A comprehensive study of CO₂ addition on soot particle formation during ethylene pyrolysis in a flow reactor

Effects of CO₂ on soot particle formation during ethylene pyrolysis were investigated in a laminar flow reactor with different CO2 additions (0-99.5% in mole fraction), with the distribution of the soot particle size examined using quantitative dilution sampling. Kinetic modeling of the gas-phase reactions including soot precursor chemistry was also conducted to analyze the experiment observations. Results show that a small amount of CO2 (0 - 10%) tends to promote soot formation by increasing the total number and volume of soot particles and reducing the soot induction delay time, whereas an increase of the CO2 mole fraction from 10% to 99.5% leads to an obvious diminution of soot nucleation and mass growth rate. The simulated mole fractions of benzene and pyrene at the reactor outlet first increase and then decrease as the CO2 concentration increases. The reaction path analysis indicates that CO2 affects soot formation mainly through generating the hydroxyl radicals. A small production of hydroxyl increases the propargyl concentration, and thereby promotes the formation of soot precursors, while excessive hydroxyl leads to more significant oxidation that inhibits soot formation.