JP7010115B2 - Chemical heat storage material and chemical heat storage system - Google Patents

Description

The present invention relates to a chemical heat storage material made of a double hydroxide and the like.

With the rise of environmental awareness, research and development aimed at saving energy and improving energy efficiency are being actively carried out. One of them is a chemical heat storage system using a chemical heat storage material that has a high heat storage density and can store heat for a long period of time without heat retention. The chemical heat storage system stores or releases a heat medium (water, etc.) from the chemical heat storage material to dissipate heat (heat generation) and store heat (heat absorption). By using a chemical heat storage system, relatively low-temperature waste heat (or waste heat) generated from various devices and plants can be effectively utilized. Descriptions related to such a chemical heat storage system can be found in the following patent documents and the like.

Japanese Unexamined Patent Publication No. 2009-186119

Abstracts of the 81st Annual Meeting of the Society of Chemical Engineers 2016/3 / 13-15 "Effects of Hydrotalcite on Dehydration and Hydration Reactions by Interlayer Anion Species and Application to Chemical Heat Storage Materials"

Patent Document 1 describes a chemical heat pump (chemical heat storage system) using a chemical heat storage material composed of a composition (composite hydroxide) in which a hygroscopic metal salt (lithium chloride, etc.) is added to magnesium hydroxide or calcium hydroxide. is suggesting. In the chemical heat storage material of Patent Document 1, since the heat storage reaction occurs in a wide temperature range, waste heat or the like in a specific temperature range cannot be effectively utilized.

Non-Patent Document 1 describes a layered double hydroxide (LDH: layered) in which CO ₃ ^2- in [Mg ₆ Al ₂ (CO ₃ ) (OH) _{16.4H 2} O] (HT: Hydrotalcite) is replaced with ^Cl- . We are proposing a chemical heat storage material consisting of double hydroxide). The chemical heat storage material of Non-Patent Document 1 also cannot effectively utilize waste heat and the like in a specific temperature range because the heat storage reaction occurs in multiple stages. Further, in the case of a layered double hydroxide containing Cl, a side reaction in which HCl is desorbed is likely to occur at the time of heat storage (during the release of water). The desorbed HCl causes corrosion of the piping of the chemical heat storage device (chemical heat storage system). In Non-Patent Document 1, the withdrawal of HCl and its countermeasures are not examined at all.

The present invention has been made in view of such circumstances, and an object of the present invention is to provide a new chemical heat storage material or the like that can be efficiently operated in a specific temperature range.

As a result of diligent research to solve this problem, the present inventor synthesizes a double hydroxide having a composition different from the conventional one, and stores the double hydroxide in a specific temperature range (endothermic) and regeneration (heat dissipation). I succeeded in making it. By developing this result, the present invention described below was completed.

《Chemical heat storage material》
(1) The chemical heat storage material of the present invention is a chemical heat storage material composed of a double hydroxide that generates or absorbs heat by storing or releasing water, and the double hydroxide is Mg ₂ when water is stored. Al (OH) ₆ Cl-nH ₂ O (n = 0 to 3.5).

(2) When the chemical heat storage material of the present invention is used, a heat storage reaction is caused in a specific temperature range (for example, 315 to 385 ° C, 325 to 370 ° C, further 335 to 355 ° C), and such a specific temperature range is generated. Can effectively utilize the waste heat in.

In addition, the present inventor suppresses the generation (desorption) of HCl by operating the double hydroxide of the present invention under an appropriate heating temperature and heating time, and endothermic (heat storage) and heat dissipation (regeneration). ) Can be repeated stably.

The reason why the chemical heat storage material of the present invention causes a heat storage reaction intensively (so-called one step) in a specific temperature range is that it has a layered structure in which Mg and Al are geometrically uniformly distributed, and ^Cl- Is presumed to be because it is held uniformly between each of these layers.

