Methanation system and methanation method

Abstract

To provide a methanation system and methanation method capable of suppressing the decline in reaction efficiency in methanation using biogas.SOLUTION: A methanation system comprises: a methane fermentation tank configured to ferment an organic waste to generate a biogas containing methane and carbon dioxide; a reactor configured to react the carbon dioxide generated in the methane fermentation tank with hydrogen supplied from a hydrogen supplier to generate methane; and a biogas introduction pipe for introducing biogas from the methane fermentation tank to the reactor, a first hydrogen supply pipe for supplying hydrogen from the hydrogen supply device to the reactor, and a second hydrogen supply pipe for supplying hydrogen from the hydrogen supply device to the methane fermentation tank; a carbon dioxide meter for measuring the amount of carbon dioxide introduced into the reactor through the biogas introduction pipe; and a controller, wherein the controller controls the amount of hydrogen supplied to the methane fermentation tank via the second hydrogen supply pipe, in accordance with the amount of carbon dioxide measured by the carbon dioxide meter.SELECTED DRAWING: Figure 1

Description

[Technical field]

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

[0002] As a facility for treating organic waste, also known as biomass, a biogasification facility equipped with a methane fermentation tank that ferments the organic waste to generate biogas containing methane and carbon dioxide is known. In order to reuse the carbon dioxide contained in the biogas generated in the methane fermentation tank, the following Patent Document 1 discloses a system equipped with a methanation device that combines hydrogen with the carbon dioxide contained in the biogas to generate methane.

[0003] In such a methanation device, the amount of hydrogen supplied to the methanation device is controlled according to the amount of carbon dioxide introduced into the methanation device. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] International Publication No. 2021 / 261417 Summary of the Invention [Problem to be solved by the invention]

[0005] Since the properties of the organic waste introduced into the methane fermentation tank are not uniform, the amount and composition ratio of the biogas generated in the methane fermentation tank constantly fluctuates. As a result, the amount and ratio of carbon dioxide in the biogas introduced into the methanation device constantly fluctuates. If the amount of carbon dioxide in the methanation device increases, the amount of reaction heat generated when the carbon dioxide reacts with hydrogen increases accordingly. If reaction heat accumulates in the methanation device, there is a risk that the reaction efficiency between carbon dioxide and hydrogen will decrease. Such problems are not mentioned in the above Patent Document 1, and there is room for improvement in methanation using biogas.

[0006] The present disclosure has been made in consideration of the above-mentioned problems, and aims to provide a methanation system and a methanation method that can suppress a decrease in reaction efficiency in methanation using biogas. [Means for solving the problem]

[0007] A methanation system according to one embodiment of the present disclosure includes a methane fermentation tank that ferments organic waste to generate biogas containing methane and carbon dioxide, a reactor that produces methane by reacting carbon dioxide generated in the methane fermentation tank with hydrogen supplied from a hydrogen supply device, a biogas inlet pipe that introduces the biogas generated in the methane fermentation tank from the methane fermentation tank to the reactor, a first hydrogen supply pipe that supplies hydrogen from the hydrogen supply device to the reactor, a second hydrogen supply pipe that supplies hydrogen from the hydrogen supply device to the methane fermentation tank, a carbon dioxide amount meter that measures the amount of carbon dioxide introduced into the reactor through the biogas inlet pipe, and a controller, wherein the controller controls the amount of hydrogen supplied to the methane fermentation tank through the second hydrogen supply pipe in accordance with the amount of carbon dioxide measured by the carbon dioxide amount meter.

[0008] A methanation method according to another aspect of the present disclosure includes fermenting organic waste in a methane fermentation tank to generate biogas containing methane and carbon dioxide, reacting hydrogen with the carbon dioxide contained in the biogas extracted from the methane fermentation tank to generate methane, measuring the amount of carbon dioxide contained in the biogas, and supplying hydrogen to the methane fermentation tank in accordance with the measured amount of carbon dioxide. Effect of the Invention

[0009] According to the present disclosure, it is possible to suppress a decrease in reaction efficiency in methanation using biogas. [Brief description of the drawings]

[0010] [Figure 1] FIG. 1 is a block diagram showing a schematic configuration of a methanation system according to an embodiment of the present disclosure. [Diagram 2] FIG. 2 is a block diagram showing a schematic configuration of a methanation system according to a modification of the embodiment shown in FIG. [Diagram 3] FIG. 3 is a block diagram showing a schematic configuration of a methanation system according to another modified example of the embodiment shown in FIG. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] Hereinafter, an embodiment will be described in detail with reference to the drawings. In the following, the same or corresponding elements are denoted by the same reference numerals throughout the drawings, and duplicated descriptions thereof will be omitted.

