Process for generating a methane-enriched gas

Abstract

The invention relates to a process for generating a methane-enriched gas using a bioreactor. The bioreactor comprises: at least one inlet for supplying hydrogen and carbon dioxide into the bioreactor; at least one inlet for supplying fresh substrate into the bioreactor; at least one outlet for removing used substrate from the bioreactor; and at least one outlet for removing methane-enriched gas formed in the bioreactor. The process for generating a methane-enriched gas comprises the steps of: providing such a bioreactor; removing from the bioreactor the methane-enriched formed in the bioreactor; determining the amount of methane formed in the bioreactor in a predetermined period of time; removing used substrate from the bioreactor; and supplying fresh substrate into the bioreactor. The amount of fresh substrate supplied into the bioreactor depends on the amount of methane formed in the bioreactor in the predetermined period of time.

Description

[0001] Process for producing a methane-enriched gas

[0002] Technical area

[0003] The invention relates to a process for producing a methane-enriched gas.

[0004] State of the art

[0005] As part of the energy transition, renewable energies are accounting for an ever-increasing share of generated energy. Since renewable energy sources such as wind power or photovoltaics are not available continuously and evenly in terms of both time and quantity, and are met with demand from consumers that also fluctuates over time, storage options for renewable energies are becoming increasingly important.

[0006] Methane as a chemical energy carrier is of central importance in this context, as it has a wide range of applications in the areas of heat generation, fuels and electricity generation, is a form of energy that can be easily stored and, unlike hydrogen, enables the use of the existing infrastructure with a natural gas network and natural gas filling stations.

[0007] Methane is produced from the starting materials carbon dioxide and hydrogen. In the vast majority of the processes described in the prior art, methanation takes place at a ratio of the reactant gases H2 to CO2 of 4:1, so that according to the reaction equation of the methanation reaction

[0008] 4 H2 + CO2CH4 + 2 H2O where there is no excess reactant gas. The hydrogen is usually produced by electrolysis of water using an electrolyzer, using surplus electricity or other inexpensive electricity. The carbon dioxide required for methane production can come from various sources such as industrial or combustion exhaust gases, but climate-friendly CO2 from renewable sources such as biomass is preferred, as is the case in biogas plants or as digester gas in sewage treatment plants. Particularly in energy conversion systems that convert excess electricity into the gaseous chemical energy carrier methane according to the "power-to-gas" principle, biological methanation is often carried out, in which biomethane is formed by methanogenic microorganisms.Compared to chemically catalytic methanation, for example according to the Sabatier process, biological methanation does not require expensive and sensitive catalysts and places lower demands on reaction conditions such as temperature and pressure as well as on the purity of the starting gases CO2 and H2.

[0009] In a biological methanation reactor, in which the desired product methane is continuously produced using an aqueous medium, it is common practice to exchange a certain proportion of the medium over time in order to maintain a suitable concentration of essential minerals over time and to remove metabolic products that would inhibit methanogenesis.

[0010] WO 2008 / 094282 A1 describes such a biological system for methane production from hydrogen and carbon dioxide using a microorganism culture in a culture medium. The carbon dioxide comes from an industrial process, while the hydrogen is obtained, among other things, through electrolysis using cheap surplus electricity. The medium is exchanged at a constant rate, meaning a constant volume of medium is exchanged per reaction time. EP 3 072 979 A1 and WO 2014 / 076062 A1 describe similar processes and also perform a constant-rate medium exchange.

[0011] Since the hydrogen for biological methanation is produced by electrolysis of water using an electrolyzer, but the electricity used for this purpose from wind power or photovoltaics is not available in constant quantities over extended periods, significantly different amounts of methane are produced in "power-to-gas" plants over time. It has been shown that a constant-rate media exchange, as described in the state of the art, does not guarantee stable methane production with good gas quality in such a system in the long term.

[0012] Therefore, there is still a need for biological methanation processes that can produce a methane-enriched gas with good product gas quality using hydrogen and CO2 in a stable process. Description of the invention

[0013] The invention, as characterized in the claims, is based on the object of providing a process for biological methanation which is suitable for producing a methane-enriched gas with good product gas quality using hydrogen and CO2 in a stable process.

[0014] This object is achieved according to the invention by the method for producing a methane-enriched gas according to claim 1 and the plant for carrying out such a method according to claim 14. Further advantageous details, aspects, and embodiments of the present invention emerge from the dependent claims and the description.

[0015] The present invention provides a method for producing a methane-enriched gas comprising the steps a) providing a bioreactor comprising

[0016] - at least one inlet for the supply of hydrogen and carbon dioxide into the bioreactor,

[0017] - at least one inlet for the supply of fresh substrate into the bioreactor,

[0018] - at least one outlet for the removal of waste substrate from the bioreactor,

[0019] - at least one outlet for the removal of methane-enriched gas formed in the bioreactor, b) removal of the methane-enriched gas formed in the bioreactor from the bioreactor, c) determining the amount of methane formed in the bioreactor in a predetermined period of time, d) removal of used substrate from the bioreactor, e) supply of fresh substrate to the bioreactor.

[0020] According to the invention, the amount of fresh substrate fed into the bioreactor depends on the amount of methane formed in the bioreactor during the predetermined period of time.

[0021] The process according to the invention is carried out using a bioreactor to produce a methane-enriched gas by biological methanation. The bioreactor can be designed in various reactor types, such as stirred tank reactors, column reactors, tubular reactors, bubble columns, trickle-bed reactors, or even as a reactor cascade. It should be made of pressure- and temperature-resistant material, such as stainless steel. Compared to the anaerobic fermenters of a biogas plant or the digestion tower of a wastewater treatment plant, the reactor volume of a reactor for biological methanation is significantly smaller to produce the same amount of biomethane, for example, in a range of 1 / 10 to 1 / 1000 of the volume of a fermenter of a biogas plant or a digestion tower of a wastewater treatment plant.

[0022] The bioreactor is fed with a carbon dioxide-containing gas and an externally generated hydrogen-containing gas. The hydrogen required for biological methanation is preferably produced by electrolysis of water using an electrolyzer. The electrolyzer is preferably powered by electricity from renewable energy sources.

[0023] Surplus electricity from the power grid is particularly preferred. The provision of surplus electricity is typically handled by a balancing energy box, allowing the biological methanation plant to participate in the balancing energy market at the level of the corresponding electrolyzer's output. Even if the electrolyzer does not participate in the balancing energy market, cheaper electricity can be used elsewhere (e.g., from special electricity tariffs). Alternatively, hydrogen from other sources such as hydrolysis gas, synthesis gas, product gas from chemical reactions, hydrogen of biological origin, such as hydrogen produced by algae, or hydrogen from photocatalysis, can also be used for biological methanation.

[0024] In the process according to the invention, the substrate exchange in the bioreactor occurs in such a way that the amount of fresh substrate fed into the bioreactor depends on the amount of methane formed in the bioreactor during the predetermined period. By exchanging the substrate, which depends on the amount of methane formed, the concentration of various salts and minerals in the bioreactor can be kept nearly constant, thus ensuring a stable methanation process.

[0025] The methane production rate achieved in the respective biological methanation process can therefore also serve as a measure of the frequency of exchange of a specific amount of substrate or as a measure of the amount of substrate exchanged. At high methane production rates, substrate exchange occurs significantly more frequently and in larger quantities than at lower methane production rates.

[0026] With the exception of the initial filling during commissioning of the bioreactor, the substrate replacement is carried out during ongoing reactor operation, depending on the amount of biomethane produced. For this replacement, old substrate is cyclically drained and then the reactor is refilled with fresh substrate. Along with the old substrate, the water resulting from the methanation reaction is also removed. In the simplest case, the substrate, if based on digested sludge or digestate, is returned to the wastewater treatment plant or biogas plant.

