Method and apparatus for producing one or more of carbon dioxide, carbon monoxide, carbon and hydrogen from a gas comprising carbon dioxide and preferably water

Abstract

A process for producing one or more of carbon dioxide, carbon monoxide, carbon and hydrogen from a gas comprising carbon dioxide and preferably water, in particular from air, comprising the following steps: in a first step, the gas comprising carbon dioxide is contacted with sodium hydroxide, preferably a sodium hydroxide solution, to absorb carbon dioxide and to form sodium carbonate, in particular sodium carbonate with water of crystallization; in a second step, the sodium carbonate is reacted with zinc oxide, in particular with an excess of zinc oxide, to release one or more of carbon dioxide, carbon monoxide and, if water is present, hydrogen.Apparatus for carrying out a process for providing one or more of carbon dioxide, carbon monoxide, carbon and hydrogen from a gas comprising carbon dioxide and preferably water, in particular from atmospheric air, comprising an absorber configured to carry out the first step and a decomposition unit configured to carry out the second step.

Description

[0001] The invention relates to a method and a device for producing one or more of carbon dioxide, carbon monoxide, carbon and hydrogen from a gas comprising carbon dioxide and preferably water, in particular from air, wherein in a first step the gas comprising carbon dioxide is brought into contact with sodium hydroxide, preferably a sodium hydroxide solution, in order to absorb carbon dioxide and to form sodium carbonate, in particular sodium carbonate with water of crystallization.

[0002] As the world continues to face the challenge of climate change, there is increasing recognition of the importance of reducing carbon emissions across all sectors of the economy. In response to this challenge, many companies are developing carbon-neutral chemical products that emit no net carbon dioxide into the atmosphere during their production, use, and disposal.

[0003] Carbon-neutral chemical products are chemicals produced using low-carbon or renewable energy sources and that emit no net carbon dioxide during their life cycle. This means that the carbon emissions associated with their production and use are offset by the removal of an equivalent amount of carbon dioxide from the atmosphere through activities such as carbon capture and storage or the use of renewable energy sources.

[0004] Furthermore, efforts are being made to produce so-called carbon-negative products. Carbon-negative products are defined as products or technologies that actively remove more carbon dioxide from the atmosphere than they emit during their production, use, and disposal. This means they have a negative net carbon footprint and can help mitigate the effects of climate change.

[0005] The decentralized provision of all components for the production of alcohols and hydrocarbons plays a key role in all concepts for the extraction of energy sources or the production of energy-intensive raw materials in remote areas. Providing a carbon source, in particular, is often complex, as carbon dioxide-containing exhaust gases are not available in regions where energy from renewable sources would be available in large quantities and is not needed locally. Such regions are areas with high, regular solar radiation, where photovoltaics and solar thermal energy operate with good efficiency. These are found around the equator in the "Earth's sunbelt," preferably in deserts or semi-deserts.The only virtually unlimited carbon source available is carbon dioxide in the air, which is present in low concentrations and is to be extracted using special processes known as direct air capture (DAC) processes. Most known DAC processes are not scalable to the extent required to replace fossil fuels or fossil feedstocks, or can only be scaled up with great effort. In addition to providing the carbon source, water consumption is also a critical factor in arid zones. Processes with high water consumption are therefore excluded, as are very energy-intensive processes and processes involving reactions that would be highly efficient but would proceed very slowly, resulting in high investment costs due to the low throughput.The constraints to be met are a closed-loop process, the avoidance of absorbents or membranes with limited storage life, and an excessive demand for rare precious metals. The process in question takes these constraints into account, which must be met to enable the economical extraction of the components carbon dioxide, carbon monoxide, and hydrogen from the air using renewable energy sources and to provide syngas in large quantities for any application in remote areas with poor infrastructure.

[0006] While the development of carbon-negative products is an important step toward reducing carbon emissions and mitigating the effects of climate change, there are several challenges associated with their production. One of the main challenges is the cost of implementing these technologies, which can be prohibitively expensive and require significant investment in research and development.

[0007] Another challenge is the scalability of these technologies, as many of the currently known carbon-negative products are still in early stages of development and may not be feasible for large-scale production. Furthermore, the energy and resource requirements for producing carbon-negative products can offset the benefits of their carbon negativity, especially if no renewable energy sources are used.

[0008] Thus, there is still a need for improved solutions that at least partially overcome the above-mentioned disadvantages.

[0009] It is the object of the invention to provide a method and a device relating to the technical field mentioned at the outset, with which one or more of carbon dioxide, carbon monoxide, carbon and hydrogen can be produced cost-effectively from a gas.

[0010] The solution of the invention is specified by the features of claim 1. According to the invention, in a second step, the sodium carbonate is reacted with zinc oxide, in particular with an excess of zinc oxide, in order to release one or more of carbon dioxide, carbon monoxide and, if water is present, hydrogen.

[0011] According to the invention, the process for producing one or more of carbon dioxide, carbon monoxide, carbon and hydrogen from a gas comprising carbon dioxide and preferably water, in particular from air, comprises the following steps: (a) the gas comprising carbon dioxide and water is brought into contact with sodium hydroxide, preferably a sodium hydroxide solution, to simultaneously absorb carbon dioxide and water to form sodium carbonate, in particular sodium carbonate with water of crystallization, and a water-enriched sodium hydroxide solution; b) sodium carbonate crystals from the solution of step a) are separated, preferably by filtration or centrifugation, optionally additional sodium carbonate may be separated by distillation of the sodium hydroxide solution to enhance crystallization; c) the sodium carbonate crystals from step b) are reacted with zinc oxide, in particular with an excess of zinc oxide, to release carbon dioxide by the formation of sodium zincate; d) the sodium zincate from step c) is reacted with water to form sodium hydroxide saturated with sodium tetrahydroxide zincate and solid zinc oxide, with at least a portion of the solid zinc oxide being separated; e) the remaining solution of sodium tetrahydroxide zincate in sodium hydroxide is used as absorbent in step a); f) the solution of step d) is used to produce metallic zinc by alkaline electroplating, in particular by converting sodium tetrahydroxide zincate into sodium hydroxide and metallic zinc, the metallic zinc being used as a reducing agent for one or more of the following processes: I. the metallic zinc is used as a reducing agent to reduce carbon dioxide obtained in step c) to carbon monoxide, zinc oxide obtained therewith being used in step c), in particular without any treatment; II. the metallic zinc is used as a reducing agent to reduce carbon monoxide recovered in step I to carbon, preferably using an iron-containing catalyst, whereby carbon and zinc oxide are separated, preferably by dissolving the zinc oxide in sodium hydroxide to form a concentrated sodium tetrahydroxide zincate solution, the concentrated sodium tetrahydroxide zincate solution being used for galvanic zinc recovery for use in step f) and recovery of sodium hydroxide solution for use as an absorbent in step a); III. the metallic zinc is used as a reducing agent to reduce water, preferably water of the water-enriched sodium hydroxide solution of step a), to hydrogen by forming a concentrated sodium tetrahydroxide zincate solution, which is preferably used for electroplating zinc and recovering more concentrated sodium hydroxide solution for use in step a).

[0012] In the following, reference is made to the steps numbered a) to f) and, where appropriate, to sub-steps I to III.

[0013] According to the invention, zinc oxide is used for the reaction with sodium carbonate. Even if the reaction could in principle be carried out with other substances, there are several advantages to using zinc oxide. Zinc is relatively abundant in the Earth's crust, and the production costs of zinc oxide are comparatively low. Zinc oxide is very common due to its wide use in various commercial applications and is therefore readily available almost everywhere in the world. This means that zinc oxide is readily available and inexpensive to obtain. Zinc oxide is generally considered non-toxic to humans and the environment, which means that in the application according to the invention, for example, it can be stored and used without special, particularly costly safety precautions. This means that workers are not exposed to any particular hazards.Furthermore, it is even biocompatible, which means that it is generally well tolerated by living tissue.

[0014] From a chemical point of view, other elements could also be used for the process. However, studies have shown that the use of zinc offers process-related advantages over the other metals. In principle, iron can also be used for the process, but this forms various oxides that are difficult to handle and can impair the function of the device. Calcium can also be used. However, calcium carbonates also have the disadvantage of adhering to the system and are therefore difficult to handle. Finally, all tests conducted have shown that zinc or zinc oxide has particularly favorable properties in the process according to the invention.

