Methane fermentation tank and methane fermentation treatment method

Abstract

Disclosed is a methane fermentation tank which comprises: an outer cylinder, the upper and lower end parts of which are closed; and at least one inner cylinder, the upper end part of which is closed and the lower end part of which is opened. The methane fermentation tank also comprises: a lower communication passage which brings the internal space of the outer cylinder and the internal space of the inner cylinder into communication with each other at lower parts, thereby making it possible for fermentation sludge to flow between the outer cylinder and the inner cylinder; a liquid level difference forming mechanism which forms 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 by supplying a biogas to the outer cylinder or the inner cylinder from the outside; and an upper communication passage which brings gas phase spaces that are formed in the upper parts of the outer cylinder and the inner cylinder into communication with each other, and which is capable of switching the communication state between an open state and a closed state. The horizontal cross-sectional area of the inner cylinder is set in the range of 0.3-0.7 times the horizontal cross-sectional area of the outer cylinder.

Description

Anaerobic digester and anaerobic digestion treatment method

[0001] The present invention relates to an anaerobic digester and an anaerobic digestion treatment method.

[0002] Patent Document 1 discloses an anaerobic digester and an anaerobic digestion treatment method for producing 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 organic waste such as paper waste and food waste contained in general waste is attracting attention as a resource circulation method that utilizes anaerobic digestion treatment as a raw material.

[0003] Patent Document 2 proposes an anaerobic digester provided with a first circulation means for generating an upward flow and a downward flow that descends outside the upward flow in the liquid in the tank to circulate the liquid, and a second circulation means for generating a horizontal swirling flow around the part of the upward flow in the liquid in the tank to circulate the liquid.

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

[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 stirs 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.

[0008] Japanese Patent Publication No. 2023-131221, Japanese Patent Publication No. 2002-263693, Japanese Patent Publication No. 2000-301116

[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 object of the present invention is to provide a methane fermentation tank and methane fermentation treatment method with a simple structure that can effectively agitate the fermentation liquid even when the solid content of the fermentation sludge is high, and which has excellent fermentation efficiency.

[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, and an upper communication path that connects the gas phase spaces formed at the top of the outer cylinder and the inner cylinder and allows switching of the communication state to open and close, 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.

[0015] The liquid level difference formation mechanism supplies biogas to the outer cylinder from the outside, and the pressurization of the outer cylinder expands the gas phase space at the top of the outer cylinder where biogas is stored. As a result, the liquid level of the fermented sludge stored in the outer cylinder falls below the liquid level of the fermented sludge stored in the inner cylinder, creating a liquid level difference. This creates a static flow of fermented sludge from the outer cylinder to the inner cylinder via the lower communication path, resulting in gentle agitation of the fermented sludge.

[0016] Conversely, when biogas is supplied to the inner cylinder from the outside, the inner cylinder is pressurized, expanding the gas phase space at the top of the inner cylinder where the biogas is stored. This causes the liquid level of the fermented sludge stored in the inner cylinder to drop below the liquid level of the fermented sludge stored in the outer cylinder, creating a liquid level difference. As a result of this liquid level difference, the fermented sludge flows statically from the inner cylinder to the outer cylinder through the lower communication path, gently agitating the fermented sludge.

[0017] Furthermore, when the upper communication path connecting the gas phase spaces formed at the top of the outer and inner cylinders is switched from a closed state to an open state, in the former case the fermented sludge flows dynamically from the inner cylinder to the outer cylinder via the lower communication path, and in the latter case the fermented sludge flows dynamically from the outer cylinder to the inner cylinder via the lower communication path, causing the fermented sludge to be rapidly agitated.

[0018] By setting the horizontal cross-sectional area of ​​the inner cylinder to 0.3 to 0.7 times the horizontal cross-sectional area of ​​the outer cylinder, the amount of biogas required to generate the same liquid level difference is approximately the same whether the outer cylinder is pressurized or the inner cylinder is pressurized. The potential energy of the fermentation sludge generated by the liquid level difference is also approximately the same. Furthermore, the stirring effect of the fermentation sludge flowing between the outer and inner cylinders is the same whether the outer cylinder is pressurized or the inner cylinder is pressurized, resulting in homogeneous stirring. Thus, a methane fermentation tank with a simple structure and high fermentation efficiency can be realized.

