methane fermentation tank

Abstract

This invention provides a methane fermentation tank with a simple structure that allows for good agitation of the fermentation liquid even when the solid content of the fermented sludge is high, resulting in excellent fermentation efficiency. [Solution] A methane fermentation tank comprising an outer cylinder with its upper and lower ends closed, and at least one inner cylinder disposed inside the outer cylinder with its upper end closed and its lower end open, the methane fermentation tank comprising: a lower communication path that connects the internal space of the outer cylinder and the internal space of the inner cylinder at the bottom, allowing fermentation sludge to flow between the outer cylinder and the inner cylinder; a liquid level difference forming mechanism that supplies biogas from the outside to the outer cylinder or the inner cylinder to form a liquid level difference between the liquid level of the fermentation sludge stored in the outer cylinder and the liquid level of the fermentation sludge stored in the inner cylinder; an upper communication path that connects the gas phase spaces formed at the top of the outer cylinder and can switch the communication state open and closed; and a swirling mechanism equipped with a plurality of stirring blades erected at the bottom of the outer cylinder corresponding to the lower communication path.

Description

Technical Field

[0001] The present invention relates to a methane fermentation tank.

Background Art

[0002] Patent Document 1 discloses a methane fermentation apparatus and a methane fermentation treatment method that generate biogas mainly composed of methane that can be used as an energy source using anaerobic microorganisms with agricultural waste generated after harvesting grains harvested in fields represented by rice straw and wheat straw as raw materials. Not only such agricultural waste but also resource recycling methods utilizing methane fermentation treatment with organic waste such as paper waste and food waste included in general waste as raw materials have attracted attention.

[0003] Patent Document 2 proposes a methane fermentation apparatus including a first circulation means for generating an upward flow in the liquid in the tank and a downward flow outside the upward flow to circulate the liquid, and a second circulation means for generating a horizontal swirling flow around the upward flow portion of the liquid in the tank to circulate the liquid.

[0004] Patent Document 3 discloses a sealed tank body, a main fermentation section formed in the tank body for methane fermentation of organic waste, a precipitation section formed in the tank body above the main fermentation section, having a methane gas discharge port at the upper part and temporarily storing granules granulated in the tank body, a digestion sludge storage section formed in the outer periphery of the precipitation section through a partition cylinder for temporarily storing the digested sludge after methane fermentation flowing in from the main fermentation section and having a discharge port for the digested sludge, a tubular mixing shaft communicating the digestion sludge storage section with below the liquid surface of the slurry stored in the main fermentation section, a center tube communicating the central part of the precipitation section and the central part of the main fermentation section and having a slurry supply port formed in the middle of the tube, and a pressure equalizing valve connected to a communication pipe communicating the upper part of the main fermentation section where methane gas generated in the main fermentation section accumulates and the upper part of the precipitation section where methane gas generated in the precipitation section accumulates. 1]

[0005] The methane fermentation tank is configured such that the tank body is divided into an inner granule granulation section and an outer main fermentation section by a center tube, and the liquid surface of the main fermentation section is pressurized using the generated gas, creating a water level difference between the inner and outer sections. When the pressure equalization valve is released, the flow of the fermentation liquid generated as the water level difference is eliminated agitates the inside of the tank.

[0006] Thus, in order to efficiently carry out methane fermentation, it is necessary to promote stirring and mixing of the methane fermentation sludge, which contains microbial cells, without allowing it to remain stagnant in the methane fermentation tank.

[0007] Therefore, several methods have been proposed to date, including mechanically stirring the fermentation liquid in the tank using agitators, supplying biogas produced by methane fermentation into the tank and using the rising gas flow to stir the fermentation liquid, circulating and stirring the fermentation liquid in the tank using a pump, and stirring the fermentation liquid by utilizing the difference in water level between areas divided into internal and external regions. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Japanese Patent Publication No. 2023-131221 [Patent Document 2] Japanese Patent Publication No. 2002-263693 [Patent Document 3] Japanese Patent Publication No. 2000-301116 [Overview of the Initiative] [Problems that the invention aims to solve]

[0009] However, the mechanical stirring method using impellers requires excessive power, making it difficult to accommodate larger fermentation tanks and higher concentrations of fermented sludge. Furthermore, the limited stirring patterns for the fermented sludge can lead to the formation of dead zones within the tank. Once dead zones form, the fermented sludge and raw materials accumulate, hindering proper fermentation and necessitating cleaning of these dead zones during maintenance.

