Polycyclic aromatic hydrocarbons from the co-pyrolysis of 1,3-butadiene and propyne

Abstract

To better understand how the interactions between fuel components and their pyrolysis products can affect the production of polycyclic aromatic hydrocarbons (PAH) during fuel pyrolysis, we have conducted co-pyrolysis experiments with the C4 hydrocarbon 1,3-butadiene and the C3 hydrocarbon propyne (supplied in equimolar amounts), in nitrogen, in an isothermal laminar-flow reactor at temperatures of 700–1000°C and a residence time of 0.3s. Analysis of the reaction products by high-pressure liquid chromatography with diode-array ultraviolet-visible absorbance detection has led to the identification of 80 PAH of three to nine rings—none of which have ever before been reported as products of 1,3-butadiene/propyne co-pyrolysis. Quantification of the products as functions of temperature shows that yields of PAH (⩾3 rings) are negligible at temperatures⩽800°C. Above 800°C, PAH yields from the co-pyrolysis experiments rise sharply with temperature and, at temperatures of 850–950°C, are five to seven times what would result from pyrolyzing the two fuels individually. Even at 1000°C, when the individual fuels’ pyrolyses produce significant yields of PAH, the factor is 2.3. Detailed examination of the temperature-dependent yields of the other hydrocarbon products reveals three principal reasons for the co-pyrolysis-induced enhancement of PAH yields: (1) accelerated conversion of propyne when in the presence of the rich radical pool supplied by 1,3-butadiene; (2) reactions between C3 and C4 species that produce toluene and benzyl radical in higher amounts and at lower temperatures than when either 1,3-butadiene or propyne is pyrolyzed individually; (3) facilitated generation, during co-pyrolysis, of aryl and arylmethyl radicals, which, in combining with abundant C2–C4 species and aromatics in the reaction environment, prove to be very effective agents of PAH growth. As the greatest beneficiaries of this facilitated growth, the largest PAH (⩾5 rings) exhibit the highest relative increases in yields.