Device of producing biogas

Abstract

To provide a device of producing biogas, capable of concurrently causing the methane fermentation and the methanation of organic waste to occur, converting hydrogen and carbon dioxide to methane in a methane fermentation tank, and realizing a more stable and efficient operation than a conventional one.SOLUTION: There is provided a biogas production device comprising a methane fermentation tank 10 being supplied with an organic substance, hydrogen supply means 20 for supplying the methane fermentation tank 10 with hydrogen and control means for controlling the operation, and being configured to be able to carry out methanation in the methane fermentation tank 10. The biogas production device comprises recovery means 40 for recovering biogas generated in the methane fermentation tank 10, gas measurement means 30 disposed in a biogas extraction passage 42, separation means 50 for separating carbon dioxide, and carbon dioxide circulation means 60 for supplying the methane fermentation tank 10 with carbon dioxide. Hydrogen supply is increased from the standard flow rate when gas concentration measured by the gas measurement means 30 is equal to or lower than a predetermined threshold, and hydrogen supply is decreased when the hydrogen concentration exceeds the threshold.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a biogas production apparatus that treats organic matter using methane fermentation.

Background Art

[0002] A technology for converting organic matter contained in sludge and food waste into biogas using methane fermentation is known. Biogas obtained by methane fermentation of organic matter mainly contains methane and carbon dioxide. In recent years, for the purpose of reducing carbon dioxide emissions and using biogas as city gas, the development of so-called biomethanation technology for converting carbon dioxide in biogas into methane by microorganisms has been promoted.

[0003] However, when carbon dioxide in the methane fermentation tank decreases due to methanation, the sludge pH in the methane fermentation tank rises, and stable methane fermentation cannot be carried out. Therefore, some carbon dioxide has to be discharged as biogas with residual carbon dioxide.

[0004] As a biomethanation technology, for example, the method described in Patent Document 1 has been proposed. The method described in Patent Document 1 is so-called in situ biomethanation, in which hydrogen gas is added to the methane fermentation process to increase the methane / carbon dioxide ratio in the generated biogas.

[0005] In addition, Patent Document 2 proposes a technology related to a biogas production system including a methane fermentation tank that generates biogas by methane fermentation and a methanation reaction unit that is connected to the methane fermentation tank and converts carbon dioxide contained in the biogas generated in the methane fermentation tank into methane.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0007] By the way, in in-situ biomethanation, as the conversion of carbon dioxide to methane by methanation proceeds, the concentration of carbon dioxide in the fermentation broth decreases, and the pH of the fermentation broth increases. Then, as the pH of the fermentation broth increases, the concentration of free ammonia in the fermentation broth increases. As a result, the activity of methanogenic bacteria decreases, and the conversion of carbon dioxide to methane by methanation becomes difficult to proceed. That is, in a treatment system using in-situ biomethanation, when methanation proceeds and the pH of the methane fermentation broth increases, the operating efficiency of the system decreases, and ultimately there is a risk that the system itself will stop functioning. In addition, when a configuration in which a methanation reaction section is provided by connecting to a methane fermentation tank is adopted, an increase in the size of the apparatus and a complication of the process are inevitable.

[0008] The present invention has been made in view of the above problems, and an object thereof is to provide a biogas production apparatus that can simultaneously cause methane fermentation and methanation of organic waste, realize the conversion of hydrogen and carbon dioxide to methane in a methane fermentation tank, and achieve a more stable and efficient operation than before.

Means for Solving the Problems

[0009] The characteristic configuration of the biogas production apparatus according to the present invention for achieving the above object is a biogas production apparatus including a methane fermentation section to which an organic substance is supplied, a hydrogen supply means for supplying hydrogen to the methane fermentation section, and a control means for controlling the operation, wherein methanation in the methane fermentation section is configured to be executable, a recovery means for recovering the biogas generated in the methane fermentation section, a separation section for separating carbon dioxide from the biogas recovered by the recovery means, It is provided with carbon dioxide circulation means for supplying the carbon dioxide separated by the separation unit to the methane fermentation unit.

[0010] According to the above characteristic configuration, hydrogen is supplied to the methane fermentation unit by the hydrogen supply means. Therefore, in the methane fermentation unit, while methane fermentation is carried out in the methane fermentation liquid, carbon dioxide in the biogas generated by the methane fermentation can be methanated by methane bacteria. On the other hand, when the methanation of carbon dioxide by methane bacteria (methanation) progresses, the carbon dioxide in the methane fermentation unit decreases, and the pH of the methane fermentation liquid increases. Just reducing the supply flow rate of hydrogen into the methane fermentation unit will take a long time for the increase of carbon dioxide in the methane fermentation unit if the amount of biogas generated by methane fermentation in the methane fermentation unit is small. The above characteristic configuration includes recovery means for recovering the biogas generated in the methane fermentation unit, a separation unit for separating the biogas recovered by the recovery means into carbon dioxide and methane, and carbon dioxide circulation means for returning carbon dioxide to the methane fermentation unit. Therefore, while generating carbon dioxide by methane fermentation in the methane fermentation liquid, carbon dioxide can be separated from the recovered biogas and returned so that the methanation of carbon dioxide by methane bacteria can be carried out, increasing the carbon dioxide in the methane fermentation unit and reducing the pH of the methane fermentation liquid. Therefore, according to the above characteristic configuration, it is possible to simultaneously cause methane fermentation of organic waste and methanation of carbon dioxide while keeping the pH of the methane fermentation liquid low, enabling more stable and efficient operation than before.

[0011] A further characteristic configuration of the biogas production apparatus according to the present invention is equipped with gas measurement means for measuring the gas concentration in the biogas, and the control means adjusts the supply amount of the hydrogen supplied from the hydrogen supply means according to the gas concentration measured by the gas measurement means.