《Chemical heat storage system》
The present invention can also be grasped as a chemical heat storage system (chemical heat storage device) using the above-mentioned chemical heat storage material. The chemical heat storage system introduces, for example, a reactor that houses a chemical heat storage material, a means (mechanism) that heats and aggregates water as a heat medium, and supplies and discharges water (water vapor) to the reactor, and waste heat. It is configured by providing means, means for opening and closing the flow path of water (steam), waste heat, etc., means for controlling their operation, and the like. At that time, it is preferable that the operation of the chemical heat storage system is controlled so that the heating temperature (T) and the holding time (Δt) of the double hydroxide constituting the chemical heat storage material are within a predetermined range. This makes it possible to suppress the generation of HCl (desorption of Cl) and efficiently absorb and dissipate heat without corroding the chemical heat storage device (system).

Specifically, it is preferable that the control of the chemical heat storage system is such that the heating temperature (T / ° C.) of the double hydroxide and its holding time (Δt / min) satisfy the following equations.
Δt <139exp (29.7-0.09T) (Equation 1)

The double hydroxide according to the present invention immediately releases HCl at 438 ° C. or higher, and is not regenerated even if it absorbs water. Therefore, the heating (holding) temperature described above is preferably 400 ° C. or lower, 385 ° C. or lower, and more preferably 365 ° C. or lower. On the other hand, if the heating temperature is too low, the heat storage reaction due to water discharge becomes very slow. Therefore, the heating (holding) temperature described above is preferably 315 ° C. or higher, 325 ° C. or higher, and more preferably 335 ° C. or higher.

"others"
Unless otherwise specified, "x to y" as used herein include a lower limit value x and an upper limit value y. A range such as "a to b" may be newly established with any numerical value included in the various numerical values or numerical ranges described in the present specification as a new lower limit value or upper limit value.

It is a list of reaction formulas of the double hydroxide which concerns on this invention. It is a temperature desorption spectrum obtained by mass spectrometry of the gas generated by heating a sample. It is a warm-up desorption spectrum obtained when the sample is held at a target temperature for a certain period of time. It is a graph which shows the relationship between the heating temperature (T) and the holding time (Δt) at which the generation of HCl is started. It is a graph which shows the relationship between the integrated value of thermal conductivity (TCD) which indicates the emission amount of _H2O , and the elapsed time from the start of heating. It is a figure which shows the preferable range of a heating temperature (T) and a holding time (Δt). It is a TG curve when the sample is thermogravimetric analysis. It is an X-ray diffraction pattern which concerns on a synthetic sample and a regenerated sample. It is a TG curve when the regenerated sample is thermogravimetric analysis. It is a schematic diagram which shows one form example of a chemical heat storage system.

One or more components arbitrarily selected from the present specification may be added to the components of the present invention described above. The contents described in this specification may appropriately apply not only to chemical heat storage materials but also to chemical heat storage systems and the like. It can be a component of a method or a component of an object. Which embodiment is the best depends on the target, required performance, and the like.

"reaction"
The double hydroxide according to the present invention is basically composed of Mg ₂ Al (OH) ₆ Cl, but in consideration of forming a hydrate in the normal temperature range, Mg ₂ Al (OH) ₆ Cl-nH It is expressed as ₂ O (n ≧ 0). However, in the present specification, those having Mg ₂ Al (OH) ₆ Cl regardless of whether they are hydrates or not (regardless of the value of n) are simply referred to as double hydroxides.

When the double hydroxide is heated from the room temperature range, it mainly undergoes a three-step reaction (reaction A → reaction B → reaction C) as shown in FIG. 1 according to the temperature range. Each reaction will be described below in sequence.

(1) First stage (reaction A)
As the double hydroxide is heated (when _QA is added), a dehydration reaction occurs in which the water molecules ( _H2O ) bound to the double hydroxide are separated (from the left side to the right side of the formula A1). Progress). This dehydration reaction occurs in a temperature range of approximately 50 to 150 ° C. Through this step, anhydrous double hydroxide (Mg ₂ Al (OH) ₆ Cl) is obtained.

The reaction at this stage is a reversible reaction. Therefore, when water is supplied, the anhydrous double hydroxide becomes a hydrate of the double hydroxide by the hydration reaction and releases heat ( _QA ) (progress from the right side to the left side of the formula A1).

