Development of a low temperature co-fired ceramic fuel processor for the micro-scale solid oxide fuel cell system

The miniaturized solid oxide fuel cells (µ-SOFCs) has become an intensively studied device for portable power generation technology due to its wide choice of hydrocarbon fuels, its high energy density and its great operation efficiency. It is being considered as a battery replacement [1]. The µ-SOFC system, which aims to provide electrical energy (≤ 10 W) at an operating temperature of ca. 550°C, consists of a fuel cell unit for the electrochemical conversion [2-3]; a fuel processing unit for the thermal start-up, fuel reforming and total oxidation of exhausts [4-5]; a system packaging that insulates the fuel cell unit from the operating temperature to the ambient as well as provides fluidic and electronic connections [6-8]; and an electronics module for regulating the power output. As a core module of the entire µ-SOFC system, various fuel processing units have been proposed and developed. Most of those modules have been based on microelectromechanical systems (MEMS), which however shows several critical limitations with regard to electrical and fluidic connections and system integration [9-10]. Here we propose a ceramic based meso-scale gas processer combined with thick film and low-temperature co-fired ceramic technology (LTCC). With an overall size of 12 × 30 × 10 mm3, the ceramic processor, made of Heraeus HeroLock 2000 LTCC materials, mainly functions as a meso-scale hotplate that has a cantilever shape to effectively decouple the heat at the hot zone produced by the start-up heater and/or exothermic fuel processing reactions from the cold zone, in which the temperature is near ambient and thus compatible with normal electrical and fluidic connections (Figure 1). Embedded cavities were integrated into the processor during the fabrication process by using a progressive lamination technique. A thick-film crack-free catalyst paste, containing rhodium-doped ceria-zirconia nanoparticles, was dispensed into the reaction chambers as packed catalytic beds for the processing reactions. An integrated thick-film platinum (Heraeus CL11-6109) heater provides the start-up energy for the exothermic reforming reaction of butane or methane as well as total oxidation reactions. Such a meso-scale monolithic ceramic reactor can carry out the gas processing in a thermally self-sustaining manner, rendering itself to be a functional packaging of the entire µ-SOFC system in the future. In this work, the fabrication process of the gas processor will be discussed in detail, and the results of the fuel processing reactions such as reforming, total oxidation and thermal start-up will be presented as well. References: [1] Bieberle-Hütter, A., Beckel, D., et al. Journal of Power Sources, 177(1), 123–130, (2008) [2] Rey-Mermet, S. and Muralt, P. Solid State Ionics, 179(27–32), 1497-1500, (2008) [3] Evans, A., Bieberle-Hütter, A., et al. Monatshefte für Chemie - Chemical Monthly, 140(9), 975–983, (2009) [4] Shao, Z., Haile, S. M., et al. Nature, 435(7043), 795–798, (2005) [5] Santis-Alvarez, A. J., Nabavi, M., et al. Energy & Environmental Science, 4(8), 3041, (2011) [6] Jiang, B., Maeder, T., Muralt, P. Proceedings, Power MEMS 2010, Leuven (BE), 2010 [7] Jiang, B., Muralt, P., et al. Sensors and Actuators B: Chemical, 175, 218–224, (2012) [8] Maeder, T., Jiang, B., et al. Proceedings, 7th International Conference on Ceramic Interconnect and Ceramic Microsystems Technologies (CICMT), San Diego (USA), 2011 [9] Hotz, N., Osterwalder, N., et al. Chemical Engineering Science, 63(21), 5193–5201, (2008) [10] Vaccaro, S., Malangone, L., Ciambelli, P. Industrial & Engineering Chemistry Research, 49(21), 10924–10933, (2010)

