Behavior of PAHs and PM in a diffusion flame of paraffin fuels

Abstract

Nano-meter size particulate matter (PM) and polycyclic aromatic hydrocarbons (PAHs) have harmful effects on human health. Fundamental research works for investigation of formation behavior of PAHs in combustion process were required for the reduction of nano-PM emission. In this paper, PAHs, soot and PM components in diffusion flames of paraffin fuels were investigated using laser diagnostics and direct sampling measurement. Methane (CH4), propane (C3H8) and hexane (C6H14) were used as test fuels. A laser induced fluorescence (LIF) method was used to obtain spatial distribution of PAHs that were precursor molecules of soot. Spatial distribution of soot was also measured by a laser induced incandescence (LII) method. A transition region from PAHs to soot was defined from the results of LIF and LII. Further, PAH concentrations in test flames were analyzed using a gas chromatography (GC). Relationship between fuel and a GC profile of PAH concentrations in these flames was discussed. PM components such as soot and soluble organic fraction (SOF) were also measured using a combustion type PM analyzer (MEXA-1370PM). These results suggested that PAH molecules rapidly grew up to soot particles in the sooting region in a flame. It was confirmed that single-ring aromatic molecules such as benzene and toluene existed in the sooting region. Concentrations of PAHs such as fluorene and pyrene in the sooting region were far lower than single-ring aromatic molecules. Further, single-ring aromatic molecules such as benzene and toluene were emitted from the flame tip. As for the fuel effect, when carbon number of fuel molecule increased, PAH formation region was shifted to the upstream region in the flame. PM concentration in the flame increased with an increase of the carbon number of fuel.