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Moderate or intense low-oxygen dilution (MILD) combustion is regarded as one of the most effective technologies to achieve extremely low NO x emissions of combustion. MILD combustion diluted by CO 2 and by N 2 are herein termed "MILD oxy-combustion" and "MILD air combustion", respectively. The present study is to investigate the difference of the two by experimental observation in a furnace of 20 kW and chemical kinetics calculation of a well-stirred reactor (WSR). Also, to identify their difference in mechanism, reaction paths of combustion diluted with N 2 and CO 2 are examined. It is revealed that the region of MILD oxy-combustion is notably larger than that of MILD air combustion for gaseous fuels, which suggests that the requirement for establishing MILD combustion is less stringent with dilution by CO 2 than by N 2 . The key reason is that the CO 2 dilution substantially lowers the temperature rise because of combustion, delays the ignition, and slows the overall reaction rate, thus facilitating the occurrence of MILD combustion. Detailed analyses show that the temperature reduction derives from the physical effect of CO 2 dilution, while the ignition delay results mainly from the chemical effect. Moreover, the investigation of reaction paths suggests that the CO 2 dilution increases the local CO production mainly through H + CO 2 → OH + CO and CO 2 + CH 2 (s) → CO + CH 2 O.

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