Use of a doped metal oxide in thermochemical cycles

Abstract

The present invention relates to the use of a doped metal oxide of the perovskite class in the form of ABO3 for direct resistive heat generation and / or heat storage upon application of a voltage. Furthermore, the present invention relates to a method for a thermochemical cycle using the metal oxide according to the invention.

Description

[0001] The present invention relates to the use of a doped metal oxide of the perovskite class in the form of ABO3 for direct resistive heat generation and / or heat storage upon application of a voltage. Furthermore, the present invention relates to a method for a thermochemical cycle using the metal oxide according to the invention.

[0002] Thermochemical cycles based on the reduction and oxidation of metal oxides are the subject of current research in a wide range of application areas. These include the splitting of H2O and CO2, and thus the production of H2 and CO2 as fuels and chemical feedstocks. Furthermore, pure nitrogen can be produced through air separation. Thermochemical redox cycles can also be used to increase the capacity of high-temperature heat storage systems.

[0003] In a two-stage thermochemical water splitting process, for example, a metal oxide (MO) is first thermally reduced so that the reduced metal oxide MO red is created and oxygen is released: MON → MON red + O2 - ΔH (I)

[0004] This reduction is endothermic, meaning it consumes energy, which is stored in the reduced metal oxide. This first step is common to all potential applications. In a second step, oxidation, the reduced metal oxide is oxidized again. The following basic reactions can be defined in relation to the application areas described above: Water splitting: MO red + H2O → MO + H2 (II-a) Carbon dioxide splitting: MO red + CO2 → MO + CO (II-b) Air separation: MO red + Air (N2 / O2) → MO + N2 + ΔH (II-c) Heat storage: MO red + O2 → MO + ΔH (II-d)

[0005] Oxidation reactions (II-a) to (II-c) typically occur approximately 200 °C - 700 °C below the reduction temperature. Reaction (II-d) is typically operated continuously within a temperature window below the reduction temperature. The lower temperature limit depends on the required temperature level of the application for which heat is being provided.

[0006] The redox cycles described above represent a method to convert heat energy 1. to convert into chemically bound energy (II-a + II-b). 2. to be used for the production of nitrogen (e.g. for fertilizer production) (II-c). 3. to store in a solid state (II-d).

[0007] The heat source is irrelevant for the chemical reaction. A particularly environmentally and climate-friendly option is the use of concentrated solar energy to heat the redox material. In this process, solar radiation is focused onto a small area using mirrors (heliostats), usually located on a so-called solar tower. This creates very high temperatures at the focus of the concentrating solar system. The concentrated radiation can be captured in a solar receiver and used to reduce the metal oxide.

[0008] Three of the most energy-intensive industries are the chemical, steel, and cement industries. Together, they account for approximately 18% of global CO2 emissions. A large portion of these emissions are attributable to primary energy consumption (IEA, Direct CO2 emissions from selected heavy industry sectors, 2019, IEA, Paris https: / / www.iea.org / data-and-statistics / charts / direct-co2-emissions-from-selected-heavy-industry-sectors-2019, IEA).

[0009] Thermochemical redox cycles powered by renewable energies can make a decisive contribution to reducing global CO2 emissions by providing emission-free heat and process gases (H2, CO, N2).

[0010] In the field of heat storage, the established method of sensible heat storage in ceramic blocks exists. Industrial waste heat is usually used as the heat source, so that the temperature level of the heat storage unit without additional firing from other heat sources (e.g., natural gas) can only be understood as heat recovery and is therefore unable to independently supply the main process with the required heat energy. To store electrical energy as heat, such sensible heat storage units are also "charged" using electric heaters. Such a system is then called "firebrick resistance-heated energy storage" (FIRES) (Daniel C. Stack, Daniel Curtis, Charles Forsberg, Performance of firebrick resistance-heated energy storage for industrial heat applications and round-trip electricity storage, Applied Energy, Vol. 242, 2019, pp. 782-796).A gas stream, usually air, is heated as a heat transfer fluid using electric heating elements and passed through the heat storage unit, consisting of ceramic blocks, where the heat is stored. For discharging, cold air is then passed through the storage unit, thus heating it. Typical materials for electric heating elements are metallic alloys of Ni-Cr and Fe-Cr-Al, as well as ceramics made of SiC or MoSi2. Ceramic heat storage units are typically made of aluminum oxide, magnesium oxide, or silicon carbide, as well as waste with a high proportion of these materials from other industries (e.g., the construction industry).

