Carbon dioxide fixation method and carbon dioxide fixation device, and method for evaluating amount of carbon dioxide fixed and system for evaluating amount of carbon dioxide fixed

Abstract

The present invention addresses the problem of providing: a carbon dioxide fixation method and a carbon dioxide fixation device with which it is possible to promote the growth of a carbonate crystal while improving carbonate precipitation reaction efficiency when fixing carbon dioxide through a carbonate precipitation reaction; and a method for evaluating the amount of carbon dioxide fixed and a system for evaluating the amount of carbon dioxide fixed which are capable of appropriately evaluating the amount of carbon dioxide fixed.　To solve the problem, provided are: a carbon dioxide fixation method comprising a concentration step for concentrating a part of water to be treated containing carbon dioxide and alkaline earth metal ions to generate concentrated water, and a mixing step for mixing the concentrated water with the water to be treated; and a carbon dioxide fixation device for performing said method.　According to the present invention, when fixing carbon dioxide through a carbonate precipitation reaction, it is possible to promote the growth of a carbonate crystal while improving carbonate precipitation reaction efficiency, by means of simple operations such as concentration and mixing.

Description

Carbon dioxide fixation method, carbon dioxide fixation device, and carbon dioxide fixation amount evaluation method and carbon dioxide fixation amount evaluation system

[0001] The present invention relates to a carbon dioxide fixation method and apparatus, and a method and system for evaluating the amount of fixed carbon dioxide. More specifically, the present invention relates to a carbon dioxide fixation method and apparatus for fixing carbon dioxide as a carbonate, and a method and system for evaluating the amount of fixed carbon dioxide that quantifies the amount of carbon dioxide fixed as a carbonate.

[0002] In recent years, reducing carbon dioxide emissions into the environment has become an urgent issue, as it is believed to have a major impact on environmental problems such as global warming. To address this issue, research is being conducted into technologies to reduce carbon dioxide emissions themselves, as well as technologies to capture and fix emitted carbon dioxide.

[0003] In particular, various methods have been studied as technologies related to the capture and fixation of carbon dioxide. For example, methods for recovering carbon dioxide from carbon dioxide-containing gases include a chemical absorption method in which carbon dioxide is dissolved in an absorption liquid such as monoethanolamine, a physical adsorption method in which carbon dioxide is adsorbed onto an adsorbent having gas adsorption capacity, and a membrane separation method using a membrane. In addition to these methods, a carbonate fixation method in which carbon dioxide is converted into carbonate by a chemical reaction is also known. Here, alkaline earth metal ions (divalent metal ions) are known as components that react with carbon dioxide to form carbonate.

[0004] For example, Patent Document 1 describes a method for immobilizing carbon dioxide, in which carbon dioxide is supplied to an aqueous solution in which specific blast furnace slag and alkali are mixed, and calcium eluted from the blast furnace slag is reacted with carbon dioxide to produce carbonate.

[0005] Japanese Patent Application Laid-Open No. 2017-214262

[0006] The carbonate fixation method has the advantage that the reaction related to carbonate precipitation proceeds spontaneously, so there is no need to supply energy from an external source for carbon dioxide fixation, and the produced carbonate can be stored stably for a long period of time. Therefore, in efforts to reduce carbon dioxide emissions into the environment, or so-called decarbonization, it is important to develop a technology that allows the reaction related to carbonate precipitation to proceed simply, efficiently, and at low cost.

[0007] In addition to chemical carbon dioxide emission reduction methods such as carbon capture and fixation, carbon pricing, a method of encouraging carbon dioxide emission reduction by pricing (taxing) emitted carbon dioxide and certifying and trading the value of carbon dioxide reduction, is gaining attention in decarbonization efforts. The application of carbon pricing requires information on the extent to which carbon dioxide emissions have been reduced. In other words, it is necessary to quantify and evaluate the amount of carbon dioxide that has been fixed. Specifically, in the case of the carbonate fixation method, which produces carbonate by reacting alkaline earth metal ions with carbon dioxide, evaluating the amount of carbon dioxide fixation requires promoting the growth of carbonate crystals so that the produced carbonate can be accurately measured as a solid.

[0008] Therefore, an object of the present invention is to provide a carbon dioxide fixation method and a carbon dioxide fixation device that can improve the efficiency of the carbonate precipitation reaction and promote the growth of carbonate crystals when carbon dioxide is fixed by the carbonate precipitation reaction, and to provide a method and system for evaluating the amount of carbon dioxide fixation that enable appropriate evaluation of the amount of carbon dioxide fixation.

[0009] As a result of extensive research into the above-mentioned problems, the present inventors have discovered that, when performing a carbonate precipitation reaction for carbon dioxide fixation using water to be treated that contains carbon dioxide and alkaline earth metal ions, mixing water to be treated that has been subjected to a concentration treatment with water to be treated that has not been subjected to a concentration treatment can improve the reaction efficiency in the carbonate precipitation reaction and promote crystal growth, and also improve the accuracy in quantifying the amount of carbon dioxide fixed in this process, enabling appropriate evaluation of the amount of carbon dioxide fixation, thereby completing the present invention. Specifically, the present invention provides the following carbon dioxide fixation method and carbon dioxide fixation apparatus, and method and system for evaluating the amount of carbon dioxide fixation.

[0010] The carbon dioxide fixation method of the present invention, which solves the above problems, is characterized by comprising a concentration step of concentrating a portion of water to be treated containing carbon dioxide and alkaline earth metal ions to produce concentrated water, and a mixing step of mixing the concentrated water with the water to be treated. As a result of extensive research, the inventors have discovered that in a carbonate fixation method for immobilizing carbon dioxide by reacting carbon dioxide with alkaline earth metal ions to produce carbonate, a concentration treatment is performed to concentrate a portion of the water to be treated when a carbonate precipitation reaction is carried out using the water to be treated containing carbon dioxide and alkaline earth metal ions, and the resulting solution (concentrated water) is mixed with water to be treated that has not been subjected to concentration treatment. This results in uneven distribution of the concentrations of components involved in carbonate formation in the solution, thereby improving the reaction efficiency of carbonate precipitation and promoting crystal growth. The carbon dioxide fixation method of the present invention is based on this discovery and can be provided as a carbon dioxide fixation method that can improve the efficiency of the carbonate precipitation reaction and promote carbonate crystal growth through simple operations such as concentration and mixing during carbon dioxide fixation by the carbonate precipitation reaction.

