| 1. |
- Agrell, Johan, 1974-
(författare)
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Development of Methanol-Reforming Catalysts for Fuel Cell Vehicles
- 2003
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Vehicles powered by proton exchange membrane (PEM) fuelcells are approaching commercialisation. Being inherently cleanand efficient sources of power, fuel cells constitute asustainable alternative to internal combustion engines to meetfuture low-emission legislation. The PEM fuel cell may befuelled directly by hydrogen, but other alternatives appearmore attractive at present, due to problems related to theproduction, transportation and handling of hydrogen.Fuelling with an alcohol fuel, such as methanol, which isoxidised directly at the anode, offers certain advantages.However, the efficiency of the direct-methanol fuel cell (DMFC)is still significantly lower than that of the conventionalhydrogen-fuelled PEM fuel cell, due to some technical problemsremaining unsolved. Hence, indirect fuelling by a reformedliquid fuel may be the most feasible option in the early stagesof the introduction of fuel cell vehicles.The work presented in this thesis concerns the developmentof catalysts for production of hydrogen from methanol bypartial oxidation, steam reforming or a combination thereof.The work contributes to the understanding of how thepreparation route affects catalyst morphology and howphysicochemical properties determine catalytic behaviour andreaction pathways.The thesis is a summary of seven papers published inscientific periodicals. The first paper (Paper I) reviews thecurrent status of catalytic hydrogen generation from methanol,focusing on the fuel cell application. Paper II investigatesthe partial oxidation of methanol over Cu/ZnO catalystsprepared in microemulsion and by a conventionalco-precipitation technique. The activity for methanolconversion in the low-temperature regime is found to besignificantly higher over the former materials and the workcontinues by determining the nature of possible Cu-ZnOinteractions in the catalysts by studying their physicochemicalproperties more thoroughly (Paper III). In Paper IV, thepathways for methanol conversion via both partial oxidation andsteam reforming are elucidated.In Paper V, partial oxidation of methanol is studied overPd/ZnO catalysts prepared by microemulsion technique and againcompared to conventional materials. This investigationdemonstrates that although possessing high methanol conversionactivity, palladium-based catalysts are not suitable forreforming in fuel cell applications due to the considerableamounts of carbon monoxide formed.In Paper VI, methanol reforming is investigated over acommercial Cu/ZnO/Al2O3 catalyst. The mechanisms for carbonmonoxide formation and strategies for its suppression arediscussed, as well as reactor design aspects. The study alsoincludes some simple kinetic modelling. Finally, Paper VIIdescribes the optimisation of catalyst composition and processconditions to reach high hydrogen production efficiency at lowoperating temperatures and with minimum carbon monoxideformation.Keywords:PEM fuel cells, hydrogen, methanol, reforming,(partial) oxidation, reaction pathways, carbon monoxide,catalyst, microemulsion, Cu/ZnO, Pd/ZnO, copper, redoxproperties, oxidation state
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| 4. |
- Andrae, Johan
(författare)
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Numerical Studies of Wall Effects of Laminar Flames
- 2001
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Numerical simulations have been done with the CHEMKINsoftware to study different aspects of wall effects in thecombustion of lean, laminar and premixed flames in anaxisymmetric boundary-layer flow. The importance of the chemical wall effects compared to thethermal wall effects caused by the development of the thermaland velocity boundary layer has been investigated in thereaction zone by using different wall boundary conditions, walltemperatures and fuel/air ratios. Surface mechanisms include acatalytic surface (Platinum), a surface that promotesrecombination of active intermediates and a completely inertwall with no species and reactions as the simplest possibleboundary condition. When hydrogen is the model fuel, the analysis of the resultsshow that for atmospheric pressure and a wall temperature of600 K, the surface chemistry gives significant wall effects atthe richer combustion case (f=0.5), while the thermal andvelocity boundary layer gives rather small effects. For theleaner combustion case (f=0.1) the thermal and velocityboundary layer gives more significant wall effects, whilesurface chemistry gives less significant wall effects comparedto the other case. For methane as model fuel, the thermal and velocity boundarylayer gives significant wall effects at the lower walltemperature (600 K), while surface chemistry gives rather smalleffects. The wall can then be modelled as chemically inert forthe lean mixtures used (f=0.2 and 0.4). For the higher walltemperature (1200 K) the surface chemistry gives significantwall effects. For both model fuels, the catalytic wall unexpectedlyretards homogeneous combustion of the fuel more than the wallthat acts like a sink for active intermediates. This is due toproduct inhibition by catalytic combustion. For hydrogen thisoccurs at atmospheric pressure, but for methane only at thehigher wall temperature (1200 K) and the higher pressure (10atm). As expected, the overall wall effects (i.e. a lowerconversion) were more pronounced for the leaner fuel-air ratiosand at the lower wall temperatures. To estimate a possible discrepancy in flame position as aresult of neglecting the axial diffusion in the boundary layerassumption, calculations have been performed with PREMIX, alsoa part of the CHEMKIN software. With PREMIX, where axialdiffusion is considered, steady, laminar, one-dimensionalpremixed flames can be modelled. Results obtained with the sameinitial conditions as in the boundary layer calculations showthat for the richer mixtures at atmospheric pressure the axialdiffusion generally has a strong impact on the flame position,but in the other cases the axial diffusion may beneglected. Keywords:wall effects, laminar premixed flames,platinum surfaces, boundary layer flow
