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To understand in more detail the formation of polycyclic aromatic hydrocarbons (PAH) from complex fuels, we have performed pyrolysis experiments in a laminar-flow reactor, using the model fuel catechol (ortho-dihydroxybenzene), a phenol-type compound representative of structural entities in tobacco, coal, and wood. Catechol pyrolysis at temperatures of 700 to 1000 °C and residence times of 0.4 to 1 s produces a range of aromatic products, which have been analyzed by gas chromatography with flame-ionization detection and by high-pressure liquid chromatography with diode-array ultraviolet-visible absorption detection. Of the 64 aromatic products identified, yields are reported for the 30 species whose yields are at least 0.005% of the mass of fed catechol, over a significant temperature range. The quantified products fall into 8 structural categories: benzene, benzenoid PAH, indene benzologues, fluoranthene benzologues, cyclopenta-fused PAH, ethynyl-substituted aromatics, alkyl-substituted aromatics, and vinyl-substituted aromatics. In general, the more prominent products within a particular structural class are more prominent at all temperatures examined, and the most prominent product in a class is usually 10 times more prevalent than other compounds in the same class. The product quantifications show that at 900-950 °C, the aromatic products account for up to 22% of the mass of fed catechol. At these higher temperatures, and all the way up to 1000 °C, there are also more quantifiable products that are the result of multiple ring-buildup steps (e.g., the larger benzenoid PAH) as well as an increase in the number and relative quantity of ethynyl-aromatics and cyclopenta-fused PAH. At the lower temperatures, indene is produced at especially high yield, indicative perhaps of a particular facility for the formation of indene from catechol. Also readily formed from catechol is benzene, the aromatic product of highest yield for the entire temperature range investigated. Experiments at different residence times show that at 800 °C the 0.4-1 s time interval is one of mostly increasing yields; at 1000 °C this same span of residence times sees mostly decreasing yields. The data reported here represent one of the most extensive quantifications of aromatic products from any fuel, and the only one for catechol.

To obtain kinetic parameters on PAH formation relevant to solid fuels combustion, pyrolysis experiments have been conducted with catechol, a model fuel representing entities in coal and biomass. Catechol pyrolysis experiments were performed in a tubular-flow reactor at temperatures of 500−1000 °C and at a residence time of 0.4 s. PAH products were identified and quantified by high-pressure liquid chromatography with ultraviolet−visible diode-array detection and by gas chromatography with flame ionization and mass spectrometric detection. A pseudo-unimolecular reaction kinetic model was used to model the experimental yield/temperature data of 15 individual aromatics and of combinations of PAH grouped by structural class and ring-number. The modeling of the individual species' yields showed that the pseudo-unimolecular model agreed very well with the experimental data. E a values ranged from 50 to 110 kcal mol-1, generally increasing as the size of the aromatic product increased from one to five aromatic rings. The pseudo-unimolecular model also performed well in modeling the experimental yields of PAH grouped by structural class and ring number. The global kinetic analysis results for PAH grouped by ring number revealed that E a values increased in the following order:  2-ring < 3-ring < 4-ring < 5-ring < 6-ring. Their yields followed the reverse order:  2-ring > 3-ring > 4-ring > 5-ring > 6-ring. These trends of increasing E a and decreasing yield, as ring number is increased, are consistent with a mechanism for PAH growth involving successive ring buildup reactions.

Knowledge of the chemical structure of young soot and its precursors is very useful in the understanding of the paths leading to soot particle inception. This paper presents analyses of the chemical functional groups, based on FT-IR and 1 H NMR spectroscopy of the products obtained in an ethylene inverse diffusion flame. The trends in the data indicate that the soluble fraction of the soot becomes progressively more aromatic and less aliphatic as the height above the burner increases. Results from 1 H NMR spectra of the chloroform-soluble soot samples taken at different heights above the burner corroborate the infrared results based on proton chemical shifts (Ha, Hα, Hβ, and Hγ ). The results indicate that the aliphatic β and γ hydrogens suffered the most drastic reduction, while the aromatic character increased considerably with height, particularly in the first half of the flame.