Process and apparatus for producing methane

Abstract

The present invention relates to a process for producing methane from a carbon dioxide-containing gas mixture comprising the steps of: - introducing a carbon dioxide-containing reactant gas stream (100) and a hydrogen-containing gas stream (101) into a methanation reactor (1), -reacting the carbon dioxide with hydrogen by anaerobic fermentation in the methanation reactor (1) to form a methane-containing product gas stream (102), - discharging the product gas stream (102) from the methanation reactor, - introducing the product gas stream (102) into a gas separation means (2) and fractionating the product gas stream (102) into an, especially substantially hydrogen-free, methane-rich fraction (103) and into a carbon dioxide-rich fraction (104), - recycling the carbon dioxide-rich fraction (104) into the reactant gas stream (100), wherein control of the molar proportions of hydrogen and carbon dioxide is effected. The invention further relates to an apparatus for producing methane from a carbon dioxide-containing gas mixture and to a process for feeding methane into a gas supply grid and further to a process for processing biomethane until suitable for feeding into a gas supply, and / or transport and / or distribution grid.

Description

[0001] Process and apparatus for producing methane

[0002] The present invention relates to a method and a device for producing methane from a carbon dioxide-containing gas mixture, having the features of the independent patent claims. In particular, the invention relates to a device and a method in which carbon dioxide is converted with hydrogen by means of methanogenic archaea to form methane. The invention also relates, where appropriate, to certain applications of the device and / or method. The invention further relates to a method for processing biomethane to the point where it is ready for feeding into a gas supply, transport, and / or distribution network.

[0003] Currently, biogas from anaerobic digestion is primarily used to generate electricity in combined heat and power plants. This produces baseload electricity and heat, which in many cases can only be used locally and only partially. Biogas plants have not yet been economically viable due to the low revenues from their products, namely electricity and heat.

[0004] Therefore, it is known to upgrade the produced biogas to natural gas quality by separating excess carbon dioxide, with membrane technology becoming the established technology of choice. In an additional step, this separated carbon dioxide can be put to further use. For example, it is known to produce methane (CH4) from carbon dioxide (CO2) and hydrogen (H2) through a process known as methanation, which can increase, in particular, almost double, the methane yield of the biogas plant.

[0005] Generally, methanation is the reaction following the Sabatier process according to the reaction equation 4 H2 + CO2— > CH4 + 2 H2O. A catalyst is required to overcome the kinetic barrier. This can be provided, for example, in the context of heterogeneous or biological / microbial catalysis.

[0006] Heterogeneous catalysis, for example, using nickel catalysts, is disadvantageous in several respects. For example, a high purity of the carbon dioxide-containing reactant gas is required, as sulfur, for example, poisons the catalyst material. Furthermore, as a result of the high operating temperatures between 200°C and 500°C, the catalyst materials are thermally degraded and lose their activity over time. Furthermore, mechanical erosion of the catalyst can occur due to abrasion or crushing of the catalyst particles.

[0007] Therefore, biological / microbial catalysis is considered a suitable alternative, which can be carried out at comparatively low operating temperatures (generally between 20°C and 100°C) and pressures (generally up to 50 bar). Methane-genic Achaea are known to be particularly suitable microorganisms.

[0008] Current technology for methanation technologies typically uses ex situ stirred tanks, and only a partial flow of carbon dioxide-containing feed gas is passed through this system. The aim is to convert the entire amount of carbon dioxide with hydrogen to methane, so that gas separation is not necessary after the methanation reaction to meet the required quality criteria for injection into a natural gas grid. However, this is only possible with a surplus of hydrogen. If the produced methane is to be injected into the gas grid, the excess hydrogen must be separated if it is not permitted to be injected into the gas grid.

[0009] If less hydrogen is available, the portion of biogas not used in methanation must continue to be used or upgraded for other purposes, which increases the investment and operating costs of a plant.

[0010] Furthermore, with existing technologies, the methanation plant is not able to respond flexibly to the daily and seasonal variations in the amount of hydrogen produced from surplus electricity.

[0011] Taking into account the aspects described above, a conflict of objectives arises which can possibly be resolved with the present invention.

[0012] The present invention therefore relates to a process for producing methane from a carbon dioxide-containing gas mixture.

[0013] In particular, the invention allows the carbon dioxide bound in biomass to be utilized, ideally completely, using hydrogen, which preferably originates from renewable sources. A device or method according to the invention therefore has a particularly negative carbon balance. Even when using fossil natural gas as a starting material for hydrogen production by pyrolysis, the device or method may have a neutral carbon balance.

[0014] Hydrogen is preferably produced via electrolysis and / or methane pyrolysis using electricity from wind and / or solar energy.

[0015] Depending on the available amount of hydrogen, the device or method according to the invention can be operated at full and partial load. This allows excess electricity available to be converted into feed-in and storable biomethane. Since many countries maintain gas storage facilities with a capacity of several months, this method also allows the excess electricity generated in the summer to be stored until periods of high energy demand in the winter. The existing gas infrastructure can be used for distribution.

[0016] A special feature of the device according to the invention or of the method according to the invention is that the operation in the methanation reactor can be reduced, if necessary, for hours, days, weeks or even months and even stopped completely and restarted very quickly and easily.

[0017] The hydrogen for the device or process can, however, come from any source and even be taken from the gas grid. The present invention also allows, in particular, the particularly simple use of hydrogen transported in mixed gas pipelines, which can be separated from the gas stream using simple means of a single-stage membrane gas treatment. Thus, almost any amount of hydrogen from the gas grid can be introduced into the methanation process. Any residual methane that may still be present does not interfere. The described device or process thus allows, in particular, the particularly simple use of hydrogen produced elsewhere in biogas plants.

[0018] Thus, with the device and method according to the invention, it is possible to combine hydrogen with the carbon dioxide originating from the biomass and convert the biomass into storable biomethane. Alternatively, the biomethane can be transported to large-scale consumers, preferably combined heat and power plants and industrial processes. This can be helpful where the entire gas infrastructure cannot or should not be converted to hydrogen.

[0019] The present invention thus solves, in particular, existing problems in energy generation (with the production of CH4, H2, and carbon black), energy distribution (feed-in / feed-out, transport of H2 to CO2 and CH4 to consumers), and energy storage (excess electricity to CH4 / H2 / carbon black, each capable of storage). Furthermore, biogas plants not only improve their carbon footprint, but may also become significantly more economical through the application of the features of the invention.

[0020] This makes it possible to build additional biogas plants to treat existing biomass streams from organic household waste and agriculture. Organic waste / biowaste is currently a major driver of global CO2 emissions because these biomass streams release large amounts of short-term climate-damaging methane into the environment through uncontrolled anaerobic degradation, which usually escapes directly into the atmosphere.

[0021] The device and method according to the invention can be retrofitted to existing biogas plants or other biomass-fired gas-emitting plants, as well as to CO2-emitting industrial plants, regardless of their size, or used in newly constructed plants. The present invention therefore has great environmental significance and significant economic benefits.

[0022] Where appropriate, the procedure includes the following steps:

[0023] - introducing a carbon dioxide-containing reactant gas stream and a hydrogen-containing gas stream into a methanation reactor,

[0024] - Formation of a methane-containing product gas stream by reacting carbon dioxide with hydrogen through anaerobic fermentation in the methanation reactor,

[0025] - Discharge of the product gas stream from the methanation reactor,

[0026] - introducing the product gas stream into a gas separation device and separating the product gas stream into a methane-rich fraction, in particular one which is essentially free of hydrogen, and a carbon dioxide-rich fraction,

[0027] - complete or partial recycling of the carbon dioxide-rich fraction to the reactant gas stream.

[0028] If necessary, control is carried out in such a way that the molar ratio of the hydrogen introduced into the methanation reactor by the hydrogen-containing gas stream n H2to the carbon dioxide n introduced into the methanation reactor by the carbon dioxide-rich fraction C02 a predetermined ratio is met.

