Lithium ortho-silicate made by Kanto Kagaku was selected for this study. In this experiment, the sample diameter was under 5 μm. Before the experiment, a reactant sample was out gassed at 500˚C for 10 h under vacuum.

This experiment for the evaluation of repetition durability for the reaction at which it absorbs is measured using a thermogravimetric method Figure 2 shows a schematic diagram of the experimental apparatus. 10 mg of the lithium ortho- silicate reactant was used, and the packed bed thickness was less than 1 mm. In the absorption step, CO₂ gas was introduced to the reactant sample at 100 kPa and 700˚C. After the absorption step, the temperature of the reactant sample was changed to 800˚C in the desorption step. When the weight change reached equilibrium, we calculated the amount of CO₂ used in the absorption/desorption reaction from the quantity of gas represented by the weight change. The reversible absorption/desorption reaction could be repeated 20 times. We evaluated the effect of absorption and desorption repetition cycles on the conversion fraction.

Figure 3 shows the experimental results. The weight of the reactant sample increased with absorption, and decreased with desorption. Figure 4 shows the reaction characteristics on repeating the cycle 20 times. From the results, the conversion fraction was decreased after the initial five repetitions. After the fifth cycle, the conversion fraction remained constant at 60%. It was observed that the reactant particles were sintered and the reactant packed bed contracted at high temperatures. The CO₂ diffusion resistance in the packed bed gradually increased. After 5 cycles, the reactant sintering converged. The expansion and contraction of the packed bed for the CO₂ reaction was stable. The experiment demonstrates that the lithium ortho-silicate and CO₂ reaction system can be repeatedly utilized in chemical heat storage technology.

The experimental apparatus for measuring the absorption rate is shown in Figure 5. This equipment consisted of a CO₂ tank, reactor, and vacuum pump. The reactor and the connecting pipe lines were maintained at a specific temperature by a thermos reactor heater. A Pt resistance thermocouple and pressure gauge were connected to a CO₂ tank, and the CO₂ gas and pressure was measured. The inner volumes of the CO₂ tank and reactor were 2.0 L and 50 mL, respectively. The weight of reactant sample was 0.1 g.

When the reactor was connected to the CO₂ tank, the tank pressure decreased with progress of the CO₂ absorption reaction. We calculated the amount of CO₂ used in the absorption reaction from the quantity of gas causing the pressure change. In this method, the tank and weight of lithium ortho-silicate sample was adjusted in order to keep the tank pressure change under 5%. After the absorption reaction step, the temperature of the reactor was raised and the CO₂ gas was desorbed. The conversion fraction was defined by

In these experiments, we evaluated the effect of reaction temperature and pressure on absorption rate.

Figure 6 shows the absorption rate as the conversion fraction approached 0.3. The experimental result shows the effect of CO₂ pressure on the absorption rate. In this experiment, the reactor temperature was maintained at 630˚C. The CO₂ pressure was set at 30, 60, 90, and 100 kPa. From the results, the same absorption rates were observed at 60, 90 and 100 kPa. The absorption rate was lower at 30 kPa compared with the other pressure conditions. The equilibrium pressure was 27.2 kPa when the reactor temperature was 630˚C. The change in CO₂ pressure at 30 kPa was very small. Therefore, the driving force for the absorption reaction was small. It is estimated that the pressure difference of 2.8 kPa was a significant influence on the absorption rate.

Figure 7 shows the effect of reactor temperature on the absorption rate. In this experiment, the CO₂ pressure was set 100 kPa. The reactor temperature was set 500, 550, 610, 630, 650, 670 and 700˚C. From the results, when the reactor temperature was high the absorption rate was large. It is revealed that the absorption rate was affected by the reactor temperature. It is estimated that this reaction system could be utilized to heat output at high temperature. The ab-

sorption rate was extremely small at 700˚C. This is why the difference of equilibrium temperature was small. Figure 8 shows the absorption rate value on each reactor temperature at a conversion fraction of 0.3. The maximum absorption rate was obtained at 670˚C in this experiment.

Figure 9 shows photos of reactant sample packed beds before and after the experiment. After the experiment, the reactant sample packed bed was sintered and contracted. It is estimated that the CO₂ diffusion rate in the packed bed decreased as the porosity of the packed bed decreased. In future, we plan to develop composite materials for a more stable configuration of reactant.