[0012] Fig. 1 is a block diagram showing a schematic configuration of a methanation system according to an embodiment of the present disclosure. The methanation system 1 in this embodiment includes a biogasification facility 2 and a methanation device 3. The biogasification facility 2 is a facility that ferments and decomposes organic waste such as food waste. The biogasification facility 2 includes a methane fermentation tank 4 into which organic waste is input and which ferments the organic waste to generate biogas containing methane and carbon dioxide.

[0013] The methanation device 3 includes a reactor 5 that produces methane by reacting carbon dioxide with hydrogen. The methane fermentation tank 4 and the reactor 5 are connected via a biogas introduction pipe 6. The biogas introduction pipe 6 introduces the biogas generated in the methane fermentation tank 4 into the reactor 5. The biogasification facility 2 may also have a treatment device that performs a predetermined treatment, such as desulfurization, on the biogas generated in the methane fermentation tank 4. In this case, the biogas introduction pipe 6 is connected between the treatment device and the reactor 5.

[0014] Furthermore, the methanation system 1 includes a hydrogen supply device 7, a first hydrogen supply pipe 8, and a second hydrogen supply pipe 9. The hydrogen supply device 7 is, for example, a hydrogen generator that generates hydrogen by electrolyzing water or a hydrogen tank that stores hydrogen. The first hydrogen supply pipe 8 is connected between the hydrogen supply device 7 and the reactor 5, and supplies hydrogen from the hydrogen supply device 7 to the reactor 5. The second hydrogen supply pipe 9 is connected between the hydrogen supply device 7 and the methane fermentation tank 4, and supplies hydrogen from the hydrogen supply device 7 to the methane fermentation tank 4.

[0015] In this embodiment, the first hydrogen supply pipe 8 and the second hydrogen supply pipe 9 have a pipe section 10 that is partially shared with each other, including a connection section to the hydrogen supplier 7. Alternatively, the first hydrogen supply pipe 8 and the second hydrogen supply pipe 9 may be configured as pipes that are entirely independent of each other.

[0016] In such a methanation system 1, biogas mainly containing methane and carbon dioxide is generated by fermenting organic waste in the methane fermentation tank 4. Since the properties of the organic waste introduced into the methane fermentation tank 4 are not uniform, the amount and composition ratio of the biogas generated in the methane fermentation tank 4 constantly fluctuates. Thereafter, the carbon dioxide contained in the biogas extracted from the methane fermentation tank 4 is supplied to the reactor 5 of the methanation device 3, where the carbon dioxide reacts with hydrogen supplied from the hydrogen supply unit 7 to produce methane. A discharge pipe 17 is connected to the reactor 5, which discharges the produced gas containing a high concentration of methane.

[0017] The methanation system 1 includes a first regulating valve 11 and a second regulating valve 12. The first regulating valve 11 is installed in the first hydrogen supply pipe 8 and adjusts the amount of hydrogen supplied to the reactor 5. The second regulating valve 12 is installed in the second hydrogen supply pipe 9 and adjusts the amount of hydrogen supplied to the methane fermentation tank 4. The first regulating valve 11 and the second regulating valve 12 are configured to be adjustable independently of each other.

[0018] The methanation system 1 includes a controller 13. The controller 13 includes a processing circuit 14 that performs various signal processing. The processing circuit 14 has a computer such as a microcontroller, a personal computer, or a PLC (Programmable Logic Controller). More specifically, the processing circuit 14 includes a processor, a memory, and peripheral circuits. The processor includes, for example, a CPU or an MPU. The memory includes a ROM, a RAM, a register, a non-volatile storage, and the like. The peripheral circuits include an input / output interface, and the like. Furthermore, the controller 13 may include an input device for a user to input operations and an output device such as a monitor that outputs a control state.

