AU2016238788B2

AU2016238788B2 - Method for the hydrogenotrophic methanogenesis of H2 and CO2 into CH4

Info

Publication number: AU2016238788B2
Application number: AU2016238788A
Authority: AU
Inventors: Diana BACKES; Stephan Bauer; Kerstin BRANDSTÄTTER-SCHERR; Alexander GUBIK; Robert KOMM; Andreas P. LOIBNER; Markus MITTEREGGER; Markus Pichler; Johanna SCHRITTER
Original assignee: RAG Austria AG
Current assignee: RAG Austria AG
Priority date: 2015-03-26
Filing date: 2016-03-24
Publication date: 2019-10-10
Anticipated expiration: 2036-03-24
Also published as: DK3280807T3; AR104102A1; AU2016238788A1; EP3280807A1; JP6666424B2; CA2979954C; CA2979954A1; EP3280807B1; HUE047068T2; ES2764722T3; WO2016151078A1; JP2018510654A

Abstract

The invention relates to a method for the hydrogenotrophic methanogenesis of H

Description

METHOD FOR THE HYDROGENOTROPHIC METHANOGENESIS OF H₂ AND

CO₂ INTO CH₄

The present invention relates to a method for the hydrogenotrophic methanogenesis of H₂ and CO₂ to CH₄ using renewable energy.

The present invention further relates to a method for the hydrogenotrophic methanogenesis of H₂ and CO₂ to form CH₄, wherein a gas mixture comprising natural gas, hydrogen generated and CO₂ having a minimum natural gas concentration of 10 %, is fed into an underground storage facility which comprises a gas zone and is stored there in the presence of methanogenic microorganisms.

In order to balance the availability of renewable energy sources that depend on the time of day and/or the weather situation, such as wind and sunlight, and to provide a corresponding adjustment to the respective power demands, storage facilities for said renewable energies are required. In this manner, it is possible to store energy produced in excess,

e.g. in times of strong wind, and to utilize it at a later point in time, e.g. in times of high power demand, or to supply it to other energetic gas applications.

Energy storage facilities serve to store energy which is to be utilized at a later point in time. If the storage of one form of energy is not favorable, e.g. due to technical difficulties, insufficient capacity or downtime losses, said energy is converted to another form of energy that is more suitable for storage, is then stored and may later be reconverted as required. One example in this context is the conversion of chemical energy (fuel) to thermal energy (heat) or the conversion of electrical energy (electricity) to chemical energy (fuel) or thermal energy (heat). In both storage and conversion of energy, losses are bound to occur.

2016238788 28 Aug 2019

The direct storage of electrical energy is very difficult to achieve and it is thus usually required to convert electrical energy to another form of energy and later reconvert it as required.

In this context, the power-to-gas approach has been developed in recent years and already been demonstrated in initial demonstration plants. In this approach, electrical energy is converted to gases and can thus be more easily stored. In Falkenhagen (Germany), for example, a plant has been established in which water is electrolytically split into hydrogen and oxygen using wind energy and is subsequently fed into the local natural gas network. Another plant in Werlte (Germany) also uses electrical energy to split water into hydrogen and oxygen, followed by a further catalytic process step in which methane is formed from hydrogen and carbon dioxide and is in turn fed into the natural gas infrastructure.

Hydrogen at a concentration of 4 to 75% forms a flammable mixture with air; an explosive mixture (oxyhydrogen) is only obtained with a hydrogen concentration of 18 % and more. Having the lowest atomic weight of all elements, hydrogen evaporates in an open environment before it can form an explosive mixture, or it already combusts at a concentration of 4% in a hot environment (unless indicated otherwise, any percentage values given in the present description or the patent claims denote percent by volume when relating to mixtures of gases and percent by mass when relating to solids and liquids).

The most frequently utilized storage form of hydrogen is the so-called pressure gas storage which is based on pressure vessels, such as gas cylinders. In this case, hydrogen is stored in suitable vessels at pressures of about 200 to 800 bar; pressure vessels with a storage pressure of up to 1,200 bar. Owing to the required stability of the pressure vessels, the storage density is about 1 kg hydrogen in 70 to 80 kg

2016238788 28 Aug 2019 vessel mass, which renders pressure gas storage highly uneconomical in terms of transport. For this reason, only about 1% of the hydrogen produced worldwide is stored and transported in this manner.

