EP4705414A1

EP4705414A1 - Processes to convert a mixture of carbon monoxide and hydrogen to methane, or to convert a carbon dioxide feedstock to methane

Info

Publication number: EP4705414A1
Application number: EP24723125.1A
Authority: EP
Inventors: Dandan Liu; Frederikus DE RINK
Original assignee: Paqell BV
Current assignee: Paqell BV
Priority date: 2023-05-02
Filing date: 2024-04-30
Publication date: 2026-03-11
Also published as: WO2024227747A1; NL2034744A; NL2034744B1

Abstract

The invention is directed to a process to convert a mixture of carbon monoxide and hydrogen to methane by performing the following steps: (a) performing a water gas shift reaction to convert carbon monoxide to carbon dioxide to obtain a shifted mixture comprising of carbon dioxide and hydrogen and (b) contacting the shifted mixture with an electron charged packed bed comprising of a carrier and a biofilm of microorganisms in an aqueous solution having a pH of above 7.5 and under anaerobic conditions.

Description

PROCESSES TO CONVERT A MIXTURE OF CARBON MONOXIDE AND HYDROGEN TO METHANE, OR TO CONVERT A CARBON DIOXIDE FEEDSTOCK TO METHANE

The invention is directed to a process to convert a mixture of carbon monoxide and hydrogen to methane.

Manufacture of methane from a mixture of carbon monoxide and hydrogen is well known. The methane as produced is also referred to as substitute natural gas (SNG). The mixture of carbon monoxide and hydrogen, also referred to as syngas, used in such a process is typically obtained by gasification of coal or biomass. Such a process is described in EP2910523. In this process a feed comprising carbon monoxide and hydrogen is contacted with a heterogeneous catalyst in a fixed bed reactor. The methanation reaction is a very exothermal reaction and the reactor outlet temperatures are about 330 °C in this publication.

A journal article titled Granular Carbon-Based Electrodes as Cathodes in Methane-Producing Bioelectrochemical Systems, Dandan Liu, Marta Roca-Puigros, Florian Geppert, Leire Caizan-Juanarena, Susakul P. Na Ayudthaya, Cees Buisman and Annemiek ter Heijne, Frontiers in Bioengineering and Biotechnology, June 2018 | Volume 6, article 78 describes a process where carbon dioxide is converted to methane in the presence of an electron charged packed bed comprising of activated carbon granules and a mixed culture microorganisms under anaerobic conditions. The CO2 was supplied as a gas to an aqueous solution having a pH of 6.5. The disclosed total salt concentration is low resulting in that non-halophilic microorganisms are expected to be present. The reported “current to methane” efficiency was 55%. The reported overall energy efficiency was 25%.

W02022079081 describes an improved process with respect to the above referred Journal article of Dandan Lui et al. In this process carbon dioxide is converted to methane by contacting an aqueous solution comprising dissolved carbon dioxide with an electron charged packed bed comprising of a carrier and a biofilm of halophile microorganisms under anaerobic conditions, wherein the dissolved carbon dioxide is present for more than 90 mol% as a bicarbonate ion and/or as a carbonate ion. It has been found that the energy efficiency is substantially improved to values of 40 % at these more saline and alkaline conditions.

A disadvantage of the traditional process to prepare methane rich product like SNG from a mixture of hydrogen and carbon monoxide as described in EP2910523 is that the reactor effluent has a high temperature and pressure. Cooling may be achieved in a boiler to generate steam. The high temperature and pressure of the process requires a minimal scale to enable an efficient operation.

The object of the present invention is to provide a process which can also be operated at a smaller scale and at more ambient conditions. This object is achieved by the following process. A process to convert a mixture of carbon monoxide and hydrogen to methane by performing the following steps:

(a) performing a water gas shift reaction to convert carbon monoxide to carbon dioxide to obtain a shifted mixture comprising of carbon dioxide and hydrogen and

(b) contacting the shifted mixture with an electron charged packed bed comprising of a carrier and a biofilm of microorganisms in an aqueous solution having a pH of above 7.5 and under anaerobic conditions.

Applicants found that this process can be operated at more ambient temperatures and at a smaller scale. This is especially advantageous when the syngas is prepared from a biomass source. Further advantages will be described when discussing the preferred embodiments below.

The mixture of carbon monoxide and hydrogen is preferably obtained by gasification of a biomass feedstock. In a gasification process the biomass feedstock is partially combusted with a non-stoichiometric amount of oxygen. The biomass feedstock may be a mixture originating from different lignocellulosic feedstocks. Furthermore, the biomass feed may comprise fresh lignocellulosic compounds, partially dried lignocellulosic compounds, fully dried lignocellulosic compounds or a combination thereof. The biomass may be subjected to a torrefaction process prior to the gasification process. Examples of suited biomass are all naturally occurring terrestrial plants such as trees, i.e. wood, bushes and grass. Waste biomass may also be used and is produced as a low value by-product of various industrial sectors such as the agricultural and forestry sector. Examples of agriculture waste biomass are corn stover, sugarcane bagasse, beet pulp, rice straw, rice hulls, barley straw, corn cobs, wheat straw, canola straw, rice straw, oat straw, oat hulls, corn fibre and manure.

Biomass gasification processes are well known and for example described in US2010269411 , US2008216405, WO13005239, WO0168789 and WO20055254. The mixture of carbon monoxide and hydrogen obtained by gasification of a biomass feedstock may comprise impurities like H2S, COS, NH3, HCN, alkali and/or halides. The molar ratio of hydrogen to carbon monoxide for a biomass derived mixture is typically around 1 :1 mol/mol.

In step (a) carbon monoxide is converted with water to hydrogen and carbon dioxide by means of a catalysed water gas shift reaction. The catalysed water gas shift reaction is well known and for example described in "Gasification" by C. Higman and M. van der Burgt, 2003, Elsevier Science, Chapter 5, pages 315-318. There exists sweet and sour water gas shift technologies. For the sweet shift technology it is important that sulphur is absent in the mixture of hydrogen and carbon monoxide. Sulphur as for example sulphide and organic sulphur components are then suitably removed prior to performing such a sweet shift. Sulphur removal is well-known for which multiple technologies exist such as for example described in "Gasification" by C. Higman and M. van der Burgt, 2003, Elsevier Science, Chapter 5, pages 298-309.

