Reaction characteristics of BaO/BaCO3 system for high temperature chemicalheat pump above 1273 K

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, e ven above 1273 K. Thermogravimetric studies on carbonation of BaO at 973-1523 K and decarbona tion of BaCO3 at 1073-1523 K were carried out under various CO2 pressures i n order to characterize the heat generating and storing rates of the CHP. I n both reactions, reaction rate and conversion increase with an increase in temperature. Compared to carbonation, decarbonation is more strongly influ enced 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 t he reaction characteristics; i.e. for carbonation with pressures around P-e and for decarbonation with all pressures lower than P-e above 1303 K, the reaction rate drops after X = 0.4-0.6 due to diffusion resistance of CO2 ca used 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 car bonation 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 equilibr ium pressure, but the heat storing operation must be carried out above 1273 K, below a quarter of P-e.