Method for obtaining hydrogen by means of water dissociation using thermochemical reactions under (quasi-)isothermal conditions and device for carrying out the method

Abstract

The present invention relates to a method for obtaining hydrogen through water molecule dissociation using thermochemical reactions under (quasi-)isothermal conditions, which comprises the following steps: placing active material (103) in the reaction chamber (109) of a reactor (101); reducing the active material (103) by supplying heat; evacuating the oxygen produced through a first outlet (106); injecting water into the reaction chamber (109); oxidising the active material (103), thereby producing hydrogen; filtering the hydrogen produced through a selective filter (104) during the oxidisation of the active material (103); and evacuating the filtered hydrogen through a second outlet (107), thereby obtaining a flow of high-purity hydrogen. The invention also relates to a device for carrying out the method.

Description

[0001] Method for obtaining hydrogen by dissociation of water through thermochemical reactions under (quasi-) isothermal conditions and device for performing it

[0002] TECHNICAL SECTOR

[0003] The present invention relates to a method and device for producing hydrogen. Specifically, the invention relates to producing hydrogen through the dissociation of water molecules by means of thermochemical reactions under isothermal (or quasi-isothermal) conditions.

[0004] BACKGROUND OF THE INVENTION

[0005] The use and exploitation of hydrogen produced through renewable energy sources (known as green hydrogen) is a priority objective in today's society to meet sustainability goals. Hydrogen is expected to be the most important energy vector in the near future, replacing natural gas and other fossil fuels, with applications in a wide range of areas: industry, transportation, and domestic activities (Turner, J. et al. Renewable hydrogen production. International Journal of Energy Research 32, 379–407, 2008).

[0006] There are several methods for producing green hydrogen. The use of electrolyzers powered by electricity from renewable sources, such as solar or wind, is the most common, due to the advanced level of the technologies involved. However, this production process currently has two major drawbacks.

[0007] First, electrolyzers require energy in the form of electricity, and although this is a widely used form, electricity production from renewable sources is limited and conditioned by market prices.

[0008] Furthermore, the final conversion efficiency from the primary source (typically solar photovoltaic energy) to hydrogen is very low due to the losses incurred during the double transformation and distribution.

[0009] For practical purposes, the operational costs of hydrogen production using electrolyzers are very high, which influences the cost of hydrogen production. Thus, the final price of hydrogen per kWh is much higher than the current price of natural gas. Alternative methods to electrolyzers include thermochemical redox processes that allow the water molecule to be split using thermal energy. To produce green hydrogen, thermal energy can be obtained directly from solar energy with greater efficiency than photovoltaic energy (A. Steinfeld, Solar thermochemical production of hydrogen — a review, Solar Energy, vol. 78, no. e 5, pp. 603-615, May 2005, doi: 10.1016 / j.solener.2003.12.012). Thermal energy from other industrial processes or high-efficiency transformations can also be used.

[0010] The thermochemical process for dissociating water is usually based on the use of an active material (typically a metal oxide) capable of supporting redox (reduction-oxidation) reactions.

[0011] The thermochemical process is usually implemented in two separate steps:

[0012] (i) First, the active material is heated, typically to a temperature between 900 S C and 1800 °C, giving rise to the reduction half-reaction, releasing oxygen in an endothermic reaction;

[0013] (i) Subsequently, the active material comes into contact with water, which gives rise to the oxidation half-reaction, releasing hydrogen in an exothermic reaction.

[0014] This two-step process has been studied extensively, and a large number of active materials are now known that can reach reduction temperatures using concentrated solar energy (Abanades, S., Charvin, P., Flamant, G. & Neveu, P. Screening of water-splitting thermochemical cycles potentially attractive for hydrogen production by concentrated solar energy. Energy 31 , 2805-2822, 2006).

[0015] The procedure based on solar thermal energy to carry out the dissociation of water through thermochemical reactions is postulated to be more efficient than the photovoltaic process combined with electrolysis or other procedures such as photolysis (Perkins, C. & Weimer, AW Solar-thermal production of renewable hydrogen. AIChE Journal 55, 286-293, 2009).

[0016] It is worth highlighting the most relevant studies, where it is discussed that it is possible to reach an efficiency of -20% using concentrated solar thermal energy (Chueh, WC et al. High-Flux Solar-Driven Thermochemical Dissociation of CO2 and H2O Using Nonstoichiometric Ceria. Science 330, 1797-1801 (2010). Muhich, CL et al. Efficient Generation of H2 by Splitting Water with an Isothermal Redox Cycle. Science 341, 540-542, 2013). This methodology has been put into practice experimentally with different configurations, most of them based on high concentration solar thermal systems (> 200 suns), such as the central tower system with heliostats with powers of more than 2 MW (Muhich, CL et al. A review and perspective of efficient hydrogen generation via solar thermal water splitting. WIREs Energy and Environment 5, 261-287, 2016).

[0017] The active material is presented as a porous monolith or in the form of particles that, upon receiving concentrated solar radiation, reach the reduction temperature and release oxygen within a period of tens of minutes. The active material is then moved or hidden from the solar source, where it cools and oxidizes with water.

[0018] There are several patented inventions with the aim of producing hydrogen from solar resources using thermochemical cycles.