Preferably a sour shift is performed. The resultant shifted gas mixture comprising for its majority of carbon dioxide and hydrogen may then still contain impurities as listed above. This gas composition may be directly used in step (b). Preferably all or some of these impurities are removed from the shifted gas prior to performing step (b). Preferably the sour shift is performed as described in WO201 0112502 where HCN and/or COS is removed from the resulting shifted gas. Preferably any hydrogen sulphide which remains in the shifted gas is removed by a sour gas absorption of at least hydrogen sulphide followed by a biological oxidation of the dissolved bisulphides to elemental sulphur as also described in WO201 0112502.

Preferably HCN and/or COS are removed in a sour water gas shift reaction step to obtain a shifted gas depleted in HCN and/or in COS and removing hydrogen sulphide by contacting this shifted gas depleted in HCN and/or in COS with an aqueous alkaline washing liquid to obtain a H2S-depleted shifted gas stream and a sulphide-comprising aqueous stream. The H2S-depleted shifted gas is suitably used in step (b).

When a sweet shift is performed and the mixture of carbon monoxide and hydrogen further comprises H2S the H2S may be removed by contacting the mixture of carbon monoxide and hydrogen with an aqueous alkaline washing liquid to obtain a H2S-depleted mixture of carbon monoxide and hydrogen and a sulphide- comprising aqueous stream before performing step (a).

The sulphide-comprising aqueous stream as obtained in the above line-ups is preferably contacted with sulphide-oxidizing bacteria in a bioreactor to obtain a sulphur slurry and a regenerated aqueous alkaline washing liquid.

From the shifted gas all or part of the hydrogen may be removed before performing step (b). Separation processes, such as membrane separation processes, may be used where some hydrogen remains in the shifted gas. The hydrogen may then be used for various applications such as hydrogenation, hydroprocessing, preparation of ammonia or as a fuel. Applicants found that step (b) can both be performed advantageously on a shifted mixture where all or part of the hydrogen is removed and advantageously on a shifted mixture comprising of carbon dioxide and hydrogen having the hydrogen and carbon monoxide compositing obtained in the water gas shift reaction.

Applicants found that in general a process of step (b) can be performed on a feedstock comprising of substantially carbon dioxide, preferably having a purity of above 90 vol%, more preferably above 95 vol% and even more preferably above 99 vol%. The carbon dioxide may be obtained in a water gas shift reaction but may also have other sources. The invention is therefore also directed to a process to convert a carbon dioxide feedstock having a purity of above 90 vol% to methane by contacting the carbon dioxide feedstock with an aqueous solution having a pH of above 7.5 and comprising more than 20 mM phosphate ions and with an electron charged packed bed comprising of a carrier and a biofilm of microorganisms under anaerobic conditions. The carbon dioxide feed is preferably directly fed to the aqueous solution, for example via an ejector. Such an embodiment does not involve an absorber. The further preferred conditions for this process are described below when discussing the preferred conditions for step (b).

In step (b) the shifted mixture is contacted with an electron charged packed bed comprising of a carrier and a biofilm of microorganisms in an aqueous solution having a pH of above 7.5 and under anaerobic conditions.

The carbon dioxide is contacted in an aqueous solution with the electron charged bed as dissolved carbon dioxide. The dissolved carbon dioxide may be present as aqueous carbon dioxide, carbonic acid, bicarbonate ions and as carbonate ions. A major part of the dissolved carbon dioxide in the aqueous solution is present as a bicarbonate ion and/or as a carbonate ion. Preferably more than 90 mol% and even more preferably more than 95 mol% of the dissolved carbon dioxide in the aqueous solution is present as a bicarbonate ion and/or as a carbonate ion. The pH conditions at which these compounds are present in an aqueous solution is above 7.5, preferably above 7.7 and more preferably above 8 and even more preferably in the range of from 8 to 10 and most preferably of from 8.5 to 9.5. These alkaline conditions may be achieved by an alkaline salt formed between a weak acid and a strong base, such as sodium bicarbonate and potassium bicarbonate. Such basic salt may be formed by adding sodium cations or sodium and potassium cations. Suitably the aqueous solution comprises between 0.3 and 4 M sodium cations or between 0.3 and 4 M sodium and potassium cations. Preferably the aqueous solution comprises between 0.4 and 2 M and even more preferably between 0.4 and 1 M sodium cations or sodium and potassium cations. The resulting aqueous solution is a buffered solution further comprising sodium carbonate and sodium bicarbonate or potassium carbonate and potassium bicarbonate or their mixtures.

Applicants found that when step (b) is performed in the presence of more than 20 mM phosphate ions a more stable process is obtained wherein the energy efficiency improves to values of around 60%. Only a small content of more than 20 mM, preferably more than 40 mM and even more preferably more than 50 mM of phosphate ions is required. It is suggested that the phosphate ions suppress microbial growth of competing microorganisms which consume electrons and form other products. For this reason only small contents are required. The upper limit will be determined by factors like scaling, which is suitably to be avoided. Contents of up to and even above 0.5 M are conceivable. For practical reasons one would operate the process at low phosphate ion contents. The phosphate ions may be added to the aqueous solution as a salt and preferably as an alkaline salt like sodium phosphate or potassium phosphate. The latter are preferred because sodium and optionally potassium ions are according to the invention present in the aqueous solution.

The aqueous alkaline solution suitably further comprises nutrients for the microorganisms. Examples of suitable nutrients are nutrients such as ammonium, vitamin and mineral elements as may be present as part of a so-called Wolfe’s mineral solution. It may be desired to add such nutrients to the aqueous alkaline solution in order to maintain active microorganisms.

