Methane gas generation system and methane gas generation method

Abstract

A methane gas generation system (1000, 1001, 1002, 1003, 1004, 1005) comprises a fluid flow generation unit (5) that generates a flow of a fermentation liquid (7) that has been generated at a fermentation tank (1) and brings the fermentation liquid (7) into contact with an anode electrode (11) and then a cathode electrode (12). As a result of the flow field of the fermentation liquid (7) being formed in one direction such that the fermentation liquid (7) contacts the anode electrode (11) and then the cathode electrode (12), hydrogen generated at the cathode electrode (12) is kept from contacting the anode electrode (11) and reverting to protons. The present invention thereby provides a methane gas generation system (1000, 1001, 1002, 1003, 1004, 1005) that can reduce the risk of increasing the power required per unit of methane generation and can thereby suppress reductions in methane gas generation efficiency.

Description

Methane gas generation system and methane gas generation method

[0001] The present disclosure relates to methane gas production systems and methods.

[0002] To recycle organic waste such as sludge, food waste, and industrial wastewater as resources, a known method involves feeding the organic waste into a digester containing anaerobic microorganisms, allowing it to undergo methane fermentation and extracting biogas. The methane in the biogas can be converted into thermal energy in a boiler or electrical energy in a generator, contributing to the recycling of organic waste and the reduction of greenhouse gas emissions by reducing fuel or electricity consumption.

[0003] The methane production process consists of two steps: one in which hydrogen-utilizing methanogens produce methane from carbon dioxide and hydrogen, and the other in which acetogenic methanogens produce methane from acetic acid. Due to the low amount of hydrogen in the fermentation liquid, the proportion of methane produced by acetogenic methanogens is higher. Therefore, the methane concentration in the extracted biogas is only about 60%, with the remaining 40% being carbon dioxide, creating the problem of a lower calorific value for biogas than that of city gas of the same volume.

[0004] For example, in the invention described in Patent Document 1, an anode electrode and a cathode electrode are brought into contact with the fermentation liquid in a digester, and a voltage is applied between the anode electrode and the cathode electrode. With this configuration, the invention described in Patent Document 1 uses microorganisms attached to the anode electrode to oxidize organic matter, generating hydrogen ions in the fermentation liquid and transferring electrons to the cathode electrode. The hydrogen ions are then reduced at the cathode electrode to produce hydrogen. This increases the amount of hydrogen in the fermentation liquid, thereby increasing the proportion of pathways through which hydrogen-utilizing methanogens produce methane. This promotes the consumption of carbon dioxide in the biogas, thereby increasing the methane concentration.

[0005] Japanese Patent Application Laid-Open No. 2005-125172

[0006] However, in the invention described in Patent Document 1, there is a risk that the hydrogen or methane generated by the reduction of hydrogen ions at the cathode electrode may be oxidized at the anode electrode and return to hydrogen ions. In this case, the amount of hydrogen or methane produced decreases, which increases the amount of electricity required per unit amount of methane produced, resulting in a problem of reduced efficiency in producing methane gas.

[0007] The present disclosure has been made to solve the above-mentioned problems, and aims to provide a methane gas generation system and a methane gas generation method that can suppress a decrease in methane gas generation efficiency.

[0008] The methane gas generation system according to the present disclosure generates methane gas using carbon dioxide contained in biogas generated from a fermentation liquid produced by fermenting organic waste with microorganisms in a fermenter, and is characterized by including an anode electrode that comes into contact with the fermentation liquid and generates protons and electrons using organic matter in the fermentation liquid, a cathode electrode that comes into contact with the fermentation liquid and generates hydrogen using the protons and electrons generated at the anode electrode, a voltage application unit that applies a voltage between the anode electrode and the cathode electrode, a control unit that controls the voltage application unit, a fluid flow generation unit that generates a flow of fermentation liquid so that the fermentation liquid contacts the anode electrode and then the cathode electrode, and a discharge unit that is provided in the fermenter and discharges the fermentation liquid that has contacted the anode electrode and then the cathode electrode to the outside of the fermenter.

[0009] Furthermore, a methane gas production method according to the present disclosure is a methane gas production method for producing methane gas using carbon dioxide in biogas generated from a fermentation liquid produced by fermenting organic waste with microorganisms in a fermenter, the method comprising the steps of: applying a voltage between an anode electrode and a cathode electrode; generating a flow of fermentation liquid so that the fermentation liquid comes into contact with the anode electrode and then the cathode electrode; producing protons and electrons using organic matter in the fermentation liquid by bringing the anode electrode into contact with the fermentation liquid that has been delivered; producing hydrogen using the protons and electrons by bringing the cathode electrode into contact with the anode electrode and then with the fermentation liquid that has been delivered; and discharging the fermentation liquid that has come into contact with the anode electrode and then the cathode electrode to the outside of the fermenter.

[0010] According to the present disclosure, a methane gas production system includes a fluid flow generating unit that generates a flow of fermentation liquid and brings the fermentation liquid produced in the fermenter into contact with an anode electrode and then a cathode electrode, thereby providing a methane gas production system and a methane gas production method that can suppress a decrease in methane gas production efficiency.

[0011] FIG. 1 is a configuration diagram of a methane gas generation system according to a first embodiment. FIG. 2 is a flowchart showing a methane gas generation method according to the first embodiment. FIG. 3 is a configuration diagram of a methane gas generation system according to a second embodiment. FIG. 4 is a flowchart showing a methane gas generation method according to the second embodiment. FIG. 5 is a configuration diagram of a methane gas generation system according to a fourth embodiment. FIG. 6 is a flowchart showing a methane gas generation method according to the fourth embodiment. FIG. 7 is a flowchart showing a methane gas generation method according to the fifth embodiment. FIG. 8 is a configuration diagram of a methane gas generation system according to a sixth embodiment. FIG. 9 is a flowchart showing a methane gas generation method according to the sixth embodiment.

[0012] Below, methane gas production systems and methane gas production methods according to embodiments will be described with reference to the drawings. The following embodiments are merely examples, and the embodiments can be combined and modified as appropriate. In the drawings, similar components are designated by the same reference numerals. Note that the relative dimensional relationships or shapes of the components in each drawing may differ from those in reality.

[0013] Embodiment 1 A methane gas generation system 1000 in embodiment 1 will be described with reference to Fig. 1. Fig. 1 is a configuration diagram of the methane gas generation system 1000 in embodiment 1. In the following description, to facilitate understanding, terms indicating directions such as "up," "down," "right," "left," "front," and "rear" will be used as appropriate, but this does not limit the embodiment.

[0014] As shown in FIG. 1 , the methane gas production system 1000 of the first embodiment includes a fermenter 1, an organic matter oxidation unit 2, a hydrogen production unit 3, a voltage application unit 4, a fluid flow production unit 5, an organic waste input unit 6, a biogas discharge unit 9, a fermentation liquid discharge unit 15, a fermentation liquid transfer unit 10, a fermentation liquid return unit 14, and a control unit 13.

[0015] The fermenter 1 holds anaerobic microorganisms inside, which are maintained under anaerobic conditions. The anaerobic microorganisms are not shown. Organic waste is crushed using a disposer or the like as needed, and then fed into the fermenter 1 from the organic waste input section 6 via input piping 101. The organic waste includes sludge, food waste, industrial wastewater, etc. The anaerobic microorganisms ferment the input organic waste to produce a fermentation liquid 7.

[0016] Fermentation using anaerobic microorganisms will now be described in detail. Anaerobic microorganisms include hydrolytic bacteria, acidogenic bacteria, acetogenic methanogens, and hydrogenogenic methanogens. The hydrolytic bacteria produce monosaccharides, amino acids, and higher fatty acids from the input organic waste. The acidogenic bacteria produce volatile fatty acids and hydrogen from the monosaccharides, amino acids, and higher fatty acids. Volatile fatty acids include valeric acid, butyric acid, propionic acid, and acetic acid. Carbon dioxide is also produced during the process of producing volatile fatty acids from the organic waste. The acetogenic methanogens produce methane from acetic acid. The hydrogenogenic methanogens produce methane from hydrogen and carbon dioxide. The produced methane and carbon dioxide are separated from the fermentation liquid 7 to form a gas phase 8 within the fermenter 1 and are discharged as biogas from the biogas discharge unit 9 via the exhaust piping 110 to the outside of the fermenter 1. The biogas discharge unit 9 is installed at the top of the fermenter 1 so as to be in contact with the gas phase 8.

[0017] The fermentation liquid transfer unit 10 is provided above the fermenter 1 so as to be in contact with the fermentation liquid 7. The fermentation liquid transfer unit 10 is connected to the organic matter oxidation unit 2 via a transfer pipe 102. The fluid flow generation unit 5 is provided on the transfer pipe 102. The fluid flow generation unit 5 is a cascade pump or the like, and generates a flow of the fermentation liquid 7. The fluid flow generation unit 5 is not particularly limited as long as it is a means capable of generating a flow of the fermentation liquid 7. The fermentation liquid 7 is sent from the fermenter 1 to the organic matter oxidation unit 2 via the transfer pipe 102 by the flow generated by the fluid flow generation unit 5.