[0010] Furthermore, the method using pumps requires setting up a circulation route to withdraw fermented sludge from the fermentation tank and return it to the tank in order to circulate the fermented sludge within the tank. However, as the fermentation tank becomes larger, it becomes difficult to set up an effective circulation route to homogenize the fermented sludge within the tank.

[0011] One example of a method using biogas to agitate fermentation sludge is to supply biogas to a draft tube and agitate it through a circulating flow generated inside and outside the draft tube as the biogas rises. However, as the fermentation tank becomes larger, the agitation range is limited, requiring an increase in the number of draft tubes. Furthermore, the behavior of the circulating flow varies depending on the properties of the fermentation sludge, making it difficult to determine the optimal arrangement.

[0012] In the methane fermentation tank described in Patent Document 3, the biogas generated in the main fermentation section, which has a larger fermentation sludge storage volume than the inner region, is stored in the space above the main fermentation section, thereby lowering the water level in the main fermentation section compared to the inner region. However, this structure requires time to form a water level difference, and in particular, when the solid content of the fermentation sludge is high and the viscosity is high, the biogas is trapped in the fermentation liquid, making it difficult to efficiently lower the liquid level. Therefore, there was room for further improvement in terms of improving stirring efficiency.

[0013] The objective of the present invention is to provide a methane fermentation tank with a simple structure that can effectively agitate the fermentation liquid even when the solid content of the fermentation sludge is high, and has excellent fermentation efficiency. [Means for solving the problem]

[0014] To achieve the above objectives, the first characteristic configuration of the methane fermentation tank according to the present invention is a methane fermentation tank comprising an outer cylinder with its upper and lower ends closed, and at least one inner cylinder disposed inside the outer cylinder with its upper end closed and its lower end open, wherein the tank comprises: a lower communication path that connects the internal space of the outer cylinder and the internal space of the inner cylinder at the bottom, allowing fermentation sludge to flow between the outer cylinder and the inner cylinder; a liquid level difference forming mechanism that supplies biogas from the outside to the outer cylinder or the inner cylinder to form a liquid level difference between the liquid level of the fermentation sludge stored in the outer cylinder and the liquid level of the fermentation sludge stored in the inner cylinder; an upper communication path that connects the gas phase spaces formed at the upper parts of the outer cylinder and the inner cylinder, and which can switch the communication state open and closed; and a swirling mechanism equipped with a plurality of stirring blades erected at the bottom of the outer cylinder corresponding to the lower communication path.

[0015] When biogas is supplied from the outside to the outer or inner cylinder by the liquid level difference formation mechanism, a liquid level difference is formed between the liquid level of the fermented sludge stored in the outer cylinder and the liquid level of the fermented sludge stored in the inner cylinder. As a result of the formation of this liquid level difference, the fermented sludge statically flows between the outer and inner cylinders through the lower communication path, causing a gentle agitation of the fermented sludge.

[0016] With the aforementioned liquid level difference formed, switching the upper communication path connecting the gas phase spaces formed at the top of the outer and inner cylinders from a closed state to an open state causes the fermented sludge at the higher liquid level to flow dynamically through the lower communication path toward the fermented sludge at the lower liquid level, resulting in rapid agitation of the fermented sludge.

[0017] In particular, in the latter embodiment, as the fermented sludge with a high liquid level descends, a swirling flow is formed in the fermented sludge by the stirring blades provided in the swirling mechanism, and it is stirred in the radial and circumferential directions, so that a good stirring effect can be obtained.

[0018] The second characteristic configuration is that, in addition to the first characteristic configuration described above, a gap is formed between the lower end of the inner cylinder and the upper end of the stirring blade.

[0019] If there is no gap formed between the lower end of the inner cylinder and the upper end of the stirring blade, there is a risk that lumps will stagnate and clog between the lower end of the inner cylinder and the upper end of the stirring blade, or that fermented sludge will accumulate between the stirring blades. However, if there is a gap formed between the lower end of the inner cylinder and the upper end of the stirring blade, when there are lumps in the fermented sludge, the lumps can easily escape through the gap, and the fermentation process can be carried out stably over a long period.

