Process and device for producing carbon from carbon dioxide, in particular from carbon dioxide obtained from biomass

Abstract

A process for producing carbon from carbon dioxide, in particular from carbon dioxide obtained from biomass, comprising the steps of: a) reacting zinc oxide with sodium hydroxide to form a solution of sodium tetrahydroxide zincate, in particular by dissolving zinc oxide in sodium hydroxide; b) using the solution from step a) to produce metallic zinc by an alkaline galvanic process in which sodium tetrahydroxide zincate is converted into sodium hydroxide and metallic zinc, preferably using sodium hydroxide obtained therefrom in step a); c) reducing the carbon dioxide, in particular from carbon dioxide obtained from biomass, to carbon monoxide with metallic zinc from step b) as a reducing agent, preferably using zinc oxide obtained therefrom in step a), in particular without treatment;d) reducing the carbon monoxide obtained in step c) and, if appropriate, the residual carbon dioxide present after step c) with metallic zinc from step b), preferably using an iron-containing catalyst, to obtain a mixture of solid carbon and zinc oxide and, if appropriate, the iron-containing catalyst; e) dissolving the zinc oxide in the mixture from step d) by adding sodium hydroxide to form a dispersion comprising sodium tetrahydroxide zincate, solid carbon; f) separating the solid carbon and, if appropriate, the iron-containing catalyst from the dispersion from step e);

Description

[0001] The invention relates to a method and a device for producing carbon from carbon dioxide, in particular from carbon dioxide obtained from biomass.

[0002] Since the beginning of the Industrial Revolution in 1800, atmospheric carbon dioxide has risen from a previously stable 280 ppmv (parts per million by volume) to over 400 ppmv. This increase is predicted to continue or even increase unless techniques are implemented to limit carbon dioxide emissions.

[0003] The main goal of the ratified Paris Agreement is to limit the increase in the global average temperature to below 2.0°C above pre-industrial levels, which requires reducing carbon dioxide emissions to zero by 2050. Proposals to limit these emissions include the use of biofuels, solar energy, and wind turbines. However, reducing carbon dioxide emissions—that is, limiting the increase in carbon dioxide levels in the atmosphere—is not sufficient in the long term to correct the imbalance between oxygen and carbon dioxide in the atmosphere caused by the overproduction of carbon dioxide. Rather, it is necessary not only to halt the increase in the carbon dioxide content of the atmosphere in the long term, but to actively reduce it.

[0004] At the same time, the demand for a secure energy supply for industry, transport, and mobility remains high. Although there are many efforts to electrify the mobility sector and use renewable energy sources to provide the required electricity, most regions of the world lack the infrastructure to deliver electricity where it's needed. Furthermore, electrification is not easy in some sectors, such as large parts of aviation and shipping, due to technological limitations.

[0005] Hydrogen, for example, is an alternative energy source of great technological interest. However, hydrogen must be produced using other energy sources (fossil energy, nuclear energy, or renewable energy) for industrial use, making it a secondary energy carrier. Thus, hydrogen is not automatically sustainable, but only as sustainable as the primary energy sources from which the hydrogen is produced.

[0006] In particular, so-called biohydrogen is of great interest. Biohydrogen is hydrogen produced biologically. For example, dark fermentation uses anaerobic fermentation to produce biohydrogen (H2), methane (CH4), and carbon dioxide (CO2) from biomass, including biological waste. The biohydrogen produced in this process is called green hydrogen. This technology is of great interest because such hydrogen is a relatively clean fuel.

[0007] DE 10 2020 116 950 A1, for example, also discloses a process for splitting hydrogen-containing gases into molecular hydrogen. The hydrogen-containing gas can be biogas. However, the known process is not carbon-neutral, as biogas is usually produced by fermenting organic waste and contains carbon dioxide.

[0008] A particular disadvantage of fermentation is the re-release of carbon dioxide that was previously present in the atmosphere and then absorbed by the organic waste. Given the goal of actively reducing the amount of carbon dioxide in the atmosphere, there is still a need for improved solutions that at least partially overcome the aforementioned disadvantages.

[0009] It is therefore an object of the invention to provide a method and a device which belong to the technical field mentioned at the outset, with which a total content of atmospheric carbon dioxide, in particular carbon dioxide obtained from biomass, can be actively reduced in the long term.

