Ammonia/Hydrogen and Cracked Ammonia Combustion

Abstract

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-NO_x 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 NO_x 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 NO_x formation by avoiding the occurrence of hot-spot regions within the reactor.