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Agglomeration Behavior of Calcium Hydroxide/calcium Oxide as Thermochemical Heat Storage Material: A Reactive Molecular Dynamics Study Min Xu, Xiulan Huai, and Jun Cai J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b08615 • Publication Date (Web): 17 Jan 2017 Downloaded from http://pubs.acs.org on January 23, 2017

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The Journal of Physical Chemistry

Agglomeration Behavior of Calcium Hydroxide/Calcium Oxide as Thermochemical Heat Storage Material: A Reactive Molecular Dynamics Study Min Xu, Xiulan Huai*, Jun Cai Institute of Engineering Thermophysics, Chinese Academy of Sciences, Beijing, 100190, China *Corresponding author. Tel./Fax.: +86 10 82543108. E-mail: [email protected];

ACS Paragon Plus Environment

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ABSTRACT: Thermochemical energy storage is a promising alternation in heat recovery application compared to phase change energy storage. However, cycling instability caused by agglomeration of the reactant particles is the main problem that hinders the application of this system. The present paper focuses on the agglomeration behavior of the calcium hydroxide/calcium oxide particles as thermochemical energy storage material at molecular level. Molecular dynamics simulations with the reactive force field were carried out to investigate the agglomeration of two nano-CaO/Ca(OH)2 particles. The results indicated that the agglomeration rate of two Ca(OH)2 particles was faster than that of two CaO particles in the presence of H2O, which was attributed to the greater spatial displacements of atoms in the reactant particles when thermochemical reaction occurred. The present of H2O could accelerate the agglomeration of the CaO particles. Moreover, the hydration of the CaO agglomeration lump was more difficult than that of fresh particles. The agglomeration of reactant particles had a negative effect on cycling stability. Finally, SiO2 particle was introduced to prevent the CaO/Ca(OH)2 particles from agglomerating during reaction occurred. The results revealed that the present of silica particle could reduce the agglomeration of reactant particles. This study might provide the guidelines on synthesizing or selecting themochemical energy storage materials with less agglomeration for Ca(OH)2/CaO reaction cycle.


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1. INTRODUCTION Large amounts of solar energy and industrial waste heat cannot be utilized effectively due to their intrinsic intermittence and fluctuation. It is an efficient way to achieve the stabilized heat output by thermal energy storage. Compared with conventionally-used phase change materials, thermochemical heat storage materials is a promising alternative because of its advantages such as high energy density, wide range of operation temperature, and low heat losses1,2. Hydroxide/oxide system (i.e. Ca(OH)2/CaO and Mg(OH)2/MgO) is considered as one of the most potential thermochemical reaction for storage of middle- and high-temperature heat. Take Ca(OH)2/CaO system for example, the dehydration of calcium hydroxide occurs in the endothermic reactor to yield calcium oxide and water vapor, and then calcium oxide hydration occurs to release heat. The reaction equation is shown as follows: Ca(OH)2

CaO + H2O ∆H = 109.3 kJ mol-1

(1)

There has been some theoretical and experimental research on thermodynamics, kinetics, heat transfer characteristics, performance enhancement and cycle stability of CaO/Ca(OH)2 and MgO/Mg(OH)2 system, in the field of thermochemical heat storage. A 10 kW demonstration unit of CaO/Ca(OH)2 thermochemical heat storage reactor was also built and evaluated by Schmidt et al3. Moreover, CaO/Ca(OH)2 and MgO/Mg(OH)2 system not only allows the waste heat to be recovered and stored, it also allows the heat to be elevated through a chemical heat pump4-8. Early in 1996, Kato et al.9 investigated the kinetic of the hydration of MgO at the reaction temperature of 373-423 K and the vapor pressure of 12.3-47.4 kPa. Azpiazu et al.10 experimentally studied the kinetic of dehydration of Ca(OH)2 in a prototype reactor. Schaube et al.11 studied the thermodynamic and kinetic of the Ca(OH)2 dehydration and rehydration at high

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H2O partial pressures. Ishitobi et al.12 studied dehydration and rehydration behavior of lithium chloride modified Mg(OH)2, and they found that the dehydration rates were raised by LiCl modification of the Mg(OH)2 surface. Yan and Zhao13,14 investigated the thermodynamic and kinetic of the dehydration of lithium modified Ca(OH)2, and then they carried out the density function theory study to verify the influences of doping on macroscopic thermochemical reaction process. The results indicated that the energy barrier was reduced from 0.40 eV to 0.11 eV when Ca(OH)2 was modified by Li, which further indicates the dehydration reaction can take place at a lower temperature in the presence of Li modification. Shkatulov and Aristov15 studied the influence of modification of magnesium and calcium hydroxides with various salts in detail. The results indicated that a strong effect of nitrates and acetates on the dehydration rate of magnesium

and

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revealed.

