New Dependence of NO Emissions on the Equivalence Ratio in Moderate or Intense Low-Oxygen Dilution Combustion

摘要

The dependence of NO emissions on the equivalence ratio (Phi) is a classic problem in combustion science. In many combustion textbooks, as Phi increases from 0.5 to 1.0, the NO formation generally increases first and then decreases, with the maximum NO emission obtained on the lean side of stoichiometry. However, through experiments and data mining of the literature, we found that there is a new trend of NO emissions in moderate or intense low-oxygen dilution (MILD) combustion. Specifically, with an increase in Phi from 0.5 to 1.0, the NO emission may decrease first and then increase, with the minimum NO emission also occurring on the lean side of stoichiometry. Systematic experiments are conducted to investigate the NO emission under MILD combustion. The experiments are conducted in a 20 kW furnace of firing methane. Detailed in-furnace thermal and species fields and exhaust NO emissions are measured. A series of MILD combustion experiments indeed record a decreasing trend of NO emissions, with an increase in Phi from 0.7 to 0.9, which is the direct opposite of traditional combustion. The experimental results are also compared with a large number of experimental data sets from the literature. Chemical kinetic calculations are performed to explain the different NO mechanisms. Moreover, the effects of local reaction factors, i.e., reaction temperature (T-r), Phi, N-2 dilution, and the mass fraction of oxygen (Y-O2), on NO emissions are further examined in detail to identify key parameters for obtaining minimal NO and CO emissions. The recommended critical reaction range for ultralow emissions of NO (= 5 ppm) and CO (= 10 ppm) is 1100 K = T-r = 1500 K, Y-O2, = 4%, and Phi near 0.85. The present study provides a more fundamental understanding of the NO emission trend with Phi for combustion science.