NON-CONVENTIONAL WASTE-DERIVED FUELS FOR MOLTEN CARBONATE FUEL CELLS
Cigolotti, Viviana (2009) NON-CONVENTIONAL WASTE-DERIVED FUELS FOR MOLTEN CARBONATE FUEL CELLS. [Tesi di dottorato] (Inedito)
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Global energy consumption is expected to increase dramatically in the next decades, driven by the rising of the standards of living and by the growth of population worldwide. The increased need of energy will require enormous growth in energy generation capacity, more secure and diversified energy sources, and a successful strategy to control and to reduce greenhouse gases emissions. There is a huge challenge to provide an everyday product, energy – that is taken absolutely for granted – in a radically different, difficult, but fundamentally improved way, at accustomed and competitive cost. Also on the demand side severe corrections have to be undertaken: product and associated waste flows have to be interpreted differently, efficiency and sustainability becoming key issues. One of the most immediate, and effective, ways to tackle this challenge is to minimize losses and waste by maximizing the exploitation efficiency of the resources that are utilised. One valid way to reduce fossil fuels dependence and demand, for example, is the use of alternative or non-conventional fuels, derived from waste or biomass. These fuels, by nature of their transient origins, are generally poor in energy content, which imposes localized deployment and maximum efficiency in their utilization in order to obtain a useful amount of work and/or heat. In the effort to maximize the energetic yield from alternative energy sources like waste or biomass, and wanting to minimize environmental impact in terms of polluting or CO2 emissions, the coupling of Molten Carbonate Fuel Cells (MCFCs) to the fuel gas produced from these sources is an attractive option. Combining these resources with fuel cell applications would provide a significant contribution to environmentally friendly, efficient energy use. Currently, biofuels from waste and biomass are mainly used in engines and turbines with fairly low efficiencies and generate significant amounts of regulated pollutants (NOx, SOx and particulates). Replacement of these conventional heat engines with MCFCs would allow a more efficient use of biofuels but, more significantly, would reduce NOx, SOx and particulates to insignificant levels and increase CO2 benefits. This work is part of an ambitious Research Project under the Agreement between ENEA (Italian National Agency for New Technologies, Energy and Sustainable Economic Development) and MSE (Italian Ministry of Economic Development) on the MCFCs Research and Development. One of the main topic of this project is the use of MCFC with biofuels (biogas from anaerobic digestion and syngas from biomass gasification). The current Ph.D. work is finalized towards the coupling of a molten carbonate fuel cell (MCFC) to an anaerobic digestion process of organic residues or sludge from a wastewater treatment plant, and it has been carried out under the supervision of the Hydrogen & Fuel Cell Project, Energy Technologies, Renewable Energy Sources and Energy Saving Department, ENEA, Rome, Italy. The biogas produced through anaerobic digestion is ideally suited for electrochemical conversion in an MCFC thanks to the large content of readily reformable methane and the necessary diluent CO2. The crucial link between these two technologies (anaerobic digestion and the MCFC), however, is formed by the gas clean-up step. This is because the raw produced biogas contains trace elements that originate from the organic nature of the feedstock, and that have detrimental effects on fuel cell performance and durability. The most common contaminant contained in the biogas is hydrogen sulphide (H2S), relevant for harmful effects, both on the fuel cell electrodes as on the reforming catalysts. A large part of the current study was dedicated to the particular effects of H2S on the MCFC anode. In the last years several studies on the effects of H2S on the MCFC anode are reported in the literature, but the knowledge is still incomplete and requires more in deep study. Based on this knowledge a full experimental study was performed looking at the accurate knowledge of the conditions which are deleterious to MCFC, in order to facilitate safe and reliable operation of the fuel cell. A systematic experimental campaign was carried out, the results of which will be presented and discussed, showing the effects and implications of cell poisoning with H2S observed at several different levels of diagnosis (chemical, electrochemical, electrical, material). The objective is to ultimately identify the true, effective tolerance limits of current MCFC materials, especially as regards different concentrations of H2S that can occur due to composition variations of the produced biogas. In order to achieve this objective, many hours of long-term experimentation has been required and different single cells have been operated in MCFC Laboratory, at Center for Fuel Cell Research, Energy & Environment Research Division, KIST – Korea Institute of Science and Technology, Seoul, South Korea. The results of this experimental study allow to identify the main effect on the MCFC anode side by H2S and also to evaluate the important role played by the Electrochemical Impedance Spectroscopy (EIS) as added value for the interpretation of this results. The impedance measurements are carried out to identify the processes which take place in the anode and to better understand the reversibility of sulphur poisoning under the regeneration processes. After the technical approach, it’s relevant to consider also the economic feasibility to understand how and when the MCFC systems fed with biogas can be competitive with other technologies currently present on the market, as Internal Combustion Engine and Gas Turbine. A Cost-Benefit model will be performed and, based on it, a technical-economical analysis will be illustrated and discussed, considering the use of biogas in a 1.4 MW MCFC plant, in order to identify the most important economic parameters that affect the use of biogas in MCFC.
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