Method for separating and utilizing co2 and ammonia from methane-fermenting digestive fluid and / or biogas

Abstract

To provide a method for separating and utilizing CO2 which contributes to promotion of removing ammonia.SOLUTION: A method for separating and utilizing CO2 and ammonia from a methane-fermented digestive fluid 3 and / or biogas containing CO2 and ammonia includes a step (S11) of decompressing an upper space 11 of the methane-fermented digestive fluid 3 by an ejector 4 and dissolving and / or mixing CO2 and ammonia in a driving fluid 5 of the ejector 4 while deaerating CO2 and ammonia from the digestive fluid 3, a step (S12) of removing ammonia in the driving fluid 5, a step (S13) of separating CO2 derived from the methane-fermented digestive fluid 3 and / or biogas, and a step of returning soil separated in the separation step to the methane-fermented digestive fluid 3 containing ammonia. And a step (S14) of controlling the pH of the methane-fermented digestive juice 3 in the range of 7.0 to 7.8. Bringing the driving fluid into contact with the biogas (S12b).SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for separating and utilizing CO2 and ammonia from methane fermentation digestate and / or biogas. [Background technology]

[0002] Conventional methods for separating CO2 from a methane fermentation tank or purifying methane include methods that use expensive solvents or large amounts of water, as well as a method that involves removing a portion of the internal liquid of the methane fermentation tank, reducing the pressure below atmospheric pressure to remove the carbon dioxide dissolved in the internal liquid, and then returning the liquid to the methane fermentation tank (Patent Document 1).

[0003] As a method for utilizing CO2, based on the knowledge that in the anaerobic methane fermentation method of sludge, the digestion rate and amount of methane generated are greatly affected by the carbon dioxide concentration in the digester gas (biogas) in the upper space inside the digester tank, there is a proposal to separate CO2 from the digester gas and inject it into the digester liquid as reflux gas (Patent Document 2). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2003-211194 [Patent Document 2] Special Publication No. 6-30797 Summary of the Invention [Problem to be solved by the invention]

[0005] The proposal in Patent Document 1 is an invention that focuses solely on the separation and purification of methane, and does not take into consideration the removal of ammonia, a fermentation inhibitor that inevitably forms in methane fermentation digested liquid. It is presumed that the pH rises as carbon dioxide is released from the partially extracted internal liquid through decompression treatment, according to the following equations (1) and (2), and that a certain amount of ammonia is also removed. However, there is a problem in that the bicarbonate ions, which are the carbon dioxide source in the methane fermentation digested liquid, gradually decrease, preventing the removal of ammonia from the extracted internal liquid. Furthermore, the pH of the methane fermentation digested liquid may rise beyond the range in which methanogen activity can be maintained. H + +HCO 3- ⇔ CO2+H2O (1) OH - +NH4 + ⇔ NH3+H2O (2) Therefore, it is considered to be insufficient in terms of promoting the removal of ammonia.

[0006] The proposal in Patent Document 2 is to control the carbon dioxide concentration in the digester gas (biogas) in the upper space inside the digester tank, with a focus on the digestion rate and the amount of methane produced, and is not to control the bicarbonate ion concentration in the digester liquid. Therefore, it is thought that there is no consideration of promoting the removal of ammonia in the digester liquid.

[0007] In view of the above-mentioned current situation, an object of the present invention is to provide a method for separating and utilizing CO2 that contributes to promoting the removal of ammonia from methane fermentation digestate. [Means for solving the problem]

[0008] The inventors investigated the difference in the solubility of ammonia, methane, and CO2 in water under normal pressure (Henry's constant solubility of ammonia: 5.75 × 10 -4 318.9 @ 20℃, Henry's law constant of methane: 3.76 × 10 -4 0.0347@20℃, Henry's law constant of carbon dioxide solubility: 0.142×10 -4 0.869@20℃) and the tendency of ammonia to dissolve in water under acidic conditions (NH3+H+ →NH4 + ), they found that ammonia removal can be promoted by dissolving CO2 and accompanying ammonia in water to separate them, appropriately removing the ammonia by reacting it with hydrogen sulfide in the biogas, and then degassing the CO2 again under reduced pressure to return it to the methane fermentation digestate, and maintaining the concentrations of dissolved ionic species of CO2, such as bicarbonate ions (also called hydrogen carbonate ions) and carbonate ions, in the methane fermentation digestate at a certain level or higher, thereby leading to the invention.

