Experimental investigation on thermodynamic and kinetic of calcium hydroxide dehydration with hexagonal boron nitride doping for thermochemical energy storage
Introduction
Due to the energy shortage and environmental pollution, the effective and large-scale use of renewable energy and industrial waste energy become a vital issue for reducing fossil fuel consumption and tackling climate change. In the energy utilization systems, thermal energy storage system should be employed to efficiently utilize solar energy and recycle the waste heat during the industrial process. Generally, thermal energy storage styles include sensible, phase change and thermochemical heat storage. In comparison to the former two methods, thermochemical heat storage can obtain larger theoretical energy storage/release density and low thermal energy losses (Alva et al., 2018, Pierre et al., 2014). Hence, thermochemical heat energy storage systems have obtained increasing attention in recent decades, which have been considered as a promising candidate to make long-term storage and long-distance transportation of thermal energy possible (Abedin and Rosen, 2012, Tatsidjodoung et al., 2013).
Thermochemical energy storage system usually is on the basis of a reversible reaction with the endothermic decomposition reaction and exothermic synthesis process between two substances, like Ca(OH)2/CaO and CaCO3/CaO. The Ca(OH)2/CaO thermochemical heat storage system is thought of one of the most potential thermochemical processes for the storage of middle- and high-temperature thermal energy (Brown et al., 1992, Irabien et al., 1990, Schaube et al., 2011, Yan et al., 2015, Zhang et al., 2016). During this system, thermal energy can be stored in the dehydration process of calcium hydroxide, which takes place at the appropriate temperature from 410 to 600 °C, and then the heat can be released from atmospheric temperature to about 500 °C during the hydration process of calcium oxide (Pierre et al., 2014). This system can be used for the utilization and recycling of industrial waste heat and the heat storage of the concentrated solar energy power plants (Hui et al., 2013, Kenisarin and Mahkamov, 2007). The recent investigations have indicated that the concentrated solar power plants coupled with thermochemical energy storage system can overcome the intermittent nature of solar energy and provide steady output power (Chen et al., 2018a, Chen et al., 2018b, Prieto et al., 2016, Schaube et al., 2011, Tian and Zhao, 2013). The above investigations have proved that Ca(OH)2/CaO system is suitable for the thermochemical heat storage. Nevertheless, the application of this system also has some limitations which can be concluded: i) poor heat conductivity; ii) easy agglomeration and sintering; and iii) unevenness of heat release rate (Dai et al., 2018, Xu et al., 2017, Yan and Zhao, 2016).
In order to solve the above mentioned problems, some investigations focus on the reactor design of Ca(OH)2/CaO thermochemical energy storage system. Yan and Zhao (2016) through developing a fixed bed reactor investigated the influence of water vapor on this calcium hydroxide/calcium oxide system, and they found that a higher vapor pressure can lead to a better heat release capacity. Schaube et al. (2013) dedicated on the performance behavior of a fixed bed reactor with the direct heat exchanger. Through varying the inlet and outlet conditions of heat exchanger, they found that both vapor flow rate and inlet temperature had great influence on the continuance time of dehydration procedure for metal hydroxide. Schmidt et al. (2014) developed a 10 kW-scale thermochemical heat storage reactor on the basis of a plate heat exchanger. There still were some investigations on fluidized bed reactor, which presented the application potential of Ca(OH)2/CaO system (Angerer et al., 2018, Pardo et al., 2014).
