Methane production system and methane production method

Abstract

To provide a methane production system that suppresses the intrusion of carbon dioxide gas into the produced methane and makes it easy to adjust a molar ratio of dissolved carbon dioxide and hydrogen in a culture solution to a ratio optimized for methane production.SOLUTION: A methane production system 1 includes: a dissolution tank 100 for dissolving carbon dioxide in an aqueous phase; a hydrogen ultra-fine bubble introduction unit 302 for introducing hydrogen ultra-fine bubbles into the system; a culture tank 200, separate from the dissolution tank, for cultivating methanogens in the presence of ultra-fine hydrogen bubbles and dissolved carbon dioxide to produce methane; and aqueous phase transfer means 103 for transferring the aqueous phase containing dissolved carbon dioxide from the dissolution tank to the culture tank.SELECTED DRAWING: Figure 1

Description

[Technical Field]

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

[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. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2013-094693 [Patent Document 2] Japanese Patent Application Laid-Open No. 2009-011999 [Patent Document 3] European Patent Application Publication No. 2675904 Summary of the Invention [Problem to be solved by the invention]

[0004] In methanation using microorganisms, a molar ratio of carbon dioxide to hydrogen dissolved in the culture solution ([CO2]:[H2]) of 1:4 is ideal for optimizing methane production. However, due to the difference in solubility between carbon dioxide and hydrogen, i.e., high solubility in water and low solubility in water, even if carbon dioxide and hydrogen gas are supplied to the culture tank at a molar ratio of 1:4, the molar ratio of the dissolved amounts of the two gases is unlikely to be 1:4. If the molar ratio of the dissolved amounts of carbon dioxide and hydrogen deviates significantly from the 1:4 balance, the methane production efficiency (the conversion efficiency from carbon dioxide and hydrogen to methane) decreases. If carbon dioxide is not consumed for methane production and becomes excess carbon dioxide gas, the excess carbon dioxide gas will be mixed with the produced methane, reducing the purity of the 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 mixes with 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. [Means for solving the problem]

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

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

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

[12] Step (c) is carried out in the aqueous phase in 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 in the hydrogen dissolution tank; (c2) The method further comprises a step of adding the aqueous phase containing dissolved ultra-fine hydrogen bubbles obtained in step (c) to the culture solution. The methane production method according to [9] above.

[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 recycling 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] (a2) introducing the gas phase in the dissolution tank into the aqueous phase in the dissolution tank via a first gas reuse flow path; and (c3) A step of 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. The methane production method according to

[12] above, further comprising:

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

[15] above, 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. [Effects of the Invention]

[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. [Brief explanation of the drawings]

[0009] [Figure 1] 1 shows the configuration of a methane production system according to a first embodiment of the present disclosure. [Figure 2] 1 shows the configuration of a methane production system according to a second embodiment of the present disclosure. [Figure 3] 10 shows the configuration of a methane production system according to a third embodiment of the present disclosure. [Figure 4] 10 shows the configuration of a methane production system according to a fourth embodiment of the present disclosure. [Figure 5] 10 shows the configuration of a methane production system according to a fifth embodiment of the present disclosure. [Figure 6] 10 shows the configuration of a methane production system according to a sixth embodiment of the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[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 dissolver for dissolving carbon dioxide into the aqueous phase; A hydrogen ultrafine bubble introduction section for introducing ultrafine hydrogen bubbles into the system; a culture 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 Aqueous phase transfer means for transferring the aqueous phase in which carbon dioxide is dissolved in the dissolution tank to the culture tank. Includes:

[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 not achieve the purpose of methanation, which is to reduce carbon dioxide emissions, and may also cause disadvantages such as reducing 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] In the methane production system of the present disclosure, 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 makes it easy to adjust 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 cultivating methanogens, and a hydrogen dissolution tank for dissolving ultra-fine hydrogen bubbles in an 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 gas into the dissolution tank, and is usually introduced into the aqueous phase by bubbling.

[0019] <<Carbon dioxide>> The carbon dioxide source may be a gas containing a sufficient amount of carbon dioxide, such as an industrial exhaust such as a combustion exhaust.

