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Wärmehaushalt einer Karbonat-Brennstoffzelle zur Wasserstoffherstellung für eine Polymerelektrolyt-Brennstoffzelle
Abstract
Die Verwendung von Karbonatbrennstoffzellen (MCFC) ist vorteilhaft, weil sie kohlenwasserstoffbasierte Treibstoffe, zum Beispiel Erdgas mit einer Energiedichte von etwa 10 kWh/m3 bei Normbedingungen, intern reformieren können und der Wirkungsgrad dabei mit circa 50% hoch ist. Jedoch ist die Leistungsdichte im Vergleich zu anderen Brennstoffzellen mit 5 kW/m3 eher gering. Die Leistungsdichte einer Polymerelektrolyt-Brennstoffzelle (PEFC) ist mit 50 kW/m3 sehr viel höher. Sie kann dabei ebenfalls mit einem Wirkungsgrad von 50% betrieben werden. Dazu benötigt sie Wasserstoff als Treibstoff, der bei Normbedingungen eine Energiedichte von 2,9 kWh/m3 aufweist. Soll Methan als Treibstoff benutzt werden, nimmt der Wirkungsgrad durch die notwendige Reformierung um circa 10 bis 15 Prozentpunkte ab und die Leistungsdichte ist mit 6 kWh/m3 etwa so hoch wie die eines MCFC-Systems. Ein gemeinsames System aus MCFC und PEFC, die so genannte CoCell, bietet darüber hinaus folgende Vorteile: Einsatz eines hochenergetischen, kohlenwasserstoffbasierten Treibstoffs, z.B. Methan Hoher elektrischer Wirkungsgrad Höhere Leistungsdichte als die jeweiligen Systeme im Einzelbetrieb mit Reformer. In der CoCell fungiert die MCFC als stromproduzierender Reformer für die PEFC. Die Abwärme der MCFC wird zur Reformierung genutzt, wodurch Wasserstoff verfügbar ist, der in der leistungsdichten PEFC weiter umgesetzt wird. Die Reformierleistung der MCFC ist durch ihren internen Wärmehaushalt begrenzt. Verbraucht die endotherme Reformierung mehr Wärme als durch die Stoffströme zugeführt und als Abwärme der Brennstoffzellen-Reaktion erzeugt wird, kühlt der Zellstapel aus. Die Leistung einer solchen kombinierten Brennstoffzellen-Anlage wurde in der vorliegenden Arbeit mit der thermodynamischen Simulations-Software ASPEN modelliert. Rechnungen zur Reduzierung der Gasnutzung in der MCFC durch verschiedene Heizverfahren zeigten, dass Wärme am effektivsten durch Erhöhung der Stromdichte in der MCFC zugeführt wird. Dadurch wird der Stapel direkt elektrisch geheizt und die Leistungsdichte des Systems durch Erhöhung der MCFC-Leistungsdichte weiter verbessert. Die Reduzierung der Gasnutzung wird durch Erhöhung der Treibstoffzufuhr erreicht. Dadurch wird der Stapel auch bei geringer Gasnutzung und geringer Luftzufuhr ausreichend gekühlt. In dieser Konguration ist die Erhöhung der Stromdichte bis auf 350 mA/cm2 möglich. Dies entspricht dem 2,5-fachen Wert von 140 mA/cm2 bei typischer, konventioneller Betriebsweise. Die Grenze der Stromdichteerhöhung wurde experimentell ermittelt. Basierend auf den vorliegenden Modellrechnungen ist die Leistungsdichte der CoCell bei einer Stromdichte der MCFC von 280 mA/cm2 mit 14 kW/m3 nahezu dreimal so hoch wie die anderer Brennstoffzellenreformersysteme (siehe oben). Die Effizienz der elektrischen Energieerzeugung ist in dieser Betriebsweise mit 38% im Nennpunkt so hoch wie die eines PEFC-Reformersystems. Molten carbonate fuel cells (MCFC) are being used in decentralised power plants, as they can reform hydrocarbon bound fuels internally, e.g. netural gas with a energy density of 10 kWh/m3 at standard conditions, and the efficiency of this mode of operation is around 50%. However in comparison to other fuel cell systems the power density is only 5 kW/m3. The power density of a polymerelectrolyte fuel cell (PEFC) ismuch higher (50 kW/m3). These systems can be run with an efficiency of 50%, too. Therefore they need hydrogen as a fuel, with an energy density of 2,9 kWh/m3 at standard conditions. Efficiency decreases to 35 to 40% using Methane as fuel, because of the reforming losses. The power density than is 6 kW/m3 and therefore as high as for a MCFC-system. Acombination of MCFC and PEFC, the so called CoCell, offers the following advantages: A highly energetic, hydrocarbon based fuel can be used, e.g. Methane. A high electrical efciency is achieved. The power density of this system is higher than for a fuel cell with reformer. In the CoCell the MCFC is working as electricity producing reformer for the PEFC. The offheat of the MCFC is used for reforming, whereby hydrogen is available, being utilised further in the powerdense PEFC. The reforming capacity of the MCFC is limited by the internal heat balance. If the endothermic reforming consumes more heat than supplied by the material streams and the fuel cell waste heat, the stack cools down. The performance of such a combined fuel cell system has been evaluated in this thesis using the thermodynamic simulation software Aspen. Calculations reducing the utilisation in the MCFC by various heating techniques showed, that additional heat is supplied most efficiently by increasing the current density of the MCFC. Thereby the stack is heated electrically and the power density of the system is increased by the improved power density of the MCFC. The reduction of the utilisation is achieved by increasing fuel supply. Thereby the stack is cooled sufficiently at low utilisation and low air supply. In this configuration the current density can be increased up to 350 mA/cm2. This is 2,5 times the value of 140 mA/cm2 of typical, conventional operation. This limitation was explored experimentally. Based on the presented model calculations the power density of the CoCell increases to 14 kW/m3 at a current density in the MCFC of 280 mA/cm2. It is thereby almost three-times as high as for other fuel cell reformer systems (see above). The electrical efficiency accounts for 38% and is as good as a PEFC-Reformer system.
