| 1. |
- Birgersson, Henrik, 1976-
(författare)
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Development and Assessment of Regeneration Methods for Commercial Automotive Three-Way Catalysts
- 2006
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Car exhaust catalysts were introduced in the early 1980’s, to limit the release of pollutants such as hydrocarbons, carbon monoxide 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 gasoline and lubricants will, after a certain time, lower the activity of the TWC. High temperatures reduce the active area by causing the noble metals to agglomerate and sinter, whereas the additives alter the activity either by fouling the pores of the support material or by interacting with the metals. The main objective of this work was to develop a method which allows for the removal of contaminants (additives) from the washcoat and enables the redispersion of the active sites (noble metals), in an effort to recover lost catalyst activity. For this purpose, regeneration experiments were carried out on a wide spectrum of different commercial car exhaust catalysts. 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 characterization methods such as SEM/TEM, XRD, BET, LA, XPS and TPR were employed to verify the effects of the regeneration treatments on the catalyst morphology (Papers I, II). The results show that 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. A wet-chemical regeneration treatment with dilute oxalic and citric acid solutions is evaluated in Paper III. These acidic solutions are able to dissolve and remove contaminants from the washcoat, thus partly restoring the catalyst activity. An investigation of the effects of an oxy-chlorine thermal treatment for regeneration of a ‘full-scale’ commercial automotive three-way catalyst was carried out (Paper IV). 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. The properties of fresh, aged and regenerated catalysts were then studied by means of labscale experiments, on a local as well as a global level using a mathematical model (Paper V). The model allows for comparison of the intrinsic properties of the active surface by deriving and tuning parameters of a fresh catalyst and verifying the activity of a regenerated or aged catalyst.
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| 2. |
- Eriksson, Sara, 1977-
(författare)
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Development of catalysts for natural gas-fired gas turbine combustors
- 2006
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Due to continuously stricter regulations regarding emissions from power generation processes, further development of existing gas turbine combustors is essential. A promising alternative to conventional flame combustion in gas turbines is catalytic combustion, which can result in ultralow emission levels of NOx, CO and unburned hydrocarbons. The work presented in this thesis concerns the development of methane oxidation catalysts for gas turbine combustors. The application of catalytic combustion to different combustor concepts is addressed in particular. The first part of the thesis (Paper I) reports on catalyst development for fuel-lean methane combustion. Supported Pd-based catalysts were investigated at atmospheric pressure. The effect on catalytic activity of diluting the reaction mixture with water and/or carbon dioxide was studied in order to simulate a combustion process with exhaust gas recirculation. The catalytic activity was found to decrease significantly in the presence of water and CO2. However, modifying the catalyst by changing support material can have a considerable impact on the performance. In the second part of this thesis (Papers II-IV), the development of rhodium catalysts for fuel-rich methane combustion is addressed. The effect of catalyst composition, oxygen-to-fuel ratio and catalyst pre-treatment on the methane conversion and the product gas composition was studied. An experimental investigation at elevated pressures of partial oxidation of methane/oxygen mixtures in exhaust gas-rich environments was also conducted. The most suitable catalyst identified for fuel-rich catalytic combustion of methane, i.e. Rh/Ce-ZrO2, showed benefits such as low light-off temperature, high activity and enhanced hydrogen selectivity. In the final part of the thesis (Paper V), a numerical investigation of fuel-rich catalytic combustion is presented. Measurements and predictions were compared for partial oxidation of methane in exhaust gas diluted mixtures at elevated pressures. The numerical model was validated for several Rh-based catalysts. The key parameter controlling the catalytic performance was found to be the noble metal dispersion.
