Methanation treatment method

Abstract

Provided is a methane fermentation treatment method that can satisfactorily stir a fermentation broth with low power even when the solid concentration of fermented sludge is high and has excellent fermentation efficiency. 【Solution means】An inner cylinder with an open lower end is provided inside an outer cylinder with closed upper and lower ends. A lower communication path that connects the outer cylinder and the inner cylinder at the bottom to allow the fermented sludge to flow, and an upper communication path that connects the gas phase spaces formed in the upper parts of the outer cylinder and the inner cylinder and can switch the communication state between open and closed are provided. Using a methane fermentation tank equipped with these, biogas is supplied from the outside to the outer cylinder or the inner cylinder, and a slow stirring step of flowing the fermented sludge between the outer cylinder and the inner cylinder while providing a liquid level difference between the outer cylinder and the inner cylinder, and by eliminating the pressure difference between the gas phase spaces formed in the outer cylinder and the inner cylinder, releasing the liquid level difference formed in the slow stirring step, and a rapid stirring step of flowing the fermented sludge in the direction opposite to the slow stirring process, and a static settling step of statically settling the fermented sludge after the rapid stirring step or the slow stirring step are repeated.

Description

Technical Field

[0001] The present invention relates to 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 generating 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 generating 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 discloses a sealed tank body, a main fermentation section partitioned in the tank body for methane fermentation of organic waste, a precipitation section partitioned 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 partitioned in the outer peripheral portion of the precipitation section via a partition cylinder for temporarily storing the digested sludge that has flowed in from the main fermentation section and having a discharge port for the digested sludge, a tubular mixing shaft communicating the digestion sludge storage section with below the liquid level of the slurry stored in the main fermentation section, a center tube communicating the central part of the precipitation section and the central part of the main fermentation section and having a slurry supply port formed in the middle of the tube, and a pressure equalizing valve connected to a communication pipe communicating the upper part of the main fermentation section where methane gas generated in the main fermentation section accumulates and the upper part of the precipitation section where methane gas generated in the precipitation section accumulates. A methane fermentation tank has been proposed.

[0005] The methane fermentation tank divides the tank body into an inner granule granulation part and an outer main fermentation area 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 area, 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 flow of the gas, circulating the fermentation liquid in the tank using a pump and stirring it, and stirring the fermentation liquid using the water level difference between the inner and outer divided areas 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 makes it 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 object to be treated, which is 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 the fermented sludge is 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 are problems such as the behavior of the circulation flow fluctuates depending on the properties of the fermented sludge, making it difficult to determine its arrangement.

[0012] In the methane fermentation tank described in Patent Document 3, biogas generated in the main fermentation section where the fermented sludge storage amount is larger than that in the inner region is stored in the upper space of the main fermentation section, so that the water level of the main fermentation section is lowered compared to 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 still 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 treatment method that can stir the fermentation liquid well with low power even when the solid concentration of the fermented sludge is high and has excellent fermentation efficiency.

Means for Solving the Problems

[0014] To achieve the above object, the first characteristic configuration of the methane fermentation treatment method according to the present invention is an outer cylinder with closed upper and lower ends, at least one inner cylinder disposed inside the outer cylinder with a closed upper end and an open lower end, a lower communication path that communicates the internal space of the outer cylinder and the internal space of the inner cylinder at the lower part and enables fermented sludge to flow between the outer cylinder and the inner cylinder, a liquid level difference forming mechanism that supplies biogas from the outside to the outer cylinder or the inner cylinder to form a liquid level difference between the liquid level of the fermented sludge stored in the outer cylinder and the liquid level of the fermented sludge stored in the inner cylinder, and an upper communication path that communicates the gas phase spaces formed in the upper parts of the outer cylinder and the inner cylinder and can switch the communication state between open and closed. A methane fermentation treatment method using a methane fermentation tank, comprising supplying biogas from the outside to the outer cylinder or the inner cylinder to provide 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 while flowing the fermented sludge between the outer cylinder and the inner cylinder in a slow stirring step; eliminating the pressure difference between the gas phase spaces formed in the outer cylinder and the inner cylinder to release the liquid level difference formed in the slow stirring step and flowing the fermented sludge in a direction opposite to the slow stirring process in a rapid stirring step; and repeating a standing step of standing the fermented sludge after the rapid stirring step or the slow stirring step. Meanwhile, the ratio of the standing step in the treatment time including the slow stirring step and the rapid stirring step is 75% or more. is at the point.

