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Improving powder bed properties for thermochemical storage by adding nanoparticles
Energy Conversion and Management 86 (2014) 93–98
Contents lists available at ScienceDirect
Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman
Improving powder bed properties for thermochemical storage by adding nanoparticles C. Roßkopf ⇑, M. Haas, A. Faik, M. Linder, A. Wörner Institute of Technical Thermodynamics, German Aerospace Center, Pfaffenwaldring 38-40, 70569 Stuttgart, Germany
a r t i c l e
i n f o
Article history: Received 20 February 2014 Accepted 2 May 2014
Keywords: Ca(OH)2 Nano particles Flowability Moving bed Thermochemical storage
a b s t r a c t Thermochemical storage offers interesting potential to store thermal energy, especially in the field of industrial waste heat utilization or for concentrated solar power (CSP) plants. However, at present the development of thermochemical storage technology is in its initial stage with investigations mainly on material aspects or small lab-scale systems. With regard to its thermodynamics and kinetics, it has been shown that the CaO/Ca(OH)2 reaction system is suitable for thermochemical heat storage at a temperature range of 400–600 °C. However, the behaviour in a small lab-scale system was mainly dominated by heat and mass transfer limitations originating from the small particle size and changes in the bulk properties. It is shown that by the addition of small amounts of additives like nanoÒ particles of SiO2 (Aerosil ), the bulk properties can be stabilized and consequently the cycling stability ensured. In addition, channelling effects can be minimized resulting in a more homogeneous flow through the reaction bed improving the overall reaction behaviour of the thermochemical storage. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Constantly rising energy prices and increasing emissions of carbon dioxide necessitate a rethinking of how energy is used. Thermal energy storage is one important cornerstone on the way to higher efficiency in industrial processes as well as for the implementation of dispatchable regenerative energy especially in concentrated solar power (CSP) plants [1,2]. Heat storage can be divided into three different technologies: sensible, latent and thermochemical. Sensible heat storage has been most researched and is already employed e.g. in solar power plants as two-tank molten salt storage systems [3]. Latent storage makes use of the phase change of the storage material. The operation of a 700 kW h storage generating steam for 3000-h has been successfully demonstrated [4,5]. In comparison to sensible and latent heat storage systems, thermochemical technologies are only in the initial phase of development but show great potential with regard to energy density and long-term storage behaviour [6–9]. The CaO/Ca(OH)2 reaction system has been investigated extensively and is appropriate for thermal energy storage in a temperature range of 400–600 °C [6,10,11].
⇑ Corresponding author. Tel.: +49 (0)711 6862 8045. E-mail address: [email protected] (C. Roßkopf). http://dx.doi.org/10.1016/j.enconman.2014.05.017 0196-8904/Ó 2014 Elsevier Ltd. All rights reserved.
The following equation shows the equilibrium reaction where DHR correspondents to the reaction enthalpy used to chemically store the thermal energy.
CaOðsÞ þ H2 OðgÞ CaðOHÞ2ðsÞ þ DHR During the charging process heat is supplied to the storage system while water vapour is drawn out. During the discharging process calcium oxide powder reacts with water vapour to calcium hydroxide thereby releasing heat. In case of a directly operated reactor, the heat is directly decoupled by a heat transfer fluid (HTF). This means that the HTF flows through the bed and is in direct contact with the powder. An advantage of this system is the large heat exchange area between the particle surface and the HTF. However, fast discharging demands a high HTF velocity leading to a high pressure drop in the reaction bed due to the low permeability of the powder bed. To avoid this problem either the particle diameter has to be increased which is difficult due to the volume change of the particles during the reaction or the porosity of the reaction bed has to be increased. The main problem occurring especially during the cycling of the thermochemical storage is the worsening of the reaction material [12,13]. After two cycles, small agglomeration lumps begin to form, growing in size from cycle to cycle and therefore constant thermophysical bed properties cannot be guaranteed. Fig. 1 shows an agglomerate of calcium hydroxide after 4 cycles in the lab-scale reactor. Due to the mechanical stability of this agglomerate, it grows continuously at each cycling step.
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minimize the contact area between particles to improve the flow behaviour of powder beds [14]. For thermochemical heat storage, a usage of nanoparticles in order to improve reaction behaviour is not known in literature. Nevertheless, Valverde et al. inserted nanoparticles to Ca(OH)2 bulk powder in order to increase the fluidization behaviour and reactivity for CO2 absorption [15–19]. Contrary to Valverde et al. who add a ratio of guest particles up to 30% and coat these nanoparticle agglomerates with Ca(OH)2, this contribution shows the coating of Ca(OH)2 with nanoparticles to preserve the high storage capacity. To this end much less material is added. This paper therefore reports on the experimental results in a reactor with direct heat transfer and analyses the effects of the addition of SiO2 nanoparticles on agglomeration and channelling. Wang also investigated the enhancement of reactivity for CO2 capture with Li4SiO4-based sorbents by adding SiO2 nanoparticles [20]. Fig. 1. Calcium hydroxide agglomerate after 4 cycles in the laboratory reactor.
