System for evaluating carbon dioxide fixation amount and method for evaluating carbon dioxide fixation amount

Abstract

The present invention addresses the problem of providing a system for evaluating a carbon dioxide fixation amount and a method for evaluating a carbon dioxide fixation amount which do not require an operation related to crystal growth when fixing carbon dioxide by a carbonate precipitation reaction, easily quantify the fixed amount of carbon dioxide, and enable appropriate evaluation of a carbon dioxide fixation amount.　In order to solve this problem, the present invention provides an evaluation system and an evaluation method for evaluating a carbon dioxide immobilization amount, the system comprising a reaction unit that performs a carbonate precipitation reaction pertaining to fixation of carbon dioxide under conditions in which calcium carbonate is preferentially precipitated, a first measurement unit and a second measurement unit that measure the calcium ion concentration before and after the reaction, and an arithmetic unit that quantifies the fixed amount of carbon dioxide on the basis of the measurement results of the first measurement unit and the second measurement unit.　The present invention makes it possible to perform a simple quantification and an appropriate evaluation of the carbon dioxide fixation amount without performing an operation related to crystal growth.

Description

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

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

[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, 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 involved in carbonate precipitation proceeds spontaneously, so there is no need to supply energy from an external source to fix carbon dioxide, and the produced carbonate can be stored stably for a long period of time.

[0007] Meanwhile, in efforts to reduce carbon dioxide emissions into the environment, or so-called decarbonization, in addition to chemical carbon dioxide emission reduction methods such as carbon capture and fixation, attention is being paid to methods that encourage carbon dioxide emission reduction by assigning a price (tax) to carbon dioxide emissions or by certifying and trading the value of carbon dioxide reductions (carbon pricing). Applying carbon pricing requires information on how much carbon dioxide emissions have been reduced. In other words, it is necessary to quantify and evaluate how much carbon dioxide has been fixed.

[0008] In the case of a carbonate fixation method in which divalent metal ions and carbon dioxide are reacted to produce carbonate, as described in Patent Document 1, the amount of carbon dioxide fixation has traditionally been evaluated by recovering all of the produced carbonate as a solid and measuring the amount recovered. In other words, unless the carbonate can be recovered as a solid and its weight can be measured, the amount of carbon dioxide fixation cannot be evaluated. Therefore, it is necessary to grow carbonate crystals so that the produced carbonate can be recovered as a solid. However, as carbon dioxide fixation progresses, the cost of using chemicals required to promote carbonate crystal growth until recovery is possible and the time required for crystal growth increase, resulting in a problem of a heavy workload associated with evaluating the amount of carbon dioxide fixation.

[0009] Therefore, an object of the present invention is to provide a system and a method for evaluating the amount of fixed carbon dioxide, which do not require any operations related to crystal growth when fixing carbon dioxide through a carbonate precipitation reaction, and which allow for easy quantification of the amount of fixed carbon dioxide and appropriate evaluation of the amount of fixed carbon dioxide.

[0010] As a result of extensive research into the above-mentioned problems, the present inventors have found that by carrying out a carbonate precipitation reaction for carbon dioxide fixation under specific conditions and determining the amount of fixed carbon dioxide based on the calcium ion concentrations before and after the reaction, it is possible to easily quantify the amount of fixed carbon dioxide and appropriately evaluate the amount of fixed carbon dioxide without performing any operations related to crystal growth, and have completed the present invention. That is, the present invention relates to the following system and method for evaluating the amount of fixed carbon dioxide.

[0011] The system for evaluating the amount of fixed carbon dioxide of the present invention, which aims to solve the above-mentioned problems, is characterized by comprising: a reaction unit in which a carbonate precipitation reaction for carbon dioxide fixation is carried out under conditions that preferentially precipitate calcium carbonate; a first measurement unit that measures the calcium ion concentration before the reaction; a second measurement unit that measures the calcium ion concentration after the reaction; and a calculation unit that quantifies the amount of fixed carbon dioxide based on the measurement results of the first and second measurement units. According to the system for evaluating the amount of fixed carbon dioxide of the present invention, by preferentially precipitating a specific carbonate (calcium carbonate) as the carbonate produced by the carbonate precipitation reaction, fluctuations in the metal ion (calcium ion) concentration before and after the reaction correlate with the amount of carbonate produced, and the amount of carbonate produced can be converted into the amount of fixed carbon dioxide. This enables simple quantification of the amount of fixed carbon dioxide and appropriate evaluation of the amount of fixed carbon dioxide without performing operations related to crystal growth. Furthermore, this system reduces the cost and time required for the chemicals required for carbonate crystal growth, thereby easing the workload involved in evaluating the amount of fixed carbon dioxide.

[0012] In one embodiment of the carbon dioxide fixation amount evaluation system of the present invention, the reaction section is characterized in that the carbonate precipitation reaction is carried out at a pH of 10.5 or less. The solubility, which serves as an index of the ease of precipitation of carbonates produced by the reaction of divalent metal ions with carbon dioxide, is pH-dependent. The present inventors have found that, by comparing the pH dependence of the solubility of various carbonates and hydroxides, there exists a pH range in which calcium carbonate preferentially precipitates. This carbon dioxide fixation amount evaluation system makes it possible to preferentially precipitate calcium carbonate even when multiple divalent metal ions are present. This makes it possible to reduce the costs associated with preparing and procuring solutions that can be used as divalent metal ion sources for carbonate precipitation.