[0029] Where appropriate, it is envisaged that the amount of hydrogen introduced into the methanation reactor via the hydrogen-containing gas stream is controlled as a function of the amount of carbon dioxide introduced into the methanation reactor by the carbon dioxide-rich fraction.

[0030] Where appropriate, it is envisaged that the amount of carbon dioxide introduced into the methanation reactor via the carbon dioxide-rich fraction is controlled as a function of the amount of hydrogen introduced into the methanation reactor by the hydrogen-containing gas stream.

[0031] If necessary, it is provided that the gas flows are controlled in such a way that the molar ratio of the hydrogen introduced into the methanation reactor by the hydrogen-containing gas flow n Hz to the carbon dioxide n introduced into the methanation reactor by the carbon dioxide-rich fraction C02 , represented by the equation: nH2 : n CO2= : 1 the condition 3 < X < 5 is met, preferably the condition 3.8 < X < 4.2, particularly preferably X = 4 is met.

[0032] Optionally, it is provided that the molar ratio of the carbon dioxide introduced into the methanation reactor by the reactant gas stream to the hydrogen introduced into the methanation reactor by the hydrogen-containing gas stream is greater than 0.2, preferably between 0.2 and 25, and particularly preferably between 0.25 and 3.

[0033] Where appropriate, the feed gas stream may contain up to 75 vol% methane. Where appropriate, the feed gas stream may comprise an externally supplied carbon dioxide-containing gas stream, originating, for example, from a biogas plant, another fermentation plant, or other sources such as biomass pyrolysis or an industrial plant containing a material stream consisting of CO, CO2, CH4, and / or H2.

[0034] Optionally, it is provided that the hydrogen-containing gas stream comprises hydrogen gas obtained by pyrolysis of an organic compound or a mixture of several organic compounds, in particular biomass and / or methane, and / or that the hydrogen-containing gas stream comprises hydrogen gas obtained from the electrolysis of water.

[0035] Where appropriate, the hydrogen-rich gas stream may comprise hydrogen gas that originates from an external hydrogen source, preferably from an existing natural gas or hydrogen transport network and can be separated therefrom using simple gas processing means, or alternatively from tanks or cylinders.

[0036] Where appropriate, it is envisaged that the hydrogen-containing gas stream comprises hydrogen gas originating from a natural gas network, the hydrogen gas being obtained by membrane separation from the natural gas produced in the natural gas network.

[0037] Where appropriate, the hydrogen-containing gas stream comprises hydrogen gas obtained by pyrolysis of methane from the methane-rich fraction.

[0038] Optionally, it is provided that the hydrogen obtained during pyrolysis and / or electrolysis has a gas pressure which is higher than the gas pressure in the methanation reactor, wherein the pressure difference between the hydrogen obtained and the methanation reactor is in particular greater than 200 mbar, preferably between 200 mbar and 1500 mbar.

[0039] If appropriate, the gas pressure in the methanation reactor is higher than the gas pressure in the gas separation device. If appropriate, the proportion of the methane-rich fraction provided for pyrolysis is controlled depending on the amount of carbon dioxide introduced into the methanation reactor via the feed gas stream.

[0040] Where appropriate, it is intended that the pyrolysis and / or electrolysis will produce a heat flow which will be fed to the methanation reactor and / or the biogas plant for heating purposes.

[0041] Optionally, it is provided that the gas separation device (2) comprises at least one separation stage with a gas separation membrane, wherein the permeability of the gas separation membrane is higher to carbon dioxide than to methane, and wherein the gas separation device has in particular two or three interconnected separation stages.

[0042] If necessary, it is provided that the retentate pressure and the permeate pressure of the entire gas separation device and / or the individual separation stages are control variables of the control system.

[0043] Optionally, it is provided that the gas separation device has two interconnected separation stages, wherein the product gas stream is introduced into a first separation stage so that a first retentate stream and a first permeate stream are obtained, wherein the first retentate stream is introduced into a second separation stage so that a second retentate stream and a second permeate stream are obtained, wherein the first permeate stream is at least partially recycled and the second permeate stream is completely recycled to the reactant gas stream.

[0044] Optionally, it is provided that the gas separation device has three interconnected separation stages, wherein the product gas stream is introduced into a first separation stage so that a first retentate stream and a first permeate stream are obtained, wherein the first retentate stream is introduced into a second separation stage so that a second retentate stream and a second permeate stream are obtained, wherein the first permeate stream is introduced into a third separation stage so that a third retentate stream and a third permeate stream are obtained, wherein the second permeate stream and the third retentate stream are completely recycled and the third permeate stream is at least partially recycled to the reactant gas stream.

[0045] If appropriate, it is provided that the pressure of the second retentate stream, the pressure of the third retentate stream and the pressure of the second permeate stream are control variables of the gas processing control system, with which the gas quality of the second retentate stream and the third permeate stream can be regulated.

[0046] If necessary, it is provided that the reactant gas stream is or can be introduced partially or completely into the product gas stream and / or into the gas separation device via a bypass stream without conversion of the carbon dioxide in the methanation reactor.

[0047] Optionally, the control is carried out in such a way that the methane content in the methane-rich fraction is at least 85 vol.%, preferably between 95 and 99.5 vol.%. Optionally, the methane content in the methane-rich fraction is at least 95 vol.%, preferably at least 97.5 vol.%.

[0048] Where appropriate, it is provided that the control is carried out in such a way that the loss of methane in the process is in any case less than 1%, preferably less than 0.5%, particularly preferably less than 0.2%.

[0049] Optionally, it is provided that the reaction in the methanation reactor takes place at a temperature between 20°C and 100°C, preferably between 40°C and 90°C, particularly preferably between 60°C and 88°C.

[0050] Where appropriate, it is provided that the amount of hydrogen introduced into the methanation reactor is adjusted based on an availability parameter, wherein the availability parameter takes into account in particular one or more of the following aspects: availability of methane; availability of electrical power; price of electrical power; availability of hydrogen from external hydrogen sources. Where appropriate, it is provided that in phases of low hydrogen availability, the liquid medium used in the methanation reactor is brought to a temperature of less than 15 °C, preferably between 4 and 10 °C. A phase of low hydrogen availability exists in particular when the availability parameter falls below a predetermined limit.

[0051] If necessary, these temperatures can be achieved primarily through structural measures with little or no external cooling effort.

[0052] If necessary, it is envisaged that by reducing the temperature in the methanation reactor, its operation can be partially or completely reduced for hours, days, weeks or months.

[0053] Where appropriate, the reaction in the methanation reactor is carried out at a gas pressure between 5 bara and 50 bara, preferably between 10 bara and 16 bara.

[0054] If necessary, it is planned that the conversion in the methanation reactor takes place using microorganisms from the group of methanogenic archaea.

[0055] Optionally, the pH of the liquid fermentation medium arranged in the methanation reactor is between 3.5 and 7, preferably between 4 and 6.

[0056] If necessary, it is provided that the gas separation device is operated at a gas pressure between 5 bara and 50 bara, preferably between 8 bara and 17 bara.

[0057] If appropriate, a nutrient supply for the methanation reactor is provided. If appropriate, the nutrient supply of the strain(s) used for methanation is controlled via a parameter that correlates with methane production, which depends on the consumption rates of individual nutrients. If appropriate, a nutrient supply for the methanation reactor is provided, with the nutrient supply being controlled based on a methane production parameter, which preferably correlates with the consumption rates of individual nutrients.

[0058] Where appropriate, it is intended that the consumption rates of the individual nutrients relevant for the regulation of nutrient supply are those of phosphate or phosphorus, sulfide or sulfur, ammonium or nitrogen, trace substances, hydrogen and CO2, concentration of the respective nutrients, as well as temperature and process pressure.

[0059] Where appropriate, the methane production parameter correlates with one or more of the following parameters: consumption rate of one or more of the following: phosphate or phosphorus, sulfide or sulfur, ammonium or nitrogen, trace elements, hydrogen, carbon dioxide; concentration of the respective nutrients; temperature of the fermentation medium; process pressure.

