Methane production system and methane production method

Abstract

A methane production system comprising: a dissolution tank that dissolves carbon dioxide in an aqueous phase; a hydrogen ultrafine bubble introduction part that introduces hydrogen ultrafine bubbles into a line; a culture tank that is a different tank from the dissolution tank and that is for culturing methane generation bacteria in the presence of hydrogen ultrafine bubbles and dissolved carbon dioxide to thereby produce methane; and a water phase transfer means for transferring, to the culture tank, the aqueous phase having dissolved therein carbon dioxide in the dissolution tank.

Description

Methane production system and methane production method

[0001] The present disclosure relates to a methane production system and a methane production method.

[0002] Methanation is a technology that produces methane from carbon dioxide and hydrogen, and is seen as a promising technology that will contribute to achieving carbon neutrality, which means reducing greenhouse gas emissions such as carbon dioxide to zero overall. Methanation methods include catalytic and microbial methods. Methanation using microorganisms involves introducing carbon dioxide and hydrogen into a culture solution and synthesizing methane using methanogens present in the culture solution.

[0003] JP 2013-094693 A JP 2009-011999 A European Patent Application Publication No. 2675904

[0004] In methanation using microorganisms, the molar ratio of carbon dioxide to hydrogen dissolved in the culture solution ([CO ２ ]:[H ２ ]) is well-balanced and ideally 1:4. However, due to the difference in solubility that carbon dioxide is easily soluble in water and hydrogen is not easily soluble in water, even if carbon dioxide gas and hydrogen gas are supplied to the culture tank at a molar ratio of 1:4, the molar ratio of the dissolved amounts of both gases is unlikely to be 1:4. If the molar ratio of the dissolved amounts of carbon dioxide and hydrogen deviates significantly from the balance of 1:4, the methane production efficiency (conversion efficiency from carbon dioxide and hydrogen to methane) decreases, and if carbon dioxide is not consumed for methane production and becomes excess carbon dioxide gas, the excess carbon dioxide gas will be mixed into the produced methane, reducing the purity of methane. Furthermore, the purpose of methanation, which is to reduce carbon dioxide emissions, will not be achieved. On the other hand, if hydrogen is not consumed for methane production and becomes excess hydrogen gas and is mixed into the produced methane, the hydrogen gas has low combustion energy efficiency per volume, resulting in a disadvantage of reduced combustion energy efficiency per volume of methane.

[0005] Therefore, there is thought to be a demand for a methane production system that achieves the purpose of methanation by suppressing the mixing of carbon dioxide gas into the produced methane, and that makes it easy to adjust the molar ratio of dissolved carbon dioxide and hydrogen in the culture solution to a ratio optimized for methane production.

[0006] An object of the present disclosure is to provide a methane production system that suppresses the incorporation of carbon dioxide gas into the produced methane and makes it easy to adjust the molar ratio of dissolved carbon dioxide and hydrogen in the culture solution to a ratio optimized for methane production.

[0007] [1] A methane production system comprising: a dissolution tank for dissolving carbon dioxide in an aqueous phase; a hydrogen ultra-fine bubble introduction section for introducing hydrogen ultra-fine bubbles into the system; a culture tank separate from the dissolution tank for culturing methanogens in the presence of hydrogen ultra-fine bubbles and dissolved carbon dioxide to produce methane; and aqueous phase transfer means for transferring the aqueous phase in which carbon dioxide is dissolved in the dissolution tank to the culture tank. [2] The methane production system according to [1] above, wherein the hydrogen ultra-fine bubble introduction section is located in the culture tank. [3] The methane production system according to [1] above, wherein the hydrogen ultra-fine bubble introduction section is located in the dissolution tank. [4] The methane production system according to [1] above, further comprising: a hydrogen dissolution tank, which is a tank separate from the dissolution tank and the culture tank, for dissolving ultra-fine hydrogen bubbles in an aqueous phase; and hydrogen-dissolved aqueous phase transfer means for transferring the aqueous phase in which hydrogen ultra-fine bubbles are dissolved in the hydrogen dissolution tank to the culture tank, wherein the hydrogen ultra-fine bubble introducing section is located within the hydrogen dissolution tank. [5] The methane production system according to [2] above, further comprising a gas recycling flow path for introducing the gas phase in the dissolution tank into the aqueous phase in the dissolution tank. [6] The methane production system according to [3] above, further comprising a gas recycling flow path for introducing the gas phase in the dissolution tank into a flow path for introducing hydrogen gas to the hydrogen ultra-fine bubble introducing section. [7] The methane production system according to [4] above, further comprising: a first gas recycling flow path for introducing the gas phase in the dissolution tank into the aqueous phase in the dissolution tank; and a second gas recycling flow path for introducing the gas phase in the hydrogen dissolution tank into a flow path for introducing hydrogen gas to the hydrogen ultra-fine bubble introducing section.(8) The methane production system according to any one of (1) to (7) above, further comprising: a flow rate adjusting unit for adjusting the amount of the aqueous phase in which carbon dioxide is dissolved in the dissolution tank to be transferred to the culture tank; a hydrogen ultra-fine bubble quantity sensor for measuring the amount of hydrogen ultra-fine bubbles introduced into the system; and a control unit for controlling the carbon dioxide supply rate adjusting unit according to the amount of hydrogen ultra-fine bubbles measured by the hydrogen ultra-fine bubble quantity sensor so that the carbon dioxide:hydrogen molar ratio in the culture tank is within the range of 1:3 to 1:5. (9) A methane production method comprising: (a) a step of dissolving carbon dioxide in an aqueous phase in the dissolution tank; (b) a step of adding the aqueous phase in which carbon dioxide is dissolved obtained in step (a) to a culture solution containing methanogens in a culture tank that is separate from the dissolution tank; (c) a step of introducing hydrogen ultra-fine bubbles into the system; and (d) a step of culturing methanogens in a culture solution containing hydrogen ultra-fine bubbles and dissolved carbon dioxide to produce methane.