[0019] The second characteristic configuration is that, in addition to the first characteristic configuration described above, a flow rate adjustment mechanism is provided in the upper communication path.

[0020] If a flow rate adjustment mechanism is provided when switching the upper communication path from a closed state to an open state while there is a liquid level difference in the fermentation sludge between the outer and inner cylinders, the flow rate of the biogas flowing through the upper communication path can be adjusted, thereby adjusting the time it takes for the liquid level difference to be resolved, adjusting the flow rate of the fermentation sludge flowing through the lower communication path, and thus adjusting the degree of agitation of the fermentation sludge at that time.

[0021] The third characteristic configuration is that, in addition to the first characteristic configuration described above, it includes a circulation path that returns the fermentation sludge withdrawn from the bottom of the methane fermentation tank back to the methane fermentation tank, and a raw material supply mechanism that supplies methane fermentation raw materials to the circulation path.

[0022] Since the methane fermentation raw materials supplied from the raw material supply mechanism are supplied to the methane fermentation tank along with the fermentation sludge that is returned to the methane fermentation tank via the circulation route, there is no need to provide a separate raw material supply mechanism in the methane fermentation tank. Furthermore, it is possible to discharge a portion of the fermentation sludge withdrawn from the methane fermentation tank via the circulation route as digested sludge outside the system.

[0023] The first characteristic configuration of the methane fermentation treatment method according to the present invention is a methane fermentation treatment method using a methane fermentation tank having any of the first to third characteristic configurations described above, wherein the method repeats the following steps: a first stirring step in which the biogas is supplied to the outer cylinder from the outside by the liquid level difference formation mechanism with the upper communication passage closed, thereby lowering the liquid level in the outer cylinder and raising the liquid level in the inner cylinder to form the liquid level difference, and then opening the upper communication passage to eliminate the liquid level difference and stir the fermentation sludge; and a second stirring step in which the biogas is supplied to the inner cylinder from the outside by the liquid level difference formation mechanism with the upper communication passage closed, thereby lowering the liquid level in the inner cylinder and raising the liquid level in the outer cylinder to form the liquid level difference, and then opening the upper communication passage to eliminate the liquid level difference and stir the fermentation sludge.

[0024] In the first stirring process, the fermented sludge is gently stirred as the liquid level in the outer cylinder is lowered below that of the inner cylinder, and then rapidly stirred when the upper communication passage is opened to eliminate the liquid level difference. In the second stirring process, the fermented sludge is gently stirred as the liquid level in the inner cylinder is lowered below that of the outer cylinder, and then rapidly stirred when the upper communication passage is opened to eliminate the liquid level difference. By repeating the first and second stirring processes, homogeneous stirring of the fermented sludge stored in the inner and outer cylinders can be achieved.

[0025] The second characteristic configuration is that, in addition to the first characteristic configuration described above, the methane fermentation tank is equipped with a circulation path that circulates and supplies the fermentation sludge withdrawn from the bottom from above the liquid surface of the inner cylinder, and in the second stirring process, when the liquid level of the inner cylinder has decreased, the fermentation sludge is circulated and supplied from above the liquid surface of the inner cylinder via the circulation path.

[0026] In the second stirring process, when the liquid level in the inner cylinder has decreased, fermented sludge is supplied from above the liquid surface in the inner cylinder via the circulation path. This causes the fermented sludge, which has a large potential energy, to collide with the liquid surface, destroying scum and other materials present on the liquid surface and preventing scum growth.

[0027] The third characteristic feature is that, in addition to the first characteristic feature described above, the concentration of evaporated residue in the fermented sludge is 5% or more.

[0028] This method can effectively promote fermentation treatment for fermented sludge with an evaporation residue concentration of 5% or higher.