[0060] Where appropriate, it is envisaged that there is a characteristic behavior of temperature and nutrient supply of the microorganisms that can be used to control productivity characteristics.

[0061] Where appropriate, it is intended that at least one basic control variable is used to characterise the productivity properties and for system control.

[0062] Optionally, a basic control variable can be the optical density or a related variable, which is preferably between 0.5 and 50, particularly preferably between 5 and 20. Optionally, a basic control variable can be the amount and composition of the individual gas streams. Optionally, a basic control variable can be the cell-specific growth and / or death rate, the uptake rate of the individual nutrients, preferably but not exclusively that of phosphate, sulfide or sulfur, ammonium or nitrogen, or trace substances. Optionally, a basic control variable can be the uptake rate of the reactants H2 and CO2. Optionally, a basic control variable can be the production rate of CH4 and H2O, where these can be the temperature and / or the process pressure and / or where these can be the temporal changes, preferably but not exclusively those mentioned.

[0063] Where appropriate, it is intended that a characterization of the productivity properties and / or a system control is carried out on the basis of one or more basic control variables.

[0064] Optionally, it is provided that the basic control variable or basic control variables are selected from one or more of the following: optical density or an associated variable of the fermentation medium, in particular between 0.5 and 50, preferably between 5 and 20; quantity and / or composition of the individual gas streams and / or media; cell-specific growth and / or death rate, nutrient uptake rate, in particular of phosphate, sulfide or sulfur, ammonium or nitrogen or trace substances; uptake rate of hydrogen and / or carbon dioxide; production rate of methane and / or water; temperature and / or process pressure in the methanation reactor; temporal change of a parameter.

[0065] Where appropriate, it is envisaged that the output of biomass and reaction water from the methanation reactor is controlled by correlation with the gas production rate, so that an essentially constant productivity of the methanation is preferably achieved.

[0066] Optionally, it is envisaged that an electricity availability parameter is used to control the process, whereby, depending on the availability of electrical power, the operating state is adapted such that the methane content in the second retentate stream is at least 85 vol.%, preferably at least 97 vol.%; a biomass stream is converted to carbon dioxide and hydrogen and then to methane; the methane-rich second retentate stream is fed into an existing gas network and / or used in pyrolysis to produce solid carbon and gaseous hydrogen, which is fed into the hydrogen-containing gas stream and thus returns to the methanation reactor; both the methane-rich second retentate stream and the hydrogen stream from the pyrolysis are fed into an existing gas network; the fed-in gases are stored in gas reservoirs.

[0067] If necessary, it is planned that the methane-rich fraction will be fed into a transport pipeline, for example a natural gas network, with or without additional pressure increase.

[0068] Where appropriate, it is envisaged that at least part of the hydrogen-containing gas stream is taken from a hydrogen-carrying gas network and / or transport network.

[0069] If necessary, it is provided that the hydrogen-containing gas stream is separated from the gas network by a membrane separation device, in particular a single-stage one.

[0070] Optionally, the invention also relates to an apparatus for producing methane from a carbon dioxide-containing gas mixture, in particular designed to carry out a process according to the invention.

[0071] Where appropriate, the device is intended to include the following components:

[0072] - a methanation reactor in which carbon dioxide is reacted with hydrogen by anaerobic fermentation,

[0073] - a feed line that introduces a carbon dioxide-containing reactant gas stream and a hydrogen-containing gas stream into the methanation reactor,

[0074] - a discharge line which discharges a methane-containing product gas stream obtained by anaerobic fermentation from the methanation reactor,

[0075] - a gas separation device which separates the product gas stream into a methane-rich fraction, in particular one which is essentially free of hydrogen, and a carbon dioxide-rich fraction, - a recirculation device which returns the carbon dioxide-rich fraction to the reactant gas stream,

[0076] - a control device which controls the molar ratio of the hydrogen introduced into the methanation reactor by the hydrogen-containing gas stream n Hzto the carbon dioxide n introduced into the methanation reactor by the carbon dioxide-rich fraction C02 to a predetermined ratio.

[0077] If necessary, a bypass device is provided which introduces at least part of the reactant gas stream into the gas separation device without converting the carbon dioxide in the methanation reactor.

[0078] Optionally, it is provided that a pyrolysis device is provided which pyrolyzes the methane-rich fraction of the product gas stream to produce hydrogen and / or that an electrolysis device is provided which electrolyzes water to produce hydrogen.

[0079] If necessary, an external hydrogen source is provided, with which hydrogen can be supplied from other sources, preferably from an existing gas network, but alternatively also from tanks or cylinders.

[0080] If appropriate, a single- or multi-stage membrane separation plant is provided that separates the hydrogen from a pipeline carrying methane and natural gas, for example, a natural gas network, and introduces it into the methanation reactor via the hydrogen-containing gas stream. Small amounts of methane in the separated hydrogen are permissible and do not interfere with the process or the device.

[0081] Optionally, it is provided that the gas separation device comprises at least one of the three membrane-based separation stages, wherein the mixed gas selectivity of each of the separation stages for the separation of carbon dioxide and methane is greater than 30, in particular greater than 50. Optionally, it is provided that the methane-rich fraction is fed into the same or a different transport network and thus made available to consumers instead of the extracted hydrogen.

[0082] The invention may also relate to a method for feeding methane into a gas supply network.

[0083] Optionally, the method comprises the step of extracting hydrogen-containing gas from a gas supply network. The gas supply network may, for example, be a natural gas network into which methane gas containing a proportion of hydrogen is pumped.

[0084] Optionally, the process comprises the step of separating hydrogen from the hydrogen-containing gas. The separation is optionally carried out using a membrane separation device. The membrane separation device can, in particular, be a single-stage device, since the resulting hydrogen may contain certain amounts of other gases.

[0085] Optionally, the process comprises the step of feeding the hydrogen, at least as part of the hydrogen-containing gas stream, into a process according to the invention and / or into a device according to the invention. In the process and / or in the device, at least part of the hydrogen can be converted to methane.

[0086] Optionally, the process includes the step of introducing the resulting methane-rich fraction into the gas supply network. This allows the resulting methane to be reintroduced into the gas supply network and transported to consumers via it.

[0087] In one step of the process according to the invention, a carbon dioxide-containing reactant gas stream can be introduced into a methanation reactor. A portion of the reactant gas stream, in particular an externally supplied carbon dioxide-containing gas stream, can originate, for example, from a biogas plant, but any other sources are also conceivable. In addition, a hydrogen-containing gas stream can be introduced into the methanation reactor in the process in order to provide the reactants for a methanation reaction. The hydrogen-containing gas stream or at least a portion of the hydrogen-containing gas stream can be obtained, for example, by electrolysis of water, wherein the electrical power used preferably originates from renewable energy sources. Alternatively or in combination, the hydrogen-containing gas stream or a portion of the hydrogen-containing gas stream can also be obtained by the pyrolysis of organic compounds, in particular by the pyrolysis of methane.Other possible additional sources of hydrogen include a pipeline network, tanks, gas cylinders, or similar. Methane may also come from the methane-rich fraction of gas processing.

[0088] In a further step of the process, the carbon dioxide contained in the reactant gas stream can be converted into methane with the introduced hydrogen in a methanation reactor. It is advantageous if the conversion is carried out by catalysis using microorganisms in an aqueous medium. The microorganisms used are in particular methanogenic archaea, with M. igneus, M. indicus, M. infernus, M. jannaschii, M. kandleri, M. lacus, M. lacustris, M. marburgensis, M. marinum, M. maripaludis, M. mazei, M. millerae, M. palustris, M. petrolearium, M. petrolearius, M. siciliae, M. soligelidi, M. spelaei, M. submarinus, M. thermaggregans, M. veterum, M. villosus, or M. vulcanius, or particularly preferably M. aeolicus, M. igneus, M. jannaschii, M. kandleri, M. marburgensis, M. maripaludis, M. submarinus, M. thermaggregans, M. villosus, or M.vulcanius or related particularly preferred species, individually or together in consortia, are suitable for use in the process according to the invention. The microorganisms used are in particular selected from the group of acidophilic methanogenic archaea.

