Conversion of carbon dioxide sources to methane

Technical field

The present invention relates to a system and method for converting a carbon dioxide source to methane, which may be particularly suitable for capturing and converting diluted carbon dioxide sources.

Background

Technologies for reducing greenhouse gas emissions from combustion of e.g. fossil fuels are receiving increasing focus due to the climate challenges. In the same context, technologies for providing alternatives to fossil fuels are of increasing importance.

The main anthropogenic greenhouse gas is carbon dioxide (CO2), which is for example released as part of the flue gas from combustion plants, such as biogas plants, petrochemical plants, and refineries. To reduce the CO2 emission into the atmosphere, the carbon may be captured from the flue gas after combustion and before the flue gas is released to the atmosphere. The carbon capture (CC) is typically obtained by use of a gas scrubber solvent which is capable of absorbing or adsorbing CO2 from the gas stream.

The captured CO2 may subsequently be stripped from the gas scrubber, such that the scrubber solvent is regenerated and a pure CO2 gas is formed. The pure CO2 may be transported and stored underground, also referred to as carbon capture and storage (CCS). The captured CO2 may also be recycled, e.g. used as a solvent or fluid (e.g. for beverages, refrigeration, dry cleaning), or converted into a higher value chemical or fuel (e.g. methanol, polymers, cement, concrete). Recycling of captured CO2 is also referred to as carbon capture and utilization (CCU).

For example, the captured CO2 may be converted into higher value fuels, such as methane (CH4) suitable for the natural gas grid. Porzse et al. describes in Energies 2021 , 14, 2297 use of pure CO2 from a regenerated scrubber as a feed into a biomethanation reactor, where the CO2 together with hydrogen (H2) is converted into methane, as described in the methanation reaction (Equation 1 below). In view of the climate challenges, there is a need for alternative as well as more efficient and flexible technologies for greenhouse gas reduction and fuel conversion.

Summary

The present disclosure provides a system for converting a carbon dioxide source to methane, such as a methane having a purity and quality suitable for the natural gas grid. The system is particularly suitable for capturing and converting diluted carbon dioxide sources, such as flue gas streams and/or air streams having low concentrations of carbon dioxide and high concentrations of oxygen. Thus, the system provides a way of capitalizing on gas streams which are otherwise difficult to utilize.

Specifically, the system provides a more simple and thus more energy efficient, flexible, and robust conversion based on a liquid carbon dioxide solvent which is contacted directly with a biological catalyst. Accordingly, the driving mechanisms for the CO2 capturing and CO2 conversion is inherently induced by the selected solvents and catalysts of the system, and the system is thus self-contained, thereby reducing the system complexity and costs. For example, expensive additional system components, such as heaters, for increasing the CO2 concentration to obtain sufficient driving forces for the CO2 conversion may be entirely avoided, even for diluted carbon dioxide sources. Hence, the system uses biological agents in the form of microorganisms for liberating and converting the CO2 from a carbon dioxide solvent to methane.

A first aspect of the disclosure relates to a system for converting a dilute carbon dioxide source comprising below 33 vol% CO2 to methane, comprising: a carbon capture reactor comprising a liquid carbon dioxide solvent and a further carbon dioxide absorbent configured for non-covalent bonding of CO2 and configured to be biocompatible with methanogens, and a liquid-based biomethanation reactor, wherein the reactors are configured to be in fluid communication such that the liquid carbon dioxide solvent is passed in a forward flow from the carbon capture reactor to the biomethanation reactor. A second aspect of the disclosure relates to a method of converting a carbon dioxide source to methane, comprising the steps of: contacting a dilute carbon dioxide source comprising below 33 vol% CO2 with a liquid carbon dioxide solvent and a further carbon dioxide absorbent configured for non-covalent bonding of CO2 and configured to be biocompatible with methanogens, and subsequently contacting the liquid carbon dioxide solvent with a biological catalyst, thereby producing a methane.

In a preferred embodiment of the disclosure, the system according to the first aspect is configured to carry out the method according to the second aspect.

In an alternative or additional embodiment of the disclosure, the method according to the second aspect is configured to be carried out in the system according to the first aspect.

Description of Drawings

The invention will in the following be described in greater detail with reference to the accompanying drawings.

Figure 1 shows a schematic embodiment of the system according to the present disclosure, where a liquid CO2 solvent is used for capturing CO2 from a flue gas, and then the liquid solvent with the captured CO2 is converted in a liquid-based biomethanation reactor through H2 mediated biomethanation. The system comprises two reactors in fluid communication (Figure 1A), which optionally may be in fluid communication via a solvent storage tank (Figure 1 B).

Figure 2 shows data from Example 3, which shows the maximum CH4 productivity rate from the pure absorbed CO2 and the H2 headspace to assess the biocompatibility for biological conversion in systems with different amine concentrations.

Figure 3 shows data from Example 4, which shows the quantity of CO2 available for biological conversion at different amine concentrations.

Figure 4 shows data from Example 5, which shows the maximum CH4 productivity rate from the conversion of CO2 absorbed from pure gas streams (i.e. synthetic gas) and from flue gas streams.

Figure 5 shows embodiments of the system according to the present disclosure comprising different reactor dimensions as further described in Example 6. The calculations show examples of the methane productivity and dilution rate based on the biomethanation reactor volume and solvent concentration, and may be used to identify whether the requirements for increased solvent concentrations for higher CO2 load capacities.

Figure 6 shows embodiments of the system according to the present disclosure comprising a solvent storage tank as further described in Example 7. The calculations show examples of a required volume for a storage tank to store 12 hours of CO2 to comply with the expansion of renewable, but intermittent, energy sources as photovoltaic panels.

Figure 7 shows an embodiment of the system according to the present disclosure, which may be operated in continuous mode as further described in Example 8.

Figure 8 shows data from Example 8, showing the biogas content as a function of operation time.

Figure 9 shows data from Example 8, showing the biomethane production rate as a function of operation time.

Figure 10 shows data from Example 9, showing the biomethane production rate for different liquid carbon dioxide solvents and concentrations.

Detailed description

The invention is described below with the help of the accompanying figures. It would be appreciated by the people skilled in the art that the same feature or component of the device are referred with the same reference numeral in different figures. A list of the reference numbers can be found at the end of the detailed description section.

Dual reactor system

Figure 1 A-B shows a schematic of a system for converting a carbon dioxide source, such as a flue gas, to methane according to the present disclosure. The system comprises two reactors: a carbon capture reactor 1 and a liquid-based biomethanation reactor 2, and may thus be referred to as a dual reactor system, where the reactors are configured to be in fluid communication 3 such that a fluid may be passed from the carbon capture reactor to the biomethanation reactor and thereby defining a flow direction.

Carbon capture reactor An example of a carbon dioxide source is a flue gas emitted from an industrial process or combustion plant 6 as illustrated in Figure 1 A-B. Instead of the flue gas being discharged into the atmosphere via a flue, the exiting gas stream is directed into a carbon capture reactor 1 according to the disclosure, as indicated by arrow.

A carbon capture reactor is generally a reactor configured for capturing CO2 from gas streams. The gas streams may be low CO2 partial pressure gas streams, such as flue gas from a plant comprising between 5-15 mol% CO2, e.g. 8 or 10 mol% CO2. The carbon capture reactor comprises a media, e.g. a solid or fluid, which is capable of capturing an amount of carbon dioxide from a gas phase into the media. For example, the media may capture the carbon dioxide by dissolving, adsorbing or absorbing the CO2. For instance, a solvent may be able to dissolve, adsorb or absorb CO2 into the liquid phase. Accordingly, when the flue gas is passed through the reactor, at least a part of the CO2 is captured by the media and retained in the media in the reactor, while the cleaned flue gas may be discharged into the atmosphere via a flue, as illustrated in Figure 1A-B.

The carbon capture media is preferably a liquid solvent 1.1 according to the present disclosure. A liquid solvent capable of capturing carbon dioxide from a gas phase may also be referred to as a “liquid carbon dioxide solvent” or a “CO2 capture solvent” or a “CO2 scrubber”. The liquid solvent may capture the carbon dioxide by the solvent being chemically adapted for dissolving, adsorbing or absorbing the CO2. For example the solvent may have a CO2 solubility comparable to water, or the solvent may include dissolved chemical agents, such as sorbents, e.g. amines, capable of adsorbing absorbing CO2. A solvent comprising amines may also be referred to as an amine scrubber. The liquid solvent may further be adapted to have improved CO2 capture capabilities, e.g. the pH of an amine scrubber may regulated by adding a base or a buffer, as follows from Equations 2 and 3 in Example 1 , and described in Example 3. For example, the pH may advantageously be between 4-10, more preferably between 6-9, such as 7 or 8. The resulting CO2 enriched liquid carbon dioxide solvent may thus have a CO2 concentration determined by the CO2 solubility and/or the CO2 sorption capacity.

