Biogas production apparatus and control method thereof

Abstract

This invention provides a biogas production apparatus and a control method thereof that simultaneously induces methane fermentation and methanation of organic waste, converting hydrogen and carbon dioxide into methane within a methane fermentation tank, thereby achieving more stable and efficient operation than conventional methods. [Solution] A biogas production apparatus 1 comprising a methane fermentation section 10 to which organic matter is supplied, a hydrogen supply means 20 to which hydrogen is supplied, a control means to control operation, and a pH measuring section 90 to measure the pH of the methane fermentate in the methane fermentation section 10, wherein the biogas production apparatus 1 is configured to enable methanation in the methane fermentation section 10, and the control means is set such that the operation control is configured such that the first duration for which the hydrogen-carbon dioxide ratio is greater than a predetermined ratio value is less than or equal to a first predetermined time, and the second duration for which the pH value is greater than 9 is less than or equal to a second predetermined time, and a control method thereof.

Description

[Technical Field]

[0001] The present invention relates to a biogas production apparatus and a control method therefor for processing organic matter using methane fermentation. [Background technology]

[0002] A technology is known that uses methane fermentation to convert organic matter contained in sludge and food waste into biogas. The biogas obtained from the methane fermentation of organic matter mainly contains methane and carbon dioxide. In recent years, with the aim of reducing carbon dioxide emissions and using biogas as city gas, so-called biomethanation technology is being developed, which converts carbon dioxide in biogas into methane using microorganisms.

[0003] However, when the amount of 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. As a result, some of the remaining carbon dioxide must be released as biogas.

[0004] As for biometanation 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 biometanation, 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 a biogas production system comprising a methane fermentation tank that generates biogas by methane fermentation, and a methanation reaction unit connected to the methane fermentation tank that 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 Publication No. 2016-108382 [Overview of the project] [Problems that the invention aims to solve]

[0007] Incidentally, in in situ biomethanation, as the conversion of carbon dioxide to methane by methanation progresses, the carbon dioxide concentration in the fermentation liquid decreases and the pH of the fermentation liquid increases. As a result of the increased pH of the fermentation liquid, the concentration of free ammonia in the fermentation liquid increases. Consequently, the activity of methane-producing archaea decreases, making it more difficult for the conversion of carbon dioxide to methane by methanation to proceed. In other words, in a processing system that utilizes in situ biomethanation, as methanation progresses and the pH of the methane fermentation liquid rises, the operating efficiency of the system decreases, and there is a risk that the system itself will eventually cease to function. Furthermore, if a configuration is adopted in which a methanation reaction unit is installed by connecting it to a methane fermenter, it is unavoidable that the equipment will become larger and the process will become more complex.

[0008] This invention has been made in view of the above-mentioned problems, and its purpose is to provide a biogas production apparatus and a control method thereof that can simultaneously cause methane fermentation and methanation of organic waste, convert hydrogen and carbon dioxide into methane in a methane fermentation tank, and achieve more stable and efficient operation than conventional methods. [Means for solving the problem]

[0009] The biogas production apparatus according to the present invention for achieving the above object includes a methane fermentation section to which organic matter is supplied, a hydrogen supply means for supplying hydrogen to the methane fermentation section, a control means for controlling the operation, and a pH measurement section for measuring the pH of the methane fermentation liquid in the methane fermentation section. The biogas production apparatus is configured to be able to perform methanation in the methane fermentation section, and its characteristic configuration is that the control of the operation in the control means is such that the hydrogen-carbon dioxide ratio obtained by dividing the hydrogen supply amount from the hydrogen supply means by at least the amount of carbon dioxide contained in the biogas generated in the methane fermentation section is set so that the first duration during which the ratio is greater than a predetermined ratio value lasts for a first predetermined time or less, and the second duration during which the pH value measured by the pH measurement section is greater than 9 lasts for a second predetermined time or less.

[0010] When the hydrogen-carbon dioxide ratio is high, the pH rises. If the high pH continues for a long time, the methane fermentation of sludge (organic matter) is inhibited, so it is better for the time with a high pH to be as short as possible.

[0011] According to this configuration, since the first duration is set to be the first predetermined time or less, the biogas production apparatus can be controlled so that the upper limit of the time during which the hydrogen-carbon dioxide ratio is greater than the predetermined ratio value is the first predetermined time. As a result, it is possible to suppress the increase in the time during which the pH rises, so it is difficult for the methane fermentation of sludge to be inhibited.

[0012] [[ID=IP11]] Also, in this configuration, since the second duration is set to be the second predetermined time or less, the biogas production apparatus can be controlled so that the upper limit of the time during which the pH value measured by the pH measuring instrument is greater than 9 is the second predetermined time. As a result, it is possible to suppress the increase in the time during which the pH is greater than 9, so it is difficult for the methane fermentation of sludge to be inhibited.

[0013] Therefore, in this configuration, it is possible to suppress the lengthening of the time during which the pH increases and the lengthening of the time during which the pH becomes greater than 9. As a result, it becomes more difficult to inhibit the methane fermentation of sludge, and it is possible to provide a biogas production apparatus in which biomethanation and the methane fermentation of sludge are stabilized.

[0014] A further characteristic configuration of the biogas production apparatus according to the present invention includes 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, and a carbon dioxide circulation means for supplying the carbon dioxide separated by the separation section to the methane fermentation section. The amount of carbon dioxide in the hydrogen-carbon dioxide ratio includes the amount of carbon dioxide supplied from the carbon dioxide circulation means.

[0015] If the amount of carbon dioxide is set as in this configuration, it is possible to calculate the hydrogen-carbon dioxide ratio in a state where the carbon dioxide in the methane fermentation section has increased.

[0016] When there are large fluctuations in the biogas generation amount, the separation ability of the separation section for separating carbon dioxide must be designed in accordance with the flow rate when the biogas generation amount is maximum, resulting in an increase in equipment costs. The biogas production apparatus of the present invention can suppress fluctuations in the biogas flow rate due to stable biomethanation, so there is no need to design the separation ability of the separation section in accordance with the flow rate when the biogas generation amount is maximum, and the equipment costs can be reduced.

[0017] A further characteristic configuration of the biogas production apparatus according to the present invention is that the operation control in the control means is set such that the supply of the organic matter is performed a plurality of times per day.

[0018] If the supply of sludge is set to be performed a plurality of times per day as in this configuration, the hydrogen-carbon dioxide ratio becomes stable and biomethanation becomes likely to be stable.

[0019] A further characteristic feature of the biogas production apparatus according to the present invention is that the predetermined ratio value is set to 4.0, the first predetermined time is set to 10 hours, and the second predetermined time is set to 9 hours.

[0020] This configuration allows for setting predetermined ratio values, upper limits for the first predetermined time, and upper limits for the second predetermined time.

[0021] A further characteristic configuration of the biogas production apparatus according to the present invention is that it includes a gas measuring means for measuring the gas concentration in the biogas, and the control means adjusts the amount of hydrogen supplied from the hydrogen supply means according to the gas concentration measured by the gas measuring means.

[0022] As methanation progresses and the methane concentration in the methane fermentation section increases, the carbon dioxide concentration decreases, the pH of the methane fermentation liquid rises, the activity of methane-producing archaea decreases, and there is a risk that the system will shut down.

[0023] However, this configuration includes a gas measuring means for measuring gas concentration, and a control means is configured to adjust the amount of hydrogen supplied from the hydrogen supply means according to the gas concentration. As a result, the rate of increase of carbon dioxide in the methane fermentation section can be accelerated, and the pH of the methane fermentate can be rapidly lowered. Therefore, with this configuration, the pH of the methane fermentate can be kept below the threshold, enabling stable and efficient operation.

[0024] A further characteristic feature of the biogas production apparatus according to the present invention is that the gas measuring means measures the concentration of at least one of methane, carbon dioxide, and hydrogen.

[0025] With this configuration, by measuring at least one of the methane, carbon dioxide, and hydrogen in the biogas generated in the methane fermentation section, the pH of the methane fermentate and the methanation efficiency of methane-producing archaea can be predicted, and the amount of hydrogen supplied from the hydrogen supply means can be adjusted. Therefore, with this configuration, the pH of the methane fermentate can be kept below a threshold, enabling stable and efficient operation.