[0089] In contrast to heterogeneous catalysis, the reaction conditions in the methanation reactor are mild. The reaction occurs at a gas pressure between 5 and 50 bara, at a temperature between 20°C and 100°C, and at a pH between 3.5 and 7. The mass transfer of hydrogen into the liquid phase in the methanation reactor can occur both above and below the bubble point.

[0090] The methane-containing product gas obtained by the reaction and derived from the methanation reactor can be fed to a gas separation device in a further step of the process, in which gas separation into a methane-rich fraction and a carbon dioxide-rich fraction can take place.

[0091] The gas separation facility can comprise one, two, or three separation stages. With a single separation stage, the purities of neither the retentate gas nor the permeate gas can be satisfactorily high for both gas streams simultaneously. Only the retentate gas stream can meet the high purity requirements of a natural gas network; however, this results in a very high methane content in the permeate stream, which must be recirculated and consumes unnecessary energy due to the necessary recompression in the compressor. To achieve satisfactory purity for both gas streams, multiple separation stages are interconnected.

[0092] Optionally, the methane-rich fraction contains at least 85 vol.%, preferably at least 95 vol.%, more preferably at least 97 vol.%, of methane. In particular, the methane-rich fraction is also substantially free of hydrogen, although residual contents of up to 1 vol.% hydrogen may be present. The methane-rich fraction may be low in carbon dioxide, i.e., in particular, contain at most 15 vol.% carbon dioxide. The methane-rich fraction may be fed at least partially into a gas network. The methane-rich fraction may also be filled into cylinders and / or tanks or subjected to pyrolysis.

[0093] The carbon dioxide-rich fraction contains, in particular, the majority of the carbon dioxide that was not converted in the methanation reactor. In one step of the process, the carbon dioxide-rich fraction can be added to the reactant gas stream and thus reintroduced into the methanation reactor. This can improve the overall reaction yield during methanation and reduce and / or avoid the generation of carbon dioxide-containing offgas streams. The carbon dioxide-rich fraction can comprise different, optionally separate, gas streams obtained from the gas separation.

[0094] The reactant gas stream may in particular comprise an externally supplied carbon dioxide-containing gas stream and at least a part of the carbon dioxide-containing fraction.

[0095] The process can provide for the amount of hydrogen introduced into the methanation reactor to be adjusted based on an availability parameter. The availability parameter can take into account various variables, such as the price of electricity or the available amount of electricity. If the hydrogen is produced by pyrolysis, for example, the control can be such that pyrolysis only occurs if the electricity used for pyrolysis does not exceed a predetermined price.

[0096] The process may provide for controlling the amount of hydrogen introduced into the methanation reactor via the hydrogen-containing gas stream and / or the amount of carbon dioxide introduced into the methanation reactor via the carbon dioxide-rich fraction.

[0097] In particular, the amount of hydrogen introduced into the methanation reactor via the hydrogen-containing gas stream can be controlled depending on the amount of carbon dioxide introduced into the methanation reactor via the carbon dioxide-rich fraction. The amount of carbon dioxide introduced into the methanation reactor via the carbon dioxide-rich fraction can also be controlled depending on the amount of hydrogen introduced into the methanation reactor via the hydrogen-containing gas stream.

[0098] This allows a specific ratio between the amounts of the two gases to be set in different scenarios, particularly in the case of fluctuations in the available hydrogen and / or carbon dioxide. In the context of the present invention, "control" means, in particular, that one or more manipulated variables are determined and / or adjusted based on one or more input variables.

[0099] Input variables of the control system can be, for example, one or more of the following variables: molar composition of the reactant gas stream and / or the carbon dioxide-containing fraction, volume flow of the reactant gas stream and / or the carbon dioxide-containing fraction, physical and / or chemical parameters from the methanation reaction, physical and / or chemical parameters from the provision of the hydrogen-containing gas stream, pressures and volume flows of the gas streams before, between and after the separation stages.

[0100] Control variables of the control system can be, for example, one or more gas pressures after and / or within the gas separation device as well as the volume flow of the product gas flow, which in turn can be controlled by the speed of the gas compression device or the compressor.

[0101] If necessary, the control is carried out in such a way that the ratio between the molar amount of hydrogen introduced into the methanation reactor via the hydrogen-containing gas stream and the molar amount of carbon dioxide introduced into the methanation reactor via the carbon dioxide-rich fraction remains constant. "Essentially constant" in this context can mean that deviations of up to 10% from the constant molar ratio are permissible.

[0102] If necessary, the control is carried out in such a way that the ratio between the amount of hydrogen introduced into the methanation reactor via the hydrogen-containing gas stream and the amount of carbon dioxide introduced into the methanation reactor via the carbon dioxide-rich fraction n Hz n cc>2 between 3:1 and 5:1. The externally supplied carbon dioxide-containing gas stream can introduce additional carbon dioxide into the reactant gas stream and thus into the methanation reactor. In particular, the carbon dioxide in the methanation reactor can then be present in a stoichiometric excess compared to the hydrogen in the methanation reactor. If necessary, the control is carried out such that the amount of hydrogen in the methanation reactor is less than four times the amount of carbon dioxide in the methanation reactor.

[0103] The molar ratio of the carbon dioxide introduced into the methanation reactor by the reactant gas stream to the hydrogen introduced into the methanation reactor by the hydrogen-containing gas stream may in particular be greater than 0.2.

[0104] The molar ratio between hydrogen and carbon dioxide can be controlled by not recycling a portion of the carbon dioxide-rich fraction to the methanation reactor. This is particularly advantageous when there is not enough hydrogen available to completely convert the carbon dioxide to methane.

[0105] Alternatively, the molar ratio between hydrogen and carbon dioxide can be controlled by recirculating the entire carbon dioxide-rich fraction to the methanation reactor and adjusting the amount of hydrogen introduced via the hydrogen-containing gas stream accordingly. This is particularly advantageous when enough hydrogen is available to completely convert the carbon dioxide to methane or when enough inexpensive electricity is available to generate a corresponding amount of hydrogen.

[0106] By appropriately controlling the molar ratio between hydrogen and carbon dioxide in the methanation reactor, essentially complete conversion of the hydrogen introduced into the methanation process can be achieved. This allows a substantially hydrogen-free product gas stream to be generated, thus avoiding the separation of hydrogen for subsequent utilization. In a synergistic manner, an excess of carbon dioxide can form a buffer in the aqueous fermentation medium. This creates an acidic pH value, i.e., a pH value <7, in the methanation reactor or its aqueous medium, which enables favorable conversion rates in the methanation reaction with the acidophilic methanogenic archaea used.

[0107] This aspect may represent a further difference to the state of the art, where methanation reactions usually take place with a stoichiometric excess of hydrogen and regulators are added so that the pH value is in the neutral or basic range.

[0108] The product gas stream can optionally be separated using a membrane-based gas separation device, which may comprise one or more gas separation modules. If multiple gas separation modules are provided, they can be interconnected in various ways, both in parallel and in series.

[0109] The methane-rich fraction obtained from gas separation can be fed at least partially into a gas network.

[0110] The methane-rich fraction obtained from gas separation can be at least partially subjected to pyrolysis to produce solid carbon and hydrogen. The hydrogen thus produced can be introduced into the methanation reactor as a hydrogen-containing gas stream or as part of the hydrogen-containing gas stream. Thus, pyrolysis offers a variant of hydrogen production for methanation. Pyrolysis can optionally be carried out using plasma pyrolysis. The heat flow obtained during pyrolysis can be integrated into the overall process and used, for example, to heat the methanation reactor or the biogas fermenter.

