Field of the invention

The present invention relates to the field of green technologies, to carbon capture and to the energy field. In particular the present invention relates to the bioindustrial manufacture of methane from carbon dioxide and hydrogen.

Background of the invention

Methane-producing microorganisms are thought to be among the earliest cellular life forms colonizing our planet, and are major contributors to the past and present global carbon cycle. Currently, all methanogens belong to the archaeal domain of life, and they utilize a variety of methanogenic metabolisms among a wide distribution of archaeal phyla.

The most commen metabolic pathways for the bacterial synthesis of methane are the hydrogenotropic, the acetoclastic and the methylotropic pathways. Hydrogenotrophic methanogens are archaea that can grow on hydrogen and carbon dioxide with the production of methane, an important intermediate in the global carbon cycle. They reduce carbon dioxide to methane and water by using hydrogen as an electron donor. The acetoclastic methanogens use acetate as an electron acceptor while the methylotropic methanogens use methanol or methylamines as an electron acceptor.

For environmental reasons plants for anaerobic digestion of various organic wastes have been implemented in many places, e.g. for treating municipal waste and organic waste from animal production etc. Such anaerobic digesters or bioreactors produce biogas and an organic residue. While the organic residue is often used as fertilizer, the biogas has a methane contents of about 60% while the remaining gas is carbon dioxide. Hence the biogas can be utilized for heating or combustion purposes due to the methane contents, methane being the hydrocarbon having the highest energy contents. However, if the biogas is not utilized in close vicinity of the anaerobic digester where it is generated, there arises a need for transporting the gas. For both the utilization of the biogas for combustion purposes and for the transportation of the biogas the high contents of carbon dioxide is a challenge. Additionally, if it is intended to liquefy the methane in the biogas the carbon dioxide must be removed since it will otherwise solidify when the biogas is cooled for liquefying the methane.

Hence, there is a need for removal of the carbon dioxide in biogas. Also, for making the process cost effective there is a need for removing the carbon dioxide in the biogas by a process converting the carbon dioxide into methane.

Also, there is a need for industrial processes to be run in a way that is more compatible with circular principles, e.g. by not releasing carbon dioxide into the atmosphere.

Summary of the invention

It has surprisingly been found that despite the very poor solubility of hydrogen in water an effective conversion of hydrogen and carbon dioxide to methane can be achieved in a pressurized bioreactor using microorganisms as catalysts.

In a first aspect the present invention provides a process for reacting hydrogen with carbon dioxide to form methane in an aqueous medium catalysed by microorganisms, where said reaction takes place inside a bioreactor, characterised in that the solubility of hydrogen in the aqueous medium is enhanced by pressure combined with a flow induced residence time enabling the dissolution and reaction of at least 80% of the gaseous hydrogen mixed with the aqueous medium.

In an embodiment the bioreactor is a U-shaped or coaxial vertical bioreactor. In another embodiment the microorganisms are archaea, i.e. hydrogenotrophic methanogens.

In a second aspect the present invention provides a bioreactor fitted for implementing the process as defined in the first aspect of the invention.

In one embodiment the bioreactor is a U-shaped or coaxial vertical bioreactor.

Brief description of the drawings

Figure 1 . Coaxial vertical bioreactor for reacting hydrogen with carbon dioxide to form methane using microorganisms as catalysts.

Figure 2. Continuously stirred tank reactor with recirculation loop for reacting hydrogen with carbon dioxide to form methane using microorganisms as catalysts.

Description of the invention

In the first aspect of the present invention is provided a process for reacting hydrogen with carbon dioxide to form methane in an aqueous medium catalysed by microorganisms, where said reaction takes place inside a bioreactor, characterised in that the solubility of hydrogen in the aqueous medium is enhanced by pressure combined with a flow induced residence time enabling the dissolution and reaction of at least 80% of the gaseous hydrogen mixed with the aqueous medium.

