The effect of the addition of different butanol isomers on the composition and optical properties of the low MW components

Biofuels are considered an environment friendly alternative to fossil fuels, helping to reduce greenhouse gas emissions. However, biofuel combustion may emit pollutants that are different from those produced by conventional fossil fuels. Therefore, by-products derived from biofuel combustion have gained through years increased attention. In particular, while small compounds can be relatively easily detected with traditional gaschromatographic techniques few studies were devoted to larger compounds as polyclycic aromatic hydrocarbons (PAHs). The present experimental research intends to make a contribution on the study of the high MW PAH prodromal to soot formation. The effect of the four butanols isomers on the formation of PAH from 200 to 500 u in rich premixed flames of ethylene has been studied. Butanols were used as substituent to ethylene in rich premixed flames (phi=2.46) up to 20% of the total carbon fed in order to directly compare the effect of oxygenated fuels. Size Exclusion Chromatography (SEC) has shown to be a useful technique for determining MW distribution of flame formed particulates in a MW range wider than other conventional techniques. This paper is focused on the partitioning of the species in the MW range typical of PAH (from 100 up to 400u) by means of SEC. Bulk particulate was collected thermophoretically on a 75 × 25 × 1 mm quartz plate used as a substrate to deposit particles. Particles were sampled by inserting the quartz plate horizontally in the flame for 2 s. This procedure was repeated several times, allowing for a cooling cycle at room temperature of 10 s after each insertion. The procedure was repeated to achieve an amount of material suitable for the structural analysis. Particulate powder, mechanically ablated from the quartz plate, was suspended in N-methyl-2-pyrrolidinone (NMP). Filtration on an Anotop filter (Whatman) of a 100 mg/L total particulate suspension was used to separate the particulate fraction of <20 nm. This fraction was analysed by SEC with a highly cross-linked "individual-pore" column to better separate the fraction in the 100-1E5 u range. Compounds from 200 to 500 u were collected after SEC separation for further examination by fluorescence and UV-Visible spectroscopy. Differences and similarities were found in terms of size and optical properties of formed PAH when different butanol isomers were burned, evidencing the role of fuel structure in the PAHs and thus particles formation.