In the present disclosure it is found that liquid carbon dioxide solvents are particularly simple, efficient, and reliable for capturing and converting diluted carbon dioxide sources, such as flue gas streams and/or air streams having low concentrations of carbon dioxide and high concentrations of oxygen. The driving mechanisms for the CO2 capturing may be efficiently controlled by the difference in chemical potential between the diluted CO2 source and the solvent. Accordingly, the solvent may be adapted to ensure a gradient in chemical potential to the gas source, and hence the carbon capture reactor may be self-contained without auxiliary components, such as pressurizers, to facilitate CO2 capture from even diluted sources.

To utilize the captured CO2, the CO2 enriched media may be regenerated by stripping the CO2 off from the media. This may conventionally be done by a high temperature desorption process, such as treating the CO2 enriched media with a water steam at 100-120 °C. The regeneration is typically limited to 120 °C to impede thermal degradation of any sorbants present, e.g. amines that are present. Upon condensation of the water, a regenerated media and a separate pure CO2 gas phase is obtained, and the upconcentrated CO2 product may be industrially utilized as a chemical reactant. An example of a carbon capture reactor with conventional stripping at high temperatures is described in Example 2.

Liquid-based biomethanation reactor

As an alternative to regeneration and upconcentration of the CO2, the CO2 enriched liquid carbon dioxide solvent 1.1 may be transferred to a liquid-based biomethanation reactor 2 as illustrated in Figure 1A-B. For example the CO2 rich absorption liquid may be transferred via a first pipe 3 as indicated by arrow. Thus, the CO2 enriched liquid solvent is used as a liquid feed (also referred to as reactant or substrate) for the biomethanation. The liquid may be transferred directly between the two reactors as shown in Figure 1A, or transferred via a solvent storage tank 7 as shown in Figure 1 B.

A biomethanation reactor is a reactor configured for converting CO2 into methane by use of a biological catalyst, such as biochemical or microbiological catalysts. For example, the biological catalyst may be methanogens, which are methane producing microorganisms, as shown in Equation (1). Thus, in the presence of methanogens and a source of reducing agents, such as reducing equivalents e.g. in the form of hydrogen, simultaneous CO2 stripping of the absorbent and conversion of the CO2 may occur within the biomethanation reactor. The produced methane may be exhausted via a pipe as indicated in Figure 1A-B. In the presence of hydrogen, the process may also be referred to as H2 mediated biomethanation. However, the conversion may also be mediated by other reducing agents, for example reducing equivalents such as hydrogen, formats and an electron source, e.g. in the form of a cathode.

In an embodiment of the disclosure, the biomethanation reactor further comprises a source of reducing agents, such as reducing equivalents selected from the group of: hydrogen, formats, electrons, and combinations thereof.

The biomethanation reactor is advantageously a liquid-based biomethanation reactor 2 such that the CO2 enriched liquid carbon dioxide solvent may be directly passed or injected from the carbon capture reactor into the biomethanation reactor, optionally via a piping system including a solvent storage tank. In a liquid-based biomethanation reactor, the methanogens 2.1 are arranged to ensure a contact interface with a liquid flow. For example, the methanogens may be fixed on a support frame or configured to have a particle size distribution facilitating particle dispersion in a liquid flow.

The CO2 concentration of the CO2 enriched liquid carbon dioxide solvent is determined by the CO2 solubility and/or the CO2 sorption capacity of the solvent, and is thus typically between 1-15 wt%, such as 5-10 or 7-10, as follows from Figure 3 and Example 4. However, the biologically induced methanation, where the methanogens extract the CO2 directly from the solvent, and convert it into methane, provides a sufficient gradient in the chemical potential and a driving force for even very diluted liquid carbon dioxide solvents.

Accordingly, the CO2 conversion is inherently induced by the solvents and catalysts of the system, and the system may thus self-contained without auxiliary driving mechanisms, such as pressurizers or heaters for upconcentrating the CO2. Specifically, it was surprisingly found that a liquid carbon dioxide solvent may be directly used as feed for a biomethanation reactor without prior separation and purification of the captured CO2 from e.g. oxygen and solvent.

Liquid carbon dioxide solvent

Carbon capture reactors based on liquid carbon dioxide solvents may be particularly efficient and reliable for capturing the CO2 of diluted carbon dioxide sources, such as flue gas streams and/or air streams having low concentrations of carbon dioxide and high concentrations of oxygen, because the CO2 capture efficiency may be tailored and/or controlled by the difference in chemical potential between the dilute gas phase and the solvent. A flue gas is a diluted gas stream of CO2 because it includes a large fraction of atmospheric air, as it is diluted in atmospheric air. Advantageously, the liquid carbon dioxide solvent may be configured for capturing the CO2 from flue gas streams or air streams with CO2 contents below 33 vol% and/or oxygen contents above 5 vol%. Further advantageously, the liquid carbon dioxide solvent is configured for capturing the CO2 and not capturing the oxygen, i.e. incapable of dissolving the oxygen due to the oxygen dissolution capacity and/or capturing the oxygen at a low dissolution rate. To reduce the amount of oxygen captured to, the gas stream advantageously comprises below 5 vol% oxygen.

In an embodiment of the disclosure, the carbon capture reactor is configured for a diluted carbon dioxide source, such as a flue gas stream and/or an air stream. In a further embodiment, the diluted carbon dioxide source comprises below 33 vol% CO2, more preferably between 5-25 vol%, and most preferably between 7-23 vol%, such as 15 vol%. In a further embodiment, the diluted carbon dioxide source comprises above 5 vol% O2, more preferably between 7-25 vol%, such as 10 or 21 vol%.

Optionally, the carbon capture reactor is configured for being detachably attached or permanently attached to the carbon dioxide source. In an embodiment of the disclosure, the carbon capture reactor is connected to a dilute carbon dioxide source.

The liquid solvent capable of capturing carbon dioxide from a gas phase may capture the carbon dioxide by the solvent being chemically adapted for dissolving, adsorbing or absorbing the CO2, and the resulting CO2 enrichment of the solvent will depend on the CO2 solubility and/or the CO2 sorption capacity.

However, the CO2 enrichment may also depend on the CO2 affinity of the solvent and e.g. how strongly and reversibly the CO2 is bonded to the solvent. Advantageously, the captured CO2 is bonded with sufficient high affinity, such that the CO2 may be non- reversibly captured by the solvent even under turbulent flow, while the affinity is still sufficiently low such that the captured CO2 may be easily and efficiently extracted by the methanogens. Further, the solvent may possess an adequate CO2 binding capacity to avoid diluting and washing out of the microbial community when employed in a continuous reactor system. Also preferably, the CO2 is captured in the solvent by non- covalent bindings to improve the CO2 bioavailability and enable a fast methanogenic consumption rate of the CO2.

A suitable CO2 affinity may be obtained by a liquid carbon dioxide solvent comprising an aqueous solution, and specifically comprising a high water concentration above 80 wt%. Accordingly, the liquid solvent may have a CO2 solubility comparable to water. In other embodiments, a suitable CO2 affinity is obtained by a non-aqueous solution or solutions comprising low water concentrations.

In an embodiment of the disclosure, the liquid carbon dioxide solvent comprises an aqueous solution. In a further embodiment, the aqueous solution has a water concentration above 80 wt%, more preferably above 85 or 90 wt%, such as 94, 96, 98 or 100 wt%.

The CO2 capture capacity of the liquid solvent may increase the efficiency of the CO2 capture process. Hence, the liquid solvent advantageously comprises a polar solvent, such as water, and/or other solvents, such as a further polar solvent, to provide a mixture of water miscible solvents, with higher CO2 solubility or CO2 absorption. However, since the liquid carbon dioxide solvent is also used as direct liquid feed for the biomethanation reactor, the solvent advantageously comprises such capacity enhancing agents in amounts that are compatible with the viability and productivity of the methanogens to ensure the efficiency of the combined dual reactors.

The viability and productivity of methanogens may decrease with increasing amounts of polar solvents and oxygen, specifically dissolved oxygen. Thus, the solvent advantageously comprises below 50 vol% alcohol, and below 1 wt% oxygen. It was further found that the viability and productivity of methanogens may be maintained or improved by the presence of a polar solvent, e.g. glycols, such as polyethylene glycol and/or diethylene glycol. For example glycols in concentrations of between 100-5000 mM, such as 1000, 2000, 3000, or 4000 mM. Specifically, the solvent may be an aqueous solution comprising a further polar solvent, such as an alcohol and/or a polyethylene glycol. In an embodiment of the disclosure, the liquid carbon dioxide solvent comprises a further polar solvent in a concentration below 50 vol%, and optionally wherein the polar solvent is an alcohol, such as methanol and/or ethanol. In an alternative and further embodiment, the liquid carbon dioxide solvent comprises a glycol, such as polyethylene glycol and/or diethylene glycol.

