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This work focuses on the investigation of the relationship between structure and oxidation behavior of particulate matter (PM) emitted from different diesel combustion systems. Commercially available graphite and carbon black materials were also studied as representing the soot with the lowest reactivity. An interpretation of the differences of soot oxidation between these materials is attempted based on the carbon morphology and microstructure; thus, various structural parameters, such as the average particle size, specific surface area, degree of nanostructural organization, average crystallite stacking height, fringe length, fringe tortuosity, and surface functional groups, have been opposed to the reactivity of the carbonaceous materials. Small structural differences were observed in between the carbonaceous samples which seem to directly affect the soot reactivity in terms of oxidation. Particulate matter composition (hydrogen-to-carbon ratio, ash content, and volatile matter) appears to have an impact on the soot reactivity.

Within the general framework of investigating novel routes for solar hydrogen production, the idea of combining a solar centrifugal particle receiver with sulphur thermochemical cycles, involving SO3 dissociation to SO2 and O2 as key step, is pursued. In this perspective, the present work concerns the synthesis, development, evaluation and characterisation of particles suitable to operate as media for direct solar irradiation absorption, transfer and storage as well as catalysts for the SO3 dissociation reaction. Commercial bauxite-based proppants were modified to incorporate raw materials with elements known for their catalytic activity with respect to the SO3 dissociation, namely iron, copper, manganese and their combinations. The catalytic activity of such modified proppants was tested in fixed bed reactor test rigs at 850 °C and ambient pressure with concentrated liquid sulphuric acid as feedstock. Extensive screening tests complemented by physicochemical properties measurements before and after catalytic testing, identified systems that at 850 °C, 1 atm and Gas Hourly Space Velocity of 11,800 h−1 could achieve high SO3 conversions (60%, corresponding to 68% of equilibrium value) for over 125 h of continuous operation. This performance was achieved without degradation of their mechanical strength which, in fact exhibited a slight increase from 53 N in the fresh state to 55 N after long-term exposure to reaction conditions. However such systems were susceptible to colour alteration, affecting adversely their absorptance in the 1000–2500 nm wavelength range. Compositions with the best combination of properties are scheduled for large-scale synthesis and on-site testing in a pilot-scale solar receiver.

Stringent regulation is endorsed to restrict global gaseous and particulate vehicle emissions by adapting advanced catalytic systems on the exhaust, incorporating costly platinum group metals (PGM) for oxidation catalysis. Porous silica and alumina particles are synthesized in the present work via a one-step aerosol spray route. The particles are doped with Pt, Ce and Na and are evaluated with respect to their pore size distribution, exhibiting surface areas between 6-269 m2/gr. The particles have been evaluated in a packed bed reactor with respect to NO oxidation in presence or absence of hydrocarbons (C2H4) in order to study the hydrocarbon inhibition effect on oxidation kinetics, showing higher conversion curves for higher surface area samples. The current work studies also the correlation between reactant accessibility to the in-pore Pt nanoparticles and catalytic activity by reaction kinetic model.

The present work relates to the synthesis of iron and iron oxide based particles via Aerosol Spray Pyrolysis; a continuous and scalable aerosol synthesis method. Different process parameters included the temperature of reactor synthesis, flow rate/type of carrier gas and post treatment temperature as well as precursor solution chemistry, concentration, solvent and reactant type and such parameters are evaluated with respect to their effect on the synthesized particles crystallinity, morphology, iron oxidation state and mean particle diameter of produced powders. Three different iron precursor sources are employed: chloride, nitrate and a dispersible gel derived from a steel industry byproduct. Iron nitrate and chloride aqueous solutions were sprayed at four different concentrations in order to evaluate the effect of the nature of the iron precursor salt on the main attributes of the particle synthesized. Particle diameter was calculated theoretically based on models from the literature describing nucleation at the droplet level, while mean diameter of the synthesized particles was experimentally measured on line at the exit of the reactor. Ex situ TEM characterization provided the necessary morphological data to elaborate on the mechanisms of produced particles formation and explain observed deviations from the theoretically calculated mean particle diameter values.

SSCI-VIDE+ING+FRM:NOG:YSC; International audience; In this study, a robust and efficient decentralized fuel processor based on the direct autothermal reforming (ATR) of biogas with a nominal production rate of 50 Nm3/h of hydrogen and a plant efficiency of about 65% was developed and tested. The ATR unit is composed of a structured catalyst support for the biogas reforming close coupled to a catalytic wall-flow filter to retain eventual soot particles.The performance of the conventional random foam and homogeneous lattice supports structures for the production of hydrogen from the ATR reaction was investigated. 15–0.05 wt%-Ni-Rh/MgAl2O4-SiSiC structured catalyst and LiFeO2-SiC monolith were selected for the conversion of biogas to hydrogen and for the syngas post-treatment process, respectively. For all the experiments, a model synthetic biogas was used and the catalytic activities were evaluated in three different experimental facilities: lab bench, pilot test rig and demonstration plant. High methane conversions (>95%) and hydrogen yields (>1.8) reached in the lab bench were also achieved in the pilot and demonstration plant operating at different GHSV.Results of duration test using a foam coupled to the filter has demonstrated that the pre-commercial processor is reliable while offering a satisfactory reproducibility and negligible pressure drop. A thermodynamic equilibrium and a cold gas efficiency of 90% were reached for an inlet temperature of 500 °C, O/C: 1.1 and S/C: 2.0, as predicted with the Aspen simulation.

To prove the usefulness and achieve penetration of microporous ceramic membranes in gas separation applications of industrial interest, their behavior needs to be validated and predicted at relatively realistic conditions. In this respect, the present study employed hybrid silica (HybSi) membranes, modified by chemical vapor infiltration (CVI) in order to render them more selective to hydrogen in hydrogen/carbon dioxide binary mixtures, which are representative to effluent streams of methane or biogas steam reforming/water gas shift processes. Experimental studies with a single modified membrane exhibited high hydrogen permeance (1.5.10−7 mol.m−2.s−1.Pa−1, at 250 °C) and H2/CO2 permselectivity (H2/CO2 = 61.3, at 250 °C). Gas separation tests with binary gas mixtures revealed that high hydrogen purity values (>99%) can be reached at different process conditions. The mathematical model, also developed in this study in order to interpret the experimental results, can reliably predict hydrogen separation process performance over a wide range of operating conditions and also serve as a tool for subsequent optimum process engineering purposes.

