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 device that processes organic matter using methane fermentation. [Background technology]

[0002] There is a known technology for converting organic matter contained in sludge and food waste into biogas using methane fermentation. Biogas obtained through methane fermentation of organic matter mainly contains methane and carbon dioxide. In recent years, development of so-called biomethanation technology, which converts carbon dioxide in biogas into methane using microorganisms, has been progressing with the aim of reducing carbon dioxide emissions and using biogas as city gas.

[0003] However, when the carbon dioxide in the methane fermentation tank decreases due to methanation, the pH of the sludge in the tank rises, making stable methane fermentation impossible, and some of the carbon dioxide must be discharged as remaining biogas.

[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 a so-called in situ biomethanation method in which hydrogen gas is added to the methane fermentation process to increase the methane / carbon dioxide ratio in the biogas produced.

[0005] Furthermore, Patent Document 2 proposes technology relating to a biogas production system that includes a methane fermentation tank that generates biogas through 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] Special Publication No. 2019-525888 [Patent Document 2] Japanese Patent Application Laid-Open No. 2016-108382 Summary of the Invention [Problem to be solved by the invention]

[0007] In in situ biomethanation, as the conversion of carbon dioxide to methane by methanation progresses, the carbon dioxide concentration in the fermentation broth decreases and the pH of the fermentation broth increases. The increase in pH of the fermentation broth then increases the free ammonia concentration in the fermentation broth. As a result, the activity of methanogens decreases, making it difficult to convert carbon dioxide to methane by methanation. In other words, in a treatment system that uses in situ biomethanation, if the pH of the methane fermentation broth increases as methanation progresses, the system's operating efficiency decreases, and the system itself may eventually cease to function. Furthermore, if a methanation reaction section is connected to a methane fermentation tank, the system inevitably becomes larger and the process becomes more complicated.

[0008] The present invention has been made in view of the above-mentioned problems, and its purpose is to provide a biogas production apparatus that simultaneously causes methane fermentation and methanation of organic waste, converts hydrogen and carbon dioxide into methane in a methane fermentation tank, and can operate more stably and efficiently than conventional apparatus. [Means for solving the problem]

[0009] The characteristic configuration of the biogas production device according to the present invention to achieve the above object is as follows: A biogas production apparatus comprising a methane fermentation unit to which organic matter is supplied, a hydrogen supply means for supplying hydrogen to the methane fermentation unit, and a control means for controlling operation, the biogas production apparatus being configured so that methanation can be performed in the methane fermentation unit, 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; a carbon dioxide circulation means for supplying the carbon dioxide separated in the separation section to the methane fermentation section, and a gas measurement means for measuring a gas concentration in the biogas, 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; the gas measurement means is configured to measure a methane concentration in the biogas; 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 less than a predetermined threshold value.

[0010] According to the above-described characteristic configuration, hydrogen is supplied to the methane fermentation unit by the hydrogen supply means, and therefore, while methane fermentation is carried out in the methane fermentation liquid in the methane fermentation unit, carbon dioxide in the biogas generated by the methane fermentation can be methanated by the methanogens. On the other hand, as methanation of carbon dioxide by methane bacteria progresses, the carbon dioxide in the methane fermentation section decreases and the pH of the methane fermentation liquid rises. If the amount of biogas generated by methane fermentation in the methane fermentation section is small, simply reducing the flow rate of hydrogen supplied to the methane fermentation section will take a long time for the carbon dioxide in the methane fermentation section to increase. The above-mentioned characteristic configuration includes a recovery means for recovering biogas generated in the methane fermentation unit, a separation unit for separating the biogas recovered by the recovery means into carbon dioxide and methane, a carbon dioxide circulation means for returning the carbon dioxide to the methane fermentation unit, and a gas measurement means for measuring the gas concentration, and the control means is configured to adjust the amount of hydrogen supplied from the hydrogen supply means in accordance with the gas concentration. Therefore, by separating and returning carbon dioxide from the recovered biogas so that carbon dioxide can be generated by methane fermentation in the methane fermentation liquid and the carbon dioxide can be methanated by methanogens, the carbon dioxide in the methane fermentation unit can be increased and the pH of the methane fermentation liquid can be lowered. Furthermore, when the methane concentration in the methane fermentation section increases and exceeds a threshold value as methanation progresses, the amount of hydrogen supplied by the hydrogen supply means can be reduced while carbon dioxide obtained by separating biogas recovered from the methane fermentation section is supplied to the methane fermentation section. This allows the pH of the methane fermentation liquid to be lowered below the threshold value more quickly than when the flow rate of hydrogen supplied to the methane fermentation section is simply reduced. This makes it easier to maintain the pH of the methane fermentation liquid below a certain value, preventing situations in which an increase in the pH of the methane fermentation liquid reduces the activity of methanogens, reduces operating efficiency, and causes the system itself to malfunction. Furthermore, the time that methanation is stopped can be shortened compared to when the flow rate of hydrogen supplied to the methane fermentation section is simply reduced.

