JP2019182924A - Chemical heat storage material, and chemical heat storage system - Google Patents

Abstract

To provide a chemical heat storage material consisting of double hydroxide capable of storing heat (heat absorption) ad regeneration (heat release) in a specific temperature range.SOLUTION: There is provided a chemical heat storage material consisting of double hydroxide which generates or absorbs heat by heat storage or release of water. The double hydroxide becomes MgAl(OH)Cl-nHO (n=0 to 3.5), when absorbs the water. When a chemical heat storage system is constituted by using the chemical heat storage material, a heating temperature of the double hydroxide (T/°C) and retention time of the temperature (Δt/min) is preferably controlled in a range satisfying Δt<139exp(29.7-0.09T). Thereby a heat storage reaction and a regeneration reaction of the double hydroxide can be repeatedly conducted while suppressing generation of HCl. For securing a practical water release speed, a heating temperature (T) is preferably set at 315°C or higher.SELECTED DRAWING: Figure 4B

Description

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

  With the heightened awareness of the environment, research and development aimed at saving energy and improving energy efficiency are being actively pursued. As one of them, a chemical heat storage system using a chemical heat storage material that has a large heat storage density and can store heat for a long period of time without keeping heat is attracting attention. The chemical heat storage system performs heat dissipation (heat generation) and heat storage (heat absorption) by causing a chemical heat storage material to store or release a heat medium (such as water). When a chemical heat storage system is used, relatively low-temperature waste heat (or exhaust heat) generated from various devices or plants can be effectively utilized. A description relating to such a chemical heat storage system can be found in the following patent documents.

JP 2009-186119 A

Abstracts of the 81st Annual Meeting of the Chemical Engineering Society of Japan 2016/3 / 13-15 “Effects of hydrotalcite on dehydration and hydration by interlayer anion species and its application to chemical heat storage materials”

  Patent Document 1 discloses 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 or the like) 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 used.

Non-Patent Document _{1, [Mg 6 Al 2 (CO} 3) (OH) 16 · 4H 2 O] (HT: Hydrotalcite) CO 3 2- and Cl in ^- substituted with layered double hydroxide (LDH: layered A chemical heat storage material consisting of double hydroxide is proposed. The chemical heat storage material of Non-Patent Document 1 also cannot effectively utilize waste heat or 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 easily occurs during heat storage (when water is released). The detached HCl causes corrosion of piping of the chemical heat storage device (chemical heat storage system). Non-Patent Document 1 does not discuss the separation of HCl and countermeasures at all.

  This invention is made | formed in view of such a situation, and it aims at providing the new chemical heat storage material etc. which can be operated efficiently in a specific temperature range.

  As a result of diligent research to solve this problem, the present inventor has synthesized a double hydroxide having a composition different from the conventional one, and stored this double hydroxide in a specific temperature region (heat absorption) and regeneration (heat dissipation). I succeeded in making it happen. By developing this result, the present invention described below has been 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 stores Mg ₂ when storing water. 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., or 335 to 355 ° C.), and such a specific temperature range is obtained. Can effectively use waste heat.

  In addition, this inventor suppresses generation | occurrence | production (detachment | desorption) of HCl by operating the double hydroxide of this invention under appropriate heating temperature and heating time, and it is heat absorption (heat storage) and heat dissipation (regeneration | regeneration). ) Can be repeated stably.

The chemical heat storage material of the present invention, the reason causing intensively (so-called one-step) thermal storage reaction in a specific temperature range, Mg and Al have a geometrically uniformly distributed layered structure, Cl ^- Is presumed to be uniformly held between 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-described chemical heat storage material. The chemical heat storage system introduces, for example, a reactor that contains a chemical heat storage material, a means (mechanism) for heating and aggregating water, which is a heat medium, and supplying and discharging water (water vapor) to the reactor, and waste heat. And a means for opening and closing a flow path such as water (water vapor) and waste heat, a means for controlling the operation thereof, 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. As a result, generation of HCl (Cl desorption) can be suppressed, and efficient heat absorption and dissipation can be performed without corroding the chemical heat storage device (system).

More specifically, the chemical heat storage system is preferably controlled such that the heating temperature (T / ° C.) of the double hydroxide and the holding time (Δt / min) satisfy the following formula.
Δt <139exp (29.7−0.09T) (Formula 1)

  The double hydroxide according to the present invention immediately releases HCl at 438 ° C. or higher, and it cannot be regenerated even when water is absorbed. Therefore, the heating (holding) temperature described above is preferably 400 ° C. or lower, 385 ° C. or lower, and 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 further 335 ° C. or higher.

