JP5112665B2 - Digestion gas purification method and purification apparatus in digestion gas utilization system - Google Patents

Description

The present invention is a part of the digestion gas obtained by anaerobic fermentation of organic waste such as sewage sludge, subtracted from the amount used as fuel for the heating equipment for heating the digestion tank. Alternatively, the present invention relates to a digestion gas purification method and a purification apparatus thereof in a digestion gas utilization system that purifies the entire amount and uses this purified gas as a fuel.

  Biomass resources are expected to be used to prevent global warming and build a recycling-oriented society (sustainable society). Food waste such as garbage, livestock manure, organic wastewater, and sewage generated at sewage treatment plants Development of a technique for generating digestion gas by subjecting organic waste such as sludge to anaerobic fermentation and using this digestion gas as energy is being promoted.

  In particular, according to the “Biomass Nippon Comprehensive Strategy” approved by the Cabinet in Japan in December 2002, biodiesel produced by anaerobic fermentation of sewage sludge generated in the first sedimentation basin and final sedimentation basin at a sewage treatment plant. Use of gas (digestion gas) is expected.

This digestion gas (sewage sludge digestion gas) is mainly composed of methane (CH ₄ ) and carbon dioxide (CO ₂ ) (methane: about 60% by volume, carbon dioxide: about 40% by volume), and sulfur-based as a trace amount of impurities. A gas containing impurities (such as H ₂ S). It is known that the digestion gas of sewage sludge in urban areas contains a siloxane compound derived from shampoo and the like.

  As a method for obtaining a purified gas from which the impurities and the siloxane compound contained in the digestion gas are simultaneously removed, the present inventors have added a raw gas containing methane, carbon dioxide and a siloxane compound and water in an amount of 0.55 to 2. The carbon dioxide is dissolved in high-pressure water and separated from the raw gas by contacting in a high-pressure state satisfying a range of 0.0 MPa, the siloxane compound is condensed and separated from the raw gas, and the methane is separated. A so-called high-pressure water absorption method for obtaining purified gas having a high concentration has already been proposed (see Patent Document 1).

  Furthermore, the inventors measured the gas pressure of the anaerobic fermenter in this high-pressure water absorption method, and the gas pressure of the anaerobic fermenter was determined based on the measurement result, which was a substantially constant set value. Thus, a biogas purification method and biogas purification facility for increasing / decreasing the rotational speed of the compressor for boosting the raw gas were proposed (see Patent Document 2).

  FIG. 5 is a diagram for explaining the digestion gas purification apparatus according to the above-described conventional technique (Patent Document 2), and is a flow diagram showing the configuration of the digestion gas purification apparatus that purifies digestion gas to obtain purified methane gas. . Hereinafter, the digestion gas purification apparatus according to the prior art (Patent Document 2) will be described in detail with reference to FIG.

  In the digestion gas refining apparatus shown in FIG. 5, the digestion gas as the raw gas is a predetermined gas higher than the atmospheric pressure by the gas compressors 402 a and 402 b connected in series after the mist and dust in the gas are removed by the mist separator 401. The pressure is raised to a pressure of ≦ 4, and introduced into the lower part of the absorption tower 404. On the other hand, water is supplied to the absorption tower 404 from the upper part in a state where the pressure is increased by the absorption water pump 408.

Thus, by maintaining the absorption tower 404 in a high pressure state and bringing the digestion gas and water into contact in the absorption tower 404 in a high pressure state, carbon dioxide contained in the digestion gas in a gaseous state and Sulfur impurities (H ₂ S and the like) are dissolved and absorbed in high-pressure water, while methane is taken out from the top of the absorption tower 404 with almost no dissolution in high-pressure water.

  At the same time, the siloxane compound contained in the digestion gas is condensed from the gas state to a droplet state due to the high pressure state, and in this droplet state, collides with the high pressure water flowing down inside the absorption tower, It collects in the bottom part of the absorption tower 404 with water.

The carbon dioxide and sulfur impurities (H ₂ S and the like) separated and removed in this manner are dissolved, and the water containing the separated siloxane compound is extracted from the bottom of the absorption tower 404 through the valve V10. And introduced into the decompression tank 411. The water in which carbon dioxide and sulfur-based impurities (such as H ₂ S) are dissolved after the methane that has been slightly dissolved in the absorption water in the decompression tank 411 is separated from the bottom of the decompression tank 411 through the valve V12. And introduced into the upper part of the stripping tower 412.

  In the stripping tower 412, water extracted from the decompression tank 411 is introduced from the top, while a stripping gas (for example, air) is introduced from the bottom by the exhaust blower 413, and both are in countercurrent contact at atmospheric pressure. To do. As a result, carbon dioxide and sulfur-based impurities dissolved in the water extracted from the decompression tank 411 move to the emission gas side and are discharged from the top of the diffusion tower 412 together with the emission gas, and flow as needed. It is led to the deodorizing equipment via the route L14.

  Then, the absorbed water regenerated by removing the dissolved gas is extracted from the bottom of the stripping tower 412, boosted by the absorbing water pump 408, and then with the brine from the chiller 410 by the heat exchanger 409. After the heat exchange between them and cooling to a predetermined temperature, the water is circulated to the upper part of the absorption tower 404 and supplied.

