| 1. |
- Birgersson, Henrik
(författare)
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Development of a regeneration procedure for commercial automotive three-wy catalysts
- 2004
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Car exhaust catalysts were introduced in the early 1980’s, to limit the release of pollutants such as hydrocarbons, carbon monoxides and nitrogen oxides. These catalysts contain noble metals such as palladium (Pd), platinum (Pt) and rhodium (Rh) and are able to simultaneously abate all three of the above-mentioned pollutants, hence the name three-way catalyst (TWC). The exposure to high temperatures (800-1000 °C) during operation and the presence of additives in petrol such as lead, calcium, silicon, magnesium, manganese, chromium, sulphur and phosphorus will after a certain time start to lower the overall effectiveness of the catalyst. These effects are either of a mechanical or a chemical nature. High temperatures reduce the active area by causing the noble metals to agglomerate and sinter whereas the additives alter the activity by either fouling the pores of the support material (phosphorus) or by interacting with the metals (sulphur and lead). The main objective of this work was to develop a method to redisperse the catalytically active sites, comprising Pd, Pt and Rh on the washcoat surface, in an effort to regain lost catalyst activity. For this purpose, a wide spectrum of different commercial car exhaust catalysts containing varying noble metal loadings, aged under various driving conditions and with mileages ranging from 30 to 100 000 km were evaluated. The influence of a thermal treatment in a controlled gas atmosphere, such as oxygen or hydrogen and a redispersing agent, e.g. chlorine, on the activity of TWC was investigated by means of laboratory-scale activity measurements. Several complementary characterisation methods such as SEM/TEM, XRD, BET and TPR were used to verify the effects of the regeneration treatments on the catalyst morphology (Paper I). Partial regeneration of catalyst activity and noble metal dispersion was achieved after thermal treatment in an oxygen-chlorine rich atmosphere at temperatures below 500 °C. Finally, an investigation of the effects of an oxy-chlorine thermal treatment for regeneration of a ‘full-scale’ commercial automotive three-way catalyst was performed. Catalyst activity and performance prior to and after the oxy-chlorine thermal treatment was measured on a test vehicle in accordance with the European driving cycle (EC2000). The catalyst surface was further characterised using XRD and EDX (Paper II). Improved catalyst activity for a high mileage catalyst could be observed, with emissions lowered by approximately 30 to 40 vol% over the EC2000 driving cycle
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| 2. |
- Elger, Ragna
(författare)
-
On the behaviour of the lithium ion battery in the HEV application
- 2004
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The lithium ion battery is today mainly used in cell phonesand laptops. In the future, this kind of battery might beuseful in hybrid electric vehicles as well. In this work, the main focus has been to gain more knowledgeabout the lithium ion battery in the hybrid electric vehicle(HEV) and more precisely to examine what processes of thebattery that are limiting at HEV currents. Both experiments andmathematical modelling have been used. In both cases, highrate, pulsed currents typical for the HEV, have been used. Two manuscripts have been written. Both of them concern thebehaviour of the battery at HEV load, but from different pointsof view. The first one concerns the electrochemical behaviourof the battery at different ambient temperatures. Theexperimental results of this paper were used to validate amathematical model of a Li-ion battery. Possiblesimplifications of the model were identified. In this work itwas also concluded that the mass transfer of the electrolyte isthe main limiting process within the battery. The mass transferof the electrolyte was further studied in the second paper,where the concentration of lithium ions was measured indirectlyusing in situ Raman spectroscopy. This study showed that themathematical description of the mass transfer of theelectrolyte is not complete. One main reason of this issuggested to be the poor description of the physical parametersof the electrolyte. These ought to be further studied in orderto get a better fit between concentration gradients predictedby experiments and model respectively.
