Abstract
The combustion of ammonia (NH3) has sparked a renewed interest in the scientific community because it is a carbon-free fuel that does not contribute to global warming. Nevertheless, some of its disadvantages include the fact that (i) it has lower laminar flame speeds when compared to hydrocarbons, and (ii) that it is prone to the formation of nitride oxides (NOx) through the fuel route. As an alternative to address the first issue, it has been proposed to mix ammonia with hydrogen, but at the expense of even higher NOx emissions. Such emissions, however, can be reduced by operating at restrictive equivalence ratios near stoichiometry, which reduces oxygen availability in the reactions for NOx formation. This is a drawback because lean combustion, a technique that has been proven to reduce emissions for most fuels, is usually avoided in NH3 combustion. Therefore, with the goal of reducing NOx, this paper presents a numerical study using perfectly stirred reactors to investigate the potential use of both lean equivalence ratios and hydrogen enrichment in the MILD combustion of NH3. Some features of the MILD regime, such as low temperature increases due to reaction and the possibility of sustaining reactions under low oxygen concentrations, could be beneficial to ammonia combustion. Simulations were carried out in open-source software Cantera. According to the results, reaction can be sustained at lean equivalence ratios like 0.6 with a 50% hydrogen enrichment, and emissions of less than 100 ppm NO at 15% O2 on a dry basis can be obtained. In conclusion, MILD is a promising strategy for the clean burning of ammonia.