[0019] Of note are inventions W02008113944A2 and CN112323092A, where the active material is transformed into a gaseous state during reduction and subsequently solidifies upon oxidation. Other inventions use particles of the active material, which circulate through different fluidized beds to perform the reduction-oxidation cycles as they move through the reactor (US20120237440A1). Alternatively, the particles are stored in tanks where they are heated and reduced by solar energy, and water is subsequently passed through the same tanks, where the reduction takes place and hydrogen is obtained (EP1 1003735A, US9399575B2). Porous monolith systems are based on solar reactors, such as Goel's invention (WO2013021397), which is used in a central tower solar thermal system.

[0020] Current methods for producing renewable hydrogen from concentrated solar energy, using thermochemical reduction-oxidation (redox) cycles, are based on two approaches: a) A thermal jump cycle such that the thermal reduction half-reaction (R1) occurs at a considerably higher temperature than the oxidation half-reaction (R2) with water. b) An isothermal (or quasi-isothermal) cycle such that the reduction half-reaction and the oxidation half-reaction take place at the same (or similar) temperature. Both approaches present difficult-to-solve efficiency problems, which have prevented the use of these reactions to produce green hydrogen industrially.

[0021] In the case of the first approach, the active material must be cooled to carry out the oxidation half-reaction at a lower temperature, to inhibit the reverse reduction reaction that occurs through the interaction of the active material with the hydrogen produced. The subsequent reheating of the reactor involves considerable energy expenditure, resulting in low overall efficiencies that are difficult to avoid, even with complex heat recovery engineering systems.

[0022] In the second case, the complete redox cycle takes place at a constant temperature, which eliminates losses due to reactor overheating. In this case, another strategy must be found to shift the equilibrium of the half-reaction of oxidation of the active material with water toward the production of hydrogen, to the detriment of the recombination of the hydrogen generated by reduction of the active material. This reduction process with hydrogen is very efficient at the high temperature used in isothermal processes. Therefore, the commonly used strategy to shift the equilibrium is to disproportionately increase the amount of reactant (water).

[0023] According to numerous studies with different materials, to shift the equilibrium clearly toward hydrogen production, the molar ratio of reactant water to hydrogen produced must be between 100:1 and 1000:1. This high water consumption entails the enormous energy expenditure required to vaporize such a quantity of water, which inevitably results in an unacceptable drop in process efficiency, which can hardly be compensated for by the use of heat recovery systems.

[0024] Hao et al. 2013 (Hao, Y., Yang, C.-K., Haile, S.M., 2013. High-temperature isothermal chemical cycling for solar-driven fuel production. Phys. Chem. Chem. Phys. 15, 17084-17092.) and Zhu et al. 2016 (Zhu, L., Lu, Y., Shen, S., 2016. Solar fuel production at high temperatures using ceria as a dense membrane. Energy 104, 53-63.) show that the efficiency does not exceed 10% for isothermal cycles below 1800 K. The authors indicate that it is necessary to recover 100% of the heat used in water vaporization to reach efficiencies of 60%.

[0025] This fact has led experts to state that thermochemical reactions under isothermal conditions cannot be practically used to produce hydrogen in future developments since the resulting efficiency is considerably lower than that obtained with thermal jump (Al-Shankiti, I., Ehrhart, BD, Weimer, AW, 2017. Isothermal redox for H2O and CO2 splitting - A review and perspective. Solar Energy, Advances in Solar Thermochemistry 156, 21-29.).

[0026] Devices used in laboratories to perform tests under isothermal conditions have even been disparagingly referred to as "steam engines" due to their high water consumption. This view among experts in the field translates into the almost unanimous proposal of cycles with a thermal jump in projects for hydrogen production through thermochemical reactions. As described below, this invention runs counter to this body of doctrine, since the method described here surprisingly allows for very high efficiencies in (quasi-) isothermal cycles. To achieve this, briefly, a hydrogen-selective filter is used capable of shifting the equilibrium of the oxidation half-reaction (R2) very sharply toward hydrogen production.

[0027] The patent (ES2932843) describes a methodology and device to improve the efficiency of thermochemical reactions using moderate solar concentration systems and at the same time reduce the degradation of the active material throughout the cycles.

[0028] The method and device described are composed of several elements, such as a vacuum reactor, fed by solar collectors with a moderate concentration factor, a heat trap, and a process control system. The method and devices allow for the optimization of reduction and oxidation half-reaction times, achieving efficient hydrogen production, due to the use of moderate temperatures, minimizing the amount of active material, and eliminating the use of a carrier gas. The method allows for operation under mild conditions, that is, avoiding high reduction mole fractions and, therefore, maintaining a low concentration of vacancies produced in the active material, which prevents material degradation. However, this device suffers from a major disadvantage: it evacuates the oxidation reaction gases, hydrogen and water vapor, through the same outlet.This limits the efficiency or production of hydrogen, in addition to requiring a subsequent process to separate the two.

[0029] Within studies involving thermal jumps between oxidation and reduction reactions, there are solutions that include membranes to separate hydrogen from other gases such as water and oxygen. Babiniec et al. 2017 (Babiniec, SM, Ambrosini, A., Miller, JE, 2017. Thermodynamic assessment of an electrically-enhanced thermochemical hydrogen production (EETHP) concept for renewable hydrogen generation. International Journal of Hydrogen Energy 42, 14380–14389) propose using electrolytic membranes to separate hydrogen from the flow that carries the active material along with the water vapor. For electrolytic separation, the input of electricity is necessary to allow the hydrogen to pass through the membrane. This separation is carried out once the active material has cooled and, therefore, has been previously oxidized. As the study indicates, the system efficiencies do not exceed 20%, although part of the heat from the thermal jump is recovered.This concept of active material carried by a flow cannot be applied to the quasi-isothermal case, since the high oxidation temperature does not allow for efficient hydrogen production.