[0015] The process according to the invention has the advantage that the zinc oxide can be recycled, i.e. the zinc oxide can be reused in the process after the carbon dioxide has been released. This means that the zinc oxide is not consumed. The process also has the advantage that the product ratios between carbon dioxide, carbon monoxide, carbon and hydrogen can be adjusted essentially freely. In particular, for example, only carbon monoxide can be produced as required. In a further variant, only carbon can be produced. In a further variant, twice as much carbon dioxide as carbon monoxide can be produced (based on mass or moles). Furthermore, equal amounts of carbon dioxide, carbon monoxide, carbon and hydrogen can be produced (based on mass or moles). Those skilled in the art will recognize that any number of other examples could be given.

[0016] Thus, a further advantage of the process is that one or more of carbon dioxide, carbon monoxide, carbon and hydrogen can be produced from air in any ratio to each other in a single process.

[0017] In particular, the gas is preferably the only consumable in the process. Thus, preferably, no other substances are consumed. However, if other products are produced from one or more of carbon dioxide, carbon monoxide, carbon, and hydrogen, it may be necessary to use other substances that are consumed.

[0018] Most preferably, the process according to the invention is a carbon dioxide-negative process. This means, in particular, that the process is carried out such that during the production of one or more of carbon dioxide, carbon monoxide, carbon, and hydrogen, the process removes more carbon dioxide from the environment, e.g., the atmosphere, than it emits during the production of the product.

[0019] Even if CO2-negative operation is naturally preferred, it is of course possible to operate the process in a carbon dioxide-nonnegative manner, e.g., in a carbon dioxide-neutral manner or in a manner that emits more carbon dioxide during the production of one or more of carbon dioxide, carbon monoxide, carbon, and hydrogen than the process removes from the environment, particularly the atmosphere. This depends in particular on whether the energy source is CO2-neutral or not.

[0020] The process according to the invention provides an effective, cost-effective, sustainable, and robust process for producing one or more of carbon dioxide, carbon monoxide, carbon, and hydrogen with a low or even negative carbon footprint. Thanks to the inventive combination of process steps, the process can be implemented without fossil raw materials and fossil energy sources.

[0021] The process according to the invention is also highly modular. This means that individual process steps can be carried out at one and the same location or the same plant, or individual process steps can be carried out at different locations. For example, step a) can be carried out, for example, in regions with high solar radiation. Step c) can be carried out at the location where one or more of carbon dioxide, carbon monoxide, carbon, and hydrogen are used. However, since zinc oxide can be reused in this process (i.e., zinc oxide is not consumed in this process), it is preferable to carry out process steps a) and c) at the same location.

[0022] The method according to the invention can be carried out particularly advantageously in the sunbelt, along the equator, between the north and south turning radii, and / or in deserts. In these locations, virtually any amount of solar energy is available, i.e., solar radiation energy that can be harnessed with photovoltaic cells and / or solar thermal energy that can be provided with solar thermal collectors.

[0023] Particularly preferably, the energy required to carry out the process step(s) is electricity and / or thermal heat. The energy required in the form of electricity is generated in particular by a photovoltaic unit and / or a wind energy unit, and / or the energy required in the form of heat is generated in particular by solar thermal collectors. However, other forms of energy can also be used.

[0024] Thus, in a further preferred embodiment, the method according to the invention comprises a further method step of generating the renewable energy, in particular with a photovoltaic unit and / or a wind energy unit and / or a solar thermal collector.

[0025] In particular, the required energy is generated at the same location, in particular in the same plant, where step a) or b) or both steps take place.

[0026] The carbon dioxide separated in step a) is at least partially used to produce carbon dioxide, carbon monoxide, and carbon. If water is present in the gas, hydrogen can also be produced. Water is adsorbed by the sodium hydroxide. Water is particularly well adsorbed by solid sodium hydroxide or by a concentrated sodium hydroxide solution.

[0027] The process preferably uses air as a source of carbon dioxide and, if present, water. The air is preferably atmospheric or ambient air. The process thus preferably removes carbon dioxide and, if present, water from the air. In the first step, the gas comprising carbon dioxide is contacted with sodium hydroxide, preferably a sodium hydroxide solution, to absorb carbon dioxide and to form sodium carbonate, in particular sodium carbonate with water of crystallization, according to the following equation: 2 NaOH + CO2 + H2O → Na2CO3.H2O

[0028] However, the water does not have to be present in exactly this ratio; in variations, there may be more or less water.

[0029] In the second step, the sodium carbonate is reacted with zinc oxide, especially with an excess of zinc oxide, to release one or more of carbon dioxide, carbon monoxide, and, if water is present, hydrogen. Since the zinc oxide is used in the cycle, it can also be used in equimolar or submolar amounts.

[0030] Preferably, the gas is air, in particular atmospheric air, and wherein after step c) zinc oxide and sodium hydroxide are recovered, in particular completely recovered for reuse in step a), and wherein in particular the water is preferably extracted from the gas, in particular to a large extent.

[0031] Once initiated, the process can essentially be maintained without additional raw materials other than the supply of air and energy, as sodium hydroxide and zinc oxide are recovered. This creates a process that can be used particularly in hard-to-reach areas. The process can thus be maintained essentially self-sufficiently. This makes the process particularly cost-effective and logistically easy to manage.

[0032] It is known to those skilled in the art that exhaust gases and the like can also be used instead of atmospheric air.

[0033] In variants, the recovery of zinc oxide or sodium hydroxide can also be at least partially omitted, so that zinc oxide and / or sodium hydroxide must be added continuously to the process, even if this is not the preferred embodiment.

[0034] In step a), the gas comprising carbon dioxide is preferably brought into contact with concentrated sodium hydroxide solution, in particular with a sodium hydroxide solution comprising at least 10 mol / l sodium hydroxide, preferably between 12 and 19 mol / l sodium hydroxide.

[0035] The advantage of using concentrated sodium hydroxide solution is that the reaction proceeds more quickly, allowing the carbon dioxide to be absorbed more efficiently. The concentrated solution also has a hygroscopic effect, allowing water to be adsorbed more efficiently from the gas in the air.

[0036] In variants, the sodium hydroxide solution can also have a concentration of less than 10 mol / l.

[0037] In step a), the sodium carbonate, especially sodium carbonate with water of crystallization, is preferably precipitated, particularly due to an excess of its solubility. The sodium carbonate typically precipitates as a monohydrate. The sodium carbonate can be separated, for example, by filtration, particularly in a chambered pressure press or similar. The sodium carbonate can be stored in this form for any length of time until further use in the process. The advantage of this process lies in the relatively low amount of energy required to separate the sodium carbonate from the solution.

[0038] In some variants, the water can also be evaporated or vaporized instead of being precipitated. This can be achieved, for example, using solar thermal energy or process waste heat.

[0039] Preferably, the sodium carbonate containing the water of crystallization is dried between steps b) and c), particularly with water recovery. The sodium carbonate is preferably dried when sufficient thermal energy is available (e.g., solar thermal energy or process waste heat). Preferably, the water is recovered and reused in process step d). Water reuse can be particularly important in areas with low water supply and low rainfall. The dried sodium carbonate can also be stored temporarily for as long as necessary.

[0040] The dried sodium carbonate can also be stored temporarily for as long as necessary. The sodium carbonate can also be dried after the wet sodium carbonate has been temporarily stored—this can be useful, for example, if sufficient solar thermal energy or process heat is not available for the production of the wet sodium carbonate.

[0041] In some variants, drying of the sodium carbonate can be omitted. Drying can also take place in the subsequent step c) (see below). It is also not absolutely necessary to reclaim the water during the drying process.

[0042] For the reaction of sodium carbonate and zinc oxide in step c), the sodium carbonate is preferably melted, in particular together with zinc oxide, with a mass ratio of sodium carbonate to zinc oxide being greater than 4:1, preferably to reduce viscosity, and with zinc oxide being added to the melt together with sodium carbonate, in particular continuously. Carbon dioxide is released in this process step.

[0043] In a first variant, the sodium carbonate can be mixed with the zinc oxide and then heated, which converts the sodium carbonate into a melt. In another variant, the sodium carbonate can be melted and then mixed with zinc oxide. The melt preferably contains an excess of sodium carbonate, especially greater than 4:1, to keep the viscosity low.

[0044] Preferably, zinc oxide is fed into the melt from above along with fresh sodium carbonate, releasing carbon dioxide. The sodium zincate product is preferably removed from the bottom of the melt and cooled.

[0045] It may also be possible to forgo melting the sodium carbonate. The mixture can also be roasted, especially at a temperature below the melting point, to release the carbon dioxide.