[0018] The organic matter oxidation unit 2 is provided in communication with the hydrogen production unit 3. The organic matter oxidation unit 2 and the hydrogen production unit 3 are provided outside the fermenter 1. An anode electrode 11 is provided in the organic matter oxidation unit 2, and the anode electrode 11 comes into contact with the fermentation solution 7 sent from the fermenter 1. A cathode electrode 12 is provided in the hydrogen production unit 3, and the cathode electrode 12 comes into contact with the fermentation solution 7 sent from the organic matter oxidation unit 2 to the hydrogen production unit 3. That is, the fermentation solution 7 comes into contact with the anode electrode 11 and then the cathode electrode 12 due to the flow generated by the fluid flow generation unit 5. That is, the fluid flow generation unit 5 forms a unidirectional flow field for the fermentation solution 7 so that the fermentation solution 7 comes into contact with the anode electrode 11 and then with the cathode electrode 12. In other words, when the organic matter oxidation unit 2 and the hydrogen production unit 3 are provided externally, the fluid flow generation unit 5 generates a flow to circulate the fermentation liquid 7 through the fermenter 1, the organic matter oxidation unit 2 provided with an anode electrode 11, the hydrogen production unit 3 provided with a cathode electrode 12, and the fermenter 1 in that order.

[0019] The anode electrode 11 and the cathode electrode 12 are connected to a voltage application unit 4 via a first wiring 103. The voltage application unit 4 applies a voltage so as to generate a constant potential difference between the anode electrode 11 and the cathode electrode 12. The applied voltage is controlled by a control unit 13.

[0020] Electric-generating bacteria are attached to the anode electrode 11. The electric-generating bacteria oxidize volatile fatty acids in the fermentation liquid 7 through metabolism and perform electrode respiration, donating electrons to the electrode. Therefore, when the fermentation liquid 7 comes into contact with the anode electrode 11, protons and electrons are generated, and the protons are released into the fermentation liquid 7. In the following description, the protons are assumed to be hydrogen ions. The generated electrons are sent from the anode electrode 11 to the cathode electrode 12 via the first wiring 103. Then, when the fermentation liquid 7 comes into contact with the cathode electrode 12 with a voltage applied, hydrogen is generated from the protons and electrons.

[0021] The fermentation liquid return unit 14 is provided at the bottom of the fermenter 1. The fermentation liquid return unit 14 is connected to the hydrogen production unit 3 via a return pipe 104. The fermentation liquid 7 is sent from the hydrogen production unit 3 to the fermenter 1 via the return pipe 104 by the flow generated by the fluid flow production unit 5.

[0022] In the fermenter 1, as described above, the hydrogen-assimilating methanogens generate methane using the hydrogen generated by the acid-producing bacteria or the hydrogen sent from the hydrogen production unit 3 and the carbon dioxide in the fermentation liquid 7. The generated methane is separated from the fermentation liquid 7 and forms a gas phase 8 in the fermenter 1.

[0023] The fermentation liquid discharge unit 15 is provided in the fermenter 1 and connected to the outside via a waste pipe 105. The fermentation liquid discharge unit 15 discharges the fermentation liquid 7, which has come into contact with the anode electrode 11 and then the cathode electrode 12, to the outside of the fermenter 1 via the waste pipe 105.

[0024] Furthermore, the fermentation liquid discharge unit 15 makes the flow rate of the fermentation liquid 7 discharged outside the fermenter 1 equal to the flow rate of the organic waste charged into the fermenter 1. This allows the methane gas production system 1000 of the first embodiment to maintain a constant amount of the fermentation liquid 7 in the fermenter 1. Here, "equal" includes the meaning of "same value," and may also include a value that is different to the extent that the same effect as in the first embodiment is achieved.

[0025] In the following description, the flow rate of the fermentation liquid 7 discharged from the fermentation liquid discharge unit 15 to the outside of the fermenter 1 is referred to as a discharge flow rate Q1 [m ３ / day], and the flow rate of the fermentation liquid 7 generated by the fluid flow generating unit 5 is set to a circulation flow rate Q2 [m ３ / day]. In this case, the relationship between Q1 and Q2 is expressed by equation (1). That is, the fluid flow generating unit 5 generates the flow of the fermentation liquid 7 so that the flow rate of the fermentation liquid 7 circulating through the fermenter 1, the anode electrode 11, the cathode electrode 12, and the fermenter 1 in this order is greater than the flow rate of the fermentation liquid 7 discharged to the outside of the fermenter 1 by the fermentation liquid discharge unit 15.

[0026]

[0027] Next, a methane gas generation method according to the first embodiment will be described. FIG. 2 is a flowchart showing the methane gas generation method according to the first embodiment. In step S101, the control unit 13 controls the voltage application unit 4 to apply a voltage between the anode electrode 11 and the cathode electrode 12. Note that step S101 is not limited to this order, and may be performed between steps S102 and S103, between steps S103 and S104, or after step S104. In step S102, the fluid flow generation unit 5 generates a flow of the fermentation liquid 7. That is, the fluid flow generation unit 5 forms a unidirectional flow field for the fermentation liquid 7 so that the fermentation liquid 7 contacts the anode electrode 11 and then the cathode electrode 12. In step S103, the anode electrode 11 contacts the fermentation liquid 7 transported by the flow generated by the fluid flow generation unit 5, thereby generating protons and electrons using organic matter in the fermentation liquid 7. In step S104, the cathode electrode 12 produces hydrogen using protons and electrons by contacting the fermentation liquid 7, which has been sent after contacting the anode electrode 11, through the flow generated by the fluid flow generating unit 5. In step S105, the fermentation liquid discharge unit 15 discharges the fermentation liquid 7, which has contacted the anode electrode 11 and then the cathode electrode 12, to the outside of the fermenter 1 via the waste pipe 105. This concludes the description of the methane gas production method in the first embodiment.

[0028] As described above, the methane gas production system 1000 of the first embodiment comprises an anode electrode 11 that comes into contact with the fermentation liquid 7 and generates protons and electrons using organic matter in the fermentation liquid 7, a cathode electrode 12 that generates hydrogen using the protons and electrons generated at the anode electrode 11, a voltage application unit 4 that applies a voltage between the anode electrode 11 and the cathode electrode 12, a control unit 13 that controls the voltage application unit 4, a fluid flow generation unit 5 that generates a flow of the fermentation liquid 7 so that the fermentation liquid 7 comes into contact with the anode electrode 11 and then the cathode electrode 12, and a discharge unit that is provided in the fermenter 1 and discharges the fermentation liquid 7 that has come into contact with the anode electrode 11 and then the cathode electrode 12 to the outside of the fermenter 1. With the above configuration, the methane gas production system 1000 of embodiment 1 forms a unidirectional flow field for the fermentation solution 7 so that the fermentation solution 7 comes into contact with the anode electrode 11 and then with the cathode electrode 12, thereby preventing hydrogen produced at the cathode electrode 12 from returning to protons upon contact with the anode electrode 11. This allows the methane gas production system 1000 of embodiment 1 to prevent a decrease in the production amount of hydrogen produced at the cathode electrode 12. Therefore, the methane gas production system 1000 of embodiment 1 can prevent a decrease in the proportion of pathways through which hydrogen-assimilating methanogens produce methane, reducing the risk of an increase in the amount of electricity required per unit of methane production, and therefore preventing a decrease in the efficiency of methane gas production.

[0029] Furthermore, the anode electrode 11 and the cathode electrode 12 are provided outside the fermenter 1, and the fluid flow generation unit 5 generates a flow of the fermentation solution 7 so that the fermentation solution 7 is circulated through the fermenter 1, the organic matter oxidation unit 2 provided with the anode electrode 11, the hydrogen production unit 3 provided with the cathode electrode 12, and the fermenter 1 in this order. With the above configuration, the fluid flow generation unit 5 can prevent the fermentation solution 7 that has come into contact with the cathode electrode 12 from flowing back and contacting the anode electrode 11 again before being sent into the fermenter 1. Therefore, the methane gas production system 1000 of the first embodiment allows hydrogen-assimilating methanogens to efficiently produce methane in the fermenter 1. Furthermore, methane and unreacted hydrogen produced at the cathode electrode 12 are difficult to dissolve in the fermentation solution 7, and are therefore separated from the fermentation solution 7 into the gas phase 8 in the fermenter 1. Therefore, the methane gas generation system 1000 of the first embodiment can prevent methane and unreacted hydrogen generated at the cathode electrode 12 from coming into contact with the anode electrode 11 and returning to protons. As a result, the methane gas generation system 1000 of the first embodiment can prevent a decrease in the proportion of pathways through which hydrogen-assimilating methanogens generate methane, and can reduce the risk of an increase in the amount of electricity required per unit amount of methane produced, thereby preventing a decrease in the efficiency of methane gas generation.

[0030] Furthermore, the fluid flow generating unit 5 generates a flow of the fermentation liquid 7 so that the flow rate of the fermentation liquid 7 circulating through the fermenter 1, the organic matter oxidizing unit 2 provided with the anode electrode 11, the hydrogen generating unit 3 provided with the cathode electrode 12, and the fermenter 1 in this order is greater than the flow rate of the fermentation liquid 7 discharged to the outside of the fermenter 1 by the fermentation liquid discharge unit 15. With the above configuration, the methane gas production system 1000 of embodiment 1 can bring the fermentation liquid 7 into contact with the anode electrode 11 and then the cathode electrode 12 by the flow generated by the fluid flow generating unit 5 before the fermentation liquid discharge unit 15 discharges the fermentation liquid 7 from the fermenter 1. This allows the methane gas production system 1000 of embodiment 1 to efficiently produce methane gas.