[0029] A fourth characteristic configuration of the methane fermentation tank according to the present invention is that it comprises a lower communication path that connects the lower spaces of the methane fermentation tank, a partition wall 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 compartment, and an upper communication path that connects the gas phase spaces formed above each compartment and can switch the communication state open and closed, wherein the liquid level difference forming mechanism is configured to be able to supply biogas to all compartments.

[0030] The methane fermentation tank is constructed such that the upper space is divided into at least two compartments by a partition wall, and the lower space is connected by a lower communication path. A liquid level difference formation mechanism is configured to supply biogas to all compartments, and by supplying biogas from the outside to at least one compartment, a liquid level difference is formed 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. At this time, the fermentation sludge in the compartment where the liquid level is decreasing flows statically through the lower space to the compartment where the liquid level is rising, resulting in gentle agitation of the fermentation sludge.

[0031] An upper communication path is configured to connect the gas phase spaces formed at the top of each section, and its connected state can be switched on and off. By connecting the gas phase spaces of sections where a liquid level difference has been formed, the liquid level difference is rapidly eliminated. In this process, the fermented sludge from the section where the liquid level is decreasing flows dynamically through the lower communication path to the section where the liquid level is rising, resulting in rapid agitation of the fermented sludge.

[0032] As described above, the present invention provides a methane fermentation tank and methane fermentation treatment method with a simple structure that allows for good stirring of the fermentation liquid even when the solid content of the fermented sludge is high and the viscosity is high, resulting in excellent fermentation efficiency.

[0033] Figure 1A is an explanatory diagram showing the configuration of a methane fermentation apparatus including a methane fermentation tank. Figure 1B is a plan view of the methane fermentation tank. Figure 2 is an explanatory diagram of the first stirring step, which is one embodiment of the methane fermentation treatment method. Figure 3 is an explanatory diagram of the second stirring step, which is another embodiment of the methane fermentation treatment method. Figure 4A is an explanatory diagram of the steps of the methane fermentation treatment method. Figure 4B is an explanatory diagram showing another embodiment of the steps of the methane fermentation treatment method. Figure 4C is an explanatory diagram showing yet another embodiment of the steps of the methane fermentation treatment method. Figure 5A is an explanatory diagram of a side view of the stirring blades provided in the methane fermentation tank. Figure 5B is an explanatory diagram of a side view showing another embodiment of the stirring blades provided in the methane fermentation tank. Figure 5C is an explanatory diagram showing the first embodiment of the stirring blades provided in the methane fermentation tank, showing the shape and arrangement in a plan view. Figure 5D is an explanatory diagram showing the second embodiment of the stirring blades provided in the methane fermentation tank, showing the shape and arrangement in a plan view. Figure 5E is an explanatory diagram showing the third embodiment of the stirring blades provided in the methane fermentation tank, showing the shape and arrangement in a plan view. Figure 5F shows a fourth configuration of the stirring blades provided in the methane fermentation tank, and is an explanatory diagram showing the shape and arrangement in a plan view. Figure 6A is an explanatory diagram of a longitudinal section showing another configuration of the methane fermentation tank. Figure 6B is an explanatory diagram of the same cross-section. Figure 7A is an explanatory diagram of a longitudinal section showing yet another configuration of the methane fermentation tank. Figure 7B is an explanatory diagram of the same cross-section.

[0034] 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.

[0035] Figures 1A and 1B illustrate a methane fermentation apparatus 1 according to the present invention. Figure 1A is an explanatory diagram showing the internal structure of the methane fermentation tank 2 and the arrangement of peripheral equipment necessary for operating the methane fermentation tank 2, and Figure 1B is a plan view explanatory diagram showing the internal structure of the methane fermentation tank 2.

[0036] 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.

[0037] 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.

[0038] 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.

[0039] Furthermore, the system includes 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.

[0040] 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 the 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.

[0041] 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.

[0042] 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.

[0043] 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 is a process in which, with the upper communication passage 6 closed, biogas is supplied to the outer cylinder 3 from the outside by the liquid level difference formation mechanism 8, 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 the upper communication passage 6 is opened to eliminate the liquid level difference and stir the fermentation sludge.