[0011] Furthermore, one embodiment of the carbon dioxide fixation method of the present invention is characterized in that carbonate seed crystals are generated in the concentration step. According to this feature, the carbonate seed crystals generated in the concentration step can be used in the subsequent mixing step, which not only improves the efficiency of the carbonate precipitation reaction but also makes it easier to further promote the growth of carbonate crystals.

[0012] In one embodiment of the carbon dioxide fixation method of the present invention, the concentration step is characterized by using a semipermeable membrane. According to this feature, concentration is performed by permeation using a semipermeable membrane, so the concentration process can proceed with less energy than when a solution is concentrated by heating and evaporating it, making it possible to fix carbon dioxide simply and at low cost.

[0013] Furthermore, one embodiment of the carbon dioxide fixation method of the present invention is characterized in that in the mixing step, stirring is performed to promote carbonate crystal growth. This feature further increases the reaction efficiency of the carbonate precipitation reaction that proceeds in the mixing step, and makes it easier to further promote the growth of carbonate crystals.

[0014] Furthermore, one embodiment of the carbon dioxide fixation method of the present invention is characterized in that the stirring in the mixing step is performed in an atmospheric air. According to this feature, carbon dioxide, which is one of the components involved in the carbonate precipitation reaction that proceeds in the mixing step, is supplied from the atmosphere as a result of the stirring, thereby making it possible to suppress a decrease in the reaction efficiency of the carbonate precipitation reaction and to further promote the growth of carbonate crystals.

[0015] The carbon dioxide fixation apparatus of the present invention, which solves the above problems, is characterized by comprising a concentrating section that concentrates a portion of water to be treated containing carbon dioxide and alkaline earth metal ions to produce concentrated water, and a mixing section that mixes the concentrated water with the water to be treated. The carbon dioxide fixation apparatus of the present invention is based on the finding that, in a carbonate fixation method in which carbon dioxide and alkaline earth metal ions are reacted to produce carbonates, concentrating a portion of the water to be treated containing carbon dioxide and alkaline earth metal ions and mixing this concentrated liquid (concentrated water) with water to be treated that has not been subjected to concentration treatment can improve the reaction efficiency of carbonate precipitation and promote crystal growth. The present invention can be provided as a carbon dioxide fixation apparatus that can improve the efficiency of the carbonate precipitation reaction and promote carbonate crystal growth through simple operations such as concentration and mixing.

[0016] The present invention provides a method for evaluating the amount of fixed carbon dioxide, which solves the above-mentioned problems, and is characterized by comprising a concentration step of concentrating a portion of water to be treated containing carbon dioxide and alkaline earth metal ions to produce concentrated water, a mixing step of mixing the concentrated water with the water to be treated, and a measurement step of measuring the amount of carbonate produced in the mixing step. The method for evaluating the amount of fixed carbon dioxide of the present invention can measure the amount of carbonate produced through a step that can improve the efficiency of the carbonate precipitation reaction and promote the growth of carbonate crystals, with respect to the amount of carbon dioxide fixed in a carbonate fixation method in which carbon dioxide and alkaline earth metal ions are reacted to produce carbonate. This enables simple quantification of the amount of fixed carbon dioxide and appropriate evaluation of the amount of fixed carbon dioxide.

[0017] The present invention provides a system for evaluating the amount of fixed carbon dioxide, which solves the above-mentioned problems, and is characterized by comprising: a concentration section that concentrates a portion of water to be treated that contains carbon dioxide and alkaline earth metal ions to produce concentrated water; a mixing section that mixes the concentrated water with the water to be treated; and a measurement section that measures the amount of carbonate produced in the mixing section. With regard to the amount of carbon dioxide fixed in a carbonate fixation method in which carbon dioxide reacts with alkaline earth metal ions to produce carbonate, the system can measure the amount of carbonate produced through a process that enables improved carbonate precipitation reaction efficiency and promotion of carbonate crystal growth. This enables simple quantification of the amount of fixed carbon dioxide and appropriate evaluation of the amount of fixed carbon dioxide.

[0018] The present invention makes it possible to provide a carbon dioxide fixation method and a carbon dioxide fixation device that can improve the efficiency of the carbonate precipitation reaction and promote the growth of carbonate crystals when fixating carbon dioxide through a carbonate precipitation reaction, as well as a carbon dioxide fixation amount evaluation method and a carbon dioxide fixation amount evaluation system that enable appropriate evaluation of the amount of carbon dioxide fixation.

[0019] FIG. 1 is a flow diagram of a carbon dioxide fixation method according to an embodiment of the present invention. FIG. 2 is a schematic explanatory diagram of a carbon dioxide fixation apparatus according to an embodiment of the present invention. FIG. 3 is a flow diagram of a method for evaluating the amount of fixed carbon dioxide according to an embodiment of the present invention. FIG. 4 is a schematic explanatory diagram of a system for evaluating the amount of fixed carbon dioxide according to an embodiment of the present invention. Photographs showing the state of solutions obtained in the concentration step as examples and comparative examples according to an embodiment of the present invention. (A) shows concentrated water, (B) shows permeate, and (C) shows water to be treated. Photographs showing the state of a solution obtained through the concentration step and the mixing step as an example according to an embodiment of the present invention, and the state of carbonates in the solution. (A) shows the solution, and (B) shows carbonates recovered from the solution by filtration. Photographs showing the state of a solution obtained through only the concentration step as a comparative example according to an embodiment of the present invention, and the state of carbonates in the solution. (A) shows the solution, and (B) shows carbonates recovered from the solution by filtration.

