NOx emission characteristics of counterflow syngas diffusion flames with airstream dilution

Syngas is produced through a gasification process using variety of fossil fuels, including coal, biomass, organic waste, and refinery residual. Although, its composition may vary significantly, it generally contains CO and H2 as the dominant fuel components with varying amount of methane and diluents. Due to its wide flexibility in fuel sources and superior pollutants characteristics, the syngas is being recognized as a viable energy source worldwide, particularly for stationary power generation. There are, however, gaps in the fundamental understanding of syngas combustion and emissions, as most previous research has focused on flames burning individual fuel components such as H2 and CH4, rather than syngas mixtures. This paper reports a numerical investigation on the effects of syngas composition and diluents on the structure and emission characteristics of syngas nonpremixed flames. The counterflow syngas flames are simulated using two representative syngas mixtures, 50%H2/50%CO and 45%H2/45%CO/10%CH4 by volume, and three diluents, N2, H2O, and CO2. The effectiveness of these diluents is characterized in terms of their ability to reduce NOx in syngas flames. Results indicate that syngas nonpremixed flames are characterized by relatively high temperatures and high NOx concentrations and emission indices. The presence of methane in syngas decreases the peak flame temperature, but increases the formation of prompt NO significantly. Consequently, while the total NO formed is predominantly due to the thermal mechanism for the 50%H2/50%CO mixture, it is due to the prompt mechanism for the 45%H2/45%CO/10%CH4 mixture. For both mixtures, CO2 and H2O are more effective than N2 in reducing NOx in syngas flames. H2O is the most effective diluent on a mass basis, while CO2 is more effective than N2. The effectiveness of H2O is due to its high specific heat that decreases the thermal NO, and its ability to significantly reduce the concentration of CH radicals, which decreases the prompt NO. The presence of methane in syngas reduces the effectiveness of all three diluents.