[10] The methane production method according to [9] above, wherein step (c) is carried out in the culture solution in the culture tank.

[11] The methane production method according to [9] above, wherein step (c) is carried out in the aqueous phase in the dissolution tank.

[12] The methane production method according to [9] above, wherein step (c) is carried out in the aqueous phase in a hydrogen dissolution tank that is a tank separate from the dissolution tank and the culture tank, thereby dissolving ultra-fine hydrogen bubbles in the aqueous phase in the hydrogen dissolution tank, and further comprising the step of: (c2) adding the aqueous phase in which ultra-fine hydrogen bubbles are dissolved obtained in step (c) to the culture solution.

[13] The methane production method according to

[10] above, further comprising the step of: (a2) introducing the gas phase in the dissolution tank into the aqueous phase in the dissolution tank via a gas reuse flow path.

[14] The method for producing methane according to the above

[11] , further comprising the step of (a2') introducing the gas phase in the dissolution tank into a flow path for introducing hydrogen gas into the hydrogen ultra-fine bubble introducing section via a gas recycling flow path.

[15] The methane production method according to any one of

[12] to

[15] , further comprising the steps of: (a2) introducing the gas phase in the hydrogen dissolution tank into the aqueous phase in the dissolution tank via a first gas reuse flow path; and (c3) introducing the gas phase in the hydrogen dissolution tank into a flow path for introducing hydrogen gas into the ultra-fine hydrogen bubble introducing section via a second gas reuse flow path.

[16] The methane production method according to any one of [9] to

[15] , wherein the molar ratio of carbon dioxide:hydrogen in the culture tank is controlled to be within a range of 1:3 to 1:5.

[0008] According to the present disclosure, it is possible to provide a methane production system that suppresses the incorporation of carbon dioxide gas into the produced methane and makes it easy to adjust the molar ratio of the dissolved amounts of carbon dioxide and hydrogen in the culture solution to a ratio optimized for methane production.

[0009] 1 shows the configuration of a methane production system according to a first embodiment of the present disclosure; FIG. 2 shows the configuration of a methane production system according to a second embodiment of the present disclosure; FIG. 3 shows the configuration of a methane production system according to a third embodiment of the present disclosure; FIG. 4 shows the configuration of a methane production system according to a fourth embodiment of the present disclosure; FIG. 5 shows the configuration of a methane production system according to a fifth embodiment of the present disclosure; FIG. 6 shows the configuration of a methane production system according to a sixth embodiment of the present disclosure.

[0010] The present invention will be described in detail below, with reference to the drawings as necessary. However, the drawings are merely examples for explaining the present invention, and the technical scope of the present invention is not limited by the examples shown in the drawings.

[0011] (Methane production system) The methane production system of the present disclosure comprises: a dissolution tank for dissolving carbon dioxide in an aqueous phase; an ultra-fine hydrogen bubble introducing section for introducing ultra-fine hydrogen bubbles into the system; a culture tank, which is a tank separate from the dissolution tank, for culturing methanogens in the presence of ultra-fine hydrogen bubbles and dissolved carbon dioxide to produce methane; and an aqueous phase transfer means for transferring the aqueous phase in which carbon dioxide is dissolved in the dissolution tank to the culture tank.

[0012] In the methane production system of the present disclosure, carbon dioxide is supplied to the methanogen culture solution in the form of a carbon dioxide solution rather than in the form of a gas. Supplying carbon dioxide in the form of a carbon dioxide solution makes it easier to adjust the amount of carbon dioxide supplied to the methanogen culture solution. Contrary to the present disclosure, if carbon dioxide is supplied to the methanogen culture solution in the form of a gas (bubbles), carbon dioxide that does not dissolve in the culture solution will be mixed into the recovered methane gas, which will prevent the methanation goal of reducing carbon dioxide emissions from being achieved and may also cause a decrease in the product value of the methane gas as a fuel. However, by supplying carbon dioxide in the form of a carbon dioxide solution as in the present disclosure, such disadvantages can be avoided.

