- Email: [email protected]
Kinetic feasibility of a chemical heat pump for heat utilization of high-temperature processes
PERGAMON
Applied Thermal Engineering 19 (1999) 239±254
Kinetic feasibility of a chemical heat pump for heat utilization of high-temperature processes Yukitaka Kato *, Naozumi Harada, Yoshio Yoshizawa Research Laboratory for Nuclear Reactors, Tokyo Institute of Technology, O-okayama, Meguro-ku, Tokyo 152-8550, Japan Received 20 October 1997
Abstract To utilize heat generated from high-temperature processes, the kinetic feasibility of a calcium oxide/ lead oxide/carbon dioxide chemical heat pump was examined experimentally by kinetic studies of CaO/ CO2 and PbO/CO2 reaction systems, which constitute the heat pump's reaction. In order to determine the optimal reaction conditions that still allow practical operation of the heat pump, both reaction systems were examined with respect to thermal drivability and reaction material durability. The heat pump was able to store heat of about 8608C and transform it to a heat of above 8808C under subatmospheric pressure without mechanical work. An applied system that combined the heat pump with a high-temperature process was proposed for high-eciency heat utilization. The scale of the heat pump in the combined system was estimated from the experimental results. # 1998 Elsevier Science Ltd. All rights reserved. Keywords: Heat pump; Calcium oxide; Lead oxide; Carbonation; Decarbonation
Nomenclature mCaO P Q r T t X
speci®c reaction amount to output, t/MW reaction pressure of CO2, atm heat amount, J temperature rise ratio, 8C/min reaction temperature, 8C reaction time, h ®xed reacted fraction, mol %
* Corresponding author. Fax: 00-81-3-5734-2959.
1359-4311/99/$ - see front matter # 1998 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 9 - 4 3 1 1 ( 9 8 ) 0 0 0 4 9 - 0
240
x DH8 Z
Y. Kato et al. / Applied Thermal Engineering 19 (1999) 239±254
reacted fraction, mol % standard reaction enthalpy, J/mol reaction amount ratio, mol%/mol% = [ÿ]
Subscripts 1 CaO/CO2 reaction system 2 PbO/CO2 reaction system a output from high temperature gas reactor for a common system b output from high temperature gas reactor for an applied system c carbonation d decarbonation r output of a high temperature gas reactor
1. Introduction High-temperature processes such as those involved in high temperature gas reactors (HTGR), iron smelting and some chemical processing, produce a large amount of hightemperature heat above 8008C. This heat may be used as the heat source for various heat utilization systems because of the high thermal quality of such heat. In order to increase the number of ways this heat can be used, heat storage and temperature transformation technology need to be improved. A chemical heat pump is one candidate for such improvements, because it can be applied to a wide temperature range by choosing the appropriate reaction system, can store heat as reactants via its chemical reaction with little heat loss for long periods of time, and under the appropriate reaction conditions, can transform the stored heat into another form over a wide temperature range. However, the use of chemical heat pumps for utilization of heat at such high temperatures has not yet been clari®ed in sucient detail. The characteristics of a particular chemical heat pump are determined by the reaction system employed. A calcium oxide/carbon dioxide reaction system (CaO/CO2) appears to be an appropriate reaction for a high-temperature heat pump above 7008C [1, 2]. The key consideration in the development of a heat pump system that employs this reaction is how to handle and store the carbon dioxide produced as the gas phase reactant. Employing a compressor, which would entail mechanical work, and using zeolites or Ca±Mg oxides as the absorbent of carbon dioxide for gas storage has been proposed [2, 3]. A practical solution for carbon dioxide storage that does not entail signi®cant mechanical work has yet to be established. In our previous study, a lead oxide/carbon dioxide reaction system (PbO/CO2) was demonstrated to be eective for storage of carbon dioxide [4]. Based on this ®nding, a heat pump that combines this reaction with the calcium oxide/carbon dioxide reaction system to form a calcium oxide/lead oxide/carbon dioxide (CaO/PbO/CO2) chemical heat pump was proposed. The heat pump uses a CaO/CO2 reaction system and a PbO/CO2 system. The equilibrium relationship of the heat pump cycle was determined from equilibrium and basic kinetic studies of both reaction systems. The heat pump eectiveness above about 8308C and under sub-atmospheric pressure was derived from the results. Next, the heat pump operability
Y. Kato et al. / Applied Thermal Engineering 19 (1999) 239±254
241
based on comprehensive kinetic studies of both reaction systems needed to be determined for heat pump realization. Thus, in the present study, the kinetics were studied with respect to thermal drivability and reaction material durability. Thermal drivability is preferable for a practical heat pump because the lack of mechanical work associated with the drivability allows simple, safe and low-cost development. This point is particularly important at such hightemperatures because of the diculty of designing a mechanical work system in that temperature range. Both reaction systems are repeated cyclic systems under heat pump operation. Reaction material durability in the repetitive cycle reaction was also examined. Based on the results of the kinetic studies, an applied system that combines a heat pump with a high-temperature process was proposed, and the scale of the applied system was discussed.