[0020] In addition to the first characteristic configuration described above, the third characteristic configuration is that the stirring blade extends from the inside to the outside of the inner cylinder in a posture intersecting the lower end of the inner cylinder in a plan view.

[0021] When the methane fermentation liquid flows due to the water level difference, it will pass through the lower end region of the inner cylinder in the lower communication path. Therefore, if the stirring blade is arranged to extend from the inside to the outside of the inner cylinder in a posture intersecting the lower end of the inner cylinder in a plan view, the fermented sludge can be efficiently stirred.

[0022] In addition to the first characteristic configuration described above, the fourth characteristic configuration is that each of the stirring blades is formed of a plate-like body.

[0023] By forming the stirring blade using a simple plate-like body, both the material cost and the processing cost can be reduced.

[0024] In addition to any one of the first to fourth characteristic configurations described above, the fifth characteristic configuration is that the water level difference forming mechanism is configured to be able to switch the supply destination of the biogas between the outer cylinder and the inner cylinder.

[0025] By supplying biogas to the outer cylinder, the liquid level of the fermented sludge in the outer cylinder can be pushed down, and the liquid level of the fermented sludge in the inner cylinder can be relatively raised. By supplying biogas to the inner cylinder, the liquid level of the fermented sludge in the inner cylinder can be pushed down, and the liquid level of the fermented sludge in the outer cylinder can be relatively raised. By switching the supply destination of the biogas, the flow direction of the sludge can be switched when the upper communication path is opened, and the stirring effect of the fermented sludge can be effectively enhanced.

[0026] The sixth characteristic configuration is that, in addition to any one of the first to fourth characteristic configurations described above, the horizontal cross-sectional area of the inner cylinder with respect to the horizontal cross-sectional area of the outer cylinder is set within the range of 0.3 to 0.7 times.

[0027] If the horizontal cross-sectional area of the inner cylinder with respect to the horizontal cross-sectional area of the outer cylinder is set within the range of 0.3 to 0.7 times, the supply amount of biogas required to generate the same water level difference is substantially equal regardless of whether it is the outer cylinder pressurization or the inner cylinder pressurization. The potential energy of the fermented sludge generated by the water level difference is also substantially equal, and the stirring effect of the fermented sludge flowing between the outer cylinder and the inner cylinder is also the same for both outer cylinder pressurization and inner cylinder pressurization, and it will be uniformly stirred. Therefore, a methane fermentation tank with a simple structure and high fermentation efficiency can be realized.

Effects of the Invention

[0028] As described above, according to the present invention, even when the viscosity of the fermented sludge is high, the fermentation liquid can be well stirred, and a simple-structured methane fermentation tank with excellent fermentation efficiency can be provided.

Brief Description of the Drawings

[0029] [Figure 1] (a) is an explanatory diagram showing the configuration of a methane fermentation device including a methane fermentation tank, and (b) is a plan view of the methane fermentation tank [Figure 2] Explanatory diagram of the first stirring step, which is an aspect of the methane fermentation treatment method [Figure 3] Explanatory diagram of the second stirring step, which is another aspect of the methane fermentation treatment method [Figure 4] (a) is an explanatory diagram of the steps of the methane fermentation treatment method, (b) is an explanatory diagram showing another aspect of the steps of the methane fermentation treatment method, and (c) is an explanatory diagram showing still another aspect of the steps of the methane fermentation treatment method [Figure 5] (a) is a side view explanatory diagram of the stirring blade provided in the methane fermentation tank, (b) is a side view explanatory diagram showing another aspect of the stirring blade provided in the methane fermentation tank, and (c) to (f) are explanatory diagrams showing the shape and arrangement of the stirring blade provided in the methane fermentation tank in plan view [Figure 6](a) is an explanatory diagram of a longitudinal section showing another configuration of a methane fermentation tank, and (b) is an explanatory diagram of a cross-section thereof. [Figure 7] (a) is an explanatory diagram of a longitudinal section showing yet another embodiment of the methane fermentation tank, and (b) is an explanatory diagram of the same in cross-section. [Modes for carrying out the invention]

[0030] The methane fermentation tank and methane fermentation method of the present invention will be explained below, using as an example the case in which rice straw, which is rice harvest residue generated in the field, is used as the fermentation raw material.