[0009] That is, one aspect of the present invention made to achieve the above-mentioned object is a method for separating and utilizing CO2 from biogas containing CO2 and methane, which comprises the following steps: a step (S1) of blowing a gas containing CO2 and methane into the first compartment or the aqueous phase immediately below the first compartment in a tank having a structure in which the gas phase is physically separated into a first compartment and a second compartment and the aqueous phase is connected, thereby releasing a gas containing methane as a main component via the first compartment; a step (S2) of transferring the aqueous phase after the methane release to the aqueous phase immediately below the second compartment; a step (S3) of reducing the air pressure in the second compartment to a level lower than that of the first compartment, thereby releasing the gas containing CO2 as a main component via the second compartment; and a step (S4) of returning the gas containing CO2 as a main component released from the second compartment to the methane-fermented digested liquid, thereby controlling the pH of the methane-fermented digested liquid to a range of 7.0 to 7.8. This method for separating and utilizing CO2 takes advantage of the difference in solubility between methane and CO2, dissolving CO2 in water to separate it from methane, increasing the concentration of methane in the biogas, which can be used as fuel.The CO2 can also be degassed again under reduced pressure and returned to the methane fermentation digestate, which can be used to promote the removal of ammonia.

[0010] Another aspect of the present invention made to achieve the above-mentioned object is a method for separating and utilizing CO2 and ammonia from a methane fermentation digestate and / or biogas containing CO2 and ammonia, the method comprising the steps of: reducing the pressure in the headspace of the methane fermentation digestate using an ejector; degassing the CO2 and ammonia from the methane fermentation digestate while dissolving and / or mixing them in a driving fluid of the ejector (S11); removing ammonia from the driving fluid (S12); separating CO2 derived from the methane fermentation digestate and / or biogas (S13); and controlling the pH of the methane fermentation digestate to a range of 7.0 to 7.8 by returning the CO2 separated in the separation step to the methane fermentation digestate containing ammonia (S14). According to this method for separating and utilizing CO2 and ammonia, the ejector is operated to promote degassing of not only CO2 but also ammonia from the methane fermentation digestate and dissolve them in the driving fluid. In addition, by returning CO2 to the methane fermentation digestate, the concentration of dissolved ionic species of CO2, such as bicarbonate ions (also called hydrogen carbonate ions) and carbonate ions, is maintained at a certain level or higher, which promotes an increase in pH due to a decarboxylation reaction in the decompression step and the resulting removal of ammonium ions in the methane fermentation digestate by ammonia gasification.

[0011] The method for separating and utilizing CO2 and ammonia preferably includes a step (S12b) of removing ammonia from the driving fluid by contacting the driving fluid with biogas prior to a step (S13) of separating CO2 derived from the methane fermentation digestate and / or biogas. This step prevents corrosion due to hydrogen sulfide and air pollution due to sulfur oxides when the biogas is combusted in a downstream gas turbine or the like to generate electricity, and also prevents sulfur degradation of the catalyst when a catalytic reaction of methane is performed in the downstream. Furthermore, this effectively prevents atmospheric release of ammonia gas, which is subject to emission restrictions and is derived from the methane fermentation digestate and / or biogas, and the resulting ammonium sulfate aqueous solution can be used as fertilizer. [Effects of the Invention]