In the natural gas industry, the storage of natural gas in underground storage facilities is state of the art. A distinction is made between natural storage formations, such as depleted oil and gas deposits and aquifers, and artificially created cavities, such as salt cavities and mining tunnels. The natural storage formations are also referred to as pore storage facilities as the gas is stored directly in the rock pores.

One indicator for the storage of gases from extraneous deposits is the storage of city gas. For example, the municipal utilities of Kiel, Germany, have operated since 1971 a gas cavity the size of 32,000 m³ for the storage of city gas having a hydrogen concentration of up to 50 %. The cavity is located at a depth of 1,330 m and the gas is stored at a pressure of 80 to 160 bar, with a total annual gas loss of only 1 to 3% of the total storage volume. Contrary to common belief, city gas storage cannot be directly compared to the storage of hydrogen or the generation of biogas because, in particular, the carbon monoxide contained in city gas is another highly reactive component which triggers further chemical processes. However, empirical values with respect to the industrial storage of hydrogen have already been obtained in Teeside (Great Britain) and Bakniev (Russia) and can be used as a comparative reference. Finally, the experience in the field of CO2supported EOR (Enhanced Oil Recovery) may also be utilized.

Accordingly, the behavior of hydrogen present underground or stored in the geological subsoil according to the state of the art can be predicted based on many years of practical experience .

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The admixture of hydrogen to natural gas is subject to the Natural Gas Standards which at present provide a decentralized regulation to allow different hydrogen concentrations in different countries. In Austria, for example, the OVGW guideline G31 defines a maximum hydrogen concentration of 4% in each part of a natural gas network. If the feed point is located in the vicinity of a natural gas fuel station, the hydrogen concentration is further limited to a maximum of only 2%. In Germany, the DVGW Worksheet G 260 'Gasbeschaffenheit' (gas quality) allows hydrogen admixtures in a one-digit percentage range, wherein the corresponding actual limit has to be determined for each case individually.

In the atmosphere, CO₂ acts as a greenhouse gas and is considered to be the principal cause of global warming. The separation of CO₂, for example from power plant exhaust gases, can be achieved using various methods, e.g. by CO₂ washing of the exhaust gas after combustion or by separation of CO₂ after coal gasification or combustion in an oxygen atmosphere. One of the suggested options for storing CO₂ that is obtained, e.g., by washing out or separation consists in geological storage, for example in geological formations such as depleted deposits of fossil oil and natural gas and in deep saltwater-bearing aquifers. The most favored option among researchers is the storage of CO₂ in deep sediment layers whose pores are filled with saltwater. At a depth of about 800 m and more, pressures prevail at which the injected CO₂ is compressed to such an extent that it remains in an over-critical state. In order to virtually exclude any resurfacing of the carbon dioxide, these layers must be covered by an impermeable top layer. The pressure prevailing in these depths compresses the CO₂ to a density approximately equal to that of saltwater and thus enables the compressed CO₂ to displace the saltwater from and occupy the pores. In this process, the saltwater is displaced in the deposit laterally with respect to the injection site.

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The lateral expansion of the pressure anomaly can be a multiple of the distribution of the CO₂ in an aquifer. Deep injection of CO₂ and displacement of saltwater performed at pressures significantly higher than the formation pressure and tensile stress of the rock may lead to the occurrence of induced earthquakes, which in some cases may even yield tremors exceeding the tactility threshold. The underground storage of CO₂ is thus problematic and, in addition, the utilization of deep aquifers competes with other utilizations, for example with the utilization of these aquifers for sustainable geothermic power generation. Moreover, the storage capacity of aquifers is limited.