[0031] Figures 1(a) and 1(b) illustrate a methane fermentation apparatus 1 according to the present invention. Figure 1(a) is an explanatory diagram showing the internal structure of methane fermentation tank 2 and the arrangement of peripheral equipment necessary to operate methane fermentation tank 2, and Figure 1(b) is a plan view explanatory diagram showing the internal structure of methane fermentation tank 2.

[0032] The methane fermentation apparatus 1 includes a methane fermentation tank 2, a gas holder 7 for storing biogas containing methane gas and carbon dioxide produced in the methane fermentation tank, a circulation path 9 for withdrawing the fermentation liquid stored in the methane fermentation tank 2, discharging a portion of it outside the system, and circulating the remainder back into the methane fermentation tank 2, a gas supply pipe 2L for pressurizing and supplying the biogas stored in the gas holder 7 to the methane fermentation tank 2, valves V1 and V2, and a blower B. The perimeter wall of the methane fermentation tank 2 is provided with an insulating jacket through which a heat transfer medium flows, maintaining the inside of the tank at approximately 55°C, which is suitable for fermentation. For example, water heated by the heat generated by a combustor that uses the biogas stored in the gas holder 7 as fuel is used as the heat transfer medium.

[0033] The main body of the methane fermentation tank 2 comprises an outer cylinder 3 with its upper and lower ends closed, and at least one inner cylinder 4 positioned inside the outer cylinder 3, with its upper end closed and its lower end open. In this example, both the outer cylinder 3 and the inner cylinder 4 are cylindrical bodies with a circular cross-section, and the horizontal cross-sectional area of ​​the inner cylinder 4 is set to 0.5 relative to the horizontal cross-sectional area of ​​the outer cylinder 3. As will be explained in detail later, the ratio of the horizontal cross-sectional areas is not limited to 0.5, but can be set in the range of 0.3 to 0.7 times, and even better if it is set in the range of 0.4 to 0.6 times.

[0034] The shapes of the outer cylinder 3 and inner cylinder 4 are not limited to cylindrical bodies; they may also be elliptical cylinders with an elliptical cross-section or rectangular cylinders with a rectangular cross-section, as long as they are formed from tubular material. Furthermore, while it is preferable that the horizontal cross-sectional area of ​​the tubular material be the same along the height direction, it may differ slightly. For example, it may be wider at the top than at the bottom, or conversely, narrower at the top. If the ratio of the horizontal cross-sectional area of ​​the inner cylinder 4 to the horizontal cross-sectional area of ​​the outer cylinder 3 differs along the height direction, the average value of the ratio of the horizontal cross-sectional areas should be set to a range of 0.3 to 0.7 times, preferably 0.4 to 0.6 times, and more preferably 0.5.

[0035] Furthermore, it is equipped with a biogas discharge pipe 3L that guides the biogas generated in the outer cylinder 3 to the gas holder 7, and a biogas discharge pipe 4L that guides the biogas generated in the inner cylinder 4 to the gas holder 7, with valves V3 and V4 provided on the biogas discharge pipes 3L and 4L.

[0036] The methane fermentation tank 2 includes an upper communication path 6 that connects the gas phase spaces 3s and 4s formed at the top of the outer cylinder 3 and inner cylinder 4, and is equipped with a valve V5 that can switch the communication state open and closed. It also includes a lower communication path 5 that connects the internal space of the outer cylinder 3 and the internal space of the inner cylinder 4 at the bottom, allowing the fermentation sludge to flow between the outer cylinder 3 and the inner cylinder 4. The valve V5 in the upper communication path 6 also functions as a flow rate adjustment mechanism that adjusts the amount of gas flowing through by adjusting the degree of opening.

[0037] Furthermore, the methane fermentation tank 2 is equipped with a liquid level difference forming mechanism 8 that supplies biogas from the outside to the outer cylinder 3 or inner cylinder 4 to form a liquid level difference between the liquid level of the fermentation sludge stored in the outer cylinder 3 and the liquid level of the fermentation sludge stored in the inner cylinder 4. The liquid level difference forming mechanism 8 is composed of the gas supply pipe 2L, valves V1 and V2, and blower B described above.

