Liquefaction system of carbon dioxide in biogas

Abstract

To provide a liquefaction system of carbon dioxide in biogas which can liquefy carbon dioxide contained in the biogas with low energy.SOLUTION: A liquefaction system 1 of carbon dioxide in biogas according to the present invention includes: a biogas generation facility 100; a liquified carbon dioxide drum 10 which reserves liquified carbon dioxide; a liquified carbon dioxide circulation line through which the liquified carbon dioxide circulates from the liquified carbon dioxide drum 10 as an origin; a liquified carbon dioxide pump 20 which is disposed on the liquified carbon dioxide circulation line; a liquified carbon dioxide cooler 30 which is disposed on the liquified carbon dioxide circulation line; an ejector 40 where, in the liquified carbon dioxide circulation line, the liquified carbon dioxide is used as driving fluid and the biogas released from the biogas generation facility 100 to a biogas receiving line is used as suction fluid; and a non-condensed gas discharge line 81 which outwardly discharges non-condensed gas in the liquified carbon dioxide drum 10.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a carbon dioxide liquefaction system for liquefying carbon dioxide, and more particularly to a carbon dioxide liquefaction system for biogas that can liquefy carbon dioxide contained in biogas with low energy consumption. [Background technology]

[0002] Under the Feed-in Tariff (FIT) system, the purchase price for electricity generated from biogas produced by methane fermentation facilities is guaranteed. However, the FIT system is based on the premise that the purchase price will be reduced in stages. Therefore, there may be cases where it is more viable to provide biogas in other forms than as electricity. For example, methane, which accounts for about 50% of biogas, could be highly purified and injected into gas pipelines to be used as city gas or various fuels. In this case, it becomes important to find a way to remove the carbon dioxide that makes up the remainder of the biogas as cheaply as possible.

[0003] In this regard, Patent Document 1 discloses a technology in which biogas and absorbing liquid are mixed into a gas-liquid mixed phase state in a mixer 5 consisting of an ejector or the like, methane is almost completely separated and recovered from this mixed liquid by a first gas-liquid separator 7, CO2 absorbing liquid is supplied to a membrane module 10 to separate carbon dioxide, and the absorbing liquid after carbon dioxide separation is recovered in an absorbing liquid storage tank. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2007-297605 Summary of the Invention [Problem to be solved by the invention]

[0005] However, the technology in Patent Document 1 only produces low-pressure carbon dioxide gas and does not take into consideration how it can be used further. For example, to transport large amounts of carbon dioxide over long distances, a liquid state is desirable, but liquefying low-pressure carbon dioxide gas requires a great deal of energy.

[0006] The present invention has been made to solve such problems, and aims to provide a system for liquefying carbon dioxide contained in biogas, which can liquefy the carbon dioxide contained in biogas with low energy consumption. [Means for solving the problem]

[0007] (1) The system for liquefying carbon dioxide in biogas according to the present invention comprises: a biogas generation facility that generates biogas containing methane and carbon dioxide by methane fermentation; a liquefied carbon dioxide drum for storing the liquefied carbon dioxide; a liquefied carbon dioxide circulation line through which liquefied carbon dioxide circulates, the liquefied carbon dioxide drum being a starting point; a liquefied carbon dioxide pump that is disposed downstream of the liquefied carbon dioxide drum in the liquefied carbon dioxide circulation line and that pressurizes and discharges the liquefied carbon dioxide stored in the liquefied carbon dioxide drum; a liquefied carbon dioxide cooler that is disposed in the liquefied carbon dioxide circulation line and cools the liquefied carbon dioxide discharged by the liquefied carbon dioxide pump by heat exchange with a refrigerant; an ejector that is disposed downstream of the liquefied carbon dioxide pump in the liquefied carbon dioxide circulation line, and that uses the liquefied carbon dioxide discharged by the liquefied carbon dioxide pump as a driving fluid and the biogas discharged from the biogas generation facility to a biogas receiving line as a suction fluid; and a non-condensable gas discharge line for discharging non-condensable gas from the gas layer of the liquefied carbon dioxide drum to the outside.

[0008] (2) Furthermore, in the above (1), it is characterized by having a biogas precooler that is arranged in the biogas receiving line and precools the biogas by the non-condensable gas flowing through the non-condensable gas discharge line.