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| 3. |
- Enbäck, Sofia
(författare)
-
The anode in a PEFC reformate system
- 2004
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Polymer electrolyte fuel cells (PEFC) constitute one ofseveral types of fuel cells that can be used to converthydrogen to electricity and heat. PEFC co-generation systemsbased on fossil fuels are on the edge of commercialisation.Biogas may be a future fuel in these kinds of systems. Thecommon denominator for these fuels is the carbonmonoxide-containing product gas from the fuel processor,poisoning the catalyst of the fuel cell.A model for the carbon monoxide (CO) oxidation mechanism onPtRu/C in Nafion® was validated by steady-state andopen-circuit potential decay experiments in a porous electrode.Furthermore, a natural gas fuelled system with an autothermalreformer and PEFC has been built from purchased components. Theconstruction was lead by ABB AB with support from KTH,especially in designing the control system. The differentcomponents of the system were evaluated at KTH.It is shown that the oxidation of CO can be described with amodel that takes into consideration the adsorption of CO on Ptand the adsorption of H2O on Ru. The adsorbed CO can be eitherlinearly or bridge-bonded to the surface. The two forms cannotbe distinguished from each other by the method used in thiswork. It is also shown that the oxidation of CO cannot bedescribed by a model describing the surface as homogeneous,where all the catalyst sites are treated equally. Therate-determining step is the oxidation step, with the otherreactions in pseudo-equilibrium up to about 0.5 V vs. RHE,where the adsorption of CO is not in equilibrium but still notrate-determining. However, at the lowest potentials, theadsorption of water might be the rate-determining step. Theopen circuit decay method confirmed the accuracy of thesteady-state measurements. This technique does not suffer fromnoise or the placement of the reference electrode since thepotential gradient in the electrolyte disappears as soon as thecurrent is interrupted. Furthermore, the model presentedpredicts the coverage of COads on the catalyst surface. Thus,the model can be used together with a model for H2 oxidation.The experimental set-up of the single cell is a useful platformfor future work on CO kinetics with different catalysts and atdifferent temperatures. It is possible to study the hydrogenoxidation in presence of CO with existing equipment but purehydrogen oxidation will still be challenging.The PEFC system is based on a fuel processor for 2-10 kWeland a fuel cell stack of 2 kWel maximum. Furthermore, itcontains a DC/AC inverter, a control system, a gas analysissystem and extensive measurement equipment, such asthermocouples and pressure meters. The research PEFC systemwith infrastructure is a useful platform for future researchalthough various components may need to be replaced.
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| 4. |
- Gamero, Rafael
(författare)
-
Mass transfer during isothermal drying of a porous solid containing multicomponent liquid mixtures
- 2004
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Mass transfer in a porous solid, partially saturated with asingle solvent and multicomponent liquid mixtures, has beenexperimentally and theoretically studied. A porous materialcontaining single liquids and mixtures of organic solvents wasisothermally dried. Experiments were performed using a jacketedwind tunnel, through which a humidity andtemperature-controlled air stream flowed. The wetted porousmaterial was placed in a cylindrical vessel, whose top isexposed to the air stream until the material became dried to acertain extent. Drying experiments with the single solventswater, methanol, ethanol and 2-propanol, were performed atdifferent temperatures and transient liquid content profileswere determined. In isothermal drying experiments with liquidmixtures,the transient concentration profiles of thecomponents along the cylindrical sample as well as the totalliquid content were determined. The liquid mixtures examinedwere water-methanolethanol and isopropanol-methanol-ethanol.Two different temperatures and initial compositions were usedin the experiments. Mathematical models that describe nonsteadystate isothermal drying of a solid containing single liquidsand multicomponent liquid mixtures were developed. In the solidwetted with a single liquid, capillary movement of the liquidwas the main mechanism responsible for mass transfer. In thesolid containing liquid mixtures, interactive diffusion inliquid phase was superimposed to the capillary movement of theliquid mixture. In addition, interactive diffusion of thevapours in empty pores was considered. The parameters todescribe the retention properties of the solid and thecapillary movement of the liquid were determined by comparingtheoretical and experimental liquid content profiles obtainedduring drying of the solid wetted with single liquids. Tosimulate the transport of the liquid mixtures these parameterswhere weighed according to liquid composition. A fairly goodagreement between theoretical and experimental liquidcomposition profiles was obtained if axial dispersion isincluded in the model when the moisture consists of amixture. Keywords:Internal mass transfer, capillary flow,multicomponent, diffusion, solvent mixtures