[0013] Furthermore, in one embodiment of the carbon dioxide fixation amount evaluation system of the present invention, the reaction unit includes a concentration unit that produces a concentrate by permeation using a semipermeable membrane, and the second measurement unit measures the calcium ion concentration in at least the concentrate. Generally, carbonate precipitation involves reacting divalent metal ions with carbon dioxide in a solution and concentrating the resulting solution to promote carbonate precipitation. According to this carbon dioxide fixation amount evaluation system, concentration by permeation using a semipermeable membrane allows the carbonate precipitation reaction to proceed with less energy than when concentrating a solution by heating and evaporating it, thereby obtaining carbonate. Furthermore, since the concentrate produced by using a semipermeable membrane is the product of the carbonate precipitation reaction, measuring the calcium ion concentration in the concentrate allows the calcium ion concentration after the reaction to be accurately and easily determined. This increases the accuracy of quantification of the amount of fixed carbon dioxide, enabling more appropriate evaluation.

[0014] The method for evaluating the amount of fixed carbon dioxide of the present invention, which aims to solve the above-mentioned problems, comprises a reaction step in which a carbonate precipitation reaction for carbon dioxide fixation is carried out under conditions in which calcium carbonate is preferentially precipitated; a first measurement step in which the calcium ion concentration before the reaction is measured; a second measurement step in which the calcium ion concentration after the reaction is measured; and a calculation step in which the amount of fixed carbon dioxide is quantified based on the results of the first and second measurement steps. According to the method for evaluating the amount of fixed carbon dioxide of the present invention, a specific carbonate (calcium carbonate) is preferentially precipitated as the carbonate produced by the carbonate precipitation reaction, and the amount of fixed carbon dioxide can be determined by observing the change in metal ion (calcium ion) concentration before and after the reaction. This enables simple quantification of the amount of fixed carbon dioxide and appropriate evaluation of the amount of fixed carbon dioxide without performing operations related to crystal growth. Furthermore, this method reduces the cost and time required for the chemicals required for carbonate crystal growth, thereby reducing the workload involved in evaluating the amount of fixed carbon dioxide.

[0015] According to the present invention, it is possible to provide a system and a method for evaluating the amount of fixed carbon dioxide, which do not require any operations related to crystal growth when fixing carbon dioxide through a carbonate precipitation reaction, allow the amount of fixed carbon dioxide to be easily quantified, and enable appropriate evaluation of the amount of fixed carbon dioxide.

[0016] FIG. 1 is a schematic explanatory diagram showing an example of an aspect of a system for evaluating the amount of fixed carbon dioxide according to a first embodiment of the present invention. FIG. 2 is a graph showing the pH dependency of the abundance ratio of each form of carbon dioxide (carbonate, bicarbonate ion, carbonate ion) in an aqueous solution. FIG. 3 is a graph showing the relationship between the pH of an aqueous solution and the solubility (logarithm) of carbonate and hydroxide produced from divalent metal ions (calcium ion, magnesium ion). FIG. 4 is a schematic explanatory diagram showing an example of another aspect of a system for evaluating the amount of fixed carbon dioxide according to a first embodiment of the present invention. FIG. 5 is a schematic explanatory diagram showing an example of an aspect of a system for evaluating the amount of fixed carbon dioxide according to a second embodiment of the present invention.

[0017] The system and method for evaluating the amount of fixed carbon dioxide of the present invention evaluate the amount of fixed carbon dioxide by obtaining information on how much carbon dioxide was fixed when carbon dioxide was fixed by a carbonate precipitation reaction, i.e., by quantifying the amount of fixed carbon dioxide. Furthermore, the system and method for evaluating the amount of fixed carbon dioxide of the present invention enable the evaluation of the amount of fixed carbon dioxide without recovering carbonate produced by the carbonate precipitation reaction, and do not require the operation of growing crystals so that the carbonate can be recovered.

[0018] 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.

[0019] Hereinafter, embodiments of the system for evaluating the amount of fixed carbon dioxide according to the present invention will be described in detail. Furthermore, the description of the method for evaluating the amount of fixed carbon dioxide according to the present invention will be replaced with a description of the operation of the system for evaluating the amount of fixed carbon dioxide according to the present invention. The system for evaluating the amount of fixed carbon dioxide and the method for evaluating the amount of fixed carbon dioxide described in the embodiments are merely examples used to explain the system for evaluating the amount of fixed carbon dioxide and the method for evaluating the amount of fixed carbon dioxide according to the present invention, and are not limited thereto.

[0020] [First embodiment] Figure 1 is a schematic explanatory diagram showing a system for evaluating the amount of carbon dioxide fixation in a first embodiment of the present invention. As shown in Figure 1, a system 100A for evaluating the amount of carbon dioxide fixation according to this embodiment (hereinafter also simply referred to as "evaluation system 100A") includes a reaction unit 10, a first measurement unit 20, a second measurement unit 30, and a calculation unit 40. Note that dashed arrows in Figure 1 indicate connections that allow input and output. Each component of the evaluation system 100A will be described below.

