Performance Improvement of Methane oxy-MILD Combustion with High Initial Oxygen Contents Using a Novel Non-premixed Oxygen/Recycled Flue Gas Jet Burner

Oxy-fuel moderate or intense low-oxygen dilution (oxy-MILD) combustion is a promising technology to control NO_x emissions while achieving large-scale CO₂ capture in gas-fired industrial furnaces. However, oxy-MILD combustion still presents challenges at high initial oxygen levels, where less satisfactory thermal uniformity and even high NO_x emissions may occur. To address these challenges, this paper reports a novel non-premixed oxygen/recycled flue gas (O₂/RFG) jet burner, whose novelty lies in that O₂ and RFG are separately supplied (where RFG serves as a “barrier gas” between fuel and oxygen jets to delay their mixing before reaction) rather than premixed with each other conventionally. Meanwhile, an improved GRI-Mech 3.0 model is proposed to predict prompt-NO formation via the NCN pathway more accurately by adding the NCN, HNCN, and HNC reaction subsets. Using the improved GRI-Mech 3.0 model, the performances of the non-premixed O₂/RFG jet burner and its difference from those of the premixed one during methane oxy-MILD combustion are evaluated at different oxygen concentrations of 25–35 vol % in a laboratory-scale furnace. In particular, NO formation and reduction via thermal, prompt, N₂O-intermediate, NNH, and reburning pathways are revealed in addition to combustion and heat transfer characteristics. Results show that, compared to the premixed one, the non-premixed O₂/RFG jet burner can sustain oxy-MILD combustion at a higher initial oxygen level, where better temperature/heat flux uniformity is obtained in a larger reaction zone. What’s more, NO emissions can be reduced by 14.8–64.9% if air leakage occurs, mainly due to less NO formation via N₂ + O → NO and N₂O + H/O → NO in the thermal and N₂O-intermediate pathways; also, the efficiency of NO reduction via HCCO/CH_i=0–3 + NO reactions is enhanced by 2.3–8.6% when doping 100–800 ppm of NO in the oxidizer. In conclusion, the non-premixed O₂/RFG jet burner is recommended to be used to help establish/sustain oxy-MILD combustion at high initial oxygen levels in industrial furnaces, where further NO emission reduction can be achieved while improving thermal uniformity.