[0011] A directly resistively heated heat storage device made of electrically conductive metal oxides is the subject of current research.[3] However, only non-reactive oxides are treated there and the process therefore represents a purely sensible heat storage device.

[0012] DE 10 2013 211249 A1 relates to a method for the thermochemical storage and provision of energy using reversible reactions. A stepwise thermochemical storage of solar heat using redox materials is disclosed. The heat storage material is charged indirectly using a heat transfer fluid.

[0013] EP 3901087 A1 relates to a process and apparatus for the electrochemically assisted thermo-chemical splitting of water and carbon dioxide.

[0014] Heat provision for the thermochemical redox cycles (II-a to IId) using concentrated solar radiation is only efficient at locations with very high levels of direct normal irradiation (DNI). In addition, other factors, such as seasonal fluctuations in DNI, atmospheric composition, humidity, and altitude, determine the techno-economic competitiveness of such a power plant and thus also the economic viability of the various thermochemical cycles.

[0015] Supplying industrial processes with high-temperature heat is particularly challenging due to the required geographical proximity of the power plant to industrial sites that require high-temperature heat for process control. Very high temperatures, such as those required in the steel industry, lead to a rapid decrease in absorber efficiency and thus in the efficiency of the overall process, and are therefore only of limited suitability for storing and feeding heat into industrial processes.

[0016] There is therefore a need for optimized heat provision in thermochemical cycles. Surprisingly, it has been shown that applying voltage to a doped metal oxide of the perovskite class in the form of ABO3 can be used for direct, resistant heat generation and / or heat storage. The inventive use of the metal oxides has the advantage that heat transfer fluids are no longer required. Energy loss is thus minimized because there are fewer heat transfer processes in the various application areas.

[0017] Furthermore, the thickness of the metal no longer limits the heat generation, as thin wires are typically required to enable the required heat generation and storage. Heat storage with conventional metals from the state of the art is limited by the thickness of the wires and is correspondingly low, as these can only store sensible heat.

[0018] Heat generation using indirectly resistively heated ceramics is also limited by the maximum operating temperature of the heating elements. Furthermore, the materials used to manufacture heating elements for high-temperature applications (SiC, MoSi2) are relatively expensive. Sensible heat storage devices can also be used exclusively for heat generation and are limited to the purely sensible heat capacity of the material, which is typically less than 1 kJ / (kg*K).

[0019] When using doped metal oxides according to the invention, the heat is determined solely by the ratio of the resistivity to the volume of the metal oxides. Furthermore, there are hardly any metals or metal oxides in the prior art that are stable at high temperatures. This leads to uneven heat development because the heat development increases or decreases the electrical resistance depending on the material used. This can result in certain zones in the component being exposed to an increased voltage drop, which leads to increased heat development. This creates so-called "hot zones" that can lead to thermal runaway and ultimately to material failure. Due to their characteristic temperature dependence of the resistivity, hot zones in the material through which current flows experience a disproportionately large voltage drop, which also increases with rising temperature.As a result, this hot zone heats up faster than the rest of the component as the temperature rises, and the material's stable temperature range is quickly exceeded in the hot zones. This overshoot usually leads to material failure. Another problem is that metal oxides do not conduct electricity well because they have a band gap that is too large. The resistivity is too low (usually less than 1 ohm mm / m). 2 ) and makes the use of conventional materials in the processes described above impractical due to insufficient efficiency.

[0020] As a result, there is a need in the prior art for the use and a corresponding method in which heat generation occurs uniformly in the material used, whereby the material is exposed to high temperatures and simultaneously generates heat efficiently and can also store it. Furthermore, there is a need for location-independent use of energy, since metal oxides of the perovskite class can currently only be used where there is sufficient solar radiation to efficiently provide heat with the aid of thermal fluids, for example, molten salts. In a first embodiment, the object underlying the present invention is achieved by the use of a metal oxide of the perovskite class in the form of ABO3 for direct resistive heat generation and / or heat storage by applying a voltage, characterized in that (i) the metal oxide is doped, and (ii) the metal oxide is in contact with a cathode and anode.