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| 5. |
- Gracin, Sandra
(författare)
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Polymorphism and Crystallization of p-Aminobenzoic Acid
- 2004
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Polymorphs are solid phases where the chemical composition isequal but the crystal structure differs. Many organic compoundsmay appear in more than one crystalline structure. The differentpolymorphs of a givensubstance may have significantly differentphysical properties (packing, thermodynamic, spectroscopic,kinetic, surface and mechanical properties). Sometimes the moststable polymorph is difficult to produce or a metastable form hasfavourable properties. This thesis focuses on the crystallization of p-aminobenzoicacid. This model compound crystallizes in two differentpolymorphic forms: 1) the α-polymorph, which isthe commercially available form and appears as long, fibrousneedles 2) theβ-polymorph, that appears in the form ofprisms. The thermodynamic stability and crystallization from differentsolvents have been studied experimentally. The system is found tobe enantiotropic with a transition temperature of 25 °C,below which theβ-form is the stable polymorph. The compoundhas been crystallized from thirteen different solvents, either byslow cooling after which the product is allowed to mature insuspension, or by rapid cooling followed by immediate isolation.Needles were obtained from all solvents by both methods. In waterand ethyl acetate below 20°C the prismaticβ-form isobtained however, often together with the needles. Theβ-formcrystals usually needed hours or days to grow at the very slowcooling used, while needles usually appeared in seconds. Bycareful control of supersaturation and temperature coolingcrystallization can be performed to produce the pure β-formin water and in ethyl acetate. The influence of the solvent isexplained by analysis of the crystal structures versus thepossible interaction of the solvent molecules with the solute insolution. The α-form structure is governed by dimers and iskinetically favoured because the dimers easily form in thesolution especially in less polar solvents. The crystal structureof the β-modification is not based on dimers but on fourmembered rings with alternating amino and carboxyl groups. Keywords:controlled crystallization, polymorphism,p-aminobenzoic acid
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| 6. |
- Luna, Fabio, 1965-
(författare)
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Drying of Multicomponent Liquid Films
- 2004
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The convective drying of thin layers of multicomponentliquid mixtures into an inert gas, and the influence ofdifferent process controlling mechanisms on drying selectivityis studied. Drying experiments under gas-phase-controlledconditions are performed by low intensity evaporation, fromfree liquid surfaces, of ternary mixtures without non-volatilesolutes. Liquid-side-controlled experiments are carried out bydrying a multicomponent polymeric solution containing twovolatile components, one non-volatile polymer and an optionalnonvolatile softening substance. Mathematical models to describe gas- andliquid-side-controlled drying based on interactive diffusion inboth liquid and gas phases as the main mechanisms for masstransfer are developed. For gas-phase-controlled drying, astability analysis of the ordinary differential equations thatdescribes the evaporation process is performed. Isothermal andnon-isothermal drying processes are considered in batch andcontinuous modes. The mathematical model to describe thecomposition profiles during batch drying of the polymeric film,considering liquid resistance, is solved numerically. Due tothe lack of experimental data, properties for this polymericsystem are estimated by using established methods. Ananalytical solution of the diffusion equation, by assuming anisothermal drying process and a constant matrix ofmulticomponent diffusion coefficients is developed. For thecontinuous case, liquid-side resistance is studied by modellingevaporation of a multicomponent falling liquid film into aninert gas including indirect heating. The results of the gas-phase-controlled model are in goodagreement with experimental results. For the polymeric film,the agreement is only qualitative since the model does notaccount for a membrane that develops on the film surface. Thestability analysis permits the prediction of trajectories andfinal state of a liquid mixture in a gas-phase-controlleddrying process. For isothermal evaporation of