Soot formation and evolution for different carbon dioxide additions at high temperature by ReaxFF molecular dynamics

The chemical effect of carbon dioxide (CO₂) addition on soot formation in flames is significant for developing cleaner combustion technologies, but its atomistic mechanisms remain elusive. This work investigates the formation and evolution of soot nanoparticles from polycyclic aromatic hydrocarbons (PAHs) in ethylene flames under different CO₂ additions using ReaxFF molecular dynamics (MD) simulations. The simulation reveals a three-stage process from PAHs to curved fullerene-like soot particles: nucleation, surface growth/coagulation, and graphitization. The results demonstrate that CO₂ effectively suppresses soot mass growth and nucleation primarily by consuming H radicals to reduce the reactivity of PAH precursors, while enhanced concentrations of oxidizers (OH and CO₂) promote the oxidation of soot nanoparticles.

The initial atomic configurations were constructed and energy-minimized using Materials Studio. All molecular dynamics simulations were performed using the LAMMPS software package with the ReaxFF reactive force field which parameters were optimized for large hydrocarbon interactions. The initial system comprised a mixture of PAH molecules, small species (C₂H₂, H, OH), and varying amounts of CO₂, based on compositions derived from chemical kinetic calculations. Simulations were conducted in the NVT ensemble at 3000 K for 1 ns. To ensure statistical reliability, each simulation case was run with three independent repetitions, and the results reported for analysis (e.g., carbon atom counts, radical populations) represent the average values. Chemical reaction pathways were analyzed using the ChemTraYzer program, and visualization was performed with VMD.