Document EP 0 963 780 Al discloses a method in which hydrogen and CO₂ are stored in a pore storage facility in the presence of methanogenic bacteria (correctly: microorganisms). It is thus already described from this document to separate CO₂from combustion exhaust gases using previously described technical means and to subsequently store said CO₂ underground. Hydrogen in pure form or, e.g., in the form of ammonia, methanogenic bacteria, bacterial substrates, catalysts and/or inhibitors can be added to the CO₂ in order to reduce CO₂ to CH₄during the underground storage period. In this process, the CO₂that has been separated from an exhaust gas using conventional means, purified, liquified and dried is rendered pumpable through a long-distance pipeline and injected into a neighboring aquifer, an artificially created underground cavity or a deposit of natural gas and fossil oil, respectively. The first two instances are preceded by an inoculation of the geological structure with corresponding bacterial cultures and the pertaining organic substrate, whereas methanogenic bacteria and organic substrates are, of course, already present in the third instance. If this teaching is adhered to, carbonic acid will form in the deposit due to the dissolution of pressureinjected CO₂ in water, which decreases the pH to about 4 at

2016238788 28 Aug 2019 least in the vicinity of the pressure injection probe, which in turn renders the methanogenic microorganisms inactive. Besides, the low pH also has the effect of irreparably damaging carbonate rock present in the deposit. The carbonic acid formed simply dissolves carbonate rock, which could pose a risk with respect to the stability of the pore storage facility. Moreover, natural gas, hydrogen and CO2 are not blended, which represents another stress factor for the methanogenic microorganisms .

Document WO 2008/128331 Al discloses a method in which hydrogen and CO2 are converted to methane by methanogenic microorganisms in an underground deposit. CO2 and hydrogen are fed to the deposit.

Most commonly, the microbial formation of methane is performed with acetate (acetoclastic methanogenesis) or H2 and CO2 (hydrogenotrophic methanogenesis) as starting materials. Low-molecular organic compounds, such as methanol, methylamine and formate, are also known as further substrates. In the process of hydrogenotrophic methanogenesis, inorganic carbon is reduced with hydrogen, which is either formed by substrate oxidation or taken up from the environment.

Methanogenic microorganisms (formerly referred to as methane bacteria) are members of the Archaea domain. They have a capacity of forming methane and are capable of obtaining energy from this metabolic process. These microorganisms are divided into the classes Methanobacteria, Methanococci, Methanomicrobia and Methanopyri, which altogether comprise six orders. ^-oxidizing methanogens utilize exergonic methanogenesis, i.e. the reduction of CO2 with molecular hydrogen to form methane and water, as an energy source. Among the obligately H2-oxidizing methanogens are, e.g., the classes of Methanococcus, Methanobacterium and Methanopyrus. These are strictly anaerobic microorganisms which are active at

2016238788 28 Aug 2019 temperatures between 0 and 70°C. In general, the metabolic activity of methanogens increases with the temperature. Some of these microorganisms can survive temperatures of more than 70 to 90°C, but the survival rate decreases with further increasing temperatures.

According to a first aspect of the present invention there is provided a method for the hydrogenotrophic methanogenesis of H₂ and CO₂ to form CH₄, wherein a gas mixture comprising natural gas, hydrogen generated and CO₂ having a minimum natural gas concentration of 10%, is fed into an underground storage facility which comprises a gas zone and is stored there in the presence of methanogenic microorganisms.

According to a second aspect of the present invention there is provided a gas mixture produced by a method according to the first aspect.

The present invention thus relates to a method of hydrogenotrophic methanogenesis, i.e. the microbial formation of CH₄ from CO₂ and H₂, characterized in that a gas mixture comprising natural gas, hydrogen and CO₂ having a minimum natural gas concentration of 10% and preferably having a ratio of hydrogen and CO₂ that is stoichiometric for the formation of CH₄, is fed into an underground storage facility which comprises a gas zone and is stored there in the presence of methanogenic microorganisms. The production of hydrogen is preferably conducted using renewable energy, particularly preferably directly at the storage location by the electrolysis of, e.g., water, followed by mixing the hydrogen thus produced with the natural gas or already present mixtures of natural gas and hydrogen and feeding the resulting mixture into the storage facility through an injection bore. The CO₂ is preferably obtained by one of the above-mentioned separation methods. The CO₂ to be converted can also be admixed with the natural gas before the addition of hydrogen. If the hydrogen is obtained by