[0026] A further characteristic feature of the biogas production apparatus according to the present invention is that the separation unit comprises one of the following: a membrane separation unit that separates the gas in the biogas using a separation membrane; a chemical absorption unit that separates the gas in the biogas by absorbing it into an absorbent liquid; and a pressure fluctuation adsorption unit that separates the gas in the biogas by adsorbing it onto an adsorbent.

[0027] This configuration allows for the efficient separation of carbon dioxide from biogas containing methane and carbon dioxide. Therefore, unreacted carbon dioxide in the biogas is recycled into the methane fermentation section and reused for methanation, thereby converting all carbon dioxide in the biogas into methane.

[0028] A further characteristic feature of the biogas production apparatus according to the present invention is that it includes a storage means for storing the carbon dioxide separated in the separation unit, and a means for supplying the carbon dioxide stored in the storage means to the methane fermentation unit.

[0029] With this configuration, even if the amount of carbon dioxide separated in the separation unit is small, the carbon dioxide can be stored in the storage means, allowing the carbon dioxide to be stably returned to the methane fermentation unit at a constant flow rate, regardless of the amount of biogas in the biogas recovery unit. Therefore, this configuration allows the pH of the methane fermentation liquid to be kept below a certain value, enabling more stable and efficient operation than conventional methods.

[0030] The control method for a biogas production apparatus according to the present invention comprises a methane fermentation section to which organic matter is supplied, a hydrogen supply means for supplying hydrogen to the methane fermentation section, a control means for controlling operation, and a pH measuring unit for measuring the pH of the methane fermentation liquid in the methane fermentation section, wherein the biogas production apparatus is configured to enable methanation in the methane fermentation section, and its characteristic configuration is that the operation control by the control means is set such that the first duration for which the hydrogen-carbon dioxide ratio, obtained by dividing the amount of hydrogen supplied from the hydrogen supply means by at least the amount of carbon dioxide contained in the biogas generated in the methane fermentation section, is greater than a predetermined ratio value is less than or equal to a first predetermined duration, and the second duration for which the pH value measured by the pH measuring unit is greater than 9 is less than or equal to a second predetermined duration.

[0031] With this configuration, since the first duration is set to be less than or equal to the first predetermined time, the biogas production device can be controlled so that the upper limit of the time during which the hydrogen-carbon dioxide ratio exceeds a predetermined ratio value is the first predetermined time. This suppresses the time during which the pH rises, making it less likely for methane fermentation of sludge to be inhibited.

[0032] Furthermore, since the second duration is set to be less than or equal to the second predetermined time, the biogas production device can be controlled so that the upper limit of the time during which the pH value measured by the pH meter 90 is greater than 9 is the second predetermined time. This suppresses the time during which the pH is greater than 9, thus making it less likely for methane fermentation of sludge to be inhibited.

[0033] Therefore, this configuration suppresses the time it takes for the pH to rise and the time it takes for the pH to exceed 9, making it less likely for methane fermentation of sludge to be inhibited, and providing a control method for a biogas production system that stabilizes biomethanation and methane fermentation of sludge. [Brief explanation of the drawing]

[0034] [Figure 1]This figure shows a schematic configuration of a biogas production apparatus according to the first embodiment. [Figure 2] This figure shows a schematic configuration of a biogas production apparatus according to the second embodiment. [Figure 3] This figure shows a schematic configuration of a biogas production apparatus according to the third embodiment. [Figure 4] This graph shows the changes in the amount of each gas, the hydrogen-carbon dioxide ratio, and the pH value up to the third day when food waste is added 12 times a day. [Figure 5] This graph shows the changes in the amount of each gas, the hydrogen-carbon dioxide ratio, and the pH value up to the third day when food waste is added once a day. [Modes for carrying out the invention]

[0035] A biogas production apparatus according to an embodiment of the present invention will be described below with reference to the drawings. In the following explanation, we will use the example of a case where the organic matter to be treated is contained in sludge (organic waste).

[0036] [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. Figure 1 is a diagram showing the schematic configuration of the biogas production apparatus 1 according to the first embodiment. As shown in Figure 1, the biogas production apparatus 1 includes a methane fermentation tank 10 (an example of a methane fermentation section) to which sludge is supplied, a hydrogen supply section 20 (an example of a hydrogen supply means) to which hydrogen is supplied into the methane fermentation tank 10, and a control device (an example of a control means) to control the operation.

[0037] Furthermore, the biogas production apparatus 1 includes a biogas recovery unit 40 (an example of a recovery means) for recovering biogas generated in the methane fermentation tank 10, a separation unit 50 for separating the recovered biogas into at least carbon dioxide and methane, a carbon dioxide circulation unit 60 (an example of a carbon dioxide circulation means) for supplying the carbon dioxide separated in the separation unit 50 into the methane fermentation tank 10, a pH meter 90 (an example of a pH measuring unit) for measuring the pH of the methane fermentation liquid in the methane fermentation tank 10, and a gas concentration meter 30 (an example of a gas measuring means) for measuring the concentration of one or more gases among methane, carbon dioxide, and hydrogen in the recovered biogas. The control device is configured to control the operation of each of these units.

[0038] In the present invention, the control of the operation in the control device is set such that the first duration for which the hydrogen-carbon dioxide ratio, obtained by dividing the amount of hydrogen supplied from the hydrogen supply unit 20 by at least the amount of carbon dioxide contained in the biogas generated in the methane fermentation tank 10, is greater than a predetermined ratio value is less than or equal to a first predetermined duration, and the second duration for which the pH value measured by the pH meter 90 is greater than 9 is less than or equal to a second predetermined duration.

[0039] As shown in Figure 1, the methane fermentation tank 10 is composed of a housing. The methane fermentation tank 10 is also configured to form a methane fermentation space 11 in which sludge supplied from outside the housing is biodegraded by methane fermentation by methane-producing archaea. 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 temperature suitable for good methane fermentation (for example, 30-37°C or 50-60°C) by the heat exchanger. In this embodiment, a sludge supply port 15 for supplying sludge containing organic matter 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.

[0040] The space above the methane fermentation space 11 (the space above the liquid surface 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 produced in the methane fermentation space 11.

[0041] In this embodiment, a hydrogen supply port 23 for supplying hydrogen from the hydrogen supply unit 20 is provided at the bottom of the housing, facing the methane fermentation space 11. In addition, on the inner wall of the housing, the inner wall where the treated water discharge port 16 is provided, a biogas outlet 41 for extracting biogas to the biogas recovery unit 40 is provided, facing the biogas collection space 12.

[0042] In this embodiment, the hydrogen supply unit 20 is composed of a hydrogen cylinder 21 in which hydrogen is stored, a hydrogen supply port 23, and a hydrogen supply passage 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 to the methane fermentation tank 10 via the hydrogen supply passage 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, with the hydrogen supply unit 20, the amount of hydrogen supplied can be kept constant at all times, or hydrogen can be supplied to the methane fermentation space 11 at any amount or timing. The hydrogen supply unit 20 is not particularly limited as long as it is capable of supplying hydrogen to the methane fermentation space 11, and for example, a hydrogen production device may be used instead of the hydrogen cylinder 21.

[0043] In this embodiment, the biogas recovery unit 40 consists of a biogas outlet 41 and a biogas extraction channel 42 connected to a booster blower 43. The biogas recovery unit 40 draws biogas from within the biogas collection space 12 via the biogas outlet 41 and the biogas extraction channel 42 and supplies it to the separation unit 50. In this embodiment, the biogas extraction channel 42 is provided with an on / off valve and a flow rate adjustment valve that can be operated by a control device. Therefore, with the biogas recovery unit 40, biogas can be recovered from within the biogas collection space 12 at any desired amount and timing. The biogas recovery unit 40 is not particularly limited as long as it is configured to recover biogas from the biogas collection space 12.