[0021] The reaction unit 10 is for carrying out a carbonate precipitation reaction related to the fixation of carbon dioxide, and further for carrying out the carbonate precipitation reaction under conditions under which calcium carbonate is preferentially precipitated. The reaction unit 10 may be any unit that can react carbon dioxide with divalent metal ions that react with carbon dioxide to produce carbonate, and can precipitate carbonate under conditions under which calcium carbonate is preferentially precipitated, and the specific means and device structure of the reaction unit 10 are not particularly limited.

[0022] 1, carbon dioxide and divalent metal ions (including calcium ions) are introduced into the reaction unit 10 via a line L1, and calcium carbonate is produced by the reaction between carbonate ions, which are a form of carbon dioxide in the aqueous solution, and calcium ions precipitate as calcium carbonate crystals. Excess calcium ions not used in the carbonate production (carbonate precipitation) are then discharged via a line L2.

[0023] The carbon dioxide and divalent metal ions (calcium ions) introduced via line L1 may be in the form of an aqueous solution containing carbonate ions and divalent metal ions (calcium ions) in the reaction section 10, and their origin is not particularly limited. For example, the carbon dioxide introduced via line L1 may be carbon dioxide gas as is, an aqueous solution containing artificially dissolved carbon dioxide, or a solution containing dissolved carbon dioxide from the beginning. Furthermore, the divalent metal ions introduced via line L1 may be an aqueous solution containing artificially dissolved substances that will become divalent metal ions (calcium ions), or a solution containing dissolved divalent metal ions (calcium ions) from the beginning. Examples of solutions containing dissolved carbon dioxide and divalent metal ions from the beginning include seawater, river water, tap water, pure water, wastewater and effluent from factories, and leachate from landfills.

[0024] The reaction unit 10 in this embodiment may include, for example, a tank (reaction tank) capable of storing an aqueous solution, and may be configured to carry out a carbonate precipitation reaction by introducing carbon dioxide gas and a divalent metal ion aqueous solution into the reaction tank, or by introducing an aqueous solution in which carbon dioxide and divalent metal ions are pre-dissolved. Here, examples of means for promoting the carbonate precipitation reaction in the reaction unit 10 include concentrating the aqueous solution in the reaction tank or adjusting the pH. Alternatively, carbon dioxide gas or the like may be introduced as fine bubbles (fine bubbles, microbubbles, etc.) into the aqueous solution contained in the reaction tank to form an extremely small gas-liquid interfacial reaction field, thereby promoting the carbonate precipitation reaction without concentrating the aqueous solution. More specifically, for example, for a reaction tank containing an aqueous solution in which carbon dioxide and divalent metal ions are pre-dissolved, the type of gas introduced as fine bubbles is not particularly limited, as long as it can form an extremely small gas-liquid interfacial reaction field between carbonate ions and calcium ions. For a reaction tank containing an aqueous solution of divalent metal ions, carbon dioxide gas may be introduced as fine bubbles. The carbon dioxide gas introduced as fine bubbles at this time functions as a carbon dioxide source in the carbonate precipitation reaction in conjunction with the formation of an extremely small gas-liquid interfacial reaction field. A detailed example of the reaction section 10 will be described later as another embodiment.

[0025] As a condition for preferentially precipitating calcium carbonate in the reaction section 10, for example, the divalent metal ion aqueous solution used in the reaction section 10 may be an aqueous solution containing only calcium ions. As a result, the carbonate precipitated in the reaction section 10 will naturally be calcium carbonate alone.

[0026] Another reaction condition in the reaction section 10 is a pH of 10.5 or less. Among multiple carbonates that can be produced, whether a certain carbonate will preferentially precipitate can be determined by its solubility. Specifically, among carbonates produced under the same conditions, those with lower solubility are more likely to precipitate as crystals, i.e., will preferentially precipitate. By comparing the pH dependence of the solubilities of various carbonates and hydroxides, the present inventors have found that there exists a pH range in which calcium carbonate is less soluble than other carbonates and hydroxides, i.e., a pH range in which calcium carbonate preferentially precipitates.

[0027] The process for deriving the pH range that provides the condition for preferential precipitation of calcium carbonate will be described below. First, for carbon dioxide dissolved in an aqueous solution, a chemical equilibrium is established as shown in Equation 1. As shown in Equation 1, carbon dioxide (CO ２ ) into water (H ２ By dissolving it in carbonic acid (H ２ CO ３ ) is produced, and some of the carbonic acid is converted to hydrogen ions (H ＋ ) and bicarbonate ions (HCO ３ － ) and hydrogen ions are further ionized from the bicarbonate ions to form carbonate ions (CO ３ ２－ The direction of reaction at this chemical equilibrium (the gradient of the equilibrium) depends on the pH of the aqueous solution.

[0028] FIG. 2 is a graph showing the pH dependence of the abundance ratio of each form of carbon dioxide (carbonic acid, bicarbonate ion, carbonate ion) in an aqueous solution. As can be seen from FIG. 2, by increasing the pH above the neutral region, the carbonate ion (CO ３ ２－ ) can be increased in abundance.