On the other hand, the purified gas having a high concentration of methane taken out from the top of the absorption tower 404 is sent to the dehumidifier 405. The purpose of dehumidification by the dehumidifier 405 is to prevent condensation even at a pressure when the purified gas is used (utilized) as fuel.
JP 2006-83156 A JP 2006-95512 A

  However, the digestion gas purification apparatus according to the above-described conventional example (Patent Document 2) is subjected to anaerobic fermentation while being heated in the digestion tank, and at least the fuel for the digester heating apparatus. When a part or the whole amount after subtracting the used amount is purified by a digestion gas purification device and applied to a digestion gas utilization system that uses this purified gas as a fuel for other uses, the following problems occur: Yes.

  That is, in the conventional example in which the absorption water discharged from the absorption tower 404 is circulated to the absorption tower 404 through the decompression tank 411 and the diffusion tower 412, the high-pressure water supplied to the absorption tower 404 is the heat exchange. The heat is exchanged with brine from the chiller 410 in the vessel 409, and then cooled to a predetermined temperature before being supplied.

  Since the solubility of carbon dioxide in water increases as the water temperature decreases, the carbon dioxide removal rate in the absorption tower can be improved by cooling the absorption water. However, since such a circulation type consumes electric power in the chiller 410, there is a problem that the operation cost per unit amount of the processing gas becomes high at the time of low load operation.

  In winter when the outside air temperature decreases, the amount of digestion gas used as fuel for the digester heating equipment increases, so the amount of digestion gas that can be supplied to the digestion gas purification device decreases, and the digestion gas purification device operates. Since the operation is low load, the operation cost per unit amount of the processing gas is increased.

  In addition, when the absorption water extracted from the absorption tower is discharged out of the system without being regenerated, when water containing a large amount of dissolved gas such as sewage treatment water is used as the absorption water, the absorption water is absorbed in the absorption tower. Dissolves carbon while releasing dissolved gas. The higher the absorbed water temperature, the lower the solubility of carbon dioxide. Therefore, a large amount of absorbed water is required to absorb a certain amount of carbon dioxide.

  On the other hand, as the amount of absorbed water increases, the amount of dissolved gas that moves from the absorbed water to the purified gas side in the absorption tower also increases. As a result, when the absorption water temperature is high in a transient manner, there is also a problem that it is difficult to maintain the methane concentration of the obtained purified gas constant (for example, 97% by volume or more).

  Therefore, the object of the present invention is to purify a digestion gas obtained by anaerobic fermentation of organic waste, at least a part or all of the digestion gas subtracted from the use as fuel for the digester heating equipment, In a digestion gas utilization system using this purified gas as a fuel for other purposes, a digestion gas purification method and a purification apparatus capable of maintaining a constant methane concentration of the resulting purified gas at a low cost throughout the year are provided. There is.

  In order to achieve the above object, the means employed by the digestion gas purification method in the digestion gas utilization system according to claim 1 of the present invention was obtained by heating an organic substance in a digestion tank and performing anaerobic fermentation. The present invention relates to the digestion gas purification method in a digestion gas utilization system that purifies a part or the whole amount of the digestion gas after subtracting at least a portion to be used as a fuel for the digester heating apparatus and uses the purified gas as a fuel for other uses. .

  At the same time, in this digestion gas purification method, the digestion gas is dissolved in the absorption water by bringing the pressurized digestion gas and the pressurized absorption water into contact with each other in an absorption tower to thereby dissolve the digestion gas. Then, the absorption water in which the carbon dioxide is dissolved is introduced into the diffusion tower to remove the dissolved carbon dioxide, and the absorption water from which the carbon dioxide has been removed is re-pressurized and supplied to the absorption tower. Further, the present invention relates to a method for obtaining a purified gas having a high concentration of methane from the absorption tower by a high-pressure water absorption method.

The digestion gas refining method in the digestion gas utilization system is configured such that when the feed water temperature supplied from outside the digestion gas purification system is lower than a preset temperature of 25 ° C. or lower , the absorption exhausted from the absorption tower. It is characterized in that the water is drained out of the system, and when the feed water temperature is higher than the set temperature , the absorption water is cooled and then switched to a circulation type that is supplied to the absorption tower. is there.

  The digestion gas purification method in the digestion gas utilization system according to claim 2 of the present invention employs the digestion gas purification method in the digestion gas utilization system according to claim 1, wherein the pressurized digestion gas and the pressurized absorption water are In the absorption tower that is in contact with the gas, the carbon dioxide contained in the digestion gas is dissolved in absorption water and separated from the digestion gas, and then the absorption water in which the carbon dioxide is dissolved is introduced into a vacuum tank. Thus, methane gas dissolved in the reduced absorption water is separated, and the separated methane is refluxed to the digestion gas pressurized in the front stage of the absorption tower.