[0011] Therefore, according to the above characteristic configuration, it is possible to simultaneously cause methane fermentation of organic waste and methanation of carbon dioxide while maintaining a low pH of the methane fermentation liquid, enabling more stable and efficient operation than conventional methods.

[0012] As methanation progresses, the methane concentration in the methane fermentation section increases, which reduces the carbon dioxide concentration and increases the pH of the methane fermentation liquid, reducing the activity of methanogens and putting the system at risk of shutting down. However, according to the above-mentioned characteristic configuration, a gas measuring means for measuring the gas concentration is provided, and the control means is configured to adjust the amount of hydrogen supplied from the hydrogen supply means in accordance with the gas concentration, thereby accelerating the rate of increase in carbon dioxide in the methane fermentation section and quickly decreasing the pH of the methane fermentation liquid. Therefore, according to the above characteristic configuration, the pH of the methane fermentation liquid can be kept low, enabling stable and efficient operation.

[0013] Further characteristic configurations of the biogas production apparatus according to the present invention include: The gas measuring means measures the concentration of at least one 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, it is possible to predict the pH of the methane fermentation liquid in the methane fermentation section and the methanation efficiency of the methane bacteria, and adjust the amount of hydrogen supplied from the hydrogen supply means. Therefore, according to the above characteristic configuration, the pH of the methane fermentation liquid can be kept low, enabling stable and efficient operation.

[0015] Further characteristic configurations of the biogas production apparatus according to the present invention include: the gas measurement means is configured to measure a methane concentration in the biogas; 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 less than a predetermined threshold value.

[0016] According to the above-described characteristic configuration, when the methane concentration in the methane fermentation unit increases and exceeds a threshold value as methanation progresses, the amount of hydrogen supplied by the hydrogen supply means is reduced while carbon dioxide obtained by separating biogas recovered from the methane fermentation unit is supplied to the methane fermentation unit. This allows the pH of the methane fermentation liquid to be lowered below the threshold value more quickly than when the flow rate of hydrogen supplied to the methane fermentation unit is simply reduced. This makes it easier to maintain the pH of the methane fermentation liquid below a certain value, preventing situations in which an increase in the pH of the methane fermentation liquid reduces the activity of methanogens, reduces operating efficiency, and causes the system itself to malfunction. Furthermore, the time during which methanation is stopped can be shortened compared to when the flow rate of hydrogen supplied to the methane fermentation unit is simply reduced. Therefore, according to the above characteristic configuration, the pH of the methane fermentation liquid can be kept lower than a certain value, enabling more stable and efficient operation than conventional methods.

[0017] Further characteristic configurations of the biogas production apparatus according to the present invention include: 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% by volume or less.

[0018] If the methane concentration in the biogas generated from the methane fermentation section exceeds 90% by volume, the carbon dioxide concentration in the biogas will be less than 10% by volume, resulting in a pH of 9 or higher in the methane fermentation liquid. This reduces the activity of methanogens, reduces operating efficiency, and poses the risk of the system itself failing to function. To mitigate this risk, the supply of hydrogen from the hydrogen supply means is reduced when the methane concentration in the biogas generated from methane fermentation exceeds 90% by volume. However, because carbon dioxide is still supplied from the carbon dioxide circulation means, the pH of the methane fermentation liquid can be lowered more efficiently than simply reducing the flow rate of hydrogen supplied to the methane fermentation section. If the methane concentration is 90% by volume or lower, the amount of hydrogen supplied from the hydrogen supply means is increased. This prevents the pH of the methane fermentation liquid from rising and keeps it below a certain value, thereby suppressing a decrease in the activity of methanogens.

[0019] Further characteristic configurations of the biogas production apparatus according to the present invention include: The gas measurement means is configured to measure a hydrogen concentration in the biogas in addition to the methane concentration in the biogas; The control means increases the supply rate of hydrogen from the 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 supply rate of hydrogen from the standard flow rate when the hydrogen concentration exceeds the threshold.