ternary mixturesinto pure gas, the solutions are trajectories in the phaseplane represented by a triangular diagram of compositions. Thepredicted ternary dynamic azeotropic points are unstable orsaddle. On the other hand, binary azeotropes are stable whenthe combination of the selectivities of the correspondingcomponents is negative. In addition, pure component singularpoints are stable when they are contained within theirrespective isolated negative selectivity zones. Undernon-isothermal conditions, maximum temperature valuescharacterise stable azeotropes. Incremental loading of the gaswith one or more of the components leads to a node-saddlebifurcation, where a saddle azeotrope and a stable azeotropecoalesce and disappear. For continuous drying, the singularpoints are infinite and represent dynamic equilibrium pointswhose stability is mainly dependent on the ratio of inletgas-to-liquid flow rates. As long as the process isgas-phasecontrolled, these results also apply to a porous solidcontaining a liquid mixture. In general, liquid-side control makes the drying processless selective but it is difficult to maintain this conditionduring the whole process. Under the influence of its owndynamics, a process starting as liquid-side-controlled tendstowards a gas-phase-controlled process. The presence ofnon-volatile components and indirect heating may delay thisdevelopment. Considering the evolution of the processcontrolling steps and its influence on selectivity, a modelaimed at describing the complete trajectory of a drying orevaporation process must include the coexistence of allrelevant mechanisms. Keywords:ternary mixture, falling film, diffusionequation, gas-phase control, liquid-phase control, selectivity,stability analysis, polymeric solution, evaporation, azeotrope,batch drying, continuous drying.
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| 7. |
- Nylén, Ulf
(författare)
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Development of ring-opening catalysts for diesel quality improvement
- 2004
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The global oil refining industry with its present shift inproduct distribution towards fuels such as gasoline and dieselwill most likely hold the fort for many years to come. However,times will change and survival will very much depend onprocessing flexibility and being at the frontiers of refiningtechnology, a technology where catalysts play leading roles.Today oil refiners are faced with the challenge to producefuels that meet increasingly tight environmentalspecifications, in particular with respect to maximum sulphurcontent. At the same time, the quality of crude oil is becomingworse with higher amounts of polyaromatics, heteroatoms(sulphur and nitrogen) and heavy metals. In order to staycompetitive, it is desirable to upgrade dense streams withinthe refinery to value-added products. For example, upgradingthe fluid catalytic cracking (FCC) by-product light cycle oil(LCO) into a high quality diesel blending component is a veryattractive route and might involve a two-step catalyticprocess. In the first step the LCO is hydrotreated andheteroatoms are removed and polyaromatics are saturated, in thesecond step naphthenic rings are selectively opened to improvethe cetane number of the final product. The present research is devoted to the second catalytic stepof LCO upgrading and was carried out within the framework of aEuropean Union project entitled RESCATS. From the patent literature it is evident that iridium-basedcatalysts seem to be good candidates for ring-opening purposes.A literature survey covering ring opening of naphthenicmolecules shows the need for extending investigations toheavier model substances, more representative of the dieselfraction than model compounds such as alkylated mono C5 and C6-naphthenic rings frequently employed in academic studies. Ring-opening catalysts, mainly Pt-Ir based, were synthesisedat KTH by two different methods: the microemulsion and theincipient wetness methods. Characterization of the catalystswas performed using a number of techniques including TPR,TEM-EDX, AFM and XPS etc. Catalytic screening at atmosphericpressure using pure indan as model substance was utilized todetect ring-opening activity and the magnitude of selectivityto desired cetane-boosting products. The development of suchring-opening catalysts is the topic of Paper I. When designing a catalytic system aimed at refiningpetroleum, it is crucial to monitor the evolution of thesulphur distribution throughout the different stages of theprocess so that catalyst properties and reaction parameters canbe optimised. The final section of this thesis and Paper II arethus devoted to high-resolution sulphur distribution analysisby means of a sulphur chemiluminescence detector (SCD). Keywords:ring opening, naphthenes, cetane numberimprovement, indan, light cycle oil (LCO), Pt-Ir catalyst,catalyst characterization, aromatic sulphur compounds, GC-SCD,distribution, analysis.