[0012] When the methane concentration in the methane fermentation section increases due to the progress of methanation, the carbon dioxide concentration decreases, the pH of the methane fermentation liquid rises, and there is a risk that the activity of methane bacteria will decrease and the system will stop. However, according to the above characteristic configuration, it is provided with gas measurement means for measuring the gas concentration, and the control means is configured to be able to adjust the hydrogen supply amount from the hydrogen supply means according to the gas concentration. Therefore, the acceleration rate of carbon dioxide in the methane fermentation section can be increased, and the pH of the methane fermentation liquid can be rapidly decreased. Therefore, according to the above characteristic configuration, the pH of the methane fermentation liquid can be kept low, and stable and efficient operation becomes possible.

[0013] A further characteristic configuration of the biogas production apparatus according to the present invention is The gas measurement means is configured to measure at least one concentration of methane, carbon dioxide, and hydrogen.

[0014] According to the above characteristic configuration, by measuring at least one of methane, carbon dioxide, and hydrogen in the biogas generated in the methane fermentation section, the pH of the methane fermentation liquid and the methanation efficiency of methane bacteria in the methane fermentation section can be predicted, and the hydrogen supply amount from the hydrogen supply means can be adjusted. Therefore, according to the above characteristic configuration, the pH of the methane fermentation liquid can be kept low, and stable and efficient operation becomes possible.

[0015] A further characteristic configuration of the biogas production apparatus according to the present invention is The gas measurement means is configured to measure the methane concentration in the biogas, The control means is configured to adjust the supply amount of the hydrogen supplied from the hydrogen supply means so that the methane concentration measured by the gas measurement means becomes equal to or lower than a predetermined threshold value.

[0016] According to the above characteristic configuration, when the methane concentration in the methane fermentation section increases due to the progress of methanation and exceeds the threshold value, while reducing the supply amount of hydrogen by the hydrogen supply means, the carbon dioxide obtained by separating the biogas recovered from the methane fermentation section is supplied to the methane fermentation section. By doing so, the pH of the methane fermentation liquid can be decreased faster until it becomes less than the threshold value than in the case of only decreasing the supply flow rate of hydrogen to the methane fermentation section. Therefore, it is easier to keep the pH of the methane fermentation liquid lower than a certain value, and it is possible to suppress the occurrence of a situation where the activity of methane bacteria decreases due to an increase in the pH of the methane fermentation liquid, the operation efficiency decreases, and the system itself stops functioning. In addition, the time during which methanation is stopped can be shortened compared to the case of only decreasing the supply flow rate of hydrogen to the methane fermentation section. Therefore, according to the above characteristic configuration, the pH of the methane fermentation liquid can be kept lower than a certain value, and a more stable and efficient operation than before is possible.

[0017] A further characteristic configuration of the biogas production apparatus according to the present invention is The control means adjusts the supply amount of the hydrogen supplied from the hydrogen supply means so that the methane concentration measured by the gas measurement means becomes 90% by volume or less.

[0018] When the methane concentration in the biogas generated from the methane fermentation section exceeds 90% by volume, the carbon dioxide concentration in the biogas is less than 10% by volume. Therefore, the pH of the methane fermentation liquid becomes 9 or more, the activity of methane bacteria decreases, the operation efficiency decreases, and there is a risk that the system itself will stop functioning. In order to suppress such a risk, when the methane concentration in the biogas generated from methane fermentation exceeds 90% by volume, the supply of hydrogen by the hydrogen supply means is reduced. On the other hand, since the supply of carbon dioxide by the carbon dioxide circulation means is performed, the pH of the methane fermentation liquid can be decreased more efficiently than in the case of only decreasing the supply flow rate of hydrogen to the methane fermentation section. When the methane concentration is 90% by volume or less, the supply amount of hydrogen by the hydrogen supply means is increased. By doing so, an increase in the pH of the methane fermentation liquid can be prevented and kept lower than a certain value, and a decrease in the activity of methane bacteria can be suppressed.

[0019] A further characteristic configuration of the biogas production apparatus according to the present invention is that the gas measuring means is configured to measure the hydrogen concentration in the biogas in addition to the methane concentration in the biogas, the control means increases the supply amount of hydrogen to be supplied beyond the standard flow rate when the hydrogen concentration measured by the gas measuring means is equal to or less than a predetermined threshold value, and reduces the supply amount of hydrogen to be supplied from the standard flow rate when the hydrogen concentration exceeds the threshold value.

[0020] When the hydrogen concentration exceeds a predetermined threshold value, the activity of the methanogenic bacteria decreases and the operation efficiency decreases, so unreacted hydrogen may remain in the biogas. In such a case, the supply amount of hydrogen by the hydrogen supply means is reduced. On the other hand, since the supply of carbon dioxide by the carbon dioxide circulation means is performed, the pH of the methanogenic fermentation liquid in the methanation section decreases efficiently, improving the activity of the methanogenic bacteria and enabling efficient methanation by the methanogenic bacteria. When the hydrogen concentration is equal to or less than a predetermined threshold value, the activity of the methanogenic bacteria is high and the hydrogen for methanation by the methanogenic bacteria is insufficient, so the supply amount of hydrogen by the hydrogen supply means is increased. By doing so, the activity of the methanogenic bacteria can be kept in a constant state, and the supply amount of hydrogen can be adjusted so that there is no unreacted hydrogen. Therefore, according to the above characteristic configuration, the pH of the methanogenic fermentation liquid is kept lower than a certain value, enabling more stable and efficient operation than before.

[0021] A further characteristic configuration of the biogas production apparatus according to the present invention is that the separation section includes any one of a membrane separation section that separates the gas in the biogas with a separation membrane, a chemical absorption section that absorbs and separates the gas in the biogas with an absorption liquid, and a pressure swing adsorption section that adsorbs and separates the gas in the biogas with an adsorbent.