The anaerobic conditions are suitably achieved by performing the process in the absence of molecular oxygen, preferably also in the absence of other oxidants such as for example nitrate. By ‘in the absence of molecular oxygen’ is meant that the concentration of molecular oxygen in the loaded aqueous solution in this process is at most 10 pM molecular oxygen, preferably at most 1 pM, more preferably at most 0.1 pM molecular oxygen. Sulfate, which may be regarded to be an oxidant, may be present at low concentrations of for example 160 pM, as part of the earlier referred Wolfe’s mineral solution. It has been found that the sulfate at these low concentrations does not negatively influence the desired conversion of carbon dioxide. The process is performed by contacting the aqueous solution with an electron charged packed bed comprising of activated carbon granules and microorganisms under anaerobic conditions wherein carbon dioxide is converted to methane. The microorganisms may be a mixed culture of microorganisms or a monoculture. The mixed culture of microorganisms is suitably obtained from an anaerobically grown culture. Suitably the mixed culture comprises hydrogenotrophic methanogens, such as for example Methanobacterium. Further microorganisms may be present, including anaerobic or facultative anaerobic bacteria, for example Proteobacteria, such as for example Deltaproteobacteria and Betaproteobacteria.

At the high salt concentration conditions of the process of this invention halophile microorganisms will dominate the culture even when the starting culture is obtained from an anaerobically grown culture which consisted of mainly nonhalophile microorganisms. Examples of halophile microorganisms which may be present in the process are slight halophiles and moderate halophiles belonging to genus level of Bathyarchaeia, Methanobacterium, Methanosaeta, Candidatus Methanogranum, Marinobacter, Balneolaceae , Desulfovibrionaceae, Acetobacterium, Acidaminococcaceae, Halothiobacillus, Spirochaetaceae, Paludibacter, Rhodobacteraceae, Desulfobacteraceae, Desulfuromonadaceae, Geobacteraceae, Solimonadaceae, Halomonadaceae, Vibrionaceae, Ectothiorhodospiraceae, Oceanospirillaceae, Lentimicrobiaceae and/or Synergistaceae.

The mixed culture microorganisms is preferably obtained from an anaerobic system, such as an anaerobically grown culture. The mixed culture may therefore be obtained from the sludge of an anaerobic bioreactor, such as an anaerobic fermenter, for example one used for anaerobic chain elongation; an anaerobic digestion reactor, for example an upflow anaerobic sludge blanket reactor (LIASB); Other suitable bioreactors for providing the sludge are expended granular sludge bed (EGSB), a sequential batch reactor (SBR), a continuously stirred tank reactor (CSTR) or an anaerobic membrane bioreactor (AnMBR). In the present context, the term sludge refers to the semi-solid flocs or granules containing a mixed culture of microorganisms. The mixed culture microorganisms may also be obtained from a natural occurring system, such as (the sediment of) a soda lake. The carrier may be any carrier which provides a surface for the biofilm and has a sufficient capacitance. The preferred carrier is biocompatible and has a 3D granular structure for attachment of the microorganisms and to enhance the mass transfer of the bulk solution and the electrode. Preferably the carrier is carbon based. Preferably the carrier is comprised of activated carbon granules.

The packed bed of the carrier suitably comprises of granules of activated carbon for example, activated biochar. Suitably the bed is a packed bed of activated carbon granules or activated biochar granules having an activated surface area of between 500 and 3000 m^/g and wherein the microorganisms are present as a biofilm on the surface of the activated surface area. The high surface area provides a surface on which the microorganisms are present. Part of the microorganisms may be present planktonically. A high surface area per volume thus provides a higher capacity to perform the desired reaction of carbon dioxide to methane per volume of reactor space.

The dimensions of the granules are suitably such that on the one hand a mass transport of the aqueous fractions is possible in the spaces between the granules without causing a high pressure drop. This means that there will be a practical lower limit with respect to the dimensions of the granules. On the other hand the granules should not be too large because this would result in long travel distances within the micropores of the activated carbon granules. The volume based diameter of the granules may be between 0.5 and 10 mm and preferably between 1 and 4 mm.

The electron charged packed bed comprising of activated carbon granules is preferably part of a biocathode in a bioelectrochemical system further comprising an anode. The biocathode suitably comprises a volume of activated carbon granules arranged in a packed bed. The packed bed contacts with a current collector, which may be a surface of a conductive electrode material, such as a carbon comprising materials such as a graphite plate or felt or a metal mesh, preferably a stainless- steel mesh. The current collector is arranged such that the packed bed may be charged with electrons from said current collector. The packed bed will further be positioned in a cathode space of the bioelectrochemical system which is fluidly connected to an anode space of the bioelectrochemical system. The cathode space may be separated from said anode space by an ion exchange membrane, for example a cation exchange membrane or an anion exchange membrane. In order to compact the packed bed of activated carbon granules it may be preferred to add inert particles, like glass beads, to the anode space such to counterbalance the pressure exercised by the packed bed on the ion exchange membrane. The ion exchange membrane functions to avoid that molecular oxygen which may be generated at the anode can flow to the cathode space. If such an oxygen cross-over can be prevented differently, for example by means of the design of the reactor in which the process is performed, a membrane between cathode and anode space may be omitted.

The aqueous solution as present at the anode is referred to as the anolyte and the aqueous solution as present at the cathode is referred to as the catholyte. Suitably a recirculation is performed where part of the catholyte is fed to the anode to become part of the anolyte and part of the anolyte is fed to the cathode to become part of the catholyte. It is found that when such a recirculation is performed a more efficient process is obtained wherein the major part of the dissolved carbon dioxide in the aqueous solution is present as a bicarbonate ion and/or as a carbonate ion.

Preferably the content of oxygen as may be present in the anolyte should be low when this is fed to the cathode to become part of the catholyte. The oxygen content may be decreased by removing oxygen from this anolyte stream by means of a gas-liquid separation or a degassing unit. Alternatively physical or chemical oxygen scavengers such as sulfite or an organic scavenger may be used to lower the oxygen content. Also the anolyte may be purged with O2 free gasses, such as N2 and/or CO2.. Oxygen may also be removed from the anolyte by electrochemical removal techniques.