Many researches focus their studies on improving the energy storage density and cycling stability on the basis of materials design (Criado et al., 2014, Criado et al., 2016, Schaube et al., 2012, Yan et al., 2015). The addition of salts (nitrates, chlorides, acetates, sulphates, bromides) and inert materials (expanded graphite, carbon nanotubes, SiO2 and Li doping) are common strategies to improve their material performances (Zamengo et al., 2014). Shkatulov et al. (2012) enhanced the performance of magnesium hydroxide through the addition of vermiculite and salts. They also investigated the influence of doping different salts on improving the decomposition dynamics and managing the decomposition temperature (Schaube et al., 2012, Shkatulov and Aristov, 2015). The dehydration reaction rates of the magnesium hydroxide were increased by the addition of metal salt due to its modification of the particle surface (Ishitobi et al., 2013, Myagmarjav et al., 2014). Hara et al. (2013) provided a synthesis routine for gaining the cemented materials. The obtained materials showed better mechanical properties account for silicates formed during reactions of these components at a high temperature. Yan and Zhao, 2014, Yan and Zhao, 2015, Yan and Zhao, 2016 investigated the thermodynamic and kinetic performances of Li-doped calcium hydroxide synthesized by ball grinding and set up a fixed reactor to investigate the reaction performances of Li-doped composite, their results show that the doping of Li can provide a faster heat storage rate. Chen et al., 2018a, Chen et al., 2018b investigated the thermodynamics, kinetics and cyclic stability of calcium carbonate doped with silicon dioxide for the thermochemical energy storage system by the thermogravimetric analysis and DSC analyzer, and the results indicated that the presence of silicon dioxide could increase the released heat and specific heat capacity, however, the doping can have tiny negative influence on the energy storage capacity of this system. Other high thermal conductivity and/or porous materials, like expanded graphite (Han et al., 2018, Zamengo et al., 2014), vermiculite (Shkatulov et al., 2012), carbon nanotubes (Mastronardo et al., 2016), graphene and silicon dioxide (Benitezguerrero et al., 2018) are usually used to improve the thermal properties of calcium-based thermochemical energy storage materials.
In the present work, hexagonal boron nitride (HBN) was first used to improve the thermochemical performance of calcium hydroxide in view of its well-known advantages of high thermal conductivity, durability at high temperature, low thermal expansion coefficient and chemically inertness (Lipp et al., 1989). The quantitative studies about the impact of HBN on calcium hydroxide/calcium oxide thermochemical heat storage system should be carried out. Hence, the influence of adding HBN on the thermodynamic properties, kinetic parameters of the decomposition process of calcium hydroxide as well as its cycling stability is studied. Purpose of this paper is aimed at (a) improving the thermal energy storage capacity of the materials, (b) obtaining the proper doped mass percentage of HBN and the related dehydration kinetic equations, and (c) verifying the possibility of enhancing multicycle activity of materials by adding HBN powder.
Section snippets
Composites preparation
Calcium hydroxide powder used in this paper was purchased from Tianjin chemical (with the purity > 95%). Hexagon boron nitride was purchased from Dandong Rijin technology Co. LTD (with the purity > 98.5%). The samples doping with HBN was obtained by steps as follows (take the 5 wt% HBN-doped as sample): 0.95 g of calcium hydroxide powder, 0.05 g of HBN powder and 50 ml of ethyl ethanol were mixed together. This mixture was sonicated for 60 min at 48 °C followed by the magnetic stirring for
Characteristics of composites
Fig. 2 shows the SEM images of pure calcium hydroxide and the composites with different contents HBN doping. Fig. 2(a) shows the SEM image of pure compound, and Fig. 2(b)–(e) represent the SEM images of different contents of HBN-doped composites. It can illustrate that the pure compound naturally tends to form the large grains with a dense layer without the presence of HBN. After adding the HBN particles, the morphologies of composites are displayed as the aggregative loose particles with
Conclusions
All in all, in this present work, the impact of HBN doping on the thermodynamic properties, kinetic equations and cycling stability of Ca(OH)2/CaO system was investigated by employing TGA, DSC, SEM and thermal constant technologies. Some conclusions can be concluded as follows:
- (1)
The TGA results show that the required time for the dehydration process of calcium hydroxide can be reduced owing to HBN doping. through comparison, the 15 wt% HBN-doped composite has better dehydration performances than
Declaration of Competing Interest
The authors declared that they have no conflicts of interest to this work. We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.
Acknowledgment
This work was supported by the National Natural Science Foundation of China (Nos. 51836009 and 51776202).
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