[0022] According to the above characteristic configuration, only carbon dioxide can be efficiently separated from biogas containing methane and carbon dioxide. Therefore, the rate of increase of carbon dioxide in the methane fermentation section can be accelerated, and the pH of the methane fermentation liquid can be rapidly decreased. Therefore, according to the above characteristic configuration, the pH of the methane fermentation liquid can be kept lower than a certain value, and more stable and efficient operation than before becomes possible.

[0023] A further characteristic configuration of the biogas production apparatus according to the present invention is that it includes storage means for storing the carbon dioxide separated by the separation section, and means for supplying the carbon dioxide stored in the storage means to the methane fermentation section.

[0024] According to the above characteristic configuration, even when the amount of carbon dioxide separated by the separation section is small, by storing carbon dioxide in the storage means, carbon dioxide can be stably returned to the methane fermentation section at a constant flow rate without depending on the amount of biogas in the biogas recovery section. Therefore, according to the above characteristic configuration, the pH of the methane fermentation liquid can be kept lower than a certain value, and more stable and efficient operation than before becomes possible.

Brief Description of the Drawings

[0025]

Figure 1

Figure 2

Figure 3

Embodiments for Carrying Out the Invention

[0026] Hereinafter, a biogas production apparatus according to an embodiment of the present invention will be described with reference to the drawings. In the following, the case where the organic substance to be treated is contained in sludge will be described as an example.

[0027] 〔Configuration of the Biogas Production Apparatus According to the First Embodiment〕 First, the configuration of the biogas production apparatus 1 according to the first embodiment will be described. FIG. 1 is a diagram showing the schematic configuration of the biogas production apparatus 1 according to the first embodiment. As shown in FIG. 1, the biogas production apparatus 1 includes a methane fermentation tank 10 (an example of a methane fermentation section) and a hydrogen supply section 20 (an example of hydrogen supply means) for supplying hydrogen into the methane fermentation tank 10.

[0028] Further, the biogas production apparatus 1 includes a biogas recovery section 40 (an example of recovery means) for recovering the biogas generated in the methane fermentation tank 10, a gas concentration meter 30 (an example of gas measurement means) for measuring the concentration of one or more of methane, carbon dioxide, and hydrogen in the recovered biogas, a separation section 50 for separating the recovered biogas into at least carbon dioxide and methane, a carbon dioxide circulation section 60 (an example of carbon dioxide circulation means) for supplying the carbon dioxide separated by the separation section 50 into the methane fermentation tank 10, and a pH meter 90 for measuring the pH of the methane fermentation liquid in the methane fermentation tank 10, and includes a control device (an example of control means) for controlling the operation of each section.

[0029] As shown in FIG. 1, the methane fermentation tank 10 is composed of a housing. Further, the methane fermentation tank 10 is configured to form a methane fermentation space 11 for biodegrading the sludge supplied from the outside of the housing by methane fermentation by methane bacteria. A heat exchanger (not shown) is provided in this methane fermentation space 11, and the methane fermentation liquid in the methane fermentation space 11 is maintained at a good temperature (for example, 30 to 37°C, or 50 to 60°C) for good methane fermentation by the heat exchanger. In this embodiment, a sludge supply port 15 for supplying sludge faces the methane fermentation space 11 and is provided on one of the two opposing inner walls of the housing. Further, a treated water discharge port 16 for discharging the treated water to the outside faces the methane fermentation space 11 and is provided on the other of the two inner walls.

[0030] The upper space of the methane fermentation space 11 (the space above the liquid level of the methane fermentation liquid in the methane fermentation tank 10) constitutes a biogas collection space 12 for collecting biogas such as methane and carbon dioxide generated in the methane fermentation space 11.

[0031] In the present embodiment, a hydrogen supply port 23 for supplying hydrogen from the hydrogen supply unit 20 is provided facing the methane fermentation space 11 at the bottom of the housing. Further, among the inner walls of the housing, a biogas outlet 41 for taking out biogas to the biogas recovery unit 40 is provided facing the biogas collection space 12 on the inner wall where the treated water discharge port 16 is provided.

[0032] In the present embodiment, the hydrogen supply unit 20 is constituted by a hydrogen cylinder 21 storing hydrogen, a hydrogen supply port 23, a hydrogen supply path 22 (hydrogen supply line) through which hydrogen flows between the hydrogen cylinder 21 and the hydrogen supply port 23, and the like. The hydrogen supply unit 20 supplies the hydrogen in the hydrogen cylinder 21 into the methane fermentation tank 10 through the hydrogen supply path 22 and the hydrogen supply port 23. In the present embodiment, the operation of an on-off valve (not shown) provided in the hydrogen cylinder 21 can be controlled by a control device. Therefore, according to the hydrogen supply unit 20, hydrogen can be supplied to the methane fermentation space 11 in an arbitrary supply amount and at an arbitrary timing. The hydrogen supply unit 20 is not particularly limited as long as it can supply hydrogen into the methane fermentation space 11. For example, a hydrogen production device may be employed instead of the hydrogen cylinder 21.

[0033] In this embodiment, the biogas recovery unit 40 is composed of a biogas outlet 41 and a biogas extraction path 42 to which a booster blower 43 is connected. The biogas recovery unit 40 sucks biogas from within the biogas collection space 12 through the biogas outlet 41 and the biogas extraction path 42 and supplies it to the separation unit 50. In this embodiment, an on-off valve and a flow control valve that can be operationally controlled by a control device are provided in the biogas extraction path 42. Therefore, according to the biogas recovery unit 40, biogas can be recovered from within the biogas collection space 12 in any extraction amount and at any timing. Note that the biogas recovery unit 40 is not particularly limited as long as it is configured to be able to recover biogas from the biogas collection space 12.

