Methanogenic digester and methanogenic treatment method

Abstract

To provide a simple-structured methane fermentation tank that can stir the fermentation broth well even when the solid concentration of the fermented sludge is high and has excellent fermentation efficiency. 【Solution means】A methane fermentation tank comprising an outer cylinder with closed upper and lower ends, and at least one inner cylinder arranged inside the outer cylinder with a closed upper end and an open lower end. The internal space of the outer cylinder and the internal space of the inner cylinder communicate with each other at the lower part, and there is a lower communication path that allows the fermented sludge to flow between the outer cylinder and the inner cylinder. There is 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. There is an upper communication path that communicates the gas phase spaces formed at the upper parts of the outer cylinder and the inner cylinder and can switch the communication state between open and closed. The horizontal cross-sectional area of the inner cylinder with respect to the horizontal cross-sectional area of the outer cylinder is set in the range of 0.3 to 0.7 times.

Description

Technical Field

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

Background Art

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

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

[0004] Patent Document 3 proposes a methane fermentation tank including: a sealed tank body; a main fermentation section formed in the tank body for methane-fermenting 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 a partition around the outer periphery of the precipitation section via a partition cylinder for temporarily storing the digestion sludge after methane fermentation flowing in from the main fermentation section and having a discharge port for the digestion sludge; a tubular mixing shaft communicating the digestion sludge storage section with below the liquid level of the slurry stored in the main fermentation section; a center tube communicating the central part of the precipitation section with 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 with the upper part of the precipitation section where methane gas generated in the precipitation section accumulates.

[0005] The methane fermentation tank partitions the tank body into an inner granule granulation part and an outer main fermentation region with a center tube, pressurizes the liquid level of the main fermentation part using the generated gas, forms a water level difference with the inner region, and is configured to stir the inside of the tank by the flow of the fermentation liquid generated as the water level difference is eliminated by releasing the pressure equalizing valve.

[0006] Thus, in order to efficiently proceed with the methane fermentation treatment, it is necessary to promote stirring and mixing without retaining the methane fermentation sludge, which is sludge containing bacteria, in the methane fermentation tank.

[0007] Therefore, methods such as mechanically stirring the fermentation liquid in the tank using stirring blades, supplying the biogas generated by methane fermentation into the tank and stirring the fermentation liquid using the upward gas flow, circulating the fermentation liquid in the tank using a pump and stirring, and stirring the fermentation liquid using the water level difference between the inner and outer divided regions have been proposed so far.

Prior Art Documents

Patent Documents

[0008]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0009] However, the method of mechanically stirring using stirring blades not only makes the power excessive and is difficult to cope with the enlargement of the fermentation tank and the increase in the concentration of the fermentation sludge, but also has a limited stirring pattern for the fermentation sludge, so there is a risk of forming a dead water area in the tank. Once a dead water area is formed, the fermentation sludge and the treatment object as the raw material will accumulate and sufficient fermentation treatment will be inhibited, and there is a problem that the dead water area needs to be cleaned during maintenance.

[0010] In addition, for the method of using a pump, it is necessary to set a circulation route for withdrawing fermented sludge from the fermentation tank and returning the withdrawn fermented sludge to the tank in order to circulate the fermented sludge in the tank. However, when the fermentation tank becomes larger, there is a problem that it is difficult to set an effective circulation route for uniformly stirring the fermented sludge in the tank.

[0011] As an example of the method of stirring fermented sludge using biogas, a method can be exemplified in which biogas is supplied to a draft tube and stirred by the circulation flow generated inside and outside the draft tube along with the movement of the biogas rising in the draft tube. However, when the fermentation tank becomes larger, not only is it necessary to increase the number of installed draft tubes because the stirring range is limited, but also there is a problem that it is difficult to determine its arrangement because the behavior of the circulation flow varies depending on the properties of the fermented sludge.