Biocompatibility

As described above, certain agents or components are not compatible with high viability and productivity of a biological catalyst. Further, certain concentrations of the agents may reduce the productivity or even be detrimental to the viability.

Thus, by the term biocompatibility as used herein is meant a material having properties being compatible with living microorganisms, such as methanogens. Different microorganism species may have different tolerances. Methanogens are mainly viable under anaerobic conditions and very low oxygen concentration conditions, such as oxygen concentrations below 3 mg/L, and their productivity decreases significantly in the presence of oxygen. Thus, methanogens are killed by normal atmospheric concentrations of oxygen, i.e. 20.95 vol%, and accordingly atmospheric air is not biocompatible with methanogens.

Thus, advantageously, the liquid carbon dioxide solvent is configured to have a high CO2 capture capacity in combination with a low O2 capture capacity or low O2 capture rate. This further facilitate the use of the system for diluted CO2 sources. In addition, or alternatively, a low oxygen concentration or low dissolved oxygen concentration, in the solvent may be facilitated by the presence of oxygen scavenging microorganisms.

In an embodiment of the disclosure, the liquid carbon dioxide solvent comprises dissolved oxygen in a concentration below 1 wt%, more preferably below 0.1 , 0.01, 0.001 , 0.0001 wt%, and most preferably is essentially free of oxygen.

The viability and productivity of the methanogens may even be improved by the presence of certain agents, such as one or more sulphur compounds. For example, the sulphur compounds may advantageously be sulphur oxides, such as SO2 and/or SO3. Accordingly, a high growth of methanogens will increase the consumption rate of CO2. This may further improve the cost-efficiency of the system, since the volumetric requirements for the reactors are reduced. The sulphur compounds may advantageously be provided via the dilute carbon dioxide source. For flue gasses, any sulphur compounds present may burn in the presence of oxygen to SO2 and/or SO3. The sulphur oxides may, similar to the CO2, be captured and dissolved by the liquid solvent and the further absorbent present in the carbon capture reactor.

In an embodiment of the disclosure, the liquid carbon dioxide solvent comprises a sulphur compound.

Sorbent agents and further carbon dioxide absorbent

To further improve the CO2 capture capacity of the liquid solvent, the liquid carbon dioxide solvent advantageously comprises chemical agents capable of adsorbing and/or absorbing CO2. Such CO2 sorbent agents may be absorbents, which have sufficient affinity for binding the CO2 and allowing extraction of the CO2 by the methanogens. Hence, the absorbent agents may be a further carbon dioxide absorbent in addition to the liquid carbon dioxide solvent as such. Further advantageously, the further carbon dioxide absorbent is soluble and/or miscible with the liquid solvent.

Further, the efficiency of the absorbent may be determined by the absorption capacity (i.e. the maximum or saturated concentration of CO2), and the tolerance to impurities (i.e. the degradation over time in operation). The higher the CO2 absorption capacity and tolerance, the more efficient the absorbent.

Efficient carbon dioxide absorbents, such as amines or sulphur compounds, are generally considered toxic to methanogens, and thus not biocompatibility with methanogens. However, it was surprisingly found that solvents with absorbents may be adapted to be biocompatible for a biomethanation reactor. Specifically, the selected combination of the solvent, such as an aqueous solution, and the absorbent, such as an amine or a sulphur compound, and an absorbent concentration between 10-400 mM, may provide a surprisingly high methanation efficiency.

In an embodiment of the disclosure, the liquid carbon dioxide solvent comprises a carbon dioxide absorbent. In a further embodiment, the absorbent concentration in the liquid carbon dioxide solvent is between 10-400 mM, more preferably between 15-200 mM or 20-140 mM, and most preferably between 25-120 mM, such as 30, 50, 70, 90 mM.

It was found that absorbents comprising toxic amines or alkylamines may be particularly efficient as absorbent, while being biocompatible when present in certain concentrations. Further advantageously for the biocompatibility and efficiency, the liquid solvent is an aqueous solution comprising amines. Examples of toxic absorbents include diethanolamine (DEA), monoethanolamine (MEA), methyldiethanolamine (MDEA), diisopropanolamine (DIPA), and aminoethoxyethanol or diglycolamine (DGA), and optionally the amines are in aqueous solution. It follows that the type of sorbents are not limited to amines and could also be based on additional or alternative substances such as sulfur compounds, carbon, or metals, which may be configured to be biocompatible with the function and activity of the biomethanation reactor.

In an embodiment of the disclosure, the absorbent comprises an amine or alkylamine, such as a tertiary amine. In a further embodiment, the absorbent is selected from the group of: diethanolamine (DEA), monoethanolamine (MEA), methyldiethanolamine (MDEA), diisopropanolamine (DIPA), and aminoethoxyethanol or diglycolamine (DGA), and any combinations thereof, and preferably is selected from the group of: DEA, MDEA, DIPA, DGA, and any combinations thereof. In a further embodiment, the absorbent is MDEA, preferably in a concentration of between 10-400 mM MDEA, more preferably between 15-200 mM or 20-140 mM, and most preferably between 25-120 mM, such as 30, 50, 70, 90, 100, 110 mM.

An example of a carbon capture reactor with an aqueous solution comprising amines are described in Example 1. Examples 3-9 further describes embodiments of the capacity and biocompatibility of aqueous amine solutions. The Examples show that specifically methanogens of the type described in Example 3 may be tolerant to the liquid carbon dioxide solvent comprising the absorbent agents as a further carbon dioxide absorbent according to the present disclosure.

The viability and particularly the productivity and selectivity of the methanogens may further be improved by the physical parameters of the liquid solvent. For example a high growth of methanogens and a high methanogen production rate may be obtained under certain temperature, pressure, pH, and chemical concentrations ranges. For example, methanogens are specifically viable and productive at a pH between 4-10 and/or temperatures between 30-75 °C. For example, a thermophilic culture of methanogens may advantageously be exposed to temperatures of 40-65 °C to enhance the methane productivity and selectivity.

To facilitate a regulated pH or temperature of the liquid solvent, the system or the liquid may comprise a pH and/or temperature control unit. For example, the pH of a liquid solvent may be controlled by a buffer, such as a chemical buffer, optionally in combination with a pH sensor control unit. Specifically, phosphate-buffered saline (PBS) solutions may be included in the liquid solvent to establish and maintain pH in the range of 4-10, which is compatible with methanogens and may facilitate the capture of CO2 by non-covalent bonding, such as bicarbonate. Similarly, the system may include a temperature controller or regulator to establish and maintain a solvent temperature of between 30-75 °C.

In an embodiment of the disclosure, the liquid carbon dioxide solvent comprises a pH controller such as a buffer configured to a pH between 4-10, more preferably between 6-9, such as 7 or 8. In a further embodiment, the liquid carbon dioxide solvent comprises a buffer, such as a phosphate-buffered saline (PBS) solution. In a further, or alternative embodiment, the system comprises a pH sensor.

In an embodiment of the disclosure, the system comprises a temperature controller for regulating the temperature of the liquid carbon dioxide solvent. In a further embodiment, the liquid carbon dioxide solvent is configured to have a temperature of between 30-75 °C, more preferably between 35-70 °C, and most preferably between 40-65 °C.

Capture agent

As further described above, the CO2 from a dilute carbon dioxide source may be captured by the liquid carbon dioxide solvent and optionally a further carbon dioxide absorbent present in the solvent, e.g. by the absorbent being dissolved or miscible in the solvent. Advantageously, the solvent and absorbents are configured to have high affinity for binding the CO2 to provide for an efficient capture of the CO2, and simultaneously the solvent and absorbents are advantageously configured to bind the CO ₂ sufficiently weak, such that the captured CO2 is bioavailable and allows for extraction and conversion by the methanogens.

The inventors have surprisingly found that a sweet spot exists for the CO ₂ bonding degree, which facilitates simultaneous efficient capture of CO2 and high methanogen CO2 extraction and conversion rate to methane.

Specifically, the liquid carbon dioxide solvent and the absorbents may bond the CO2 in such a specific adequate degree that provides efficient capturing of CO2 from dilute carbon dioxide sources and that further ensures that CO2 remains bonded during turbulent flow or flow rates related to continuous reactor operation. For example, the carbon dioxide solvents and further carbon dioxide absorbents according to the present disclosure are advantageously configured for efficiently capturing the CO2 from dilute carbon dioxide sources, such as flue gas streams or air streams with CO2 contents below 33 vol% and/or oxygen contents above 5 vol%. Specifically, the liquid carbon dioxide solvent and absorbent agents may be configured for efficiently capturing the CO2 and not capturing the oxygen efficiently. Accordingly, the carbon dioxide is efficiently captured and bonded by the liquid carbon dioxide solvent and the sorbent present within the solvent. Due to the CO2 bonding and the related chemical potential between a dilute CO2 source and the solvent and absorbents, an efficient CO2 capture and/or low oxygen capture rate or oxygen capacity, from even dilute sources may be obtained. Accordingly, an efficient carbon capture reactor may be obtained which is self-contained without auxiliary components, such as pressurizers.

