Soot Formation Stages
Soot formation consists of four main stages viz,
- Inception and nucleation,
- Surface growth,
- Coagulation and Agglomeration, and
- Oxidation.
Soot Inception and Nucleation
Soot nucleation is a non-equilibrium process and is not yet fully understood. The nucleation process appears to consists of several processes viz.,
- Thermal cracking or pyrolysis of long chain fuel molecules in an environment of oxygen deficiency. During this process the large fuel molecules are broken into smaller molecules forming soot precursors.
- Condensation reactions and polymerization that result in larger molecules.
- The third process is dehydrogenation process that increases C/H ratio of fuel molecules enroute to soot formation.
The path to soot nucleation and formation also depends on temperature as below;
- At T < 1800 K, condensation reactions of aromatics and pyrolysis of other highly unsaturated hydrocarbons are likely to form soot.
- At intermediate temperatures, 2800 > T > 1800 K, the thermal cracking leading to fragmentation of fuel molecules to smaller unsaturated hydrocarbon molecules followed by polymerization dominates soot nucleation. Acetylene, poly acetylenes and poly unsaturated hydrocarbon radicals are formed due to fragmentation reactions that lead to formation of soot.
- At still higher temperatures T > 3000K the vapour phase condensation of molecules occurs to form soot nuclei.
The aromatic condensation route for soot nucleation is a more direct and faster route. The fragmentation to smaller molecules and polymerization to soot nuclei is a slower route.
Of all the products of fragmentation reactions and pyrolysis the acetylenes and poly aromatic hydrocarbons are considered the most likely precursors leading to soot formation. These gas-phase species on condensation reactions produce soot nuclei. Considering acetylene as an important soot precursor the formation of large ring structures, i.e., PAHs leading finally to soot formation has been proposed that involves the following three types of reactions known as Hydrogen-abstraction-C2H2 -addition (HACA) mechanism:
Reaction 1:
Hydrogen atom is removed from an aromatic molecule (AR) by a free hydrogen atom to form H2 and aromatic radical (AR*);
Reaction 2:
Addition of an acetylene to AR*;
Reaction 3:
Cyclization to form aromatic ring
Large PAH molecules are formed as this process repeats many times. The various PAHs formed polymerize to form a solid phase soot nuclei of 1 to 2 nm diameter. These soot nuclei on further surface growth produce soot particles.
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