Experimental and Numerical Study of NO_x Formation From the Lean Premixed Combustion of CH₄ Mixed With CO₂ and N₂

This paper describes an experimental and numerical study of the emission of nitrogen oxides (NO_x) from the lean premixed (LPM) combustion of gaseous fuel alternatives to typical pipeline natural gas in a high intensity, single-jet, stirred reactor (JSR). In this study, CH₄ is mixed with varying levels CO₂ and N₂. NO_x measurements are taken at a nominal combustion temperature of 1800K, atmospheric pressure, and a reactor residence time of 3 ms. The experimental results show the following trends for NO_x emissions as a function of fuel dilution: (1) more NO_x is produced per kg of CH₄ consumed with the addition of a diluent, (2) the degree of increase in emission index is dependent on the chosen diluent; N₂ dilution increases NO_x production more effectively than equivalent CO₂ dilution. Chemical kinetic modeling suggests that NO_x production is less effective for the mixture diluted with CO₂ due to both a decrease in N₂ concentration and the ability of CO₂ to deplete the radicals taking part in NO_x formation chemistry. In order to gain insight on flame structure within the JSR, three dimensional computational fluid dynamic (CFD) simulations are carried out for LPM CH₄ combustion. A global CH₄ combustion mechanism is used to model the chemistry. While it does not predict intermediate radicals, it does predict CH₄ and CO oxidation quite well. The CFD model illustrates the flow-field, temperature variation, and flame structure within the JSR. A 3-element chemical reactor network (CRN), including detailed chemistry, is constructed using insight from spatial measurements of the reactor, the results of CFD simulations, and classical fluid dynamic correlations. GRI 3.0 is used in the CRN to model the NO_x emissions for all fuel blends. The experimental and modeling results are in good agreement and suggest the underlying chemical kinetic reasons for the trends.