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Soot Oxidation
Oxidation of soot takes place during each stage of soot formation i.e., during formation of precursors, nuclei, primary soot particles and aggregates. A large fraction of soot that forms early in the combustion process is oxidized within the cylinder. Reactions with O, O2, OH, CO2 and H2O oxidize soot. But, O, O2 and OH are the main oxidants. OH radical plays an important role in soot oxidation in fuel rich conditions that exist in diffusion combustion systems. The OH radical is present in significant concentrations in the reaction zone of diffusion flames and oxidizes soot in the flame region. In the post combustion gases, O2 is present in abundance and is the main oxidant. Rate of oxidation is also influenced by the shape and size of the soot particles. As the particles grow in size they tend to agglomerate instead of coagulation, and form clusters or chains. The oxidation of chain structure by O2 is much slower than by OH.
Most widely used soot oxidation model in engines is that by Nagle and Strickland-Constable (NS-C). The NS-C soot oxidation mechanism was obtained from studies on oxidation of graphite by oxygen at 1000 to 2000º C. The NS-C mechanism assumes presence of two types of oxidation sites on soot particle surface. Site A is a nascent surface not covered by surface oxides and is more reactive. The site B is covered with oxides and is less reactive. The sites A change to sites B depending upon temperature and reaction time. The surface area fraction, x occupied by the sites A is calculated based on a rate constant, kD. The NS-C mechanism for soot oxidation is given by
where;
From the soot oxidation rate, w the rate at which radius of a spherical particle is reduced may be calculated as below,
 , the density of soot may be taken equal to 1.8 g/cm3.
From this relation the time required to completely burn a spherical particle can be calculated. Using the NS-C mechanism as above, at 1500 K it would take about 30 ms to completely burn a particle of 30 nm diameter.
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