[0019] It should be noted that the functions of the elements disclosed herein can be performed using circuits or processing circuits, including general purpose processors, special purpose processors, integrated circuits, ASICs (Application Specific Integrated Circuits), conventional circuits, and / or combinations thereof, configured or programmed to perform the disclosed functions. A processor is considered a processing circuit or circuit, since it includes transistors and other circuits. In this specification, a circuit, unit, means, or part is hardware that performs the recited functions or hardware that is programmed to perform the recited functions. The hardware may be hardware disclosed in this specification, or other known hardware that is programmed or configured to perform the recited functions. In the case where the hardware is a processor, which is considered a type of circuit, the circuit, unit, or means is a combination of hardware and software, and the software is used to configure the hardware and / or the processor.

[0020] The memory stores a control program. The processor reads out the control program from the memory and generates command values ​​for controlling the first regulating valve 11 and the second regulating valve 12, which are objects to be controlled, based on the control program. In this way, the controller 13 controls the amount of hydrogen supplied to the reactor 5 through the first hydrogen supply pipe 8 and the amount of hydrogen supplied to the methane fermentation tank 4 through the second hydrogen supply pipe 9.

[0021] For such control, the methanation system 1 is equipped with a carbon dioxide amount meter 15. The carbon dioxide amount meter 15 is installed in the biogas inlet pipe 6 and measures the amount A1 of carbon dioxide introduced from the methane fermentation tank 4 through the biogas inlet pipe 6 to the reactor 5. For example, the carbon dioxide amount meter 15 has a flowmeter that measures the flow rate of carbon dioxide and a concentration meter that measures the concentration of carbon dioxide. The carbon dioxide amount meter 15 may calculate the amount of carbon dioxide from the flow rate and concentration of carbon dioxide, or the controller 13 may calculate the amount of carbon dioxide from the acquired flow rate and concentration.

[0022] The controller 13 controls the amount of hydrogen supplied to the reactor 5 through the first hydrogen supply pipe 8 in accordance with the amount of carbon dioxide measured by the carbon dioxide amount meter 15. At this time, the controller 13 determines the amount of hydrogen supplied to the reactor 5 so that the ratio of carbon dioxide to hydrogen in the reactor 5 is constant.

[0023] In the methanation carried out in the reactor 5, carbon dioxide and hydrogen react to produce methane and water as follows. CO2+4H2→CH4+2H2O

[0024] In the above reaction, the molar ratio of carbon dioxide to hydrogen is carbon dioxide:hydrogen=1:4. Therefore, ideally, 4 moles of hydrogen are required to be supplied per mole of carbon dioxide supplied to the reactor 5. The amount of hydrogen to be supplied to the reactor 5 is determined based on this ratio.

[0025] By controlling the hydrogen supplied to reactor 5 so that the ratio of carbon dioxide to hydrogen in reactor 5 is constant, it is possible to prevent a decrease in the reaction efficiency in reactor 5 even if the amount of carbon dioxide introduced into reactor 5 fluctuates.

[0026] However, if the amount of carbon dioxide supplied to the reactor 5 increases due to a change in the amount of biogas or the composition ratio of the biogas supplied to the reactor 5, the amount of reaction heat generated when the carbon dioxide reacts with hydrogen increases accordingly. If the reaction heat accumulates in the reactor 5, there is a risk that the efficiency of the reaction between carbon dioxide and hydrogen will decrease.

[0027] Therefore, in this embodiment, in addition to the first hydrogen supply pipe 8 that supplies hydrogen to the reactor 5, a second hydrogen supply pipe 9 that supplies hydrogen to the methane fermentation tank 4 is provided. In addition, the controller 13 controls the amount of hydrogen supplied to the methane fermentation tank 4 through the second hydrogen supply pipe 9 in accordance with the amount of carbon dioxide measured by the carbon dioxide amount meter 15.

[0028] More specifically, the controller 13 increases the amount of hydrogen supplied to the methane fermentation tank 4 when the amount of carbon dioxide measured by the carbon dioxide amount meter 15 increases, and decreases the amount of hydrogen supplied to the methane fermentation tank 4 when the amount of carbon dioxide measured by the carbon dioxide amount meter 15 decreases.

[0029] According to this configuration, when the amount of carbon dioxide introduced into the reactor 5 increases, the amount of hydrogen supplied to the methane fermentation tank 4 increases. By supplying hydrogen to the methane fermentation tank 4, the carbon dioxide contained in the biogas generated in the methane fermentation tank 4 reacts with the hydrogen in the methane fermentation tank 4 to produce methane. In other words, the amount of carbon dioxide contained in the biogas generated in the methane fermentation tank 4 decreases by supplying hydrogen.

