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

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

PAH formation characteristics in hydrogen-enriched non-premixed hydrocarbon flames

The utilisation of hydrogen with conventional hydrocarbons offers an excellent opportunity to decarbonise current energy systems without significant hardware upgrades. However, this presents fresh scientific challenges, one of which is the difficulty in effective control of pollutant soot emissions due to complex reaction kinetics of hydrogen enriched flames. This paper focuses on polycyclic aromatic hydrocarbons (PAHs), which are the building blocks of soot and responsible for its carcinogenicity. Detailed understanding of the effect of on the underlying processes of PAH formation and growth is important for the development of effective strategies to curtail PAH formation and hence, reduce soot emissions from combustion systems. In this study, an experimental methodology was employed to analyse PAH formation and growth characteristics of laminar inverse diffusion flames of various hydrocarbon fuels (alkanes and alkenes) enriched with using simultaneous planar laser induced fluorescence (PLIF) imaging of PAHs and hydroxyl radicals (OH). OH PLIF was used to indicate peak temperature locations in the flame (flame front), while PAH PLIF was used to determine PAH formation characteristics. Methane () was also separately added to the same hydrocarbon fuels to study effects of carbon-bound hydrogen addition, in comparison to addition. It was observed that only the addition of to showed significant variation in the magnitude of PAH reduction levels as the length along the flame front, Lf increased. The results also showed that while the addition of was more effective in reducing the rate of PAH fluorescence signal increase (indicative of concentration growth) when compared to addition, both fuels showed two distinct regions in the PAH growth curve; a steep growth region followed by a slower growth region. This is potentially indicative of the self-limiting nature of PAH formation and growth. The study concluded that the growth rate of PAHs lies within a narrow band irrespective of the fuel bonding, molecular structure and the H:C ratio of the fuel mixtures tested

Investigation of the effect of hydrogen addition on soot and PAH formation in ethylene inverse diffusion flames by combined LII and PAH LIF

It is shown that signals from both laser-induced incandescence (LII) of soot and laser-induced fluorescence (LIF) of polycyclic aromatic hydrocarbons (PAHs) decrease with hydrogen addition (in volume fractions of 3%, 6% and 9% with respect to the fuel mixture) to ethylene–air inverse diffusion flames (IDFs). The structure of the IDF suppresses soot oxidation and the effect of hydrogen addition under these conditions has been studied. Experiments were performed using a frequency-doubled, pulsed dye laser to perform planar LIF of PAH, and a pulsed fibre laser to perform LII. In relative terms, the LII signal decreases more sharply than the PAH LIF signal. This would be consistent with the dependence of soot inception on PAH concentration as well as the suppression of soot growth via the reduced concentration of PAH and perhaps other precursors such as acetylene. Similar trends in relative signal decrease are observed at a range of heights above the burner, despite the measurement locations encompassing a wide range of absolute signal levels. As a comparison, the influence of adding methane to the IDF in the same volume fractions was also studied and found to suppress PAH LIF and LII signals but to a far lesser extent than in the case of hydrogen

Experimental Characterisation of the Dynamics of Partially Premixed Hydrogen Flames in a Lean Direct Injection (LDI) Combustor

