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.

Investigation of NO production and flame structures in ammonia-hydrogen flames

Ammonia/hydrogen fuel blends have recently emerged as a promising solution to the de-carbonization of the energy and transport sectors. However, concerns over performance and, more importantly, NOx emissions have impeded their progress so far. Before effective NOx mitigation strategies can be developed, the fundamental chemical mechanisms involved in NOx production in NH3/H2 flames must be well understood. Although NOx formation in hydrocarbons and the oxidative processes involved in NH3 combustion have been well studied, there is a significant lack of such information for NH3/H2 flames. Key insights on NO formation mechanisms and flame structures for NH3/H2 mixtures are required to develop and improve chemical kinetic models. In this work, laminar Bunsen NH3/H2 flames with 65/35 NH3/H2 volume fraction were tested at two equivalence ratios (rich and lean). For all test conditions, the adiabatic flame temperature was kept constant. Measurements of simultaneous OH/NO-PLIF and OH-PLIF/chemiluminescence (of NH*, NH2* and OH*) were conducted and compared to computational results of four reaction mechanisms (available in literature) applicable to NH3/H2 flames. The maximum OH-PLIF signal gradient was used as a spatial reference point for each simultaneous measurement and one-dimensional line profiles were determined for each species of interest. The simultaneous OH/NO-PLIF images show that the NO signal intensity in the NH3/H2 flames were up to 100 times more than a CH4 reference flame. OH* and NH* chemiluminescence results showed a good spatial correlation with the maximum OH-PLIF signal gradient for both test conditions. NH* also showed a positive correlation with the computed HRR values from lean to rich, indicating it is a promising candidate for direct HRR measurement but warrants further investigation over a wide range of equivalence and mixture ratios. The results also indicate that all the mechanisms underestimate the profile widths of the measured species. Similarly, the experimental results showed a much higher relative increase in NH2 production from lean to rich compared to the computed profiles. The data analysis approach employed in this paper, based on simultaneous measurements, could be further used for optimizing chemical kinetics mechanisms for NH3/H2 flames

Heat release rate estimation in laminar premixed flames using laser-induced fluorescence of CH2O and H-atom

The present work demonstrates the feasibility of heat release rate imaging using the laser-induced fluorescence (LIF) of atomic hydrogen (H-atom) and formaldehyde (CH2O) in laminar premixed flames. The product of H-atom LIF and CH2O LIF signals is evaluated on a pixel-by-pixel basis and is compared with that of the OH × CH2O technique. These results for equivalence ratio ranging from 0.8 to 1.1 are compared with computations of one-dimensional freely-propagating flames. The performance of these markers is studied based on the following two aspects: the spatial accuracy of the local heat release rate and the trend in the total heat release rate with equivalence ratio. The measured trend in the spatial distribution of radicals and the deduced heat release rate agree well with the computational values. The variation in the spatially integrated heat release rate as a function of equivalence ratio is also investigated. The results suggest that the trend in the variation of the integrated heat release rate and the spatial location of heat release rate can be evaluated by either of these markers. The OH-based marker showed certain sensitivity to the chemical mechanism as compared to the H-atom based marker. Both the OH-based and H-atom based techniques provide close estimates of heat release rate. The OH based technique has practical advantage when compared to the H-atom based method, primarily due to the fact that the H-atom LIF is a two-photon process

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

Use of portable air purifiers to reduce aerosols in hospital settings and cut down the clinical backlog

SARS-CoV-2 has severely affected capacity in the NHS, and waiting lists are markedly increasing due to downtime of up to 50 minutes between patient consultations/procedures, to reduce the risk of infection. Ventilation accelerates this air cleaning, but retroactively installing built-in mechanical ventilation is often cost-prohibitive. We investigated the effect of using portable air cleaners (PAC), a low-energy and low-cost alternative, to reduce the concentration of aerosols in typical patient consultation/procedure environments. The experimental setup consisted of an aerosol generator, which mimicked the subject affected by SARS-CoV-19, and an aerosol detector, representing a subject who could potentially contract SARS-CoV-19. Experiments of aerosol dispersion and clearing were undertaken in situ in a variety of rooms with 2 different types of PAC in various combinations and positions. Correct use of PAC can reduce the clearance half-life of aerosols by 82% compared to the same indoor-environment without any ventilation, and at a broadly equivalent rate to built-in mechanical ventilation. In addition, the highest level of aerosol concentration measured when using PAC remains at least 46% lower than that when no mitigation is used, even if the PAC’s operation is impeded due to placement under a table. The use of PAC leads to significant reductions in the level of aerosol concentration, associated with transmission of droplet-based airborne diseases. This could enable NHS departments to reduce the downtime between consultations/procedures

Spatiotemporal droplet dispersion measurements demonstrate face masks reduce risks from singing

COVID-19 has restricted singing in communal worship. We sought to understand variations in droplet transmission and the impact of wearing face masks. Using rapid laser planar imaging, we measured droplets while participants exhaled, said ‘hello’ or ‘snake’, sang a note or ‘Happy Birthday’, with and without surgical face masks. We measured mean velocity magnitude (MVM), time averaged droplet number (TADN) and maximum droplet number (MDN). Multilevel regression models were used. In 20 participants, sound intensity was 71 dB for speaking and 85 dB for singing (p  85% reduction wearing face masks. Droplet transmission varied widely, particularly for singing. Masks decreased TADN by 99% (p < 0.001) and MDN by 98% (p < 0.001) for singing and 86–97% for other tasks. Masks reduced variance by up to 48%. When wearing a mask, neither singing task transmitted more droplets than exhaling. In conclusion, wide variation exists for droplet production. This significantly reduced when wearing face masks. Singing during religious worship wearing a face mask appears as safe as exhaling or talking. This has implications for UK public health guidance during the COVID-19 pandemic

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