Biogas generator

Abstract

To enhance methane fermentation efficiency without enlarging a fermenter and to reduce the emission of carbon dioxide into the atmosphere.SOLUTION: In the bio-methane generation system 10, microorganisms for generating organic matter by photosynthesis are bred in a fermentation tank 20, and a part of the biogas is refluxed to the fermentation tank 20 from the downstream side of a desulfurization device 24, so that the microorganisms generate organic matter by photosynthesis. As a result, it is possible to produce biogas from the organic matter produced by the microorganisms in addition to the fermentation raw material, and it is possible to increase the methane fermentation efficiency without increasing the size of the fermenter. In addition, since a part of the biogas is refluxed to the fermenter 20 and photosynthesis is performed using carbon dioxide contained in the biogas in the fermenter 20, it is possible to circulate carbon dioxide in the bio-methane generation system 10, and it is possible to reduce the emission amount of carbon dioxide.SELECTED DRAWING: FIG. 1a

Description

[Technical Field]

[0001] The present invention relates to a biogas generation system that includes a fermenter that anaerobic ferments fermentation raw materials containing organic matter to produce biogas containing methane gas, carbon dioxide, and hydrogen sulfide, and a desulfurization device that is connected downstream of the fermenter and removes hydrogen sulfide from the biogas. [Background technology]

[0002] As described in the following patent documents, there have been proposed techniques for anaerobic fermentation of a fermentation raw material containing organic matter to produce a biogas containing methane gas, carbon dioxide, and hydrogen sulfide, and for removing the hydrogen sulfide from the biogas. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 2023-59838 Summary of the Invention [Problem to be solved by the invention]

[0004] The technology described in the above patent document requires a large fermenter to increase the efficiency of methane fermentation of the fermentation feedstock. It is also desirable to reduce the amount of carbon dioxide emitted into the atmosphere during methane fermentation.

[0005] Therefore, an object of the present invention is to increase the efficiency of methane fermentation without increasing the size of the fermenter and to reduce the amount of carbon dioxide released into the atmosphere. [Means for solving the problem]

[0006] In order to achieve the above-mentioned object, one aspect of the present invention is a biogas generation apparatus that includes a fermentation tank that anaerobic fermentation of fermentation raw materials containing organic matter to produce biogas containing methane gas, carbon dioxide, and hydrogen sulfide, and a desulfurization device that is connected downstream of the fermentation tank and removes hydrogen sulfide from the biogas. In this biogas generation apparatus, microorganisms that produce organic matter by photosynthesis are made to live in the fermentation tank, and a reflux path is provided that refluxes a portion of the biogas from the downstream side of the desulfurization device to the fermentation tank, so that the biogas is produced not only from the fermentation raw materials but also from the organic matter produced by the microorganisms. [Effects of the Invention]

[0007] In a biogas generator having the above configuration, microorganisms that produce organic matter through photosynthesis are allowed to live in the fermenter, and a portion of the biogas is returned to the fermenter from the downstream side of the desulfurization device, allowing the microorganisms to produce organic matter through photosynthesis. This makes it possible to produce biogas from the organic matter produced by the microorganisms in addition to the fermentation raw material, thereby increasing the efficiency of methane fermentation without increasing the size of the fermenter. Furthermore, by returning a portion of the biogas to the fermenter and performing photosynthesis in the fermenter using the carbon dioxide contained in the biogas, it is possible to circulate carbon dioxide in the biogas generator, thereby reducing the amount of carbon dioxide emitted. [Brief explanation of the drawings]

[0008] [Figure 1a] 1 is a schematic diagram showing a biogas generation device according to a first embodiment. [Figure 1b] FIG. 6 is a schematic diagram showing a biogas generation device according to a second embodiment. [Figure 2] FIG. 1 is a diagram showing the relationship between light absorptance and wavelength. [Figure 3] FIG. 10 is a diagram showing the amount of biogas generated depending on the presence or absence of microbubbles according to the second embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0009] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A biomethane production system 10 according to an embodiment of the present invention will be described in detail below with reference to the drawings.