[0020] Hereinafter, embodiments of the carbon dioxide fixation method, carbon dioxide fixation device, and carbon dioxide fixation amount evaluation method and carbon dioxide fixation amount evaluation system according to the present invention will be described in detail. Note that the carbon dioxide fixation method, carbon dioxide fixation device, carbon dioxide fixation amount evaluation method and carbon dioxide fixation amount evaluation system described in the embodiments are merely examples for the purpose of explaining the carbon dioxide fixation method, carbon dioxide fixation device, carbon dioxide fixation amount evaluation method and carbon dioxide fixation amount evaluation system according to the present invention, and the present invention is not limited to these.

[0021] In the present invention, "carbon dioxide" refers to gaseous carbon dioxide (CO ２ ), as well as carbonic acid (H ２ CO ３ ), bicarbonate ion (HCO ３ － ), carbonate ions (CO ３ ２－ ) In the following, when referring specifically to gaseous carbon dioxide, the term "carbon dioxide" will be used.

[0022] [Carbon Dioxide Fixation Method and Carbon Dioxide Fixation Apparatus] First, an embodiment of the carbon dioxide fixation method and carbon dioxide fixation apparatus according to the present invention will be described. The carbon dioxide fixation method and carbon dioxide fixation apparatus of the present invention are based on the carbonate fixation method, and when a carbonate precipitation reaction is carried out using water to be treated containing carbon dioxide and alkaline earth metal ions to immobilize carbon dioxide, the reaction efficiency of the carbonate precipitation reaction from the water to be treated containing carbon dioxide and alkaline earth metal ions is improved, while promoting the growth of the resulting carbonate crystals. More specifically, the carbon dioxide fixation method and carbon dioxide fixation apparatus of the present invention are based on the finding that, when performing a carbonate fixation method in which carbon dioxide reacts with alkaline earth metal ions to produce carbonate, concentrating a portion of the water to be treated containing carbon dioxide and alkaline earth metal ions and mixing this concentrated liquid (concentrated water) with water to be treated that has not been subjected to concentration can improve the reaction efficiency of carbonate precipitation and promote the growth of carbonate crystals. The present invention provides a carbon dioxide fixation method and carbon dioxide fixation apparatus that can improve the carbonate precipitation reaction efficiency and promote the growth of carbonate crystals through simple operations such as concentration and mixing.

[0023] (Carbon dioxide fixation method) Fig. 1 is a flow diagram showing each operation (step) involved in a carbon dioxide fixation method in an embodiment of the present invention. As shown in Fig. 1, the carbon dioxide fixation method in this embodiment includes a concentration step in which concentrated water is produced by concentrating a portion of the water to be treated that contains carbon dioxide and alkaline earth metal ions, and a mixing step in which the concentrated water is mixed with the water to be treated. Each operation (step) involved in the carbon dioxide fixation method in this embodiment will be described below.

[0024] First, the source of the water to be treated that is applied to the carbon dioxide fixation method of this embodiment is not particularly limited, as long as it is an aqueous solution containing carbon dioxide and alkaline earth metal ions. For example, the water may be an aqueous solution in which substances that become carbon dioxide gas or alkaline earth metal ions are artificially dissolved, or an aqueous solution in which carbon dioxide and alkaline earth metal ions are dissolved from the beginning. Specific examples of the water to be treated in this embodiment include seawater, river water, tap water, pure water, wastewater and effluent from factories, and leachate from landfills.

[0025] A portion of the water to be treated that is applied to the carbon dioxide fixation method is taken from its source (or supply source) and introduced into a concentration step where concentration treatment is performed, and the remaining water to be treated (the water to be treated that is not introduced into the concentration step) is introduced into a mixing step.

[0026] In this case, the means for separating a portion of the water to be treated is not particularly limited. For example, a pipe for transporting the water to be treated from the generation source (or supply source) may be branched, or a flow path switching mechanism for separating the water may be provided on the pipe. Furthermore, in the stage of collecting and transporting the water to be treated that is applied to the carbon dioxide fixation method, the water to be introduced into the concentration step and the water to be introduced into the direct mixing step may be separated.

[0027] The concentration step involves concentrating the separated water to be treated to produce concentrated water. The specific means for this concentration step are not particularly limited. Examples include concentration by thermal evaporation of the water to be treated, or concentration using a semipermeable membrane, as shown in the carbon dioxide fixation device 100 described below. In particular, the concentration step in this embodiment is preferably performed by permeation using a semipermeable membrane. This allows the concentration process to proceed with less energy than when the water to be treated is concentrated by thermal evaporation, and carbon dioxide fixation can be performed simply and at low cost. Details of the concentration process using a semipermeable membrane will be described later in conjunction with the description of the carbon dioxide fixation device 100.

[0028] The concentration step in this embodiment is sufficient as long as it produces concentrated water from the water to be treated, but preferably also produces carbonate seed crystals. In other words, the concentration step in this embodiment is preferably performed to satisfy the conditions for producing carbonate seed crystals from the water to be treated. The conditions are determined or set depending on the concentration means used in the concentration step. Examples of such conditions include adjusting the time required for the concentration step, adjusting the amount of water to be treated, and selecting or setting treatment conditions (such as temperature, pressure, and the type of mechanism and components used in the concentration step) that result in a predetermined rate of change in the volume and component concentration of the water to be treated due to the concentration step. Furthermore, the concentration step in this embodiment is sufficient as long as it promotes the formation of carbonate crystals, and no particular operation is required to promote the growth of carbonate crystals.

[0029] The size of the carbonate seed crystals produced in the concentration step is not limited to a specific numerical value as long as they function as crystal nuclei involved in the crystal growth of the carbonate produced in the subsequent mixing step. Note that, in crystal growth promotion using seed crystals, seed crystals of the order of μm or larger are generally used, but the carbonate seed crystals produced in the concentration step of this embodiment can also be those of the order of μm or smaller, i.e., those of a size that makes recovery by ordinary filtration or the like difficult. For example, those of 0.1 μm or smaller can also be used as seed crystals.