Microorganisms and feed

Microorganisms used for methane synthesis according to the present invention are so- called hydrogenotropic methanogens. Hydrogenotropic methanogens derive metabolic energy from the reaction : Methanogens presently all belong to the archae bacteria and they are found ubiquitu- ous in nature and off course in anaerobic digesters where they often establish as mixed cultures in more or less equilibrium with the feed streams of the digester and its operating mode.

Non-limiting examples of hydrogenotropic methanogens are Methanopyrus kandleri, Mathanococcus maripaludis, Methanobacterium thermoautotrophium, Methanosarcina mazei, Methanospirillum hungatei, and Methanocella paludicola.

In an embodiment the process of the present operation is conducted using specific methanogenic bacteria as inoculum.

In another embodiment the bioreactor is inoculated with methanogenic bacteria obtained from an anaerobic digester, such as the anaerobic digester producing a biogas used as a source of carbon dioxide.

In an embodiment the process of the present invention is a continuous process where a microbial feed continuously is fed to the bioreactor, e.g. from an anerobic digester such as a municipal waste water treatment facility or an animal effluent processing facility. The fluid exit stream from the bioreactor may be discarded or it may be wholly or partly recycled to the anaerobic digester wherefrom it came in the first instance.

The carbon dioxide can be supplied as pure carbon dioxide or as part of the exit gas from an industrial process. Likewise, the hydrogen gas may be supplied as pure gas from e.g. electrolysis, or supplied from the exit gas of an industrial process.

In a preferred embodiment the carbon dioxide is fed to the bioreactor as part of a gas which is the biogas from an anaerobic digestion, e.g. having about 40% of carbon dioxide and 60% of methane, while the hydrogen gas is fed as a pure gas, e.g. 95-100% hydrogen. The hydrogen gas may be prepared on-site by e.g. electrolysis.

In a preferred embodiment the feed of hydrogen gas is regulated so as to match on a molar basis the feed of carbon dioxide in the biogas from an anerobic digestion. Typically, this feed ratio is about 2.5 volumes of biogas per 1.0 volume of hydrogen gas.

It is to be understood that the carbon dioxide and hydrogen fed to the bioreactor when supplied from two different gasses may either be mixed prior to being fed to the reactor or may be fed as separate gasses that are mixed inside the bioreactor. Bioreactors

The bioreactor used for the process converting carbon dioxide and hydrogen into methane may be a U-shaped or coaxial bioreactor or it may be a pressurized continuous stirred tank reactor.

The U-shaped or coaxial vertical bioreactor is a continuous bioreactor having the inlets for liquid substrate, microorganisms and gaseous substrates (carbon dioxide and hydrogen) in the first end of the bioreactor and the exit for spent liquid and reacted gasses at the second end of the bioreactor.

In one embodiment the U-shaped or coaxial vertical bioreactor has partial recycling of the exit gas from the bioreactor. In one embodiment the amount of the exit gas being recycled may be about 25%, about 35%, about 50% or about 75%. In another embodiment the amount of the exit gas being recycled to the bioreactor is from about 25% to 75%, from about 30% to 60%, from about 40% to about 50%.

In another embodiment the U-shaped or coaxial vertical bioreactor has partial recycling of the exit gas from the bioreactor, where the amount of gas being recycled is controlled in response to a measurement or estimation of the amount of carbon dioxide in the exit gas.

In another embodiment the U-shaped or coaxial vertical bioreactor has partial recycling of the exit liquid. This may serve to stabilize the microorganisms in the continuous bioreactor and make the microorganisms more robust.

The height of the U-shaped or coaxial vertical bioreactor sets a minimum pressure that will be present at the bottom of the bioreactor where the part having downwards flow turns to the part having upward flow. For every 10 meters of bioreactor level (liquid height) the pressure is additionally approx. 1 bar as compared to the top of the bioreactor. The U-shaped or coaxial bioreactor may also be pressurized at the inlet at the top, but if the reactor has a sufficient height it can have a sufficiently high hydrostatic pressure in the bottom part to quantitative drive the dissolution of gaseous hydrogen to dissolved hydrogen. The solubility of hydrogen in water is much lower than the solubility of other gasses, such as oxygen, carbon dioxide (see Table 1). For instance, the solubility of hydrogen at 293 K is only 0.16 mg per 100 g of water, which amounts to 7% of the solubility of methane, and to 0.1% of the solubility of carbon dioxide.

