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					Proceedings International Hydrogen Energy Congress and Exhibition IHEC 2005 Istanbul, Turkey, 13-15 July 2005

Hydrogen Generation from High-Temperature Thermal Solar Energy
A. Castro1, V. Gallardo1, E. Moreno1, V. Fernández2, M. Romero3, M. J. Marcos3
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Hynergreen Technologies, S.A., Spain; 2 Solúcar Energía, S.A., Spain; 3 Ciemat, Spain africa.castro@hynergreen.abengoa.com

ABSTRACT As described in a study by the European Commision, global energy supply is expected to increase by 1.6 % per year due to population increase and rapid growth in some regions of the world. The future energy supply will depend on the magnitude of demand growth, changing objectives and social priorities and the technologies to meet them. On the other hand, to burn fossil fuels means to liberate carbon dioxide gas with undesirables effects: CO2 is toxic to life, it is the primary cause of the greenhouse effect and fossil fuels sources are not renewable. The Sustainable Development objective makes it necessary to reduce the emission of greenhouse gases during the next years. The increment of the non-fossil fuel energy sources (biomass, wind and solar thermal or photovoltaic) will contribute to this reduction. Hydrogen has always been followed as a further secondary energy carrier, in addition to electricity. Cost-effective solutions for generation, storage, transport, distribution and application may help to introduce a new energy concept with hydrogen playing an essential role as such a carrier. In the currently most part of hydrogen production (97 %) is produced from fossil fuels with thermal processes, burning an extra fossil fuel in order to obtain the energy that these endotermic reactions need. The electrolytic water decomposition using electricity as the energy source is another way to obtain hydrogen. Solar energy will contribute, both to electric and thermal based processes and it is one of the most representative examples of the increasing interest in the usage of renewable energy. Among the benefits of using solar thermal energy, avoiding the growth of pollutants and increasing the energy independence are the most relevant, thus, solar thermal energy and its use for a clean and renewable H2 production emerge specifically as a promising solution. SolterH aims the design and development of a solar thermochemical plant for hydrogen production. For this purpose a detailed study is being carried out in order to decide the most convenient method, using economical and technical criteria, to generate hydrogen using solar thermal energy. Several advantages and disadvantages are being analyzed for the different potential termochemical methods for hydrogen production, such as gasification, cracking, thermolisis, reforming or thermochemical dissociation, as well as the different fluids suitable for each one of them. In SolterH we will study two concentrate solar technologies (power tower and dish engine) able to provide the temperature level and energy required by the chemicals processes chosen.. Keywords: High Temperature Solar, Thermal Energy, Hydrogen Production, Thermochemical Process

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1. INTRODUCTION The high average annual solar radiation existing in Spain, 1600 kWh/m2 per year, together with the studies carried out by the IDAE (Institute for Energy Diversification and Saving) on the forecasts in the Plan for Promotion of Renewable Energies, through which a possible 200 MW of installed electricity from thermal solar energy was set for 2010, merely confirm the existence of the sun as an inexhaustible resource and the technical viability for the use of this renewable energy to supply most of the demand for energy which exists in this country. The objective of this project is the design, development and evaluation of a system which is capable of producing hydrogen from high-temperature thermal solar energy. To do this, the high temperatures reached using this technology will be used to obtain hydrogen from processing of a fuel which could be water, a hydrocarbon, etc. for use as an energy carrier in other applications related with fuel cells. In this way, given the continuous growth in the demand for energy and the permanent dependence on fossil fuels for supplying it, the aim is to contribute to the increase in the energy supply from a cleaner and more efficient environment using the sun as an inexhaustible and renewable source. 2. PRELIMINARY STUDY Throughout this task, a study was made of the art of obtaining hydrogen from high-temperature solar energy. To do this, the different technologies existing at present were compared for making a choice at a later stage, essentially taking into account both technical and economic requirements. 2.1 High Temperature Solar Technology At present, given the increasing concern about the environment and the search for energy independence with respect to fossil fuels, the use of solar energy for the direct production of electricity is spreading every day; whether through photovoltaic solar technology or thermal solar energy, see Figure 1

