JP5743686B2 - Biogas purification equipment - Google Patents

Description

  The present invention relates to a biogas purification apparatus, and more particularly, to a biogas purification apparatus that diffuses and purifies biogas in digested liquid after methane fermentation.

  In recent years, attention has been focused on methane fermentation that produces methane that can be used as biogas by fermenting livestock manure and wastes generated from the region, in view of energy issues and a resource recycling society.

  Biogas contains hydrogen sulfide derived from carbon dioxide and sulfur-containing components in the fermentation raw material as impurities. Carbon dioxide is an incombustible gas, and when the concentration in biogas increases, the concentration of methane, which is a relative fuel, decreases. As a result, the methane concentration in the biogas usually varies in the range of 55 to 65%, sometimes causing problems such as deactivation of the gas engine or non-ignition of the gas boiler, and lack of reliability as a fuel. . In addition, hydrogen sulfide is precipitated as a sulfur component in the drive unit in the energy conversion device, and there is a risk of reducing energy conversion efficiency.

  Conventionally, as a technique for increasing the concentration of methane in biogas, Patent Document 1 discloses that biogas is diffused as microbubbles and nanobubbles in an absorbing solution such as water and wastewater to dissolve and absorb carbon dioxide and hydrogen sulfide. A technique for obtaining purified biogas containing high purity methane is disclosed.

  However, since the solubility of carbon dioxide and hydrogen sulfide in the absorbing solution decreases early, in order to maintain the biogas purification efficiency, the absorbing solution must be replaced at high speed, and the pump power There is a problem that the cost of the waste liquid increases and a problem that the amount of waste liquid increases and the cost of waste liquid treatment increases. Furthermore, due to fluctuations in the solubility of carbon dioxide and hydrogen sulfide, there was a problem that the methane concentration in the purified biogas was likely to fluctuate.

  On the other hand, Patent Document 2 discloses that in a biological desulfurization tower, a fermentation liquid in which carbon dioxide has been heated and diffused in advance is sprinkled from the top, and biogas is introduced from the bottom to supply gas and liquid in a state where oxygen is supplied. A technique is disclosed in which carbon dioxide in biogas is dissolved and absorbed in a fermentation broth by contacting it, and hydrogen sulfide is oxidized and removed as sulfuric acid by biological desulfurization by an aerobic sulfur-oxidizing bacterium.

  The technology of Patent Document 2 has realized efficient removal of carbon dioxide and hydrogen sulfide from biogas.

  However, oxygen supply in biological desulfurization is usually performed by introducing air, and nitrogen in the air and excess oxygen dilute the methane concentration in the biogas. Since the hydrogen sulfide concentration in biogas is not constant, it is desirable to supply the minimum amount of air according to the varying hydrogen sulfide concentration to prevent dilution, but such control is difficult. Therefore, there is room for further improvement.

JP 2008-255209 A JP 2008-13649 A

  In the technique described in Patent Document 1, there is a problem that the solubility of carbon dioxide and hydrogen sulfide in the absorbing solution is lowered at an early stage. The inventor considers the reason as follows.

  FIG. 4 shows the pH dependence of the presence form of carbon dioxide (carbonic acid).

Above pH 8, it dissolves and becomes a state of hydrogen carbonate ions (HCO ₃ ⁻ ) and carbonate ions (CO ₃ ²⁻ ), and there is almost no free carbon dioxide (gas), but as the pH of the absorbing solution decreases, Free carbon dioxide (gas) is stabilized, bicarbonate ions (HCO ₃ ⁻ ) and carbonate ions (CO ₃ ²⁻ ) are reduced, and at pH 4, free carbon dioxide accounts for 100%. Thus, the solubility of carbon dioxide in the absorption liquid greatly depends on the pH of the absorption liquid.

Since hydrogen sulfide is ionized into hydrosulfide ions (HS ⁻ ) and hydrogen ions (H ⁺ ) in the absorbing solution and shows weak acidity, it exhibits the same behavior as carbon dioxide.

  Therefore, in the technique described in Patent Document 1, the pH of the absorbing liquid is likely to decrease due to absorption of carbon dioxide and hydrogen sulfide, so that the solubility of carbon dioxide and hydrogen sulfide is significantly reduced early.

  Based on such analysis of the prior art, the present inventor has studied the use of digested liquid after methane fermentation as the absorbing liquid.

  The digestive fluid contains ammonium chloride in addition to ammonia. Therefore, when ammonia, which is a weak base, and ammonium chloride, which is a salt thereof, coexist, an ammonia-ammonium chloride buffer solution is obtained.

  In addition to this, since several types of pH buffer action expression systems such as a system composed of other digestive fluid components, for example, a weak acid such as an organic acid such as acetic acid and a salt thereof are involved, it has a high buffering property.

