Role of NNH in Low-NOx Hydrogen Combustion

Abstract

The role of the NNH mechanism in NOx formation is investigated using a jet stirred reactor (JSR). Two sets of experiments are performed over a wide range of fuel-air equivalence ratios (0.8-1.3) for combustion of hydrogen. Each set of experiments uses a different diluent to maintain the temperature of the JSR at a constant value while the fuel-air equivalence ratio is varied from lean to rich. Nitrogen and argon are the diluents used to maintain the temperature of the JSR at 1635K and 1525K, respectively. The two sets of experiments show that NOx decreases as fuel-air equivalence ratio is increased from 0.8 to 1.3 Chemical kinetic modeling is used to explain the trends seen in the experiments. Different chemical reactor networks and different chemical kinetic mechanisms for the hydrogen and nitrogen chemistry are used to model the NOx emission from the JSR using CHEMKIN 17.2. Chemical kinetic modeling shows reasonable agreement with both sets of experimental data. The sensitivity analysis of this modeling shows that NNH is the major contributor for NOx production on the rich region, for both the diluents. Pathway analysis is done to understand the important reactions that contribute to NOx. The pathway analysis is done for different H2-N2 mechanism combinations and it is observed that all the mechanisms give the same trends in NOx with some minor differences. The H2 mechanism of Li et al. coupled with the nitrogen mechanism of Glarborg et al. or Klippenstein et al. give close agreement to the measurements of NOx. A CFD analysis is also done as a 2D axisymmetric problem using ANSYS Fluent 17 and 18. The analysis is done for both lean and rich combustion using the mechanism combination which gives a reasonable agreement with the CRN modeling. The goal of the CFD simulation is to understand radical formation inside the JSR, especially analyzing the change in NNH contours upon moving from lean to rich combustion. Moreover, the recirculation zone NOx predicted by CFD is in close agreement with the results predicted by CRN modeling. The trends in species like H, O, NNH and N2O between the jet flame and recirculation zone agreeing between the CFD and CRN modeling imply that the CRN modeling gives a good insight of species formation.