Experimental efforts have shown that thermo-chemical heat storage through cyclic hydration/dehydration of the CaO/Ca(OH)2 couple requires efficient CaO-based particles, in terms of both hydration capacity and structural robustness. Acknowledging the challenge caused by high fragmentation of pure CaO particles during multi-cyclic operation in bed reactors, the development of CaO-based materials with enhanced mechanical properties is essential. Promising results have been obtained for nearly spherical structured formulations using kaolinite as binder. The combination of natural limestone with 25 wt% kaolinite rendered mechanically strong materials with a hydration capacity of up to 50% compared to the maximum hydration capacity of pure CaO. These formulations remained intact and showed stable reactivity in the course of 10 hydration/dehydration cycles. In order to obtain valid conclusions for the suitability of these materials for the here suggested reaction scheme, examination upon multiple hydration/dehydration cycles was necessary. The current work is related to the assessment of CaO-kaolinite composite materials under three different evaluation protocols, as well as to the examination of their mechanical properties through crushing strength measurements and attrition tests. For most of the materials examined, more satisfactory results were obtained for hydration and dehydration reactions in vapor-rich atmosphere at 450 and 550 °C, respectively. Multi-cyclic evaluation (50–200 cycles) confirmed the initial findings of the 10-cycle tests. The final selected composition constituted a promising material for the suggested reaction scheme and was qualified as in-principle suitable for long-term operation in pilot scale units.

Solar driven hybrid sulfur-ammonia water splitting cycle (HySA) integrates a solar-photocatalytic hydrogen, H 2 , production step (H 2 sub-cycle) with a high-temperature solar thermochemical oxygen, O 2 , evolution step (O 2 sub-cycle), implementing efficient thermal energy storage as part of the cycle operation. Previous studies of the cycle omitted intermediate products, such as ammonium bisulfate, from the O 2 sub-cycle and, thus, neglected their potential impact on the cycle's chemistry. Also, there are discrepancies in reported literature for the thermodynamic properties of ammonium sulfate, (NH 4 ) 2 SO 4, and ammonium bisulfate, NH 4 HSO 4 . In this study, thermal analysis experiments were conducted in order to determine the phase transition temperatures and enthalpies, and the heat capacity temperature dependence of the ammonium sulfate, (NH 4 ) 2 SO 4, and ammonium bisulfate, NH 4 HSO 4 . Our experimentally determined values for these parameters agree well with the data reported in DIPPR Project 801 database. Moreover, an exploratory thermodynamic analyses was performed using AspenPlus © and FactSage © , that included all potential reaction products, in order to identify critical parameters for an optimum O 2 sub-cycle. A methodology is proposed and evaluated to mitigate AspenPlus © 's deficiency to handle solid phase changes. The thermodynamic analyses demonstrate that the NH 4 HSO 4 inclusion in the O 2 sub-cycle reduces the overall process energy requirements, and allows its use as an energy storage medium. Finally, we show that the use of molten salts, in combination with their interactions, significantly affects the efficiency and the operating conditions of the process, as well as the state of the mixtures.

Solar-driven hybrid sulfur–ammonia water splitting cycle (HySA) is a promising technology for energy and environment applications. The advantage of the proposed cycle is the utilization of both solar photon and thermal radiation in a series of reaction steps from ambient temperature to less than 900 °C. It uses molten salts as reagents to control products of each step and as potential thermochemical energy storage. The use of a solar aerosol based reactor for the thermochemical steps of the cycle appears promising, therefore a particle model is required. Reliable thermodynamic data are necessary to develop an efficient conceptual particle model. Therefore, in this present study, we perform thermal analysis experiments and thermodynamic calculations for the related compounds and their reciprocal mixtures. Based on the experimental and numerical findings, we discuss the conceptual particle model according to the thermochemical steps of the hybrid sulfur–ammonia water splitting cycle.

This study aims at developing a kinetic model that can adequately describe solar thermochemical water and carbon dioxide splitting with nickel ferrite powder as the active redox material. The kinetic parameters of water splitting of a previous study are revised to include transition times and new kinetic parameters for carbon dioxide splitting are developed. The computational results show a satisfactory agreement with experimental data and continuous multicycle operation under varying operating conditions is simulated. Different test cases are explored in order to improve the product yield. At first a parametric analysis is conducted, investigating the appropriate duration of the oxidation and the thermal reduction step that maximizes the hydrogen yield. Subsequently, a non-isothermal oxidation step is simulated and proven as an interesting option for increasing the hydrogen production. The kinetic model is adapted to simulate the production yields in structured solar reactor components, i.e. extruded monolithic structures, as well.

Production of fuel-cell grade hydrogen via reforming of gaseous hydrocarbon fuels requires the design and implementation of efficient formulations, which incorporate high performance materials for the removal of undesired impurities; notably H2S. To this respect, the present study relates to the preparation and parametric evaluation of small-scale honeycomb structured flow-through reactors, coated with an in-house synthesized ZnO/CuO, 50%mol/50%mol, sorbent. Experimental tests involved quantification of the H2S trapping capacity and efficiency of the coated monoliths by monitoring the evolution of H2S concentration at the reactor outlet for known H2S feeds at the reactor inlet. The de-H2S performance of the coated monoliths was examined at reactor temperatures ranging from 160 °C to 400 °C, under two different synthetic mixtures containing a) 30%vol H2, 32%vol H2O, N2 balance and b) 30%vol H2, 32%vol H2O, 6.5%vol CO2, 4.5%vol CO, N2 balance. Space velocities at the reactor ranged from 20,000 to 60,000 h−1 for the experiments designed to examine trapping capacity and from 10,000 to 90,000 h−1 for the experiments designed to examine trapping efficiency. The breakthrough curves obtained proved that the particular formulation can provide a very efficient solution, particularly for the case of decentralized applications, for which system compactness is of prime importance. Total capture capacities up to a targeted H2S breakthrough value of 0.1 ppmv at the reactor outlet varied between 5 and 25 (gsulphur/100 × gsorbent). This value depended on reactor temperature and flow conditions, gas composition, monolith loading and monolith cell density.

Being a relatively new but quite promising field in CSP, thermochemical energy storage could be the next generation solution towards the implementation of baseload cost-effective solar thermal plants. The present work, taking place within the framework of EU-funded project PEGASUS, aims at synthesizing, shaping and experimentally evaluating the catalytic activity and main physicochemical attributes of Fe2O3−based particles. These particles can be exploited as both heat transfer fluid of the CSP plant and highly active catalysts for the key step of the SO3 dissociation reaction to produce SO2 and O2 in the framework of a solid sulfur thermochemical energy storage cycle. The experimental campaign takes place in a purpose-built setup used to evaluate the materials in a fixed bed reactor formulation at a temperature of 850°C and ambient pressure while concentrated liquid sulfuric acid (H2SO4) was used as feedstock. Based on the findings, pure Fe2O3 particles sintered at 950°C offered the most promising compromise between high SO3 conversion on the one hand and crushing strength value (i.e. a preliminary measure of thermomechanical stability) and particles’ color before and after exposure at reaction conditions on the other hand.