[0030] For this reason, the amount of carbon dioxide introduced into the reactor 5 decreases, and the amount of heat generated in the reactor 5 decreases. When the amount of carbon dioxide introduced into the reactor 5 decreases, the amount of hydrogen supplied to the methane fermentation tank 4 also decreases accordingly. Note that when control is performed to reduce the amount of hydrogen, the amount of hydrogen may be set to zero, that is, the supply of hydrogen to the methane fermentation tank 4 may be stopped.

[0031] Even during this control, hydrogen is supplied to the reactor 5 in an amount corresponding to the amount of carbon dioxide introduced into the reactor 5. This allows the ratio of carbon dioxide to hydrogen in the reactor 5 to be controlled to a constant value. That is, when the amount of carbon dioxide introduced into the reactor 5 increases, the amount of hydrogen supplied to the reactor 5 also increases, and when the amount of carbon dioxide introduced into the reactor 5 decreases due to the supply of hydrogen to the methane fermentation tank 4, the amount of hydrogen supplied to the reactor 5 also decreases. This allows the ratio of carbon dioxide to hydrogen in the reactor 5 to be maintained constant.

[0032] As described above, in this embodiment, the amount of carbon dioxide contained in the generated biogas is measured, and hydrogen is supplied to the methane fermentation tank 4 according to the measured amount of carbon dioxide. This suppresses fluctuations in the amount of carbon dioxide supplied to the reactor 5. This makes it possible to prevent an increase and accumulation of reaction heat in the reactor 5 due to an increase in the amount of carbon dioxide supplied to the reactor 5. This makes it possible to suppress a decrease in reaction efficiency in methanation using biogas, in which the amount or ratio of carbon dioxide introduced into the reactor 5 can constantly fluctuate.

[0033] This makes it possible to control the amount of carbon dioxide supplied from the methane fermentation tank 4 to the reactor 5 without providing a separate facility for storing biogas between the methane fermentation tank 4 and the reactor 5. Therefore, the methanation system 1 can be made compact.

[0034] In addition, the hydrogen supplied to the reactor 5 is also controlled according to the measured amount of carbon dioxide, so that the ratio of carbon dioxide to hydrogen in the reactor 5 can be maintained at an appropriate ratio for the methanation reaction. Moreover, although they have in common the point of controlling based on the amount of carbon dioxide, the amount of hydrogen supplied to the methane fermentation tank 4 and the amount of hydrogen supplied to the reactor 5 are controlled independently of each other, so that control of one does not interfere with control of the other.

[0035] Furthermore, in this embodiment, the controller 13 also controls the amount of hydrogen supplied to the methane fermentation tank 4 in accordance with the temperature T inside the reactor 5. For this purpose, the methanation system 1 is equipped with a temperature measuring instrument 16 that measures the temperature inside the reactor 5. The controller 13 increases the amount of hydrogen supplied to the methane fermentation tank 4 when the temperature T measured by the temperature measuring instrument 16 exceeds a predetermined threshold value.

[0036] When the temperature T in the reactor 5 exceeds the threshold value, it is assumed that the amount of carbon dioxide introduced into the reactor 5 is increasing. At this time, as described above, the controller 13 generates a command value for increasing the amount of hydrogen supplied to the methane fermentation tank 4 according to the amount of carbon dioxide. The amount of hydrogen in the command value is set to A2. When the temperature T in the reactor 5 exceeds the threshold value, the controller 13 corrects the command value so as to increase the amount of hydrogen A2 by a predetermined amount. For example, when the temperature T in the reactor 5 exceeds the threshold value, the controller 13 sets the amount of hydrogen in the command value to an amount A3=A2+Ac, which is the amount of hydrogen in the command value based on the amount of carbon dioxide, A2, plus a correction value Ac.

[0037] This makes it possible to quickly reduce the amount of carbon dioxide introduced into the reactor 5 when the reaction heat increases transiently due to an increase in carbon dioxide in the reactor 5. When the temperature T in the reactor 5 falls below the threshold value, the controller 13 controls the amount of hydrogen supplied to the methane fermentation tank 4 using only a command value based on the amount of carbon dioxide.