(2) Second stage (reaction B)
When the double hydroxide is further heated (when _QB is added), a heat storage reaction (original endothermic reaction) in which water is separated from the hydroxyl group in the double hydroxide occurs. (Progress from the left side to the right side of equations B1 and B2). This heat storage reaction occurs in a temperature range of approximately 315 to 400 ° C. Depending on the degree of dehydration, the reaction represented by the formula B1 or the formula B2 will occur.

Since the reaction at this stage is also a reversible reaction, when water is supplied, heat ( _QB ) is released by the regeneration reaction (heat dissipation reaction) and returns to Mg ₂ Al (OH) ₆ Cl (formulas B1 and B2). From the right side to the left side of).

(3) Third stage (reaction C)
When the double hydroxide is further heated (when _QC is added), a desorption reaction occurs in which Cl becomes HCl and is removed from the double hydroxide (progress from the left side to the right side of the formulas C1 to C3). ). This elimination reaction occurs in a temperature range of about 415 ° C. or higher. It should be noted that the reaction as shown in the formulas C1 to C3 is shown depending on the heating history and the heating temperature. Since the reaction at this stage is an irreversible reaction, the original double hydroxide is not regenerated only by supplying water (formulas C1 to C3 do not proceed from the right side to the left side).

However, such a release reaction of HCl can occur even before the temperature range described above is reached. For example, when the double hydroxide is held for a long time even in the temperature range of 400 ° C. or lower, an irreversible reaction as shown in the formulas C1 to C3 may occur in parallel with the heat storage reaction shown in the formulas B1 and B2.

However, if the holding time (Δt) is set within a predetermined range according to the heating temperature (T), the generation of HCl can be suppressed. For example, the heating temperature (T) and the holding time (Δt) of the double hydroxide may satisfy the above-mentioned formula 1.

A double hydroxide (sample) was synthesized and its characteristics were clarified by various experiments. The present invention will be described in more detail based on these specific examples.

《Manufacturing of samples》
A sample (Mg ₂ Al (OH) ₆ Cl-nH ₂ O) was synthesized by the following steps. In addition, in order to avoid mixing of CO ₂ from the atmosphere, all the steps shown below were carried out in an inert gas (Ar) atmosphere.

(1) Solution preparation process As raw materials, MgCl 2.6H ₂ O (manufactured by Wako Pure Chemical Industries, Ltd./purity 98%), AlCl _{3.6H 2} O (manufactured by Wako Pure Chemical Industries, _Ltd./purity 98%) and ammonia Water (manufactured by Wako Pure Chemical Industries, Ltd./concentration 25%) was prepared. These were used as purchased. In addition, deionized water from which dissolved oxygen was removed in advance by Ar aeration was also prepared.

MgCl 2.6H ₂ O: 20.3 g (0.1 mol) and AlCl _3.6 H ₂ O: 12.1 g ₍ 0.05 mol) were dissolved in deionized water: 500 mL to prepare a mixed aqueous solution. Further, the above-mentioned ammonia water (concentration 25%): 160 mL was diluted with deionized water: 500 mL to obtain dilute ammonia water.

(2) Mixing stabilization step (generation step)
The entire amount of the mixed aqueous solution was added dropwise to the dilute ammonia water being stirred with a magnetic stirrer at a rate of about 8 mL / min. After the completion of the dropping, stirring was continued at room temperature for about 4 hours. In this way, a mixed liquid in a solid-liquid coexisting state was obtained.

(3) Solid-liquid separation and cleaning process (extraction process)
The solid was separated from the mixture by centrifugation (15000 rpm x 10 min). The solid was dispersed in 1000 mL of deionized water and washed. The dispersion was similarly centrifuged to extract a solid.

(4) Drying step The solid was heated on a hot plate at 150 ° C. for 5 hours. The sample was synthesized in this way.