[0012] In the methane fermentation tank described in Patent Document 3, biogas generated in the main fermentation part where the amount of fermented sludge stored is larger than that in the inner region is stored in the upper space of the main fermentation part, so that the water level of the main fermentation part is lower than that in the inner region. Therefore, it takes time to form a water level difference. In particular, when the solid concentration of the fermented sludge is high and the viscosity is high, there is a problem that the biogas is trapped in the fermentation liquid and the liquid level cannot be efficiently lowered, and there is room for further improvement from the viewpoint of improving the stirring efficiency.

[0013] An object of the present invention is to provide a methane fermentation tank and a methane fermentation treatment method having a simple structure that can stir the fermentation liquid well even when the solid concentration of the fermented sludge is high and have excellent fermentation efficiency.

Means for Solving the Problems

[0014] To achieve the above object, the first characteristic configuration of the methane fermentation tank according to the present invention is a methane fermentation tank comprising an outer cylinder with closed upper and lower ends, and at least one inner cylinder disposed inside the outer cylinder with a closed upper end and an open lower end, wherein the internal space of the outer cylinder and the internal space of the inner cylinder communicate with each other at the lower part, a lower communication path enabling fermentation sludge to flow between the outer cylinder and the inner cylinder, a liquid level difference forming mechanism for supplying 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 for communicating the gas phase spaces formed at the upper parts of the outer cylinder and the inner cylinder and capable of switching the communication state between open and closed, and the horizontal cross-sectional area of the inner cylinder with respect to the horizontal cross-sectional area of the outer cylinder is set in the range of 0.3 to 0.7 times.

[0015] Due to the external supply of biogas to the outer cylinder by the outer cylinder pressurization in which the liquid level difference forming mechanism supplies biogas to the outer cylinder from the outside, the gas phase space where biogas is stored in the upper part of the outer cylinder expands, and the liquid level of the fermentation sludge stored in the outer cylinder drops below the liquid level of the fermentation sludge stored in the inner cylinder, forming a liquid level difference. Along with the formation of the liquid level difference, the fermentation sludge flows statically from the outer cylinder to the inner cylinder through the lower communication path, thereby gently stirring the fermentation sludge.

[0016] Conversely, due to the inner cylinder pressurization in which biogas is supplied to the inner cylinder from the outside, the gas phase space where biogas is stored in the upper part of the inner cylinder expands, and the liquid level of the fermentation sludge stored in the inner cylinder drops below the liquid level of the fermentation sludge stored in the outer cylinder, forming a liquid level difference. Along with the formation of the liquid level difference, the fermentation sludge flows statically from the inner cylinder to the outer cylinder through the lower communication path, thereby gently stirring the fermentation sludge.

[0017] Furthermore, when the upper communication path for communicating the gas phase spaces formed at the upper parts of the outer cylinder and the inner cylinder is switched from the closed state to the open state, in the former case, the fermentation sludge flows dynamically from the inner cylinder to the outer cylinder through the lower communication path, and in the latter case, the fermentation sludge flows dynamically from the outer cylinder to the inner cylinder through the lower communication path, and the fermentation sludge is rapidly stirred.

[0018] If the horizontal cross-sectional area of the inner cylinder is set in the range of 0.3 to 0.7 times that of the outer cylinder, the supply amount of biogas required to generate the same liquid level difference is almost equal regardless of whether the outer cylinder is pressurized or the inner cylinder is pressurized. The potential energy of the fermented sludge generated by the liquid level difference is also approximately equal, and there is no difference in the stirring effect of the fermented sludge flowing between the outer cylinder and the inner cylinder between outer cylinder pressurization and inner cylinder pressurization, and it will be stirred evenly. Therefore, 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] When switching the communication state of the upper communication path from a closed state to an open state with a liquid level difference created in the fermented sludge between the outer cylinder and the inner cylinder, if a flow rate adjustment mechanism is provided, the flow rate of the biogas flowing through the upper communication path can be adjusted, so that the liquid level difference elimination time is adjusted, the flow rate of the fermented sludge flowing through the lower communication path is adjusted, and the degree of stirring of the fermented sludge at that time can be adjusted.