[0012] The method for separating and utilizing CO2 and the method for separating and utilizing CO2 and ammonia of the present invention promotes the removal of ammonia, a fermentation inhibitor in methane fermentation digestate, and can bring about stable fermentation even with raw materials that are prone to producing ammonia. [Brief explanation of the drawings]

[0013] [Figure 1] 1 is a schematic diagram illustrating one embodiment of an ammonia removal system, with dashed lines indicating gas flow and solid lines indicating liquid flow. [Figure 2] FIG. 2 is a schematic cross-sectional view showing an embodiment of an ejector. DETAILED DESCRIPTION OF THE INVENTION

[0014] The following defines the terms used in this specification. In this specification, a "tank" refers to one or more tanks capable of storing a liquid. Therefore, a "tank" does not necessarily refer to a single, physically independent tank at first glance, but may refer to a group of multiple tanks connected by their liquid phase portions. As used herein, a "compartment" is a defined spatial region occupied by a gas phase. As used herein, the term "physically separate" means that the compartments are isolated from each other so that no material is transferred between them. In this specification, the term "methane fermentation digestate" refers to a liquid in which methanogens survive and methane fermentation is currently progressing. Therefore, the liquid present in a tank in which a portion of the methane fermentation digestate is extracted from the main digester and subjected to predetermined treatment such as ammonia removal may also be considered a methane fermentation digestate. Even in such cases, as long as methanogens survive, it is often assumed that the liquid after the predetermined treatment will be returned to the main digester. As used herein, "biogas" generally refers to one or more gases produced from methane fermentation digestate, regardless of composition. Biogas typically contains methane, carbon dioxide, and hydrogen sulfide. In this specification, the term "ejector" refers to a device that utilizes the principle of sucking in gas at a relatively low pressure by using the force of a driving fluid. Any device that utilizes this principle may be called an "ejector," such as a device called an aspirator. In this specification, the term "driving fluid" refers to a fluid flowing through a single circulation path, which, when passed through a convergent nozzle to reduce the cross-sectional area of ​​the fluid flow and increase the flow rate, has the effect of drawing in a fluid at a lower pressure from the surrounding area.

[0015] The ammonia removal system 1 according to one embodiment of the present invention shown in FIG. 1 includes an ammonia removal tower 2 having digested liquid circulation piping systems 33a, 33b that circulate a methane fermentation digested liquid 3 between the ammonia removal tower 2 and a main digester 32 via a digested liquid circulation pump 20; an ejector 4 that can suck degassed components from an upper space 11 of the ammonia removal tower 2 and mix and / or dissolve them in a driving fluid 5; a first wet desulfurization tower 6 that can cause the driving fluid 5 from the ejector 4 to flow downward while introducing biogas from the main digester 32; a driving fluid transport pump 8 that can extract the driving fluid 5 from the bottom of the first wet desulfurization tower 6 and return it to the ejector 4; and a pump that can introduce desulfurized gas extracted from the top of the first wet desulfurization tower 6 and extract methane-enriched gas that does not dissolve in the stored water 13. the methane concentrator 10, which can extract stored water 13 and supply it to a fuel tank; a vacuum desorption tower 12, which is connected to the methane concentrator 10 via a connecting line 25 at the bottom and shares stored water 13 with the methane concentrator 10, but has a second compartment 29 that is physically separated from the first compartment 27 of the methane concentrator 10; a stored water pump 22, which provides power to extract water from the aqueous phase of the vacuum desorption tower 12 and send it to the aqueous phase of the methane concentrator 10; an ejector 34, which draws in and mixes desulfurized gas extracted from the top of the first wet desulfurization tower 6 with the water supplied from the stored water pump 22 to the aqueous phase of the methane concentrator 10; a vacuum pump 14, which can suck CO2 gas from the top of the vacuum desorption tower 12; and a conduit 16, which can supply CO2 gas sucked by the vacuum pump 14 to the main digester 32.