[0020] If the hydrogen concentration exceeds a predetermined threshold, the activity of the methane bacteria decreases, reducing operational efficiency, and there is a possibility that unreacted hydrogen remains in the biogas. In such a case, the amount of hydrogen supplied by the hydrogen supply means is reduced. Meanwhile, carbon dioxide is supplied by the carbon dioxide circulation means, which efficiently reduces the pH of the methane fermentation liquid in the methane fermentation section, improving the activity of the methane bacteria and allowing efficient methanation by the methane bacteria. If the hydrogen concentration is below a predetermined threshold, the activity of the methane bacteria is high and there is a shortage of hydrogen for methanation by the methane bacteria, so the amount of hydrogen supplied by the hydrogen supply means is increased. In this way, the activity of the methane bacteria can be maintained at a constant level, and the amount of hydrogen supplied can be adjusted so that there is no unreacted hydrogen. Therefore, according to the above characteristic configuration, the pH of the methane fermentation liquid can be kept lower than a certain value, enabling more stable and efficient operation than conventional methods.

[0021] Further characteristic configurations of the biogas production apparatus according to the present invention include: The separation unit includes one of a membrane separation unit that separates the gas in the biogas using a separation membrane, a chemical absorption unit that absorbs the gas in the biogas into 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.

[0022] According to the above-mentioned characteristic configuration, it is possible to efficiently separate only carbon dioxide from biogas containing methane and carbon dioxide, thereby accelerating the rate at which carbon dioxide increases in the methane fermentation section and quickly lowering the pH of the methane fermentation liquid. Therefore, according to the above characteristic configuration, the pH of the methane fermentation liquid can be kept lower than a certain value, enabling more stable and efficient operation than conventional methods.

[0023] Further characteristic configurations of the biogas production apparatus according to the present invention include: The methane fermentation system includes 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.

[0024] According to the above characteristic configuration, even if the amount of carbon dioxide separated in the separation section is small, by storing the carbon dioxide in the storage means, carbon dioxide can be returned to the methane fermentation section stably at a constant flow rate regardless of 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, enabling more stable and efficient operation than conventional methods. [Brief explanation of the drawings]

[0025] [Figure 1] 1 is a diagram showing a schematic configuration of a biogas production device according to a first embodiment. [Figure 2] FIG. 10 is a diagram showing a schematic configuration of a biogas production device according to a second embodiment. [Figure 3] FIG. 10 is a diagram showing a schematic configuration of a biogas production device according to a third embodiment. DETAILED DESCRIPTION OF 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, an example will be described in which the organic matter to be treated is contained in sludge.

[0027] [Configuration of the biogas production device according to the first embodiment] First, the configuration of a biogas production apparatus 1 according to the first embodiment will be described. Fig. 1 is a diagram showing a 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 a hydrogen supply means) that supplies hydrogen into the methane fermentation tank 10.

[0028] The biogas production apparatus 1 also includes a biogas recovery section 40 (an example of a recovery means) that recovers the biogas generated in the methane fermentation tank 10, a gas concentration meter 30 (an example of a gas measurement means) that measures the concentration of one or more gases of methane, carbon dioxide, and hydrogen in the recovered biogas, a separation section 50 that separates the recovered biogas into at least carbon dioxide and methane, a carbon dioxide circulation section 60 (an example of a carbon dioxide circulation means) that supplies the carbon dioxide separated in the separation section 50 into the methane fermentation tank 10, and a pH meter 90 that measures the pH of the methane fermentation liquid in the methane fermentation tank 10, as well as a control device (an example of a control means) that controls the operation of each section.

[0029] As shown in FIG. 1, the methane fermentation tank 10 is composed of a housing. The methane fermentation tank 10 is configured to form a methane fermentation space 11 in which sludge supplied from outside the housing is biodegraded through 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 by the heat exchanger at a temperature suitable for efficient methane fermentation (for example, 30 to 37°C or 50 to 60°C). In this embodiment, a sludge supply port 15 for supplying sludge is provided on one of the two opposing inner walls of the housing, facing the methane fermentation space 11. A treated water discharge port 16 for discharging treated water to the outside is provided on the other of the two inner walls, facing the methane fermentation space 11.

[0030] The space above the methane fermentation space 11 (the space above the liquid level of the methane fermentation liquid in the methane fermentation tank 10) forms a biogas collection space 12 that collects biogas such as methane and carbon dioxide produced in the methane fermentation space 11.