Many

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dedicated their study to bettering the performance of the thermochemical heat storage systems by materials design, such as mixed hydroxides16, addition of expanded graphite17,18 and expanded vermiculite19, and mixed calcium oxide-alumina compositions20. Reactor design and heat transfer characteristics analyses were also discussed in literature21,22. Detailed description of reactor design for thermochemical heat storage systems could be found in the review of Solé et al1. Cycling stability was the other concern for thermochemical heat storage systems. Schaube et al.11,23 observed the refinement of Ca(OH)2 particles after 10 cycles but no further significant varyings of the spherical grains was found between the 10th and the 100th cycle. They also found the BET surface of Ca(OH)2 particles reduced from 16.0 m2/g to 11.3 m2/g after only 3 cycles, and then decreased a little to 9.9 m2/g after 25 cycles. The mean Ca(OH)2 particle diameter grew from 5.3 µm to 11.1 µm and after 25 cycles to 17.6 µm. Roβopf et al.24 also found that an obvious sintering of Ca(OH)2 occurred after only 4 cycles in the lab-scale reactor.

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Agglomeration behavior of particles may lead to diffusion limitations and thus destroy cycling stability. Roβopf et al.25 used nanoparticles (nano-silica) as additive for Ca(OH)2 particles to stabilize the surface structure of the particles and improve the cycling performance. Although the experimental observations were reported in the literature, to our best knowledge, the agglomeration behavior of CaO/Ca(OH)2 particles at molecular level has not been revealed so far. Understanding of connection between microcosmic reaction mechanism and agglomeration behavior would help better explain the experimental phenomena and provide a guide for developing desired materials for the thermochemical heat storage application. To address the above issues, this paper aims to i) conduct the reactive molecular dynamics simulations to study the agglomeration behavior during CaO hydration and Ca(OH)2 dehydration; ii) compare the sintering rate of Ca(OH)2 particles dehydration and CaO particles hydration, and thus ascertain the controlling step and main reasons of reactant particles agglomeration; iii) investigate the effect of agglomeration on cycle performances of the thermochemical heat storage materials; iv) verify the possibility of preventing against agglomeration by adding nano-silica. 2. SIMULATION DETAILS ReaxFF was developed by the group of van Duin26,27. The bond formation and charge transfer for reactive systems can be described by using this empirical reactive force field, which divides the system energy into various partial energy contributions, and it can be given as: U system = U bond + U angle + U lp + U over + U under + U coulomb + U vdW

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where, Ubond means bond energies, Uangle means valence angle, Ulp means lone pair, and Uover/Uunder represents over/under conjugation energy corrections to describe the valence forces

arising from molecular orbitals. All of them belong to the bond-order dependent terms. The nonbond terms relate to vdW and Coulomb interactions(Uover and Uunder). In this work, the Si/Al/Ca/O/H ReaxFF reactive force field parameters developed by Pitman and van Duin28 was used, which has revealed good agreements with density function theory methods in reproducing the potentials of calcium hydroxide, calcium oxide, silica and water. This force field was also verified by the results of Manzano et al.29, which indicated that the simulation results obtained from DFT agree well with that from reactive force field for calcium oxide hydration. Therefore, this force field was used for MD simulations of the CaO hydration and Ca(OH)2 dehydration without further modification. The spherical particles of CaO, Ca(OH)2 and SiO2 were used in our simulations, which were produced from the original unit cells of various materials by using Materials Studio. The radius of the grains was set as 1.2 nm. Two particles were placed vertically in a simulation box (10 nm ×10 nm×10 nm) with the distance between them of 0.3 nm. For CaO hydration, the simulation box contains 100 randomly distributed H2O molecules. The resulting initial packing of CaO in the presence of H2O contains 1270 atoms. In a typical simulation, the energy of the initial structure was first minimized to wipe off any possible overlaps and close contact by an equilibrium run at the condition of the fixed bond order for about 10 picoseconds (ps) . The equilibrium system was then run at NVT ensembles (starting at 293K, 600 K and 800 K) for 200 ps with a MD time step of 0.25 femtoseconds (fs). The resulting trajectories of system were recorded at each 1000 steps (each 0.25 ps).

