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

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 (T-wall). Results show that as T-wall 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 T-wall is reduced from 1800 to 950 K in regardless of combustion modes. Lowering T-wall 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 T-wall is limited below 950 K, while the conventional combustion can be sustained at cold state (T-wall = 300 K). Furthermore, MILD combustion has a larger total heat flux comparing to the conventional mode, mainly from the higher radiative heat transfer.