[0016] A pressure gauge 15a and a biogas inlet pipe 31a are attached to the top of the ammonia removal tower 2, and the methane fermentation digestate 3 is stored in the space below, into which seven parallel tubes made of hydrophobic porous membranes (hereinafter referred to as hydrophobic tubes 17) are immersed. The acidic liquid inlet and outlet sides of the seven hydrophobic tubes 17 are combined into a single acidic liquid inlet pipe 19 and an acidic liquid outlet pipe 21 within the methane fermentation digestate 3. The acidic liquid inlet pipe 19 is connected to the bottom of the second wet desulfurization tower 7 via a heater 23, and the acidic liquid outlet pipe 21 is connected to the top of the second wet desulfurization tower 7 via an ammonia water pump 18.

[0017] The ejector 4 shown in FIG. 2 typically includes a convergent nozzle 41 from which the driving fluid 5 is injected from an outlet at the tip, a housing 43 that accommodates the convergent nozzle 41, and a diffuser 45 that passes the driving fluid 5 injected from the tip of the convergent nozzle 41. The housing 43 is provided with an inlet 46 that communicates with the upper space 11 of the ammonia removal tower 2 and a nozzle inlet 48 that introduces the driving fluid 5. The driving fluid 5 is a high-pressure fluid that circulates between the first wet desulfurization tower 6 and the ejector 4, but is not particularly limited to this, and may be water, steam, or nitrogen. In this embodiment, water (circulating water) is used.

[0018] The first wet desulfurization tower 6 and the second wet desulfurization tower 7 house the microorganism-immobilized carrier 24 at a position higher than the liquid levels of the driving fluid 5 and the acidic liquid 9 stored at the bottom of the tower, and each tower has a gas inlet on the lower side wall of the microorganism-immobilized carrier 24 and a gas outlet on the upper side wall. On the surface of the microorganism immobilization carrier 24, sulfur-oxidizing bacteria form a biofilm.

[0019] The first wet desulfurization tower 6 and the second wet desulfurization tower 7 are connected in series. That is, biogas is introduced through a biogas inlet pipe 31b provided on the lower side wall of the microorganism immobilized carrier 24a of the second wet desulfurization tower 7, and the gas extracted from the upper side wall of the microorganism immobilized carrier 24a of the second wet desulfurization tower 7 is introduced through the lower side wall of the microorganism immobilized carrier 24b of the first wet desulfurization tower 6 and extracted from the top.

[0020] The methane concentrating tower 10 has an inlet at the top for introducing the gas discharged from the first wet desulfurization tower 6, and a methane outlet for discharging gas having a higher methane concentration than the introduced gas.The methane concentrating tower 10 is a tower equipped with a methane outlet valve 26 on a pipe connecting the methane outlet to a gas holder (not shown), and further equipped with a pressure gauge 15b at the top.

[0021] The vacuum desorption tower 12 has a CO2 outlet at the top, and is equipped with a CO2 exhaust valve 28 and a vacuum pump 14, in that order, on the piping connecting the CO2 outlet to the main digestion tank 32, and is further equipped with a pressure gauge 15c at the top.