[0031] In this embodiment, a hydrogen supply port 23 for supplying hydrogen from the hydrogen supply unit 20 is provided at the bottom of the casing, facing the methane fermentation space 11. Furthermore, a biogas outlet 41 for extracting biogas to the biogas recovery unit 40 is provided on the inner wall of the casing, on the inner wall on which the treated water outlet 16 is provided, facing the biogas collection space 12.

[0032] In this embodiment, the hydrogen supply unit 20 is made up of a hydrogen cylinder 21 in which hydrogen is stored, a hydrogen supply port 23, and a hydrogen supply channel 22 (hydrogen supply line) through which hydrogen flows between the hydrogen cylinder 21 and the hydrogen supply port 23. The hydrogen supply unit 20 supplies hydrogen from the hydrogen cylinder 21 into the methane fermentation tank 10 via the hydrogen supply channel 22 and the hydrogen supply port 23. In this embodiment, the operation of an on-off valve (not shown) provided on the hydrogen cylinder 21 can be controlled by a control device. Therefore, the hydrogen supply unit 20 can supply hydrogen to the methane fermentation space 11 in any amount and at any timing. The hydrogen supply unit 20 is not particularly limited as long as it is configured to be able to supply hydrogen to the methane fermentation space 11. For example, a hydrogen production device may be used instead of the hydrogen cylinder 21.

[0033] In this embodiment, the biogas collection unit 40 is composed of a biogas outlet 41 and a biogas extraction path 42 connected to a booster blower 43. The biogas collection unit 40 sucks biogas from the biogas collection space 12 via the biogas outlet 41 and the biogas extraction path 42 and supplies it to the separation unit 50. In this embodiment, the biogas extraction path 42 is provided with an on-off valve and a flow rate adjustment valve whose operation can be controlled by a control device. Therefore, the biogas collection unit 40 can collect biogas from the biogas collection space 12 in any amount and at any timing. The biogas collection unit 40 is not particularly limited as long as it has a configuration that allows it to collect biogas from the biogas collection space 12.

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

[0035] Furthermore, the separation method used in the separation device 51 is not particularly limited as long as it can separate methane and carbon dioxide, but the following methods can be used alone or in combination: membrane separation (an example of a membrane separation section) in which gases in biogas are separated using a separation membrane; chemical absorption (an example of a chemical absorption section) in which gases in biogas are separated by absorbing them into an absorption liquid; pressure swing adsorption (an example of a pressure swing adsorption section) in which gases in biogas are separated by adsorbing them onto an adsorbent; and cryogenic separation.

[0036] Membrane separation is a method of separating gases by utilizing differences in the size and speed of gas molecules, their solubility, and differences in diffusion rate within a membrane. Both organic and inorganic membranes can be used. Organic membranes include polymer membranes, facilitated transport membranes, and ionic liquid-containing membranes. Inorganic membranes include zeolite membranes, silica membranes, and carbon membranes. Polymer membranes include polyimide, cellulose acetate, polysulfone, and polycarbonate. Facilitated transport membranes include molecular gate membranes using polyamidoamine (PAMAM) dendrimers as CO2 carriers and amine-supported nanogel particle membranes. The shape of the separation membrane is not particularly limited, and any shape, including tubular, hollow fiber, monolithic, and honeycomb, can be used. A particularly preferred separation membrane is a polyimide hollow fiber gas separation membrane.

[0037] Chemical absorption is a method in which biogas is brought into contact with an absorbing solution to selectively absorb carbon dioxide, and then the absorbing solution is heated to separate the carbon dioxide. Monoethanolamine or methyldiethanolamine can be used as the absorbing solution.

[0038] Pressure swing adsorption is a method in which carbon dioxide is adsorbed onto an adsorbent under high pressure to obtain highly concentrated methane, and then the carbon dioxide is separated by desorption under low pressure. Zeolite and molecular sieving carbon (MSC) can be used as adsorbents.

[0039] Cryogenic separation is a method in which biogas is pressurized and cooled to liquefy it, and the components of the biogas are separated by distillation, taking advantage of the difference in temperature when the gas condenses.

[0040] 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. 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 in the separation device 51 into the methane fermentation space 11 via 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, the carbon dioxide circulation unit 60 can supply carbon dioxide generated in the methane fermentation tank 10 to the methane fermentation space 11 in any amount and at any timing.