[0015] Due to the pressurization of the outer cylinder in which biogas is supplied from the outside to the outer cylinder by the liquid level difference forming mechanism, the gas phase space in which biogas is stored in the upper part of the outer cylinder expands, and the liquid level of the fermented sludge stored in the outer cylinder drops below the liquid level of the fermented sludge stored in the inner cylinder, forming a liquid level difference. Along with the formation of the liquid level difference, the fermented sludge flows statically from the outer cylinder to the inner cylinder through the lower communication path, thereby gently stirring the fermented sludge, that is, the slow stirring step is executed. At this time, instead of pressurizing with biogas generated from the liquid in the methane fermentation liquid staying in the outer cylinder, pressurization is performed with biogas supplied from the outside, so that an appropriate liquid level difference can be formed within an appropriate time.

[0016] Conversely, by pressurizing the inner cylinder with biogas supplied from the outside to the inner cylinder, the gas phase space where biogas is stored at the upper part of the inner cylinder is expanded, and the liquid level of the fermented sludge stored in the inner cylinder drops below the liquid level of the fermented sludge stored in the outer cylinder, forming a liquid level difference. Along with the formation of the liquid level difference, the fermented sludge flows statically from the inner cylinder to the outer cylinder through the lower communication path, and gentle agitation of the fermented sludge, that is, a slow agitation process, is executed. Similarly, instead of pressurizing with biogas spontaneously generated from the liquid of the methane fermentation liquid staying in the inner cylinder, it is forcedly pressurized with biogas supplied from the outside, so that an appropriate liquid level difference can be formed within an appropriate time.

[0017] Furthermore, when the upper communication path connecting 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 fermented sludge flows dynamically from the inner cylinder to the outer cylinder through the lower communication path, and in the latter case, the fermented sludge flows dynamically from the outer cylinder to the inner cylinder through the lower communication path, and rapid agitation of the fermented sludge, that is, a rapid agitation process, is executed.

[0018] After the bacterial flora, raw materials, and organic acids, which are decomposition products of the raw materials, etc. in the fermentation liquid are agitated by the above-mentioned rapid agitation process or slow agitation process, a standing process of standing the fermented sludge is executed, and the methane fermentation treatment by the bacterial flora is effectively promoted. Then, by repeating the rapid agitation process and the slow agitation process, and the standing process of standing the fermented sludge after the rapid agitation process or the slow agitation process, the methane fermentation treatment proceeds efficiently.

[0019] And Taking the processing time required for the rapid agitation process and the slow agitation process, and the standing process of standing the fermented sludge after the rapid agitation process or the slow agitation process as a unit cycle, by setting the ratio of the standing process in the unit cycle to 75% or more, good fermentation efficiency can be realized.

[0020] The same second characteristic configuration is A methane fermentation treatment method using a methane fermentation tank comprising an outer cylinder with closed upper and lower ends, at least one inner cylinder disposed inside the outer cylinder with a closed upper end and an open lower end, a lower communication path that communicates the internal space of the outer cylinder and the internal space of the inner cylinder at the lower part to enable fermented sludge to flow between the outer cylinder and the inner cylinder, a liquid level difference forming mechanism that supplies biogas from the outside to the outer cylinder or the inner cylinder to form a liquid level difference between the liquid level of the fermented sludge stored in the outer cylinder and the liquid level of the fermented sludge stored in the inner cylinder, and an upper communication path that communicates the gas phase spaces formed in the upper parts of the outer cylinder and the inner cylinder and can switch the communication state between open and closed. In the slow stirring step of supplying biogas from the outside to the outer cylinder or the inner cylinder to provide 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 while flowing the fermented sludge between the outer cylinder and the inner cylinder, and by eliminating the pressure difference between the gas phase spaces formed in the outer cylinder and the inner cylinder, the liquid level difference formed in the slow stirring step is released, and in the rapid stirring step of flowing the fermented sludge in the direction opposite to the slow stirring process, and a standing step of standing the fermented sludge after the rapid stirring step or the slow stirring step are repeated, and according to the amount of biogas generated from the methane fermentation tank, at least one of the time of the standing step, the time of the slow stirring step, and the time of the rapid stirring is adjusted.

[0021] In addition to the above-mentioned actions, By making it possible to adjust any of the times of the slow stirring step, the rapid stirring step, and the standing step, good fermentation efficiency can be obtained. Each time can be appropriately adjusted based on the target fermentation efficiency.

[0022] The third characteristic configuration is that, in addition to the first or second characteristic configuration described above, the evaporation residue concentration of the fermented sludge is 5% or more.

[0023] The fermentation treatment can be effectively promoted for fermented sludge with an evaporation residue concentration of 5% or more.

