Ultrafine (<100 nm) particles related to traffic are of high environmental and human health concern, as they are supposed to be more toxic than larger particles. In the present study transmission ...electron microscopy (TEM) is applied to obtain a concrete picture on the nature, morphology and chemical composition of non-volatile ultrafine particles in the exhaust of state-of-the-art, Euro 6b, Gasoline and Diesel vehicles. The particles were collected directly on TEM grids, at the tailpipe, downstream of the after-treatment system, during the entire duration of typical driving cycles on the chassis dynamometer. Based on TEM imaging coupled with Energy Dispersive X-ray (EDX) analysis, numerous ultrafine particles could be identified, imaged and analyzed chemically. Particles <10 nm were rarely detected. The ultrafine particles can be distinguished into the following types: soot, ash-bearing soot and ash. Ash consists of Ca, P, Mg, Zn, Fe, S, and minor Sn compounds. Most elements originate from lubricating oil additives; Sn and at least part of Fe are products of engine wear; minor W ± Si-bearing nearly spherical particles in Diesel exhaust derive from catalytic coating material. Ultrafine ash particles predominate over ultrafine soot or are nearly equal in amount, in contrast to emissions of larger sizes where soot is by far the prevalent particle type. This is probably due to the low ash amount per volume fraction in the total emissions, which does not favor formation of large ash agglomerates, opposite to soot, which is abundant and thus easily forms agglomerates of sizes larger than those of the ultrafine range. No significant differences of ultrafine particle characteristics were identified among the tested Gasoline and Diesel vehicles and driving cycles. The present TEM study gives information also on the imaging and chemical composition of the solid fraction of the unregulated sub-23 nm size category particles.
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•Ultrafine (<100 nm) solid particle emissions from modern Gasoline & Diesel vehicles occur as soot, ash-bearing soot and ash.•Among solid emissions <100 nm, ash particles are more abundant or nearly equal to soot.•Solid particle emissions <10 nm were rarely identified by TEM; they consist predominantly of ash.•At least part of the sub-23 nm particle size category in the exhaust are solid and derive from the engine..
Ultrafine (<100 nm) Diesel and Gasoline emissions (Euro 6b) include soot, ash-bearing soot & ash, mainly >10 nm; ultrafine ash particles predominate over ultrafine soot for Diesel vehicles and are nearly equal or predominate for GDI and MPI vehicles, opposite to emissions >100 nm where ash is subordinate.
•Methane conversion pathway is analyzed in engine exhaust and model gas reactor.•CH4 conversion efficiency is more than fivefold higher in transient than in steady.•In steady states, direct oxidation ...is the only CH4 conversion pathway.•Transient high conversion is due to activation of steam reforming.•Methane steam reforming diminishes with time due to surface carbonaceous species.
Methane abatement pathways in Pd/Rh three-way catalysts have been investigated in three scales ranging from a vehicle application size catalyst, a model gas reactor and the catalyst in powder form. A special test rig was designed for the investigation of vehicle size catalysts, allowing sampling along the catalyst at discrete spatial locations, which are subject to different feed compositions. Dependent on the history of chemical environment of the catalyst, significant differences in methane conversion rate at identical feed have been identified. At steady state methane conversion rate was low and the reaction pathway was identified as limited to only direct oxidation by oxygen. Following a rich-to-lean transition, the catalyst exhibited more than 8 times higher methane conversion rates compared to steady state. The high methane conversion rates have been identified and attributed to the activation of methane steam reforming (SR) related to transient reduction of ceria. Methane SR efficiency decreased with time and the conversion rate finally converged to steady state levels. The findings were validated using a model gas reactor enabling analysis under well-defined feed compositions. The deactivation of SR was further analyzed with infrared spectroscopy (DRIFTS). Evidences from DRIFTS measurements showed that the deactivation was linked to the formation of carbonaceous species on the catalyst surface, most likely carbonates. The coherent results from engine exhaust analysis, model gas reactor and DRIFTS study give important insights in the activation and deactivation of methane reaction pathways. The results of this study suggest that catalyst formulation and operation strategies of methane conversion should focus on the stimulation of SR and the maintenance of catalyst activity towards SR through targeted periodic lean/rich transitions.