  Furthermore, the pH of the digestive fluid is preferably increased to 7-9, more preferably 8-9, due to the presence of ammonia nitrogen and the like, and this high pH state is strong due to the above-mentioned properties as a buffer solution. Therefore, it is possible to maintain a high solubility in carbon dioxide and hydrogen sulfide over a long period of time by using such digestive fluid as an absorbing solution for dissolving and removing carbon dioxide and hydrogen sulfide. This is because I thought.

  However, since digestive fluid contains high-concentration SS, when biogas is diffused as microbubbles or nanobubbles as in the past, SS precipitates in the biogas refining tank and the diffuser vent The problem that the gas-liquid contact between the bubbles and the digestive fluid becomes unstable due to the closure or the diffusion of the bubbles into the digestive fluid is inhibited by the SS.

  Further, when the digestive fluid is circulated between the biogas purification tank and the digestive fluid storage tank that stores the digestive fluid supplied to the biogas purification tank, the SS trapped in the bubbles is the biogas purification tank. It stays inside and SS is concentrated in the biogas refining tank, and the above-mentioned gas-liquid contact is further destabilized.

  When the gas-liquid contact becomes unstable, a serious problem occurs that the methane concentration in the purified biogas becomes unstable over time.

  Furthermore, from the point of view of energy, in the biogas purification apparatus having a configuration in which the digestive juice is circulated between the digestive juice storage tank and the biogas purification tank, the energy consumed by the liquid feed pump for circulation is used. However, it is an important issue from the viewpoint of practical application of the apparatus and from the viewpoint of environmental protection.

  Then, this invention makes it a subject to provide the biogas refinement | purification apparatus which is excellent in temporal stability of the methane density | concentration in refinement | purification biogas, and can implement | achieve energy saving.

  Other problems of the present invention will become apparent from the following description.

  The above problems are solved by the following inventions.

Invention of Claim 1 consists of a digestive-solution storage tank main body and a cover body which store the digested liquid after the methane fermentation from a methane fermenter, and is equipped with the digestive-solution discharge port which discharges digestive juice in the lower part of this main body. , A digestive fluid storage tank provided with a digestive fluid inlet into which the digested fluid after biogas purification flows into the upper part of the main body,
It consists of a biogas refining tank main body that stores the digested liquid after methane fermentation, and a lid that seals the upper part of the main body, and is provided with a digestive liquid inlet for flowing the digested liquid after methane fermentation into the lower part of the main body, The upper part of the main body is provided with a digestion liquid outlet for discharging the digested liquid after biogas purification, and a biogas containing at least methane gas, carbon dioxide gas, and hydrogen sulfide gas sent from the methane fermentation tank is introduced into the lower part of the main body. Provided with a biogas inlet, provided with a biogas outlet for taking out the purified biogas above the level of the digestive liquid stored in the main body, and from the biogas inlet under the main body It consists of a biogas refining tank equipped with a bubble forming means for bubbling the introduced biogas into the digestive juice,
The digestive fluid inlet provided in the biogas purification tank and the digestive fluid outlet provided in the digestive liquid storage tank are connected by a supply pipe, and the digestive fluid outlet provided in the biogas purification tank, The digestive fluid inlet provided in the digestive fluid storage tank is connected by a return pipe,
A biogas purification apparatus for purifying biogas by circulating a digestion liquid between the digestion liquid storage tank and the biogas purification tank,
The bubble diameter generated by the bubble forming means is in the range of 300 μm to 3 mm,
Both the digestive fluid inlet and the digestive fluid outlet provided in the biogas purification tank are disposed below the liquid level of digestive liquid stored in the biogas purification tank body, and the digestive fluid inlet Is disposed at the bottom of the body, the digestive juice outlet is disposed at the top of the body,
Both the digestive fluid inlet and the digestive fluid outlet provided in the digestive fluid storage tank are disposed below the level of the digestive fluid stored in the digestive fluid reservoir main body, and the digestive fluid inlet Is disposed at the top of the body, the digestive juice outlet is disposed at the bottom of the body,
Due to the density difference generated between the digestive liquid containing bubbles inside the biogas purification tank main body and the digestive liquid not containing bubbles newly supplied from the inside of the digestive liquid storage tank main body through the supply pipe, It is a biogas purification apparatus characterized by generating a digestive fluid circulation force between the digestive juice storage tank and the biogas purification tank.

  The invention according to claim 2 is the biogas purification apparatus according to claim 1, wherein the SS concentration of the digestive liquid inside the biogas purification tank main body is in the range of 5000 to 40000 mg / L.

According to a third aspect of the invention, the biogas purification device of said 1 or 2, wherein the biogas purification tank oxygen supply to the internal body is dripping off.