[0030] On the other hand, systems with active materials that change to vapor phase when they are oxidized (FesO FeO) have been theoretically studied with direct conversion using chemical reactions, electrolytic processes and thermochemical cycles always with a thermal jump. Mallapragada and Agrawal 2013 (Mallapragada, DS, Agrawal, R., 2014. Limiting and achievable efficiencies for solar thermal hydrogen production. International Journal of Hydrogen Energy 39, 62-75.) study a closed system, where the active material and products have no direct outlets. The system only consists of an inlet for water vapor. The authors propose ideal membranes to separate hydrogen and oxygen from the gases produced during the reactions of the active material in the closed system. These membranes only act by separating the gases that have been previously formed, during the chemical, electrolytic, or reduction / oxidation reaction of the active material.The study concludes that ideal direct conversion processes can yield efficiencies above 60%, such as electrolytic processes, while two-step cycles using the thermal jump approach are less efficient, as they lose energy in latent heat.

[0031] As is the case with the vast majority of studies, this study does not consider the isothermal approximation and is in fact incompatible with it, since outlets are necessary to evacuate gases produced in the system that are not present in the closed system developed.

[0032] Therefore, it is necessary to improve the equipment and methods for obtaining high-purity hydrogen from thermochemical reactions under (quasi-) isothermal conditions. In this invention, we focus on achieving the two-step thermochemical reaction (reduction and oxidation) under (quasi-) isothermal conditions, despite the general belief that this is a discarded option. The reason, as described below, lies in the fact that, using the developed method, very high efficiencies can be achieved in the devices presented, since the problem of excessive water consumption is solved.

[0033] SUMMARY OF THE INVENTION

[0034] The present invention solves the problems associated with obtaining hydrogen through thermochemical cycles under (quasi-) isothermal conditions. Despite the current state-of-the-art trend, the present solution provides very high efficiency, employing a device under (quasi-) isothermal conditions that avoids the high water consumption typical of prior art solutions.

[0035] In an isothermal development, the intrinsic efficiency per cycle, for a water / hydrogen molar ratio of 1:1, is greater than 85%, with the real efficiency being a function of the rest of the elements of the process and device, such as the heating system, the thermal insulation systems, the pumping systems and other plant systems.

[0036] In a first aspect, the present invention relates to a process for dissociating water by means of thermochemical reduction-oxidation reactions under (quasi-) isothermal conditions with high efficiency.

[0037] The procedure consists of two-step cycles under (quasi-) isothermal conditions:

[0038] 1. Reduction of active material (redox) placed in a reaction volume by means of heat input with evacuation of the oxygen produced in the reduction through a first outlet.

[0039] 2. Oxidation of the active material by injecting water into the reaction volume through a dedicated inlet, keeping the first outlet closed, and evacuating the hydrogen produced during the oxidation through a hydrogen-selective filter via a second outlet. The hydrogen thus obtained is highly pure.

[0040] Specifically, the method according to the present invention comprises the following steps: a. placing the active material in a reaction volume of a reactor; b. reducing the placed active material by adding heat; c. removing the oxygen produced through a first outlet; d. injecting water into the reaction volume; e. oxidizing the active material to produce hydrogen; f. filtering the hydrogen produced through a selective filter during the oxidation process of the active material; and g. removing the filtered hydrogen through a second outlet, configured to selectively remove the hydrogen produced during the oxidation of the active material in the reaction volume, obtaining high-purity hydrogen.

[0041] Oxygen and hydrogen are removed through a first and second outlet, which are two independent reactor outlets. Additionally, the flows at the inlet and the first and second outlets of each redox cycle are regulated and synchronized by a control system.

[0042] In this way, the solid active material, placed in a reaction volume of limited size, in which gases are introduced or evacuated through the inlets and outlets in a controlled manner, can be reduced. The reduction is carried out by heating the active material and maintaining it at a (quasi-) isothermal operating temperature, either under vacuum conditions or in the presence of an inert gas. Once reduced, the active material is oxidized in the presence of water, maintaining the previous temperature. The water can be injected into the reaction volume once the first outlet is closed, coming into contact with the active material. Water injection can occur in the form of vapor, in liquid form (when operating under vacuum), or mixed with an inert gas.

[0043] The selective filter separates hydrogen from the water vapor present in the reaction volume. The result of selective removal is an effective shift in the reaction equilibrium toward hydrogen production, increasing process efficiency.

[0044] In a second aspect of the invention, the solution relates to a device for obtaining hydrogen by dissociation of water through thermochemical reactions under (quasi-) isothermal conditions.

[0045] Said device comprises: a. a reactor with a reaction volume inside it configured to contain an active material and carry out the thermochemical cycles of water dissociation; b. an energy source configured to heat, in use, the active material arranged in the reaction volume of the reactor. c. an inlet, in connection with a water injection system, configured to introduce water into the reaction volume of the reactor; d. a first outlet configured for oxygen evacuation; e. a selective hydrogen filter, connecting the reaction volume with a second outlet, configured to selectively evacuate, in use, the hydrogen produced in the reaction volume of the reactor.

