Influence of carbon number of C1-C7 hydrocarbons on PAH formation

The influence of carbon number of seven hydrocarbons (methane, ethane, propane, n-butane, i-butane, heptane and toluene) on PAH formation was investigated in a laminar tube reactor. The hydrocarbons underwent oxygen-free pyrolysis within the temperature range of 1050-1350 degrees C at a fixed carbon concentration of 10,000 ppm on C-1 basis. Particulate and gas phase PAHs were collected at the outlet of the reactor at pyrolysis temperature intervals of 100 degrees C. The particulates generated were characterised at sub-micron levels in terms of size, number and mass using a differential mobility spectrometer (DMS-500). PAHs from both the gas and particulate samples were extracted using an accelerated solvent extractor (ASE) and the extracts analysed using gas chromatography coupled to mass spectrometry (GCMS). The PAHs studied were the US EPA 16 priority PAHs with particular attention given to group B2, which are possible human carcinogens. The experimental results showed that increase in temperature of the reactor from 1050 to 1350 degrees C decreased the total PAH concentrations regardless of the carbon number of the hydrocarbon investigated. Increasing the carbon number of C-1-C-7 hydrocarbons decreased the gas phase (GP) PAH concentrations at a temperature of 1350 degrees C, while the particulate phase (PP) PAH concentrations (as well as those of Group B2 PAHs) decreased at a temperature of 1150 degrees C. There was increasing and decreasing trends of total PAH concentrations with increasing carbon number of the hydrocarbons at temperatures of 1050 degrees C and 1350 degrees C respectively. Benzenoid and five-membered ring PAHs of 2-4 rings were detected in roughly similar concentrations irrespective of the carbon number of the hydrocarbon. Soot propensities, abundance of particle phase PAHs and carcinogenicity of soot particles increased substantially at a temperature of 1050 degrees C due to isomerisation in the case of the C-4 hydrocarbons and aromatisation in the case of C-7 hydrocarbons. PAHs from toluene and propane had the highest weighted carcinogenicities at a temperature of 1050 degrees C per unit volume of gas and per unit soot mass respectively. The weighted carcinogenicity (soot mass basis) decreased with increasing carbon number at temperature of 1150 degrees C. Potential implication of these observations is that hydrocarbons known to produce substantial particulate mass in combustion systems such as an internal combustion engines, could also have low toxicity.