2. Material and methods Accordingly, a homogeneous flow of the reaction and HTF gases through the fixed reaction bed is not possible. A large fraction of the HTF passes the reaction bed through wide channels within the bed. Therefore an optimum mass and mainly heat transfer is not ensured. Transfer of the HTF into the agglomerates is insufficient due to their low permeability. Fig. 2 shows a powder bed of Ca(OH)2 at ambient temperature while N2 flows through it from bottom to top within a glass tube. Even though the material has not reacted yet, the channels demonstrate the cohesiveness of this powder. In order to solve the problem of agglomeration one option is to reduce the attractive forces between the particles. This can be achieved on one hand by pelletizing the fine powder which is difficult due to the volume change of the single particles during the reaction of the material. Another possibility to minimize these forces is to increase the surface roughness by coating the surface of the particles with additives. In bulk industries nanoparticles are used in order to increase these roughness and consequently
2.1. Lab-scale reactor A directly heated lab-scale reactor is used for cycling experiments and analysing the reaction behaviour. The flow chart of the test bench is presented in Fig. 3 with additional details given in [12]. A nitrogen flow (0–75 Nl/min) is heated up to the required temperature of 600 °C. Distilled water (0–1000 g/h) can be evaporated, overheated and added to the nitrogen flow in a mixing chamber. This mixture flows either through the thermochemical reactor from bottom to top or through a bypass. A dew point meter after the reactor outlet measures the steam content in the gas mixture whereby the conversion during the reaction process can be determined. The H2O pressure can be calculated by the equation of Sonntag which is described in detail in [12]. A maximum theoretical error of 0.1 K can be assumed with this measurement procedure and is taken into account in the standard deviation of the conversion. By means of a back pressure regulator the absolute pressure within the reactor can be controlled. Fig. 4 shows a picture of the thermochemical reactor and the positions of the Typ K thermocouples with a diameter of 1 mm and a maximum measuring inaccuracy of 0.1 °C inside. The diameter of the reaction zone within the reactor is 54.5 mm and its height measures 158 mm. The reactor is equipped with thermocouples which are placed at the central axis and close to the reactor wall within the reaction bed. The reactor is filled with 60 g of Ca(OH)2 which correspondents to a chemically stored capacity of about 21 W h [13]. In order to investigate the influence of the additives on the powder, the discharging process is considered. Here, the reactor is preheated to the starting temperature of 300 °C. By means of the back-pressure regulator the absolute pressure in the reactor is set in advance. The valve in front of the reactor is closed and the gas flow is directed through the bypass. Then, the desired water vapour/nitrogen mixture of nH2O = 0.4 is adjusted. As soon as the dew point meter outputs a constant signal, the valve in front of the reactor is opened and the gas mixture flows through the reactor defining the starting point t = 0 for the experiment. 2.2. CaO/Ca(OH)2
Fig. 2. Channeling effects within the powder bed of Ca(OH)2 at ambient temperature.
For the investigation of the influence of the additives white hydrated lime Ca(OH)2 by HeidelbergCement, Germany is used. White hydrated lime is a fine powder whose main commercial application is in the production of mortar. The most important key data are the true density of 2200 kg/m3, the bulk density of
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Fig. 3. Set-up of the test bench.
Fig. 4. Lab-scale reactor, photographic image (left) and position of powder bed and thermocouple (right).
450 kg/m3, the mean diameter d50 of 5 lm and the purity of 97.8%. The hydroxide form of the material Ca(OH)2 was chosen as starting material due to the less harmful handling compared to CaO and to start the cycling with a shrinking reaction bed instead of an expanding reaction bed.
similar to the one of calcium hydroxide [23]. The nanoparticles are deposited on the surface of calcium hydroxide by means of a magnetic stirrer with agitator in a dry process. The standard mixing time is 30 min. 3. Results and discussion
2.3. Additive As outlined above, the attractive forces of the reacting particles are decreased by increasing the roughness of their surface [21,22]. In order to resist the high thermal stresses and to avoid side reactions as well as transformations, fumed silica SiO2 that is thermally stable up to 1000 °C and moderately reactive is used. In bulk material technology and pharmaceutical industries, silica nanoparticles are commonly used to reduce adhesive forces in Ò bulks of fine particles. In this work the hydrophilic Aerosil 300 is used for experiments in the thermochemical storage reactor. The mean size of the primary particles is about 7 nm. The surface of Ò Aerosil 300 is smooth and nonporous and the solid density is
In order to assess the flow-through characteristics, the reaction bed is first investigated in a glass tube at ambient temperature (Fig. 5). The velocity of the nitrogen flowing through the bed from bottom to top corresponds to the velocity in the lab-scale reactor and is assumed not leading to a fluidization of the bed. The strong channelling effects observed in Fig. 2 already disappear by the addition of 2 wt% of nanoparticles. However, the porosÒ ity of the powder bed rises with increasing amount of Aerosil as highlighted in [18]. In the unmodified powder bed, most part of the nitrogen flow passes the bed through these wide channels which present less resistance to the flow. With the minimization of the channels the flow is forced to pass the whole reaction bed
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Fig. 5. Powder bed with addition of two percent of Aerosil nanoparticles at ambient temperature.