The present study relates to the preparation and evaluation of small-scale honeycomb structures as compact reactors/heat exchangers via exploitation of the cobalt/cobaltous oxide (Co3O4/CoO) cyclic reduction–oxidation (redox) heat storage scheme. The structures considered included in-house extruded monoliths (pure cobalt oxide and cobalt oxide/alumina composites) and commercial cordierite substrates coated with Co3O4. The samples were subjected to multi-cyclic redox operation under air flow, in the temperature range of 700–1000 °C. Reduction occurred during heating up to 1000 °C, while oxidation took place during cooling. Redox performance was evaluated on the basis of on-line oxygen release/consumption measurements, while continuous monitoring of imposed air flow reactor inlet/outlet temperatures facilitated the preliminary estimation of heat dissipation in the duration and after completion of the exothermic reaction (oxidation). For all samples, redox performance remained stable in the course of multi-cyclic exposure. In terms of heat transfer, there is strong indication that both composition and the geometry of the honeycomb are important. The pure Co3O4 extruded honeycomb exhibited the highest heat dissipation efficiency but suffered from severe deformation upon multi-cyclic operation. The addition of a small amount of alumina in the aforementioned composition (10% on the basis of total initial mass of oxides), particularly when combined with an increase of the honeycomb wall thickness, substantially improved macro-structural stability upon thermal/redox cycling. The Co3O4-coated cordierite monoliths showed essentially the same normalised redox performance with the pure Co3O4 extruded honeycomb, however the overall heat dissipation achieved was lower. Regarding the effect of redox cycling on the structural stability of studied formulations, pure Co3O4 samples exhibited notable swelling. In the case of the extruded body, this resulted to structural collapse while for the coated cordierite honeycomb, expansion of the coating layer led to partial channels blocking. Based on relevant morphological and structural post-analysis, it was concluded that formation of cobalt aluminate largely reduced swelling intensity.

The present work aims to establish a generic reforming reaction scheme to evaluate the performance of catalytic reforming systems with the aid of a one-dimensional heterogeneous dynamic model. The novelty of the numerical model stems from the direct inclusion of interphase (fluid-to-particle surface), intraparticle (within particle), and intrareactor heat and mass transport resistances under transient conditions. The developed model accounts for the multicomponent gas mixture physicochemical properties and correlations for calculating mass and heat transfer coefficients. Effective macroscopic properties within the particle are calculated by incorporating diffusivities and conductivities of the porous network characteristics accounting for Knudsen and molecular transport as well as tortuosity and porosity of the overall porous path. The industrial case of a steam-methane reforming multitubular reactor was studied as the most representative case of the generic reaction scheme, with all mass/energy resistances present under severe pressure and temperature conditions. It was shown that there are notable diffusional limitations within the particle, whereas there are also temperature and partial pressure gradients due to the heat and mass transport resistances in the particle film layer. It is further demonstrated that the proposed model can be utilized as a versatile design tool for catalytic reactor development and optimization.

Nanoparticles (NPs), due to their increased application and production, are being released into the environment with unpredictable impact on the physiology of marine organisms, as well as on entire ecosystems and upcoming effects on human health. The aim of the present study was to evaluate and compare the oxidative responses of the mussel Mytilus galloprovincialis after exposure to iron oxide NPs and to iron oxide NPs incorporated into zeolite for 1, 3 and 7 days. Our results showed that both effectors induced changes on animal physiology by causing oxidative stress in hemocytes of exposed mussels compared to control animals. This was shown by the significant increase in reactive oxygen species (ROS) production, protein carbonylation, lipid peroxidation, ubiquitin conjugates and DNA damage. In addition an increase in prooxidant levels as measured by the prooxidant-antioxidant balance (PAB) assay was observed in exposed mussels' hemolymph. The results show that ROS, DNA damage, protein and lipid oxidation, ubiquitin conjugates and PAB could constitute, after further investigation, reliable biomarkers for the evaluation of pollution or other environmental stressors. In addition, more studies are needed in order to ensure the safety of these NPs on various biomedical applications, since it is critical to design NPs that they meet the demands of application without causing cellular toxicity.

The thermal reduction step of substituted ferrites (MFe2O4 where M = Fe, Ni, Co, Gd) and doped ceria (MxCe1-xO2, where M = Ce, Zr, Hf and x = 0.25) in two-step thermochemical cycles for H2O and CO2 splitting is investigated within the DFT+U framework. This thermal reduction step is described as the oxygen vacancy formation energy (reduction enthalpy), i.e. the energy required to create an oxygen vacancy in the crystal lattice. Oxides with a lower oxygen vacancy creation energy are easier to reduce. A Bader charge analysis of the reduction mechanism is carried out providing the charge distribution of the bulk and reduced ions, enabling interrelations of the substitute ions and the resulting reduction energies. Based on the approach presented here, interesting solar fuels producing materials are CoFe2O4, NiFe2O4 and Hf0.25Ce0.75O2.

The present study relates to the preparation of mixed calcium oxide–alumina compositions as candidate materials for a cyclic thermochemical hydration–dehydration scheme at moderate to high temperatures (e.g. 400–600 °C) that can offer the possibility of short and long term energy storage, particularly suitable for concentrated solar power installations. The synthesized materials were assessed in terms of their cyclic hydration–dehydration performance in the temperature range of 200–550 °C. Acknowledging the fact that the particular thermochemical scheme has been identified to result in substantial cycle-to-cycle fragmentation of pure CaO particles which is detrimental to particle reactor bed concepts, one of the main purposes of using Al as additive is related to materials structural enhancement. To this respect, preliminary studies related to macro-structural integrity assessment were also conducted. In addition, the performance of synthesized material is compared to the one of natural lime (benchmark material). The additive content spanned over a wide range of Ca/Al molar ratios, namely from 95/5 to 52/48, while two different calcium oxide precursors, i.e. calcium nitrate and calcium acetate, were employed. Fresh and hydrated compositions were characterized in-detail with respect to their physicochemical properties in order to correlate different behaviors with certain attributes of the materials. Synthetic materials, both calcium nitrate and calcium acetate derived, favored the formation of Ca/Al mixed phases. The latter led to materials with higher surface areas and, for a given Ca/Al ratio, resulted to higher hydration/dehydration performance. Mixed oxides, although capable of being hydrated at ambient temperature, did not participate in the reaction scheme at temperatures ⩾200 °C and thus presence of such phases resulted in considerable decrease of hydration/dehydration capacity versus the one of natural lime. On the other hand, the presence of such mixed compositions improved, albeit not dramatically, macro-structural integrity. A relatively good combination of hydration–dehydration performance with better-than-natural lime structural integrity was achieved for the mixed materials with a Ca/Al molar ratio equal to 89/11 and 81/19 molar ratio. The Ca-precursor used in these materials slightly affected their cyclic performance with the ex-CaN ones presenting better behavior.

