Abstract
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 °C at a fixed carbon concentration of 10,000 ppm on C1 basis. Particulate and gas phase PAHs were collected at the outlet of the reactor at pyrolysis temperature intervals of 100 °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 °C decreased the total PAH concentrations regardless of the carbon number of the hydrocarbon investigated. Increasing the carbon number of C1–C7 hydrocarbons decreased the gas phase (GP) PAH concentrations at a temperature of 1350 °C, while the particulate phase (PP) PAH concentrations (as well as those of Group B2 PAHs) decreased at a temperature of 1150 °C. There was increasing and decreasing trends of total PAH concentrations with increasing carbon number of the hydrocarbons at temperatures of 1050 °C and 1350 °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 °C due to isomerisation in the case of the C4 hydrocarbons and aromatisation in the case of C7 hydrocarbons. PAHs from toluene and propane had the highest weighted carcinogenicities at a temperature of 1050 °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 °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.
Highlights
Atmospheric air quality is continually degraded by particulate emissions from different combustion sources, and stringent global particulate legislation has largely been enacted due to the adverse health effects of these emissions [1]
There is incomplete information on whether increasing carbon number has an influence on the carcinogenic Group B2 Polycyclic aromatic hydrocarbons (PAHs) when they are adsorbed onto soot particles or when they are available in the gas phase
This paper reports the analysis of particulate and gas phase PAHs of a homologous series of C1–C7 hydrocarbons generated in a homogenous tube reactor
Summary
Atmospheric air quality is continually degraded by particulate emissions from different combustion sources, and stringent global particulate legislation has largely been enacted due to the adverse health effects of these emissions [1]. Systematic studies which investigate the influence of increasing carbon number on PAH formation of a homologous series of these fuels are rare. And Pfefferle [30] reported increase in yield sooting index (YSI) for several hydrocarbons when the carbon number was increased It is evident from the foregoing that there are relatively few experimental PAH studies of the individual hydrocarbons in the homologous series of C1 to C7 in a tube reactor, and only limited published information is available on particulate characterisation and soot formation in a tube reactor for this homologous series. There is incomplete information on whether increasing carbon number has an influence on the carcinogenic Group B2 PAHs when they are adsorbed onto soot particles or when they are available in the gas phase. The PAH studied were the US EPA 16 priority PAHs shown in Table 1, with particular attention being paid to the group B2 PAHs, which are possible human carcinogens
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