  The means employed by the digestion gas refining apparatus in the digestion gas utilization system according to claim 3 of the present invention is at least the digestion tank of digestion gas obtained by heating an organic substance in an digestion tank and subjecting it to anaerobic fermentation. The present invention relates to a digestion gas refining apparatus for purifying a part or the whole of a heating device, subtracting the amount used as fuel, and obtaining the purified gas in a digestion gas utilization system using this purified gas as a fuel for other uses. .

  At the same time, the digestion gas purifier includes a gas pressure boosting means for boosting the digestion gas, a liquid pressure boosting means for boosting the absorption water that absorbs the digestion gas, and the digestion gas and the absorption water boosted by these pressure boosting means. In the tower, the carbon dioxide contained in the digestion gas is dissolved in the absorption water and separated from the digestion gas to obtain a purified gas having a high concentration of methane, and the carbon dioxide A stripping tower that introduces dissolved absorbed water and removes dissolved carbon dioxide, and a water supply flow path that is supplied to the digestion gas purification device from outside the digestion gas purification device are provided.

  A flow path for absorbing water extracted from the absorption tower, wherein the absorption water extracted from the absorption tower is circulated to the absorption tower via the diffusion tower, and the absorption water is A drainage channel for draining outside the system, a first channel switching means provided on the primary side or secondary side of the diffusion tower for switching between the circulation channel and the drainage channel, and the pressure of the absorbed water In the suction side flow path of the liquid pressurizing means, provided on the secondary side of the diffusion tower for switching between the circulation path of the absorption water from which carbon dioxide has been removed and the water supply path supplied from outside the system. A second flow path switching means, a cooler for cooling the absorbed water in the circulation flow path, and a water supply flow path supplied to the digestion gas purification apparatus from outside the digestion gas purification apparatus. A water supply temperature detector for detecting the water temperature of the water supply is provided.

Further, the digestion gas purifying apparatus incorporates an arithmetic circuit for comparing the detected feed water temperature with a preset temperature of 25 ° C. or lower and a control circuit for controlling the first and second flow path switching means. And when the feed water temperature is lower than the set temperature, the first flow path switching means is on the drain flow path side and the second flow path switching means is on the water supply flow path side. Moreover, when the feed water temperature is higher than the set temperature, the first and second flow path switching means are switched to the circulation flow path side.

  The means employed by the digestion gas purification apparatus in the digestion gas utilization system according to claim 4 of the present invention is the digestion gas purification apparatus in the digestion gas utilization system according to claim 3, wherein the absorbed water extracted from the absorption tower is used. A circulation path for circulating the methane gas dissolved in the absorbed water is provided in the circulation path for circulation, and a reflux path for refluxing the separated gas to the gas booster that is boosted in the front stage of the absorption tower. It is provided.

  The means employed by the digestion gas purification apparatus in the digestion gas utilization system according to claim 5 of the present invention is the digestion gas purification apparatus in the digestion gas utilization system according to claim 3 or 4, wherein the first flow path switching means and The two-channel switching means is a three-way valve.

  A digestion gas purification method in a digestion gas utilization system according to claim 1 of the present invention obtains a purified gas having a high concentration of methane by a high-pressure water absorption method from the absorption tower and is supplied from outside the digestion gas purification system. When the feed water temperature is low, the absorption water discharged from the absorption tower is drained out of the system, and when the feed water temperature is high, the carbon dioxide dissolved in the absorption water is removed. Therefore, the digestion gas can be purified at low cost throughout the year, and the methane concentration of the resulting purified gas can be maintained constant.

  Moreover, the digestion gas purification method in the digestion gas utilization system according to claim 2 of the present invention, after the carbon dioxide contained in the digestion gas is dissolved in absorption water and separated from the digestion gas in the absorption tower, Next, by introducing the absorption water in which the carbon dioxide is dissolved into the decompression tank, the methane gas dissolved in the reduced absorption water is separated, and the separated methane is pressurized in the front stage of the absorption tower. Since the gas is refluxed to the digestion gas, the methane dissolved in the absorbed water is recovered again to improve the methane recovery rate.

  Furthermore, the digestion gas refining apparatus in the digestion gas utilization system according to claim 3 of the present invention is configured to circulate the absorption water extracted from the absorption tower to the absorption tower through the diffusion tower, and the absorption A drainage channel for draining water out of the system, a first channel switching means provided on the primary side or secondary side of the diffusion tower for switching between the circulation channel and the drainage channel, and the absorbed water In the suction side flow path of the liquid pressure boosting means for increasing the pressure, on the secondary side of the diffusion tower for switching between the circulation path of the absorption water from which the carbon dioxide has been removed and the water supply path supplied from outside the system It comprises a second flow path switching means provided, a cooler for cooling the absorbed water, and a feed water temperature detector for detecting the feed water temperature.

  At the same time, there is provided a controller incorporating an arithmetic circuit for comparing the detected feed water temperature with a preset temperature, and a control circuit for controlling the first and second flow path switching means, When the feed water temperature is lower than the set temperature, the first channel switching means is on the drain channel side, the second channel switching means is on the feed channel side, and the feed water temperature is the set temperature. When the temperature is higher, the first and second flow path switching means are switched to the circulation flow path side, so that the transient type and the circulation type can be automatically switched at a preset temperature.