<Others>
Unless otherwise specified, “x to y” in this specification includes a lower limit value x and an upper limit value y. A range such as “a to b” can be newly established with any numerical value included in 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 according to the present invention. It is a temperature programmed desorption spectrum obtained by mass spectrometry of a gas generated by heating a sample. 2 is a temperature programmed desorption spectrum obtained when a 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) at which generation | occurrence | production of HCl is started, and holding time (Δt). It is a graph showing the relationship between the elapsed time of the release of H 2 _O from the accumulated value and the heating start of thermal conductivity which indicates (TCD). It is a figure which shows the preferable range of heating temperature (T) and holding time ((DELTA) t). It is a TG curve when a sample is subjected to thermogravimetric analysis. It is an X-ray diffraction pattern concerning a synthetic sample and a reproduction sample. It is a TG curve when a regenerated sample is subjected to thermogravimetric analysis. It is a schematic diagram which shows one example of a chemical heat storage system.

  One or two or more components arbitrarily selected from the present specification may be added to the above-described components of the present invention. The contents described in this specification can be appropriately applied not only to chemical heat storage materials but also to chemical heat storage systems. It can be a component related to a method or a component related to 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 basically consists of Mg ₂ Al (OH) ₆ Cl, but considering that it forms a hydrate in the normal temperature range, Mg ₂ Al (OH) ₆ Cl-nH. ₂ O (n ≧ 0). However, in this specification, regardless of whether or not it is a hydrate (regardless of the value of n), one having Mg ₂ Al (OH) ₆ Cl is simply referred to as a double hydroxide.

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

(1) First stage (Reaction A)
When the double hydroxide is heated (when Q _A is added), a dehydration reaction occurs in which water molecules (H ₂ O) bonded to the double hydroxide are separated (from the left side of Formula A1 to the right side). Progress). This dehydration reaction generally occurs in the temperature range of 50 to 150 ° C. Through this stage, anhydrous double hydroxide (Mg ₂ Al (OH) ₆ Cl) is obtained.

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

(2) Second stage (Reaction B)
As you further heating the double hydroxide (Q _B is applied when), heat storage reaction in which water from the hydroxyl group of the double in the hydroxide leaving (original endothermic reaction) occurs. (Progress from left side to right side of Formula B1 and Formula B2). This heat storage reaction occurs in a temperature range of approximately 315 to 400 ° C. Depending on the degree of dehydration, the reaction is represented by Formula B1 or Formula B2.

Since the reaction at this stage is also a reversible reaction, when water is supplied, heat (Q _B ) is released by a regeneration reaction (heat release reaction) and returned to Mg ₂ Al (OH) ₆ Cl (formulas B1, B2 From right to left).

(3) Third stage (Reaction C)
As you further heating the double hydroxide (Q when _C is applied), elimination reaction Cl from double in the hydroxide is disengaged becomes HCl occurs (progression to right from the left side of the equation C1~C3 ). This elimination reaction occurs in a temperature range of approximately 415 ° C. or higher. In addition, reaction like Formula C1-C3 is shown with a heating history and 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 HCl elimination reaction may occur before reaching the above-described temperature range. For example, when the double hydroxide is held for a long time even in a 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), generation of HCl can be suppressed. For example, it is preferable that the heating temperature (T) and the holding time (Δt) of the double hydroxide satisfy Expression 1 described above.

  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.

<Production of sample>
Samples _{(Mg 2 Al (OH) 6} Cl-nH 2 O) was synthesized by the following process. In addition, in order to avoid mixing of CO ₂ from the atmosphere, all the steps shown below were performed in an inert gas (Ar) atmosphere.

(1) Solution preparation step As raw materials, MgCl ₂ · 6H ₂ O (Wako Pure Chemical Industries, Ltd./purity 98%), AlCl ₃ · 6H ₂ O (Wako Pure Chemical Industries, Ltd./purity 98%) and ammonia Water (Wako Pure Chemical Industries, Ltd./concentration 25%) was prepared. These were used as purchased. Moreover, deionized water from which dissolved oxygen was previously removed by aeration with Ar was also prepared.