[0038] [Variation 1] Fig. 2 is a block diagram showing a schematic configuration of a methanation system according to a modified example of the embodiment shown in Fig. 1. In the configuration of this modified example, the same components as those in Fig. 1 are given the same reference numerals, and the description thereof will be omitted. This modified example differs from the configuration in Fig. 1 in that the methanation system 1B includes a gas purifier 18 in the rear stage of the reactor 5.

[0039] The gas purifier 18 is connected to the discharge pipe 17 of the reactor 5. As a result, the product gas containing methane discharged from the reactor 5 is introduced into the gas purifier 18. The gas purifier 18 extracts methane from the product gas containing methane generated in the reactor 5.

[0040] As described above, by controlling the amount of hydrogen supplied to the methane fermenter 4 and the reactor 5 according to the amount A1 of carbon dioxide introduced into the reactor 5, the reaction efficiency of carbon dioxide and hydrogen in the reactor 5 can be maintained high, and the product gas discharged from the reactor 5 is expected to contain a high concentration of methane. However, when the product gas, which is mainly composed of methane, is reused, for example, when the product gas is injected into a city gas supply pipe as city gas, there are cases in which the methane concentration in the product gas needs to be equal to or higher than a standard value. In such cases, the methane concentration in the product gas discharged from the reactor 5 is not necessarily equal to or higher than the standard value.

[0041] Therefore, in this modification, the generated gas discharged from the reactor 5 is purified by the gas purifier 18. The gas purifier 18 includes, for example, a separation membrane. The generated gas introduced into the gas purifier 18 is permeated through the separation membrane. As a result, the generated gas is separated into methane and other remaining gases due to the difference in the permeation speed for permeating the separation membrane, and the methane is discharged from the discharge pipe 19 of the gas purifier 18. The remaining gas mainly contains carbon dioxide and hydrogen that did not react in the reactor 5. The methane concentration is increased by extracting methane from the generated gas in the gas purifier 18. As a result, the final methane concentration can be made equal to or higher than the reference value.

[0042] A methane concentration meter may be installed in the exhaust pipe 19 of the gas purifier 18. When the methane concentration measured by the methane concentration meter is below a reference value, the gas exhausted from the gas purifier 18 may be returned to the gas purifier 18 again.

[0043] Furthermore, in this modification, the methanation system 1B includes a reintroduction pipe 20 that introduces the residual gas obtained by extracting methane from the generated gas in the gas purifier 18 into the reactor 5. As a result, unreacted carbon dioxide and hydrogen are returned to the reactor 5 through the reintroduction pipe 20. This makes it possible to increase the utilization rates of carbon dioxide and hydrogen.

[0044] In this modification, the biogas introduction pipe 6 and the reintroduction pipe 20 are configured as pipes independent of each other. Alternatively, the reintroduction pipe 20 may be connected between the gas purifier 18 and a portion of the biogas introduction pipe 6 upstream of the carbon dioxide amount meter 15. In this case, the carbon dioxide amount meter 15 measures the amount of carbon dioxide introduced from the methane fermentation tank 4 plus the amount of carbon dioxide introduced from the gas purifier 18.

[0045] In addition, a dehydrator for removing water vapor from the generated gas may be connected between the reactor 5 and the gas purifier 18.

[0046] [Variation 2] Fig. 3 is a block diagram showing a schematic configuration of a methanation system according to another modified example of the embodiment shown in Fig. 1. In the configuration of this modified example, the same components as those in Fig. 1 are given the same reference numerals, and the description thereof will be omitted. This modified example differs from the configuration in Fig. 1 in that the methanation system 1C includes a gas purifier 18 located downstream of the methane fermenter 4 and upstream of the reactor 5.

[0047] In this modification, the biogas introduction pipe 6 includes a first pipe 6a and a second pipe 6b. The first pipe 6a is connected between the methane fermentation tank 4 and the gas purifier 18. The second pipe 6b is connected between the gas purifier 18 and the reactor 5. The carbon dioxide amount meter 15 is installed in the second pipe 6b.