Fourth aspect of the present invention, only by the digestive juices circulation force, the one of the 1 to 3, wherein that you circulate the digestive juices between said digested liquid reservoir and the biogas purification tank The biogas purification apparatus as described.

  ADVANTAGE OF THE INVENTION According to this invention, the biogas refinement | purification apparatus which is excellent in temporal stability of the methane density | concentration in refinement | purification biogas, and can implement | achieve energy saving can be provided.

Schematic which shows an example of the biogas manufacturing system provided with the biogas purification apparatus which concerns on this invention Schematic showing an embodiment without a liquid feed pump that circulates digestive juice Schematic showing a mode in which a liquid feed pump for circulating digestive fluid is provided on the bypass line Diagram showing the pH dependence of the existence form of carbon dioxide (carbonic acid)

  Hereinafter, embodiments for carrying out the present invention will be described with reference to the drawings.

  FIG. 1 is a schematic diagram illustrating an example of a biogas production system including a biogas purification apparatus according to the present invention.

  In FIG. 1, 1 is a methane fermenter and 2 is a biogas purification apparatus. The biogas purification device 2 includes a digestive juice storage tank 3 and a biogas purification tank 4.

  The methane fermentation tank 1 performs methane fermentation of organic waste (biomass), and examples of biomass include food waste, livestock manure, sewage treatment sludge, and septic tank sludge.

  Biomass is put into the methane fermentation tank 1 and methane fermentation is performed under anaerobic conditions. The biogas generated by methane fermentation contains at least methane gas and carbon dioxide and hydrogen sulfide which are impurities.

  Moreover, although the digestive liquid after methane fermentation in the methane fermenter 1 contains high concentration SS, when using as an absorption liquid, the range whose SS density | concentration is 5000-40000 mg / L is preferable, More preferably It is the range of 7000-30000 mg / L.

  The digested liquid after methane fermentation is introduced into the digested liquid storage tank 3 by the liquid supply pump 11 via the liquid supply pipe 12, while the biogas generated from the methane fermentation tank 1 is temporarily stored in the gas bag 15. Then, it is introduced into the biogas purification tank 4 by the blower 13 through the blower pipe 14.

  The digestive fluid storage tank 3 includes a digestive fluid storage tank main body 30a and a lid 30b for storing the digested liquid after methane fermentation from the methane fermentation tank 1.

  In the illustrated example, the digestive fluid storage tank 3 includes a digestive fluid discharge port 31 for discharging the digestive fluid at the lower part of the digestive fluid storage tank body 30a, and the biogas in the biogas purification tank 4 at the upper part of the digestive fluid storage tank body 30a. A digestive fluid inlet 32 into which digested fluid after gas purification flows is provided.

  On the other hand, the biogas purification tank 4 includes a biogas purification tank body 40a for storing digested liquid after methane fermentation, and a lid body 40b for sealing the upper part of the biogas purification tank body 40a.

  In the illustrated example, the biogas refining tank 4 includes a digestive fluid inlet 41 through which the digested liquid after methane fermentation from the digested liquid storage tank 3 flows into the lower part of the biogas refining tank main body 40a, and the biogas refining tank main body The upper part of 40a is equipped with the digestive-solution discharge port 42 which exhausts the digested liquid after biogas refinement | purification.

  The digestive fluid inlet 41 provided in the biogas purification tank 4 and the digestive fluid outlet 31 provided in the digested liquid storage tank 3 are connected by a supply pipe 51. In addition, the digestive juice outlet 42 provided in the biogas purification tank 4 and the digestive fluid inlet 32 provided in the digested liquid storage tank 3 are connected by a return pipe 52.

  In the illustrated example, the supply pipe 51 is provided with a liquid feed pump 50.

  Thereby, between the digestive fluid storage tank 3 and the biogas purification tank 4, the digestive fluid storage tank main body 30a → the digestive fluid discharge port 31 → the supply pipe 51 → the digestive fluid inlet 41 → the biogas purification tank main body 40a → the digestion. The digestive fluid is circulated in the order of the liquid discharge port 42 → the return pipe 52 → the digestive fluid inlet 32 → the digestive fluid storage tank body 30a.

  The biogas refining tank 4 includes a biogas introduction port 43 for introducing biogas including at least methane gas, carbon dioxide gas, and hydrogen sulfide gas sent from the methane fermentation tank 1 to the lower portion of the biogas refining tank body 40a.

  In addition, below the inside of the biogas refining tank main body 40a, an aeration means 43a that bubbles the biogas introduced from the biogas introduction port 43 is provided.

  That is, in the present invention, the bubbling means 43a diffuses the bubble-like biogas into the digested liquid so as to bring it into gas-liquid contact.

  On the other hand, a biogas outlet 44 for taking out biogas after purification (after gas-liquid contact) is provided above the liquid level of the digested liquid stored in the biogas purification tank body 40a (headspace H). ing.