[0024] The fourth characteristic configuration is that, in addition to the first or second characteristic configuration described above, a swirling mechanism equipped with stirring blades is provided at the bottom of the fermentation tank so as to correspond to the lower communication path, and in the rapid stirring step, a swirling flow of the fermented sludge is formed by the swirling mechanism.

[0025] When the swirling mechanism 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 swirling mechanism, the fermented sludge is stirred in the radial and circumferential directions during rapid stirring, so that a good stirring effect can be obtained.

Advantages of the Invention

[0026] As described above, according to the present invention, even when the viscosity of the fermented sludge is high, the fermentation liquid can be well stirred with low power, and a methane fermentation treatment method with excellent fermentation efficiency can be provided.

Brief Description of the Drawings

[0027]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Embodiments for Carrying Out the Invention

[0028] Hereinafter, the methane fermentation tank and the methane fermentation treatment method of the present invention will be described by taking the case where rice straw, which is a rice harvesting residue generated in the field as a raw material, is used as a fermentation raw material as an example

[0029] In FIGS. 1(a) and (b), the methane fermentation apparatus 1 according to the present invention is illustrated FIG. 1(a) is an explanatory view 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 plan view showing the internal structure of the methane fermentation tank 2

[0030] The methane fermentation apparatus 1 includes a methane fermentation tank 2, a gas holder 7 for retaining 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 thereof outside the system, and circulating and supplying 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 medium 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 heat generated by a combustor using the biogas stored in the gas holder 7 as fuel is used as the heat medium

[0031] 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 disposed 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 circular cross-sections, 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 within the range of 0.3 to 0.7 times, and more preferably within the range of 0.4 to 0.6 times.

[0032] The shapes of the outer cylinder 3 and the inner cylinder 4 are not limited to cylindrical bodies, and may be elliptical cylinders with elliptical cross-sections or rectangular cylinders with rectangular cross-sections, 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 below to above, 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 varies along the height direction, the average value of the ratio of the horizontal cross-sectional areas may be set within the range of 0.3 to 0.7 times, preferably within the range of 0.4 to 0.6 times, and more preferably to 0.5.

[0033] Furthermore, it includes 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, and valves V3 and V4 are arranged on the biogas discharge pipes 3L and 4L.

[0034] The methane fermentation tank 2 includes an upper communication path 6 having a valve V5 that communicates the gas phase spaces 3s and 4s formed in the upper parts of the outer cylinder 3 and the inner cylinder 4 and can switch the open / closed state 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 to enable 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.

[0035] Furthermore, the methane fermentation tank 2 is provided with 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 fermentation sludge stored in the outer cylinder 3 and the liquid level of the fermentation sludge stored in the inner cylinder 4. The gas supply pipe 2L, valves V1, V2, and blower B described above constitute the liquid level difference forming mechanism 8.

[0036] The circulation path 9 includes a drawing pipe 9A that draws out fermentation sludge from the lower part of the methane fermentation tank 2 and a supply pipe 9B that adds a methane fermentation raw material to the drawn fermentation sludge and supplies it to the methane fermentation tank 2. The drawing pipe 9A is provided with a drawing pump P2, and the supply pipe 9B is provided with a supply pump P1. Further, a raw material supply mechanism 10 is provided in the path connecting the drawing pipe 9A and the supply pipe 9B. The raw material supply mechanism 10 includes a mixer that mixes the cut rice straw serving as the raw material and the fermentation sludge, and may further add and mix dilution water.

[0037] 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, in the first stirring step, with the upper communication path 6 closed, biogas is supplied from the outside to the outer cylinder 3 by the liquid level difference forming mechanism 8, 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. In this state, the upper communication path 6 is opened to eliminate the liquid level difference, thereby stirring the fermentation sludge.

[0038] As shown in FIG. 3, the second stirring step is 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 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.

[0039] 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. However, the tip of the gas supply pipe 2L may be arranged to be located in the fermentation liquid so as to supply biogas into the respective fermentation liquids.

[0040] During the process of forming the liquid level difference, a slow stirring process 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 process 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.

[0041] 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 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.

[0042] As shown in Fig. 4(a), 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 causing the fermentation sludge to flow between the outer cylinder 3 and the inner cylinder 4.

[0043] The rapid stirring step is a stirring step in which the pressure difference between the gas phase spaces formed in the outer cylinder and the inner cylinder is eliminated, the liquid level difference formed in the slow stirring step is released, and the fermentation sludge is caused to flow in the direction opposite to the slow stirring process. The standing step is a step of standing the fermentation sludge after the rapid stirring step or the slow stirring step.