[0046] Preferably, the reaction of sodium carbonate and zinc oxide in step c) takes place at a temperature higher than 600°C, in particular at a temperature between 700°C and 1000°C, more preferably between 800°C and 900°C. It is also preferred to react the sodium carbonate and zinc oxide at a temperature between 850 and 900°C. Both dried and moist sodium carbonate can be used. It has been shown that an optimal reaction rate relative to the energy input is achieved in this temperature range.

[0047] In some variants the temperature can be below 600 °C.

[0048] Preferably, the reaction of sodium carbonate and zinc oxide in step c) takes place in a rotary kiln. Rotary kilns have proven particularly suitable for this process step because of their ability to circulate solids particularly effectively.

[0049] However, other suitable devices for reacting sodium carbonate with zinc oxide are also known to those skilled in the art. In particular, a stirred tank may also be used.

[0050] In step c), one or more of the following reactions preferably take place: (i) carbon dioxide is produced by drying the sodium carbonate and heating it, in particular with zinc oxide, to a temperature above 850 °C; ii) carbon monoxide is produced by drying the sodium carbonate and, in particular, adding an excess of metallic zinc and preferably heating it to a temperature above 900 °C, or carbon monoxide is produced from carbon dioxide by bringing carbon dioxide from i) into contact with liquid or gaseous zinc; (iii) carbon dioxide and hydrogen are produced by adding an equimolar amount of zinc in proportion to an amount of water present and, in particular, by heating to a temperature below 900 °C; (iv) carbon monoxide and hydrogen are produced by adding an excess of metallic zinc to sodium carbonate and water, in particular to wet sodium carbonate, and preferably heating to a temperature above 900 °C; v) Carbon is produced by reducing CO2 or CO with metallic zinc, in particular with an iron catalyst.

[0051] Preferably, the sodium hydroxide and zinc circuit in step f)II) is separated from the sodium hydroxide and zinc circuit in the absorption circuit in a) in order to avoid the carryover of iron or carbon.

[0052] These process steps can optionally be performed in addition to the reaction of sodium carbonate with zinc oxide. The sodium carbonate produced in step a) can thus be fed to a first part of the reaction with zinc oxide in step c), while a second part can be fed to one or more of the above reactions. The result is a process in which one or more of carbon dioxide, carbon monoxide, carbon, and hydrogen can be produced variably and in any desired proportions. The process is thus particularly dynamic and can be adapted to current (market) needs.

[0053] The special common feature of the above reactions is that each of these reactions can be carried out using zinc or zinc oxide, and the zinc or zinc oxide can be recovered after the reaction has taken place.

[0054] Preferably, in step c), a sodium zincate, in particular Na2ZnO2, is obtained in addition to carbon dioxide, wherein in particular the sodium zincate is extracted from the melt.

[0055] The reaction preferably takes place according to the following reaction equation: Na2CO3 + ZnO → Na2ZnO2 + CO2↑

[0056] However, other possible reaction pathways are also known to the person skilled in the art.

[0057] Preferably, the reaction of sodium carbonate and zinc oxide is carried out with an excess of zinc oxide. Sodium carbonate is preferably mixed with zinc oxide powder, and additional zinc powder is preferably used as a separating agent. However, the zinc powder as a separating agent can be omitted. The sodium carbonate thus dispersed in zinc oxide is preferably melted, e.g., in a rotary kiln, thereby preferably forming spherical particles of sodium zincate coated with excess zinc oxide. After the subsequent process steps, the excess zinc oxide is separated off along with the zincate produced during the decomposition process.

[0058] Preferably, sodium zincate / zinc oxide particles, after optional mechanical separation of zinc oxide dust by sieving, are dissolved in hot water as sodium tetrahydroxidozincate and then preferably held at temperature for several hours to recover zinc oxide and sodium hydroxide. This recovers zinc oxide for the process step of reacting zinc oxide with sodium carbonate, as well as highly concentrated sodium hydroxide solution for absorbing carbon dioxide and water from the air. To achieve a high concentration of the sodium hydroxide solution, preferably more sodium zincate is added to the water than is dissolved (saturated solution with sediment). Solid zinc oxide is preferably separated from the concentrated sodium hydroxide solution, e.g., by filtration in a chamber filter press.

[0059] Preferably, the sodium zincate obtained in step c) is added to water, preferably at a temperature above 80°C, wherein at least a portion of the sodium zincate is hydrolyzed to sodium tetrahydroxide zincate and wherein the tetrahydroxide zincate is at least partially decomposed to sodium hydroxide solution and zinc oxide, wherein preferably solid zinc oxide is separated, in particular precipitated, and wherein in particular an amount of the added water is selected such that the resulting sodium hydroxide solution has a concentration between 12 mol / l and 19 mol / l, in particular for reuse in step a). Even more preferably, the resulting sodium hydroxide solution has a concentration between 17 and 19 mol / l. After decomposition, the solution is preferably cooled to reduce the solubility of zinc oxide, preferably until the amount of zinc oxide in the solution is below 10 wt%.

[0060] Preferably, the unreacted sodium carbonate is filtered off in this way together with the precipitated zinc oxide, dried, and reused in the step in which sodium carbonate is mixed with zinc oxide. The sodium carbonate still dissolved in the sodium hydroxide solution is returned to the absorber and is therefore not lost.

[0061] The water temperature can also be below 80 °C. Even more preferably, the tetrahydroxide zincate is completely decomposed into sodium hydroxide solution and zinc oxide. However, due to the equilibrium reaction, this is usually hardly possible directly. Therefore, the zinc oxide is preferably removed during the process to shift the equilibrium toward decomposition. This can be done continuously or discontinuously.

[0062] By selecting the amount of water, the sodium hydroxide solution obtained by decomposition of the sodium zincate can be used directly in step a) of the process to absorb carbon dioxide from a gas, in particular from the air.

[0063] However, the person skilled in the art is also aware of other possible reaction pathways capable of releasing sodium hydroxide from sodium zincate.

[0064] Preferably, the water is obtained by distilling water of crystallization from the sodium carbonate of step a) and / or by drying the sodium carbonate.

[0065] The water preferably comes from the respective gas in the air or was obtained during the drying of the sodium carbonate. This makes the process largely independent of water sources. However, in variants, the water can also come from other sources.

[0066] Preferably, metallic zinc is extracted from the remaining tetrahydroxide zincate in the sodium hydroxide solution by electroplating. This significantly reduces the zinc content of the sodium hydroxide solution, so that the sodium hydroxide solution, when subsequently used in the absorber, deposits sodium carbonate with little or no zinc oxide, preventing zinc oxide deposits in the absorber and increasing the solution's capacity for CO2 capture. Preferably, the separation takes place according to the following reaction equation: 4 OH - -4e - → O2 + 2 H2O anode: 2 Zn 2+ + 4e - → 2 Zn cathode:

[0067] The energy required for this is preferably generated by photovoltaics. However, experts are aware that other energy sources can also be used. Electroplating zinc is particularly advantageous because elemental zinc can be used as a reducing agent (see below).

[0068] However, galvanic reduction to zinc can also be omitted, especially in cases where neither carbon monoxide, carbon, nor hydrogen are produced. This eliminates the need to produce metallic zinc, which in the present process is used exclusively as a reducing agent for the production of one or more of carbon monoxide, carbon, and hydrogen. This eliminates the need for galvanization.

[0069] Preferably, the sodium hydroxide solution remaining after electroplating is reused in step a). This makes the process largely independent of material suppliers. This can be omitted in some variants of the invention.

[0070] Preferably, the metallic zinc is at least partially reacted in one or more of the following processes: (i) metallic zinc is brought into contact with water, in particular with water from the air, and converted into zinc oxide and hydrogen; ii) metallic zinc is heated to a temperature of at least 400 °C, in particular to a temperature between 600 °C and 1 000 °C, and reacted with carbon dioxide, in particular with carbon dioxide from step c), to form zinc oxide and carbon monoxide; iii) metallic zinc is reacted with carbon dioxide and / or carbon monoxide in the presence of a catalyst, in particular iron oxides, to form zinc oxide and carbon; (iv) metallic zinc is reacted with oxygen to form zinc oxide, in particular with oxygen obtained from the electroplating process, the energy released by the reaction being converted into electrical energy or thermal energy.