[0010] Fig. 1a is a schematic diagram showing a biogas generator according to a first embodiment. The biomethane generation system 10 generates methane through methane fermentation, a biochemical reaction in which organic matter is broken down into methane and carbon dioxide through the metabolic action of methane-fermenting microorganisms under anaerobic conditions. As shown in Fig. 1a, the biomethane generation system 10 includes a fermenter 20, lighting 22, a desulfurization device 24, a methane concentration device 26, and a reflux path 28.

[0011] The fermenter 20 anaerobicly ferments a fermentation raw material containing organic matter to produce biogas containing methane gas, carbon dioxide, and hydrogen sulfide. The fermentation raw material is, for example, livestock manure, food waste, or sewage sludge, and contains biodegradable high molecular weight organic matter such as carbohydrates, proteins, and fats. The fermentation raw material is then introduced into the fermenter 20. Methane-fermenting microorganisms are also introduced into the fermenter 20, and the methane-fermenting microorganisms live in the fermenter 20. The methane-fermenting microorganisms include hydrolytic and acid-fermenting bacteria, symbiotic acetogenic bacteria, homoacetogenic bacteria, methanogenic archaea, and sulfate-reducing bacteria. During methane fermentation using the methane-fermenting microorganisms, the biodegradable high molecular weight organic matter contained in the fermentation raw material is decomposed to produce biogas containing methane and other substances.

[0012] The decomposition process of biodegradable high molecular weight organic matter in methane fermentation includes a hydrolysis process in which soluble organic monomers such as monosaccharides, amino acids, and higher fatty acids are produced from the biodegradable high molecular weight organic matter; an acid production process in which organic acids such as formic acid, acetic acid, propionic acid, and butyric acid, as well as alcohols, are produced from the soluble organic monomers; an acetic acid production process in which acetic acid and hydrogen are produced from volatile fatty acids such as propionic acid and butyric acid; and a methane production process in which methane and carbon dioxide are produced from hydrogen, acetic acid, and the like.

[0013] The hydrolysis and acid production processes are carried out by hydrolytic and acid-fermenting bacteria, the acetic acid production process is carried out by symbiotic acetogenic bacteria and homoacetogenic bacteria, and the methanogenic process is carried out by methanogenic archaea. Hydrolytic and acid-fermenting bacteria include Eubacterium and Clostridium. Symbiotic acetogenic bacteria include Syntrophobacter and Syntrophomonas. Homoacetogenic bacteria include Acetobacterium and Clostdum. Methanogenic archaea include Methanobacterium, Methanospirillum, Methanococcus, Methanosarcina, and Methanosaeta.

[0014] For example, when glucose is produced as a monosaccharide from a biodegradable organic polymer, methane and carbon dioxide are produced according to the following reaction formula. Specifically, acetic acid, propionic acid, butyric acid, etc. are produced from glucose according to the following three reaction formulas. C6H12O6 → C3H7COOH + 2CO2 + 2H2 C6H12O6→4 / 3C2H5COOH+2 / 3CH3COOH+2 / 3CO2+2 / 3H2 C6H12O6+2H2O→2CH3COOH+2CO2+4H2 Next, acetic acid and hydrogen are produced from fatty acids such as propionic acid and butyric acid according to the following three reaction formulas. CH3(CH2)14COOH+12H2O→8CH3COOH+12H2 C3H7COOH+2H2O→2CH3COOH+2H2 C2H5COOH+2H2O→CH3COOH+3H2+CO2 Then, methane and carbon dioxide are produced from acetic acid and hydrogen according to the following two reaction formulas: CH3COOH → CH4 + CO2 4H2+CO2→CH4+2H2O In other words, methane fermentation produces methane through two pathways: one in which methane is produced from acetic acid, and the other in which methane is produced by reducing carbon dioxide with hydrogen. In normal methane fermentation, 70-80% of methane is produced from acetic acid, and the remaining 20-30% is produced by reducing carbon dioxide with hydrogen.

[0015] In methane fermentation, sulfate-reducing bacteria, such as Desulfovibrio and Desulfotomaculum, produce hydrogen sulfide from the sulfur components contained in the fermentation raw material.