[0022] The function and operation of the ammonia removal system 1 will be described below. (A) When the ammonia water pump 18 is operated in the ammonia removal tower 2, the acidic liquid 9 (pH = 5) present inside the hydrophobic tube 17 begins to flow through the acidic liquid discharge pipe 21, the second wet desulfurization tower 7, and the acidic liquid introduction pipe 19 in that order, and begins to circulate. (B) By driving the heater 23 and heating the acidic solution 9 to approximately 60°C, heat is transferred to the methane fermentation digested solution 3 via the hydrophobic tube 17. As a result, the methane fermentation digested solution 3 is heated to approximately 55°C, promoting the CO2 desorption reaction of the above formula (1), which has a higher temperature sensitivity than the above formula (2). When CO2 desorption begins, the pH rises, promoting the production of ammonia gas of the above formula (2). The ammonia gas permeates through the surface of the hydrophobic tube 17 and dissolves in the flowing acidic solution 9, or is desorbed into the upper space 11 of the ammonia removal tower 2. Here, the pressure in the upper space 11 reaches -1 kPa to -80 kPa (gauge pressure). (C) When the driving fluid transport pump 8 is operated, the driving fluid 5 is extracted from the bottom of the first wet desulfurization tower 6 and returned to the ejector 4, enters through the nozzle inlet 48, and is injected from the injection port. Then, as it passes through the diffuser 45, it sucks in the CO2 gas and ammonia gas that have been desorbed into the upper space 11 of the ammonia removal tower 2 through the suction port 46, and the CO2 gas and ammonia gas are mixed and / or dissolved in the driving fluid 5. At this point, the pressure in the upper space 11 drops to -20 kPa to -50 kPa (gauge pressure). (D) Biogas is introduced from the main digestion tank 32 into the biogas inlet 31b on the lower side wall of the second wet desulfurization tower 7. Hydrogen sulfide in the ascending biogas is oxidized by sulfur-oxidizing bacteria attached to the surface of the microorganism-immobilizing carrier 24a according to the following formula (3): H2S+2O2→H2SO4(3) This sulfuric acid is converted into sulfuric acid by the catalyst. This sulfuric acid undergoes a neutralization reaction with the acidic liquid 9 containing ammonia and ammonium ions that flows down from the top of the tower, as shown in the following formula (4), to produce an aqueous ammonium sulfate solution. 2NH3+H2SO4→2(NH4)2SO4(4) (E) When the gas leaves the gas outlet on the upper side wall of the microorganism immobilized carrier 24a of the second wet desulfurization tower 7, the gas from which most of the hydrogen sulfide and ammonia has been removed is introduced into the gas inlet on the lower side wall of the first wet desulfurization tower 6, where the oxidation reaction shown in the above formula (3) is catalyzed by the sulfur-oxidizing bacteria attached to the surface of the microorganism immobilized carrier 24b, and the neutralization reaction shown in the above formula (4) occurs with the driving fluid 5 containing ammonia and ammonium ions that has flowed down from the top of the tower, thereby removing the remaining hydrogen sulfide and ammonia, and the gas leaves the gas outlet at the top. (F) Because ammonium sulfate and ammonia accumulate in the driving fluid 5 accumulating at the bottom of the first wet desulfurization tower 6 and the acidic solution 9 accumulating at the bottom of the second wet desulfurization tower 7, the driving fluid 5 is diluted appropriately with industrial water and returned to the ejector 4 using the driving fluid transport pump 8, and the acidic solution 9 is also diluted appropriately with industrial water and sent to the acidic solution inlet pipe 19. The driving fluid 5 and the acidic solution 9 are drained while their water levels are still lower than the gas inlet and are stored in the seal pot 30. (G) The first wet desulfurization tower 6 and the second wet desulfurization tower 7 are connected in series, so that hydrogen sulfide and ammonia can be removed without any leakage, and depending on the ammonia content in the methane fermentation digestate 3 or the amount of hydrogen sulfide in the biogas, it is possible to stop the operation of either the driving fluid transport pump 8 or the ammonia water pump 18 and operate only one of the wet desulfurization towers. This makes it possible to save on pump power energy according to fluctuations (busy or slow periods) in the composition and amount of the input raw material. The gas exiting the gas outlets (H) and (E) is introduced into the methane concentrator 10 via an inlet at the top of the methane concentrator 10 and via the ejector 34, which is pumped by the water pump 22 from the aqueous phase of the vacuum desorption column 12 to the aqueous phase of the methane concentrator 10. The gas is primarily composed of methane and CO2 (typically 60 mol% methane and 40 mol% CO2). By filling the gas phase 27 with the methane discharge valve 26 (actually a backpressure valve) closed until the pressure reaches 0 kPa to 40 kPa, CO2 dissolves primarily into the water 13 via the liquid surface, increasing the molar fraction of methane in the gas phase to approximately 65% ​​to 75%. Once the methane molar fraction has increased, the methane discharge valve 26 is opened to reduce the pressure in the gas phase to atmospheric pressure, and the methane discharge valve 26 is closed again. The methane-enriched gas that has passed through the methane discharge valve 26 is stored in a gas holder (not shown). (I) The stored water 13 is shared with the vacuum desorption tower 12 through the communication line 25, so that the CO 2 concentration contained in the stored water 13 of the vacuum desorption tower 12 also becomes the same as that of the methane concentration tower 10. (J) With the CO2 discharge valve 28 of the vacuum desorption tower 12 open, the vacuum pump 14 is driven to reduce the pressure in the gas phase to -10 kPa to -60 kPa. This causes CO2 to be desorbed from the liquid surface of the stored water 13 and blown through the CO2 discharge port into the conduit 16 and into the methane fermentation digested liquid 3 in the main digester 32, thereby maintaining the bicarbonate ion concentration in the methane fermentation digested liquid 3.

