The thermal DeNOx process is a widely used NOx emission control technique, but its chemical kinetic description still lacks accuracy. In the present work, two key kinetic parameters of the thermal DeNOx process were investigated: the branching fraction (α) of the NH₂ + NO reaction, and the rate coefficient of the unimolecular decomposition of NNH (inverse lifetime of NNH, τ_NNH). Values of these rate parameters were determined using a mechanism optimization method that utilizes both direct and indirect data and minimizes the value of an error function. Data were collected from the literature and available in data files on the ReSpecTh site (https://ReSpecTh.hu). Indirect experimental data used as optimization targets were NO mole fractions measured in tubular flow reactors. The most recent nitrogen chemistry mechanism of Glarborg and coworkers (2024) was used as the initial mechanism. Inconsistency was found between the indirect experimental data, and therefore mechanism optimization was not feasible using all indirect data. Using a new algorithm, a consistent subset of indirect data was identified. The optimized value of τ_NNH (8.5 ∙ 10⁻¹¹ s) is approximately an order of magnitude smaller than in the initial mechanism (10⁻⁹ s), but consistent with theoretical calculations. The posterior uncertainty of τ_NNH is significantly smaller than its prior uncertainty. The optimized value of the branching fraction is different from its initial value by less than 2%, but due to the very large sensitivity of the simulation results to α, this small change improves the performance of the mechanism noticeably. The width of the posterior uncertainty range of α is approximately half that of its prior uncertainty range, estimated using only direct measurements. This is a significant improvement, but more accurate indirect experimental data are needed to further increase the accuracy of the determination of α.

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