[0010] The solution of the invention is defined by the features of claim 1. In particular, the invention relates to a process for producing carbon from carbon dioxide, in particular from carbon dioxide obtained from biomass, comprising the steps: (a) reacting zinc oxide with sodium hydroxide to form a solution of sodium tetrahydroxide zincate, in particular by dissolving zinc oxide in sodium hydroxide; b) using the solution from step a) to produce metallic zinc by an alkaline galvanic process in which sodium tetrahydroxide zincate is converted into sodium hydroxide and metallic zinc, preferably using sodium hydroxide obtained therefrom in step a) and / or e); c) reducing the carbon dioxide, in particular carbon dioxide obtained from biomass, to carbon monoxide with metallic zinc from step b) as a reducing agent, preferably using zinc oxide obtained therewith in step a), in particular without treatment; d) reducing the carbon monoxide obtained in step c) and, if appropriate, the residual carbon dioxide present after step c) with metallic zinc from step b), preferably using an iron-containing catalyst, to obtain a mixture of solid carbon and zinc oxide and, if appropriate, the iron-containing catalyst; e) dissolving the zinc oxide in the mixture from step d) by adding sodium hydroxide, in particular sodium hydroxide obtained in step b), to form a dispersion comprising sodium tetrahydroxide zincate, solid carbon and optionally the iron-containing catalyst; f) separating the solid carbon and optionally the iron-containing catalyst from the dispersion from step e), wherein preferably the remaining solution of sodium tetrahydroxide zincate is reused in step b), in particular without treatment.

[0011] The invention has several advantages. In particular, the process according to the invention has the advantage that the zinc oxide can be used in a circular process, i.e., the zinc oxide can be reused in the process after the carbon has been produced. This means that the zinc oxide is not consumed.

[0012] Therefore, the process according to the invention can be implemented as a cyclic process, with carbon dioxide being the only consumable in the process. Preferably, no other substances are consumed. However, if other products are produced from the carbon in an additional step, it may be necessary to use other consumed substances.

[0013] In particular, once initiated, the process can essentially be maintained without additional raw materials, apart from the input of carbon dioxide and energy, since 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.

[0014] The process according to the invention is particularly preferably a carbon dioxide-negative process. This means, in particular, that the process is carried out in such a way that the process removes more carbon dioxide from the environment, e.g., the atmosphere, during the production of the carbon than it emits during the production of the product.

[0015] Even if carbon-negative operation is obviously preferred, it is possible to operate the process in a non-negative manner, e.g., in a carbon-neutral manner or in a manner that emits more carbon dioxide during carbon production than the process removes from the environment, particularly the atmosphere. This depends, in particular, on whether the energy source is carbon-neutral or not.

[0016] Furthermore, the process according to the invention can actively remove carbon dioxide while at the same time obtaining a product that is suitable for various applications.

[0017] In particular, carbon dioxide produced in chemical processes, e.g., during biomass fermentation, is converted into solid carbon instead of being released into the atmosphere. Therefore, the process according to the invention effectively captures the carbon dioxide, so that it no longer contributes to the greenhouse effect. This helps improve the carbon dioxide balance of fermentation processes, such as those used for the production of biohydrogen.

[0018] Furthermore, the carbon produced is a solid substance. As such, it can be stored without the need for sophisticated storage media, such as those required for gaseous products. Furthermore, the process according to the invention makes it possible to produce the carbon required for many industrial and agricultural processes and / or products in a cost-effective manner. This is particularly important because it has not been economically feasible to recycle carbon until now.

[0019] It is particularly beneficial if the resulting carbon is introduced into the soil, particularly the seabed, for medium- or long-term storage and / or used as a soil conditioner. Use as a soil conditioner has the further advantage of returning the carbon to a cycle that promotes the reduction of atmospheric carbon dioxide in the long term. As a soil conditioner, carbon is very effective in accelerating plant growth. Carbon is therefore advantageously used as a soil conditioner for the reforestation of plants that are highly susceptible to carbon dioxide. As a soil conditioner, carbon thus contributes to an improved and overall accelerated uptake of atmospheric carbon dioxide by plants, i.e., organic waste, by accelerating plant growth.From this organic waste, the process according to the invention can in turn be used to produce carbon as an active carbon dioxide reducing soil conditioner.

[0020] Furthermore, the carbon can be used to produce and / or incorporated into an industrial product. The industrial product is preferably intended for recycling and / or disposal. More specifically, the industrial product is preferably not incinerated. This way, the produced carbon is stored in the industrial product, at least in the medium term, and thus does not pollute the atmosphere.

[0021] Overall, the process according to the invention provides an effective, cost-effective, sustainable, and robust process for carbon production 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.