Preferably the content of methane as may be present in the catholyte should be low when this is fed to the anode to become part of the anolyte. The methane content may be decreased by removing methane from this anolyte stream by means of a gas-liquid separation or by stripping with a suitable gas, such as for example nitrogen.

The packed bed of activated carbon granules may be charged in such a system by applying a potential to the bioelectrochemical system resulting in a current between biocathode and anode such that electrons are donated at the anode and at the cathode electrons are supplied to the packed bed. At the anode an oxidation reaction, such as water oxidation, takes place providing the required electrons. The potential may be achieved by an external power supply generating electricity, like for example power generated by wind and/or solar. Alternatively the electrons may be partially donated by another chemical reaction at the anode than water oxidation. An example of such a chemical reaction is the organic matter (i.e. COD) oxidation as described in Cerrillo, M., Vi as, M. and Bonmati, A. (2017) Unravelling the active microbial community in a thermophilic anaerobic digester-microbial electrolysis cell coupled system under different conditions. Water Research 110, 192-201.

The anode will be placed in the anode space and may be made of a material suited for the oxidation of the chosen electron donor. Preferred materials for water as the electron donor are platinum, ruthenium, titanium, tantalum coated with iridium and their mixtures. An example of a suitable anode material is a iridium-tantalum- coated titanium plate. Preferably the anode is a titanium mesh coated with iridium, ruthenium and/or tantalum, for example a titanium mesh coated with ruthenium or a titanium mesh coated with ruthenium-iridium. It has been found that the electrochemically catalytic property for water oxidation of the titanium mesh coated with ruthenium-iridium is higher than a platinum based anode. The experimental results have shown that the required anode potential for water splitting is much lower at the same current density observed in previous experiments with the platinum based anode material. A lower anode potential requires less energy input. When the current density was increased to 10 A/m2, it was expected that the anode potential would increase. However, the actual increase of the anode potential was negligible. The charged packed bed is suitably charged to a capacitance of between 10 to 100 F/g. Preferably charging is performed in a bioelectrochemical system comprising a biocathode, an anode and a cation exchange membrane. The electron charged packed bed is part of the biocathode. The packed bed is charged by applying a voltage/current to the bioelectrochemical system resulting in a current between biocathode and anode for a certain time resulting in that the packed bed is loaded with electrons. Preferably the packed bed is charged by applying a current density to the cathode electrode of between 2 and 200 A/m^ and preferably between 2.5 and 120 A/m^. The packed bed may also be charged by applying a potential to the bioelectrochemical system by controlling the cathode potential of the biocathode, preferably at a value which is less negative than the hydrogen evolution potential. The cathode potential at which hydrogen evolution occurs is depending on the pH at which the process is operated. Preferably, as the pH ranges typically between 7.5 and 9.0, the range of the cathode potential varies between -0.50 and -0.74V vs. Ag/AgCI (3M KCI).

The electron charged packed bed does not necessarily have to be connected to an external power supply such that no electrons are supplied when performing the process. When the packed bed is sufficiently charged with electrons it is found that the process performs for a prolonged period of time. For example the process may be performed for between 0.03 and 12 hours, preferably between 0.05 and 10 hours, in a situation wherein no electrons are supplied to the electron charged packed bed. This is advantageous because this allows the use of a non-continuous power supply generating electricity, preferably a sustainable and renewable external power supply. Such a renewable power and thus electrons supply may be generated by solar and/or wind. The capability of the process to operate when such a non-continuous power supply is temporally not available is advantageous.

The process may be performed using an electron charged packed bed as part of the above described bioelectrochemical system wherein no electrons are supplied to the electron charged packed bed of the bioelectrochemical system. In such an embodiment the packed bed is charged before performing the process as described above by applying a potential to the bioelectrochemical system resulting in a current between biocathode and anode. The process may also be performed when the packed bed is charged as described above. Also possible is that the process is performed wherein the packed bed is alternatingly charged and not charged because of the absence of an external power supply. In this embodiment some net charging will take place when performing the process. The system will then be connected to an external power supply to supply power.

The process may also be performed in more than one bioelectrochemical system, each system comprising of the biocathode and an anode, and wherein one bio electrochemical system performs the process and another bio electrochemical system is charged. The system performing the process may be performed while no electrons are supplied to the electron charged packed bed. To the bio electrochemical system which is charged electrons are supplied such that the packed bed is charged with electrons. Optionally a further bioelectrochemical system of the more than one bioelectrochemical system performs the process while the packed bed is charged by applying a potential/current to the bioelectrochemical system resulting in that electrons are supplied to the packed bed.

Without wanting to be limited to the following theory applicants believe that electrons stemming from the anode water oxidation reaction, traverse through an electroactive biofilm on the outer surface of the activated carbon granules to be stored within the micro and macro-pores of the activated carbon granules. These stored electrons, are utilized by the electroactive biofilm to convert CO2 into CH4 according to this invention. Due to the ability of the activated carbon granules to store electrons, also called capacitance, the activated carbon granules essentially function as a rechargeable battery, facilitating both charging and discharging processes. This capability imbues the system with notable flexibility, enabling operation under intermittent power supply conditions and enhancing system robustness for turn-down scenarios. It has been found that the activated carbon granules can be charged by an external power supply such that the process can later operate for more than 10 hours without being connected to an external power supply. This feature of the invented process offers a valuable solution as a grid balancing service by charging activated carbon granules when there is an abundance of, especially renewable, electrical power and the stop of charging when there is a shortage of electrical power on the grid. Further the invention offers a process to convert carbon dioxide to a chemical energy carrier making effective use of the above mentioned surplus of electrical power. Such a process is crucial for the transition from fossil fuels to renewable energy sources, such as wind and solar power. The obtained methane can be readily transported and stored using existing natural gas infrastructures. Moreover, in the event of electricity shortages, the invented process can operate in low-power or turndown mode. The obtained and optionally stored methane can even be used to generate additional electricity for the grid as feed in for example gas turbine power stations in the event of electricity shortages.