It was surprisingly found that liquid carbon dioxide solvents comprising a further carbon dioxide absorbent configured for non-covalent bonding of CO2 may provide a CO2 bonding degree, which facilitates simultaneous efficient capture of CO2 and high methanogen CO2 extraction and conversion rate to methane. Specifically, liquid carbon dioxide solvents and absorbents configured to bond the captured CO2 via hydrogen bonds and/or by conversion to bicarbonate (HCOT) may facilitate both efficient CO2 capture and extraction. Accordingly, solvents and/or absorbents that are proton acceptors may particularly efficient convert dilute gaseous CO2 into aqueous bicarbonate, corresponding to the captured CO2 being bonded as bicarbonate. In an embodiment of the disclosure, the further carbon dioxide absorbent is configured for non-covalent bonding of the CO2. In a further embodiment, the further carbon dioxide absorbent is configured to bond CO2 via hydrogen bonds and/or by conversion to bicarbonate.

Examples of carbon dioxide absorbents configured for non-covalent bonding of CO2 are listed in Table 3 below. It was found that absorbents configured for bonding the captured CO2 by conversion to bicarbonate facilitate both high capture efficiency and high extraction efficiency. Examples of such absorbents include: tertiary amines, sterically hindered amines, carbonic anhydrases, hydroxide solutions such as NaOH and/or KOH solutions, and amine blends, such as blends comprising tertiary and primary or secondary amines, and diamines.

In an embodiment of the disclosure, the further carbon dioxide absorbent is selected from the group consisting of: tertiary amines, sterically hindered amines, carbonic anhydrases, hydroxide solutions such as NaOH and/or KOH solutions, and amine blends, such as blends comprising tertiary and primary or secondary amines, diamines, and combinations thereof.

Table 3. Carbon dioxide absorbents with non-covalent bonding of CO2.

It was found that the liquid non-covalent bonding carbon dioxide solvents in combination with the further absorbent agents according to the present disclosure may provide a surprisingly high bioavailability and efficient extraction and extraction rate by e.g. the methanogens of the type described in Example 3 and Example 9.

From Example 9 it follows that absorbents which are configured to bond CO2 by conversion to bicarbonate are specifically bioavailable and may further capture CO2 efficiently. A particular adequate CO2 binding capacity and bioavailability may be seen for absorbents selected from the group of: tertiary amines, sterically hindered amines, carbonic anhydrases, and amine blends, such as blends comprising tertiary and primary or secondary amines, and diamines, which facilitate bonding of the captured CO2 by conversion to bicarbonate. Depending on the dimensions of the system and the efficiency of the absorbent, above 50 vol% of the captured CO2 may be converted to bicarbonate by the selected absorbents, such as above 60, 70, 80, 90, or 100%.

Specifically it is seen that the further absorbent advantageously comprises one or more amines for efficiently capturing CO2, and present in concentrations providing bioavailability. Advantageously, the one or more amines are selected from the group of amines with structural resemblance to tertiary amines, and specifically tertiary alkanol amines, such as heterocyclic amines and sterically hindered amines. Examples of heterocyclic amines and sterically hindered amines are described in Table 3.

In an embodiment of the disclosure, the further absorbent comprises one or more amines selected from the group of: tertiary amines, tertiary alkanol amines, heterocyclic amines, sterically hindered amines, and combinations thereof.

In a further embodiment of the disclosure, the sterically hindered amines are selected from the group of: AMP, DIPA, TBA, AHPD, and combinations thereof. In a further embodiment of the disclosure, the heterocyclic amines are selected from the group of: piperidine, piperidine derivatives, methylpiperidine, piperazine, piperazine derivatives, N-Methylpiperazine, pyridine, pyridine derivatives, N,N-dimethylaminepyridine, pyrrolidine, pyroiodine derivatives, morpholine, morpholine derivatives, N-Methyl morpholine.

It is further seen that tertiary alkanol amines are particularly efficient for binding CO2 by conversion to bicarbonate, as well as providing efficient bioavailability, in the form of high methane production rate. Specifically it is seen that N-Methyl diethanolamine (MDEA) and the MDEA derivatives may be particularly efficient. Examples of MDEA derivatives are described in Table 3.

In a further embodiment of the disclosure, the one or more tertiary alkanol amines are selected from the group of: MDEA, MDEA derivatives, such as alkyl substituted diethanolamines, alcohol substituted alkyl diethanolamines, N,N-disubstituted alkanolamines, and combinations thereof.

In addition or alternatively, the carbon dioxide absorbent may be configured to bond the captured CO2 via hydrogen bonds. Examples of such absorbents include: glycols and polyethylene glycols, such as dimethylethers of polyethylene glycols (PEG). By the term “polyethylene glycols” (PEG) is meant a polyether with a structure expressed as H-(O-CH2-CH2)n-OH, and where the chain length determines the molar weight (MW). For example, the average molar weight may be 200 g/mol, and the compound referred to as PEG-200. The PEG may further facilitate high bioavailability if the concentration of the PEG within the liquid carbon dioxide solvent is between 1-180 mM, for example above 50 mM, as further described in Example 9.

In an embodiment of the disclosure, the further carbon dioxide absorbent comprises one or more dimethylethers of polyethylene glycol, such as PEG-200, PEG-300, PEG- 400, and combinations thereof, optionally in concentrations between 1-180 mM to further ensure sufficient biocompatibility.

By the term “tertiary amines” is meant an amine, where the basic nitrogen atom has three organic substituents. Examples of tertiary amines include MDEA, DMAE, DEEA, DBEA, TEA, DIPA, DMAH, 3DEA1P, as mentioned in Table 3 above. The tertiary amines facilitate efficient capture of CO2 from dilute CO2 sources by being a proton acceptor, which converts the dilute gaseous CO2 into aqueous bicarbonate, which is also referred to as the captured CO2 being bonded as bicarbonate. The tertiary amines may further facilitate high bioavailability if the concentration of the tertiary amines within the liquid carbon dioxide solvent is between 1-300 mM, for example between 20-150 mM, as further described in Example 9.

Tertiary amines differ from primary amines, where only one of the three hydrogen atoms is replaced by an alkyl. Examples of primary amines include MEA. Secondary amines are amines with two organic substituents, such as C12-DMPA. The primary amines and secondary amines may bond the captured CO2 covalently as carbamate.

By the term “carbonic anhydrases” (CAs) is meant the enzymes that catalyse the conversion of CO2 to HCOT in e.g. microbes and fungi. Examples of engineered carbonic anhydrases include: CA from Sulfurihydrogenibium yellowstonense, and CA from Desulfovibrio vulgaris.

By the term “sterically hindered amines” is meant amines, where the nitrogen atom of the amine molecule is shielded by neighboring groups, such that larger molecules cannot easily react with the nitrogen. Examples of such absorbents include AMP, DIPA, TBA, as mentioned in Table 3 above.

By the term “hydroxides” is meant hydroxide solutions, such as NaOH and/or KOH solutions.

It was surprisingly found that a liquid carbon dioxide solvent may be directly used as feed for a biomethanation reactor without prior separation and purification of the CO2, and that the methanogens of the biomethanation reactor may convert the CO2 into methane with a high efficiency and selectivity. Accordingly, the product of the biomethanation reactor may be a gas with a high concentration of methane. For example, the product advantageously has a methane concentration above 80 vol% and a purity and quality suitable for the natural gas grid. For example, methane having a quality suitable for the natural gas grid comprises maximum impurity levels, such as a maximum of 1-8 vol% CO2.

Liquid-based biomethanation reactors may facilitate a particularly efficient and selective methane product, optionally due to a particularly homogeneous distribution of the methanogens in the reactor. This may for example be obtained in trickle-bed reactors and generally stirred reactors, as well as up-flow anaerobic sludge bed reactors (LIASB), membrane reactors, and a bioelectrochemical reactor system, where the reducing equivalents is supplied from an electrode, e.g. electrons and H+ protons.

In an embodiment of the disclosure, the liquid-based biomethanation reactor is selected from the group of: trickle-bed gas phase reactors, continuous stirred-tank reactors (CSTR), up-flow anaerobic sludge bed reactors (LIASB), membrane reactors, and bioelectrochemical reactors.