[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] In particular, the energy required to carry out the method step(s) according to the invention 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 turbine, 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 process step of generating the renewable energy, in particular with a photovoltaic unit and / or a wind turbine and / or a solar heat collector.

[0025] In particular, the required energy is generated at the same location, in particular in the same plant, where steps a), b), c), d), e) and / or f) take place.

[0026] Specifically, carbon dioxide is carbon dioxide extracted from biomass. Likewise, carbon produced is carbon extracted from biomass. Biomass refers to substances from recently living organisms, especially plants, algae, and / or animals. Specific examples of biomass include wood, wood residues, crops, agricultural residues, and / or organic waste from industry and / or households.

[0027] Carbon dioxide obtained from biomass can be determined by the proportion of 14 C atoms from carbon dioxide extracted from fossil materials. The same applies to a carbon product.

[0028] Specifically, carbon dioxide derived from biomass and carbon derived from biomass 14 C atoms. By determining the 14C content, it is possible to clearly determine whether and to what extent the carbon dioxide or the extracted carbon is derived from biomass. Specifically, carbon dioxide or carbon extracted from biomass differs from non-biobased or fossil-based carbon dioxide or carbon by a measurable proportion of 14 C carbon isotope.

[0029] The amount of carbon present in the atmosphere 14 C isotope has a half-life of about 5730 years and is introduced into living biological organisms. A fresh organic sample contains about 1 ppt (parts per trillion, 10 -12 ) 14 C atoms, relative to the sum of all carbon atoms. In dead organisms, the amount of bound radioactive 14C atoms decay according to the law of decay, but this decrease is only measurable over long periods of time. Organic compounds produced from fossil raw materials are not derived from biomass and do not exhibit any measurable 14 C content.

[0030] The 14 The carbon content of a sample can be determined analytically. This content allows us to determine the proportion of carbon dioxide and / or carbon that comes from biomass. Preferably, the 14 C content and the content of carbon dioxide or carbon extracted from biomass were determined according to ASTM D6866 “Standard Test Methods for Determining the Biobased Content of Solid, Liquid, and Gasous Samples Using Radiocarbon Analysis”.

[0031] The share of 14C carbon atoms in the carbon dioxide and / or in the carbon produced is preferably more than 0.1 ppt, in particular more than 0.25 ppt, more than 0.5 ppt or more than 0.8 ppt, based on the sum of all carbon atoms contained in the carbon dioxide and / or in the carbon produced.

[0032] In preferred embodiments, the carbon dioxide used in the process and / or the carbon produced is obtained from biomass to an extent of at least 10 wt.%, in particular to an extent of more than 25 wt.%, more than 50 wt.%, more than 75 wt.%, more than 90 wt.%, more than 95 wt.%, more than 98 wt.% or 100 wt.%, based on the total weight of the carbon dioxide used.

[0033] In a further preferred embodiment, the process according to the invention comprises a step of producing carbon dioxide from biomass, in particular in a fermentation unit. The fermentation unit can be configured to use bacteria and / or heat to decompose the biomass.

[0034] Preferably, a gas comprising methane and carbon dioxide is produced from the biomass and the carbon dioxide is separated from the methane in a separation unit.

[0035] If desired, the methane can then be converted into hydrogen and carbon in a conversion unit, in particular a pyrolysis unit.

[0036] The sodium tetrahydroxide zincate formed in step a) is understood to mean in particular Na2Zn(OH)4.

[0037] In step a), zinc oxide is preferably dissolved in sodium hydroxide to form a concentrated sodium tetrahydroxide zincate solution. The concentrated sodium tetrahydroxide zincate solution can then be used for electroplating zinc.

[0038] In step b), metallic zinc is extracted from the sodium tetrahydroxide zincate, particularly the sodium tetrahydroxide zincate in the sodium hydroxide solution, by electroplating or galvanic zinc deposition. The separation preferably takes place according to the following reaction equation: Anode: 4 OH - - 4 e - → O2 + 2 H2O Cathode: 2 Zn 2+ + 4 e - → 2 Zn

[0039] In addition to metallic zinc, oxygen (O2) is released.

[0040] The energy required for this reaction is preferably generated by photovoltaics. However, experts are aware that other energy sources can also be used. Electroplating of zinc is particularly advantageous because elemental zinc can be used as a reducing agent.

[0041] Preferably, the sodium hydroxide produced in this way and / or remaining after electroplating is reused in step a) and / or e). This makes the process largely independent of material suppliers. This can be omitted in some variants of the invention.