2016238788 28 Aug 2019 the electrolysis of water, the oxygen recovered as a waste product can also be utilized for the production of particularly pure CO2. The presence of methanogens can be ensured by injecting such microorganisms into the storage facility, either before or after filling said storage facility, or during the filling process by adding the methanogenic microorganisms to the gas mixture to be stored. Alternatively, the addition of methanogenic microorganisms may also be omitted if such organisms are already present in the storage facility in a sufficient amount. In the method according to the present invention, it was surprisingly found that in the operation of an underground storage facility, in particular of an underground natural gas storage facility, it is possible to convert CO2 and H2 to CH₄ simultaneously with the normal gas storage operation. The method according to the present invention has significant advantages over the prior art, which are obvious from the increased productivity of the methanogenic microorganisms with the given specific ratio of natural gas (of course, methane and/or methane-rich gas mixtures similar to natural gas can also be employed in the method according to the present invention instead of natural gas), hydrogen and CO2. Surprisingly, the relatively high concentration of natural gas in the feed gas does not cause end product inhibition with respect to methanogenesis; on the contrary, the presence of the specified minimum proportion of natural gas rather increases productivity. Preferably, the minimum natural gas proportion is at least 20%, 30 %, 40%, 50%, 60%, 70%, 80% or 87.5%. In case of a stoichiometric ratio of hydrogen and CO2 for the formation of CH₄, the minimum proportion of natural gas can also be calculated based on values lying between the above-mentioned minimum proportions; correspondingly, the same applies to the amounts of hydrogen and CO2 specified in the following. The method according to the present invention allows the conversion of CO2 separated from waste gas to form natural gas with the

2016238788 28 Aug 2019 aid of H2. In a particular embodiment, the present invention also allows the purification and C0₂-depietion of a CO2contaminated natural gas directly at a deposit or storage site thereof. In such a case the ratio of hydrogen and CO2 in the gas mixture to be fed needs to be adapted such that the stoichiometric ratio of hydrogen and CO2 that is required for the formation of CH₄ is only achieved with the addition of the CO2 which is already present in the deposit and contaminates the stored natural gas. The purified natural gas, i. e. CH₄, can then be withdrawn from the storage facility on demand through a withdrawal bore.

According to a preferred embodiment, the present invention is characterized in that the residence time of the gas mixture in the underground storage facility is determined by the distance between injection and withdrawal bore such that natural gas having a content of less than 18%, preferably less than 10%, particularly preferably less than 5%, of the injected CO2 is withdrawn at the withdrawal bore. It has surprisingly turned out - provided that the distance between injection and withdrawal bore is suitably selected depending on the geology of the underground storage facility - that already after a few duty cycles, i.e. cycles of conversion of CO2 and H2 to CH4 by hydrogenotrophic methanogenesis, the reaction rate of the conversion, even without the addition of a substrate, is already high enough to allow a continuous conversion of CO2 and H2 to CH4 by hydrogenotrophic methanogenesis, i.e. that the CO2 and H2 fed into the storage facility through the injection bore is already fully converted to CH4 upon reaching the withdrawal bore. As will be shown in the following Examples, the duty cycle period is shortened by a factor of three and more as soon as a sufficiently large population of methanogenic microorganisms has formed in the underground storage facility. In this context, it has also turned out that the hydrogen required for the formation of CH4 does not originate from the

2016238788 28 Aug 2019 water contained in the pores but rather from the added gaseous H2, which contradicts the teaching of prior art document EP 0 963 780 Al.

In the fields of geology, hydrogeology and pedology, the porosity denotes the ratio of the total volume of all cavities of a porous soil or rock to the outer volume thereof. It is a measure of the space within a given volume that is filled by the rock based on its grain size or jointing, respectively of the cavities it leaved within said volume. The pores or capillaries are usually filled with air and/or water. The porosity (also referred to as pore space or pore volume) refers to the volume and is usually expressed as a percentage or fraction (fractions of 1 = 100%) and designated with the Greek letter Φ in formulas.