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| 8. |
- Persson, Katarina
(författare)
-
Bimetallic palladium catalysts for catalytic combustion of methane
- 2004
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Catalytic combustion is a promising combustion technique in gas turbines, which results in ultra low levels of NOx, CO and unburned hydrocarbons. Due to the low combustion temperature achieved in catalytic combustion almost no thermal NOx is formed. The focus in this thesis will be on the first stage in a catalytic combustion chamber, i.e. the ignition catalyst. The catalyst used for this application is often a supported palladium-based catalyst due to its excellent activity for methane combustion. However, this type of catalyst has a serious drawback; the methane conversion decreases with time during operation. The unstable activity will result in increasing difficulties to ignite the fuel. The objective of the work presented in this thesis has been to improve the catalytic performance of supported palladium catalysts, with focus on stabilizing the methane conversion. The first part gives a general background to gas turbines and catalytic combustion. The second part concerns the monometallic palladium catalysts; their behaviour during methane combustion is addressed. The third part describes different bimetallic catalysts, which all have palladium as one of the active components. Results from the activity tests of methane combustion showed that it is possible to stabilize the activity by adding certain co-metals into the palladium catalyst. The morphology of the various bimetallic catalysts has been studied to gain a better understanding of the various combustion behaviours. Finally, the influence of pressure on the catalytic performance is evaluated. The catalysts were tested under more realistic conditions for gas turbines, with elevated pressure, in a high-pressure test facility with a 100 kW fuel power.
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| 9. |
- Pino-García, Osvaldo, 1966-
(författare)
-
Influence of Admixtures on Crystal Nucleation of Vanillin
- 2004
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Admixtures like reactants and byproducts are solublenon-crystallizing compounds that can be present in industrialsolutions and affect crystallization of the main substance.This thesis provides experimental and molecular modellingresults on the influence of admixtures on crystal nucleation ofvanillin (VAN). Seven admixtures: acetovanillone (AVA),ethylvanillin (EVA), guaiacol (GUA), guaethol (GUE), 4-hydroxy-acetophenone (HAP), 4-hydroxy-benzaldehyde (HBA), andvanillic acid (VAC) have been used in this study. Classicalnucleation theory is used as the basis to establish arelationship between experimental induction time andsupersaturation, nucleation temperature, and interfacialenergy. A novel multicell device is designed, constructed, andused to increase the experimental efficiency in thedetermination of induction times by using 15 nucleation cellsof small volumes simultaneously. In spite of the largevariation observed in the experiments, the solid-liquidinterfacial energy for each VAN-admixture system can beestimated with an acceptable statistical confidence. At 1 mole% admixture concentration, the interfacial energy is increasedin the presence of GUA, GUE, and HBA, while it becomes lower inthe presence of the other admixtures. As the admixtureconcentration increases from 1 to 10 mole %, the interfacialenergy also increases. The interfacial energies obtained are inthe range 7-10 mJ m-2. Influence of admixtures on metastable zone widthand crystal aspect ratio of VAN is also presented. Theexperimental results show that the admixtures studied arepotential modifiers of the nucleation of VAN. Molecularmodelling by the program Cerius2 is used to identify the likelycrystal growth faces. Two approaches, the surface adsorptionand the lattice integration method, are applied to estimatequantitatively the admixture-crystal interaction energy on thedominating crystal faces of VAN,i.e., {0 0 1} and {1 0 0}. However, a simple and clearcorrelation between the experimental values of interfacialenergy and the calculated interaction energies cannot beidentified. A qualitative structural analysis reveals a certainrelationship between the molecular structure of admixtures andtheir effect on nucleation. The determination of the influenceof admixtures on nucleation is still a challenge. However, themolecular and crystal structural approach used in this thesiscan lead to an improved fundamental understanding ofcrystallization processes. Keywords: Crystallization,nucleation, vanillin, admixtures, additives, impurities,induction time, interfacial energy, molecular modelling,interaction energy.