[0111] Another option for producing hydrogen for methanation is water electrolysis. This option is particularly advantageous when a sufficient amount of inexpensive electricity is available. The hydrogen contained in the hydrogen-containing gas stream can, if necessary, be obtained simultaneously from different sources.

[0112] The hydrogen-containing gas stream obtained from pyrolysis and / or electrolysis can have a gas pressure higher than the gas pressure in the methanation reactor. This avoids the need for gas compression, enabling further energy savings.

[0113] Another possibility for providing hydrogen is a gas network from which the hydrogen is obtained, or separated from a gas stream therein using simple gas processing means, or provision in liquid form in bottles or tanks.

[0114] If insufficient or no hydrogen is available and / or cannot be provided, it may be provided that at least part or all of the reactant gas stream is passed via a bypass gas stream past the methanation reactor and directly into the gas separation device and / or into the product gas stream.

[0115] If necessary, the product gas stream from the methanation is mixed with the bypass gas stream before being introduced into the gas separation device.

[0116] If appropriate, the product gas stream is subjected to one or more treatment steps before being introduced into the gas separation device, which are optionally selected from one or more of the following steps: gas purification, in particular by particle filter and / or activated carbon absorption; gas drying by cold drying and droplet separation; gas thermostatting.

[0117] The present invention also relates to a device for producing methane from a carbon dioxide-containing gas mixture. Optionally, the device is configured to carry out a process according to the invention. The device may comprise a methanation reactor configured to receive a liquid fermentation medium. A feed line may be configured to introduce a carbon dioxide-containing reactant gas stream and a hydrogen-containing gas stream into the methanation reactor. A discharge line may be configured to discharge a product gas stream from the methanation reactor.

[0118] The device may further comprise a gas separation device configured to separate the product gas stream into different fractions. In particular, the gas separation device is configured to separate a mixed gas containing methane and carbon dioxide into a methane-rich fraction and a carbon dioxide-rich fraction. Optionally, the gas separation device comprises at least one gas separation module with a gas separation membrane, wherein the gas separation membrane may have a mixed gas selectivity of at least 30 for the separation of carbon dioxide and methane.

[0119] The device may comprise a recirculation device which is designed to recirculate at least a portion of the product gas stream separated in the gas separation device into the methanation reactor.

[0120] If necessary, a control device is provided which is designed to control the gas flows depending on the amounts of hydrogen and / or carbon dioxide contained.

[0121] If necessary, a first gas compression device is provided which compresses the carbon dioxide-containing reactant gas stream after supply of the carbon dioxide-containing fraction from the gas processing before mixing with the hydrogen-containing gas stream.

[0122] If necessary, a second gas compression device is provided to compress the hydrogen-containing fraction if it is not sufficiently pressurized for introduction into the methanation reactor. If necessary, a third gas compression device is provided to compress the carbon dioxide-rich fraction if it is not sufficiently pressurized for introduction into the methanation reactor, particularly if the capacity of the first gas compression device is insufficient.

[0123] A further advantage of the present invention is that the coupling of methanation and upgrading enables particularly flexible operation without large buffer and intermediate storage facilities. This may potentially enable the efficient conversion of hydrogen or renewable electricity to biomethane. With an ideal combination of inexpensive raw materials, efficient methanation, and the associated high methane yield, this could significantly reduce the currently required high indirect subsidies via electricity feed-in tariffs for biogas plants, as the plants can be largely profitable without subsidies through revenues from the sale of other products such as biomethane and carbon black.

[0124] Without methanation, half of the carbon captured by the plants in biogas escapes back into the atmosphere, possibly unused, in the form of carbon dioxide separated during gas upgrading. With the additional complete conversion of the carbon dioxide present in the externally supplied carbon dioxide-containing gas stream (e.g., present in biogas at up to 50 vol.%) to methane, up to twice as much carbon can be removed from the atmosphere in the medium term and used for material and / or energy purposes.

[0125] The present invention potentially provides a fully integrated solution for the methanation of green carbon dioxide from biogas plants using renewable hydrogen. The methane yield of the biogas plant may be increased, thus improving its economic efficiency.

[0126] The invention can be installed and operated as a container solution at any existing biogas plant, where possible, to feed the produced biomethane into an existing natural gas grid, or to use it in methane pyrolysis for hydrogen and carbon production. The capacity is scalable and can be expanded modularly, depending on the size of the biogas plant. The methanation capacity can be continuously adjusted, depending on the amount of renewable hydrogen available.

[0127] Gas processing is integrated where appropriate, and its capacity can be continuously adjusted. If necessary, it can also be adapted to different mass flow rates and compositions, such as those that may occur with different operating modes of the methanation reactor when different amounts of hydrogen are processed.

[0128] Further features of the invention emerge from the patent claims, the figures and the description of the embodiments.

[0129] The present invention is explained in detail below using exemplary, non-limiting embodiments.

[0130] Fig. 1 shows a simplified process diagram of a first embodiment of the present invention; and

[0131] Fig. 2 shows a simplified process diagram of a second embodiment of the present invention.

[0132] Unless otherwise indicated, the following components or material flows are shown in the figures: Methanization reactor 1, gas separation device 2, biogas plant 3, first separation stage 4, second separation stage 5, third separation stage 6, gas separation membrane 7, bypass device 8, feed line 9, discharge line 10, recirculation device 11, gas network 12, pyrolysis device 13, electrolysis device 14, gas compression device 15, gas cooling device 16, gas drying device 17, gas cleaning device 18, external hydrogen source 19, membrane separation device 20, reactant gas stream 100, hydrogen-containing gas stream 101, product gas stream 102, methane-rich fraction 103, carbon dioxide-rich fraction 104, first retentate stream 105, first permeate stream 106, second retentate stream 107, second Permeate stream 108, third retentate stream 109, third permeate stream 110, bypass stream 111, solid carbon 112, externally supplied carbon dioxide-containing gas stream 113, exhaust gas stream 114. Fig.1 shows a simplified process diagram of a first embodiment of the present invention.

[0133] A carbon dioxide-containing reactant gas stream 100 is compressed by a first compression device 15', then mixed with a hydrogen-containing gas stream 101, and subsequently introduced into a methanation reactor 1 via a feed line 9. The methanation reactor 1 contains a liquid fermentation medium and microorganisms of the species Methanothermobacter marburgensis. In this embodiment, the temperature in the methanation reactor is approximately 65°C, and the gas pressure is approximately 10 bar.

[0134] Under these conditions, the carbon dioxide introduced into the methanation reactor 1 is converted by the microorganisms with the hydrogen to form methane. This forms a methane-containing product gas stream 102, which is discharged from the methanation reactor 1 via a discharge line 10.

[0135] In this embodiment, the product gas stream 102 is subjected to several treatment steps before further processing and is passed through a gas cleaning device 18, a gas cooling device 16 and a gas drying device 17, in particular a droplet separator.

[0136] The product gas stream 102 thus treated is introduced into a gas separation device 2, which is designed to separate the product gas stream 102 into a methane-rich fraction 103 and a carbon dioxide-rich fraction 104.

[0137] In this exemplary embodiment, the gas separation device 2 comprises three separation stages 4, 5, 6, each containing a gas separation membrane 7. The separation stages 4, 5, 6 can be designed in a known manner and, in this exemplary embodiment, have a CO2 / CH4 selectivity of approximately 60, i.e., a permeance with respect to carbon dioxide that is 60 times higher than the permeance with respect to methane. The separation stages 4, 5, 6 are arranged such that the methane content of the gas is higher on the retentate side than on the permeate side. In this exemplary embodiment, the three separation stages 4, 5, 6 are interconnected such that the product gas stream 102 is introduced into the first separation stage 4. This forms a methane-rich first retentate stream 105 and a carbon dioxide-rich first permeate stream 106. The first retentate stream 105 is introduced into the second separation stage 5, whereby a second retentate stream 107 and a second permeate stream 108 are formed.The first permeate stream 106 is introduced into the third separation stage 6 to form a third retentate stream 109 and a third permeate stream.