The “holy grail” of solar chemistry and solar engineering, is the technical storage of solar energy into a more easily transformable and transportable form, namely an energy carrier such as H2. The two-step redox-based solar thermochemical H2O splitting cycle is considered to be among the most promising approaches for the production of H2 from entirely renewable sources (solar energy and water). In this redox cycle an active material is initially reduced thermally under inert atmosphere and at the next step it is oxidized from H2O producing H2. The materials that have been in the core of solar chemistry research are metal oxides such as ferrites, cerium oxides, perovskites, etc. The reactor types that are being investigated for the redox thermochemical splitting of H2O are either powder-particle reactors or structured reactors. In the current work Ni-ferrite and Ce-oxide structured into different monolithic bodies (honeycombs, foams) were evaluated w.r.t. their redox activity. Based on this investigation, the most promising structure was further scaled-up for the construction of the full-scale reactors of the HYDROSOL-PLANT solar plant installation.The “holy grail” of solar chemistry and solar engineering, is the technical storage of solar energy into a more easily transformable and transportable form, namely an energy carrier such as H2. The two-step redox-based solar thermochemical H2O splitting cycle is considered to be among the most promising approaches for the production of H2 from entirely renewable sources (solar energy and water). In this redox cycle an active material is initially reduced thermally under inert atmosphere and at the next step it is oxidized from H2O producing H2. The materials that have been in the core of solar chemistry research are metal oxides such as ferrites, cerium oxides, perovskites, etc. The reactor types that are being investigated for the redox thermochemical splitting of H2O are either powder-particle reactors or structured reactors. In the current work Ni-ferrite and Ce-oxide structured into different monolithic bodies (honeycombs, foams) were evaluated w.r.t. their redox activity. Based on this investigation,...

The present work is an experimental investigation of commercial diesel fuel liquid desulfurization via adsorption under mild conditions. The sorbent employed was a commercial high surface area activated carbon (AC), and studies involved both laboratory- and pilot-scale experiments performed in dedicated fixed-bed setups. Under laboratory-scale conditions, maximum sulfur removal measured exceeded 90%, while according to breakthrough curves the total sulfur content remained below 2 ppmw for up to 20–22 mL of processed diesel per gram of sorbent. Process scaling-up by a factor of 15 showed a moderate negative effect, with the respective breakthrough fuel amount (total sulfur ≤2 ppmw) being ∼15–17 mL processed fuel/g sorbent. Several sorbent regeneration strategies were studied under laboratory-scale conditions. The one with the highest restoration of initial (i.e., fresh state) AC performance involved heating under vacuum (200 mbara) up to 200 °C and subsequent washing of the material with a binary organic s...

Catalysts for direct soot oxidation in catalyzed diesel particulate filters (CDPFs) consist typically of various mixed oxide compositions (frequently with CeO2 as the dominant component) that assist soot oxidation by enhancing the supply of oxygen from the catalyst to the soot. Apart from the composition, the material morphological characteristics may also contribute to the catalytic activity and this is the motivation for the present study. Different CeO2 nanoparticle catalysts have been obtained employing aerosol-based synthesis (ABS) and sol–gel methods. The obtained catalyst particles have been characterized with respect to their physical and morphological properties as well as with respect to their catalytic soot oxidation activity. The results have been analyzed with the aid of a multi-population kinetics model where soot is found to consist of three fractions reacting with different activation energies, namely 120, 180, and 240 kJ/mol. The occurrence of these three fractions is attributed to the formation of distinct families of surface oxygen complexes (SOCs) on the carbon surface which are subsequently gasified and hence cause soot oxidation, in agreement with accepted mechanisms of soot oxidation in the literature. The CeO2 nanoparticles oxidize catalytically all three fractions of soot, but with different “enhancement factors,” while the activation energies during catalytic oxidation remain the same. A comparison of the catalytic pre-exponentials to those of plain soot shows, in most cases, enhancements, which for some catalysts, can be up to ∼4.5, 6.5, and 2 times larger than those of plain soot, reflecting the relative capacity of the catalyst to generate more of the respective SOCs. The developed approach provides a more detailed but tractable way to describe soot oxidation (plain and catalytic), which can be readily incorporated into simulations of actual emission control systems to increase their reliability.

The present study deals with the mechanical properties of structured reactors/heat exchangers, for high temperature heat storage via the cobalt oxide cyclic redox scheme. Two different structures (i.e. honeycomb and perforated block) and two different compositions (i.e. 100% cobalt oxide and 90 wt% cobalt oxide – 10 wt% aluminium oxide) were evaluated. During thermal cycling in the range of 800-1000 o C, different loads were applied to the sampleswhile monitoring their length variation. The integrity of the samples was assessed after every cycle. It was found that mechanical strength was substantially improvedupon addition of 10 wt% aluminium oxide. The cobalt oxide/alumina composite presented lower maximal expansion during cycling and exhibited higher integrity, already after one thermal cycle. Another important result is that, for both the honeycomb and the perforated block, the load decreases the over-all sample net expansion. Moreover, the perforated block exhibited lower expansion and better mechanical strength as compared to the honeycomb. Due to the better chemical performance expected to be achieved by the honeycomb structure, a compromise between these two structureshas to be chosen (e.g. honeycomb structure with thicker walls). The results are used for building a thermochemical storage system prototype, implemented for the first time in an existing concentrated solar power facility (STJ).

The Co 3 O 4 /CoO redox system has been recently proposed and is currently under consideration by several research groups as a promising thermochemical heat storage (THS) scheme to be coupled with high temperature Concentrated Solar Power plants. The present work is an investigation of cobalt oxide based honeycomb structures as candidate reactors/heat exchangers in relevant compact and efficient THS systems. The formulations studied included extruded bodies from pure cobalt oxide and two different cobalt oxide/alumina composites (i.e. 95/5 wt% and 90/10 wt%) as well as cobalt oxide-coated cordierite honeycombs with two different loadings (i.e. 28% and 65%). The structures were evaluated with respect to their redox performance in the course of 5 successive cycles in the temperature window of 800-1000 o C and under air flow. Based on measured oxygen evolution profiles, honeycombs from pure cobalt oxide and cobalt oxide-coated cordierites exhibited very similar normalized (i.e. μmol O 2 /g Co 3 O 4 ) redox performance. On the other hand, the addition of alumina had a moderately negative effect on normalized redox performance versus the two aforementioned formulations but contributed to the substantial increase of the honeycombs structural stability when compared to the extruded pure cobalt oxide monolith. The particular finding was based on performing, in a separate experimental setup, 10 redox cycles under idealized (i.e. no imposed loads) conditions. Pre- (i.e. fresh/calcined) and post-characterization (i.e. after 10 redox cycles) of extruded structures via mercury porosimetry revealed measurable decrease of bulk density and increase of mean pore size, indicating a net structure expansion/‘swelling’ effect.

Ce-based materials, that have been extensively investigated for catalytic applications, such as in automotive emission control and more specifically in the oxidation of diesel soot particles, were evaluated with respect to their performance on CO2 splitting. DFT calculations assisted towards the synthesis of two Zr-doped Ce-oxide formulations (Zr content: 20% and 40% respectively) via the LPSHS method. The materials were evaluated with respect to their CO2 splitting activity and compared with other Ce-Zr-based reference oxides. The most active materials were further evaluated under different CO2 splitting temperatures in the range of 800-1480oC in the course of successive redox cycles. The effect of thermal reduction temperature (T: 1480oC and 1600oC) on the most active material was also assessed. The Ce-based oxide formulation with the best performance (Ce0.8Zr0.2O2) was further investigated with respect to its activity towards H2O splitting. In addition, a porous flow-through structure consisting entirely of Ce0.8Zr0.2O2 was manufactured and the resulted body was also evaluated and compared to the respective powder in terms of its CO2 and H2O splitting ability.