Table 1. Solubility of gasses in water at 293 K (from Kaye and Laby, “Tables of Physical and Chemical Constants” 15 ^th ed, Longman NY, 1986, p 219).

Additionally, the solubility of gasses in water decreases with the temperature and since most hydrogenotropic methanogens are thermophiles, the solubility of hydrogen is very low and consequently the mass transfer of gaseous hydrogen to soluble hydrogen becomes the rate limiting step.

It is to be understood that the U-shaped or coaxial vertical bioreactor may have any suitable shape as long as the unidirectional flow of the liquid and the gaseous phase are maintained. In one embodiment the U-shaped or coaxial vertical bioreactor has a coaxial design, i.e. one vertical tubular structure placed inside a larger vertical structure having fluid and gaseous connection only in the bottom of the bioreactor.

The dimensions of the two parts of the U-shaped or coaxial vertical bioreactor must be appropriate in relation to the flow of aqueous and gaseous phases respectively. Determination of these dimension may be done by the governing fluid engineering known in the art. Hence, in one embodiment the U-shaped or coaxial vertical bioreactor has the dimension of a pipe starting from the inlet end at the top, going vertically down to the buttom and turning up towards the exit also at the top, i.e. a U-shape. In a further embodiment the cross sectional area of the downward part of the bioreactor is the same as the cross sectional area of the upward part of the bioreactor. In another embodiment the cross sectional area of the downward part of the bioreactor is smaller than the cross sectional area of the upward part of the bioreactor.

In another embodiment the U-shaped or coaxial vertical bioreactor has a coaxial design, i.e. one vertical tubular structure placed inside a larger vertical structure having fluid and gaseous connection only in the bottom of the bioreactor. In one embodiment the cross sectional area of the downward part of the bioreactor is the same as the cross sectional area of the upward part of the bioreactor. In another embodiment the cross sectional area of the downward part of the bioreactor is smaller than the cross sectional area of the upward part of the bioreactor. The latter configuration may be favourable when the bioreactor configuration and the liquid and gaseous flow rates are matched to create a substantial unidirectional plug-flow.

The bioreactor for implementing the process for reacting hydrogen with carbon dioxide to form methane may also be a continuous stirred tank reactor (CSTR) or a counterflow bioreactor which is pressurized.

In one embodiment the counterflow bioreactor or CSTR is operated at a pressure of at least 2 bar, at least 3 bar, at least 6 bar or at least 10 bar.

In one embodiment the bioreactor is a counterflow bioreactor wherein the gaseous phase moves upwards and the aqueous phase moves downwards.

In another embodiment the bioreactor is a CSTR. In another embodiment, the CSTR reactor has a recirculation loop which recycles a part of the exit gas to the bioreactor. In one embodiment the amount of the exit gas being recycled to the bioreactor is from about 25% to 75%, from about 30% to 60%, from about 40% to about 50%. In a further embodiment the amount of the exit gas being recycled is about 25%, about 35%, about 50% or about 75%.

The following items further describe the present invention: 1. A process for reacting hydrogen with carbon dioxide to form methane in an aqueous medium catalysed by microorganisms, where said reaction takes place inside a bioreactor, characterised in that the solubility of hydrogen in the aqueous medium is enhanced by pressure combined with a flow induced residence time enabling the dissolution and reaction of at least 80% of the gaseous hydrogen mixed with the aqueous medium.

2. The process according to item 1, wherein said microorganisms are archaea bacteria.

3. The process according to any of items 1-2, wherein said microorganisms are hydrogenotrophic methanogens.

4. The process according to any of the preceding items, wherein said pressure in at least part of the bioreactor is at least 3 bar, at least 6 bar or at least 10 bar.

