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Sakellariou K.G.,Aerosol and Particle Technology Laboratory | Sakellariou K.G.,Aristotle University of Thessaloniki | Criado Y.A.,CSIC - National Coal Institute | Tsongidis N.I.,Aerosol and Particle Technology Laboratory | And 3 more authors.
Solar Energy | Year: 2017

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. © 2017 Elsevier Ltd

Melas A.D.,European Commission - Joint Research Center Ispra | Melas A.D.,Aristotle University of Thessaloniki | Isella L.,European Commission | Konstandopoulos A.G.,Aristotle University of Thessaloniki | And 2 more authors.
Journal of Colloid and Interface Science | Year: 2014

The relationship between geometric and dynamic properties of fractal-like aggregates is studied in the continuum mass and momentum-transfer regimes. The synthetic aggregates were generated by a cluster-cluster aggregation algorithm. The analysis of their morphological features suggests that the fractal dimension is a descriptor of a cluster's large-scale structure, whereas the fractal prefactor is a local-structure indicator. For a constant fractal dimension, the prefactor becomes also an indicator of a cluster's shape anisotropy. The hydrodynamic radius of orientationally averaged aggregates was calculated via molecule-aggregate collision rates determined from the solution of a Laplace equation. An empirical expression that relates the aggregate hydrodynamic radius to its radius of gyration and the number of primary particles is proposed. The suggested expression depends only on geometrical quantities, being independent of statistical (ensemble-averaged) properties like the fractal dimension and prefactor. Hydrodynamic radius predictions for a variety of fractal-like aggregates are in very good agreement with predictions of other methods and literature values. Aggregate dynamic shape factors and DLCA individual monomer hydrodynamic shielding factors are also calculated. © 2013 Elsevier Inc.

Kostoglou M.,Aerosol and Particle Technology Laboratory | Kostoglou M.,Aristotle University of Thessaloniki | Bissett E.J.,Aristotle University of Thessaloniki | Konstandopoulos A.G.,Aerosol and Particle Technology Laboratory | Konstandopoulos A.G.,Aristotle University of Thessaloniki
Industrial and Engineering Chemistry Research | Year: 2012

Wall-flow monolithic (WFM) catalytic reactors occupy an ever increasing important position in environmental and industrial catalysis as well as in energy applications. Their performance is very frequently determined by transport (momentum, energy, and mass) limitations, driven by the market needs for lower pressure drop, efficient heat exploitation, and miniaturization. In the present problem we address the problem of deriving the appropriate single channel equations that describe heat transfer in a wall-flow monolithic (WFM) reactor with porous channels of square-cross section. The first step of the study involves setting up a self-similar hydrodynamic problem for the two-dimensional flow field in the channel cross section. This flow field depends only on the so-called wall Reynolds number. It is shown that the self-similarity fails for large values of wall Reynolds number. The second step involves setting up the Graetz problem for the flow velocity profile found in the first step and solving for the asymptotic Nusselt number. This Nusselt number depends on the Prandtl number in addition to the wall Reynolds dependence through the flow-field. Correlations for the Nusselt number as a function of wall Reynolds and Prandtl numbers are given to facilitate the inclusion of these effects into standard practice. © 2012 American Chemical Society.

Kostoglou M.,Aerosol and Particle Technology Laboratory | Kostoglou M.,Aristotle University of Thessaloniki | Lorentzou S.,Aerosol and Particle Technology Laboratory | Konstandopoulos A.G.,Aerosol and Particle Technology Laboratory | Konstandopoulos A.G.,Aristotle University of Thessaloniki
International Journal of Hydrogen Energy | Year: 2014

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. Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

Bissett E.J.,Gamma Technologies Inc. | Kostoglou M.,Aerosol and Particle Technology Laboratory | Kostoglou M.,Aristotle University of Thessaloniki | Konstandopoulos A.G.,Aerosol and Particle Technology Laboratory | Konstandopoulos A.G.,Aristotle University of Thessaloniki
Chemical Engineering Science | Year: 2012