Further, liquid-based biomethanation reactors may generally include methanogens, and/or may include hydrogenotrophic methanogens, which further facilitate a more efficient and selective conversion of the CO2 into methane according to Equation 1. Thus, the product of the biomethanation reactor may be a gas with a particularly high concentration of methane and low concentration of CO2.

In an embodiment of the disclosure, the liquid-based biomethanation reactor comprises hydrogenotrophic methanogens.

The H2 mediated biomethanation includes a hydrogen supply 5 as illustrated in Figure 1A-B. The hydrogen may be supplied form a renewable hydrogen source, such as an electrolyzer operated by renewable energy, such as wind generated electricity, as sketched in Figure 1A-B. An electrolyzer is an electrochemical device which uses electricity to split water and/or steam into hydrogen. Thus, an electrolyzer may function as a hydrogen supply unit or hydrogen source to the biomethanation reactor as indicated by arrow in Figure 1A-B. Advantageously, the electrolyzer is operated on renewable energy such as solar, wind, or wave energy, and the hydrogen supply unit may be referred to as a renewable hydrogen source. Thus the system according to the present disclosure may include a combination of carbon capture and utilization (CCU based on a carbon capture reactor and archaeal biomethanation) and with power-to-X (PtX).

In addition, or alternatively, the hydrogen supply may be blue hydrogen, i.e. a hydrogen source obtained from steam reforming of e.g. methane.

In an embodiment of the disclosure, the biomethanation reactor further comprises a hydrogen supply unit. In a further embodiment, the hydrogen supply unit is a renewable hydrogen source, such as an electrolyzer, and/or a blue hydrogen source, such as a steam reformer.

Optionally the methane product of the biomethanation reactor may be further cleaned, or upconcentrated, e.g. to obtain a purity and quality suitable for the natural gas grid. A methane gas from a biomethanation reactor with higher impurity levels may be used for combustion or upgraded in a methanation reactor to reach the required purity and concentrations of methane.

In an embodiment of the disclosure, the operation further comprises a step of cleaning and/or upconcentrating the methane.

The reactors of the system according to the present disclosure are configured to be in fluid communication such that a fluid may be passed from the carbon capture reactor to the biomethanation reactor, as sketched in Figure 1A-B. The liquid solvent flow defines a forward flow direction. The fluid communication may be facilitated by a piping system comprising at least one first pipe 3 extending between at least one outlet of the carbon capture reactor, and at least one inlet of the biomethanation reactor, as shown in Figure 1A. In addition or alternatively, the first pipe is part of a piping system comprising a solvent storage tank 7 as shown in Figure 1 B. This may be advantageous for systems including a renewable energy source as further described in Example 7.

In an embodiment of the disclosure, the fluid communication comprises a piping system. In a further embodiment, the piping system comprises a solvent storage tank.

Optionally the piping system may be further configured for passing a fluid reversibly between the carbon capture reactor 1 and the liquid-based biomethanation reactor 2. Accordingly, the CO2 enriched liquid carbon dioxide solvent from the carbon capture reactor may be transferred to the biomethanation reactor where the CO2 is stripped or extracted by the methanogens, and the CO2 stripped solvent e.g. a regenerated absorbant, may then be transferred back to the carbon capture reactor and recycled. It follows that the liquid solvent may be recycled for any number of cycles between the two reactors.

In an embodiment of the disclosure, the piping system is configured for passing the liquid carbon dioxide solvent in a forward flow and/or in a reverse flow between the carbon capture reactor and the biomethanation reactor.

The reversible flow between the carbon capture reactor 1 and the liquid-based biomethanation reactor 2. may be obtained by the at least one first pipe 3, as shown in Figure 1 A-B. Thus, the first pipe extends between an outlet of the carbon capture reactor, and an inlet of the biomethanation reactor in forward flow configuration, and in reverse flow configuration the inlet becomes outlet and vice versa.

To improve the system efficiency and facilitate continuous flow operation the piping system may comprise at least one first pipe 3 adapted for forward flow, and at least one second pipe 4 adapted for reverse flow, as shown in Figure 1A-B. Thus, the first pipe(s) and/or second pipe(s) are advantageously unidirectional flow pipe(s), where the first pipe connects an outlet of the carbon capture reactor and an inlet of the biomethanation reactor, and the second pipe connects an outlet of the biomethanation reactor and an inlet of the carbon capture reactor.

In an embodiment of the disclosure, the piping system comprises one or more unidirectional flow pipe(s). In a further embodiment, the piping system comprises at least one first pipe connecting an outlet of the carbon capture reactor and an inlet of the biomethanation reactor, and at least one second pipe connecting an outlet of the biomethanation reactor and an inlet of the carbon capture reactor. In a further embodiment, the first and second pipes are configured to form a flow loop for recycling the liquid carbon dioxide solvent between the carbon capture reactor and the biomethanation reactor.

The system may additionally comprise one or more pumps, flow regulators, flow direction regulators, and valves for providing a forward flow and/or reverse flow, as known to the skilled person.

The system efficiency, including the efficiency of the CO2 capture and utilization of the captured CO2, will depend on the carbon dioxide capacity of the liquid solvent, as well as the quantity of the solvent volume and the recirculation rate, as further described in Example 6 and Figure 5. Further, it follows that a higher efficiency may be obtained the smaller the system dimensions. For example, a solvent storage tank with a minor volume may imply lower costs for the system operation, and that more energy may be stored. It is seen that a high efficiency may be obtained for a biomethanation reactor volume per CO2 emitted below 3.50 m ³ mco2’ ³ d’ ¹ , Further a high efficiency may be obtained for a dilution rate between 0.05-0.35 days ^-1 .

In an embodiment of the disclosure, the volume of the biomethanation reactor is below 3.50 m ³ mco2' ³ d' ¹ , more preferable below 1.25 m ³ mco2' ³ d' ¹ , and most preferable below 1.00 m ³ m _C o2’ ³ d- ¹ , such as 0.75, 0.50, 0.25, 0.20, 0.15, or 0.10 m ³ m _C o2' ³ d' ¹ .

In an embodiment of the disclosure, the dilution rate of the system is between 0.05- 0.35 days ^-1 , more preferably between 0.10-0.30, and most preferably between 0.15- 0.25.

A further embodiment of a system operated under continuous operation is described in Example 8 and related Figures 7-9.

Examples

The invention is further described by the examples provided below. Example 1 - Carbon capture reactor with aqueous solution of tertiary amines Carbon capture reactors based on tertiary amines, such as methyl diethanolamine (MDEA), are advantageously used for the present disclosure. The tertiary amines do not form stable carbamates by the reaction with CO2, and thus CO2 absorbed into the liquid carbon dioxide absorbent will mainly be in the form of bicarbonate, as follows from the CO2 absorption (Equation 2) below. Hence, during the capture of CO2 from the flue gas in the aqueous solution by tertiary amines as MDEA, the CO2 may be converted to bicarbonate via base-catalyzed hydration.

(Eq. 2) C0 ₂ + H ₂ 0 H ⁺ + HCO3

The bicarbonate form of the CO2 in the aqueous solution makes the CO2 particularly suitable for microbial consumption.

Further advantageously, a tertiary amine (R3N) may further promote the CO2 absorption as seen in Equation 3A below. CO2 reacts reversibly with the tertiary amine to form bicarbonate in a hydrolysis reaction. For example, the tertiary amine may be MDEA as shown in Equation 3B.

(Eq. 3A) C0 ₂ + R3N + H ₂ 0 -> HCO3 + R3NH +

(Eq. 3B) C0 ₂ + MDEA + H ₂ 0 -> HCO3 + MDEAH ⁺

The CO2 absorption reaction is exothermic in nature, resulting in elevated temperatures within the absorption or carbon capture reactor. The temperature of a typical CO2 absorption reactor is between 40-65 °C.

Example 2 - Conventional carbon capture reactor with stripping

The CO2 enriched liquid absorbent of Example 1 may be regenerated into the original aqueous solution of tertiary amines and pure carbon dioxide gas, as shown in Equation 4 below.

(Eq. 4) HCO3 + MDEAH ⁺ -> C0 ₂ + MDEA + H ₂ 0 This is done by a high temperature desorption process, such as treating the CO2 enriched media with a water steam at 100-120 °C. amine solutions

The microbial biocompatibility to convert absorbed CO2 from an aqueous amine solution was quantified at 20 different MDEA concentrations in the range of 0 to 500 mM.

CO2 absorption set-up

Batch reactors constructed in borosilicate serum bottles with rubber-butyl stoppers with a total volume of 117.2 mL were filled with 44 mL of an aqueous amine solution combined with a phosphate-buffered saline (PBS) solution. The PBS solution of 8 g L' ¹ of NaCI, 0.2 g L' ¹ of KCI, 1 .44 g L' ¹ of Na ₂ HPO ₄ and 0.24 of g L' ¹ of KH ₂ PO ₄ was added to each batch reactor to provide a minor pH buffer capacity to compensate for the fast pH changes during the microbial consumption of the absorbed CO2.

