Thermodiffusive instabilities and nitrogen oxides in lean ammonia/hydrogen/nitrogen-air laminar premixed flames

Though a promising zero-carbon hydrogen carrying fuel, ammonia suffers from poor combustion properties. One way to improve the combustion properties of ammonia is to partially crack the fuel into a mixture of ammonia, hydrogen, and nitrogen. Under fuel-lean conditions, pure hydrogen flames are thermodiffusively unstable, which has implications for not only flame propagation but also the formation of nitrogen oxides. However, much less is known about the onset of thermodiffusive instabilities when hydrogen is part of a multi-component fuel also containing ammonia. In this work, detailed two-dimensional simulations of laminar premixed flames are conducted to understand the onset of thermodiffusive instabilities in laminar premixed flames of ammonia/hydrogen/nitrogen mixtures and air. The degree of ammonia cracking, that is, the relative amount of ammonia to hydrogen/nitrogen, is varied to understand the onset of thermodiffusive instabilities as a function of fuel composition. Additionally, the influence of the fuel mixture and thermodiffusive instabilities on the formation of nitrogen oxides is analyzed. This understanding is critically required for ammonia to be a practical zero-carbon alternative fuel.