MgCl ₂ · 6H ₂ O: 20.3 g (0.1 mol) and AlCl ₃ · 6H ₂ O: 12.1 g (0.05 mol) were dissolved in deionized water: 500 mL to obtain a mixed aqueous solution. Further, the above-described ammonia water (concentration 25%): 160 mL was diluted with deionized water: 500 mL to obtain diluted ammonia water.

(2) Mixing stabilization process (generation process)
The whole amount of the mixed aqueous solution was dropped at a rate of about 8 mL / min into dilute ammonia water being stirred with a magnetic stirrer. After completion of the dropwise addition, stirring was continued at room temperature for about 4 hours. Thus, a mixed liquid in a solid-liquid coexistence state was obtained.

(3) Solid-liquid separation and washing process (extraction process)
The solid was separated from the mixture by centrifugation (15000 rpm × 10 min). This solid was dispersed in 1000 mL 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. A sample was thus synthesized.

When the obtained sample was analyzed by X-ray diffraction (XRD: Cu—Kα, λ = 1.5418Å), it was confirmed to be Mg ₂ Al (OH) ₆ Cl—nH ₂ O (sample A in FIG. 6). reference). Note that the n value indicating the number of water adsorbed in a molecular form varies depending on the heating conditions during the drying process. The n value is specified, for example, by determining the weight reduction rate during the dehydration reaction (reaction A in FIG. 1) by thermogravimetric analysis (TG). In the sample described above, 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 a rate of 5 ° C./min in an air flow of He: 60 mL / min, and the generated gas was analyzed with a mass spectrometer (REGA Ptype manufactured by ULVAC, Inc.). The temperature programmed desorption spectrum obtained at this time is shown in FIG. m / e = 18 represents H ₂ O, and m / e = 36 represents HCl. The signal intensity shown in FIG. 1 is an arbitrary unit, and the vertical axis of HCl is shown by enlarging the vertical axis of H ₂ O by about 6 times.

(2) As is clear from FIG. 2, as the sample is heated, the dehydration reaction (reaction A in FIG. 1) → the heat storage reaction (reaction B in FIG. 1) → the elimination reaction of HCl (reaction C in FIG. 1). It turns out that it arises sequentially. It was also found that HCl was generated before the completion of the heat storage reaction.

When the generation of HCl (Cl detachment) occurs, the sample cannot return to the original composition (Mg ₂ Al (OH) ₆ Cl—nH ₂ O) only by supplying H ₂ O. For this reason, in order to repeat heat generation and heat absorption by occlusion and release of H ₂ O (heat medium), water absorption / release with respect to the double hydroxide is performed under the condition that generation of HCl (Cl separation) does not occur. Need to do

<Inhibition of HCl generation / upper limit of heating temperature>
As in the case described above, the sample (20 mg) was heated while being heated (5 ° C./min), and after reaching the target temperature (Te), the sample (20 mg) was held at that temperature for a certain period of time. The gas generated during the heating was detected using a mass spectrometer. The temperature programmed 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 after about 90 minutes had elapsed after reaching that temperature. Further, when Te = 397 ° C., HCl was generated after about 5 minutes had elapsed after reaching that temperature. In either case, the release of water (reaction B) was not completed at the start of HCl generation.

  Thus, it was found that HCl was generated in a shorter time as the temperature maintained was higher. 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 retention temperature (T) obtained in this way and the retention time (Δt) from reaching that temperature to the start of HCl generation 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, generation of HCl is accompanied. It was found that H ₂ O was released.
Δt <139exp (29.7−0.09T) (Formula 1)
This mathematical formula was calculated by approximating with experimental data and curve fitting.

<Start 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 H ₂ O from the sample (Mg ₂ Al (OH) ₆ Cl) starts is about 200 ° C. Therefore, the lower limit value of the heating temperature necessary for the start of the heat storage reaction (reaction B) is about 200 ° C. However, around 200 ° C., the release rate of H ₂ O is very slow. Therefore, it is necessary to set a temperature considering the practicality of the chemical heat storage material as the start temperature of the heat storage reaction.

(1) As in the case described above, the sample (20 mg) is heated while being heated (5 ° C./min), and after reaching the target temperature (Te), the temperature is maintained for a certain period of time. The product gas was detected with a mass spectrometer. The integrated value of the thermal conductivity (TCD) of the product 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 increase.