[0041] The gas concentration meter 30 is installed in the biogas extraction path 42 and is configured to measure the concentration of gases generated in the methane fermentation tank 10 at regular intervals and transmit the measurement results to the control device. The gas measurement means used in 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 electrochemical sensors, semiconductor sensors, thermal conduction sensors, and non-dispersive infrared sensors can be used. Furthermore, methane, carbon dioxide, and hydrogen may be detected by one gas sensor, or each gas may be detected by multiple sensors.

[0042] The pH measuring device 90 is installed in the methane fermentation tank 10 and is configured to measure the pH of the methane fermentation liquid at regular intervals and send the measured value 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 is more than 90% by volume. 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. The pH of the methane fermentation liquid is monitored, and the amount of hydrogen supplied by the hydrogen supply means can be controlled, so that the pH of the methane fermentation liquid can be maintained 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. The carbon dioxide returned to the methane fermentation space 11 and hydrogen supplied from the hydrogen supply line 22 are used for methanation by methanogens in the methane fermentation liquid. Furthermore, the tank is provided with a means for adjusting the amount of hydrogen supplied based on the gas concentration measured by the gas concentration meter 30, enabling stable and efficient control of methane production.

[0044] [Process flow of the biogas production device according to the first embodiment] Next, we will explain the process of treating sludge using the biogas production apparatus 1 having the above configuration. First, we will explain the effect on the operation of the system of the conversion of carbon dioxide to methane by methanation in the methane fermentation tank 10.

[0045] As the conversion of carbon dioxide to methane by methanation progresses in the methane fermentation tank 10, the carbon dioxide concentration in the methane fermentation liquid decreases, the pH of the methane fermentation liquid increases, and the free ammonia concentration in the methane fermentation liquid increases. Because free ammonia reduces the activity of methanogens, as the conversion of carbon dioxide to methane by methanation progresses, the activity of the methanogens decreases, making it difficult for methanation to proceed. Therefore, if the pH of the methane fermentation liquid increases as methanation progresses, the operating efficiency of the system decreases, and ultimately the system itself may cease to function.

[0046] To enable stable and efficient operation of the organic matter treatment system, it is effective to take measures to adjust the pH of the methane fermentation liquor so as not to generate so much free ammonia that the activity of methane bacteria is significantly reduced. Specifically, when the methane concentration in the methane fermentation tank increases, it is effective to reduce the amount of hydrogen supplied to the methane fermentation tank 10 to suppress the decrease in carbon dioxide, or to take measures to supply carbon dioxide to the methane fermentation tank 10. In this way, the increased pH of the methane fermentation liquor 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 this embodiment, the concentration of the gas generated in the methane fermentation tank 10 is measured by the gas concentration meter 30, and the flow rate of hydrogen supplied 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 a threshold value (90% by volume in this example), hydrogen is supplied to the methane fermentation tank 10 at a flow rate (standard flow rate) that is necessary and sufficient for converting carbon dioxide to methane. On the other hand, when the methane concentration exceeds the threshold value, the supply of hydrogen to the methane fermentation tank 10 is reduced. Furthermore, in this embodiment, after the hydrogen supply is reduced, the supply of hydrogen at the standard flow rate is resumed when the methane concentration subsequently drops to 90% by volume or lower.

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

[0049] Next, in the methane fermentation space 11, methanogens in the methane fermentation liquid methanate part of the carbon dioxide in the generated biogas using hydrogen supplied from the hydrogen supply unit 20. The biogas, from which part of the carbon dioxide has been methanated, is supplied to the separation unit 50 via the biogas outlet 41 and the biogas outlet path 42. The carbon dioxide separated in the separation unit 51 is returned to the methane fermentation tank 10 via the carbon dioxide circulation unit 60. The separated methane is stored in the tank 52 via the methane outlet 53 and the methane outlet path 54. Meanwhile, the treated water produced by methane fermentation is discharged to the outside from the treated water outlet 16.

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

[0051] This suppresses the decrease in carbon dioxide due to methanation, and the carbon dioxide separated in the separator 51 is returned by the carbon dioxide circulation unit 60, and biogas containing carbon dioxide is generated by methane fermentation, which causes the rate at which carbon dioxide increases in the methane fermentation tank 10 and the pH of the methane fermentation liquid to decrease quickly. This makes it possible to maintain the pH of the methane fermentation liquid below a certain value.