[0044] As shown in Fig. 3, the second stirring step refers to a step of stirring the fermented sludge by closing the upper communication path 6 and supplying biogas from the outside to the inner cylinder 4 by the liquid level difference forming mechanism 8 to lower the liquid level of the inner cylinder 4 and raise the liquid level of the outer cylinder 3 to form a liquid level difference, and then opening the upper communication path 6 in that state to eliminate the liquid level difference. "Outside" means the outside of the methane fermentation tank 2, and in this embodiment, the biogas stored in the gas holder 7 is used. Instead of pressurizing with the biogas spontaneously generated from the fermented liquid staying in the methane fermentation tank 2, forced pressurization is performed with the biogas supplied from the outside via the blower B, so that an appropriate liquid level difference can be formed within an appropriate time.

[0045] In Figs. 2 and 3, the gas is selectively supplied from the gas supply pipes 2L connected to the ceiling parts of the outer cylinder 3 and the inner cylinder 4 via the valves V1 and V2. However, the tip of the gas supply pipe 2L may be arranged to be located in the fermented liquid so as to supply biogas into each fermented liquid.

[0046] During the process of forming the liquid level difference, a slow stirring step is executed in which the fermented sludge stored in the outer cylinder 3 and the inner cylinder 4 flows statically through the lower communication path 5. During the process of eliminating the liquid level difference, a rapid stirring step is executed in which the fermented sludge stored in the outer cylinder 3 and the inner cylinder 4 flows dynamically through the lower communication path 5.

[0047] The step of supplying biogas to the outer cylinder 3 is called the outer cylinder pressurization step, and the step of supplying biogas to the inner cylinder 4 is called the inner cylinder pressurization step. When the horizontal cross-sectional area of the inner cylinder 4 with respect to the horizontal cross-sectional area of the outer cylinder 3 is set to 0.5 and the liquid level differences generated in the outer cylinder pressurization step and the inner cylinder pressurization step are the same, the liquid volume of the fermented liquid corresponding to the liquid level difference is an equivalent value, that is, the potential energy generated by the liquid level difference is an equivalent value, and the stirring force caused by the liquid level difference is an equivalent 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 above-mentioned range.

[0048] As shown in FIG. 4A, the methane fermentation treatment method is a method for effectively generating 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 step is a stirring step in which biogas is forcibly supplied from the outside to the outer cylinder 3 or the inner cylinder 4 by a biogas supply means such as a blower B, and a liquid level difference is provided 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 while flowing the fermentation sludge between the outer cylinder 3 and the inner cylinder 4.

[0049] The rapid stirring step is a stirring step in which the liquid level difference formed in the slow stirring step is released by eliminating the pressure difference in the gas phase space formed between the outer cylinder and the inner cylinder, and the fermentation sludge is caused to flow in the opposite direction to the slow stirring process. The standing step is a step of allowing the fermentation sludge to stand after the rapid stirring step or the slow stirring step.

[0050] For example, if the capacity of the methane fermentation tank 2 is about 40 L, the time required for the slow stirring step is about several tens of seconds to several minutes, the time required for the rapid stirring step is about 0.1 second to 1 minute, and the time required for the standing step is set to about 30 minutes to 1 hour. The first stirring step and the second stirring step are alternately repeated including the standing step. In order to obtain good fermentation efficiency, it is preferable that the ratio of the standing step in the treatment time including the slow stirring step and the rapid stirring step is set to 75% or more. The time required for each step is not limited to this value and is appropriately set based on the target fermentation efficiency.

[0051] In addition to being performed after the rapid stirring step, the standing step may be performed after the rapid stirring step as shown in FIG. 4B, or the standing step may be performed after each of the slow stirring step and the rapid stirring step as shown in FIG. 4C. 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 including a standing step between the first stirring step and the second stirring step, or during each step (between the slow stirring step and the rapid stirring step).

[0052] 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.

[0053] 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 enabling efficient contact between methanogenic bacteria and 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.

[0054] 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, which allows 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 aggregates of methane-fermenting bacteria, 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 the 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.

[0055] 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 the 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.

[0056] 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.

[0057] 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.

[0058] 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 is limited 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.