through very narrow passages leading to a rising pressure drop compared to the unmodified bed. In order to investigate the influence on the storage the effects on the bulk material are investigated within the lab-scale reactor. In the following experiments the discharging process of the lab-scale reactor is considered. The procedure is carried out as mentioned above. Fig. 6(left) shows the temperature development at T7 within the powder bed of the hydration process in the lab-scale reactor. The reactor is heated up uniformly to a temperature of 300 °C and the pressure within the reaction chamber is adjusted to 2 bar g. After switching the mixed flow from the bypass to the reactor, the hydration process starts (at t = 0). The temperature of the whole reaction bed jumps instantaneously to the equilibrium temperature of 500 °C. The duration of the complete reaction process can be approximated by the temperature development within the reaction bed (T7 in Fig. 6(left)). The process seems to be completed when T7 equals the starting temperature. However,
by taking the reaction rate (Fig. 6(right)) measured via the dew point meter into account, it can be seen that most of the conversion is already completed after 2700 s. At the beginning the reaction takes place in the whole bed until the equilibrium temperature is reached. At this point, the conversion rate reaches its maximum of 125 g H2O/h. Afterwards, the reaction continues only at the section of the reaction bed where the heat is dissipated. After 2700 s the rate of reaction is almost negligible as only sensible heat is still stored within the reaction bed. The decline of the temperature therefore correspondents to the cooling of the bulk without chemical reaction. By integrating this curve the overall conversion of the reaction can be determined (compare [12]). The absorbed mass of H2O is 13.32 g where the standard deviation was found to be 0.37 g. Taking the standard deviation and the impurity of the material into consideration, full conversion of the material can be assumed [12]. After investigating the reference case, the reaction of Ca(OH)2 Ò with addition of Aerosil is analysed. The conditions are exactly Ò the same, except that 0.5 wt% of Aerosil is added to the calcium hydroxide powder by means of a dry coating process. Fig. 7(left) and (right) shows corresponding to Fig. 6(left) and (right) the temperature development and the reaction rate of the discharging process for the modified calcium hydroxide. The immediate rise of the bulk temperature to the equilibrium temperature can also be observed in the modified bed. However, the reaction is faster since the initial temperature is reached in about half the time. The development of the reaction rate also confirms the faster reaction behaviour since after 100 s 11.06 g have reacted compared to 9.44 g H2O for the unmodified material. The absorbed mass of water at the end of reaction equals 13.28 g with a standard deviation of 0.37 g confirming a full conversion. Comparing the reference material with the modified one, the considerably faster reaction behaviour as well as the faster cooling of the bulk is notable since only the material has been changed. It can be concluded that a reduction of channelling was also possible in the lab-scale reactor leading to a more homogeneous distribution of the HTF inside the bulk. Therefore, with the addition of nanoparticles the heat transfer into the gas is clearly increased resulting in faster cooling of the reaction bed and overall faster dynamics of the reactor. In addition to the promising results of the thermochemical storage behaviour of the modified material, the cycling stability of this effect is analysed. Fig. 8 shows the temperature developments of the hydration processes of eight consecutive cycles. These cycling processes are carried out using the same procedure as in the previous experiments. For the dehydration of the material the reactor is heated up to 500 °C and the reactor is purged by nitrogen. After 10 cycles, the reaction material is taken
Fig. 6. Temperature development (left) and reaction rate (right) during hydration of the unmodified material with T0 = in = 300 °C, P0 = out = 2 bar g and fH2O = 0.4.
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Fig. 7. Temperature development (left) and reaction rate (right) during hydration with T0 = in = 300 °C, P0 = out = 2 bar g and fH2O = 0.4 with addition of 0.5 wt% Aerosil.
out of the reactor in its hydrated state. The mass loss after treatment in an oven (nitrogen atmosphere) at 600 °C amounts to 13.1 g with a measure uncertainty of the scales of 0.01 g which means that the whole material has been fully converted. The temperature developments of the single cycles remain almost unchanged leading to the assumption that the nanoparticles are stable on the particle surface. To confirm this observation a SEM picture of the surface after this cycling is taken (Fig. 9). In the SEM view the nanoparticles can be clearly seen on the surface (branched structures). No change of structure or coating behaviour can be noticed. Based on the cycling stability in the reacÒ tor and the visual confirmation by the SEM, the Aerosil is assumed to be stable. Nevertheless this stability has to be proven by other material characterization methods at significantly more cycles. Summarizing, it can be noted that agglomeration effects within fine powder bulks can be minimized by addition of small amounts of nano scaled additives (Fig. 10) and cycling stability during thermochemical operations has been demonstrated for 10 cycles. Additionally, due to reduced adhesive forces between the particles cohesiveness within the powder bed is reduced leading to improved flow-through characteristics. Consequently, the mass and heat transfer in the reaction bed as well as the applicability for thermochemical storage are enhanced. Nevertheless, further extensive investigations of processes during chemical reaction and influences for thermochemical storage behaviour have to be performed.