  Furthermore, the digestion gas purification apparatus in the digestion gas utilization system according to claim 4 of the present invention separates the methane gas dissolved in the absorption water into a circulation channel for circulating the absorption water extracted from the absorption tower. Since the depressurization tank is provided and the reflux flow path for recirculating the separated gas to the gas pressurizing means is provided, the methane in the absorbed water is re-recovered and the flow path switching is automated. It can be operated continuously throughout the year at low cost and with the methane concentration of the refined gas kept constant.

  In the digestion gas purification system in the digestion gas utilization system according to claim 5 of the present invention, since the first flow path switching means and the second flow path switching means are three-way valves, automation of flow path switching can be implemented reliably. It becomes.

  Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. 1 and 2 are flowcharts showing an overall configuration example of a digestion gas utilization system to which a digestion gas purification method and a purification apparatus according to Embodiment 1 of the present invention are applied. FIG. 3 is a flowchart for explaining the first embodiment of the digestion gas purification method and purification apparatus in the digestion gas utilization system exemplified in FIGS. 1 and 2.

  In this Embodiment 1, the digestion gas purification equipment is provided in the sewage treatment plant, and as shown in FIG. 1, the digestion tank 1 as an anaerobic fermentation tank and the digestion gas generated from this digestion tank 1 are desulfurized. The desulfurization tower 2 is provided, a gas holder 3 for temporarily storing the desulfurized digestion gas, and a digestion gas purification device 4 for purifying the digestion gas supplied from the gas holder 3.

  In addition, the digestion gas purification apparatus 4 constituting the digestion gas purification equipment does not need to be supplied with digestion gas via the desulfurization tower 2 and the gas holder 3, and as shown in FIG. The generated raw material digestion gas may be directly supplied and purified. Alternatively, the desulfurization tower 2 and the gas holder 3 shown in FIGS. 1 and 2 may be omitted.

  And the digestion gas utilization system is comprised by such digestion gas refinement | purification equipment and the following digestion gas consumption equipment. That is, the digestion gas consuming equipment is a purified gas comprising a hot water boiler 5 which is a digester heating equipment, a surplus gas utilization device 6, and a gas engine (cogeneration system) or a gas filling equipment (natural gas stand), for example. And a utilization device 9.

  For example, in the digestion gas utilization system illustrated in FIG. 1 or FIG. 2, desulfurization is performed instead of purifying the total amount of digestion gas obtained by heating in the digestion tank 1 and performing anaerobic fermentation. The digestion gas desulfurized by the tower 2 is temporarily stored in the gas holder 3. At the same time, a part of the digestion gas supplied from the gas holder 3 is used as fuel for a hot water boiler 5 as a digester heating device, and the hot water heated by the hot water boiler 5 is used as a heat exchanger 7. And used as a heating source for the heating medium enclosed in the heating jacket of the digester 1.

  Further, another part of the digestion gas is used as a fuel for the above-described surplus gas utilization device 6, and a digestion gas utilization system in which the remaining digestion gas is purified by the digestion gas purification device 4 is configured. The digestion gas purification apparatus 4 is supplied with the digested gas after desulfurization from the gas holder 3 through the flow path L1 as shown in FIG. 1, or alternatively, the digestion gas before desulfurization is supplied through the flow path L1 as shown in FIG. Supplied directly.

  And after refine | purifying as high concentration methane by this digestion gas refiner | purifier 4, it connects so that it may supply to the refined gas tank 8 via each flow path L2. The digestion gas purification method and the purification apparatus thereof according to the present invention relate to the digestion gas purification method and the purification apparatus 4 thereof in such a digestion gas utilization system.

  Next, the digestion gas purification method and its purification apparatus 4 according to Embodiment 1 of the present invention will be described with reference to FIG. In this digestion gas purification apparatus 4, digestion gas is supplied from the gas holder or digestion tank through the flow path L1, and after the mist (water droplets) and dust in the gas are removed by the mist separator 401, the gas connected in series. The pressure is increased to a predetermined pressure higher than the atmospheric pressure by the gas compressors 402a and 402b which are pressure increasing means.

  The digestion gas pressurized by the gas compressors 402 a and 402 b is introduced into the lower part of the absorption tower 404. On the other hand, the absorption tower 404 is circulated and supplied as described later in a state where water is pressurized from the upper part by an absorption water pump 408 which is a liquid booster.

  As described above, the digestion gas is boosted by the gas compressors 402a and 402b which are gas boosting means and fed into the absorption tower 404 from below, and the water is boosted by the absorption water pump 408 which is the liquid boosting means to absorb the absorption tower. The inside of the absorption tower 404 is maintained in a high pressure state that satisfies the range of 0.55 to 2.0 MPaG by feeding it into the upper part 404, and the digestion gas and water are maintained in the high pressure state that satisfies the pressure range in the absorption tower 404. To make contact. The absorption tower 404 is filled with a packing such as Raschig ring in order to bring the digestion gas and water into sufficient contact.

