Hydrogen, an Energy Carrier of the Future
Abstract: The climate is changing, something that many scientists correlate to increased emissions of greenhouse gases. The effluent that is considered to have the highest impact on global warming is carbon dioxide, when effect and quantity are taken into account. Most projections however, show that the effect is reversible if the amount of carbon dioxide emissions discharged to the atmosphere is decreased. There are several ways of reducing the carbon dioxide discharge, e.g. using so called bio-fuels such as bio-diesel or bio-ethanol. Another way is to better utilize the fossil fuels that are being used today in a more efficient way, e.g. by using hybrid engines or hydrogen fuel cells. Using hydrogen fuel cells will better utilize the energy in a fuel, and hence require less fuel for doing the same amount of work. However, the hydrogen has to be manufactured, stored and transported to the point of consumption, something that requires energy input. There are three generally accepted roads to hydrogen production when scale is considered. Large centralized production, distributed production e.g. in a refuelling context and on-board production, where the large centralized production enjoy economics of scale while the other two can offer advantages of modularity and proximity of production. The use of fuel cells, however, places exceptional demands on the hydrogen purity and especially the carbon monoxide and sulphur content. In this thesis, two different reactor systems for small-scale hydrogen production have been investigated. Both systems are steam-reforming-based and contain a water-gas shift section, but the mode of carbon monoxide clean-up is different. The size of the systems is in the 3-20 kW of hydrogen output range. The first system is a pressurized system with pressure swing adsorption purification, delivering pure hydrogen. This system has been operated in stand-alone mode and integrated with hydrogen storage, low pressure hydrides and regular tank storage, 4 kWe fuel cells and 30 kWe photovoltaic installations. The system was operated on a synthetic Fischer-Tropsch diesel and performed satisfactorily compared to the design specifications. The reformer system including the pressure swing adsorption had almost 60 % conservation of lower heating value, and produced 99.9998 % pure hydrogen with less than 1 ppm of carbon monoxide. This, together with the fuel cells gave an electricity energy conversion of almost 30 % of the fuel heating value and another 30 % of the inlet energy can be used for space or water heating. The system shows good properties from a hybridization perspective as well. The second system is an atmospheric system and utilizes a preferential oxidation clean-up of the carbon monoxide. This system does not provide pure hydrogen, but a mixture of hydrogen and carbon dioxide that requires a different kind of fuel cell. This system was operated on an array of different fuels like methane, ethane, propane, kerosene, methanol and ethanol. The system performed satisfactorily at several loads and during 500 h operation. Both systems have advantages and disadvantages and the preferred system in any given situation depends on the requirements of the implementation. The systems described in brief above require catalysts to perform in a satisfactory way. This thesis reports the work performed in finding suitable catalysts for the fuel processing operations. The steam-reforming catalyst is noble-metal-based and several different configurations have been tested and reported. The experiments also include tests of sulphur resistance and regeneration of several different kinds of catalysts, both supported and hexa-alumina based. The water-gas shift catalyst is a noble metal supported on ceria. A number of stabilizations have been done on that particular catalyst, either by promoting the active phase or by doping the carrier phase. The sulphur resistance has been investigated as well. The catalyst used for the preferential oxidation is a noble metal mounted on a transition metal supported on an alumina carrier. Experiments show the effects of different active phases and transition metals used for the mounting. The most active catalyst was investigated further, using transition electron microscopy and the activity was characterized before and after 1,000 h of operation. To get a truly sustainable hydrogen society the hydrogen generated has to originate from renewable energy sources like wind, sun or biomass. One way to generate a sufficient amount of bio-fuel that can be used in distributed hydrogen generation is to gasify woody bio-mass and liquefy it. In this way it is possible to produce different kinds of fuel ranging from methanol to Fischer-Tropsch diesel. In this thesis, different pathways to producing liquid fuels and hydrogen from bio-mass from gasification have been investigated. Different process configurations for production of methanol and Fischer-Tropsch diesel, both stand-alone and co-production of synthetic natural gas have been investigated. An alternative route to oxygen generation to the gasifier, based on electrolysis, has been economically evaluated and the deactivation of steam reforming catalysts by biomass-derived aerosols downstream from the gasifier has been investigated. The investigations have shown that generating hydrogen from a wide range of fuels, both liquid and gaseous, is feasible. Both of the systems used were proven to be suitable for hydrogen generation. What system to use in a specific situation depends on the requirements in each individual case. The steam-reforming catalyst used is stable over time, and can handle a variety of fuels as well as low to medium sulphur levels. The water-gas shift catalyst is very active initially and by various means of stabilization the high activity was retained to a higher degree over time. A catalyst consisting of platinum mounted on cobalt and supported on alumina is both active and selective for the selective oxidation of carbon monoxide in hydrogen-rich streams. The platinum was shown to be present on both the alumina and on the cobalt, which led to the conclusion that it is the cobalt-supported platinum that has the high activity. The major deactivation of this catalyst type after 1,000 h was a loss of selectivity, while the activity in fact increased slightly, which might be explained by an increase in noble metal particle size. The production of methanol and Fischer-Tropsch diesel was shown to be significantly favoured by co-production with synthetic natural gas, and the use of an alternative oxygen source in the gasification did not prove to give any cost advantages to regular oxygen generation. The cost of hydrogen production with both the alternative oxygen- generation method and regular oxygen generation was shown to be at least twice the cost of production of natural-gas-based hydrogen. The presence of biomass-generated aerosols did not prove to be a major source of deactivation, but there might be long-term effects. The exposure also changes the catalyst morphology, including the metallic particle size and specific surface area.
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