The process is further advantageous in that it can be turned down for extensive periods, eg in terms of months, after it can be successfully restarted. It appears that the microorganisms at the biocathode can survive such extensive periods.

Suitably the packed bed comprising of carrier and a biofilm of microorganisms is obtained in an activation step. The activation step is performed at the pH ranges described above for the process steps (i) and (ii) and under anaerobic conditions and by supplying a current at a cathode potential which is more positive than the theoretical hydrogen evolution potential at -0.71 V vs Ag/AgCI (3M KCI) at pH of 8.5 to the packed bed comprising of carrier and a biofilm of microorganisms from a sludge of an anaerobic wastewater treatment plant. The theoretical hydrogen evolution potential is pH dependent. For example, at a pH of 7 the theoretical hydrogen evolution potential is -0.61 V vs Ag/AgCI (3M KCI). It has been found that the resulting packed bed, especially comprising of activated carbon granules or activated biochar and a mixed culture microorganisms, is more robust and avoids hydrogen evolution at the cathode when compared to when such an activation does not take place. The activation is preferably performed until a stable and optimal potential is obtained after turning on the current supply.

The process can be reactivated by supplying an amount of current such that the cathode potential is more positive than the theoretical hydrogen evolution potential at -0.71 V vs Ag/AgCI (3M KCI) at a pH of 8.5 under anaerobic conditions and at a pH of greater than 7.5.

The shifted mixture may be directly fed to the aqueous solution. Depending on the gas pressure and composition, this is done via for example an ejector or an absorber. The shifted gas as obtained in step (a) may have a higher pressure which is preferably reduced to about ambient pressures before using the shifted gas in step (b). Such a reduction in pressure may be performed by means of a flash operation, for example performed in a flash vessel.

The shifted mixture or the substantially pure carbon dioxide feed as described above may be directly contacted with the electron charged bed. For example the gaseous shifted mixture or carbon dioxide may be added via for example a sparger to a zone below the electron charged bed such that the resulting gas bubbles can dissolve in the aqueous solution while traveling through the aqueous solution and the electron charged bed. More preferred is to first dissolve the gaseous carbon dioxide and optionally hydrogen in the aqueous solution before contacting the aqueous solution with the electron charged bed. Such a separate dissolving step may be performed using an ejector or an absorber. The choice between an ejector or an absorber may depend on the desired end products, the composition of the gas and/or the pressure.

For example a shifted gas mixture of step (a), also referred to as step (a1 ), may be contacted in an absorber with the aqueous solution in a step (a2). The aqueous solution may be part of the catholyte which is discharged from a vessel in which the electron charged bed is present to the absorber. In such an absorption step the majority of the carbon dioxide and some hydrogen will dissolve in the aqueous solution which in turn is contacted with the electron charged bed according to this invention. Methane which may be present in the catholyte will be stripped by the shifted gas. The resulting treated shifted gas will be enriched in hydrogen and may contain the formed methane. When the pressure of the shifted gas is high it is preferred to perform the absorption at the corresponding pressure. The aqueous solution containing the dissolved carbon dioxide and hydrogen is then preferably reduced in pressure, suitably by a flash operation, before contacting the aqueous solution with the electron charged bed.

The absorption process step is typically performed in an absorption or contacting column where gas and liquid flow counter-currently. Suitably the absorption process step is performed in a vertical column wherein continuously the shifted gas mixture is fed to the column at a lower position of the column and the aqueous solution is continuously fed to a higher position of the column such that a substantially upward flowing gaseous stream contacts a substantially downwards flowing liquid stream. The column is further provided with an outlet for the loaded aqueous solution at its lower end and an outlet for the gas having a lower content of carbon dioxide at its upper end.

The absorption is preferably performed such that no oxygen is dissolved in the loaded aqueous solution. This may be achieved by starting with a carbon dioxide gas having a low oxygen content. If the gas contains oxygen some pretreatment may be required. Traces of oxygen are allowed as traces of oxygen will also enter the cathode compartment via the membrane from the anode where oxygen is formed in one preferred embodiment.

For example, when the substantially pure carbon dioxide feed is the feed it is preferred that an ejector is used to dissolve carbon dioxide in the aqueous solution before contacting the aqueous solution with the electron charged packed bed. When an ejector is used substantially all of the gas feed is dissolved in the aqueous solution.

Figure 1 shows part of a possible process scheme for the process of this invention. A shifted mixture comprising of carbon dioxide and hydrogen (1) is counter currently contacted in absorption column (3) with an aqueous solution (2) having a pH of above 7.5 as obtained in reactor (4). In column (3) the shifted mixture (1 ) strips the methane from the aqueous solution (2). Preferably part or all of the methane as formed in reactor (4) is separated as a gas (2d) in separator (2c) from aqueous reaction mixture (2a). In the obtained aqueous solution (6) a major part of the dissolved carbon dioxide is present as a bicarbonate ion and/or as a carbonate ion. This aqueous solution (6) is cooled in heat exchanger (7) and fed to an electron charged packed bed (8) comprising of a carrier and a biofilm of microorganisms under anaerobic conditions. In the electron charged packed bed (8) carbon dioxide as the bicarbonate ion and/or as the carbonate ion reacts to methane. It is believed that this is achieved without the formation of hydrogen as an intermediate reaction product. The electron charged packed bed (8) is part of a biocathode (8a) in a bioelectrochemical reactor (4) further comprising an anode (9) and an ion exchange membrane (10) to avoid oxygen as may be formed at the anode (9) to flow to the biocathode (8a).

Part of the aqueous reaction mixture (2e) as obtained in separator (2c) is directly fed to column (3) via line (2f) and part of the aqueous reaction mixture (2e) is fed to column (3) via a mixture vessel (13) via line (2g) to form combined aqueous solution (2). To mixture vessel (13) make up water (14), make up caustic (15) and make up nutrients and vitamins (16) may be added. Because the aqueous reaction mixture (2e) originates from the catholyte the contents of mixture vessel (13) is also referred to as the catholyte. A catholyte bleed stream (17) discharges part of the catholyte from the process.

