Formation of Soot and Elemental Carbon

Carbonaceous particles are a byproduct of the combustion of liquid or gaseous fuels. Particles formed this way consist of both EC and OC and are known as soot. Soot particles are agglomerates of small roughly spherical elementary carbonaceous particles. While the size and morphology of the clusters vary widely, the small spherical elementary particles are remarkably consistent from one to another. These elementary particles vary in size from 20 to 30 nm and cluster with each other, forming straight or branched chains. These chains agglomerate and form visible soot particles that have sizes up to a few micrometers. 

Atmospheric Chemistry and Physics From Air Pollution to Climate Change, Second Edition, by John H. Seinfeld and Spyros N. Pandis. Copyright 2006 John Wiley Sons, Inc. 

The formation of soot depends critically on the carbon/oxygen ratio in the hydrocarbon-air mixture. Let us assume for the moment that the mixture has insufficient oxygen to form CO2 and that CO is the combustion product of the fuel CmH . Then the combustion stoichiometry is 

Soot forms in a flame as the result of a chain of events starting with the oxidation and/or pyrolysis of the fuel into small molecules. Acetylene, C2H2, and polycyclic aromatic hydrocarbons (PAHs) are considered the main molecular intermediates for soot formation and growth (McKinnon and Howard 1990). The growth of soot particles involves first the formation of soot nuclei and then their rapid growth due to surface reactions (Harris and Weiner 1983a,b). 

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