  In the present invention, the diameter of bubbles generated by the bubble forming means 43a is in the range of 300 μm to 3 mm, preferably 500 μm to 2 mm, more preferably 800 μm to 1500 μm.

  The bubble generating means 43a is not particularly limited as long as it can generate bubbles having a bubble diameter of 300 μm to 3 mm, and a plate or a membrane filter in which pores are formed can be preferably used. Among them, a membrane diffuser is preferable. is there.

  When biogas from the methane fermentation tank 1 is introduced into the biogas purification apparatus 2 of the present invention having the above-described configuration, the biogas is contained in the digestion liquid by the aeration means 43a in the biogas purification tank body 40a. The air bubbles are diffused as bubbles in the range of 300 μm to 3 mm, preferably 500 μm to 2 mm, more preferably 800 μm to 1500 μm.

  Thereby, the biogas and the digestive liquid are brought into gas-liquid contact, and the carbon dioxide and hydrogen sulfide in the biogas are dissolved and removed in the digestive liquid in the biogas refining tank body 40a.

  On the other hand, methane gas having a low solubility is not dissolved but is degassed and released from the liquid level of the digested liquid in the biogas purification tank main body 40a to the headspace H, and then recovered as purified biogas via the biogas outlet 44. The

  The digested liquid after the gas-liquid contact that has absorbed carbon dioxide and hydrogen sulfide is returned to the digested liquid storage tank 3 via the return pipe 52. Along with this return, a new digestive fluid is supplied from the digestive fluid storage tank 3 to the biogas purification tank 4 via the supply pipe 51.

  In this way, the digestion liquid is circulated between the digestion liquid storage tank 3 and the biogas purification tank 4 to purify the biogas.

  It is preferable to circulate the digestion liquid between the digestion liquid storage tank 3 and the biogas purification tank 4 so that the pH of the digestion liquid in the biogas purification tank body 40a is maintained at a predetermined value, preferably pH 7 or higher. .

  As described above, in the present invention, gas-liquid contact is performed with 300 μm to 3 mm bubbles which are orders of magnitude larger than conventionally used bubbles of several tens of μm or less classified as microbubbles and nanobubbles.

  If a person skilled in the art uses 300 μm to 3 mm bubbles that are extremely large, the contact area and residence time will be greatly reduced, and the removal efficiency of carbon dioxide and hydrogen sulfide by gas-liquid contact will be significantly impaired. I should have thought.

  On the other hand, as described above, the inventor firstly increased the pH of the digested liquid to 7-9, more preferably 8-9, due to the presence of ammonia nitrogen and the like. Since the high pH state is strongly maintained by the properties of the digestive juice as a buffer, the digestive fluid can maintain high solubility in carbon dioxide and hydrogen sulfide over a long period of time.

  Therefore, even if bubbles of 300 μm to 3 mm, which are extremely large, are used, biogas purification efficiency is suitably supplemented by the high solubility of digestive fluid in carbon dioxide and hydrogen sulfide, especially in the case of gas boilers and gas engines. If the required methane concentration is relatively low (preferably 60 to 70%), a satisfactory purified biogas can be provided.

  In addition, when the digestion liquid is diffused with bubbles of 300 μm to 3 mm, SS is precipitated in the biogas purification tank main body 40a, which is observed when the bubbles are diffused with microbubbles or nanobubbles, and the bubbles are dispersed in the bubble generating means 43a. There is no problem that the mouth is closed or that the diffusion of bubbles into the digestive juice is hindered by SS.

  Furthermore, even if the digestive juice is circulated between the digestive juice storage tank 3 and the biogas purification tank 4, the SS is not easily captured by bubbles, and the SS is retained and concentrated in the biogas purification tank body 40a. It is prevented. As a result, according to the present invention, the gas-liquid contact between the bubbles and the digestive liquid is extremely stabilized, and it is possible to continuously provide purified biogas having a methane concentration of 60% or more over a long period of time (over time) To stabilize).

  In particular, in the present invention, the property of the digestive juice as a buffer solution greatly contributes to the temporal stability of the methane concentration. That is, even if carbon dioxide and hydrogen sulfide are absorbed, the pH variation of the digestive fluid is small, and it is easily maintained at pH 7 or higher over a long period of time. Therefore, the removal amount of carbon dioxide and hydrogen sulfide is stabilized, and the methane concentration in the purified biogas is stabilized with time.

  That is, according to the present invention, the stabilization of gas-liquid contact between the bubbles and the digestive fluid and the pH stability of the digestive fluid interact synergistically and synergistically, and the stability over time of the methane concentration is extremely high. It is possible to provide purified biogas with high reliability and high reliability. In particular, when purified biogas is supplied to a gas boiler or a gas engine, the effect of suitably preventing the deactivation of the gas engine or poor ignition of the gas boiler can be obtained.