A methodology is introduced, the Collision Rate Method (CRM), to calculate the friction coefficient of power-law aggregates across the entire momentum-transfer regime. The friction coefficient is calculated via the ratio of two fictitious particle-aggregate collision rates evaluated in the continuum and slip-flow regimes. The effective collision rates are obtained from the numerical solution of the Laplace equation with Robin boundary condition. The methodology was justified by comparing the slip correction factor of straight-chains composed of up to 50 spherical monomers with literature results. We determined the validity of the CRM to lie in an extended slip-flow regime, the maximum monomer Knudsen number being 2. We calculated the adjusted-sphere radius, the radius of a sphere with the same slip correction factor as the chain, in slip flow. We found it to be weakly dependent on flow conditions, as specified by the carrier-gas mean free path, a dependence that leads to a weak effect on calculated slip correction factors. The CRM was combined with the Adjusted-Sphere Method to extend its validity to all Knudsen numbers. Excellent agreement of calculated, straight-chain, slip correction factors with literature values was obtained for monomer Knudsen numbers up to 100. Various characteristic length scales, geometric (equivalent volume radius, radius of gyration) and dynamic (equivalent hydrodynamic and mobility radii), were calculated.

The influence of geometric properties and particle size on mobility properties of fractal-like aggregates was studied in the mass and momentum-transfer transition regimes. Two methodologies were investigated. The Collision Rate Method (CRM) that determines the slip correction factor through the ratio of two fictitious Brownian particle-aggregate effective collision rates, and the Adjusted Sphere Method (ASM) that assumes the existence of a virtual, flow-independent adjusted sphere with the same slip correction factor as the aggregate over the entire transition regime. The simulated fractal-like aggregates were synthetic as they were generated via a cluster–cluster agglomeration algorithm. The CRM was used to calculate the adjusted-sphere radius of various aggregates: we found it to be weakly dependent on the monomer Knudsen number for Kn greater than 0.5. Numerical expressions for the aggregate orientationally-averaged projected area and the adjusted-sphere radius are proposed. Both expressions depend on geometric, non-ensemble averaged quantities: the radius of gyration and the number of monomers. The slip correction factor and the mobility radius of DLCA and RLCA aggregates were calculated using the ASM: for a given number of monomers, fractal dimension and prefactor, and Knudsen number their values were approximately constant (averaged over aggregate realizations). A fractal-like scaling law based on the mobility radius was found to hold. The mobility fractal dimension and prefactor were determined for different aggregates. The hydrodynamic radius, proportional to the friction coefficient, and the dynamic shape factor of DLCA and RLCA aggregates were also calculated.

The present work provides an overview of activities performed in the framework of the EU-funded collaborative project RESTRUCTURE, the main goal of which was to develop and validate a compact structured reactor/heat exchanger for thermochemical storage driven by 2-step high temperature redox metal oxide cycles. The starting point of development path included redox materials qualification via both theoretical and lab-scale experimental studies. Most favorable compositions were cobalt oxide/alumina composites. Preparation of small-scale structured bodies included various approaches, ranging from perforated pellets to more sophisticated honeycomb geometries, fabricated by extrusion and coating. Proof-of-concept of the proposed novel reactor/heat exchanger was successfully validated in small-scale structures and the next step included scaling up of redox honeycombs production. Significant challenges were identified for the case of extruded full-size bodies and the final qualified approach related to preparation of cordierite substrates coated with cobalt oxide. The successful experimental evaluation of the pilot reactor/heat exchanger system constructed motivated the preliminary techno-economic evaluation of the proposed novel thermochemical energy storage concept. Taking into account experimental results, available technologies and standard design aspects a model for a 70.5 MWe CSP plant was defined. Estimated LCOE costs were calculated to be in the range of reference values for Combined Cycle Power Plants operated by natural gas. One of main cost contributors was the storage system itself, partially due to relatively high cost of cobalt oxide. This highlighted the need to identify less costly and equally efficient to cobalt oxide redox materials.

Samples of surface carbonates were collected from three different areas of the Qatar peninsula. We employed material characterization techniques to examine the morphology and composition of the samples, while their CO2 capture capacity was assessed via multiple successive calcination-carbonation cycles. Our samples were mainly calcite and dolomite based. Calcite samples showed higher initial capacity of around 11 mmol CO2 g-1 which decayed rapidly to less than 2 mmol CO2 g-1. On the other hand, dolomite samples showed an excellent stability (∼15 cycles) with a capacity of 6 mmol CO2 g-1. The performance of the dolomite samples is better compared to other similar natural samples, from literature. A promising result for future studies towards improving their performance by physical and chemical modification.

SSCI-VIDE+ING+FRM:NOG:YSC; International audience; The present work deals with the modelling and simulation of a biogas Demo-processor for green hydrogen production via Autothermal reforming (ATR) process aimed at covering a wide span of potential applications, from fuel cells feed up to the production of pure hydrogen. The biogas ATR unit is composed of a structured catalyst support close coupled to a wall-flow filter that retain soot particles that can be formed during the ATR reaction. Modelling and simulation (CFD and FEM) were carried out to select the innovative catalyst support with promising results for the fuel processor. 3D digital sample reconstruction was performed for the selection of the appropriate porous structures commercially available for the soot filtration and furthermore, 2D CFD analysis was also used to examine flow uniformity issues due to soot trap integration downstream to the ATR. Moreover, the inherent flexibility of the model performed allowed its application in the assessment of the Demonstration plant operating in real conditions. Besides, Aspen simulation has demonstrated that the ATR process is the most promising process to hydrogen production compared to other types of reforming process.

In a previous work of the authors ( AIChE Journal 2013; 59(4): 1213-1225 ) on the characterization of the performance of redox material compositions during two-step thermochemical splitting of water, it was observed that fitting of the obtained hydrogen and oxygen concentration profiles with a reaction model based on simple first order reaction rates could describe adequately only the first part of the evolution curves. This suggested that more complicated reaction models taking into account the structure of the redox material are needed to describe the whole extent of the experimental data. Based on the above, a minimum set of experiments for water splitting thermochemical cycles over a Nickel-ferrite was deigned and performed involving an increased duration of the reaction steps. A new extended model was derived for the water splitting and thermal reduction reactions, which considers two oxygen storage regions of the redox material communicating to each other by a solid state diffusion mechanism. The inclusion of two state variables instead of one has a significant effect on the reaction dynamics and renders the model capable to explain the dynamics of the convergence of the thermochemical cycles to a periodic steady state, observed experimentally in the previous work.

