Morphology and mobility diameter of carbonaceous aerosols during agglomeration and surface growth

Agglomeration and surface growth of fractal-like carbonaceous aerosols in the absence of oxidation and condensation of volatiles are investigated by Discrete Element Modeling (DEM) from the free molecular to transition regime, accounting for primary particle polydispersity and chemical bonding (aggregation). That way, carbon black or soot formation is elucidated in plasma reactors, quenched flames and combustion engines running at high exhaust gas recirculation where oxidation of carbon is limited or non-existent. During nascent soot formation polydisperse spheres and aggregates are formed, attaining an asymptotic geometric standard deviation of 1.2 ± 0.01, in agreement with experiments in premixed, diffusion flames and wood combustion. When surface growth stops, mature soot grows by agglomeration of these polydisperse spheres and aggregates before oxidation and condensation take over. The evolution from nascent to mature soot morphology quantified by its fractal dimension, D_f, and mass-mobility exponent, D_fm, is in good agreement with measurements in premixed, diffusion flames and the Cast soot generator. The mobility diameter, d_m, can be related to the average number of primary particles per agglomerate by n_va = (d_m/d_va)^0.45 and the relative effective density of soot by ρ_eff/ρ_s = (d_m/d_va)^−0.78, where d_va is the primary particle diameter and ρ_s the soot bulk density.