Khan, Zuhaib Ali; (2022) Decomposition of some hydrocarbon and oxygenated fuel molecules and subsequent formation of PAHs under pyrolytic conditions in a flow reactor. Doctoral thesis (Ph.D), UCL (University College London).

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Text PhD thesis, Zuhaib Ali Khan, SN 18046471.pdf - Accepted Version Download (150MB) | Preview

Abstract

Air pollution has reached critical levels in many major industrial cities, endangering public health, deteriorating the environment, and causing harm to property and landscape. The particulate emissions (PM) which contribute to air pollution vary greatly in size and composition, conveying carcinogenic polyaromatic hydrocarbons (PAHs) present on the particle surface. The toxicity of particulate matter from internal combustion systems is a topic of scientific and practical relevance. Although it has been found that replacing fossil fuels with renewable oxygen-bearing fuels reduces the mass of PM released, not much is known on how this change in fuel composition affects soot levels, PAH production, and toxicity during the pyrolysis process. However, despite the increasing use of renewable fuel, necessitates further examination of the decomposition pathways of biofuel and hydrocarbon fuels, investigating the potential precursor to the formation of benzene and PAHs. This study was conducted on a laminar flow reactor. The pyrolysis of potential biofuels and collection of soot-bound and gas-phase PAH was carried out at a fixed carbon atom concentration in nitrogen. Accelerated solvent extraction (ASE) was used to extract sootbound PAH and gaseous species captured on XAD resin, with speciation and quantification of PAHs undertaken using Gas Chromatography coupled with Mass Spectrometry (GCMS). Intermediate gaseous species were collected at different residence times during pyrolysis of alcohol and hydrocarbon fuels of premixed fuel and nitrogen with the help of a specially designed high-temperature ceramic sampling probe. A Gas Chromatography-Flame Ionization Detector (GC-FID) was used to identify and quantify intermediate species. A variety of fuels, containing diverse functional groups, such as alcohols, esters, ether, and ketone, were evaluated for their contribution to PAHs and soot. It was found that increasing fuel oxygen to carbon ratio significantly reduced the soot and PAHs emission, for example, methyl acetate produced the least soot and PAHs as compared to propanol and acetone. A greater abundance of toxic PAHs was present at low temperatures, despite lower soot mass concentrations at these conditions. Moreover, the investigation of intermediate species showed that increasing carbon chain length and the presence of a hydroxyl group both influenced on the initial breakdown and relative abundance of C1 to C4 intermediate species. Common to the pyrolysis of both straight chain hydrocarbons and alcohols was the detection of acetylene, suggesting the dominant pathway for benzene formation and PAHs growth to have been via the HACA mechanism.

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