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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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A more straightforward combustion map for identifying moderate or intense low-oxygen dilution (MILD) combustion regime in a well-stirred reactor (WSR) using initial inlet temperature (T in) and oxygen mole fraction (X O2) has been proposed based on previous mathematical criteria provided by Cavaliere and de Joannon (Prog. Energy Combust. Sci.200430329366). Furthermore, the detailed evolution of different combustion regimes under the nonadiabatic condition has been comprehensively examined. Results show that there exists a critical X O2 (X O2*), below which MILD combustion can be established unconditionally as long as T in exceeds the self-ignition point (T si) and beyond which T in needs to be remarkably promoted to fulfill the mathematical criteria of MILD combustion. Thus, the two regions are termed unconditional MILD combustion (UMC) and conditional MILD combustion (CMC), respectively. For the adiabatic condition, X O2 * is calculated to be 9.7%, indicating that MILD combustion will be more easily achieved with an oxygen-diluted oxidizer than the oxygen-enriched counterpart. Interestingly, X O2 * is found to climb as the heat loss ratio (HLR) increases, suggesting that enhancing the HLR of the WSR would help expand the UMC region, namely, more readily establishing MILD combustion. In addition, high-temperature combustion (HTC) can shift to CMC or even UMC by just enlarging HLR, providing a potential solution to realize MILD combustion in practical applications. However, the combustion regime would further shift to unsteady combustion (USC) or even no reaction (NR) regions once the heat is overextracted. Hence, it would be a challenge for MILD combustion application in intense heat extraction scenarios, such as boilers. Interestingly, higher T in and lower X O2 are found able to widen the UMC region under larger HLR conditions. Moreover, CO2 or H2O dilution would result in a wider UMC region compared to N2 dilution, while it is more pronounced for CO2 due to its highest X O2 *. Besides, the shifting of the combustion regime from HTC to MILD combustion by heat extraction would be more effective with CO2 dilution than either N2 or H2O dilution.