Hydrogen continues to show significant promise as a zero-carbon energy carrier in the pursuit of global decarbonisation targets. Hydrogen has wide flammability limits which means it can operate at considerably leaner conditions for reduced NOx emissions. However, fuel-lean operation makes these systems more susceptible to thermoacoustic instabilities and flame blow-off. Combustor configurations such as jet-in-crossflow are gaining popularity in industry for 100% hydrogen as they can help mitigate risk of flashback, but detailed characterisation of flame dynamics is still necessary. In this study, the combustion dynamics of partially premixed hydrogen flames in a lean direct injection (LDI) multi-cluster combustor were investigated at atmospheric conditions. The combustor inlet consisted of nine circular air channels, with hydrogen injected inwards through two diametrically opposite holes into each air channel. Dynamic pressure and OH* chemiluminescence measurements were employed to study the effect of varying key parameters, such as Reynolds number and global equivalence ratio, on combustor dynamics. High-speed OH-PLIF imaging was conducted to understand flame dynamics. The results showed that self-excited oscillations were observed at all tested conditions and the dynamical behaviour of the combustor was complex with strong dependency on global equivalence ratio and bulk velocity conditions. The magnitude of self-excited thermoacoustic oscillations initially increased with a decrease in global equivalence ratio, but subsequently decreased at leaner conditions (below global equivalence ratio 0.3). Similar observations were noted for all bulk velocities. High speed OH-PLIF imaging indicated that the heat release oscillations were influenced by vortex-flame roll up and possible global lean extinction events. The results from this work have the potential to inform design efforts towards development of new architectures for stable, low-emission 100% hydrogen combustors

Experimental Investigation of Combustion Instabilities in a Laboratory-Scale, Multi-Can Gas Turbine Combustor

Can and can-annular (can-type) combustors are widely employed in stationary gas turbines. While majority of combustion instability research so far has focused on single can combustors, studying combustion dynamics in multi-can configurations holds more practical relevance. In can-type combustors, the annular gap between the transition ducts and first stage nozzles, known as the ‘cross-talk’ region, promotes strong can-to-can acoustic interactions. In this study, coupled interactions between neighbouring cans are experimentally investigated in a laboratory-scale, atmospheric, two-can combustor rig. Results demonstrate that an out-of-phase longitudinal mode, characterised by pressure anti-nodes in the cans and a pressure node in the cross-talk area, is preferentially excited in the multi-can combustor. Investigation into the effects of combustor flow rate and cross-talk geometry on the can-to-can dynamics revealed an increase in the amplitude of the limit cycle oscillations with mass flow rate, cross-talk volume and cross-talk exit area reduction. However, these factors exerted a weak influence on the frequency of the oscillations and did not impact the mode shape of the resonant can-to-can mode. These findings would help better understand the complex dynamical interactions that occur in multi-can systems towards model validation and tool development, while highlighting the significance of factoring in thermoacoustic aspects in the design of the combustor-turbine interface

Assessment of the Dynamical Behaviour of Hydrogen Combustors With Recurrence Quantification Analysis (RQA)

Hydrogen has immense potential as a future energy vector for power and propulsion applications. However there are several challenges, including flame stability, flashback and emissions, that impede its widespread use in modern energy systems. Hydrogen systems are required to be operated at fuel-lean conditions to avoid high NOₓ emissions and flashback issues, however lean operation makes systems more susceptible to thermoacoustic instabilities and lean blow-off (LBO). Understanding these dynamical behaviours is key to develop robust operational strategies. Recurrence Quantification Analysis (RQA) is an effective tool to study nonlinear dynamics, and has been applied in this study to characterise hydrogen flame behaviour in industry-relevant laboratory-scale combustion systems. RQA was conducted on two different configurations — hydrogen lean direct injection (LDI) combustor and a swirl-stabilised combustor operated on methane-hydrogen blends. RQA was carried out on time-resolved data of (i) pressure measurements in the combustor and (ii) integrated OH* chemiluminescence signals. These commonly used measurements in combustion diagnostics were selected to develop a tool which can be applied easily to other industrial systems. The effect of H₂ blend, bulk velocity, and global equivalence ratio (ϕg) were investigated in this study, and four quantifiable parameters such as Shannon entropy, recurrence rate, determinism, and laminarity were extracted from the RQA, to determine different dynamical behaviours — stable, intermittent, and limit-cycle. High values of Shannon entropy and recurrence rate indicated stable conditions for the swirl-stabilised combustor, while a combination of Shannon entropy and determinism were found to best capture the hydrogen flame dynamics in the LDI combustor. The outcome of this work will allow development of efficient predictive tools for nonlinear data analysis