When the obtained sample was analyzed by X-ray diffraction (XRD: Cu-Kα, λ = 1.5418 Å), it was confirmed that it was Mg ₂ Al (OH) ₆ Cl-nH ₂ O (Sample A in FIG. 6). reference). The n value indicating the number of adsorbed water in the form of a molecule changes depending on the heating conditions during the drying step. The n value is specified by, for example, thermogravimetric analysis (TG) to determine the rate of weight loss during the dehydration reaction (reaction A in FIG. 1). In the above-mentioned sample, n≈1.5. Hereinafter, unless otherwise specified, the sample will be described as Mg ₂ Al (OH) ₆ Cl-1.5H ₂ O.

《Endothermic reaction》
(1) The above-mentioned sample: 20 mg was heated at 5 ° C./min under an air flow of He: 60 mL / min, and the generated gas was analyzed by a mass spectrometer (REGA Ptype manufactured by ULVAC, Inc.). The temperature-increased elimination spectrum obtained at this time is shown in FIG. m / e = 18 indicates H ₂ O, and m / e = 36 indicates HCl. The signal strength shown in FIG. 1 is an arbitrary unit, and the vertical axis of HCl is shown by enlarging the vertical axis of _H2O by about 6 times.

(2) As is clear from FIG. 2, as the temperature of the sample rises, the dehydration reaction (reaction A in FIG. 1) → the heat storage reaction (reaction B in FIG. 1) → the withdrawal reaction of HCl (reaction C in FIG. 1). It can be seen that they occur sequentially. It was also found that HCl was generated even before the heat storage reaction was completed.

When the generation of HCl (withdrawal of Cl) occurs, the sample cannot return to the original composition (Mg ₂ Al (OH) ₆ Cl-nH ₂ O) only by supplying H ₂ O. Therefore, in order to be able to repeatedly generate heat and absorb heat by storing and releasing _H2O (heat medium), water absorption and desorption for double hydroxides are performed under conditions where HCl is not generated (Cl is separated). Need to do

<< Suppression of HCl generation / Upper limit of heating temperature >>
In the same manner as described above, the sample (20 mg) was heated while raising the temperature (5 ° C./min), and after reaching the target temperature (Te), the sample (Te) was held at that temperature for a certain period of time. The gas generated during the heating was detected using a mass spectrometer. The warm-up desorption spectrum when Te = 338 ° C. or 397 ° C. is also shown in FIG. 3A.

As can be seen from FIG. 3A, when Te = 338 ° C., HCl was generated about 90 minutes after the temperature was reached. Further, when Te = 397 ° C., HCl was generated about 5 minutes after the temperature was reached. In each case, the release of water (reaction B) was not completed at the start of the generation of HCl.

As described above, it was found that the higher the holding temperature, the shorter the time it takes to generate HCl. Similarly, for other target temperatures (holding temperatures) between 338 ° C and 397 ° C, the holding time until HCl was generated was investigated. The correlation between the holding temperature (T) thus obtained and the holding time (Δt) from reaching that temperature to the start of generation of HCl is summarized in FIG. 3B.

From the results shown in FIG. 3B, when the heating temperature (T) and the holding time (Δt) after reaching the temperature are controlled to be within the range satisfying the following equation 1, HCl is generated. It was found that H ₂ O was released without.
Δt <139exp (29.7-0.09T) (Equation 1)
This formula was calculated by approximating the experimental data with curve fitting.

<< Starting temperature of heat storage reaction / Lower limit of heating temperature >>
It can be seen from FIG. 2 that the temperature at which the release of _H2O starts from the sample (Mg ₂ Al (OH) ₆ Cl) is about 200 ° C. Therefore, the lower limit of the heating temperature required to start the heat storage reaction (reaction B) is about 200 ° C. However, at around 200 ° C., the release rate of _H2O is very slow. Therefore, it is necessary to set a temperature in consideration of the practicality of the chemical heat storage material as the start temperature of the heat storage reaction.

(1) In the same manner as described above, the sample (20 mg) is heated while raising the temperature (5 ° C./min), and after reaching the target temperature (Te), the sample (Te) is held at that temperature for a certain period of time during heating. The produced gas was detected by a mass spectrometer. The integrated value of the thermal conductivity (TCD) of the produced gas obtained when Te = 327 ° C. or 338 ° C. is shown in FIG. 4A. The integrated value is proportional to the amount of water released from the start of temperature rise.

