MILD combustion of a premixed NH₃/air jet flame in hot coflow versus its CH₄/air counterpart

Moderate or intense low-oxygen dilution (MILD) combustion is suitable for solving the problems of unstable flames and high NOx emissions (E-NO(x)) of ammonia fuels; however, studies on this are rare. This paper numerically investigates the MILD combustion characteristics of a premixed NH3/air jet flame in hot coflow (JHC) under different jet equivalence ratios (Phi(J)), coflow temperatures (T-C), and oxygen levels (X-O2,X-C). For comparison, similar CH4/air MILD-JHC flame characteristics are calculated. The results show that the NH3 flame generally has a lower temperature increase and heat release, as well as a larger reaction zone, than CH4. This suggests that the NH3 flame can develop into a MILD regime more easily than CH4. E-NO(x) of the NH3 flame is two orders of magnitude higher than that of CH4 flame at all T-C and X-O2,X-C values for Phi(J) < 1. At Phi(J) > 1, E-NO(x) of the NH3 flame rapidly decreases to nearly zero, but unburned NH3 emissions (ENH3) and H-2 emissions (E-H2) are significantly high (similar to 1000 ppm) and the combustion efficiency is low. Moreover, E-NO(x) increases rapidly (gradually) with X-O2,X-C (T-C), whereas E-NH3 and E-H2 are considerably low at T-C >= 1300 K and X-O2,X-C >= 1% for the NH3 flame. E-NO(x) primarily originates from NO; however, N2O is also important at low Phi(J) and T-C values. Therefore, considering E-NO(x), E-NH3, E-H2 and the burning efficiency, the optimal conditions for NH3 MILD combustion are Phi(J) = 1, T-C >= 1500 K, and X-O2,X-C = 1%-3%. In addition, compared with conventional flames, NH3 MILD combustion can reduce E-NO(x) by one to two orders of magnitude.