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Four other simulations were also carried out to investigate the effect of particle sizes and separation distances for the sintering of Ca(OH)2 particles. To investigate the cycle performance of sintered reactant particles, MD simulations of the rehydration of the sintered CaO particles were carried out. The particles that had been sintered during previous 200 ps MD simulations of two CaO particles in the absence of H2O molecules were used. 3. RESULTS AND DISCUSSION 3.1 Agglomeration during Ca(OH)2 dehydration Fig. 1 - 3 show successive snapshots of two Ca(OH)2–Ca(OH)2 particles at t= 0, 10.0, 50.0, 100.0, 150.0, and 200.0 ps at 800 K, 600 K and 293 K (some desorption water molecular are not displayed). These images clearly show that the sintering between the Ca(OH)2 particles occurs during the dehydration process within a short time. When comparing the snapshot images obtained at different temperatures, we can find that the sintering is the most noticeable at 800 K, and the extent of agglomeration decreases at the lower temperature. The two Ca(OH)2 particles totally fuse together to form a spherical particle at about 100.0 ps for the 800 K case(See Fig.1), and a elliptical spherical particle is observed at 200.0 ps for the 600 K case(See Fig.2). Nevertheless, a sintered Ca(OH)2–Ca(OH)2 particles is not formed at 200.0 ps for the 293 K case(See Fig.3). Even though volume of Ca(OH)2 particles may decrease during dehydration process, intense spatial changes of the individual atoms in the Ca(OH)2 particles should contribute to the agglomeration process. Fig.4 show the displacements of each of the atoms in the Ca(OH)2 particles at t= 200 ps from their starting positions at t=0 ps at three different temperatures, respectively. It can be seen that the desorption water molecular present the greatest spatial changes, the displacement of other atoms is less than 40 Å. In comparison to the mean

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displacements of every atoms in the Ca(OH)2 particles for 600 and 293 K simulation case, those for 800 K simulation case are about 1.5 and 3.7 times larger. The results indicate that the dehydration temperature play an significant role in agglomeration behavior of the Ca(OH)2 particles.

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Figure 1 Snapshots (t=0,10,50,100,150 and 200 ps) from reactive molecular dynamic simulation at 800K for two Ca(OH)2- Ca(OH)2 spherical particles (r=1.2nm) color codes: Ca=pink; O=cyan; H=purple.

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Figure 4 Spatial displacements of the individual atoms within the Ca(OH)2 particles after 200 ps MD simulation: (a) at 800 K, (b) at 600 K and (c) at 293 K from MD simulations. The atom IDs of the upper Ca(OH)2 particle rangs from 1 to 668 and that of the lower rangs from 669 to 1336.

3.2 Agglomeration during CaO hydration In order to examine the reactant agglomeration behavior under different conditions, MD simulations of two CaO particles are first performed in the absence and presence of H2O

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molecules. Fig. 5 show successive snapshots of two CaO–CaO particles in the absence of H2O at t= 0, 50.0, 100.0, and 200.0 ps at 800 K, 600 K and 293 K, respectively. It can be seen that only a narrow neck between two CaO–CaO particles is observed after 200.0 ps even at 800K. The results indicate that the sintering between the CaO particles is not obvious in the absence of H2O molecules. Fig.6 shows the displacement of each of the atoms within the CaO particles at t= 200 ps at three different temperatures, respectively. It can be seen that all of the displacement of atoms in the CaO particles is less than 10 Å for simulation at 800 K and 6 Å for simulation at 600 K and 293 K, which is significantly lower than that of the individual atoms in the Ca(OH)2 particles at the same conditions.