[0021] The third characteristic configuration is that, in addition to the first characteristic configuration described above, a circulation path for returning the fermented sludge drawn from the lower part of the methane fermentation tank to the methane fermentation tank is provided, and a raw material supply mechanism for supplying methane fermentation raw materials to the circulation path is provided.

[0022] Since the methane fermentation raw materials supplied from the raw material supply mechanism together with the fermented sludge returned to the methane fermentation tank through the circulation path are supplied to the methane fermentation tank, there is no need to provide a separate raw material supply mechanism in the methane fermentation tank. In addition, a part of the fermented sludge drawn from the methane fermentation tank through the circulation path can be discharged out of the system as digested sludge.

[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 one of the first to third characteristic configurations described above. With the upper communication path closed, biogas is supplied from the outside to the outer cylinder by the liquid level difference forming mechanism, thereby lowering the liquid level of the outer cylinder and raising the liquid level of the inner cylinder to form the liquid level difference. In this state, the upper communication path is opened to eliminate the liquid level difference, thereby stirring the fermented sludge in the first stirring step. In the second stirring step, with the upper communication path closed, biogas is supplied from the outside to the inner cylinder by the liquid level difference forming mechanism, thereby lowering the liquid level of the inner cylinder and raising the liquid level of the outer cylinder to form the liquid level difference. In this state, the upper communication path is opened to eliminate the liquid level difference, thereby stirring the fermented sludge. The point is to repeat these steps.

[0024] In the first stirring step, when the liquid level of the outer cylinder is lowered from the liquid level of the inner cylinder, gentle stirring of the fermented sludge is performed. When the upper communication path is opened to eliminate the liquid level difference, rapid stirring of the fermented sludge is performed. In the second stirring step, when the liquid level of the inner cylinder is lowered from the liquid level of the outer cylinder, gentle stirring of the fermented sludge is performed. When the upper communication path is opened to eliminate the liquid level difference, rapid stirring of the fermented sludge is performed. By repeating the first stirring step and the second stirring step, uniform stirring of the fermented sludge stored in the inner cylinder and the outer cylinder can be achieved.

[0025] The second characteristic configuration is that, in addition to the first characteristic configuration described above, the methane fermentation tank is provided with a circulation path for circulating and supplying the fermented sludge drawn from the lower part above the liquid level of the inner cylinder. In the state where the liquid level of the inner cylinder has dropped in the second stirring process, the fermented sludge is circulated and supplied from above the liquid level of the inner cylinder through the circulation path.

[0026] In the state where the liquid level of the inner cylinder has dropped in the second stirring process, by supplying the fermented sludge through the circulation path from above the liquid level of the inner cylinder, the fermented sludge with a large potential energy collides with the liquid surface, destroying scum and the like existing on the liquid surface, and preventing the growth of scum.

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

[0028] For fermented sludge with an evaporation residue concentration of 5 Weight % or more, the fermentation treatment can be effectively promoted.

[0029] The fourth characteristic configuration of the methane fermentation tank according to the present invention is to form a lower communication path through which the lower space of the methane fermentation tank communicates, a partition wall that partitions the upper space into at least two compartments, and at least one of the compartments is supplied with biogas from the outside to provide a liquid level difference between the liquid level of the fermented sludge stored in the compartment and the liquid level of the fermented sludge stored in other compartments. A liquid level difference forming mechanism, and an upper communication path that communicates the gas phase spaces formed in the upper parts of the respective compartments and can switch the communication state between open and closed. The liquid level difference forming mechanism is configured to be able to supply biogas to all the compartments.

[0030] The upper space of the methane fermentation tank is partitioned into at least two compartments by a partition wall, and the methane fermentation tank is formed such that the lower space communicates through a lower communication path. By the liquid level difference forming mechanism configured to be able to supply biogas to all the compartments, by supplying biogas from the outside to at least one compartment, a liquid level difference is formed between the liquid level of the fermented sludge stored in the compartment and the liquid level of the fermented sludge stored in other compartments. At this time, the fermented sludge in the compartment where the liquid level drops flows statically through the lower space into the compartment where the liquid level rises, thereby gently stirring the fermented sludge.