[0016] In this way, biogas containing methane, carbon dioxide, and hydrogen sulfide is produced by methane fermentation in the fermenter 20. The components of the biogas produced in the fermenter 20 are 60% (±5%) methane and 40% (±5%) carbon dioxide. The hydrogen sulfide concentration is several hundred ppm to several thousand ppm.

[0017] A desulfurization device 24 is connected downstream of the fermentation tank 20. The desulfurization device 24 is a device that removes hydrogen sulfide from the biogas. The desulfurization device 24 removes hydrogen sulfide by using iron oxide.

[0018] A methane concentrator 26 is connected downstream of the desulfurization unit 24. The methane concentrator 26 is a device that separates carbon dioxide contained in biogas using various methods, such as chemical absorption, physical absorption, membrane separation, cryogenic separation, physical adsorption (pressure swing adsorption, thermal swing adsorption), oxyfuel combustion, and chemical looping combustion. This enables the biomethane generation system 10 to produce highly concentrated methane, which can be used in power generation devices, fuel cells, etc.

[0019] In conventional biomethane generation systems, the carbon dioxide separated from the biogas by the methane concentrator 26 is released into the atmosphere or used to cultivate plants in a greenhouse. On the other hand, the biomethane generation system 10 is provided with a return path 28, and the carbon dioxide separated from the biogas by the methane concentrator 26 is returned to the fermenter 20 via the return path 28.

[0020] Specifically, the reflux path 28 is composed of a first reflux path 28a for circulating the slurry in the fermenter 20, a second reflux path 28b for supplying carbon dioxide to the first reflux path 28a, a Venturi tube 30, and a pump 32. The pump 32 is connected to one end of the Venturi tube 30 inserted in the first reflux path 28a, and causes the slurry in the fermenter 20 to flow into the Venturi tube 30. The other end of the Venturi tube 30 is connected to the fermenter 20, and the slurry that has flowed into the Venturi tube 30 is returned to the fermenter 20. As a result, the slurry in the fermenter 20 is circulated via the first reflux path 28a and the Venturi tube 30 by operation of the pump 32. Note that the amount of slurry circulated per unit time is 20 to 40 L / min for the fermenter 20, which has a diameter of 284 mm, a length of 2590 mm, and a volume of 164 L. The circulation of the slurry causes the interior of the fermenter 20 to be agitated. A second reflux path 28b is connected between the longitudinal center of the Venturi tube 30 and the methane concentrator 26, and carbon dioxide separated from the biogas in the methane concentrator 26 is introduced into the Venturi tube 30. This causes the carbon dioxide to be converted into microbubbles inside the Venturi tube 30 and mixed into the slurry. Therefore, the carbon dioxide converted into microbubbles in the Venturi tube 30 is returned to the fermenter 20 through the first reflux path 28a. The particle size of the microbubbled carbon dioxide is preferably 500 μm or less, and more preferably 100 μm or less. The microbubbled carbon dioxide may also have a particle size of nanometers or less.

[0021] In this way, when the microbubbled carbon dioxide is returned to the fermenter 20, organic matter is produced from hydrogen sulfide and light energy through photosynthesis in the fermenter 20. More specifically, in addition to methane fermentation microorganisms, microorganisms that produce organic matter through photosynthesis (hereinafter referred to as "photosynthetic microorganisms") are also introduced into the fermenter 20 and live there. Photosynthetic microorganisms include cyanobacteria and photosynthetic bacteria. Note that the photosynthetic bacteria are at least one of purple sulfur bacteria and green sulfur bacteria. The fermenter 20 is made of a transparent material, and lighting 22 such as LEDs is arranged around the fermenter 20. Therefore, in the fermenter 20, the photosynthetic microorganisms produce organic matter through photosynthesis using the light energy irradiated by the lighting 22.

[0022] Specifically, cyanobacteria produce glucose according to the following reaction scheme: 12H2O + 6CO2 + light energy → C6H12O6 + 6O2 + 6H2O In addition, photosynthetic bacteria produce glucose according to the following reaction formula: 12H2S + 6CO2 + light energy → C6H12O6 + 12S + 6H2O

[0023] In this way, organic matter is produced by photosynthesis in the fermenter 20, and biogas is produced by methane fermentation microorganisms from the organic matter produced by photosynthesis. That is, in the fermenter 20, biogas is produced not only from the fermentation raw material but also from the organic matter produced by the photosynthetic microorganisms. This makes it possible to increase the efficiency of methane fermentation. In other words, the efficiency of methane fermentation can be increased without increasing the size of the fermenter 20.

