Formation of polycyclic aromatic hydrocarbons and soot in fuel-rich oxidation of methane in a laminar flow reactor

Abstract

Conversion of methane to higher hydrocarbons, polycyclic aromatic hydrocarbons (PAHs), and soot was investigated under fuel-rich conditions in a laminar flow reactor. The effects of stoichiometry, dilution, and water vapor addition were studied at temperatures between 1073 and 1823 K. A chemical kinetic mechanism was established for methane oxidation, with emphasis on formation of higher hydrocarbons and PAH. A submodel for soot formation was adopted from the work of Frenklach and co-workers without changes. Modeling predictions showed good agreement with experimental results. Reactants, stable intermediates, and PAH compounds are generally well predicted. In sooting systems, the calculations indicate that the cyclopentadienyl radical has a significant influence on growth of PAH, particularly in the formation of naphthalene and phenanthrene. Both naphthalene and phenanthrene appear to be important intermediates in PAH growth, and they have a significant influence on the formation of pyrene, which subsequently results in formation of soot in the applied model. The soot model generally predicts soot volume fractions to be two to three orders of magnitude lower than those obtained experimentally, and the model predicts inception of soot to occur approximately 100 K below experimental observations. Addition of water vapor has a considerable effect on the measured acetylene concentration and on soot formation at 1500 K and above. In this temperature regime, concentrations of both acetylene and soot decrease with increasing addition of water vapor. The effect is described qualitatively by the reaction mechanism. The enhanced oxidation of acetylene is attributed to higher levels of hydroxyl radicals, formed from the reaction between the water vapor and hydrogen atoms.