[0138] Depending on the operating scenario, the second permeate stream 108 and the third retentate stream 109 are completely recycled into the reactant gas stream 100, and the third permeate stream 110 is partially or completely recycled as a carbon dioxide-rich fraction 104.

[0139] The methane-rich fraction 103, formed by the second retentate stream 107, contains at least 97 vol.% methane in this exemplary embodiment and is essentially free of hydrogen. If the specifications of a local gas grid permit, the methane-rich fraction 103, or at least a portion of the methane-rich fraction 103, can be introduced into a gas grid 12.

[0140] Alternatively or additionally, the methane-rich fraction 103 or a portion of the methane-rich fraction 103 can be introduced into a pyrolysis device 13. In the pyrolysis device 13, gaseous hydrogen and solid carbon 112 can be formed from the methane contained in the methane-rich fraction 103. The hydrogen can be fed into the hydrogen-containing gas stream 101 and fed back into the methanation reactor 1 via this stream. The solid carbon 112 can be used as a process end product for further use. The thermal energy required for pyrolysis can be provided, for example, by a microwave plasma powered by electricity.

[0141] A heat flow can be diverted from the pyrolysis unit 13 and returned to the process. This heat flow can be used to heat the methanation reactor 1. Depending on the operating conditions, parts of the process can thus be operated in a thermally self-sufficient manner.

[0142] Alternatively or additionally, the hydrogen in the hydrogen-containing gas stream 101 can also originate from other sources. In this exemplary embodiment, an electrolysis device 14 is provided as an additional hydrogen source, which is used, in particular, when inexpensive, sustainably generated electrical power is available to produce hydrogen for the electrolysis of water. In this exemplary embodiment, the hydrogen obtained during electrolysis and pyrolysis has a gas pressure approximately 250 mbar higher than the gas pressure in the methanation reactor 1. This leads to energy savings, since no compression of the hydrogen or the hydrogen-containing gas stream 101 is required.

[0143] Alternatively, the hydrogen in the hydrogen-containing gas stream 101 can also originate from an external hydrogen source 19. In this exemplary embodiment, an external hydrogen source 19 is provided as an external supply from a gas network or from cylinders and / or tanks.

[0144] The carbon dioxide-rich fraction 104 can be recycled back into the reactant gas stream 100 via a recirculation device 11, which in this embodiment also has a mixing function. In this embodiment, an externally supplied carbon dioxide-containing gas stream 113 is mixed with the carbon dioxide-rich fraction 104 in the recirculation device 11 to form the reactant gas stream 100.

[0145] The externally supplied carbon dioxide-containing gas stream 113 can in particular originate from a biogas plant 3 and contain carbon dioxide and methane.

[0146] In this exemplary embodiment, a control device is provided that controls the gas flows in the process. In particular, the gas flows are controlled such that the molar ratio of hydrogen from the hydrogen-rich gas stream 101 and carbon dioxide from the carbon dioxide-rich fraction 104 is approximately 4:1. This provides as much hydrogen as is required for the methanation of the carbon dioxide contained in the carbon dioxide-rich fraction 104. In addition, the control is carried out such that additional carbon dioxide is introduced into the reactant gas stream 100 and thus into the methanation reactor 1 via the externally supplied carbon dioxide-containing gas stream 113. The gas flows are controlled such that the molar ratio between the hydrogen introduced into the methanation reactor 1 and the carbon dioxide introduced into the methanation reactor 1 is approximately 2:1.As a result, less hydrogen is available in the methanation reactor 1 than would be required for the complete conversion of the introduced carbon dioxide. This allows a product gas stream 102 to be obtained that is essentially hydrogen-free, thus eliminating the need to remove excess hydrogen. In addition, the excess carbon dioxide in the liquid fermentation medium in the methanation reactor 1 maintains a pH value between 4 and 5, with the dissolved carbon dioxide providing a buffering effect. This pH value allows for advantageous conversion rates in the methanation reaction. In particular, a larger amount of CO2 results in a lower pH value.

[0147] In order to adjust the described ratio between carbon dioxide and hydrogen in the methanation reactor 1, the control device also controls which portion of the methane-rich fraction 103 is introduced into the pyrolysis device 13, and / or which amount of water is electrolyzed in the electrolysis device 14, and / or which amount of hydrogen is drawn from the external hydrogen source 19, for example, a gas network or cylinders and / or tanks. Furthermore, it controls which portion of the carbon dioxide-rich fraction 104 is returned to the reactant gas stream 100. The amount of electricity available for the electrolysis can also be taken into account within the scope of the control system.

[0148] A bypass stream 111 can be bypassed past the methanation reactor 1 via a bypass device 8 if the carbon dioxide contained in the reactant gas stream is not to be converted into methane. This bypass stream 111 can be recirculated to the product gas stream 102 before the product gas stream 102 is introduced into the gas separation device 2. In this exemplary embodiment, the gas compositions of the second retentate stream 107 are controlled based on the gas pressure of the second retentate stream 107 and the second permeate stream 108, and those of the third permeate stream 110, as well as the gas pressure of the third retentate stream 109, as manipulated variables. Input variables for the control system are the composition and volume flow of the reactant gas stream 100, parameters from the methanation and parameters from the hydrogen supply, as well as pressures, temperatures, and volume flows before, between, and after the separation stages 4, 5, and 6.

[0149] In this embodiment, a second gas compression device 15" is provided in the second permeate stream 108.

[0150] In combination with the control, different control scenarios are possible in this embodiment.

[0151] The externally supplied carbon dioxide-containing gas stream 113 can continuously supply a certain amount of carbon dioxide-containing gas with a certain composition.

[0152] In a first scenario, no inexpensive electricity is available to generate hydrogen in the electrolysis device 14, and external supply from other sources is also not possible. The reactant gas stream 100 is then passed via the bypass device 8 or the bypass stream 111 past the methanation reactor 1 into the gas separation device 2, where it is separated into the methane-rich fraction 103 and the carbon dioxide-rich fraction 104. The methane-rich fraction 103 can then be fed into the gas grid 12. The carbon dioxide-rich fraction 104 is released into the environment after further treatment to remove greenhouse gas methane. In this scenario, the system's net carbon dioxide emissions are reduced by the equivalent of the portion fed into the gas grid 12 in the form of methane.

[0153] In a second scenario, there is insufficient cheap electricity available to completely convert the carbon dioxide contained in the externally supplied carbon dioxide-containing gas stream 113 into methane. The entire externally supplied carbon dioxide-containing gas stream 113 is then compressed to the operating pressure of the methanation reactor 1 by means of the first gas compression device 15'. Hydrogen, also pressurized, is supplied via the hydrogen-containing gas stream 101. The gas flows are controlled by the control device such that the biological methanation in the methanation reactor 1 proceeds with an excess of carbon dioxide under acidophilic high-pressure conditions. The resulting product gas stream 102 is passed through a gas purification device 18 with an activated carbon filter for desulfurization and a gas drying device 17 with a droplet separator and then introduced into the gas separation device 2.

[0154] The second permeate stream 108 and the third retentate stream 109 obtained by the gas separation device 2 are completely recycled to the reactant gas stream 100 as part of the carbon dioxide-rich fraction 104. A partial stream of the third permeate stream 110, the amount of which is determined by the desired excess of carbon dioxide, is also recycled to the reactant gas stream 100 as part of the carbon dioxide-rich fraction 104. Thus, the carbon dioxide-rich fraction is fed back into the methanation reactor 1. The remaining partial stream of the third permeate stream 110 is post-treated as exhaust gas stream 114 and released to the environment.

[0155] The resulting methane-rich fraction 103, which in this embodiment corresponds to the second retentate stream 107, can either be fed into the gas network 12 or split into hydrogen and solid carbon in the pyrolysis unit 13. The resulting hydrogen can be reintroduced into the hydrogen-rich gas stream 101. The recovered solid carbon 112 represents a valuable commodity for sale. The system's net carbon dioxide emissions are reduced by the portion that is fed into the gas network 12 in the form of methane or separated in the pyrolysis unit 13 as solid carbon 112.