[0013] Furthermore, in the methane production system disclosed herein, hydrogen is supplied to the methanogen culture solution in the form of ultrafine bubbles (bubbles with a diameter of approximately 1 μm or less). Ultrafine bubbles have high retention in the aqueous phase (long retention time, uniform dispersion), and can be treated equivalently to a stable gas-dissolved solution. Supplying hydrogen in the form of ultrafine bubbles facilitates adjustment of the amount of hydrogen supplied to the methanogen culture solution. Conversely, if hydrogen is supplied to the methanogen culture solution in the form of bubbles with a diameter larger than that of the ultrafine bubbles, hydrogen that does not dissolve in the culture solution will be mixed into the recovered methane gas. The low combustion energy efficiency per volume of hydrogen gas results in a disadvantage of reduced combustion energy efficiency per volume of recovered methane gas. Supplying hydrogen in the form of ultrafine bubbles as disclosed herein can avoid these disadvantages.

[0014] <Configuration of Methane Production System> In the methane production system of the present disclosure, the dissolution tank for dissolving carbon dioxide in the aqueous phase and the culture tank for culturing methanogens must be separate tanks. Examples of the methane production system of the present disclosure include a two-tank type and a three-tank type.

[0015] In one embodiment of the two-tank methane production system, the ultra-fine hydrogen bubble introducing section is located in the culture tank, thereby allowing the culture tank to both introduce ultra-fine hydrogen bubbles into a methanogen culture solution and cultivate the methanogens.In another embodiment of the two-tank methane production system, the ultra-fine hydrogen bubble introducing section is located in the dissolution tank, thereby allowing the dissolution tank to both dissolve carbon dioxide in the aqueous phase and introduce ultra-fine hydrogen bubbles into the aqueous phase.Each two-tank methane production system includes an aqueous phase transfer means for transferring the aqueous phase in the dissolution tank to the culture tank.

[0016] The three-tank methane production system includes a dissolution tank for dissolving carbon dioxide in an aqueous phase, a culture tank for culturing methanogens, and a hydrogen dissolution tank for dissolving ultra-fine hydrogen bubbles in the aqueous phase, each of which is a separate tank. The three-tank methane production system further includes a first aqueous phase transfer means for transferring the aqueous phase in the dissolution tank to the culture tank, and a second aqueous phase transfer means for transferring the aqueous phase in the hydrogen dissolution tank to the culture tank.

[0017] The methane production system of the present disclosure may further include a gas reuse flow path (sometimes referred to as a "first gas reuse flow path" in a three-vessel methane production system) for introducing the gas phase in the dissolution tank into a flow path for introducing gas into the dissolution tank. The three-vessel methane production system may also include a second gas reuse flow path for introducing the gas phase in the hydrogen dissolution tank into a flow path for introducing gas into the hydrogen dissolution tank. The inclusion of a gas reuse flow path in the methane production system of the present disclosure can improve the efficiency of carbon dioxide and hydrogen consumption.

[0018] <Dissolution Tank> The dissolution tank is a tank for introducing carbon dioxide into an aqueous phase to dissolve the carbon dioxide in the aqueous phase. Carbon dioxide is introduced into the dissolution tank via a flow path for introducing a gas into the dissolution tank, and is usually introduced into the aqueous phase by bubbling.

[0019] <<Carbon Dioxide>> As a carbon dioxide supply source, a gas containing a sufficient amount of carbon dioxide can be used, for example, industrial exhaust gas such as combustion exhaust gas.

[0020] <<Aqueous Phase>> The aqueous phase is not particularly limited, but examples thereof include water, physiological saline, and the like. From the viewpoints of reducing the workload and improving the lifespan of the equipment, the aqueous phase is preferably water. Furthermore, from the viewpoint of suppressing stress on the methanogens due to osmotic pressure fluctuations when the culture solution is added to the aqueous phase, the aqueous phase is preferably physiological saline.

[0021] <Hydrogen Ultrafine Bubbles> "Fine bubbles" are bubbles with a diameter of 100 μm or less. Fine bubbles are classified according to their diameter into "microbubbles" with a diameter of approximately 1 to 100 μm and "ultrafine bubbles (UFB)" (formerly known as nanobubbles) with a diameter of approximately 1 μm or less. Microbubbles rise very slowly in the aqueous phase, while ultrafine bubbles move mainly by Brownian motion in the aqueous phase and remain there for a long time. Therefore, in the present invention, hydrogen is supplied as hydrogen ultrafine bubbles. Furthermore, because ultrafine bubbles are negatively charged, the bubbles repel each other, suppressing their aggregation and merging, and maintaining the ultrafine bubble state.