Figure 1: DISTAL I, Almeria Solar Platform (PSA). However, the use of the solar energy–hydrogen vector pairing allows accumulation of the advantages involved in the sum of both technologies. One of the approaches which appears in an increasingly insistent manner is that of the possibility of using the hydrogen vector as an energy buffer. In this way, taking into the account the random nature of solar energy, the production of hydrogen from it in periods of excess sunlight could make up for its lack at times when the solar energy does not respond to the needs required.
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In any case, SolterH has as its objective the production of hydrogen from high-temperature thermal solar energy, which allows use to be made of the high temperatures reached in the installation for production of hydrogen from chemical processes such as the reforming of other fuels or to favour the dissociation of water molecules without the need to make use of electricity. Systems based on the high temperature thermal route may also make it possible to make use of solar energy in the form of electricity, but following an indirect method; thus, these systems would absorb the solar energy in the form of heat through a thermal sensor and they would then transform it into electricity via a thermodynamic machine. The steps for transforming high-temperature solar energy into hydrogen would be: first, absorption and conversion of the solar radiation into heat, this would be achieved through the use of a solar collector. Next, to reach higher temperatures and achieve better use of the energy, it is essential to concentrate solar radiation. There are optical procedures with lens devices but they are extremely expensive and they have only been used for research into photovoltaic, not thermal conversion, processes. The reflection and concentration of direct sunlight are achieved by mirrors or solar collectors called heliostats. The concentration of the solar radiation takes place in the receiver.

Hightemperature solar radiation

Concentrator

Hot air

Fuel or water

Receiver Renewable hydrogen Chemical transformation method

Figure 2: Transformation of high-temperature solar energy into hydrogen The main solar receivers available in high-temperature solar technology are: • Cylinder_Parabolic Concentrator The cylinder-parabolic concentrator, Figure 3, consists of a cylinder-parabolic mirror which reflects the solar radiation received onto a glass tube placed along the focal line of the mirror, inside which is the absorbent surface which is in contact with the heat-bearing fluid; this fluid is heated to up to 400 ºC.

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Receiver

Trough concentrator

Figure 3: Diagram of cylinder-parabolic concentrator • Central Receiver or Power Tower The central receiver or power tower, Figure 4, is used with a circular group of heliostats, large mirrors with individual traction of between 20 and 50 square metres in area, so that a heat transfer device in this central receiver absorbs the highly concentrated radiation reflected by the heliostats. High temperatures can be achieved with this technology, around 2000 degrees centigrade

Receiver

Tower

Heliostat

Figure 4: Diagram showing tower concentrator • Parabolic Dish Parabolic dish, Figure 5, is the name given to the parabolic reflector in the form of a dish which allows sunlight to be concentrated into a receiver situated on its focal point. This dish absorbs the energy reflected by the concentrators, making the fluid in the receiver heat up to around 750 ºC. The high optical efficiency and the low heat loss caused during start-up convert the parabolic dishes into very efficient systems. In addition, their modular design makes them suitable for covering needs at remote points and for larger applications.