[0031] The upper communication path that communicates the gas phase spaces formed in the upper parts of the respective compartments is configured to be able to switch the communication state between open and closed. By communicating the gas phase spaces of the compartments where the liquid level difference is formed, the liquid level difference is rapidly eliminated, and in the process, the fermented sludge in the compartment where the liquid level drops through the lower communication path flows dynamically through the lower space into the compartment where the liquid level rises, thereby rapidly stirring the fermented sludge.

Effects of the Invention

[0032] As described above, according to the present invention, it has become possible to provide a simple-structured methane fermentation tank and a methane fermentation treatment method that can satisfactorily stir the fermentation broth even when the solid concentration of the fermented sludge is high and the viscosity is large, and that are excellent in fermentation efficiency.

Brief Description of the Drawings

[0033]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Embodiments for Carrying Out the Invention

[0034] Hereinafter, the methane fermentation tank and the methane fermentation treatment method of the present invention will be described by taking as an example the case where rice straw, which is a rice harvesting residue generated in the field as a raw material, is used as the fermentation raw material.

[0035] In FIGS. 1(a) and (b), a methane fermentation apparatus 1 according to the present invention is illustrated. Fig. 1(a) 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 Fig. 1(b) is an explanatory diagram in a plan view 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, carbon dioxide, etc. generated in the methane fermentation tank, a circulation path 9 for withdrawing the fermentation liquid stored in the methane fermentation tank 2, discharging a part of it out of the system, and circulating the remaining part to 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, V2, and a blower B, etc. A heat-insulating jacket through which a heat medium flows is provided on the peripheral wall of the methane fermentation tank 2, and the inside of the tank is maintained at about 55 °C suitable for fermentation. For example, water heated by the heat generated by a combustor using the biogas stored in the gas holder 7 as fuel is used as the heat medium.

[0037] The main body of the methane fermentation tank 2 includes an outer cylinder 3 with closed upper and lower ends, and at least one inner cylinder 4 arranged inside the outer cylinder 3 with a closed upper end and an open lower end. 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 relative to the horizontal cross-sectional area of the outer cylinder 3 is set to 0.5. As will be described in detail later, the ratio of the horizontal cross-sectional areas is not limited to 0.5, and it may be set in the range of 0.3 to 0.7 times, and more preferably in the range of 0.4 to 0.6 times.

[0038] The shapes of the outer cylinder 3 and the inner cylinder 4 are not limited to cylindrical bodies, and may be elliptical cylinders with an elliptical cross-section or rectangular cylinders with a rectangular cross-section, as long as each is formed of a tubular body. Also, it is preferable that the horizontal cross-sectional area of the tubular body is the same along the height direction, but it may be slightly different. For example, it may expand from the bottom to the top, or conversely, it may narrow. When the ratio of the horizontal cross-sectional area of the inner cylinder 4 to the horizontal cross-sectional area of the outer cylinder 3 is different along the height direction, the average value of the ratio of the horizontal cross-sectional areas may be set in the range of 0.3 to 0.7 times, preferably in the range of 0.4 to 0.6 times, and more preferably 0.5.

[0039] Furthermore, a biogas discharge pipe 3L for guiding the biogas generated in the outer cylinder 3 to the gas holder 7 and a biogas discharge pipe 4L for guiding the biogas generated in the inner cylinder 4 to the gas holder 7 are provided, and valves V3 and V4 are arranged on the biogas discharge pipes 3L and 4L.

[0040] The methane fermentation tank 2 includes an upper communication path 6 that communicates the gas phase spaces 3s and 4s formed at the upper parts of the outer cylinder 3 and the inner cylinder 4 and is provided with a valve V5 capable of switching the opening and closing of the communication state, and a lower communication path 5 that communicates the internal space of the outer cylinder 3 and the internal space of the inner cylinder 4 at the lower part and enables the fermented sludge to flow between the outer cylinder 3 and the inner cylinder 4. The valve V5 provided in the upper communication path 6 also functions as a flow rate adjustment mechanism for adjusting the amount of gas flowing through by adjusting the opening degree.

