Numerical and experimental study of product gas characteristics in premixed ammonia/methane/air laminar flames stabilised in a stagnation flow

The adoption of ammonia/hydrocarbon fuel blends can be viewed as an intermediate step towards a hydrogen economy, hence the characterization of methane/ammonia flame product gas trends is essential for designing combustors for a broader range of low-carbon fuel blends while fulfilling strict NOx requirements. This paper describes the product gas content of laminar premixed ammonia/methane flames for a range of equivalence ratios and ammonia heat ratios ranging from 10% to 60%, using a strain stabilized burner at atmospheric pressure and room temperature. The optimal condition to reduce NOx emissions while maintaining below 100 ppm of unburnt NH3 emissions was found to be at equivalence ratio of 1.20 for higher ammonia ratios, moving incrementally closer over 1.35 as the methane fuel content was increased. Meanwhile, the highest measured NO values were ∼6,950 ppm at an equivalence ratio of 0.9, peaking at heat ratios of 30% to 40% at this equivalence ratio. Detailed reaction mechanisms were evaluated against the experimental data and rate constants of NO production/consumption steps featuring both NH and HNO intermediates and thermal NOx reactions were updated for Okafor's mechanism. Changes in reaction rate constants improved the mechanism accuracy for NO emissions in lean to stoichiometric flames. Meanwhile, in the rich region, modelled NO values were less responsive to changes in reaction constants, suggesting the need for an alternative approach to improve NO predictions for rich, high methane content flames. However, N2O performance in the rich region could be improved, highlighting the significance of the HNO+CO→NH+CO2 reaction.