[0023] A first embodiment of the present invention is a method for separating and utilizing CO2 from biogas containing CO2 and methane.

[0024] The method according to the first embodiment includes a step (S1) of releasing a gas primarily composed of methane via the first compartment 27 by blowing a gas containing CO2 and methane into the first compartment 27 or the aqueous phase portion immediately below the first compartment 27 in a tank having a structure in which the gas phase is physically separated into a first compartment 27 and a second compartment 29 and the aqueous phase is connected. The gas introduced into the first compartment 27 or the aqueous phase portion immediately below the first compartment 27 is a gas composed mainly of methane and CO2 (typically 60 mol% methane: 40 mol% CO2). However, by blowing gas into the first compartment 27 until the pressure in the first compartment 27 reaches 0 kPa to 40 kPa, CO2 is mainly dissolved in the aqueous phase portion in the form of bicarbonate ions via the liquid surface, and the molar fraction of methane contained in the first compartment 27 increases relatively to approximately 65% ​​to 75%.

[0025] The method according to the first embodiment includes a step (S2) of transferring the aqueous phase after releasing methane to the aqueous phase portion immediately below the second compartment 29. Here, "transfer" does not necessarily involve any operation, and in this embodiment, the method simply involves providing a communication path 25 between the aqueous phase immediately below the first compartment 27 and the aqueous phase immediately below the second compartment 29.

[0026] The method according to the first embodiment includes a step (S3) of releasing the gas mainly composed of CO2 via the second compartment 29 by reducing the pressure in the second compartment 29 to a level lower than the pressure in the first compartment 27. By reducing the pressure in the second section 29 to -10 kPa to -60 kPa, the gas mainly composed of CO 2 can be desorbed from the liquid surface of the aqueous phase directly below the second section 29.

[0027] The method according to the first embodiment includes a step (S4) of controlling the pH of the methane fermentation digested liquid 3 to a range of 7.0 to 7.8 by returning the gas mainly composed of CO2 released from the second section 29 to the methane fermentation digested liquid 3. If step (S4) is not performed, the pH of the methane fermentation digested liquid 3 will rise to about 9 due to the consumption of bicarbonate ions, but by returning CO2, the reaction of formula (1) above will proceed to the left, generating dissolved ion species of CO2 such as bicarbonate ions and carbonate ions, and lowering the pH to a range of 7.0 to 7.8. A pH below 7.0 may be undesirable from the perspective of maintaining the activity of methanogens, while a pH above 7.8 may be undesirable from the perspective of ammonia removal. The lower limit of the pH is preferably above 7.2, more preferably 7.3. In this specification, the pH of the methane fermentation digested liquid is a value measured using a glass electrode pH meter or the like.