Thus, by holding the inside of the absorption tower 404 in a high pressure state satisfying the range of 0.55 to 2.0 MPaG, and bringing the digestion gas and water into contact in the absorption tower 404 in a high pressure state satisfying the pressure range. Carbon dioxide and sulfur impurities (H ₂ S, etc.) contained in the digestion gas in a gaseous state are dissolved and absorbed in high-pressure water, while methane is almost dissolved in high-pressure water. And is taken out from the top of the absorption tower 404.

  At the same time, the siloxane compound contained in the sewage sludge digestion gas is condensed from the gas state into a droplet state due to the high pressure state, and collides with the high pressure water flowing down inside the absorption tower in this droplet state. Then, it collects at the bottom of the absorption tower 404 together with water.

Thus, when refine | purifying sewage sludge digestion gas, it is preferable to make digestion gas and water contact in the high pressure state which satisfy | fills the range of 0.55-2.0 MPaG. In an atmosphere at a lower pressure than this range, carbon dioxide, sulfur-based impurities (H ₂ S, etc.) and siloxane compounds are not sufficiently separated and removed, and even in an atmosphere at a higher pressure than this range, carbon dioxide and sulfur-based impurities (H _{2 S,} etc.), without removal rate much improved siloxane compound, operating costs and is not preferred in view of increased cost of the apparatus due to high pressure specifications. In addition, it is more preferable to make digestion gas and water contact in the high-pressure state which satisfy | fills the range of 0.7 MPaG or more and less than 1.0 MPaG from the point of removal of a siloxane compound, an operating cost, and an apparatus cost.

The carbon dioxide and sulfur impurities (H ₂ S, etc.) separated and removed in this manner are dissolved, and water containing the separated siloxane compound is discharged as absorbed water from the bottom of the absorption tower 404, and the valve V10 is discharged. And introduced into the upper part of the stripping tower 412.

  Here, the flow path of the absorption water extracted from the absorption tower 404 and introduced into the diffusion tower 412 through the valve V10 absorbs the absorbed water through the diffusion tower 412 on the primary side of the diffusion tower 412. It is branched into a circulation channel L15a that circulates to the tower 404 and a drain channel L16 that drains the absorbed water out of the system. The branch channel is provided with a first channel switching means 421 for switching between the circulation channel L15a and the drain channel L16, and is configured to switch the channel according to conditions described later.

  In the diffusion tower 412, the absorption water extracted from the absorption tower 404 and passing through the circulation flow path L15a is introduced from the upper part, while the emission gas (for example, the atmosphere) is introduced from the lower part by the exhaust blower 413. Both are in countercurrent contact at atmospheric pressure. Thereby, the carbon dioxide, sulfur impurities and siloxane compound contained in the absorption water extracted from the absorption tower are transferred to the emission gas side and are discharged from the top of the emission tower 412 together with the emission gas. It is led to deodorizing equipment as needed via L14.

  At the same time, the absorbed water regenerated by removing carbon dioxide and the like in the stripping tower 412 is extracted from the bottom of the stripping tower 412. The absorption water flow path includes a circulation flow path L15b for repressurizing the absorption water by an absorption water pump 408 as a liquid pressure increase means and supplying the absorption water to the absorption tower 404, and a water supply flow for supplying absorption water from outside the system. Branches to the road L17a. The branch flow path is provided with a second flow path switching means 422 for switching between the circulation flow path L15b and the water supply flow path L17a, and is configured to switch the flow path according to conditions described later.

  When the second flow path switching means 422 is switched to the circulation flow path L15b side, the absorbed water is boosted again by the absorption water pump 408, which is a liquid boosting means, and passes through the circulation flow paths L15c and L15d to the absorption tower 404. Supplied. In addition, when the water supply channel L17a is switched, the absorption water is replenished from the water supply tank 406 by the water supply pump 407, the pressure is increased by the absorption water pump 408 which is a liquid pressure increasing means, and the circulation channels L15c and L15d are passed through. It is supplied to the absorption tower 404.

  Although the first flow path switching means 421 is shown in the example provided in the circulation flow path L15a on the primary side of the diffusion tower 412, the second flow path switching means on the circulation flow path L15b on the secondary side of the diffusion tower 412 is shown. 422 may be provided upstream.

  On the other hand, a water supply tank 406 for supplying absorbed water from outside the digestion gas purifier is provided with a water supply temperature detector 424 for detecting the temperature of the water supply. When the feed water temperature detected by the feed water temperature detector 424 is lower than a preset temperature preset in an arithmetic circuit in a controller (not shown), the first flow path switching means is generated by a command signal from the controller. 421 is operated to drain the absorbed water from the drainage channel L16 to the outside of the system, and the second channel switching means is operated to switch the absorbed water to the feed water from outside the digestion gas purification system (transient type). .

  As said preset temperature, it is preferable to set it as 25 degreeC, for example, and it is still more preferable to set it as 20 degreeC. The reason for this is that, based on the experimental data on the absorbed water temperature and the purified gas methane concentration, the purified gas methane concentration is approximately 97% by volume or higher when the absorbed water temperature is 25 ° C, and is approximately 98% by volume or higher when the absorbed water temperature is 20 ° C. Because.