[0071] Electroplating zinc is particularly advantageous because elemental zinc can be used as a reducing agent according to the following reaction equations: Zn + 2 NaOH + 2 H2O → Na2Zn(OH)4 + H2↑

[0072] This reaction uses zinc as a reducing agent to produce hydrogen. Sodium tetrahydroxidozincate, a byproduct of hydrogen production, can in turn be used in electroplating. Zn + CO2 → ZnO + CO↑

[0073] In this reaction, zinc is used as a reducing agent to produce carbon monoxide. Zinc oxide as a by-product can be reused in step c) of the process according to the invention. In a particularly preferred embodiment, carbon monoxide or a mixture of carbon dioxide and carbon monoxide can be produced simultaneously in step c) by reacting the sodium carbonate with zinc oxide to release carbon dioxide, the process being modified by the addition of metallic zinc to the zinc oxide and sodium carbonate. Initially bound free carbon dioxide is immediately reduced by metallic zinc to carbon monoxide in an amount corresponding to the added metallic zinc. Zn + CO → ZnO + C

[0074] This reaction allows the carbon monoxide to be further reduced to carbon. Zinc oxide can again be reused as a byproduct in step c) of the process according to the invention.

[0075] All of the above reactions have the advantage that no substances are consumed, but can be recycled back into the process. The individual products, such as carbon dioxide, carbon monoxide, coal, and hydrogen, can be produced in different ratios as needed.

[0076] Additionally, zinc can be used as a central reagent for all the above reactions, which is inexpensive, non-toxic, readily available and, last but not least, can be recycled after each reaction.

[0077] However, none of the above reactions need to be performed. In some variants, only carbon dioxide can be produced.

[0078] Preferably, all gases and water used in the process are extracted from the air, in particular from atmospheric air, and in particular intermediate gases and water are recycled in the process.

[0079] This results in a process for producing one or more of carbon dioxide, carbon monoxide, carbon, and hydrogen that is particularly independent of material suppliers. All materials can preferably be recycled and reused in the process.

[0080] The invention further relates to a device for carrying out a method as described above for providing one or more of carbon dioxide, carbon monoxide, carbon, and hydrogen from a gas comprising carbon dioxide and preferably water, in particular from atmospheric air. The device comprises an absorber configured to carry out step a) and a decomposition unit configured to carry out step c).

[0081] There are several ways to bring the gas into contact with the sodium hydroxide. Basically, a device for conveying the gas, in particular the ambient air, is provided, which guides the gas to the sodium hydroxide where the reaction takes place. In a first variant, the gas can be blown into a sodium hydroxide solution, in particular below the surface. In a second variant, the sodium hydroxide solution can be circulated in the presence of the gas in order to achieve an efficient conversion to sodium carbonate. In a preferred embodiment, a spray device is provided which sprays the sodium hydroxide solution in the presence of the gas. Such spray reactors are generally known to those skilled in the art. These have the advantage that the sodium hydroxide solution in the spray mist has a very large surface area, with which the carbon dioxide can be efficiently absorbed.Other devices are also known to those skilled in the art by which carbon dioxide can be extracted from the gas using sodium hydroxide solution.

[0082] The decomposition unit preferably comprises a heating device with which the mixture of zinc oxide and sodium carbonate can be heated to release the carbon dioxide. Many possibilities are also known to those skilled in the art.

[0083] Particularly preferably, the device comprises an arrangement in which the absorber is arranged above the decomposition unit. This allows the sodium hydroxide solution to be sprayed in the presence of the gas in the spray reactor and collected at the lower end of the spray reactor. The solution, which now contains sodium carbonate, can then be directly processed further, for example, by precipitating the sodium carbonate and then reacting it with the zinc oxide, preferably in a furnace, to release the carbon dioxide. This arrangement results in a particularly compact system.

[0084] In variants, the absorber can also be arranged next to the decomposition unit. Other arrangements of the device are also known to those skilled in the art.

[0085] Preferably, the absorber and / or decomposition unit are designed as a roof-mounted system. This design allows the device to be installed in populated areas or industrial zones where particularly high levels of carbon dioxide are produced. In this case, no subsoil is required for the system.

[0086] The expert is aware that the device does not necessarily have to be designed as a roof-mounted system, but can also be installed in other ways. In particular, the system may be so large that a building's floor area must be built to accommodate a particularly large system.

[0087] Preferably, the absorber and / or decomposition unit comprises one or more absorber modules and / or decomposition unit modules. This has the advantage that the device can be flexibly and cost-effectively adapted to local requirements.

[0088] In some variants, the modular structure can also be omitted.

[0089] In a particular embodiment, the absorber unit, in particular a module of the absorber unit, can comprise a frame that is encased to obtain a cylindrical module that can be permanently stacked high. An upper module can be equipped with a spray device with which the sodium hydroxide solution is sprayed to absorb carbon dioxide from the air. Other ways of constructing an absorber unit in a modular and cost-effective manner are also known to those skilled in the art. For example, the sodium hydroxide solution can be sprinkled over filler material, in particular packing, to increase the contact area between the air and the sodium hydroxide solution.

[0090] The decomposition unit preferably comprises a rotary kiln. A rotary kiln is a cylindrical furnace-like structure that rotates around its axis. It is commonly used in various industrial processes such as cement production, mineral calcination, and thermal treatment of materials. Within the kiln, materials are fed into one end and gradually move toward the other end as the kiln rotates.

[0091] Rotary kilns are particularly suitable for drying the sodium carbonate and roasting it together with the zinc oxide. The rotary kiln also has the advantage that the process can be carried out continuously. This continuous operation increases productivity and efficiency in the processes. Rotary kilns can reach and maintain high temperatures, making them particularly preferred for the process according to the invention. Furthermore, the rotary motion of the kiln ensures even heat distribution and good mixing throughout the material being processed, resulting in efficient and therefore economical energy consumption.

[0092] The residence time of the materials within the rotary kiln can be controlled by adjusting the rotation speed and angle of the kiln, which allows precise control of the process parameters, which in turn leads to efficient and economical operation of the process.

[0093] Rotary kilns have a relatively compact footprint compared to other types of industrial reactors, making them suitable for installation in space-limited industrial facilities, especially on a rooftop.

[0094] Overall, rotary kilns offer numerous advantages in terms of versatility, continuous operation, temperature capability, heat distribution, residence time control, feed material handling, emission reduction, and footprint size, making them a preferred choice for the present process.

[0095] However, other suitable decomposition systems are also known to experts. For example, shaft kilns are a conventional technology used for thermal decomposition. These are vertical kilns in which raw materials are fed from the top and move downward through the kiln. Shaft kilns can operate in various modes, such as wet or dry processes.

[0096] Overall, a process is thus provided for producing synthesis gases from carbon dioxide and water, wherein carbon dioxide and water are preferably obtained from the air. The synthesis gases can be used in a known manner to produce various substances, in particular hydrocarbons and / or alcohols, which can be used as energy sources or raw materials. These substances can be produced with a neutral or even negative CO2 balance.

[0097] The process preferably provides a process for the direct production of highly concentrated sodium hydroxide solution as an absorbent in the circuit.

[0098] The water for the synthesis is preferably obtained at least partially, and in a particularly preferred embodiment even or almost completely, from the air.

[0099] Preferably, renewable energy (e.g. photovoltaics, wind energy, solar thermal energy, etc.) is used partially, particularly preferably exclusively, in the process.

[0100] Preferably, the electrochemical processes are carried out without membranes. This has the advantage of reducing costs, improving durability, and reducing maintenance. The process is therefore independent of the availability of membranes. Galvanic zinc deposition is preferred over water electrolysis.

[0101] The produced gases CO2, O2, CO, and H2 can be produced in separate (spatially separated) process steps or in defined mixtures for subsequent syntheses. In particular, elemental zinc can be added during the reaction between zinc oxide and sodium carbonate to produce carbon monoxide and carbon dioxide. The ratio between carbon dioxide and carbon monoxide can be arbitrarily controlled by adding zinc—in extreme cases, only carbon monoxide can be produced. If the mixture of zinc oxide, zinc, and sodium carbonate still contains water, hydrogen can be produced in the same process. The process is therefore particularly versatile, using a small number of inexpensive reagents.

[0102] The separate process steps for the production of carbon monoxide and carbon by reduction with elemental zinc and the reduction of water with zinc make it possible to produce the gases CO2, O2, CO and H2 separately in time and space.

[0103] Due to their stability, intermediates can be stored for any length of time with minimal effort, enabling flexible process control depending on the changing availability of energy (renewable energy sources) and water: sodium carbonate, sodium zincate, zinc oxide, zinc, water, and caustic soda can be easily stored for any length of time. The time-consuming intermediate storage of gases under pressure can be reduced to a minimum. Gases for subsequent syntheses of alcohols or hydrocarbons are preferably produced directly on demand using the described process, largely eliminating the need for gas storage.