Mass flow controllers, Brooks SLA 5850 series, were used to supply a gas stream of N2 and CO2 in an 80:20 ratio at a total rate of 150 mL min ^-1 . The gas stream was bubbled through the aqueous amine solutions with a cylindrical aquarium bubble stone diffuser to increase the bubble surface area and attain a rapid and adequate scrubbing of the CO2. Concurrently with the scrubbing, the pH would decrease as a result of the CO2 absorption, and its conversion into carbamate compounds. Thus, the pH was measured before and during the CO2 scrubbing with a pH meter (Mettler-Toledo, Five Easy), and the scrubbing was continued until the pH had remained stable for at least 5 minutes to confirm that the amine scrubbing liquid was fully saturated with CO2.

Biomethanation set-up

Fresh inoculum was supplied from a 1200 m ³ thermophilic manure-based anaerobic digester (Foulum, Denmark), and the batch bottles were inoculated a few hours after the extraction to maintain a high activity. The fresh inoculum was screened with a 1 mm mesh-sized sieve to remove coarse biomass residues, and hereafter transferred to the batch bottles, where it constituted 4.3% of the total liquid volume (2 mL).

It follows that the resulting methanogens present in the biomethanation reactor will be a mixed methanogen culture, since the methanogens originate from extracted fluids from a digester or biogas plant. Such mixed cultures are advantageously used due to their cost-efficiency. Mixed cultures may be cheaper and facilitate simpler reactor designs, since they are more robust towards impurities and sterile working conditions in certain steps may be avoided.

On exposure to the solvent from the carbon capture reactor, the starter culture will be selectively grown to the most robust and active microorganism culture under the exposed conditions. The cultivated microorganisms advantageously include: hydrogenotrophic methanogens, such as Methanothermobacter, and hydrolytic, acidogenic and acetogenic bacteria for improving the culture stability by driving breakdown of impurities, oxygen, and excess biomass.

The headspace of the batch reactors was flushed with bottled H2 (Air Liquide) and afterwards pressurized to an average total pressure of 1.56 ± 0.06 bar. The overall gas to liquid volume ratio thereby constituted 1.66 in the batch reactors at the beginning of the inoculation.

The batch reactors were incubated in an incubator (Memmert, Model 30 - 1060), where the temperature was maintained at thermophilic conditions of 54°C in an incubator to accommodate the thermophilic archaeal culture. A high rate of magnetic stirring of 500 RPM were applied to the batch reactors to ensure a continuous availability of H2 in the headspace to the entire culture without the poor H2 mass transfer becoming the limiting factor for the conversion. The pressure of the batch reactors was continuously logged in a customized LabView interface throughout the entire experimental period with analogue pressure sensors (MPX4250AP, CASE 867B-04, NPX) with an A/D-converter (MCP3423E/SL, Microchip).

Biocompatibility testing

The biocompatibility experiments were conducted over 4 periods with a randomized mix of different MDEA concentrations for each period, and fresh new extractions of inoculum from the anaerobic digester had been used for each period of the experiments. Additionally, a positive control with H2 and CO2 in the headspace together with a biogas production control were included for each period to verify the activity of the inoculum and to subtract the CH4 produced from the degradation of residual biomasses from the mass balances. Results

The alkaline amines have a relatively high pH of 10.63 at 20 mM and 11.08 at 500 mM, and a PBS buffer was therefore employed to maintain the isotonicity and reduce the pH to 9.62 and 10.51 respectively. The pH was likewise lower after CO2 scrubbing constituting a pH of 6.71 (wo. PBS buffer) and 6.67 (w. PBS buffer) at 20 mM MDEA and 7.57 (wo. PBS buffer) and 7.50 (w. PBS buffer) at 500 mM MDEA.

The microbial biocompatibility to different MDEA amine concentrations in the scrubber solution was evaluated in batch bottles, where CO2 had been scrubbed from pure gas streams of CO2 and N2 in a ratio of 20:80.

The results are shown in Figure 2, showing the maximum CH4 productivity rate from the pure absorbed CO2 and the H2 headspace to assess the biocompatibility for biological conversion in systems with different amine concentrations. The CH4 productivity rate is measured in NmL (normal milliliter per reactor liter) per hour.

Evaluating the biocompatibility of the microbes to convert the absorbed CO2 and injected H2 into CH4 revealed an immediate improvement of 36.81 %, when applying low concentrations of 20 mM MDEA for the scrubbing compared to not using any amine in the scrubber solution.

The CH4 productivity rate increased until it reached 63.6 ± 2.5 NmLcnu L' ¹ _C uiture h ^-1 with a MDEA concentration of 30 mM. Increasing the concentration further gradually decreased the microbial biocompatibility, until a concentration of 400 mM, where the CH4 productivity had been diminished to 2.1 ± 0.4 NmLcnu L' ¹ _C uiture h’ ¹ , and entirely inhibited at 500 mM.

During the biocompatibility experiments, excluding the distinctly inhibited samples at 300 to 500 mM, an average yield of CH4 from the injected H2 was found corresponding to 93.8% of the theoretical yield.

Common aqueous amine scrubber solutions of MDEA for CO2 capture have a high concentration of MDEA corresponding to 1 ,5M to 2.5M. It follows from Figure 2 that these MDEA concentration levels would inhibit the methanogenic activity, and it would therefore be necessary for the system to operate with lower MDEA concentrations for the absorption unit.

Example 4 - Carbon dioxide capacity of aqueous amine solutions

The bioavailable CO2 in aqueous amine solutions was evaluated. A complete mass balance of the conversion of headspace H2 and absorbed CO2 to CH4 and acetate was conducted to quantify the bioavailable CO2 in the amine solutions.

The bioavailable CO2 in the amine solutions were evaluated in a setup resembling the biocompatibility experiments. The liquid to gas volume ratio was increased by a 5.4-fold to 9.2 (Vuq /gas) to ensure excess volumes of H2 readily available for conversion. The gas to liquid ratio increment was achieved by using batch bottles (Borosilicate serum bottles with rubber-butyl stoppers) with a volume of 250 mL, while the total mixture volume of the amine and inoculum was scaled down to 22 mL.

The pressure and headspace gas concentration were evaluated in the beginning and in the end of each experimental run to determine the required CO2 consumption for the associated quantified production of CH4.

The amine scrubber solution had a pH in a range of 9.5 to 11 .7 depending on the MDEA concentrations ranging from 20 to 500 mM. After CO2 scrubbing, the pH had got reduced to 6.6 (At 20 mM MDEA) to 7.8 (At 500 mM MDEA) as a result of the CO2 conversion described in Equations 3.

After the microbial consumption of the produced carbonate compounds (Equation 4), the pH of the scrubber medium rose, but only an average of 92.76 ± 1.28 % of the pH was recovered. A full regeneration of the absorption liquid was not achievable, which indicated that not all of the absorbed CO2 was available for microbial conversion.

According to the base-catalyzed hydration of CO2 (Equations 3), the theoretical maximum capacity of CO2 absorption by the MDEA amine is limited to 1 mol CO2 per mol of MDEA. In addition, the PBS buffer would also be able to promote the absorption of additional CO2. The concentration of bioavailable CO2, which was consumed to produce CH4 was quantified in batch reactors with excessive amounts of H2, as shown in Figure 3.

Figure 3 shows the quantity of CO2 available for biological conversion at different amine concentrations.

It follows that a more efficient CO2 absorption may be obtained for MDEA concentrations between 20-140 mM MDEA.

Example 5 - Biocompatibility with scrubbed flue gas streams

The biocompatibility of converting CO2 from scrubbed flue gas was tested with the same method as for the testing the biocompatibility of converting scrubbed CO2 from pure bottled gas streams in Examples 3-4.

MDEA concentrations of 30 mM and 60 mM were applied for the flue gas scrubbing based on the results from the methane productivity in Example 3.

The flue gas was supplied from a 625 kW biogas engine and generator (Jenbacher JMS 312 GS-B.L), where a fraction of the exhaust flue gas was redirected from the chimney to the aqueous amine solutions for CO2 scrubbing.

The flue gas was analyzed for the main components of N2, CO2 and O2 with a gas chromatograph (Agilent Technologies 7890A, USA) equipped with a thermal conductivity detector and a CTR1 double column (Alltech, USA), where argon was used as carrier gas.

The aqueous amine solutions were kept at a temperature of 48°C with a water bath to reduce the solubility of O2, since the dissolved oxygen could interfere with the anaerobic environment that is required for the methanogenic archaea to sustain the activity. The dissolved oxygen concentration was measured for each flue gas batch reactor before and after operation with an optical dissolved oxygen probe (Vernier Go Direct®).

