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

Effect of hydrogen-diesel fuel co-combustion on exhaust emissions with verification using an in–cylinder gas sampling technique

AbstractThe paper presents an experimental investigation of hydrogen-diesel fuel co-combustion carried out on a naturally aspirated, direct injection diesel engine. The engine was supplied with a range of hydrogen-diesel fuel mixture proportions to study the effect of hydrogen addition (aspirated with the intake air) on combustion and exhaust emissions. The tests were performed at fixed diesel injection periods, with hydrogen added to vary the engine load between 0 and 6 bar IMEP. In addition, a novel in–cylinder gas sampling technique was employed to measure species concentrations in the engine cylinder at two in–cylinder locations and at various instants during the combustion process.The results showed a decrease in the particulates, CO and THC emissions and a slight increase in CO2 emissions with the addition of hydrogen, with fixed diesel fuel injection periods. NOx emissions increased steeply with hydrogen addition but only when the combined diesel and hydrogen co-combustion temperatures exceeded the threshold temperature for NOx formation. The in–cylinder gas sampling results showed higher NOx levels between adjacent spray cones, in comparison to sampling within an individual spray cone

Planar Interferometric Tracking of droplets in evaporating conditions

An effective Lagrangian Planar Interferometric Tracking (PIT) processor is proposed to track the size and path of multiple droplets, with spray droplet diameters (20–150 µm) and volumetric concentrations (≈ 300 drops/cm3) consistent with industrial applications, produced by an ultrasonic atomiser in evaporating conditions. A test facility was developed where liquid droplets are exposed to a temperature gradient in a co-axial air flow, where the outer stream is preheated to the desired temperature (288–550 K). The PIT method builds on a TSI Global Sizing Velocimtery measurement technique and allows to detect, size and follow the path of droplets which were otherwise discarded or mis-analysed by the commercial software. The methodology was first tested under non-evaporating conditions, and multiple sources of errors, some common to most planar interferometric techniques, were identified and their order of magnitude and impact on final droplet measurement assessed. The main source of error is related to the out-of-plane motion of the droplets and the time they spend in the measurement volume. For non-evaporating conditions, measured data can be processed to filter out this source of error. In evaporating conditions, a novel method for assessing the impact of measurement error with respect to droplet evaporation and measurement timescales is defined. The PIT method allowed tracking of individual methanol droplets entrained within an airflow heated to 495 K and determining their size reduction under evaporating conditions. Measured droplet evaporation rates were then compared against those predicted by an iterative evaporation model, and a very good agreement was found between the modelled and measured estimates. Graphical abstract: [Figure not available: see fulltext.

Thermoacoustic Instability Considerations for High Hydrogen Combustion in Lean Premixed Gas Turbine Combustors: A Review

Hydrogen is receiving increasing attention as a versatile energy vector to help accelerate the transition to a decarbonised energy future. Gas turbines will continue to play a critical role in providing grid stability and resilience in future low-carbon power systems; however, it is recognised that this role is contingent upon achieving increased thermal efficiencies and the ability to operate on carbon-neutral fuels such as hydrogen. An important consideration in the development of gas turbine combustors capable of operating with pure hydrogen or hydrogen-enriched natural gas are the significant changes in thermoacoustic instability characteristics associated with burning these fuels. This article provides a review of the effects of burning hydrogen on combustion dynamics with focus on swirl-stabilised lean-premixed combustors. Experimental and numerical evidence suggests hydrogen can have either a stabilising or destabilising impact on the dynamic state of a combustor through its influence particularly on flame structure and flame position. Other operational considerations such as the effect of elevated pressure and piloting on combustion dynamics as well as recent developments in micromix burner technology for 100% hydrogen combustion have also been discussed. The insights provided in this review will aid the development of instability mitigation strategies for high hydrogen combustion

Influence of Combustion Characteristics and Fuel Composition on Exhaust PAHs in a Compression Ignition Engine

This paper reports an experimental investigation into the effects of fuel composition on the exhaust emission of toxic polycyclic aromatic hydrocarbons (PAHs) from a diesel engine, operated at both constant fuel injection and constant fuel ignition modes. The paper quantifies the US EPA (United State Environmental Protection Agency) 16 priority PAHs produced from combustion of fossil diesel fuel and several model fuel blends of n-heptane, toluene and methyl decanoate in a single-cylinder diesel research engine based on a commercial light duty automotive engine. It was found that the level of total PAHs emitted by the various fuel blends decreased with increasing fuel ignition delay and premixed burn fraction, however, where the ignition delay of a fuel blend was decreased with use of an ignition improving additive the level of particulate phase PAH also decreased. Increasing the level of toluene present in the fuel blends decreased levels of low toxicity of two to four ring PAH, while displacing n-heptane with methyl decanoate increased particulate phase adsorbed PAH. Overall, the composition of the fuels investigated was found to have more influence on the concentration of exhaust PAHs formed than that of combustion characteristics, including ignition delay, peak heat release rate and the extent of the premixed burn fractions

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 Forced Intermittency in Swirl-Stabilised Turbulent Methane-Air Flames

Thermoacoustic oscillations continue to be a major problem affecting combustor performance and operation in gas turbines. Acoustic forcing is often employed to simulate dynamical states in a combustor, however reproducing intermittent-like behaviour can be difficult. This paper, for the first time, reports a forced intermittent behaviour in a laboratory-scale, swirl-stabilised combustor burning methane-air mixtures that mimics the configuration of an industrial gas turbine. A frequency sweep using loudspeakers to generate velocity fluctuations identified 90, 220 and 320 Hz as the dominant frequencies of the burner. Amplitude sweeps were performed at these same frequencies and the flame response was obtained. At low forcing amplitude the acoustic flame response was in the linear range and the dominant frequency of the response matched the forcing frequency. Intermittency was observed beyond an amplitude threshold at 320 Hz, consisting of bursts of high-amplitude oscillations corresponding to broadband excitation around the forcing frequency. Further amplification resulted in excitation of a low frequency band. Wavelet analysis showed intermittent bursts to have a frequency signature matching the forcing frequency, and pressure and heat release oscillations were found to be coupled. In low amplitude regions between bursts, signals were found to be decoupled and response at forcing frequency was weak. Phase-space reconstruction showed acoustic modes to be consistent with those of self-excited flames found in literature. The outcomes of this work could provide further understanding on the causes of intermittency and its relationship with limit cycle oscillations

Demonstrating Clean Burning Future Fuels at a Public Engagement Event

Sustainable future fuels are likely to be produced by a wide range of processes, and there exists the opportunity to engineer these fuels so that they burn more efficiently and produce fewer harmful emissions. Such potential is especially important within the context of reducing the emissions of both greenhouse gases (GHG) and toxic pollutants that adversely impact air quality and human health. To illustrate how fuel design on a molecular level may be exploited to reduce these emissions, the combustion and emission properties of three potential future fuels, geraniol, diethyl carbonate, and a biodiesel (soy methyl ester), were evaluated along with a fossil diesel. The fuels were assessed using “smoke point” tests and a Stirling engine. The purpose of the demonstration was to highlight to a general audience several burning characteristics of some possible future fuels, and thus the potential for the development of clean burning “designer” fuels. During the 15 min demonstration, significant differences in the combustion properties of the different fuels were shown. For example, the conventional fossil diesel fuel produced a significant amount of soot in flame tests, whereas diethyl carbonate, which is a potential second-generation biofuel, produced visibly lower amounts of soot