The reduced kinetic description of lean premixed combustion

Abstract

Lean premixed methane-air flames are investigated in an effort to facilitate the numerical description of CO and NO emissions in LP (lean premixed) and LPP (lean premixed prevaporized) combustion systems. As an initial step, the detailed mechanism describing the fuel oxidation process is reduced to a four-step description that employs CO, H2, and OH as intermediates not following a steady-state approximation. It is seen that, under conditions typical of gas-turbine combustion, this mechanism can be further simplified to give a two-step reduced description, in which fuel is consumed and CO is produced according to the fast overall step CH4 + 3/2 O2 CO + 2H2O, while CO is slowly oxidized according to the overall step CO + 1/2 O2 CO2. Because of its associated fast rate, fuel consumption takes place in a thin layer where CO, H2, and OH are all out of steady state, while CO oxidation occurs downstream in a distributed manner in a region where CO is the only intermediate not in steady state. In the proposed description, the rate of fuel consumption is assigned a heuristic Arrhenius dependence that adequately reproduces laminar burning velocities, whereas the rate of CO oxidation is extracted from the reduced chemistry analysis. Comparisons with results obtained with detailed chemistry indicate that the proposed kinetic description not only reproduces well the structure of one-dimensional unstrained and strained flames, including profiles of CO, temperature, and radicals, but can also be used to calculate NO emissions by appending an appropriate one-step reduced chemistry description that includes both the thermal and the N2O production paths. Although methane is employed in the present study as a model fuel, the universal structure of the resulting CO oxidation region, independent of the fuel considered, enables the proposed formulation to be readily extended to other hydrocarbons.