[0030] The concentrated water produced through the concentration step is introduced into the mixing step.

[0031] The mixing step involves mixing the concentrated water with the remaining water to be treated (water not introduced into the concentration step) after the separation described above. Furthermore, during the mixing step, a carbonate precipitation reaction, which is a reaction between carbon dioxide contained in the concentrated water and the water to be treated and alkaline earth metal ions, proceeds. In particular, during the mixing step in this embodiment, the concentrated water is mixed with water to be treated that has not been subjected to concentration treatment, resulting in uneven distribution of the concentrations of components involved in carbonate formation within the mixed solution. Various phenomena and reactions (e.g., diffusion of components and carbonate precipitation reaction) then proceed in a direction that eliminates this uneven distribution of concentrations, thereby improving the reaction efficiency of carbonate precipitation and promoting crystal growth. Furthermore, by generating carbonate seed crystals in the concentrated water during the concentration step, the mixing step in this embodiment can further promote the growth of carbonate crystals in addition to improving the reaction efficiency of carbonate precipitation.

[0032] The mixing step in this embodiment is not particularly limited as long as the carbonate precipitation reaction proceeds by mixing the concentrated water and the water to be treated, producing a carbonate-containing solution. For example, as shown in the carbon dioxide fixation apparatus 100 described below, a treatment tank capable of accommodating the concentrated water and the water to be treated is provided, and the concentrated water and the water to be treated are introduced into the treatment tank.

[0033] Furthermore, the mixing step in this embodiment may include a means for increasing the mixing efficiency of the concentrated water and the water to be treated. In particular, the mixing step in this embodiment preferably includes stirring to promote carbonate crystal growth. In other words, the mixing step in this embodiment preferably includes stirring at a stirring intensity and duration that are sufficient to create and maintain conditions favorable for promoting carbonate crystal growth. This further increases the reaction efficiency of the carbonate precipitation reaction that occurs in the mixing step, and facilitates further promotion of carbonate crystal growth.

[0034] Furthermore, when stirring is performed in the mixing step in this embodiment, it is preferable to perform the stirring in an atmospheric atmosphere. As the carbonate precipitation reaction progresses during the mixing step, the amount (concentration) of carbon dioxide contained in the water to be treated (and the concentrated water) decreases over time, resulting in a decrease in the reaction efficiency of the carbonate precipitation reaction. On the other hand, by performing the stirring in an atmospheric atmosphere, carbon dioxide contained in the air is supplied to the mixed solution of the concentrated water and the water to be treated, which suppresses a decrease in the reaction efficiency of the carbonate precipitation reaction and makes it easier to further promote the growth of carbonate crystals.

[0035] Although the size of the carbonate crystals produced in the mixing step is not particularly limited, it is preferably 0.1 μm or more in order to smoothly and accurately carry out the operation related to the evaluation of the amount of fixed carbon dioxide, which will be described later. This makes it easy to recover the produced carbonate crystals, and enables quantification and appropriate evaluation of the amount of fixed carbon dioxide from the amount of carbonate produced (recovered amount).

[0036] As the carbonate precipitation reaction progresses in the mixing step, a carbonate-containing solution is produced, and carbon dioxide in the water to be treated is thereby fixed.

[0037] (Carbon Dioxide Fixation Apparatus) Next, an example of a carbon dioxide fixation apparatus for carrying out the carbon dioxide fixation method of this embodiment will be described. FIG. 2 is a schematic diagram showing the carbon dioxide fixation apparatus of this embodiment. As shown in FIG. 2, the carbon dioxide fixation apparatus 100 of this embodiment includes a concentrating section 10 and a mixing section 20. Water W to be treated containing carbon dioxide and alkaline earth metal ions (hereinafter simply referred to as "water W to be treated") is supplied to the carbon dioxide fixation apparatus 100 of this embodiment via line L1. Line L1 branches into lines L2 and L3. Line L2 connects to the mixing section 20 and supplies the water W to be treated, while line L3 connects to the concentrating section 10 and concentrates a portion of the water W to obtain concentrated water Wc. The resulting concentrated water Wc is supplied to the mixing section 20 via line L4, and the carbonate-containing solution S produced in the mixing section 20 is discharged to the system via line L6. Each component of the carbon dioxide fixation apparatus 100 will be described below.

[0038] The concentrating unit 10 is for performing the concentration step described above, and is supplied with a portion of the water to be treated W via lines L1 and L3 to produce concentrated water Wc. The concentrating unit 10 may be any unit that can perform concentration treatment on the water to be treated W to obtain concentrated water Wc, and as described above, there are no particular limitations on the means for the concentration treatment.

[0039] 2, an example of the concentrating unit 10 in this embodiment includes a treatment tank 11 and a semipermeable membrane 12 provided to separate the interior of the treatment tank 11. A flow rate adjusting mechanism (such as a valve or a liquid feed pump) for adjusting the flow rate of the water W to be treated supplied into the concentrating unit 10 may be provided on a line L3 connected to the treatment tank 11.

[0040] The treatment tank 11 may be made of any material, shape, or size as long as it is capable of storing the water to be treated W introduced via lines L1 and L3. The treatment tank 11 is separated into a first chamber 11a and a second chamber 11b by a semipermeable membrane 12 (described later). The water to be treated W is introduced into the first chamber 11a, and concentrated water Wc is produced in the first chamber 11a. Meanwhile, permeated water Wp that has passed through the semipermeable membrane 12 flows into the second chamber 11b. The concentrated water Wc is then supplied to the mixing section 20 via line L4 provided on the first chamber 11a side. The permeated water Wp may be discharged outside the system, or, as shown in FIG. 2, may be supplied to the mixing section 20 together with the concentrated water Wc via line L5 provided on the second chamber 11b side.