Fig. 9. SEM picture of Ca(OH)2 particle mixed with 5 wt% of Aerosil after three cycles in the lab scale-reactor.
Fig. 10. Calcium hydroxide powder with 0.5 wt% Aerosil after 4 cycles in the labscale reactor.
4. Conclusions Fig. 8. Temperature developments during eight hydration cycles with addition of 0.5% Aerosil.
Thermochemical systems offer high energy densities and longterm storage possibilities due to the chemically stored thermal
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energy. However, due to the generally fine powdered material and its potentially cohesive character, an application is hindered due to an easy formation of bulk inhomogeneity like e.g. agglomeration. It has been shown that the addition of a small amount of hydrophilic Ò Aerosil prevents agglomeration during the cycling of CaO/Ca(OH)2 without having a negative influence on the reaction dynamics. Thus, the cycling stability of the powder bed within 8 cycles is demonstrated and confirmed by SEM pictures. This leads to a significantly improved heat and mass transfer within the experimental reactor and the charging and discharging times of the storage are considerably reduced. Therefore, the possibility to improve the cycling stability of bulk properties during thermochemical cycles could be transferred to different applications dealing with fine-powdered material. However, due to the chemical reactions and the changing atmosphere investigations of cycling stability have to be extended. References [1] Abedin AH, Rosen MA. Assessment of a closed thermochemical energy storage using energy and exergy methods. Appl Energy 2012;93:18–23. [2] Neveu P, Tescari S, Aussel D, Mazet N. Combined constructal and exergy optimization of thermochemical reactors for high temperature heat storage. Energy Convers Manage 2013;71:186–98. [3] Dincer I, Rosen M. Thermal energy storage: systems and applications. Wiley; 2010. [4] Laing D, Bahl C, Bauer T, Lehmann D, Steinmann W-D. Thermal energy storage for direct steam generation. Sol Energy 2011;85:627–33. [5] Bahl C, Laing D, Hempel M, Stückle A. Concrete thermal energy storage for solar thermal power plants and industrial process heat. SolarPaces 2009. [6] Schaube F, Wörner A, Tamme R. High temperature thermochemical heat storage for concentrated solar power using gas–solid reactions. J Sol Energy Eng 2011;133. [7] Fujii I, Tsuchiya K, Higano M, Yamada J. Studies of an energy storage system by use of the reversible chemical reaction: CaO + H2O Ca(OH)2. Sol Energy 1985;34:367–77.
[8] Brown DR, La Marche JL, Spanner GE. Chemical energy storage system for solar electric generating system (SEGS) solar thermal power plant. J Sol Energy Eng 1992;114:212–8. [9] Ogura H, Shimojyo R, Kage H, Matsuno Y, Mujumdar AS. Simulation of hydration/dehydration of CaO/Ca(OH)2 chemical heat pump reactor for cold/ hot heat generation. Dry Technol 1999;17:1579–92. [10] Fujimoto S, Bilgen E, Ogura H. CaO/Ca(OH)2 chemical heat pump system. Energy Convers Manage 2002;43:947–60. [11] Matsuda H, Ishitu T, Lee SK, Hasatani M. Kinetic study of Ca(OH)2/CaO reversible thermochemical reaction for thermal energy storage by means of chemical reaction. J Kagaku Kogaku Ronbunshu 1985;11:542–8. [12] Schaube F, Kohzer A, Schütz J, Wörner A, Müller-Steinhagen H. De- and rehydration of Ca(OH)2 in a reactor with direct heat transfer for thermochemical heat storage. Part A: experimental results. Chem Eng Res Des 2012. [13] Kerskes H, Bertsch F, Mette B, Wörner A, Schaube F. Thermochemische Energiespeicher. Chem Ing Tech 2011;83:2014–26. [14] Kojima T, Elliott JA. Effect of silica nanoparticles on the bulk flow properties of fine cohesive powders. Chem Eng Sci 2013;101:315–28. [15] Pontiga F, Valverde JM, Moreno H, Duran-Olivencia FJ. Dry gas–solid carbonation in fluidized beds of Ca(OH)2 and nanosilica/Ca(OH)2 at ambient temperature and low CO2 pressure. Chem Eng J 2013;222:546–52. [16] Perez-Vaquero J, Valverde JM, Quintanilla MAS. Flow properties of CO2 sorbent powders modified with nanosilica. Powder Technol 2013;249:443–55. [17] Valverde JM, Duran FJ, Pontiga F, Moreno H. CO2 capture enhancement in a fluidized bed of a modified Geldart C powder. Powder