[0048] As in the configuration of FIG. 1, the biogas generated in the methane fermentation tank 4 contains methane and carbon dioxide. This biogas is introduced into the gas purifier 18 through the first piping 6a. As in the example of FIG. 2, the gas purifier 18 includes, for example, a separation membrane. The biogas introduced into the gas purifier 18 is permeated through the separation membrane. As a result, the biogas is separated into methane and the remaining biogas due to the difference in permeation speed for permeating the separation membrane, and the methane is discharged from the discharge piping 19 of the gas purifier 18. The remaining biogas mainly contains carbon dioxide.

[0049] The remaining biogas is introduced through the second pipe 6b into the reactor 5. The controller 13 controls the amount of hydrogen supplied to the methane fermentation tank 4 and the reactor 5 according to the amount of carbon dioxide passing through the second pipe 6b.

[0050] According to this modification, by extracting in advance the methane generated in the methane fermentation tank 4, it is possible to reduce the amount of biogas introduced into the reactor 5. As a result, the capacity of the reactor 5 can be reduced, and the methanation system 1 can be made more compact.

[0051] [Other embodiments] Although the embodiments of the present disclosure have been described above, the present disclosure is not limited to the above-described embodiments, and various improvements, changes, and modifications are possible without departing from the spirit and scope of the present disclosure.

[0052] For example, the above-mentioned modified example 1 may be combined with the above-mentioned modified example 2. For example, the methanation system may include a first gas purifier connected between the methane fermenter 4 and the reactor 5, and a second gas purifier connected downstream of the reactor 5.

[0053] Furthermore, in the above embodiment, an example has been given of a mode in which both the amount of hydrogen supplied to the methane fermentation tank 4 and the amount of hydrogen supplied to the reactor 5 are controlled. Alternatively, only the amount of hydrogen supplied to the methane fermentation tank 4 may be controlled. For example, a constant amount of hydrogen may be supplied to the reactor 5, and the controller 13 may control the amount of hydrogen supplied to the methane fermentation tank 4 so that the ratio between the amount of carbon dioxide supplied to the reactor 5 and the amount of hydrogen supplied to the reactor 5 approaches a predetermined ratio.

[0054] In the above embodiment, the amount of hydrogen supplied to the methane fermentation tank 4 and the amount of hydrogen supplied to the reactor 5 are controlled independently of each other. Alternatively, the amount of hydrogen supplied to the methane fermentation tank 4 and the amount of hydrogen supplied to the reactor 5 may be controlled by a common command value.

[0055] For example, instead of the first adjusting valve 11 and the second adjusting valve 12, an adjusting valve may be provided in the piping portion 10 which is a common portion with the first hydrogen supply piping 8 and the second hydrogen supply piping 9. The controller 13 may perform control to increase the amount of hydrogen supplied to the methane fermentation tank 4 and the reactor 5 when the amount of carbon dioxide introduced into the reactor 5 increases, and to decrease the amount of hydrogen supplied to the methane fermentation tank 4 and the reactor 5 when the amount of carbon dioxide introduced into the reactor 5 decreases. In this case, the first hydrogen supply piping 8 and the second hydrogen supply piping 9 may be configured so that the amount of hydrogen supplied to the methane fermentation tank 4 and the amount of hydrogen supplied to the reactor 5 are constant regardless of the opening degree of the adjusting valve.

[0056] Furthermore, in the above embodiment, an example has been given of a mode in which the controller 13 constantly controls the amount of hydrogen supplied to the methane fermentation tank 4 in accordance with the amount of carbon dioxide introduced into the reactor 5. Alternatively, the controller 13 may control the amount of hydrogen supplied to the methane fermentation tank 4 in accordance with the amount of carbon dioxide when the amount of carbon dioxide introduced into the reactor 5 is within a predetermined control application range.

[0057] For example, when the amount of carbon dioxide introduced into the reactor 5 is equal to or greater than a first threshold value and less than a second threshold value greater than the first threshold value, the controller 13 may control the amount of hydrogen supplied to the methane fermentation tank 4 in accordance with the amount of carbon dioxide. In this case, the controller 13 may fix the amount of hydrogen supplied to the methane fermentation tank 4 to 0 or a first value when the amount of carbon dioxide is less than the first threshold value. The first value may be set to a value equal to or less than a lower limit value of the amount of hydrogen controlled in accordance with the amount of carbon dioxide. Similarly, the controller 13 may fix the amount of hydrogen supplied to the methane fermentation tank 4 to a second value when the amount of carbon dioxide is equal to or greater than a second threshold value. The second value may be set to a value equal to or greater than an upper limit value of the amount of hydrogen controlled in accordance with the amount of carbon dioxide.