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

[0053] As described above, in the biogas production apparatus 1 according to this embodiment, carbon dioxide is separated from the biogas generated in the methane fermentation tank 10 by the separator 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. If the gas concentration in the methane fermentation tank 10 exceeds a threshold value as methanation progresses, the supply of hydrogen to the methane fermentation tank 10 is reduced, and the methane concentration in the methane fermentation tank 10 is reduced to below the threshold value. This prevents a situation in which an increase in the pH of the methane fermentation liquid reduces the activity of methanogens, resulting in a decrease in operating efficiency and failure of the system itself, or in which unreacted hydrogen remains. Therefore, in the biogas production apparatus 1, methane fermentation and methanation can occur simultaneously in the methane fermentation section, enabling stable and efficient operation.

[0054] [Modification of the biogas production device according to the first embodiment] In the above embodiment, the gas concentration meter 30 measures the concentration of methane 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 methane concentration. In addition, in a modified example, the hydrogen supply flow rate 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 a threshold and the hydrogen supply flow rate is at a standard flow rate, if the hydrogen concentration is below a 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, if the hydrogen concentration exceeds the threshold, the supply of hydrogen into the methane fermentation tank 10 is reduced. Furthermore, when the methane concentration is below the threshold and the hydrogen supply flow rate is at the standard flow rate, if the carbon dioxide concentration exceeds the threshold (for example, 10% by volume), the hydrogen flow rate into the methane fermentation tank 10 is increased from the standard flow rate. On the other hand, if the carbon dioxide concentration is below the threshold, the supply of hydrogen into the methane fermentation tank 10 is reduced.

[0055] [Configuration of the biogas production device according to the second embodiment] Next, the configuration of a biogas production apparatus 70 according to a second embodiment will be described. FIG. 2 is a diagram showing a 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 differs from the first embodiment mainly in that it includes a carbon dioxide storage section 80 (carbon dioxide storage means). The biogas production apparatus 70 according to the second embodiment will be described below, but a description of the same configuration as the biogas production apparatus 1 according to the first embodiment will be omitted.

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

[0057] In this embodiment, the carbon dioxide storage unit 80 is composed of a tank 81 (storage means) and a carbon dioxide supply channel 82. One end of the carbon dioxide supply channel 82 is connected to the tank 81, and the other end is connected to the bottom of the methane fermentation tank 10. The separator 51 is composed of a pressure swing adsorption unit that adsorbs carbon dioxide. 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 supply channel 82 can be controlled by a control device. Therefore, the carbon dioxide storage unit 80 allows the biogas stored in the tank 81 to be supplied to the methane fermentation space 11 in any amount and at any timing.

[0058] The tank 81 is provided separately from the tank 52 that constitutes the separation unit 50. In this embodiment, the tank 81 stores the carbon dioxide separated by the separation device 51.

[0059] [Process flow of the biogas production device according to the second embodiment] Next, a process of treating sludge using the biogas production apparatus 70 having the above configuration will be described.

[0060] To enable stable and efficient operation of the biogas production apparatus, it is effective to return carbon dioxide to the methane fermentation tank 10. However, the flow rate of carbon dioxide separated in the separation unit 50 may fluctuate, and there is a possibility that the amount of carbon dioxide that can be returned to the methane fermentation tank 10 may be insufficient. In such cases, less carbon dioxide is returned to the methane fermentation tank 10, which raises the risk of the pH of the methane fermentation liquid increasing and causing the system to shut down. There is also a possibility that hydrogen supplied to the methane fermentation tank 10 may remain unreacted.

[0061] Therefore, in this embodiment, a tank 81 is provided to store the carbon dioxide separated by the separator 51. The stored carbon dioxide is returned to the methane fermentation tank 10 via a carbon dioxide supply path 82.

[0062] As a result, similarly to the first embodiment, the rate of increase of carbon dioxide in the methane fermentation tank 10 increases, and the pH of the methane fermentation liquid decreases more efficiently, compared to when the supply of hydrogen is simply stopped. Therefore, in the biogas production apparatus 70 according to this embodiment, carbon dioxide can be returned to the methane fermentation tank 10 at a constant flow rate, and stable methanation can be performed.