Technol 2012;224:247–52. [18] Valverde JM, Pontiga F, Soria-Hoyo C, Quintanilla MAS, Moreno H, Duran FJ, et al. Improving the gas–solids contact efficiency in a fluidized bed of CO2 adsorbent fine particles. Phys Chem Chem Phys 2011;13:14906–9. [19] Valverde JM, Sanchez-Jimenez PE, Perejon A, Perez-Maqueda LA. Constant rate thermal analysis for enhancing the long-term CO2 capture of CaO at Ca-looping conditions. Appl Energy 2013;108:108–20. [20] Wang K, Zhao P, Guo X, Li Y, Han D, Chao Y. Enhancement of reactivity in Li4SiO4-based sorbents from the nano-sized rice husk ash for hightemperature CO2 capture. Energy Convers Manage 2014;81:447–54. [21] Pahl MH, Wicke R. Haftkräfte im kompaktierten Schüttgütern. AufbereitungsTechnik 1988;29:371–80. [22] Rumpf H. Die Wissenschaft des Agglomerierens. Chem Ing Tech 1974;46:1–11. [23] Mathias J, Wannemacher G. Basic characteristics and applications of aerosil: 30. The chemistry and physics of the aerosil Surface. J Colloid Interface Sci 1988;125:61–8.
Contents lists available at ScienceDirect
Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman
Improving powder bed properties for thermochemical storage by adding nanoparticles C. Roßkopf ⇑, M. Haas, A. Faik, M. Linder, A. Wörner Institute of Technical Thermodynamics, German Aerospace Center, Pfaffenwaldring 38-40, 70569 Stuttgart, Germany
a r t i c l e
i n f o
Article history: Received 20 February 2014 Accepted 2 May 2014
Keywords: Ca(OH)2 Nano particles Flowability Moving bed Thermochemical storage
a b s t r a c t Thermochemical storage offers interesting potential to store thermal energy, especially in the field of industrial waste heat utilization or for concentrated solar power (CSP) plants. However, at present the development of thermochemical storage technology is in its initial stage with investigations mainly on material aspects or small lab-scale systems. With regard to its thermodynamics and kinetics, it has been shown that the CaO/Ca(OH)2 reaction system is suitable for thermochemical heat storage at a temperature range of 400–600 °C. However, the behaviour in a small lab-scale system was mainly dominated by heat and mass transfer limitations originating from the small particle size and changes in the bulk properties. It is shown that by the addition of small amounts of additives like nanoÒ particles of SiO2 (Aerosil ), the bulk properties can be stabilized and consequently the cycling stability ensured. In addition, channelling effects can be minimized resulting in a more homogeneous flow through the reaction bed improving the overall reaction behaviour of the thermochemical storage. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Constantly rising energy prices and increasing emissions of carbon dioxide necessitate a rethinking of how energy is used. Thermal energy storage is one important cornerstone on the way to higher efficiency in industrial processes as well as for the implementation of dispatchable regenerative energy especially in concentrated solar power (CSP) plants [1,2]. Heat storage can be divided into three different technologies: sensible, latent and thermochemical. Sensible heat storage has been most researched and is already employed e.g. in solar power plants as two-tank molten salt storage systems [3]. Latent storage makes use of the phase change of the storage material. The operation of a 700 kW h storage generating steam for 3000-h has been successfully demonstrated [4,5]. In comparison to sensible and latent heat storage systems, thermochemical technologies are only in the initial phase of development but show great potential with regard to energy density and long-term storage behaviour [6–9]. The CaO/Ca(OH)2 reaction system has been investigated extensively and is appropriate for thermal energy storage in a temperature range of 400–600 °C [6,10,11].
⇑ Corresponding author. Tel.: +49 (0)711 6862 8045. E-mail address: [email protected] (C. Roßkopf). http://dx.doi.org/10.1016/j.enconman.2014.05.017 0196-8904/Ó 2014 Elsevier Ltd. All rights reserved.
The following equation shows the equilibrium reaction where DHR correspondents to the reaction enthalpy used to chemically store the thermal energy.