[0041] Furthermore, the methane fermentation tank 2 includes a liquid level difference forming mechanism 8 that supplies biogas from the outside to the outer cylinder 3 or the inner cylinder 4 to form 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. The liquid level difference forming mechanism 8 is constituted by the above-described gas supply pipe 2L, valves V1 and V2, and blower B.

[0042] The circulation path 9 includes a withdrawal pipe 9A for withdrawing the fermented sludge from the lower part of the methane fermentation tank 2 and a supply pipe 9B for adding a methane fermentation raw material to the withdrawn fermented sludge and supplying it to the methane fermentation tank 2. The withdrawal pipe 9A is provided with a withdrawal pump P2, and the supply pipe 9B is provided with a supply pump P1. Also, a raw material supply mechanism 10 is provided in the path connecting the withdrawal pipe 9A and the supply pipe 9B. The raw material supply mechanism 10 includes a mixer for mixing the cut rice straw as the raw material and the fermented sludge, and may further add dilution water for mixing.

[0043] The methane fermentation treatment method using the above-described methane fermentation tank 2 will be described. The methane fermentation treatment method includes a first stirring step shown in FIG. 2 and a second stirring step shown in FIG. 3. As shown in Fig. 2, the first stirring step means a step of stirring the fermented sludge by supplying biogas from the outside to the outer cylinder 3 by the liquid level difference forming mechanism 8 with the upper communication path 6 closed, thereby lowering the liquid level of the outer cylinder 3 and raising the liquid level of the inner cylinder 4 to form a liquid level difference, and in that state, opening the upper communication path 6 to eliminate the liquid level difference.

[0044] As shown in Fig. 3, the second stirring step means a step of stirring the fermented sludge by supplying biogas from the outside to the inner cylinder 4 by the liquid level difference forming mechanism 8 with the upper communication path 6 closed, thereby lowering the liquid level of the inner cylinder 4 and raising the liquid level of the outer cylinder 3 to form a liquid level difference, and in that state, opening the upper communication path 6 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 biogas generated naturally from the liquid in the fermentation liquid staying in the methane fermentation tank 2, forced pressurization is performed with 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 portions of the outer cylinder 3 and the inner cylinder 4 via the valves V1 and V2, but the tip of the gas supply pipe 2L may be arranged to be located in the fermentation liquid so as to supply biogas into each fermentation liquid.

[0046] During the process of forming the liquid level difference, a slow stirring step is performed in which the fermented sludge stored in the outer cylinder 3 and the inner cylinder 4 flows statically through the lower communication path 5, and during the process of eliminating the liquid level difference, a rapid stirring step is performed 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 process of supplying biogas to the outer cylinder 3 is referred to as the outer cylinder pressurization process, and the process of supplying biogas to the inner cylinder 4 is referred to as the inner cylinder pressurization process. When the horizontal cross-sectional area of the inner cylinder 4 relative 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 process and the inner cylinder pressurization process are the same, the liquid volume of the fermentation 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-described range.

[0048] As shown in Fig. 4(a), the methane fermentation treatment method is a method for effectively generating biogas by repeating a slow stirring process, a rapid stirring process, and a standing process 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 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 causing the fermentation sludge to flow between the outer cylinder 3 and the inner cylinder 4.

[0049] The rapid stirring process is a stirring process in which the liquid level difference formed in the slow stirring process 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 direction opposite to the slow stirring process. The standing process is a process of standing the fermentation sludge after the rapid stirring process or the slow stirring process.

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

[0051] Except that the standing process is carried out after the rapid stirring process, as shown in FIG. 4(b), the standing process may be carried out after the rapid stirring process. As shown in FIG. 4(c), the standing process may be carried out after each of the slow stirring process and the rapid stirring process. In other words, the methane fermentation treatment method is a treatment method that repeats the first stirring process and the second stirring process, and is also a treatment method that includes a standing process between the first stirring process and the second stirring process, or during each process (between the slow stirring process and the rapid stirring process).