At the anode water is oxidised and the oxygen as formed is discharged via (18) to an anolyte buffer vessel (19). In this vessel molecular oxygen is separated as (21 ).The anolyte compartment of the reactor (9) is fed with fresh anolyte via (20). Make up water (22) and make up caustic (23) is added and an anolyte bleed stream (24) discharges part of the aqueous solution from the process. The formed oxygen (21 ) may be advantageously used when preparing the mixture of carbon monoxide and hydrogen in a gasification process.

Part of the anolyte (12) is fed to the mixture vessel (13) to become part of the catholyte and part of the catholyte (11) is fed to the anolyte buffer vessel (19) to become part of the anolyte. These streams (11 ,12) may be treated to lower the content of oxygen and methane as described above. A hydrogen rich stream (25), for example a rest stream of another process, for example a downstream process, may be supplied to biocathode (8a) of bioelectrochemical reactor (4). This hydrogen will be converted with carbon dioxide to methane in the bioelectrochemical reactor (4).

When the shifted mixture comprising of carbon dioxide and hydrogen (1 ) also comprises carbon monoxide a shifted mixture (5) may be obtained having a higher hydrogen to carbon monoxide molar ratio that the ratio of the feed mixture of the water gas shift reaction. This shifted mixture (5) may then advantageously be used as a feed to processes requiring such a higher hydrogen to CO molar ratio, such as Fischer-Tropsch synthesis, especially cobalt catalysed Fischer-Tropsch synthesis, methanol manufacturing and DME manufacturing.

The separately obtained methane stream (2d) may be added to a natural gas grid, as a fuel in the process or as an additional hydrocarbon source to prepare the mixture of carbon monoxide and hydrogen in a gasification process such to increase the carbon efficiency of the process.

Figure 2 shows an alternative for the process scheme of Figure 1 . In this process scheme the aqueous reaction mixture (2e) as obtained in separator (2c) is directly fed to column (3) and not via mixing vessel (13). Part of the aqueous solution (6) is cooled in heat exchanger (7) and fed to the electron charged packed bed (8). Another part (6a) of the aqueous solution (6) is fed to mixture vessel (13). Part of the anolyte (12) is fed to the mixture vessel (13) to become part of the catholyte and part of the catholyte (11) is fed to the anolyte buffer vessel (19) to become part of the anolyte. Another part (6b) of the catholyte in mixture vessel (13) is supplied to the electron charged packed bed (8).

The invention is illustrated by the following non-limiting examples. In these examples the energy efficiency of the process is shown. This energy efficiency is defined as follows. In general, the energy efficiency of an electron driven process as the process according to this invention is described as the external electrical energy that ends up in the aimed end-product methane. The energy efficiency is calculated as Equation 1 . energy ^— 0 product ^x 0 voltage (Eq. 1 )

For the CH4 producing process of this invention, q_Product is the current-to-methane efficiency. This is described as the efficiency of capturing electrons from the electric current in the form of CH4, which is calculated as shown in Equation 2.

Where NcH4 is the amount of methane produced (in mole) during a certain amount of time (t); 8 is the amount of electrons required to produce 1 molecule of CH4; F is the Faraday constant (96485 C/mol e-); I is the current (A).

The voltage efficiency (F|voltage) is described as the part of the energy input (i.e. the required cell voltage to run the system) which ends up in CH4, which is calculated as shown in Equations.

Voltage

In this equation AGCH4 '^S the change in Gibb’s free energy of oxidation of CO2 to CH4 (890 x 10³ J/mol CH4); Ecell is the applied cell voltage (V); 8 is the amount of electrons required to producel molecule of CH4; F is the Faraday constant (96485 C/mol e-).

Example 1

A biocathode was operated at halo alkaline medium, containing 0.6M carbonate/bicarbonate buffer with a conductivity of around 40 mS/cm (Na:K of 4:1 ). The medium contained 0.2 g/L NH4CI, 1 mL/L Wolfe’s vitamin solution and 1 mL/L Wolfe’s modified mineral solution. The BES setup is similar to the BES setup described in Liu, Dandan, Marta Roca-Puigros, Florian Geppert, Leire Caizan- Juanarena, Na Ayudthaya, P. Susakul, Cees Buisman, and Annemiek Ter Heijne. "Granular carbon-based electrodes as cathodes in methane-producing bioelectrochemical systems." Frontiers in bioengineering and biotechnology 6 (2018): 78. The cathode electrode was 10.3 g of granular activated carbon (GAC), which was fully packed in the cathodic chamber. A plain graphite plate was used as a current collector. An anodic chamber and a cathodic chamber with a flow channel of 33 cm^ each _Cmx2 cmxl .5 cm). The anodic chamber and cathodic chamber were separated by a cation exchange membrane with a projected surface area of 22 cm2 (1 icmx2cm). The catholyte and anolyte were recirculated over a catholyte and anolyte recirculation bottles. The total volume of anolyte and catholyte were 500 mL and 330 mL, respectively. In order to remove O2 produced at the anode electrode, a high anolyte flow rate of 94 mL/min was used. Also, N2 was continuously bubbled at the rate of 80 mL/min in the anolyte recirculation bottle. The catholyte recirculation rate was 11 mL/min.

The cathodic chamber was inoculated with 30 mL of anaerobic sludge from an upflow anaerobic sludge blanket (UASB) digestor in Eerbeek, The Netherlands. The volatile suspended solids of the inoculated anaerobic sludge was 30.6 g/L. The methane-producing BES was galvanostatically controlled (fixed current) by a potentiostat (with a current density 4 A/m2). In addition, the cell voltage was manually monitored via a multimeter. Liquid samples for pH and conductivity measurements were taken twice per week for both anolyte and catholyte. After a start-up period (not shown), the following results were obtained.