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Receiver

Dish

Figure 5: Diagram of parabolic dish

2.2 Methods for Hydrogen Production Below is brief description of the different methods existing for hydrogen production. These methods have been classified into two major groups according to the type of fluid in question: water or another type of fuel likely to be transformed into hydrogen. Production of hydrogen from water presents a basic difference in relation to respecting the environment if compared with production from other fuels. This basic difference lies in the nonemission of gases which contaminate the atmosphere during its processing, given that its molecule consists of one atom of oxygen and two of hydrogen. On the other hand, the use of other fuels such as hydrocarbons, alcohols or natural gas involves emission in larger or smaller amounts of certain gases likely to accentuate the so-called greenhouse effect. The advantage of using different fuels other than water for hydrogen production is reflected in production costs, which are currently lower through the use of technologies for processing hydrocarbons or natural gas. Specifically, a more detailed description is given of hydrogen production from water through thermochemical cycles to it being finally the method selected for integration into a hightemperature solar installation; this selection is justified in section 2.2. 2.2.1 Production from water • Electrolysis This is the only process from those described here which would require the direct application of electricity for it to be carried out. This method is used when the hydrogen produced is required, not in high quantities but with a high level of purity, and as long as electricity is an available resource. The electrochemical breakdown of water or electrolysis consists of making its ions react on polarized electrodes using direct current, obtaining as a result molecular hydrogen in a gaseous state in the cathode area and oxygen in the same conditions in the anode area. There are three types of industrial hydrogen production via electrolysis. Two of them are based on an aqueous potassium hydroxide solution used for its high conductivity and they are known as alkaline electrolysers; the difference between them is that one is bipolar and the other unipolar. The third type uses a solid polymer electrolyte and it is known as proton exchange membrane electrolyser. The reactions which take place in the electrodes are stated below:
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Cathode: 2 H2O + 2 e-→ H2 + 2 OHAnode: 2 H2O → O2 + 4 H++ 4 eOnce the molecule has been dissociated, in the case of pure water, a specific voltage is required to separate the hydrogen from the oxygen according to the water temperature and pressure. The lowest quantity of electricity required to carry out electrolysis of one molecule of water is 65.3 Wh at 25ºC. The efficiency reached with this method is 75 – 85 %. In any case, in spite of this being a completely clean hydrogen production process, the disadvantage is found in the high cost in comparison with other technologies. Specifically, the costs associated with generation are almost as much as the installation costs, which does not make potential escalation of this process excessively viable. • Water Thermolysis Direct water thermolysis is a thermochemical process through which direct separation of the water into hydrogen and oxygen takes place. This separation requires temperatures of higher than 2500 ºC at atmospheric pressure for significant generation of hydrogen. H2O → H2 + ½ O2 (2500 ºC min) At this temperature, only 10% of the water is decomposed and it is also very easy for the hydrogen to recombine with the oxygen. Moreover, the high reaction temperature poses problems associated with the use of high concentration optical systems and the use of highly refractory materials. In any case, the tests carried out so far do not show efficiency rates higher than 1 – 2 %. • Thermochemical Dissociation of Water Thermochemical dissociation of water, a process also known by the name of “thermochemical cycles”, allows dissociation of the water molecule in several stages at moderate temperatures. After an in-depth bibliographical study of the numerous thermochemical cycles and taking into account parameters such as cost, efficiency and technical viability for its connection with a solar plant, below are the characteristics of the four most representative cycles encountered to date. • Westinghouse-General Atomics Cycle

The Westinghouse cycle is based on the endothermic reaction of decomposition of sulphuric acid at a temperature of 900 ºC[1]
1 H2SO 4 → H2O + SO 2 + 2 O 2

850º C

This process was proposed by Bowman and later researched in-depth by the Westinghouse company. This cycle is closed with a simple electrolytic stage.
SO 2 + 2H2O → H2SO 4 + H2

80º C (Electrolysis)

The only disadvantage relating to this cycle comes from its status as a hybrid process, since it conserves all the problems inherent in this type of process. The advantages, however, are based on its simplicity, the absence of secondary reactions and the experience accumulated over all these years of testing carried out by the Westinghouse company and the European Commission Joint Research Centre (ISPRA) [4] each on pilot plants.

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•

UT-3 Cycle

The process known as UT-3 was developed at the University of Tokyo and studied by a large number of Institutions, including the Japanese Atomic Energy Research Institute (JAERI). The research carried out by the JAERI, after taking into consideration several variants in the process concluded that the UT-3 cycle operated adiabatically was the most appropriate for final escalation [2]. Generally, the efficiencies estimated for this process variant gave values between 35 and 50%, depending on the efficiency of the separation membranes, which are still being developed. The biggest disadvantage lies in the use of solids, although this aspect appears to have been solved with the use of fixed beds and operation in gas phase. • Zinc-Zinc Oxide Cycle

An example of this two-stage process would be represented by the reaction for dissociation of ZnO:

ZnO → Zn + CO
The dissociation stage is used for storing solar energy in chemical form. For thermodynamic reasons, this oxide dissociation stage requires very high temperatures (< 2000 K). Finally, the energy stored in the zinc can be used in Zn/air fuel cells or batteries, or to produce hydrogen by direct hydrolysis of the zinc through an exothermic reaction.