[0020] <<Aqueous phase>> The aqueous phase is not particularly limited, but examples thereof include water, physiological saline, etc. From the viewpoints of reducing the workload and extending the life 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] <Ultra-fine hydrogen bubbles> "Fine bubbles" are bubbles with a diameter of 100 μm or less. Fine bubbles are classified according to their diameter into "microbubbles" with diameters of approximately 1 to 100 μm and "ultrafine bubbles (UFB)" (formerly known as nanobubbles) with diameters 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 this invention, hydrogen is supplied as ultrafine hydrogen bubbles. Furthermore, because ultrafine bubbles are negatively charged, they repel each other, preventing aggregation and merging of the bubbles, 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> Methanogens include Methanobacterium alcaliphilum, Methanobacterium bryantii, Methanobacterium congolense, Methanobacterium defluvii, Methanobacterium espanolae, Methanobacterium formicicum, Methanobacterium ivanovii, Methanobacterium palustre, Methanobacterium thermaggregans, and Methanobacterium uliginosum. uliginosum, Methanobrevibacter acididurans, Methanobrevibacter arboriphilicus, Methanobrevibacter gottschalkii, Methanobrevibacter olleyae, Methanobrevibacter ruminantium, Methanobrevibacter smithii, Methanobrevibacter woesei, Methanobrevibacter wolinii, Methanothermobacter marburgensis marburgensis, Methanothermobacter thermoautotrophicusMethanothermobacter thermoautotrophicus, Methanobacterium thermoautotrophicus, Methanothermobacter thermoflexus, Methanothermobacter thermophilics, Methanothermobacter wolfeii, Methanothermus sociabilis, Methanocorpusculum bavaricum, Methanocorpusculum parvum, Methanoculleus chikuoensis, Methanoculleus submarinus, Methanogenium frigidum Examples of such bacteria include Methanogenium frigidum, Methanogenium liminatans, Methanogenium marinum, Methanomicrobium mobile, Methanocaldococcus jannaschii, Methanococcus aeolicus, Methanococcus maripaludis, Methanococcus vannielii, Methanococcus voltaei, and Methanothermococcus thermolithotrophicus.

[0026] <Culture solution> The culture medium may be a conventional medium used for culturing methanogens. 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, but 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 molar ratio of carbon dioxide to hydrogen 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 an adjustment means include: a flow rate adjusting unit for adjusting the amount of the aqueous phase containing dissolved carbon dioxide in the dissolution tank transferred to the culture tank; a flow meter for measuring the amount of aqueous phase transferred from the dissolution tank to the culture tank; a flow rate adjusting unit for adjusting the amount of aqueous phase in the hydrogen dissolution tank transferred to the culture tank (if a hydrogen dissolution tank is present); a flow meter 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 meter to measure the amount of aqueous phase transferred from the hydrogen dissolver to the culture tank (if a hydrogen dissolver 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 to measure the concentration of hydrogen ultra-fine bubbles in the aqueous phase of the dissolution tank (if hydrogen ultra-fine bubbles are introduced into the dissolution tank); A hydrogen ultra-fine bubble quantity sensor to measure the concentration of ultra-fine hydrogen bubbles in the aqueous phase of the hydrogen dissolution tank (if a hydrogen dissolution tank is present); A hydrogen ultra-fine bubble quantity sensor for measuring the amount of hydrogen ultra-fine bubbles in the aqueous phase (culture solution) in a culture tank; A control unit for controlling the flow rate regulators in the dissolution tank-culture tank flow path and between the hydrogen dissolution tank and culture tank (if a hydrogen dissolution tank is present) and / or the flow rate regulators for regulating the amount of hydrogen gas introduced into the methane production system, so that the carbon dioxide:hydrogen molar ratio in the culture tank is 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 ultra-fine hydrogen bubble concentration in each tank, the amount of transfer through the flow path between the dissolution tank and culture tank, and the amount of transfer through the flow path between the hydrogen dissolution tank and culture tank (if a hydrogen dissolution tank is present). The above means may be a combination of at least one of the above.

[0031] The flow rate regulator may be, for example, an on-off valve. A general flow meter may be used as the flow meter. A non-dispersive infrared (NDIR) sensor, a pH meter, or the like may be used as the dissolved carbon dioxide concentration sensor. A diaphragm-type polarographic electrode-type dissolved hydrogen meter, a pH meter, or the like may be used as the ultra-fine hydrogen bubble amount sensor.

[0032] The carbon dioxide:hydrogen molar ratio in the fermenter tank adjusted by the control unit may use the measured dissolved carbon dioxide concentration in the fermenter tank as the amount of carbon dioxide in the fermenter tank, or an estimated value based on the measured dissolved carbon dioxide concentration in the dissolution tank and the measured 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 ultra-fine hydrogen bubble concentration in the fermenter tank as 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 ultra-fine hydrogen bubble concentration in the hydrogen dissolution tank and the measured 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 automatically by program control.

[0033] <Example of methane production system configuration> The methane production system (1) of the present disclosure may be configured, for example, in the following embodiments.

[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) has a carbon dioxide inlet section (102) connected to a carbon dioxide inlet path (101). The culture tank (200) has 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 (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 junction (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 large quantities and with long-term retention. Hydrogen passes through the 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 the ultrafine hydrogen bubble introduction section (302). The carbon dioxide solution (aqueous phase) (111) produced in the dissolution tank (100) passes through the 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 ultra-fine bubble amount sensor. The aqueous phase flow channel (103) and the hydrogen inlet channel (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. In addition, 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 1:4 ratio, the ratio of the carbon dioxide and hydrogen mixture introduced is preferably 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 includes: A hydrogen dissolution tank (300) 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 channel (303)) for transferring the aqueous phase in which ultra-fine hydrogen bubbles are dissolved in the hydrogen dissolution tank to the culture tank. further comprising An ultrafine hydrogen bubble introduction section (302) is present in the hydrogen dissolution tank (300). In this embodiment, the methane production system (1) of the present disclosure is composed of three tanks: a dissolution tank (100), a culture tank (200), and a hydrogen dissolution tank (300), and ultrafine 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 has the following features: a first gas recycling channel (a gas phase outlet (104) of the dissolution tank (100), a junction (106) of the carbon dioxide inlet channel (101), and a carbon dioxide recycling channel (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 introduction part. (Figure 6).