[0027] In none of the prior art processes for biological methanation is the rate at which substrate is exchanged in the bioreactor related to the amount of methane formed as a reaction product. In known cases, the exchange rate [h -1] refers to the reaction time of methanation, whereby the proportion of the reactor volume that is exchanged per unit of time is usually specified. Also related to the reaction time of methanation is the feed rate [I h -1 ], Here, the volume of the reactor contents is taken into account and the volume of substrate added per unit time is specified. In a few cases, the rate of substrate exchange is also related to the doubling time of the methanogenic microorganisms responsible for methane production, with the substrate exchange occurring at a rate reciprocal to the doubling time.

[0028] A key advantage of the process according to the invention compared to other processes known in the prior art is that it increases economic efficiency, as the methane formation rate is significantly increased. At the same time, establishing the process according to the invention requires virtually no investment in existing plants, as existing infrastructure and technology can be utilized.

[0029] The bioreactor contains an aqueous reaction medium that serves as a substrate or nutrient medium for the methanogenic microorganisms and thus fulfills the function of a methanation medium. The medium in which the methanation reaction takes place is either a synthetic culture medium for methanogenic microorganisms, as described in the prior art (e.g., WO 2008 / 094282 A1, EP 2 675 904 B1), or a complex nutrient medium based on biomass is used, which essentially consists of a digestate as a substrate and is optionally supplemented with nutrients and / or trace elements (e.g., EP 2 982 740 A1).

[0030] The more nutrients and trace elements already present in a fresh substrate, the lower the amount of additives required can be. For example, the digestate from a biogas plant, especially the digestate from a later fermentation stage or a secondary digester, can be used as a substrate. The digestate can also come from a final repository. Likewise, in principle, any type of raw sludge and / or digested sludge from a wastewater treatment plant is suitable as a substrate for the bioreactor for biological methanation.

[0031] Methanogenic microorganisms, particularly hydrogenotrophic methanogenic microorganisms, are added to a synthetic culture medium in the form of a pure culture or a mixed culture of suitable microorganisms. When digestate is used as a substrate, methanogenic microorganisms are usually already present, but additional methanogenic microorganisms, particularly hydrogenotrophic methanogenic microorganisms, can optionally be added. Thermophilic methanogens of the genus Methanothermobacter are particularly suitable.

[0032] According to a particularly preferred embodiment of the present invention, the amount of fresh substrate (MPhSubstrat) fed into the bioreactor depends on the amount of methane (VMethan) formed in the bioreactor during the predetermined period and on an exchange rate (ATR). It satisfies the equation (MPhSubstrat) = VMethan • ATR, where the exchange rate (ATR) satisfies the condition (ATR > 0). This embodiment makes it possible to specify a defined exchange rate for the biological methanation process. A stable methanation process with good product gas quality is achieved.

[0033] Preferably, the amount of fresh substrate [kg] fed into the bioreactor depends on the amount of methane VMethan [Nm 3 ] formed in the bioreactor in the predetermined period of time. 3 ] and an exchange rate ATR [kg / Nm 3 ] and satisfies the equation mprischsubstrat [kg] = VMethan [Nm 3 ] • ATR [kg / Nm 3], where the exchange rate ATR of the condition 0.01 kg / Nm 3 < ATR < 10 kg / Nm 3 is enough.

[0034] Particularly preferably, the exchange rate ATR satisfies the condition 1 kg / Nm 3 < ATR < 6 kg / Nm 3 For example, an exchange rate of 4 kg per Nm 3 of methane produced in the bioreactor. This means that 4 kg of fresh substrate is added to the bioreactor as soon as one standard cubic meter of pure methane has been produced.

[0035] It is clear to the person skilled in the art that, given a known substrate density, all mass quantities can be replaced by volume quantities. The unit for the exchange rate can therefore be specified, for example, in kilograms of fresh substrate per cubic meter of methane formed, or in liters of fresh substrate per cubic meter of methane formed. It should be noted that, in the context of this text, the term "amount of methane formed in the bioreactor in a predetermined period of time" actually refers to the exact amount of methane newly formed in the bioreactor in a predetermined period of time. If the reactant gas for the methanation reaction already contains methane because, for example, raw biogas or sewage gas is used as the carbon dioxide source, this must be taken into account when evaluating the measurement results, which reflect the amount of methane in the product gas.When calculating the amount of substrate to be exchanged, only the amount of methane newly formed in the bioreactor within a predetermined period of time may be taken into account.

[0036] The fresh substrate supplied in step e) of the process according to the invention is preferably a biological substrate with increased viscosity or a culture medium. Particularly preferably, the biological substrate with increased viscosity is digestate from a biogas plant or sewage sludge. Methanization reactors are often constructed near biogas plants or sewage treatment plants, which offers particular advantages with regard to the availability of fresh substrate and the associated low costs.

[0037] According to a preferred embodiment, the culture medium contains one or more of microorganisms, nutrients, and trace elements. According to this embodiment, nutrients and / or trace elements are also added to the bioreactor depending on the amount of biomethane produced. This results in a stable methanation process with excellent product gas quality.

[0038] In the processes known from the prior art, the addition of nutrients depends on the needs of the cultured cells and the consumption of nutrients in the bioreactor. Addition of nutrients depending on the amount of methane produced is not described in the prior art.

[0039] Particularly preferably, after step d) and before, together with, or after step e), step f) is carried out, adding one or more auxiliary substances to the bioreactor, wherein the one or more auxiliary substances are nutrients, trace elements, defoamers, acids, alkalis, or mixtures thereof. As already mentioned, the salt and mineral concentrations in the bioreactor can be kept virtually constant by substrate exchange depending on the amount of methane formed, thereby ensuring a stable methanation process. The salt and mineral concentrations in the bioreactor can be kept constant in a particularly advantageous manner if an adapted nutrient and trace element addition is carried out at the same time, which also avoids both a deficiency in and an overdose of nutrients and trace elements.

[0040] In principle, the amount of additives added, such as acid or alkali for pH control, defoamers, trace element, or nutrient solutions, influences the volume of the reactor contents, as does sampling, which leads to a reduction in the reactor contents. However, all additives are usually added in such a concentrated form that they only constitute a fraction of the reactor contents and can therefore be disregarded in volume calculations.

[0041] The removal of old substrate from the bioreactor and the supply of fresh substrate into the bioreactor can in principle be carried out continuously or discontinuously in a batch-wise substrate exchange at time intervals.

[0042] Continuous substrate exchange has the advantage that a nearly constant substrate level can be maintained in the bioreactor. Gas flow rates also remain essentially constant at a given methane production rate. Compared to batchwise substrate exchange, the pumps that supply fresh substrate and / or remove used substrate can be smaller, as they continuously exchange a smaller amount of substrate. Steps d) and e) of the process according to the invention are therefore preferably carried out continuously.

[0043] Particularly preferably, the removal of used substrate from the bioreactor in step d) and the supply of fresh substrate into the bioreactor in step e) are carried out in such a way that the total amount of substrate present in the bioreactor remains substantially constant, wherein the total amount of substrate present in the bioreactor corresponds to a predetermined target amount of substrate.

[0044] In a batchwise substrate exchange, the old substrate is first drained and then fresh substrate is added until a predetermined target value for the set reactor volume is reached. Batchwise substrate exchange has the advantage that the substrate pumps used for the substrate exchange do not have to run continuously, thus reducing wear. Steps d) and e) of the process according to the invention are therefore preferably carried out batchwise.