[0022] The method for generating ultrafine bubbles is not particularly limited, and can be performed using a known method, for example, by bubbling in an aqueous phase using an ultrafine bubble generating nozzle.

[0023] The hydrogen source for generating ultra-fine hydrogen bubbles can be a gas containing a sufficient amount of hydrogen, such as industrial exhaust gases from aluminum production.

[0024] <Culture Tank> The culture tank is a tank for culturing methanogens in a culture solution containing ultra-fine hydrogen bubbles and dissolved carbon dioxide to produce methane.

[0025] <Methane-producing bacteria> Methane-producing bacteria include Methanobacterium alcaliphilum, Methanobacterium bryantii, Methanobacterium congolense, Methanobacterium defluvii, Methanobacterium espanolae, Methanobacterium formicicum, Methanobacterium ivanovii, Methanobacterium palustre, Methanobacterium thermaggregans, and Methanobacterium uriginosum. uliginosum), Methanobrevibacter acididurans, Methanobrevibacter arboriphilicus, Methanobrevibacter gottschalkii, Methanobrevibacter olleyae, Methanobrevibacter ruminantium, Methanobrevibacter smithii, Methanobrevibacter woesei, Methanobrevibacter wolinii, Methanothermobacter marburgensis marburgensis, Methanothermobacter thermoautotrophicusMethanothermobacter thermoautotrophicus, Methanothermobacter thermoflexus, Methanothermobacter thermophilics, Methanothermobacter wolfeii, Methanothermus sociabilis, Methanocorpusculum bavaricum, Methanocorpusculum parvum, Methanoculleus chikuoensis, Methanoculleus submarinus, Methanogenium frigidum, Methanogenium liminatans Examples of the fungal pathogen include Methanogenium marinum, Methanomicrobium mobile, Methanocaldococcus jannaschii, Methanococcus aeolicus, Methanococcus maripaludis, Methanococcus vannielii, Methanococcus voltaei, Methanothermococcus thermolithotrophicus, and the like.

[0026] <Culture Solution> A typical culture medium used for culturing methanogens can be used as the culture solution. Examples of medium components include sugars, nucleic acids, proteins, protein hydrolysates (e.g., amino acids, peptides, etc.), ocean water, lake water, marine sediments, lake sediments, salts, pH adjusters, etc. These medium components may be contained in the form of, for example, milk or meat juice, or hydrolysates thereof, yeast, or yeast extract.

[0027] <Culture Temperature> The culture temperature may be, for example, 35° C. or higher, preferably 40° C. or higher, more preferably 45° C. or higher, or may be, for example, 65° C. or lower, preferably 60° C. or lower, more preferably 55° C. or lower. However, the optimal culture temperature may vary depending on the type of methanogen.

[0028] <Aqueous Phase Transfer Means> The aqueous phase transfer means is not particularly limited, and examples thereof include an aqueous phase flow path connecting the dissolution tank and the culture tank, and a container for temporarily holding the aqueous phase to be pumped from the dissolution tank and introduced into the culture tank.

[0029] <Carbon dioxide:hydrogen molar ratio and means for adjusting it> The carbon dioxide:hydrogen molar ratio in the culture tank is preferably within the range of 1:3 to 1:5, more preferably within the range of 1:3.5 to 1:4.5, and most preferably 1:4.

[0030] In order to adjust the carbon dioxide:hydrogen molar ratio within the above range, the methane production system of the present disclosure preferably includes a means for adjusting the carbon dioxide:hydrogen molar ratio. Examples of such adjusting means include: a flow rate adjusting unit for adjusting the amount of the aqueous phase in the dissolution tank containing dissolved carbon dioxide transferred to the culture tank; a flow meter for measuring the amount of the aqueous phase in the dissolution tank transferred to the culture tank; a flow rate adjusting unit for adjusting the amount of the aqueous phase in the hydrogen dissolution tank transferred to the culture tank (if a hydrogen dissolution tank is present); a flow rate adjusting unit for measuring the amount of hydrogen gas introduced into the methane production system; a flow rate adjusting unit for adjusting the amount of hydrogen gas introduced into the methane production system; a flow rate adjusting unit for measuring the amount of the aqueous phase in the hydrogen dissolution tank transferred to the culture tank (if a hydrogen dissolution tank is present); a dissolved carbon dioxide concentration sensor for measuring the dissolved carbon dioxide concentration in the aqueous phase in the dissolution tank; a dissolved carbon dioxide concentration sensor for measuring the dissolved carbon dioxide concentration in the aqueous phase (culture solution) in the culture tank; a hydrogen ultra-fine bubble quantity sensor for measuring the concentration of ultra-fine hydrogen bubbles in the aqueous phase in the dissolution tank (if hydrogen ultra-fine bubbles are introduced into the dissolution tank); a hydrogen ultra-fine bubble quantity sensor for measuring the concentration of ultra-fine hydrogen bubbles in the aqueous phase in the hydrogen dissolution tank (if a hydrogen dissolution tank is present); and a control unit for controlling flow rate regulators and / or flow rate regulators for regulating the amount of hydrogen gas introduced into the methane production system in the flow path between the dissolution tank and the culture tank and between the hydrogen dissolution tank and the culture tank (if a hydrogen dissolution tank is present), so that the carbon dioxide:hydrogen molar ratio in the culture tank falls within a desired range, according to a combination of at least one of the measured values ​​of the dissolved carbon dioxide concentration in each tank, the hydrogen ultrafine bubble concentration in each tank, the amount of transfer through the flow path between the dissolution tank and the culture tank, and the amount of transfer through the flow path between the hydrogen dissolution tank and the culture tank (if a hydrogen dissolution tank is present).