(2) As is clear from FIG. 4A, for example, when releasing H ₂ O equivalent to 1.0 × 105 ⁽ μV × min), when Te = 327 ° C, it is higher than when Te = 338 ° C. It will be longer for about 100 minutes. Therefore, it is preferable that the heating temperature (T) and the holding time (Δt) after reaching the temperature are adjusted within the range satisfying the following formula 2.
232exp (30.6-0.1T) <Δt (Equation 2)
This formula was calculated by modifying the above-mentioned formula 1 based on the experimental data.

(3) Equation 1 and Equation 2 are shown together in FIG. 4B. By keeping the heating temperature (T) and holding time (Δt) of the double hydroxide within the range determined by both equations (within the hatched region of FIG. 4B), the release of HCl is suppressed and its practicality is ensured. Is possible. An example of a form of a chemical heat storage system utilizing this is schematically shown in FIG.

In this chemical heat storage system, first, among the heat storage bodies, the maximum point (A) of the temperature rise rate and the minimum point (B) of the temperature rise rate are detected, respectively. Next, at each point, the temperature at point A (Ta) and the temperature at point B (Tb) both satisfy the above-mentioned equations 1 and 2 at the same time (that is, within the range of FIG. 4). Control the temperature (T) or holding time (Δt).

It should be noted that points A and B can be specified, for example, by installing a plurality of thermocouples inside the heat storage container and detecting their temperature changes. Further, the temperature of each point can be controlled as follows, for example. As shown in FIG. 8, a plurality of heat medium (water vapor and the like) flow paths are provided, and valves are provided in each flow path (particularly on the upstream side). Depending on the temperature distribution of the heat storage body, each valve opens and closes each flow path or adjusts the flow rate of the heat medium flowing through each flow path. By such temperature control, it is possible to construct a chemical heat storage system that satisfies both equations 1 and 2 at the same time.

《Heat storage reaction / release of water》
The heat storage amount of the double hydroxide (Mg ₂ Al (OH) ₆ Cl) is proportional to the number of molecules of water that can be absorbed and released (the number of mols of water with respect to 1 mol of the double hydroxide). Therefore, the number of molecules of water that can be absorbed and released was clarified as follows within the range that does not generate HCl.

(1) Sample: 18 mg was heated at 5 ° C./min under an air flow of N ₂ : 100 mL / min, held at 325 ° C. for 180 minutes, then further heated at the same rate, and 60 at 900 ° C. Held for minutes. The result of thermogravimetric analysis of the mass change during this heating process is shown in FIG.

(2) The weight loss due to the water released in the heat storage reaction (reaction B in the figure) was 14.4%. This corresponds to 1.9 molecules (≈2 molecules) of water per ₆ Cl of Mg ₂ Al (OH). Therefore, it was confirmed that the heat storage reaction shown in the formula B1 occurred under the condition that HCl was not generated.

《Regeneration reaction / occlusion of water》
It was confirmed by producing various regenerated samples that the double hydroxide can be regenerated by absorbing water after being discharged. Both steps were carried out in an inert gas atmosphere.

(1) Production of regenerated sample (i) The double hydroxide (referred to as "Sample A") synthesized by the method described above was pulverized in an agate mortar. The powder was heated (327 ° C. × 3 hours) using the same mortar. In this way, a double hydroxide (referred to as "Sample B10") after the heat treatment corresponding to the heat storage reaction was prepared.

Sample B10 (0.1 g) was placed in a vial (5 mL) and dispersed in degassed deionized water (4 mL). After that, the sealed vial was immersed in an ultrasonic cleaner and irradiated with ultrasonic waves for 1 hour. The obtained dispersion was transferred to a beaker, and the water was evaporated while heating and stirring (150 ° C. × 1 hour). The solid double hydroxide (referred to as "Sample B1") regenerated in this manner was obtained.