[0024] Furthermore, carbon dioxide separated from the biogas by the methane concentrator 26 is returned to the fermenter 20 via the return path 28, where the returned carbon dioxide is consumed by photosynthesis. This allows the carbon dioxide separated from the biogas to be circulated within the biomethane generation system 10, thereby reducing the amount of carbon dioxide released into the atmosphere. Furthermore, since carbon dioxide is consumed by photosynthesis in the fermenter 20, the proportion of carbon dioxide in the biogas generated in the fermenter 20 decreases. As described above, biogas is also generated from organic matter produced by photosynthetic microorganisms, which increases the efficiency of methane fermentation and increases the proportion of methane in the biogas generated in the fermenter 20. In this way, the proportion of carbon dioxide in the biogas generated in the fermenter 20 decreases and the proportion of methane in the biogas generated in the fermenter 20 increases, making it possible to increase the concentration of methane in the biogas generated in the fermenter 20.

[0025] Furthermore, the wavelength of the lighting 22 can be adjusted, and light of a wavelength appropriate for photosynthetic bacteria, rather than cyanobacteria, is irradiated. Specifically, cyanobacteria contain chlorophyll a as a photosynthetic pigment, while photosynthetic bacteria contain bacteriochlorophyll as a photosynthetic pigment. As shown in FIG. 2, chlorophyll a absorbs light with a wavelength of 400 to 500 nm, and bacteriochlorophyll absorbs light with a wavelength of 750 to 800 nm. Therefore, by irradiating light with a wavelength of 750 to 800 nm, the lighting 22 irradiates light with a wavelength appropriate for photosynthetic bacteria, rather than cyanobacteria. As a result, photosynthesis is mainly performed by photosynthetic bacteria, with cyanobacteria only performing a small amount of photosynthesis.

[0026] In this way, by having photosynthetic bacteria mainly perform photosynthesis, it is possible to reduce the ratio of hydrogen sulfide in the biogas produced in the fermenter 20. In other words, when photosynthetic bacteria perform photosynthesis, they consume hydrogen sulfide and produce glucose, as shown in the reaction formula above, and therefore it is possible to reduce the ratio of hydrogen sulfide in the biogas produced in the fermenter 20. This makes it possible to reduce the size of the desulfurization device 24.

[0027] On the other hand, cyanobacteria only perform a small amount of photosynthesis, making it possible to maintain anaerobic conditions in the fermenter 20. In other words, when cyanobacteria perform photosynthesis, they also produce oxygen along with glucose, as shown in the reaction formula above, so when cyanobacteria perform photosynthesis, it becomes difficult to maintain anaerobic conditions. For this reason, anaerobic conditions in the fermenter 20 can be maintained by suppressing photosynthesis by cyanobacteria.

[0028] Furthermore, because the carbon dioxide returned from the reflux path 28 to the fermenter 20 is microbubbled, the amount of biogas produced in the fermenter 20 increases. Specifically, in methane fermentation, methane is not only produced from acetic acid, but also by reducing carbon dioxide with hydrogen. Therefore, when methane is produced by reducing carbon dioxide with hydrogen, the microbubbled carbon dioxide reacts at the cellular level of organic matter. Furthermore, when carbon dioxide is microbubbled and its surface area increases, the reactivity of reducing carbon dioxide with hydrogen increases. Therefore, when carbon dioxide is microbubbled, the amount of biogas produced in the fermenter 20 increases. Furthermore, when carbon dioxide is microbubbled and its surface area increases, the reactivity of photosynthesis in the fermenter 20 increases, the amount of glucose produced increases, and the amount of biogas produced in the fermenter 20 increases. In addition, the microbubbles generated by the carbon dioxide returned from the return path 28 to the fermenter 20 accelerate the decomposition rate of the fermentation raw materials containing organic matter and shorten the hydraulic retention time of the fermentation raw materials in the fermenter 20, thereby enabling the fermenter 20 to be made smaller.