[0031] The methane gas production method of the first embodiment also includes the steps of applying a voltage between the anode electrode 11 and the cathode electrode 12, generating a flow of the fermentation liquid 7 using the fluid flow generation unit 5 so that the fermentation liquid 7 contacts the anode electrode 11 and then the cathode electrode 12, producing protons and electrons from organic matter in the fermentation liquid 7 by bringing the anode electrode 11 into contact with the fermentation liquid 7 sent by the flow generated by the fluid flow generation unit 5, producing hydrogen from the protons and electrons by bringing the cathode electrode 12 into contact with the fermentation liquid 7 sent by the flow generated by the fluid flow generation unit 5, and discharging the fermentation liquid 7 that has contacted the anode electrode 11 and then the cathode electrode 12 to the outside of the fermenter 1. With the above configuration, the methane gas production method of the first embodiment can suppress a decrease in the proportion of pathways in which hydrogen-assimilating methanogens produce methane, reducing the risk of an increase in the amount of power required per unit of methane production, and thereby suppressing a decrease in the efficiency of methane gas production.

[0032] The circulation flow rate Q2 is not particularly limited and is set appropriately so that the methane concentration in the biogas exceeds 60% and so that unreacted hydrogen is not mixed into the biogas as much as possible. For example, the circulation flow rate Q2 is set so that it falls within the turbulent flow range based on the Reynolds number of the flow generated by the fluid flow generating unit 5. In this case, the turbulent flow range refers to the flow rate range in which the fermentation liquid 7 becomes turbulent when it contacts the anode electrode 11 and the cathode electrode 12. As a result, the methane gas generation system 1000 of the first embodiment can increase the probability of contact between the anode electrode 11 and the cathode electrode 12 and the fermentation liquid 7, thereby efficiently generating hydrogen.

[0033] Furthermore, in the first embodiment, the organic matter oxidation unit 2 and the hydrogen production unit 3 are described as being provided outside the fermenter 1, and the fermentation solution 7 is described as being circulated, but this is not limited to this. That is, as long as the fluid flow generation unit 5 can generate a flow of the fermentation solution 7 so that the fermentation solution 7 contacts the anode electrode 11 and then the cathode electrode 12, the anode electrode 11 and the cathode electrode 12 may be provided inside the fermenter 1, for example. In this case, the fluid flow generation unit 5 is, for example, a stirrer with stirring blades and an adjustable rotation speed. Furthermore, for example, the anode electrode 11 is provided in the center of the fermenter 1, and the cathode electrode 12 is provided on the side wall side of the fermenter 1.

[0034] At this time, the fluid flow generating unit 5 generates a flow of the fermentation liquid 7 in one direction from the center toward the side wall of the fermenter 1. As a result, the fluid flow generating unit 5 can generate a flow of the fermentation liquid 7 so that the fermentation liquid 7 contacts the anode electrode 11 and then the cathode electrode 12, thereby reducing the risk of an increase in the amount of electricity required per unit amount of methane produced and suppressing a decrease in the efficiency of methane gas production.

[0035] The material of the anode electrode 11 is not particularly limited, but is preferably a metal material such as carbon, gold, or iron oxide. By using such a material, the anode electrode 11 is less susceptible to decomposition by microorganisms and is highly compatible with the power-generating bacteria.

[0036] The structure of the anode electrode 11 is not particularly limited, and may be a sheet, a tube, a brush, or other similar structure. The anode electrode 11 preferably has a large surface area so that a large amount of power-generating bacteria can adhere to it. For example, the anode electrode 11 may be a porous body with a high porosity. The surface of the anode electrode 11 may be subjected to treatments such as the formation of millimeter- or micrometer-order irregularities or surface modification to improve biocompatibility.

[0037] The material of the cathode electrode 12 is not particularly limited, but is preferably a metal material such as stainless steel or copper. By using such a material, the cathode electrode 12 has low electrical resistance, high corrosion resistance, and low biocompatibility. This allows the cathode electrode 12 to have improved electrical energy efficiency and durability.

[0038] The structure of the cathode electrode 12 is not particularly limited, and may be a sheet, a tube, a mesh, or other structure.

[0039] Furthermore, the voltage applied by the voltage application unit 4 is not particularly limited, but is preferably 0.5 V or more and 2.5 V or less, and more preferably 1.0 V or more and 2.0 V or less. If the applied voltage is lower than the above range, there is a risk that the cathode electrode 12 will not be able to generate sufficient hydrogen. If the applied voltage is higher than the above range, there is a risk that water will be electrolyzed at the anode electrode 11, or that harmful chlorine will be generated due to oxidation of chloride ions in the fermentation liquid 7.

[0040] The volume of the fermenter 1 is not particularly limited, but is set based on the flow rate of the organic waste introduced into the fermenter 1 so that the solid retention time (SRT) of the organic waste in the fermenter 1 is a predetermined value. ３ ], the solid concentration of the fermentation liquid 7 is C1 [g / m ３ ], the flow rate of organic waste fed into the fermenter 1 is Q3 [m ３ / day], and the solid concentration of organic waste is C2 [g / m ３ ], the solids retention time T [days] in the fermenter 1 is expressed by the following formula (2): That is, the solids retention time T [days] in the fermenter 1 is the value obtained by dividing the product of the volume of the fermenter 1 and the solids concentration in the fermenter 1 by the product of the flow rate of the organic waste and the solids concentration of the organic waste.

[0041]

[0042] Furthermore, the positions of the fermentation liquid transfer unit 10 and the fermentation liquid return unit 14 are not particularly limited as long as the fluid flow generating unit 5 can generate a flow of the fermentation liquid 7 so that the fermentation liquid 7 contacts the anode electrode 11 and then the cathode electrode 12. The fermentation liquid transfer unit 10 in the methane gas production system 1000 of embodiment 1 is preferably provided so as to contact the fermentation liquid 7 at an upper portion of the fermenter 1. The fermentation liquid return unit 14 is preferably provided at a lower portion of the fermenter 1 than the position at which the fermentation liquid transfer unit 10 is provided. With the above configuration, the fermentation liquid return unit 14 is provided at a position away from the gas phase 8 formed at an upper portion of the fermenter 1. As a result, the methane gas production system 1000 of embodiment 1 can prevent hydrogen produced at the cathode electrode 12 and sent from the fermentation liquid return unit 14 to the fermenter 1 from separating from the fermentation liquid 7 into the gas phase 8 before it is converted into methane by hydrogen-assimilating methanogens in the fermentation liquid 7.

[0043] As described above, the organic matter oxidizing unit 2 and the hydrogen generating unit 3 are preferably provided in communication with each other without being separated by a hydrogen ion exchange membrane, a partition wall, or the like. This allows the fluid flow generating unit 5 to efficiently generate a flow of the fermentation liquid 7 so that the fermentation liquid 7 contacts the anode electrode 11 and then the cathode electrode 12. Furthermore, with the above configuration, the methane gas generating system 1000 can reduce the solution resistance between the anode electrode 11 and the cathode electrode 12, thereby lowering the voltage that needs to be applied by the voltage applying unit 4.

[0044] The structures of the organic matter oxidation unit 2 and the hydrogen generation unit 3 are not particularly limited. For example, the organic matter oxidation unit 2 and the hydrogen generation unit 3 may be formed in a tank shape and configured as a continuous tank reactor. Alternatively, the organic matter oxidation unit 2 and the hydrogen generation unit 3 may be formed in a tubular shape and configured as a tubular reactor using extrusion flow.

[0045] Furthermore, when the organic matter oxidation unit 2 and the hydrogen production unit 3 are configured as tubular reactors, protrusions for blocking the flow may be provided on the side surfaces of the tubes. With the above configuration, the methane gas production system 1000 of embodiment 1 can easily generate turbulent flow in the fermentation liquid 7. This increases the probability of contact between the anode electrode 11 and the cathode electrode 12 and the fermentation liquid 7, thereby enabling the methane gas production system 1000 of embodiment 1 to efficiently produce hydrogen.

[0046] Embodiment 2 A methane gas production system 1001 in embodiment 2 will be described with reference to Figure 3. Figure 3 is a configuration diagram of the methane gas production system 1001 in embodiment 2. The methane gas production system 1001 in embodiment 2 differs from the methane gas production system 1001 in embodiment 1 in that the control unit 13 is connected to the fluid flow production unit 5 via the second wiring 106, and the control unit 13 controls the flow rate of the fermentation liquid 7. Components similar to those in embodiment 1 are denoted by the same reference numerals. Further, detailed description of components similar to those in embodiment 1 will be omitted, and components different from embodiment 1 will be mainly described.

[0047] 3, the methane gas production system 1001 in the second embodiment includes a second wiring 106 that connects the control unit 13 and the fluid flow generation unit 5. This allows the control unit 13 to control the circulation flow rate Q2 by controlling the fluid flow generation unit 5.

[0048] As described above, the fluid flow generating unit 5 generates a flow of the fermentation liquid 7 at a circulation flow rate Q2 that is set so as to fall within the turbulent range based on the Reynolds number of the flow to be generated. Furthermore, when the discharge flow rate Q1 fluctuates in accordance with fluctuations in the flow rate of the organic waste input from the organic waste input unit 6 and the circulation flow rate Q2 becomes equal to or less than the discharge flow rate Q1, the control unit 13 controls the circulation flow rate Q2 so that the circulation flow rate Q2 is greater than the discharge flow rate Q1.

[0049] Furthermore, when the circulation flow rate Q2 deviates from the turbulent flow range, the control unit 13 controls the circulation flow rate Q2 so that it falls within the turbulent flow range.