[0031] Examples of the flow rate regulator include an on-off valve. A general flow meter can be used as the flow meter. Examples of the dissolved carbon dioxide concentration sensor include a non-dispersive infrared (NDIR) sensor and a pH meter. Examples of the ultra-fine hydrogen bubble amount sensor include a diaphragm-type polarographic electrode dissolved hydrogen meter and a pH meter.

[0032] The carbon dioxide:hydrogen molar ratio in the fermenter tank adjusted by the control unit may use the measured value of the dissolved carbon dioxide concentration in the fermenter tank as is for the amount of carbon dioxide in the fermenter tank, or an estimated value based on the measured value of the dissolved carbon dioxide concentration in the dissolution tank and the measured value of the amount of hydrogen transferred through the flow path between the dissolution tank and the fermenter tank.The carbon dioxide:hydrogen molar ratio in the fermenter tank adjusted by the control unit may use the measured value of the ultra-fine hydrogen bubble concentration in the fermenter tank as is for the amount of hydrogen in the fermenter tank, or an estimated value based on the amount of hydrogen gas introduced into the methane production system.If a hydrogen dissolution tank is present, an estimated value based on the measured value of the ultra-fine hydrogen bubble concentration in the hydrogen dissolution tank and the measured value of the amount of hydrogen transferred through the flow path between the hydrogen dissolution tank and the fermenter tank may be used.The control unit may be operated by automatic control using a program.

[0033] <Configuration Example of Methane Production System> The configuration of the methane production system (1) of the present disclosure may be, for example, the following embodiment.

[0034] In a first embodiment, an ultra-fine hydrogen bubble introduction section (302) is present in a culture tank (200) ( FIG. 1 ). In this embodiment, a methane production system (1) of the present disclosure is composed of two tanks, a dissolution tank (100) and a culture tank (200), and the dissolution tank (100) and the culture tank (200) are connected by an aqueous phase flow path (103). The dissolution tank (100) is equipped with a carbon dioxide inlet section (102) connected to a carbon dioxide inlet path (101). The culture tank (200) is equipped with an ultra-fine hydrogen bubble introduction section (302) connected to a hydrogen inlet path (301) and a gas phase outlet (201) connected to a gas phase outlet path (202).

[0035] In the second embodiment, in addition to the first embodiment, the methane production system (1) of the present disclosure further includes a gas recycling flow path for introducing the gas phase in the dissolution tank into the aqueous phase in the dissolution tank ( FIG. 2 ). The second embodiment further includes a gas phase outlet (104) of the dissolution tank (100), a confluence (106) of the carbon dioxide inlet flow path (101), and a carbon dioxide recycling flow path (105) connecting the gas phase outlet (104) and the confluence (106).

[0036] The methanation processes of the first and second embodiments will be described. First, a carbon dioxide solution is produced in the dissolution tank (100). Carbon dioxide-containing gas passes through the carbon dioxide inlet flow path (101) and is introduced into the aqueous phase (111) in the dissolution tank (100) at the carbon dioxide inlet section (102). Because carbon dioxide has high solubility in water, most of the carbon dioxide introduced into the aqueous phase (111) dissolves in the aqueous phase (111), producing a carbon dioxide solution. Excess carbon dioxide that does not dissolve in the aqueous phase (111) is released into the gas phase (112) in the dissolution tank (100). In the second embodiment, the excess carbon dioxide-containing gas phase (112) flows out from the gas phase outlet (104), passes through the carbon dioxide reuse flow path (105), and is combined with the carbon dioxide-containing gas in the carbon dioxide inlet flow path (101) at the confluence section (106) for reuse.