[0045] Regardless of the substrate type (digested sludge, digestate, artificial medium), the pressure in the reactor is kept constant. This results in significant changes in the product gas volume flow during substrate exchange, which decreases during the discharge of used substrate and increases during substrate supply. For this reason, a downstream gas treatment plant must be designed for a higher gas volume flow than the gas volume flow specified by the respective reactant gas volume flow. This effect is significantly more pronounced in batchwise substrate exchange than in continuous substrate exchange, since a higher amount of substrate is exchanged per unit time in batchwise exchange.

[0046] Particularly preferably, the amount of fresh substrate m fresh substrate [kg] fed into the bioreactor per batch depends on a predetermined target amount of substrate m target substrate [kg] present in the bioreactor and satisfies the condition 0.01 m target substrate < m fresh substrate 0.1 m target substrate. The amount of fresh substrate m fresh substrate [kg] fed into the bioreactor per batch can be calculated knowing the amount of substrate contained in the respective bioreactor. A practical solution has proven to be to exchange the substrate when approximately 5% of the substrate volume is to be exchanged. Exchanges of up to 10% of the substrate volume can also be tolerated without the changes in the fill level in the reactor having an excessive impact on the biological methanation process.

[0047] Since in the methanation reaction according to the equation

[0048] CO2 + 4 H2CH4 + 2 H2O, in addition to methane, water is also formed, the amount of methane formed in the bioreactor in a predetermined period of time can be determined by determining the amount of water formed in the bioreactor in the predetermined period of time. According to a further preferred embodiment of the present invention, the amount of methane VMethane formed in the bioreactor in a predetermined period of time in step c) is therefore determined by determining the amount of water VWater formed in the bioreactor in the predetermined period of time. Under standard conditions (atmospheric pressure, 0 °C), 1.6 liters of water are formed per standard cubic meter of methane produced. An amount of 1 Nm 3The methane formed in this case corresponds to an amount of 1.6 l of water. The formation of water is also the reason why the amount of used substrate discharged from the reactor is usually not identical to the amount of fresh substrate newly introduced into the reactor. In the presence of carbon monoxide in the reactant gas, methane formation can also occur according to the equation 3 H2 + CO CH4 + H2O. However, the amounts of carbon monoxide in the reactant gas are usually so small that this effect can be neglected.

[0049] The present invention also relates to a plant for carrying out one of the above-described methods, wherein the plant comprises at least one bioreactor. The bioreactor has at least one inlet for supplying hydrogen and carbon dioxide to the bioreactor, at least one inlet for supplying fresh substrate to the bioreactor, at least one outlet for removing used substrate from the bioreactor, and at least one outlet for removing methane-enriched gas formed in the bioreactor.

[0050] The plant according to the invention for carrying out one of the methods described above also comprises a device for determining the amount of methane formed in the bioreactor in a predetermined period of time and a computer unit, wherein the computer unit is designed and configured to calculate the amount of fresh substrate to be fed into the bioreactor as a function of the amount of methane formed in the bioreactor in the predetermined period of time.

[0051] The bioreactor of the plant according to the invention is fed with a carbon dioxide-containing gas and with an externally generated hydrogen-containing gas. The hydrogen required for biological methanation is preferably generated by an electrolyzer. The hydrogen is preferably generated in the electrolyzer using surplus electricity from the power grid through the electrolysis of water. The provision of surplus electricity is usually handled by a control energy box, so that the biological methanation plant can participate in the control energy market to the extent of the output of the corresponding electrolyzer. Even if the electrolyzer does not participate in the control energy market, cheaper electricity can be used elsewhere (e.g., from special electricity tariffs). Alternatively, hydrogen can also be generated from other sources such as hydrolysis gas, synthesis gas, product gas from chemical reactions, H2 of biological origin, such asH2 produced by algae or H2 from photocatalysis can be used for biological methanation.

[0052] The bioreactor for biological methanation has at least one inlet for the supply of hydrogen and carbon dioxide into the bioreactor. This is advantageously a suitable system for introducing the CO2-containing gas and the hydrogen-containing gas into the liquid reactor medium. The CO2-containing gas and the hydrogen-containing gas are preferably introduced into the bioreactor via supply lines. Preferably, the two gases are mixed in a gas mixing chamber before being introduced into the reactor and then fed into the reactor via a common reactant gas line. The H2- and CO2-containing reactant gases are fed, for example, directly into the bioreactor via at least one supply line, preferably in the lower region of the bioreactor.

[0053] The reactor for biological methanation has at least one outlet for the removal of methane-enriched gas formed in the bioreactor. The methane-enriched gas formed is preferably discharged from the bioreactor via a product gas line, which is preferably arranged at the upper end of the bioreactor.

[0054] Before the product gas can be further utilized, it is usually subjected to gas processing including gas drying and cooling, and if necessary, desulfurization, gas scrubbing to separate out other components such as ammonia and / or a ^ separation and / or CCh separation. The methane-enriched gas that leaves the bioreactor as product gas is thus converted into processed SNG (Substitute Natural Gas) or biomethane, which can be fed into a public gas grid. In addition to feeding SNG into the natural gas grid, it can also be used as a fuel, for example in the form of a biomethane filling station, or for material use. Another possible utilization option in times of insufficient electricity supply is the reconversion of biomethane to electricity with simultaneous heat generation, for example via a CHP plant. In this case, an intermediate storage facility for the produced biomethane is required.However, this as well as a CHP unit are often already integrated into corresponding systems and can therefore be used advantageously.

[0055] The bioreactor is advantageously also equipped with a control valve for pressure regulation within the bioreactor and correspondingly pressure-stable components, allowing a reactor pressure higher than atmospheric pressure, in particular an overpressure of up to 30 bar, preferably an overpressure of up to 16 bar, to be set. The bioreactor is optionally equipped with sensors for measuring temperature, pressure, and pH. A system for pH control or for dosing alkali or acid can also be provided.

[0056] To technically regulate substrate exchange, the quality of the product gas is determined after leaving the methanation reactor. Devices for measuring the CCh content of the product gas, as well as the CH2p content and the H2 content of the product gas, are generally already present in a methanation system and can thus be used advantageously. The device for product gas analysis is preferably located directly downstream of the methanation reactor or in the gas treatment area. In this way, the amount of methane formed in the bioreactor is continuously measured in the methane-enriched gas withdrawn from the bioreactor and can be determined for any predetermined period of time.

[0057] From the measured values ​​thus obtained, the amount of fresh substrate to be fed into the bioreactor is then calculated by the computer unit provided in the plant according to the invention as a function of the amount of methane formed in the bioreactor in the predetermined period of time.

[0058] A suitable computer, for example, can be used as the computing unit. This computer, of course, does not necessarily have to be located directly next to the methanation reactor, but can also be connected to it via cables or remote control. Advantageously, the measured values ​​obtained by the device for determining the amount of methane produced in the bioreactor over a predetermined period of time are transmitted to the computing unit via cables or wirelessly.

[0059] The computing unit is preferably a programmable logic controller (PLC), which is particularly preferably located directly at the methanation plant. The measured values ​​are transferred to the PLC, which then calculates the amount of fresh substrate to be fed into the bioreactor based on the amount of methane produced in the bioreactor during the predetermined period. The PLC thus calculates when a substrate change is due and triggers this change by controlling certain actuators, such as a substrate pump.

[0060] Particularly preferably, the computer unit calculates the amount of fresh substrate (mprischsubstrat) to be fed into the bioreactor as a function of the amount of methane (VMethan) formed in the bioreactor and an exchange rate (ATR) according to the equation (mprischsubstrat) = (MTethan) • (ATR), where the exchange rate (ATR) satisfies the condition (ATR > 0). This embodiment makes it possible to specify a defined exchange rate for the biological methanation process. A stable methanation process with good product gas quality is achieved.