[0044] In this embodiment, the separation unit 50 consists of a separation device 51, a tank 52, a methane outlet 53, and a methane outlet passage 54 through which methane flows between the tank 52 and the methane outlet 53. The methane separated by the separation device 51 is stored in the tank 52 via the methane outlet 53 and the methane outlet passage 54. The separation device 51 is capable of separating 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 on-off valves (not shown) and flow rate control valves (not shown) provided in the separation device 51 can be controlled by a control device. Therefore, with 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 at any desired amount and timing. In addition, hydrogen, which is present in small amounts in the biogas generated from the methane fermentation tank 10, is separated together with carbon dioxide in the separation device 51 and supplied to the methane fermentation tank 10 by the carbon dioxide circulation unit 60.

[0045] Furthermore, the separation method used in the separation device 51 is not particularly limited as long as it can separate methane and carbon dioxide. However, membrane separation methods (an example of a membrane separation unit) that separate gas in biogas using a separation membrane, chemical absorption methods (an example of a chemical absorption unit) that separate gas in biogas by absorbing it into an absorbent liquid, pressure fluctuation adsorption methods (an example of a pressure fluctuation adsorption unit) that separate gas in biogas by adsorption onto an adsorbent, and cryogenic separation methods can be used individually or in combination.

[0046] Membrane separation is a method of separating gases by utilizing differences in the size and velocity of gas molecules, the solubility of gas molecules, and the difference in diffusion rates within a membrane. Both organic and inorganic membranes can be used. Organic membranes include polymer membranes, accelerated transport membranes, and ionic liquid-containing membranes, while inorganic membranes include zeolite membranes, silica membranes, and carbon membranes. Polymer membranes that can be used include polyimide, cellulose acetate, polysulfone, and polycarbonate. Accelerated transport membranes that can be used include molecular gate membranes using polyamidoamine (PAMAM) dendrimers as CO2 carriers and amine-supported nanogel particle membranes. Furthermore, the shape of the separation membrane is not particularly limited, and any shape can be adopted, such as tubular, hollow fiber, monolithic, and honeycomb. A particularly preferred separation membrane is the polyimide hollow fiber gas separation membrane.

[0047] Even if unreacted hydrogen is mixed into the biogas, it can be separated into carbon dioxide, and the hydrogen can be reused for methanation along with the carbon dioxide. Therefore, if unreacted hydrogen is mixed into the biogas, it can be reused, for example, by applying a membrane separation method.

[0048] Chemical absorption is a method in which biogas is brought into contact with an absorbent solution to selectively absorb carbon dioxide into the solution, and then the absorbent solution is heated to separate the carbon dioxide. Monoethanolamine and methyldiethanolamine can be used as the absorbent solution.

[0049] Pressure fluctuation adsorption is a method of separating carbon dioxide by adsorbing it onto an adsorbent under high pressure to obtain a high concentration of methane, and then desorbing the adsorbed carbon dioxide from the adsorbent under low pressure. Zeolites and molecular sieve carbon (MSCs) can be used as adsorbents.

[0050] Cryogenic separation is a method that involves pressurizing and cooling biogas to liquefy it, and then using the temperature difference during condensation to separate the biogas components through distillation.

[0051] In this embodiment, the carbon dioxide circulation unit 60 consists of a carbon dioxide supply port 62 and a carbon dioxide circulation path 61. 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 located at the bottom of the housing, facing 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 via the carbon dioxide circulation path 61 and the carbon dioxide supply port 62. In this embodiment, the operation of the on / off valve (not shown) and 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 into the methane fermentation space 11 at any desired amount and timing.

[0052] The pH meter 90 is installed inside the methane fermentation tank 10 and is configured to measure the pH of the methane fermentation liquid at regular intervals and transmit the results to the control device. If 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 greater than 90% by volume. If 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 system is configured to monitor the pH of the methane fermentation liquid and control the amount of hydrogen supplied by the hydrogen supply means to maintain the pH of the methane fermentation liquid at a constant value.

[0053] The gas concentration meter 30 is installed in the biogas extraction channel 42 and is configured to measure the gas concentration 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. In addition, methane, carbon dioxide, and hydrogen may be detected by one gas sensor, or each gas may be detected by a separate sensor.

[0054] In this embodiment, a gas concentration meter 30 for measuring gas concentration is exemplified as the gas measurement means, but the system is not limited to this, and the gas measurement means may also include means for measuring the amount of gas generated. In this case, the system may include at least one of the gas concentration meter 30 and means for measuring the amount of gas generated.

[0055] In the methane fermentation tank 10 with 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 in the biogas generated by the decomposition of sludge and the carbon dioxide returned to the methane fermentation space 11, along with hydrogen supplied from the hydrogen supply channel 22, are used to produce methane by methane-producing archaeon in the methane fermentation liquid. Furthermore, by providing a means to adjust the amount of hydrogen supplied based on the gas concentration measured by the gas concentration meter 30, stable and efficient control of methane production is possible.

[0056] Biogas production is highest immediately after sludge input and gradually decreases. When the hydrogen supply is kept constant in biomethanation, if the sludge supply per predetermined period (e.g., one day) is the same, a higher frequency of sludge input results in less fluctuation in the ratio of hydrogen supplied from the hydrogen supply unit 20 to the amount of carbon dioxide generated (H2 / CO2 ratio: hydrogen-carbon dioxide ratio), and thus more stable biomethanation. When the hydrogen-carbon dioxide ratio is high, the pH rises. Since prolonged periods of high pH inhibit methane fermentation of the sludge, it is considered desirable to minimize the time spent with high pH.

[0057] In this invention, the control of the operation of the control device is set such that the first duration for which the hydrogen-carbon dioxide ratio exceeds a predetermined ratio value is less than or equal to a first predetermined time, and the second duration for which the pH value measured by the pH meter 90 is greater than 9 is less than or equal to a second predetermined time. In the first duration and the second duration, "duration" refers to a continuous period of time.

[0058] The control of this operation is set so that, within a predetermined period, the first duration is less than or equal to the first predetermined time, and the second duration is less than or equal to the second predetermined time. The predetermined period can be set to any number of days between 1 day (24 hours) and 15 days. However, it is not limited to this period and may be set to more than 15 days. Various periods can be set depending on the sludge condition, weather, and season.

[0059] The hydrogen-carbon dioxide ratio is the ratio obtained by dividing the amount of hydrogen supplied from the hydrogen supply unit 20 by the amount of carbon dioxide contained in the biogas generated in the methane fermentation tank 10. The biogas production apparatus of this embodiment includes a carbon dioxide circulation unit 60 that supplies carbon dioxide separated in the separation unit 50 into the methane fermentation tank 10. Therefore, in this embodiment, the amount of carbon dioxide will be described in a case that includes the amount of carbon dioxide contained in the biogas generated in the methane fermentation tank 10 and the amount of carbon dioxide supplied from the carbon dioxide circulation unit 60.

[0060] By setting the amount of carbon dioxide in this way, it is possible to calculate the hydrogen-carbon dioxide ratio when the amount of carbon dioxide in the methane fermentation section increases.

[0061] As described above, the amount of hydrogen supplied from the hydrogen supply unit 20 can be adjusted by controlling the operation of an on-off valve (not shown) provided on the hydrogen cylinder 21 using a control device. Similarly, as described above, the amount of carbon dioxide supplied from the carbon dioxide circulation unit 60 can be adjusted by controlling the operation of an on-off valve (not shown) and a flow control valve (not shown) provided on the separation device 51 using a control device.

[0062] The concentrations of hydrogen and carbon dioxide can be measured at regular intervals by a gas concentration meter 30 installed in the biogas extraction channel 42, and the measurement results can be transmitted to a control device. The control device can be configured to calculate the hydrogen-carbon dioxide ratio based on the measured concentrations of hydrogen and carbon dioxide. The control device can also be configured to store the calculated hydrogen-carbon dioxide ratio and calculate a first duration during which the hydrogen-carbon dioxide ratio exceeds a predetermined set ratio value.

[0063] Alternatively, the carbon dioxide concentration may be determined by extracting a portion of the biogas from the biogas extraction channel 42, performing a component analysis by gas chromatography, and calculating the amount of carbon dioxide generated per hour based on that data.

[0064] The pH of the methane fermentation liquid can be measured at regular intervals by a pH meter 90 installed inside the methane fermentation tank 10 and transmitted to a control device. The control device can be configured to store the transmitted pH values ​​and calculate a second duration during which the pH value remains above 9.