CaOðsÞ þ H2 OðgÞ CaðOHÞ2ðsÞ þ DHR During the charging process heat is supplied to the storage system while water vapour is drawn out. During the discharging process calcium oxide powder reacts with water vapour to calcium hydroxide thereby releasing heat. In case of a directly operated reactor, the heat is directly decoupled by a heat transfer fluid (HTF). This means that the HTF flows through the bed and is in direct contact with the powder. An advantage of this system is the large heat exchange area between the particle surface and the HTF. However, fast discharging demands a high HTF velocity leading to a high pressure drop in the reaction bed due to the low permeability of the powder bed. To avoid this problem either the particle diameter has to be increased which is difficult due to the volume change of the particles during the reaction or the porosity of the reaction bed has to be increased. The main problem occurring especially during the cycling of the thermochemical storage is the worsening of the reaction material [12,13]. After two cycles, small agglomeration lumps begin to form, growing in size from cycle to cycle and therefore constant thermophysical bed properties cannot be guaranteed. Fig. 1 shows an agglomerate of calcium hydroxide after 4 cycles in the lab-scale reactor. Due to the mechanical stability of this agglomerate, it grows continuously at each cycling step.
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C. Roßkopf et al. / Energy Conversion and Management 86 (2014) 93–98
minimize the contact area between particles to improve the flow behaviour of powder beds [14]. For thermochemical heat storage, a usage of nanoparticles in order to improve reaction behaviour is not known in literature. Nevertheless, Valverde et al. inserted nanoparticles to Ca(OH)2 bulk powder in order to increase the fluidization behaviour and reactivity for CO2 absorption [15–19]. Contrary to Valverde et al. who add a ratio of guest particles up to 30% and coat these nanoparticle agglomerates with Ca(OH)2, this contribution shows the coating of Ca(OH)2 with nanoparticles to preserve the high storage capacity. To this end much less material is added. This paper therefore reports on the experimental results in a reactor with direct heat transfer and analyses the effects of the addition of SiO2 nanoparticles on agglomeration and channelling. Wang also investigated the enhancement of reactivity for CO2 capture with Li4SiO4-based sorbents by adding SiO2 nanoparticles [20]. Fig. 1. Calcium hydroxide agglomerate after 4 cycles in the laboratory reactor.
2. Material and methods Accordingly, a homogeneous flow of the reaction and HTF gases through the fixed reaction bed is not possible. A large fraction of the HTF passes the reaction bed through wide channels within the bed. Therefore an optimum mass and mainly heat transfer is not ensured. Transfer of the HTF into the agglomerates is insufficient due to their low permeability. Fig. 2 shows a powder bed of Ca(OH)2 at ambient temperature while N2 flows through it from bottom to top within a glass tube. Even though the material has not reacted yet, the channels demonstrate the cohesiveness of this powder. In order to solve the problem of agglomeration one option is to reduce the attractive forces between the particles. This can be achieved on one hand by pelletizing the fine powder which is difficult due to the volume change of the single particles during the reaction of the material. Another possibility to minimize these forces is to increase the surface roughness by coating the surface of the particles with additives. In bulk industries nanoparticles are used in order to increase these roughness and consequently
2.1. Lab-scale reactor A directly heated lab-scale reactor is used for cycling experiments and analysing the reaction behaviour. The flow chart of the test bench is presented in Fig. 3 with additional details given in [12]. A nitrogen flow (0–75 Nl/min) is heated up to the required temperature of 600 °C. Distilled water (0–1000 g/h) can be evaporated, overheated and added to the nitrogen flow in a mixing chamber. This mixture flows either through the thermochemical reactor from bottom to top or through a bypass. A dew point meter after the reactor outlet measures the steam content in the gas mixture whereby the conversion during the reaction process can be determined. The H2O pressure can be calculated by the equation of Sonntag which is described in detail in [12]. A maximum theoretical error of 0.1 K can be assumed with this measurement procedure and is taken into account in the standard deviation of the conversion. By means of a back pressure regulator the absolute pressure within the reactor can be controlled. Fig. 4 shows a picture of the thermochemical reactor and the positions of the Typ K thermocouples with a diameter of 1 mm and a maximum measuring inaccuracy of 0.1 °C inside. The diameter of the reaction zone within the reactor is 54.5 mm and its height measures 158 mm. The reactor is equipped with thermocouples which are placed at the central axis and close to the reactor wall within the reaction bed. The reactor is filled with 60 g of Ca(OH)2 which correspondents to a chemically stored capacity of about 21 W h [13]. In order to investigate the influence of the additives on the powder, the discharging process is considered. Here, the reactor is preheated to the starting temperature of 300 °C. By means of the back-pressure regulator the absolute pressure in the reactor is set in advance. The valve in front of the reactor is closed and the gas flow is directed through the bypass. Then, the desired water vapour/nitrogen mixture of nH2O = 0.4 is adjusted. As soon as the dew point meter outputs a constant signal, the valve in front of the reactor is opened and the gas mixture flows through the reactor defining the starting point t = 0 for the experiment. 2.2. CaO/Ca(OH)2
Fig. 2. Channeling effects within the powder bed of Ca(OH)2 at ambient temperature.
