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To improve moderate or intense low-oxygen dilution (MILD) combustion characteristics in oxygen-enriched conditions, this study reported the design of a nonpremixed air/oxygen jet burner that is different from a premixed one. Implementing the premixed and nonpremixed air/oxygen jet burners, the performance of oxygen-enriched natural gas MILD combustion was numerically investigated under different oxygen concentrations (19.5–36%) and velocities (47.16–261.18 m/s). The results show that MILD combustion yields a higher peak temperature and NO emission when the oxygen concentration is enriched for both burners due to the lack of desirable mixing of reactants with flue gases resulting from the reduced oxidizer velocity. Compared to the premixed air/oxygen jet, the nonpremixed oxygen/air jet reduces the in-furnace maximum temperature with higher thermal uniformity regardless of the oxygen concentration and thus the net NO emission at 25, 30.5, and 36% oxygen concentration is mitigated from 43.3, 73.4, and 112.9 ppm@6%O2 to 29.4, 47.1, and 71.7 ppm@6%O2, respectively, mainly due to the thermal NO reduction. Unlike the oxygen-enrichment effect, increasing the oxygen jet velocity while maintaining a fixed air velocity can further improve oxygen-enriched MILD combustion and its NO emission behaviors. Overall, the nonpremixed air/oxygen jet burner can provide low NO emission if combustion air with low oxygen contents is employed, while the usage of the nonpremixed air/oxygen jet burner together with high oxygen velocity is suggested to avoid a high NO emission level in highly oxygen-enriched conditions. Moreover, the total heat flux enhances as the oxygen concentration is increased, mainly from the radiative heat transfer enhancement; however, higher convective and lower radiative heat fluxes are observed in the case with high oxygen jet velocity.

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