[0029] On the other hand, the solubility of a compound is determined by the solubility product K ｓｐ For example, the divalent metal ion (M ２＋) and carbonate ions (CO ３ ２－ ) the solubility product K of carbonate ｓｐ The relationship between α and solubility Cs is expressed by Equations 2 to 4. Note that α is the fraction of carbonate ions relative to the total carbon dioxide concentration in the aqueous solution.

[0030]

[0031]

[0032]

[0033] From Equation 4, it can be seen that the solubility of carbonate is correlated with α, which is the fraction (abundance ratio) of carbonate ions relative to the total carbon dioxide concentration in an aqueous solution. As shown in Figure 2 above, the value of α is pH-dependent, and therefore the solubility of carbonate also exhibits pH dependence.

[0034] In addition, by adjusting the pH of the aqueous solution, hydroxides can be produced simultaneously with carbonates, so the solubility of hydroxides produced from divalent metal ions must also be taken into consideration. ２＋ ) and hydroxide ions (OH － ) The solubility product K of the hydroxide ｓｐ is expressed by Equation 5.

[0035]

[0036] Here, taking the logarithm of Equation 5 gives Equation 6.

[0037] The logarithm of the hydroxide ion concentration in Equation 6 is expressed using pH as shown in Equation 7.

[0038] Here, the solubility product K ｓｐ is a characteristic value (constant), and the divalent metal ion concentration in Equation 5 corresponds to the hydroxide solubility. Therefore, when Equation 7 is substituted into Equation 6 and the logarithm of the divalent metal ion concentration is shifted to the left side, it can be seen that the logarithm of the hydroxide solubility shows a correlation with pH.

[0039] 3 is a graph showing the relationship between the pH of an aqueous solution and the solubility (logarithm) of carbonates and hydroxides formed from divalent metal ions (calcium ions and magnesium ions). More specifically, FIG. 3 shows the pH dependence of the solubility (logarithm) of calcium carbonate, magnesium carbonate, calcium hydroxide, and magnesium hydroxide when calcium ions and magnesium ions are used as divalent metal ions.

[0040] As shown in Figure 3, the solubility of magnesium hydroxide is lowest when the pH exceeds 10.5, while the solubility of calcium carbonate is lowest when the pH is 10.5 or less. That is, by performing the carbonate precipitation reaction in the reaction unit 10 at a pH of 10.5 or less, calcium carbonate can be preferentially precipitated even when multiple divalent metal ions are present. This facilitates reducing the costs associated with preparing and procuring solutions that can be used as aqueous divalent metal ion solutions. In particular, by simply adjusting the pH, calcium carbonate can be preferentially precipitated from solutions that contain dissolved carbon dioxide and divalent metal ions and are readily available at low cost, such as seawater, river water, tap water, pure water, industrial wastewater and effluent, and landfill leachate.

[0041] Although not shown in Figure 3, when both calcium ions and magnesium ions are present, dolomite (CaMg(CO ３ ) ２) may be produced. Even if the solubility of dolomite is lower than that of calcium carbonate over the entire pH range, the reaction unit 10 may proceed with the carbonate precipitation reaction under conditions under which calcium carbonate preferentially precipitates (pH 10.5 or lower). As will be described later, the evaluation system 100A in this embodiment evaluates the amount of carbon dioxide fixation based on the calcium ion concentrations before and after the reaction in the reaction unit 10, and evaluates the amount of carbon dioxide fixation using the difference between the total amount of calcium ions before and after the reaction. Therefore, the amount of calcium ions used in the dolomite precipitation reaction and the amount of calcium ions used in the calcium carbonate precipitation reaction are both included in the total amount of calcium ions used in the carbonate precipitation reaction, and therefore do not affect the evaluation of the amount of carbon dioxide fixation.

[0042] The first measuring unit 20 is for measuring the calcium ion concentration before the reaction. More specifically, the first measuring unit 20 measures the calcium ion concentration in the solution before the solution is introduced into the reaction unit 10 as the free calcium ion concentration, rather than the calcium concentration regardless of the form of calcium present. In other words, the first measuring unit 20 determines the total amount of calcium ions present as ions before the carbonate precipitation reaction.

[0043] In this embodiment, the first measurement unit 20 may be provided with a measuring device capable of measuring calcium ion concentration on the line L1 provided upstream of the reaction unit 10. ２＋ Examples include those that use colorimetric or spectroscopic analysis using detection reagents and those that use ion-selective electrodes.

[0044] The second measurement unit 30 is for measuring the calcium ion concentration after the reaction. More specifically, the second measurement unit 30 measures the calcium ion concentration in the solution discharged from the reaction unit 10 as the free calcium ion concentration, rather than the calcium concentration regardless of the form of calcium present. That is, the second measurement unit 30 determines the total amount of calcium ions present as ions after the carbonate precipitation reaction. As the second measurement unit 30 in this embodiment, a measuring device capable of measuring calcium ion concentration may be provided on a line L2 provided downstream of the reaction unit 10.