[0046] Specifically, the first and second outputs of the device are two independent outputs of the reactor. Additionally, the device (100) comprises a process control system configured to regulate the stages of the redox cycle, synchronizing the opening and closing of the water injection system and the output of produced gases.

[0047] Thus, thermochemical water dissociation cycles can be carried out inside the reactor. The energy source comprises a thermal system. The thermal system must be capable of capturing energy through thermal contact (e.g., a resistive heater), through the absorption of electromagnetic waves, through the use of a heat transfer fluid, or through the induction of eddy currents in the case of using an induction system. It must also be capable of transmitting the thermal energy necessary to heat the active material and maintain the temperature during the reduction and oxidation cycles under (quasi-) isothermal conditions. The inlet is configured to introduce a controlled amount of water into the reaction volume of the reactor, and the independent outlets allow the gases generated in the reactor to be selectively evacuated.

[0048] The filter is mounted so that hydrogen can easily reach it and be evacuated from the system. Depending on the reactor type, the filter may be located inside the reactor, attached to the reactor in an area close to the reactor, as part of the reactor, or in a combination of these options. The filter can be heated to a high temperature (2500 K) to facilitate its operation, improving the selective filtration of hydrogen and, therefore, the efficiency of the process.

[0049] The use of a hydrogen-selective filter to rapidly remove the hydrogen produced as it is produced effectively shifts the R2 half-reaction toward hydrogen production via oxidation, rather than the reverse reduction reaction in the presence of hydrogen, without the need to add a large excess of water, as required by current solutions. Thus, the selective removal of hydrogen through the selective filter not only allows for effective operation without excess reactant water, but also provides high-purity hydrogen and minimizes recombination with oxygen in the reactor.Furthermore, the designed configuration of the device of the present invention allows taking advantage of the high temperature of the reactor to heat the filter to a temperature equal to or different from that of the reactor to maximize the selective extraction of hydrogen (preventing other gases from escaping) in appropriate materials such as, for example, but not limited to, nickel and niobium.

[0050] The result of using a device such as the one described is a surprising increase in the efficiency of the device and the process in (quasi-)isothermal cycles. Therefore, the solution proposed in the present invention can replace or function complementary (as appropriate) to the combination of photovoltaic energy and electrolyzers to generate hydrogen. Furthermore, it replaces all alternative forms of generating green hydrogen, such as photolysis processes, biological systems, etc.

[0051] In this sense, the invention can work in a complementary way to photovoltaic energy, since hydrogen is an energy carrier that allows energy to be stored for on-demand use, something that is difficult to achieve with electrical energy. This allows the use of fuel cells to replace batteries in vehicles, especially in heavy-duty vehicles and for long-distance (highway) travel.

[0052] Harnessing solar energy through hydrogen generation is particularly useful for road transport and industrial supplies, where it can replace natural gas as a fuel. It can also be used as a chemical reagent in numerous relevant processes (e.g., fertilizer manufacturing). Furthermore, an advantage of the invention is that it can be used for on-site hydrogen generation through a distributed generation paradigm, as opposed to the current centralized energy production paradigm. This is no small matter, considering that the biggest challenge in moving to a hydrogen-based economy is the difficulty of transporting hydrogen cheaply and effectively, as it cannot be liquefied at room temperature.

[0053] Furthermore, various embodiments of devices powered by energy sources other than solar energy can replace current electrolyzers with numerous advantages, as any heat-convertible energy source can be used to produce renewable hydrogen.

[0054] The following references have been used in the figures:

[0055] 100. Device

[0056] 101. Reactor

[0057] 102. Energy source

[0058] 103. Active material

[0059] 104. Selective filter

[0060] 105. Entrance

[0061] 106. First exit

[0062] 107. Second exit

[0063] 108. Container

[0064] 109. Reaction volume

[0065] 110. Insulator

[0066] 111. Heater

[0067] 112. Absorbent

[0068] BRIEF DESCRIPTION OF THE FIGURES

[0069] Figure 1 shows a diagram of an embodiment of a device for obtaining hydrogen through thermochemical reactions equipped with a selective hydrogen flow filter located inside the reactor.

[0070] Figure 2 shows a diagram of an embodiment of a device for obtaining hydrogen through thermochemical reactions equipped with a selective hydrogen flow filter coupled to the reactor.

[0071] Figure 3 shows a diagram of an embodiment of a device for obtaining hydrogen through thermochemical reactions equipped with a selective filter for the passage of hydrogen, where the filter is one of the walls of the reaction volume of the reactor.

[0072] Figure 4 shows a diagram of an embodiment of a device with tubular geometry comprising a tubular reactor defining a reaction volume and a selective filter, where the selective filter is arranged: a) within the reaction volume coupled with the second outlet, b) outside the reaction volume, and c) as part of the reaction volume. Figure 5 shows a diagram of an embodiment of a device comprising a reactor defining a reaction volume, a selective filter and a thermal system formed by a heater and an insulator, where said selective filter is arranged: a) within the reaction volume coupled with the second outlet, b) outside the reaction volume at the second outlet, and c) as part of the reaction volume.