Results - Flue gas characterization Most of the heat in the flue gas produced by the biogas engine was recovered in a high temperature heat exchanger followed by a low temperature heat exchanger. The flue gas was then collected at the entrance to the chimney, just before it was exhausted. The temperature and composition of the flue gas was measured twice with an interval of 3 hours. The temperature of the flue gas constituted 44.5 ± 1 ,6°C, and the major constituents in the dry flue gas composition are shown in Table 1.

Table 1. The major constituents in the composition of the dry flue gas from the biogas engine used for biomethanation.

Component CO ₂ [%] N ₂ [%] O ₂ [%]

Concentration [Vol. %] 14.85 ± 0.15 75.22 ± 0.08 9.93 ± 0.24

Additionally, the NOx and CO emissions in the flue gas were measured periodically externally and were last reported to constitute a concentration of 141 ppm of NO _X and 491 ppm of CO. Lastly, the flue gas composed of a large fraction of water vapor.

The composition and quantity of flue gas highly depend on the specific industrial applications, where the CO2 concentration are primarily based on the type of fuel and the excess addition of atmospheric air, which determines the combustion conditions of the fuel. Examples of typical CO2 concentrations in exhaust flue gas from a selection of different application are shown in Table 2.

Table 2. Typical CO2 concentrations in flue gas from different industrial processes.

Source CO2 concentration [vol. %]

Coal-fired power plant 12.0 - 15.0

Oil-fired boilers 8.0 - 13.0

Natural gas power plant 7.4 - 8.6

Integrated gasification combined cycle (IGCC) 8.9 - 13.2

Cement production 11.5 - 33.0

Steel production (Blast furnace gas) 21.3 - 23.0

Results - Biocompatibility Figure 4 shows the maximum CH4 productivity rate from the conversion of CO2 absorbed from pure gas streams and from flue gas streams.

It was found that the MDEA solutions scrubbed with flue gas had a consistently lower CH4 productivity compared to using lab-graded gas of N2:CC>2 in a ratio of 80:20. The CH4 productivity rate from CO2 in scrubbed flue gas accounted to 83.5% of the synthetic gas stream at 30 mM MDEA and 72.6% at 60 mM MDEA.

This may be explained by the oxygen concentration. One of the largest barriers associated with the amine scrubbing and biomethanation of diluted CO2 sources as flue gas is the presence of oxygen. The oxygen-containing flue gas induces an oxidative degradation of the amine. For ordinary carbon capture systems with amine scrubbing, the extent of oxidative degradation will be greatly promoted by the increased temperatures during stripping. However, the removal of the CO2 stripping unit should therefore reduce the oxidative degradation of the amines.

Meanwhile, the presence of oxygen also limits the possibility to directly convert the CO2 into CH4 biologically as the hydrogenotrophic methanogens are strictly anaerobic. The average dissolved oxygen (DO) concentration in flue gas samples accounted for 3.01 mg L’ ¹ , while the batch bottles scrubbed with 80:20 of pure N2:CO2 had a DO concentration of 0.74 mg L’ ¹ . However, because of the mixed microbial community in the applied sludge, the dissolved oxygen level was reduced to an average of 0.22 mg L- ¹ .

The parasitic oxygen consumption is sufficient for creating an anaerobic environment promoting the methanogenic conversion of H2 and CO2 to CH4. However, the parasitic consumption of oxygen could mainly be based on hydrogen oxidation, which induces losses of energy from the reacted H2. According to the mass balances, the loss would account for 0.051 ± 0.006% of the total consumed H2 at a MDEA concentration of 60 mM. Additionally, the potential of utilizing higher MDEA concentration and the appertaining higher CO2 absorption, the lower would the fraction be between the O2 and CO2 molecules transported to the biomethanation reactor.

Example 6 - Reactor dimensioning Full capture and utilization of the captured CO2 may be obtained by considering the system integration of the PtX biomethanation in the carbon capture technology of e.g. amine scrubbing as the CO2 stripper.

The efficiency of the amine scrubbing or capturing is limited by the CO2 loading capacity in the liquid solvent, which may be regulated by the solvent concentration, e.g. the amine concentration. Increases in the solvent concentration can be accomplished within the range of biological activity, as e.g. described in Example 3.

Afterwards, the only method to ensure sufficient carbon capture is by adjusting the quantity of the solvent volume and the recirculation rate, also referred to as the dilution rate, having the unit “recirculation per day”.

The biomethanation process requires a certain hydraulic retention time of the CC>2-rich liquid in the biomethanation reactor to: (1) fit the CH4 productivity to ensure sufficient conversion of the CO2 and, (2) provide a low dilution rate to avoid washing out the microbes.

The relation between the biomethanation reactor volume and the CO2 loading capacity of the liquid solvent, the biomethanation rate and the dilution rate is shown in Figure 5. CH4 productivities up to 15.4 m ³ nr ³ d ^-1 and maximum dilution rates of 0.18 days ^-1 are known, and are used as reference for the calculations in Figure 5.

From Figure 5 it follows that the volume of the biomethanation reactor is mainly governed by the dilution rate, which is controlled by the CO2 loading capacity of the solvent. Hence, the same amount of methane may be produced in a smaller dimensioned tank if either the concentration of the amine is increased or the recirculation is increased.

Example 7 - Solvent storage tank

The increasing green transition and share of renewable, but fluctuating, energy sources such as wind turbines and photovoltaic panels is increasing, challenges the stability of the electrical grid and the security of supply. Thus advantageously, the Power-to-X technologies convert electrical energy into other energy carriers that can balance the electricity grid and support long-term storage. The energy required for the generating the reducing equivalents used for liberating and converting the absorbed CO2 should therefore be based on renewable energy sources, as e.g. shown in Figure 1A, and which further introduces the option of intermittency to the system, as shown in Figure 1 B. For example, the carbon capture unit will most likely be producing CO2 continuously, and a storage tank 7 may store the CO2 in the liquid solvent until renewable based reducing equivalents are available.

With an availability of renewable power corresponding to 12 hours daily, as in an photovoltaic solar cell park, a 300 MW natural gas-fired power plant as i.e. the Pio Pico Energy Center in California that at average emitted 146.6 tons CO2 daily in 2020 (corresponding to 78400 mco2 ³ day ^-1 ) would require a storage tank volume of 2041 m ³ if an MDEA concentration of 1M was used.

Example 8 - Continuous operation

A system according to the present disclosure was operated under continuous operation. Figure 7 shows an embodiment of the system, comprising a carbon capture reactor 1 containing a liquid carbon dioxide solvent comprising 50 mM MDEA as capture agent or sorbent agent. The carbon capture reactor is supplied or flushed with a feed gas from one or more feed tanks 6 with a carbon dioxide source as illustrated in Figure 7. Optionally, the feed gas is supplied via a gas manifold system connected to a multiple of feed tanks, such that the reactor may be supplied with different carbon dioxide sources. For example, the carbon capture reactor may be supplied with different gas sources during different phases in continuous operation. In a first start-up phase (PI) the reactor may be supplied with N2/CO, in a second phase (PH) the reactor may be supplied with synthetic N2/CO2, and in a third phase (PHI) the reactor may be supplied with flue gas.

The CO2 enriched MDEA solution from the carbon capture reactor is forwarded and fed into a liquid-based biomethanation reactor 2 via a first pipe 3. The biomethanation reactor may also be supplied with hydrogen from a hydrogen supply 5, illustrated as a hydrogen tank, which may be green hydrogen produced from surplus electricity. The system may additionally include a controller 8, such as a microcontroller for adjusting the pH of the liquid carbon dioxide solvent to be between e.g. 4-10. For example, the microcontroller may control a pump, which is activated at pH > 8.1.

Figure 8 shows the resulting gas product content as a function of the operation time and the different phases, where the symbols indicate H2 (triangles), CO2 (squares), and CH4 (circles). A similar and consistent high methane content above 80% was seen for all three carbon dioxide sources.

Figure 9 shows the resulting methane production rate (VMPR) and yield as a function of operation time and the different phases, where the symbols indicate the VMPR (diamonds) and yield (stars). A similar and consistent high methane production rate and yield was seen for all three carbon dioxide sources.

Example 9 - Carbon dioxide absorbents

It was surprisingly found that liquid non-covalent bonding carbon dioxide solvents and absorbent agents may provide the specific adequate CO2 bonding degree. Specifically, the solvents or sorbent agents may advantageously bond the CO2 via hydrogen bonds and/or by conversion to bicarbonate (HCOT).

Figure 10 shows the biomethane production rate for different carbon dioxide absorbents and absorbent concentrations. The absorbents are the primary amine MEA, the tertiary amine MDEA, the diamine C12-DMAPA, and PEG-200.