2. Principle and background of the heat pump 2.1. Principle of the heat pump The following CaO/CO2 and PbO/CO2 reaction systems were used in the CaO/PbO/CO2 chemical heat pump. CaO s CO2 g CaCO3 s; DH1 ÿ178:321kJ=mol;
1
PbO s CO2 g PbCO3 s; DH2 ÿ88:271kJ=mol:
2
The principle of a heat transformation type heat pump is shown in Fig. 1. A heat that has a higher temperature than the original heat source can be obtained using this operation. The system consists of CaO and PbO reactors. The operation settings of the heat pump include: (a) a heat storage mode and (b) a heat supply mode. Initially, CaCO3 and PbO are charged into each reactor. In the storage mode, the CaO reactor receives heat (Qd1) from a heat source at temperature Td1. Subsequently, CaO and CO2 are formed as a result of the decarbonation of CaCO3. The CO2 is reacted with PbO in the PbO reactor at some pressure (Pc2) and the exothermic heat of the carbonation is recovered at temperature Tc2, yielding PbCO3. In the heat supply mode, the decarbonation of PbCO3 proceeds in the PbO reactor using heat at temperature Td2, which is higher than Tc2. The CO2 formed at pressure Pd2, which is higher than Pc2, is introduced into the CaO reactor. Carbonation of CaO then proceeds and heat (Qc1) is generated exothermically in the reactor at a temperature of Tc1, which is higher than Td1, due to the higher reaction pressure. 2.2. Equilibrium relationship The equilibrium relationship of the heat pump operation is shown in Fig. 2. The solid and broken sloping lines indicate the reaction equilibrium of CaO/CO2 [5] and PbO/CO2. PbO/CO2 equilibrium consists of three equilibriums as follows [6]: Step A :
2PbCO3 Pb PbCO3 CO2 ;
3
242
Y. Kato et al. / Applied Thermal Engineering 19 (1999) 239±254
Fig. 1. Principle of CaO/PbO/CO2 chemical heat pump of heat transformation type operation; (a) heat storage mode, and (b) heat output mode.
Step B :
3 PbO PbCO3 2 2PbO PbCO3 CO2 ;
4
Step C :
2PbO PbCO3 3PbO CO2 :
5
The PbO/CO2 equilibrium line in Fig. 2 shows step C equilibrium. Step C is the equilibrium of heat pump operation, as determined in our previous study [7]. PbCO3 decarbonation and PbO carbonation are repeated reversibly during heat pump operation. Initial PbCO3 as a precursor is decarbonated through these three reaction equilibriums, and changes to PbO. The following carbonation is saturated at step C equilibrium, and the next repetitive reaction occurs under step C.
Y. Kato et al. / Applied Thermal Engineering 19 (1999) 239±254
243
Fig. 2. Heat pump cycle of reaction equilibrium.
2.3. Thermal drivability of the heat pump Preferably, as shown in Fig. 1 and mentioned in Section 1, the heat pump should be thermally driven without any mechanical work. To successfully realize the thermal drive operation, the choice of reaction pressure is important, as it ensures the reactivity of each reaction and the safety of the reactor design. In a previous study, the basic kinetics of each reaction was measured over a wide range of reaction pressures [7]. The four gray areas in Fig. 2 show the optimal reaction condition areas for thermal drive operation derived from that study. The number of each operation in Fig. 2 corresponds to the same numbered operations in Fig. 1. According to these operations, about 0.4 atm of reaction pressure is optimal for the heat storage mode [(1)±(2) in Figs. 1 and 2], and 1.0 atm or more reaction pressure is suitable for the heat output mode [(4)±(3) in Figs. 1 and 2]. However, the reactivity of each reaction is
244
Y. Kato et al. / Applied Thermal Engineering 19 (1999) 239±254
required to be high enough for practical operation of the heat pump and is aected by the reaction temperature. Thus, comprehensive kinetic studies of the reaction areas shown in Fig. 2 were required to determine the reaction conditions which would realize practical heat pump operation.