[0104] Sodium and zinc are preferably in a closed cycle so that these elements do not need to be replenished once the process has started.

[0105] Non-recyclable waste heat that can be generated in the individual processes is preferably used to heat the absorption solution and the absorber.

[0106] Further advantageous embodiments and combinations of features emerge from the following detailed description and the entirety of the claims.

[0107] The drawings used to explain the embodiments show: Fig. 1a a schematic representation of the entire reaction steps of the process; Fig. 1b a schematic representation of the reaction steps concerning the carbon dioxide pathway; Fig. 1c a schematic representation of the reaction steps concerning the carbon monoxide pathway; Fig. 1d a schematic representation of the reaction steps concerning the carbon pathway; Fig. 1e a schematic representation of the reaction steps concerning the hydrogen pathway; and Fig. 2 a schematic representation of an exemplary device for carrying out the method.

[0108] In the figures, identical components are provided with identical reference symbols.

[0109] Fig. Figure 1 shows a reaction diagram illustrating the individual reaction steps for the production of carbon dioxide, carbon monoxide, coal, hydrogen and oxygen.

[0110] The process begins with step 1: Water and carbon dioxide from the air are absorbed in a sodium hydroxide solution. The sodium hydroxide solution has a concentration of 40-50 wt.% and is preheated with process waste heat. The sodium hydroxide solution is distributed over tower packing to increase the contact area with the air. The carbon dioxide reacts with the sodium hydroxide solution to form sodium carbonate. The absorption solution is circulated to continuously increase the sodium carbonate concentration and also absorb more and more water.

[0111] In this case, absorption can also be achieved using a spray reactor, in which the sodium hydroxide solution is sprayed vertically above a spray device, while the air is passed countercurrently from bottom to top through the spray reactor. Spraying the sodium hydroxide solution creates a particularly large surface area over which the carbon dioxide can be absorbed, forming sodium carbonate and water. In an alternative variant, the sodium carbonate is formed within a sodium hydroxide solution by blowing air into the sodium hydroxide solution. This first step, represented by step 1, is a DAC process (Direct Air Capture); such processes are known per se. The sodium carbonate is formed according to the following reaction equation: 2 NaOH + CO2 → Na2CO3.H2O

[0112] In a preferred embodiment, the preferred concentration of the sodium hydroxide solution is 17-19 mol / l at the inlet from plant section 7 and from plant section 2; due to water absorption from the air and precipitation of sodium carbonate, the concentration of the sodium hydroxide solution in the absorber drops to 12-15 mol / l according to the invention. It is clear that the sodium hydroxide solution or sodium hydroxide from step 7 is therefore added to the absorption step of step 1 to keep the concentration high. The sodium hydroxide, as well as the other substances except for the carbon dioxide and water obtained from the air, are recirculated, i.e., treated and reused in the process. It is clear that the process must first be started by adding sodium hydroxide (as well as zinc oxide, see step 4 below).

[0113] After the absorption step, an aqueous solution containing sodium carbonate and unused sodium hydroxide is obtained.

[0114] In step 2, the solution, which comprises sodium carbonate and unused sodium hydroxide, is fed into a crystallization vessel and heated with process heat. The evaporating or vaporizing water is preferably collected from the gas phase and condensed for further use.

[0115] In this process, sodium carbonate monohydrate (Na2CO3·H2O) precipitates. The precipitate is separated, e.g., by filtration in a chamber filter press or similar. The sodium carbonate monohydrate can be stored, for example, to bridge periods when insufficient energy is available for further process steps.

[0116] The remaining solution, depleted of sodium carbonate and water, is preferably recycled to step 1, the absorption step. Alternatively, a portion of the remaining solution can be mixed with metallic zinc for hydrogen recovery (see step 8 below).

[0117] If sufficient thermal energy is available (solar thermal energy or process waste heat), the sodium carbonate can be dried in step 3. Here, too, the water of crystallization and / or residual moisture is recovered and reused in subsequent steps (see below). The anhydrous sodium carbonate can also be stored for later processing. This step can also be skipped if necessary.

[0118] In the next step, the dried sodium carbonate from step 3 or directly the moist sodium carbonate from step 2 (see below) is mixed with zinc oxide to release carbon dioxide at a temperature of 800°C to 900°C. There are basically two ways to do this. In a first variant, the sodium carbonate is reacted with the zinc oxide as a melt to release carbon dioxide. In a second variant, the sodium carbonate is roasted together with zinc oxide, particularly in a rotary kiln, to release the carbon dioxide. In this step, sodium zincate is formed according to the following reaction equation: Na2CO3 + ZnO → Na2ZnO2 + CO2↑

[0119] In the process, a mixture of 2-5 parts fine zinc oxide powder and 1 part coarse-grained sodium carbonate is preferably placed in the upper part of a rotary kiln. The kiln is preferably heated to approximately 900 °C with constant rotation, melting the sodium carbonate. Due to the rotation, the melting particles become spheres coated with zinc oxide. Zinc oxide dissolves within the spheres, while carbon dioxide is released. The product of the roasting process preferably leaves the rotary kiln as particles consisting of sodium zincate (Na2ZnO2) coated with zinc oxide. Furthermore, additional excess zinc oxide powder is preferably present in the kiln, which acts as a release agent to prevent molten sodium carbonate from adhering to the kiln. Scrapers or similar mechanical devices in the kiln also ensure that no deposits occur.The mixture of spheres and powder is preferably sieved in a first step, and the powder is preferably returned to the roasting process with fresh sodium carbonate. Preferably, the larger molten particles are processed in the subsequent step 5 (see below).

[0120] The carbon dioxide can be used directly or reduced to carbon monoxide or carbon in further reaction steps with metallic zinc (see below).

[0121] In step 5, the sodium zincate is dissolved in hot water according to the following reaction equation: Na2ZnO2 + 2 H2O → Na2Zn(OH)4

[0122] Sodium tetrahydroxidozincate is formed by hydrolysis.

[0123] The spheres, consisting of sodium zincate and zinc oxide, are dissolved in hot water, where the spheres disintegrate, the zincate hydrolyzes, and dissolves as sodium tetrahydroxide zincate. Zinc oxide also partially dissolves, but immediately separates again due to supersaturation. Dissolution is assisted by mechanical devices such as stirrers or in drums. The entire suspension of sodium hydroxide solution with dissolved sodium tetrahydroxide zincate and solid zinc oxide is then fed to the subsequent step 6.

[0124] In step 6, the supersaturated solution is kept just below boiling temperature for a few hours, during which most of the sodium tetrahydroxide zincate decomposes into sodium hydroxide solution and zinc oxide powder: Na2Zn(OH)4 → ZnO + 2 NaOH

[0125] The conversion to zinc oxide can be further increased by slow cooling to room temperature. With or without a cooling phase, depending on whether there is a higher demand for zinc oxide or whether a higher content of sodium tetrahydroxide zincate is preferred for the subsequent electroplating of zinc, the zinc oxide is separated by filtration, preferably in a chamber filter press, or by centrifugation in a continuous-flow centrifuge. Sodium carbonate not converted in the roasting process partially precipitates along with the zinc oxide. After drying with water recovery, the resulting zinc oxide is reused in the roasting process in step 4.The concentration of the sodium hydroxide solution produced during the hydrolysis of sodium zincate is 10-20 mol / l, preferably 17-19 mol / l, the sodium hydroxide solution is saturated with sodium tetrahydroxidozincate, the absolute zinc concentration in the sodium hydroxide solution is adjusted by the temperature during equilibration before the separation of the ZnO.

[0126] Caustic soda used in this process typically also contains sodium carbonate, which does not harm any of the subsequent steps. Sodium carbonate-saturated caustic soda serves as the absorbent, the substrate for electrodeposition, and the substrate for hydrogen production by adding zinc metal to the water-enriched absorbent. Even if the concentration could be reduced by cooled crystallization, this is not necessary to successfully carry out the inventive recycling process.

[0127] The remaining concentrated sodium hydroxide solution, saturated with sodium tetrahydroxide zincate, is used in step 7 for the electrodeposition of metallic zinc. This occurs according to the following redox equations: 4 OH - -4e - → O2 + 2 H2O anode: 2 Zn 2+ + 4e - → 2 Zn cathode:

[0128] In addition to metallic zinc, oxygen is also produced. Since there are no requirements regarding the structure or shape of the zinc deposit at the cathode, as the zinc formed is then completely decomposed or melted through reactions, the solution from system step 6 can be used directly without any further additives. The higher the temperature, the more easily oxygen evolves at the anode, which is why a working temperature just below the boiling point is preferred.