[0073] Figure 6 shows a diagram of an embodiment of a device comprising a cylindrical reactor, a selective filter and a thermal system formed by an absorbent and an insulator, where the selective filter is arranged: a) within the reaction volume coupled with the second outlet, b) outside the reaction volume at the second outlet, and c) as part of the reaction volume.

[0074] DETAILED DESCRIPTION OF THE INVENTION

[0075] The objective of the present invention is to provide a solution to the problems currently existing in the production of hydrogen, specifically, in the processes for producing hydrogen using thermochemical reactions under isothermal (or quasi-isothermal) conditions. The present invention, unlike the solutions known in the state of the art, does not require the addition of excess reactant water in the half-reaction with water (R2) to shift it significantly towards the production of hydrogen, but rather employs a selective filtration process to extract the hydrogen as it is produced.

[0076] In a first aspect of the invention, the solution relates to a process for obtaining hydrogen, achieving high efficiency, mainly due to the low water consumption in the process. In a preferred embodiment, the water / hydrogen molar ratio is less than 10:1, with a water / hydrogen molar ratio of less than 2:1 being more preferred, and a 1:1 molar ratio being even more preferred, whose intrinsic efficiency for isothermal cycles is greater than 85%.

[0077] The process according to the present invention obtains hydrogen by means of thermochemical water dissociation cycles under (quasi-) isothermal conditions from an active material (103) capable of sustaining redox cycles, with hydrogen production in each oxidation half-cycle, where said material is partially reduced in the reduction half-cycle and oxidized in the oxidation half-cycle, reacting with water. Thus, said process comprises the following steps: a. placing the active material (103) in a reaction volume (109) of a reactor (101); b. reducing the active material (103) placed therein, by means of heat input; c. evacuating the oxygen produced through a first outlet (106); d. injecting water into the reaction volume (109); e. oxidizing the active material (103) producing hydrogen; f. selectively filtering the hydrogen produced through a selective filter (104) during the oxidation process of the active material (103); and g.evacuate the filtered hydrogen through the second outlet (107) obtaining high purity hydrogen.

[0078] The oxidation and production of hydrogen according to this procedure is clearly favored by selectively evacuating hydrogen from the reaction volume (109) as it is produced, to the detriment of the reverse reduction reaction.

[0079] In this way, a very effective shift of the equilibrium of the half-reaction R2 towards the production of hydrogen occurs without the need to add excess water, as required by current solutions for (quasi-)isothermal cycles.

[0080] Furthermore, it is important to note that selective hydrogen filtration not only significantly increases hydrogen production without excess reactant water, but also provides a highly pure product. To this end, in a preferred embodiment, selective hydrogen filtration is carried out at a temperature range of 600-2000 K.

[0081] For its part, the reduction of the active material (103) in step b is preferably carried out at high temperature depending on the active material (103) selected, with the temperature range 1300 - 2500 K being recommended.

[0082] In a preferred embodiment, the process steps are automated, synchronizing the input and output of the compounds, thereby reducing water consumption.

[0083] In order to carry out this process, a second aspect of the invention describes a device (100) for obtaining hydrogen. Said device (100) comprises a reactor (101 ), where the thermochemical cycles of water dissociation are carried out under (quasi-) isothermal conditions in the presence of an active material (103). The reactor (101 ) comprises a reaction volume (109) configured to contain an active material (103) and carry out said thermochemical cycles. A preferred embodiment of the reactor (101 ) has, in use, a ratio between the free volume and the total volume, or reaction volume (109), of less than 1:10. In this way, it is possible to reduce water consumption in the process and the water pressure in the reactor (101 ) is maximized, optimizing the kinetics of the reaction.

[0084] The active material (103) is a material capable of supporting redox reactions without losing its properties over numerous successive cycles. It is located inside the reactor (101), in its reaction volume (109). Typically, metal oxides are used, such as silica, with or without modifications to improve their properties by including additives, nanostructuring, catalysts, etc. The use of any material with good properties is not ruled out, for example, other metal oxides such as FesC, ZnO, CO3O4, Fe2O, SnO2, Mn2C>3, BI2O3, NI0, CuO, BaO2, PbsC, or Ag2O or newer materials such as, for example, perovskites with promising properties.

[0085] The device (100) additionally comprises a gas inlet (105) and two gas outlets (106, 107).

[0086] The inlet (105) is connected to a water injection system. Its function is to introduce a controlled amount of water into the reaction volume (109) of the reactor (101) during the oxidation half-cycles of the active material (103) to obtain hydrogen.

[0087] The water can be introduced in the form of steam, using a vaporizer as an injection system, or in liquid form if operating under vacuum. In addition, an inert carrier gas can also be introduced. For this purpose, in a preferred embodiment, the inlet is connected to an inert gas injection system in addition to the water injection system.

[0088] For their part, unlike other previous solutions, the first and second outlets (106, 107) are two independent outlets of the reactor (101). These outlets can be part of an evacuation system. The evacuation system allows the reactor volume (109) to be evacuated, for example, by means of an external pumping system. Additionally, the evacuation system can be configured to inject a flow of inert gas to induce the reduction process of the active material.

[0089] The first gas outlet (106) is configured for the evacuation of gases, mainly the oxygen generated during the reduction step of an active material (103) arranged in the reaction volume (109). However, depending on the operating conditions, the water vapor remaining from the oxidation half-cycle, mixed with a small amount of unfiltered hydrogen, may also be evacuated through this first outlet (106). Consequently, in a further embodiment, said unfiltered hydrogen may be recovered from this waste stream.