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| 3. |
- Ji, Xiaoyan, 1971-
(författare)
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Thermodynamic properties of humid air and their application in advanced power generation cycles
- 2006
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Water or steam is added into the working fluid (often air) in gas turbines to improve the performance of gas turbine cycles. A typical application is the humidified gas turbine that has the potential to give high efficiencies, high specific power output, low emissions and low specific investment. A heat recovery system is integrated in the cycle with a humidifier for moisturizing the high-pressure air from the compressor as a kernel. Based on today’s gas turbines, the operating temperature and pressure in the humidifier are up to about 523 K and 40 bar, respectively. The operating temperature of the heat exchanger after the humidifier is up to 1773 K. The technology of water or steam addition is also used in the process of compressed air energy storage (CAES), and the operating pressure is up to 150 bar. Reliable thermodynamic properties of humid air are crucial for the process simulation and the traceable performance tests of turbomachinery and heat exchanger in the cycles. Several models have been proposed. However, the application range is limited to 400 K and 100 bar because of the limited experimental data for humid air. It is necessary to investigate the thermodynamic properties of humid air at elevated temperatures and pressures to fill in the knowledge gap. In this thesis, a new model is proposed based on the modified Redlich-Kwong equation of state in which a new cross interaction parameter between molecular oxygen and water is obtained from the fitting of the experimental data of oxygen-water system. The liquid phase is assumed to follow Henry’s law to calculate the saturated composition. The results of the new model are verified by the experimental data of nitrogen-water and oxygen-water systems from ambient temperature and pressure to 523 K and 200 bar, respectively. Properties of air-water system are predicted without any additional parameter and compared with the available experimental data to demonstrate the reliability of the new model for air-water system. The results of air-water system predicted using the new model are compared with those calculated using other real models. The comparison reveals that the new model has the same calculation accuracy as the best available model but can be used to a wider temperature and pressure range. The results of the new model are also compared with those of the ideal model and the ideal mixing model from ambient temperature and pressure to 1773 K and 200 bar to investigate the effect of the models on the thermodynamic properties of humid air. To investigate the impact of thermodynamic properties on the simulation of systems and their components, different models (ideal model, ideal mixing model and two real models) are used to calculate the thermodynamic properties of humid air in the simulation of the compressor, humidification tower, and heat exchanger in a humidified gas turbine cycle. The simulation reveals that a careful selection of a thermodynamic property model is crucial for the cycle design. The simulation results provide a useful tool for predicting the performance of the system and designing the humidified cycle components and systems.
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| 4. |
- Nylén, Linda
(författare)
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Critical potential and oxygen evolution of the chlorate anode
- 2006
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In the chlorate process, natural convection arises thanks to the hydrogen evolving cathode. This increases the mass transport of the different species in the chlorate electrolyte. There is a strong connection between mass transport and the kinetics of the electrode reactions. A better knowledge about these phenomena and their interactions is desirable in order to understand e.g. the reasons for deactivation of anode coatings and what process conditions give the longest lifetime and the highest efficiency. One of the aims of his work was to understand how the chlorate process has to be run to avoid exceeding the critical anode potential (Ecr) in order to keep the potential losses low and to achieve a long lifetime of the DSAs. At Ecr anodic polarisation curves in chlorate electrolyte bend to higher Tafel slopes, causing increasing potential losses and accelerated ageing of the anode. Therefore the impact on the anode potential and on Ecr of different electrolyte parameters and electrolyte impurities was investigated. Additionally, the work aimed to investigate the impact of an addition of chromate on oxygen evolution and concentration profiles under conditions reminiscent of those in the chlorate process (high ionic strength, 70 °C, ruthenium based DSA, neutral pH), but without chloride in order to avoid hypochlorite formation. For this purpose a model, taking into account mass transport as well as potential- and concentration-dependent electrode reactions and homogeneous reactions was developed. Water oxidation is one of the side reactions considered to decrease the current efficiency in chlorate production. The results from the study increase the understanding of how a buffer/weak base affects a pH dependent electrode reaction in a pH neutral electrolyte in general. This could also throw light on the link between electrode reactions and homogeneous reactions in the chlorate process. It was found that the mechanism for chloride oxidation is likely to be the same for potentials below Ecr as well as for potentials above Ecr. This was based on the fact that the apparent reaction order as well as αa seem to be of the same values even if the anode potential exceeds Ecr. The reason for the higher slope of the polarisation curve above Ecr could then be a potential dependent deactivation of the active sites. Deactivation of active ruthenium sites could occur if ruthenium in a higher oxidation state were inactive for chloride oxidation. Concentration gradients of H+, OH-, CrO4 2- and HCrO4 - during oxygen evolution on a rotating disk electrode (RDE) were predicted by simulations. The pH dependent currents at varying potentials calculated by the model were verified in experiments. It was found that an important part of the chromate buffering effect at high current densities occurs in a thin (in the order of nanometers) reaction layer at the anode. From comparisons between the model and experiments a reaction for the chromate buffering has been proposed. Under conditions with bulk pH and chromate concentration similar to those in the chlorate process, the simulations show that the current density for oxygen evolution from OH- would be approximately 0.1 kA m-2, which corresponds to about 3% of the total current in chlorate production.