For the investigation of the influence of the additives white hydrated lime Ca(OH)2 by HeidelbergCement, Germany is used. White hydrated lime is a fine powder whose main commercial application is in the production of mortar. The most important key data are the true density of 2200 kg/m3, the bulk density of
C. Roßkopf et al. / Energy Conversion and Management 86 (2014) 93–98
95
Fig. 3. Set-up of the test bench.
Fig. 4. Lab-scale reactor, photographic image (left) and position of powder bed and thermocouple (right).
450 kg/m3, the mean diameter d50 of 5 lm and the purity of 97.8%. The hydroxide form of the material Ca(OH)2 was chosen as starting material due to the less harmful handling compared to CaO and to start the cycling with a shrinking reaction bed instead of an expanding reaction bed.
similar to the one of calcium hydroxide [23]. The nanoparticles are deposited on the surface of calcium hydroxide by means of a magnetic stirrer with agitator in a dry process. The standard mixing time is 30 min. 3. Results and discussion
2.3. Additive As outlined above, the attractive forces of the reacting particles are decreased by increasing the roughness of their surface [21,22]. In order to resist the high thermal stresses and to avoid side reactions as well as transformations, fumed silica SiO2 that is thermally stable up to 1000 °C and moderately reactive is used. In bulk material technology and pharmaceutical industries, silica nanoparticles are commonly used to reduce adhesive forces in Ò bulks of fine particles. In this work the hydrophilic Aerosil 300 is used for experiments in the thermochemical storage reactor. The mean size of the primary particles is about 7 nm. The surface of Ò Aerosil 300 is smooth and nonporous and the solid density is
In order to assess the flow-through characteristics, the reaction bed is first investigated in a glass tube at ambient temperature (Fig. 5). The velocity of the nitrogen flowing through the bed from bottom to top corresponds to the velocity in the lab-scale reactor and is assumed not leading to a fluidization of the bed. The strong channelling effects observed in Fig. 2 already disappear by the addition of 2 wt% of nanoparticles. However, the porosÒ ity of the powder bed rises with increasing amount of Aerosil as highlighted in [18]. In the unmodified powder bed, most part of the nitrogen flow passes the bed through these wide channels which present less resistance to the flow. With the minimization of the channels the flow is forced to pass the whole reaction bed
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Fig. 5. Powder bed with addition of two percent of Aerosil nanoparticles at ambient temperature.
through very narrow passages leading to a rising pressure drop compared to the unmodified bed. In order to investigate the influence on the storage the effects on the bulk material are investigated within the lab-scale reactor. In the following experiments the discharging process of the lab-scale reactor is considered. The procedure is carried out as mentioned above. Fig. 6(left) shows the temperature development at T7 within the powder bed of the hydration process in the lab-scale reactor. The reactor is heated up uniformly to a temperature of 300 °C and the pressure within the reaction chamber is adjusted to 2 bar g. After switching the mixed flow from the bypass to the reactor, the hydration process starts (at t = 0). The temperature of the whole reaction bed jumps instantaneously to the equilibrium temperature of 500 °C. The duration of the complete reaction process can be approximated by the temperature development within the reaction bed (T7 in Fig. 6(left)). The process seems to be completed when T7 equals the starting temperature. However,
by taking the reaction rate (Fig. 6(right)) measured via the dew point meter into account, it can be seen that most of the conversion is already completed after 2700 s. At the beginning the reaction takes place in the whole bed until the equilibrium temperature is reached. At this point, the conversion rate reaches its maximum of 125 g H2O/h. Afterwards, the reaction continues only at the section of the reaction bed where the heat is dissipated. After 2700 s the rate of reaction is almost negligible as only sensible heat is still stored within the reaction bed. The decline of the temperature therefore correspondents to the cooling of the bulk without chemical reaction. By integrating this curve the overall conversion of the reaction can be determined (compare [12]). The absorbed mass of H2O is 13.32 g where the standard deviation was found to be 0.37 g. Taking the standard deviation and the impurity of the material into consideration, full conversion of the material can be assumed [12]. After investigating the reference case, the reaction of Ca(OH)2 Ò with addition of Aerosil is analysed. The conditions are exactly Ò the same, except that 0.5 wt% of Aerosil is added to the calcium hydroxide powder by means of a dry coating process. Fig. 7(left) and (right) shows corresponding to Fig. 6(left) and (right) the temperature development and the reaction rate of the discharging process for the modified calcium hydroxide. The immediate rise of the bulk temperature to the equilibrium temperature can also be observed in the modified bed. However, the reaction is faster since the initial temperature is reached in about half the time. The development of the reaction rate also confirms the faster reaction behaviour since after 100 s 11.06 g have reacted compared to 9.44 g H2O for the unmodified material. The absorbed mass of water at the end of reaction equals 13.28 g with a standard deviation of 0.37 g confirming a full conversion. Comparing the reference material with the modified one, the considerably faster reaction behaviour as well as the faster cooling of the bulk is notable since only the material has been changed. It can be concluded that a reduction of channelling was also possible in the lab-scale reactor leading to a more homogeneous distribution of the HTF inside the bulk. Therefore, with the addition of nanoparticles the heat transfer into the gas is clearly increased resulting in faster cooling of the reaction bed and overall faster dynamics of the reactor. In addition to the promising results of the thermochemical storage behaviour of the modified material, the cycling stability of this effect is analysed. Fig. 8 shows the temperature developments of the hydration processes of eight consecutive cycles. These cycling processes are carried out using the same procedure as in the previous experiments. For the dehydration of the material the reactor is heated up to 500 °C and the reactor is purged by nitrogen. After 10 cycles, the reaction material is taken
Fig. 6. Temperature development (left) and reaction rate (right) during hydration of the unmodified material with T0 = in = 300 °C, P0 = out = 2 bar g and fH2O = 0.4.