  Furthermore, in the present invention, the digestive fluid can maintain the solubility in carbon dioxide and hydrogen sulfide over a long period of time, so that the circulation of the digestive fluid between the digestive fluid storage tank 3 and the biogas purification tank 4 is significantly slowed down. Can be That is, there is an effect that energy saving can be realized by greatly reducing the power of the liquid feed pump 50. This is a practically significant effect in view of the fact that biogas technology was originally developed mainly for environmental protection (energy conservation). This effect will be described in detail later.

  In the example of FIG. 1, the case where the liquid feed pump 50 is provided in the supply pipe 51 has been described. However, the present invention is not limited to this, and may be provided in the return pipe 52.

  Moreover, in the example of FIG. 1, although the digestive fluid discharge port 31 was provided in the lower part of the digestive fluid storage tank main body 30a and the digestive fluid inlet 32 was provided in the upper part, the digestive fluid outlet 31 and the digestive fluid inlet were demonstrated. The position 32 may be either the upper part or the lower part of the digestive fluid storage tank main body 30a.

  Furthermore, the position of the digestive fluid inlet 32 of the digestive fluid reservoir main body 30a may be disposed below the level of the digestive fluid in the digestive fluid reservoir main body 30a as shown in FIG. , May be disposed above.

  Similarly, in the example of FIG. 1, an example in which the digestion fluid inlet 41 is provided in the lower part of the biogas purification tank main body 40 a and the digestion fluid outlet 42 is provided in the upper part has been described. The position of the outlet 42 may be either the upper part or the lower part of the biogas purification tank main body 40a, and the position of the digestion liquid inlet 41 is also below the liquid level of the digestive liquid in the biogas purification tank main body 40a. It may be arranged or may be arranged above.

  In the present invention, oxygen supply to the inside of the biogas refining tank body 40a is substantially cut off in order to draw out the properties of the digestive fluid as a buffer solution more efficiently and make maximum use. Is preferred.

  In this specification, the oxygen supply to the inside of the biogas purification tank main body 40a is substantially cut off. Specifically, for example, an aeration process or aeration process with air to the biogas purification tank main body 40a is performed. This means that the oxygen supply is substantially cut off as long as dissolved oxygen is present in the digested liquid supplied into the biogas refining tank body 40a, for example. . Moreover, since the biogas produced by methane fermentation, which is anaerobic fermentation, does not contain oxygen, even if this is introduced, the state where the oxygen supply is substantially cut off is maintained.

  Since the gas-liquid contact between the biogas and the digestive fluid is performed in a state where the oxygen supply is cut off because the oxygen supply into the biogas purification tank main body 40a is cut off, the sulfur-oxidizing bacteria that are aerobic bacteria Activity is disturbed. Therefore, the oxidation reaction of hydrogen sulfide by sulfur-oxidizing bacteria is inhibited, and the production of sulfuric acid, which is a strong acid, can be suppressed.

  In the present invention, since the gas-liquid contact is performed by aerating the biogas in the digestive liquid, the generation of sulfuric acid is prevented compared to the biodesulfurization apparatus that sprays the digestive liquid into the biogas under aerobic conditions. However, since the oxygen supply into the biogas refining tank main body 40a is substantially cut off as described above, the generation of sulfuric acid is further remarkably prevented.

  By preventing the generation of a strong acid, it is avoided that the pH buffering ability of the digestive fluid reaches the buffer limit early, and the pH buffering property of the digestive fluid is made to act only for dissolving and removing carbon dioxide and hydrogen sulfide. It is possible to extract the properties of the digestive fluid as a buffer solution very efficiently. As a result, the pH variation of the digestive juice is further reduced, and the pH drop is further prevented.

  In the present invention, the pH of the digestive fluid is prevented from being lowered, and the digestive fluid can maintain the solubility in carbon dioxide and hydrogen sulfide over a long period of time, so that the power of the liquid feeding pump 50 can be reduced as described above. However, with the configuration described below, the effect of significantly reducing the power of the liquid feed pump 50 can be obtained.

  That is, as shown in FIG. 1, first, both the digestive fluid inlet 41 and the digestive fluid outlet 42 provided in the biogas purification tank 4 are more than the liquid level of the digestive liquid stored in the biogas purification tank main body 40a. Place it below. And the digestive-solution inlet 41 is arrange | positioned under the biogas refinement | purification tank main body 40a, and the digestive-solution discharge port 42 is arrange | positioned above the biogas refinement | purification tank main body 40a. Furthermore, the digestive fluid inlet 32 and the digestive fluid outlet 31 provided in the digestive fluid storage tank 3 are both disposed below the liquid level of the digestive fluid stored in the digestive fluid storage tank body 30a. And a digestive-solution inflow port is arrange | positioned above the digestive-solution storage tank main body 30a, and the digestive-solution discharge port 31 is arrange | positioned under the digestive-solution storage tank main body 30a.