[0052] During rapid stirring, which is the stirring of the fermentation broth by releasing the liquid level difference, since the entire fermentation broth is forced to flow, solids such as sludge and organic acids dissolved in the liquid can be forced to move and mix. On the other hand, during slow stirring, solids such as sludge containing a large amount of methane bacteria remain in a retained state, and the highly fluid liquid component in which organic acids are dissolved flows between the solids, so it is assumed that methane bacteria and organic acids are efficiently in contact.

[0053] Therefore, it is assumed that by slow stirring, the liquid in which organic acids are dissolved can penetrate into the sludge retention part 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, so that methane bacteria and organic acids can be efficiently brought into contact. In the slow stirring process, if the air volume of the blower B can be controlled, the degree of contact between the sludge and the organic acid can be controlled, and the amount of biogas generated can be adjusted to be maximized. For example, the blower B may be configured to be controlled by an inverter circuit.

[0054] In the rapid stirring process, the moving speed of the liquid is very high compared to slow stirring, and since the stirring efficiency of the fermentation broth in the tank is high, the substrate can be dispersed throughout the tank. If the stirring force in the rapid stirring process is strong, there is a risk of destroying the aggregates of methane fermenting bacteria, and if the stirring force is weak, there is a risk of insufficient dispersion of the substrate in the fermentation broth. Therefore, by adjusting the opening degree of the valve V5 provided in the upper communication path 6, the stirring force in the rapid stirring process can be configured to be adjustable, and the amount of biogas generated can be adjusted to be maximized.

[0055] Therefore, the adjustment of the air volume of the blower B and the opening degree of the valve V5 may be adjusted based on the amount of biogas generated measured based on the stirring cycle including the first stirring step and the second stirring step including the standing step. That is, it is preferable to adjust at least one of the time of the standing step, the time of the slow stirring step, and the time of the rapid stirring according to the amount of biogas generated from the methane fermentation tank 2. In addition, as a method of adjusting the stirring force in the rapid stirring step, in addition to adjusting the opening degree of the valve V5, the liquid level difference set in the slow stirring step may be adjusted.

[0056] Regarding the circulation supply of the fermented sludge to the fermentation tank 2 through the circulation path, it is preferable to set the fermented sludge to be circulated and supplied from above the liquid level of the inner cylinder 4 in a state where the liquid level of the inner cylinder 4 has dropped in the second stirring process. The fermented sludge having a large potential energy supplied through the circulation path collides with the liquid surface, and scum and foams floating on the liquid surface are destroyed, so that the growth of the scum can be prevented.

[0057] The solid concentration of the fermented sludge to which the present invention is applied is preferably high, and the evaporation residue concentration (usually also referred to as "TS") is 5 Weight % or more is preferable. The methane fermentation sludge tends to exhibit non-Newtonian fluid properties when the evaporation residue concentration reaches 5 Weight % or more. The non-Newtonian fluid changes its viscosity according to the shear force, that is, the viscosity becomes high when the applied shear force is small. Therefore, in gas stirring or pump stirring, the range in which the shear force can be applied is limited, and the entire tank cannot be stirred. However, in this stirring method, since the liquid level difference is forcibly created, a shear force can be applied to the entire sludge, so that the entire tank can be stirred well.

[0058] When the evaporation residue concentration is less than 5 Weight %, mechanical stirring that rotates the stirring blades can also stir the fermented sludge. However, when the evaporation residue concentration is 5 Weight % or more, the stirring action only reaches the stirring blades and the vicinity thereof, and an excessive stirring facility is required to stir the entire tank. Also, when the evaporation residue concentration is 5 WeightIf the above-described water tank stirring (slow, rapid, static) is carried out at % or more, the contact between the methanogenic bacteria and the organic acid (slow) and the overall stirring of the fermentation broth (rapid) can be effectively exerted, and even if the static step is sufficiently ensured, the methane fermentation treatment can be carried out with high efficiency as a whole.

