Reduced kinetics and coupling functions for calculating CO and NO emissions in gas-turbine combustion

Abstract

A reduced chemical-kinetic mechanism consisting of two global steps for fuel oxidation and an additional step for NO production is proposed as the minimal chemistry description for calculating CO and NO emissions in gas-turbine combustion. Carbon monoxide is seen to emerge as the main intermediate during the fuel-oxidation process, which takes place in two steps: fast partial hydrocarbon oxidation to give CO and H2O in a relatively thin fuel-consumption layer and slow CO oxidation to CO2 in a much larger region. All relevant intermediates but CO follow a steadystate approximation in the CO-oxidation region, so that the associated steady-state expressions can be employed to accurately compute the CO-oxidation rate. Since steady states for radicals and H2 fail in the fuel-consumption layer, fuel consumption cannot be computed with acceptable accuracy from the reduced kinetics, a limitation that motivates the introduction of a heuristic Arrhenius law for the fuel-consumption rate. Production of oxides of nitrogen is represented by a single overall step that considers both the thermal and the nitrous oxide mechanisms but neglects the effects of the Fenimore and reburn contributions. As a preliminary step to facilitate computations, the conservation equations corresponding to the resulting three-step mechanism are written in terms of appropriate coupling functions, different in premixed and nonpremixed systems. Preliminary calculations of methane-air flames, including both freely propagating premixed flames as well as counterflow nonpremixed flames, indicate that the proposed reduced kinetics produces good accuracy over a wide range of conditions.