[0038] The circulation path 9 includes an extraction pipe 9A for extracting fermentation sludge from the bottom of the methane fermentation tank 2, and a supply pipe 9B for supplying the extracted fermentation sludge to the methane fermentation tank 2 with added methane fermentation raw materials. The extraction pipe 9A is equipped with an extraction pump P2, and the supply pipe 9B is equipped with a supply pump P1. A raw material supply mechanism 10 is also provided in the path connecting the extraction pipe 9A and the supply pipe 9B. The raw material supply mechanism 10 is equipped with a mixer for mixing the chopped rice straw and fermentation sludge, which are the raw materials, and may also be configured to add dilution water for further mixing.

[0039] The methane fermentation treatment method using the methane fermentation tank 2 described above will now be explained. The methane fermentation treatment method includes a first stirring step shown in Figure 2 and a second stirring step shown in Figure 3. As shown in Figure 2, the first stirring step involves supplying biogas from the outside to the outer cylinder 3 via the liquid level difference formation mechanism 8 while the upper communication passage 6 is closed, thereby lowering the liquid level in the outer cylinder 3 and raising the liquid level in the inner cylinder 4 to form a liquid level difference, and then opening the upper communication passage 6 to eliminate the liquid level difference and stir the fermented sludge.

[0040] As shown in Figure 3, the second stirring step involves supplying biogas from the outside to the inner cylinder 4 via the liquid level difference formation mechanism 8 with the upper communication passage 6 closed, thereby lowering the liquid level in the inner cylinder 4 and raising the liquid level in the outer cylinder 3 to form a liquid level difference, and then opening the upper communication passage 6 to eliminate the liquid level difference and stir the fermentation sludge. "External" refers to the outside of the methane fermentation tank 2, and in this embodiment, biogas stored in the gas holder 7 is used. Instead of pressurizing with biogas that is naturally generated from the fermentation liquid remaining in the methane fermentation tank 2, pressurization is forced by biogas supplied from the outside via the blower B, so that an appropriate liquid level difference can be formed within an appropriate time.

[0041] In Figures 2 and 3, gas is selectively supplied from gas supply pipes 2L connected to the ceilings of the outer cylinder 3 and inner cylinder 4 via valves V1 and V2. However, the tip of the gas supply pipe 2L may be positioned in the fermentation liquid to supply biogas to each of the fermentation liquids.

[0042] During the process in which a liquid level difference is formed, a slow stirring process is performed in which the fermented sludge stored in the outer cylinder 3 and inner cylinder 4 flows statically through the lower communication path 5. During the process in which the liquid level difference is eliminated, a rapid stirring process is performed in which the fermented sludge stored in the outer cylinder 3 and inner cylinder 4 flows dynamically through the lower communication path 5.

[0043] The process of supplying biogas to the outer cylinder 3 is called the outer cylinder pressurization process, and the process of supplying biogas to the inner cylinder 4 is called the inner cylinder pressurization process. When the horizontal cross-sectional area of ​​the inner cylinder 4 is set to 0.5 relative to the horizontal cross-sectional area of ​​the outer cylinder 3, and the liquid level difference generated in the outer cylinder pressurization process and the liquid level difference generated in the inner cylinder pressurization process are the same, then the volume of fermentation liquid corresponding to the liquid level difference will be the same value, meaning that the potential energy generated by the liquid level difference will be the same value, and the stirring force caused by the liquid level difference will be the same value. Note that the ratio of the horizontal cross-sectional areas is not limited to 0.5, and there is no particular problem as long as it is within the range described above.

[0044] As shown in Figure 4(a), the methane fermentation treatment method is a method that effectively generates biogas by repeating a slow stirring step, a rapid stirring step, and a standing step in a predetermined order. As described above, the slow stirring process is a stirring process in which biogas is forcibly supplied from the outside to the outer cylinder 3 or inner cylinder 4 by a biogas supply means such as a blower B, thereby creating a liquid level difference between the liquid level of the fermented sludge stored in the outer cylinder 3 and the liquid level of the fermented sludge stored in the inner cylinder 4, and causing the fermented sludge to flow between the outer cylinder 3 and the inner cylinder 4.

