NO formation rates for hydrogen combustion in stirred reactors

The combustion of lean, stoichiometric and rich hydrogen–air mixtures in well-stirred reactors has been modeled in the temperature range 1500–2200 K employing a detailed H/N/O reaction scheme, and a H/O reaction scheme together with algebraic expressions for the NO production rate as a function of local gas composition and temperature. Thermal-NO, nitrous oxide, and NNH mechanisms of nitric oxide formation have been taken into account. It is demonstrated that explicit expressions of the NO formation rate correctly reproduce the prediction of the detailed mechanism for residence times longer than about 100 μs at all temperatures of this study in rich, stoichiometric and lean mixtures. With time increase the reduced mechanism deviates from the full one in rich mixtures at residence times longer than about 10 ms. The evaluation of the NNH route showed that this pathway is important at all temperatures at short residence times. At temperatures of 2100 K and higher the thermal-NO mechanism becomes dominant after about 1 ms. Sensitivity analysis clearly demonstrates the validity of the assumptions employed in the development of the explicit expressions for the instantaneous NO formation rates in hydrogen combustion in the range of residence times from 1 to 25 ms. A possible new route for NO formation in rich hydrogen mixtures via N2H3 radicals is identified.