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Figure 5 Snapshots (t=0, 50,100 and 200 ps) from reactive molecular dynamic simulation at 800K(top), 600K(middle) and 293K(bottom) for two CaO- CaO spherical particles (r=1.2nm) without H2O. color codes: Ca=pink; O=cyan.

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Figure 6 Displacements of the individual atoms within the CaO particles after 200 ps MD simulation: (a) at 800 K, (b) at 600 K and (c) at 293 K from MD simulations in the absence of H2O. The atom IDs of the upper CaO particle ranges from 1 to 485 and the lower ranges from 486 to 970. Fig. 7 show successive snapshots of two CaO–CaO particles in the presence of H2O at t= 0, 50.0, 100.0, and 200.0 ps at 800 K, 600 K and 293 K, respectively. It can be seen that many water molecules are adsorbed on CaO particles. A narrow neck between two CaO–CaO particles is observed at 50ps, and then grows thicker during reaction occurs for simulations starting at 800

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K. However, the narrower neck between two CaO–CaO particles is present for simulation case at 600 K and 293K. Compared with the agglomeration behavior of the CaO particles in the absence of H2O, the sintering of the CaO particles in the presence of H2O was more obvious. This is due to the expansion of the CaO particles in the presence of H2O during the hydration reaction occurs as demonstrated by Manzano et al29. The similar results were found by the study of Zhang et al.30 about sintering of CaO during CO2 chemisorption.

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Figure 7 Snapshots (t=0, 50,100 and 200 ps) from reactive molecular dynamic simulation at 800K(top), 600K(middle) and 293K(bottom) for two CaO- CaO spherical particles (r=1.2nm) with H2O. color codes: Ca=pink; O=cyan; H=purple.

When comparing the snapshot images obtained from Ca(OH)2 dehyration simulation case and CaO hydration simulation case, we can find that the sintering of Ca(OH)2 particles was more obvious at the same condition. A detailed analysis on displacements of the individual atoms within the two CaO particles after 200 ps MD simulation at 800 K and 600 K in the presence of H2O molecules shown in Fig. 8. In comparison to the results in Fig.6, the mean displacements of the individual atoms within the two CaO particles in the presence of H2O are about 1.6 times larger than those in the absence of H2O. Also, when we compared the results in Fig. 3 and Fig. 8, the conclusion that the smaller variations of atoms in CaO particles (less than 20 Å at 800K and less than10 Å at 600 K) than those of atoms in Ca(OH)2 particles at the same temperature can also be revealed. It may indicate that the Ca(OH)2 dehydration process was the main controlling step of the thermochemical heat storage system to influence the cycling stability. It is interesting that the sintering rate of CaO particles with obvious expansion in volume is lower than that of

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Ca(OH)2 particles with reduction in volume. This phenomenon may be due to the fact that crystal distortion resulting from dehydration process of Ca(OH)2 is more obvious. In view of the fact that the sintering of Ca(OH)2 particles is the controlling step of the agglomeration behavior, more detailed investigations are carried out to study the effect of particle sizes and separation distances. The results are shown in Supporting Information. Fig.1S and 2S show successive snapshots of two Ca(OH)2–Ca(OH)2 particles with the radius of 1.8 and 2.5 nm at t= 0, 25.0, 50.0, and 100.0 ps at 800 K. It can be found that the sintering rate decreases as the particle size increases, which may be due to the fact that the atoms at the core of the bigger granules undergo the smaller spatial variations. Fig.3S and 4S show successive snapshots of two Ca(OH)2–Ca(OH)2 particles with a separation distance of 0.5 and 1.0 nm at t= 0, 25.0, 50.0, and 100.0 ps at 800 K. As seen, we can find that the sintering becomes unnoticeable when larger separation distance between the two Ca(OH)2– Ca(OH)2 particles is used. It indicates the separation distance is a key adjusting parameter to prevent the thermochemical heat storage materials from sintering. In this study, a separation distance of 1.0 nm between the two Ca(OH)2–Ca(OH)2 particles decreases the sintering rate effectively.

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Figure 8 Displacements of the individual atoms in the CaO particles after 200 ps MD simulation: (a) at 800 K, (b) at 600 K and (c) at 293 K from MD simulations in the presence of H2O. The atom IDs of the upper CaO particle ranges from 1 to 485, and that of the lower ranges from 486 to 970 and H2O has atom IDs from 971 to 1270.