In the first period (25 days), the catholyte contained trace amounts of dissolved phosphate, i.e. <0.1 mM. The coulombic efficiency in this period was between 35% and 60%. That means that the remaining electrons which were supplied to the biocathode ended up in biological growth and other end-products. It was found that acetate was one dominating by-product, which is typically formed by acetogenesis. No significant amount of H2 was detected. To suppress the formation of acetate, at day 25 the phosphate concentration was increased to 5 mM in the catholyte. While an initial increase in coulombic efficiency could be noticed, following days did not show improvement in the coulombic efficiency. In contrast, more supplied electrons ended up in acetate. Hence, at day 50, the phosphate concentration was increased to 50 mM. An immediate increase in CH4 formation was observed, bringing the coulombic efficiency towards >80%.

During the entire period of operation, the voltage efficiency did not change. This is because i) the applied potential at the biocathode always remained between -0.73 and -0.65 V, ii) the applied potential at the anode remained constant and iii) applied current density remained constant. Hence, a change in coulombic efficiency directly resulted in a change in energy efficiency. While in the period of day 0-50 the energy efficiency fluctuated between 20-50%, at higher phosphate levels the energy efficiency increased to 67%.

See also Figure 3 where the current to methane efficiency is represented by the dotted line, the voltage efficiency by the solid line and the energy efficiency by the dashed line.

Example 2

A biocathode was operated at high saline medium, containing 0.6M carbonate/bicarbonate buffer with a conductivity of around 40 mS/cm (Na:K of 4:1 ). The medium contained 0.2 g/L NH4CI, 1 mL/L Wolfe’s vitamin solution and 1 mL/L Wolfe’s modified mineral solution. Additionally, the phosphate concentration was 50 mM. Similar setup and process control was applied as in Example 1 .

The methane-producing BES was galvanostatically controlled (fixed current) by a potentiostat (with a current density of 2.5 A/m²). During a 70 days period of operation, energy efficiency was monitored over time. While maintaining the phosphate concentration at 50 mM, also the energy efficiency over the entire period was between 55 to 67%, except for day 24 (49%). The latter might be explained as an outlier. Thus a stable process is illustrated as also shown in Figure 4 where the current to methane efficiency is represented by the open circles, the voltage efficiency by the open diamonds and the energy efficiency by the crosses and connecting line. Example 3

This biocathode was used to investigate the effect of intermittent current supply on the system performance. The example 3 was performed three times as Example 3a, 3b and 3c with different durations of current-on periods and current-off periods as listed in Table 1 . An overview of the experimental conditions and average current-to- methane efficiencies can be seen in table 1. To conclude, similar performance, in terms of current-to-methane efficiency, for biocathodes under intermittent current supply and constant current supply (at same current density) are found.

Table 1.

Example 4

Example 3a was repeated wherein after the duration on-period the biocathode was controlled at off current (0 A rrr²) and methane concentration was measured approximately every hour in the headspace of the absorber. The off-period lasted 12 hours, during which the cathode potential became less negative, and the anode potential became less positive with time as shown in Figure 5A. The methane concentration did not seem to deviate a lot from the starting concentration throughout the first eight hours of current off period, even though the feed gas rate of CO2 stayed the same in the absorber for 12 hours as shown in Figure 5B. Therefore, the methane production rate stabilized throughout this period. Although applied current density was zero (0 A m^_2), methane production occurred by utilizing stored electrons in the cathode material (GAC). Consequentially, while electrons (stored in the GAC) are used up, the reductive power of the electrode decreased and therefore the cathode potential increased.

Example 5

After performing Example 3c the system was stopped at current off conditions. After 2 months the system was restarted by switching on the current and the system immediately started to convert CO2 to methane. This indicated that biocathode with GACs as cathode materials can efficiently cope with intermittent and fluctuating power sources from hours to months’ time scales.

Claims

1 . A process to convert a mixture of carbon monoxide and hydrogen to methane by performing the following steps:

2. Process according to claim 1 , wherein the aqueous solution comprises more than 20 mM phosphate ions.

3. Process according to claim 1 , wherein the aqueous solution comprises between 0.3 and 4 M sodium cations or between 0.3 and 4 M sodium and potassium cations.

4. A process according to any one of claims 1-2, wherein the aqueous solution comprises between 0.4 and 2 M sodium cations or between 0.4 and 2 M sodium and potassium cations.

5. A process according to any one of claims 1-4, wherein the carrier is comprised of activated carbon granules.

6. A process according to any one of claims 1-5, wherein no electrons are supplied to the electron charged packed bed.

7. A process according to claim 6, wherein the electron charged packed bed is part of a biocathode in a bioelectrochemical system further comprising an anode, optionally an ion exchange membrane, and a cathode wherein the packed bed is charged by applying a potential to the bioelectrochemical system resulting in a current between biocathode and anode for a certain time.

8. A process according to any one of claims 1-5, wherein the electron charged packed bed is part of a biocathode in a bioelectrochemical system further comprising an anode and wherein at one moment in time the process is performed when the packed bed is charged by applying a potential to the bioelectrochemical system resulting in a current between biocathode and anode and wherein at another moment in time the process is performed when no electrons are supplied to the electron charged packed bed.

9. A process according to any one of claims 7-8, wherein the process is performed in more than one bioelectrochemical system, each system comprising of the biocathode and an anode, and wherein in one or more bioelectrochemical systems the process is performed while no electrons are supplied to the electron charged packed bed of these one or more bioelectrochemical systems and wherein electrons are supplied to the packed bed of one or more other bioelectrochemical system of the more than one bioelectrochemical systems such that these packed beds are charged with electrons while the process is not performed.

10. A process according to any one of claims 7-8, wherein the process is performed for between 0.03 and 12 hours when no electrons are supplied to the electron charged packed.

11. A process according to any one of claims 7-10, wherein the power supply generating the potential is electricity generated by solar and/or wind.

12. A process according to any one of claims 7-11 , wherein the packed bed is charged by applying a current density to the cathode electrode of between 2 and 200 A/m^ or by controlling the cathode potential of the biocathode at a value which is less negative than the hydrogen evolution potential.