[0059] As shown in Fig. 5(a), in the methane fermentation tank 2, corresponding to the lower communication path 5 that fluidly connects the internal space of the outer cylinder 3 and the internal space of the inner cylinder 4 at the lower part, a plurality of stirring blades 11A are provided, and it is preferable that a turning mechanism 11 in an open state without side plates at the upper part is erected at the bottom of the outer cylinder 3. And it is preferable that a gap is formed between the upper end of the stirring blade 11A constituting the turning mechanism 11 and the lower end of the inner cylinder 4.

[0060] When the stirring blade 11A is not provided, during rapid stirring, the fermented sludge mainly moves in the radial direction of the cylindrical body, and no stirring force in the circumferential direction is generated, so there is a risk of insufficient stirring performance. However, by providing the above-described turning mechanism 11, the fermented sludge is stirred in the radial and circumferential directions during rapid stirring, so that a good stirring effect can be obtained.

[0061] As shown in Fig. 5(b), it is also possible to set the height of the stirring blade 11A to the same height as the lower communication path 5. However, in that case, clogging may occur when there are lumps in the fermented sludge, or there is a risk that the fermented sludge accumulates between the stirring blades 11A. Therefore, depending on the properties of the fermented sludge, the mode of Fig. 5(a) is preferable, and it is preferable to set the height of the stirring blade 11A in the range of 40 to 60% of the height of the lower communication path 5.

[0062] The shape of each stirring blade 11A is a flat plate body with a pressure receiving surface perpendicular to the bottom surface of the outer cylinder 3, and it may be arranged in a posture slightly inclined in the same direction with respect to the radial direction of the outer cylinder 3. As shown in FIGS. 5(c) and (e), it is preferable that each stirring blade 11A is arranged to extend from the inside to the outside of the inner cylinder 4 in a posture intersecting the lower end of the inner cylinder 4 in a plan view. However, as shown in FIG. 5(d), each stirring blade 11A may be arranged inside the lower end of the inner cylinder 4 in a plan view. Further, as shown in FIG. 5(f), the pressure receiving surface of each stirring blade 11A may be an arcuate plate body perpendicular to the bottom surface of the outer cylinder 3.

[0063] As shown in FIGS. 5(c) to (f), for example, when the fermented sludge dynamically flows from the outer cylinder 3 side to the inner cylinder 4 side in the above-described rapid stirring process, the flow is deflected by the turning mechanism 11, and a left-handed swirling flow indicated by a one-dot chain line in the figure is formed, enhancing the stirring effect on the fermented sludge. Also, when the fermented sludge dynamically flows from the inner cylinder 4 side to the outer cylinder 3 side, the flow is deflected by the turning mechanism 11, and a right-handed swirling flow indicated by a two-dot chain line in the figure is formed, similarly enhancing the stirring effect on the fermented sludge.

[0064] In the above-described embodiment, the methane fermentation tank 2 in which the inner cylinder 4 is concentrically arranged inside the outer cylinder 3 has been described. However, as shown in FIGS. 6(a) and (b), a plurality of inner cylinders 4 may be evenly and dispersedly arranged inside the outer cylinder 3. The number of inner cylinders 4 is not limited to four, and may be two, three, or five. Also in this case, 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 may 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.

[0065] Figures 7(a) and 7(b) show still another embodiment of the methane fermentation tank 2. A lower communication path 5 that communicates with the lower space of the methane fermentation tank 2 is formed, a partition wall W that partitions the upper space into at least two compartments, and biogas is supplied from the outside to at least one of the compartments to create a liquid level difference between the liquid level of the fermented sludge stored in the compartment and the liquid level of the fermented sludge stored in the other compartments. A liquid level difference forming mechanism, and an upper communication path that communicates the gas phase spaces formed above each compartment and can switch the communication state between open and closed are provided. The liquid level difference forming mechanism is configured to be able to supply biogas to all compartments.

