0 . The NNH contribution has negative pressure sensitivity for Φ = 0.5 but positive sensitivity for Φ = 1.5 .