[0065] As described above, the pH rises when the hydrogen-carbon dioxide ratio is high. Therefore, in this invention, the control of the operation of the control device is set so that the first duration for which the hydrogen-carbon dioxide ratio exceeds a predetermined ratio value is less than or equal to the first predetermined time.

[0066] Furthermore, as mentioned above, since it is desirable to minimize the time spent with a high pH, ​​the control device is configured to ensure that the second duration during which the pH value is greater than 9 is less than or equal to the second predetermined duration.

[0067] The control device controls the amount of hydrogen supplied from the hydrogen supply unit 20 and the amount of carbon dioxide supplied from the carbon dioxide circulation unit 60 so that, within a predetermined period, the first duration is less than or equal to the first predetermined time, and the second duration is less than or equal to the second predetermined time.

[0068] In this configuration, the first duration is set to be less than or equal to the first predetermined time, so the biogas production device 1 can be controlled so that the upper limit of the time during which the hydrogen-carbon dioxide ratio exceeds a predetermined ratio value is the first predetermined time. This suppresses the time during which the pH rises, making it less likely for methane fermentation of sludge to be inhibited.

[0069] Furthermore, in this configuration, since the second duration is set to be less than or equal to the second predetermined time, the biogas production device 1 can be controlled so that the upper limit of the time during which the pH value measured by the pH meter 90 is greater than 9 is the second predetermined time. This suppresses the time during which the pH is greater than 9, making it less likely for methane fermentation of sludge to be inhibited.

[0070] Therefore, this configuration suppresses the time it takes for the pH to rise and the time it takes for the pH to exceed 9, making it less likely for methane fermentation of sludge to be inhibited, and thus stabilizing biometanation and methane fermentation of sludge.

[0071] The predetermined ratio value should preferably be 4.0. However, it is not limited to this value and can be set to any value between 0.1 and 4.0, for example. Of these, it is preferable to set it to a value within the range of 2.7 to 3.7.

[0072] The first predetermined time should ideally be 10 hours. However, it is not limited to this value; it can be set between 0 and 10 hours, for example, 9 hours, 8 hours, 7 hours, 6 hours, or 5 hours.

[0073] The second prescribed time should ideally be 9 hours. However, it is not limited to this value; it can be set between 0 and 9 hours, for example, 8 hours, 7 hours, 5 hours, 5 hours, and 4 hours.

[0074] In this embodiment, we will describe the case where the predetermined ratio value is 4.0, the first predetermined time is 10 hours, and the second predetermined time is 9 hours.

[0075] In this configuration, a predetermined ratio value, a first predetermined time, and a second predetermined time can be set as upper limits.

[0076] The control system should be configured to supply sludge multiple times per day.

[0077] In other words, the supply of sludge to the methane fermentation tank 10 via the sludge supply port 15 is controlled by the control means to occur multiple times per day. This supply may be controlled to occur multiple times per day at predetermined time intervals, or the time interval may be shortened during periods when the amount of sludge supplied increases and lengthened during periods when the amount of sludge supplied decreases. In this embodiment, the case in which the supply is controlled to occur multiple times per day at predetermined time intervals will be described. In this case, the predetermined time interval may be set to any of the intervals from 1 to 12 hours, such as every hour, every 2 hours, every 3 hours, every 4 hours, every 6 hours, or every 12 hours, but it is not limited to these intervals.

[0078] As in this configuration, setting the sludge supply to occur multiple times a day stabilizes the hydrogen-carbon dioxide ratio and makes biomethanation more stable.

[0079] [Processing flow of the biogas production apparatus according to the first embodiment] Next, we will explain the process of treating sludge using the biogas production apparatus 1 equipped with the above configuration. First, we will explain the impact of the conversion of carbon dioxide to methane by methanation in the methane fermentation tank 10 on the operation of the system.

[0080] In the methane fermentation tank 10, as the conversion of carbon dioxide to methane by methanation progresses, the carbon dioxide concentration in the methane fermentate decreases, the pH of the methane fermentate increases, and the free ammonia concentration in the methane fermentate increases. Free ammonia reduces the activity of methanogenic archaea, so as the conversion of carbon dioxide to methane by methanation progresses, the activity of methanogenic archaea decreases, making it more difficult for methanation to proceed. Therefore, as the pH of the methane fermentate increases due to the progress of methanation, the operating efficiency of the system decreases, and ultimately the system itself may cease to function.

[0081] 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 fermentate so that the amount of free ammonia generated does not significantly reduce the activity of methane-producing archaea. Specifically, when the methane concentration in the methane fermenter rises, it is effective to reduce the amount of hydrogen supplied to the methane fermenter 10 to suppress the decrease in carbon dioxide, or to supply carbon dioxide to the methane fermenter 10. In this way, the pH of the elevated methane fermentate can be lowered, and the decrease in the activity of methane-producing archaea can be suppressed.

[0082] For example, a case is described in which the gas concentration generated in the methane fermentation tank 10 is measured by a gas concentration meter 30, and the hydrogen supply flow rate from the hydrogen supply unit 20 is adjusted according to the measured gas concentration. Specifically, when the methane concentration is below a threshold (90 volume%) in this example, hydrogen is supplied into the methane fermentation tank 10 at a flow rate (standard flow rate) that is necessary and sufficient for the conversion of carbon dioxide to methane. On the other hand, when the methane concentration exceeds the threshold, the supply of hydrogen into the methane fermentation tank 10 is reduced. Furthermore, in this embodiment, if the hydrogen supply is reduced, the supply of hydrogen at the standard flow rate is resumed when the methane concentration subsequently falls below 90 volume%.

[0083] Therefore, in a biogas production apparatus 1 of this type, first, sludge is supplied to the methane fermentation tank 10, and hydrogen is supplied to 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 undergoes methane fermentation in the methane fermentation tank 10, and biogas is generated.

[0084] Next, within the methane fermentation space 11, the generated biogas is used by methanogenic archaea in the methane fermentation liquid to convert some of the carbon dioxide in the biogas into methane (methanation) using hydrogen supplied from the hydrogen supply unit 20. The biogas, with some of the carbon dioxide converted into methane, is supplied to the separation unit 50 via the biogas outlet 41 and the biogas outlet 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 and reused for methanation. The separated methane is stored in the tank 52 via the methane outlet 53 and the methane outlet 54. Meanwhile, the treated water produced by methane fermentation is discharged to the outside from the treated water outlet 16.

[0085] As methanation progresses within the methane fermentation space 11, the concentration of methane increases as the amount of carbon dioxide in the biogas decreases. When the gas concentration measured by the gas concentration meter 30 exceeds a threshold (in this embodiment, the methane concentration is 90% by volume), the hydrogen supply from the hydrogen supply unit 20 is reduced.

[0086] This suppresses the reduction of carbon dioxide due to methanation, and by returning the carbon dioxide separated by the separation device 51 to the carbon dioxide circulation unit 60 for reuse in methanation, all the carbon dioxide in the generated biogas can be converted into methane.

[0087] Subsequently, when the methane concentration measured by the gas concentration meter 30 falls below 90% by volume, the hydrogen supply from the hydrogen supply unit 20 to the methane fermentation tank 10 is increased. This stabilizes the methane concentration within the methane fermentation space 11.

[0088] As described above, according to the biogas production apparatus 1, 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 a threshold due to the progress of methanation, the supply of hydrogen to the methane fermentation tank 10 is reduced, and the methane concentration in the methane fermentation tank 10 is lowered to below the threshold. Therefore, it is possible to prevent situations where the activity of methane-producing archaea decreases due to an increase in the pH of the methane fermentate, leading to a decrease in operating efficiency and the system itself ceasing to function, as well as situations where unreacted hydrogen remains. Thus, in the biogas production apparatus 1, methane fermentation and methanation can occur simultaneously in the methane fermentation section, enabling stable and efficient operation.

[0089] For example, if sludge is supplied once a day, biogas production will be high 2-3 hours after supply, but will decrease 23 hours later. In this case, the hydrogen supply may be controlled frequently, but if the hydrogen supply is controlled after the methane concentration (and simultaneously the pH) has risen, there is a risk that the activity of methane-producing archaea will decrease until the pH returns to an acceptable level (below 9.0). Considering this situation, it may be preferable to operate with a constant hydrogen supply.

