A numerical study of accelerated MILD combustion stability for methane in a lab-scale furnace by off-stoichiometric combustion technology

Abstract Moderate or intense low-oxygen dilution (MILD) combustion has become a promising low-NOX emission technology, while the delayed mixing of reactants and slower oxidation rate could potentially cause ignition instability in some scenarios. This paper proposes a new idea for enhancing the ignition stability for methane MILD combustion by combining with off-stoichiometric combustion (OSC), and its performances have been numerically assessed through a comparison against the original MILD combustion burner. The results reveal although non-premixed pattern has the lowest NO emission, it suffers from a larger liftoff distance, thus less ignition stability. Contrarily, both partially-premixed and fully premixed patterns exhibit excellent ignition stability. Among the considered OSC conditions, the pattern of Inner ultra-rich and Outer lean produces the lowest NO emission while maintains a high ignition stability. Furthermore, the enhancement of the combustion stability by implementing OSC to the original MILD combustion burner is shown by comparing the operational range of furnace wall temperature (Tf), CO and NO emissions, as well as the evolution of chemical flame. The comparison reveals that OSC can extend the lowest operational Tf from 900 K to 800 K. More importantly, OSC can significantly improve the ignition stability in the whole range of Tf as compared to the original MILD combustion burner.