  Conversely, when the feed water temperature detected by the feed water temperature detector 424 exceeds the set temperature, the first flow path switching means 421 and the second flow path switching means 422 are actuated by a command signal from the controller. The absorbed water is switched to the circulation flow paths L15a and L15b, respectively, and introduced into the absorbed water pump 408.

  The absorption water is boosted by the absorption water pump 408 and reaches the cooler 414 from the circulation flow path L15c. The heat exchanger 409 and the chiller 410 constituting the cooler 414 allow the chiller to move to the chiller. After the heat exchange with the brine from 410 is cooled to a predetermined temperature, it is supplied to the upper part of the absorption tower 404 via the circulation flow path L15d (circulation type).

  In the above, in order to maintain the quality of the absorption water supplied to the absorption tower 404, it is desirable to open the valve V13 periodically. As a result, a part of the absorbed water is extracted, and the extracted water is sent to the wastewater treatment facility via the flow path L13. When the amount of absorbed water becomes equal to or less than a predetermined amount due to this extraction, the water supply pump 407 opens the valve V14 on the water supply flow path L17b to supply the insufficient water from the water supply tank 406.

  The water used at this time includes treated water obtained by treating wastewater such as sewage, and in this embodiment, from the sand filtration facility of treated water provided downstream of the final sedimentation basin of the sewage treatment plant. The sand filtration water is used. It is also possible to use tap water or well water.

  On the other hand, the purified gas having a high concentration of methane taken out from the top of the absorption tower 404 is sent to the dehumidifier 405. In this embodiment, the dehumidifier 405 is a dehumidifier that adsorbs and removes moisture by a pressure swing adsorption method (PSA method), and uses synthetic zeolite as an adsorbent. The purpose of dehumidification by the dehumidifier 405 is to prevent condensation even at a pressure when the purified gas is used (utilized) as fuel.

  The pressure of the purified gas taken out from the absorption tower 404 is, for example, 0.9 MPaG (when the pressure in the absorption tower 404 is 0.9 MPaG), and the purified gas is used, for example, as fuel for a natural gas vehicle. The dew point is -60 ° C or less, more preferably -70 ° C or less, and particularly preferably -80 ° C or less in terms of the dew point at atmospheric pressure so that no dew condensation occurs even at a pressure of 19.6 MPaG. Dehumidification by the dehumidifier 405 is performed.

  As described above, the purified gas dehumidified by the dehumidifier 405 so as not to condense even at the pressure when used as fuel is sent to the purified gas tank 8 shown in FIG. 1 or 2 through the flow path L2. It is like that.

  As described above, the digestion gas purification method and the purification apparatus thereof in the digestion gas utilization system according to Embodiment 1 of the present invention obtain purified gas having a high concentration of methane by a high-pressure water absorption method from the absorption tower, and the digestion gas When the feed water temperature supplied from outside the gas purification system is low, the absorption water discharged from the absorption tower is drained outside the system, and when the feed water temperature is high, the feed water is cooled. After that, because the digestion gas purification method and its purification apparatus are switched to the circulation type to be supplied to the absorption tower, when the temperature is low, the digestion gas purification apparatus has a small amount of digestion gas that can be supplied to the digestion gas purification apparatus. Through the year, the digestion gas can be purified at a low cost, and the methane concentration of the purified gas obtained can be maintained constant.

  Next, a digestion gas purification method and a purification apparatus thereof in a digestion gas utilization system according to Embodiment 2 of the present invention will be described below with reference to FIG. FIG. 4 is a flowchart for explaining a digestion gas purification method and a purification apparatus according to Embodiment 2 in the digestion gas utilization system exemplified in FIGS. 1 and 2.

  The second embodiment of the present invention is different from the first embodiment in that the absorbed water discharged from the absorption tower 404 is post-treated in the decompression tank 411, and the rest is the same configuration. Therefore, the same components as those in the first embodiment are denoted by the same reference numerals, and different points will be described below.

  That is, in the digestion gas purification method and purification apparatus in the digestion gas utilization system according to Embodiment 2 of the present invention, as shown in the flowchart of FIG. 4, the absorbed water in which carbon dioxide is dissolved is fed from the bottom of the absorption tower 404. After being extracted, it is introduced into the decompression tank 411 through the valve V10. The pressure in the decompression tank 411 is reduced compared to that in the absorption tower 404.

For the purpose of increasing the methane recovery rate, the methane slightly dissolved in the absorption water extracted from the bottom of the absorption tower 404 is separated as a gas and gas is supplied from the top of the decompression tank 411 through the valve V11. The gas is returned to the intermediate stage of the gas compressors 402a and 402b constituting the pressure increasing means and merged with the digested gas from the gas compressor 402a, that is, recirculated to the digested gas whose pressure is increased in the front stage of the absorption tower. ing. Meanwhile, the methane is separated and recovered, and the absorbed water in which carbon dioxide and sulfur impurities (H ₂ S and the like) are dissolved and decompressed is supplied from the bottom of the decompression tank 411 to the first flow path switching means 421 through the valve V12. (If the first flow path switching means 421 is provided on the secondary side of the diffusion tower 412, it reaches the diffusion tower 412).

