Numerical study of methane combustion under moderate or intense low-oxygen dilution regime at elevated pressure conditions up to 8 atm

Abstract

This paper investigates the effect of operating pressure (up to 8 atm) on the moderate or intense low-oxygen dilution (MILD) combustion of methane. Computational fluid dynamics (CFD) modeling was firstly conducted for a laboratory-scale combustor to obtain the distributions of temperature and minor species (CH2O and OH), and pollutants (CO and NO) emission under elevated operating pressures. To provide further insight into the heat release characteristics, flame structure, CO formation mechanism as well as extinction limit, a counterflow diffusion flame with hot oxidizer diluted oxidizer configuration was also modeled using detailed chemistry. The CFD modeling results show that increasing the operating pressure tends to increase both peak value and spatial gradient of the combustion temperature, and reduce the width of the reaction zone, indicating the departure of MILD combustion regime when using the identical burner. However, enhancing the internal flue gas recirculation by minimizing the air nozzle diameter can help sustain the MILD combustion regime. The kinetic modeling shows that MILD combustion features only positive heat release region, while traditional combustion exists both negative and positive heat release regions, where multiple peaks are observed. In addition, MILD combustion shows a dual oxygen consumption stage regardless of operating pressures. Simultaneously, MILD combustion exhibits a stretched S-curve of flame response without extinction and re-ignition processes, while traditional combustion presents obvious extinction and ignition points. These unique phenomena can be used for identifying MILD combustion regime in future work.