[0045] The rapid stirring process is a stirring process that eliminates the pressure difference in the gas phase space formed between the outer and inner cylinders, thereby releasing the liquid level difference created in the slow stirring process and causing the fermented sludge to flow in the opposite direction to the slow stirring process. The settling process is a process in which the fermented sludge is allowed to settle after either the rapid stirring process or the slow stirring process.

[0046] For example, if the capacity of methane fermentation tank 2 is about 40L, the time required for the slow stirring process is set to be about several tens of seconds to a few minutes, the time required for the rapid stirring process is set to be about 0.1 seconds to 1 minute, and the time required for the standing process is set to be about 30 minutes to 1 hour. The first stirring process and the second stirring process are repeated alternately, including the standing process. In order to obtain good fermentation efficiency, it is preferable that the proportion of the standing process in the total processing time, including the slow stirring and rapid stirring processes, is set to 75% or more. Note that the time required for each process is not limited to these values ​​and should be set appropriately based on the target fermentation efficiency.

[0047] In addition to being performed after the rapid stirring step, the settling step may also be performed after the rapid stirring step, as shown in Figure 4(b), or after the slow stirring step and the rapid stirring step, respectively, as shown in Figure 4(c). In other words, the methane fermentation treatment method is a treatment method that repeats the first stirring step and the second stirring step, and is also a treatment method that includes a settling step between the first stirring step and the second stirring step, or within each step (between the slow stirring step and the rapid stirring step).

[0048] During rapid stirring, which involves agitating the fermentation liquid by opening the liquid level difference, the entire fermentation liquid is forced to flow, allowing for the forced movement and mixing of solids such as sludge and organic acids dissolved in the liquid. On the other hand, during slow stirring, solids such as sludge containing many methanogenic bacteria remain stagnant, while the highly fluid liquid containing dissolved organic acids flows through the solids, allowing for efficient contact between the methanogenic bacteria and organic acids.

[0049] Therefore, it is assumed that slow stirring allows the liquid containing dissolved organic acids to permeate the sludge accumulation area near the boundary between the outer cylinder 3 and the inner cylinder 4, through which the liquid always passes in the lower communication path 5, thereby efficiently bringing the methanogenic bacteria into contact with the organic acids. In the slow stirring process, if the airflow of blower B can be controlled, the degree of contact between the sludge and organic acids can be controlled, and the amount of biogas generated can be adjusted to maximize the amount. For example, blower B can be configured to be controlled by an inverter circuit.

[0050] In the rapid stirring process, the liquid movement speed is much higher than in slow stirring, resulting in high stirring efficiency of the fermentation liquid in the tank, thus allowing the substrate to be dispersed throughout the tank. If the stirring force in the rapid stirring process is too strong, there is a risk of destroying the methane fermentation bacteria aggregates, and if the stirring force is too weak, there is a risk of insufficient dispersion of the substrate in the fermentation liquid. Therefore, by adjusting the opening of valve V5 provided in the upper communication path 6, the stirring force in the rapid stirring process can be adjusted to maximize the amount of biogas generated.

[0051] Therefore, the airflow of blower B and the opening of valve V5 should be adjusted based on the amount of biogas produced, measured using the stirring cycle, which includes the first stirring process (including the standing process) and the second stirring process. In other words, it is preferable to adjust at least one of the following: the time of the standing process, the time of the slow stirring process, and the time of the rapid stirring process, according to the amount of biogas produced from methane fermentation tank 2. In addition to adjusting the opening of valve V5, the liquid level difference set in the slow stirring process may also be adjusted as a method of adjusting the stirring force in the rapid stirring process.

[0052] Regarding the circulation supply of fermented sludge to the fermentation tank 2 via the circulation path, it is preferable to set the fermented sludge to be circulated and supplied from above the liquid surface of the inner cylinder 4 when the liquid level in the inner cylinder 4 has decreased in the second stirring process. This allows the fermented sludge, which has a large potential energy and is supplied via the circulation path, to collide with the liquid surface, destroying scum and foam floating on the liquid surface and preventing scum growth.