[0156] In a third scenario, the entire required amount of hydrogen can be provided by external supply, electrolysis, or pyrolysis to convert the carbon dioxide present in the externally supplied carbon dioxide-containing gas stream 113 into methane. Operation proceeds as in the second scenario described above. However, the entire third permeate stream 110 is recycled into the reactant gas stream 100, and there is no exhaust gas stream 114. The net carbon dioxide emissions of the system are reduced by the portion that is fed into the gas network 12 in the form of methane or that is separated as solid carbon during pyrolysis. In the case of pyrolysis, this corresponds to the entire amount of carbon in the externally supplied carbon dioxide-containing gas stream 113.

[0157] In a fourth scenario, all or part of the required hydrogen can be provided from a gas grid as an external hydrogen source 19. The Austrian gas grid already contains a certain amount of hydrogen. This can be separated from the natural gas stream using a particularly simple membrane gas separation process and made available for methanation. In addition, various suppliers offer stationary tanks filled with liquid, cryogenic, or highly compressed hydrogen, which can be used as an external hydrogen source 19. Hydrogen from these external hydrogen sources 19 can form part or all of the hydrogen-containing gas stream 101.

[0158] Fig. 2 shows a simplified process diagram of a second embodiment of the present invention.

[0159] This second embodiment largely corresponds to the first embodiment in terms of its function and is therefore not described in detail. Compared to the first embodiment, the gas separation device 2 in this second embodiment is designed with only two separation stages 4, 5.

[0160] The first separation stage 4 and the second separation stage 5 are connected in series. The product gas stream 102 is introduced into the first separation stage 4, forming a methane-rich first retentate stream 105 and a carbon dioxide-rich first permeate stream 106. The first retentate stream 105 is introduced into the second separation stage 5, forming a methane-rich second retentate stream 107 and a carbon dioxide-rich second permeate stream 108. The second retentate stream 107 forms the methane-rich fraction 107. The first permeate stream 106 and the second permeate stream 108 can be recycled to the methanation reactor 1 as the carbon dioxide-rich fraction 104. Within the scope of the control system, the first permeate stream 106 can, if necessary, only be partially recycled. An offgas stream 114 is then discharged into the environment.

Claims

Patent claims 1. A process for producing methane from a carbon dioxide-containing gas mixture comprising the following steps: - Introducing a carbon dioxide-containing reactant gas stream (100) and a hydrogen-containing gas stream (101) into a methanization reactor (1 ), - Formation of a methane-containing product gas stream (102) by reacting the carbon dioxide with hydrogen by anaerobic fermentation in the methanization reactor (1 ), - Diverting the product gas stream (102) from the methanization reactor, - Introducing the product gas stream (102) into a gas separation device (2) and separating the product gas stream (102) into a methane-rich fraction (103), in particular essentially hydrogen-free, and into a carbon dioxide-rich fraction (104), - partial or complete recycling of the carbon dioxide-rich fraction (104) to the reactant gas stream (100), whereby a control is carried out such that the molar ratio of the hydrogen introduced into the methanation reactor (1) by the hydrogen-containing gas stream (101) n Hz to the carbon dioxide introduced into the methanization reactor (1) by the carbon dioxide-rich fraction (104) COz a predetermined ratio is fulfilled.

2. Method according to claim 1, characterized in that the molar ratio of the hydrogen introduced into the methanization reactor (1) by the hydrogen-containing gas stream (101) is n Hz to the carbon dioxide introduced into the methanization reactor (1) by the carbon dioxide-rich fraction (104) COz , represented by the equation: nH2 : n co2= ^ :1 the condition 3 < X < 5 is satisfied, preferably the condition 3.8 < X < 4.2, especially preferably the condition X = 4, is satisfied.

