Impact of Oxygen Enrichment and CO2–H2O Dilution on Stability and Pollutant Emissions of Non-Premixed Swirling Turbulent Flames

Abstract

The aim of this work is to investigate the effect of exhaust gas recirculation (EGR: water vapor and CO2), with and without O2 enrichment, on non-premixed turbulent flames stabilized on a swirl burner. The motivation includes CO2 capture applications using O2 and CO2, combustion of biogas that containing CO2 and the use of EGR or H2O in certain industrial applications to reduce pollutant emissions. Experiments were carried out on a coaxial swirl burner placed in a combustion chamber of 25 kW nominal power. The oxidant (air-O2, + H2O, + CO2) is introduced in the annular part though a swirler. The fuel (CH4) is fed though the central tube and injected radially at the exit section. The study focuses on laminar burning velocity, pollutant emissions, flame stability, and flow fields measurements with different fractions of O2, H2O and CO2 in the mixture. The fraction of diluents is varied from 0 to 20%, O2 concentration from 21 to 25% (in vol.) and the swirl number from 0.8 to 1.4. Different measurements are recorded: OH* chemiluminescence to locate the flame front, Stereo-PIV to analyze the flow field, pollutant emissions analysis (NOx and CO) and temperatures in the combustion chamber. Results show that dilution significantly influences flame characteristics. Dilution increases the lift-off height and reduces flame stability especially with high fractions (16–20%), whereas O2 enrichment decreases lift-off height and enhances flame stability. The increase dilution reduces NOx and increases CO emissions. Stereo-PIV measurements reveals the turbulent coherent structure of the swirling flow as well as the effect of dilution on the corresponding axial and tangential velocities. The effect of dilution on the underlying laminar burning velocity were determined by 1D calculation using COSILAB with GRI3.0 mechanism.