[0037] In the culture tank (200), methanogens are cultured in the presence of hydrogen and carbon dioxide in a culture solution (aqueous phase) (211) to produce methane. Hydrogen is less soluble in water than carbon dioxide, and dissolved hydrogen alone may not be sufficient for methane production. Therefore, hydrogen is introduced in the form of ultrafine hydrogen bubbles. When introduced in the form of ultrafine hydrogen bubbles, hydrogen can be introduced into the culture solution (aqueous phase) (211) in a large amount and with long-term retention. Hydrogen passes through a hydrogen inflow channel (301) and is introduced in the form of ultrafine hydrogen bubbles into the culture solution (aqueous phase) (211) in the culture tank (200) at a hydrogen ultrafine bubble introduction section (302). The carbon dioxide solution (aqueous phase) (111) produced in the dissolution tank (100) passes through an aqueous phase flow channel (103) and is transferred to the culture solution (aqueous phase) (211) in the culture tank (200). Methane produced by the methanogens is released into the gas phase (212) in the culture tank (200) and is discharged from the gas phase outlet (201) through the gas phase outlet channel (202).

[0038] The dissolution tank (100) and the culture tank (200) may be equipped with a dissolved carbon dioxide concentration sensor. The culture tank (200) may be equipped with a hydrogen ultrafine bubble amount sensor. The aqueous phase flow path (103) and the hydrogen inlet path (301) may be equipped with a flow rate regulator and a flow meter. The methane production system (1) may be equipped with a control unit for regulating the carbon dioxide:hydrogen molar ratio in the culture tank within a desired range (e.g., ).

[0039] As an effect of the first and second embodiments, by combining hydrogen bubbling and carbon dioxide solution introduction, it is possible to control the dissolved concentrations of both gases and supply them at an optimal ratio (e.g., 1:4) for the methane synthesis reaction. Furthermore, since the generation of hydrogen and carbon dioxide into the gas phase of the culture tank can be suppressed, pure methane gas can be obtained.

[0040] The third embodiment is the same as the first embodiment except that the ultra-fine hydrogen bubble introduction section (302) is located in the dissolution tank (100') (FIG. 3). In this embodiment, the methane production system (1) of the present disclosure is composed of two tanks: the dissolution tank (100') and the culture tank (200), and ultra-fine hydrogen bubbles are introduced into the aqueous phase in the dissolution tank (100').

[0041] In this case, carbon dioxide and hydrogen are dissolved in a single dissolution tank to a desired ratio (e.g., 1:4), and the solution is introduced into the culture tank. The carbon dioxide and hydrogen are dissolved as a gas mixture by bubbling with a single fine bubble generating nozzle. Carbon dioxide is 20 times more soluble in water than hydrogen, so for example, to dissolve carbon dioxide and hydrogen in a ratio of 1:4, it is preferable that the ratio of the gas mixture of carbon dioxide and hydrogen introduced be 1:80.

[0042] As an effect of this embodiment, it is possible to suppress the generation of hydrogen and carbon dioxide into the gas phase of the culture tank, and therefore it is possible to obtain pure methane gas.

[0043] In the fourth embodiment, the methane production system (1) of the present disclosure is the same as the third embodiment except that the carbon dioxide inlet flow path and the hydrogen inlet flow path are combined into a hydrogen / carbon dioxide inlet flow path (101'), the carbon dioxide inlet section and the ultra-fine hydrogen bubble introducing section are combined into a hydrogen / carbon dioxide fine bubble introducing section (102'), and the system further includes a gas recycling flow path (a gas phase outlet (104') of the dissolution tank (100'), a junction (106') of the hydrogen / carbon dioxide inlet flow path (101'), and a hydrogen / carbon dioxide recycling flow path (105') connecting the gas phase outlet (104') and the junction (106')) for introducing the gas phase in the dissolution tank into a flow path for introducing hydrogen gas to the ultra-fine hydrogen bubble introducing section (FIG. 4). An advantage of this embodiment is that the cost of the bubbling port can be reduced.

[0044] In a fifth embodiment, the methane production system (1) of the present disclosure further includes: a hydrogen dissolution tank (300) that is a tank separate from the dissolution tank (100) and the culture tank (200) for dissolving ultra-fine hydrogen bubbles in an aqueous phase; and a hydrogen-dissolved aqueous phase transfer means (aqueous phase flow path (303)) for transferring the aqueous phase in which ultra-fine hydrogen bubbles are dissolved in the hydrogen dissolution tank to the culture tank, wherein a hydrogen ultra-fine bubble introduction section (302) is located in the hydrogen dissolution tank (300). In this embodiment, the methane production system (1) of the present disclosure is composed of three tanks: the dissolution tank (100), the culture tank (200), and the hydrogen dissolution tank (300), and ultra-fine hydrogen bubbles are introduced into the aqueous phase (311) in the hydrogen dissolution tank (300) ( FIG. 5 ).