[0050] Next, a methane gas generation method according to the second embodiment will be described. FIG. 4 is a flowchart illustrating the methane gas generation method according to the second embodiment. Steps S201, S204, S205, and S206 are similar to steps S101, S103, S104, and S105 of the first embodiment, and therefore detailed description thereof will be omitted. In step S202, the control unit 13 sets the flow rate of the fermentation liquor 7 generated by the fluid flow generation unit 5. That is, when the discharge flow rate Q1 fluctuates in accordance with fluctuations in the flow rate of the organic waste input from the organic waste input unit 6 and the circulation flow rate Q2 becomes equal to or less than the discharge flow rate Q1, the control unit 13 controls the circulation flow rate Q2 so that the circulation flow rate Q2 is greater than the discharge flow rate Q1. Furthermore, when the circulation flow rate Q2 deviates from the turbulent flow range, the control unit 13 controls the circulation flow rate Q2 so that it falls within the turbulent flow range. Note that step S202 is not limited to this order and may be performed before step S201. In step S203, the fluid flow generating unit 5 is controlled by the control unit 13 to generate a flow of the fermentation liquid 7 at the flow rate set in step S202. That is, the fluid flow generating unit 5 forms a unidirectional flow field for the fermentation liquid 7 so that the fermentation liquid 7 comes into contact with the anode electrode 11 and then with the cathode electrode 12. This completes the description of the methane gas production method in embodiment 2.

[0051] As in the first embodiment, the methane gas production system 1001 of the second embodiment includes an anode electrode 11 that comes into contact with the fermentation liquid 7 and generates protons and electrons using organic matter in the fermentation liquid 7, a cathode electrode 12 that generates hydrogen using the protons and electrons generated at the anode electrode 11, a voltage application unit 4 that applies a voltage between the anode electrode 11 and the cathode electrode 12, a control unit 13 that controls the voltage application unit 4, a fluid flow generation unit 5 that generates a flow of the fermentation liquid 7 so that the fermentation liquid 7 comes into contact with the anode electrode 11 and then the cathode electrode 12, and a discharge unit that is provided in the fermenter 1 and discharges the fermentation liquid 7 that has come into contact with the anode electrode 11 and then the cathode electrode 12 to the outside of the fermenter 1. With the above configuration, the methane gas production system 1001 of Embodiment 1 forms a unidirectional flow field for the fermentation solution 7 so that the fermentation solution 7 comes into contact with the anode electrode 11 and then with the cathode electrode 12, thereby preventing hydrogen produced at the cathode electrode 12 from returning to protons upon contact with the anode electrode 11. This allows the methane gas production system 1001 of Embodiment 2 to prevent a decrease in the production amount of hydrogen produced at the cathode electrode 12. Therefore, the methane gas production system 1001 of Embodiment 2 can prevent a decrease in the proportion of pathways through which hydrogen-assimilating methanogens produce methane, reducing the risk of an increase in the amount of power required per unit of methane production, and therefore preventing a decrease in the efficiency of methane gas production.

[0052] Furthermore, in the methane gas generation system 1001 of the second embodiment, the control unit 13 can control the circulation flow rate Q2 by controlling the fluid flow generation unit 5. With the above configuration, the methane gas generation system 1001 of the second embodiment can adjust the contact time between the anode electrode 11 and the cathode electrode 12 and the fermentation solution 7. Furthermore, the methane gas generation system 1001 of the second embodiment can adjust the amount of hydrogen generated at the cathode electrode 12. This prevents unreacted hydrogen from being sent from the fermentation solution transfer unit 10 to the organic matter oxidation unit 2, coming into contact with the anode electrode 11, and then returning to protons. Therefore, the methane gas generation system 1001 of the second embodiment can reduce the risk of an increase in the amount of power required per unit amount of methane generated, thereby preventing a decrease in the methane gas generation efficiency.

[0053] Furthermore, when the discharge flow rate Q1 fluctuates in accordance with fluctuations in the flow rate of organic waste input from the organic waste input unit 6 and the circulation flow rate Q2 becomes equal to or less than the discharge flow rate Q1, the control unit 13 of the second embodiment controls the circulation flow rate Q2 so that the circulation flow rate Q2 is greater than the discharge flow rate Q1. With the above configuration, the methane gas production system 1001 of the second embodiment can bring the fermentation liquid 7 into contact with the anode electrode 11 and then the cathode electrode 12 by the flow generated by the fluid flow generation unit 5 before the fermentation liquid discharge unit 15 discharges the fermentation liquid 7 from the fermenter 1. This allows the methane gas production system 1001 of the second embodiment to efficiently produce methane gas.

[0054] Furthermore, when the circulation flow rate Q2 deviates from the turbulent flow range, the control unit 13 of the second embodiment controls the circulation flow rate Q2 so that it falls within the turbulent flow range. By adopting the above configuration, the methane gas production system 1001 of the second embodiment can increase the probability of contact between the anode electrode 11 and the cathode electrode 12 and the fermentation liquid 7, thereby enabling efficient production of hydrogen.

[0055] Furthermore, the methane gas production method of the second embodiment includes the steps of applying a voltage between the anode electrode 11 and the cathode electrode 12; setting, by the control unit 13, the flow rate of the flow of fermentation liquid 7 produced by the fluid flow generation unit 5; generating, by the fluid flow generation unit 5, a flow of fermentation liquid 7 at the set flow rate so that the fermentation liquid 7 comes into contact with the anode electrode 11 and then the cathode electrode 12; producing protons and electrons from organic matter in the fermentation liquid 7 by bringing the anode electrode 11 into contact with the fermentation liquid 7 delivered by the flow generated by the fluid flow generation unit 5; producing hydrogen from the protons and electrons by bringing the cathode electrode 12 into contact with the fermentation liquid 7 delivered by the flow generated by the fluid flow generation unit 5; and discharging the fermentation liquid 7 that has come into contact with the anode electrode 11 and then the cathode electrode 12 to the outside of the fermenter 1. By adopting the above configuration, the methane gas production method of Embodiment 2 can suppress a decrease in the proportion of pathways in which hydrogen-assimilating methanogens produce methane, and can reduce the risk of an increase in the amount of electricity required per unit amount of methane produced, thereby suppressing a decrease in methane gas production efficiency, as in Embodiment 1. Furthermore, by adopting the above configuration, the methane gas production method of Embodiment 2 can adjust the contact time between the anode electrode 11 and the cathode electrode 12 and the fermentation liquid 7.

[0056] Embodiment 3. A methane gas generation system 1002 in embodiment 3 will be described with reference to FIG. 5 . FIG. 5 is a configuration diagram of the methane gas generation system 1002 in embodiment 3. The methane gas generation system 1002 in embodiment 3 differs from the above-described embodiments in that it includes a current measurement unit 16 that measures the value of the current flowing between the anode electrode 11 and the cathode electrode 12, a first property measurement unit 17 that is provided on the transfer piping 102 and that measures the property of the fermentation liquor 7 sent from the fermenter 1 to the organic matter oxidation unit 2, and a second property measurement unit 18 that is provided on the return piping 104 and that measures the property of the fermentation liquor 7 sent from the hydrogen production unit 3 to the fermenter 1. Components similar to those in the above-described embodiments are denoted by the same reference numerals. Further, detailed description of the components similar to those in the above-described embodiments will be omitted, and the following description will focus mainly on the components that differ from those in the above-described embodiments.

[0057] As shown in Figure 5, the methane gas generation system 1002 in the third embodiment includes a current measurement unit 16, a first property measurement unit 17, and a second property measurement unit 18. The current measurement unit 16 is connected to the control unit 13 via a third wiring 107. The first property measurement unit 17 is connected to the control unit 13 via a fourth wiring 108. The second property measurement unit 18 is connected to the control unit 13 via a fifth wiring 109.

[0058] The current measuring unit 16 measures the value of the current flowing through the first wiring 103 between the anode electrode 11 and the cathode electrode 12. The current measuring unit 16 also inputs the measured current value to the control unit 13. The control unit 13 calculates the amount of electrons flowing between the anode electrode 11 and the cathode electrode 12 from the input current value using Faraday's law. This allows the control unit 13 to grasp the progress of the oxidation reaction of volatile fatty acids by the power-generating bacteria at the anode electrode 11 and the hydrogen production reaction at the cathode electrode 12.

[0059] The control unit 13 controls the voltage applied by the voltage application unit 4 in accordance with the current value measured by the current measurement unit 16. For example, if the current value input from the current measurement unit 16 is less than a predetermined value, the control unit 13 estimates that the amount of hydrogen generated at the cathode electrode 12 is less than a reference value, and controls the voltage applied by the voltage application unit 4 to be increased. Furthermore, if the current value input from the current measurement unit 16 exceeds a predetermined value, the control unit 13 estimates that the amount of hydrogen generated at the cathode electrode 12 is more than the reference value, and controls the voltage applied by the voltage application unit 4 to be decreased.

[0060] The first property measurement unit 17 is provided on the transfer pipe 102 and measures the property of the fermentation liquid 7 sent from the fermenter 1 to the organic matter oxidation unit 2. That is, the first property measurement unit 17 measures the property of the fermentation liquid 7 before the fermentation liquid 7 comes into contact with the anode electrode 11. The first property measurement unit 17 inputs the measured property of the fermentation liquid 7 to the control unit 13. The second property measurement unit 18 is provided on the return pipe 104 and measures the property of the fermentation liquid 7 sent from the hydrogen production unit 3 to the fermenter 1. That is, the second property measurement unit 18 measures the property of the fermentation liquid 7 after the fermentation liquid 7 comes into contact with the cathode electrode 12. The second property measurement unit 18 inputs the measured property of the fermentation liquid 7 to the control unit 13. This allows the control unit 13 to estimate the amount of volatile fatty acids in the fermentation liquid 7 based on the properties of the fermentation liquid 7 measured by the first property measurement unit 17 and the second property measurement unit 18. Since the amount of volatile fatty acids in the fermentation liquid 7 depends on the activity levels of the acid-producing bacteria and the electricity-generating bacteria, the control unit 13 can estimate the activity levels of the acid-producing bacteria and the electricity-generating bacteria by estimating the amount of volatile fatty acids in the fermentation liquid 7.