[0045] The measurement results from the first measurement unit 20 and the second measurement unit 30 are input to the calculation unit 40. The first measurement unit 20 and the second measurement unit 30 may be any unit capable of measuring calcium ion concentrations before and after the reaction, and may be, but are not limited to, units in which measuring devices are permanently installed on lines L1 and L2 as shown in FIG. 1 . For example, the first measurement unit 20 and the second measurement unit 30 may be units in which measuring devices are detachably installed on lines L1 and L2, allowing measurements only when necessary. Furthermore, the first measurement unit 20 and the second measurement unit 30 may measure calcium ion concentrations in samples collected from lines L1 and L2, rather than installing measuring devices on lines L1 and L2.

[0046] Here, in the evaluation system 100A of this embodiment, the first measurement unit 20 and the second measurement unit 30 are not limited to being provided independently as shown in FIG. 1 . FIG. 4 is a schematic explanatory diagram showing another embodiment of the evaluation system 100A of this embodiment. As shown in FIG. 4 , another embodiment of the evaluation system 100A of this embodiment includes a system in which one measuring device M capable of measuring calcium ion concentration is provided in the reaction unit 10, and this measuring device M functions as both the first measurement unit 20 and the second measurement unit 30. Additionally, one example of a means for promoting and accelerating the carbonate precipitation reaction in the reaction unit 10 is a system in which a line L3 for introducing a reaction accelerator into the reaction unit 10 is provided, and the calcium ion concentration in the reaction unit 10 before and after the addition of the reaction accelerator is measured by the measuring device M. In this way, the calcium ion concentration measured by the measuring device M before the addition of the reaction accelerator corresponds to the calcium ion concentration before the reaction measured by the first measurement unit 20, while the calcium ion concentration measured by the measuring device M after the addition of the reaction accelerator corresponds to the calcium ion concentration after the reaction measured by the second measurement unit 30. Examples of the reaction accelerator introduced into reaction unit 10 via line L3 for promoting the carbonate precipitation reaction include a pH adjuster and fine bubbles for forming an extremely small gas-liquid interfacial reaction field. The timing of calcium ion concentration measurement by measuring device M may be any timing corresponding to before or after the carbonate precipitation reaction. For example, calcium ion concentration measurement by measuring device M may be performed before and after the application of a means for promoting the carbonate precipitation reaction (such as concentrating an aqueous solution) in addition to before and after the addition of the reaction accelerator via line L3 shown in Figure 4.

[0047] The calculation unit 40 is configured to quantify the amount of immobilized carbon dioxide based on the measurement results of the first measurement unit 20 and the second measurement unit 30. More specifically, the calculation unit 40 calculates the total amount of calcium ions utilized in the carbonate precipitation reaction in the reaction unit 10 from the difference between the calcium ion concentrations measured by the first measurement unit 20 and the second measurement unit 30. Because the first measurement unit 20 and the second measurement unit 30 measure the free calcium ion concentrations, this difference in calcium ion concentration (the total amount of calcium ions after the reaction minus the total amount of calcium ions before the reaction) corresponds to the total amount of calcium ions utilized in the carbonate precipitation reaction. Because this total amount of calcium ions correlates with the amount of calcium carbonate produced and because calcium carbonate is preferentially precipitated in the reaction unit 10, the calculation unit 40 calculates the amount of calcium carbonate produced from the total amount of calcium ions utilized in the carbonate precipitation reaction and converts this amount into the amount of carbon dioxide fixation, thereby enabling the amount of carbon dioxide fixation to be quantified.

[0048] The calculation unit 40 may include calculations and operations by an operator, for example, 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 fixed carbon dioxide. This makes it possible to quickly and accurately evaluate the amount of fixed carbon dioxide.

[0049] Conventionally, in order to quantitatively evaluate how much carbon dioxide has been fixed in carbon dioxide fixation by a carbonate precipitation reaction, it has been necessary to separate and recover all of the precipitated carbonate as a solid fraction and measure the weight of the separated and recovered solid fraction. However, in this case, in order to separate and recover the carbonate as a solid fraction, it is necessary to carry out an operation related to carbonate crystal growth, which increases the cost of using chemicals and the time required for crystal growth. In particular, when fine carbonate crystals are precipitated in the carbonate precipitation reaction, since they are precipitated as carbonate, even though carbon dioxide fixation has occurred, it is difficult to recover them as a solid fraction, and therefore it is not possible to evaluate the amount of carbon dioxide fixation.

[0050] On the other hand, in the evaluation system 100A of this embodiment, a specific carbonate (calcium carbonate) is preferentially precipitated as the carbonate produced by the carbonate precipitation reaction. This allows the change (difference) in calcium ion concentration before and after the reaction to correlate with the amount of carbonate produced, and further allows the amount of carbonate produced to be converted into the amount of carbon dioxide fixation. In other words, the amount of carbonate produced can be easily determined without separating and recovering the carbonate as a solid, and further allows for appropriate evaluation of the amount of carbon dioxide fixation. Therefore, the evaluation system 100A of this embodiment enables simple quantification of the amount of immobilized carbon dioxide and appropriate evaluation of the amount of carbon dioxide fixation without performing operations related to crystal growth. Furthermore, it is possible to reduce the cost and time required for using chemicals required for carbonate crystal growth, thereby reducing the workload involved in evaluating the amount of carbon dioxide fixation.