[0041] The semipermeable membrane 12 may be any membrane capable of transmitting water molecules in the aqueous solution (the water to be treated W in this embodiment). However, it is preferable to use a membrane that is impermeable or difficult to transmit ions necessary for the carbonate precipitation reaction, such as carbonate ions and alkaline earth metal ions, contained in the aqueous solution (the water to be treated W in this embodiment). The semipermeable membrane 12 concentrates the water to be treated W, containing dissolved carbon dioxide and alkaline earth metal ions, by allowing water molecules to pass through and reducing the concentration of the water to be treated, while preventing or making it difficult for carbonate ions present in the water to be treated W to pass through, thereby increasing the carbonate ion concentration in the first chamber 11a (the concentrated water Wc side). This allows the reaction between carbonate ions and alkaline earth metal ions (carbonate precipitation reaction) to proceed in the first chamber 11a (the concentrated water Wc side), facilitating the production of carbonate (carbonate seed crystals). The carbonate produced in the concentrating unit 10 may be any suitable carbonate seed crystal, and it is not essential that it promotes the growth of carbonate crystals in the concentrating unit 10.

[0042] Furthermore, when water to be treated W having a pH exceeding 7 is concentrated using the semipermeable membrane 12, the concentrated water Wc concentrated by the semipermeable membrane 12 has a reduced number of water molecules and an increased concentration of hydroxide ions. That is, the pH of the concentrated water Wc increases. It is known that the abundance ratio of each form of carbon dioxide (carbonate, bicarbonate ion, carbonate ion) in an aqueous solution depends on the pH, and the abundance ratio of carbonate ions increases at high pH. That is, increasing the pH of the concentrated water Wc facilitates the production of carbonate ions, resulting in the effect of more efficiently producing and precipitating carbonates (carbonate seed crystals).

[0043] Furthermore, the semipermeable membrane 12 is preferably permeable to carbon dioxide gas dissolved in an aqueous solution (the water to be treated W in this embodiment). When the semipermeable membrane 12 allows carbon dioxide gas dissolved in the water to be treated W to pass through, the carbon dioxide gas concentration does not increase on the side of the concentrated water Wc (the first chamber 11a side) obtained by concentrating the water to be treated W. Therefore, the carbon dioxide gas contained in the water to be treated W is ionized to generate hydrogen ions, which prevents an unintended decrease in the pH of the concentrated water Wc. This facilitates the generation of carbonate ions on the first chamber 11a side, making it easier to maintain a pH range conducive to carbonate precipitation. In particular, when an aqueous solution with a pH exceeding 7 is used as the water to be treated W, the synergistic effect of the pH increase due to the increase in hydroxide ion concentration and the suppression of an increase in the carbon dioxide gas concentration results in the more easily generated carbonate ions.

[0044] In the concentrating unit 10 of this embodiment, concentrating the water W to be treated using the semipermeable membrane 12 allows for the concentration of the water W using less energy than concentrating the water W by thermal evaporation (such as a method known as simple distillation). Furthermore, by selecting the function of the semipermeable membrane 12 (the types of substances and ions that do or do not permeate the semipermeable membrane 12), carbonate seed crystals can be effectively formed in the concentrated water Wc. In particular, when a nanofiltration membrane (NF membrane) is used as the semipermeable membrane 12, the permeation resistance is reduced (almost equivalent to increasing the pore size) compared to a reverse osmosis membrane (RO membrane), thereby reducing the energy required for the concentration process (energy saving). Furthermore, as described above, the crystals (seed crystals) generated in the concentrating unit 10 (concentration step) may be of the size of microcrystals (on the order of μm or less) that pass through to the permeate (permeate Wp) side, making clogging of the semipermeable membrane 12 less likely to occur.

[0045] The concentrated water Wc produced in the concentration section 10 is supplied to the mixing section 20 via line L4. A flow rate control mechanism or a pressure control mechanism, such as a valve, may be provided on line L4 to adjust the amount or timing of supply of the concentrated water Wc to the mixing section 20. As described above, the permeated water Wp produced in the concentration section 10 may be discharged to the system via line L5. Alternatively, as shown in FIG. 2 , line L5 may be connected to the mixing section 20, and the permeated water Wp may also be supplied to the mixing section 20. This allows a sufficient amount of carbonate seed crystals to be supplied to the mixing section 20, even if some of the carbonate seed crystals permeate the semipermeable membrane 12, thereby promoting carbonate crystal growth in the mixing section 20. In other words, all of the carbonate crystals (microcrystals) produced in the concentration section 10 can be supplied as seed crystals to the mixing section 20 in the subsequent process, thereby maximizing the effect of promoting carbonate crystal growth in the mixing section 20.

[0046] The mixing unit 20 is used to perform the above-mentioned mixing step, and mixes the concentrated water Wc supplied via line L4 with the water to be treated W supplied via line L2. The mixing unit 20 may be any unit that can mix the concentrated water Wc and the water to be treated W, and as described above, there are no particular limitations on the means for the mixing process.

[0047] As an example of the mixing section 20 in this embodiment, as shown in FIG. 2, there is one including a treatment tank 21 and a stirring mechanism 22 provided inside the treatment tank 21.

[0048] The treatment tank 21 may be made of any material, shape, or size as long as it can accommodate the concentrated water Wc supplied via line L4, the water to be treated W supplied via line L2, and the permeated water Wp supplied via line L5. The connection between line L5 and the treatment tank 21 may be omitted, for example, when the permeated water Wp is discharged outside the system. Hereinafter, the mixing section 20 will be described with respect to the mixing of the concentrated water Wc and the water to be treated W. However, when the permeated water Wp is supplied to the treatment tank 21 via line L5, the permeated water Wp will naturally also be mixed.

[0049] The agitation mechanism 22 is one of the means for increasing the mixing efficiency of the concentrated water We and the water to be treated W contained in the treatment tank 21, and its specific structure is not particularly limited. Specific examples of the agitation mechanism 22 include a mechanism equipped with a shaft and an agitation blade that rotates the agitation blade, and a mechanism that supplies an agitation fluid (gas or liquid that does not inhibit the carbonate precipitation reaction) to the treatment tank 21. Furthermore, flow rate adjustment mechanisms may be provided on the lines L2 and L4 connected to the treatment tank 21, so that the concentrated water We and / or the water to be treated W supplied to the treatment tank 21 function as the agitation fluid.

