Effects of reactants stratification and pre-heating on the stabilization and emissions of partially cracked ammonia swirl flames

Ammonia is an easy solution for the transportation and storage of hydrogen. To achieve combustion properties similar to those of methane, ammonia can be partially cracked on site to introduce hydrogen and nitrogen in the fuel mixture. In this work, an atmospheric pressure dual-swirl ammonia-hydrogen burner is used to study different configurations of stratified flames of ammonia cracked at 28 %. First, flame stabilization is evaluated in terms of the overall equivalence ratio and the distribution of air between the hydrogen and ammonia streams. This is done for both cases where either hydrogen or ammonia occupies the central part of the burner or its periphery. The configuration of the burner is adjusted in such a way that several stratification levels are scrutinized, depending on where the two streams meet. Seven types of flames are identified and described. A stability map is measured. The results show that the configuration with ammonia flowing centrally and hydrogen occupying the periphery enhances stability. Second, measurements of NOx, N₂O, unburnt ammonia, and unburnt hydrogen in the exhaust gases are performed. Full stratification reduces NOx emissions, but both lean overall equivalence ratio and lean ammonia equivalence ratio increase N₂O emissions. The flame with ammonia in the center and hydrogen at the periphery with an overall equivalence ratio of 0.55 gives the best results in terms of stability and low pollutant emissions. This condition is further investigated by changing the reactants temperature. The reactants preheating is beneficial for N₂O emissions but comes with a strong NOx penalty.