[0063] [Configuration of the biogas production device according to the third embodiment] Next, the configuration of a biogas production apparatus 100 according to the third embodiment will be described. This differs from the first and second embodiments in that the methane fermentation section includes a methane fermentation tank 10 and a methanation tank 110. The methanation tank 110 is connected downstream of the methane fermentation tank 10. The biogas production apparatus 100 according to the third embodiment will be described below, but descriptions of the same configuration as 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, and a methanation tank 110 connected downstream of the methane fermentation tank 10, and the biogas and digested sludge generated in the methane fermentation tank 10 are supplied to the methanation tank 110. It also includes a hydrogen supply unit 20 that supplies hydrogen into the methanation tank 110, a biogas recovery unit 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 unit 60 that supplies the carbon dioxide separated in the separation unit 50 into the methanation tank 110, and is equipped with a control device (an example of a control means) that controls the operation of each unit.

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

[0066] The space above the methanation space 111 (the space above the liquid level of the methane fermentation liquid in the methanation tank 110) forms a biogas collection space 112 that collects biogas such as methane and carbon dioxide produced in the methanation space 111.

[0067] In this embodiment, a hydrogen supply port 23 for supplying hydrogen from the hydrogen supply unit 20 is provided at the bottom of the methanation tank 110, facing the methanation space 111. Furthermore, a biogas outlet 41 for extracting biogas to the biogas recovery unit 40 is provided on the inner wall of the casing, on the inner wall on which the methanation-treated water outlet 116 is provided, facing 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. The carbon dioxide supply port 62 is provided at the bottom of the housing so as to face the methanation space 111. The carbon dioxide circulation unit 60 supplies the carbon dioxide separated in the separation device 51 into the methanation space 111 via 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, the carbon dioxide circulation unit 60 can supply carbon dioxide generated in the methanation tank 110 to the methanation space 111 in any supply amount and at any timing.

[0069] [Treatment flow of the biogas production device according to the third embodiment] Next, a process of treating sludge using the biogas production apparatus 100 having the above configuration will be described.

[0070] In this embodiment, biogas and digested sludge generated by methane fermentation in the methane fermentation tank 10 are supplied to the methanation tank 110. Subsequently, hydrogen supplied by the hydrogen supply unit 20 is used in the methanation tank 110 to methanate a portion of the carbon dioxide in the biogas by methanogens in the methanation tank 110. The biogas, from which a portion of the carbon dioxide has been methanated, is supplied to the separation unit 50 via the biogas outlet 41 and the biogas outlet path 42. The carbon dioxide separated in the separation unit 51 is returned to the methanation tank 110 via the carbon dioxide circulation unit 60, and the separated methane is stored in the tank 52 via the methane outlet 53 and the methane outlet path 54. Meanwhile, treated water produced by methane fermentation is discharged to the outside from the methanation-treated water outlet 116.

[0071] In this way, in the biogas production apparatus 100 according to this embodiment, methane fermentation and methanation are carried out in separate tanks, which prevents an increase in the pH of the methane fermentation liquid caused by a decrease in the carbon dioxide concentration as methanation progresses, thereby enabling the biogas production apparatus 100 to operate more stably and efficiently.

[0072] [Simulation of the amount of hydrogen required for methanation] The amount of methane obtained and the amount of hydrogen required when carbon dioxide is recycled and when it is not recycled were simulated using the example in Figure 1 under the following preconditions and definitions. [Prerequisites] Prerequisite 1: It is assumed that methane and carbon dioxide are completely separated in the separation section after the methane fermentation tank. Assumption 2: The hydrogen supplied to the methane fermenter is assumed to react completely with carbon dioxide and be converted into methane. [Definition] 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, of which the methane concentration is C1 (volume %) and the carbon dioxide concentration is C'1 (volume %). The amount of carbon dioxide recovered as the main component in the gas separation section downstream of the methane fermenter is Q2, of which the methane concentration is C2 (0% by volume) and the carbon dioxide concentration is C'2 (100% by volume). The amount of methane recovered as the main component in the separation section downstream of the methane fermenter is Q3, of which the methane concentration is C3 (100% by volume) and the carbon dioxide concentration is C'3 (0% by volume). The amount of hydrogen required to convert carbon dioxide to methane is Q4. The amount of biogas finally generated at the outlet of the methane fermentation tank is Q, of which the methane concentration is C and the carbon dioxide concentration is C'.

[0073] <When carbon dioxide is not circulated> The amount of hydrogen required to produce methane is four times the amount of carbon dioxide. Therefore, the amount of hydrogen required to methanate to a given concentration can be calculated using the following formula: Q1×(C'1-C')×4…(I)

[0074] <When circulating carbon dioxide> The mass balance for methane and carbon dioxide in the separation section is as follows: Q = Q2 + Q3...(b) Methane: From premise 1, Q×C=Q3×C3 Here, C3 is 100% by volume, or 1, so Q3 = Q × C. Carbon dioxide: Q × C' = Q2 × C'2 Here, C'2 is 100% by volume, or 1, so Q2 = Q × C'.