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| 10. |
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| 11. |
- Rydstrand, Magnus
(författare)
-
Heat driven cooling in district energy systems
- 2004
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Abstract The threat of global warming, caused by increasingemissions of carbon dioxide (CO2), is one reason why cooling supply systems mustwork more efficiently in the world today. An increasing share,currently between 10 and 20%, of the global output ofelectricity is consumed to produce cooling. The majority of thecoming increase in electricity production will be based on theconsumption of fossil fuels, implying everincreasing CO2-emissions. Vapor compressor chillers are currentlythe predominant provider of cooling and consume large amountsof electricity as well as leak refrigerants that harm theenvironment. This thesis focuses on the production of cooling from adistrict energy system perspective, with focus on heat-drivencooling. Cooling technologies, CHP production and thermalstorage are discussed in order to find a cost effective andenvironmentally sound way to meet today’s increasingcooling demand. Heat-driven cooling technologies withenvironmentally friendly refrigerants can give a net output ofelectricity if used in combination with combined heat and power(CHP) production. There is a net electricity output from asystem including heat-driven cooling. Also fuel is saved forthe production of cooling since heat-driven cooling is moreeffective as compared to vapor compressor chillers that consumeelectricity. It is found that cost effective solutions for district heatdriven chillers and/or the combined production of electricityand district cooling can be found in all climates with a highenough density of heating and cooling demands. In dry climates,with low latent cooling loads, district cooling has a largepotential and absorption cooling will give a high fuelutilization, as seen from a system perspective. District heatdriven chillers are believed to be very energy efficient inwarm and humid climates since desiccant systems are aneffective way of handling latent cooling loads. The choicebetween district heat driven chillers and district coolingdepends very much on the availability of a cost effective heatsink and the available space that can be used for coolingequipment. Low cost heat, a requirement for heat-driven cooling, can besupplied e.g. from CHP (back-pressure steam turbine), wasteincineration or from flue gas condensation. Inlethumidification in combination with enhanced latent flue gasheat recovery is found to be energy effective both for thesupply of heat and cooling using absorption chillers. Thermal storage can improve the performance, lower thecapacity requirements (cost of capital), and increase theseasonal efficiency of cooling equipment. CHP, coolingproduction and thermal storage are three fields in the districtenergy system that need to be integrated and adapted to localconditions in order to find a cost and energy effectivesolution to meet the increasing cooling demand of today. Key words:District heating, district cooling, thermalstorage, humidified gas turbines, combined heat and power,cooling
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| 12. |
- Vidlund, Anna
(författare)
-
Sustainable production of bio-energy products in the sawmill industry
- 2004
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- One of the great challenges facing society is to convert theglobal energy system to a sustainable process. Currently, 80%of the world´s energy is supplied through the combustionof fossil fuels. Not only are the fossil resources limited, theutilisation also increases the level of greenhouse gases in theatmosphere. The convertion to a sustainable energy system isproblematic since the technology needed to exploit mostnon-fossil energy sources is not yet fully developed, e.g.solar energy. Biofuel is an available renewable energy sourcewhich is already widely used in many countries. If an effectiveswitch-over from fossil fuels to biofuels is to be realised,biofuels must be viewed as a limited resource. Consequently, itis important that the handling, upgrading and utilisationprocesses involving biofuels are efficient so that itspotential can be fully exploited. This thesis considers efficient biofuel utilisation andupgrading within the sawmill industry. The goal has been toanalyse not only the technical opportunities for energy savingsin the sawmill industry, but also to analyse the costeffectiveness and environmental impact of studied measures. Theheat demand of the sawmill industry is almost completelycovered by its own by-products; primarily bark, sawdust andwood chips. The increased demand and improved economic value ofwoody biofuels on the market is thus an incentive for thesawmill industry to place more focus on energy issues. Thesawmill industry also has a more or less constant heat loadover the year, which is a beneficial factor for integrationwith district heating networks, biofuel upgrading plants andcombined heat and power plants. The conclusion of the study is that a variety of energyproducts such as heat, unrefined biofuel, pellets andelectricity can be efficiently produced in the sawmill industryand sold for profit to external customers. The payback periodsfor the proposed investments are moderate and both theemissions of volatile organic compounds and global CO2 aredecreased. Should the proposed measures be fully implemented atSwedish sawmills, about 2.8 TWh of biofuel could be savedannually, 0.5 TWh of waste heat could be sold as districtheating and 0.8 TWh of green electricity could be produced.Language: English Keywords:Sawmill industry, energy efficiency, heatrecovery, integration, biofuel, upgrading, district heating,fuel pellets, CHP, VOC, CO2
