Experimental and numerical study of MILD combustion characteristics in the long-narrow confined space with different types of flue gas dilution

MILD combustion is extensively applied in wide confined spaces as a new ultralow NOx emission technology but is rarely reported in long-narrow confined spaces. To address this, as exemplified by a coke oven heat flue, a flue gas dual-dilution system in a long-narrow confined space was constructed to achieve MILD combustion, where combustion characteristics such as temperature distribution, flow field, flame morphology and NOx emission were assessed through experiments and numerical simulations. The results indicate that when the dilution rate reaches 25%, single-fuel dilution can achieve unstable MILD combustion, while single-air dilution is still in diffusion combustion, and the corresponding NOx emissions are reduced to 371 ppm and 325 ppm, respectively. When the fuel/air dilution ratio is 10/15 in flue gas dual-dilution, the flame front disappears completely, the flame volume ratio reaches 41.9%, and MILD combustion is established. The corresponding NOx emission is reduced to 58 ppm, which is 95.0% lower than conventional combustion and achieves ultralow NOx emission. An in-depth analysis proves that flue gas dual dilution is the key to achieving MILD combustion in a long-narrow confined space, which is irrelevant to the construction of the flue gas internal recirculation rate.