[0050] Furthermore, when the agitation mechanism 22 requires power (energy) for operation, it is preferable to use renewable energy such as solar, wind, or wave power, or surplus electricity from other facilities, as its power supply means (energy supply means). This makes it possible to reduce the energy used in carbon dioxide fixation. In particular, adopting a power supply means (energy supply means) that uses renewable energy that does not emit carbon dioxide when generating electricity has the effect of promoting the reduction of carbon dioxide emissions.

[0051] Furthermore, when stirring is performed in the mixing section 20 in this embodiment, it is preferable to perform the stirring in an atmospheric atmosphere. Specific means for performing the stirring in an atmospheric atmosphere are not particularly limited. For example, the volume (introduction amount) of the mixed solution of concentrated water and the water to be treated contained in the treatment tank 21 may be adjusted to provide sufficient space above the treatment tank 21 for the presence of atmospheric air, or the upper part of the treatment tank 21 may be opened to allow the mixed solution of concentrated water and the water to be treated contained in the treatment tank 21 to come into contact with the atmospheric air, or a means for supplying atmospheric air from the outside to the treatment tank 21 may be provided. From the viewpoint of promoting carbon dioxide fixation at low cost, stirring in an atmospheric atmosphere is preferable. However, from the viewpoint of improving the efficiency of the carbonate precipitation reaction, stirring may be performed by introducing a gas having a higher carbon dioxide concentration than atmospheric air into the treatment tank 21.

[0052] As the carbonate precipitation reaction progresses in the mixing section 20, a carbonate-containing solution S is produced, which immobilizes the carbon dioxide in the water to be treated W. The produced carbonate-containing solution S is discharged to the outside of the system via line L6.

[0053] [Method and system for evaluating the amount of fixed carbon dioxide] Next, an embodiment of the method and system for evaluating the amount of fixed carbon dioxide of the present invention will be described. The method and system for evaluating the amount of fixed carbon dioxide of the present invention utilize the carbon dioxide fixation method and carbon dioxide fixation device of the present invention described above to obtain information on how much carbon dioxide has been fixed when carbon dioxide is fixed by a carbonate precipitation reaction, i.e., to quantify the amount of fixed carbon dioxide, thereby evaluating the amount of fixed carbon dioxide.

[0054] (Method for evaluating the amount of fixed carbon dioxide) Fig. 3 is a flow diagram showing each operation (step) involved in the method for evaluating the amount of fixed carbon dioxide in an embodiment of the present invention. As shown in Fig. 3, the method for evaluating the amount of fixed carbon dioxide in this embodiment includes a concentration step of concentrating a portion of the water to be treated containing carbon dioxide and alkaline earth metal ions to produce concentrated water, a mixing step of mixing this concentrated water with the water to be treated, and a measurement step of measuring the amount of carbonate produced in the mixing step. Below, each operation (step) involved in the method for evaluating the amount of fixed carbon dioxide in this embodiment will be described. Note that explanations of parts common to the carbon dioxide fixation method in this embodiment described above will be omitted.

[0055] First, in the method for evaluating the amount of fixed carbon dioxide of this embodiment, a carbonate-containing solution is obtained by applying the concentration step and mixing step according to the carbon dioxide fixation method of this embodiment described above to the water to be treated.

[0056] Next, a measuring step is carried out. The measuring step measures the amount of carbonate produced in the mixing step, and more specifically, as shown in Fig. 3, the carbonate is recovered from the carbonate-containing solution and weighed. This makes it possible to quantify the amount of carbon dioxide that has been fixed by the carbonate precipitation reaction in relation to the carbon dioxide contained in the water to be treated.

[0057] The means for recovering carbonate in the measurement step is not particularly limited. As described above, since the size of carbonate crystals produced through the mixing step exceeds 0.1 μm, a suitable example is, for example, filtration using a filter having openings (mesh size) that can capture (recover) particles of 0.1 μm or more, but the method is not limited thereto.

[0058] Furthermore, the means for measuring the recovered carbonate in the measurement step is not particularly limited. For example, a suitable example is to measure the carbonate captured (recovered) on a filter by filtration using a weighing scale such as an electronic balance, but the present invention is not limited thereto.

[0059] The method for evaluating the amount of fixed carbon dioxide according to this embodiment may be any method capable of quantifying the amount of carbonate produced by the carbonate fixation method. Therefore, it is not essential to recover the carbonate from the carbonate-containing solution in the measurement step, and if the carbonate can be measured in the state of the carbonate-containing solution, the operation for recovering the carbonate may be omitted.

[0060] By obtaining information on the amount of carbonate produced through the concentration step and the mixing step in the measurement step, it becomes possible to quantify the amount of carbon dioxide contained in the treated water that has been fixed by the carbonate precipitation reaction, thereby enabling an appropriate evaluation of the amount of fixed carbon dioxide.

[0061] (System for Evaluating the Amount of Carbon Dioxide Fixation) Next, an example of a system for evaluating the amount of carbon dioxide fixation for carrying out the method for evaluating the amount of carbon dioxide fixation according to this embodiment will be described. FIG. 4 is a schematic diagram illustrating the system for evaluating the amount of carbon dioxide fixation according to this embodiment. As shown in FIG. 4, the system for evaluating the amount of carbon dioxide fixation according to this embodiment includes a concentrating section 10, a mixing section 20, and a measuring section 30. Furthermore, to the system for evaluating the amount of carbon dioxide fixation according to this embodiment, water to be treated W containing carbon dioxide and alkaline earth metal ions is supplied via line L1. Line L1 branches into lines L2 and L3. Line L2 is connected to the mixing section 20 to supply the water to be treated W, while line L3 is connected to the concentrating section 10 to concentrate a portion of the water to be treated W to obtain concentrated water Wc. The obtained concentrated water Wc is supplied to the mixing section 20 via line L4, where a carbonate-containing solution S is produced. Furthermore, the permeated water Wp produced when the semipermeable membrane 12 is used as the concentrating section 10 may be discharged outside the system, or, as shown in FIG. 4 , may be supplied to the mixing section 20 together with the concentrated water Wc via a line L5 provided on the second chamber 11b side, and the carbonate-containing solution S may be produced by mixing the water to be treated W, the concentrated water Wc, and the permeated water Wp. The produced carbonate-containing solution S is then introduced into the measuring section 30 via a line L6, where the amount of carbonate in the carbonate-containing solution S is measured. Below, each component of the carbon dioxide fixation amount evaluation system 200 will be described. Note that a description of parts common to the carbon dioxide fixation apparatus of the present embodiment described above will be omitted.

