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

• NH 3 flame has much lower temperature rise and larger reaction zone than CH 4. • NO x emission of NH 3 flame is two orders higher than CH 4 at equivalence ratio < 1. • NO x emissions of both flames increase rapidly with coflow oxygen concentration. • NO x emission mainly comes from NO while N 2 O is important at low temperature. • MILD combustion can reduce the NO x emission of NH 3 flame by 1–2 orders. Moderate or intense low-oxygen dilution (MILD) combustion is suitable for solving the problems of unstable flames and high NO x emissions (E NO x) of ammonia fuels; however, studies on this are rare. This paper numerically investigates the MILD combustion characteristics of a premixed NH 3 /air jet flame in hot coflow (JHC) under different jet equivalence ratios (Φ J), coflow temperatures (T C), and oxygen levels (X O2,C). For comparison, similar CH 4 /air MILD-JHC flame characteristics are calculated. The results show that the NH 3 flame generally has a lower temperature increase and heat release, as well as a larger reaction zone, than CH 4. This suggests that the NH 3 flame can develop into a MILD regime more easily than CH 4. E NO x of the NH 3 flame is two orders of magnitude higher than that of CH 4 flame at all T C and X O2,C values for Φ J < 1. At Φ J > 1, E NO x of the NH 3 flame rapidly decreases to nearly zero, but unburned NH 3 emissions (E NH3) and H 2 emissions (E H2) are significantly high (∼1000 ppm) and the combustion efficiency is low. Moreover, E NO x increases rapidly (gradually) with X O2,C (T C), whereas E NH3 and E H2 are considerably low at T C ≥ 1300 K and X O2,C ≥ 1% for the NH 3 flame. E NO x primarily originates from NO; however, N 2 O is also important at low Φ J and T C values. Therefore, considering E NO x , E NH3 , E H2 and the burning efficiency, the optimal conditions for NH 3 MILD combustion are Φ J = 1, T C ≥ 1500 K, and X O2,C = 1%–3%. In addition, compared with conventional flames, NH 3 MILD combustion can reduce E NO x by one to two orders of magnitude. [ABSTRACT FROM AUTHOR]