[0021] The materials used according to the invention do not undergo phase transformation like conventional materials described in the prior art. This is achieved through doping, as this ensures electrical conductivity while simultaneously maintaining the lattice structure and the resulting dimensional stability when electrical energy is supplied. This ensures a higher heat storage capacity. Furthermore, the use according to the invention circumvents the limitation of the specific heat capacity, as the doped metal oxides of the perovskite class ABO3 can store heat sensitively and chemically. This results in a significantly higher heat capacity of the materials.

[0022] The present invention represents an alternative way of providing heat for the operation of these thermochemical cycles compared to the prior art. It allows the use of the heat from the thermochemical cycles (II-a to II-d).

[0023] The invention differs significantly from the prior art in the way it provides heat. The invention utilizes the electrical conductivity of the active material, the metal oxide, for direct resistive heat generation. The heat is thus generated in-situ in the material, which reactively participates in the process. This makes the invention largely location-independent and can be operated anywhere electrical power, i.e., electricity, is available. In particular, the ability to transport electricity over long distances with low losses enables geographical separation between power generation and process control. This makes the invention particularly suitable for integrating renewable electrical energy from a variety of sources, such as photovoltaics, wind power, hydropower, etc.

[0024] In contrast to the prior art heat provision using concentrated solar radiation, the invention thus enables continuous operation of the processes as long as electricity is available. Furthermore, the invention offers the possibility of absorbing electricity production peaks around midday, which are to be expected with the increasing expansion of photovoltaics. The heat required for partial reaction I can be generated and temporarily stored at any time until the main process is operated with partial reaction II. This allows electricity consumption and process control to be separated from each other in time.

[0025] Preferred embodiments of the use and a method are further described below, wherein all features can be combined with one another in any desired manner and do not limit the use and the method of the present invention.

[0026] According to the invention, a metal oxide of the perovskite class in the form ABO3 is understood to mean metal oxides which comprise a divalent ion A from the class of alkaline earth metals (Ca, Mg, Sr, Ba, Be) and a tetravalent ion B of a transition metal. According to the invention, transition metals are understood to mean elements which have an incomplete d-subshell or form ions with an incomplete d-subshell, wherein according to the invention the elements with a d 1 Configuration as a pure substance are not included. The transition metals are preferably selected from Fe, Mn, Ti, Cr, Co, Ni, Zr, Mo, Tc, Ru, Rh, Pd, Hf, W, Re, Os and / or Ir, most preferably from Fe, Mn, Ti and / or Cr.

[0027] In a further preferred embodiment of the use according to the invention, it is also possible to use a perovskite as metal oxide, which has the form AB 1 B 2O3. In this form, the metal oxide consists of a mixed oxide comprising two transition metals from the above-mentioned elements.

[0028] In another preferred embodiment, it is also possible to use a perovskite of form A 1 A 2 BO3, where two alkaline earth metals are included in the metal oxide. Furthermore, a combination of A 1 A 2 B 1 B 2 O3 is possible, where both two alkaline earth metals and two transition metals are included in the metal oxide. The lattice sites are divided into a certain proportion of A 1 by A 2 and B 1 by B 2 replaced.

[0029] By replacing the lattice sites, further thermodynamic and kinetic properties of the metal oxide can be fine-tuned.

[0030] The synthesis is carried out by solid-state synthesis, as described, for example, by Pein et al. in Adv. Energy Mater., 2022, 12(10):2102882, from alkaline earth carbonates and transition metal oxides and doping elements. Furthermore, the synthesis can be achieved using sol-gel processes, as described, for example, by Danks et al. in Mater. Horiz., 2016, 3, 91-112.

[0031] According to the invention, the metal oxides used are doped. Doping is understood to mean the mixing of an element into the corresponding metal oxide, wherein the introduced element, as an ion, has an oxidation state that is either 1 higher or lower than that of the transition metal B in the metal oxide. According to the invention, the elements to be doped (also called doping elements) are selected from the transition metals with a d 1 electron configuration, lanthanides and / or actinides.

[0032] There may be p- or n-doping, depending on whether the doping element has a higher or lower oxidation state than the element B or B 1 and B 2 has.

[0033] In a preferred embodiment of the use according to the invention, the metal oxides are doped with one of the elements selected from the group consisting of Y, Sc, La, Ac, and / or Ce, particularly preferably with La, Ce, and / or Y. In this preferred embodiment, it is possible to achieve particularly advantageous electrical properties of the material.