5. The process according to any of the preceding items, wherein said reaction takes place at a temperature in the range from about 30 °C to about 70 °C, from about 45 °C to about 70 °C, or from about 50 °C to about 70 °C.

6. The process according to any of the preceding items, wherein the mean residence time of the liquid phase in the bioreactor is at least 10 seconds, at least 30 seconds, at least 1 minute, at least 5 minutes, at least 20 minutes, or at least 60 minutes.

7. The process according to any of the preceding items, wherein said reactor is fed liquid medium comprising microorganisms from a biogas manufacturing facility or a waste treatment facility, and optionally wherein the liquid medium exiting said reactor is returned to said biogas manufacturing facility or waste treatment facility.

8. The process according to any of the preceding items, wherein part of the exit gas is recycled into said bioreactor, such as from about 25% to 75%, from about 30% to 60%, or from about 40% to about 50% of the exit gas being recycled into said bioreactor. The process according to any of the preceding items, wherein at least 85%, at least 90% or at least 95% of the hydrogen in the inlet gas to said bioreactor is transformed into methane. The process according to any of items 1-7 and 9, wherein no exit gas is recycled into said bioreactor. The process according to any of the preceding items, wherein said reactor is selected from a U-shaped or a coaxial vertical reactor having a height of at least 10 meters, at least 50 meter or at least 80 meters. The process according to any of items 1-9, wherein said reactor is a counterflow bioreactor or a continuously stirred tank reactor, the latter optionally having a recirculating loop for recirculating at least part of the gaseous phase. The process according to any of the preceding items, wherein said microorganisms comprise a microorganism selected from Methanopyrus kandleri, Mathanococcus maripaludis, Methanobacterium thermoautotrophium, Methano- sarcina mazei, Methanospirillum hungatei, and Methanocella paludicola. A bioreactor fitted for implementing the process as defined in any of items 1-13. The bioreactor according to item 14, wherein the inlet of aqueous medium comprising microorganisms and the inlet for hydrogen and carbon dioxide are both at the upper part of the bioreactor, whereby a downward flow mixes and transports the fluid and gas phases down past the buttom of the bioreactor and subsequently upward towards the outlet end of said bioreactor which is also at the upper part of the bioreactor. The bioreactor according to item 14, wherein said reactor is a continuously stirred tank reactor having a recirculating loop for recirculating at least part of the gaseous phase. Examples

Example 1 - U-shaped vertical reactor

A U-shaped vertical bioreactor in the form of a tubular reactor having an inlet for liquid digestate obtained from a waste treatment facility containing hydrogenotropic methanogens, c.f. Figure 1 . Also at the inlet end of the bioreactor two separate gasses are fed into the digestate, the first gas supplying the hydrogen and the second gas supplying the carbon dioxide.

The first part of the tubular bioreactor is a vertical part going into the ground and after the turn at the buttom where the hydrostatic pressure is at maximum, another vertical tubular part of the bioreactor goes back above ground level where the liquid phase and the methane rich gas phase are separated.

Example 2 - pressurized counterflow bioreactor

A counterflow bioreactor produces methane from a feed gas being made by mixing hydrogen with biogas from a waste treatment facility (Figure 2). The mixed inlet gas containing hydrogen and carbon dioxide is blown into a sparger ring at the buttom of the bioreactor and moving upwards through the liquid phase. The liquid phase contains hydrogenotropic methanogens which catalyse the reaction forming methane. The liquid phase moves downward against the flow of the gas phase, and at the bottom of the bioreactor the liquid phase is recirculated back to the top of the bioreactor. The counterflow bioreactor is pressurized to enhance the rate of dissolution of the hydrogen gas into the aqueous phase. The exit gas from the top of the bioreactor is a methane enriched biogas having a much lower carbon dioxide contents relative to the 40% CO2 in the biogas fed to the bioreactor. Also, the exit gas contains traces of remnant hydrogen.