| 47741. |
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| 47742. |
- Johansson, Marcus, 1975
(författare)
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Screening of oxygen-carrier particles based on iron-, manganese-, copper- and nickel oxides for use in chemical-looping technologies
- 2007
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Capture and storage of carbon dioxide from combustion will likely be used in the future as a method of reducing emissions of greenhouse gases and thus be part of the overall strategy to stabilize the atmospheric levels of CO2. Chemical-looping combustion is a method of combustion where CO2 is inherently separated from the non-condensable components in the flue gas without the need for an energy intensive air separation unit. This is because nitrogen from the combustion air is never mixed with the fuel. Instead, oxygen carriers, in the form of metal oxide particles, circulate between two interconnected fluidized reactors and transfer oxygen from the air to the fuel through heterogeneous gas-solid redox reactions. The technology could also be adapted for the production of hydrogen from fossil fuels with CO2 separation, i.e. chemical-looping reforming.108 different oxygen-carriers based on iron-, manganese-, copper- and nickel oxides have been investigated. These carriers are prepared with inert material to increase the lifetime and performance of the particles. All particles but one have been produced by a freeze-granulation method. In order to optimize the performance of the particles, the sintering temperature of the particles was varied between 950°C and 1600°C. Normally particles of the size range of 125-180 m have been used for the reactivity investigations. Screening tests were performed in a laboratory fluidized-bed reactor of quartz placed in a furnace. The particles were exposed to an environment simulating a real chemical-looping combustor, by alternating between reducing (50% CH4 / 50 % H2O) and oxidizing conditions (5% O2 in N2). The temperature was varied in the range 600 – 950°C with most experiments conducted at 950°C. In addition the particles were characterized with respect to strength, physical appearance and chemical structure before and after the experiments. Some suitable oxygen carriers were investigated in more detail in the fluidized bed, and parameters such as reaction temperature, particle size, reducing gas and experimental method were varied. With respect to reactivity with methane, the different oxygen carriers can generally be ranked in the order nickel> copper> manganese> iron whereas the crushing strength roughly follows the opposite order. Several types of inert material were used in this work, and this was found to be a very important parameter. It was found that inert material based on alumina and zirconia in general resulted in promising oxygen carriers, whereas titania, silica and magnesia were less promising with respect to reactivity or lifetime of the particles. Using a low sintering temperature in preparation is associated with a high reactivity, but also a low strength. This is because the higher temperatures provoke a breakdown of the internal porous structure which also makes them denser. Twelve out of the initial 108 particles were not useful for different reasons, including melting, lack of structure and lack of reactivity due to formation of non-reducible species.The majority of the investigated oxygen carriers are well suited for chemical-looping combustion taking into consideration the important criteria of reactivity, high conversion of the fuel, relatively high strength and ability to withstand de-fluidization, agglomeration, fragmentation and abrasion.
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| 47743. |
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| 47744. |
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| 47745. |
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| 47746. |
- Johansson, P., et al.
(författare)
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Primary energy use for heating in the Swedish building sector - Current trends and proposed target
- 2007
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Ingår i: Energy Policy. - : Elsevier BV. - 0301-4215. ; 35:2, s. 1386-1404
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Tidskriftsartikel (refereegranskat)abstract
- One goal of the Swedish energy policy is to reduce the amount of electricity used for heating in the building sector. This means to reduce the primary energy used for heating which in this paper is analyzed in the context of various heating technologies and CO2 emissions. The analysis is applied to a region in Sweden (southern Sweden) for which detailed information on the energy infrastructure (the capital stock of the buildings and heating systems together with geographical variations in heat intensity) is available from a previous work [Johansson, P., Nylander, A., Johnsson, F., 2005. Electricity dependency and CO2 emissions from heating in the Swedish building sector-current trends in conflict with governmental policy? Energy policy] and which is large enough to be assumed representative for Sweden as a whole. The detailed mapping of the energy infrastructure allows a good estimate on the rate at which the energy system can be expected to be replaced with respect to economical lifetime of the capital stock (the year 2025 in this case). Two scenarios are investigated; a: target scenario for which energy savings are employed (e.g. improving climate shell in buildings) and oil and most of the electricity used for heating purposes are phased out and a second for which the current trend in the heating market continues. In the target scenario it is shown that although only applying commercially competitive heating technologies, it is possible to achieve a 47% reduction in primary energy use for heating with a 34% decrease in heat demand together with significant reduction in CO2 emissions. However, the scenario which continues the current trends on the heating market instead yields an increase (of about 10%) in primary energy use (reduction in conversion efficiency) of the heating system of the region over the period studied, in spite of a slight decrease in heat demand (9%, mainly due to energy efficiency measures) as well as in CO2 emissions. In light of the recently proposed introduction of energy performance certification of buildings in Sweden (and in EU), it can be concluded that such a certification system must take into account the primary energy use and local conditions. A politically acceptable way to do so would be to propose municipalities to develop "best strategies" on development of the heating market for various house types and districts within the municipality which can then be included as recommendations in the energy performance certification of the buildings. (c) 2006 Elsevier Ltd. All rights reserved.
