Modelling NO-NO2 Conversion in Counter-Flow Diffusion Flames

As emission regulations become more stringent there is increasing interest in the formation of NO2 in combustion products where it is in higher concentration than if slowly formed from NO in the atmosphere. It is common knowledge that NO2 is significantly more toxic than NO. The chemistry of NO2 formation in combustion processes is simple in comparison to that of NO. Indeed, all NO2 is formed from oxidation of NO mainly by reaction with HO2 radicals with its conversion back to NO resulting from reactions involving O and H atoms. Since consumption and formation of NO2 always occur simultaneously, although with unbalanced kinetic rates leading to local super-equilibrium concentrations, parameters such as temperature, velocity and species concentrations fields can drastically affect the degree of conversion of NO to NO2 in combustion applications. It is not well known what these conditions are and in certain circumstances, such as aircraft engine reheat systems, the emission of NO2 is clearly visible under the form of brown fumes. A comprehensive numerical simulation was undertaken to investigate the NO-NO2 relationship in a counter-flow diffusion flame. The CHEMKIN II suite of software (Kee et al., 1989) in conjunction with the opposed diffusion flame code OPPDIF (Lutz et al, 1997) was run using the Gas Research Institute’s (GRI’s) methane reaction mechanism v.3.0. A number of different strain rates using boundary conditions typical in a gas turbine exhaust were investigated. A rate of production and sensitivity analysis was made in determining which reactions were important in the NO-NO2 conversion process.