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The assignment of numerical values to physical quantities underlies all quantitative statements in engineering and the physical sciences. This assignment is achieved by the process of measurement. The physical quantity being measured and the precision required in the numerical value determines the instrumentation to be used. The design of a measurement system therefore involves the analysis of the attribute to be measured, the means available for its detection and the verification that the measurement system performs as intended and can achieve the desired accuracy and precision.
In this book Figliola and Beasley first discuss in general terms the basics of measurement, analogue and digital data acquisition systems and signal processing, the statistics of measurement and the analysis of error and uncertainty. In successive chapters they concentrate on the instruments and their physical basis in the areas of electricity, temperature, fluid flow, elastic strain and mechanics (displacement, motion, force and power). The coverage is directed towards measurements in various branches of engineering, with numerous worked examples and problems for students (approximately 30 to 40) at the end of each chapter. Since it is an American engineering text, the book uses both SI and English units.
Unfortunately, the text is flawed by numerous errors. Some of the more egregious are that in chapter 1, `dimension' is used in place of `unit', the definitions given for the ampere and the ohm are in terms of `international' units that were abandoned in 1948 and derived units are expressed, for example, as `' in place of the standard forms (SI, ISO, ANSI) of `' or `'. There are furthermore numerous minor numerical errors and inconsistencies. One of the more serious flaws is the failure to distinguish between bias errors and uncertainty due to bias in the discussion of chapter 5. There is also a misuse of the student in the evaluation of uncertainty (although this error is not exclusively Figliola and Beasley's, since it occurs in ANSI documents on fluid flow measurement). Given the estimate for variance, with degrees of freedom, an uncertainty interval at confidence level p is properly , while the uncertainty for a combined quantity is
where is evaluated from the Welch - Satterthwaite expression
In spite of its shortcomings, the book collects a great deal of material in one place and, in the hands of a careful instructor who is aware of its flaws, could be useful as a supplementary text on measurement.
E Richard Cohen
Optimal use of energy in the chemical industry. Energy costs represent a considerable proportion of manufacturing costs in the chemical industry. Optimization of energy consumption is therefore an important task, but supply and removal of energy also should be considered in the field. Rising oil prices and a predictable lack of steam from power plants required a new concept of the energy supply at the Ludwigshafen site of BASF. Saving energy turned out to be the best solution, considering the oil price, the costs of investment in power plants and the risks of further developments. Some examples explain the procedure and the results of this concept. Though production has increased by 60% since 1970, the consumption of primary energy has been reduced to less than 50%. Investments in power stations have been avoided, but capital had to be spent in energy saving measures and modifications of supply structures. The processes of productions have been improved, air pollution has decreased, the environment has been relieved. Energy saving remains an important task for the future in spite of present low oil prices.
Commercialization of molten carbonate fuel cells (MCFCs) requires a lifetime of 40,000 h. In order to achieve this goal, the main decay mechanisms must be understood and controlled. Limiting factors affecting the lifetime of MCFC monocells have been investigated in endurance tests totaling more than 15,000 h using an automated monocell test facility installed at CNR-TAE. The cells were assembled with tape-casted NiCr anodes and NiO cathodes separated by LiAIO2 matrices impregnated with a mixture of Li2CO3 and K2CO3. The tests have been carried out at temperatures ranging from 873 to 923K.The results of these investigations have indicated that the main limiting factors for long-term operations must be correlated with an insufficient carbonate inventory in the tile and with the electrolyte redistribution amongst the active cell components. In operating cells, the electrolyte losses occur through four basic mechanisms: lithiation of the anode and cathode, hardware corrosion, volatilization and electrolyte migration (creepage). Post-mortem analysis by atomic adsorption spectroscopy, scanning electron microscopy and Hg-intrusion porosimetry has been employed to study these phenomena. Particularly, it has been observed that the tile structure changed drastically. A further problem was the dissolution of the cathode, that is the effect of the small solubility of the NiO in the electrolyte. The decay effects appear more drastic in the high-temperature test, probably due to the faster kinetics of the parasitic reactions.