[0053] The fermentation sludge to which this invention applies preferably has a high solid content, and preferably a evaporation residue concentration (usually also referred to as "TS") of 5% or more. Methane fermentation sludge tends to exhibit non-Newtonian fluid properties more strongly when the evaporation residue concentration is 5% or more. Non-Newtonian fluids change viscosity in response to shear force; that is, viscosity increases when the applied shear force is small. Therefore, gas stirring or pump stirring limits the range in which shear force can be applied, making it impossible to stir the entire tank. However, this stirring method forcibly creates a liquid level difference, allowing shear force to be applied to the entire sludge, thus enabling good stirring of the entire tank.

[0054] If the evaporation residue concentration is less than 5%, the fermentation sludge can be stirred by mechanical stirring using agitator blades. However, if the evaporation residue concentration is 5% or higher, the stirring action only extends to the agitator blades and their vicinity, requiring excessive stirring equipment to stir the entire tank. Furthermore, if the aforementioned tank stirring (slow, rapid, and static) is performed when the evaporation residue concentration is 5% or higher, the contact between methanogenic bacteria and organic acids (slow) and the overall stirring of the fermentation liquid (rapid) can be effectively utilized, allowing for a sufficiently static stage while still enabling highly efficient methane fermentation treatment overall.

[0055] As shown in Figure 5(a), it is preferable that the methane fermentation tank 2 has a swirling mechanism 11, which is composed of multiple stirring blades 11A and has no side plates at the top, erected at the bottom of the outer cylinder 3, corresponding to a lower communication path 5 that allows fermentation sludge to flow between the internal space of the outer cylinder 3 and the internal space of the inner cylinder 4 at the bottom. Furthermore, it is preferable that the upper ends of the stirring blades 11A constituting the swirling mechanism 11 are arranged to form a gap between them and the lower ends of the inner cylinder 4.

[0056] If the stirring blades 11A are not provided, during rapid stirring, the fermented sludge will mainly move radially within the cylindrical body, and no stirring force will be generated in the circumferential direction, which may result in insufficient stirring performance. However, by providing the aforementioned swirling mechanism 11, the fermented sludge will be stirred radially and circumferentially during rapid stirring, thus achieving a good stirring effect.

[0057] As shown in Figure 5(b), it is also possible to set the height of the stirring blades 11A to the same height as the lower communication path 5. However, in that case, clogging may occur if there are lumps in the fermented sludge, or fermented sludge may accumulate between the stirring blades 11A. Therefore, depending on the properties of the fermented sludge, the configuration shown in Figure 5(a) is preferable, and it is preferable to set the height of the stirring blades 11A to within the range of 40-60% of the height of the lower communication path 5.

[0058] Each stirring blade 11A has a flat, plate-like shape with its pressure-receiving surface perpendicular to the bottom surface of the outer cylinder 3, and can be positioned at a slight inclination in the same direction as the radial direction of the outer cylinder 3. As shown in Figures 5(c) and (e), it is preferable that each stirring blade 11A extends from the inside to the outside of the inner cylinder 4 in a position that intersects with the lower end of the inner cylinder 4 in a plan view, but as shown in Figure 5(d), each stirring blade 11A may be positioned inside the lower end of the inner cylinder 4 in a plan view. Furthermore, as shown in Figure 5(f), each stirring blade 11A may be an arc-shaped plate-like shape with its pressure-receiving surface perpendicular to the bottom surface of the outer cylinder 3.

[0059] As shown in Figures 5(c) to (f), for example, when the fermented sludge flows dynamically from the outer cylinder 3 to the inner cylinder 4 during the rapid stirring process described above, the swirling mechanism 11 deflects the flow, forming a left-handed swirling flow indicated by the dashed line in the figure, thereby enhancing the stirring effect on the fermented sludge. Similarly, when the fermented sludge flows dynamically from the inner cylinder 4 to the outer cylinder 3, the swirling mechanism 11 deflects the flow, forming a right-handed swirling flow indicated by the dashed line in the figure, thereby enhancing the stirring effect on the fermented sludge.

[0060] In the embodiment described above, a methane fermentation tank 2 was described in which inner cylinders 4 are arranged concentrically inside an outer cylinder 3. However, as shown in Figures 6(a) and 6(b), multiple inner cylinders 4 may be evenly distributed inside the outer cylinder 3. The number of inner cylinders 4 is not limited to 4; it may be 2, 3, or 5. In this case as well, the ratio of the sum of the horizontal cross-sectional areas of the inner cylinders 4 to the horizontal cross-sectional area of ​​the outer cylinder 3 should be set in the range of 0.3 to 0.7 times, more preferably in the range of 0.4 to 0.6 times, and most preferably 0.5.

