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

Abstract

Decarbonisation efforts for 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, Tu, 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 Tu ∈[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 N2O contributions for α = 1.0 depending on operating conditions. For Φ = 0.5, the N2O 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.