[0090] However, the second gas outlet (107) is configured to selectively evacuate the hydrogen produced during the oxidation of the active material (103) in the reaction volume (109).

[0091] For this purpose, the device (100) comprises a selective hydrogen filter (104) which connects the reaction volume (109) with the second outlet (107). In this way, the flow exiting through the second outlet (107) is a flow of hydrogen with a high purity.

[0092] The selective filter (104) can be made of any material with good thermomechanical behavior, good structural integrity, low chemical reactivity and high hydrogen permeability, preventing the passage of other gases.

[0093] The reactor (101) preferably operates at a high temperature, so that the heat existing in the reaction volume (109) can be used to also raise the temperature of the selective filter (104) and promote the permeability of the selective filter (104).

[0094] Thus, materials with selective permeability at high temperatures, such as nickel, are a preferred option, although there are also other alternatives such as palladium, niobium, titanium, vanadium, tantalum, iron and alloys of these or other compounds such as, for example, nickel titanium-niobium-zirconium-tantalum-cobalt alloys (in proportions of 0 to 80%) or palladium-silver copper alloys (in proportions of 0 to 80%), among others, which can also present good results. Alternatively, mixtures and alloys of materials for the formation of the selective filter (104), as well as multilayer systems, are also acceptable. The connection of said selective filter (104) with the reaction volume (109) must be carried out in the manner that most favors the passage of hydrogen.However, it can have different configurations, differentiating the arrangement of said selective filter (104) or the amount of contact with the reactor (101), in order to control other aspects, such as the optimization of the operating temperature of the selective filter (104) used.

[0095] Examples of such embodiments are shown in Figures 1-3.

[0096] Figure 1 shows a schematic of an embodiment of a device (100) equipped with a hydrogen selective filter (104). In this embodiment, the selective filter (104) is located inside the reactor (101), in the reaction volume (109) so that the generated hydrogen can easily reach the selective filter (104). In this embodiment, the selective filter (104) can reach the same temperature as the active material present in the reaction volume (109) of the reactor (101).

[0097] Alternatively, Figure 2 shows a diagram of a second embodiment of a device (100) with a selective filter (104) passing through the hydrogen. In this case, the selective filter (104) is coupled to the reactor (101). As can be seen, there is a connection between the reaction volume (109) and the selective filter (104), which must be crossed by the hydrogen to reach the selective filter (104). Although initially this alternative presents less contact with hydrogen, it has the advantage of allowing the temperature at which the selective filter (104) operates to be adjusted independently of the temperature of the active material (103), which allows the operation of the former to be optimized.

[0098] Additionally, Figure 3 shows a schematic of a third embodiment of a device

[0099] (100) for obtaining hydrogen. In this schematic embodiment, the selective filter (104) corresponds to one of the walls that limit the reaction volume (109) of the reactor.

[0100] (101 ). That is, the selective filter (104) can correspond partially or completely with at least one of the walls of the reactor (101 ), maximizing contact with the reaction volume (109).

[0101] In a particular embodiment, the filter (104) may comprise a combination of the previous embodiments, for example, in the case of using a large filter partially inserted into the reactor, which would result in a non-uniform temperature in the filter. Figure 4 shows an embodiment of a reactor (101) with a tubular body, where the reaction volume (109) is linear, from an inlet (105) to a first outlet (106). The reaction volume (109) may contain active material (103) inside, arranged along the entire length of the reaction volume (109).

[0102] In case a) of Figure 4, a selective filter (104) is shown located inside the reactor tube (101) occupying part of the reaction volume (109). The selective filter (104) is connected to the second outlet (107), through which the generated hydrogen can be evacuated. Thus, the filter (104) can take the tubular form to adapt to the body of the tubular reactor (101). In case b) of Figure 4, a selective filter (104) is shown coupled to the reaction volume (109). In this case, the selective filter (104) is arranged on a structure associated with the reaction volume (109) such that the generated hydrogen can reach the filter, passing through it and being evacuated through the second outlet (107). As indicated above, this configuration is especially relevant in those processes where the selective filter (104) has an optimal operating temperature different from that of the active material (103).In case c) of Figure 4, the selective filter (104) is the reactor wall.

[0103] (101) or part of it. In this way the filter delimits the reaction volume (109), and is connected to the second outlet (107), through which the generated hydrogen can be evacuated.

[0104] Therefore, the selective filter (104) allows the selective permeation of hydrogen through some or all of the walls that delimit the reaction volume (109) of the reactor (101), increasing the speed of the hydrogen extraction process, and preventing it from recombining by means of the reverse reaction to the oxidation of the active material (103) in the presence of water. In a preferred embodiment, regardless of the position where said selective filter (104) is located, it operates at a temperature in the range 600 - 2000 K.

[0105] The device (100) further comprises a power source (102). Said power source

[0106] (102) is configured to heat, in use, the reactor (101) which must transmit the heat to an active material (103) arranged in the reaction volume (109) to reach a temperature such that thermochemical cycles can be induced in the active material (103). Depending on the final application, the device may have a power source (102) with a different configuration. Thus, the power source (102) comprises a thermal system that can be formed by one or more elements as will be described below.