[0034] In this embodiment, the separation unit 50 is composed of a separation device 51, a tank 52, a methane outlet 53, a methane extraction path 54 through which methane flows between the tank 52 and the methane outlet 53, and the like. The methane separated by the separation device 51 is stored in the tank 52 through the methane outlet 53 and the methane extraction path 54. In the separation device 51, the recovered biogas can be separated into at least carbon dioxide and methane. The separated carbon dioxide is supplied to the methane fermentation tank 10 by the carbon dioxide circulation unit 60. Note that in this embodiment, the operation of an on-off valve (not shown) and a flow control valve (not shown) provided in the separation device 51 can be controlled by a control device. Therefore, according to the separation unit 50, methane can be stored in the tank 52 and carbon dioxide can be supplied to the carbon dioxide circulation unit 60 in any supply amount and at any timing. Also, hydrogen slightly present in the biogas generated from the methane fermentation tank 10 is separated together with carbon dioxide by the separation device 51 and supplied to the methane fermentation tank 10 by the carbon dioxide circulation unit 60.

[0035] In addition, the separation method used for the separation device 51 is not particularly limited as long as it can separate methane and carbon dioxide. Examples include the membrane separation method (an example of a membrane separation unit) that separates the gas in biogas using a separation membrane, the chemical absorption method (an example of a chemical absorption unit) that absorbs the gas in biogas with an absorbent liquid for separation, the pressure swing adsorption method (an example of a pressure swing adsorption unit) that adsorbs the gas in biogas to an adsorbent for separation, and the cryogenic separation method. These methods can be used alone or in combination.

[0036] The membrane separation method refers to a method of separating gases by utilizing the size and velocity difference of gas molecules, the solubility of gas molecules, and the diffusion velocity difference in the membrane. Organic membranes and inorganic membranes can be used. Examples of organic membranes include polymer membranes, facilitated transport membranes, and ionic liquid-containing membranes, while examples of inorganic membranes include zeolite membranes, silica membranes, and carbon membranes. As polymer membranes, polyimide, cellulose acetate, polysulfone, polycarbonate, etc. can be used. As facilitated transport membranes, molecular gate membranes using polyamidoamine (PAMAM) dendrimer as a CO 2 carrier, amine-supported nanogel particle membranes, etc. can be used. The shape of the separation membrane is not particularly limited, and any shape such as tubular, hollow fiber, monolith, honeycomb, etc. can be adopted. A particularly preferably used separation membrane is a polyimide hollow fiber gas separation membrane.

[0037] The chemical absorption method refers to a method in which biogas is brought into contact with an absorbent liquid to selectively absorb carbon dioxide into the absorbent liquid, and then the absorbent liquid is heated to separate carbon dioxide. As the absorbent liquid, monoethanolamine, methyldiethanolamine, etc. can be used.

[0038] The pressure swing adsorption method refers to a method in which carbon dioxide is adsorbed to an adsorbent under high pressure to obtain high-concentration methane, and then the carbon dioxide adsorbed to the adsorbent is desorbed under low pressure to separate carbon dioxide. As the adsorbent, zeolite, molecular sieve carbon (MSC), etc. can be used.

[0039] The cryogenic separation method refers to a method in which biogas is pressurized and cooled to be liquefied, and the components of biogas are separated by distillation using the difference in temperature when the gas condenses.

[0040] In this embodiment, the carbon dioxide circulation unit 60 includes a carbon dioxide supply port 62, a carbon dioxide circulation path 61, and the like. One end of the carbon dioxide circulation path 61 is connected to the separation device 51, and the other end is connected to the carbon dioxide supply port 62. Further, the carbon dioxide supply port 62 is provided at the bottom of the housing so as to face the methane fermentation space 11. The carbon dioxide circulation unit 60 supplies the carbon dioxide separated by the separation device 51 into the methane fermentation space 11 through the carbon dioxide circulation path 61 and the carbon dioxide supply port 62. In this embodiment, the operation of an on-off valve (not shown) and a flow rate adjustment valve (not shown) provided in the carbon dioxide circulation path 61 can be controlled by a control device. Therefore, according to the carbon dioxide circulation unit 60, the carbon dioxide generated in the methane fermentation tank 10 can be supplied into the methane fermentation space 11 at an arbitrary supply amount and at an arbitrary timing.

[0041] The gas concentration meter 30 is installed in the biogas extraction path 42, configured to measure the gas concentration generated in the methane fermentation tank 10 at regular intervals, and to be able to transmit the measurement results to the control device. The gas measurement means used for the gas concentration meter 30 is not particularly limited as long as it can detect methane, carbon dioxide, and hydrogen, and gas sensors such as an electrochemical sensor, a semiconductor sensor, a thermal conductivity sensor, and a non-dispersive infrared sensor can be used. Further, one gas sensor may detect methane, carbon dioxide, and hydrogen, or each gas may be detected by a plurality of sensors respectively.

[0042] The pH meter 90 is installed in the methane fermentation tank 10, configured to measure the pH of the methane fermentation liquid at regular intervals and transmit it to the control device. When the pH of the methane fermentation liquid exceeds 9, the carbon dioxide concentration in the biogas generated from the methane fermentation tank 10 is less than 10% by volume, and the methane concentration exceeds 90% by volume. Also, when the pH of the methane fermentation liquid is 9 or less, the carbon dioxide concentration in the biogas generated from the methane fermentation tank 10 is 10% by volume or more, and the methane concentration is 90% by volume or less. It is configured to monitor the pH of the methane fermentation liquid and be able to control the hydrogen supply amount by the hydrogen supply means so as to maintain the pH of the methane fermentation liquid at a constant value.

[0043] In the methane fermentation tank 10 having the above configuration, carbon dioxide separated from the biogas generated in the methane fermentation space 11 is returned to the methane fermentation space 11. It is methanated by methane bacteria in the methane fermentation liquid using the carbon dioxide returned to the methane fermentation space 11 and the hydrogen supplied from the hydrogen supply passage 22. Furthermore, by providing means for adjusting the hydrogen supply amount according to the gas concentration measured by the gas concentration meter 30, stable and efficient control of methane production is made possible.

