MILD Combustion of Ammonia, from Kinetics to Applications

Ammonia is recognized as one of the most effective hydrogen carriers and as an excellent candidate for supporting the decarbonization of the energy sector. Nevertheless, its combustion characteristics make it hard to be used in standard combustion processes. For example, its low reactivity, that leads to a very low laminar flame speed, does not allow to easily stabilize processes based neither on deflagrative nor diffusive flame structure. At the same time, it yields to the formation of a relevant amount of nitrogen oxides (NOx), not acceptable for their environmental impact. On this basis, it is striking to break the mold and consider processes that go beyond the stabilization mechanisms of standard combustion processes, thus avoiding the main issues related to ammonia combustion. From this point of view, one of the most promising conversion technologies of ammonia is Moderate or Intense Level of Dilution (MILD) combustion, already proven to be really fuel flexible. In this chapter, the main characteristic of ammonia combustion in MILD condition will be explored based on the knowledge available in the literature. Firstly, kinetics of ammonia will be discussed in relation to this combustion regime, highlighting reaction subsets active in the temperature range characteristic of MILD combustion. In this context, it will be also discussed the crucial role of ammonia in third-body efficiency reactions. Moreover, possible burner configurations for MILD combustion of ammonia and its impact on stability range and NOx emissions will be discussed, showing the fundamental role of such a combustion regime in the energy transition.