A Computational Study of Nitric Oxide Reduction Efficiency in a Chemically Non-Equilibrium Methane–Air Diffusion Flame

This study investigates the effect of chemical non-equilibrium in a methane–air diffusion flame at moderate pressures and initial reactant temperatures on the reduction of nitric oxide (NO) via reactions known as “Reburn NO”. Based on the detailed GRI-Mech 3.0 mechanism for methane oxidation kinetics, an analysis is performed of the distribution of the primary NO-reducing components (CH, CH $$_{2}$$ , CH $$_{3}$$ , and HCCO) involved in reburn reactions, as well as the precursors for NO formation via Zeldovich reactions, across the thickness of a laminar diffusion flame. The analysis utilizes a numerical solution of the Peters–Kuznetsov equation system within the thin flame front model (flamelet model). By representing the flame as a chain of isolated perfectly stirred reactors distributed across the flame thickness, the NO concentration is estimated using an exact analytical formula for the reburn reaction rates according to three detailed kinetic mechanisms: GRI-Mech 3.0, Glarborg, and Miller–Bowman. A comparative assessment of the efficiency of these mechanisms in reducing the NO concentration is conducted; the maximum achieved reductions are 13.3, 23, and 30.4%, respectively. The estimated influence of diffusion shows a change in the relative concentration of NO profiles of no more than 0.06%. It is also demonstrated that, as the system deviates from chemical equilibrium, reburn reactions gradually begin to occur on the lean side of the flame (as fuel radicals diffuse across the stoichiometric boundary) and the operating range of reburn reactions in terms of the gas mixture composition expands more than twofold. The contribution of reburn reactions from the lean side of the flame increases monotonically and can reach up to 56% of the total reburn efficiency, which is of practical significance for systems with a high degree of combustion non-equilibrium. The primary contributions to NO reduction on the lean side are from reactions with CH (up to 75% for the GRI-Mech 3.0 mechanism) and reactions with HCCO (up to 80% for the Glarborg mechanism).