[0044] 〔Processing Flow of Biogas Production Apparatus According to First Embodiment〕 Subsequently, the process of treating sludge by the biogas production apparatus 1 having the above configuration will be described. First, the influence of the conversion of carbon dioxide to methane by methanation in the methane fermentation tank 10 on the operation of the system will be described.

[0045] In the methane fermentation tank 10, when the conversion of carbon dioxide to methane by methanation proceeds, the carbon dioxide concentration in the methane fermentation liquid decreases, the pH of the methane fermentation liquid rises, and the concentration of free ammonia in the methane fermentation liquid increases. Since free ammonia reduces the activity of methane bacteria, when the conversion of carbon dioxide to methane by methanation proceeds, the activity of methane bacteria decreases, making it difficult for methanation to proceed. Therefore, when the pH of the methane fermentation liquid rises due to the progress of methanation, the operating efficiency of the system decreases, and ultimately the system itself may stop functioning.

[0046] In order to enable the stable and efficient operation of the organic matter treatment system, it is effective to take measures to adjust the pH of the methane fermentation liquid so that free ammonia that significantly reduces the activity of methane bacteria does not occur. Specifically, when the methane concentration in the methane fermentation tank increases, it is effective to reduce the supply amount of hydrogen into the methane fermentation tank 10 to suppress the decrease in carbon dioxide, or to supply carbon dioxide into the methane fermentation tank 10. By doing so, the increased pH of the methane fermentation liquid can be lowered, and the decrease in the activity of methane bacteria can be suppressed.

[0047] Therefore, in the biogas production apparatus 1 according to the present embodiment, the gas concentration meter 30 measures the concentration of the gas generated in the methane fermentation tank 10, and the hydrogen supply flow rate by the hydrogen supply unit 20 is adjusted according to the measured gas concentration. Specifically, when the methane concentration is equal to or lower than the threshold value (90% by volume in this example), hydrogen is supplied into the methane fermentation tank 10 at a flow rate (standard flow rate) sufficient for the conversion of carbon dioxide to methane. On the other hand, when the methane concentration exceeds the threshold value, the supply of hydrogen into the methane fermentation tank 10 is reduced. Further, in the present embodiment, when the supply of hydrogen is reduced, the supply of hydrogen at the standard flow rate is restarted when the methane concentration becomes 90% by volume or less thereafter.

[0048] Therefore, in the biogas production apparatus 1 according to the present embodiment, first, sludge is supplied to the methane fermentation tank 10, and hydrogen is supplied to the inside of the methane fermentation tank 10 at a standard flow rate by the hydrogen supply unit 20. As a result, the sludge supplied to the methane fermentation tank 10 is methane-fermented in the methane fermentation tank 10, and biogas is generated.

[0049] Subsequently, the generated biogas uses hydrogen supplied from the hydrogen supply unit 20 in the methane fermentation space 11, and methane bacteria in the methane fermentation liquid methanate (methanation) a part of the carbon dioxide in the biogas. The biogas in which a part of the carbon dioxide has been methanated is supplied to the separation unit 50 through the biogas outlet 41 and the biogas outlet passage 42. The carbon dioxide separated by the separation device 51 is returned to the inside of the methane fermentation tank 10 through the carbon dioxide circulation unit 60. The separated methane is stored in the tank 52 through the methane outlet 53 and the methane outlet passage 54. On the other hand, the treated water generated by methane fermentation is discharged to the outside from the treated water outlet 16.

[0050] As methanation proceeds in the methane fermentation space 11 and the amount of carbon dioxide contained in the biogas decreases, the concentration of methane increases. Then, when the gas concentration measured by the gas concentration meter 30 exceeds the threshold value (in this embodiment, the methane concentration is 90% by volume), the supply of hydrogen by the hydrogen supply unit 20 is decreased.

[0051] Thereby, the decrease in carbon dioxide due to methanation is suppressed, and the return of the carbon dioxide separated by the separation device 51 by the carbon dioxide circulation unit 60 and the generation of biogas containing carbon dioxide by methane fermentation cause the acceleration rate of carbon dioxide in the methane fermentation tank 10 to be high, and the pH of the methane fermentation liquid to decrease quickly. Therefore, it becomes possible to maintain the pH of the methane fermentation liquid lower than a certain value.

[0052] After that, when the methane concentration measured by the gas concentration meter 30 becomes 90% by volume or less, the supply of hydrogen into the methane fermentation tank 10 by the hydrogen supply unit 20 is increased. As a result, in the methane fermentation space 11, the methanation of carbon dioxide in the biogas is performed again.

[0053] Thus, according to the biogas production apparatus 1 according to the present embodiment, carbon dioxide is separated from the biogas generated in the methane fermentation tank 10 by the separation device 51, the separated carbon dioxide is returned to the methane fermentation tank 10, and methanation is performed using the hydrogen supplied to the methane fermentation tank 10. When the gas concentration in the methane fermentation tank 10 exceeds the threshold due to the progress of methanation, the supply of hydrogen into the methane fermentation tank 10 is decreased, and the methane concentration in the methane fermentation tank 10 can be decreased until it becomes below the threshold. Therefore, it is possible to suppress the occurrence of a situation where the activity of methane bacteria decreases due to an increase in the pH of the methane fermentation liquid, the operation efficiency decreases, and the system itself no longer functions, or a situation where unreacted hydrogen remains. Therefore, in the biogas production apparatus 1, methane fermentation and methanation can occur simultaneously in the methane fermentation section, and stable and efficient operation is possible.

