(522c) Basic Study on Development of Thermal Storage Technology at 500 °C Using CaO /H2O Hydration Reaction

1.  Introduction

Recently, global warming has become a big problem due to the massive consumption of fossil fuels. Effective use of exhaust heat produced in various industrial processes is a very helpful way to reduce the amount of energy consumption. For example, exhaust gas higher than 500 °C, especially from high temperature process such as firing furnace of ceramics, is emitted. As a countermeasure, we focused on a chemical thermal storage which can store and reuse high temperature heat. A chemical thermal storage by using reversible thermochemical reaction can solve the problem in imbalance between the heat supply and heat demand.

     In this study, CaO/H₂O reversible chemical reaction was selected for its reaction system, since it has appropriate thermal operation range, high heat storage capacity and remarkable environmental affinity.

CaO(s) + H₂O(g) <-> Ca(OH)₂(s) + 104.2 kJ/mol

In this reaction system, exothermic reaction occurs by hydration of CaO, and reverse endothermic reaction occurs by dehydration of Ca(OH)₂. 

Chemical thermal storage using this reaction system consists of a reactor filled with CaO particle, evaporator, and condenser. Theoretical relation between temperature (T) of the reactor and water vapor pressure (P) of evaporator/condenser is able to describe by Clausius-Clapeyron equation.

lnP = -DH/RT + DS/R

DH and DS are enthalpy and entropy in the course of the reaction, and R is gas constant. At the heat storage step, the heat of 380 °C or more is stored during dehydration reaction when the vapor pressure of the condenser is 3.2 kPa. At the heat release step, the heat of 500 °C or more is released along with the hydration reaction when the vapor pressure of the evaporator is over 70 kPa. In addition, the transport rate of water vapor in the packed bed of CaO/Ca(OH)₂ will be accelerated when the vapor is under pressurized condition. Thus, this reaction system can store the heat at the target temperature of higher than 500 °C, and release the heat at the same temperature.

In this study, two experiments are carried out to discuss with applicability of CaO/H₂O reversible chemical reaction.

2.  Evaluation of durability of the CaO/H₂O reversible chemical reaction

Degradation of the reactivity due to sintering at high temperature is one of the main concerns of the chemical thermal heat storage. Therefore, durability of the cycle of hydration (heat release) and dehydration (heat storage) was evaluated by a repetitive reaction experiment.

A thermo-gravimetric analysis (TG) was used for this experiment. CaO particles of 20~25 μm in diameter were prepared as sample, and 10 mg of the sample was placed in a sample cell. The repetitive reaction was carried out under the temperature condition ranging from 250 °C to 500 °C. The rate of temperature rise and fall was 5 °C per minutes. Vapor pressure was constant at 4.2 kPa, under N₂ gas flow during the cycle of the reaction. Hydration and dehydration reaction was repeated 50 times. The durability of the reaction was evaluated from the conversion ratio which was calculated from the change in the amount of the sample weight.

As a result, the behaviour of hydration or dehydration was the same over 50time repetitions. The final conversion ratio of the hydration reaction was 0.95. After 50 times of hydration and dehydration reaction cycles, the conversion ratio of the hydration reaction decreased only 0.05 compared to that of the first time. This result shows that almost no degradation due to sintering has occurred.

It is expected that the CaO/H₂O reversible chemical reaction has ability to endure heat release and storage cycle for tens to hundreds of repetitions.

3.  Observation of ability to release heat on the level of 500 °C 

      A closed system thermal storage apparatus was used to demonstrate the heat release at the temperature on the level of 500 °C which is equal to the targeted heat storage temperature. This apparatus consists of packed bed type reactor and evaporator. 700 g of CaO particles was placed into the reactor and the packed bed was formed. The packed bed of CaO was in the form of cylinder with 106 mm in diameter and 104 mm in height. A vapor flow channel of 10 mm in outside diameter is located in the center of the cylindrical packed bed so as to transport vapor from the center in the direction of the diameter during the hydration reaction. The temperature of the packed bed type reactor and the evaporator was controlled at 230 °C and 110 °C by an electric furnace and a thermostat chamber respectively. The equilibrium pressure of the evaporator is 150 kPa at 110 °C to supply steam under pressurized condition. The thermocouples were placed inside the cylindrical packed bed at 74 mm in the height, and at 30, 40, 50 mm in the direction of the diameter from the axis. Hydration reaction began after the valve between the reactor and the evaporator was opened.

    After the reaction was started, the increase in the temperature of each measuring point was observed and the exothermic reaction was demonstrated. The temperature of the CaO packed bed at 30 mm from the axis reached maximum at 500 °C in about 35 minutes from the reaction started. Thus, the heat release on the level of 500°C was demonstrated under pressurized vapor condition. Different behavior of the temperature was shown at 40 mm and 50 mm in the direction of the diameter from the center. At 40 mm from the axis, the temperature reached maximum at 470 °C in about 20 minutes after started and sudden drop in temperature was observed after that. At 50 mm from the centerline, increase rate of the temperature was slower than those of other point and peak temperature was about 420 °C. The different behavior of three measurement points means that the heat transfer in the packed bed is slower than the heat release to the outside of the reactor.

4.  Conclusion

     CaO/H₂O reversible chemical reaction was selected for chemical thermal storage which could release and store the heat over 500 °C. In this study, two experiments were carried out to evaluate the durability of the cycle of hydration and dehydration and to demonstrate the heat release on the level of 500 °C.

The repetitive reaction experiment was carried out using TG analysis. The conversion ratio of the hydration reaction was 0.95 and decreased only 0.05 compared to that of the first time after 50 times of hydration and dehydration reaction cycle. So, it means that almost no degradation has occurred at 500 °C or less due to sinter.

      A closed system thermal storage apparatus was used to demonstrate the heat release at the temperature on the level of 500 °C under pressurized condition. Rise in the temperature up to 500 °C of the CaO packed bed inside the reactor was observed.

 It is suggest that, we may conclude that this reaction system is promised to realize a technology of heat storage and release on the level of 500°C.