[0028] A second embodiment of the present invention is a method for separating and utilizing CO2 and ammonia from a methane fermentation digested liquid 3 and / or biogas containing CO2 and ammonia.

[0029] The method according to the second embodiment includes a step (S11) of reducing the pressure in the headspace 11 of the methane fermented digested liquid 3 by the ejector 4, and dissolving and / or mixing CO2 and ammonia in the driving fluid 5 of the ejector 4 while degassing them from the methane fermented digested liquid 3. First, the pressure is reduced to -1 kPa to -80 MPa by the ejector 4, causing carbon dioxide, which was originally more susceptible to desorption due to heating, to desorb from the methane fermentation digested liquid 3. As a result, the hydrogen ion concentration of the methane fermentation digested liquid 3 decreases according to the above formula (1), and the hydroxide ion concentration increases relatively (pH increases). As a result, the reaction of the above formula (2) proceeds to the right, and ammonia gas, which has low solubility, escapes into the upper space 11 above the methane fermentation digested liquid 3. Due to the high solubility of ammonia in water, the reaction proceeds to the left of formula (2), and the pH of the driving fluid 5 rises to approximately pH 7.5 to 10.0. As a result, it is believed that some carbon dioxide also becomes dissolved. The term "dissolving and / or mixing" does not necessarily mean that complete dissolution into the driving fluid 5 has occurred, but rather that simple mixing or microbubbling, or partial dissolution or partial mixing are acceptable.

[0030] The method according to the second embodiment includes a step (S12) of removing ammonia from the driving fluid 5. The means for removing ammonia from the driving fluid 5 in step (S12) are not limited, as long as they can remove the ammonia from the driving fluid 5. Examples of such means include passing the driving fluid 5 through an aqueous hydrochloric acid solution or an aqueous nitric acid solution, directly or indirectly contacting the driving fluid 5 with biogas, electrodialysis using a cation exchange membrane, and filtration using a reverse osmosis membrane. However, as described above, this embodiment employs a step (S12b) of removing ammonia from the driving fluid 5 by indirectly contacting the driving fluid 5 with biogas. Specifically, the hydrogen sulfide in the biogas is first oxidized by microorganisms immobilized on carriers 24 to produce sulfuric acid, which is then reacted with the ammonia in the driving fluid 5.

[0031] The method according to the second embodiment includes a step (S13) of separating CO2 derived from the methane fermentation digestate 3 and / or biogas, and the step (S13) includes a step (S13-1) of directly or indirectly contacting the gas or biogas degassed from the digestate with water to dissolve the CO2, and a step (S13-2) of degassing the CO2 under reduced pressure from the stored water 13 in which the CO2 has been dissolved. Step (S13-1) is typically realized by steps (S1) and (S2), but in other embodiments, it can also be realized by the microbubble generation method described in JP 2021-151640 A, the gas pressurization and countercurrent contact method described in JP 2004-83542 A, or the method of introducing degassed gas or biogas from digestive fluid into hollow fibers that come into contact with an absorption liquid described in JP 2002-363581 A. Step (S13-2) is typically realized in process (S3). In another embodiment, step (S13) can also be realized by a method using a polymer gas separation membrane, as in Patent Document 2.

[0032] The method according to the second embodiment includes a step (S14) of controlling the pH of the methane fermentation digested liquid 3 to a range of 7.0 to 7.8 by returning the CO2 separated in the separation step to the methane fermentation digested liquid 3 containing ammonia. If step (S14) is not performed, the pH of the methane fermentation digested liquid 3 will rise to about 9 due to the consumption of bicarbonate ions, but by returning CO2, the reaction of formula (1) above will proceed to the left, generating dissolved ion species of CO2 such as bicarbonate ions and carbonate ions, and lowering the pH to a range of 7.0 to 7.8. A pH below 7.0 may be undesirable from the perspective of maintaining the activity of methanogens, while a pH above 7.8 may be undesirable from the perspective of ammonia removal. The lower limit of the pH is preferably above 7.2, more preferably 7.3.