Zn + H2O → ZnO + H2
The thermal decomposition of the Zn oxide takes place endothermically (∆H298K = 478 kJ) and the temperature is 2350 K for a free energy variation which is equal to zero. In practice, the Zn formation output basically depends on the mechanics of dissociation and adequate separation of the gaseous products, which should be sufficiently rapid to avoid recombining. The Paul Scherrer Institute (PSI) in Switzerland studied the process in order to analyze its technical and economic viability. Palumbo et al. [3] encountered very high efficiency values for this cycle with values of 50%. The high temperatures required limit the practical viability of the process, which is why some variants have been studied for carrying out the reaction at more moderate temperatures. • Mixed Oxide/Ferrite Cycle

The experiments carried out so far with this type of cycle indicate low conversions. There is not enough thermodynamic data relating to ferrites as pure products or their solid solutions. This aspect is important given that the forming of new phases would avoid the reactions being completed efficiently. Generally, these two-stage processes are based on the thermal activation of these compounds at temperatures of 600 – 800 ºC by the forming of oxygen vacants, which would then recover their initial status at temperatures of 1200 ºC, producing H2 in the presence of H2O. This cycle has the disadvantage that the thermal activation stage, consisting of producing a material with oxygen vacants, has low efficiency and usually only a small fraction of these vacants are created in the material. However, the reduction stage has efficiency which is closer to required values [4].

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2.2.2 Production from other fuels • Catalytic Reforming Catalytic reforming is a process used for producing hydrogen from hydrocarbons or alcohols in the presence of a catalyser. This reforming can be carried out in three ways: steam reforming, partial oxidation or autothermal reforming. Light hydrocarbon steam reforming: endothermic reaction which takes place at around 750-950 °C, pressures in the range of 2-30 atm and at low spatial velocity (GHSV: 3000 to 8000 h-1). CnHm + n H2O → n CO + (n + m/2) H2 AH <0

Partial catalytic oxidation: exothermic reaction where the fuel reacts with oxygen, producing a lower quantity of hydrogen per reagent molecule supplied than in the previous case; it takes place at higher temperatures and pressures 1010-1.500 ºC and 1-300 atm, but with the advantage that no external heat source is required. CnHm + n/2 O2 → n CO + m/2 H2 AH >0

Unlike catalytic steam reforming, reforming through partial oxidation is used basically for heavy hydrocarbons such as petrol, diesel or heavy oils. Autothermal reforming: the fuel reacts with water and oxygen at the same time, which is why the two previous reactions occur. CnHm + n H2O → n CO + (n + m/2) H2 CnHm + n/2 O2 → n CO + m/2 H2 AH<0 AH>0