In the 1D modeling of flow in the channels of wall-flow monoliths used in diesel particulate filters for engine exhaust emissions control, it is common to use friction coefficients and Nusselt numbers from idealized 2D/3D channel flows with zero wall flow. This practice implicitly makes the additional approximation that the actual velocity and scalar (temperature or species concentrations) profiles within the channels are little affected by nonzero wall flow. There is extensive related research in the filtration literature for the simpler geometries of circular tubes and parallel planes that exposes much more complex and interesting effects as the wall Reynolds number, Re w, increases. Here we extend these results to the 3D geometry of square channels appropriate for wall-flow monoliths. We solve for the fully developed laminar flow, and heat transfer, within long square channels with porous walls and uniform wall velocity. Results are generated for the appropriate parameter range applicable for the diesel particulate filter application which provide the corrected friction coefficients and Nusselt numbers for nonzero Re w. Furthermore, we confirm the observation, from prior work on the simpler geometries that there exists a limiting Re w beyond which there is no fully developed flow for the inlet channels (wall suction). Implications for modeling diesel particulate filters are discussed. © 2012 Elsevier Ltd.

Lee K.S.,Aerosol and Particle Technology Laboratory | Jung J.H.,Korea Institute of Science and Technology | Keel S.I.,Environmental Systems Research Division | Yun J.H.,Environmental Systems Research Division | And 2 more authors.
Science of the Total Environment | Year: 2012

The oxy-fuel combustion system is a promising technology to control CO 2 and NO X emissions. Furthermore, sulfation reaction mechanism under CO 2-rich atmospheric condition in a furnace may lead to in-furnace desulfurization. In the present study, we evaluated characteristics of calcium carbonate (CaCO 3) sorbent particles under different atmospheric conditions. To examine the physical/chemical characteristics of CaCO 3, which is used as a sorbent particle for in-furnace desulfurization in the oxy-fuel combustion system, they were injected into high temperature drop tube furnace (DTF). Experiments were conducted at varying temperatures, residence times, and atmospheric conditions in a reactor. To evaluate the aerosolizing characteristics of the CaCO 3 sorbent particle, changes in the size distribution and total particle concentration between the DTF inlet and outlet were measured. Structural changes (e.g., porosity, grain size, and morphology) of the calcined sorbent particles were estimated by BET/BJH, XRD, and SEM analyses. It was shown that sorbent particles rapidly calcined and sintered in the air atmosphere, whereas calcination was delayed in the CO 2 atmosphere due to the higher CO 2 partial pressure. Instead, the sintering effect was dominant in the CO 2 atmosphere early in the reaction. Based on the SEM images, it was shown that the reactions of sorbent particles could be explained as a grain-subgrain structure model in both the air and CO 2 atmospheres. © 2012.

Pagliaro M.,CNR Institute of Nanostructured Materials | Konstandopoulos A.G.,Aerosol and Particle Technology Laboratory | Ciriminna R.,CNR Institute of Nanostructured Materials | Palmisano G.,CNR Institute of Nanostructured Materials
Energy and Environmental Science | Year: 2010

Renewable hydrogen produced using solar energy to split water is the energy fuel of the future. Accelerated innovation in both major domains of solar energy (photovoltaics and concentrated solar power) has resulted in the rapid fall of the solar electricity price, opening the route to a number of practical applications using solar H2. Referring to several examples as well as to new technologies, this article provides insight into a crucial technology for our common future. © 2010 The Royal Society of Chemistry.

