Formation of soot particles in methane and ethylene combustion: A reactive molecular dynamics study

A numerical analysis of the complete soot formation process was conducted in the combustion condition with methane and ethylene as initial fuel and oxygen as oxidant by applying the reactive molecular dynamics. The relative time of different periods during soot formation was quantitatively analyzed by a normalized time (t/tg), and the effects of fuel type, temperature, and equivalent ratio on soot formation were further studied. The simulation results showed that there is little difference of the normalized time (t/tg) of the different periods during soot formation between CH4 and C2H4 combustion. The normalized time of the periods for fuel pyrolysis and initial PAHs formation, soot nucleation, and soot surface growth and particle coalescence are 27.7%, 12.3%, and 60%, respectively. The carbon atom number and C/H ratio of the final soot particles in ethylene combustion were approximately twice that of methane combustion, which was due to the high acetylene concentration during ethylene combustion promoting the formation of initial polycyclic aromatic hydrocarbons (PAHs). Increasing temperature significantly accelerated soot formation mainly by promoting soot coalescence process, while excessive temperature inhibited soot formation by accelerating soot fragmentation. Decreasing equivalence ratio mainly inhibited soot formation by reducing the concentrations of acetylene and PAHs during fuel pyrolysis and initial PAHs formation period. Assuming CH3 and C2H3 free radical as the initial fuel could speed up soot formation process, while the hydroxyl (OH) had no obvious effect on soot formation process.