[0090] In the control method for the biogas production apparatus 1 of the present invention, the operation control of the control device is set such that the first duration for which the hydrogen-carbon dioxide ratio exceeds a predetermined ratio value is less than or equal to a first predetermined time, and the second duration for which the pH value measured by the pH meter 90 is greater than 9 is less than or equal to a second predetermined time.

[0091] In this case, it is best to keep the hydrogen supply constant and set the sludge supply to occur multiple times per day.

[0092] The control device controls the amount of hydrogen supplied from the hydrogen supply unit 20 and the amount of carbon dioxide supplied from the carbon dioxide circulation unit 60 so that the first duration is less than or equal to the first predetermined time and the second duration is less than or equal to the second predetermined time.

[0093] In this embodiment, since the first duration is set to be less than or equal to the first predetermined time, the biogas production apparatus 1 can be controlled so that the upper limit of the time during which the hydrogen-carbon dioxide ratio exceeds a predetermined ratio value is the first predetermined time. This suppresses the time during which the pH rises, making it less likely for methane fermentation of sludge to be inhibited.

[0094] Furthermore, in this embodiment, since the second duration is set to be less than or equal to the second predetermined time, the biogas production apparatus 1 can be controlled so that the upper limit of the time during which the pH value measured by the pH meter 90 is greater than 9 is the second predetermined time. This suppresses the time during which the pH is greater than 9, making it less likely for methane fermentation of sludge to be inhibited.

[0095] Therefore, in the control method for the biogas production apparatus 1 of this embodiment, methane fermentation and methanation can be carried out simultaneously in the methane fermentation section, enabling stable and efficient operation.

[0096] [Modified example of a biogas production apparatus according to the first embodiment] In the above embodiment, in the case where the methane concentration generated in the methane fermentation tank 10 is measured by the gas concentration meter 30 and the hydrogen supply flow rate from the hydrogen supply unit 20 is adjusted according to the measured methane concentration, in the modified version, 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 the standard flow rate, if the hydrogen concentration is below the threshold (for example, 0.1 volume% (1000 ppm)), the amount of hydrogen supplied to the methane fermentation tank 10 is increased beyond the standard flow rate. On the other hand, if the hydrogen concentration exceeds the threshold, the supply of hydrogen to the methane fermentation tank 10 is decreased. 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 volume%), the hydrogen supply to the methane fermentation tank 10 is increased beyond the standard flow rate. On the other hand, when the carbon dioxide concentration is below the threshold, the hydrogen supply to the methane fermentation tank 10 is reduced.

[0097] [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. Figure 2 is a diagram showing the schematic configuration of the biogas production apparatus 70 according to the second embodiment. As shown in Figure 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 unit 80 (carbon dioxide storage means). The biogas production apparatus 70 according to the second embodiment will be described below, but the same configuration as the biogas production apparatus 1 according to the first embodiment will not be described.

[0098] As shown in Figure 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 unit 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.

[0099] In this embodiment, the carbon dioxide storage unit 80 consists 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 separation device 51 consists of a pressure fluctuation 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 is controlled by a control device. Therefore, the carbon dioxide storage unit 80 can supply biogas stored in the tank 81 into the methane fermentation space 11 at any desired amount and timing.

[0100] Furthermore, tank 81 is provided separately from tank 52, which constitutes the separation unit 50. In this embodiment, tank 81 stores carbon dioxide separated by the separation device 51.

[0101] [Processing flow of the biogas production apparatus according to the second embodiment] Next, we will explain the process of treating sludge using the biogas production apparatus 70 equipped with the above configuration.

[0102] To enable stable and efficient operation of the biogas production system, 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 the amount of carbon dioxide that can be returned to the methane fermentation tank 10 may not be sufficient. In such cases, the amount of carbon dioxide returned to the methane fermentation tank 10 will decrease, increasing the pH of the methane fermentate and posing a risk of system shutdown. Furthermore, there is a possibility that unreacted hydrogen supplied to the methane fermentation tank 10 may remain.

[0103] Therefore, in this embodiment, a tank 81 is provided to store the carbon dioxide separated by the separation device 51. The stored carbon dioxide is returned to the methane fermentation tank 10 via the carbon dioxide supply passage 82.

[0104] This allows all the carbon dioxide in the biogas produced to be converted into methane. 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, enabling stable methanation.

[0105] [Configuration of the biogas production apparatus according to the third embodiment] Next, the configuration of the biogas production apparatus 100 according to the third embodiment will be described. It differs from the first and second embodiments in that the methane fermentation section comprises 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 the same configuration as the biogas production apparatuses 170 according to the first and second embodiments will not be described.

[0106] As shown in Figure 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 downstream 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. 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 outlet 42, and a carbon dioxide circulation unit 60 that supplies 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.

[0107] As shown in Figure 3, the methanation tank 110 is composed of a housing. The methanation tank 110 is configured to biodegrade at least a portion of the digested sludge supplied from the methane fermentation tank 10 by methane-producing archaea, and to form a methanation space 111 in which carbon dioxide 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 temperature suitable for good methanation (for example, 30-37°C or 50-60°C) by the heat exchanger.

[0108] In this embodiment, one of the two opposing inner walls of the housing is provided with a digested sludge supply port 115 facing the methanation space 111 for supplying sludge generated in the methane fermentation tank 10, and a biogas supply port 113 facing the methanation space 111 for supplying biogas generated in the methane fermentation tank 10. The other of the two inner walls is provided with a methanation treated water discharge port 116 facing the methanation space 111 for discharging treated water to the outside.

[0109] The space above 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 produced in the methanation space 111.

[0110] 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. In addition, on the inner wall of the housing, the inner wall where the methanation treated water outlet 116 is provided, a biogas outlet 41 for extracting biogas to the biogas recovery unit 40 is provided, facing the biogas collection space 112.

[0111] In this embodiment, the carbon dioxide circulation unit 60 consists of a carbon dioxide supply port 62 and a carbon dioxide circulation path 61. 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, 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 via the carbon dioxide circulation path 61 and the carbon dioxide supply port 62. In this embodiment, the operation of the on / off valve (not shown) and flow rate control 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 into the methanation space 111 at any desired amount and timing.

[0112] [Processing flow of the biogas production apparatus according to the third embodiment] Next, we will explain the process of treating sludge using the biogas production apparatus 100 equipped with the above configuration.

[0113] 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, within the methanation tank 110, hydrogen supplied by the hydrogen supply unit 20 is used to methanate a portion of the carbon dioxide in the biogas by methane-producing archaea in the methanation tank 110. The biogas, in which a portion of the carbon dioxide has been converted to methane, is supplied to the separation unit 50 via the biogas outlet 41 and the biogas outlet 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 54. Meanwhile, the treated water produced by methane fermentation is discharged to the outside from the methanation treated water outlet 116.

[0114] Thus, in the biogas production apparatus 100 according to this embodiment, methane fermentation and methanation are carried out in separate tanks, which prevents the pH of the methane fermentation liquid from rising due to the decrease in carbon dioxide concentration as methanation progresses. Therefore, the biogas production apparatus 100 can be operated more stably and efficiently.

[0115] [Simulation of the amount of hydrogen required for methanation] The amount of methane obtained and the amount of hydrogen required, under the conditions of recycling carbon dioxide versus not recycling, were simulated using the case shown in Figure 1 as an example, with the following assumptions and definitions.

[0116] [Prerequisites] Prerequisite 1: It is assumed that the separation section downstream of the methane fermentation tank completely separates the gas into methane and carbon dioxide. Prerequisite 2: The hydrogen supplied to the methane fermentation tank is assumed to react completely with carbon dioxide and be converted into methane.