[0129] To achieve the greatest possible deposition of dissolved zinc, three-dimensional electrodes in the form of fillings, wire mesh, three-dimensional structures, etc. are preferably used. The cathode material can consist of zinc itself or another conductive material with a higher melting point than zinc, preferably steel or stainless steel. The zinc-loaded electrodes are either used directly in the process, e.g., by reacting them with water-laden sodium hydroxide absorbent from the absorber to produce hydrogen, or, preferably, the resulting zinc (melting point 419.5 °C) is extracted from the cathodes by melting.

[0130] The caustic soda solution remaining after zinc electroplating is highly concentrated and low in zinc and is reused as the absorption solution in step 1. Due to its high concentration, this increases the rate of water absorption from the atmosphere. This is achieved by lowering the vapor pressure above the absorption solution.

[0131] The oxygen can in turn be used to generate electrical or thermal energy by oxidizing metallic zinc - the metallic zinc and oxygen therefore form an energy storage medium that can be used, for example, for night-time operation.

[0132] In a preferred embodiment, the anodes for zinc electroplating are separated in open-bottom tubes that capture rising oxygen generated at the anode to prevent oxygen from mixing with hydrogen, which is developed as a byproduct during electroplating. The electroplating bath is a closed unit in which hydrogen is collected. This way, hydrogen does not represent an energy loss but rather a useful reagent or product.

[0133] The metallic zinc can be used for one or more of the reduction reactions described below: - At least a portion of the metallic zinc may be used in step 10 to reduce carbon dioxide to carbon monoxide in a separate plant section according to the following reaction equation: Zn + CO2 → ZnO + CO↑

[0134] The resulting zinc oxide is again used in step 4 to release carbon dioxide from sodium carbonate.

[0135] Reduction with gaseous zinc proceeds gently at atmospheric pressure. The higher the temperature, the faster the conversion. In a preferred design, a rotary tube reactor is used, half of which is charged with carbon dioxide and half with liquid zinc. The lower half of the tube reactor is heated with a jacket heater, so that gas temperatures of preferably 900–950°C are generated in the lower third of the reactor (zinc boiling point 907°C). The actual reaction with the deposition of zinc oxide takes place only in the quarter below the half of the tube reactor. The upper part of the tube reactor is cooled, so that liquid zinc dominates in the middle section. Packing elements with a large surface area are preferably introduced into the middle section, which facilitates the condensation of liquid zinc. To achieve complete deposition of the zinc, the upper section must be cooled to approximately 100°C.The metallic zinc accumulates on the surfaces and must be melted at regular intervals by heating the entire reactor. No gas is transported through the reactor during the melting process. In the lowest quarter, the reaction product, zinc oxide, is separated from the liquid zinc by evaporation. The zinc oxide is continuously transported downward and expelled into a gas-tight container, where it cools or is immediately fed hot into plant step 4. - At least a portion of the metallic zinc can be used in step 9 to reduce carbon monoxide (and / or carbon dioxide, not shown) to carbon according to the following reaction equation: Zn + CO → ZnO + C

[0136] The direct conversion of carbon dioxide to solid carbon is preferably carried out in the gas phase by reduction with zinc using catalysts, particularly iron oxide. In a preferred embodiment, metallic zinc in liquid form is heated to a temperature of 900–1000°C in the center of a top-closed rotary kiln to generate reactive zinc vapor. At the same level, carbon dioxide containing fine iron oxide particles is blown upwards. In a first step, zinc oxide and carbon monoxide are produced. The carbon monoxide further reacts with iron oxide in the presence of zinc vapor as a reducing agent to form elemental carbon. This reaction takes place in the gas phase above the inlet on the iron oxide catalyst particles. The iron oxide particles, together with the resulting zinc oxide and the carbon formed, fall onto the tube wall, where they are transported downwards.Excess metallic zinc evaporates from the bottom of the tube and returns to the top as a gas. Reaction completeness is achieved by pumping only gas into the reaction chamber, but preventing any gaseous product from leaving the reaction chamber. Only solids are discharged, meaning gases remain in the reaction chamber until they are completely converted to solids. Increasing the pressure in the furnace is one way to increase the reaction rate. The resulting mixture of zinc oxide, carbon, and iron oxide is worked up by dissolving it in caustic soda, which dissolves the zinc oxide and separates the iron oxide and carbon as solids. The separation of iron oxide and carbon is based on density—lighter carbon floats in water, while iron oxide sinks.Iron oxide residues adhering to the carbon can be dissolved with acid if they interfere with the use of the carbon products. Alternatively, simple closed reactors can also be used in batch operation, where zinc and carbon dioxide are introduced with iron oxide particles, also in the center of the reactor. The solid products simply settle at the bottom of the reactor and are removed after a defined runtime. It is also possible to feed this plant with mixtures of carbon dioxide and carbon monoxide, or preferably only with carbon monoxide (which was produced in plant section 8). This variant has the advantage that only half the amount of zinc oxide is produced in this process step, which facilitates the processing of the resulting solid mixture.The zinc dust produced in plant section 8 is pure zinc oxide, which does not require any processing, while the zinc oxide produced in this step 9 must be separated from iron oxide and carbon. - At least part of the metallic zinc can be used in step 8 to reduce water to hydrogen according to the following reaction equation: Zn + 2 NaOH + 2 H2O → Na2Zn(OH)4 + H2↑

[0137] The reaction of sodium hydroxide solution with zinc is a well-known process. Zinc powder, particles, plates, cathodes, etc., are placed in sodium hydroxide solution, and hydrogen gas is spontaneously released. The special feature of this design is that the reaction is carried out with an absorption solution, whereby water absorbed from the atmosphere reacts directly with zinc, dehydrating the sodium hydroxide solution. The product is hydrogen gas. The second product is sodium tetrahydroxide zincate, which dissolves in the dehydrated sodium hydroxide solution. If the process is carried out with larger amounts of metallic zinc, sodium tetrahydroxide zincate decomposes to zinc oxide and caustic soda.The same reaction takes place in plant section 6, so that in a complete plant, the resulting solution can be fed into the circuit in step 6 when more zinc oxide is needed, or fed directly into the zinc electroplating in step 7 to produce metallic zinc again as quickly as possible.

[0138] In a particularly preferred embodiment, hydrogen evolution and electroplating are carried out at different times in the same reactor. When no current is applied, the zinc coating of the cathodes dissolves and hydrogen is released. The flow direction is from the crystallization system section 2 into a tank that temporarily stores the sodium tetrahydroxide zincate / NaOH solution. After a large portion of the zinc has been consumed, the flow direction is reversed so that the solution from the intermediate tank passes through the electroplating system into the absorber 1. Current is applied, causing zinc to redeposit and oxygen to be released.

[0139] The sodium hydroxide solution required for this can be obtained from step 2 (sodium carbonate-depleted absorption solution). Water previously absorbed from the air in the absorber is consumed in the process. The resulting sodium tetrahydroxide zincate in sodium hydroxide solution can be fed to step 6 for the decomposition of the sodium tetrahydroxide zincate to zinc oxide and sodium hydroxide, or directly to the zinc electrodeposition process in step 7.

[0140] From steps 8-10 above, it is evident that metallic zinc can be used in a variety of ways, allowing the production of various products from carbon dioxide or water. The unique feature of this process is that the resulting byproducts (particularly zinc oxide and sodium tetrahydroxide zincate) can be reused in steps 4 and 7, respectively. It is also noteworthy that the products carbon dioxide, carbon monoxide, carbon, and hydrogen can be produced in any desired ratio, making the process particularly easy to adapt to current market requirements.

[0141] Although all steps 8 - 10 are optional, it should be noted that step 8 (reduction of water to hydrogen) is feasible but not preferred due to the high effort required, particularly the need for platinum group metals and membranes.

[0142] However, hydrogen is needed to produce alcohols or hydrocarbons from carbon monoxide. This, in turn, requires water. Water is absorbed in concentrated sodium hydroxide solution along with carbon dioxide from the atmosphere. Some of this water precipitates as water of crystallization in sodium carbonate crystals and is recovered as condensate during the drying process in step 3. When a highly concentrated sodium hydroxide solution is used, the resulting product is predominantly sodium carbonate monohydrate. This means that one mole of water per mole of sodium hydroxide can be obtained from the drying process in step 3. Additional water can be distilled directly from the absorption solution of step 2. Preferably, sodium carbonate is precipitated in step 2 by heating the absorption solution with waste heat or solar heat.During this step, the absorption solution is at its warmest, and water removal promotes and completes the precipitation of sodium carbonate. Therefore, it is preferable to recover water from the absorption solution by distillation during precipitation.