3. Experimental procedure The kinetics of the CaO/CO2 and PbO/CO2 reaction systems were studied using a thermobalance system under a closed system. Based on our previous study, 0.4 atm of reaction pressure for the heat storage mode [(1)±(2) in Fig. 2] and 1.0 atm for the heat output mode [(3)±(4) in Fig. 2] were appropriate reaction conditions. Reaction reactivity of both the CaO/ CO2 and PbO/CO2 systems under these conditions and under repetitive reaction were measured. Reaction samples were set in a sample cell placed on a balance. During each reaction, sample weight was monitored continuously and the reacted fraction was calculated based on the change in weight. Temperature was measured directly at the bottom of the cell using a thermocouple. This temperature was considered to be the reaction temperature and was maintained using an electric heater. After residual inert gas in the apparatus was removed using a vacuum pump at the beginning of each experiment, reaction pressure was controlled by introducing CO2 gas, and was measured using a manometer. The experiments were carried out at a reaction temperature between 300 and 9008C and under sub-atmospheric reaction pressure. The starting reactants were 45 mg of lead carbonate (PbCO3) and 18 mg of calcium carbonate (CaCO3) in 99.9% grade powder form (Kojundo Chemical Laboratory Co, Ltd). The dierent sample weights employed ensured identical sample volumes. Experimental analysis of the carbonation and decarbonation of PbO and the repetitive reaction was performed under the following process. PbCO3 ÿÿÿÿ4 PbO ÿÿÿÿ4 PbCO3 ÿÿÿÿ4 PbO . . . Carbonation Repetition Precursor Decarbonation Exp: Exp: Exp:
6
The reactivity of the carbonation and decarbonation of CaO was analyzed in a similar manner as the PbO/CO2 system.
4. Experimental results 4.1. PbO/CO2 reaction experiment 4.1.1. Decarbonation of PbCO3 The original PbCO3 sample was used for the PbCO3 decarbonation experiment. As mentioned in Section 2, the initial decarbonation reaction proceeds successively through the three steps of Eqs. (3)±(5) with a rise in temperature and loss of CO2, with 50, 16.7 and 33.3 mol% of reacted fraction amount, respectively. The step C reaction is only used as the heat pump cycle. The temperature dependence on decarbonation under a reaction pressure of
Y. Kato et al. / Applied Thermal Engineering 19 (1999) 239±254
245
Fig. 3. Temperature dependency of decarbonation of PbCO3.
1.0 atm is shown in Fig. 3. The sample was heated at 508C/min to the target decarbonation temperatures and was held at the temperature after 0 min in Fig. 3. Each reaction proceeds via step A and B during initial heating until about 4008C. This is con®rmed by the initial change in reacted fraction being about 67%, which is equal to the sum of 50 and 16.7 mol% for steps A and B, before 0 min in Fig. 3 and by our previous study. The reaction then attains step C. At 4208C, step C does not proceed because the reaction equilibrium temperature is higher than 4208C. The higher the reaction temperature was raised above 4308C, the faster the reaction rate increased. The reactions at 4408C and 4508C were complete at about 60 and 30 min, respectively, which indicates that decarbonation temperatures of 4408C or 4508C are optimal for the carbonation of a practical heat pump operation with respect to reaction rate.
4.1.2. Carbonation of PbO At the beginning of the carbonation experiment, the PbO sample for the carbonation was prepared by decarbonation of the original PbCO3 under ®xed conditions of 3508C and 0.01 atm. These conditions were employed to ensure completion of the decarbonation. The temperature dependence on carbonation reactivity under a reaction pressure of 0.4 atm is shown in Fig. 4. The carbonation proceeds in the reaction range of step C; the reaction is expected to be saturated at 33 mol% of the reacted fraction. However, a fairly long reaction time is required for saturation in Fig. 4 because of the low reactivity caused by the low reaction pressure. The reactivity is maximal around 3008C. Although the reaction rate increases along with reaction temperature rise, the exothermic carbonation approaches the reaction equilibrium of step C, and apparent reactivity decreases near the equilibrium. Therefore, the
246
Y. Kato et al. / Applied Thermal Engineering 19 (1999) 239±254
Fig. 4. Reactivity change in carbonation of PbO in response to change in reaction temperature.