[0058] The control application range is not limited to the range equal to or greater than the first threshold and less than the second threshold. For example, the control application range may be equal to or greater than the first threshold, i.e., no upper limit value is set, or may be less than the second threshold, i.e., no lower limit value is set.

[0059] In the above embodiment, the amount of hydrogen supplied to the methane fermenter 4 is increased when the temperature of the reactor 5 exceeds a predetermined threshold value. However, such control does not have to be performed. In other words, the methanation system 1 does not have to include the temperature measuring device 16 that measures the temperature inside the reactor 5.

[0060] In the above embodiment, a gas holder for storing biogas is not provided between the methane fermentation tank 4 and the reactor 5. Alternatively, the gas holder may be provided. Even in this case, if carbon dioxide exceeding the capacity of the gas holder is generated in the methane fermentation tank 4, the amount of carbon dioxide supplied from the gas holder to the reactor 5 may vary. Increasing the capacity of the gas holder leads to an increase in the size of the methanation system 1. Therefore, by controlling the amount of hydrogen supplied to the methane fermentation tank 4 in response to such fluctuations in the amount of carbon dioxide, it is possible to suppress a decrease in reaction efficiency in methanation using biogas, as described above. In addition, since the capacity of the gas holder can be reduced, the methanation system 1 can be made smaller.

[0061] [Summary of this disclosure] [Item 1] A methanation system according to one embodiment of the present disclosure includes a methane fermentation tank that ferments organic waste to generate biogas containing methane and carbon dioxide, a reactor that produces methane by reacting carbon dioxide generated in the methane fermentation tank with hydrogen supplied from a hydrogen supply device, a biogas inlet pipe that introduces the biogas generated in the methane fermentation tank from the methane fermentation tank to the reactor, a first hydrogen supply pipe that supplies hydrogen from the hydrogen supply device to the reactor, a second hydrogen supply pipe that supplies hydrogen from the hydrogen supply device to the methane fermentation tank, a carbon dioxide amount meter that measures the amount of carbon dioxide introduced into the reactor through the biogas inlet pipe, and a controller, wherein the controller controls the amount of hydrogen supplied to the methane fermentation tank through the second hydrogen supply pipe in accordance with the amount of carbon dioxide measured by the carbon dioxide amount meter.

[0062] According to the above configuration, the amount of carbon dioxide contained in the generated biogas is measured, and hydrogen is supplied to the methane fermentation tank according to the measured amount of carbon dioxide. This makes it possible to prevent an increase and accumulation of reaction heat in the reactor due to an increase in the amount of carbon dioxide supplied to the reactor. Therefore, it is possible to suppress a decrease in reaction efficiency in methanation using biogas.

[0063] [Item 2] In the methanation system of item 1, the controller may increase the amount of hydrogen supplied to the methane fermentation tank when the amount of carbon dioxide measured by the carbon dioxide amount meter increases, and may decrease the amount of hydrogen supplied to the methane fermentation tank when the amount of carbon dioxide measured by the carbon dioxide amount meter decreases.

[0064] [Item 3] The methanation system of item 1 or 2 may include a temperature measuring device that measures the temperature inside the reactor, and the controller may increase the amount of hydrogen supplied to the methane fermentation tank when the temperature measured by the temperature measuring device exceeds a predetermined threshold value. This makes it possible to quickly reduce the amount of carbon dioxide introduced into the reactor when the reaction heat increases transiently due to an increase in carbon dioxide in the reactor.

[0065] [Item 4] In the methanation system of any one of items 1 to 3, the controller may control the amount of hydrogen supplied to the reactor through the first hydrogen supply pipe in accordance with the amount of carbon dioxide measured by the carbon dioxide amount meter so that the ratio of carbon dioxide to hydrogen in the reactor is constant. This allows the hydrogen supplied to the reactor to be controlled in accordance with the measured amount of carbon dioxide. Therefore, the ratio of carbon dioxide to hydrogen in the reactor can be maintained at an appropriate ratio for carrying out the methanation reaction.

[0066] [Item 5] The methanation system according to any one of items 1 to 4 may include a gas purifier that extracts methane from the methane-containing product gas generated in the reactor. By extracting methane from the product gas in the gas purifier, the methane concentration can be increased.