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Fig. 7. Temperature development (left) and reaction rate (right) during hydration with T0 = in = 300 °C, P0 = out = 2 bar g and fH2O = 0.4 with addition of 0.5 wt% Aerosil.
out of the reactor in its hydrated state. The mass loss after treatment in an oven (nitrogen atmosphere) at 600 °C amounts to 13.1 g with a measure uncertainty of the scales of 0.01 g which means that the whole material has been fully converted. The temperature developments of the single cycles remain almost unchanged leading to the assumption that the nanoparticles are stable on the particle surface. To confirm this observation a SEM picture of the surface after this cycling is taken (Fig. 9). In the SEM view the nanoparticles can be clearly seen on the surface (branched structures). No change of structure or coating behaviour can be noticed. Based on the cycling stability in the reacÒ tor and the visual confirmation by the SEM, the Aerosil is assumed to be stable. Nevertheless this stability has to be proven by other material characterization methods at significantly more cycles. Summarizing, it can be noted that agglomeration effects within fine powder bulks can be minimized by addition of small amounts of nano scaled additives (Fig. 10) and cycling stability during thermochemical operations has been demonstrated for 10 cycles. Additionally, due to reduced adhesive forces between the particles cohesiveness within the powder bed is reduced leading to improved flow-through characteristics. Consequently, the mass and heat transfer in the reaction bed as well as the applicability for thermochemical storage are enhanced. Nevertheless, further extensive investigations of processes during chemical reaction and influences for thermochemical storage behaviour have to be performed.
Fig. 9. SEM picture of Ca(OH)2 particle mixed with 5 wt% of Aerosil after three cycles in the lab scale-reactor.
Fig. 10. Calcium hydroxide powder with 0.5 wt% Aerosil after 4 cycles in the labscale reactor.
4. Conclusions Fig. 8. Temperature developments during eight hydration cycles with addition of 0.5% Aerosil.
Thermochemical systems offer high energy densities and longterm storage possibilities due to the chemically stored thermal
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energy. However, due to the generally fine powdered material and its potentially cohesive character, an application is hindered due to an easy formation of bulk inhomogeneity like e.g. agglomeration. It has been shown that the addition of a small amount of hydrophilic Ò Aerosil prevents agglomeration during the cycling of CaO/Ca(OH)2 without having a negative influence on the reaction dynamics. Thus, the cycling stability of the powder bed within 8 cycles is demonstrated and confirmed by SEM pictures. This leads to a significantly improved heat and mass transfer within the experimental reactor and the charging and discharging times of the storage are considerably reduced. Therefore, the possibility to improve the cycling stability of bulk properties during thermochemical cycles could be transferred to different applications dealing with fine-powdered material. However, due to the chemical reactions and the changing atmosphere investigations of cycling stability have to be extended. References [1] Abedin AH, Rosen MA. Assessment of a closed thermochemical energy storage using energy and exergy methods. Appl Energy 2012;93:18–23. [2] Neveu P, Tescari S, Aussel D, Mazet N. Combined constructal and exergy optimization of thermochemical reactors for high temperature heat storage. Energy Convers Manage 2013;71:186–98. [3] Dincer I, Rosen M. Thermal energy storage: systems and applications. Wiley; 2010. [4] Laing D, Bahl C, Bauer T, Lehmann D, Steinmann W-D. Thermal energy storage for direct steam generation. Sol Energy 2011;85:627–33. [5] Bahl C, Laing D, Hempel M, Stückle A. Concrete thermal energy storage for solar thermal power plants and industrial process heat. SolarPaces 2009. [6] Schaube F, Wörner A, Tamme R. High temperature thermochemical heat storage for concentrated solar power using gas–solid reactions. J Sol Energy Eng 2011;133. [7] Fujii I, Tsuchiya K, Higano M, Yamada J. Studies of an energy storage system by use of the reversible chemical reaction: CaO + H2O Ca(OH)2. Sol Energy 1985;34:367–77.
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