[0033] It should be noted that the present invention is not limited to the above-described embodiments, and not all of the configurations described in the above-described embodiments are necessarily essential requirements of the present invention. The present invention may be modified in various ways within the scope of the technical concept thereof. For example, the ejector 34 may be replaced by other means capable of mixing gas and liquid under pressure. The vacuum pump 14 may be replaced with other means capable of blowing air, such as a blower (fan motor). [Industrial Applicability]

[0034] The method of the present invention has the advantage of promoting the removal of ammonia, a fermentation inhibitor that is inevitably produced in the methane fermentation digestate 3, and of bringing about stable fermentation even when raw materials have a high proportion of manure or other substances that are prone to producing ammonia, and therefore has great industrial applicability. [Explanation of symbols]

[0035] 1. Ammonia removal system 2. Ammonia removal tower 3. Methane fermentation digestate 4, 34 Ejector 5 Driving fluid 6 First wet desulfurization tower 7 Second wet desulfurization tower 8. Drive fluid transport pump 9 Acidic liquid 10 Methane concentrator 11 Upper space 12 Vacuum desorption tower 13. Reservoir water 14 Vacuum pump 15a, 15b, 15c Pressure gauge 16 Conduit 17 Hydrophobic tubing 18 Ammonia water pump 19 Acidic liquid introduction pipe 20 Digestive fluid circulation pump 21 Acidic liquid discharge pipe 22 Storage water pump 23 Heating heater 24, 24a, 24b Microbial immobilization carrier 25 Access Road 26 Methane exhaust valve 27 Section 1 (gas phase part of methane concentration tower) 28 CO2 exhaust valve 29 Section 2 (gas phase part of vacuum desorption tower) 30 Seal Pot 31a, 31b Biogas inlet pipe 32 Main digester 33a,33b Digestive fluid circulation piping system 41 Tapered nozzle 43 Housing 45 Diffuser 46 Intake port 48 Nozzle inlet

Claims

1. CO ２ and CO from methane-containing biogas ２ A method for separating and utilizing In a tank having a structure in which the gas phase is physically divided into a first compartment and a second compartment and the aqueous phase is connected, CO is introduced into the first compartment or the aqueous phase portion immediately below the first compartment. ２ and a step (S1) of blowing a gas containing methane into the first compartment to release a gas mainly composed of methane through the first compartment; A step (S2) of transferring the aqueous phase after releasing methane to an aqueous phase portion immediately below the second compartment; By reducing the pressure in the second compartment to a level lower than the pressure in the first compartment, CO ２ A step (S3) of releasing a gas mainly composed of the above through the second compartment; CO released from the second compartment ２ a step (S4) of returning the gas mainly composed of the above to the methane fermentation digested liquid to control the pH of the methane fermentation digested liquid to a range of 7.0 to 7.8; CO ２ How to separate and utilize.

2. CO ２ and CO from methane fermentation digestate and / or biogas containing ammonia ２ and a method for separating and utilizing ammonia, The space above the methane fermentation digested liquid is decompressed by an ejector, and CO is extracted from the methane fermentation digested liquid. ２ and a step (S11) of dissolving and / or mixing ammonia in the driving fluid of the ejector while degassing the ammonia; a step (S12) of removing ammonia from the driving fluid; CO derived from methane fermentation digestate and / or biogas ２ a step (S13) of separating the The methane fermentation digested liquid containing ammonia is mixed with the CO separated in the separation step. ２ and (S14) controlling the pH of the methane fermentation digested liquid to a range of 7.0 to 7.8 by returning the

3. 3. The method of claim 2, wherein the step (S12) of removing ammonia in the driving fluid includes the step (S12b) of contacting the driving fluid with biogas. ２ and a method for separating and utilizing ammonia.

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