Applicability Of Carbonation/Decarbo nation Reactions For Storing Thermal Energy From Nuclear Reactors

摘要

Heat energy extracted from nuclear reactors so far has been used only for power generation. As it is known, there are industrial processes such as iron manufacturing, coal gasification and so on, which mainly consist of thermal energy process. Recently, cleaner fuels such as hydrogen and methanol are highly expected to resolve global warming and oil shortage problems. From the view point of best energy mixing or highly developed energy system, it is important to study the possibility of making effective use of thermal energy from the nuclear reactors for different kinds of industrial applications and for producing cleaner fuels.rnIn order to realize the usage of high temperature heat (around 1023-1273 K) from the nuclear reactors, development of thermal energy utilization system like heat storage, heat transportation, heat pump, etc. are essential. However, it is very difficult to store large amount of heat energy at very high temperatures. Among the thermal energy storage methods, thermochemical energy storage seems very appropriate for storing high temperature heat since chemical energy storage, in principle, needs a minimum of thermal insulation as the thermal energy is converted into chemical bonds and is stored as chemical bonding energy. Moreover thermochemical storage can store large amount of energy per unit mass and can also perform as a heat transformer.rnAs research works on the chemical reactions for heat energy storage are rather scant, simple thermal dissociation reactions have been investigated for energy storage and are compared in terms of their thermodynamic, reaction equilibrium and physical properties of the involved reactants.rnAmong them CaO/CO_2 reaction system seems very promising because (i) its energy storage density is very high (～0.42 kcal/kg) (ii) CaCO_3, dissociates at 1123 K under 101 kPa of CO_2 pressure (iii) the reaction products are free from toxicity and corrosiveness (iv) the CaO-CO_2 reversible reaction has no side reaction and (v) the CaCO_3 raw material is abundantly available.rnDissociation of CaCO_3 produces CO_2 gas which has to be stored (1) as compressed gas (2) in the form of other metal carbonates by letting the CO_2 gas reacts with other metal oxides or (3) as adsorbed CO_2 by adsorbing the CO_2 gas with appropriate adsorbents like zeolite. Each corresponding exergy values, COP (coefficient of performance) and the upgraded temperature in heat upgradingrnconfiguration was evaluated.rnRepeated carbonation/decarbonation of CaO was carried out by TGA (thermogravimetric analysis technique) to study the reactivity of CaO when it underwent repeated heat absorbing/releasing processes. It is found that under 101 kPa of CO_2 pressure, CaCO_3 completely decomposes at 1123 K and that reactivity and conversion of CaO is faster and higher with increase in temperature for temperatures lower than that of the decomposition temperature.rnThe ability to generate high temperature heat around 1273 K by CaO-CO_2 reaction is confirmed by means of carbonating CaO in a lab-scale adiabatic reactor. During these carbonation experiments, pseudo-steady state conditions prevailed in which the reactant bed temperatures are, e.g. about 1220 K for loaded CO_2 pressure of 200 kPa and about 1300 K for CO_2 pressure of 600 kPa, etc. From this, it is concluded that the CaO-CO_2 reversible reaction can be used not only for energy storage but also for transforming heat at a certain temperature to a relatively higher temperature; e. g. heat at 1123 K will be used for decomposing CaCO_3 at 101 kPa pressure, whereas, heat at about 1220 K will be generated when CaO is made to react with CO_2 of 200 kPa.rnSuch issues as the enhancement of reactivity of CaO, heat and mass transfer in the reactor packed with CaO particles are now being studied for realizing this CaO/CO_2 heat storage system from a practical point of view.