Effects of CO2 Dilution and O2 Enrichment on Non-premixed Turbulent CH4-Air Flames in a Swirl Burner

Effects of oxygen enrichment and CO2 dilution on the characteristics of non-premixed methane-air turbulent flames in a coaxial swirl burner are investigated in this article. The work primarily focuses on pollutant emissions (NOx and CO), flame structure, and stability investigations. The experiments are conducted using a 1-m high, 0.5-m wide, 25-kW parallelepiped combustion chamber, cooled by outside water. The burner configuration consists of two concentric tubes in which a swirler is placed inside the annular part for air or air-O-2-CO2 supply that can allow for the rotation of the oxidant. Fuel is injected radially from the central tube. OH* chemiluminescence measurements are performed to describe the structure and stability of the flame providing information on the flame lengths and lift-off heights. The measurements are conducted with the oxygen concentrations ranging from 21% to 35%, CO2 concentrations ranging from 0 to 20%, swirl numbers ranging from 0.8 to 1.4, and global equivalence ratios ranging from 0.8 to 1. The lift-off heights, the fluctuations of the flame base, and the flame lengths are determined as a function of these parameters. The results show that oxygen enrichment stabilizes better the flame, promotes the formation of NOX and CO2, and decreases the formation of CO. The dilution by CO2 changes significantly the flame shape and its behavior. The flame becomes longer, less intense, and unstable. CO2 dilution reduces considerably the flame temperature which obviously reduces the NOx formation, but it is observed that CO2 and CO concentrations in the flue gases are increased.