[0046] In the fifth and sixth embodiments, carbon dioxide and hydrogen are dissolved in 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 includes: (a) dissolving carbon dioxide in an aqueous phase in a dissolver; (b) adding the aqueous phase having 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 ultrafine hydrogen bubbles into the system; (d) A step of culturing methanogens in a culture solution containing ultra-fine hydrogen bubbles and dissolved carbon dioxide to produce methane. The methane production method of the present disclosure can be carried out using the methane production system of the present disclosure.

[0050] Step (c) may be performed in the culture medium 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 recycling passage (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 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 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. [Industrial Applicability]

[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. [Explanation of symbols]

[0055] 1. Methane production system 100 Dissolution tank 101 Carbon dioxide inlet channel 102 Carbon dioxide inlet 103 Water phase flow channel 104 Gas phase outlet 105 Carbon dioxide reuse flow path 106 Junction 111 Water phase 112 Gas Phase 100' dissolution tank 101' Hydrogen / Carbon Dioxide Inlet Channel 102' Hydrogen / carbon dioxide fine bubble introduction section 103' Water phase flow section 104' Gas phase outlet 105' Hydrogen / Carbon Dioxide Recycle Path 106' Junction 200 culture tank 201 Gas phase outlet 202 Gas phase outflow channel 211 Water phase 212 Gas Phase 300 Hydrogen dissolving tank 301 Hydrogen inlet channel 302 Hydrogen ultra-fine bubble introduction section 303 Water phase flow channel 304 Gas phase outlet 305 Hydrogen reuse channel 306 Junction 311 Water phase 312 Gas Phase

Claims

1. a dissolver for dissolving carbon dioxide in an aqueous phase; an ultrafine hydrogen bubble introducing section for introducing ultrafine hydrogen bubbles into the system; a culture vessel, separate from the dissolution vessel, for culturing methanogens in the presence of ultra-fine hydrogen 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.

1. A methane production system comprising:

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

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

4. 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. further comprising The ultra-fine hydrogen bubble introduction section is present in the hydrogen dissolution tank. The methane production system according to claim 1 .

5. 3. 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. 4. The methane production system according to claim 3, further comprising a gas recycling passage for introducing the gas phase in the dissolution tank into a passage for introducing hydrogen gas into the ultra-fine hydrogen bubble introducing section.

7. a first gas recycle passage for introducing the gas phase in the dissolution tank into the aqueous phase in the dissolution tank; and a second gas reuse flow path for introducing the gas phase in the hydrogen dissolution tank into a flow path for introducing hydrogen gas into the hydrogen ultra-fine bubble introducing portion; The methane production system of claim 4, further comprising:

8. a flow rate adjusting unit for adjusting the amount of the aqueous phase in which carbon dioxide is dissolved in the dissolution tank transferred to the culture tank; A hydrogen ultrafine bubble quantity sensor for measuring the amount of hydrogen ultrafine bubbles introduced into the system; a control unit for controlling the carbon dioxide supply amount adjustment unit according to the amount of ultra-fine hydrogen bubbles measured by the ultra-fine hydrogen bubble amount sensor so that the molar ratio of carbon dioxide to hydrogen in the culture tank is within a range of 1:3 to 1:

5. The methane production system according to any one of claims 1 to 7, further comprising:

9. (a) dissolving carbon dioxide in an aqueous phase in a dissolver; (b) adding the aqueous phase having 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 ultrafine hydrogen bubbles into the system; (d) Cultivating methanogens in a culture solution containing ultra-fine hydrogen bubbles and dissolved carbon dioxide to produce methane.

1. A method for producing methane, comprising:

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

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

12. Step (c) is carried out in an aqueous phase in 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 in the hydrogen dissolution tank, (c2) The method further comprises a step of adding the aqueous phase containing dissolved ultra-fine hydrogen bubbles obtained in step (c) to the culture solution. The method for producing methane according to claim 9.

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

14. 12. The method for producing methane according to claim 11, further comprising the step of (a2') introducing the gas phase in the dissolution tank through a gas reuse flow path into a flow path for introducing hydrogen gas into the ultra-fine hydrogen bubble introducing section.

15. (a2) introducing the gas phase in the dissolution tank into the aqueous phase in the dissolution tank via a first gas recycle flow path; and (c3) A step of introducing the gas phase in the hydrogen dissolving tank into a flow path for introducing hydrogen gas into the ultra-fine hydrogen bubble introducing section via a second gas reuse flow path.

13. The method for producing methane according to claim 12, further comprising:

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 a range of 1:3 to 1:5.

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