3.3 Rehydration of sintered CaO-CaO particles As discussed in Introduction, particles agglomeration is the major factors leading to degradation of thermochemical heat storage materials. In the above section, the sintered CaO particles from the MD simulations at 800 K and 600 K for 200 ps (Fig. 5) in the absence of H2O were obtained. To investigate the effect of agglomeration on cycle performance, the sintered CaO-CaO particles derived from the above-mentioned simulation are exposed to 100 H2O molecules and the MDNVT simulations are performed for 200 ps at 600 K. As a comparison, the case of two CaO-CaO particles with a separated distance of 1.5 nm (there is not any contact between these two particles during the whole simulation) are also performed at the same condition. Fig. 9 show the amount of water molecules adsorbed on CaO particles from the MD simulations at 600 K for sintered

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and fresh CaO-CaO particles. H2O adsorption increased during the exposure time. However, the fresh CaO particles adsorbed more H2O moleculers than the sintered CaO-CaO particles. As shown in Fig. 8, the the amount of H2O adsorbed on the fresh CaO particles is about 64, and nevertheless about 51 H2O on the CaO-CaO particles sintered at 600 K and 38 H2O on CaO-CaO particles sintered at 800 K are adsorbed. The results indicate that the sintered CaO particles lose tremendous rehydration capacity under these conditions, which is due to the decreasing surface area.

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3.4 Effect of SiO2 In order to examine the effect of inert dopant material on reactant agglomeration behavior under different conditions, MD simulations are carried out for a CaO/ Ca(OH)2 particle and a SiO2 particle (r = 1.2 nm) separated by a distance of 0.3 nm. Fig. 10 show successive snapshots of the 20


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Ca(OH)2–SiO2 particles at t= 0, 50.0, 100.0, and 200.0 ps obtained from MD-NVT simulations starting at 800 K and 600 K, respectively. It can be seen that deformation of Ca(OH)2 particle occurs at the surface of SiO2 particle during the proceeding of dehydration at 800K, and only little volume variation of SiO2 particle is observed. But these two particles cannot fuse together, which verifies the feasibility of inert dopant material. The MD simulation case at 600 K shows the smaller deformation and fusion of the particles as shown in Fig.10 (the bottom row). The displacements of the individual atoms in the Ca(OH)2 and SiO2 particles after 200 ps MD simulation shown in Fig.10 also reveal that molecules movement of the Ca(OH)2 particle is less than that of the Ca(OH)2 particle in the absence of nano-SiO2.

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Figure 9 Snapshots (t=0, 50,100 and 200 ps) from reactive molecular dynamic simulation at 800K(top) and 600K(bottom) for two Ca(OH)2-SiO2 spherical particles (r=1.2nm). color codes: Ca=pink; O in Ca(OH)2 =cyan; H=purple; Si=yellow; O in SiO2=red.

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Figure 10 Displacement of every atoms in the Ca(OH)2 and SiO2 particles after 200 ps MD simulation: (a) at 800 K and (b) at 600 K from MD simulations. The atom IDs of the Ca(OH)2 particle ranges from 1 to 668 and that of the SiO2 ranges from 669 to 1255.

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Fig. 11 show successive snapshots of the CaO-SiO2 particles in the presence of H2O at t= 0, 50.0, 100.0, and 200.0 ps obtained from MD-NVT simulations starting at 800 K and 600 K, respectively. It can be seen that expansion of CaO particle occurs at the surface of SiO2 particle during the proceeding of hydration at 800 K and 600 K, and only little volume variation of SiO2 particle is also observed. Less sintering is found in the CaO-SiO2 system than that in the CaOCaO system during hydration occurs, which also verifies the feasibility of inert dopant material. The displacements of every atoms in the Ca(OH)2 and SiO2 particles after 200 ps MD simulation shown in Fig.12 also reveal that molecules movement of the CaO particleis is less than that of the CaO particle in the absense of nano-SiO2. It is noted that the gaps between CaO/Ca(OH)2 particles are formed due to the addition of nanoSiO2 particles. The CaO/Ca(OH)2 particles reduces the opportunity to interact with each other due to these gaps, which is the reason why the sintering of the CaO/Ca(OH)2 can be prohibited after adding SiO2. However, silica may take place reaction with Ca(OH)2 to form calcium silicate according to the study of Roßkopf31. So some more investigations should be carried out to reveal the formation mechanism of calcium silicate and its influence on thermochemical cycle, and thus to test the effectiveness of SiO2 as the inert dopant material.