With a proportion of pore water in the underground storage facility of preferably at least 15% of the pore space, it has surprisingly turned out that the conversion of CO₂ and H₂ to CH₄by hydrogenotrophic methanogenesis can be performed in a continuous operation of the storage facility. A continuous operation is defined such that the gas volume withdrawn from the storage facility corresponds to the gas volume fed to the storage facility. A specific spatial distance between injection and withdrawal bore is not required in this case.

According to a preferred embodiment of the present invention, the pH value in the gas zone of the storage facility is regulated by means of a controlled addition of hydrogen and/or CO₂ and is kept within a range that is optimal with respect to the productivity of the methanogenic microorganisms. This range varies depending on the microorganism population present and is in general between pH 5 and pH 10, preferably between pH 6 and pH 9 and particularly preferably between pH 7 and pH 8. The pH value is adjusted as follows: if the pH value is too high, the supply of CO₂ to the gas zone is increased,

2016238788 28 Aug 2019 which causes a pH reduction, and if the pH value is too low, the supply of molecular hydrogen is increased, which causes a pH increase. Optionally, the supply of CO2 and/or hydrogen may be automated to maintain the pH value within a predetermined range based on the evaluation of data obtained by a pH probe that is located in the gas zone of the storage facility.

According to another preferred embodiment of the present invention, the methanogenic microorganisms are provided by means of inoculating the storage facility with deposit water from other storage facilities that are already in operation. The deposit water from a second storage facility already in operation has proved to be advantageous due to the population of microorganisms suitable for methanation that has already formed in said deposit water. After the inoculation, this population also propagates more rapidly in the new storage facility than a new population would have been able to. During the growth period in the new storage facility the composition of the population will, if necessary, undergo changes and adapt to the prevailing conditions. In general, the bacteria populations of the storage facilities have basically identical compositions; however, the compositions may differ, in particular with respect to the Archaea that are capable of performing hydrogenotrophic methanogenesis, but in most cases this difference will only relate to the percentages.

The microbial community in the reactors used in the evaluation of the method according to the present invention may be considered as an example of a population present in a storage facility. In a total of eight reactors used, the microbial consortium contained identical genera in all reactors .

In ffi-exposed reservoirs there are bacteria and archaea that are responsible for microbial processes such as methanogenesis, homoacetogenesis as well as sulfate and iron

2016238788 28 Aug 2019 reduction. Methanobacteriaceae are the only family hitherto known to be capable of generating methane from carbon dioxide or monoxide and hydrogen. Moreover, the individual species are

capable of	adapting	to a	wide range of	environmental
conditions,	such	as	pH value, temperature	and	nutrient
availability.	In	the	microb	ial community present	in the
reactors, a	low	proportion	(0.3 to 1.2%)	of the genus
Methanobacterium	was	found,	which is known	to	have an

obligately anaerobic and chemolithotrophic metabolism.

Also of significance is the Thermotogaceae family, which is known for its ability to degrade oil. In the microbial community present in the reactors used, this family is represented by the genera Kosmotoga (detected in percentages of 1.7 to 6.4%) and Petrotoga (detected in percentages of 2.4 to

6.9 %) . Both genera are typical for anaerobic oil reservoirs and occur within a wide range of temperatures from mesophilic (20 to 45°C) to thermophilic (45 to 80°C) . They have been described as fermentative bacteria which are also capable of reducing elemental sulfur to H2S. Furthermore, a synergistic interaction between methanogenic bacteria and hydrogenic bacteria is known.

Another important family, the Peptococcaceae family (detected in percentages of 12.7 to 17.1%) is present in the microbial community in the reactors and is represented, e.g., by the following genera: Desulfotomaculum (about 1.2%), Desulfurispora (about 0.6%), Dehalobacter (about 0.3%), Desulfitibacter (about 1.7%) and further Peptococcaceae which are still uncultivated (14.03 to 7.92%) at present. The genus Desulfotomaculum represents mesophilic, obligately anaerobic bacteria that have a capacity for the sulfate reaction and the formation of H2S. They are predominantly found in microbial biofilms, in which they often form synergistic consortia with fermentative bacteria.