Nakamura K.,Ibiden Co. | Vlachos N.,Aerosol and Particle Technology Laboratory | Konstandopoulos A.,CERTH CPERI | Iwata H.,Ibiden Co. | Kazushige O.,Ibiden Co.
SAE Technical Papers | Year: 2012

Nowadays diesel particulate filters (DPFs) with catalyst coatings have assumed one of the most significant roles for road vehicle emission control. DPFs made of re-crystallized SiC (SiC-DPFs) have guaranteed the soot filtration efficiency for the current regulation. In order to further enhance their filtration efficiency, even though a higher porosity and larger pore size must be adopted for sufficient catalyst coating capacity, we developed the concept of a filtration layer on the DPF inlet channel walls and researched its performance both theoretically and experimentally. First of all, models of the new filtration layer, closely resembling the real one made in the laboratory, were digitally reconstructed and soot deposition simulations were conducted. According to the results, the pore size of the filtration layer providing the target filtration efficiency is found to be between the characteristic soot particle size (of order 100 nm) and the nominal DPF wall pore size (of order 10 μm). Additionally, it is shown that the optimum spatial distribution of filtration layer thickness along DPF length should be matched to the filtration velocity distribution. Finally we experimentally verified the performances of SiC-DPF with filtration layer by engine bench tests. We found that very high filtration efficiency is attained while it is shown that the concept presented can bring 3 significant advantages through its use: a high initial filtration efficiency of 97 % in spite of a higher porosity SiC-DPF wall, an 18 % decrease of transient pressure drop at 4 g/L soot mass in DPF without increased initial pressure drop due to a deep-bedding of soot, and a repeatability of transient pressure drop after several loading-regeneration cycles. Copyright © 2012 SAE International.

Sarvi A.,Åbo Akademi University | Lyyranen J.,VTT Technical Research Center of Finland | Jokiniemi J.,VTT Technical Research Center of Finland | Jokiniemi J.,Aerosol and Particle Technology Laboratory | Zevenhoven R.,Åbo Akademi University
Fuel Processing Technology | Year: 2011

This paper addresses particulate matter (PM) size distributions in large-scale diesel engine exhaust. The test engines were multivariable large-scale turbo-charged, after-cooled medium speed (~ 500 rpm, ~ 1 MW power per cylinder) direct injection diesel engines. Emissions measurements were carried out while burning heavy fuel (HFO) and light fuel (LFO) oils. Test modes for investigation were propulsion mode (marine) and generator mode (power plant), with load varying from 25 to 100%. PM was measured using a gravimetric impactor with four impactor stages plus a filter, classifying particles between 0.005 and 2.5 μm (aerodynamic diameter). The results show that HFO firing produces significantly higher PM emissions (more than factor of ~three on mass bases for high load operation) compared to LFO, especially for particles smaller than 0.5 μm. This is mainly due to higher ash-forming elements and sulphur content of HFO. For HFO, the fraction of the finest particles increases with load, more strongly for generator mode than for propulsion mode, with generator mode giving ~ 50% higher PM emissions than propulsion mode. With LFO firing, the largest amount of fine PM was emitted at the lowest load, for propulsion mode being lower and almost independent of load at higher loads, while for generator mode a steady decrease in emissions with increasing load is seen for all PM size classes measured. © 2011 Elsevier B.V. All rights reserved.

Konstandopoulos A.G.,Aerosol and Particle Technology Laboratory | Kostoglou M.,Aerosol and Particle Technology Laboratory
SAE Technical Papers | Year: 2010

In the present work we derive analytical solutions for the problem of convection, diffusion and chemical reaction in wall-flow monoliths. The advantage of having analytical instead of numerical treatments is clear as the analytical solutions not only can be exploited to bring full scale simulations of diesel particulate filters to the real time domain, but also they enable efficient implementations on computationally limited engine control units (ECUs) for on-board management and control of emission control systems. The presentation describes the mathematical problem formulation, the governing dimensionless parameters and the corresponding assumptions. Then the analytical solution is derived and several asymptotic (for limiting values of the parameters) and approximating solutions are developed, corresponding to different physical situations. Reactant distributions in the filter are presented and discussed for several values of the parameters. The conclusion is that the classic single channel model for DPF simulation can for all practical conditions accommodate diffusive phenomena with no added computational cost and without significantly altering the structure of existing code implementations. Copyright © 2010 SAE International.

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