[0061] Preferably, the computer unit calculates the amount of fresh substrate to be fed into the bioreactor mprischsubstrat [kg] as a function of the amount of methane VMethan [Nm 3 ] formed in the bioreactor in the predetermined period of time. 3 ] and the exchange rate ATR [kg / Nm 3 ] according to the equation m Fr ischsubstrate [kg] = V Me than [Nm 3 ] • ATR [kg / Nm 3], where the exchange rate ATR of the condition 0.01 kg / Nm 3 < ATR < 10 kg / Nm 3 , preferably the condition 1 kg / Nm 3 < ATR < 6 kg / Nm 3 , is sufficient. For example, an exchange rate of 4 kg substrate per Nm 3 of methane produced in the bioreactor. This means that 4 kg of fresh substrate is added to the bioreactor as soon as one standard cubic meter of pure methane has been produced.

[0062] Particularly preferably, the system also comprises at least one substrate pump, wherein the at least one substrate pump is designed and configured to convey fresh substrate to the inlet for supplying fresh substrate to the bioreactor and / or to remove used substrate from the bioreactor. A substrate pump represents the simplest and most failure-resistant device for technically implementing the substrate exchange.

[0063] According to a particularly preferred embodiment, the computer unit is designed and configured to control the at least one substrate pump, wherein the control of the at least one substrate pump is carried out in such a way that fresh substrate is supplied to the bioreactor according to the equation fresh substrate = VMethane • ATR. This embodiment enables fully automatic operation of the bioreactor with continuous or discontinuous substrate exchange.

[0064] Particularly preferably, the computer unit is designed and configured to control the at least one substrate pump, wherein the control of the at least one substrate pump is carried out in such a way that fresh substrate is continuously supplied to the bioreactor in such an amount m Frischsubstrat [kg] is supplied such that the total amount mcesamtsubstrat [kg] of substrate present in the bioreactor remains substantially constant, wherein the total amount of substrate present in the bioreactor corresponds to a predetermined target amount of substrate msoiisubstrat [kg].

[0065] Definitions:

[0066] Biogas: Biogas is a gas produced by anaerobic fermentation in a biogas plant under the influence of various microorganisms. Its main components are methane and carbon dioxide. It also contains water vapor and possibly small amounts of hydrogen, nitrogen, oxygen, hydrogen sulfide, and ammonia. This type of biogas is also known as raw biogas and can, for example, be fed into a combined heat and power plant to generate electricity and heat. If the raw biogas is further processed, for example, to be subsequently fed into a natural gas grid, carbon dioxide, among other components, is separated. The separated carbon dioxide-rich gas from biogas processing can be used as a carbon dioxide-containing feedstock for biological methanation. However, raw biogas is also suitable as a carbon dioxide-containing feedstock for biological methanation.A biogas production facility can, for example, be a biogas plant, where biogas is produced from biomass. Furthermore, a biogas production facility can also be a wastewater treatment plant that has a digestion tower. The biogas produced in a wastewater treatment plant is also referred to as digester gas or sewage gas.

[0067] Educt gas: Educt gases for methanation are hydrogen-containing gas and carbon dioxide-containing gas or hydrogen and carbon dioxide.

[0068] Methane-enriched gas = product gas = biomethane: A methane-enriched gas is a gas formed by the action of hydrogenotrophic methanogenic microorganisms in an anaerobic bioreactor with the addition of hydrogen-containing and carbon dioxide-containing gas. The methane-enriched gas is also the product gas formed from the reactant gases in the bioreactor during the biological methanation process and leaves the bioreactor via the product gas line. In addition to CF, the methane-enriched gas can also contain components from the reactant gases introduced into the bioreactor for methanation, for example, unreacted hydrogen and carbon dioxide, or other gases present in smaller quantities such as nitrogen, hydrogen sulfide, or ammonia, as found, for example, in raw biogas. Therefore, the methane-enriched gas does not have to consist of 100% methane.However, methane is the main component of the methane-enriched gas. The methane-enriched gas produced in the present application is also a biomethane and can also be considered methane-enriched biogas if raw biogas is used as the carbon dioxide-containing gas. The term biomethane is to be understood as a distinction from synthetic methane, which is formed during chemically catalytic methanation.

[0069] Biomethane and SNG (substitute natural gas): Biomethane is a methane gas that can be fed into the natural gas grid according to the applicable guidelines (in Germany, for example, DVGW guidelines G260, G262) and was produced through biological methanation using CO2 of biogenic origin. The term SNG, which is an abbreviation for "synthetic natural gas," is essentially used synonymously. It is a technically produced substitute for natural gas that can be used like natural gas and fed into a natural gas grid as a replacement gas.Biomass: Biomass in the sense of the invention means all types of fermentable renewable raw materials such as corn, grain, grass, silphium, sugar beet, but also animal excrement such as cattle or pig slurry, horse manure or chicken manure, but also municipal or industrial wastewater as well as biowaste or organic waste from food production or processing as well as any mixtures of these fermentation substrates.

[0070] Digestate: Digestate or digestate is the residue of fermentation substrate that remains after the fermentation of biomass in a biogas plant. Digestate, for example, refers to the residue after the last temperature-controlled fermentation stage (e.g., secondary digester) in a biogas plant. Likewise, digestate is digested sludge from the digestion towers of wastewater treatment plants.

[0071] Bioreactor = methanation reactor: Reactor with a methanation medium including methanogenic microorganisms in which the product gas methane is formed from carbon dioxide and hydrogen.

[0072] Methanization: Methanization is the formation of methane starting from the gaseous substances hydrogen and carbon dioxide as reactant gases. Biological methanation describes the formation of biomethane with the help of hydrogenotrophic methanogenic microorganisms in an aqueous medium. It essentially follows the chemical equation: 4 H2 + CO2 CH4 + 2 H2O. If one of the reactant gases contains carbon monoxide, methane formation can also occur according to the equation: 3 H2 + CO CH4 + H2O. Other routes of methane formation, for example from acetate or methyl compounds, cannot be ruled out depending on the methanation medium and reaction conditions used, but they play a minor role because carbon dioxide- and hydrogen-containing gases are added in predominant quantities.

[0073] Methanization medium: Refers to the reactor contents of the bioreactor suitable for biological methanation. The methanation medium is an aqueous reaction medium and represents the nutrient medium for the methanogenic microorganisms. It contains all nutrients, trace elements, and other components necessary for the growth of corresponding hydrogenotrophic methanogenic microorganisms. The methanation medium is either a synthetic culture medium for methanogenic microorganisms or a complex nutrient medium based on biomass, which essentially consists of digestate as a substrate and is optionally supplemented with nutrients and / or trace elements. The composition of the methanation medium changes during methanation. Firstly, water is formed during the reaction of carbon dioxide and hydrogen to form methane.In addition, the addition of acids, alkalis, and / or defoamers may be necessary during methanation. During bacterial growth, some substances contained in the methanation medium are consumed, while others are formed. These processes are the reason why used substrate must be repeatedly replaced with fresh substrate during the methanation process.

[0074] Substrate: The substrate can be a biological substrate with increased viscosity (e.g., digestate, sewage sludge) or a culture medium for methanogenic microorganisms. The substrate may be sufficient for biological methanation on its own, but it can also be supplemented with additional additives.

[0075] Fresh substrate: Fresh substrate is the material that is added to the bioreactor and thus forms a component of the methanation medium.

[0076] Old substrate: Old substrate refers to material in the bioreactor in which a methanation reaction has already taken place over a certain period of time. Old substrate is removed from the bioreactor at certain times.