  In the biogas refining tank main body 40a, while the aerated biogas continues to stay in the digestive liquid, the relative density of the digestive liquid containing bubbles decreases. For this reason, a density difference arises with the digestive liquid which does not contain the bubble newly supplied from the digestive liquid storage tank 3 via the supply piping 51. FIG. Due to this density difference, a liquid flow toward the biogas refining tank 4 is generated in the supply pipe 51, and a force for generating a liquid flow toward the digestive liquid storage tank 3 is generated in the return pipe 52. That is, a digestive fluid circulation force is generated between the digestive fluid storage tank 3 and the biogas purification tank 4.

  By obtaining the digestive fluid circulation force, the burden on the liquid feed pump 50 that circulates the digestive fluid between the digestive fluid storage tank 3 and the biogas purification tank 4 is reduced, and the power cost can be further reduced. The effect which becomes.

  As a more preferable mode, the liquid feeding pump 50 in FIG. 1 is stopped or the liquid feeding pump 50 is not provided as shown in FIG. You may make it circulate a digestion liquid between the gas purification tanks 4. FIG.

  The circulation speed obtained only by the digestive fluid circulation force is generally inferior to the circulation speed by the liquid feed pump 50. However, in the present invention, as described above, the pH of the digestive liquid is remarkably prevented and the pH of the digestive liquid in the biogas refining tank body 40a can be kept high even with a slight circulation. Even with a low digestive fluid circulation force, it is possible to suitably cover the digestive fluid circulation supply, and it is possible to completely eliminate the cost for the power of the liquid feed pump 50.

  In the present invention, as shown in FIG. 3, it is also preferable that the liquid feed pump 50 is provided not on the supply pipe 51 but on the bypass line 51 a of the supply pipe 51.

  In the biogas purification apparatus 2 having the configuration shown in FIG. 3, in normal operation, the liquid feed pump 50 is stopped, the valve 51 a ′ provided in the bypass line 51 a is closed, and the valve 51 provided in the supply pipe 51 is closed. 'Is opened, and circulation is performed only through the digestive fluid circulation force described above via the supply pipe 51.

  And when pH of the digestive liquid in the biogas refinement | purification tank main body 40a falls below predetermined value (preferably pH7), valve | bulb 51 'provided in the supply piping 51 is closed, and it was provided in the bypass line 51a. The valve 51a ′ is opened, the liquid feed pump 50 is operated, and switching is performed so as to circulate through the bypass line 51a.

  As a result of circulation by the liquid feed pump 50, when the pH of the digested liquid in the biogas purification tank main body 40a exceeds a predetermined value and is restored to an ideal value (preferably pH 7.5 or more), the normal operation described above is switched.

  Such switching is performed by connecting the pH detecting means 45 for detecting the pH of the digested liquid in the biogas purification tank main body 40a, the liquid feed pump 50, the valve 51 'and the valve 51a' to the control unit 6, respectively, and measuring the pH. You may comprise so that it may be automated based on the detection result from 45.

  In this way, there is an effect that the biogas purification can be more reliably performed with the minimum pump power.

  In the example of FIG. 3, the case where the liquid feed pump 50 is provided in the bypass line 51 a of the supply pipe 51 has been described. However, the present invention is not limited to this, and the liquid feed pump 50 is provided in the bypass line of the supply pipe 52 to perform the same switching. You may go.

  Further, in the present invention, the digestive fluid storage tank 3 includes a deodorizing device (not shown) for processing biogas-derived hydrogen sulfide contained in the gas released from the digestive fluid in the digestive fluid storage tank body 30a. Also good. Although it does not specifically limit as a deodorizing apparatus, What absorbs and removes hydrogen sulfide in an activated carbon packed bed can be used preferably.

  In the present invention, the purified biogas recovered from the biogas outlet 44 is preferably subjected to a mist separator to separate mist (water vapor).

  In addition, the purified biogas recovered from the biogas outlet 44 can be used in a desulfurization apparatus such as biological desulfurization to further remove hydrogen sulfide in the biogas. In the present invention, hydrogen sulfide can be removed to a high degree in the biogas refining tank 4, and therefore, when a desulfurization device is provided, the load on the desulfurization device can be reduced. Furthermore, when the concentration of hydrogen sulfide in the biogas generated from the methane fermentation tank 1 is low due to the raw material biomass, etc., the biogas refining tank 4 can perform sufficient removal, eliminating the need for a desulfurization device. It becomes.

