Experimental and Numerical Study of the Effect of Fuel/Air Mixing Modes on NOx and CO Emissions of MILD Combustion in a Boiler Burner

Abstract

The Moderate or Intense Low-oxygen Dilution (MILD) combustion is characterized by low emissions, stable combustion and low noise for various kinds of fuel, which has great potential in the industry. The aim of this study is to investigate the effect of fuel/air mixing modes on NOx and CO emissions of MILD combustion in a boiler burner by experiments and numerical simulations. Three types of fuel/air mixing modes (premixing mode, diffusion mode and hybrid mode) have been considered in this study. The realizable k-e turbulent model and the Eddy Dissipation Concept (EDC) combustion model were used in numerical simulations. In addition to the temperature near the burner head, the calculation results match very well with the axial temperature distribution at the furnace center. The flow pattern under different mixing modes is similar, while the hybrid mode has a higher OH concentration near the diffusive fuel nozzle than the premixing mode, and the corresponding position of the peak OH concentration is closer to the rear half of the furnace. The distribution of temperature is extremely uniform for the premixing mode in the main reactive zone, which is typical for MILD combustion. There is a distinct area where the reaction temperature is higher than 1600 K for the hybrid mode. Moreover, in the main reactive zone, the gas recirculation ratio is high enough to ensure flue gas recirculation, which is beneficial to achieve MILD combustion at local areas. At the location where the axial distance is greater than 0.2 m, the gas recirculation ratio of the premixing mode is larger than that of the hybrid mode, which strengthens the entrainment of hot flue gas into the recirculated gas. The experimental results show that when the thermal intensity is less than 1.67 MWm−3, the NOx emissions are less than 15×10−6@3.5%O2 in near stoichiometric ratio in the premixing mode, and the CO emissions are less than 10×10−6@3.5%O2 under the same conditions. In the diffusion mode, the NOx emissions are less than 30×10−6@3.5%O2. In order to keep NOx and CO emissions low, the hybrid modes with optimized fuel distribution ratio are found under different thermal intensities.