[0009] (3) Furthermore, in the above (1) or (2), the biogas receiving system is characterized in that it comprises a carbon dioxide separator disposed in the non-condensable gas discharge line for separating carbon dioxide from the non-condensable gas, and a carbon dioxide supply line for supplying the carbon dioxide gas separated by the carbon dioxide separator to the biogas receiving line. [Effects of the Invention]

[0010] According to the present invention, carbon dioxide contained in biogas can be liquefied with low energy consumption. [Brief explanation of the drawings]

[0011] [Figure 1] 1 is a schematic diagram showing a carbon dioxide liquefaction system according to a first embodiment of the present invention. [Figure 2] FIG. 4 is a schematic diagram showing a carbon dioxide liquefaction system according to a second embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0012] [Embodiment 1] First, the configuration and functions of the first embodiment of the present invention will be described with reference to FIG. The carbon dioxide liquefaction system 1 according to the first embodiment of the present invention includes a liquefied carbon dioxide drum 10, a liquefied carbon dioxide pump 20, a liquefied carbon dioxide cooler 30, an ejector 40, a biogas cooler 50, and a biogas generation facility 100.

[0013] The liquefied carbon dioxide drum 10 is a vertical cylindrical drum that temporarily stores the liquefied carbon dioxide received from the ejector outlet line 74 .

[0014] The liquefied carbon dioxide pump 20 is a centrifugal pump that receives liquefied carbon dioxide temporarily stored in the liquefied carbon dioxide drum 10 from a liquefied carbon dioxide pump inlet line 71, pressurizes the liquefied carbon dioxide, and discharges it to a liquefied carbon dioxide pump outlet line 72. The amount of pressure increase by the liquefied carbon dioxide pump 20 corresponds to the pressure loss in the system including the liquefied carbon dioxide cooler 30 and the ejector 40.

[0015] The liquefied carbon dioxide cooler 30 is a typical shell-and-tube heat exchanger, and cools the liquefied carbon dioxide received from the liquefied carbon dioxide pump outlet line 72 using a low-temperature refrigerant. The refrigerant is supplied from a refrigerant supply line 76 and is discharged to a refrigerant discharge line 77 after heat exchange. In FIG. 1, the liquefied carbon dioxide cooler 30 is disposed downstream of the liquefied carbon dioxide pump 20, but it may be disposed upstream of the liquefied carbon dioxide pump 20.

[0016] The ejector 40 is a general ejector, which creates a reduced pressure state using liquefied carbon dioxide supplied from an ejector inlet line 73 as a driving fluid, and receives carbon dioxide-containing gas from a biogas receiving line 75 .

[0017] The biogas cooler 50 is a typical shell-and-tube heat exchanger, which cools the biogas flowing through the biogas receiving line 75 by heat exchange with the non-condensable gas in the liquefied carbon dioxide drum 10 .

[0018] The biogas generation facility 100 includes a raw material tank 110 , a crushing and sorting machine 120 , a methane fermentation tank 130 , a desulfurization facility 140 , a gas holder 150 , and a dryer 160 .

[0019] Raw material tank 110 is a vertical cylindrical drum that receives organic waste from raw material receiving line 171, mixes it, and discharges it to mixed raw material discharge line 172. Crushing and sorting machine 120 is a crushing and sorting machine equipped with rotary blades and a screen, and crushes the mixed raw material received from mixed raw material discharge line 172 into small pieces, passes organic materials suitable for fermentation through the screen, and discharges them to fermentation raw material discharge line 173, while materials unsuitable for fermentation, such as packaging paper, are discharged to the outside via a line not shown.

[0020] The methane fermentation tank 130 is a vertical cylindrical drum in which the fermentation material received from the fermentation material discharge line 173 is retained for a certain period of time, and methane bacteria decompose the organic matter to produce methane, carbon dioxide, and water. The liquid water is discharged through a line not shown. The methane, carbon dioxide, and water vapor are discharged as gases through the raw biogas discharge line 174.