[0061] The first property measurement unit 17 and the second property measurement unit 18 are, for example, pH sensors. That is, in this case, the property of the fermentation liquor 7 measured by the first property measurement unit 17 and the second property measurement unit 18 is pH.

[0062] The control unit 13 controls the voltage applied by the voltage application unit 4 based on the current value measured by the current measurement unit 16 and the property measured by the first property measurement unit 17. For example, if the pH value measured by the first property measurement unit 17 is smaller than a predetermined value, the control unit 13 estimates that the amount of volatile fatty acids in contact with the anode electrode 11 is greater than a reference value, and controls the voltage applied by the voltage application unit 4 to be increased. On the other hand, if the pH value measured by the first property measurement unit 17 is larger than a predetermined value, the control unit 13 estimates that the amount of volatile fatty acids in contact with the anode electrode 11 is smaller than a reference value, and controls the voltage applied by the voltage application unit 4 to be decreased.

[0063] Furthermore, for example, even if the current value input from the current measurement unit 16 is less than a predetermined value, if the pH value measured by the first property measurement unit 17 is greater than the predetermined value, the control unit 13 does not control the voltage applied by the voltage application unit 4 to be increased. In the above case, the control unit 13 may also control the voltage applied by the voltage application unit 4 to be increased. In this case, the control unit 13 controls the voltage application unit 4 to apply a lower voltage than when the pH value measured by the first property measurement unit 17 is smaller than the predetermined value.

[0064] Furthermore, the control unit 13 controls the voltage applied by the voltage application unit 4 based on the current value measured by the current measurement unit 16, the property measured by the first property measurement unit 17, and the property measured by the second property measurement unit 18. For example, if the current value input from the current measurement unit 16 is less than a predetermined value, the pH value measured by the first property measurement unit 17 is smaller than a first predetermined value, and the pH value measured by the second property measurement unit 18 is smaller than a second predetermined value, it may be assumed that the activity of the electricity-generating bacteria is reduced, and control may be implemented to increase the voltage applied by the voltage application unit 4. Here, the second predetermined value is higher than the first predetermined value.

[0065] Furthermore, the measured property of the fermentation liquor 7 may be any physical property value that can estimate the amount of volatile fatty acids in the fermentation liquor 7. However, it is preferable that the first property measurement unit 17 and the second property measurement unit 18 in the third embodiment measure the same property. In this case, the control unit 13 in the third embodiment can control the voltage applied by the voltage application unit 4 based on the difference between the property measured by the first property measurement unit 17 and the property measured by the second property measurement unit 18. For example, if the difference between the pH value measured by the first property measurement unit 17 and the pH value measured by the second property measurement unit 18 is smaller than a predetermined value, the control unit 13 may infer that the activity of the electricity-generating bacteria has decreased, and may control the voltage applied by the voltage application unit 4 to be increased.

[0066] Next, a methane gas generation method according to the third embodiment will be described. FIG. 6 is a flowchart showing the methane gas generation method according to the third embodiment. Steps S302 to S305 are similar to steps S102 to S105 of the first embodiment, and therefore detailed description thereof will be omitted. In step S301, the control unit 13 controls the voltage application unit 4 to apply a voltage between the anode electrode 11 and the cathode electrode 12 based on the current value measured by the current measurement unit 16, the property measured by the first property measurement unit 17, and the property measured by the second property measurement unit 18. Alternatively, in step S301, the control unit 13 may apply a voltage between the anode electrode 11 and the cathode electrode 12 based on the current value measured by the current measurement unit 16 and the property measured by the first property measurement unit 17. Furthermore, in step S301, the control unit 13 may apply a voltage between the anode electrode 11 and the cathode electrode 12 based on the current value measured by the current measurement unit 16. Note that step S301 is not limited to this order, and may be performed between step S302 and step S303, between step S303 and step S304, or after step S304. This completes the description of the methane gas production method in embodiment 3.

[0067] As in the first embodiment, the methane gas production system 1002 of the third embodiment includes an anode electrode 11 that comes into contact with the fermentation liquid 7 and generates protons and electrons using organic matter in the fermentation liquid 7, a cathode electrode 12 that generates hydrogen using the protons and electrons generated at the anode electrode 11, a voltage application unit 4 that applies a voltage between the anode electrode 11 and the cathode electrode 12, a control unit 13 that controls the voltage application unit 4, a fluid flow generation unit 5 that generates a flow of the fermentation liquid 7 so that the fermentation liquid 7 comes into contact with the anode electrode 11 and then the cathode electrode 12, and a discharge unit that is provided in the fermenter 1 and discharges the fermentation liquid 7 that has come into contact with the anode electrode 11 and then the cathode electrode 12 to the outside of the fermenter 1. With the above configuration, the methane gas production system 1002 of embodiment 3 forms a unidirectional flow field for the fermentation solution 7 so that the fermentation solution 7 comes into contact with the anode electrode 11 and then with the cathode electrode 12, thereby preventing hydrogen produced at the cathode electrode 12 from returning to protons upon contact with the anode electrode 11. This allows the methane gas production system 1002 of embodiment 3 to prevent a decrease in the production amount of hydrogen produced at the cathode electrode 12. Therefore, the methane gas production system 1002 of embodiment 3 can prevent a decrease in the proportion of pathways through which hydrogen-assimilating methanogens produce methane, reducing the risk of an increase in the amount of power required per unit of methane production, and therefore preventing a decrease in the efficiency of methane gas production.

[0068] Furthermore, the control unit 13 of the third embodiment controls the voltage applied by the voltage application unit 4 in accordance with the current value measured by the current measurement unit 16. With the above configuration, the methane gas production system 1002 of the third embodiment can adjust the amount of hydrogen produced at the cathode electrode 12 within a desired range. This makes it possible for the methane gas production system 1002 of the third embodiment to prevent unreacted hydrogen from being sent from the fermentation solution transfer unit 10 to the organic matter oxidation unit 2, coming into contact with the anode electrode 11, and then returning to protons. Therefore, the methane gas production system 1002 of the third embodiment can more accurately control the voltage applied by the voltage application unit 4, thereby reducing the risk of an increase in the amount of power required per unit amount of methane produced and suppressing a decrease in the efficiency of methane gas production.

[0069] Furthermore, the control unit 13 of the third embodiment controls the voltage applied by the voltage application unit 4 based on the current value measured by the current measurement unit 16 and the property measured by the first property measurement unit 17. With the above configuration, the methane gas production system 1002 of the third embodiment can grasp the progress of the oxidation reaction of volatile fatty acids by the power-generating bacteria at the anode electrode 11 and the hydrogen production reaction at the cathode electrode 12 relative to the amount of volatile fatty acids in the fermentation liquor 7. Therefore, the methane gas production system 1002 of the third embodiment can more accurately control the voltage applied by the voltage application unit 4. With the above configuration, the methane gas production system 1002 of the third embodiment can prevent the voltage applied by the voltage application unit 4 from becoming excessively high when the amount of volatile fatty acids in the fermentation liquor 7 is lower than a reference value and the oxidation reaction of volatile fatty acids by the power-generating bacteria is suppressed. As a result, the methane gas generation system 1002 of the third embodiment can suppress the electrolysis of water at the cathode electrode 12 or the generation of chlorine due to the reduction of chloride ions in the fermentation liquid 7. Furthermore, with the above configuration, the methane gas generation system 1002 of the third embodiment can estimate the amount of volatile fatty acids that come into contact with the anode electrode 11, and therefore can more accurately control the voltage applied by the voltage application unit 4.

[0070] Furthermore, the control unit 13 of the third embodiment controls the voltage applied by the voltage application unit 4 based on the current value measured by the current measurement unit 16, the property measured by the first property measurement unit 17, and the second property measurement unit 18. With the above configuration, the methane gas production system 1002 of the third embodiment can grasp the progress of the oxidation reaction of volatile fatty acids by the power-generating bacteria at the anode electrode 11 and the hydrogen production reaction at the cathode electrode 12 relative to the amount of volatile fatty acids in the fermentation liquor 7. Therefore, the methane gas production system 1002 of the third embodiment can more accurately control the voltage applied by the voltage application unit 4. With the above configuration, the methane gas production system 1002 of the third embodiment can adjust the amount of hydrogen produced at the cathode electrode 12 within a desired range in cases where the oxidation reaction of volatile fatty acids by the power-generating bacteria is suppressed due to reduced activity of the power-generating bacteria even though the amount of volatile fatty acids in the fermentation liquor 7 is sufficient. Furthermore, by adopting the above configuration, the methane gas generation system 1002 of embodiment 3 can estimate the amount of hydrogen generated from the cathode electrode 12, and therefore can more accurately control the voltage applied by the voltage application unit 4.

[0071] Furthermore, the control unit 13 of the third embodiment controls the voltage applied by the voltage application unit 4 based on the difference between the pH value measured by the first property measurement unit 17 and the pH value measured by the second property measurement unit 18. With the above configuration, the methane gas generation system 1002 of the third embodiment can grasp the progress of the oxidation reaction of volatile fatty acids by the electricity-generating bacteria at the anode electrode 11 and the hydrogen production reaction at the cathode electrode 12 relative to the amount of volatile fatty acids in the fermentation liquor 7. Therefore, the methane gas generation system 1002 of the third embodiment can more accurately control the voltage applied by the voltage application unit 4. With the above configuration, the methane gas generation system 1002 of the third embodiment can adjust the amount of hydrogen produced at the cathode electrode 12 within a desired range.