  And the temperature signal of the feed water temperature detected by the feed water temperature detector 424 is transmitted to the controller, and when the feed water temperature is lower than the preset temperature set by the arithmetic circuit built in the controller, In response to a command signal from the controller, the first flow path switching means 421 is operated to discharge the absorbed water from the drain flow path L16 to the outside of the system, and the second flow path switching means 422 is operated to supply the absorbed water. Switch to water supply from outside the system (one-time type). The absorbed water discharged from the drainage channel L16 to the outside of the system is gas-liquid separated in the gas-liquid separation tank 425, the separated gas is exhausted from the exhaust channel L18, and the liquid is drained from the drainage channel L19. .

  On the other hand, when the feed water temperature exceeds the preset set temperature, the first flow path switching means 421 is actuated by a command signal from the controller to introduce the absorbed water into the upper portion of the diffusion tower 412. In addition to switching to the path L15a side, the second flow path switching means 422 is operated, and finally the absorbed water supplied to the absorption tower 404 through the circulation flow paths L15b, L15c, and l15d is regenerated in the diffusion tower 412. Switch to the absorbed water (circulation type).

  In the above, when the absorption water discharged from the decompression tank 411 is discharged out of the system from the drainage channel L16 and not used as absorption water to the absorption tower 404 (transient type), or the absorption water 412 In the case of using the water as the absorption water of the absorption tower 404 (circulation type), the set temperature serving as a reference for switching the second flow path switching means 422 is selected as the reference temperature. This is the same as the set temperature of the one flow path switching means 421. This is because if the switching of the first flow path switching means 421 and the second flow path switching means 422 is not performed under the same conditions, an excess or shortage of absorbed water immediately occurs in the system, and the operation of the purification apparatus cannot be continued.

  In addition, when the flow path switching means 421 and 422 are switched temporarily, it is naturally preferable that the operation of the chiller 410 constituting the cooler is also stopped at the command of the controller. In this way, when the feed water temperature is low at a time when the outside air temperature is low and the amount of digestion gas that can be supplied to the digestion gas purification device 4 is low, the operation of the digestion gas purification device 4 is switched from the circulation type to the transient type. As a result, the operation of unnecessary equipment is stopped to reduce the power consumption, and the operation cost can be reduced throughout the year.

  The flow path switching means 421 and 422 are preferably three-way valves as shown in FIG. 3 and FIG. 4, but automatic open / close valves are provided in the drain flow path L16, the circulation flow paths L15a and L15b, and the water supply flow path L17a, respectively. The flow path switching means 421 and 422 may be configured, and the flow of the flow path may be switched by an opening / closing command from the controller.

  Further, such flow path switching is not automatically switched by a command from the controller based on the temperature signal detected by the feed water temperature detector 424. For example, when the water temperature is low from November to April. It may be configured to be a transient type, and manually switch over manual valves provided in the drainage flow path L16, the circulation flow paths L15a and L15b, and the water supply flow path L17a, respectively, during a period when the water temperature is high from May to October. .

  As described above, according to the digestion gas purification method and purification apparatus in the digestion gas utilization system according to the present invention, a purified gas having a high concentration of methane by a high-pressure water absorption method is obtained from an absorption tower, and from outside the digestion gas purification system. When the supplied water temperature is low, the absorption water discharged from the absorption tower is drained out of the system, and when the supply water temperature is high, the absorption water discharged from the absorption tower is discharged. Since it is switched to a circulation system that is regenerated and cooled and then supplied to the absorption tower, it is possible to purify the digestion gas at a low cost throughout the year and to maintain a constant methane concentration in the resulting purified gas.

  Further, according to the digestion gas purification method and the purification apparatus, after the carbon dioxide contained in the digestion gas is dissolved in absorption water and separated from the digestion gas in the absorption tower, the carbon dioxide is then removed. By introducing the dissolved absorption water into the vacuum tank, the methane gas dissolved in the reduced absorption water is separated, and the separated methane is refluxed to the digestion gas pressurized in the front stage of the absorption tower, Since the absorbed water after methane gas separation is introduced into a diffusion tower to remove dissolved carbon dioxide, the methane dissolved in the absorbed water is recovered again to improve the recovery rate of methane.

It is a flowchart which shows the example of whole structure of the digestion gas utilization system to which the digestion gas refinement | purification method and its refiner | purifier which concern on Embodiment 1 of this invention are applied. It is a flowchart which shows the other whole structure example of the digestion gas utilization system to which the digestion gas refinement | purification method and its refiner | purifier which concern on Embodiment 1 of this invention are applied. It is a flowchart for demonstrating Embodiment 1 of the digestion gas purification method and its refinement | purification apparatus in a digestion gas utilization system. It is a flowchart for demonstrating Embodiment 2 of the digestion gas purification method in the digestion gas utilization system, and its refinement | purification apparatus. It is a figure for demonstrating the digestion gas refinement | purification apparatus which concerns on a prior art (patent document 2), Comprising: It is a flowchart which shows the structure of the digestion gas refinement | purification apparatus which refine | purifies digestion gas and obtains refined methane gas.

