Active Sites in Soot Growth

Abstract

Soot surface growth rates in premixed flames can be described in terms of acetylene addition to active sites on the soot surface. These sites are conserved in the growth process but are lost with increasing age of the particles. Factors influencing the number of active sites available for the addition of acetylene have been investigated for a range of ethylene/air flames at atmospheric pressure. No correlation is found between local conditions, especially the concentration of hydrogen atoms and the local temperature, and the number of active sites for surface growth. Rather, the number of sites appears to be determined by the number present on the soot particles as they are first formed and by the loss of these sites by purely thermal “tempering” processes. The initial number of active sites is such that a constant fraction of the soot aerosol area is occupied by active sites, over a wide range of conditions. Comparison with other studies reveals this to be true for pressures from 1·10⁴Pa to 1·10⁵Pa; temperatures from 1600 to 2000 K; and fuels such as methane, ethylene, propane, and acetylene. Despite this common factor, it is not possible to predict the initial number of active sites. The failure to establish a correlation between active site density and the local reaction conditions, especially with regard to the initial number of sites, means that the prediction of soot surface growth rates in a diffusion flame environment remains highly uncertain. Further work is required to establish the factors influencing the initial site density and also to investigate the possibility that surface growth by addition of species of higher molecular weight might be significant.