(2) As is clear from FIG. 4A, for example, in the case of releasing H ₂ O corresponding to 1.0 × 10 ⁵ (μV × min), when Te = 327 ° C. than when Te = 338 ° C. Increases about 100 minutes. Therefore, it is preferable that the heating temperature (T) and the holding time (Δt) after reaching the temperature are adjusted within a range satisfying the following Expression 2.
232exp (30.6−0.1T) <Δt (Formula 2)
This mathematical formula was calculated by modifying the above-described formula 1 based on experimental data.

(3) Formula 1 and Formula 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 formulas (in the hatched area in FIG. 4B), the release of HCl is prevented and the practicality is ensured. Is possible. One example of a chemical heat storage system using this is schematically shown in FIG.

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

  In addition, A point and B point can be specified by, for example, installing a plurality of thermocouples inside the heat storage body container and detecting their temperature changes. Moreover, temperature control of each point can be performed as follows, for example. As shown in FIG. 8, a plurality of flow paths for the heat medium (water vapor or the like) are provided, and a valve is provided for each flow path (especially the upstream side). Depending on the temperature distribution state of the heat storage body, each valve is used to open and close each flow path or adjust the flow rate of the heat medium flowing through each flow path. By such temperature control, a chemical heat storage system that satisfies Equation 1 and Equation 2 at the same time can be constructed.

<Heat storage reaction / Water release>
The amount of heat stored in the double hydroxide (Mg ₂ Al (OH) ₆ Cl) is proportional to the number of water molecules that can be absorbed and released (the number of moles of water relative to 1 mol of the double hydroxide). Therefore, the number of water molecules that can be absorbed and released within the range where HCl is not generated was clarified as follows.

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

(2) Weight loss due to 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 Mg ₂ Al (OH) ₆ Cl. Therefore, it was confirmed that the heat storage reaction shown in Formula B1 occurred under the condition where HCl was not generated.

《Regeneration Reaction / Occlusion of Water》
It was confirmed by manufacturing various regenerated samples that the double hydroxide can be regenerated by absorbing water after being discharged. All steps were performed 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. Thus, a double hydroxide after heat treatment corresponding to the heat storage reaction (referred to as “sample B10”) was prepared.

  Sample B10 (0.1 g) was placed in a vial (5 mL) and dispersed in degassed deionized water (4 mL). Thereafter, 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 water was evaporated while heating and stirring (150 ° C. × 1 hour). A solid double hydroxide (referred to as “sample B1”) thus regenerated was obtained.

  (Ii) The sample A was pulverized by a ball mill (P-4 manufactured by Fritsch). The grinding with the ball mill was performed 4 times at 200 rpm × 15 min. A double hydroxide (referred to as “sample B20”) obtained by subjecting the obtained powder to the same heat treatment as (i) described above was obtained.

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

  (Iii) The sample B20 was also regenerated by refluxing. That is, sample B20 (0.1 g) was placed in a round bottom flask (100 mL) and dispersed in degassed deionized water (20 mL). This 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 (i) described above to evaporate water. A solid double hydroxide (referred to as “Sample B3”) thus regenerated was obtained.

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

As apparent from FIG. 6, the XRD patterns of the sample A and the samples B1 to B3 are basically the same, and any of the regenerated samples B1 to B3 is a double hydroxide as in the sample A at the time of synthesis. _{(Mg 2 Al (OH) 6} Cl-nH 2 O) and going on it, it was confirmed that that is being played.

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

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

If the reactions B1 and B2 and 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 .2% (theoretical value). Since the weight of the double hydroxide increases with the progress of the regeneration reaction, the weight reduction rate of thermogravimetric analysis and the regeneration rate of the double hydroxide have a positive correlation. Therefore, when the regeneration rate of each sample was obtained from the ratio between the theoretical value of the weight reduction rate and the weight reduction rate of each sample obtained from thermogravimetric analysis, it was as follows. In any case, it was also found that a sufficiently high regeneration rate was obtained.
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 is sufficiently absorbed by water absorption. It was also confirmed that it was possible to reproduce at a high rate.

Claims (3)

A chemical heat storage material consisting of double hydroxides that generate or absorb heat by occlusion or release of 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 a heating temperature (T / ° C.) of the double hydroxide and a holding time (Δt / min) of the heating temperature satisfy the following formula.
Δt <139exp (29.7−0.09T) (Formula 1)   The chemical heat storage system according to claim 2, wherein the heating temperature (T) is 315 ° C or higher.