[0107] In a particular embodiment, the thermal system is formed by a heater (11 1 ) and an insulator (1 10), partially or completely surrounding the reactor (101 ). The heater (1 11 ) can be an electric heater, which typically has a resistance of tungsten, tantalum, silicon carbide or molybdenum disilicide, without excluding others, embedded in ceramic material. For its part, the insulator (110) is an alumina wool type insulator (typically composed of aluminum, silicon and / or zirconium oxides with variable composition depending on the operating temperature). As can be seen in Figure 5, where a diagram of an embodiment of a device comprising a reactor (101 ), a selective filter (104) and a thermal system is shown. The filter (104) may be located inside the reactor (101) (case a), coupled to the reactor (101) in a nearby location (case b), or the filter (104) may form the reactor or part of the reactor (case c).Furthermore, the reactor (101) may be made of alumina or other refractory ceramic.

[0108] Alternatively, in another particular embodiment, the thermal system comprises an energy absorber (112) and an insulator (11 ) partially or completely surrounding the reactor (101 ). Figure 6 shows a diagram of an embodiment of a device with cylindrical geometry composed of the container (108), the reactor (101 ) with its inlets (105) and outlets (106, 107) of gases that define the reaction volume (109), in addition to a selective filter (104). The selective filter (104) is arranged: a) inside the reaction volume (109) coupled with the second outlet (107), b) outside the reaction volume (109) at the second outlet (107), and c) as part of the reaction volume (109). The absorbent (1 12) is located near the walls that make up the reaction volume (109), and may be in contact with the reactor (101 ), so that its temperature increases by capturing energy, transmitting the captured energy to the reactor (101 ) by conduction.The absorber (112) may be a particular material or a combination of vapors, in order to maximize energy absorption.

[0109] Thus, in the case of using electromagnetic radiation, the absorber (1 12) comprises a high absorption coefficient in the electromagnetic band of the source used. In the case of using electrical energy through a resistance, the absorber comprises a high thermal conductivity to transmit heat from the resistance to the reactor. In the case of using a hot fluid, the absorbent medium (112) comprises a high convection coefficient for the fluid in question. In the case of using a magnetic induction system, the absorber (112) must be capable of generating induced eddy currents inside it in order to heat up to operating temperatures.

[0110] Regardless of the method used, the transmission of the thermal energy captured by the absorbent (1 12) to the reactor (101 ) must be optimal in order to heat the active material (103) and maintain the temperature during the reduction and oxidation cycles in (quasi-) isothermal conditions.

[0111] The consequence of the configuration of the device (100) according to the present invention is a very marked shift in the equilibrium towards the production of hydrogen in the oxidation half-reaction with water, without the need to add excess reactant water, which allows the final efficiency of the process to be substantially improved.

[0112] In one embodiment, the device comprises a container (108). In a particular embodiment, the container (108) is a hermetic container. Additionally, in a particular embodiment, the container (108) can be designed to effectively reflect electromagnetic radiation. Thus, in another preferred embodiment, conduction and convection losses are reduced by using a hermetic container (108) of low thermal conductivity, configured to contain at least the reaction volume (109) inside, reducing heat losses. Furthermore, losses are reduced when the hermetic container (108) and the reactor (101) operate in a vacuum, then, preferably, the hermetic container (108) is a vacuum container, to prevent heat losses by conduction and convection.

[0113] The container (108) does not need to be airtight if the outlet (107) has direct access to the reactor by means of the filter (104). However, it does need to be airtight and operate in a vacuum if the filtered hydrogen exits directly into the container from where it must be evacuated. In a particular embodiment, the presence of the insulator (110) and, where appropriate, the combination with a container (108) designed as an electromagnetic radiation reflector ensure that the surface of the container (108) is very close to room temperature even under operating conditions of the reactor (101) at very high temperatures.

[0114] Therefore, the geometry of the reactor (101) can vary to suit the thermal system as well as the dimensions of the container (108).

[0115] Additionally, the device (100) comprises a process control system configured to regulate the stages of the redox cycle. Specifically, said control system comprises a set of valves and pumps to regulate the flows at the inlet (105) and the outlets (106, 107), such that a correct admission of the exact amount of water (or other fluids) required in the oxidation half-cycles and for the evacuation of the gases produced, hydrogen and oxygen, during the procedure is achieved. In this way, the synchronization of the opening and closing of the injection system at the inlet (105) and the evacuation of the gases produced through the two outlets (106, 107) is achieved.

[0116] Thanks to the present invention, it is possible to shift the equilibrium of the water dissociation reaction towards the production of hydrogen, favoring the reoxidation of the active material (103) to the detriment of the reverse reaction, where the hydrogen produced again reduces the active material (103).

[0117] An exemplary embodiment of the method may be one that uses ceria, acting as the active material (103), in powder form at a temperature <2000 K. Additionally, a selective filter (104) of nickel, niobium (or alloys of these elements) with a small thickness, between 50 pm and 2 mm, may be used. This selective filter (104) may be at a temperature equal to or different from that of the active material (103). Thus, the selective filter (104) is preferably at a temperature between 1000 - 2000 K.