  In the above description, the digestion liquid and the biogas introduced into the biogas purification apparatus 2 are both generated from the same methane fermentation tank 1, but are not limited thereto. In the present invention, the digested liquid and the biogas introduced into the biogas purification apparatus 2 may be generated from different methane fermentation tanks.

  The digestive fluid stored in the digestive fluid storage tank 3 can be appropriately extracted and used as fertilizer or the like. Depending on the amount extracted, a new digestive juice can be appropriately supplemented.

  Examples of the present invention will be described below, but the present invention is not limited to such examples.

Example 1
<Test sample>
Biogas A to be treated: Biogas produced from a methane fermentation tank implemented in K city was collected and used as biogas A to be treated. The carbon dioxide concentration in the biogas A to be treated detected by the detector tube was 40%, and the hydrogen sulfide concentration was 170 ppm. The temperature is room temperature.

  Test solution (digestion solution A): The digestion solution after completion of fermentation was collected from the same methane fermenter and left in the atmosphere for 2 hours to obtain digestion solution A. The pH of the digestive fluid A measured by a pH meter was 7.94. The temperature is room temperature. The SS concentration was 12000 mg / L.

<Test method>
100 ml of the biogas A to be treated and 100 ml of the digestive liquid A are poured into a white hard syringe and shaken for 2 minutes in a sealed state (at a state where oxygen supply is substantially cut off) at room temperature and normal pressure. It was.

  After shaking, the pH of the liquid phase (digested liquid) was measured with a pH meter, and the carbon dioxide concentration and hydrogen sulfide concentration in the gas phase (biogas) were further measured with a detector tube.

  In addition, the decrease rate of the carbon dioxide concentration and the hydrogen sulfide concentration in the biogas before and after the test was calculated, and further, the methane concentration in the biogas after the test was calculated based on this result. The results are shown in Table 1.

(Example 2)
<Test sample>
Biogas B to be treated: Biogas was collected from the same methane fermentation tank as in Example 1 on another day in the same manner as in Example 1 to obtain Biogas B to be treated. The carbon dioxide concentration in the treated biogas B measured by the detector tube was 35%. The temperature is room temperature.

  Test solution (digestion solution B): On the other day from the same methane fermentation tank as in Example 1, the digestion solution was collected in the same manner as in Example 1, and left in the atmosphere for 2 hours to obtain Digestion Solution B. The pH of the digestive juice B measured with a pH meter was 8.01. The temperature is room temperature. The SS concentration was 12000 mg / L.

<Test method>
100 ml of the biogas B to be treated and 100 ml of the digestive liquid B were injected into a white syringe, and were shaken for 2 minutes in a sealed state at normal temperature and normal pressure to make gas-liquid contact.

  After shaking, the pH of the liquid phase (digested liquid) was measured with a pH meter, and the carbon dioxide concentration in the gas phase (biogas) was further measured with a detector tube.

  Moreover, the decrease rate of the carbon dioxide concentration in the biogas before and after the test was calculated, and further, the methane concentration in the biogas after the test was calculated based on this result. The results are shown in Table 1.

(Comparative Example 1)
<Test sample>
Biogas A to be treated: The same as that used in Example 1.

  Test liquid (tap water): Tap water was used as the test liquid. The pH of tap water measured by a pH meter was 7.18. The temperature is room temperature.

<Test method>
100 ml of the biogas A to be treated and 100 ml of the tap water were poured into a white hard syringe and shaken for 2 minutes in a sealed state at normal temperature and normal pressure to make gas-liquid contact.

  After shaking, the pH of the liquid phase (tap water) was measured with a pH meter, and the carbon dioxide concentration and hydrogen sulfide concentration in the gas phase (biogas) were further measured with a detector tube.

  In addition, the decrease rate of the carbon dioxide concentration and the hydrogen sulfide concentration in the biogas before and after the test was calculated, and further, the methane concentration in the biogas after the test was calculated based on this result. The results are shown in Table 1.

<Evaluation>
In Examples 1 and 2 in which the digested liquid after methane fermentation and the biogas produced by methane fermentation of biomass were brought into gas-liquid contact in a state where the oxygen supply was substantially cut off, water was used instead of the digested liquid. It can be seen that, compared to Comparative Example 1 using water, a drop in pH due to gas-liquid contact is remarkably prevented and the removal efficiency of carbon dioxide and hydrogen sulfide is excellent.

(Example 3)
Biogas purification was performed by gas-liquid contact between biogas and digestive fluid using a biogas purification tank having a diameter of 150 mm and a height of 2050 mm (volume: 36 L).

  The biogas was introduced while the gas from the gas bag (fresh gas) was bubbled by a membrane diffuser provided at the bottom of the biogas purification tank. The gas flow rate was 6 L / min.

