Effects of space-confined ratio on the MILD combustion of coke oven gas

Abstract

The objective of this study is to investigate the effects of the spatial structure on MILD combustion characteristics of coke oven gas via numerical simulation. The space-confined ratio, defined as the ratio of the furnace height to the longest diagonal of the transverse section, is used to characterize the variation in spatial structure. An experimental study in a long-narrow confined furnace (LNCF) was initially conducted to investigate the conditions for achieving MILD combustion. Subsequently, the effects of spatial structure on MILD combustion characteristics were thoroughly investigated through numerical simulation by altering the space-confined ratio of the LNCF. The experimental results show that the transition from conventional combustion to MILD combustion is achieved at a dilution rate of 50 %, which improves the temperature uniformity and reduces the NO emission by 96.5 % to 3 ppm. The numerical results show that reducing the space-confined ratio enhances the flue gas internal recirculation and the temperature uniformity, especially the maximum temperature is significantly reduced. Moreover, reducing the space-confined ratio achieves nearly zero NO emissions, but an excessively space-confined ratio may result in incomplete combustion, consequently causing a significant rise in CO emissions. To guarantee complete combustion, the optimal furnace structure is achieved at a space-confined ratio of 6.4, ensuring not only temperature uniformity but also ultra-low emissions of NO and CO. Therefore, this study provides a theoretical and practical foundation for the ultra-low emission of pollutants in space-confined combustion systems.