[0117] [Definition] Flow rate:Q(m 3 / h), methane concentration: C (volume %), carbon dioxide concentration: C' (volume %) Let Q1 be the amount of biogas generated from sludge (before hydrogen reaction) in the methane fermentation tank, of which C1 (volume %) is the methane concentration and C'1 (volume %) is the carbon dioxide concentration. In the gas separation section downstream of the methane fermentation tank, the amount recovered as the main component of carbon dioxide is Q2, of which the methane concentration is C2 (0 vol%) and the carbon dioxide concentration is C'2 (100 vol%). In the separation section downstream of the methane fermentation tank, the amount recovered as the main component is Q3, of which the methane concentration is C3 (100 vol%) and the carbon dioxide concentration is C'3 (0 vol%). Let Q4 be the amount of hydrogen required to react carbon dioxide with methane. Let Q be the total amount of biogas ultimately produced from the outlet of the methane fermentation tank, with C being the methane concentration and C' being the carbon dioxide concentration.

[0118] <If carbon dioxide is not recycled> The amount of hydrogen required to produce methane is four times the amount of carbon dioxide. Therefore, the amount of hydrogen required to perform methanation to a predetermined concentration can be calculated using the following formula. Q1 × (C'1 - C') × 4 ... (i)

[0119] <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, which is 1, so Q2 = Q × C'.

[0120] The mass balance for methane at the outlet and separation section of the methane fermentation tank is as follows: Q = Q1 + Q2 ... (ha) From (b) and (c), Q3 can be calculated using the following formula. Q1 = Q3…(2) Since the amount of methane at the outlet of the methane fermentation tank and the amount of methane separated in the separation section are the same, Q3 × C3 = Q × C Here, since Q1 = Q3 and C3 is 100% by volume, Q1 = (Q1 + Q2) × C Therefore, Q2 can be calculated using the following formula. Q2 = (1 - C) × Q1 / C ... (Ho)

[0121] The amount of CO2 present in the methane fermentation tank is It is Q1 × C'1 + Q2 × C'2. The amount of hydrogen required to perform methanation to a predetermined concentration can be calculated using the following formula. (Q1×C'1+Q2×C'2-Q×C')×4…(へ)

[0122] Here, using the following conditions as an example, we calculated the amount of hydrogen required and the amount of methane produced when carbon dioxide is not recycled and when it is recycled. <Condition> Q1: 100 (m 3 / h), C1:60 (volume %), C'1:40 (volume %) C:C' = 85 (volume %):15 (volume %)

[0123] <If carbon dioxide is not recycled> The required amount of hydrogen is given by equation (i) 100 × (0.40 - 0.15) × 4 = 100 (m 3 It is / h). The amount of methane obtained is 100 × 0.85 = 85 (m 3 / h)

[0124] <When circulating carbon dioxide> The amount of methane obtained is given by equation (ii) Q3 = 100 (m 3 It is / h). The amount of carbon dioxide recovered in the gas separation section is calculated using equation (e). Q2 = 17.65 (m 3 It is / h). The flow rate Q is given by equation (b) 100 + 17.65 = 117.65 (m 3 It is / h). The required amount of hydrogen is given by equation (f) (100×0.4+17.65×1-117.65×0.15)×4=160(m 3 / h)

[0125] Thus, by employing a system that separates and recycles carbon dioxide, it is possible to increase methane production compared to a system that does not recycle carbon dioxide.

[0126] <Another Embodiment> In the embodiments described above, sludge was treated as organic waste, but the invention is not limited to this, and methods for treating food waste, bioplastic hydrolysates, etc., may also be used. Furthermore, a mixture of these organic wastes may be treated.

[0127] In the embodiments described above, 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, facing the methane fermentation space 11. However, the embodiment is not limited to this configuration. For example, the hydrogen supply port 23 and the carbon dioxide supply port 62 may be provided at the ceiling of the housing, facing the biogas collection space 12.

[0128] In the above embodiment, an configuration in which the hydrogen supply line 22 and the carbon dioxide circulation unit 60 are connected separately to the methane fermentation tank 10 (or methanation tank 110) has been described, but the embodiment is not limited to this. For example, the hydrogen supply line 22 and the carbon dioxide circulation unit 60 may be combined, and the hydrogen and carbon dioxide may be mixed before being supplied to the methane fermentation tank 10 (or methanation tank 110). Furthermore, although the above embodiment describes an embodiment in which the hydrogen supply channel 22 and the carbon dioxide supply channel 82 are connected separately to the methane fermentation tank 10, the invention is not limited to this embodiment. For example, the hydrogen supply channel 22 and the carbon dioxide supply channel 82 may be combined, and the hydrogen and carbon dioxide may be mixed before being supplied to the methane fermentation tank 10.

[0129] Furthermore, the configurations disclosed in the above embodiments (including other embodiments, the same applies hereinafter) can be applied in combination with configurations disclosed in other embodiments, provided that no inconsistencies arise. Moreover, the embodiments disclosed herein are illustrative, and the embodiments of the present invention are not limited thereto, and can be modified as appropriate without departing from the object of the present invention. [Examples]

[0130] [Example 1] In the biogas production device 1, as the methane fermentation tank 10, a jar fermenter (NBC-3000: manufactured by Mitsuwafrontier Co., Ltd.) with a capacity of 3.0 L was used, and a hydrogen supply port 23 was installed. An air stone (Ibuki air stone round series 10φ round: manufactured by King Grinding Co., Ltd.) was attached to the tip of the hydrogen supply port 23. 2.0 L of high-temperature anaerobic digestion sludge collected from a general food waste methane fermentation facility was introduced, and after purging the headspace for 5 minutes with a nitrogen flow rate of 1.0 L / min, the lid was closed, and it was heated to 55°C to construct a laboratory-scale methane fermentation tank 10.

[0131] As a substrate for methane fermentation, water was added to food waste made into a paste with a mixer so that the solid content concentration was 4%, and the prepared material was added to the methane fermentation tank 10 at 33.5 mL / L every 12 hours so that the residence time in the methane fermentation tank 10 was 15 days 消化汚泥 / day (input twice a day), and the excess sludge was also withdrawn at 33.5 mL / L every 12 hours 消化汚泥 / day. The stirring of the methane fermentation tank 10 was set to 500 rpm with the attached agitator. Under these conditions, the methane fermentation tank 10 (biogas production device 1) was operated for 15 days, and after confirming that the biogas generation amount was stable at 2.0 L / L 消化汚泥 / day, methane concentration 55%, and carbon dioxide concentration 45%, hydrogen was started to be injected by the hydrogen supply unit 20 at 2.4 L / L 消化汚泥 / day. Hydrogen was injected in a constant amount at all times.

[0132] Since it was assumed that the biogas generation amount would change over time after the food waste was input, on the first to third days of operation under the above conditions, the biogas flow meter (BPC go: manufactured by BPC Instruments), which is the gas concentration meter 30, was used to measure the biogas generation amount every hour.

[0133] The obtained biogas was analyzed for its components every other day using gas chromatography (990 microGC: GL Sciences Co., Ltd.). Based on this data, the amount of carbon dioxide generated each hour was calculated, and the effective hydrogen-carbon dioxide ratio each hour was then calculated using a control device. The average daily hydrogen-carbon dioxide ratio during this period was calculated to be 2.7.

[0134] Furthermore, the pH was measured using a pH meter (AS700: manufactured by AS ONE Corporation), which is a pH measuring instrument 90.

[0135] This operation was carried out for 15 days, the same as the residence time of the main methane fermentation tank 10, and the amount of gas generated and the gas composition were checked on the 15th day.

[0136] Table 1 shows the concentrations of each gas (methane, carbon dioxide, and hydrogen) on day 1 and day 15.

[0137] [Table 1]

[0138] In this invention, a control device calculates the first duration during which the hydrogen-carbon dioxide ratio exceeds a predetermined ratio value. In this embodiment 1, the first duration was calculated to be 5 hours when the predetermined ratio value was set to 4.0. The total time during which the hydrogen-carbon dioxide ratio exceeds 4.0 in a day was 10 hours.

[0139] Furthermore, the control device calculated the second duration during which the pH value was greater than 9, which was found to be 4 hours. The total duration during which the pH value was greater than 9 was 8 hours per day.

[0140] The operating status of biogas production unit 1 over a 15-day period (biogas generation amount and methane concentration on day 15) was evaluated according to the following criteria. Grade A: The percentage decrease in biogas production on day 15 compared to day 1 is less than 10%, and the percentage decrease in methane concentration on day 15 compared to day 1 is less than 5%. Grade B: The decrease in biogas production on day 15 compared to day 1 is within the range of 10-20%, and the decrease in methane concentration on day 15 compared to day 1 is less than 5%. Grade C: The percentage decrease in biogas production on day 15 compared to day 1 is greater than 20%, or the percentage decrease in methane concentration on day 15 compared to day 1 is 5% or more.