[0143] Another way to obtain water from the absorption solution is to perform classical alkaline electrolysis to obtain hydrogen with the absorber solution. The disadvantage of classical electrolysis compared to hydrogen production with zinc is that hydrogen and oxygen are produced simultaneously and in close proximity. This requires membranes, or at least diaphragms, to prevent the formation of explosive mixtures of oxygen and hydrogen gas and to prevent oxygen from entering the hydrogen product stream. In the preferred zinc process according to the invention, oxygen is formed during electrodeposition at the anode, while hydrogen is formed only during the reaction of metallic zinc with caustic soda, which is completely separated in time and space.This process can therefore be carried out in simple, even very large, reactors on an industrial scale and without membranes. At the same time, oxygen-free hydrogen is guaranteed, providing optimal conditions for further syntheses of hydrocarbons and alcohols without further purification steps. Nevertheless, alkaline electrolysis is also a viable option for obtaining hydrogen directly from the absorption solution, not least because it is a proven standard process.

[0144] In this process, therefore, there are several ways to utilize the water from the air bound in the sodium hydroxide solution in the absorber. The distillation of the crystallization water from sodium carbonate and the distillation of water from the absorption solution in the crystallization vessel yield liquid water, which can be used, for example, to dissolve sodium zincate. The other two routes directly yield hydrogen gas, which can be used for the subsequent synthesis of alcohols or hydrocarbons. Overall, even in desert locations, a sufficient water supply for the entire process is made possible solely by absorbing the atmospheric moisture in the caustic soda solution. However, if water is available at the site at low cost and in sufficient quantities (e.g., rainwater), the use of these external sources as partial or sole water supply can lead to energy savings and thus cost savings at some locations.In addition to costs, the impacts of water extraction on the environment must also be examined to avoid collateral damage.

[0145] In a further variant, carbon dioxide can be released in step 4 and simultaneously reduced to carbon monoxide in a combined process (not shown in step 4). A mixture of zinc oxide and metallic zinc is used for this purpose. Part of the metallic zinc can come from the sodium hydroxide solution obtained from step 7. In addition to the above reaction equation, in which sodium carbonate is reacted with zinc oxide to form sodium zincate and carbon dioxide, the carbon dioxide is further reacted with the metallic zinc to form carbon monoxide and zinc oxide: Zn + CO2 → ZnO + CO↑

[0146] The resulting zinc oxide can in turn be reacted with sodium carbonate to form sodium zincate and carbon dioxide.

[0147] The entire process can take place in the same reactor, allowing carbon dioxide and carbon monoxide to be produced simultaneously. The ratio of carbon monoxide to carbon dioxide can be controlled by the amount of metallic zinc added during the roasting process or in the melt. If metallic zinc is used in excess, almost complete conversion to carbon monoxide is possible in a single step.

[0148] In this case, the plant is operated so that the gas phase has a temperature above 900 °C, preferably between 900 and 1000 °C. Zinc is gaseous at these temperatures and reacts particularly efficiently with the released gaseous CO2. Zinc oxide is produced as a dust and bound by the addition of fresh sodium carbonate. Zinc contained in the product gas is separated by cooling and returned to the process.

[0149] In another preferred embodiment of the process, the moist sodium carbonate from step 2 is used directly in step 4. The moist sodium carbonate is mixed with metallic zinc and zinc oxide and subjected to the roasting process. The drying process in step 3 is thus omitted or only partially carried out, so that the sodium carbonate is not completely dried. In other variants, additional water can also be added to influence the product ratios.

[0150] According to the following reaction equation, zinc forms hydrogen with water in the gas phase of the rotary kiln, which precipitates zinc oxide powder: Zn + H2O → ZnO + H2↑

[0151] In this way, the composition of the released gas is defined by the ratio of sodium carbonate to water to zinc in order to provide an optimal composition of the synthesis gas for subsequent reactions. Zinc oxide is always maintained in excess. Mixtures of H2 and CO in any ratio are directly accessible via this process variant and thus form a classic synthesis gas. Mixtures that also contain CO2 are possible by adding small amounts of metallic zinc. If excess water is used, drying of the gas may be necessary due to incomplete conversion and depending on the intended use of the synthesis gas. In any case, the use of zinc as a reducing agent instead of the classic reduction with hydrogen gas massively reduces the need for drying.

[0152] In practice, the process is carried out in a rotary kiln; the solids sodium carbonate, zinc oxide, and zincate are fed downward, preferably in countercurrent, to the resulting product gas. Continuous charging of the furnace with zinc and sodium carbonate occurs approximately halfway through, drawing the zinc vapor upward with the carbon dioxide generated in the lower half, reacting to form carbon monoxide and depositing zinc oxide along the way. This zinc oxide is transported downward in the rotary kiln and mixed with the added sodium carbonate, transporting them further downward while slowly converting to sodium zincate with the release of carbon dioxide. When the ratio of sodium carbonate to metallic zinc is at least equimolar, excess zinc oxide can be added to optimize transport and prevent sticking, caking, etc., without affecting the product gas.The product gas is carbon monoxide or a mixture of carbon monoxide and carbon dioxide, or a mixture of carbon monoxide and hydrogen when using wet sodium carbonate, or carbon monoxide, carbon dioxide, and hydrogen when using wet sodium carbonate and less metallic zinc than would be required for all reduction processes combined. The product gas is extracted at the top of the rotary kiln. When conducting the process with metallic zinc, it must be taken into account that zinc is very mobile in the gas phase; cooling and separation devices for zinc must be provided at the kiln outlet. Optimum furnace temperatures in the lower part are 900 °C, while slightly higher temperatures of 950–1000 °C lead to good conversion in the upper part. Alternatively, conversion in a stationary furnace with continuous or discontinuous charging and discharging is possible.

[0153] The addition of additional alkali or alkaline earth carbonates to lower the melting point and thus reduce the viscosity of the melt is possible. In particular, the use of potassium carbonate and / or lithium carbonate. These variants are not favored for cost reasons, although their technical feasibility is clearly stated at this point. In special cases, e.g. for salt mixtures produced by processing brine from freshwater extraction into an absorption solution and therefore naturally containing elements that lower the melting point, this variant can be considered. As a rule, the addition of elements that lower the melting point is not necessary, since it is almost inevitable that the added elements will occur throughout the circuit in all parts of the system, especially in the absorber, where large quantities are required.

[0154] The following Fig. 1b to Fig. 1e show parts of the overall reaction scheme, each focusing on the preparation of a substance and the reaction steps not relevant for this process b.

[0155] Fig. Figure 1b shows a schematic representation of the reaction steps related to the carbon dioxide pathway. Steps 1 to 6 are sufficient for the production of carbon dioxide. In particular, this process eliminates the need for galvanic zinc deposition.

[0156] In the following process steps, carbon monoxide and carbon, zinc is oxidized to zinc oxide, which can be returned to the process in step 4 and thus remains in the cycle.

[0157] Fig. Figure 1c shows a schematic representation of the reaction steps related to the carbon monoxide pathway. In addition to the removal of carbon dioxide, the carbon dioxide must be reduced to carbon monoxide using zinc as the reducing agent, which is produced electrolytically in step 7. The reduction occurs in step 10, meaning that steps 8 and 9 can be omitted.

[0158] Fig. Figure 1d shows a schematic representation of the reaction steps related to the carbon pathway. To further reduce carbon monoxide to carbon, metallic zinc from step 7 is used together with an iron catalyst in step 9. This means that only step 8 is omitted.

[0159] Fig. Figure 1e shows a schematic representation of the reaction steps related to the hydrogen pathway. Water from step 2 is reduced using zinc as the reducing agent. The resulting sodium tetrahydroxide zincate can be recycled back into the process in step 7, thus remaining in the cycle. In the hydrogen pathway, the roasting process of step 4 still produces CO2, which can be further processed, e.g., to produce methane or methanol by catalytic reaction with the produced hydrogen.

