Ammonia/Hydrogen and Cracked Ammonia Combustion

Ammonia is a promising energy carrier for energy system decarbonization, although several drawbacks affect its combustion process. Coupling moderate or intense low-oxygen dilution (MILD) combustion with the use of high reactivity fuels allows to improve NH3 combustion. In particular, H2 addition may be a feasible strategy, considering the high proportion of H2 achievable by NH3 partial cracking. The present study focuses on MILD combustion effectiveness in ensuring high stability and low-NOx emissions for NH3/H2 blends. Influence of both equivalence ratio and H2 addition was experimentally investigated in a cyclonic reactor. Furthermore, the results were directly compared with those obtained with cracked NH3 mixtures (NH3/H2/N2). Results for NH3/H2 blends strengthen the fuel flexibility of the cyclonic reactor, which allows total conversion of the fuel mixtures by ensuring operating temperatures always lower than 1400 K, independently of the equivalence ratio and the fuel blend composition. In particular, H2 addition increases NH3 reactivity, whereas increasing NOx emissions with respect to pure ammonia. Instead, for pure H2 and pure NH3, they always stay lower than 40 and 100 ppm, respectively. For cracked NH3 mixtures, the fuel dilution content by N2 does not affect the NH3/H2 combustion behavior under MILD conditions. Instead, for 100% NH3 cracking (75%H2-25%N2 mixture), H2 dilution by N2 entails a more uniform reaction zone than not diluted H2 case, further limiting NOx formation by avoiding the occurrence of hot-spot regions within the reactor.