Quantitative evaluation of NO formation and destruction routes during methane MILD combustion using an improved calculation method

•The existing NO calculation methods are evaluated for methane MILD combustion.•An improved calculation method to quantify the contribution of each NO route is proposed.•The improved calculation method involves thermal, prompt, N2O-intermediate, NNH, SNCR, and NO-reburning routes.•The NO formation and destruction route of methane MILD combustion is evaluated using the improved method with two mechanisms.•Two mechanisms perform differently in predicting NO formation via the prompt, N2O-intermediate, and NNH routes. Moderate or intense low-oxygen dilution (MILD) combustion has become increasingly attractive because of its ultra-low nitric oxide (NO) emission. Although many efforts have been made to investigate the NO formation and destruction pathways of MILD combustion, their relative contribution still fails to reach an agreement when using previously-developed NO calculation methods. For this reason, these NO calculation methods are first evaluated for methane MILD combustion by the kinetic modeling of a well-stirred reactor (WSR). Then, an improved calculation method for quantifying the contribution of NO sub-routes is proposed, which involves thermal, prompt, N2O-intermediate, NNH, selective non-catalytic reduction of NO, and NO-reburning routes. Also, this improved method allows the application of a more comprehensive mechanism (Progress in Energy and Combustion Science, 2018, 67: 31–68) other than GRI-Mech 2.11. Using the improved method with the more comprehensive mechanism, the NO formation and destruction routes during methane MILD combustion are quantitatively investigated. The results show that in the WSR at 1300 K with 5% O2, the contribution of the N2O-intermediate route accounts for about 80% at an equivalent ratio (Φ) of 0.8, and it gradually decreases as Φ increases. Once Φ is >1.0, the prompt route surpasses the N2O-intermediate route to dominate the NO generation entirely. The NNH mechanism is found to be of little importance at Φ ranging from 0.5 to 1.5. Additionally, the NO-reburning reaction reduces