Explanation of symbols

1 ... digester, 2 ... desulfurization tower, 3 ... gas holder,
4 ... digestion gas purifier, 5 ... digester heating equipment (hot water boiler),
6 ... surplus gas utilization device, 7 ... heat exchanger, 8 ... purified gas tank,
9 ... Purified gas utilization device,
401 ... mist separator,
402a, 402b ... Gas pressure raising means (gas compressor),
404 ... Absorption tower, 405 ... Dehumidifier, 406 ... Water tank,
407 ... Water supply pump, 408 ... Liquid pressure raising means (absorption water pump),
409 ... heat exchanger, 410 ... chiller, 411 ... decompression tank,
412 ... diffusion tower, 413 ... exhaust blower, 414 ... cooler,
421 ... first flow path switching means (three-way valve), 422 ... second flow path switching means (three-way valve),
424 ... Feed water temperature detector, 425 ... Gas-liquid separation tank

Claims (5)

The digestion gas obtained by heating the organic matter in the digestion tank and anaerobic fermentation is purified at least partly or entirely after subtracting the amount used as fuel for the digester heating equipment. In a digestion gas utilization system using gas as a fuel for other purposes, carbon dioxide contained in the digestion gas is dissolved in absorption water by bringing the pressurized digestion gas and the pressurized absorption water into contact with each other in an absorption tower. To obtain a purified gas having a high concentration of methane separated from the digestion gas, and when the feed water temperature supplied from outside the digestion gas purification system is lower than a preset temperature of 25 ° C. or less , the absorption through once to discharge the absorbed water discharged from the column out of the system, when the water-supply water temperature is higher than the set temperature, and dissolved carbon dioxide is discharged from the absorption tower absorption The dissolved carbon dioxide is removed by introducing the gas into the stripping tower, and the absorption water regenerated by removing the carbon dioxide is re-pressurized and cooled, and then switched to a circulation type that is supplied to the absorption tower. A digestion gas purification method in a digestion gas utilization system.   In the absorption tower in which the pressurized digestion gas and the pressurized absorption water are brought into contact with each other, carbon dioxide contained in the digestion gas is dissolved in absorption water and separated from the digestion gas. By introducing the dissolved absorption water into the vacuum tank, the methane gas dissolved in the reduced absorption water is separated, and the separated methane is refluxed to the digestion gas pressurized in the front stage of the absorption tower. The digestion gas refinement | purification method in the digestion gas utilization system of Claim 1 characterized by these. The digestion gas obtained by heating the organic matter in the digestion tank and anaerobic fermentation is purified at least partly or entirely after subtracting the amount used as fuel for the digester heating equipment. In a digestion gas utilization system that uses gas as a fuel for other purposes, a digestion gas purification device that obtains the purified gas comprises gas boosting means for boosting the digestion gas, and absorption water that absorbs carbon dioxide contained in the digestion gas. By bringing the liquid pressurizing means to pressurize, and the digestion gas pressurized by these pressurizing means and the absorbed water in contact with each other in the tower, carbon dioxide contained in the digestive gas is dissolved in the absorbed water, and from the digested gas. Absorption tower that obtains purified gas with high concentration of methane by separation, and stripping tower that removes dissolved carbon dioxide by introducing absorbed water in which this carbon dioxide is dissolved A circulation flow path for supplying absorption water extracted from the absorption tower to the absorption tower via the diffusion tower, and a water supply flow path supplied to the digestion gas purification apparatus from outside the digestion gas purification apparatus A drainage channel for draining the absorbed water out of the system; a first channel switching means provided on the primary side or secondary side of the diffusion tower for switching between the circulation channel and the drainage channel; In the suction side flow path of the liquid pressure boosting means for boosting the absorption water, the two of the diffusion towers are switched to switch between the absorption water circulation path from which the carbon dioxide has been removed and the water supply path supplied from outside the system. A second flow path switching means provided on the secondary side, a cooler for cooling the absorbed water in the circulation flow path, and a feed water flow supplied from outside the digestion gas purification apparatus to the digestion gas purification apparatus Water temperature detection that detects the temperature of this water supply on the road When provided a controller incorporating a control circuit for controlling the operation circuit and said first and second channel switching means for comparing said detected set the water coolant temperature in advance has been 25 ° C. below the set temperature, When the feed water temperature is lower than the set temperature by the controller, the first channel switching means is on the drain channel side, the second channel switching means is on the water channel side, and the feed water temperature is When the temperature is higher than the set temperature, the digestion gas purification apparatus in the digestion gas utilization system is configured to switch the first and second flow path switching means to the circulation flow path side.   A decompression tank for separating methane gas dissolved in the absorption water is provided in a circulation channel for circulating the absorption water extracted from the absorption tower, and the pressure of the separated gas is increased in the upstream stage of the absorption tower. The digestion gas refining apparatus in a digestion gas utilization system according to claim 3, wherein a reflux flow path for refluxing is provided in the gas pressurizing means.   The digestion gas purification apparatus in a digestion gas utilization system according to claim 3 or 4, wherein the first flow path switching means and the second flow path switching means are three-way valves.

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