[0045] In the sixth embodiment, in addition to the fifth embodiment, the methane production system (1) of the present disclosure further includes: a first gas reuse flow path (a gas phase outlet (104) of the dissolution tank (100), a junction (106) of the carbon dioxide inlet flow path (101), and a carbon dioxide reuse flow path (105) connecting the gas phase outlet (104) and the junction (106)) for introducing the gas phase in the dissolution tank into the aqueous phase in the dissolution tank; and a second gas reuse flow path (a gas phase outlet (304) of the hydrogen dissolution tank (300), a junction (306) of the hydrogen inlet flow path (301), and a hydrogen reuse flow path (305) connecting the gas phase outlet (304) and the junction (306)) for introducing the gas phase in the hydrogen dissolution tank into a flow path for introducing hydrogen gas into the ultra-fine hydrogen bubble introducing section ( FIG. 6 ).

[0046] In the fifth and sixth embodiments, carbon dioxide and hydrogen are dissolved in the respective dissolution tanks, and the solution is introduced into the culture tank so that the carbon dioxide and hydrogen ratio is the desired ratio (e.g., 1:4). The concentrations of carbon dioxide and hydrogen can be controlled to be maximized in the dissolution tank (100) and the hydrogen dissolution tank (300). In the sixth embodiment, the gas phase from each of the dissolution tank (100) and the hydrogen dissolution tank (300) can be recovered and redissolved.

[0047] The effect of the sixth embodiment is that carbon dioxide and hydrogen can be recovered and reused separately.

[0048] In the seventh embodiment, hydrogen can be generated by a water electrolysis device. In this case, carbon dioxide may be exhaust gas from combustion in a boiler or the like. In the case of exhaust gas, the carbon dioxide content is about 10%, so when applied to the fourth embodiment, the mixture ratio can be exhaust gas:hydrogen = 1:8.

[0049] (Methane production method) The methane production method of the present disclosure comprises the steps of: (a) dissolving carbon dioxide in an aqueous phase in a dissolution tank; (b) adding the aqueous phase with dissolved carbon dioxide obtained in step (a) to a culture solution containing methanogens in a culture tank that is a tank separate from the dissolution tank; (c) introducing ultra-fine hydrogen bubbles into the system; and (d) culturing methanogens in the culture solution containing ultra-fine hydrogen bubbles and dissolved carbon dioxide to produce methane. The methane production method of the present disclosure can be performed using the methane production system of the present disclosure.

[0050] Step (c) may be performed in the culture solution in the culture tank (corresponding to the first embodiment described above). In this case, the methane production method of the present disclosure may further include the step (a2) of introducing the gas phase in the dissolution tank into the aqueous phase in the dissolution tank via a gas reuse flow path (corresponding to the second embodiment described above).

[0051] Step (c) may be carried out in the aqueous phase in the dissolution tank (corresponding to the third embodiment). In this case, the methane production method of the present disclosure may further include the step (a2') of introducing the gas phase in the dissolution tank into a channel for introducing hydrogen gas into the ultra-fine hydrogen bubble introducing section via a gas recycling channel (corresponding to the fourth embodiment).

[0052] Step (c) may be carried out in the aqueous phase of a hydrogen dissolution tank, which is a tank separate from the dissolution tank and the culture tank, thereby dissolving ultra-fine hydrogen bubbles in the aqueous phase of the hydrogen dissolution tank. The methane production method of the present disclosure may further include (c2) adding the aqueous phase containing dissolved hydrogen bubbles obtained in step (c) to the culture solution (corresponding to the fifth embodiment described above). In this case, the methane production method of the present disclosure may further include (a2) introducing the gas phase of the hydrogen dissolution tank into the aqueous phase of the dissolution tank via a first gas reuse flow path; and (c3) introducing the gas phase of the hydrogen dissolution tank into a flow path for introducing hydrogen gas to the ultra-fine hydrogen bubble inlet via a second gas reuse flow path (corresponding to the sixth embodiment described above).

[0053] In the methane production method of the present disclosure, the carbon dioxide:hydrogen molar ratio in the culture tank may be controlled to be within the desired range.

[0054] According to the present disclosure, it is possible to provide a methane production system that suppresses the incorporation of carbon dioxide gas into the produced methane and makes it easy to adjust the molar ratio of the dissolved amounts of carbon dioxide and hydrogen in the culture solution to a ratio optimized for methane production.

[0055] 1 Methane production system 100 Dissolution tank 101 Carbon dioxide inlet flow path 102 Carbon dioxide inlet section 103 Aqueous phase flow path 104 Gas phase outlet 105 Carbon dioxide reuse flow path 106 Confluence section 111 Aqueous phase 112 Gas phase 100' Dissolution tank 101' Hydrogen / carbon dioxide inlet flow path 102' Hydrogen / carbon dioxide fine bubble introduction section 103' Aqueous phase flow section 104' Gas phase outlet 105' Hydrogen / carbon dioxide reuse flow path 106' Confluence section 200 Culture tank 201 Gas phase outlet 202 Gas phase outlet flow path 211 Aqueous phase 212 Gas phase 300 Hydrogen dissolution tank 301 Hydrogen inlet flow path 302 Hydrogen ultra-fine bubble introduction section 303 Aqueous phase flow path 304 Gas phase outlet 305 Hydrogen reuse flow path 306 Confluence section 311 Water phase 312 Gas phase

Claims

1. A methane production system comprising: a dissolution tank for dissolving carbon dioxide in an aqueous phase; a hydrogen ultra-fine bubble introduction section for introducing ultra-fine hydrogen bubbles into the system; a culture tank, separate from the dissolution tank, for cultivating methanogens in the presence of ultra-fine hydrogen bubbles and dissolved carbon dioxide to produce methane; and an aqueous phase transfer means for transferring the aqueous phase in which carbon dioxide is dissolved from the dissolution tank to the culture tank.