[0141] The operating status of the biogas production apparatus 1 in Example 1 was evaluated and received a rating of B.

[0142] [Example 2] In this Example 2, the input conditions for the substrate, which is food waste, were changed to 5.6 mL / L every two hours, as in Example 1. 消化汚泥 The conditions are set to 12 times per day, with excess sludge being removed every 2 hours at a rate of 5.6 mL / L. 消化汚泥 Biogas production device 1 was operated for 15 days under the same conditions as the previous operation (per day).

[0143] In Example 2, the average daily hydrogen-carbon dioxide ratio was calculated to be 2.7. Table 1 shows the concentrations of each gas (methane, carbon dioxide, and hydrogen) on day 1 and day 15.

[0144] Furthermore, in Example 2, the first duration during which the hydrogen-carbon dioxide ratio exceeds the predetermined ratio was calculated, with a predetermined ratio value of 4.0, and it was found to be 0 hours. The total time during which the hydrogen-carbon dioxide ratio exceeds 4.0 in a day was 0 hours.

[0145] Furthermore, when calculating the second duration during which the pH value was greater than 9, it was found to be 0 hours. In addition, the total amount of time during the day when the pH value was greater than 9 was 0 hours.

[0146] Figure 4 shows the changes in gas amounts, hydrogen-carbon dioxide ratio, and pH value up to the third day when food waste was added 12 times a day. In other words, even with the same daily food waste input and hydrogen supply, the pH did not exceed 9.0 for longer periods compared to when organic waste was added once a day (Comparative Example 1 described later), indicating that operation was more stable and biometanation was more stable.

[0147] The operating status of the biogas production apparatus 1 in Example 2 was evaluated and received an A rating.

[0148] [Example 3] In this Example 3, the input conditions for the substrate, which is food waste, were changed to 5.6 mL / L every two hours, as in Example 1. 消化汚泥 The conditions are set to 12 times per day, with excess sludge being removed every 2 hours at a rate of 5.6 mL / L. 消化汚泥 The process is carried out at a rate of 3.4 L / L per day, with hydrogen supplied by the hydrogen supply unit 20. 消化汚泥 Biogas production device 1 was operated for 15 days under the same conditions except for the injection rate per day.

[0149] In Example 3, the average daily hydrogen-carbon dioxide ratio was calculated to be 3.7. Table 1 shows the concentrations of each gas (methane, carbon dioxide, and hydrogen) on day 1 and day 15.

[0150] Furthermore, in Example 3, the first duration during which the hydrogen-carbon dioxide ratio exceeds the predetermined ratio was calculated using a predetermined ratio value of 4.0, and it was found to be 1 hour. The total duration during which the hydrogen-carbon dioxide ratio exceeds 4.0 in a day was 12 hours.

[0151] Furthermore, the second duration during which the pH value was greater than 9 was calculated to be 1 hour. The total time during which the pH value was greater than 9 in a day was 12 hours.

[0152] The operating status of the biogas production apparatus 1 in Example 3 was evaluated and received an A rating.

[0153] [Example 4] In this Example 4, the input conditions for the substrate, which is food waste, were changed to 2.8 mL / L every hour, as in Example 1. 消化汚泥 The conditions are set to 24 injections per day, with excess sludge being removed every hour at a rate of 2.8 mL / L. 消化汚泥 Biogas production device 1 was operated for 15 days under the same conditions as the previous operation (per day).

[0154] In Example 4, the average daily hydrogen-carbon dioxide ratio was calculated to be 2.7. Table 1 shows the concentrations of each gas (methane, carbon dioxide, and hydrogen) on day 1 and day 15.

[0155] Furthermore, in Example 4, the first duration during which the hydrogen-carbon dioxide ratio exceeds the predetermined ratio was calculated using a predetermined ratio value of 4.0, and it was found to be 0 hours. The total duration during which the hydrogen-carbon dioxide ratio exceeds 4.0 in a single day was 0 hours.

[0156] Furthermore, when calculating the second duration during which the pH value was greater than 9, it was found to be 0 hours. In addition, the total amount of time during the day when the pH value was greater than 9 was 0 hours.

[0157] The operating status of the biogas production apparatus 1 in Example 4 was evaluated and received an A rating.

[0158] [Example 5] In this Example 5, the input conditions for the substrate, which is food waste, were changed to 2.8 mL / L every hour, as in Example 1. 消化汚泥 The conditions are set to 24 injections per day, with excess sludge being removed every hour at a rate of 2.8 mL / L. 消化汚泥 The process is carried out at a rate of 3.4 L / L per day, with hydrogen supplied by the hydrogen supply unit 20. 消化汚泥 Biogas production device 1 was operated for 15 days under the same conditions except for the injection rate per day.

[0159] In Example 5, the average daily hydrogen-carbon dioxide ratio was calculated to be 3.7. Table 1 shows the concentrations of each gas (methane, carbon dioxide, and hydrogen) on day 1 and day 15.

[0160] Furthermore, in Example 5, the first duration during which the hydrogen-carbon dioxide ratio exceeds the predetermined ratio was calculated using a predetermined ratio value of 4.0, and it was found to be 0 hours. The total duration during which the hydrogen-carbon dioxide ratio exceeds 4.0 in a single day was 0 hours.

[0161] Furthermore, when calculating the second duration during which the pH value was greater than 9, it was found to be 0 hours. In addition, the total amount of time during the day when the pH value was greater than 9 was 0 hours.

[0162] The operating status of the biogas production apparatus 1 in Example 5 was evaluated and received an A rating.

[0163] [Example 6] In Example 6, the input conditions for the substrate, which is food waste, were changed to 33.5 mL / L every 12 hours, as in Example 1. 消化汚泥 The conditions are set to 1 / day (addition twice a day), and excess sludge is removed every 12 hours at a rate of 33.5 mL / L. 消化汚泥 The process is carried out at a rate of 2.8 L / L per day, with hydrogen supplied by the hydrogen supply unit 20. 消化汚泥 Biogas production device 1 was operated for 15 days under the same conditions except for the injection rate per day.

[0164] In Example 6, the average daily hydrogen-carbon dioxide ratio was calculated to be 3.1. Table 1 shows the concentrations of each gas (methane, carbon dioxide, and hydrogen) on day 1 and day 15.

[0165] Furthermore, in Example 6, the first duration during which the hydrogen-carbon dioxide ratio exceeds the predetermined ratio was calculated using a predetermined ratio value of 4.0, and it was found to be 9 hours. The total time during which the hydrogen-carbon dioxide ratio exceeds 4.0 in a day was 18 hours.

[0166] Furthermore, the second duration during which the pH value was greater than 9 was calculated to be 8 hours. The total time during which the pH value was greater than 9 in a day was 16 hours.

[0167] The operating status of the biogas production apparatus 1 in Example 6 was evaluated and received a rating of B. The biogas output on day 15 was 1.6 L / L. 消化汚泥 The decrease in biogas production per day was 20%. Therefore, the operating condition is acceptable if the second duration is around 8 hours (rated B), but if it exceeds 9 hours, the decrease in biogas production is expected to exceed 20%. For this reason, it is considered best to limit the second duration to 9 hours or less.

[0168] [Comparative Example 1] In this Comparative Example 1, the input conditions for the substrate, food waste, were changed to once a day at 67.0 mL / L, as in Example 1. 消化汚泥 The conditions are as follows: Excess sludge is removed once a day, at a rate of 67.0 mL / L. 消化汚泥 Biogas production device 1 was operated for 15 days under the same conditions as the previous operation (per day).

[0169] In Comparative Example 1, the average daily hydrogen-carbon dioxide ratio was calculated to be 2.7. Table 1 shows the concentrations of each gas (methane, carbon dioxide, and hydrogen) on day 1 and day 15.

[0170] In Comparative Example 1, the first duration during which the hydrogen-carbon dioxide ratio exceeds the predetermined ratio was calculated using a predetermined ratio value of 4.0, and it was found to be 16 hours. The total duration during which the hydrogen-carbon dioxide ratio exceeds 4.0 in a single day was 16 hours.

