A numerical study of accelerated moderate or intense low-oxygen dilution (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-NO X 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 ( T f ), CO and NO emissions, as well as the evolution of chemical flame. The comparison reveals that OSC can extend the lowest operational T f from 900 K to 800 K. More importantly, OSC can significantly improve the ignition stability in the whole range of T f as compared to the original MILD combustion burner.