3. Method according to claim 1 or 2, characterized in that furthermore, control is provided such that the molar ratio of the The ratio of the carbon dioxide introduced into the methanation reactor (1) in the reactant gas stream (100) to the hydrogen introduced into the methanation reactor (1) by the hydrogen-containing gas stream (101) is greater than 0.2, preferably between 0.2 and 25, and particularly preferably between 0.25 and 3. A process according to any one of claims 1 to 3, characterized in that the reactant gas stream (100) comprises an externally supplied carbon dioxide-containing gas stream (113), which originates, for example, from a biogas plant (3), from another fermentation plant, or from another gas source that provides a mass stream comprising carbon monoxide, carbon dioxide, methane, and / or hydrogen.A process according to any one of claims 1 to 4, characterized in that the hydrogen-containing gas stream (101) comprises hydrogen gas obtained by pyrolysis of an organic compound or a mixture of several organic compounds, in particular biomass and / or methane, and / or that the hydrogen-containing gas stream (101) comprises hydrogen gas obtained from the electrolysis of water. A process according to any one of claims 1 to 5, characterized in that the hydrogen-containing gas stream (101) comprises hydrogen gas obtained by pyrolysis of methane from the methane-rich fraction.A process according to claim 5 or 6, characterized in that the hydrogen obtained during pyrolysis and / or electrolysis has a gas pressure that is higher than the gas pressure in the methanation reactor (1), wherein the pressure difference between the hydrogen obtained and the methanation reactor (1) is in particular greater than 200 mbar, preferably between 200 mbar and 1500 mbar. A process according to any one of claims 1 to 7, characterized in that the hydrogen-containing gas stream (101) comprises hydrogen gas that originates from an external hydrogen source (19), for example a gas supply network or storage facilities such as tanks and / or cylinders. A method according to any one of claims 1 to 8, characterized in that the hydrogen-containing gas stream (101) comprises hydrogen gas originating from a natural gas network, wherein the hydrogen gas is obtained from the natural gas extracted from the natural gas network by membrane separation. A method according to any one of claims 1 to 9, characterized in that the amount of hydrogen introduced into the methanization reactor (1) is adjusted based on an availability parameter, wherein the availability parameter takes into account, in particular, one or more of the following aspects: availability of carbon dioxide; availability of electricity; price of electricity; availability of hydrogen from external hydrogen sources. A method according to any one of claims 1 to 10, characterized in that the gas pressure in the methanization reactor (1) is higher than the gas pressure in the gas separation device (2).A process according to any one of claims 5 to 11, characterized in that the proportion of the methane-rich fraction (103) provided for pyrolysis is controlled depending on the amount of carbon dioxide introduced into the methanation reactor (1) by the reactant gas stream (100). A process according to any one of claims 5 to 12, characterized in that a heat flow is obtained by the pyrolysis and / or by the electrolysis, which is supplied to the methanation reactor (1) and / or the biogas plant (3) for heating purposes. A process according to any one of claims 1 to 13, characterized in that the gas separation device (2) comprises at least one separation stage (4, 5, 6) with a gas separation membrane (7), wherein the permeability of the gas separation membrane (7) to carbon dioxide is higher than to methane, and wherein the gas separation device (2) in particular comprises two or three interconnected separation stages (4, 5, 6). A method according to claim 14, characterized in that the retentate pressures and / or the permeate pressures of the entire gas separation device (2) and / or the individual separation stages (4, 5, 6) are control variables. A method according to claim 14 or 15, characterized in that the gas separation device (2) comprises two interconnected separation stages (4, 5, 6). - wherein the product gas stream (102) is introduced into a first separation stage (4) so ​​that a first retentate stream (105) and a first permeate stream (106) are obtained, - wherein the first retentate stream (105) is introduced into a second separation stage (5) so that a second retentate stream (107) and a second permeate stream (108) are obtained, - wherein the first permeate stream (106) is at least partially and the second permeate stream (108) is completely recycled to the reactant gas stream (100). Method according to claim 14 or 15, characterized in that the gas separation device (2) has three interconnected separation stages (4, 5, 6), - wherein the product gas stream (102) is introduced into a first separation stage (4) so ​​that a first retentate stream (105) and a first permeate stream (106) are obtained, - wherein the first retentate stream (105) is introduced into a second separation stage (5) so that a second retentate stream (107) and a second permeate stream (108) are obtained, - wherein the first permeate stream (106) is introduced into a third separation stage (6) so that a third retentate stream (109) and a third permeate stream (110) are obtained, - wherein the second permeate stream (108) and the third retentate stream (109) are completely and the third permeate stream (110) is at least partially recycled to the reactant gas stream (100). Method according to claim 17, characterized in that the pressure of the second retentate stream (107), the pressure of the third retentate stream (109) and the pressure of the second permeate stream (108) are control variables. A process according to any one of claims 1 to 18, characterized in that the reactant gas stream (100) is introduced, or can be introduced, partially or completely into the product gas stream (102) and / or into the gas separation device (2) via a bypass stream (111) without conversion of the carbon dioxide in the methanation reactor (1). A process according to any one of claims 1 to 19, characterized in that the control is such that the methane content in the methane-rich fraction (103) is at least 85 vol.%, preferably 95 vol.%, particularly preferably 97.5 vol.%. A process according to any one of claims 1 to 20, characterized in that - that the reaction takes place in the methanization reactor (1) at a temperature between 20°C and 100°C, preferably between 40°C and 90°C, particularly preferably between 60°C and 88°C, - and / or that the conversion in the methanization reactor (1) takes place at a gas pressure between 5 bara and 50 bara, preferably between 10 bara and 16 bara, - and / or that the reaction in the methanization reactor (1) is carried out using microorganisms from the group of methanogenic archaea. A process according to any one of claims 10 to 21, characterized in that the liquid fermentation medium arranged in the methanization reactor (1) is brought to a temperature of below 15°C, preferably between 4 and 10°C, when the availability parameter falls below a predetermined limit value and thus no hydrogen is available for the reaction. A process according to any one of claims 1 to 22, characterized in that the pH value of the liquid fermentation medium arranged in the methanization reactor (1) is between 3.5 and 7, preferably between 4 and 6. A process according to any one of claims 1 to 23, characterized in that the gas separation device (2) is operated at a gas pressure between 5 bar and 50 bar, preferably between 10 bar and 17 bar. A process according to any one of claims 1 to 24, characterized in that the liquid fermentation medium arranged in the methanization reactor (1) comprises microorganisms in pure culture or in mixed culture, wherein the microorganisms are preferably selected from the group of methanogenic archaea, particularly preferably from one or more species of the following group: M. igneus, M. indicus, M. infernus, M. jannaschii, M. kandleri, M. lacus, M. lacustris, M. marburgensis, M. marinum, M. maripaludis, M. mazei, M. millerae, M. palustris, M. petrolearium, M. petrolearius, M. siciliae, M. soligelidi, M. spelaei, M. submarinus, M. thermaggregans, M. veterum, M. villosus, or M. vulcanius, more preferably from one or more species of the following Group: M. aeolicus, M. igneus, M. jannaschii, M. kandleri, M. marburgensis, M. maripaludis, M. submarinus, M. thermaggregans, M. villosus, M. vulcanius.A method according to any one of claims 1 to 25, characterized in that a nutrient supply to the methanization reactor (1) is provided, wherein the nutrient supply is controlled based on a methane production parameter. A method according to claim 26, characterized in that the methane production parameter correlates with one or more of the following parameters: consumption rate of one or more of the following: phosphate or phosphorus, sulfide or sulfur, ammonium or nitrogen, trace elements, hydrogen, carbon dioxide; concentration of the respective nutrients; temperature of the fermentation medium; process pressure. A method according to any one of claims 1 to 27, characterized in that the productivity characteristics and / or system control are performed based on one or more basic control variables.Method according to claim 28, characterized in that the basic control variable or basic control variables are selected from one or more of the following:. - optical density or a related parameter of the fermentation medium, in particular between 0.5 and 50, preferably between 5 and 20, - Quantity and / or composition of the individual gas streams, - cell-specific growth and / or death rate, nutrient uptake rate, especially of phosphate or phosphorus, sulfide or sulfur, ammonium or nitrogen or trace substances, - Uptake rate of hydrogen and / or carbon dioxide, - Production rate of methane and / or water, - Temperature and / or process pressure in the methanization reactor (1 ), - temporal change of a parameter. A method according to any one of claims 1 to 29, characterized in that the discharge of biomass and reaction water from the methanization reactor (1) is controlled by correlation with a gas production rate, such that a substantially constant methanization productivity is preferably achieved. A method according to any one of claims 1 to 30, characterized in that a power availability parameter is used to control the method, wherein, depending on the availability of electrical current, the operating state is adjusted such that - methane is produced from carbon dioxide and hydrogen using the methanization reactor (1 ). - the methane content in the second retentate stream (107) from the gas separation device (2) is at least 85 vol.%, preferably at least 97 vol.%, - a biomass stream is converted into carbon dioxide and hydrogen and subsequently into methane - the methane-rich second retentate stream (107) is fed into an existing gas network (12) and / or used in pyrolysis to produce solid carbon and gaseous hydrogen, - both the methane-rich second retentate stream (107) and the hydrogen stream from pyrolysis are fed into an existing gas network (12), - the injected gases are stored in gas storage facilities. A method according to any one of claims 1 to 31, characterized in that the methane-rich fraction (103) is fed into a transport pipeline, for example a natural gas network, with or without additional pressure increase. A method according to any one of claims 1 to 32, characterized in that at least a portion of the hydrogen-containing gas stream (101) is extracted from a hydrogen-bearing gas network (12). A method according to claim 33, characterized in that the hydrogen-containing gas stream (101) is separated from the gas network (12) by a membrane separation device (20), in particular a single-stage device. A device for producing methane from a carbon dioxide-containing gas mixture, in particular configured for carrying out a method according to any one of claims 1 to 34, wherein the device comprises the following components: - a methanization reactor (1) in which carbon dioxide is reacted with hydrogen by anaerobic fermentation, - a feed line (9) that introduces a carbon dioxide-containing reactant gas stream (100) and a hydrogen-containing gas stream (101) into the methanization reactor (1), - a derivative (10) that derives a methane-containing product gas stream (102) obtained by anaerobic fermentation from the methanization reactor, - a gas separation device (2) which separates the product gas stream (102) into a methane-rich fraction (103), in particular essentially hydrogen-free, and into a carbon dioxide-rich fraction (104), - a recirculation device (11) that recirculates the carbon dioxide-rich fraction (104) back to the reactant gas stream (100), - a control device that determines the molar ratio of the hydrogen introduced into the methanization reactor (1) by the hydrogen-containing gas stream (101 ). Hz to the carbon dioxide introduced into the methanization reactor (1) by the carbon dioxide-rich fraction (104) C02controls a predetermined ratio. Device according to claim 35, characterized in that the gas separation device (2) comprises at least one membrane-based separation stage (4, 5, 6), wherein the mixed gas selectivity of the separation stage (4, 5, 6) for the separation of carbon dioxide and methane is greater than 30, in particular greater than 50. Device according to claim 35 or 36, characterized in that a bypass device (8) is provided which introduces at least a portion of the reactant gas stream (100) into the gas separation device (2) without conversion of the carbon dioxide in the methanation reactor (1). Device according to any one of claims 35 to 37, characterized in that a pyrolysis device (13) is provided which pyrolyzes the methane-rich fraction (103) of the product gas stream (102) to produce hydrogen, and / or that an electrolysis device (14) is provided which electrolyzes water to produce hydrogen, and / or that an external hydrogen source is provided.Device according to any one of claims 35 to 38, characterized in that the produced methane is fed into a transport pipeline, for example a natural gas network, with or without additional pressure increase. Device according to any one of claims 35 to 39, characterized in that the hydrogen introduced into the methanization reactor is taken from a hydrogen-bearing gas network (12). Device according to claim 40, characterized in that the separation of the hydrogen from the hydrogen-containing gas is carried out by a membrane separation device, in particular a single-stage device.

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