13. A process according to any one of claims 7-12, wherein the anode is a titanium mesh coated with indium, ruthenium and/or tantalum.

14. A process according to any one of claims 7-10, wherein the power supply is generated by a chemical reaction at the anode.

15. A process according to any one of claims 1-14, wherein the packed bed is a packed bed of activated carbon granules having an activated surface area of between 500 and 3000 m2/g and wherein the microorganisms are present as a biofilm on the surface of the activated surface area.

16. A process according to any one of claims 1-15, wherein the mixture of carbon monoxide and hydrogen further comprises H2S, HCN and/or COS, wherein HCN and/or COS are removed in a sour water gas shift reaction step (a) to obtain a shifted gas depleted in HCN and/or in COS and removing hydrogen sulphide by contacting this shifted gas depleted in HCN and/or in COS with an aqueous alkaline washing liquid to obtain a H2S-depleted shifted gas stream and a sulphide-comprising aqueous stream and using the H2S-depleted shifted gas in step (b) as the shifted gas.

17. A process according to any one of claims 1 -15, wherein the mixture of carbon monoxide and hydrogen further comprises H2S, wherein H2S is removed by contacting the mixture of carbon monoxide and hydrogen with an aqueous alkaline washing liquid to obtain a H2S-depleted mixture of carbon monoxide and hydrogen and a sulphide-comprising aqueous stream before performing step (a).

18. A process according to any one of claims 16-17, wherein the sulphide- comprising aqueous stream is contacted with sulphide-oxidizing bacteria in a bioreactor to obtain a sulphur slurry and a regenerated aqueous alkaline washing liquid.

19. A process to convert a mixture of carbon monoxide and hydrogen to methane by performing the following steps:

(a1) performing a water gas shift reaction to convert carbon monoxide to carbon dioxide to obtain a shifted mixture comprising of carbon dioxide and hydrogen, (a2) contacting the shifted mixture with an aqueous solution having a pH of above

7.5 in an absorber wherein carbon dioxide and part of the hydrogen dissolves in the aqueous solution to obtain a loaded aqueous solution and a treated gas enriched in hydrogen and

(b) contacting the loaded aqueous solution with an electron charged packed bed comprising of a carrier and a biofilm of microorganisms where carbon dioxide is converted to methane under anaerobic conditions.

20. Process according to claim 19, wherein the aqueous solution in step (a2) and the loaded aqueous solution comprises more than 20 mM phosphate ions.

21 . Process according to any one of claims 19-20, wherein the aqueous solution in step (a2) and the loaded aqueous solution comprises between 0.3 and 4 M sodium cations or between 0.3 and 4 M sodium and potassium cations.

22. A process according to any one of claims 19-21 , wherein the aqueous solution in step (a2) and the loaded aqueous solution comprises between 0.4 and 2 M sodium cations or between 0.4 and 2 M sodium and potassium cations.

23. A process according to any one of claims 19-22, wherein the carrier is comprised of activated carbon granules.

24. A process according to any one of claims 19-23, wherein no electrons are supplied to the electron charged packed bed.

25. A process according to claim 24, wherein the electron charged packed bed is part of a biocathode in a bioelectrochemical system further comprising an anode, optionally an ion exchange membrane, and a cathode wherein the packed bed is charged by applying a potential to the bioelectrochemical system resulting in a current between biocathode and anode for a certain time.

26. A process according to any one of claims 19-23, wherein the electron charged packed bed is part of a biocathode in a bioelectrochemical system further comprising an anode and wherein at one moment in time the process is 1 performed when the packed bed is charged by applying a potential to the bioelectrochemical system resulting in a current between biocathode and anode and wherein at another moment in time the process is performed when no electrons are supplied to the electron charged packed bed.

27. A process according to any one of claims 25-26, wherein the process is performed in more than one bioelectrochemical system, each system comprising of the biocathode and an anode, and wherein in one or more bioelectrochemical systems the process is performed while no electrons are supplied to the electron charged packed bed of these one or more bioelectrochemical systems and wherein electrons are supplied to the packed bed of one or more other bioelectrochemical system of the more than one bioelectrochemical systems such that these packed beds are charged with electrons while the process is not performed.

28. A process according to any one of claims 25-26, wherein the process is performed for between 0.03 and 12 hours when no electrons are supplied to the electron charged packed.

29. A process according to any one of claims 25-28, wherein the power supply generating the potential is electricity generated by solar and/or wind.

30. A process according to any one of claims 25-29, wherein the packed bed is charged by applying a current density to the cathode electrode of between 2 and 200 A/m^ or by controlling the cathode potential of the biocathode at a value which is less negative than the hydrogen evolution potential.

31 . A process according to any one of claims 25-30, wherein the anode is a titanium mesh coated with iridium, ruthenium and/or tantalum.

32. A process according to any one of claims 25-28, wherein the power supply is generated by a chemical reaction at the anode.

33. A process according to any one of claims 19-32, wherein the packed bed is a packed bed of activated carbon granules having an activated surface area of between 500 and 3000 m^/g and wherein the microorganisms are present as a biofilm on the surface of the activated surface area.

34. A process to convert a carbon dioxide feedstock having a purity of above 90 vol% to methane by contacting the carbon dioxide feedstock with an aqueous solution having a pH of above 7.5 and comprising more than 20 mM phosphate ions and with an electron charged packed bed comprising of a carrier and a biofilm of microorganisms under anaerobic conditions.

35. A process according to claim 34, wherein the aqueous solution comprises between 0.3 and 4 M sodium cations or between 0.3 and 4 M sodium and potassium cations.

36. A process according to any one of claims 34-35, wherein the aqueous solution comprises between 0.4 and 2 M sodium cations or between 0.4 and 2 M sodium and potassium cations.

37. A process according to any one of claims 34-36, wherein the carrier is comprised of activated carbon granules.

38. A process according to any one of claims 34-37, where an ejector is used to dissolve carbon dioxide in the aqueous solution before contacting the aqueous solution with the electron charged packed bed.