The presence of water in the reagents makes the proportion of hydrogen increase, since it comes both from the fuel and the water itself, and introducing oxygen in turn makes the reaction selfsufficient, without it requiring any external energy source. However, hydrogen production continues to be small-scale compared with that obtained from steam reforming. As an example of steam reforming, the specific case and the one most used to demonstrate methane reforming is highlighted. The working temperature for this is situated in the range of 700-925 ºC and efficiency of 65 – 75 % can be reached in the process. Reforming with natural gas vapour appears as one of the most representative cases given that most bibliographical sources coincide in confirming that around half of the world hydrogen demand is currently met by using this method. In fact, it is the most economic method for obtaining hydrogen from all those existing to date, and it is also the cost of the natural gas itself which represents 60 –70 % of the total cost of production. • Carbon Gasification and Biomass The process used for obtaining hydrogen from different sources such as carbon, biomass, etc. Requires the use of pure oxygen and works at temperatures of around 1100 – 1300 ºC. During gasification, partial oxidation takes place of the hydrocarbons which produce synthesis gas (a mixture of carbon monoxide and hydrogen), which may be used as fuel for electricity generation, as raw material for manufacturing methane, OXO alcohol ammonias or even as a reducing agent for steel production in blast furnaces. It has the advantage, compared to other chemical recycling procedures, of being able to admit for supply the entire flow of municipal waste, without the need for prior separation of plastics.
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Gasification offers one of the cleanest and most efficient methods for producing synthesis gas from impure carbon, petroleum coke, fuels with a high content of sulphur or waste materials. The gas may be used to replace natural gas in the generation of electricity, or as a producer of liquid and chemical fuels. In the case of carbon gasification, in spite of it generating around 18% of world hydrogen production, it has the added disadvantage of carbon dioxide emission to the atmosphere. Gasification of biomass, on the other hand, involves decomposition and partial oxidation, producing a gaseous mixture which may be refined subsequently. However, although the technology used is well-known and viable it has not been functionally tested on a large scale to date. • Thermal Cracking Process by which methane and other hydrocarbons are decomposed at temperatures which vary between 700-980 ºC and, without the presence of air, they form hydrogen and carbon according to the following reaction: CH4 → C + 2H2 (75 kJ/mol) In spite of the advantage of not producing CO2, this is normally considered an undesirable reaction due to the formation of carbon. Cracking has been studied in both continuous flow and pulse catalytic reactors. Variations in operational parameters produced different mixtures of CH4H2 in proportions which varied in range from 30 to 98 %. The Kvaener process allows thermal non-catalytic decomposition of hydrocarbons without CO2 emissions. It consists of the carrying out of the process of pyrolysis from plasma. The reaction products have a high H2 content as well as black carbon. The specific energy demand is 1.01 kWh/Nm3. 2.3 Selection of Method and Integration with Solar Energy Steam hydrocarbon reforming, carbon gasification or water electrolysis currently constitute the main hydrogen production methods at international level, given the current state of progress of the technology. However, the hydrogen production method selected at SolterH is based on the use of thermochemical cycles. This method is seen as promising since it uses water processing for production of clean hydrogen, with high energy efficiency (direct use of the thermal energy from a heat source). However, this technology is still being developed at laboratory research level. This makes the final decision difficult regarding the use of one of the up to 2000 different cycles studied to date. Both the bibliography available and the operating data existing for both cycles, General Atomics and UT-3, reflect excessive complexity, both in terms of the number of reactions involved and in the development of suitable technology for solving some basic problems. This, linked to the problems inherent in the operation of a thermosolar plant would complicate the operation extraordinarily, since there would also have to be integration of control of the heliostat field, with several receivers, basic separation, purification operations, etc. This interaction between the different components in the field would be even more critical in the presence of transients or even at start-up and stoppage times. However, hybrid cycles such as the Westinghouse cycle, although less efficient and with two clearly differentiated reactions, could be a viable alternative for a solar plant.

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Figure 6: Solar furnace concentrator disc (left) and receiver in the focal zone (right) Regarding the simplest cycles, the Zn-ZnO cycle is still at the study phase due to insufficient knowledge of the materials technology which will allow operation at high temperatures (2000 K). Finally, the Ferrite cycle has all the qualities for being an ideal cycle, namely simplicity and relatively high temperatures, although it has the disadvantage of the lack of reliable operating data. In short, the Westinghouse cycle and the ferrite reduction cycle would, in an initial approximation, be the two cycles with the most possibilities of success with a view to future implementation in a solar plant. Elsewhere, although it may still seem premature to mention it, it seems that a parabolic dish could be the best option as the final solar plant, due to its scalability and the high temperatures reached in it. However, before reaching a prototype of this nature, evaluation and testing of the final cycle selected shall take place in a solar furnace, Figure 6. Solar furnaces are those which reach the highest energy levels which can be obtained with a concentration solar system, with concentrations of over 10,000 suns having been reached. Its field of application mainly includes tests on materials, both in ambient conditions and in controlled atmospheres or vacuums, and solar chemistry experiments through receivers associated with chemical reactors. They basically consist of a flat heliostat which carries out continuous following of the sun, a parabolic concentrator mirror, an attenuator or louver sheet and the test area situated on the focal point of the concentrator. The flat sensor mirror reflects parallel and horizontal rays of sunshine onto the parabolic dish, which reflects them again, concentrating them on its focal point. The quantity of incident light is regulated through the attenuator situated between the concentrator and the heliostat. Under the focal point is the test table which has three-dimensional spatial movement and is used for positioning the glass cylinders with high precision on the focal point. 3. SYSTEM DESIGN The difficulty when it comes to making a final decision regarding the thermochemical cycle to be developed, has led to a theoretical approach at the expense of its final solution. As has already been mentioned in section 2.2, prior to reaching a definitive solution in terms of design of the solar installation, the use of a solar furnace will be opted for, which will facilitate experimentation until arriving at the definition of optimum parameters which are suitable in
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relation to both the reaction or reactions to take place and the most appropriate reactor which complies with this transformation. In any case, below is a brief description of the design methodology to be followed once the testing stage in the furnace has been completed.

