The role of CO2 dilution on soot formation and combustion characteristics in counter-flow diffusion flames of ethylene

The effect on soot formation of carbon dioxide addition as diluent in ethylene/O2/Ar laminar counter flow diffusion flames has been examined both experimentally and with the help of a multi-sectional kinetic model. Different concentrations of CO2 have been used in the oxidizer stream to replace argon, whereas the amount of oxygen has been kept constant. Optical techniques have been adopted to measure particulate matter: laser-induced fluorescence (LIF) to detect carbon nanoparticles and laser-induced incandescence (LII) to detect soot. Significant reductions of both soot and carbon nanoparticles have been observed with the addition of CO2: by increasing the amount of CO2, the reduction of particulate matter increases. In the case of 100% CO2 as diluent, soot particles are completely depleted, whereas carbon nanoparticles are still formed but at a lower rate. Therefore, the net result of CO2 addition is the reduced emission of total particulate matter enriched in carbon nanoparticles. On the basis of the results of the kinetic model, particle reduction has been mostly attributed to a thermal effect due to the reduced adiabatic flame temperatures at increasing amounts of CO2 added and only partially to the chemical effect of CO2 in enhancing OH radical formation.