[0051] Furthermore, in the evaluation system 100A of this embodiment, even when carbonate precipitates as fine crystals through the carbonate precipitation reaction, it is possible to evaluate that carbon dioxide fixation has occurred without directly measuring the weight of the carbonate, thereby enabling highly accurate evaluation of the amount of carbon dioxide fixation. Examples of carbonate precipitation reactions in which carbonate precipitates as fine crystals include those in which the degree of supersaturation of ions necessary for the carbonate precipitation reaction is high, making it easy to generate fine crystals that serve as nuclei. More specifically, examples include a carbonate precipitation reaction at an extremely small gas-liquid interface reaction field using fine bubbles of carbon dioxide gas, and a carbonate precipitation reaction by concentration using a semipermeable membrane, which will be described later.

[0052] [Second Embodiment] Figure 5 is a schematic diagram illustrating a carbon dioxide fixation amount evaluation system according to a second embodiment of the present invention. The carbon dioxide fixation amount evaluation system 100B (hereinafter also referred to simply as "evaluation system 100B") according to this embodiment is the same as the evaluation system 100A according to the first embodiment described above, except that the reaction unit 10 includes a concentration unit 11 that produces a concentrate Wc by permeation using a semipermeable membrane 12, and the second measurement unit 30 includes a concentrate measurement unit 31 that measures the calcium ion concentration in the concentrate Wc and a permeate measurement unit 32 that measures the calcium ion concentration in the permeate Wp that has permeated the semipermeable membrane 12. The combined measurement results of the concentrate measurement unit 31 and the permeate measurement unit 32 are input into the calculation unit 40 of the evaluation system 100B as the measurement result of the second measurement unit 30. Note that the components of the evaluation system 100B according to this embodiment that are the same as those of the evaluation system 100A according to the first embodiment will not be described again.

[0053] In the evaluation system 100B of this embodiment, the reaction unit 10 includes a concentration unit 11 that performs concentration by permeation using a semipermeable membrane 12, and the carbonate precipitation reaction proceeds by concentrating the aqueous solution containing carbon dioxide and divalent metal ions introduced into the reaction unit 10 in this concentration unit 11. Hereinafter, in this embodiment, the carbon dioxide and divalent metal ions introduced via line L1 will be described as an example in the form of an aqueous solution in which both are dissolved in advance (hereinafter referred to as "introduced solution W0").

[0054] The concentration section 11 in this embodiment includes a semipermeable membrane 12 and a treatment tank 13 whose interior is partitioned by the semipermeable membrane 12 .

[0055] The treatment tank 13 may be made of any material, shape, or size as long as it is capable of storing the feed solution W0 introduced via line L1. The treatment tank 13 is divided into a first chamber 13a and a second chamber 13b, separated by a semipermeable membrane 12 (described later). The feed solution W0 is introduced into the first chamber 13a, and a concentrated solution Wc is produced in the first chamber 13a. Meanwhile, an aqueous solution containing at least carbon dioxide and calcium ions (not shown) is introduced into the second chamber 13b, and permeated water Wp that has passed through the semipermeable membrane 12 is introduced into the second chamber 13b. The concentrated solution Wc is then discharged to the outside of the system via line L3 provided on the first chamber 13a side, and the permeated water Wp (including the aqueous solution originally introduced into the second chamber 13b side) is discharged to the outside of the system via line L4 provided on the second chamber 13b side.

[0056] The semipermeable membrane 12 is a membrane that is permeable to water molecules in the aqueous solution (introducing solution W0 in this embodiment) but impermeable or difficult to permeate ions necessary for the carbonate precipitation reaction, such as carbonate ions and divalent metal ions, contained in the aqueous solution (introducing solution W0 in this embodiment). The semipermeable membrane 12 concentrates the introducing solution W0, which contains dissolved carbon dioxide and divalent metal ions, by allowing water molecules to pass through and reducing the concentration, while impermeable or difficult to permeate carbonate ions present in the introducing solution W0, thereby increasing the carbonate ion concentration. This allows carbonate ions and divalent metal ions to react efficiently in the first chamber 13a (concentrated solution Wc), efficiently producing and precipitating carbonate. Furthermore, when the introducing solution W0, which has a pH greater than 7, is concentrated using the semipermeable membrane 12, the concentrated solution Wc concentrated by the semipermeable membrane 12 has a reduced concentration of water molecules and an increased concentration of hydroxide ions. That is, the pH of the concentrate Wc increases, which facilitates the production of carbonate ions and allows the production and precipitation of carbonate to proceed more efficiently, as shown in Fig. 2. In the reaction section 10 of this embodiment, it is also preferable to allow the carbonate precipitation reaction to proceed at a pH of 10.5 or less, thereby providing conditions under which calcium carbonate is preferentially precipitated.

[0057] Furthermore, the semipermeable membrane 12 is preferably permeable to carbon dioxide gas dissolved in an aqueous solution (the introduce solution W0 in this embodiment). When the semipermeable membrane 12 allows carbon dioxide gas dissolved in the introduce solution W0 to pass through, the carbon dioxide gas concentration does not increase on the side of the concentrate Wc (the first chamber 13a side) obtained by concentrating the introduce solution W0. Therefore, the carbon dioxide gas contained in the introduce solution W0 ionizes to generate hydrogen ions, which prevents an unintended decrease in the pH of the concentrate Wc. This facilitates the generation of carbonate ions and the maintenance of a pH range in which calcium carbonate preferentially precipitates. In particular, when an aqueous solution with a pH above 7 is used as the introduce solution W0, 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 facilitates the generation of carbonate ions.