[0034] In another preferred embodiment of the use of the metal oxide ABO3 according to the invention, this is characterized in that the metal oxide ABO3 has a heat capacity in the temperature range 300 °C - 1200 °C of > 1 Jg -1 K -1 , especially from > 1.1 Jg -1 K -1 , preferably > 1.2 Jg -1 K -1Such high heat capacities are not usually achievable for metal oxides, but this is made possible by the inventive use of doped metal oxides.

[0035] A further advantage of the metal oxides used is that the use of a heat transfer medium can be dispensed with to utilize the generated heat or energy, as described in the prior art. Therefore, in a preferred embodiment of the inventive use of the metal oxide ABO3, no heat transfer medium is used when utilizing the generated heat.

[0036] Furthermore, in a preferred embodiment, it is possible to use the metal oxides at very high temperatures. As already described, the heat is generated via electrical energy, which leads to heat development in the material. The metal oxides can be applied for such a long time or at such high voltages that they can be used at an operating temperature of up to 2000 °C, preferably 1600 °C, and most preferably 1400 °C.

[0037] In a further preferred embodiment, the energy produced according to the invention can be used in the form of heat for chemical water splitting, carbon dioxide splitting, air separation and / or heat storage, which have already been described via equations II ad.

[0038] In a further preferred embodiment, the metal oxide ABO3 is used according to the invention, wherein the electrodes are in contact with the metal oxide in such a way that the current generated by the applied voltage travels the greatest possible distance. This is made possible by placing the electrodes at the end and beginning points of the deposited metal oxide. This ensures maximum electrical resistance, which leads to the greatest possible heat generation.

[0039] In another preferred embodiment, the metal oxide is designed to be open-pored when used according to the invention. This ensures that, when used in a manner that allows the generated / stored heat to be transferred directly to the reacting substance in the form of gases and / or liquids. For example, heat can be transferred more efficiently due to greater surface contact in one of the reactions II ad.

[0040] In a further preferred embodiment of the use according to the invention, the metal oxide can be configured to have a hexagonal structure, a foam, a cylindrical shape with channels, and / or simple block structures that are stacked offset so that a gas can flow through them. If the material is hexagonal, the core of the metal oxide is not filled, so that the largest possible surface contact with the gas or liquid can be achieved.

[0041] Furthermore, in a preferred embodiment, the metal oxide can be monolithic or modular, preferably modular. A monolithic design means that the metal oxide was manufactured and / or synthesized from a single piece. A modular design means that the metal oxide consists of multiple parts and has been assembled. When used according to the invention, in a preferred modular design, it must be ensured that all modular parts of the metal oxide are in contact with one another so that electrical current can flow through them when voltage is applied.

[0042] When used according to the invention, the metal oxides must be in contact with an anode and cathode so that a voltage can be applied. In a preferred embodiment, the electrode material is selected so that it can be used at the operating temperatures of the preferred embodiment described above without phase transformation. Accordingly, in a preferred embodiment, the electrode material is selected from titanium, tungsten, platinum, tantalum, and / or boron, particularly preferably titanium and / or platinum.

[0043] Furthermore, in another preferred embodiment, it is possible to use the metal oxide ABO3 in such a way that an insulating material is applied to the outer walls of the metal oxide, which can function as both an electrical and thermal insulation material. A material based on aluminum oxide is particularly preferably used as such an insulating material.

[0044] In a further preferred embodiment, the current or voltage for use according to the invention is provided from renewable energies. Renewable energies are understood to mean wind turbines, photovoltaic systems, and / or hydropower plants, which generate so-called green electricity.

[0045] Furthermore, the present invention encompasses a process for carrying out a thermochemical cycle using the metal oxide according to the invention. The thermochemical process comprises: 1) a reduction step in which the metal oxide as redox material is converted from its oxidized state ABO3 to a reduced state ABO 3-δ is reduced with the release of oxygen, and 2) an oxidation step in which the redox material is converted from its reduced state ABO 3-dis oxidized to its oxidized state ABO3, wherein step 2 is carried out at a temperature in the range of 300 °C to 1800 °C, preferably 600 °C to 1400 °C, most preferably 800 °C to 1200 °C and the amount of heat required for step 1 is introduced as resistive heat by applying current to the redox material.