[0044] For example, if the capacity of the methane fermentation tank 2 is about 40 L, the time required for the slow stirring step is set to about several tens of seconds to several minutes, the time required for the rapid stirring step is set to 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 including the standing step are alternately repeated. 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. Note that the time required for each step is not limited to this value and is appropriately set based on the target fermentation efficiency.

[0045] In addition to being executed after the rapid stirring step, the standing step may be executed after the rapid stirring step as shown in Fig. 4(b), or the standing step may be executed after each of the slow stirring step and the rapid stirring step as shown in Fig. 4(c). In other words, the methane fermentation treatment method is a treatment method that repeats the first stirring step and the second stirring step, and is also a treatment method 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).

[0046] During rapid agitation, which is the agitation 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 agitation, solids such as sludge containing a large amount of methanogens remain in a retained state, and the highly fluid liquid component in which organic acids are dissolved flows between the solids, assuming that the methanogens and organic acids are efficiently in contact.

[0047] Therefore, it is assumed that by slowly agitating and allowing the liquid in which organic acids are dissolved to 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, the methanogens and organic acids can be efficiently brought into contact. In the slow agitation step, if the air volume of the blower B can be controlled, the degree of contact between the sludge and the organic acids can be controlled, and the generation amount of biogas can be adjusted to be maximized. For example, the blower B may be configured to be controlled by an inverter circuit.

[0048] In the rapid agitation step, the moving speed of the liquid is very high compared to slow agitation, and since the agitation efficiency of the fermentation broth in the tank is high, the substrate can be dispersed throughout the tank. If the agitation force in the rapid agitation step is strong, there is a risk of destroying the aggregates of methanogenic bacteria, and if the agitation 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 agitation force in the rapid agitation step can be configured to be adjustable, and the generation amount of biogas can be adjusted to be maximized.

[0049] 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 generation amount of biogas measured based on the agitation cycle including the first agitation step and the second agitation 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 agitation step, and the time of the rapid agitation according to the generation amount of biogas from the methanogenic fermentation tank 2. In addition to adjusting the opening degree of the valve V5, as a method of adjusting the agitation force in the rapid agitation step, the liquid level difference set in the slow agitation step may be adjusted.

[0050] Regarding the circulating supply of fermented sludge to the fermentation tank 2 through the circulation path, it is preferable to set it so that the fermented sludge is 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 with a large potential energy supplied through the circulation path collides with the liquid surface, destroying scum, foams, etc. floating on the liquid surface, and it becomes possible to prevent the growth of scum.

[0051] The solid concentration of the fermented sludge to which the present invention is applied is preferably high, and the concentration of evaporation residue (usually also referred to as "TS") is preferably 5% or more. When the concentration of evaporation residue of the methanogenic sludge reaches 5% or more, it tends to show the properties of a non-Newtonian fluid more strongly. A non-Newtonian fluid has a viscosity that changes according to the shear force, that is, when the applied shear force is small, the viscosity becomes high. 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 a 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.

[0052] If the concentration of evaporation residue is less than 5%, it is possible to stir the fermented sludge even by mechanical stirring that rotates the stirring blades. However, when the concentration of evaporation residue reaches 5% 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, if the concentration of evaporation residue is 5% or more and the above-described water tank stirring (slow, rapid, static) is carried out, the contact between methanogens and organic acids (slow) and the overall stirring of the fermentation broth (rapid) act effectively, and even if the static process is sufficiently ensured, the methanation treatment can be carried out with high efficiency as a whole.

[0053] As shown in Fig. 5(a), in the methane fermentation tank 2, corresponding to a 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 turning mechanism 11 in an open state composed of a plurality of stirring blades 11A and without side plates at the upper part is preferably 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.

[0054] 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.

[0055] As shown in Fig. 5(b), it is also possible to set the height of the stirring blade 11A to be the same as the height of 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.

[0056] The shape of each stirring blade 11A is a plate-like body with a pressure-receiving surface perpendicular to the bottom surface of the outer cylinder 3 and flat, 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 5(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), each stirring blade 11A may be an arc-shaped plate-like body with a pressure-receiving surface perpendicular to the bottom surface of the outer cylinder 3.

[0057] 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 agitation step, 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 agitation effect on the fermented sludge. Further, 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 agitation effect on the fermented sludge.

[0058] 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 dispersed and 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.

[0059] FIGS. 7(a) and (b) show still another aspect of the methane fermentation tank 2. A lower communication path 5 that communicates with the lower space of the methane fermentation tank 2, a partition wall W 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 the other compartments. And an upper communication path that communicates the gas phase spaces formed above each compartment and can switch the open / closed state of the communication state. The liquid level difference forming mechanism is configured to be able to supply biogas to all compartments.