[0021] The desulfurization equipment 140 has a cylindrical container filled with a desulfurization agent such as iron oxide, and desulfurizes the biogas by adsorbing the hydrogen sulfide contained in the biogas as iron sulfide. The desulfurized biogas is discharged to the desulfurized biogas discharge line 175.

[0022] The gas holder 150 is a dome-shaped container that temporarily stores the desulfurized biogas received from the desulfurized biogas discharge line 175. The stored desulfurized biogas is discharged from the desulfurized biogas supply line 176.

[0023] The dryer 160 has a cylindrical container containing a zeolite-based moisture absorbent, and dehumidifies by adsorbing the water vapor contained in the desulfurized biogas received from the desulfurized biogas supply line 176. The dehumidified biogas is discharged to the biogas receiving line 75.

[0024] Next, the operation of the carbon dioxide liquefaction system 1 according to this embodiment will be described. The carbon dioxide liquefaction system 1 circulates liquefied carbon dioxide within the system by operating the liquefied carbon dioxide pump 20 . Specifically, the liquefied carbon dioxide in the liquefied carbon dioxide drum 10 is sent to the liquefied carbon dioxide pump inlet line 71, passes through the liquefied carbon dioxide pump 20, the liquefied carbon dioxide pump outlet line 72, the liquefied carbon dioxide cooler 30, the ejector inlet line 73, the ejector 40 and the ejector outlet line 74, and is returned to the liquefied carbon dioxide drum 10. Therefore, the above-mentioned devices and lines constitute the liquefied carbon dioxide circulation line of the present invention.

[0025] In the liquefied carbon dioxide drum 10, the liquefied carbon dioxide is stored at, for example, 0.9 MPaG and −50° C., and is pressurized by the liquefied carbon dioxide pump 20 to, for example, 1.6 MPaG.

[0026] The liquefied carbon dioxide pressurized by the liquefied carbon dioxide pump 20 is cooled in the liquefied carbon dioxide cooler 30 to a temperature range where it will not solidify, for example, -53°C, by a refrigerant supplied by cold heat generation equipment (e.g., a refrigerator) not shown.

[0027] The liquefied carbon dioxide cooled by the liquefied carbon dioxide cooler 30 is supplied to the ejector 40, and flows as a high-speed flow through the small diameter portion inside the ejector 40, thereby creating a reduced pressure state using Bernoulli's theorem.

[0028] As a result, biogas is supplied to the ejector 40 from the biogas receiving line 75. The composition of the biogas is, for example, 50% carbon dioxide and 50% methane.

[0029] Inside the ejector 40, the biogas supplied from the gas receiving line 75 mixes with the high-speed flow of liquefied carbon dioxide. As the diameter inside the ejector 40 expands and the flow rate decreases, the pressure recovers to, for example, 0.9 MPaG, and the carbon dioxide mixed at a ratio of 50% is compressed to a partial pressure of 0.45 MPaG, cooled, and condensed. In addition, methane gas is compressed and cooled as the pressure recovers.

[0030] At this time, the carbon dioxide and methane gases are compressed while coming into contact with the liquefied carbon dioxide and being cooled, and therefore the compression process of these gases is close to isothermal compression, resulting in high compression efficiency.

[0031] In the downstream of the ejector 40, liquefied carbon dioxide flows through the ejector outlet line 74 towards the liquefied carbon dioxide drum 10, but if there is carbon dioxide gas that has not been completely condensed inside the ejector 40, this will also be included, and uncondensed methane will also be included in the gaseous state, resulting in a multiphase flow.

[0032] The liquefied carbon dioxide that has flowed into the liquefied carbon dioxide drum 10 becomes saturated at 0.9 MPaG and separates into a gas layer and a liquid layer. The amount of carbon dioxide held in the system increases as carbon dioxide gas is supplied from the ejector outlet line 74. As a result, liquefied carbon dioxide is discharged from the liquefied carbon dioxide drum 10 to the outside via the liquefied carbon dioxide discharge line 78 in an amount corresponding to the supplied carbon dioxide gas.

[0033] Furthermore, since methane gas does not condense, it accumulates in the gas layer of the liquefied carbon dioxide drum 10. For this reason, the gas layer of the liquefied carbon dioxide drum 10 is discharged to the outside of the drum by the non-condensed gas discharge flow rate control valve 83.