A novel fuel processing platform for micro-scale solid oxide fuel cells

Bo Jiang, Thomas Maeder, Paul Muralt, Dimos Poulikakos

Recently, the development of micro-scale solid oxide fuel cells (µ-SOFCs) has become a promising research topic in the area of portable energy production. A µ-SOFC system, which can provide 1 to 2 W electrical power under an operating temperature of 550°C 1-2, mainly consists of a fuel processor, an electrochemical power generator, and a post combustor. The role of the fuel processor is to generate a hydrogen-rich product stream that is fed to the power generation module. In previous works, various micromachined fuel reformers based on microelectromechanical system (MEMS) technology have been demonstrated to achieve high-yield syngas generation from liquid hydrocarbons 3-5. However, MEMS-based micro-reformers require time-consuming and expensive fabrication processes, and face critical issues concerning electrical and fluidic interconnects. Conversely, 'traditional' thick film-technology is a simple and low-cost fabrication route, allows integration of a wide palette of materials by a convenient printing technique, and has a proven track record in harsh environments 6. Here, we propose to apply thick-film technology to a fuel-processing platform for the development of µ-SOFC systems. The thick-film based fuel processor consists of a self-sensing heating element, a fluidic carrier comprising a catalyst chamber, and a ceramic substrate. The heating element consists of two independent thick-film platinum conductor meanders, which are screen printed at the bottom of the fluidic carrier, and provide relatively homogeneous heating of the catalytic reforming zone up to 700°C 7. Due to their temperature dependence of resistance, the thick-film Pt heaters double as temperature sensors and thus allow integrated temperature control of the fuel processing. The fluidic carrier was made of two pieces of (12 mm × 75 mm × 0.7 mm) borosilicate glass (Schott AF32), bonded by a screen-printed glass frit seal (Ferro IP760c), which also patterns the fluidic channels in the carrier. Multiple glass paste prints allow for building up the channel height (i.e. distance between glass plates) up to ca. 150 µm. The catalyst is placed into an open chamber on the top plate of the fluidic carrier by dispensing, and is capped by a piece of AF32 glass (12 mm x 13.8 mm) using the same glass frit bonding technique. The elongated shape of the fluidic carrier and low thermal conductivity of the glass efficiently decouples the heat generated in the "hot" catalyst area from the other "cold" end of the carrier, allowing conventional low-temperature electrical and fluidic interconnections. With a heating power below 8 W, the platform is able to quickly heat the active zone to 700°C, while maintaining the electrical and fluidic connections below 50°C. The performance of isobutane reforming was evaluated, studying the impact of design parameters such as the catalyst chamber dimension, and geometry of the thick-film Pt resistor. The talk will present and discuss the studied fabrication processes including the glass frit and catalyst paste formulation, screen printing and dispensing processes, and show the performance of the platform with results obtained on thermal characterization and gas reforming. The heat output of exothermic reactions is well observed. Keywords solid oxide fuel cell, fuel processer, hydrogen production Reference [1] S. Rey-Mermet and Paul Muralt, Solid State Ionics, 179 (2008) 1497 -1500 [2] A. Bieberle-Hütter et al., Journal of Power Source, 177 (2008) 123-130 [3] J. D. Holladay et al., Chemical Reviews, 104(2004) 4767-4789 [4] K. Yoshida et al., Journal of Micromechanical and Microengineering, 16 (2006) S191-S197 [5] A. J. Santi-Alvarez et al., Energy & Environmental Science, 4 (2011) 3041-3050 [6] T. Maeder et al., Proceedings of 7th CICMT, San Diego (USA), 2011 [7] B. Jiang et al., Sensors and Actuators B, 2012, under review

2012

Production d'hydrogène dans un réacteur microstructuré

Pierre Reuse

The aim of this work was to operate the steam reforming of methanol and the total oxidation of methanol in a microstructured two-passage reactor. The steam reforming enables the production of hydrogen at relatively low temperatures (250 °C) with only a small amount of carbon monoxide (~ 1%). After a cleaning step (CO preferential oxidation, for example), this hydrogen would be a good feed for a fuel cell to produce electricity for mobile applications. Steam reforming is an endothermic reaction. The energy needed to run this reaction is produced by the total oxidation, a strongly exothermic reaction. To allow an efficient heat transfer between these two reactions, a microstructured reactor was used. These structures, with dimensions in the micron range, are widely known to have far better heat transfer capacity than traditional heat exchangers. Furthermore, these structures have some unique features with respect to classical reactors. A narrow residence time distribution, close to that of a plug flow reactor, can be obtained, keeping selectivity high for complex reactions. Microstructured reactors also have small residence time, allowing very quick variations in the feed and experimental conditions. In chapter 4, the concepts relating to the channel coatings will be presented. A Cu/Zn/Al catalyst provided by Süd-Chemie was transformed into a suspension of nanoparticles in isopropanol by using an attrition grinding apparatus. A thin layer (~5 µm) of catalyst was formed on the microchannel walls. Several tests were performed to characterize the adhesion of this layer. The influence of the layer on the residence time distribution was also investigated; it was possible to maintain Bodenstein numbers as high as 50. Chapter 5 is focused on the steam reforming of methanol, the reaction that generates hydrogen. A kinetic analysis in differential and integral modes established that the rate of the reaction is inhibited by hydrogen, the main product of the reaction; the partial order is - 0.2. The influence of water on the rate of reaction is negligible (partial order of 0.1), and methanol has a positive partial order of 0.7. A kinetic constant of 5.57 · 10-6 mol0.4 · (m3)0.6 · gcat-1 · s-1 at 200 °C and an activation energy of 73.9 kJ · mol-1 complete the description of the kinetics of the steam reforming. Throughout the comparisons, we showed that the catalyst coated in the microchannels has an initial rate for the steam reforming 30% higher than the original catalyst in fixed bed. We can therefore conclude that the catalyst modification procedure does not diminish the catalyst activity. This was also shown by comparing our results with results available in the scientific literature for similar catalysts. CO2 selectivity remains high and constant then decreases as 90% conversion is approached. Working with an excess of water with respect to methanol contributes to keeping the selectivity high. A molar ratio of 1.2 is a good compromise between selectivity and increased energy costs due to the vaporization of the additional water. It's also beneficial to work at conversions below 90% to be in the high selectivity domain and to diminish the inhibiting influence of hydrogen. Finally, we showed that this catalyst deactivates over 100 hours at a temperature of 300 °C. A kinetic model for the deactivation at temperatures between 300 °C and 330 °C was established. In chapter 6 the total oxidation of methanol was studied. A cobalt based catalyst was developed to allow the synchronization of the two reactions temperatures. The minimum temperature for complete conversion for the total oxidation reaction must clearly not exceed an acceptable working temperature for the steam reforming catalyst (i.e. a temperature that results in an acceptable catalyst lifetime). A kinetic analysis determined partial orders for methanol and oxygen as 0.67, the kinetic constant at 230 °C as 5.45 · 10-3 mmol-0.33 · m4 · (gcat · s)-1 and the activation energy as 130 kJ · mol-1. The copper based catalyst used for the steam reforming of methanol was also active for the total oxidation of methanol. Unfortunately the minimal temperature to ensure a total conversion was not compatible with the working temperature of the steam reforming. In chapter 7, the results concerning the coupling of the two previously studied reactions are presented. The numerous fittings and cables necessary for the operation of the reactor didn't allow the reactor to be run in an autothermal mode. Therefore constant, not regulated, heating was used to compensate the thermal losses. By coupling the two reactions and by studying the dynamic response time of the reactor (temperature and conversion of the steam reforming reaction) at the time of stopping and starting the oxidation reaction, we've seen that the temperature increase was around +10 °C within 60 seconds and that the conversion for the steam reforming of the methanol fitted well with the temperature increase. This reactor also has good dynamics and responds quickly to feed variations. When the coupling mode was changed from co-current to counter-current we noticed a significant increase in conversion. Furthermore, selectivity in counter-current mode remained high, even for conversion approaching 100%.