[0072] Furthermore, the methane gas production method of the third embodiment includes the steps of: applying a voltage between the anode electrode 11 and the cathode electrode 12 based on the current value measured by the current measurement unit 16, the property measured by the first property measurement unit 17, and the property measured by the second property measurement unit 18; generating a flow of the fermentation liquid 7 by the fluid flow generation unit 5 so that the fermentation liquid 7 comes into contact with the anode electrode 11 and then the cathode electrode 12; producing protons and electrons from organic matter in the fermentation liquid 7 by bringing the anode electrode 11 into contact with the fermentation liquid 7 sent by the flow generated by the fluid flow generation unit 5; producing hydrogen from the protons and electrons by bringing the cathode electrode 12 into contact with the fermentation liquid 7 sent by the flow generated by the fluid flow generation unit 5; and discharging the fermentation liquid 7 that has come into contact with the anode electrode 11 and then the cathode electrode 12 to the outside of the fermenter 1. By adopting the above configuration, the methane gas production method of Embodiment 3 can suppress a decrease in the proportion of pathways in which hydrogen-assimilating methanogens produce methane, as in Embodiment 1, and can reduce the risk of an increase in the amount of electricity required per unit of methane production, thereby suppressing a decrease in methane gas production efficiency. Furthermore, by adopting the above configuration, the methane gas production method of Embodiment 3 can grasp the progress of the oxidation reaction of volatile fatty acids by the power-generating bacteria at the anode electrode 11 and the hydrogen production reaction at the cathode electrode 12 relative to the amount of volatile fatty acids in the fermentation liquor 7.

[0073] Although the first property measurement unit 17 and the second property measurement unit 18 in the third embodiment have been described as pH sensors, they are not limited to this. That is, the property of the fermentation liquor 7 to be measured may be any physical property value that can estimate the amount of volatile fatty acids in the fermentation liquor 7. For example, the first property measurement unit 17 may be a known sensor such as a COD (Chemical Oxygen Demand) sensor. Furthermore, the second property measurement unit 18 may be a known sensor such as a hydrogen gas sensor.

[0074] Furthermore, the first property measurement unit 17 and the second property measurement unit 18 are not limited to one type of sensor each, and may be configured by providing a plurality of types of sensors.

[0075] Fourth Embodiment A methane gas generation system 1003 in a fourth embodiment will be described with reference to FIG. 7. FIG. 7 is a configuration diagram of the methane gas generation system 1003 in the fourth embodiment. The methane gas generation system 1003 in the fourth embodiment differs from the above-described embodiments in that it includes a gas amount measurement unit 20 that measures the amount of biogas generated. The same components as those in the above-described embodiments are assigned the same reference numerals. Further, detailed description of the same components as those in the above-described embodiments will be omitted, and the description will mainly focus on the components that differ from those in the above-described embodiments.

[0076] As shown in FIG. 7 , the methane gas production system 1003 in the fourth embodiment includes a gas storage unit 19, a gas amount measurement unit 20, and a generator 21. The gas storage unit 19 is connected to the biogas discharge unit 9 via an exhaust pipe 110. The biogas discharged via the exhaust pipe 110 is stored in the gas storage unit 19. The gas amount measurement unit 20 is provided between the fermenter 1 and the gas storage unit 19 and measures the amount of biogas generated. The gas amount measurement unit 20 is connected to the control unit 13 via a sixth wiring 111. The gas amount measurement unit 20 inputs the measured amount of biogas generated to the control unit 13. The generator 21 is provided downstream of the gas storage unit 19. The generator 21 generates electricity using the biogas as a raw material.

[0077] The control unit 13 controls the voltage applied by the voltage application unit 4 in accordance with the amount of biogas generated measured by the gas amount measurement unit 20. For example, if the input amount of biogas generated is smaller than a predetermined value, the control unit 13 estimates that the calorific value of the biogas is smaller than a reference value, and controls the voltage applied by the voltage application unit 4 to be higher. Also, for example, if the input amount of biogas generated is larger than a predetermined value, the control unit 13 estimates that the calorific value of the biogas is larger than the reference value, and controls the voltage applied by the voltage application unit 4 to be lower.

[0078] Next, a methane gas generation method according to the fourth embodiment will be described. FIG. 8 is a flowchart showing the methane gas generation method according to the fourth embodiment. Steps S402 to S405 are similar to steps S102 to S105 according to the first embodiment, and therefore detailed description thereof will be omitted. In step S401, the control unit 13 controls the voltage application unit 4 to apply a voltage between the anode electrode 11 and the cathode electrode 12 in accordance with the amount of biogas generated measured by the gas amount measurement unit 20. Note that step S401 is not limited to this order, and may be performed between steps S402 and S403, between steps S403 and S404, or after step S404. This concludes the description of the methane gas generation method according to the fourth embodiment.

[0079] As in the first embodiment, the methane gas production system 1003 of the fourth embodiment includes an anode electrode 11 that comes into contact with the fermentation liquid 7 and generates protons and electrons using organic matter in the fermentation liquid 7, a cathode electrode 12 that generates hydrogen using the protons and electrons generated at the anode electrode 11, a voltage application unit 4 that applies a voltage between the anode electrode 11 and the cathode electrode 12, a control unit 13 that controls the voltage application unit 4, a fluid flow generation unit 5 that generates a flow of the fermentation liquid 7 so that the fermentation liquid 7 comes into contact with the anode electrode 11 and then the cathode electrode 12, and a discharge unit that is provided in the fermenter 1 and discharges the fermentation liquid 7 that has come into contact with the anode electrode 11 and then the cathode electrode 12 to the outside of the fermenter 1. With the above configuration, the methane gas production system 1003 of embodiment 4 forms a unidirectional flow field for the fermentation solution 7 so that the fermentation solution 7 comes into contact with the anode electrode 11 and then with the cathode electrode 12, thereby preventing hydrogen produced at the cathode electrode 12 from returning to protons upon contact with the anode electrode 11. This allows the methane gas production system 1003 of embodiment 4 to prevent a decrease in the production amount of hydrogen produced at the cathode electrode 12. Therefore, the methane gas production system 1003 of embodiment 4 can prevent a decrease in the proportion of pathways through which hydrogen-assimilating methanogens produce methane, reducing the risk of an increase in the amount of power required per unit of methane production, and therefore preventing a decrease in the efficiency of methane gas production.

[0080] Furthermore, the control unit 13 of the fourth embodiment controls the voltage applied by the voltage application unit 4 in accordance with the amount of biogas generated measured by the gas amount measurement unit 20. With the above configuration, the methane gas generation system 1003 of the fourth embodiment can vary the voltage applied by the voltage application unit 4 in accordance with fluctuations in the amount of biogas generated. As a result, when the amount of biogas generated fluctuates due to fluctuations in the amount of organic waste input and seasonal fluctuations in the properties of the organic waste, the methane gas generation system 1003 of the fourth embodiment can adjust the methane concentration of the biogas so that the calorific value of the biogas falls within a desired range. That is, the methane gas generation system 1003 of the fourth embodiment can enhance the effect of increasing the methane concentration of the biogas when the amount of biogas generated is less than a predetermined value. Furthermore, the control unit 13 can weaken the effect of increasing the methane concentration of the biogas when the amount of biogas generated is greater than a predetermined value. Thus, the control unit 13 can level out the calorific value of the biogas regardless of fluctuations in the amount of biogas generated.

[0081] Furthermore, with the above configuration, the methane gas generation system 1003 of embodiment 4 can level out the calorific value of the biogas within a range in which the power generation efficiency and power generation amount of the generator 21 are stable. This allows the methane gas generation system 1003 of embodiment 4 to reduce the risk of the methane concentration in the biogas and the amount of hydrogen generated at the cathode electrode 12 becoming excessive. Therefore, the methane gas generation system 1003 of embodiment 4 can reduce the risk of an increase in the amount of power required to generate methane gas.

[0082] Furthermore, the methane gas production method of the fourth embodiment includes the steps of: applying a voltage between the anode electrode 11 and the cathode electrode 12 in accordance with the amount of biogas generated measured by the gas amount measurement unit 20; generating a flow of the fermentation liquid 7 by the fluid flow generation unit 5 so that the fermentation liquid 7 comes into contact with the anode electrode 11 and then the cathode electrode 12; producing protons and electrons from organic matter in the fermentation liquid 7 by bringing the anode electrode 11 into contact with the fermentation liquid 7 sent by the flow generated by the fluid flow generation unit 5; producing hydrogen from the protons and electrons by bringing the cathode electrode 12 into contact with the fermentation liquid 7 sent by the flow generated by the fluid flow generation unit 5; and discharging the fermentation liquid 7 that has come into contact with the anode electrode 11 and then the cathode electrode 12 to the outside of the fermenter 1. By adopting the above configuration, the methane gas production method of Embodiment 4 can suppress a decrease in the proportion of pathways in which hydrogen-assimilating methanogens produce methane, as in Embodiment 1, and can reduce the risk of an increase in the amount of electricity required per unit of methane production, thereby suppressing a decrease in methane gas production efficiency. Furthermore, by adopting the above configuration, the methane gas production method of Embodiment 4 can level out the calorific value of the biogas within a range in which the power generation efficiency and power generation amount of the generator 21 are stable.

[0083] The methane gas generation system 1003 of embodiment 4 may include a means for measuring the composition of the biogas, such as gas chromatography, in addition to measuring the amount of biogas generated by the gas amount measurement unit 20. With the above configuration, the methane gas generation system 1003 of embodiment 4 can measure the methane concentration in the biogas and more accurately determine the calorific value of the biogas. Therefore, the methane gas generation system 1003 of embodiment 4 can more accurately control the voltage of the voltage application unit 4 in accordance with the calorific value of the biogas and the amount of biogas generated.