[0034] The non-condensable gas discharged from the liquefied carbon dioxide drum 10 is supplied to the biogas precooler 50 through the non-condensable gas discharge line 81, where the biogas is cooled and then discharged to the outside.

[0035] According to the carbon dioxide liquefaction system 1 of this embodiment, the reduced pressure created by the ejector 40 can be used to take in biogas and liquefy the carbon dioxide, thereby reducing the energy required to separate the carbon dioxide.

[0036] Furthermore, according to the carbon dioxide liquefaction system 1 of this embodiment, the biogas can be compressed during the pressure recovery process downstream of the ejector 40, so the compressor power required to pressurize the carbon dioxide can be reduced to zero or significantly reduced.

[0037] Furthermore, according to the carbon dioxide liquefaction system 1 of this embodiment, the pressure recovery process downstream of the ejector 40 described above is carried out while in contact with low-temperature liquefied carbon dioxide, which is close to isothermal compression, allowing for efficient compression.

[0038] In this embodiment, the desulfurization equipment 140 is described as being filled with iron oxide, but the present invention is not limited to this, and a biological desulfurization equipment using sulfur-oxidizing bacteria or the like may also be used.

[0039] [Embodiment 2] Next, the configuration and functions of the second embodiment will be described with reference to FIG. The same numbers are used for components having the same configurations and functions as those in the first embodiment. The carbon dioxide liquefaction system 200 according to the second embodiment has a carbon dioxide separation membrane 210 that separates carbon dioxide from non-condensed gases.

[0040] The carbon dioxide separation membrane 210 is a membrane separation unit formed by stacking carbon dioxide separation membranes 210 made of a polymer material, and preferentially permeates carbon dioxide from the non-condensed gas supplied from the carbon dioxide separation membrane inlet line 82 and sends it to the permeated carbon dioxide supply line 272.

[0041] Next, the operation of the carbon dioxide liquefaction system 200 according to this embodiment will be described. The explanation of the same functions as those in the first embodiment will be omitted. Inside the liquefied carbon dioxide drum 10, methane accumulates in the gas layer of the liquefied carbon dioxide drum 10, causing an increase in internal pressure. For this reason, the non-condensable gas discharge flow rate control valve 83 is used to discharge the non-condensable gas from the gas layer of the liquefied carbon dioxide drum 10 while adjusting the pressure and flow rate.

[0042] During this process, of the low-temperature methane and carbon dioxide supplied to the carbon dioxide separation membrane 210 , carbon dioxide preferentially permeates with a pressure difference of, for example, about 100 kPa, and is discharged to the permeated carbon dioxide supply line 272 .

[0043] The high-concentration carbon dioxide gas discharged to the permeated carbon dioxide supply line 272 has its flow rate adjusted and its pressure reduced by the permeated carbon dioxide flow control valve 273, and is then merged with the biogas flowing in the biogas receiving line 75 and supplied to the ejector 40. The gas that did not permeate the carbon dioxide separation membrane 210 contains less carbon dioxide and an even higher concentration of methane, and is then discharged to the methane discharge line 271.

[0044] At this time, the pressure is reduced from about 0.9 MPaG to about atmospheric pressure in the permeated carbon dioxide flow control valve 273, and the temperature drops by several degrees Celsius to several tens of degrees Celsius due to the Joule-Thomson effect that accompanies the pressure reduction.

[0045] According to the carbon dioxide liquefaction system 200 of this embodiment, carbon dioxide can be separated from the non-condensed gas discharged from the liquefied carbon dioxide drum 10 and supplied to the ejector again, thereby maximizing the efficiency of separation, recovery, and liquefaction of carbon dioxide from biogas.

[0046] Furthermore, according to the carbon dioxide liquefaction system 200 of this embodiment, carbon dioxide can be separated from the non-condensable gas discharged from the liquefied carbon dioxide drum 10, and the highly concentrated methane gas can be discharged to the outside at a pressure equivalent to the liquefaction pressure of the carbon dioxide in the liquefied carbon dioxide drum 10, thereby minimizing the power required to boost pressure when used in city gas pipelines or as fuel for gas turbines.