(Ii) Sample A was pulverized with a ball mill (P-4 manufactured by Fritzch). Milling with a ball mill was performed at 200 rpm × 15 min four times. The obtained powder was subjected to the same heat treatment as in (i) described above to obtain a double hydroxide (referred to as "Sample B20").

A double hydroxide (referred to as "Sample B2") was obtained by subjecting the sample B20 to the same regeneration treatment as in (i) described above. However, in the reproduction process at this time, the ultrasonic irradiation time was extended from 1 hour to 3 hours.

(Iii) The sample B20 was also regenerated by reflux. That is, sample B20 (0.1 g) was placed in a round bottom flask (100 mL) and dispersed in degassed deionized water (20 mL). The flask was immersed in an oil bath and heated and stirred with a hot stirrer (105 ° C. × 8 hours). The obtained dispersion was transferred to a beaker and heated and stirred in the same manner as in (i) above to evaporate the water. The solid double hydroxide (referred to as "Sample B3") regenerated in this manner was obtained.

(2) X-ray diffraction The crystal structures of Sample A and Samples B1 to B3 were analyzed by X-ray diffraction (Cu—Kα, λ = 1.5418 Å). The obtained results are summarized in FIG.

As is clear from FIG. 6, the XRD patterns of the sample A and the samples B1 to B3 are basically the same, and all the regenerated samples B1 to B3 have the same double hydroxide as the sample A at the time of synthesis. It was confirmed that it was (Mg ₂ Al (OH) ₆ Cl-nH ₂ O), that is, it was regenerated.

(3) Regeneration rate Samples B1 to B3 were reheated, and the regeneration rate by the above-mentioned regeneration treatment was examined. Reheating was performed by reheating each sample (about 18 mg) in an air stream of N ₂ (100 mL / min) at a heating rate of 5 ° C./min and holding for 60 minutes after reaching 900 ° C. The mass change during reheating was thermogravimetric analysis. As an example, the TG curve for sample B3 is shown in FIG.

The number of water molecules (n) bound to the double hydroxide after the regeneration treatment can change depending on the drying conditions. Therefore, attention was paid to the weight loss rate generated in the reaction B and the reaction C after the completion of the reaction A. The weight loss rate of each sample is as follows.
Sample B1: 33.8%, Sample B2: 34.0%, Sample B3: 35.6%

Assuming that the reactions B1 and B2 and the reactions C1 and C2 shown in FIG. 1 occur and Mg ₂ Al (OH) ₆ Cl changes to Mg ₂ Al (O) _3.5 , the weight loss rate at that time is 38. It becomes 0.2% (theoretical value). Since the weight of the double hydroxide increases as the regeneration reaction progresses, the weight loss rate of the thermogravimetric analysis and the regeneration rate of the double hydroxide have a positive correlation. Therefore, the regeneration rate of each sample was obtained from the ratio of the theoretical value of the weight loss rate to the weight loss rate of each sample obtained from the thermogravimetric analysis as follows. It was also found that a sufficiently high reproduction rate can be obtained in either case.
Sample B1: 88%, Sample B2: 89%, Sample B3: 93%

From the above, when the double hydroxide according to the present invention is used, heat can be stored by discharging water in a specific temperature range without generating HCl, and the double hydroxide after the heat storage can be sufficiently absorbed by water absorption. It was also confirmed that it was reproducible at a high rate.

Claims (3)

A chemical heat storage material consisting of a double hydroxide that generates or absorbs heat by storing or releasing water.
The double hydroxide is a chemical heat storage material that becomes Mg ₂ Al (OH) ₆ Cl-nH ₂ O (n = 0 to 3.5) when water is occluded. A chemical heat storage system using the chemical heat storage material according to claim 1.
A chemical heat storage system in which the heating temperature (T / ° C.) of the double hydroxide and the holding time (Δt / min) of the heating temperature satisfy the following equations.
Δt <139exp (29.7-0.09T) (Equation 1) The chemical heat storage system according to claim 2, wherein the heating temperature (T) is 315 ° C. or higher.

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