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| 13. |
- Wallmark, Cecilia, 1974-
(författare)
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Design and evaluation of stationary polymer electrolyte fuel cell systems
- 2004
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The objectives of this doctoral thesis are to give a basisincluding methods for the development of stationary polymerelectrolyte fuel cell (PEFC) systems for combined heat andpower production. Moreover, the objectives include identifyingprerequisites, requirements and possibilities for PEFC systemsproducing heat and power for buildings in Sweden. The PEFCsystem is still in a pre-commercial state, but low emissionlevels, fast dynamics and high efficiencies are promisingcharacteristics. A thermodynamic model to simulate stationary PEFC systemshas been constructed and pinch technology and exergy analysesare utilised to design and evaluate the system. The finalsystem configuration implies a high total efficiency ofapproximately 98 % (LHV). A flexible test facility was built in connection with theresearch project to experimentally evaluate small-scalestationary PEFC systems at KTH. The research PEFC system hasextensive measurement equipment, a rigorous control system andallows fuel cell systems from approximately 0.2 to 4 kWel insize to be tested. The simulation models of the fuel processorand the fuel cell stack are verified with experimental datataken from the test facility. The initial evaluation andsimulation of the first residential installation of a PEFCsystem in Sweden is also reported. This PEFC system, fuelled bybiogas and hydrogen, is installed in an energy system alsoincluding a photovoltaic array, an electrolyser and hydrogenstorage. Technical aspects of designing a fuel cell system-basedenergy system, including storages and grid connections, whichprovides heat and power to a building are presented in thisthesis. As a basis for the technical and economic evaluations,exemplifying energy systems are constructed and simulated. Fuelcell system installations are predicted to be economicallyunviable for probable near-term conditions in Sweden. The mainfactor in the economic evaluations is the fuel price. However,fuel cell system installations are shown to have a higher fuelutilisation than the conventional method of energy supply. The methods presented in this thesis serve as a collectedbasis for continued research and development in the area. Keywords:Small-scale, stationary, fuel cell system,polymer electrolyte fuel cell, PEFC system, reformer,thermodynamic modelling, pinch technology, exergy analyses,system configuration, test facility, experiments, application,simulation, installation, energy system, energy storage, heatand power demand.
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| 14. |
- Wolf, Jens, 1972-
(författare)
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CO2 mitigation in advanced power cycles
- 2004
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis encompasses CO2 mitigation using three different processes: i) natural gas-fired combined cycle with chemical looping combustion (CLC), ii) trigeneration of electrical power, hydrogen and district heating with extended CLC, iii) steam-based gasification of biomass integrated in an advanced power cycle. In CLC, a solid oxygen carrier circulates between two fluidised-bed reactors and transports oxygen from the combustion air to the fuel; thus, the fuel is not mixed with air and an inherent CO2 separation occurs. In this thesis, CLC has been studied as an alternative process for CO2 capture in a natural gas-fired combined cycle (NGCC). The potential efficiency of such a process using a turbine inlet temperature of 1200 °C and a pressure ratio of 13 is between 52 and 53 % when including the penalty for CO2 compression to 110 bar. It is shown that this efficiency cannot be further improved by including an additional CO2 turbine. Two conceivable reactor designs for CLC in an NGCC are presented. Top-firing has been studied as an option to overcome a temperature limitation in the CLC reactor system. The degree of CO2 capture is shown versus the temperature in the CLC reactor and its combustion efficiency. CLC has the potential to reach both a higher efficiency and a higher degree of CO2 capture than conventional post combustion CO2 capture technique. However, further research is needed to solve technical problems as, for example, temperature limitations in the reactor to reach this potential. Extended CLC (exCLC) is introduced, in which hydrogen is not only produced but also inherently purified. The potential efficiency of a novel tri-generation process for hydrogen, electricity and district heating using exCLC for CO2 capture is investigated. The results show that a thermal efficiency of about 54% might be achieved. A novel power process named evaporative biomass air turbine (EvGT-BAT) for biomass feedstock is presented. This process contains a steam-based gasification of biomass, which is integrated in an externally fired gas turbine cycle with top-firing. In the EvGT-BAT process, the steam-based gasification is conducted in an entrained-flow tubular reactor that is installed in the SFC as a heat exchanger. The EvGT-BAT process has the potential to generate electrical power from biomass with an efficiency of 41 %.
