Experimental and Numerical Study of NOx 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, CH 4 is mixed with varying levels CO 2 and N 2 . 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 C H 4 consumed with the addition of a diluent, (2) the degree of increase in emission index is dependent on the chosen diluent; N 2 dilution increases NO x production more effectively than equivalent CO 2 dilution. Chemical kinetic modeling suggests that NO x production is less effective for the mixture diluted with CO 2 due to both a decrease in N 2 concentration and the ability of CO 2 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 CH 4 combustion. A global CH 4 combustion mechanism is used to model the chemistry. While it does not predict intermediate radicals, it does predict CH 4 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 constricted 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.