The role of reactive PAH dimerization in reducing soot nucleation reversibility

Nucleation of incipient soot or carbon black nanoparticles has significant scientific and industrial importance because it influences the particle size distribution, morphology and composition, hence its health and environmental impact as well as functional properties. Reversible Polycyclic Aromatic Hydrocarbon (PAH) Clustering (RPC) with van der Waals forces (vdW) was proposed recently as opposed to Irreversible PAH Clustering (IPC) for soot nucleation (Eaves et al., Proc. Combust. Inst, 35 (2015) 1787) to relax the assumption of stable dimer formation with physical bonds at flame temperatures. Here, the necessity of considering chemical bond formation between PAHs in a dimer for reducing soot nucleation reversibility in ethylene coflow diffusion flames with a wide range of nitrogen dilution ratios is demonstrated. An RPC model with Chemical Bond Formation (RPC-CBF) is developed and its performance is compared to those of RPC and IPC models. Only the RPC-CBF model with low reversibility for PAH addition on the surface of soot primary particles can predict soot volume fraction, average primary particle diameter, primary particle number density and the bimodal size distribution of soot agglomerates on the flame centerline within experimental uncertainty. While the IPC model overpredicts soot concentrations by a factor of five with increased dilution, the RPC model underpredicts it by more than two orders of magnitude for the flame with 68% nitrogen dilution. The RPC model fails to predict the observed bimodality of agglomerate particle size distribution in flames with low dilution because its predicted nucleation rate is much weaker compared to that of growth. Accounting for chemical bond formation for dimers is essential to form enough nuclei with increasing dilution and temperature. Also, low reversibility for PAH addition is required to predict the balance between nucleation and surface growth, hence the average size of soot primary particles. Crown Copyright (C) 2018 Published by Elsevier Inc. on behalf of The Combustion Institute. All rights reserved.