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Experimental investigation of polycyclic aromatic hydrocarbons (PAHs) in hydrogen-enriched diffusion flames

Ezenwajiaku, Chinonso Emma; (2021) Experimental investigation of polycyclic aromatic hydrocarbons (PAHs) in hydrogen-enriched diffusion flames. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Polycyclic aromatic hydrocarbons (PAHs) are the carcinogenic components of soot. Detailed understanding of the underlying processes of PAH formation and growth is required for the development of effective strategies to curtail PAH formation and reduce soot emissions from combustion systems. One such approach is the use of hydrogen (H_2) as an alternative energy vector which is in-line with the global push for transition towards low carbon energy systems. Therefore, understanding the effects of H_2 addition in hydrocarbon fuels on PAH formation process is key to its full utilisation in combustion devices to reduce pollutant emissions. This thesis presents an experimental methodology to analyse PAH formation and growth characteristics of laminar inverse diffusion flames of various H_2-enriched hydrocarbon fuel mixtures using simultaneous planar laser induced fluorescence (PLIF) imaging of PAHs and hydroxyl radicals (OH). Methane was also separately added to the same hydrocarbon fuels to study effects of fuel (carbon-hydrogen) composition in comparison to H_2 addition. Additionally, argon, nitrogen, and carbon dioxide were used as control diluents to study the diluting effects of H_2 addition in hydrocarbon flames. PAH fluorescence intensity values were observed to increase with increasing length along the flame front, L_f, for all conditions tested, however this rate of increase reduced with H_2 addition. The reduction in PAH with H_2 addition might be attributable to an anticipated reduction in acetylene and propargyl concentrations, and reduced H-atom abstraction rates, which reduced the availability of active sites for PAH growth. Furthermore, the addition of both H_2 and 〖CH〗_4 was found to reduce the growth rate of PAH, with H_2 demonstrating higher reductions. The PAH growth rate in this thesis refers to the rate of increase of the PAH LIF signal as the length along the flame front, L_f, increases. For both fuel additions (〖CH〗_4 and H_2), two distinct regions in the PAH growth curve were observed; a steep growth region followed by a slower growth region. This suggests that for PAH and soot formation, though first ring formation plays a significant role, it is not the only important step for PAH and soot formation. Other PAH growth processes could be playing significant roles as well.

Type: Thesis (Doctoral)
Qualification: Ph.D
Title: Experimental investigation of polycyclic aromatic hydrocarbons (PAHs) in hydrogen-enriched diffusion flames
Event: UCL (University College London)
Language: English
Additional information: Copyright © The Author 2021. Original content in this thesis is licensed under the terms of the Creative Commons Attribution-NonCommercial 4.0 International (CC BY-NC 4.0) Licence (https://creativecommons.org/licenses/by-nc/4.0/). Any third-party copyright material present remains the property of its respective owner(s) and is licensed under its existing terms. Access may initially be restricted at the author’s request.
UCL classification: UCL
UCL > Provost and Vice Provost Offices
UCL > Provost and Vice Provost Offices > UCL BEAMS
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Engineering Science
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Engineering Science > Dept of Mechanical Engineering
URI: https://discovery.ucl.ac.uk/id/eprint/10130693
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