[0084] Furthermore, for example, if the first property measurement unit 17 is a pH sensor, the control unit 13 of the fourth embodiment may perform machine learning using the pH value measured by the first property measurement unit 17 and the methane concentration value in the biogas as learning data, and obtain the methane concentration in the biogas as output data. A correlation exists between the pH value measured by the first property measurement unit 17 and the amount of volatile fatty acids in the fermentation liquid 7. A correlation also exists between the amount of volatile fatty acids in the fermentation liquid 7 and the carbon dioxide concentration in the biogas. Furthermore, because biogas is basically composed of methane and carbon dioxide, the control unit 13 can estimate the methane concentration from the carbon dioxide concentration. Therefore, with the above configuration, the control unit 13 of the fourth embodiment can estimate the methane concentration in the biogas by determining the carbon dioxide concentration from the pH value measured by the first property measurement unit 17. Therefore, the methane gas generation system 1003 of the fourth embodiment can more accurately control the voltage of the voltage application unit 4 in accordance with the methane gas concentration and the amount of biogas generated.

[0085] Fifth Embodiment A methane gas generation system 1004 in the fifth embodiment will be described with reference to FIG. 9. FIG. 9 is a configuration diagram of the methane gas generation system 1004 in the fifth embodiment. The methane gas generation system 1004 in the fifth embodiment differs from the above-described embodiments in that it includes a renewable energy power generation facility 22 that generates power using renewable energy. Components similar to those in the above-described embodiments are designated by the same reference numerals. Furthermore, detailed descriptions of components similar to those in the above-described embodiments will be omitted, and components different from those in the above-described embodiments will be mainly described.

[0086] As shown in Figure 9, the methane gas production system 1004 in the fifth embodiment includes a renewable energy power generation facility 22. The renewable energy power generation facility 22 generates electricity by using renewable energy such as solar, wind, and nuclear power. The renewable energy power generation facility 22 is connected to the control unit 13 via an eighth wiring 113. The generator 21 is also connected to the control unit 13 via a seventh wiring 112.

[0087] The control unit 13 causes the voltage application unit 4 to apply a voltage using the power generated by the generator 21 or the renewable energy power generation facility 22. For example, during a time period when the renewable energy power generation facility 22 can operate, the control unit 13 causes the voltage application unit 4 to apply a voltage using the power generated by the renewable energy power generation facility 22. Furthermore, for example, during a time period when the renewable energy power generation facility 22 cannot operate, the control unit 13 causes the voltage application unit 4 to apply a voltage using the power generated by the generator 21.

[0088] Next, a methane gas generation method according to the fifth embodiment will be described. FIG. 10 is a flowchart showing the methane gas generation method according to the fifth embodiment. Steps S502 to S505 are similar to steps S102 to S105 of the first embodiment, and therefore detailed description thereof will be omitted. In step S501, the control unit 13 controls the voltage application unit 4 to apply a voltage between the anode electrode 11 and the cathode electrode 12 using power generated by the generator 21 or the renewable energy power generation facility 22. Note that step S501 is not limited to this order, and may be performed between steps S502 and S503, between steps S503 and S504, or after step S504. This concludes the description of the methane gas generation method according to the fifth embodiment.

[0089] As in the first embodiment, the methane gas production system 1004 of the fifth embodiment includes an anode electrode 11 that comes into contact with the fermentation liquid 7 and generates protons and electrons using organic matter in the fermentation liquid 7, a cathode electrode 12 that generates hydrogen using the protons and electrons generated at the anode electrode 11, a voltage application unit 4 that applies a voltage between the anode electrode 11 and the cathode electrode 12, a control unit 13 that controls the voltage application unit 4, a fluid flow generation unit 5 that generates a flow of the fermentation liquid 7 so that the fermentation liquid 7 comes into contact with the anode electrode 11 and then the cathode electrode 12, and a discharge unit that is provided in the fermenter 1 and discharges the fermentation liquid 7 that has come into contact with the anode electrode 11 and then the cathode electrode 12 to the outside of the fermenter 1. With the above configuration, the methane gas production system 1004 of embodiment 5 forms a unidirectional flow field for the fermentation solution 7 so that the fermentation solution 7 comes into contact with the anode electrode 11 and then with the cathode electrode 12, thereby preventing hydrogen produced at the cathode electrode 12 from returning to protons upon contact with the anode electrode 11. This allows the methane gas production system 1004 of embodiment 5 to prevent a decrease in the production amount of hydrogen produced at the cathode electrode 12. Therefore, the methane gas production system 1004 of embodiment 5 can prevent a decrease in the proportion of pathways through which hydrogen-assimilating methanogens produce methane, reducing the risk of an increase in the amount of power required per unit amount of methane produced, and therefore preventing a decrease in the efficiency of methane gas production.

[0090] Furthermore, the control unit 13 in the fifth embodiment applies a voltage to the voltage application unit 4 using the power generated by the generator 21 or the renewable energy power generation facility 22. With the above configuration, the methane gas generation system 1004 in the fifth embodiment can suppress the power required from outside the system when converting carbon dioxide into methane, thereby reducing power costs. As a result, the methane gas generation system 1004 in the fifth embodiment can reduce the risk of an increase in the power required from outside the system per unit amount of methane generated.

[0091] Furthermore, the methane gas production method of the fifth embodiment includes the steps of applying a voltage between the anode electrode 11 and the cathode electrode 12 using power generated by the generator 21 or the renewable energy power generation facility 22; generating a flow of the fermentation liquid 7 by the fluid flow generation unit 5 so that the fermentation liquid 7 comes into contact with the anode electrode 11 and then the cathode electrode 12; producing protons and electrons from organic matter in the fermentation liquid 7 by bringing the anode electrode 11 into contact with the fermentation liquid 7 sent by the flow generated by the fluid flow generation unit 5; producing hydrogen from the protons and electrons by bringing the cathode electrode 12 into contact with the fermentation liquid 7 sent by the flow generated by the fluid flow generation unit 5; and discharging the fermentation liquid 7 that has come into contact with the anode electrode 11 and then the cathode electrode 12 to the outside of the fermenter 1. By adopting the above configuration, the methane gas production method of embodiment 5 can suppress a decrease in the proportion of pathways in which hydrogen-assimilating methanogens produce methane, as in embodiment 1, and can reduce the risk of an increase in the amount of electricity required per unit of methane production, thereby suppressing a decrease in methane gas production efficiency. Furthermore, by adopting the above configuration, the methane gas production method of embodiment 5 can reduce the risk of an increase in the amount of electricity required from outside the system per unit of methane production.

[0092] The time period during which the renewable energy power generation facility 22 can operate is set as appropriate. For example, the control unit 13 may set a predetermined time and switch between controlling the voltage application unit 4 with the power generated by the generator 21 and controlling the voltage application unit 4 with the power generated by the renewable energy power generation facility 22. The time period during which the renewable energy power generation facility 22 can operate does not need to be set in advance. For example, the control unit 13 may switch to controlling the voltage application unit 4 with the power generated by the generator 21 when the power generated by the renewable energy power generation facility 22 falls below a predetermined value.

[0093] Sixth Embodiment A methane gas generation system 1005 in the sixth embodiment will be described with reference to FIG. 11. FIG. 11 is a configuration diagram of the methane gas generation system 1005 in the sixth embodiment. The methane gas generation system 1005 in the sixth embodiment differs from the above-described embodiments in that it includes a carbon dioxide separation and concentration unit 23 that separates and concentrates carbon dioxide. Components similar to those in the above-described embodiments are designated by the same reference numerals. Furthermore, detailed descriptions of components similar to those in the above-described embodiments will be omitted, and components different from those in the above-described embodiments will be mainly described.

[0094] 11 , the methane gas generation system 1005 in the sixth embodiment includes a carbon dioxide separation and concentration unit 23. The carbon dioxide separation and concentration unit 23 is provided on the carbon dioxide supply pipe 114. The carbon dioxide separation and concentration unit 23 is connected to the hydrogen generation unit 3 via the carbon dioxide supply pipe 114.

[0095] The gas storage unit 19 sends the stored biogas to the carbon dioxide separation and concentration unit 23. The carbon dioxide separation and concentration unit 23 separates and concentrates the carbon dioxide contained in the biogas sent from the gas storage unit 19. The carbon dioxide separation and concentration unit 23 uses known separation techniques such as chemical absorption, adsorption, and membrane separation. The carbon dioxide separation and concentration unit 23 also sends the separated and concentrated carbon dioxide to the vicinity of the cathode electrode 12.

[0096] Next, a methane gas generation method according to the sixth embodiment will be described. FIG. 12 is a flowchart showing the methane gas generation method according to the sixth embodiment. Steps S601 to S605 are similar to steps S101 to S105 of the first embodiment, and therefore detailed description thereof will be omitted. In step S606, the carbon dioxide separation and concentration unit 23 separates and concentrates carbon dioxide in the biogas. In step S607, the carbon dioxide separation and concentration unit 23 sends the separated and concentrated carbon dioxide to the vicinity of the cathode electrode 12. Note that step S606 may be performed before step S601, between steps S601 and S602, between steps S602 and S603, between steps S603 and S604, or between steps S604 and S605. Furthermore, as long as step S607 is performed after step S606, it may be performed before step S601, between step S601 and step S602, between step S602 and step S603, between step S603 and step S604, or between step S604 and step S605. This completes the description of the methane gas production method in embodiment 6.