The present study addresses the feasibility to synthesize aluminium nanoparticles (NPs) from micron-sized aluminium powder with the use of a customized atmospheric plasma spraying (APS) technique. Using APS, nanoparticle synthesis can be achieved via rapid melting and vaporization of the initial micrometric particles and their subsequent re-nucleation. A custom mantle system was designed and developed with the aid of relevant simplified CFD simulations. The mantle provided the necessary inert environment (argon), at ambient pressure, in order to avoid any oxidation of the metal during plasma spraying while promoted rapid quenching of the gasified metal. The particles formed were collected with the aid of a quartz filter downstream of the plasma flame and the production rate achieved was 2 g min−1. Ex situ post-characterization of the particles via X-ray diffraction, specific surface area measurement (BET), transmission electron microscopy (TEM) coupled with energy dispersive spectrometry (EDS) and thermogravimetric analysis (TGA) under air revealed that the powders obtained primarily comprised of monocrystalline metallic aluminium nanoparticles of almost spherical shape. The NPs possessed a 2–5 nm oxide coating layer. By regulating the conditions inside the mantle, a variety of different size distributions were obtained.

The current work relates to the development of synthetic calcium oxide (CaO) based compositions as candidate materials for energy storage under a cyclic carbonation/decarbonation reaction scheme. Although under such a cyclic scheme the energy density of natural lime based CaO is high (∼ 3MJ/kg), the particular materials suffer from notable cycle-to-cycle deactivation. To this direction, pure CaO and CaO/Al2O3 composites have been prepared and preliminarily evaluated under the suggested cyclic carbonation/decarbonation scheme in the temperature range of 600-800°C. For the composite materials, Ca/Al molar ratios were in the range between 95/5 and 52/48 and upon calcination the formation of mixed Ca/Al phases was verified. The preliminary evaluation of materials studied was conducted under 3 carbonation/decarbonation cycles and the loss of activity for the case of natural CaO was obvious. Synthetic materials with superior stability/capture c.f. natural CaO were further subjected to multi-cyclic carbonation/decarbonation, via which the positive effect of alumina addition was made evident. Selected compositions exhibited adequately high CO2 capture capacity and stable performance during multi-cyclic operation. Moreover, this study contains preliminary experiments referring to proof-of-principle validation of a concept based on the utilization of a CaO-based honeycomb reactor/heat exchanger preliminary design. In particular, cordierite monolithic structures were coated with natural CaO and in total 11 cycles were conducted. Upon operation, clear signs of heat dissipation by the imposed flow in the duration of the exothermic reaction step were identified.

Solar methane decomposition reactors are a novel technology for the production of carbon neutral hydrogen; however, the impact of this technology depends greatly on the ability to co-produce carbon black particles of commercial grade in order to offset the cost of hydrogen production and, therefore, the control of the reactor is very important. To this end, the seeding of indirect heating concept reactors using the product particles themselves could be used to control heat transfer inside the reactor. In this work, a previously developed one-dimensional reactor – particle population model was used to simulate the effect of seeding on the hydrogen and carbon particle production rates in the absorber tubes of a 10 MW indirect heating concept solar reactor. It was found that seed particle feed rates less than 10% of the methane-contained carbon feed rate allowed the hydrogen and fresh particle production rates to be doubled while keeping the rate of carbon growth on the tube walls constant. It was also found that similar seed fee rates could be used to maintain the hydrogen and particle production rates constant, given variations in the absorber tube wall temperature within a 100 °C range, for example due to cloud passage. Furthermore, it was found that the size characteristics of the freshly produced particles were not affected at these seed feed rates. Thus, seeding could be an effective means for increasing and controlling the hydrogen and carbon particle production rates in industrial scale indirect heating concept solar methane decomposition reactors, while also reducing carbon growth on the walls of the absorber tubes.

Redox-pair-based thermochemical cycles are considered as a very promising option for the production of hydrogen via renewable energy sources like concentrated solar energy and raw materials like water. This work concerns the synthesis of various spinel materials of the iron and aluminum families via combustion reactions in the solid and in the liquid-phase and the testing of their suitability as redox-pair materials for hydrogen production by water splitting via thermochemical cycles. The effects of reactants' stoichiometry (fuel/oxidizer) on the combustion synthesis reaction characteristics and on the products' phase composition and properties were studied. By fine-tuning the synthesis parameters, a wide variety of single-phase, pure and well crystallized spinels could be controllably synthesized. Post-synthesis, high-temperature calcination studies under air and nitrogen at the temperature levels encountered during solar-aided thermochemical cyclic operation have eliminated several material families due to phase composition instabilities and identified among the various compositions synthesized NiFe 2 O 4 and CoFe 2 O 4 as the two most suitable for cyclic water splitting – thermal reduction operation. First such thermochemical cyclic tests between 800 and 1400 °C with NiFe 2 O 4 and CoFe 2 O 4 in powder form in a fixed bed laboratory reactor have demonstrated capability for cyclic operation and alternate hydrogen/oxygen production at the respective cycle steps for both materials. Under the particular testing conditions the two materials exhibited hydrogen/oxygen yields of the same magnitude and similar temperatures of oxygen release during thermal reduction.

This work is a follow-up of previous efforts reported on the synthesis of various single and mixed oxide materials and their evaluation as catalysts for the sulfuric acid dissociation reaction for the production of SO 2 and O 2 . The current work concerns the comparative assessment of Fe 2 O 3 , CuO, Cu–Fe, Fe–Cr, Cu–Al and Cu–Fe–Al mixed oxides coated as catalysts on silicon carbide monolithic honeycomb structures, with respect to sulfuric acid decomposition reaction conditions for 100 h at 850 °C and ambient pressure, as well as their ex-situ characterization after such operation. The exposure conditions are representative to a potential future real application. The exposure time, although of relatively short-term, is adequate to extract safe conclusions on the stability and therefore to a large extent also on the suitability of the candidate oxide-based catalysts. All catalytic systems tested exhibited high SO 3 conversions reaching or exceeding 70%, for space velocities in the range of 5–35 h −1 . For some of the samples, the relatively high initial activity decreased by about 5–10 percentage points in the course of the 100 h testing, reaching stable mean values. It was concluded that Fe 2 O 3 , CuO and Fe–Cr mixed oxide retained their chemical and structural stability after exposure to reaction conditions, while the other three mixed oxides studied suffered from significant phase decomposition phenomena. Based on the fact that the initial catalytic activity of the Fe–Cr mixed oxide, as identified in a previous comparative study among several materials, was found higher than the ones of Fe 2 O 3 and CuO and relatively close to the one of the highly active but costly Pt/Al 2 O 3 catalyst, the particular mixed oxide is considered a promising catalyst for the SO 3 dissociation reaction.

A two-dimensional model of methane thermal decomposition reactors is developed which accounts for coupled radiative heat and polydisperse carbon particle nucleation, growth, and transport. The model uses the Navier–Stokes equations for the fluid dynamics, the radiative transfer equation for methane and particle species radiation absorption, the advection–diffusion equation for gas and particle species transport, and a sectional method for particle species nucleation, heterogenous growth, and coagulation. The model is applied to a tubular laminar flow reactor. The simulation results indicate the development of a reaction boundary layer inside the reactor, which results in significant variation of the local particle size distribution across the reactor. © 2011 American Institute of Chemical Engineers AIChE J, 58: 2545–2556, 2012

The hydrogen production based on the exploitation of the water splitting reaction and of solar energy on monolith type reactors is a novel and very promising process. Several trade offs between reaction efficiency and material stability appear during the operation of the process rendering necessary advanced optimization and control using appropriate mathematical models. The complexity of the geometric and operating features of the process prevents from the use of a general model for the optimization studies so a model splitting in several size scales procedure is proposed. It is shown that in case of silicon carbide as monolith material the one dimensional single channel model is the most appropriate to be used for the process operation optimization studies. The above general principles are described in detail in the present paper to shed light to the particular modeling problem with the emphasis given in the work performed in our laboratory.

