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

ABSTRACT

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-O₂-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 NO_X 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.

KEYWORDS:

• CO₂ dilution
• OH chemiluminescence
• Oxygen enrichment
• Pollutant emissions
• Swirling flame
• Turbulent flame

Acknowledgments

This work is supported by the ANR (Agence Nationale de la Recherche), the CNRS, and the University of Orléans. The help and useful discussions with Marc MORLAT from the University of Orleans are gratefully acknowledged.