[0042] Particularly preferably, a first part of the sodium hydroxide obtained in step b) is used in step a) and a second part of the sodium hydroxide obtained in step b) is used in step e).

[0043] In step c), the metallic zinc is reacted with the carbon dioxide to obtain carbon monoxide. Zn + CO2 → ZnO + CO↑

[0044] In this reaction, zinc is used as a reducing agent to produce carbon monoxide. Zinc oxide, as a byproduct, can be reused in step a) of the process according to the invention.

[0045] In step d), the carbon monoxide obtained in step c) can be further reduced to carbon. Zn + CO → ZnO + C

[0046] Again, metallic zinc is used as a reducing agent to produce solid carbon and zinc oxide as the byproduct. This allows the residual carbon dioxide present after step c) to also be converted into carbon monoxide and carbon.

[0047] In particular, step d) is carried out with an iron-containing catalyst, in particular an iron oxide catalyst.

[0048] Step d) results in a mixture of solid carbon and zinc oxide, and optionally the iron-containing catalyst. In particular, the mixture is in the form of a particulate material, especially a powdered material.

[0049] In particular, in step c) and / or d), the 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 the carbon dioxide and / or carbon monoxide. The reaction can be carried out, for example, in a rotary tube reactor and / or a rotary kiln. Rotary tube reactors and rotary kilns have proven particularly useful in this process step because of their particularly good ability to circulate solids. However, other suitable devices are also known to those skilled in the art.

[0050] Subsequently, in step e), zinc oxide is dissolved in the mixture from step d) by adding sodium hydroxide, in particular sodium hydroxide obtained in step b). This forms a dispersion comprising sodium tetrahydroxide zincate, solid carbon, and optionally the iron-containing catalyst. In particular, the iron-containing catalyst is also a solid substance.

[0051] In step f), the solid carbon and, if present, the iron-containing catalyst are then separated from the dispersion from step e). This can be done by filtration, in particular using a filter press. Preferably, the remaining sodium tetrahydroxide zincate solution is reused in step b), in particular without further treatment.

[0052] If desired, the iron-containing catalyst can additionally be separated from the carbon, e.g. by treatment with an acid, and optionally recovered for further use in the process according to the invention.

[0053] However, it is also possible to dispense with the separation of the iron-containing catalyst. In this case, the iron-containing catalyst is a consumable of the process according to the invention. Since the proportion of iron-containing catalyst in the carbon is usually small, it will hardly have a negative impact on the properties of the carbon. In particular, iron-containing catalysts, especially iron oxides, are biocompatible, thus yielding a more usable end product compared to methane decomposition.

[0054] Preferably, after step c), zinc oxide is recovered, in particular completely recovered, for reuse in step a), and after step f), the remaining solution of sodium tetrahydroxide zincate sodium is reused in step b).

[0055] In particular, steps c) and d) can be carried out simultaneously or sequentially.

[0056] It is a great advantage that zinc can be used as the central reagent for the above reactions. Zinc is inexpensive, non-toxic, readily available, and, last but not least, can be recycled after each reaction. Furthermore, unlike other hydrogen- and methane-based processes, the zinc-based process according to the invention produces products free of polycyclic aromatic hydrocarbons (PAHs; toxic and carcinogenic substances). This is because, thanks to the zinc, no hydrogen-containing reagents are required to obtain the carbon product.

[0057] All of the above reactions also have the advantage that, apart from carbon dioxide and possibly the iron-containing catalyst, no substances need to be consumed, but can be recycled into the process.

[0058] Therefore, the process according to the invention is preferably carried out in such a way that carbon dioxide and optionally the iron-containing catalyst is / are the only consumable in the process.

[0059] Most preferably, the process is carried out as a cyclic process.

[0060] A further aspect of the present invention relates to an apparatus for carrying out a method according to the invention for providing carbon from carbon dioxide, in particular from carbon dioxide obtained from biomass, comprising an electroplating unit configured to carry out step b), a reduction unit configured to carry out step c) and / or d), a dissolution unit configured to carry out step e), and a separation unit configured to carry out step f). The separation unit is in particular a filter unit, in particular a filter press.

[0061] Furthermore, a reaction unit configured to carry out step a) may be present. However, the dissolution of zinc oxide in sodium hydroxide can also be carried out in the electrodeposition unit.