[0029] 1b is a schematic diagram showing a biogas generation apparatus according to a second embodiment. The second embodiment is the same as the first embodiment except that a second return path 28b is connected between the desulfurization device 24 and the methane concentration device 26. In the second embodiment, the biogas from which hydrogen sulfide has been removed by the desulfurization device 24 is returned to the fermentation tank 20 via a return path 28 connected downstream of the desulfurization device 24. In the second embodiment, the concentration of carbon dioxide returned to the fermentation tank 20 via the return path 28 is reduced, but, as in the first embodiment, organic matter is produced by photosynthesis in the fermentation tank 20, and biogas is produced from the organic matter produced by photosynthesis by methane fermentation microorganisms. Thus, in the fermenter 20 of the biomethane generation system 10, biogas is generated not only from the fermentation raw material but also from organic matter produced by photosynthetic microorganisms. Furthermore, because the carbon dioxide returned to the fermenter 20 is microbubbled, the amount of biogas generated increases. FIG. 3 is a diagram showing the amount of biogas generated depending on the presence or absence of microbubbles according to the second embodiment. Specifically, when the biomethane generation system 10 was operated continuously from August 23 to September 15, carbon dioxide was returned to the fermenter 20 only from August 30 to September 3, and carbon dioxide was not returned to the fermenter 20 from August 23 to August 29 and from September 4 to September 15. In other words, from August 30 to September 3, carbon dioxide was introduced into the Venturi tube 30 via the second return path 28b connected downstream of the desulfurization device 24, and microbubbled carbon dioxide was returned to the fermenter 20. On the other hand, during the periods from August 23 to August 29 and from September 4 to September 15, carbon dioxide separated in methane concentrator 26 was not flowed into Venturi tube 30, and carbon dioxide was not returned to fermenter 20. The amount of biogas generated (L / day) when microbubbled carbon dioxide was returned to fermenter 20 was 30.0 to 41.2 L / day, and the amount of biogas generated (L / day) when carbon dioxide was not returned to fermenter 20 was 6.2 to 30.8 L / day. In this way, by returning microbubbled carbon dioxide to fermenter 20, the amount of biogas produced in fermenter 20 clearly increases.

[0030] As described above in detail, the biomethane generation system 10 according to this embodiment includes a fermenter 20 that anaerobicly ferments a fermentation feedstock to produce biogas containing methane gas, carbon dioxide, and hydrogen sulfide, and a desulfurization device 24 connected downstream of the fermenter 20 and that removes hydrogen sulfide from the biogas. Photosynthetic microorganisms inhabit the fermenter 20, and a portion of the biogas is returned to the fermenter 20 from the downstream side of the desulfurization device 24 via a return path 28. Therefore, photosynthesis occurs in the fermenter 20, and the photosynthetic microorganisms produce glucose. Biogas is thus produced in the fermenter 20 not only from the fermentation feedstock but also from the glucose produced by the photosynthetic microorganisms. This makes it possible to increase the efficiency of methane fermentation without increasing the size of the fermenter. Furthermore, by returning a portion of the biogas to the fermenter and performing photosynthesis in the fermenter using the carbon dioxide contained in the biogas, it is possible to circulate carbon dioxide in the biomethane generation system 10, thereby reducing the amount of carbon dioxide emitted.

[0031] The biomethane generation system 10 also includes a methane concentrator 26 connected downstream of the desulfurizer 24 to separate carbon dioxide contained in the biogas, and a return path 28 returns the carbon dioxide separated from the biogas by the methane concentrator 26 to the fermenter 20. This allows high-concentration carbon dioxide to be returned to the fermenter 20.