[0075] The mass balance for methane at the outlet of the methane fermenter and the separation section is as follows: Q = Q1 + Q2...(c) From (b) and (c), Q3 can be calculated using the following formula. Q1=Q3…(d) Since the amount of methane at the outlet of the methane fermentation tank is the same as the amount of methane separated in the separation section, Q3×C3=Q×C Here, Q1 = Q3, and C3 is 100% by volume, so Q1 = (Q1 + Q2) × C Therefore, Q2 can be calculated using the following formula: Q2 = (1-C) × Q1 / C…(E)

[0076] The amount of CO2 present in the methane fermentation tank is Q1×C'1+Q2×C'2. The amount of hydrogen required to methanate to a predetermined concentration can be calculated using the following formula: (Q1×C'1+Q2×C'2-Q×C')×4…(to)

[0077] Here, the amount of hydrogen required and the amount of methane produced were calculated for cases where carbon dioxide is not recycled and where it is recycled, using the following conditions as examples. <Condition> Q1:100(m 3 / h), C1: 60 (volume%), C'1: 40 (volume%) C:C' = 85 (volume%):15 (volume%)

[0078] <When carbon dioxide is not circulated> The amount of hydrogen required is given by equation (a). 100×(0.40-0.15)×4=100(m 3 / h). The amount of methane obtained is 100×0.85=85(m 3 / h).

[0079] <When circulating carbon dioxide> The amount of methane obtained is given by equation (d): Q3 = 100 (m 3 / h). The amount of carbon dioxide recovered in the gas separation section is calculated from equation (E). Q2 = 17.65 (m 3 / h). The flow rate Q is calculated from equation (b) 100+17.65=117.65(m 3 / h). The amount of hydrogen required is given by equation (f): (100×0.4+17.65×1-117.65×0.15)×4=160(m 3 / h).

[0080] In this way, when a system that separates and circulates carbon dioxide is adopted, the amount of methane produced can be increased compared to when carbon dioxide is not circulated.

[0081] <Another embodiment>

[0082] In each of the above embodiments, sludge is treated, but the present invention is not limited to this and may be applied to treating food waste or the like.

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

[0084] Furthermore, the configurations disclosed in the above embodiments (including other embodiments, the same applies below) can be applied in combination with configurations disclosed in other embodiments, as long as no contradictions arise. Furthermore, the embodiments disclosed in this specification are examples, and the embodiments of the present invention are not limited to these, and can be modified as appropriate within the scope that does not deviate from the purpose of the present invention. [Industrial Applicability]

[0085] INDUSTRIAL APPLICABILITY The present invention can be used in a biogas production device that processes organic matter by utilizing methane fermentation. [Explanation of symbols]

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

Claims

1. A biogas production apparatus comprising a methane fermentation unit to which organic matter is supplied, a hydrogen supply means for supplying hydrogen to the methane fermentation unit, and a control means for controlling operation, the biogas production apparatus being configured so that methanation can be performed in the methane fermentation unit, 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; a carbon dioxide circulation means for supplying the carbon dioxide separated in the separation section to the methane fermentation section, and a gas measurement means for measuring a gas concentration in the biogas, 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; the gas measurement means is configured to measure a methane concentration in the biogas; 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.

2. The biogas production device according to claim 1 , wherein the gas measurement means measures the concentration of at least one of methane, carbon dioxide, and hydrogen.

3. 2. The biogas production apparatus according to claim 1, 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% by volume or less.

4. The gas measurement means is configured to measure a hydrogen concentration in the biogas in addition to the methane concentration in the biogas; 2. The biogas production apparatus according to claim 1, wherein the control means increases the supply rate of hydrogen 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 supply rate of hydrogen from the standard flow rate when the hydrogen concentration exceeds the threshold.

5. 3. The biogas production apparatus according to claim 1, wherein the separation unit comprises one of a membrane separation unit that separates gases in the biogas using a separation membrane, a chemical absorption unit that absorbs gases in the biogas into an absorption liquid and separates the gases, and a pressure swing adsorption unit that adsorbs gases in the biogas into an adsorbent and separates the gases.

6. 3. The biogas production apparatus according to claim 1, 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.

Images