[0077] Excipients: Excipients supplement the substrate in the bioreactor. These can be nutrients, trace elements, microorganisms, acids, alkalis, and defoamers. Excipients are an essential part of the substrate. They may already be present in a biological substrate or culture medium in quantities optimal for methanation. In this case, only fresh substrate is added to the bioreactor. Nutrients and / or trace elements are often added as excipients via separate dosing stations containing concentrated solutions of these substances. This is usually done using suitable pumps. The addition of the nutrients and / or trace elements is then carried out separately from the addition of the fresh substrate, but the quantity is linked to the amount of fresh substrate added.Microorganisms are also added separately as auxiliary agents for methanation, particularly methanogenic microorganisms, especially hydrogenotrophic methanogenic microorganisms. The addition of microorganisms is preferably carried out via an inoculum, which is added once at the beginning of a methanation process. The addition of acids, alkalis, and defoamers is usually carried out via dedicated dosing stations with appropriate pumps, separate from the addition of the fresh substrate. Brief description of the drawings.

[0078] The invention will be explained in more detail below using exemplary embodiments in conjunction with the drawings. It is understood that the information provided in connection with the exemplary embodiments is not intended to limit the invention. They show

[0079] Fig. 1 shows a graphic representation of measured values ​​of the product gas composition and process parameters when carrying out an embodiment of the process according to the present invention with sewage sludge as fresh substrate at a constant exchange rate and different methane formation rates;

[0080] Fig. 2 shows a graphical representation of measured values ​​of the product gas composition and process parameters when carrying out an embodiment of the process according to the present invention with sewage sludge as fresh substrate at a constant methane formation rate and different exchange rates;

[0081] Fig. 3A, 3B show, in graphical representation, measured values ​​of the product gas composition and process parameters when carrying out an embodiment of the method according to the present invention with a synthetic culture medium at a constant exchange rate and different methane formation rates, wherein the composition of the synthetic culture medium in the experiment on which the measured values ​​of Figure 3A are based differs from the composition of the synthetic culture medium in the experiment on which the measured values ​​of Figure 3B are based;

[0082] Fig. 4A, 4B graphically depict measured values ​​of the product gas composition and process parameters when conducting biological methanation according to a prior art process using sewage sludge as the fresh substrate with constant exchange times, independent of the methane formation rate. Ways of implementing the invention

[0083] Example 1

[0084] A bioreactor equipped with a gassing agitator and a volume of 80 kg of substrate was operated with digested sludge from a wastewater treatment plant. The exchange rate was 4 kg of substrate per Nm 3 of methane formed. This means that 4 kilograms of fresh substrate were added as soon as one standard cubic meter of pure methane was formed.

[0085] During methane production in the bioreactor, the amount of product gas produced was continuously measured. In conjunction with an analysis of the respective product gas quality, the amount of methane produced was determined based on the methane content in the product gas.

[0086] Since the substrate exchange was carried out in batches, a quantity of new substrate was determined to be exchanged per substrate exchange in the bioreactor. This can be referred to as the target mass for the fresh substrate. The target mass for the fresh substrate was set at 5 percent of the target value of the bioreactor contents. The pilot-scale bioreactor used has a target value for the substrate mass of 80 kilograms. The lower limit of the substrate mass is 76 kilograms for an exchange of 5 percent of the bioreactor mass, resulting in a target mass for the fresh substrate of 4 kilograms per exchange.

[0087] To ensure stable operation of the methanation reactor and optimize the methane production rate, a supply of auxiliary materials was planned. These included defoamers, alkalis or acids to adjust the pH if necessary, as well as nutrients and trace elements. A separate dosing station with its own line into the bioreactor was provided for each of the auxiliary materials. The dosing stations were designed with the expected consumption quantities and the required storage quantities in mind.

[0088] Since the exchange rate is 4 kg / Nm 3formed methane, the substrate exchange was triggered as soon as one standard cubic meter of methane had been formed since the last substrate exchange. For the substrate exchange, the mass of the bioreactor contents was first drained to the lower limit for the substrate mass in the bioreactor of 76 kilograms. The amount of substrate drained was more than 4 kilograms, since a corresponding amount of 1.6 liters of water was formed during the formation of one standard cubic meter of methane. Subsequently, 4 kilograms of fresh substrate were pumped into the bioreactor via a substrate pump, so that the substrate mass in the bioreactor again reached the target value of 80 kilograms.

[0089] With each substrate exchange, additives were added along with the fresh substrate according to previously defined target values. The addition of additives such as nutrients and / or trace elements ensures that these substances are always available to the methanogenic microorganisms in sufficient quantities. The addition of these additives was also adjusted depending on the amount of methane produced, thus ensuring an appropriate supply of nutrients and trace elements to the methanogens regardless of the bioreactor size.

[0090] Example 2

[0091] Figure 1 shows graphically, depending on the test time [h] from top to bottom

[0092] - Measured values ​​for the reactant gas volume flows of hydrogen and carbon dioxide in standard litres per hour,

[0093] - Measured values ​​for the product gas composition as volume fractions of methane, carbon dioxide and hydrogen, expressed as a percentage of the product gas,

[0094] - Measured values ​​of methane formation rates, expressed in standard cubic metres of methane produced per cubic metre of reactor volume per day

[0095] - Measured values ​​for the filling quantity in the bioreactor [kg] and for the pressure in the reactor in bar overpressure.

[0096] The biological methanation was carried out in a bioreactor containing digested sludge from a municipal wastewater treatment plant, to which individual trace elements and nutrients were added, as the methanation medium. The biological methanation was carried out without the addition of a methanogenic microorganism. The methane-enriched gas was produced in a stirred tank reactor with an 80-liter reactor capacity at a temperature of 65 °C and a pressure of 7 bar.

[0097] The reactant gases H2 and CO2 were added from gas bundles containing pure H2 and CO2 in pressurized gas cylinders. The respective gas flow rates in standard liters per hour [Nl / h] were controlled by Bronkhorst mass flow controllers. The hydrogen flow rate (V_H2) was specified depending on the desired methane formation rate. The exact value for the supplied carbon dioxide flow rate (V_CO2) was determined by measuring the actual carbon dioxide value and calculating the control deviation from the manipulated variable output by a CCh controller in conjunction with the specified hydrogen flow rate.

[0098] Over the entire period, biological methanation was carried out at an exchange rate (ATR) of 4 kilograms of fresh substrate per standard cubic meter of methane produced [kg / Nm 3] was carried out. The exchange of fresh substrate was carried out batchwise as described in Example 1. At a constant methane formation rate, a portion of the reactor contents was drained at regular intervals and then replenished with fresh substrate or methanation medium, allowing a certain exchange with fresh material. This is evident in the bottom diagram of Figure 1 by small downward deflections and a subsequent increase in mass.

[0099] After substrate exchange, individual nutrients and trace elements were added to the reactor via a pump from a reservoir of concentrated stock solutions over a short period of time. The respective amount depended on the amount of substrate exchanged and was calculated based on predefined target values ​​for the individual nutrients and trace elements.

[0100] The bioreactor was adjusted to a methane production rate (MBR) of 80 standard cubic meters of methane per cubic meter of reactor volume per day [Nm 3 / (m 3 d)] and then operated for 54 hours at this MBR. A methane-enriched gas with a quality of over 98% methane, 1% carbon dioxide, and less than 1% hydrogen was obtained. The gas quality of the product gas is shown in the second diagram from the top. The CFU values ​​can be read off the left scale of the y-axis, and the values ​​for CO2 and H2 can be read off the right scale of the y-axis.

[0101] In the bottom diagram, the measured values ​​for the filling quantity in the bioreactor (scale on the left) show that with an MBR of 80 Nm 3 / (m 3d) every 3.75 hours, a substrate exchange of 4 kg of used substrate for fresh substrate was carried out, which corresponds to 5% of the bioreactor's capacity. This results in pressure fluctuations in the bioreactor, which can be seen in the measured values ​​for the internal reactor pressure (scale on the right).