Fast fillings of hydrogen vehicles require proper control of the temperature to ensure the integrity of the storage tanks. This study presents an analysis of heat transfer during filling of a ...hydrogen tank. A conjugate heat transfer based on energy balance is introduced. The numerical model is validated against fast filling experiments of hydrogen in a Type IV tank by comparing the gas temperature evolution. The impact of filling parameters, such as initial temperature, inlet nozzle diameter and filling time is then assessed. For the considered Type IV tank, the results show that both a higher and lower tank shell thermal conductivity results in lower inner wall peak temperatures. The presented model provides an analytical description of the temperature evolution in the gas and in the tank shell, and is thus a useful tool to explore a broad range of parameters, e.g., to determine new hydrogen filling protocols.
•An analytical model is developed to predict the tank shell temperature distribution.•The analytical model is validated against experimental results.•The influence of filling parameters on the tank wall peak temperature is assessed.•The tank materials heat diffusivity is the critical parameter for fast fillings.
Modern direct injection gasoline engines (GDI) generate considerably higher soot particulate matter (PM) emissions than conventional, port fuel gasoline, as well as diesel cars with particle filter. ...Soot PM generated by a typical state-of-the-art GDI (Euro VI) vehicle during two standard international driving cycles (NEDC, WLTC), as well as during a short experimental sub-cycle was investigated by electron microscopy. The study reveals primary particles between ∼4–55nm in diameter, the majority being <20nm during all driven cycles; sub-20nm particles are more abundant in the more dynamic WLTC. Monodisperse agglomerates made of small (<20nm), medium (∼25nm) and large (∼35nm) primary particles are distinguished; they usually form parts of polydisperse agglomerates but also occur as distinct entities. The particle groups of different sizes are probably derived under diverse operating conditions, the larger ones being likely associated with the cold start phase. This inference is supported by simultaneously ran FMPS measurements, which reveal in addition that the cold start phase seems to account for ∼25% of the particle agglomerate emissions of the entire cycle. All particles, especially the medium and large fractions, exhibit low degrees of crystallinity indicating relatively high reactivity. The predominance of sub-20nm primary particles render GDI soot PM potentially hazardous to human health, especially considering the higher surface and surface/volume ratio compared to larger particles of the same total volume. The engine operating parameters seem to be of prime importance for the resulting morphological features of soot.
Brake particle emissions number (PN) and mass (PM) of a light-duty hybrid-electric vehicle have been assessed under realistic driving patterns on a chassis dynamometer. Therefore, the front-right ...disc brake was enclosed in a specifically designed casing featuring controlled high scavenging air ventilation. The WLTC cycle was chosen for most measurements. Different scavenging flow rates have been tested assessing their influence on the measured particles as well as on the temperature of the braking friction partners. Particle transport efficiencies have been assessed revealing scavenging flow rates with losses below 10%. During the performed cycle, most brake particle emissions occurred during braking. There were also isolated emission peaks during periods with no brakes in use, especially during vehicle accelerations. Sequential WLTC cycles showed a continuous decrease in the measured PN and PM emissions; however, size-number and size-mass distributions have been very similar. The measured PN emission factors (>23 nm) at the right front wheel over the WLTC cycle lie at 5.0 × 1010 1/km, whereas the PM emission factor lies at 3.71 mg/km for PM < 12 µm and 1.58 mg/km for PM < 2.5 µm. These values need to roughly triple in order to obtain the brake particle emission of all four brakes and wheels of the entire vehicle. Thus, the brake PN emissions factors have been in the same order of magnitude as the tailpipe PN of a Euro 6 light-duty vehicle equipped with a particle filter. Finally, differences between brake particle emissions in hybrid and all-electric operating modes have been assessed by a series of specific measurements, demonstrating the potential of all-electric vehicle operation in reducing brake particles by a factor of two.
Human-induced biodiversity decline has been on the rise for the past 250 years, due to various causes. What is equally troubling, is that we are unaware which plants are threatened and where they ...occur. Thus, we are far from reaching Aichi Biodiversity Target 2, i.e., assessing the extinction risk of most species. To that end, based on an extensive occurrence dataset, we performed an extinction risk assessment according to the IUCN Criteria A and B for all the endemic plant taxa occurring in Greece, one of the most biodiverse countries in Europe, in a phylogenetically-informed framework and identified the areas needing conservation prioritization. Several of the Greek endemics are threatened with extinction and fourteen endemics need to be prioritized, as they are evolutionary distinct and globally endangered. Mt. Gramos is identified as the most important conservation hotspot in Greece. However, a significant portion of the identified conservation hotspots is not included in any designated Greek protected area, meaning that the Greek protected areas network might need to be at least partially redesigned. In the Anthropocene era, where climate and land-use change are projected to alter biodiversity patterns and may force many species to extinction, our assessment provides the baseline for future conservation research, ecosystem services maintenance, and might prove crucial for the timely, systematic and effective aversion of plant extinctions in Greece.