[0067] [Item 6] The methanation system of item 5 may include a reintroduction pipe for introducing the residual gas obtained by extracting methane from the generated gas in the gas purifier into the reactor. This allows unreacted carbon dioxide and hydrogen to be returned to the reactor through the reintroduction pipe. This makes it possible to increase the utilization rate of carbon dioxide and hydrogen.

[0068] [Item 7] The methanation system of any one of items 1 to 6 may include a gas purifier that extracts methane from the biogas generated in the methane fermentation tank, and the biogas inlet pipe may introduce the remaining biogas after methane is extracted from the biogas in the gas purifier into the reactor. In this way, the amount of biogas introduced into the reactor can be reduced by extracting the methane generated in the methane fermentation tank in advance. As a result, the capacity of the reactor can be reduced, and the methanation system can be made more compact.

[0069] [Item 8] A methanation method according to another aspect of the present disclosure includes fermenting organic waste in a methane fermentation tank to generate biogas containing methane and carbon dioxide, reacting hydrogen with the carbon dioxide contained in the biogas extracted from the methane fermentation tank to generate methane, measuring the amount of carbon dioxide contained in the biogas, and supplying hydrogen to the methane fermentation tank in accordance with the measured amount of carbon dioxide. [Explanation of symbols]

[0070] 1,1B,1C Methanation System 4. Methane fermentation tank 5. Reactor 6,6C Biogas introduction piping 7 Hydrogen supply 8. First hydrogen supply pipe 9 Second hydrogen supply pipe 13 Controller 15 Carbon dioxide measurement device 16 Temperature measuring instrument 18 Gas Purifier 20 Re-introduction piping

Claims

1. a methane fermenter that ferments organic waste to generate biogas containing methane and carbon dioxide; a reactor for producing methane by reacting the carbon dioxide generated in the methane fermentation tank with hydrogen supplied from a hydrogen supply device; A biogas introduction pipe for introducing the biogas generated in the methane fermentation tank from the methane fermentation tank to the reactor; a first hydrogen supply pipe for supplying hydrogen from the hydrogen supply device to the reactor; a second hydrogen supply pipe for supplying hydrogen from the hydrogen supply device to the methane fermentation tank; A carbon dioxide amount meter that measures the amount of carbon dioxide introduced into the reactor through the biogas introduction pipe; A controller, The controller controls the amount of hydrogen supplied to the methane fermentation tank through the second hydrogen supply pipe in accordance with the amount of carbon dioxide measured by the carbon dioxide amount meter.

2. The methanation system according to claim 1, wherein the controller increases the amount of hydrogen supplied to the methane fermentation tank when the amount of carbon dioxide measured by the carbon dioxide amount meter increases, and decreases the amount of hydrogen supplied to the methane fermentation tank when the amount of carbon dioxide measured by the carbon dioxide amount meter decreases.

3. A temperature measuring device is provided for measuring a temperature inside the reactor, The methanation system according to claim 1 or 2, wherein the controller increases the amount of hydrogen supplied to the methane fermentation tank when the temperature measured by the temperature measuring device exceeds a predetermined threshold value.

4. 3. The methanation system according to claim 1, wherein the controller controls the amount of hydrogen supplied to the reactor through the first hydrogen supply pipe in accordance with the amount of carbon dioxide measured by the carbon dioxide amount meter so that a ratio of carbon dioxide to hydrogen in the reactor is constant.

5. The methanation system according to claim 1 or 2, further comprising a gas purifier that extracts methane from a product gas containing methane produced in the reactor.

6. The methanation system according to claim 5, further comprising a reintroduction pipe for introducing residual gas obtained by extracting methane from the generated gas in the gas purifier into the reactor.

7. A gas purifier is provided for extracting methane from the biogas generated in the methane fermentation tank. The methanation system according to claim 1 or 2, wherein the biogas inlet pipe introduces the remaining biogas after methane is extracted from the biogas in the gas purifier into the reactor.

8. fermenting the organic waste in a methane fermenter to generate biogas containing methane and carbon dioxide; generating methane by reacting hydrogen with carbon dioxide contained in the biogas extracted from the methane fermentation tank; Measure the amount of carbon dioxide contained in biogas, A methanation method, comprising supplying hydrogen to the methane fermentation tank in accordance with the measured amount of carbon dioxide.

Images