[0102] Immediately afterwards, the bioreactor was heated to an MBR of approximately 155 Nm 3 / (m 3 d) before the feed gas supply was stopped. This increase was easily achieved. The measured values ​​for the reactor fill level clearly indicate that the substrate exchange was performed every 2 hours, as the exchange rate depends on the amount of methane produced.

[0103] During the so-called "cold standby" period, the bioreactor was not stirred or heated for 64 hours, so that the operating temperature for methanation dropped from 65 °C to room temperature of approximately 25 °C. This can be seen from the pressure in the bioreactor dropping. No substrate or auxiliary materials were added. After a short warm-up phase, biological methanation could be restarted immediately with an MBR of 155 Nm 3 / (m 3 d) be continued. After 8 hours of operation at an MBR of 155 Nm 3 / (m 3 d) another "cold standby" was carried out for 12 h followed by a long methanation phase at an MBR of 155 Nm 3 / (m 3 d) over 48 h, after which the MBR is reduced to 80 Nm 3 / (m 3 d) was returned.

[0104] After another "cold standby" phase of 69 h, the bioreactor was operated for a total period of 30 hours with methane production rates of 210, 175 and 200 Nm 3 / (m 3 d). Only immediately after the biological methanation was restarted after a long shutdown was the gas quality slightly worse, at approximately 3% hydrogen and approximately 96% methane.

[0105] Subsequently, a so-called "hot standby" was carried out three times for 12 to 14 hours, interrupted each time for a methanation phase with a methanation rate of 200 Nm 3 / (m 3 d), followed by a "hot standby" for 65 hours followed by an 8-hour methanation phase with a methane formation rate of 200 Nm 3 / (m 3 d). In a "hot standby," the bioreactor continues to stir and heat without adding substrate or additives. At an MBR of 200 Nm 3 / (m 3d) the substrate exchange of 4 kilograms per standard cubic meter of methane produced took place every one and a half hours.

[0106] The test results show that when the method according to the present invention is applied with an exchange rate at which old substrate is exchanged for fresh substrate depending on the amount of methane formed and further auxiliary substances such as nutrients and trace elements are added accordingly, a bioreactor with different methane formation rates can be operated stably and, in addition, a start-stop operation of the bioreactor is possible.

[0107] The bioreactor operated stably throughout the entire test period without pH control by measuring the pH value in the reactor and adding alkali or acid accordingly. The pH value was determined randomly in samples from the drained old substrate and ranged between pH 7.5 and pH 8.3 throughout the entire test period. Furthermore, the measurement data showed that, without further gas treatment, a methane-enriched gas was obtained as the product gas. This gas had a very high methane content of at least 95%, a carbon dioxide concentration of approximately 1%, and a hydrogen concentration of less than 4%.

[0108] A process according to the present invention can therefore achieve different methane production rates and also tolerate different downtimes of the methanation process. These facts represent enormous advantages due to the fluctuating availability of renewable or inexpensive electricity for electrolysis to produce hydrogen, as well as the fluctuating amount of sewage gas as a carbon dioxide source.

[0109] Example 3

[0110] Figure 2 shows, in four diagrams, measured values ​​as a function of the test time [h] for the same parameters as described in Example 2 with reference to Figure 1.

[0111] Biological methanation was carried out in a bioreactor containing digested sludge from a municipal wastewater treatment plant as the methanation medium, to which individual trace elements and nutrients were added. The biological methanation was carried out with the addition of methanogenic microorganisms from the order Methanobacteriales, which were added as inoculum at the beginning of the experiment. No methanogenic microorganisms were added during the further course of the experiment. The methane-enriched gas was produced in a 75-liter stirred-tank reactor at a temperature of 65 °C and a pressure of 7 bar.

[0112] In contrast to Example 2, biological methanation was carried out over the entire period at a constant methane formation rate (MBR) of 80 standard cubic meters of methane per cubic meter of reactor content per day [Nm 3 / (m 3d)], varying the exchange rate (ATR). The exchange of fresh substrate was carried out batchwise as described in Example 1.

[0113] After substrate exchange, individual nutrients and trace elements were added to the reactor via a pump from a reservoir of concentrated stock solutions over a short period of time. The respective amount depended on the amount of substrate exchanged and was calculated based on predefined target values ​​for the individual nutrients and trace elements.

[0114] For the first six exchange processes shown in Figure 2, substrate was used with an exchange rate (ATR) of 4 kg fresh substrate per standard cubic meter of methane formed [kg / Nm 3] was added. From a test period of 108 hours up to a test period of 232 hours, the substrate exchange was carried out with an ATR of 2 kg of fresh substrate per standard cubic meter of methane produced, thereafter with an ATR of 1 kg of fresh substrate per standard cubic meter of methane produced. The corresponding time periods are marked in the bottom diagram for the different ATRs.

[0115] From a test time of 210 hours, biological methanation was carried out in start / stop operation with 8 hours of gassing and 4 hours of "hot standby". Since the exchange process was triggered as soon as 5% of the reactor volume had to be exchanged at the corresponding methane formation rate, the substrate exchange took place at an ATR of 4 kg / Nm 3 twice as often as with an ATR of 2 kg / Nm 3 and four times as often as with an ATR of 1 kg / Nm 3 . At the exchange rates shown of 4 kg / Nm 3 , 2 kg / Nm 3and 1 kg / Nm 3 Accordingly, it took approximately 3 hours, 6 hours, or 12 hours until the next substrate change. In start / stop mode, the downtime was 4 hours in each case.

[0116] As can be seen from the experimental results, stable methanation was achieved at various exchange rates. The gas qualities remained virtually constant, with a methane content of 95% to 96%, a hydrogen content of 2% to 3%, and a carbon dioxide content of approximately 1%. The pH value also remained stable in a range between 7.9 and 8.1 without the addition of acid or alkali.

[0117] Example 3 shows that different amounts of substrate can be exchanged per amount of methane produced. With each substrate exchange, the same amount of nutrients, trace elements, or, if necessary, other additives is added. If the fresh substrate for biological methanation is an organic substrate with increased viscosity, such as sewage sludge or digestate, a different exchange rate can be used as a technical means to influence the viscosity in the bioreactor.

[0118] Since biological methanation produces 1.6 liters of water per standard cubic meter of methane, the substrate in the bioreactor is diluted significantly more at a lower exchange rate than at a higher one. Starting with a viscosity of approximately 0.03 Pa-s at an ATR of 4 kg / Nm 3 In Example 3, the viscosity decreased to a value of about 0.003 Pa-s at an ATR of 1 kg / Nm 3This can be used as a technical feature for the methanation process in individual cases, e.g., when a fresh substrate has a relatively high viscosity, which would require high agitator performance or results in high energy consumption during agitation. If an organic substrate already contains high amounts of essential trace elements and nutrients, it is advantageous to use higher exchange rates so that as few or, ideally, no additional nutrients and trace elements need to be added.

[0119] Example 4

[0120] Figures 3A and 3B show, in four diagrams each, measured values ​​as a function of the test time [h] for the same parameters as described in Example 2 with reference to Figure 1.

[0121] Figures 3A and 3B show measured values ​​of the product gas composition and process parameters when carrying out an embodiment of the method according to the present invention with a synthetic culture medium at a constant exchange rate and different methane formation rates, wherein the composition of the synthetic culture medium in the experiment on which the measured values ​​of Figure 3A are based differs from the composition of the synthetic culture medium in the experiment on which the measured values ​​of Figure 3B are based.

[0122] Biological methanation was carried out in a bioreactor with a gassing stirrer. The methanation medium contained a synthetic culture medium, as used in the prior art for hydrogenotrophic methanogenic archaea (see, for example, WO 2008 / 094282 A1 or WO 2012 / 110257 A1). The medium was inoculated with a methanogenic strain from the genus Methanothermobacter. The methane-enriched gas was produced in a 60-liter stirred-tank reactor at a temperature of 65 °C and a pressure of 7 bar.