[0054] 〔Modification Example of Biogas Production Apparatus According to First Embodiment〕 In the above embodiment, the methane concentration generated in the methane fermentation tank 10 is measured by the gas concentration meter 30, and the supply flow rate of hydrogen by the hydrogen supply unit 20 is adjusted according to the measured methane concentration. In addition to that, in the modification example, the supply flow rate of hydrogen is adjusted according to the hydrogen concentration and carbon dioxide concentration measured by the gas concentration meter 30. Specifically, when the methane concentration is below the threshold and the hydrogen supply flow rate is at the standard flow rate, if the hydrogen concentration is below the threshold (for example, 0.1% by volume (1000 ppm)), the hydrogen supplied into the methane fermentation tank 10 is increased from the standard flow rate. On the other hand, when the hydrogen concentration exceeds the threshold, the supply of hydrogen into the methane fermentation tank 10 is decreased. Also, when the methane concentration is below the threshold value and the hydrogen supply flow rate is at the standard flow rate, if the carbon dioxide concentration exceeds the threshold value (for example, 10% by volume), the hydrogen supply to the methane fermentation tank 10 is increased from the standard flow rate. On the other hand, when the carbon dioxide concentration is below the threshold value, the supply of hydrogen to the methane fermentation tank 10 is decreased.

[0055] 〔Configuration of the biogas production apparatus according to the second embodiment〕 Next, the configuration of the biogas production apparatus 70 according to the second embodiment will be described. FIG. 2 is a diagram showing the schematic configuration of the biogas production apparatus 70 according to the second embodiment. As shown in FIG. 2, the biogas production apparatus 70 according to the second embodiment is different from the first embodiment in that it mainly includes a carbon dioxide storage unit 80 (carbon dioxide storage means). Hereinafter, the biogas production apparatus 70 according to the second embodiment will be described, but the description of the same configuration as that of the biogas production apparatus 1 according to the first embodiment will be omitted.

[0056] As shown in FIG. 2, the biogas production apparatus 70 according to the second embodiment includes a carbon dioxide storage unit 80 that stores the carbon dioxide separated by the separation device 51 in the tank 81 via the carbon dioxide circulation path 61 and supplies the stored carbon dioxide into the methane fermentation tank 10 through the carbon dioxide supply port 62, and includes a control device (an example of control means) that controls the operation of each part.

[0057] In the present embodiment, the carbon dioxide storage unit 80 is composed of a tank 81 (storage means), a carbon dioxide supply path 82, and the like. One end of the carbon dioxide supply path 82 is connected to the tank 81, and the other end is connected to the bottom of the methane fermentation tank 10. Further, the separation device 51 is composed of a pressure swing adsorption unit that adsorbs carbon dioxide. In the present embodiment, the operation of an on-off valve (not shown) and a flow rate adjustment valve (not shown) provided in the carbon dioxide supply path 82 can be controlled by the control device. Therefore, according to the carbon dioxide storage unit 80, the biogas stored in the tank 81 can be supplied into the methane fermentation space 11 at an arbitrary supply amount and an arbitrary timing.

[0058] Furthermore, the tank 81 is provided separately from the tank 52 that constitutes the separation unit 50. In the present embodiment, carbon dioxide separated by the separation device 51 is stored in the tank 81.

[0059] 〔Treatment Flow of Biogas Production Apparatus According to Second Embodiment〕 Subsequently, the process of treating sludge by the biogas production apparatus 70 having the above configuration will be described.

[0060] In order to enable stable and efficient operation of the biogas production apparatus, it is effective to return carbon dioxide into the methane fermentation tank 10. However, the flow rate of carbon dioxide separated by the separation unit 50 may fluctuate, and the amount of carbon dioxide that can be returned into the methane fermentation tank 10 may be insufficient. In such a case, the amount of carbon dioxide returned into the methane fermentation tank 10 decreases, and there is a risk that the pH of the methane fermentation liquid increases and the system stops. Also, there is a possibility that unreacted hydrogen remaining in the methane fermentation tank 10.

[0061] Therefore, in the present embodiment, a tank 81 is provided to store carbon dioxide separated by the separation device 51. The stored carbon dioxide is returned into the methane fermentation tank 10 via the carbon dioxide supply line 82.

[0062] Thereby, similar to the case of the first embodiment, the increasing speed of carbon dioxide in the methane fermentation tank 10 becomes faster compared to the case where only the supply of hydrogen is stopped, and the pH of the methane fermentation liquid decreases more efficiently. Therefore, in the biogas production apparatus 70 according to the present embodiment, carbon dioxide can be returned into the methane fermentation tank 10 at a constant flow rate, and stable methanation can be performed.

[0063] 〔Configuration of Biogas Production Apparatus According to Third Embodiment〕 Next, the configuration of the biogas production apparatus 100 according to the third embodiment will be described. The methane fermentation section includes a methane fermentation tank 10 and a methanation tank 110, which is different from the first and second embodiments. The methanation tank 110 is connected to the subsequent stage of the methane fermentation tank 10. Hereinafter, the biogas production apparatus 100 according to the third embodiment will be described, but the description of the same configurations as those of the biogas production apparatuses 1 and 70 according to the first and second embodiments will be omitted.

[0064] As shown in FIG. 3, the biogas production apparatus 100 according to the third embodiment includes a methane fermentation tank 10 to which sludge is supplied, a methanation tank 110 connected to the subsequent stage of the methane fermentation tank 10, and supplies the biogas and digested sludge generated in the methane fermentation tank 10 to the methanation tank 110. Further, it includes a hydrogen supply section 20 that supplies hydrogen into the methanation tank 110, a biogas recovery section 40 that recovers the biogas generated in the methanation tank 110, a gas concentration meter 30 installed in the biogas extraction path 42, and a carbon dioxide circulation section 60 that supplies the carbon dioxide separated by the separation section 50 into the methanation tank 110, and includes a control device (an example of control means) that controls the operations of each section.