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t=0ps

t=50ps

t=100ps

t=200ps

t=0ps

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t=200ps

Figure 11 Snapshots (t=0, 50,100 and 200 ps) from reactive molecular dynamic simulation at 800K(top) and 600K(bottom) for two CaO-SiO2 spherical particles (r=1.2nm) with H2O. color codes: Ca=pink; O in CaO and H2O=cyan; H=purple; Si=yellow; O in SiO2=red.

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Figure 12 Displacements of each of the atoms in the CaO and SiO2 particles after 200 ps MD simulations: (a) at 800 K and (b) at 600 K from MD simulations in the presence of H2O. The CaO particle has atom IDs ranging from 1 to 485, the SiO2 has atom IDs from 486 to 1072 and H2O has atom IDs from 1073 to 1372.

4. CONCLUSION Focusing on cycling stability of thermochemical heat storage materials, we investigated the agglomeration behavior of calcium hydroxide/calcium oxide by using reactive molecular dynamics simulations for the first time. The sintering of two solid calcium hydroxide/calcium oxide particles at 293, 600 and 800 K and particle separation distances of 0.3, 0.5 and 1.0 nm was studied. The connection between agglomeration behavior and cycling stability was also investigated. Finally, we verified the possibility of preventing against agglomeration by adding nano-silica. The following major conclusions can be drawn. (1) The sintering of two Ca(OH)2 particles was very faster than that of two CaO particles when the thermochemical reaction occurs. The present of H2O could accelerate the agglomeration of 25



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the CaO particles. The increasing temperature caused more expansion and sintering of the particles. The results indicated that the Ca(OH)2 dehydration process was the main controlling step of the thermochemical heat storage system to influence the cycling stability. (2) The sintered CaO particles showed the lower hydration reaction rate than the fresh ones, which indicated that the agglomeration of reactant particles had a significantly negative effect on cycling stability. (3)The effect of particle sizes and separation distances is obvious during the sintering process of two Ca(OH)2 particles. The bigger particle diameters and separation distances could slow down the agglomeration rate of the Ca(OH)2 particles. (4) The addition of nano-silica as a potential barrier material for the CaO/Ca(OH)2 heat storage system reduced the agglomeration rate of the CaO/Ca(OH)2 particles during thermochemical reaction occurs. ASSOCIATED CONTENT Supporting Information. Snapshots of the simulations at 800K for two Ca(OH)2- Ca(OH)2 spherical particles with various sizes and various separation distances. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *Corresponding author(X.L. Huai). Tel./Fax.: +86 10 82543108. E-mail: [email protected];

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Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (No. 21306192, 51276181)

and

the

National

Key Research

and

Development

Program

–

China

(2016YFB0601203). The first author (Min Xu) is grateful to Prof. Adri van Duin at Pennsylvania State University for sharing the Si/Al/Ca/O/H reactive force field. REFERENCES (1)Solé, A.; Martorell, I.; Cabeza, L.F. State of The Art on Gas–Solid Thermochemical Energy Storage Systems And Reactors for Building Applications. Renew. Sust. Energ. Rev. 2015, 47, 386-398. (2)Solé, A.; Fontanet, X.; Barreneche, C.; Fernández. A.I.; Martorell, I.; Cabeza, L.F. Requirements to Consider When Choosing A Thermochemical Material for Solar Energy Storage. Sol. Energy 2012, 93, 261-267. (3) Schmidt, M.; Szczukowski, C.; Roßkopf, C.; Linder, M.; Wörner, A. Experimental Results of A 10 kW High Temperature Thermochemical Storage Reactor Based on Calcium Hydroxide. Appl. Therm. Eng. 2014, 62, 553-559.

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(31)Roßkopf, C.; Entwicklung eines Reaktorkonzepts mit bewegtem Reaktionsbett f ü r Thermochemische Energiespeicher. Dissertation, Universität Stuttgart. 2015.

Table of Contents Graphic

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Calcium Oxide as

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