Abstract:
The effect of chemical nonequilibrium in the combustion process of a diffusion methane-air flame at moderate pressures and temperatures of the initial reactants on the reduction of nitrogen oxide in reactions known as NO reburn was studied. Based on the detailed mechanism of chemical kinetics of methane oxidation GRI-Mech 3.0, an analysis was performed of the distribution of the main NO-reducing substances (CH, CH$_2$, CH$_3$, HCCO) in reburn reactions, as well as the initial substances forming NO in Zeldovich reactions, across the thickness of a diffusion laminar flame based on a numerical solution of the Peters-Kuznetsov system of equations using the thin flame front model (flamlet model). In the flame representation as a chain of isolated ideally mixed reactors distributed across the flame thickness, the NO concentration was estimated using a precise analytical formula representing the rates of reburn reactions in accordance with three detailed kinetic mechanisms: GRI-Mech 3.0, Glarborg, and Miller–Bowman. A comparative assessment of the efficiency of these mechanisms for reducing NO concentration was performed; the maximum reduction was 13.3, 23, and 30.4%, respectively. An assessment of the influence of diffusion showed a change in the relative concentration of NO profiles by no more than 0.06%. It was also shown that, as the chemical equilibrium deviates, reburn reactions gradually begin to occur on the “lean” side of the flame (as fuel radicals diffuse across the stoichiometric boundary), and the region of reburn reactions in terms of the composition of the gas mixture expands more than twofold. The contribution of reburn reactions on the lean side of the flame increases steadily and can reach 56% of the total reburn efficiency, which is of practical significance in systems with a high degree of combustion nonequilibrium. The main contributions to NO reduction on the lean side of the flame are reactions with CH (up to 75% via the GRI-Mech 3.0 mechanism) and reactions with HCCO (up to 80% via the Glarborg mechanism).