[0061] Figures 7(a) and 7(b) show yet another embodiment of the methane fermentation tank 2. The methane fermentation tank 2 comprises a lower communication path 5 through which the lower spaces of the methane fermentation tank 2 are interconnected, a partition wall W that divides the upper space into at least two compartments, a liquid level difference forming mechanism that supplies biogas from the outside to at least one of the compartments to create a liquid level difference between the liquid level of the fermentation sludge stored in that compartment and the liquid level of the fermentation sludge stored in the other compartments, and an upper communication path that connects the gas phase spaces formed above each compartment and can switch the connected state on and off, and the liquid level difference forming mechanism is configured to be able to supply biogas to all compartments.

[0062] If there are two sections, the first and second stirring steps shown in Figure 4(a) will be repeated alternately for the left and right sections. If there are four or more sections, any multiple sections can be grouped into two groups, and the first and second stirring steps will be repeated alternately for each group. When grouping, it is preferable that adjacent sections belong to different groups.

[0063] In the embodiments described above, the case in which rice straw, which is the harvest residue of rice, is used as the fermentation raw material was explained. However, as a suitable fermentation raw material for the fermentation tank according to the present invention, agricultural waste generated after the harvest of grains harvested in the field, such as wheat straw, can be suitably used. In addition to agricultural waste, organic waste such as paper waste and food waste, or organic waste such as sewage sludge and livestock waste can also be used.

[0064] The various embodiments described above are merely examples of the present invention, and the scope of the invention is not limited by this description. It goes without saying that the design can be modified as appropriate within the scope in which the effects and advantages of each invention are achieved. [Explanation of symbols]

[0065] 1: Methane fermentation apparatus 2: Methane fermentation tank 2L: Gas supply pipe 3: Outer cylinder 3L: Biogas discharge pipe 4: Inner cylinder 4L: Biogas discharge pipe 5: Lower connecting route 6: Upper connecting route 6A: Stirring blade 6B: Electric motor 6C: Rotation axis 7: Gas holder 8: Liquid level difference formation mechanism 10: Raw material supply mechanism (mixer) 11: Swivel mechanism 11A: Stirring blade B: Blower V1~V5: Valve

Claims

1. A methane fermentation tank comprising an outer cylinder with its upper and lower ends closed, and at least one inner cylinder disposed inside the outer cylinder, with its upper end closed and its lower end open, A lower connecting path connects the internal space of the outer cylinder and the internal space of the inner cylinder at the bottom, allowing fermented sludge to flow between the outer cylinder and the inner cylinder, A liquid level difference forming mechanism that supplies biogas from the outside to the outer cylinder or the inner cylinder to form a liquid level difference between the liquid level of the fermented sludge stored in the outer cylinder and the liquid level of the fermented sludge stored in the inner cylinder, An upper communication path connects the gas phase spaces formed in the upper part of the outer cylinder and the upper part of the inner cylinder, and the communication state can be switched open or closed. A swirling mechanism comprising a plurality of stirring blades erected at the bottom of the outer cylinder corresponding to the lower communication path, A methane fermentation tank equipped with a methane fermentation tank.

2. The methane fermentation tank according to claim 1, wherein a gap is formed between the lower end of the inner cylinder and the upper end of the stirring blade.

3. The methane fermentation tank according to claim 1, wherein the stirring blade extends from the inside to the outside of the inner cylinder in a position that intersects with the lower end of the inner cylinder in a plan view.

4. The methane fermentation tank according to claim 1, wherein each of the stirring blades is formed of a plate-like body.

5. The methane fermentation tank according to any one of claims 1 to 4, wherein the liquid level difference formation mechanism is configured to allow switching between the outer cylinder and the inner cylinder as the destination for the biogas supply.

6. A methane fermentation tank according to any one of claims 1 to 4, wherein the horizontal cross-sectional area of ​​the inner cylinder is set in the range of 0.3 to 0.7 times the horizontal cross-sectional area of ​​the outer cylinder.

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