[0123] Over the entire period, biological methanation was carried out at an exchange rate (ATR) of 4 kilograms of fresh substrate per standard cubic meter of methane produced [kg / Nm 3] was carried out. The exchange of fresh substrate was carried out batchwise as described in Example 1. A mineral culture medium was used as the fresh substrate. Nutrients and trace elements were added to the reactor from a concentrated stock solution over a short period of time. The respective amount depended on the amount of substrate exchanged and was calculated according to predetermined target values ​​for the individual nutrients and trace elements. As additional excipients, sodium hydroxide solution was repeatedly added to stabilize the pH value, as well as defoamers if necessary. The addition of the lye and defoamers took place independently of the substrate change.

[0124] In the experiment shown in Figure 3 A, the bioreactor was operated for 2 days with a methane production rate of 50 Nm 3 / (m 3d). The methane formation rate was then increased to 200 Nm 3 / (m 3 d) and produced methane-enriched gas for just over a day. At an MBR of 50 Nm 3 / (m 3 d) every 6 hours, a fresh substrate exchange of 5% of the reactor target mass was carried out, with an MBR of 200 Nm 3 / (m 3 d) the same amount of fresh substrate was exchanged every 1.5 hours. In the experiment shown in Figure 3 B, the bioreactor was operated for 10 days with increasing methane production rates of 100 Nm 3 / (m 3 d), then 145 Nm 3 / (m 3 d) and finally 200 Nm 3 / (m 3 d). The exchange intervals lasted 3 hours, approximately 2 hours, and 1.5 hours, respectively. The product gas had a constant carbon dioxide content of approximately 1% at all methane formation rates. While the methane content at an MBR of 50 Nm 3 / (m 3d) was about 97% and the hydrogen content was about 2%, the methane content at an MBR of 145 Nm 3 / (m 3 d) still 93.5% and the hydrogen content about 5.5%, with an MBR of 200 Nm 3 / (m 3 d) finally, the methane content is 90% and the hydrogen content is 9% of the product gas.

[0125] Example 4 shows that, when applying an embodiment of the process according to the present invention, stable biological methanation is achieved even when using pure cultures in synthetic media, with an exchange rate dependent on the amount of methane formed and varying methane formation rates. A methane-enriched gas with a very high methane content is obtained, which can be upgraded to feed-in biomethane by gas processing with hydrogen separation. Comparative Example

[0126] Figures 4A and 4B show, in four diagrams each, measured values ​​as a function of the test time [h] for the same parameters as described in Example 2 with reference to Figure 1.

[0127] In this comparative example, the substrate exchange was not carried out depending on the amount of methane produced, but rather a certain amount of substrate and auxiliary materials was exchanged depending on the time.

[0128] The biological methanation was carried out in a bioreactor containing digested sludge from a municipal wastewater treatment plant, to which individual trace elements and nutrients were added, as the methanation medium. The biological methanation was carried out without the addition of a methanogenic microorganism. The methane-enriched gas was produced in a stirred tank reactor with an 80-liter reactor capacity at a temperature of 65 °C and a pressure of 7 bar.

[0129] As shown in Figure 4A, starting with a test time of 680 hours in the bioreactor, the dosage of the reactant gas volume flows of carbon dioxide and hydrogen was adjusted so that over a period of 24 hours a methane formation rate (MBR) of 40 standard cubic meters of methane per cubic meter of reactor content per day [Nm 3 / (m 3 d)] with a gas quality of 99% CH4, 1% CO2 and 0% H2. Corresponding to an exchange rate (ATR) of 4 kilograms of fresh substrate per standard cubic meter of methane formed [kg / Nm 3 ] a substrate exchange of 5% of the reactor volume was carried out approximately every 8 hours.

[0130] Subsequently, the methane production rate was to be increased without simultaneously increasing the number of substrate exchanges. To this end, the reactant gas flow was increased after the third substrate exchange to achieve an MBR of 88 Nm 3 / (m 3 d) Based on an MBR of 88 Nm 3 / (m3 d) after 3 hours the feed gas supply was increased so that an MBR of 120 Nm 3 / (m 3 d). However, this was not possible because the gas quality deteriorated significantly to values ​​of 76% CH4, but 10% CO2 and 14% H2.

[0131] After a test period of 710 hours, the bioreactor was again operated for 18 hours with an MBR of 40 Nm 3 / (m 3 d). From the test time of 729 hours, the amount of reactant gas was increased again to achieve an MBR of 88 Nm 3 / (m 3 d). However, the exchange times were not adjusted according to the increased methane production, but instead a substrate change was only carried out every 8 hours. When the reactant gas addition was significantly increased after 26 hours to achieve an MBR of 120 Nm 3 / (m 3d) at full conversion, there was again a sharp drop in gas quality, so that the increase in the feed gas supply was reduced to the amount corresponding to an MBR of 80 Nm 3 / (m 3 d) corresponds.

[0132] Two further exchanges of 4 kg of fresh substrate were carried out after

[0133] 8 hours, as originally for an MBR of 40 Nm 3 / (m 3d). After a test period of 774 hours, the product gas quality decreased rapidly without the feed gas supply being increased. A likely explanation is that the supply of nutrients and trace elements was no longer sufficient, as methanation had been carried out over a longer period at a higher methane formation rate. After a substrate exchange of 20 kg, which corresponds to a quarter of the reactor content, 4 kg of fresh substrate were exchanged every 3 h 20 min and the corresponding nutrients and trace elements were added. This is converted to the corresponding amount of methane formed, equivalent to the amount that would be produced at an MBR of 40 Nm 3 / (m 3 d) was added at a lower number of exchange cycles. With this increased addition of fresh substrate, methanation could be continued at an MBR of 88 Nm 3 / (m 3 d) continue again with a very good gas quality of 99% CF , 1% CO2 and 0% H2.

[0134] After 68 h, another attempt was made to increase the gas supply so that an MBR of 120 Nm 3 / (m 3 d) could be achieved. However, this again led to a drop in gas quality. Starting at a test time of 854 h, the number of substrate exchanges at an MBR of 88 Nm 3 / (m 3 d) reduced again to the original value at an MBR of 40 Nm 3 / (m 3 d) were carried out. During the test period of 870 h, there was a brief interruption of methanation due to a technical malfunction at the gas station. Overall, biological methanation functioned well with the lower number of exchanges at an MBR of 88 Nm 3 / (m 3d) over a period of approximately 38 hours. However, after the test period of 890 hours, there was a very sharp decline in both the gas quality and the amount of methane produced. By reducing the gas supply, methanation could be resumed with an MBR of 40 Nm 3 / (m 3 d) with an exchange of 4 kg of substrate every 8 hours, the process can be continued with good gas quality.

[0135] In a further experiment, the measured values ​​of which are shown graphically in Figure 4B in four diagrams as a function of the test time [h] for the same parameters as described in Example 2 with reference to Figure 1, the substrate exchange was again not carried out as a function of the amount of methane formed, but rather a certain amount of substrate and auxiliary materials was exchanged as a function of time.

[0136] The biological methanation was carried out in a bioreactor containing digested sludge from a municipal wastewater treatment plant, to which individual trace elements and nutrients were added, as the methanation medium. The biological methanation was carried out without the addition of a methanogenic microorganism. The methane-enriched gas was produced in a stirred tank reactor with an 80-liter reactor capacity at a temperature of 65 °C and a pressure of 7 bar.

[0137] Starting from a test time of 136 hours, the feed gas supply of hydrogen and carbon dioxide into the bioreactor was adjusted to achieve an MBR of about 87 Nm 3 / (m 3 d) with a very good gas quality of 98% CH4, 1% CO2, and 1% H2. Corresponding to an exchange rate (ATR) of 4 kilograms of fresh substrate per standard cubic meter of methane produced, a substrate exchange of 5% of the reactor volume was carried out approximately every 4 hours.