[0032] Furthermore, the reflux path 28 includes a Venturi tube 30, which refluxes the carbon dioxide in the reflux path 28 as fine bubbles to the fermenter 20. Therefore, when methane is produced by reducing carbon dioxide with hydrogen, the carbon dioxide in the fine bubbles reacts at the cellular level of organic matter. Furthermore, when carbon dioxide is turned into fine bubbles and the surface area of ​​the carbon dioxide increases, the reactivity of reducing carbon dioxide with hydrogen increases. Therefore, when carbon dioxide is turned into fine bubbles, the amount of biogas produced in the fermenter 20 increases. Furthermore, when carbon dioxide is turned into fine bubbles and the surface area of ​​the carbon dioxide increases, the reactivity of photosynthesis in the fermenter 20 increases, the amount of glucose produced increases, and the amount of biogas produced in the fermenter 20 increases.

[0033] The photosynthetic microorganisms are at least one of purple sulfur bacteria and green sulfur bacteria, which produce organic matter from hydrogen sulfide and carbon dioxide. This makes it possible to reduce the amount of hydrogen sulfide contained in the biogas, and the desulfurization device 24 can be made smaller.

[0034] The biomethane production system 10 also includes lighting 22 that promotes photosynthesis by photosynthetic microorganisms in the fermenter 20. This allows photosynthesis to be carried out favorably in the fermenter 20. Furthermore, for example, photosynthesis can be carried out over a long period of time.

[0035] The present invention is not limited to the above-described embodiment, and it goes without saying that various improvements and modifications are possible within the scope of the present invention. For example, in the first embodiment, carbon dioxide separated in the methane concentrator 26 is returned to the fermenter 20, and in the second embodiment, biogas from which hydrogen sulfide has been removed in the desulfurizer 24 is returned to the fermenter 20. However, carbon dioxide separated in the methane concentrator 26 and biogas from which hydrogen sulfide has been removed in the desulfurizer 24 may also be returned to the fermenter 20.

[0036] Furthermore, in this embodiment, carbon dioxide in the form of fine bubbles is returned to the fermenter 20, but carbon dioxide that is not in the form of fine bubbles may also be returned to the fermenter 20.

[0037] Furthermore, in this embodiment, photosynthesis is carried out using the light emitted by the lighting 22, but photosynthesis may also be carried out using sunlight.

[0038] Furthermore, in this embodiment, the methane concentrator 26 is provided and carbon dioxide is separated by the methane concentrator 26 to produce high-concentration methane, but the methane concentrator 26 does not have to be provided. That is, high-concentration methane is supplied in the biomethane generation system 10, but biogas from which hydrogen sulfide has been removed may be supplied in a system that does not include the methane concentrator 26. Note that in a system that does not include the methane concentrator 26, biogas from which hydrogen sulfide has been removed is returned to the fermenter 20.

[0039] In this embodiment, the wavelength of the light source 22 can be adjusted, but the light intensity may also be adjustable. If the light source has adjustable light intensity, it can irradiate light with an intensity appropriate for the photosynthetic bacteria. [Explanation of symbols]

[0040] 10... Biomethane production system (biogas generator), 20... Fermenter, 22... Lighting (light source), 24... Desulfurization device, 26... Methane concentration device, 28... Reflux path, 30... Venturi tube

Claims

1. a fermenter that anaerobic ferments a fermentation raw material containing organic matter to produce biogas containing methane gas, carbon dioxide, and hydrogen sulfide; a desulfurization device connected downstream of the fermenter and configured to remove hydrogen sulfide from the biogas; A biogas generator comprising: A biogas generation device that generates biogas not only from the fermentation raw material but also from the organic matter produced by the microorganisms by causing microorganisms to live in the fermentation tank and providing a return path for returning a portion of the biogas from the downstream side of the desulfurization device to the fermentation tank.

2. a methane concentrator connected downstream of the desulfurization device to separate carbon dioxide contained in the biogas; The biogas generating apparatus according to claim 1 , wherein the return path returns the carbon dioxide separated from the biogas by the methane concentrator to the fermenter.

3. 3. The biogas generator according to claim 1, wherein the return passage includes a Venturi tube, and the biogas in the return passage is returned to the fermenter in the form of fine bubbles.

4. 2. The biogas generating apparatus according to claim 1, wherein the microorganisms are at least one of purple sulfur bacteria and green sulfur bacteria that produce organic matter from hydrogen sulfide and carbon dioxide.

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