The Clostridiaceae family, which has also been detected (in percentages of 1.10 to 1.96%) in the microbial community present in the reactors used, is capable of reducing amorphous iron (III) oxide by means of peptide fermentation. This family is represented by the genera Natronincola and Proteiniclasticum. Further bacteria that have a capacity to reduce iron and may thus be present in the microbial community of a storage facility when performing the method according to the present invention are Shewanella, Desulfovibrio and Desulfuromonas. These bacteria, however, have an additional capacity for the reduction of manganese.

The family of Thermoanaerobacteraceae (detected in percentages of up to 5%), represented by Gelria (about 1.40%), Moorella (about 2.35%) and Syntrophaceticus (about 1.15%), is typically found in CCg-reducing anaerobic conditions. Several species of the genus Moorella are known that are capable of producing hydrogen and directly transferring it to methanogens.

Pseudomonas (detected in percentages of 14.26 to 30.71%) are members of a group of bacteria which are most versatile with respect to metabolism and habitat. Pseudomonas stutzeri, for example, is capable of reducing nitrate to nitrite and further forming molecular nitrogen from said nitrite under anaerobic conditions.

As already mentioned, the legal regulations in Austria limit the maximum hydrogen concentration in natural gas to 4%, while Germany, for example, allows 5%. Thus, the hydrogen concentration provided according to the present invention in

the	gas	mixture to	be injected	into	the storage	facility
depends	on the legal	regulations	in the	respective	countries
and	may	be lower than	70%, 40%, 30-	%, preferably lower	than 25%,
24%,	23b	b, 22%, 21%,	20%, 19%, 18	%, 17%,	16%, 15%,	14%, 13%,
12%,	lib	b, 10%, 9%, 8	%, 7%, 6%, 5	%, 4%,	3% or lower	than 2%.

Such a gas mixture (containing, e.g., in Austria less than 4%

2016238788 28 Aug 2019 hydrogen in the natural gas) may be transported via the existing infrastructure of the gas network and then be depleted of hydrogen during a comparatively short storage period, wherein simultaneously CO2, which is present, e.g., in the form of a separated fraction of a waste gas, is advantageously converted to methane. In connection with the storage facilities used, the method according to the present invention can also provide a homogenization of fluctuating renewable energy by infiltrating the hydrogen produced using renewable energy into the natural gas network and then methanating said hydrogen on demand while simultaneously utilizing excess CO2 in the storage facility.

According to a preferred embodiment of the present invention, it is envisaged that the concentration of CO2 in the gas mixture to be stored is less than 20%, preferably less than 19%, 18%, 17%, 16%, 15%, 14%, 13%, 12%, 11%, 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, 1% or less than 0.5%. The CO2 concentration can be provided by feeding CO2 into a mixture of natural gas and hydrogen which is already present in a storage facility. Alternatively, a desired CO2 content can be adjusted during the storage process using the method according to the present invention, wherein defined amounts of hydrogen produced using renewable energy are added to a stored natural gas having a known concentration of CO2 according to the chemical equation:

CO₂ + 4H₂ - CH₄ + H₂O

In the method according to the present invention, hydrogen can preferably be transported in an already existing natural gas network in the form of an admixture with natural gas according to the legal regulations, and the resulting mixture of natural gas, hydrogen and CO₂ is preferably stored in underground pore storage facilities. Subsequently to withdrawing the stored gas mixture from the deposit and prior to feeding it into the natural gas network, the gas mixture may

2016238788 28 Aug 2019 optionally be subjected to a membrane management is conceivable. For instance, the residence time of the gas mixture in the underground storage facility may be increased as is required for its complete conversion; alternatively, a circulating operation may be provided, in which excess CO2/H2 is separated from the withdrawn gas using membranes. At the present stage, the ideal system is expected to establish itself after 3 to 4 duty cycles.