[0065] As shown in FIG. 3, the methanation tank 110 is composed of a housing. The methanation tank 110 is configured to form a methanation space 111 in which the digested sludge supplied from the methane fermentation tank 10 is biodegraded by methanogenic bacteria and methane and hydrogen are methanated. A heat exchanger (not shown) is provided in this methanation space 111, and the digested sludge in the methanation space 111 is maintained at a favorable temperature (for example, 30 to 37°C, or 50 to 60°C) for good methanation by the heat exchanger. In this embodiment, on one of the two opposing inner walls of the housing, a digested sludge supply port 115 for supplying the sludge generated in the methane fermentation tank 10 faces the methanation space 111, and a biogas supply port 113 for supplying the biogas generated in the methane fermentation tank 10 faces the methanation space 111. Also, on the other of the two inner walls, a methanation treated water discharge port 116 for discharging the treated water to the outside faces the methanation space 111.

[0066] The upper space of the methanation space 111 (the space above the liquid level of the methane fermentation liquid in the methanation tank 110) constitutes a biogas collection space 112 for collecting biogas such as methane and carbon dioxide generated in the methanation space 111.

[0067] In this embodiment, at the bottom of the methanation tank 110, a hydrogen supply port 23 for supplying hydrogen from the hydrogen supply unit 20 faces the methanation space 111. Also, on the inner wall of the housing where the methanation treated water discharge port 116 is provided, a biogas extraction port 41 for extracting biogas to the biogas recovery unit 40 faces the biogas collection space 112.

[0068] In this embodiment, the carbon dioxide circulation unit 60 is composed of a carbon dioxide supply port 62, a carbon dioxide circulation path 61, and the like. One end of the carbon dioxide circulation path 61 is connected to the separation device 51, and the other end is connected to the carbon dioxide supply port 62. Further, the carbon dioxide supply port 62 is provided at the bottom of the housing facing the methanation space 111. The carbon dioxide circulation unit 60 supplies the carbon dioxide separated by the separation device 51 into the methanation space 111 through the carbon dioxide circulation path 61 and the carbon dioxide supply port 62. In this embodiment, the operation of an on-off valve (not shown) and a flow rate adjustment valve (not shown) provided in the carbon dioxide circulation path 61 can be controlled by a control device. Therefore, according to the carbon dioxide circulation unit 60, the carbon dioxide generated in the methanation tank 110 can be supplied into the methanation space 111 at an arbitrary supply amount and at an arbitrary timing.

[0069] 〔Treatment Flow of Biogas Production Apparatus According to Third Embodiment〕 Subsequently, the process of treating sludge by the biogas production apparatus 100 having the above configuration will be described.

[0070] In this embodiment, the biogas and digested sludge generated by methane fermentation in the methane fermentation tank 10 are supplied to the methanation tank 110. Subsequently, using the hydrogen supplied by the hydrogen supply unit 20 in the methanation tank 110, a part of the carbon dioxide in the biogas is methanated by the methane bacteria in the methanation tank 110. The biogas in which a part of the carbon dioxide has been methanated is supplied to the separation unit 50 through the biogas outlet 41 and the biogas outlet path 42. The carbon dioxide separated by the separation device 51 is returned into the methanation tank 110 through the carbon dioxide circulation unit 60, and the separated methane is stored in the tank 52 through the methane outlet 53 and the methane outlet path 54. On the other hand, the treated water generated by methane fermentation is discharged to the outside from the methanation treated water outlet 116.

[0071] Thus, in the biogas production apparatus 100 according to the present embodiment, since methane fermentation and methanation are performed in separate tanks, it is possible to prevent the pH of the methane fermentation liquid from rising due to a decrease in the carbon dioxide concentration as methanation progresses. Therefore, the biogas production apparatus 100 can operate more stably and efficiently.

[0072] 〔Simulation of the amount of hydrogen required for methanation〕 Regarding the amount of methane and the amount of hydrogen required when carbon dioxide is circulated and when it is not circulated, the following preconditions and definitions were set, and the case of FIG. 1 was simulated as an example. 〔Preconditions〕 Precondition 1: It is assumed that in the separation section after the methane fermentation tank, it is completely separated into methane and carbon dioxide. Precondition 2: It is assumed that the hydrogen supplied to the methane fermentation tank completely reacts with carbon dioxide and is converted into methane. 〔Definitions〕 Flow rate: Q (m 3 / h), methane concentration: C (volume %), carbon dioxide concentration: C’ (volume %) · The amount of biogas derived from sludge (before hydrogen reaction) generated in the methane fermentation tank is Q1, and among them, the methane concentration is C1 (volume %), and the carbon dioxide concentration is C’1 (volume %). · In the gas separation section after the methane fermentation tank, the amount recovered with carbon dioxide as the main component is Q2, and among them, the methane concentration is C2 (0 volume %), and the carbon dioxide concentration is C’2 (100 volume %). · In the separation section after the methane fermentation tank, the amount recovered with methane as the main component is Q3, and among them, the methane concentration is C3 (100 volume %), and the carbon dioxide concentration is C’3 (0 volume %). · The amount of hydrogen required to react carbon dioxide to methane is Q4. · The amount of biogas finally generated from the outlet of the methane fermentation tank is Q, and among them, the methane concentration is C, and the carbon dioxide concentration is C’.

[0073] <When carbon dioxide is not circulated> The amount of hydrogen required for methane production is four times the amount of carbon dioxide. Therefore, the amount of hydrogen required to perform methanation to a predetermined concentration is calculated by the following formula. Q1×(C’1 - C’)×4…(A)

[0074] <When carbon dioxide is circulated> The material balance of methane and carbon dioxide in the separation section is as follows. Q = Q2 + Q3…(B) Methane: From the prerequisite 1, Q×C = Q3×C3 Here, since C3 is 100% by volume, that is, 1, Q3 = Q×C. Carbon dioxide: Q×C’ = Q2×C’2 Here, since C’2 is 100% by volume, that is, 1, Q2 = Q×C’.