[0062] In addition, the device according to the invention preferably comprises a feed system, in particular comprising lines and / or pumps, which is configured to return the sodium hydroxide and / or the zinc oxide obtained as by-products from step b) and / or c) to the unit(s) in which step a) and / or e) takes place, in particular the reaction unit, the electrodeposition unit and / or the dissolution unit, and / or the feed system is configured to return the sodium tetrahydroxide zincate from step f) to the unit in which step b) takes place, in particular the electrodeposition unit.

[0063] Further advantageous embodiments and combinations of features emerge from the following detailed description and the entirety of the claims.

[0064] The drawings used to explain the embodiments show: Fig. 1 a schematic representation of the entire reaction steps of the process.

[0065] In the figures, the same components are provided with the same reference numerals.

[0066] Fig. Figure 1 shows a reaction diagram illustrating the individual reaction steps for producing carbon.

[0067] The process begins with step 1: zinc oxide is dissolved in sodium hydroxide to form a concentrated solution of sodium tetrahydroxide zincate (Na2Zn(OH)4).

[0068] In step 2, the solution from step 1 is used to produce metallic zinc through an alkaline electroplating process. Specifically, metallic zinc is extracted from the sodium tetrahydroxide zincate solution by electroplating. The energy required for this reaction is generated, for example, by photovoltaics. The separation occurs according to the following reaction equation: Anode: 4 OH - - 4 e - → O2 + 2 H2O Cathode: 2 Zn 2+ + 4 e - → 2 Zn

[0069] In addition to the metallic zinc, oxygen (O2) is released.

[0070] The sodium hydroxide (NaOH) produced and / or remaining after electroplating is recycled and reused in step 1 and step 5 (see below).

[0071] In step 3, the metallic zinc obtained in step 2 is reacted with carbon dioxide to obtain carbon monoxide. Zn + CO2 → ZnO + CO↑

[0072] For example, the metallic zinc is heated to a temperature between 900 °C and 1,000 °C and reacted with carbon dioxide. In this reaction, zinc is used as a reducing agent to produce carbon monoxide.

[0073] The carbon dioxide used in step 3 is, for example, biomass-derived carbon dioxide obtained in a fermentation reaction from biomass, such as wood, wood residues, crops, agricultural residues and / or organic waste from industry and / or households.

[0074] In particular, reduction with gaseous zinc proceeds smoothly at atmospheric pressure. The higher the temperature, the faster the conversion. In a preferred embodiment, 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 (boiling point of zinc: 907 °C). The actual reaction with the deposition of zinc oxide only takes place 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. Zinc oxide, the byproduct from step 3, is continuously transported downward and expelled into a gas-tight container, where it cools or is immediately fed hot into step 1. In step 4, the carbon monoxide obtained in step 3, as well as any remaining carbon dioxide from step 3, is further reduced to carbon using the metallic zinc as the reducing agent and an iron oxide catalyst (optional). Zn + CO → ZnO + C

[0075] In particular, the conversion of carbon dioxide / carbon monoxide to solid carbon is preferably carried out in the gas phase by reduction with zinc using catalysts, especially 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 / carbon monoxide is blown upwards with fine iron oxide particles. Carbon dioxide is converted to zinc oxide and carbon monoxide. The carbon monoxide 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 to the tube wall, where they are transported downwards.Excess metallic zinc is evaporated from the bottom of the tube and returns to the top as a gas. The completeness of the reaction is achieved by pumping only gas into the reaction chamber, but there is no possibility of any gaseous product leaving the reaction chamber. Only solids are discharged, meaning gases remain in the reaction chamber until they are completely converted to solids. One way to increase the reaction rate is to increase the pressure in the furnace.

[0076] Step 4 results in a solid mixture of carbon, zinc oxide, and the iron-containing catalyst. Specifically, the mixture is in the form of a powdered material.

[0077] In step 5, the zinc oxide is dissolved in the mixture from step 4 by adding sodium hydroxide obtained in step 2. This forms a dispersion comprising sodium tetrahydroxide zincate, solid carbon, and the solid iron-containing catalyst.

[0078] Then, the solid carbon and the solid iron-containing catalyst are separated from the dispersion from step 5 using a press filter, and the remaining solution of sodium tetrahydroxide zincate is reused in step 2.

[0079] In step 6, the iron-containing catalyst can be separated from the carbon, e.g. by treatment with an acid, and optionally recovered for further use in the process according to the invention.

[0080] The fixed carbon obtained in step 6 can, for example, be stored for further use and / or incorporated into the soil.