The present work describes the realisation and successful test operation of a 100 kW pilot plant for two-step solar thermo-chemical water splitting on a solar tower at the Plataforma Solar de Almeria, which aims at the demonstration of the feasibility of the process on a solar tower platform under real conditions. The process applies multi-valent iron based mixed metal oxides as reactive species which are coated on honeycomb absorbers inside a receiver–reactor. By the introduction of a two-chamber reactor it is possible to run both process concepts in parallel and thus, the hydrogen production process in a quasi-continuous mode. In summer 2008 an exhaustive thermal qualification of the pilot plant took place, using uncoated ceramic honeycombs as absorbers. Some main aspects of these tests were the development and validation of operational and measurement strategy, the gaining of knowledge on the dynamics of the system, in particular during thermal cycling, the determination of the controllability of the whole system, and the validation of practicability of the control concept. The thermal tests enabled to improve, to refine and finally to prove the process strategy and showed the feasibility of the control concept implemented. It could be shown that rapid changeover between the modules is a central benefit for the performance of the process. In November of 2008 the absorber was replaced and honeycombs coated with redox material were inserted. This allowed carrying out tests of hydrogen production by water splitting. Several hydrogen production cycles and metal oxide reduction cycles could be run without problems. Significant concentrations of hydrogen were produced with a conversion of steam of up to 30%.

The present work concerns the synthesis of various single and mixed oxide materials and their study as catalysts for the sulfuric acid dissociation reaction via which the production of SO2 and O2 is achieved. This is the most energy intensive step of sulfur-based thermochemical cycles for the production of hydrogen. Commercial (i.e. FeO, Fe3O4 ,F e2O3, CuO, Cr2O3, g-Al2O3, Pt/g-Al2O3) and in-house binary and ternary compositions of the CueFeeAl system as well as FeeCr mixed oxide materials prepared by the Solution Combustion Synthesis (SCS) technique were comparatively tested. The materials were studied in

The solar thermal decomposition of methane is a promising route for the large scale production of hydrogen and carbon black with zero CO2 emissions, however careful control of the reactor is required to ensure product particles of specific sizes. A one-dimensional model employing a sectional method is developed to simulate the evolution of polydisperse fresh and seed particle populations in an indirectly heated solar reactor. The model accounts for the homogeneous nucleation of fresh particles, the heterogeneous growth of the fresh and seed particles, particle coagulation, and the growth of carbon on the walls of the reactor from heterogeneous reaction and particle deposition. The heat transport mechanisms modelled include wall–gas convection, wall–particle radiation exchange, particle–gas convection and heat release from chemical reaction. The model is validated in terms of methane conversion against a 10 kW experimental solar reactor and used to extract kinetic parameters for the homogeneous and heterogeneous reaction paths. The model shows promise as a quick and simple tool for the design and control of industrial scale solar reactors.

The present study deals with Aerosol Spray Pyrolysis (ASP) synthesis of two families of nanostructured redox materials targeted to two different environmental applications: transition-metal-doped ferrites and base metal-doped cerium oxide, used for hydrogen production through solar-assisted water splitting and for catalytic soot oxidation, respectively. The synthesized powders were characterized with respect to their phase composition, morphology and particle size distribution by XRD, SEM and TEM analysis, which have shown their nanostructured character. Doped ferrites were evaluated, with respect to their hydrogen production activity from water dissociation, in an in-house built water-splitting testing rig. ASP materials proved to be very active water splitters demonstrating higher water conversion and hydrogen yields than materials of the same composition synthesized through Solid State Synthesis (SSS), with material performance depending on the dopants’ kind and stoichiometry. Base metal-doped cerium oxides were evaluated with respect to their direct soot oxidation activity, via Thermogravimetric Analysis (TGA), as well as on a diesel engine bench under realistic conditions. It was found that doping improves their activity and that they enhance soot oxidation at lower temperatures compared to materials synthesized through Liquid Phase Self-propagating High temperature Synthesis (LPSHS).

The present work focuses on the development of efficient desulphurization processes for multi-fuel reformers for hydrogen production. Two processes were studied: liquid hydrocarbon desulphurization and H 2 S removal from reformate gases. For each process, materials with various chemical compositions and microporous structures were synthesized and characterized with respect to their physicochemical properties and desulphurization ability. In the case of liquid phase desulphurization, the adsorption of sulphur compounds contained in diesel fuel under ambient conditions was studied employing as sorbents, zeolite-based materials, i.e. NaY, HY and metal ion-exchanged NaY and HY, as well as a high-surface area activated carbon (AC), for three different diesel fuels with sulphur content varying between 5 and 180 ppmw. Among all sorbents studied, AC showed the best desulphurization performance followed by cerium ion-exchanged HY. The gas phase desulphurization experiments involved the evaluation of zinc-based mixed oxides, synthesized by non-conventional (combustion synthesis) techniques on high steam content reformate gas mixtures.

Aerosol spray pyrolysis (ASP) was employed for the synthesis of oxygen-deficient doped ferrite systems to be used as redox materials for the production of solar Hydrogen from water via a two-step thermochemical water-splitting cycle. In the first step (water splitting) the reduced state of a metal oxide is oxidized by taking oxygen from water and producing hydrogen; in the second step (regeneration) it is reduced again by delivering some of its lattice oxygen. Redox materials of the iron spinel family doped with other bivalent metals (Zn, Ni, Mn) were synthesized via ASP, characterized and evaluated with respect to their water-splitting activity. Organic additives, like citric acid, in the precursor solutions were found to result in products with finer particle size and to enhance the water-splitting activity of the synthesized materials. Material performance (water splitting activity, hydrogen yield, regeneration capability) was found to depend on the dopants’ kind and stoichiometry; in particular high percentages of Zn dopant seem to enhance the overall materials’ performance. ASP synthesized materials have demonstrated higher water conversion and hydrogen yields than materials of the same composition synthesized through solid-state routes. The ASP synthesis process was scaled-up successfully maintaining the favorable characteristics of the synthesized materials.

The present work summarizes the recent activities of our laboratory in the field of solar-aided hydrogen production with structured monolithic solar reactors. This reactor concept, “transferred” from the well-known automobile exhaust catalytic after-treatment systems, employs ceramic supports optimized to absorb effectively solar radiation and develop sufficiently high temperatures, that are coated with active materials capable to perform/catalyze a variety of “solar-aided” reactions for the production of hydrogen such as water splitting or natural gas reforming. Our work evolves in an integrated approach starting from the synthesis of active powders tailored to particular hydrogen production reactions, their deposition upon porous absorbers, testing of relevant properties of merit such as thermomechanical stability and hydrogen yield and finally to the design, operation simulation and performance optimization of structured monolithic solar hydrogen production reactors. This approach, among other things, has culminated to the world's first closed, solar-thermochemical cycle in operation that is capable of continuous hydrogen production employing entirely renewable and abundant energy sources and raw materials – solar energy and water, respectively – without any CO 2 emissions and holds, thus, a significant potential for large-scale, emissions-free hydrogen production, particularly for regions of the world that lack indigenous resources but are endowed with ample solar energy.