[0059] As shown in Figure 5A, it is preferable that the methane fermentation tank 2 has a swirling mechanism 11, which is composed of multiple stirring blades 11A and has an open state without 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 so as to form a gap between them and the lower end of the inner cylinder 4.

[0060] 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.

[0061] As shown in Figure 5B, 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 in Figure 5A is preferable, and it is preferable to set the height of the stirring blades 11A to a range of 40 to 60% of the height of the lower communication path 5.

[0062] 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 5C and 5E, 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 5D, 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 5F, each stirring blade 11A may be an arc-shaped plate-like body with its pressure-receiving surface perpendicular to the bottom surface of the outer cylinder 3.

[0063] As shown in Figures 5C to 5F, for example, when the fermented sludge flows dynamically from the outer cylinder 3 to the inner cylinder 4 in the rapid stirring process described above, the flow is deflected by the swirling mechanism 11, forming a left-handed swirling flow as shown 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 flow is deflected by the swirling mechanism 11, forming a right-handed swirling flow as shown by the double-dashed line in the figure, thereby enhancing the stirring effect on the fermented sludge.

[0064] In the embodiment described above, a methane fermentation tank 2 was described in which an inner cylinder 4 is arranged concentrically inside an outer cylinder 3. However, as shown in Figures 6A and 6B, multiple inner cylinders 4 may be evenly distributed inside the outer cylinder 3. The number of inner cylinders 4 is not limited to four; it may be two, three, or five. 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 times.

[0065] Figures 7A and 7B 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 tank are connected, 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.

[0066] If there are two sections, the first and second stirring steps shown in Figure 4A 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.

[0067] 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.

[0068] 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.

[0069] 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 communication path 6: Upper communication path 6A: Agitator blade 6B: Electric motor 6C: Rotating shaft 7: Gas holder 8: Liquid level difference formation mechanism 10: Raw material supply mechanism (mixer) 11: Swirling mechanism 11A: Agitator blade B: Blower V1-V5: Valves

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, the 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; and an upper communication path that connects the gas phase spaces formed at the top of the outer cylinder and the inner cylinder, and allows switching the communication state to open or close, 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.

2. The methane fermentation tank according to claim 1, wherein a flow rate adjustment mechanism is provided in the upper communication path.

3. The methane fermentation tank according to claim 1, comprising a circulation path that returns the fermentation sludge withdrawn from the lower part of the methane fermentation tank to the methane fermentation tank, and a raw material supply mechanism that supplies methane fermentation raw materials to the circulation path.

4. A methane fermentation treatment method using a methane fermentation tank according to any one of claims 1 to 3, comprising: a first stirring step of supplying the biogas from the outside to the outer cylinder by the liquid level difference formation mechanism with the upper communication passage closed, thereby lowering the liquid level in the outer cylinder and raising the liquid level in the inner cylinder to form the liquid level difference, and then opening the upper communication passage to eliminate the liquid level difference and stir the fermentation sludge; and a second stirring step of supplying the biogas from the outside to the inner cylinder by the liquid level difference formation mechanism with the upper communication passage closed, thereby lowering the liquid level in the inner cylinder and raising the liquid level in the outer cylinder to form the liquid level difference, and then opening the upper communication passage to eliminate the liquid level difference and stir the fermentation sludge, repeating these steps.

5. The methane fermentation treatment method according to claim 4, wherein the methane fermentation tank is provided with a circulation path for circulating and supplying the fermented sludge withdrawn from the bottom from above the liquid surface of the inner cylinder, and the fermented sludge is circulated and supplied from above the liquid surface of the inner cylinder via the circulation path when the liquid level of the inner cylinder has decreased in the second stirring process.

6. The methane fermentation treatment method according to claim 4, wherein the evaporation residue concentration of the fermented sludge is 5% or more.

7. A methane fermentation tank comprising: a partition wall that divides the upper space into at least two compartments, forming a lower communication path through which the lower spaces of the methane fermentation tank are interconnected; 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 allows switching the communication state to open or close, wherein the liquid level difference forming mechanism is configured to supply biogas to all compartments.

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