2. PYROLYSIS EXPERIMENTS, RESULTS AND PROPOSED REACTION MECHANISMS
2.4 Mechanisms of Formation of Polycyclic Aromatic and High Molecular Weight Hydrocarbons
The fonnation of soot during the thermal degradation of hydrocarbons is the result of competition between destruction mechanisms involving free radicals and molecular weig~t growth. In pyrolysis systems. the presence of chlorine, e.g. in PCB's, increases the conversion of carbon to soot [McKinnon & Howard, 1990].
The soot fonnation process consists of four basic mechanisms:' mass growth, coagulation, oxidation and pyrolysis. each of which is dependent on the free radical concentration. Polycyclic aromatic hydrocarbons (PAH's) are considered the molecular intennediates to soot. This is supponed by the rapid rise in the PAH's concentration prior to soot nucleation. The chemical structure of soot is also very similar to that of PAH on an atomic level. The main difference between soot particles and large PAH molecules is not chemical structure, but the discreet changes in molecular weight and geometrical structure. These differences result from the reactive coagulation of large PAH molecules.
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subsequent growth of aromatic rings as the two basic parts of PAH formation. Acetylene is regarded as the species causing mass addition to the PAH and soot system. In systems where no aromatic structures exist, the first aromatic ring is usually formed by the addition of acetylene to vinyl, resulting in vinylacelylene and a hydrogen atom at high temperature:
Abstraction of a hydrogen atom from vinylacetylene occurs. The vinylacetylene radical, C .. H;h has been proposed as the key intermediate in the formation of aromatic hydrocarbons through addition to C2H2, followed by cyc1isation to phenyl radical.
Benzene and phenyl molecules are converted from one to the other by the abstraction or addition of a hydrogen molecule:
Hydrogen abstraction from an aromatic ring activates the aromatic molecule for acetylene addition, which propagates molecular growth as well as cyclisation to PAH. Several mechanisms have been identified in which acetylene can react to form an aromatic ring and then continue to fonn larger aromatic structures. The following mechanism for ring Closure was presented [Frenklach &
Wamatz, 1987}:
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-~+H
~o · +C2~ -H ~ [ J ?
+ HI- H
•
0:: . ~
CX) .. • + C2f-2 [ .
Figure 2-2: Ring closure mechanism in soot formation model
Direct condensation of aromatic rings is also important. In the high temperature pyrolysis of benzene, the reaction sequence below dominates (he initial stages ofPAH growth.
+H •
'.
+H
•+f-2
0>--< +H
•Figure 2-3; Aromatic ring condensation
Feedstock benzenoid molecules decompose primarily into acetylene as the reaction progresses, and when the concentration of acetylene approaches that of benzene, PAH growth occurs via the acetylene addition mechanism for non-aromatic fuels, as discussed above. The free energy change for the formation of certain PAH molecules such as coronene, pyrene and anthracene is so large
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PAH's persist.
Tirey er al [1990] have devised a series of olefinic and acetylenic pathways for the formation of higher molecular weight aromatic species. Molecular growth pathways involving secondary C4
radicals are also presented by Tay tar et af [1994].
Investigations into the effect of chlorine on PAH and soot formation have also been carried out by McKinnon and Howard [1990] and Marr et at [1992}. During pyrolysis, chlorine present in the reactant can be liberated as hydrogen chloride, or can be lost through attack by hydrogen atoms:
As mentioned earlier, chlorine abstraction by chlorine is unlikely due to the large endothermicity and the high strength of the C-CI bond.
It was observed that larger amounts of soot formed from chlorinated hydrocarbons'than from non- chlorinated hydrocarbons. Sooting tendency and PAH production increased with increasing chlorine: hydrogen ratio. This trend, based on kinetic analysis and modeling was attributed to enhanced Chlorine-catalysed molecular degradation. This promotes the formation of aromatic ring compounds and the large concentration of chlorine atoms accelerates the abstraction of H from stable PAH molecules, thus activating them for further growth. These results were mirrored by Marr et af [1992J and McKinnon and Howard [1990]. Marr er at [J 992) also found that the yields of cyclopenta(cd)pyrene and benzo(a)pyrene, two PAH's that are major contributors to biological activity in human cell mutagenicity and carcinogenicity testing, are greatly suppressed with chlorine present.
Ritter and Bozzelli [1990] carried out experiments on the pyrolysis of chlorobenzene in hydrogen and helium make-up gases. It was suggested that the reason for chlorobenzene's acceleration of soot formation is the role of the chlorine product in the reaction system. The ability of soot to capture high molecular weight PAH is increased in the presence of chlorine. Kern er af [1992]
came to the same conclusion in their study of chlorobenzene pyrolysis.
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It is thus conclusive that, during the high temperature treatment of organic waste, higher
chlorine: hydrogen ralios promote saoting and PAH fonnation. It follows that chlorine: hydrogen ratios less than one are required to inhibit sooling.