[0081] In summary, it should be noted that the invention provides a process in which a large number of substances can be provided from any source containing carbon dioxide, in particular biomass or atmospheric air, with particularly few reagents, wherein substantially all reagents can be used in the cycle. List of reference symbols 1-6 process steps 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 2020 116 950 A1

[0007]

Claims

[1] Process for producing carbon from carbon dioxide, in particular from carbon dioxide obtained from biomass, comprising the steps: a) Reacting zinc oxide with sodium hydroxide to form a solution of sodium tetrahydroxide zincate, in particular by dissolving zinc oxide in sodium hydroxide; b) The solution from step a) is used to produce metallic zinc by an alkaline electroplating process in which sodium tetrahydroxide zincate is converted into sodium hydroxide and metallic zinc, preferably using the sodium hydroxide obtained thereby in step a) and / or e); c) Reducing carbon dioxide, in particular carbon dioxide obtained from biomass, to carbon monoxide using metallic zinc from step b) as a reducing agent, preferably using zinc oxide obtained in step a), in particular without treatment; d) Reducing the carbon monoxide obtained in step c) and optionally the remaining carbon dioxide present after step c) with metallic zinc from step b), preferably using an iron-containing catalyst, to obtain a mixture of solid carbon and zinc oxide and optionally the iron-containing catalyst; e) Dissolving the zinc oxide in the mixture from step d) by adding sodium hydroxide, in particular sodium hydroxide obtained in step b), to form a dispersion comprising sodium tetrahydroxide zincate, solid carbon and optionally the iron-containing catalyst; f) Separating the solid carbon and optionally the iron-containing catalyst from the dispersion from step e), preferably reusing the remaining solution of sodium tetrahydroxide zincate in step b), in particular without treatment. [2] The method of claim 1, wherein the carbon dioxide is carbon dioxide obtained from biomass. [3] Method according to any of the preceding claims, further comprising a step of producing carbon dioxide from biomass, in particular in a fermentation unit. [4] Method according to claim 3, wherein a gas comprising methane and carbon dioxide is produced from the biomass and the carbon dioxide is separated from the methane in a separation unit. [5] Method according to claim 4, wherein the methane is converted into hydrogen and carbon in a conversion unit, in particular a pyrolysis unit. [6] Method according to any one of the preceding claims, wherein in step a) zinc oxide is dissolved in sodium hydroxide to form a concentrated sodium tetrahydroxide zincate solution. [7] Method according to any of the preceding claims, wherein after step b) sodium hydroxide is recovered for reuse in step a) and / or e), in particular is completely recovered. [8] Method according to any of the preceding claims, wherein in step c) the 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 is reacted with the carbon dioxide to obtain the zinc oxide and the carbon monoxide. [9] Method according to any of the preceding claims, wherein step d) is carried out with an iron-containing catalyst, in particular an iron oxide catalyst. [10] Method according to any of the preceding claims, wherein after step c) zinc oxide is recovered for reuse in step a), in particular is completely recovered, and after step f) the remaining solution of sodium tetrahydroxide zincate sodium is reused in step b). [11] Method according to any of the preceding claims, wherein a first part of the sodium hydroxide obtained in step b) is used in step a) and a second part of the sodium hydroxide obtained in step b) is used in step e). [12] Method according to any of the preceding claims, wherein the method is carried out such that carbon dioxide and optionally the iron-containing catalyst is / are the only consumable(s) in the process. [13] A method according to any of the preceding claims, wherein the method is carried out as a cyclic process. [14] Method according to any of the preceding claims, wherein the total energy required to carry out the steps is electricity and / or heat, wherein the energy required in the form of electricity is generated by a photovoltaic unit and / or a wind turbine, and / or the energy required in the form of heat is generated by solar thermal collectors. [15] Method according to any of the preceding claims, comprising a further process step of generating energy with a photovoltaic unit and / or a wind turbine and / or a solar thermal collector. [16] Apparatus for carrying out a method according to any one of claims 1 to 15, for providing carbon from carbon dioxide, in particular from carbon dioxide obtained from biomass, comprising a galvanic deposition unit configured to carry out step b), a reduction unit configured to carry out step c) and / or d), a dissolution unit configured to carry out step e), and a separation unit configured to carry out step f), and optionally a reaction unit configured to carry out step a). [17] Device according to claim 16, further comprising a feed system, in particular comprising lines and / or pumps, configured to return the sodium hydroxide and / or the zinc oxide obtained as by-products from step b) and / or c) to the unit(s) in which step a) and / or e) takes place, and / or the feed system is configured to return the sodium tetrahydroxide zincate from step f) to the unit in which step b) takes place.

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