Analysis of the performance of kinetic reaction mechanisms in estimating N2O mole fractions in 70/30 vol% NH3/H2 premixed flames

To address the environmental concerns associated with fossil fuels, this study explores ammonia (NH3) blended with hydrogen (H2) as an alternative fuel. While offering reduced CO2 emissions and leveraging existing infrastructure, NH3-H2 combustion notably leads to the production of nitrogen oxides (NOx), including the greenhouse gas nitrous oxide (N2O). Understanding the flame structure and chemistry responsible for N2O formation and consumption is crucial. This study comprehensively investigates various kinetic reaction mechanisms, focusing on accurately estimating N2O mole fractions and identifying mechanisms that align closely with experimental data. Sixty-seven chemical kinetic mechanisms have been numerically analysed across various equivalence ratios (φ) ranging from 0.57 to 1.4, utilizing the Premixed Stagnation Flame Model via Chemkin-Pro software. Simulations in a Perfectly Stirred Reactor were also conducted for the kinetic models that demonstrated high accuracy within the burner-stabilized stagnation flame model and closely matched experimental measurements with minimal discrepancies. This was done to determine whether the tested models, which perform accurately, maintain their performance across different combustion configurations. A preliminary assessment was carried out using the Normalized Error Approach, taking into account the uncertainty of experimental measurements, to compare the numerical results with experimental data. This method is significant in determining whether the discrepancies between the model's calculations and the experimental results, considering the experimental uncertainties, are within an acceptable range of error. The sensitivity analysis along with rate of production/consumption of N2O investigation at several conditions of equivalence ratio (0.6,1,1.4) has been conducted to check the discrepancies among the mechanisms and shed light on the reactions that dominate the formation/consumption of N2O at different conditions. The study revealed that the kinetic model developed by Klippenstein et al. (2018) demonstrates remarkable accuracy in predicting N2O mole fractions across a range of conditions, specifically within the equivalence ratio range of 0.6 to 1.4. In this range, the normalised error values were observed to be less than 1, signifying that the experimental values align closely with the numerical expectations, considering the uncertainty. However, it is noteworthy that the model's accuracy appears to decrease in lean flame scenarios, particularly when the equivalence ratio falls between 0.57 and 0.585. In these conditions, higher normalised error values exceeding 1 were recorded, suggesting a possible deviation between numerical predictions and experimental observations. Along with that the rate of production/consumption analysis revealed the NH + NO ⇌ N2O + H reaction has a dominant role in the formation of N2O for all studied conditions, while the consumption of N2O is dominated by reactions N2O + H ⇌ N2 + OH and N2O (+M) ⇌ N2 + O(+M) at all investigated conditions.