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| 5. |
- Persson, Katarina, 1977-
(författare)
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Bimetallic Palladium Catalysts for Methane Combustion in Gas Turbines
- 2006
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Catalytic combustion is a promising combustion technology for gas turbines, which results in ultra low emission levels of nitrogen oxides (NOx), carbon monoxide (CO) and unburned hydrocarbons (UHC). Due to the low temperature achieved in catalytic combustion almost no thermal NOx is formed. This thesis is concentrated 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 severely with time during operation. The unstable activity will result in increasing difficulties to ignite the fuel. The parameters that govern the poor stability and other features of the palladium catalysts are discussed in the thesis. The objective of the work is to improve the catalytic performance of supported palladium catalysts, with focus on stabilising the methane conversion. A large number of different bimetallic palladium catalysts have been evaluated, where the influence of co-metals, molar ratio and support material is addressed. Results from the activity tests of methane combustion showed that it is possible to stabilise the activity by adding certain co-metals into the palladium catalyst. An extensive characterisation study has been carried out on the various bimetallic catalysts in order to gain a better understanding of how their morphology and physicochemical properties determine the various patterns of combustion behaviour. The environment inside a gas turbine combustor is very harsh for a catalyst. Since the stability of the catalyst is of great importance for ignition catalysts, it is essential to evaluate the risk of deactivation. In this work special emphasis has been given to thermal deactivation, water inhibition and sulphur poisoning. It was found that a bimetallic Pd Pt catalyst is significantly more tolerant to the various deactivation processes investigated than the monometallic palladium catalyst. Finally, the influence of pressure on the catalytic performance has been investigated. The catalysts were assessed at more realistic conditions for gas turbines, in a high-pressure test facility with 100 kW fuel power.
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| 6. |
- Randström, Sara, 1978-
(författare)
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Characterisation of materials for use in the molten carbonate fuel cell
- 2006
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Fuel cells are promising candidates for converting chemical energy into electrical energy. The Molten Carbonate Fuel Cell (MCFC) is a high temperature fuel cell that produces electrical energy from a variety of fuels containing hydrogen, hydrocarbons and carbon monoxide. Since the waste heat has a high temperature it can also be used leading to a high overall efficiency. Material degradation and the cost of the components are the problems for the commercialisation of MCFC. Although there are companies around the world starting to commercialise MCFC some further cost reduction is needed before MCFC can be fully introduced at the market. In this work, alternative materials for three different components of MCFC have been investigated. The alternative materials should have a lower cost compared to the state-of-the-art materials but also meet the life-time goal of MCFC, which is around 5 years. The nickel dissolution of the cathode is a problem and a cathode with lower solubility is needed. The dissolution of nickel for three alternative cathode materials was investigated, where one of the materials had a lower solubility than the state-of-the-art nickel oxide. This material was also tested in a cell and the electrochemical performance was found to be comparable with nickel oxide and is an interesting candidate. An inexpensive anode current collector material is also desired. For the anode current collector, the contact resistance should be low and it should have good corrosion properties. The two alternative materials tested had low contact resistance, but some chromium enrichment was seen at the grain boundaries. This can lead to a decreased mechanical stability of the material. In the wet-seal area, the stainless steel used as bipolar/separator plate should be coated. An alternative process to coat the stainless steel, that is less expensive, was evaluated. This process can be a suitable process, but today, when the coating process is done manually there seems to be a problem with the adherence. This work has been a part of the IRMATECH project, which was financed by the European Commission, where the partners have been universities, research institutes and companies around Europe.
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| 7. |
- Wesselmark, Maria, 1977-
(författare)
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Methanol and formic acid oxidising anodes in zinc electrowinning
- 2006
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Electrowinning is an energy demanding process, in which the electrolysis accounts for the major part of the energy consumption. The anodic reaction in the electrolysis of zinc from sulphate based electrolytes is oxygen evolution, which has a standard potential of 1.23 V at 25 ºC. The lead anodes used in zinc electrowinning today have a high overpotential for oxygen evolution and the operating anode potential is as high as 2 V vs NHE. Since the lead anodes are not stable some of the lead can dissolve and incorporate in the zinc metal on the cathode. The lead anodes are also undesired because of health concerns. By introducing formic acid or methanol oxidation as anode reaction, the anode potential could be lowered significantly. This would reduce the energy consumption and also enable the use of new types of electrodes. The general aim of this thesis was to investigate how depolarised anodes for methanol and formic acid oxidation can be used in electrowinning processes. Several electrodes were shown to be suitable as anodes in zinc electrowinning. A high activity was obtained with porous electrodes, whereas a platinum coated titanium was the most stable electrode. The choice of electrode will however always be a compromise between activity, stability and costs. All electrodes were deactivated with time and the best method for reactivation tested in this study, was periodic current reversal (PCR). An operating procedure was established for methanol and formic acid oxidation on high surface area electrodes, Pt-TySAR®. With the use of PCR at process current density, a potential lower than 0.7 V vs NHE could be maintained for 36 hour in synthetic electrolyte. The use of formic acid oxidation resulted in lower potentials than the use of methanol oxidation. The activities for methanol and formic acid oxidation in industrial electrolyte were very low and chloride impurities were shown to cause the major part of the activity reduction. Even small amounts of chloride (10-6-10-5 M) affected the activity of the electrodes. The chloride impurities are considered as the main problem to overcome in order to introduce the oxidation of formic acid or methanol in the zinc electrowinning process.
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