Abstract
A chemical heat pump system using BaO/BaCO3 reversible reaction is proposed for high temperature heat storage and temperature upgrading above 1273 K, because the BaO/BaCO3 reaction can be controlled under moderate pressure, even above 1273 K. Thermogravimetric studies on carbonation of BaO at 973-1523 K and decarbonation of BaCO3 at 1073-1523 K were carried out under various CO2 pressures in order to characterize the heat generating and storing rates of the CHP. In both reactions, reaction rate and conversion increase with an increase in temperature. Compared to carbonation, decarbonation is more strongly influenced by reaction temperature as well as pressure of CO2. It is also found that the eutectic reaction between BaO and BaCO3 has a great influence on the reaction characteristics; i.e. for carbonation with pressures around Pe and for decarbonation with all pressures lower than Pe above 1303 K, the reaction rate drops after X = 0.4-0.6 due to diffusion resistance of CO2 caused by the eutectic reaction. A rate expression similar to that of the CaO/CaCO3 reaction is found to be applicable for the BaO/BaCO3 reaction in the initial phase-boundary controlled period. Assuming that conversions of carbonation and decarbonation have to reach over 0.6 within 30 and 60 min, the heat releasing step can be operated at 1273 K or higher above the equilibrium pressure, but the heat storing operation must be carried out above 1273 K, below a quarter of Pe.