[0046] The voltage required to generate the heat depends on the redox material itself and the doping. Depending on this, household current with a voltage of up to 240 V may be sufficient. However, three-phase current with a voltage of up to 415 V may also be used or necessary. QUOTES CONTAINED IN THE DESCRIPTION

[0000] This list of documents submitted by the applicant was generated automatically and is included solely for the convenience of the reader. This list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions. Cited patent literature

[0000] DE 10 2013 211249 A1

[0012] EP 3901087 A1

[0013] Cited non-patent literature

[0000] Daniel C. Stack, Daniel Curtis, Charles Forsberg, Performance of firebrick resistance-heated energy storage for industrial heat applications and round-trip electricity storage, Applied Energy, Vol. 242, 2019, p. 782-796

[0010] Pein et al. in Adv. Energy Mater., 2022, 12(10):2102882

[0030] Danks et al. in Mater. Horiz., 2016,3, 91-112

[0030]

Claims

[1] Use of a metal oxide of the perovskite class in the form ABO3 for direct resistive heat generation and / or heat storage by applying a voltage, characterized by , that (i) the metal oxide is doped, and (ii) the metal oxide is in contact with a cathode and anode. [2] Use of a metal oxide ABO3 according to claim 1, wherein A is selected from an element which is a divalent ion in the metal oxide, preferably selected from alkaline earth metals. [3] Use of a metal oxide ABO3 according to claim 1 or 2, characterized by that the metal oxide ABO3 has a heat capacity in the temperature range 300 °C - 1200 °C of > 1 Jg -1 K -1 , especially from > 1.1 Jg -1 K -1 , preferably > 1.2 Jg -1 K -1 has. [4] Use of a metal oxide ABO3 according to at least one of claims 1 to 3, wherein B is selected from an element which is present in the metal oxide as a tetravalent ion, preferably from a transition metal. [5] Use of a metal oxide ABO3 according to at least one of claims 1 to 4, wherein no heat transfer medium is used when utilizing the generated heat. [6] Use of a metal oxide ABO3 according to at least one of claims 1 to 5, wherein the metal oxide is used at a maximum operating temperature of 2000 °C, preferably of 1400 °C, most preferably of 1000 °C. [7] Use of a metal oxide ABO3 according to at least one of claims 1 to 6, wherein the energy produced, in the form of heat, is used in chemical water splitting, carbon dioxide splitting, air separation and / or heat storage. [8] Use of a metal oxide ABO3 according to at least one of claims 1 to 7, wherein the metal oxide has been doped with an element of the lanthanides or actinides, preferably with lanthanum, cerium or yttrium. [9] Use of a metal oxide ABO3 according to at least one of claims 1 to 8, wherein the metal oxide is open-pored. [10] Use of a metal oxide ABO3 according to claim 9, wherein the metal oxide has one of the structures selected from a hexagonal structure, a foam, a cylindrical shape with channels and / or simple block structures which are stacked in a staggered manner so that a gas can flow therethrough. [11] Use of a metal oxide ABO3 according to at least one of claims 1 to 10, wherein the metal oxide is monolithic or modular, preferably modular. [12] Use of a metal oxide ABO3 according to at least one of claims 1 to 11, wherein the electrodes are in contact with the metal oxide in such a way that the current resulting from the applied voltage travels the greatest possible distance. [13] Use of a metal oxide ABO3 according to at least one of claims 1 to 12, wherein the electrodes consist of a temperature-stable electrical conductor, preferably platinum. [14] Use of a metal oxide ABO3 according to at least one of claims 1 to 13, wherein the metal oxide is insulated on the outer walls with an insulating material which can function both as an electrical and thermal insulating material, preferably based on aluminum oxide. [15] Use of a metal oxide ABO3 according to at least one of claims 1 to 14, characterized by that the applied voltage is provided from renewable energies. [16] Thermochemical cycle using a metal oxide as defined in at least one of claims 1 to 15, comprising 1) a reduction step in which the metal oxide as redox material is converted from its oxidized state ABO3 to a reduced state ABO 3-δ is reduced with the release of oxygen, and 2) an oxidation step in which the redox material is converted from its reduced state ABO 3-d is oxidized to its oxidized state ABO3, wherein step 2 is carried out at a temperature in the range of 300 °C to 1800 °C, preferably 600 °C to 1400 °C, most preferably 800 °C to 1200 °C and the amount of heat required for step 1 is introduced as resistive heat by applying current to the redox material.