[0058] In this embodiment, the reaction section 10 is provided with a concentration section 11, and the feed solution W0 is concentrated using a semipermeable membrane 12. This allows the aqueous solution to be concentrated and the carbonate precipitation reaction to proceed with less energy than when concentrating an aqueous solution by heating and evaporating it in conventional carbonate precipitation (such as a method called simple distillation).

[0059] Furthermore, by concentrating the feed solution W0 using the concentrating section 11 as the reaction section 10 in this embodiment, the ion concentration required for the carbonate precipitation reaction increases near the semipermeable membrane 12, maintaining a high supersaturation state. In this case, while many fine crystals that serve as nuclei for carbonate crystallization are produced, the crystal growth rate decreases. Therefore, recovering these carbonate crystals as a solid by a separation operation such as filtration requires the use of chemicals and time for crystal growth. However, in the evaluation system 100B in this embodiment, there is no need to recover carbonate crystals as a solid. Therefore, even if fine carbonate crystals are produced in the reaction section 10 in this embodiment, it is possible to easily quantify and appropriately evaluate the amount of carbon dioxide fixation.

[0060] In the concentrating section 11 of this embodiment, it is preferable to introduce a solution having a salt concentration into both the concentrating side (first chamber 13a side) and the diluting side (second chamber 13b side) of the treatment tank 13. Furthermore, it is more preferable that the salt concentration of the solution introduced into the diluting side is the same as the salt concentration of the introduce solution W0 introduced into the concentrating side. And it is even more preferable that the salt concentration of the solution introduced into the diluting side is higher than the salt concentration of the introduce solution W0 introduced into the concentrating side. By introducing a solution into the diluting side having a salt concentration, the difference in salt concentration between the introduce solution W0 introduced into the concentrating side and the introduce solution W0 introduced into the diluting side is reduced, allowing the introduce solution W0 to be concentrated with less energy and the carbonate precipitation reaction related to carbon dioxide fixation to proceed. Furthermore, by making the salt concentration of the solution introduced into the diluting side higher than the salt concentration of the introduce solution W0 introduced into the concentrating side, osmotic pressure is generated, allowing the introduce solution W0 introduced into the concentrating side to be concentrated with less energy.

[0061] The reaction section 10 in this embodiment may be provided with a pressurizing means as the concentrating section 11. The pressurizing means is not particularly limited as long as it is a means for applying pressure to the introduced solution W0 and the semipermeable membrane 12 in the first chamber 13a. This allows concentration without adjusting the salt concentration of each solution introduced into the treatment tank 13. Furthermore, the pressurizing means may be used in conjunction with adjusting the salt concentration of each solution introduced into the treatment tank 13. This allows for a reduction in the cost of using chemicals required for adjusting the salt concentration as well as a reduction in the energy required for the pressurizing means.

[0062] If the feed solution W0 contains other salts that are not involved in the carbonate precipitation reaction, has a pH of 7 or higher, and contains carbonate ions, the semipermeable membrane 12 may be a membrane that is permeable to water molecules, carbon dioxide gas, and hydrogen ions, but is impermeable to or difficult to permeate other ions (except hydrogen ions) necessary for the carbonate precipitation reaction. In this case, hydrogen ions can be prevented from remaining in the first chamber 13a (concentrated liquid Wc) and increasing in concentration, thereby preventing a decrease in pH. This increases the concentration of carbonate ions in the concentrated liquid Wc, facilitating the formation and precipitation of carbonate.

[0063] In the evaluation system 100B of this embodiment, the calcium ion concentrations before and after the reaction are measured by the first measurement unit 20 and the second measurement unit 30, and the total amounts of calcium ions present as ions before and after the carbonate precipitation reaction are determined.

[0064] The first measuring unit 20 in this embodiment is similar to the content shown in the evaluation system 100A described above, and therefore a description thereof will be omitted.

[0065] In this embodiment, the second measuring unit 30 includes a concentrate measuring unit 31 that measures the calcium ion concentration on the concentrate Wc side, and a permeate measuring unit 32 that measures the calcium ion concentration on the permeate Wp side that has permeated the semipermeable membrane 12. More specifically, as shown in Fig. 5, the concentrate measuring unit 31 may be provided with a measuring device capable of measuring calcium ion concentration on a line L3 provided downstream of the reaction unit 10, and the permeate measuring unit 32 may be provided with a measuring device capable of measuring calcium ion concentration on a line L4 provided downstream of the reaction unit 10.

[0066] The concentrated solution Wc produced by using the semipermeable membrane 12 and discharged to the outside of the system through line L3 is a product obtained after the carbonate precipitation reaction, so that the calcium ion concentration after the reaction can be determined easily and accurately by providing a concentrated solution measuring unit 31 and measuring the calcium ion concentration contained in the concentrated solution Wc. This increases the accuracy of the quantification of the amount of fixed carbon dioxide, enabling more appropriate evaluation.

