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

Abstract

This paper describes an experimental and numerical study of the emission of nitrogen oxides (NOx) 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. NOx 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 NOx emissions as a function of fuel dilution: (1) more NOx 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 NOx production more effectively than equivalent CO2 dilution. Chemical kinetic modeling suggests that NOx 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 NOx 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 NOx emissions for all fuel blends. The experimental and modeling results are in good agreement and suggest the underlying chemical kinetic reasons for the trends.