In the experiments according to Figures 2 and 3, the above-mentioned eight bioreactors were used, each containing a microbial community according to the above description. According to Figure 2, a gas mixture of 20% of H₂, 5% of CO₂ and 75% of CH₄ was used; according to Figure 3, a mixture of 40% of H₂, 10% of CO₂ and 50% of CH₄ was used. Surprisingly, a higher hydrogen content in the stored gas has a negative effect on the conversion rate as calculated from the linear region of the hydrogen partial pressure progression after the onset of the experiment (Figure 4) . The conversion rate with an initial content of 10% of H₂ (2.5% of CO₂, 87.5% of CH₄) is 0.54 bar/d, with 20% of H₂ (5% of CO₂, 75% of CH₄) it is 0.44 bar/d and with 40% of H₂ (10% of CO₂, 50% of CH₄) it is 0.34 bar/d. In the absence of methane (80% of H₂, 20% of CO₂) no hydrogenotrophic methanogenesis could be observed at all, which led to the surprising conclusion that the presence of methane in the initial gas mixture is mandatory in the method according to the present invention.

Where any or all of the terms comprise, comprises, comprised or comprising are used in this specification (including the claims) they are to be interpreted as specifying the presence of the stated features, integers, steps or

15a

2016238788 28 Aug 2019 components, but not precluding the presence of one or more other features, integers, steps or components.

A reference herein to a patent document or any other matter identified as prior art, is not to be taken as an admission that the document or other matter was known or that the information it contains was part of the common general knowledge as at the priority date of any of the claims.

Claims

The claims defining the invention are as follows:

1. Method for the hydrogenotrophic methanogenesis of H2 and CO2 to form CH₄, wherein a gas mixture comprising natural gas, hydrogen generated and CO2 having a minimum natural gas concentration of 10%, is fed into an underground storage facility which comprises a gas zone and is stored there in the presence of methanogenic microorganisms.
2. Method according to claim 1 wherein the ratio of hydrogen and CO2 is stoichiometric for the formation of CH₄.
3. Method according to claim 1 or claim 2, wherein the residence time of the gas mixture in the underground storage facility is determined by the distance between injection and withdrawal bore such that natural gas having a content of less than 18% of the injected CO2 is withdrawn at the withdrawal bore .
4. Method according to claim 3 wherein the gas mixture having a natural gas content of less than 10% of the injected CO2 is withdrawn at the withdrawal bore.
5. Method according to claim 4 wherein the gas mixture having a natural gas content of less than 5% of the injected CO2 is withdrawn at the withdrawal bore.
6. Method according to any one of claims 1 to 5, wherein the underground storage facility has a water content of at least 15% of the pore space of the reservoir rock and the conversion occurs during the continuous operation of the storage facility.
7. Method according to any one of claims 1 to 6, wherein the pH value in the gas zone of the storage facility is regulated by means of a controlled addition of hydrogen and/or CO2 and is kept within a range that is optimal with respect to the productivity of the methanogenic microorganisms.

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8. Method according to any one of claims 1 to 7, wherein the methanogenic microorganisms are provided by means of inoculating the storage facility with deposit water from other storage facilities that are already in operation.
9. Method according to any hydrogen concentration in the than 72%.

one of claims 1 to 8, wherein the gas mixture to be stored is lower
10. Method according to claim 9 wherein the hydrogen concentration in the gas mixture is lower than 40%.
11. Method according to claim 9 wherein the hydrogen concentration in the gas mixture is lower 30%, 20%, 13%, 12%, 11%, 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3% or lower than 2%.
12. Method according to any one of claims 1 to 11, wherein the

CO2 concentration in the gas mixture to be stored is lower than

18 %.
13. Method according to claim 12 wherein the CO2 concentration in the gas mixture to be stored is lower than 10%, 9%, 8%, 7%,

6%, 5%, 4%, 3%, 2%, 1% or lower than 0.5%
14. Method according to any one of claims 1 to 12, wherein the bacteria is selected from the classes of Methanobacteria, Methanococci, Methanomicrobia and Methanopyri as well as mixtures thereof are used.
15. Method according to any one of claims 1 to 14, wherein the underground storage facility is a pore storage facility.
16. Method according to any one of claims 1 to 15 when used for reducing the CO2 content in the gas zone of a naturally occurring fossil oil or natural gas deposit, either before or after the extraction of fossil oil from the deposit.

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17. Method according to any one of claims 1 to 15 when used for regulating the CO₂ and/or hydrogen content in a natural gas .
18. A gas mixture produced by a method according to any one of claims 1 to 17.