  The digestive fluid was circulated in an upward flow (supplied from the lower part of the biogas refining tank and returned to the digestive liquid storage tank from the upper part of the biogas refining tank) with a digestive liquid of 100 L having an SS concentration of 12000 mg / L stored for one month or more. . The circulation flow rate of the digestive fluid was 6 L / min.

  The pH of the digested liquid was measured with a pH meter (portable pH meter HM manufactured by Toa DKK Corporation). Fresh gas and biogas purification tank outlet gas (purified gas) were measured for methane and hydrogen sulfide concentrations. The methane and hydrogen sulfide concentrations were measured by sampling the gas and measuring it later by gas chromatographic analysis. The results are shown in Table 2.

  In Table 2, the gas-liquid ratio is a value obtained by dividing the gas flow rate in the biogas purification tank by the digestion flow rate in the biogas purification tank.

<Evaluation>
The contact time with the digestive fluid (from when biogas was introduced into the biogas refining tank until it reached the headspace) was about 5 seconds, but the methane gas concentration of the purified gas could be increased to 70% or more. . The concentration of hydrogen sulfide could also be reduced to 200 ppm or less.

1: Methane fermentation tank 11: Liquid feed pump 12: Liquid feed pipe 13: Blower 14: Air blow pipe 15: Gas bag 2: Biogas purification device 3: Digestive liquid storage tank 30a: Digestive liquid storage tank body 30b: Lid 31 : Digestive fluid outlet 32: Digestive fluid inlet port 4: Biogas purification tank 40a: Biogas purification tank body 40b: Lid 41: Digestive fluid inlet port 42: Digestive fluid outlet port 43: Biogas inlet port 43a: Bubble formation Means 44: Biogas outlet 45: pH detection means 50: Liquid feed pump 51: Supply pipe 51 ': Valve 51a: Bypass line 51a': Valve 52: Return pipe 6: Control unit

Claims (4)

A digestive fluid storage tank main body for storing digested liquid after methane fermentation from a methane fermentation tank and a lid, and a digestive fluid outlet for discharging the digestive fluid at the lower part of the main body, and a biogas at the upper part of the main body A digestive fluid storage tank having a digestive fluid inlet into which the purified digestive fluid flows, and
It consists of a biogas refining tank main body that stores the digested liquid after methane fermentation, and a lid that seals the upper part of the main body, and is provided with a digestive liquid inlet for flowing the digested liquid after methane fermentation into the lower part of the main body, The upper part of the main body is provided with a digestion liquid outlet for discharging the digested liquid after biogas purification, and a biogas containing at least methane gas, carbon dioxide gas, and hydrogen sulfide gas sent from the methane fermentation tank is introduced into the lower part of the main body. Provided with a biogas inlet, provided with a biogas outlet for taking out the purified biogas above the level of the digestive liquid stored in the main body, and from the biogas inlet under the main body It consists of a biogas refining tank equipped with a bubble forming means for bubbling the introduced biogas into the digestive juice,
The digestive fluid inlet provided in the biogas purification tank and the digestive fluid outlet provided in the digestive liquid storage tank are connected by a supply pipe, and the digestive fluid outlet provided in the biogas purification tank, The digestive fluid inlet provided in the digestive fluid storage tank is connected by a return pipe,
A biogas purification apparatus for purifying biogas by circulating a digestion liquid between the digestion liquid storage tank and the biogas purification tank,
The bubble diameter generated by the bubble forming means is in the range of 300 μm to 3 mm,
Both the digestive fluid inlet and the digestive fluid outlet provided in the biogas purification tank are disposed below the liquid level of digestive liquid stored in the biogas purification tank body, and the digestive fluid inlet Is disposed at the bottom of the body, the digestive juice outlet is disposed at the top of the body,
Both the digestive fluid inlet and the digestive fluid outlet provided in the digestive fluid storage tank are disposed below the level of the digestive fluid stored in the digestive fluid reservoir main body, and the digestive fluid inlet Is disposed at the top of the body, the digestive juice outlet is disposed at the bottom of the body,
Due to the density difference generated between the digestive liquid containing bubbles inside the biogas purification tank main body and the digestive liquid not containing bubbles newly supplied from the inside of the digestive liquid storage tank main body through the supply pipe, A biogas purification apparatus that generates a digestive fluid circulation force between the digestive juice storage tank and the biogas purification tank.   The biogas purification apparatus according to claim 1, wherein the SS concentration of the digestive liquid inside the biogas purification tank main body is in the range of 5000 to 40000 mg / L.   The biogas purification apparatus according to claim 1 or 2, wherein oxygen supply to the biogas purification tank main body is cut off.   The biogas purification apparatus according to any one of claims 1 to 3, wherein the digestion liquid is circulated between the digestion liquid storage tank and the biogas purification tank only by the digestive liquid circulation force.

Images