[0171] Furthermore, the second duration during which the pH value was greater than 9 was calculated to be 15 hours. The total time during which the pH value was greater than 9 in a single day was 15 hours.

[0172] Figure 5 shows the changes in the amount of each gas, the hydrogen-carbon dioxide ratio, and the pH value up to the third day when food waste was added once a day. Specifically, in this comparative example, the first duration was 16 hours, and the time during which the pH exceeded 9.0 was long (second duration was 16 hours), so it was found that the operation was prone to instability.

[0173] When the operating condition of Biogas Production System 1 in Comparative Example 1 was evaluated, it received a rating of C.

[0174] [Comparative Example 2] In this comparative example 2, the input conditions for the substrate, food waste, were changed to 2.8 mL / L every hour, as in Example 1. 消化汚泥 The conditions are set to 24 injections per day, with excess sludge being removed every hour at a rate of 2.8 mL / L. 消化汚泥 The process is carried out at a rate of 3.8 L / L per day, with hydrogen supplied by the hydrogen supply unit 20. 消化汚泥 Biogas production device 1 was operated for 15 days under the same conditions except for the injection rate per day.

[0175] In Comparative Example 2, the average daily hydrogen-carbon dioxide ratio was calculated to be 4.2. Table 1 shows the concentrations of each gas (methane, carbon dioxide, and hydrogen) on day 1 and day 15.

[0176] In Comparative Example 2, the first duration during which the hydrogen-carbon dioxide ratio exceeds the predetermined ratio was calculated using a predetermined ratio value of 4.0, and it was found to be 24 hours. The total duration during which the hydrogen-carbon dioxide ratio exceeds 4.0 in a day was 24 hours.

[0177] Furthermore, the duration of the hydrogen-carbon dioxide ratio exceeding 4.0 was 10 hours, and at this time, the decrease in biogas production exceeded 20%. Therefore, it is considered best to set the first predetermined time to 10 hours or less.

[0178] Furthermore, when calculating the second duration during which the pH value is greater than 9, it was determined to be 24 hours. The total time during which the pH value is greater than 9 in a day was 24 hours.

[0179] When the operating condition of Biogas Production System 1 in Comparative Example 2 was evaluated, it received a rating of C.

[0180] [Reference example 1] In this Reference Example 1, the input conditions for the substrate, food waste, in Example 1 were set to 67.0 mL / L once a day. 消化汚泥 The conditions are as follows: Excess sludge is removed once a day, at a rate of 67.0 mL / L. 消化汚泥 The biogas production device 1 was operated for 15 days under the same conditions as above, except that the procedure was performed daily and no hydrogen was injected.

[0181] In Reference Example 1, no hydrogen injection was performed, so the hydrogen-carbon dioxide ratio was not calculated. Table 1 shows the concentrations of each gas (methane, carbon dioxide, and hydrogen) on day 1 and day 15.

[0182] When the operating status of Biogas Production System 1 in Reference Example 1 was evaluated, it received an A rating.

[0183] [Reference example 2] In this Reference Example 1, the input conditions for the substrate, food waste, in Example 1 were set to 2.8 mL / L every hour. 消化汚泥 The conditions are set to 24 injections per day, with excess sludge being removed every hour at a rate of 2.8 mL / L. 消化汚泥 The biogas production device 1 was operated for 15 days under the same conditions as above, except that the procedure was performed daily and no hydrogen was injected.

[0184] In Reference Example 2, no hydrogen injection was performed, so the hydrogen-carbon dioxide ratio was not calculated. Table 1 shows the concentrations of each gas (methane, carbon dioxide, and hydrogen) on day 1 and day 15.

[0185] When the operating status of Biogas Production System 1 in Reference Example 2 was evaluated, it received an A rating.

[0186] [Reference example 3] In this Reference Example 3, in Example 1, the input conditions for the substrate, food waste, were set to 67.0 mL / L once a day. 消化汚泥The conditions are as follows: Excess sludge is removed once a day, at a rate of 67.0 mL / L. 消化汚泥 The process is carried out at a rate of 1.8 L / L per day, with hydrogen supplied by the hydrogen supply unit 20. 消化汚泥 Biogas production device 1 was operated for 15 days under the same conditions except for the injection rate per day.

[0187] In Reference Example 3, the average daily hydrogen-carbon dioxide ratio was calculated to be 2.0. Table 1 shows the concentrations of each gas (methane, carbon dioxide, and hydrogen) on day 1 and day 15.

[0188] Furthermore, in Reference Example 3, the first duration during which the hydrogen-carbon dioxide ratio exceeds the predetermined ratio was calculated using a predetermined ratio value of 4.0, and it was found to be 9 hours. The total time during which the hydrogen-carbon dioxide ratio exceeds 4.0 in a day was 9 hours.

[0189] Furthermore, the second duration during which the pH value was greater than 9 was calculated to be 8 hours. The total time during which the pH value was greater than 9 was 8 hours in a single day.

[0190] Biogas production on day 15 was 1.7 L / L 消化汚泥 The biogas output decreased by 20% per day. When the operating status of Biogas Production System 1 in Reference Example 3 was evaluated, it received a rating of B. [Industrial applicability]

[0191] This invention can be used in a biogas production device that processes organic matter using methane fermentation. [Explanation of symbols]

[0192] 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 unit (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 section to which organic matter is supplied, a hydrogen supply means for supplying hydrogen to the methane fermentation section, a control means for controlling operation, and a pH measuring unit for measuring the pH of the methane fermentation liquid in the methane fermentation section, wherein methanation in the methane fermentation section is performed, A biogas production apparatus in which the operation control in the control means is set such that the first duration for which the hydrogen-carbon dioxide ratio, obtained by dividing the amount of hydrogen supplied from the hydrogen supply means by at least the amount of carbon dioxide contained in the biogas generated in the methane fermentation section, is greater than a predetermined ratio value is less than or equal to a first predetermined duration, and the second duration for which the pH value measured by the pH measuring unit is greater than 9 is less than or equal to a second predetermined duration.

2. A recovery means for recovering biogas generated in the methane fermentation section, A separation unit for separating carbon dioxide from the biogas recovered by the aforementioned recovery means, The system includes a carbon dioxide circulation means for supplying the carbon dioxide separated in the separation unit to the methane fermentation unit, The biogas production apparatus according to claim 1, wherein the amount of carbon dioxide in the hydrogen-carbon dioxide ratio includes the amount of carbon dioxide supplied from the carbon dioxide recycling means.

3. The biogas production apparatus according to claim 1 or 2, wherein the control of the operation in the control means is set to supply the organic matter multiple times per day.

4. The biogas production apparatus according to claim 1 or 2, wherein the predetermined ratio value is 4.0, the first predetermined time is 10 hours, and the second predetermined time is 9 hours.

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

6. The biogas production apparatus according to claim 5, wherein the gas measuring means measures the concentration of at least one of methane, carbon dioxide, and hydrogen.

7. The biogas production apparatus according to claim 2, wherein the separation unit comprises one of the following: a membrane separation unit that separates the gas in the biogas using a separation membrane; a chemical absorption unit that separates the gas in the biogas by absorbing it into an absorbent liquid; and a pressure fluctuation adsorption unit that separates the gas in the biogas by adsorbing it onto an adsorbent.

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

9. A control method for a biogas production apparatus comprising: a methane fermentation section to which organic matter is supplied; a hydrogen supply means for supplying hydrogen to the methane fermentation section; a control means for controlling operation; and a pH measuring unit for measuring the pH of the methane fermentation liquid in the methane fermentation section, wherein the apparatus is configured to enable methanation in the methane fermentation section, A control method for a biogas production apparatus, wherein the operation control in the control means is set such that the first duration for which the hydrogen-carbon dioxide ratio, obtained by dividing the amount of hydrogen supplied from the hydrogen supply means by at least the amount of carbon dioxide contained in the biogas generated in the methane fermentation section, is greater than a predetermined ratio value is less than or equal to a first predetermined duration, and the second duration for which the pH value measured by the pH measuring unit is greater than 9 is less than or equal to a second predetermined duration.

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