Transportation sector CO2 emissions contribute to global warming. Methanol generated from clean energy sources has been proposed as a transportation fuel as an alternative to gasoline or diesel to reduce emissions. Catalytic methanol-steam reformers can be combined with high temperature polymer electrolyte membrane (HTPEM) fuel cell systems to create compact electrical power modules which run on liquid methanol. These modules combine the efficiency of a fuel cell system with the convenience of using a traditional, liquid hydrocarbon fuel. Catalytic methanol-steam reformers require a heat source as the methanol-steam reforming process is endothermic. The heat source for this system will initially be from the catalytic combustion of either pure methanol, during startup, or from HTPEM fuel cell anode waste gas during system operation. Efficient use of catalyst requires effective premixing of the fuel and air. This study will investigate parameters affecting premixing and their effect on temperature distributions and emissions.

Catalytic combustion is an efficient method to reduce pollutant emissions produced by a variety of fuels. In this thesis, the use of iron pentacarbonyl (Fe(CO)5) as a catalyst precursor in the combustion of natural gas is experimentally studied. The counter-flow diffusion flame burner is employed as the experimental apparatus. The products of combustion are analyzed by using a Gas Chromatograph (GC) to quantitate the effects of adding the catalyst. The experimental setup is such that a mixture of methane (CH4) and nitrogen (N2) is fed from the bottom burner while a mixture of oxygen (O2) and air is supplied from the top burner. The combustion of natural gas without catalyst is first characterized. The oxidizer and fuel flow parameters are set up so that a stable, flat blue flame is formed close to the centre plane between the two burners upon ignition. The experimental results agree with the literature data and the numerical predictions from CHEMKIN software. To investigate and evaluate the performance of iron-containing catalysts on emission reduction, a small amount of separated nitrogen flow is used to carry iron pentacarbonyl into the flame through the central port of the fuel-side burner. Catalytic combustion produces an orange flame. Compared with the non-catalytic combustion data, it is found that carbon monoxide (CO) and soot precursor acetylene (C2H2) are reduced by 80% to 95% when 7453ppm iron pentacarbonyl is added.

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The catalytic combustion of methane in excess of O2 over Pd catalysts was studied on model catalysts, including polycrystalline palladium foil and palladium single crystals. The kinetics of this reaction could be measured at conditions not accessible to supported catalysts and, thus, the issues of structure sensitivity, mechanism, hysteresis on oxidation, and deactivation could be studied in detail. Methane oxidation on PdO was insensitive to the original metal surface structure which PdO grew from, with turnover rates in the range of 1.3-4.7 s-1 on (111), (100) and (110) single crystals at 160 Torr O2, 16 Torr CH-4, 1 Torr H2O and 598 K. Methane oxidation on Pd metal was also insensitive to the original surface structure, with the turnover rate in the range of 2.0-2.8 s-1 on the three single crystals at 2.3 Torr O2, 0.46 Torr CH4, 0.05 Torr H2O and 973 K. Since there is no support effect and the surface purity could be certified, these turnover rates for this reaction can be used as a benchmark. The turnover rate for methane oxidation was found to decrease 95% when PdO decomposed to Pd metal at 888 K, showing that PdO was more active than Pd metal for methane combustion at this temperature. Water inhibition to the reaction was not observed at a temperature above 813 K on both PdO and Pd metal, while it was observed at 598 K on PdO. The activation energy on PdO was 32 kJ mol-1 in the range of 783-873 K, while it was 125 kJ mol-1 in the range of 568-623 K. The activation energy on Pd metal was 125 kJ mol-1 in the range of 930-980 K. The change of reaction orders and activation energies suggests that the reaction mechanism is a function of temperature and palladium chemical states. We propose that adsorbed water, the most abundant surface intermediate at 598 K, was not present in significant quantities at temperatures above 783 K. This change in surface inhibition by water is the reason for lower activation energy at temperatures above 783 K. Interaction between the catalyst and support, or presence of impurities, is one of the factors for catalyst deactivation. The interaction between oxidized silicon and palladium was investigated on a polycrystalline palladium foil and on supported Pd/SiO2 catalysts. During methane oxidation, oxidized silicon covered the palladium oxide surface as observed by TEM on Pd/SiO2 catalysts and by XPS on palladium foil. On Pd foil, the source of silica was a silicon impurity, common on bulk metal samples. The migration of oxidized silicon onto PdO deactivated the catalysts by blocking the active sites for methane oxidation. Silicon oxide overlayers were also observed covering the Pd surface after reduction of Pd/SiO2 by H2 at 923 K.