[0062] The concentrating section 10 and the mixing section 20 in the carbon dioxide fixation amount evaluation system 200 in this embodiment are the same as those shown in the carbon dioxide fixation device 100 in this embodiment described above, and a carbonate-containing solution S is obtained from a mixed solution of the treated water W and the concentrated water Wc (which may contain the permeated water Wp).

[0063] The obtained carbonate-containing solution S is introduced into the measurement unit 30 via line L6. The measurement unit 30 is used to perform the measurement step described above, and measures the amount of carbonate produced in the mixing unit 20. More specifically, the measurement unit 30 measures the amount of carbonate C in the carbonate-containing solution S produced in the mixing unit 20.

[0064] As shown in Fig. 4, one example of the measurement unit 30 in this embodiment includes a recovery unit 31 and a weighing unit 32. The recovery unit 31 recovers carbonate C from the carbonate-containing solution S. The means for recovering the carbonate is not particularly limited, but a preferred example is filtration using a filter, as described above. The weighing unit 32 weighs the carbonate C recovered in the recovery unit 31. The means for measuring the carbonate is not particularly limited, but a preferred example is weighing using a weighing scale, as described above. This provides information on the amount of carbonate produced by the carbonate immobilization method, and conversion into the amount of carbon dioxide fixation based on this information enables the amount of carbon dioxide fixation to be quantified.

[0065] As mentioned above, the measurement unit 30 only needs to be capable of quantifying the amount of carbonate C contained in the carbonate-containing solution S, and if the weighing unit 32 is capable of measuring the carbonate C while it is still in the carbonate-containing solution S, the recovery unit 31 may be omitted.

[0066] Furthermore, the quantification (evaluation) of the amount of carbon dioxide fixation by the measurement unit 30 may include calculations and operations by an operator, but it is preferable to use a calculation device that has a data input / output function for acquiring information and that uses a processor such as a CPU to execute a program for performing calculations related to conversion into the amount of carbon dioxide fixation. This makes it possible to quickly and accurately evaluate the amount of carbon dioxide fixation.

[0067] Conventionally, in the fixation of carbon dioxide by a carbonate precipitation reaction, in order to quantitatively evaluate how much carbon dioxide was fixed, it was necessary to spend the cost of using chemicals and the time required for crystal growth, and to carry out operations related to carbonate crystal growth.

[0068] On the other hand, the system 200 for evaluating the amount of fixed carbon dioxide in this embodiment measures carbonate obtained through a process that enables improvement of the reaction efficiency of the carbonate precipitation reaction and promotion of carbonate crystal growth simply and at low cost. Therefore, the system 200 for evaluating the amount of fixed carbon dioxide in this embodiment can measure the amount of carbonate produced simply and at low cost, and further enables appropriate evaluation as the amount of fixed carbon dioxide.

[0069] The carbon dioxide fixation method, carbon dioxide fixation apparatus, carbon dioxide fixation amount evaluation method, and carbon dioxide fixation amount evaluation system of the present invention will be described below, particularly by showing examples and comparative examples of the results of performing each operation (step) related to the carbon dioxide fixation method and the carbon dioxide fixation amount evaluation method. Note that the present invention is not limited to these examples.

[0070] In the example, seawater (pH 8.1, alkaline earth metal ion (calcium ion) concentration: 440 ppm) was used as the water to be treated, and a concentration step using a semipermeable membrane (Film Tec NF270, manufactured by DuPont Water Solutions) was performed to obtain concentrated water, followed by a mixing step in which the concentrated water and the water to be treated were stirred and mixed. On the other hand, in the comparative example, the same water to be treated as in the example was used, and only the concentration step using the same semipermeable membrane as in the example was performed.

[0071] The operating conditions for the concentration step were: permeate volume: 50.6%, circulation pressure: 0.5 MPa, circulation flow rate: 0.2 L / min, and concentration treatment time (operating time): approximately 3 hours. The operating conditions for the mixing step were: after obtaining a mixed solution by mixing the concentrated water and the water to be treated at a weight ratio of 1:2, this mixed solution was stirred for a predetermined time using a stirrer (rotation speed: 400 rpm). In the comparative example, instead of performing the mixing step, the concentrated water obtained in the concentration step was stirred for a predetermined time using a stirrer (rotation speed: 400 rpm).

[0072] 5A and 5B are photographs showing the state of the solutions obtained in the concentration step in the examples and comparative examples, respectively showing the state of the concentrated water (FIG. 5A), the permeated water (FIG. 5B), and the water to be treated (FIG. 5C). Also, FIG. 5 shows the state of each solution obtained in the concentration step, which was stored in a beaker, left in the atmosphere for a predetermined time, and then irradiated with laser light from the center right edge of the photograph. The dehydration rate (concentration rate) of each solution at this time was approximately 60%.

[0073] As shown in Figure 5, the untreated water (Figure 5C) exhibits little laser light scattering, indicating that almost no solid matter (carbonate crystals) is present. On the other hand, the concentrated water (Figure 5A) exhibits strong laser light scattering. This indicates that solid matter (carbonate crystals) is present in the concentrated water as a result of the concentration step, and that these solid matter is not visible to the naked eye. Furthermore, the permeated water (Figure 5B) exhibits laser light scattering that is intermediate between that of the untreated water (Figure 5C) and the concentrated water (Figure 5A). Therefore, it can be seen that some solid matter (carbonate crystals) is also present in the permeated water.

