NNH mechanism in low-NOx hydrogen combustion: Experimental and numerical analysis of formation pathways

Abstract

The role of the NNH mechanism in NOx formation is investigated using an atmospheric-pressure jet-stirred reactor (JSR). Two sets of hydrogen combustion experiments are performed over a range of fuel–air equivalence ratios (ϕ = 0.8–1.3). Each experimental series uses a different diluent to maintain the temperature of the JSR recirculation zone while varying the fuel–air equivalence ratio. Nitrogen and argon were used as diluents to control the reactor temperature at 1635 K and 1525 K, respectively. In the fuel-lean region, the NOx increases with the fuel–air ratio until the stoichiometric condition (ϕ = 1) is reached, and then decreases in the fuel-rich region with a further increase in the equivalence ratio. A 2D axisymmetric CFD model is used to examine temperature and species distribution in the JSR and to guide the choice of chemical reactor network (CRN). The CFD modeling shows the NNH is formed in the flame brush for all ϕ, the region of high NNH concentration spreads into the recirculation zone for the fuel-rich conditions. Three different CRN schemes are evaluated to model NOx formation. The nitrogen chemistry of Klippenstein et al. (2011) and Glarborg et al. (2018) show the best agreement with the NOx data. The CRN analysis suggests that NNH is the main contributor in the NOx formation during the fuel-rich combustion. For the fuel-lean conditions, the contribution from the nitrous oxide mechanism is significant.