Experimental Investigation of Conventional and MILD Combustion of Coke Oven Gas Under Different Dilution Types and Preheating Temperatures in a Long-Narrow Confined Furnace

Moderate or intense low-oxygen dilution (MILD) combustion has garnered significant attention for its high temperature uniformity and ultra-low NOx emissions in recent years. To achieve ultra-low NOx emissions in the coking industry, a laboratory-scale long-narrow confined furnace (LNCF) system based on the coke oven heat flue was established to investigate the combustion characteristics of conventional and MILD combustion of coke oven gas (COG) in a long-narrow confined space. Different operating parameters (air excess ratio, dilution type and preheating temperature [T_ox]) were carried out. The results show that the optimal temperature uniformity is achieved at an air excess ratio of 1.2 in conventional combustion. A comparative analysis of different dilution types revealed that single-fuel dilution (SFD) and flue gas dual-dilution (FGDD) are more effective than single-air dilution (SAD) in achieving MILD combustion, and reducing the T_ox can result in a reduction in the critical dilution ratio (θ). The temperature uniformity ratio is increased in conventional combustion but reduced in MILD combustion by increasing the T_ox, indicating that increasing the T_ox effectively enhances the temperature uniformity in MILD combustion. The optimal temperature uniformity in MILD combustion can be obtained through FGDD at T_ox = 873 K and θ = 50%. FGDD is more effective for mixing intensity than SFD and SAD, promoting complete reactions between reactants and effectively reducing CO emissions. Compared with SAD, SFD, and FGDD are more conducive to achieving clean NO emissions in the MILD combustion regime. Clean MILD combustion can be achieved at 573 K ≤ T_ox ≤ 673 K and θ = 50%, regardless of the dilution type.