[0075] The material balance of methane at the outlet of the methanation tank and in the separation section is as follows. Q = Q1 + Q2…(C) From (B) and (C), Q3 is obtained by the following formula. Q1 = Q3…(D) Since the amount of methane at the outlet of the methanation tank is the same as the amount of methane separated in the separation section, Q3×C3 = Q×C Here, since Q1 = Q3 and C3 is 100% by volume, Q1 = (Q1 + Q2)×C Therefore, Q2 is obtained by the following formula. Q2 = (1 - C)×Q1 / C…(E)

[0076] The amount of CO present in the methanation tank 2 is Q1×C’1 + Q2×C’2. The amount of hydrogen required to perform methanation to a predetermined concentration is obtained by the following formula. (Q1×C’1 + Q2×C’2 - Q×C’)×4…(F)

[0077] Here, taking the following conditions as an example, the hydrogen amount required and the methane amount produced were calculated for the case where carbon dioxide is not circulated and the case where it is circulated. <Condition> Q1: 100 (m 3 / h), C1: 60 (volume %), C’1: 40 (volume %) C:C’ = 85 (volume %): 15 (volume %)

[0078] <Case where carbon dioxide is not circulated> The required hydrogen amount is, from formula (i) 100×(0.40 - 0.15)×4 = 100 (m 3 / h). The methane amount obtained is 100×0.85 = 85 (m 3 / h).

[0079] <Case where carbon dioxide is circulated> The methane amount obtained is, from formula (ii) Q3 = 100 (m 3 / h). The carbon dioxide amount recovered in the gas separation section is, from formula (ho) Q2 = 17.65 (m 3 / h). The flow rate Q is, from formula (b) 100 + 17.65 = 117.65 (m 3 / h). The required hydrogen amount is from formula (he) (100×0.4 + 17.65×1 - 117.65×0.15)×4 = 160 (m 3 / h).

[0080] Thus, when adopting a system that separates and circulates carbon dioxide, the methane production amount can be increased compared to the case where carbon dioxide is not circulated.

[0081] <Another embodiment>

[0082] In each of the above embodiments, sludge was treated, but it is not limited thereto, and a mode of treating food waste or the like may also be used.

[0083] In each of the above embodiments, the hydrogen supply port 23 and the carbon dioxide supply port 62 are provided so as to face the methane fermentation space 11 at the bottom of the housing constituting the methane fermentation tank 10. However, the present invention is not limited to this. For example, the hydrogen supply port 23 and the carbon dioxide supply port 62 may be provided so as to face the biogas collection space 12 on the ceiling of the housing.

[0084] In addition, the configurations disclosed in the above embodiments (including other embodiments, the same applies hereinafter) can be applied in combination with the configurations disclosed in other embodiments as long as no contradiction occurs. Further, the embodiments disclosed in this specification are examples, and the embodiments of the present invention are not limited thereto, and can be appropriately modified within the scope not departing from the object of the present invention.

Industrial Applicability

[0085] The present invention can be used in a biogas production apparatus that treats organic substances using methane fermentation.

Explanation of Reference Numerals

[0086] 1, 70, 100: Biogas production apparatus 10: Methane fermentation tank (methane fermentation section) 20: Hydrogen supply section (hydrogen supply means) 30: Gas concentration meter (gas measurement means) 40: Biogas recovery section (recovery means) 50: Separation section 60: Carbon dioxide circulation section (carbon dioxide circulation means) 80: Carbon dioxide storage section (storage means) 90: pH meter 110: Methanation tank (methane fermentation section)

Claims

1. A biogas production apparatus comprising a methane fermentation section to which organic matter is supplied, a hydrogen supplying means for supplying hydrogen to the methane fermentation section, and a control means for controlling operation, the biogas production apparatus being configured to be capable of methanation in the methane fermentation section, A recovery means for recovering the biogas generated in the methane fermentation section; A separation unit that separates carbon dioxide from the biogas recovered by the recovery means; and a carbon dioxide circulation means for supplying the carbon dioxide separated in the separation section to the methane fermentation section.

2. A gas measuring means for measuring a gas concentration in the biogas is provided, 2. The biogas production apparatus according to claim 1, wherein the control means adjusts the amount of hydrogen supplied from the hydrogen supply means in accordance with the gas concentration measured by the gas measurement means.

3. The biogas producing apparatus according to claim 2 , wherein the gas measuring means measures a concentration of at least one of methane, carbon dioxide, and hydrogen.

4. The gas measurement means is configured to measure a methane concentration in the biogas; 4. The biogas production apparatus according to claim 2 or 3, wherein the control means adjusts the amount of hydrogen supplied from the hydrogen supply means so that the methane concentration measured by the gas measurement means is equal to or lower than a predetermined threshold value.

5. 5. The biogas production apparatus according to claim 4, wherein the control means adjusts the amount of hydrogen supplied from the hydrogen supply means so that the methane concentration measured by the gas measurement means is 90 volume % or less.

6. The gas measurement means is configured to measure a hydrogen concentration in the biogas in addition to the methane concentration in the biogas; 5. The biogas production apparatus according to claim 4, wherein the control means increases the amount of hydrogen supplied from a standard flow rate when the hydrogen concentration measured by the gas measurement means is equal to or lower than a predetermined threshold, and decreases the amount of hydrogen supplied from the standard flow rate when the hydrogen concentration exceeds the threshold.

7. 4. The biogas production apparatus according to claim 2 or 3, wherein the separation unit comprises any one of a membrane separation unit that separates gas in the biogas using a separation membrane, a chemical absorption unit that absorbs the gas in the biogas in an absorption liquid and separates it, and a pressure swing adsorption unit that adsorbs the gas in the biogas into an adsorbent and separates it.

8. 4. The biogas production apparatus according to claim 2, further comprising a storage means for storing the carbon dioxide separated in the separation section, and a means for supplying the carbon dioxide stored in the storage means to the methane fermentation section.

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