[0118] In conclusion, the set of solutions described according to the present invention resolves the limitations existing in the state of the art related to the low efficiency of the processes and devices for obtaining hydrogen through thermochemical cycles under (quasi-) isothermal conditions. The devices described here can replace or function complementarily with the combination of photovoltaic energy and electrolyzers to generate hydrogen. Furthermore, they replace all alternative forms of generating green hydrogen (photolysis, biological systems, etc.).

[0119] In this sense, the hydrogen produced can act as an energy vector that allows energy to be stored for use on demand. This capacity of the hydrogen produced, characterized by a high purity process, allows its use in fuel cells, replacing batteries in vehicles, especially for heavy vehicles and over long distances. Additionally, the ability to take advantage of any form of energy convertible to heat through the energy source (102) employed allows the use of the method and device (100) according to the present invention to recover residual energy from other industrial processes, which translates into a substantial improvement compared to the limitations presented by other known solutions.

Claims

CLAIMS 1. A process for obtaining hydrogen by means of thermochemical water dissociation cycles under (quasi-) isothermal conditions from an active material capable of sustaining redox cycles, with hydrogen production in each oxidation half-cycle, wherein said active material is partially reduced in the reduction half-cycle and reoxidized in the oxidation half-cycle induced by interaction with water, characterized in that said process comprises the following steps: a. placing the active material (103) in a reaction volume (109) of a reactor (101); b. reducing the active material (103) so placed by means of heat input; c. evacuating the oxygen produced through a first outlet (106); d. injecting water into the reaction volume (109); e. oxidizing the active material (103) to produce hydrogen; f. filtering the hydrogen produced through a selective filter (104) during the oxidation process of the active material (103); and g.evacuate the filtered hydrogen through a second outlet (107), configured to selectively evacuate the hydrogen produced during the oxidation of the active material (103) in the reaction volume (109), obtaining high purity hydrogen; where the first and second outlets (106, 107) are two independent outlets of the reactor (101) and where the flows of the inlet (105) and of the first and second outlets (106, 107) of each redox cycle are regulated and synchronized by means of a control system.

2. The process for obtaining hydrogen according to claim 1, wherein the selective filtration of hydrogen is carried out at a temperature in the range 600 - 2000 K.

3. The process for obtaining hydrogen according to any one of claims 1 to 2, wherein the reduction of the active material (103) is carried out at a temperature in the range 1300-2000 K.

4. The process for obtaining hydrogen according to any one of claims 1 to 3, wherein the water / hydrogen molar ratio is less than 10:

1.

5. A device for obtaining hydrogen by means of thermochemical water dissociation cycles under (quasi-) isothermal conditions, from an active material capable of sustaining redox cycles, with hydrogen production in each oxidation half-cycle, wherein said active material is partially reduced in the reduction half-cycle and reoxidized in the oxidation half-cycle induced by interaction with water, characterized in that said device (100) comprises: a. a reactor (101) with a reaction volume (109) therein configured to contain an active material (103) and carry out thermochemical water dissociation cycles; b. an energy source (102) configured to heat, in use, an active material (103) arranged in the reaction volume (109) of the reactor (101); c. an inlet (105), in connection with a water injection system, configured to introduce a controlled amount of water into the reaction volume (109) of the reactor (101); d.a first outlet (106) configured for gas evacuation; and e. a selective hydrogen filter (104), connecting the reaction volume (109) with a second outlet (107), configured to evacuate, in use, hydrogen produced in the reaction volume (109) of the reactor (101) where the first and second outlets (106, 107) are two independent outlets of the reactor (101), and where the device (100) comprises a process control system comprising a set of valves and pumps configured to regulate the stages of the redox cycle, synchronizing the opening and closing of the water injection system and output of produced gases.

6. The device for obtaining hydrogen according to claim 5, wherein the device (100) comprises a process control system comprising a set of valves and pumps configured to regulate the stages of the redox cycle, synchronizing the opening and closing of the water injection system and output of produced gases.

7. The device for obtaining hydrogen according to claim 6, wherein the process control system comprises an automatic means for controlling the set of valves and pumps.

8. The device for obtaining hydrogen according to claim 5; wherein the selective filter (104) comprises an operating temperature in the range 600 - 2000 K.

9. The device for obtaining hydrogen according to any one of claims 5 to 6; wherein the selective filter (104) is located inside the reactor (101), corresponds partially or completely to at least one of the walls of the reaction volume (109) of the reactor (101) or is coupled to the reactor (101).

10. The device for obtaining hydrogen according to any one of claims 5 to 7, wherein the selective filter (104) is a filter of nickel, niobium or an alloy of these compounds. 1 1. The device for obtaining hydrogen according to claim 8, wherein the selective filter (104) has a thickness between 50 pm and 2 mm, the operating temperature being between 1000 - 2000 K.

12. The device for obtaining hydrogen according to any one of claims 5 to 9, wherein the water injection system is a vaporizer (1 10).

13. The device for obtaining hydrogen according to any one of claims 5 to 10, wherein the ratio, in use, of the free volume and the reaction volume (109) is less than 1:

10.

14. The device for obtaining hydrogen according to any one of claims 5 to 11, wherein the device comprises an electric heater (111).

15. The device for obtaining hydrogen according to any one of claims 5 to 11, wherein the device comprises an absorbent (112).

16. The device for obtaining hydrogen according to any one of claims 5 to 13, wherein the device comprises an insulator (110).

17. The device for obtaining hydrogen according to any one of claims 5 to 14; wherein the device comprises a container (108).

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