[0067] The second measurement unit 30 is configured to perform measurements using at least the concentrate measurement unit 31, and the permeate measurement unit 32 may be omitted. Whether the permeate measurement unit 32 can be omitted depends on the ion permeability of the semipermeable membrane 12. When a semipermeable membrane 12 that is completely impermeable to calcium ions is used, the second measurement unit 30 can measure the calcium ion concentration in the concentrate Wc to determine the total amount of calcium ions after the reaction, and therefore the permeate measurement unit 32 can be omitted. On the other hand, when a semipermeable membrane 12 that is poorly permeable to calcium ions is used, some of the excess calcium ions generated during the carbonate precipitation reaction in the reaction unit 10 will migrate to the dilution side (the second chamber 13b side). In this case, to determine the total amount of calcium ions after the reaction, the second measurement unit 30 must measure the calcium ion concentration in the concentrate Wc as well as the calcium ion concentration in the permeate Wp. Therefore, the permeate measurement unit 32 is not omitted and functions as the second measurement unit 30.

[0068] The measurement results from the first measuring unit 20 and the second measuring unit 30 (the concentrate measuring unit 31 and the permeate measuring unit 32) are input to the calculation unit 40. The calculation unit 40 quantifies the amount of immobilized carbon dioxide based on the measurement results from the first measuring unit 20 and the second measuring unit 30. Here, the details related to the quantification of the amount of immobilized carbon dioxide are the same as those shown in the evaluation system 100A described above, and therefore a description thereof will be omitted.

[0069] In the evaluation system 100B of this embodiment, similar to the evaluation system 100A described above, the amount of carbonate produced can be easily determined without separating and recovering the carbonate as a solid fraction, and further, it becomes possible to appropriately evaluate the amount of carbon dioxide fixation. Furthermore, in the evaluation system 100B, concentration is performed by permeation using a semipermeable membrane, so that carbonate can be obtained by proceeding with the carbonate precipitation reaction with less energy than when concentrating by heating and evaporating a solution. During this process, fine crystals may precipitate as carbonate. However, as described above, when evaluating the amount of carbon dioxide fixation using the evaluation system 100B, there is no need to recover the carbonate as a solid fraction, and therefore it becomes possible to simply quantify and appropriately evaluate the amount of carbon dioxide fixation.

[0070] The above-described embodiments show examples of the system and method for evaluating the amount of fixed carbon dioxide. The system and method for evaluating the amount of fixed carbon dioxide according to the present invention are not limited to the above-described embodiments, and the system and method 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.

[0071] Furthermore, the carbon dioxide fixation amount evaluation system and carbon dioxide fixation amount evaluation method of the present invention can be applied to various equipment and facilities (systems) that require quantitative evaluation of the carbon dioxide fixation amount. Specifically, they can be applied to 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.

[0072] The system and method for evaluating the amount of fixed carbon dioxide of the present invention enable the quantification of the amount of fixed carbon dioxide (amount of fixed carbon dioxide) in the fixation of carbon dioxide by a carbonate precipitation reaction, and are suitable for use in evaluating the amount of fixed carbon dioxide. Furthermore, the system and method for evaluating the amount of fixed carbon dioxide of the present invention do not require crystal growth of carbonate or direct weight measurement of carbonate, and are therefore particularly suitable for use in cases where carbonate precipitates as fine crystals in the carbonate precipitation reaction.

[0073] 100A, 100B Carbon dioxide fixation amount evaluation system, 10 Reaction section, 11 Concentration section, 12 Semipermeable membrane, 13 Treatment tank, 13a First chamber, 13b Second chamber, 20 First measurement section, 30 Second measurement section, 31 Concentrate measurement section, 32 Permeate measurement section, 40 Calculation section, L1 to L4 Lines, M Measuring device, W0 Intake solution, Wc Concentrate, Wp Permeate

Claims

1. A system for evaluating an amount of fixed carbon dioxide, comprising: a reaction unit that performs a carbonate precipitation reaction for fixation of carbon dioxide under conditions in which calcium carbonate preferentially precipitates; a first measurement unit that measures a calcium ion concentration before the reaction; a second measurement unit that measures a calcium ion concentration after the reaction; and a calculation unit that quantifies the amount of fixed carbon dioxide based on measurement results from the first measurement unit and the second measurement unit.

2. The system for evaluating the amount of fixed carbon dioxide as described in claim 1, wherein the reaction section carries out a carbonate precipitation reaction at a pH of 10.5 or less.

3. The system for evaluating the amount of carbon dioxide fixation described in claim 1 or 2, characterized in that the reaction unit includes a concentration unit that produces a concentrated liquid by permeation using a semipermeable membrane, and the second measurement unit measures at least the calcium ion concentration on the concentrated liquid side.

4. A method for evaluating an amount of fixed carbon dioxide, comprising: a reaction step of carrying out a carbonate precipitation reaction for fixation of carbon dioxide under conditions in which calcium carbonate preferentially precipitates; a first measurement step of measuring a calcium ion concentration before the reaction; a second measurement step of measuring a calcium ion concentration after the reaction; and a calculation step of quantifying the amount of fixed carbon dioxide based on the measurement results of the first measurement step and the second measurement step.

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