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| 5. |
- Andrae, Johan, 1973-
(författare)
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Wall Related Lean Premixed Combustion Modeled with Complex Chemistry
- 2002
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Increased knowledge into the physics and chemistrycontrolling emissions from flame-surface interactions shouldhelp in the design of combustion engines featuring improvedfuel economy and reduced emissions. The overall aim of this work has been to obtain afundamental understanding of wall-related, premixed combustionusing numerical modeling with detailed chemical kinetics. Thiswork has utilized CHEMKIN®, one of the leading softwarepackages for modeling combustion kinetics. The simple fuels hydrogen and methane as well as the morecomplex fuels propane and gasified biomass have been used inthe model. The main emphasis has been on lean combustion, andthe principal flow field studied is a laminar boundary layerflow in two-dimensional channels. The assumption has been madethat the wall effects may at least in principle be the same forlaminar and turbulent flames. Different flame geometries have been investigated, includingfor example autoignition flames (Papers I and II) and premixedflame fronts propagating toward a wall (Papers III and IV).Analysis of the results has shown that the wall effects arisingdue to the surface chemistry are strongly affected by changesin flame geometry. When a wall material promoting catalyticcombustion (Pt) is used, the homogeneous reactions in theboundary layer are inhibited (Papers I, II and IV). This isexplained by a process whereby water produced by catalyticcombustion increases the rate of the third-body recombinationreaction: H+O2+M ⇔ HO2+M. In addition, the water produced at higherpressures increases the rate of the 2CH3(+M) ⇔ C2H6(+M) reaction, giving rise to increased unburnedhydrocarbon emissions (Paper IV). The thermal coupling between the flame and the wall (theheat transfer and development of the boundary layers) issignificant in lean combustion. This leads to a sloweroxidation rate of the fuel than of the intermediatehydrocarbons (Paper III). Finally in Paper V, a well-known problem in the combustionof gasified biomass has been addressed, being the formation offuel-NOx due to the presence of NH3 in the biogas. A hybridcatalytic gas-turbine combustor has been designed, which cansignificantly reduce fuel-NOx formation. Keywords:wall effects, premixed flames, flamequenching, numerical modeling, CHEMKIN, boundarylayerapproximation, gasified biomass, fuel-NOx, hybrid catalytic combustor.
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| 7. |
- Anheden, Marie
(författare)
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Analysis of gas turbine systems for sustainable energy conversion
- 2000
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Increased energy demands and fear of global warming due tothe emission of greenhouse gases call for development of newefficient power generation systems with low or no carbondioxide(CO2) emissions. In this thesis, two different gasturbine power generation systems, which are designed with theseissues in mind, are theoretically investigated and analyzed.Inthe first gas turbine system, the fuel is combusted using ametal oxide as an oxidant instead of oxygen in the air. Thisprocess is known as Chemical Looping Combustion (CLC). CLC isclaimed to decrease combustion exergy destruction and increasethe power generation efficiency. Another advantage is thepossibility to separate CO2without a costly and energy demanding gasseparation process. The system analysis presented includescomputer-based simulations of CLC gas turbine systems withdifferent metal oxides as oxygen carriers and different fuels.An exergy analysis comparing the exergy destruction of the gasturbine system with CLC and conventional combustion is alsopresented. The results show that it is theoretically possibleto increase the power generation efficiency of a simple gasturbine system by introducing CLC. A combined gas/steam turbinecycle system with CLC is, however, estimated to reach a similarefficiency as the conventional combined cycle system. If thebenefit of easy and energy-efficient CO2separation is accounted for, a CLC combined cyclesystem has a potential to be favorable compared to a combinedcycle system with CO2separation.In the second investigation, a solid, CO2-neutral biomass fuel is used in a small-scaleexternally fired gas turbine system for cogeneration of powerand district heating. Both open and closed gas turbines withdifferent working fluids are simulated and analyzed regardingthermodynamic performance, equipment size, and economics. Theresults show that it is possible to reach high power generationefficiency and total (power-and-heat) efficiency with thesuggested system. The economic analysis reveals that the costof electricity from theEFGT plant is competitive with the moreconventional alternatives for biomass based cogeneration in thesame size range (<10 MWe).Keywords:power generation, Chemical Looping Combustion,CO2separation, oxygen carrier, biomass fuel, closedcycle gas turbine, externally fired gas turbine
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| 8. |
- Arnerdal, Klas
(författare)
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Transport of Heavy Metals in Soil Subjected to an Electric Field -A Study of the Transport of Copper in Bentonite Clay
- 2002