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| 15. |
- Yu, Ziyun, 1961-
(författare)
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Fluid and suspension hydrodynamics in the impeller discharge flow of stirred tanks
- 2004
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The hydrodynamics of an agitated tank have been studied byphase-Doppler anemometry. The focus is on the impeller anddischarge region of a 45o pitched blade turbine (PBT). Thestudy includes agitation of pure water as well as of a dilutesuspension of process particles. A three-dimensionalphase-Doppler anemometer is used to measure local,instantaneous, three-dimensional velocities of the fluid and ofthe suspended particles. A shaft encoding technique is used toresolve the turbulent fluctuations from the periodic velocityfluctuation due to the impeller blades, and to provide moredetailed information about the variations relative to theimpeller blade. Velocity bias is corrected for by the total3-dimensional velocity.The mean flow field, the fluctuating velocities, and thecomplete Reynolds stress tensor, are reported for the liquidphase flow. The periodic fluctuations in the flow that aregenerated by the impeller blades are eliminated in theexamination of the turbulence. The anisotropy of the turbulenceis assessed by the invariants of the anisotropy tensor. Thetrailing vortex structure is demonstrated to be associated withhigh kinetic energy and strong anisotropy of the turbulence.The vortex is still observable 130-140 degrees behind theblade. It gradually moves down from the impeller blade but thelocation in radial direction remains essentially unchanged. Theinfluence of the periodic fluctuations is examined and it isshown that the turbulence appears more isotropic when theperiodic fluctuations are not eliminated.The solid particle concentration is low below the impellerand is high above the impeller tip. The particles diverge fromthe liquid flow mean direction, especially below the agitatorclose to the tip where the strongest turbulence is found.Periodic fluctuations in the particle concentration relate tothe variations found in the angle-resolved mean velocity andfluctuating velocity. The ratio of the maximum to the minimumconcentration is about 2.0 in the present study.The baffles influence on the conditions in the impellerregion, and this influence can be observed on the fluid meanvelocity field, the angle-resolved velocities, the kineticenergy, and on the behavior of larger process particles. In theimpeller region the highest kinetic energies are about 15%higher upstream of the baffle than at the middle plane betweenthe baffles. The highest energy level in the middle plane isactually the lowest value and is therefore not representativewhen rotation symmetry is assumed.Local energy dissipation rates have been investigated, andthe integration of the local energy dissipation rates overdifferent control volumes has been compared with macroscopicenergy balance calculations. The discrepancy is significant.Different reasons have been analyzed and recommendations forfurther investigation are given. In the outflow region there is a significant variation alsoin the direction of the instantaneous velocity, which may leadto direction bias in the case of non-spherical measurementvolume. In order to account for this direction bias, amathematical model is developed to estimate the projected areaof the measurement volume in LDA or PDA. It is shown that theprojected area variation can lead to a significant directionbias in determination of time averaged values and localparticle concentration in a highly turbulent stirred tank flow.This bias is however negligible for an orthogonal optical setup, as is used in the present study.Keywords:Hydrodynamics, phase-Doppler anemometer,suspension, pitched-blade turbine, anisotropy, turbulence,Reynolds stresses, trailing vortex, kinetic energy, stirredtank
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