[0060] If the number of compartments is two, the first agitation step and the second agitation 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 may be grouped into two groups, and the first agitation step and the second agitation step may be alternately repeated in units of groups. When grouping, it is preferable to group them so that adjacent compartments belong to different groups.

[0061] In the above-described embodiments, the case where rice straw, which is a rice harvest residue, is used as a fermentation raw material has been described. However, as a fermentation raw material suitable for the fermenter according to the present invention, agricultural wastes generated after harvesting cereals harvested in the field, such as wheat straw, can be preferably used. In addition to agricultural wastes, organic wastes such as paper waste and food waste, and organic wastes such as sewage sludge and livestock waste can also be used.

[0062] 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 functions and effects of each aspect of the present invention are achieved.

Explanation of Reference Numerals

[0063] 1: Methanation apparatus 2: Methanation 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: Stirring blade 6B: Electric motor 6C: Rotating shaft 7: Gas holder 8: Liquid level difference forming mechanism 10: Raw material supply mechanism (mixer) 11: Swinging mechanism 11A: Stirring blade B: Blower V1~V5: Valves

Claims

1. An outer cylinder with closed upper and lower ends, at least one inner cylinder disposed inside the outer cylinder with a closed upper end and an open lower end, a lower communication path that communicates the internal space of the outer cylinder and the internal space of the inner cylinder at the lower part, enabling fermented sludge to flow between the outer cylinder and the inner cylinder, a liquid level difference forming mechanism that supplies biogas from the outside to the outer cylinder or the inner cylinder to form a liquid level difference between the liquid level of the fermented sludge stored in the outer cylinder and the liquid level of the fermented sludge stored in the inner cylinder, an upper communication path that communicates the gas-phase spaces formed in the upper parts of the outer cylinder and the inner cylinder and can switch the communication state between open and closed, A methane fermentation treatment method using a methane fermentation tank comprising: a slow stirring step of supplying biogas from the outside to the outer cylinder or the inner cylinder, providing 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, and flowing the fermented sludge between the outer cylinder and the inner cylinder, a rapid stirring step of eliminating the pressure difference between the gas-phase spaces formed in the outer cylinder and the inner cylinder, releasing the liquid level difference formed in the slow stirring step, and flowing the fermented sludge in the direction opposite to the slow stirring process, a standing step of standing the fermented sludge after the rapid stirring step or the slow stirring step, repeating, A methane fermentation treatment method in which the ratio of the standing step in the treatment time including the slow stirring step and the rapid stirring step is 75% or more.

2. An outer cylinder with closed upper and lower ends, at least one inner cylinder disposed inside the outer cylinder with a closed upper end and an open lower end, a lower communication path that communicates the internal space of the outer cylinder and the internal space of the inner cylinder at the lower part, enabling fermented sludge to flow between the outer cylinder and the inner cylinder, a liquid level difference forming mechanism that supplies biogas from the outside to the outer cylinder or the inner cylinder to form a liquid level difference between the liquid level of the fermented sludge stored in the outer cylinder and the liquid level of the fermented sludge stored in the inner cylinder, an upper communication path that communicates the gas-phase spaces formed in the upper parts of the outer cylinder and the inner cylinder and can switch the communication state between open and closed, A methane fermentation treatment method using a methane fermentation tank comprising: a slow stirring step of supplying biogas from the outside to the outer cylinder or the inner cylinder, providing 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, and flowing the fermented sludge between the outer cylinder and the inner cylinder, a rapid stirring step in which the pressure difference between the gas phase spaces formed in the outer cylinder and the inner cylinder is eliminated to release the liquid level difference formed in the slow stirring step, thereby causing the fermented sludge to flow in a direction opposite to that of the slow stirring step; a standing step of standing the fermented sludge after the rapid stirring step or the slow stirring step; Repeating this, A methane fermentation treatment method, wherein at least one of the time of the standing step, the time of the slow stirring step, and the time of the rapid stirring step is adjusted depending on the amount of biogas generated from the methane fermentation tank.

3. 3. The method for methane fermentation treatment according to claim 1, wherein the concentration of evaporation residue in the fermented sludge is 5% or more.

4. a rotating mechanism equipped with stirring blades is provided at the bottom of the fermenter in correspondence with the lower communication path; 3. The methane fermentation treatment method according to claim 1, wherein a swirling flow of the fermented sludge is formed by the swirling mechanism in the rapid stirring step.

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