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This licentiate thesis addressesphosphorus release from sewage sludge, which is the first stepin phosphorus recovery. The concept ofphosphorus recovery maybe seen as a part of sustainable sludge handling in that it isa means of recycling resources without supplying harmfulsubstances to humans or the environment. Independent of whichdirection the future development in the water and wastewatersector turns, phosphorus recovery as a part of the wholewastewater system is presumably to be included.In order to obtain moreinformation about different ways of phosphorus release fromsewage sludge experiments have been carried out. The results ofthe experiments on phosphorus release using acid and base arepresented. Fractionation of sludge is an important step torelease different components from the sludge, includingphosphorus. With the addition of acid, a large fraction of theinorganic material can be dissolved, and thereby also sludgebound phosphorus compounds. In particular, fractionation byusing acid treatment on digested sludge releases a considerableamount of phosphorus.The method of phosphorus removalin a conventional wastewater treatment plant has an impact onthe chemicals required for phosphorus recovery. A comparison oftechnology for the phosphorus recovery systems is highlydependent on the composition of the sludge, which is the rawmaterial for recovery of phosphorus, and different localconditions.The results of the performedexperiments concerning chemical demand for phosphorus releaseare compared with calculations based on a study of chemicaldemand of different technology systems for phosphorus recoveryat present operational conditions at Henriksdal wastewatertreatment plant. Overall, experiment and theory seem to be ingood agreement. Thermal technology and addition of acid torelease phosphorus lead to high chemical demand. The essentialfactor is the incoming iron content of the sludge, as given asthe quotient iron/phosphorus.Another possible method torelease phosphorus may be to use supercritical water oxidation,followed by phosphorus recovery. Studies of the Aqua Reciprocess, which uses this principle, have recently begun and theprocess is under development. Experiments have been performedto give information about recovery of phosphorus from sewagesludge, with application to digested sludge from biologicaltreatment and chemical precipitation with iron. Experimentsperformed on sludge residue from supercritical water oxidationare presented in this thesis.The results show that phosphatewas leached out more easily with acid (hydrochloric acid) atroom temperature than with base (sodium hydroxide), howeverheavy metals show slightly higher leaching percentage with acidthan with base at room temperature. Also, the leachingexperiment showed that the phosphate is leached out before theiron, in both cases. The iron content seems to be leached outto a somewhat greater degree with acid than with base. This maydisturb the further process to obtain a valuable purephosphorus product. The results also indicate that phosphorusis more easily leached out with acid from the sludge residuefrom supercritical water oxidation than from ash viaco-incineration with sewage sludge and household wastes.Supercritical water oxidation followed by phosphorus recoverymay be seen as an option to incineration of sludge andphosphorus recovery from ash.Keywords:Chemical demand, phosphorus recovery, sludgefractionation, supercritical water oxidation, sustainablesludge handling
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| 10. |
- Bartlett, Michael, 1975-
(författare)
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Developing Humidified Gas Turbine Cycles
- 2002
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- As a result of their unique heat recovery properties,Humidified Gas Turbine (HGT) cycles have the potential todeliver resource-effective energy to society. The EvaporativeGas Turbine (EvGT) Consortium in Sweden has been studying thesetypes of cycles for nearly a decade, but now stands at acrossroads, with commercial demonstration remaining. Thisthesis binds together several key elements for the developmentof humidified gas turbines: water recovery and air and waterquality in the cycle, cycle selection for near-term, mid-sizedpower generation, and identifying a feasible niche market fordemonstration and market penetration. Moreover, possiblesocio-technical hinders for humidified gas turbine developmentare examined.Through modelling saltcontaminant flows in the cycle andverifying the results in the pilot plant, it was found thathumidification tower operation need not endanger the hot gaspath. Moreover, sufficient condensate can be condensed to meetfeed water demands. Air filters were found to be essential tolower the base level of contaminant in the cycle. This protectsboth the air and water stream components. By capturing airparticles of a similar size to the air filters, the humidifieractually lowers air stream salt levels. Measures to minimisedroplet entrainment were successful (50 mg droplets/kg air) andmodels predict a 1% blow down from the water circuit issufficient. The condensate is very clean, with less than 1 mg/lalkali salts and easily deionised.Based on a core engine parameter analysis for three HGTcycle configurations and a subsequent economic study, asteam-cooled steam injected cycle complemented with part-flowhumidification is recommended for the mid-size power market.This cycle was found to be particularly efficient at highpressures and turbine inlet temperatures, conditions eased bysteam cooling and even intercooling. The recommended HGT cyclegives specific investment costs 30- 35% lower than the combinedcycles and cost of electricity levels were 10-18% lower.Full-flow intercooled EvGT cycles give high performances, butseem to be penalised by the recuperator costs, while stillbeing cheaper than the CC. District heating is suggested as asuitable niche market to commercially demonstrate the HGTcycle. Here, the advantages of HGT are especially pronounceddue their very high total efficiencies. Feasibility prices forelectricity were up to 35% lower than competing combinedcycles. HGT cycles were also found to effectively include wasteheat sources.Keywords:gas turbines, evaporative gas turbines,humidification, power generation, combined heat and powergeneration.
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