[0138] Starting at a test duration of 150 hours, 4 kilograms of fresh substrate were replaced every 8 hours, and the corresponding amount of nutrients and trace elements was added. At a test duration of 166 hours, a technical malfunction occurred at the gas station, resulting in no gas supply for one hour, and consequently no product gas entering the line.

[0139] After the fourth substrate change, which was performed in a cycle of 8 hours per substrate change, the gas quality deteriorated significantly after 182 hours of testing. After 187 hours of testing, the gas quality plummeted, so the reactant gas supply was throttled to a level that, at full gas conversion, resulted in an MBR of 40 Nm. 3 / (m 3d). Under these conditions, with a substrate exchange every 8 hours, biomethane of very good gas quality (99% CH4, 1% CO2, less than 0.2% H2) could be produced. When the feedstock gas supply was increased again at the 207-hour test time, the gas quality declined again.

[0140] Since the pH remained constant in a range of 7.0 to 7.3 without major fluctuations throughout the entire period of the experiments described in Figures 4 A and 4 B, a changing pH cannot be the reason for the deterioration in biological methanation. Rather, the comparative example shows that a time-dependent exchange of a specific amount of substrate, as well as nutrients and trace elements, produces significantly poorer biological methanation results than the substrate exchange according to the invention, depending on the amount of methane produced.

Claims

Claims 1. A method for producing a methane-enriched gas comprising the steps a) providing a bioreactor comprising - at least one inlet for the supply of hydrogen and carbon dioxide into the bioreactor, - at least one inlet for the supply of fresh substrate into the bioreactor, - at least one outlet for the removal of waste substrate from the bioreactor, - at least one outlet for the removal of methane-enriched gas formed in the bioreactor, b) removal of the methane-enriched gas formed in the bioreactor from the bioreactor, c) determining the amount of methane formed in the bioreactor in a predetermined period of time, d) removal of used substrate from the bioreactor, e) supplying fresh substrate to the bioreactor, wherein the amount of fresh substrate supplied to the bioreactor depends on the amount of methane formed in the bioreactor in the predetermined period of time.

2. Method according to claim 1, characterized in that the amount of fresh substrate mFreshsubstrat fed into the bioreactor depends on the amount of methane VMethan formed in the bioreactor in the predetermined period of time and an exchange rate ATR and the equation mFreshsubstrat = VMethane • ATR. is sufficient, whereby the exchange rate ATR satisfies the condition ATR > 0.

3. Method according to claim 2, characterized in that the amount of fresh substrate mFresh substrate [kg] fed into the bioreactor depends on the amount of methane VMethan [Nm 3 ] formed in the bioreactor in the predetermined period of time. 3 ] and an exchange rate ATR [kg / Nm 3 ] and the equation mFresh substrate [kg] — VMethane [Nm 3 ] • ATR [kg / Nm 3 ], where the exchange rate ATR satisfies the condition 0.01 kg / Nm 3 < ATR < 10 kg / Nm 3 is enough.

4. Method according to claim 3, characterized in that the exchange rate ATR of the condition 1 kg / Nm 3 < ATR < 6 kg / Nm 3 is enough.

5. The method according to any one of claims 1 to 4, characterized in that the fresh substrate supplied in step e) is a biological substrate with increased viscosity or a culture medium.

6. The method according to claim 5, characterized in that the biological substrate with increased viscosity is digestate from a biogas plant or sewage sludge.

7. Process according to claim 5 or 6, characterized in that the culture medium contains one or more of microorganisms, nutrients, trace elements.

8. The method according to any one of claims 1 to 7, characterized in that after step d) and before, together with or after step e), step f) feeding one or more auxiliary substances into the bioreactor is carried out, wherein the one or more auxiliary substances are nutrients, trace elements, defoamers, acids, alkalis, or mixtures thereof.

9. Process according to one of claims 1 to 8, characterized in that steps d) and e) are carried out continuously.

10. The method according to claim 9, characterized in that the removal of used substrate from the bioreactor in step d) and the supply of fresh substrate to the bioreactor in step e) are carried out in such a way that the total amount of substrate present in the bioreactor remains substantially constant, wherein the total amount of substrate present in the bioreactor corresponds to a predetermined target amount of substrate.

11. Process according to one of claims 1 to 8, characterized in that steps d) and e) are carried out batchwise.

12. The method according to claim 11, characterized in that the amount of fresh substrate [kg] per batch fed into the bioreactor depends on a predetermined target amount of substrate msoiisubstrat [kg] present in the bioreactor and satisfies the condition 0.01 msoiisubstrat — mfresh substrate — 0.1 msoiisubstrat is sufficient.

13. Method according to one of claims 1 to 12, characterized in that the determination of the amount of methane VMethane formed in the bioreactor in a predetermined period of time in step c) is carried out by determining the amount of water Vwater formed in the bioreactor in the predetermined period of time, wherein an amount of 1 Nm 3 The methane formed corresponds to an amount of 1.6 l of water.

14. Plant for carrying out a process according to one of claims 1 to 13, wherein the plant comprises at least one bioreactor, wherein the bioreactor - at least one inlet for the supply of hydrogen and carbon dioxide into the bioreactor, - at least one inlet for the supply of fresh substrate into the bioreactor, - at least one outlet for the removal of waste substrate from the bioreactor and - has at least one outlet for the removal of methane-enriched gas formed in the bioreactor, the plant also having - a device for determining the amount of methane formed in the bioreactor in a predetermined period of time and - a computer unit, wherein the computer unit is designed and arranged to calculate the amount of fresh substrate to be fed into the bioreactor as a function of the amount of methane formed in the bioreactor in the predetermined period of time.

15. Plant according to claim 14, wherein the computer unit for calculating the amount of fresh substrate mFreshsubstrat to be fed into the bioreactor as a function of the amount of methane VMethane formed in the bioreactor and an exchange rate ATR according to the equation mFreshsubstrat = VMethan • ATR is designed and set up, whereby the exchange rate ATR satisfies the condition ATR > 0.

16. Plant according to claim 15, wherein the computer unit for calculating the amount of fresh substrate mFresh substrate [kg] to be fed into the bioreactor as a function of the amount of methane VMethan [Nm 3 ] and the exchange rate ATR [kg / Nm 3 ] according to the equation mFresh substrate [kg] = VMethane [Nm 3 ] • ATR [kg / Nm 3 ] is designed and arranged, with the exchange rate ATR of the condition 0.01 kg / Nm 3 < ATR < 10 kg / Nm 3 , preferably the condition 1 kg / Nm 3 < ATR < 6 kg / Nm3 , is sufficient.

17. Plant according to one of claims 14 to 16, wherein the plant further comprises at least one substrate pump, wherein the at least one substrate pump is designed and arranged to convey fresh substrate to the inlet for the supply of fresh substrate into the bioreactor and / or to remove used substrate from the bioreactor.

18. Plant according to claim 17, wherein the computer unit is designed and arranged to control the at least one substrate pump, wherein the control of the at least one substrate pump is carried out in such a way that fresh substrate is supplied to the bioreactor according to the equation mFreshsubstrat = VMethan • ATR is supplied.

19. Plant according to claim 17 or 18, wherein the computer unit is designed and arranged to control the at least one substrate pump, wherein the control of the at least one substrate pump is carried out in such a way that fresh substrate is continuously supplied to the bioreactor in such a quantity mfreshsubstrat [kg] that the total quantity rncesamtsubstrat [kg] of substrate present in the bioreactor remains substantially constant, wherein the total quantity of substrate present in the bioreactor corresponds to a predetermined target quantity of substrate msoiisubstrat [kg].

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