Numerical study of heat release rate markers in laminar premixed Ammonia-methane-air flames

Ammonia has the potential of being a future fuel for carbon-free combustion to reduce greenhouse gases (GHG) emission. The heat release rate (HRR) information of ammonia flames is desired if the ammonia fuel is applied in real combustion devices, but direct measurement of HRR is almost impossible. In the present work, the HRR markers of ammonia-methane-air flames were investigated through the simulation of one-dimensional freely propagating flames (FPF) with a validated chemistry mechanism. Various HRR markers were proposed. The performance of the target HRR markers was tested with the defined criteria. It was found that the profiles of NH and HCO agree well with the HRR profiles, but only NH is a suitable HRR marker because the mole fraction of HCO could be too low for laser induced fluorescence (LIF) measurement. The products of two species, such as [OH][CH₂O] and [NO][NH₂], were then proposed to circumvent the low mole fraction issues. Product [OH][CH₂O], which was usually used as HRR marker in hydrocarbon flames, was found to have the best performance in identifying the HRR location and be the only one that has a positive correlation with the amount of HRR among the proposed products. Moreover, the performance of [OH][CH₂O] is valid over a wide range of conditions, including the very high ammonia fuel fraction (X_NH3 = 95%), without suffering from the low mole fraction issues. The observations were further analyzed by looking into the reaction pathways and examining the reactions that contributes the largest amount to the total HRR. The present findings provide a meaningful reference for future indirect measurement of HRR in ammonia-methane-air flames.