EPFL2003

Thermo-Economic Optimisation of Solar Tower Thermal Power Plants

James Spelling

Amidst a backdrop of rapidly increasing world-wide electricity demand, solar thermal power generation shows great potential for supplying the electricity needs of numerous countries in the Sun-belt regions of the world. In these regions, the absence of significant biomass, hydrological or geothermal reserves makes solar thermal power the most promising solution for meeting that most fundamental of demands: energy. Amidst the different options available for harnessing the Sun’s power, solar thermal technologies have been shown capable of producing electricity at the most economically viable rates [32]. However, all currently existing solar thermal power plants have been based around the use of Rankine, or steam turbine, cycles which are limited in the efficiencies they can achieve. In order to make better use of the Sun’s energy, higher efficiency cycles should be used. With their high concentration ratios, solar thermal power tower systems are amongst the best options for making use of the Sun’s potential. As the energy delivery temperature of central receiver systems is higher, they harness the solar radiation at a higher exergy level [4]. This higher temperature also opens the road to the use of more advanced thermodynamic cycles. Hybrid fossil-fuel – solar systems are also imaginable [13], as the energy delivery temperature is compatible with standard gas turbine cycles. Hybrid systems can help mitigate the requirements of thermal energy storage, as well as reducing the perceived risk when investing in new solar technology, which stimulates research and development, as well as providing employment. Recent developments in the field of high temperature volumetric receivers [10] along with rock- based packed-bed storage systems have opened up an interesting possibility. High temperature receivers allow the use of higher-efficiency combined-cycle setups, whereas packed-bed units offer the possibility of cheap storage. Larger storage volumes allow pure-solar systems to extend their power production into the night, making hybrid options less attractive. This is compounded by the common practice of guaranteeing a preferred tariff for electricity sales from pure-solar sources With this in mind, a complete dynamic model of a power plant based on the pure-solar concept has been elaborated. The performance of the setup is evaluated over a range of days, using solar insolation profiles obtained from satellite data. In order to examine different design options and their impact on the performance of the power plant, a multi-objective, thermo-economic optimisation of both the power plant superstructure and operating conditions was performed using the new, dynamic models. By means of an evolutionary algorithm [17], a family of Pareto-optimal points were obtained, representing the trade-off between increased power plant efficiency, and lower levelised energy costs. Through use of optimisation, it was shown that exergetic efficiencies in the region of 21-27% can be achieved, for relatively low power outputs of between 3 and 11 MWe. These configurations correspond with levelised electricity costs in the region of 14-21 UScts/kWhe. In most regions, the use of solar power is encouraged by the guarantee of a preferential tariff for electricity sales from renewable sources. Sale of electricity at a price of around 24 UScts/kWhe [32] should therefore be imaginable, ensuring the economic viability of solar thermal power tower systems. It can be concluded that, when properly designed, solar thermal power plants based on combined cycles are both economically and thermodynamically promising. By raising the efficiency of the plant, the size of the solar collector field is diminished, increasing the otherwise low energy densities of solar power systems. By reducing the levelised electricity costs, solar thermal power plants become more economically viable, accelerating their construction and thus, hopefully, reducing our dependence on fossil-fuels.

2009