[0097] As in the first embodiment, the methane gas production system 1005 of the sixth embodiment includes an anode electrode 11 that comes into contact with the fermentation liquid 7 and generates protons and electrons using organic matter in the fermentation liquid 7, a cathode electrode 12 that generates hydrogen using the protons and electrons generated at the anode electrode 11, a voltage application unit 4 that applies a voltage between the anode electrode 11 and the cathode electrode 12, a control unit 13 that controls the voltage application unit 4, a fluid flow generation unit 5 that generates a flow of the fermentation liquid 7 so that the fermentation liquid 7 comes into contact with the anode electrode 11 and then the cathode electrode 12, and a discharge unit that is provided in the fermenter 1 and discharges the fermentation liquid 7 that has come into contact with the anode electrode 11 and then the cathode electrode 12 to the outside of the fermenter 1. With the above configuration, the methane gas production system 1005 of embodiment 6 forms a unidirectional flow field for the fermentation solution 7 so that the fermentation solution 7 comes into contact with the anode electrode 11 and then with the cathode electrode 12, thereby preventing hydrogen produced at the cathode electrode 12 from returning to protons upon contact with the anode electrode 11. This allows the methane gas production system 1005 of embodiment 6 to prevent a decrease in the production amount of hydrogen produced at the cathode electrode 12. Therefore, the methane gas production system 1005 of embodiment 6 can prevent a decrease in the proportion of pathways through which hydrogen-assimilating methanogens produce methane, reducing the risk of an increase in the amount of power required per unit of methane production, and therefore preventing a decrease in the efficiency of methane gas production.

[0098] Furthermore, the carbon dioxide separation and concentration unit 23 of the sixth embodiment sends the separated and concentrated carbon dioxide to the vicinity of the cathode electrode 12. With the above configuration, in the methane gas production system 1005 of the sixth embodiment, carbon dioxide dissolves in the fermentation liquid 7 to form bicarbonate ions, which have a buffering effect. As a result, the methane gas production system 1005 of the sixth embodiment can prevent positively charged ions, such as calcium ions and magnesium ions, in the fermentation liquid 7 from being electrodeposited onto the cathode electrode 12 when the vicinity of the cathode electrode 12 becomes excessively alkaline due to hydrogen production. Therefore, the methane gas production system 1005 of the sixth embodiment can prevent an increase in the voltage required to produce methane gas and can reduce the risk of an increase in the power required per unit amount of methane produced, thereby preventing a decrease in the efficiency of methane gas production.

[0099] Furthermore, the methane gas production method of the sixth embodiment includes the steps of applying a voltage between the anode electrode 11 and the cathode electrode 12 using power generated by the generator 21 or the renewable energy power generation facility 22; generating a flow of the fermentation liquid 7 by the fluid flow generation unit 5 so that the fermentation liquid 7 comes into contact with the anode electrode 11 and then the cathode electrode 12; producing protons and electrons from organic matter in the fermentation liquid 7 by bringing the anode electrode 11 into contact with the fermentation liquid 7 sent by the flow generated by the fluid flow generation unit 5; producing hydrogen from the protons and electrons by bringing the cathode electrode 12 into contact with the fermentation liquid 7 sent by the flow generated by the fluid flow generation unit 5; and discharging the fermentation liquid 7 that has come into contact with the anode electrode 11 and then the cathode electrode 12 to the outside of the fermenter 1. By adopting the above configuration, the methane gas production method of embodiment 6 can suppress a decrease in the proportion of pathways in which hydrogen-assimilating methanogens produce methane, as in embodiment 1, and can reduce the risk of an increase in the amount of electricity required per unit of methane production, thereby suppressing a decrease in methane gas production efficiency. Furthermore, by adopting the above configuration, the methane gas production method of embodiment 6 can suppress an increase in the voltage required for methane gas production, and can reduce the risk of an increase in the amount of electricity required per unit of methane production, thereby suppressing a decrease in methane gas production efficiency.

[0100] Although the carbon dioxide separation and concentration unit 23 has been described as separating and concentrating carbon dioxide in the biogas, the present invention is not limited to this. In other words, the carbon dioxide separation and concentration unit 23 may simply send the separated and concentrated carbon dioxide to the vicinity of the cathode electrode 12. For example, the carbon dioxide separation and concentration unit 23 may separate and concentrate carbon dioxide contained in factory exhaust gas, boiler exhaust gas, or the like.

[0101] 1000, 1001, 1002, 1003, 1004, 1005 Methane gas production system, 1 Fermenter, 2 Organic matter oxidation section, 3 Hydrogen production section, 4 Voltage application section, 5 Fluid flow production section, 6 Organic waste input section, 7 Fermentation liquid, 8 Gas phase, 9 Biogas discharge section, 10 Fermentation liquid transfer section, 11 Anode electrode, 12 Cathode electrode, 13 Control section, 14 Fermentation liquid return section, 15 Fermentation liquid discharge section, 16 Current measurement section, 17 First property measurement section, 18 Second property measurement section, 19 Gas storage section, 20 Gas amount measurement section, 21 Generator, 22 Renewable energy power generation equipment, 23 Carbon dioxide separation and concentration section, 101 Input piping, 102 Transfer piping, 103 First wiring, 104 Return piping, 105 Disposal piping, 106 Second wiring, 107; Third wiring, 108; Fourth wiring, 109; Fifth wiring, 110; Exhaust pipe, 111; Sixth wiring, 112; Seventh wiring, 113; Eighth wiring, 114; Carbon dioxide supply pipe

Claims

1. A methane gas production system that produces methane gas using carbon dioxide in biogas generated from a fermentation liquid produced by fermenting organic waste with microorganisms in a fermenter, comprising: an anode electrode that comes into contact with the fermentation liquid and produces protons and electrons using organic matter in the fermentation liquid; a cathode electrode that comes into contact with the fermentation liquid and produces hydrogen using the protons and electrons produced at the anode electrode; a voltage application unit that applies a voltage between the anode electrode and the cathode electrode; a control unit that controls the voltage application unit; a fluid flow generation unit that generates a flow of the fermentation liquid so that the fermentation liquid comes into contact with the anode electrode and then the cathode electrode; and a discharge unit that is provided in the fermenter and discharges the fermentation liquid that has come into contact with the anode electrode and then the cathode electrode to the outside of the fermenter.

2. The methane gas production system according to claim 1, wherein the anode electrode and the cathode electrode are provided outside the fermenter, and the fluid flow generating unit generates a flow of the fermentation liquid so as to circulate the fermentation liquid through the fermenter, the organic matter oxidation unit provided with the anode electrode, the hydrogen production unit provided with the cathode electrode, and the fermenter in that order.

3. The methane gas production system according to claim 1 or 2, characterized in that the control unit controls the flow rate of the fermentation liquid flow produced by the fluid flow production unit.

4. A methane gas generation system as described in any one of claims 1 to 3, further comprising: a current measurement unit that measures the value of the current flowing between the anode electrode and the cathode electrode; and the control unit controls the voltage applied by the voltage application unit according to the current value measured by the current measurement unit.

5. The methane gas generation system described in claim 4, further comprising: a first property measurement unit that measures the properties of the fermentation liquid before the fermentation liquid comes into contact with the anode electrode; and the control unit controls the voltage applied by the voltage application unit based on the current value measured by the current measurement unit and the property measured by the first property measurement unit.

6. The methane gas generation system described in claim 5, further comprising: a second property measurement unit that measures the properties of the fermentation liquid after the fermentation liquid comes into contact with the cathode electrode; and the control unit controls the voltage applied by the voltage application unit based on the current value measured by the current measurement unit, the property measured by the first property measurement unit, and the property measured by the second property measurement unit.

7. The methane gas generation system described in claim 6, characterized in that the property of the fermentation liquid measured by the first property measurement unit and the second property measurement unit is pH, and the control unit controls the voltage applied by the voltage application unit based on the difference between the pH value measured by the first property measurement unit and the pH value measured by the second property measurement unit.

8. A methane gas generation system as described in any one of claims 1 to 7, further comprising: a gas amount measurement unit that measures the amount of biogas generated; and the control unit controls the voltage applied by the voltage application unit according to the amount of biogas generated measured by the gas amount measurement unit.

9. A methane gas generation system as described in any one of claims 1 to 3, further comprising: a gas amount measurement unit that measures the amount of biogas generated; and a first property measurement unit that measures the property of the fermentation liquid before the fermentation liquid comes into contact with the anode electrode, wherein the control unit estimates the concentration of methane gas contained in the biogas based on the property measured by the first property measurement unit, and controls the voltage applied by the voltage application unit based on the amount of biogas generated and the concentration of methane gas.

10. A methane gas generation system as described in any one of claims 1 to 9, further comprising: a generator that generates electricity using the biogas as a feedstock; and a renewable energy power generation facility that generates electricity using renewable energy, wherein the control unit applies voltage to the voltage application unit using the electricity generated by the generator or the renewable energy power generation facility.

11. A methane gas generation system as described in any one of claims 1 to 10, further comprising: a carbon dioxide separation and concentration unit that separates and concentrates carbon dioxide contained in at least one of the biogas and the exhaust gas, and the carbon dioxide separation and concentration unit sends the separated and concentrated carbon dioxide to the vicinity of the cathode electrode.

12. A method for producing methane gas by using carbon dioxide in biogas generated from a fermentation liquid produced by fermenting organic waste with microorganisms in a fermenter, the method comprising the steps of: applying a voltage between an anode electrode and a cathode electrode; generating a flow of the fermentation liquid so that the fermentation liquid comes into contact with the anode electrode and then the cathode electrode; producing protons and electrons using organic matter in the fermentation liquid by bringing the anode electrode into contact with the fermentation liquid that has been fed; producing hydrogen using the protons and electrons by bringing the cathode electrode into contact with the anode electrode and then with the fermentation liquid that has been fed; and discharging the fermentation liquid that has come into contact with the anode electrode and then the cathode electrode to the outside of the fermenter.

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