•Micro-CTs of real ceramic and metallic foams for exact determination of foam structural properties.•Micro-CTs of real foams for deriving equivalent Kelvin cell lattices.•CFD analysis of momentum and ...mass transfer through CT’s of real foams and their Kelvin cell equivalent.•The trade off between Mass transfer and pressure drop is generally 10–15% higher in Kelvin cell structures compare to real foams.
Open cell foams are considered promising catalytic substrates providing high surface area and a tortuous structure resulting in enhanced mass transfer characteristics. CFD analysis, recently, has focused in pointing structures with favourable reactivity-flow resistance characteristics. In order to reduce the geometrical complexity and computational efforts, foams have been modelled as regular (polyhedric) open cell structures. In this study a comprehensive comparison of real foams with equivalent Kelvin cell lattices is performed in CFD. Therefore 4 typical foams (two ceramic and two metallic) have been chosen. Geometric properties have been accessed with Micro-Tomography scans. Randomised Kelvin cell lattices have been generated matching porosity and specific surface area of the scanned real foams. Geometric, momentum and mass transfer characteristics of real foams and Kelvin cell lattices are analysed with CFD. Kelvin cell lattices showed similar behaviour in respect to their real foam equivalents, had though clearly better reactivity-pressure drop trade-offs. Based on the results presented best performances as a catalyst can be expected by 3D printed, additive manufactured, high porosity polyhedric structures.
To address energy transition in the immediate future, synthetic energy carriers are among the most promising options for decarbonization due to their capability to store renewable energy in the long ...term and rely on existing infrastructure. Methane, as a substitute for natural gas, can be produced at high purity through sorption-enhanced processes.
These processes are discontinuous, they require sorbent regeneration once saturation is reached. Therefore, optimal operation of these complex systems requires the control of many time-dependent variables. In this framework, the rapid analyses that can be carried out via dynamic mathematical models could be greatly beneficial in lowering costs at the design stage.
In this study, a model of a sorption-enhanced methanation system was developed and validated with experimental data. The proposed model allows the dynamic behavior of the process to be captured during production and sorbent regeneration. Insights into the process show the behavior of the sorbent bed while retaining water, demonstrating how the bed saturation occurs and how to effectively exploit it.
The model potential was assessed by analyzing the alternating operation of two reactors for continuous production. The importance of bed drying was underlined to ensure the enhancement of the reaction and maximize the conversion.
The reactivity of soot at different sites of the exhaust after-treatment system of a diesel engine (upstream and downstream of the diesel oxidation catalyst (DOC), downstream of the diesel ...particulate filter (DPF), as well as inside the DPF) was investigated on the basis of morphology and structure of primary soot particles by high resolution transmission electron microscopy (HRTEM). The results indicate that combustion-formed soot particles are susceptible to further transformations of their morphology within the exhaust system. The same primary soot particles can possess both oxidation-promoting and oxidation-inhibiting morphological features, the particle cores being highly reactive. Most reactivity-promoting features are encountered in pre-DOC and post-DOC primary particles, suggesting that soot can be more easily oxidised before it enters the DPF. The residence time of soot in the DPF contributes to modification of its reactivity by affecting size distribution and nanostructure of primary particles. Partial NO2 oxidation and high temperatures during active regeneration modify the morphology of outer particle shells, thus rendering post-DOC and post-DPF primary soot particles less reactive in this respect. Primary soot particles that pass through the DPF and reach the atmosphere are characterised by the highest graphitisation degree and sizes larger than those entering the DPF. Complementary Near-Edge X-ray Absorption Fine Structure (NEXAFS) analyses proved not as relevant regarding soot reactivity but indicate higher chemical inhomogeneity of pre-DOC than of post-DOC and post-DPF soot and high contents of carboxyl carbon in post-DPF particles.
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