2. The methane production system according to claim 1, wherein the ultra-fine hydrogen bubble introduction section is located within the culture tank.

3. The methane production system according to claim 1, wherein the ultra-fine hydrogen bubble introduction section is located within the dissolution tank.

4. The methane production system according to claim 1, further comprising: a hydrogen dissolution tank, which is a tank separate from the dissolution tank and the culture tank, for dissolving ultra-fine hydrogen bubbles in an aqueous phase; and a hydrogen-dissolved aqueous phase transfer means for transferring the aqueous phase in which ultra-fine hydrogen bubbles are dissolved in the hydrogen dissolution tank to the culture tank, wherein the hydrogen ultra-fine bubble introduction section is located within the hydrogen dissolution tank.

5. The methane production system according to claim 2, further comprising a gas recycling passage for introducing the gas phase in the dissolution tank into the aqueous phase in the dissolution tank.

6. The methane production system according to claim 3, further comprising a gas recycling flow path for introducing the gas phase in the dissolution tank into a flow path for introducing hydrogen gas into the hydrogen ultra-fine bubble introducing section.

7. The methane production system according to claim 4, further comprising: a first gas recycling passage for introducing the gas phase in the hydrogen dissolution tank into the aqueous phase in the dissolution tank; and a second gas recycling passage for introducing the gas phase in the hydrogen dissolution tank into a passage for introducing hydrogen gas into the ultra-fine hydrogen bubble introducing section.

8. The methane production system according to any one of claims 1 to 7, further comprising: a flow rate adjusting unit for adjusting the amount of the aqueous phase in which carbon dioxide is dissolved in the dissolution tank that is transferred to the culture tank; a hydrogen ultra-fine bubble amount sensor for measuring the amount of hydrogen ultra-fine bubbles introduced into the system; and a control unit for controlling the carbon dioxide supply amount adjusting unit according to the amount of hydrogen ultra-fine bubbles measured by the hydrogen ultra-fine bubble amount sensor so that the carbon dioxide:hydrogen molar ratio in the culture tank falls within a range of 1:3 to 1:

5.

9. A method for producing methane, comprising the steps of: (a) dissolving carbon dioxide in an aqueous phase in a dissolution tank; (b) adding the aqueous phase with dissolved carbon dioxide obtained in step (a) to a culture solution containing methanogens in a culture tank that is a tank separate from the dissolution tank; (c) introducing ultra-fine hydrogen bubbles into the system; and (d) culturing methanogens in a culture solution containing ultra-fine hydrogen bubbles and dissolved carbon dioxide to produce methane.

10. The method for producing methane according to claim 9, wherein step (c) is carried out in the culture medium in the culture tank.

11. The method for producing methane according to claim 9, wherein step (c) is carried out in the aqueous phase in the dissolver.

12. The methane production method according to claim 9, wherein step (c) is carried out in an aqueous phase in a hydrogen dissolution tank that is a tank separate from the dissolution tank and the culture tank, thereby dissolving ultra-fine hydrogen bubbles in the aqueous phase in the hydrogen dissolution tank, and the method further comprises the step of (c2) adding the aqueous phase in which the ultra-fine hydrogen bubbles obtained in step (c) have been dissolved to the culture solution.

13. The methane production method according to claim 10, further comprising the step of (a2) introducing the gas phase in the dissolution tank into the aqueous phase in the dissolution tank via a gas recycling flow path.

14. A methane production method according to claim 11, further comprising the step of (a2') introducing the gas phase in the dissolution tank via a gas recycling flow path into a flow path for introducing hydrogen gas into the hydrogen ultra-fine bubble introduction section.

15. The methane production method according to claim 12, further comprising: (a2) introducing the gas phase in the dissolution tank into the aqueous phase in the dissolution tank via a first gas recycling flow path; and (c3) introducing the gas phase in the hydrogen dissolution tank into a flow path for introducing hydrogen gas into the ultra-fine hydrogen bubble introducing section via a second gas recycling flow path.

16. The method for producing methane according to any one of claims 9 to 15, wherein the molar ratio of carbon dioxide to hydrogen in the culture tank is controlled to be within the range of 1:3 to 1:5.

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