Effects of Preheating and CO₂ Dilution on Oxy-MILD Combustion of Natural Gas

Oxy-moderate or intense low-oxygen dilution (MILD) combustion, which is a novel combination of oxy-fuel technology and MILD regime, is numerically studied in the present work. The effects of external preheating and CO₂ dilution level on the combustion field, emission, and CO formation mechanisms are investigated in a recuperative laboratory-scale furnace with a recirculating cross-flow. Reynolds-averaged Navier–Stokes (RANS) equations with eddy dissipation concept (EDC) model are employed to perform a 3-D simulation of the combustion field and the turbulence–chemistry interactions. In addition, a well-stirred reactor (WSR) analysis is conducted to further examine the chemical kinetics of this combination when varying the target parameters. The simulations used the skeletal USC-Mech II, which has been shown to perform well in the oxy-fuel combustion modeling. Results show that with more preheating, the uniformity of temperature distribution is noticeably enhanced at the cost of higher CO emission. Also as inlet temperature increases, the concentration of minor species rises and CO formation through the main path (CH₄→CH₃→CH₂O→HCO→CO→CO₂) is strengthened, while heavier hydrocarbons path (C₂H₂→CO) is suppressed. Meanwhile, greater CO₂ addition notably closes the gap between maximum and exhaust temperatures. In a highly CO₂-diluted mixture, chain-branching reactions releasing CH₂O are strengthened, while chain-terminating reactions are weakened. CH₂O production through CH₃O is accelerated compared with the straight conversion of methyl to formaldehyde. When diluting the oxidant, methylene CH₂(s) plays a more influential role in CO formation than when pure oxygen is used, contributing to higher CO emission.