A Reduced Mechanism for Low-Heating-Value Gas Combustion in a Perfectly Stirred Reactor

We have begun an effort to accurately model NO, formation from the combustion of coal-derived fuels in turbine combustors. Both turbulent mixing and the chemical kinetics of ammonia oxidation are expected to have important influences upon NOx formation rates. This paper concentrates upon the development of a model for the kinetics. Previous empirical, kinetic mechanisms have inaccurately assumed equilibrium OH concentrations and ignored the chemistry of HCN, an important intermediate. We have developed a reduced mechanism by applying simplifying assumptions to a full, detailed mechanism for methane combustion with nitrogen chemistry. The mechanism contains 7 rates for 10 non-steady-state species, a single partial equilibrium assumption, and steady-state relations for 18 species. The Zeldovich and Fenimore mechanisms of NO formation are modeled, as is the NO recycle mechanism by which NO is converted to HCN. Nitric oxide formation from N20 is also included. Two oxidation routes for NHJ are included: the first describes NH3 conversion to N, and then to NO; the second describes HNO formation, and final conversion of HNO to NO. Stirred reactor calculations were performed for three cases: (1) methane-air combustion with no nitrogenated species in the reactants, (2) methane-air combustion with 1000 ppmV NO in the reactants, and (3) methane-air combustion with 1000 ppmV NH3 in the reactants. The reactor temperature (1300 to 2000 K) and residence time (10-4 to 10-1 s) were varied. Both the reduced and skeletal mechanism calculations agree very well with calculations using the detailed mechanism of Miller and Bowman, except for fuel-rich combustion at low temperatures (less than 1500 K), where results from the skeletal mechanism begin to deviate due to neglect of C2 chemistry. Future work will test the performance of the reduced and skeletal mechanisms in models of laminar, counterflow flames, which are meant to represent flamelet combustion in a turbulent flame.