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Catalytic combustion is an environmentally friendlytechnique to combust fuels in e.g. gas turbines. Introducing acatalyst into the combustion chamber of a gas turbine allowscombustion outside the normal flammability limits. Hence, theadiabatic flame temperature may be lowered below the thresholdtemperature for thermal NOXformation while maintaining a stable combustion.However, several challenges are connected to the application ofcatalytic combustion in gas turbines. The first part of thisthesis reviews the use of catalytic combustion in gas turbines.The influence of the fuel has been studied and compared overdifferent catalyst materials. The material section is divided into two parts. The firstconcerns bimetallic palladium catalysts. These catalysts showeda more stable activity compared to their pure palladiumcounterparts for methane combustion. This was verified both byusing an annular reactor at ambient pressure and a pilot-scalereactor at elevated pressures and flows closely resembling theones found in a gas turbine combustor. The second part concerns high-temperature materials, whichmay be used either as active or washcoat materials. A novelgroup of materials for catalysis, i.e. garnets, has beensynthesised and tested in combustion of methane, a low-heatingvalue gas and diesel fuel. The garnets showed some interestingabilities especially for combustion of low-heating value, LHV,gas. Two other materials were also studied, i.e. spinels andhexaaluminates, both showed very promising thermal stabilityand the substituted hexaaluminates also showed a good catalyticactivity. Finally, deactivation of the catalyst materials was studied.In this part the sulphur poisoning of palladium, platinum andthe above-mentioned complex metal oxides has been studied forcombustion of a LHV gas. Platinum and surprisingly the garnetwere least deactivated. Palladium was severely affected formethane combustion while the other washcoat materials were mostaffected for carbon monoxide and hydrogen. <b>Keywords:</b>catalytic combustion, catalyst materials,palladium, platinum, bimetallic, garnet, spinel, hexaaluminate,deactivation, sulphur, poisoning, diesel, methane,hydrocarbons

Catalytic combustion is a promising technology for heat and power applications, especially gas turbines. By using catalytic combustion ultra low emissions of nitrogen oxides (NOX), carbon monoxide (CO) and unburned hydrocarbons (UHC) can be reached simultaneously, which is very difficult with conventional combustion technologies. Besides achieving low emission levels, catalytic combustion can stabilize the combustion and thereby be used to obtain stable combustion with low heating-value gases. This thesis is focused on the high-temperature part of the catalytic combustor. The level of performance demanded on this part has proven hard to achieve. In order to make the catalytic combustor an alternative to the conventional flame combustor, more stable catalysts with higher activity have to be developed. The objective of this work was to develop catalysts with higher activity and stability, suitable for the high-temperature part of a catalytic combustor fueled by natural gas. Two template-based preparation methods were developed for this purpose. One method was based on soft templates (microemulsion) and the other on hard templates (carbon). Supports known for their stability, magnesia and hexaaluminate, were prepared using the developed methods. Catalytically active materials, perovskite (LaMnO3) and ceria (CeO2), were added to the supports in order to obtain catalysts with high activities and stabilities. The supports were impregnated with active materials by using a conventional technique as well as by using the microemulsion technique. It was shown that the microemulsion method can be used to prepare catalysts with higher activity compared to the conventional methods. Furthermore, by using a microemulsion to apply active materials onto the support a significantly higher activity was obtained than when using the conventional impregnation technique. Since the catalysts will operate in the catalytic combustor for extended periods of time under harsh conditions, an aging study was performed on selected catalysts prepared by the microemulsion technique. The stability of the catalysts was assessed by measuring the activity before and after aging at 1000 C in humid air for 100 h. One of the most stable catalysts reported in the literature, LMHA (manganese-substituted lanthanum hexaaluminate), was included in the study for comparative purposes. The results showed that LMHA deactivated much more strongly compared to several of the catalysts consisting of ceria supported on lanthanum hexaaluminate prepared by the developed microemulsion method. Carbon templating was shown be a very good technique for the preparation of high-surface-area hexaaluminates with excellent sintering resistance. It was found that the pore size distribution of the carbon used as template was a crucial parameter in the preparation of hexaaluminates. When a carbon with small pores was used as template, the formation of the hexaaluminate crystals was strongly inhibited. This resulted in a material with poor sintering resistance. On the other hand, if a carbon with larger pores was used as template, it was possible to prepare materials with hexaaluminate as the major phase. These materials were, after accelerated aging at 1400 C in humid air, shown to retain surface areas twice as high as reported for conventionally prepared materials. / QC 20100719

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. / QC 20110125