Understanding the reactivity of organic molecules in pure superheated water is developing from studies aimed at explaining how organic matter (kerogen) forms and then breaks down, in low temperature (<150°C) natural environments, into energy source materials. In natural systems where kerogens are depolymerized to generate petroleum, superheated water is ubiquitous. Organic compounds, especially those with oxygen-containing functionality, can participate in reactions such as ionic condensations, additions, cleavages and hydrolyses. These reactions are discussed with an awareness to their facilitation by changes in the chemical and physical properties of water as temperature increases. It is emphasized that these changes make the solvent properties of liquid water at high temperature similar to those of polar organic solvents at room temperature, thus favoring ionic versus thermal free radical solution phase reactions with organic compounds. Examples illustrate that in this aquathermolysis chemistry, water can participate in one or more roles: as a catalyst, reactant, and solvent. It is often not necessary to add acid, base, or other catalysts. We point out how an understanding of the aqueous chemistry of a wide range of organic molecules may lead to environmentally friendly potential applications in areas as diverse as recycling of plastics, syntheses of chemicals, and generation of liquid fuels from natural resource materials.
This paper reports on microkinetic measurements of cathodic oxygen reduction at immersed flag electrodes in Li/K carbonate melts and macrokinetic measurements of the same reaction in laboratory fuel cells with eutectic Li/K and Li/Na carbonate electrolyte. The microkinetic measurements confirm that at 650°C oxygen reduction is a relatively fast reaction with exchange current densities of the order of 10−2 A cm−2 at gold, lithiated nickel oxide and lithium cobaltite electrodes, so that charge transfer hindrance might be of little relevance for the macrokinetics of molten carbonate fuel cell (MCFC) cathodes. O2 reduction seems to be faster on Lithium cobaltate than on lithiated nickel oxide. The results on gold electrodes indicate that in Li/K carbonate melts the species which transports oxygen may be peroxocarbonate. Macrokinetic measurements show that mass transfer of oxygen determines current–voltage curves, in particular at low oxygen partial pressures but also at low carbon dioxide partial pressures of the order of 10−1 bar and less, which are relevant to the technical operating conditions of MCFCs. CO2 transport bears sizeably on the cathodic current–voltage correlation at low partial pressures. Since the oxygen solubility is lower in Li/Na than in Li/K carbonate melts, oxygen overpotentials become much higher with lean gas mixtures at higher current densities with Li/Na carbonate melts. Therefore in the practical sense the Li/K carbonate melt seems to be overwhelmingly superior to the Li/Na carbonate eutectic in MCFC technology. If the morphology of MCFC cathodes can be significantly improved to remove oxygen and carbon dioxide mass transfer hindrance, or if pressurised cells are used in which O2 and CO2 partial pressures exceed 0.25 bar even for lean gases, the less volatile and more conductive the Li/Na carbonate melt will be a superior alternative to the Li/K carbonate eutectic as an MCFC electrolyte.
Water near or above its critical point (374 C, 218 atm) is attracting increased attention as a medium for organic chemistry. Most of this new attention is driven by the search for more green or environmentally benign chemical processes. Using near-critical or supercritical water (SCW) instead of organic solvents in chemical processes offers environmental advantages and may lead to pollution prevention. Interest in doing chemistry in SCW is not entirely new, however. There has been much previous research in this area with applications in synthetic fuels production, biomass processing, waste treatment, materials synthesis, and geochemistry. Water near its critical point possesses properties very different from those of ambient liquid water. The dielectric constant is much lower, and the number and persistence of hydrogen bonds are both diminished. As a result, high-temperature water behaves like many organic solvents in that organic compounds enjoy high solubilities in near-critical water and complete miscibility with SCW. Moreover, gases are also miscible in SCW so employing a SCW reaction environment provides an opportunity to conduct chemistry in a single fluid phase that would otherwise occur in a multiphase system under more conventional conditions. The paper discusses the use of supercritical water in the following reactions: hydrogenation/dehydrogenation; C-C bond formation; rearrangements; hydration/dehydration; elimination; hydrolysis; partial oxidation; H-D exchange; decomposition; and oxidation.
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