[0160] Fig.Figure 2 shows a schematic representation of a rooftop arrangement of such a system. The system 101 is arranged on the roof of a high-rise building 100. The system comprises a modular absorber unit—in this case, this comprises three absorber modules 102. In the absorber unit, carbon dioxide and water from the ambient air are absorbed with a sodium hydroxide solution, forming an absorption solution containing sodium carbonate. The absorber unit is arranged on a crystallization unit 103 so that the absorption solution can be fed directly to the crystallization unit 103 by gravity. The sodium carbonate is precipitated from the absorption solution in the crystallization unit 103. In addition to the crystallization unit 103, a rotary kiln 104 is also schematically shown, in which the sodium carbonate is heated with zinc oxide to release carbon dioxide.The system 101 further comprises solar thermal or photovoltaic elements 105 for supplying energy to the system 101.

[0161] In summary, it should be noted that the invention provides a method in which a large number of substances can be prepared from the air using particularly few reagents, whereby all reagents can be used in the circuit and the consumables are obtained exclusively from the air. List of reference symbols 1-10 process steps 100 buildings 101 Systems 102 absorber modules 103 Crystallization unit 104 rotary kiln 105 solar thermal or photovoltaic elements

Claims

[1] A process for producing one or more of carbon dioxide, carbon monoxide, carbon and hydrogen from a gas comprising carbon dioxide and preferably water, in particular from air, comprising the following steps: a) the gas comprising carbon dioxide and water is brought into contact with sodium hydroxide, preferably a sodium hydroxide solution, to simultaneously absorb carbon dioxide and water to form sodium carbonate, in particular sodium carbonate with water of crystallization, and a water-enriched sodium hydroxide solution; b) Sodium carbonate crystals from the solution of step a) are separated, preferably by filtration or centrifugation, optionally additional sodium carbonate can be separated by distillation of the sodium hydroxide solution to enhance crystallization; c) the sodium carbonate crystals from step b) are reacted with zinc oxide, in particular with an excess of zinc oxide, to release carbon dioxide by forming sodium zincate; d) the sodium zincate from step c) is reacted with water to form sodium hydroxide saturated with sodium tetrahydroxide zincate and solid zinc oxide, with at least some of the solid zinc oxide being separated; e) the remaining solution of sodium tetrahydroxide zincate in sodium hydroxide is used as an absorbent in step a); f) The solution from step d) is used to produce metallic zinc by alkaline electroplating, in particular by converting sodium tetrahydroxide zincate to sodium hydroxide and metallic zinc, the metallic zinc being used as a reducing agent for one or more of the following processes: I. The metallic zinc is used as a reducing agent to reduce carbon dioxide obtained in step c) to carbon monoxide, whereby the zinc oxide obtained thereby is used in step c), in particular without any treatment; II. The metallic zinc is used as a reducing agent to reduce carbon monoxide obtained in step I to carbon, preferably using an iron-containing catalyst, wherein carbon and zinc oxide are separated, preferably by dissolving the zinc oxide in sodium hydroxide to form a concentrated sodium tetrahydroxide zincate solution, wherein the concentrated sodium tetrahydroxide zincate solution is used for electroplating zinc for use in step f) and for recovering sodium hydroxide solution for use as an absorbent in step a); III. The metallic zinc is used as a reducing agent to reduce water, preferably water of the water-enriched sodium hydroxide solution from step a), to hydrogen by forming a concentrated sodium tetrahydroxide zincate solution, which is preferably used for electroplating zinc production and recovery of higher concentrated sodium hydroxide solution for use in step a). [2] Method according to claim 1, wherein the gas is air, in particular atmospheric air, and wherein after step c) zinc oxide and sodium hydroxide are recovered, in particular completely recovered for reuse in step a), and wherein in particular the water is extracted from the gas, preferably to a large extent. [3] The method of claim 2, wherein the gas is the only consumable in the method. [4] Method according to one of the preceding claims, wherein in step a) the gas comprising carbon dioxide is brought into contact with concentrated sodium hydroxide solution, in particular with a sodium hydroxide solution comprising at least 10 mol / l sodium hydroxide, preferably between 12 and 19 mol / l sodium hydroxide. [5] Method according to one of the preceding claims, wherein in step a) the sodium carbonate, in particular sodium carbonate with water of crystallization, is precipitated, in particular due to exceeding the solubility. [6] Method according to one of the preceding claims, wherein between step a) and step c) the sodium carbonate comprising water of crystallization is dried, in particular with water recovery. [7] Method according to one of the preceding claims, wherein for the reaction of sodium carbonate and zinc oxide in step c) the sodium carbonate is melted, in particular together with zinc oxide, wherein preferably a mass ratio between sodium carbonate and zinc oxide is greater than 4:1, preferably to reduce viscosity, and wherein zinc oxide is added to the melt together with sodium carbonate, in particular continuously. [8] Method according to one of the preceding claims, wherein the reaction of sodium carbonate and zinc oxide in step c) takes place at a temperature higher than 600 °C, in particular at a temperature between 700 °C and 1000 °C, more preferably between 800 °C and 900 °C. [9] Method according to any of the preceding claims, wherein the reaction of sodium carbonate and zinc oxide in step c) takes place in a rotary kiln. [10] Method according to any of the preceding claims, wherein in step c) one or more of the following reactions take place: i) Carbon dioxide is produced by drying the sodium carbonate and, in particular, heating it with zinc oxide to a temperature above 850 °C; ii) Carbon monoxide is produced by drying the sodium carbonate and in particular adding an excess of metallic zinc and preferably heating to a temperature above 900 °C, or carbon monoxide is produced from carbon dioxide by bringing carbon dioxide from i) into contact with liquid or gaseous zinc; iii) Carbon dioxide and hydrogen are produced by adding an equimolar amount of zinc in relation to an amount of water present and, in particular, by heating to a temperature below 900 °C; iv) Carbon monoxide and hydrogen are produced by adding an excess of metallic zinc to sodium carbonate and water, in particular to wet sodium carbonate, and preferably heating to a temperature above 900 °C; v) Carbon is produced by reducing CO2 or CO with metallic zinc, in particular with an iron oxide catalyst. [11] Method according to one of the preceding claims, wherein in step c) a sodium zincate, in particular Na2ZnO2, is obtained in addition to carbon dioxide, wherein in particular the sodium zincate is extracted from the melt. [12] The method of claim 11, wherein the sodium zincate obtained in step c) is added to water, preferably at a temperature above 80 °C, wherein at least a part of the sodium zincate is hydrolyzed to sodium tetrahydroxide zincate and wherein the tetrahydroxide zincate is at least partially decomposed to sodium hydroxide solution and zinc oxide, wherein preferably solid zinc oxide is separated, in particular precipitated, and wherein in particular an amount of the added water is selected such that the sodium hydroxide solution obtained has a concentration between 12 mol / l and 19 mol / l, in particular for reuse in step a). [13] Method according to claim 12, wherein the water is obtained by distillation of water of crystallization from the sodium carbonate of step a) and / or by drying the sodium carbonate. [14] Method according to claim 12 or 13, wherein metallic zinc is extracted from the remaining tetrahydroxide zincate in the sodium hydroxide solution by electroplating. [15] Method according to claim 14, wherein the sodium hydroxide solution remaining after electroplating is reused in step a). [16] Method according to one of claims 14 or 15, wherein the metallic zinc is at least partially converted in one or more of the following methods: i) metallic zinc is brought into contact with water, especially water from air, and reacted to form zinc oxide and hydrogen; ii) metallic zinc is heated to a temperature of at least 400 °C, in particular to a temperature between 600 °C and 1000 °C, and reacted with carbon dioxide, in particular with carbon dioxide from step c), to form zinc oxide and carbon monoxide; iii) metallic zinc is reacted with carbon dioxide and / or carbon monoxide in the presence of a catalyst, in particular iron oxides, to form zinc oxide and carbon; iv) Metallic zinc is reacted with oxygen to form zinc oxide, in particular with oxygen obtained from the electroplating process according to claim 6, wherein the energy released by the reaction is converted into electrical energy or thermal energy. [17] Method according to any of the preceding claims, wherein all gases and water used in the method are extracted from the air, in particular from atmospheric air, and in particular intermediate gases and water are recycled in the method. [18] Apparatus for carrying out a method according to any one of claims 1 to 17 for providing one or more of carbon dioxide, carbon monoxide, carbon and hydrogen from a gas comprising carbon dioxide and preferably water, in particular from atmospheric air, comprising an absorber configured to carry out step a) and a decomposition unit configured to carry out step c). [19] Device according to claim 18, wherein the absorber and / or the decomposition unit are designed as a roof-mounted system. [20] Device according to claim 18 or 19, wherein the absorber and / or the decomposition unit comprises one or more absorber modules and / or decomposition unit modules. [21] Device according to claim 19 or 20, wherein the decomposition unit comprises a rotary kiln.

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