[0066] If the number of compartments is two, the first stirring step and the second stirring step shown in Fig. 4(a) are alternately repeated for the left and right compartments. If the number of compartments is four or more, any plurality of compartments can be grouped into two groups, and the first stirring step and the second stirring step can be alternately repeated in units of groups. When grouping, it is preferable to group them so that adjacent compartments belong to different groups.

[0067] In the above-described embodiment, the case of using rice straw, which is a rice harvesting residue, as a fermentation raw material was described. However, as a fermentation raw material suitable for the fermentation tank according to the present invention, agricultural waste generated after harvesting grains such as wheat straw harvested in the field can be preferably used. In addition to agricultural waste, organic waste such as paper waste and food waste, and organic waste such as sewage sludge and livestock waste can also be used.

[0068] The various embodiments described above are examples of the present invention, and it goes without saying that the scope of the present invention is not limited by the description and can be appropriately modified and designed within the scope where the effects of each aspect of the present invention are achieved.

Description of Reference Numerals

[0069] 1: Methane fermentation device 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 forming mechanism 10: Raw material supply mechanism (mixer) 11: Swiveling mechanism 11A: Agitator blade B: Blower V1~V5: Valve

Claims

1. A methane fermentation tank comprising an outer cylinder having upper and lower ends closed, and at least one inner cylinder disposed inside the outer cylinder and having an upper end closed and a lower end open, a lower communication path that communicates an internal space of the outer cylinder with an internal space of the inner cylinder at a lower part thereof and allows fermentation sludge to flow between the outer cylinder and the inner cylinder; a liquid level difference generating mechanism that supplies biogas from the outside to the outer cylinder or the inner cylinder to generate 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 that communicates between gas phase spaces formed at the upper portions of the outer cylinder and the inner cylinder and that can switch between an open and closed state; Equipped with A methane fermentation tank, 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. 2. The methane fermentation tank according to claim 1, wherein a flow rate adjusting mechanism is provided in the upper communication passage.

3. 2. The methane fermentation tank according to claim 1, further comprising a circulation path through which the fermented sludge withdrawn from the lower portion of the methane fermentation tank is returned to the methane fermentation tank, and a raw material supply mechanism for supplying raw material for methane fermentation to the circulation path.

4. A methane fermentation method using the methane fermenter according to any one of claims 1 to 3, a first stirring step in which the biogas is supplied from the outside to the outer cylinder by the liquid level difference generating mechanism while the upper communication path is closed, thereby lowering the liquid level in the outer cylinder and raising the liquid level in the inner cylinder to generate the liquid level difference, and in this state, the upper communication path is opened to eliminate the liquid level difference, thereby stirring the fermented sludge; a second stirring step in which the biogas is supplied from the outside to the inner cylinder by the liquid level difference generating mechanism while the upper communication path is closed, thereby lowering the liquid level in the inner cylinder and raising the liquid level in the outer cylinder to generate the liquid level difference, and in this state, the upper communication path is opened to eliminate the liquid level difference, thereby stirring the fermented sludge; This is a methane fermentation processing method in which the process is repeated.

5. The methane fermentation tank includes a circulation path for circulating the fermented sludge drawn from a lower portion of the methane fermentation tank from above the liquid level in the inner cylinder, 5. The methane fermentation treatment method according to claim 4, wherein, in a state where the liquid level in the inner cylinder is lowered in the second stirring process, the fermented sludge is circulated and supplied from above the liquid level in the inner cylinder via the circulation path.

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

7. a partition wall that forms a lower communication path through which the lower space of the methane fermentation tank communicates and that divides the upper space into at least two compartments; a liquid level difference generating mechanism that supplies biogas from an outside to at least one of the compartments to generate a liquid level difference between the liquid level of the fermented sludge stored in the compartment and the liquid levels of the fermented sludge stored in the other compartments; an upper communication path that communicates the gas phase spaces formed at the upper portions of the compartments and is capable of switching between an open and closed state; The liquid level difference generating mechanism is a methane fermentation tank configured to be able to supply biogas to all of the compartments.

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