Nucleation rate of carbonaceous nanoparticles by n-heptane pyrolysis at high pressure and temperature via molecular dynamics simulations

Chemical nucleation of carbonaceous nanoparticles is investigated during pyrolysis of n-heptane at high temperature (2200-2600 K) by reactive molecular dynamics (MD) simulations. The MD-derived n-heptane consumption rate is in agreement with kinetic modeling, validating the present work at high temperature and high fuel concentration conditions. The critical nucleus size is quantified by the free formation energy at 2200-2600 K for n-heptane concentrations ranging from 3 x 1020-9 x 1020 #/cm3. Increasing temperature leads to smaller critical size, starting from 59 +/- 7 carbon atoms at 2200 K that decreases down to 33 +/- 3 carbon atoms at 2600 K, while the fuel concentration hardly affects the critical nucleus size. The onset time of nucleation decreases exponentially with temperature, consistent with previous shock tube pyrolysis experiments. The nucleation rate is obtained by the rate of formation of critical and supercritical hydrocarbon molecules. An Arrhenius-type relationship between the nucleation rate and the process temperature is proposed, exhibiting a first-order dependency to the initial fuel concentration. The number density of carbon nuclei derived by this nucleation rate is four to five orders of magnitude higher than that obtained by kinetic models for soot nucleation by reactive polyaromatic hydrocarbon (PAH) dimerization. The present MD-derived nucleation rate provides a computationally efficient pathway to model carbonaceous nanoparticle formation dynamics without relying on individual chemical reaction rate constants or on computationally expensive PAH-based models.