The effects of equivalence ratio on the formation of polycyclic aromatic hydrocarbons and soot in premixed methane flames

Abstract

The formation of polycyclic aromatic hydrocarbons (PAH) and soot has been investigated in atmospheric pressure, laminar, methane/oxygen/argon premixed flames as a function of mixture equivalence ratio. Mole fraction profiles of major products, trace aromatic, substituted aromatic, and PAH were quantified by direct gas chromatography/mass spectrometry. In addition, soot-particle diameters, number densities, and volume fractions were determined using classical light scattering. The dependencies of flame species on equivalence ration, using the expression X_i^max=A_iⁿⁱ were also determined; results reveal that the sensitivity parameter n for stable aliphatic species exhibit the following rank order: c-C₅H₆ (4.2)>C₃H₄ (3.9)>C₄H₆ (3.3)>C₄H₄ (2.8)>C₄H₂ (2.2)>C₂H₂ (1.8). For aromatic species, the values of n were in the following order: phenylacetylene (10.8)>benzene (10.2)>indene (7.5)>toluene (5.5). In comparison, PAH species were extremely sensitive to flame equivalence ratios, with the following n values: acenaphthylene (14.2)>fluoranthene (14.1)>pyrene (13.2)>anthracene (13.1)>phenanthrene (11.7)>naphthalene (10.8). Similarly, the sensitivity of soot volume fraction to equivalence ratio was about 13. These results suggest that PAH and soot formation are closely related and their levels are not strongly influenced by the levels of acetylene present in the flame under the conditions investigated.