Figure 7: Solar Reactor Design Methodology The process of producing hydrogen from the energy obtained from the sun goes through the following stages schematically:

Figure 8: Diagram of optimization of solar field
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Solar Radiation → Reflector/Collector → Receiver/Reactor → Heat (working fluid) → Reagents (water, CO2, carbon, methane, etc) and/or catalyser (metal oxides, ZnO) → Regeneration of metal oxide → Solar fuel (H2,CO,C2H4). With all this, once the quantity of hydrogen to be generated by the plant, which is expressed as the power of the plant, has been defined, as well as the specifications in terms of thermal flow required, its concentration and temperature, design of the solar reactor will begin. Figure 7 shows a layout of the general methodology which has been chosen for the project for design of this solar reactor. Thermodynamic analysis determines when and in what conditions chemical reaction is possible, kinetic analysis sets the speed and performance of the reaction according to the operating conditions (pressure, temperature, volume, flow, etc.), the design of the reactor should achieve the best possible integration between the chemical requirements of the process and the solar part which supplies it with energy. Once the reactor has been designed, the solar installation will be designed and optimized following the outline shown in Figure 8. 4. CONCLUSIONS To conclude the description of the work carried out so far at SolterH we can state; on the one hand, that we are approaching the final solution which has been arrived at when it comes to selecting the method for hydrogen production which is most suitable for use from high-temperature solar energy. The final selection, given the current status of the technology and, as commented previously, is between two possible thermochemical cycles, the Zn-ZnO cycle and the ferrite cycle, although some study is still required by the work groups involved. On the other hand, the design of the solar plant to conform to this production method is being gradually profiled, although there are still some tasks pending, not only at the expense of the method which is finally selected, but of certain optical simulation processes, as well as optimizations in terms of layout and direction of the installation, among many other important decisions to be taken. Generally, major progress and developments are expected in the technology involved in this Project through which Spain, and in particular Spanish companies, could reach the technology level of other countries which stand out in this area, such as the US. REFERENCES Bowman, M.G. and Onstrot, E.I. (1974) “Hydrogen production by low vol electrolysis in a combined thermochemical and electrochemical cycle” Proc. Electrochemical society Meeting, New York, 1974Koku, H., Eroğlu, I., Gündüz, U., Yücel, M., Türker, L.: Kinetics of biological hydrogen production by the photosynthetic bacterium Rhodobacter sphaeroides O.U. 001, International Journal of Hydrogen Energy, 29, (2003), 381-388. <Reference Style> Brown, L.C.; Funk, J.F. and Showalter, S.K. “High efficiency generation of hydrogen fuels using nuclear power” Annual Report of the U.S. Department of Energy. GA_A23451. July 2000 Palumbo, R.; Lede, J.; Boutin, O.; Steinfeld, A.; Móller, S.; Weidenkaff, A. ; Fletcher, E. and and Bielicki, J. I. The scientific framework of the process. Chem. Engng. Sci. 1998; 53 (14): 250317 Tamaura, Y., Ueda, Y, Matsuanmi, J., Hasegawa, N, Nezuka, Sano, T. And Tsuji, M. “Solar hydrogen production by ferrites” Solar Energy vol. 65, Nº 1, pp. 55-57, 1999

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