Combustion Models and the prediction of Polycyclic Aromatic Hydrocarbons (PAH) in laminar flames of simple hydrocarbons: Methane, Methyl Chloride and, 1,1 Dichloroethane

Detailed chemical kinetic models (DCKM) have been assembled for the combustion of methane, methyl chloride and1,1 dichloroethane in fuel-rich premixed laminar flames. The methane model consists of over 100 species and 600 elemetary chemical reactions, this model was incorporated into the chlorinated hydrocarbon (CHC) model resulting in over 150 species and more than 1500 elementary reactions. The models incorporate known and new chemistry for the formation of polycyclic aromatic hydrocarbons (PAH) in flames, a major objective. Because of the hierarchical nature of combustion chemistry these models are very important building blocks for DCKM of more complex fuels.

The DCKM were "optimized" for major, intermediate, trace species and PAH using experimental data generated at UCLA . The optimization procedure involved reaction path and sensitivity analysis for the species of interest. The thermochemistry of PAH and kinetic coefficients for their reaction were estimated using computational quantum chemistry.

The most important findings of the study can be summarized as follows:

1. The combustion models capture well the major features of combustion of methane, methyl chloride, and 1,1 DCE in laminar flames under fuel-rich conditions. Due to the large size of the CHC model more testing is required for its optimization. No attempt was made to simplify the model.

2. For the flames under consideration formation of benzene and naphthalene can not be explained by acetylene addition chemistry alone. To fully explain benzene formation and growth to naphtahlene reactions involving propargyl radical, a 3-carbon molecule,must be included in the model. New chemistry was developed for this purpose.

3. Aryl radicals are involved in the formation of PAH, and they constitute a new path for PAH formation. The chemistry was developed in the study.

4. Because of low concentration, aromatic growth involving a 4-carbon molecule such as C4H3 for example is not significant. Chemical paths are proposed.

5. For the formation of cyclopentadiene, a 5-carbon cyclic molecule, reactions involving the phenoxy radical produced by oxidation of phenyl result in the most significant contribution to its formation. Formation of cyclopentadiene via reaction of propargyl radical with acetylene and from benzyl radical are minor paths.

6. Hydrogen atom additions to the aromatic ring, catalize the isomerization of naphthalene to azulene, and isomerization of anthracene to phenanthrene. Hydrogen addition to benzene with the subsequent isomerization to cyclopentadiene is a minor path for cyclopentadiene formation in the flame. The chemistry was developed in the study.



Major features of a Premixed Methane Flame



1-ring Aromatic Concentrations in a Premixed Methane Flame



2-ring PAH concentration profiles in a Methane Premixed Flame

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