[0074] 6A and 6B are photographs showing the solution obtained through the concentration step and the mixing step and the state of carbonates in the solution, as an example. More specifically, FIG. 6A shows a photograph of the solution (carbonate-containing solution) obtained by stirring and mixing the concentrated water and the water to be treated, and FIG. 6B shows a photograph of the solids (carbonate crystals) recovered from the solution by filtration. The filtration process in the example (FIG. 6B) was performed using a filter with a mesh size of 0.1 μm.

[0075] On the other hand, Fig. 7 shows photographs of a comparative example showing a solution obtained through only the concentration step and the state of carbonate in the solution. More specifically, Fig. 7 shows a photograph (Fig. 7A) of concentrated water and a photograph (Fig. 7B) of solids (carbonate crystals) recovered from the concentrated water by filtration. Note that the filtration process for the comparative example (Fig. 7B) was carried out using a filter with 0.1 µm mesh, as in the example (Fig. 6B).

[0076] As shown in FIG. 6 , carbonates could be recovered by filtration from the solution (carbonate-containing solution) obtained through the concentration and mixing steps in the Example, and the recovered carbonates grew to a size that was visible to the naked eye. The amount of immobilized carbon dioxide could be determined by weighing the recovered carbonates. On the other hand, as shown in FIG. 7 , carbonates could also be recovered from the solution obtained through only the concentration step in the Comparative Example, but almost none of the recovered carbonates were large enough to be visually identified. Furthermore, the amount of recovered carbonates weighed was significantly less than that in the Example ( FIG. 6 ) (approximately 30% of the Example), making it difficult to properly evaluate the amount of immobilized carbon dioxide. In other words, the results in FIGS. 6 and 7 demonstrate that the Example of the present invention can easily and inexpensively improve the reaction efficiency of the carbonate precipitation reaction and promote the growth of the resulting carbonate crystals, and also enable proper evaluation of the amount of carbon dioxide immobilization.

[0077] The above-described embodiments show examples of the carbon dioxide fixation method, carbon dioxide fixation device, and method and system for evaluating the amount of fixed carbon dioxide. The carbon dioxide fixation method, carbon dioxide fixation device, method and system for evaluating the amount of fixed carbon dioxide according to the present invention are not limited to the above-described embodiments, and the carbon dioxide fixation method, carbon dioxide fixation device, method and system for evaluating the amount of fixed carbon dioxide according to the above-described embodiments may be modified within the scope of the gist of the claims.

[0078] Furthermore, the carbon dioxide fixation method, carbon dioxide fixation device, and carbon dioxide fixation amount evaluation method and carbon dioxide fixation amount evaluation system of the present invention can be applied to various existing facilities and systems that require quantitative evaluation of the carbon dioxide fixation amount.Specific examples include facilities that directly capture and separate carbon dioxide from the atmosphere and store it underground (DACCS systems), carbon dioxide capture and storage facilities (CCS systems) attached to thermal power plants, and carbon dioxide fixation facilities installed on ships or at sea to fix carbon dioxide using seawater.

[0079] The carbon dioxide fixation method and carbon dioxide fixation device of the present invention are suitable for use in fixating carbon dioxide from water to be treated that contains carbon dioxide and alkaline earth metal ions through a carbonate precipitation reaction. Furthermore, the carbon dioxide fixation amount evaluation method and carbon dioxide fixation amount evaluation system of the present invention enable the quantification of the amount of carbon dioxide fixed (carbon dioxide fixation amount) in relation to carbon dioxide fixation through the carbonate precipitation reaction, and are suitable for use in evaluating the amount of carbon dioxide fixation.

[0080] 100 Carbon dioxide fixation device, 200 Evaluation system for carbon dioxide fixation amount, 10 Concentration section, 11 Treatment tank, 11a First chamber, 11b Second chamber, 12 Semipermeable membrane, 20 Mixing section, 21 Treatment tank, 22 Stirring mechanism, 30 Measurement section, 31 Recovery section, 32 Metering section, L1 to L6 Lines, C Carbonate, S Carbonate-containing solution, W Water to be treated, Wc Concentrated water, Wp Permeated water

Claims

1. A carbon dioxide fixation method comprising: a concentration step of concentrating a portion of water to be treated that contains carbon dioxide and alkaline earth metal ions to produce concentrated water; and a mixing step of mixing the concentrated water with the water to be treated.

2. The carbon dioxide fixation method according to claim 1, wherein seed crystals of carbonate are generated in the concentration step.

3. The carbon dioxide fixation method according to claim 1, wherein the concentration step utilizes a semipermeable membrane.

4. The carbon dioxide fixation method according to claim 1, wherein the mixing step involves stirring to promote carbonate crystal growth.

5. The carbon dioxide fixation method according to claim 4, wherein the stirring in the mixing step is carried out in an atmospheric environment.

6. A carbon dioxide fixation device comprising: a concentration unit that concentrates a portion of water to be treated that contains carbon dioxide and alkaline earth metal ions to produce concentrated water; and a mixing unit that mixes the concentrated water with the water to be treated.

7. A method for evaluating the amount of fixed carbon dioxide, comprising: a concentration step of concentrating a portion of water to be treated containing carbon dioxide and alkaline earth metal ions to produce concentrated water; a mixing step of mixing the concentrated water with the water to be treated; and a measurement step of measuring the amount of carbonate produced in the mixing step.

8. A system for evaluating the amount of carbon dioxide fixation, comprising: a concentration unit that concentrates a portion of water to be treated that contains carbon dioxide and alkaline earth metal ions to produce concentrated water; a mixing unit that mixes the concentrated water with the water to be treated; and a measurement unit that measures the amount of carbonate produced in the mixing unit.

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