[0047] In this embodiment, the carbon dioxide separation membrane 210 is exemplified as an example of a carbon dioxide separation device, which allows carbon dioxide to preferentially permeate from non-condensable gas, which is a mixed gas of methane and carbon dioxide. However, the carbon dioxide separation device of the present invention is not limited to a carbon dioxide separation membrane as long as it can separate carbon dioxide and methane. For example, it may be a PSA (pressure swing adsorption) device filled with a methane adsorbent or a TSA (temperature swing adsorption) device.

[0048] Furthermore, in the present embodiment, it has been described that the high-concentration carbon dioxide gas that has permeated through the carbon dioxide separation membrane 210 is decompressed to approximately atmospheric pressure and supplied to the ejector 40, but an ejector that can receive high-concentration carbon dioxide gas at a maintained pressure without decompressing it and a circulation line may be provided separately. In this case, if the pressure of the suction fluid is high, the differential pressure of the ejector can be reduced, and therefore the total head of the pump corresponding to the ejector can be reduced, thereby reducing power costs. [Industrial Applicability]

[0049] The present invention can be used as a system for liquefying carbon dioxide contained in biogas, which can liquefy carbon dioxide contained in biogas with low energy consumption. [Explanation of symbols]

[0050] 1. Carbon dioxide liquefaction system 10 liquefied carbon dioxide drums 20 Liquefied carbon dioxide pump 30 Liquid carbon dioxide cooler 40 Ejector 50 Biogas Cooler 71 Liquefied carbon dioxide pump inlet line 72 Liquefied carbon dioxide pump outlet line 73 Ejector inlet line 74 Ejector outlet line 75 Biogas receiving line 76 Refrigerant supply line 77 Refrigerant discharge line 78 Liquefied carbon dioxide delivery line 81 Non-condensable gas discharge line 82 Carbon dioxide separation membrane inlet line 83 Non-condensable gas discharge flow control valve 100 Biogas generation equipment 110 Raw material tank 120 Crushing and sorting machine 130 Methane fermentation tank 140 Desulfurization equipment 150 Gas Holder 160 Dryer 171 Raw material receiving line 172 Mixed raw material delivery line 173 Fermentation raw material delivery line 174 Untreated biogas discharge line 175 Desulfurized biogas output line 176 Desulfurized biogas supply line 200 Carbon dioxide liquefaction system (embodiment 2) 210 Carbon dioxide separation membrane 271 Methane Discharge Line 272 Permeate carbon dioxide supply line 273 Permeation carbon dioxide flow control valve

Claims

1. a biogas generation facility that generates biogas containing methane and carbon dioxide through methane fermentation; a liquefied carbon dioxide drum for storing the liquefied carbon dioxide; a liquefied carbon dioxide circulation line through which liquefied carbon dioxide circulates, the liquefied carbon dioxide drum being a starting point; a liquefied carbon dioxide pump that is disposed downstream of the liquefied carbon dioxide drum in the liquefied carbon dioxide circulation line and that pressurizes and discharges the liquefied carbon dioxide stored in the liquefied carbon dioxide drum; a liquefied carbon dioxide cooler that is disposed in the liquefied carbon dioxide circulation line and cools the liquefied carbon dioxide discharged by the liquefied carbon dioxide pump by heat exchange with a refrigerant; an ejector that is disposed downstream of the liquefied carbon dioxide pump in the liquefied carbon dioxide circulation line, and that uses the liquefied carbon dioxide discharged by the liquefied carbon dioxide pump as a driving fluid and the biogas discharged from the biogas generation facility to a biogas receiving line as a suction fluid; a non-condensable gas discharge line that discharges non-condensable gas from the gas layer of the liquefied carbon dioxide drum to the outside.

2. 2. The system for liquefying carbon dioxide in biogas according to claim 1, further comprising: a biogas precooler disposed in the biogas receiving line and precooling the biogas with non-condensable gas flowing through the non-condensable gas discharge line.

3. 3. The system for liquefying carbon dioxide in biogas according to claim 1, further comprising: a carbon dioxide separator disposed in the non-condensable gas discharge line for separating carbon dioxide from the non-condensable gas; and a carbon dioxide supply line for supplying the carbon dioxide gas separated by the carbon dioxide separator to the biogas receiving line.

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