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| 47747. |
- Johansson, Robert, 1977, et al.
(författare)
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Influence of intraparticle gradients in modeling of fixed bed combustion
- 2007
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Ingår i: Combustion and Flame. ; 149, s. 49-62
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Tidskriftsartikel (refereegranskat)abstract
- The impact of using a porous media approximation, neglecting intraparticle gradients, for modeling fixed bed combustion has been investigated. This has been done by comparing the results from a bed model using the porous media approximation with the results from the same model, where the intraparticle gradients have been taken into account by a two-dimensional particle model. The particle model provides information on the internal heating of the solid particles and the internal rate of drying and devolatilization. The surface temperature of the particles, given by the porous bed model, is the boundary condition for the particle model. Fuel particles of sizes between 5 and 40 mm, consisting of wood, and inlet gas velocities ranging from 0.05 to 0.2 m/s are used. For large bed particles there are considerable differences between the two modeling approaches in the rates of release of volatiles and moisture. The temperature profile in the bed is also affected. For the smaller bed particles in the size range investigated, the differences are small. However, the difference in ignition rate and maximum temperature are small even for the largest particles, compared to uncertainties in the models and scattering of experimental data.
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| 47748. |
- Johansson, Robert, 1977, et al.
(författare)
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Sensitivity Analysis of a Fixed Bed Combustion Model
- 2007
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Ingår i: Energy & Fuels. ; 21:3, s. 1493-1503
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Tidskriftsartikel (refereegranskat)abstract
- The ability to model the combustion of biofuels in a fixed bed is evaluated by a sensitivity analysis. Theanalysis treats the uncertainty of model parameters related to heat transport, reaction rates, and composition ofvolatiles. The scatter of the parameters is estimated from the differences between several published correlations.The results are compared with measurement data and possible model simplifications are discussed. It is shownthat the bed model is able to reproduce the ignition rate and the maximum temperature. Prediction of theseproperties is relatively insensitive to the uncertainty of most of the parameters. Gas concentrations within thebed are more difficult to predict. They are greatly influenced by the composition of the volatile gas releasedduring devolatilization. Also the composition of the volatile gas has a significant influence on the ignition ofthe gas in the model, affecting the ignition rate, particularly at low airflows. Moreover, the investigation showsthat treatment of radiation can be simplified, and the number of gas species included in the model can berestricted without significant losses of model generality.
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| 47749. |
- Johansson, Sven, 1981
(författare)
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Multiscale Modeling of Interfaces
- 2007
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Understanding strength and stability of interfaces between dissimilar materials or phases is a highly active field of contemporary materials research. Knowledge of interface structure and energetics is important for modeling of complex materials both on a mechanical and thermodynamical level as well as for many functional properties. Interfaces is a collective term including surfaces, interphase boundaries and grain boundaries. A general interface can be sharp or diffuse, planar or corrugated. The interface between two crystals is usually quite sharp, and can be classified as coherent, semicoherent or incoherent depending on the mismatch between the adjoining crystals.The goal of this Thesis is to obtain a better understanding of interfacial structures and energetics, in particular of semicoherent interfaces. Previous first-principles work in the field has often excluded the effect of misfit. Therefore, we present a simple model combining the interfacial interaction from first-principles methods with a continuum description to account for the elastic displacements. The accuracy of the model has been satisfactorily tested against atomistic modelling.We investigate the effect of misfit on interfaces in two classes of materials: steels and cemented carbides. In the first case, we study the interface between Fe and a VN precipitate. The results show that even a small misfit (2%) has a large influence on the interface energy. In the second case, we apply the method to the semicoherent Sigma=2 twist boundary found in both tungsten carbide powder and sintered cemented carbides. The results are discussed in the context of grain boundary evolution during sintering.
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| 47750. |
- John, Wolfgang, 1978, et al.
(författare)
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Analysis of Internet Backbone Traffic and Header Anomalies Observed
- 2007
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Ingår i: IMC '07: Proceedings of the 7th ACM SIGCOMM conference on Internet measurement. - 9781595939081 ; , s. 111-116
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Konferensbidrag (refereegranskat)abstract
- The dominating Internet protocols, IP and TCP, allow some flexibility in implementation, including a variety of optional features. To support research and further development of these protocols, it is crucial to know about current deployment of protocol specific features and accompanying anomalies. This work is intended to reflect the current characteristics of Internet backbone traffic and point out misbehaviors and potential problems. On 20 consecutive days in April 2006 bidirectional traffic was collected on an OC-192 backbone link. The analysis of the data provides a comprehensive summary about current protocol usage including comparisons to prior studies. Furthermore, header misbehaviors and anomalies were found within almost every aspect analyzed and are discussed in detail. These observations are important information for designers of network protocols, network application and network attack detection systems.
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