Effects of wall temperature on methane MILD combustion and heat transfer behaviors with non-preheated air

Abstract

Owing to its ultra-low NO emission, moderate or intense low-oxygen dilution (MILD) combustion is expected to be applied in the boilers or kilns under a strong wall heat extraction condition, especially with non- or lowly-preheated air. In this paper, the combustion stability and heat exchange behaviors for the conventional and MILD combustion modes were numerically investigated with non-preheated air in a lab-scale furnace; simultaneously, the effect of wall heat transfer on MILD combustion also was discussed by gradually decreasing wall temperature (Twall). Results show that as Twall reduces, an enhanced wall heat transfer, a longer ignition delay time and a slower kinetic rate of R99 (CO2 + H ↔ CO + OH) are produced under MILD combustion. Interestingly, CO emission decreases firstly and then increases as Twall is reduced from 1800 to 950 K in regardless of combustion modes. Lowering Twall would help to achieve MILD combustion in a certain degree due to the extended reaction region; however, MILD combustion stability becomes highly poor and eventually the flame extinguishes if Twall is limited below 950 K, while the conventional combustion can be sustained at cold state (Twall = 300 K). Furthermore, MILD combustion has a larger total heat flux comparing to the conventional mode, mainly from the higher radiative heat transfer.