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The influence of injection conditions on NO emission during moderate or intense low-oxygen dilution (MILD) combustion was investigated through experiments and numerical simulations. The detailed GRI-Mech 2.11 mechanism and finite-rate chemistry effects were considered in simulations. The model was systematically validated using detailed in-furnace and exhaust emission measurements. The initial conditions investigated included the equivalence ratio (Φ), oxidant preheating temperature (T O ), and thermal input (P). Notable modeling results were found: under MILD combustion, with Φ increasing from 0.5 to 1.2, NO production first decreases and then increases, with the minimum NO emission obtained at Φ ≈ 0.8. Moreover, this trend is obtained at various P and T O , although NO emission increases monotonically with increasing P or T O . Furthermore, we found that CO emission is generally extremely low and can be ignored when Φ ≤ 0.85 but is much higher when Φ > 0.9. Consequently, for the furnace considered here, an optimal equivalence ratio (Φ) exists to minimize the NO and CO emissions of MILD combustion; this equivalence ratio is approximately 0.8, regardless of P and T O . These findings were confirmed by our experiments; in fact, nearly zero NO emission was achieved at this optimal Φ in the present experiment, which is significantly lower than that of previous studies on MILD combustion with typical NO emissions of 15−40 mg/m 3 (at 15% O 2 ). Therefore, the present study improves MILD combustion technology significantly to relatively low or even nearly zero NO emission when using the optimal equivalence ratio. Our detailed analysis of NO mechanisms further reveals that all NO formation routes (particularly the N 2 O-intermediate route and the prompt route) are strongly suppressed at this optimal Φ, thus minimizing NO emission.

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