For comparison, Figure 10 also shows the biomethane production rate for a liquid carbon dioxide solvent without a further absorbent agent (i.e. 0 mM absorbtion agent concentration). The methane production rate is in this case ca. 36 NmL (normal milliliter per reactor liter) per hour. However, the solvent without a further absorbent is less efficient for capturing CO2, especially from dilute carbon dioxide sources. Thus, even if the CO2 is available to the methanogens and may be converted to methane at the rate of 36 NmLcnu L' ¹ h’ ¹ , then the absolute amount of captured and available CO2 will be low.

The addition of a further absorbent will generally increase the CO2 capture yield, however the presence of the absorbent may simultaneously inhibit the methanogens production rate, since the absorbent may be toxic to the methanogens or bond the CO2 such that it is not available for the methanogens. For example, the addition of the diamine C12-DMAPA in even small concentrations of between 40-100 mM is seen to complete inhibit the methanogen production rate. C12-DMAPA is believed to bond the captured CO2 covalently as carbamate, and the covalently bonded CO2 is not bioavailable.

However, it was surprisingly seen that the addition of 50-300 mM MDEA, or 45-180 mM PEG resulted in relatively high bioavailability and methane production. Thus, despite the presence of absorbents which may be toxic to the methanogens, the captured CO2 is seen to be available and convertible by the methanogens. Even increased methane production rates may be obtained for tertiary amines, such as MDEA in concentrations of 1-300 mM, such as 20-150 mM, e.g. 50 or 100 mM. Accordingly, both a high CO2 capture yield and efficiency, and a high CO2 extraction yield and efficiency may be obtained.

Reference numbers

1 - Carbon capture reactor

1.1 - Liquid carbon dioxide solvent

2 - Liquid-based biomethanation reactor

2.1 - Methanogens

3 - First pipe

4 - Second pipe

5 - Hydrogen supply

6 - Combustion plant or carbon dioxide source

7 - Solvent storage tank

8 - Controller

Items

The presently disclosed may be described in further detail with reference to the following items.

1. A system for converting a carbon dioxide source to methane, comprising: a carbon capture reactor comprising a liquid carbon dioxide solvent configured to be biocompatible with methanogens, and a liquid-based biomethanation reactor, wherein the reactors are configured to be in fluid communication such that the liquid carbon dioxide solvent is passed in a forward flow from the carbon capture reactor to the biomethanation reactor. The system according to item 1 , wherein the carbon capture reactor is configured for a diluted carbon dioxide source, such as a flue gas stream and/or an air stream. The system according any of the preceding items, wherein the carbon capture reactor is connected to a dilute carbon dioxide source. The system according to any of items 2-3, wherein the diluted carbon dioxide source comprises below 33 vol% CO2, more preferably between 5-25 vol%, and most preferably between 7-23 vol%, such as 15 vol%. The system according to any of items 2-4, wherein the diluted carbon dioxide source comprises above 5 vol% O2, more preferably between 7-25 vol%, such as 10 or 21 vol%. The system according to any of the preceding items, wherein the liquid carbon dioxide solvent comprises an aqueous solution. The system according to item 6, wherein the aqueous solution has a water concentration above 80 wt%, more preferably above 85 or 90 wt%, such as 94, 96, 98 or 100 wt%. The system according to any of the preceding items, wherein the liquid carbon dioxide solvent comprises a further polar solvent in a concentration below 50 vol%, and optionally wherein the polar solvent is a glycol, such as polyethylene glycol or diethylene glycol. The system according to any of the preceding items, wherein the liquid carbon dioxide solvent comprises oxygen, such as dissolved oxygen, in a concentration below 1 wt%, more preferably below 0.1 , 0.01 , 0.001 , 0.0001 wt%, and most preferably is essentially free of oxygen. 10. The system according to any of the preceding items, wherein the liquid carbon dioxide solvent comprises a sulphur compound.

11. The system according to any of the preceding items, wherein the liquid carbon dioxide solvent comprises a further carbon dioxide absorbent.

12. The system according to item 11, wherein the further carbon dioxide absorbent is configured for non-covalent bonding of the CO2.

13. The system according to any of items 11-12, wherein the further carbon dioxide absorbent is configured to bond CO2 via hydrogen bonds and/or by conversion to bicarbonate.

14. The system according to any of items 11-13, wherein the further absorbent concentration in the liquid carbon dioxide solvent is between 10-400 mM, more preferably between 15-200 mM or 20-140 mM, and most preferably between 25-120 mM, such as 30, 50, 70, 90, 100, 110 mM.

15. The system according to any of items 11-14, wherein the further absorbent is selected from the group consisting of: tertiary amines, sterically hindered amines, carbonic anhydrases, hydroxide solutions such as NaOH and/or KOH solutions, and amine blends, such as blends comprising tertiary and primary or secondary amines, diamines, and combinations thereof.

16. The system according to any of items 11-15, wherein the absorbent comprises an amine or alkylamine, such as a tertiary amine.

17. The system according to any of items 11-16, wherein the further absorbent comprises one or more amines selected from the group of: tertiary amines, tertiary alkanol amines, heterocyclic amines, sterically hindered amines, and combinations thereof.

18. The system according to item 17, wherein the one or more tertiary alkanol amines are selected from the group of: MDEA, MDEA derivatives, such as alkyl substituted diethanolamines, alcohol substituted alkyl diethanolamines, N,N- disubstituted alkanolamines, and combinations thereof. 19. The system according to item 16, wherein the absorbent is selected from the group of: diethanolamine (DEA), monoethanolamine (MEA), methyldiethanolamine (MDEA), diisopropanolamine (DIPA), and aminoethoxyethanol or diglycolamine (DGA), and any combinations thereof.

20. The system according to any of items 11-19, wherein the absorbent is MDEA, preferably in a concentration of between 10-400 mM MDEA, more preferably between 15-200 mM or 20-140 mM, and most preferably between 25-120 mM, such as 30, 50, 70, 90 mM.

21. The system according to any of the preceding items, wherein the liquid carbon dioxide solvent comprises a pH controller such as a buffer and is configured to a pH between 4-10, more preferably between 6-9, such as 7 or 8.

22. The system according to item 21 , wherein the liquid carbon dioxide solvent comprises a buffer, such as a phosphate-buffered saline (PBS) solution.

23. The system according to any of items 21-22, comprising a pH sensor.

24. The system according to any of the preceding items, further comprising a temperature controller for regulating the temperature of the liquid carbon dioxide solvent.

25. The system according to item 24, wherein the liquid carbon dioxide solvent is configured to have a temperature of between 30-75 °C, more preferably between 35-70 °C, and most preferably between 40-65 °C.

26. The system according to any of the preceding items, wherein the liquid-based biomethanation reactor is selected from the group of: trickle-bed gas phase reactors, continuous stirred-tank reactors (CSTR), up-flow anaerobic sludge bed reactors (LIASB), membrane reactors, and bioelectrochemical reactors.

27. The system according to any of the preceding items, wherein the liquid-based biomethanation reactor comprises hydrogenotrophic methanogens.

28. The system according to any of the preceding items, wherein the biomethanation reactor further comprises a source of reducing agents, such as reducing equivalents selected from the group of: hydrogen, formate, electrons, and combinations thereof. The system according to item 28, wherein the source of reducing agents is a renewable hydrogen source, such as an electrolyzer, and/or a blue hydrogen source, such as a steam reformer. The system according to any of the preceding items, wherein the fluid communication comprises a piping system, and optionally wherein the piping system comprises a solvent storage tank. The system according to item 30, wherein the piping system is configured for passing the liquid carbon dioxide solvent in a forward flow and/or in a reverse flow between the carbon capture reactor and the biomethanation reactor. The system according to any of items 30-31 , wherein the piping system comprises one or more unidirectional flow pipe(s). The system according to any of items 30-32, comprising at least one first pipe connecting an outlet of the carbon capture reactor and an inlet of the biomethanation reactor, and at least one second pipe connecting an outlet of the biomethanation reactor and an inlet of the carbon capture reactor. The system according to item 33, wherein the first and second pipes are configured to form a flow loop for recycling the liquid carbon dioxide solvent between the carbon capture reactor and the biomethanation reactor. The system according to any of the preceding items, configured to carry out the method according to any of items 36-38. A method of converting a carbon dioxide source to methane, comprising the steps of: contacting a carbon dioxide source with a liquid carbon dioxide solvent configured to be biocompatible with methanogens, and subsequently contacting the liquid carbon dioxide solvent with a biological catalyst, thereby producing a methane. The method according to item 36, further comprising a step of cleaning and/or upconcentrating the methane. The method according to any of items 36-37, configured to be carried out in the system according to any of items 1-35.