Ceria-based fuel additives in diesel engines when dosed at above a certain concentration into the fuel have been shown to lead into bimodal exhaust particle size distributions in a previous study. In order to model this complex problem it is assumed that the soot aggregate size distribution (where each aggregate consists of individual primary particles) evolves fast toward a constant total surface area (determined by the “open” non-coalesced fractal aggregate morphology) and this surface represents a sink for the ceria nuclei. The latter undergo kinetic aggregation among them as well as are simultaneously scavenged by the soot particles. The mathematical model is formulated in terms of an aerosol population balance equation for the ceria particles. The governing parameter in the resulting dimensionless population balance equation is the ratio of the total surface area of the soot aerosol over the initial additive surface area. The latter is proportional to the total additive mass if the ceria critical nuclei are assumed to consist of one molecule, an appropriate assumption for high surface tension, metal oxides like ceria. The experimental results show that the critical additive concentration at the onset of the bimodal shape of the exhaust size distribution scales linearly with the exhaust soot mass fraction, hence the soot aggregates must have a quite “open” fractal structure in order for their total area to be proportional to their total mass. Although simple population balance models may provide some insight into the problem of interest, the experimental results show that models accounting for more complex interactions of additive and soot particles (potentially involving incomplete accommodation) must be investigated in the future.

Porous monolithic multi-channeled silicon carbide (SiC) honeycombs employed as open volumetric receivers of concentrated solar radiation, were evaluated with respect to their porous structure and thermomechanical properties before and after long-time operation. Proper “tuning” of porosity, pore size distribution and microstructure can provide SiC honeycombs with improved mechanical properties (higher bending and compressive strength) in the “as-manufactured” state. Exposure under solar irradiation was found to affect both their pore structure and their mechanical characteristics. During the first stages of exposure, a re-structuring of the porous structure takes place shifting the mean pore size to higher values and slightly decreasing the total porosity; this re-structuring ceases after some “characteristic” exposure time. After solar exposure the honeycombs become harder and exhibit significantly higher compressive strength. Extension of anticipated lifetime can be achieved by materials with enhanced mechanical properties like silicon-infiltrated (siliconized) SiC.

In this study, explicit criteria are derived for the applicability of the uniform temperature approximation (UTA) to unsteady heat transfer through the solid phase in assemblies of spheres. The sources of unsteadiness studied are temperature variation and contact area variation. It is shown that, for the case of temperature variation, the UTA cannot be used for a certain range of frequency of the fluctuations. For the case of contact area variation, the UTA is shown to be applicable in all cases, provided that the contact radius is smaller than 10% of the particle radius. Following the assessment of its validity, the UTA is employed to illustrate the effect of aggregate and deposit morphology on the aggregate thermal relaxation behavior, considering the limiting case of linear aggregates and simple two-dimensional on-lattice deposits.

[EN] The prominent aim of this investigation was the examination of the in-principle easibility of aluminum combustion under internal combustion engine (ICE)-like conditions. This study was performed in the framework of recent consideration of metallic nanoparticles as alternative fuels for ICE engines. Aluminum nanoparticles of different morphologies and sizes were studied with respect to their fundamental oxidation characteristics via thermogravimetric analysis under various nitrogen−oxygen environments and by spark-ignition of Al nanopowder strips under controlled airflow at ambient pressure. The ICE-like tests included measurements performed in two different arrangements; namely, a shock-tube setup and a constantvolume combustion vessel. A set of engine tests was also performed in a single-cylinder compression-ignition engine with a customized, single-shot aerosol injection system. Burned powder samples were, in all cases, examined via in situ and ex situ techniques for the identification of products and their morphologies. The results largely verified that combustion of aluminum particles in an engine environment is indeed feasible. Nonetheless, prominent differences, in terms of the products formed and their morphologies/structures, were identified among the various oxidation/combustion techniques employed., We thank the European Commission for partially funding of this work through the Project "COMETNANO" (FP7-NMP4-SL-2009-229063).

HycycleS was a cooperation of nine European partners and further non-European partners and aimed at the qualification and enhancement of materials and components for key steps of solar and nuclear powered thermochemical cycles for hydrogen generation from water. The focus of HycycleS was the decomposition of sulphuric acid (H2SO4) which is the central step of the sulphur-based family of those processes. Emphasis was put on materials and components for H2SO4 evaporation, decomposition, and sulphur dioxide separation. The suitability of materials and components was demonstrated by decomposing H2SO4 and separating its decomposition products in scalable prototypes. Silicon Carbide (SiC) turned out as the material of choice for the components facing the most corrosive environment of the process: the sulphuric acid evaporator and decomposer. Candidate catalysts for the high temperature reduction of sulphur trioxide have been screened and analysed. Cr-Fe mixed oxide (Fe 0.7Cr1.3O3) was the most promising material among the ones examined. Based on the use of the highlighted construction and catalyst materials prototype decomposers have been developed and tested. The successful fabrication and testing of a large size heat exchanger/reactor prototype composed of SiC plates shows promise with respect to its use for H2SO4 decomposition in the SI and HyS cycle. A solar specific decomposer prototype was developed, realised and successfully tested on sun in a solar furnace. A novel approach of using dense oxygen transport membranes, made from complex ceramics, for oxygen removal from the H 2SO4 decomposition product in order to shift the equilibrium in favour of increased decomposition was investigated. The membranes stability and suitability for carrying out this separation was investigated experimentally. Parallel to this, a conventional oxygen separator, a low-temperature wet scrubbing system, was investigated as well. Scale-up scenarios of components and of the process were addressed. Copyright © 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

Diesel engine emission particle filters are often placed at exhaust outlets to remove particles from the exhaust. The use of filters results in the exposure to a reduced number of nanometer-sized particles, which might be more harmful than the exposure to a larger number of micrometer-sized particles. An in vitro exposure system was established to expose human alveolar epithelial cells to freshly generated exhaust. Computer simulations were used to determine the optimal flow characteristics and ensure equal exposure conditions for each well of a 6-well plate. A selective particle size sampler was used to continuously deliver diesel soot particles with different particle size distributions to cells in culture. To determine, whether the system could be used for cellular assays, alterations in cytokine production and cell viability of human alveolar A549 cells were determined after 3h on-line exposure followed by a 21-h conventional incubation period. Data indicated that complete diesel engine emission slightly affected pre-stimulated cells, but naive cells were not affected. The fractions containing large or small particles never affected the cells. The experimental set-up allowed a reliable exposure of the cells to the complete exhaust fraction or to the fractions containing either large or small diesel engine emission particles.