Abstract
Polycyclic aromatic hydrocarbons (PAH) formed during the pyrolysis and combustion of solid fuels like coal, wood, and biomass are widespread environmental pollutants. Since some PAH are known to exhibit carcinogenic and mutagenic activity, understanding the chemical reactions responsible for PAH formation is of utmost importance. To better understand the reactions leading to the formation of PAH from complex solid fuels, pyrolysis and oxidation experiments have been performed in an isothermal laminar-flow reactor, using the model fuel catechol (ortho-dihydroxybenzene), a phenol-type compound representative of structural entities in coal, wood, and biomass. Catechol pyrolysis experiments have also been performed in the presence of: (1) acetylene, a major growth species in combustion environments, and (2) 1,3-butadiene, a major product of the pyrolysis of coal, wood, and biomass. Experiments have been conducted over a temperature range of 500-1000 oC and at a fixed residence time of 0.3 s. The pyrolysis products are analyzed by high-pressure liquid chromatography with diode-array ultraviolet-visible absorbance detection and mass spectrometric detection, by gas chromatography with flame-ionization and mass spectrometric detection, and by nondispersive infrared analysis. Analysis of the catechol pyrolysis products has led to the identification of 13 C1-C6 non-aromatic species, 5 one-ring aromatics, 7 oxygen-containing organics, and 104 PAH. Of these, 50 (including 47 PAH) have never before been reported as products of catechol or any phenol-type fuel. A new set of “rules” has been developed for the identification of methylene-bridged PAH. Employing these “rules,” two C25H14 methylene-bridged PAH have been identified that have never before been reported as products of any fuel. Product quantification reveals that catechol’s relatively labile O-H bond and capacity for generating oxygen-containing radicals accelerate both fuel conversion and the pyrolysis reactions leading to 1- and 2-ring aromatics and PAH. Among the C1-C5 species, 1,3-butadiene appears to be the most important intermediate in PAH formation from catechol. The results are consistent with the C2 and C4 radicals being the dominant growth species. Reactions responsible for the formation of the C1-C10 products from catechol, under pyrolytic and oxidative conditions, are discussed. A tentative PAH formation mechanism during catechol pyrolysis is presented.
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