reactivity has a maximal eect on the reaction temperature. This result shows that a reaction temperature of 3008C is optimal for the carbonation. 4.1.3. Repetitive reaction of the PbO/CO2 system The heat pump is operated by repetitive carbonation and decarbonation of PbO. The reactivity under the repetitive reaction was measured under the optimal reaction conditions described above. Fig. 5 shows the result of six repetitions under a decarbonation temperature, pressure and time of 4508C, 1.0 atm and 0.5 h, respectively, and a carbonation temperature, pressure and time of 3008C, 0.4 atm and 4 h, respectively. Although an initial decrease in reactivity was measured during the initial four cycles, reactivity was maintained over the next two cycles. The PbO sample is expected to maintain reactivity in further repetitive cycles after the initial reactivity decrease. 4.2. CaO/CO2 reaction experiment 4.2.1. Decarbonation of CaCO3 The original CaCO3 sample was used for the CaCO3 decarbonation experiment. The temperature dependency of the decarbonation under the target decarbonation pressure of 0.4 atm is shown in Fig. 6. The sample was heated at 508C/min to each objective decarbonation temperature and was held at the temperature after 0 min in Fig. 6. The higher the reaction temperature increased, the faster the reaction rate increased. The result indicates that over 8608C is the optimal temperature for the decarbonation of a practical heat pump operation, because the reaction is completed within 30 min of the reaction time.
Y. Kato et al. / Applied Thermal Engineering 19 (1999) 239±254
Fig. 5. Repetitive reaction of PbO/CO2 reaction system under derived optimal reaction conditions.
Fig. 6. Temperature dependency of decarbonation of CaCO3.
247
248
Y. Kato et al. / Applied Thermal Engineering 19 (1999) 239±254
4.2.2. Carbonation of CaO The temperature dependency of carbonation reactivity under a reaction pressure of 1.0 atm is shown in Fig. 7. For the carbonation, the precursor CaCO3 sample was decarbonated completely under a pressure of 0.01 atm during an increase in temperature before the reaction temperature attained the carbonation temperature. After the attainment of the carbonation temperature, CO2 gas was introduced into the reactor at 0 min, and the reactor maintained at the target pressure of 1.0 atm. The reactivity was observed up to 8808C. When the reaction temperature increases, the reaction rate decreases because the exothermic carbonation approaches the reaction equilibrium. As higher temperature output generated from the carbonation is preferred for the heat pump, a reaction temperature of 8808C is the optimal temperature for carbonation during practical heat pump operation. 4.2.3. Repetitive reaction of the CaO/CO2 system The reactivity under the repetitive reaction was measured under the optimal reaction conditions described above. Fig. 8 shows the result of seven repetitions under a decarbonation temperature, pressure and time of 8608C, 0.4 atm and 0.25 h, respectively, and a carbonation temperature, pressure and time of 8808C, 1.0 atm and 2 h, respectively. A continuous decrease
Fig. 7. Reactivity change in the carbonation of CaO in response to change in reaction temperature.
Y. Kato et al. / Applied Thermal Engineering 19 (1999) 239±254
249
Fig. 8. Repetitive reaction of CaO/CO2 reaction system under derived optimal reaction conditions.
in reaction amount during repetition was observed in contrast with the repetition result of the PbO/CO2 system (Fig. 5). The CaO sample is expected to lose reactivity in further repetitive cycles. A similar reduction in reactivity upon repetition was measured in a magnesium oxide/ water chemical heat pump, however, this reduction has already been overcome using a newly designed reaction material [8]. Thus, development of a highly-durable material for the CaO/ CO2 reaction is a realistic possibility. 5. Discussion 5.1. Optimal reaction conditions for heat pump operation The optimal reaction conditions for heat pump operation derived from the discussion in Section 4 are shown as four black dots in Fig. 2. The results show that the reaction systems can realize a heat transformation type operation, which means that the output temperature (Tc1) is higher than the input temperature (Td1), under thermal driving conditions without mechanical work. With respect to practical operation, the pressure gradient between reactors at (1) and (2), and between (3) and (4) in the ®gure are required to ensure the mobility of CO2 reactant between reactors. Thus, some adjustment of the reaction conditions would be required according to the heat pump design. Lower temperature input [process (1) in Figs. 1 and 2] and higher output [process (3)] are preferred for this type of heat pump. Higher temperature output is realized by higher temperature carbonation of CaO [process (3)] which is induced by the high pressure CO2 generated by the higher temperature decarbonation of PbCO3 [process (4)].
250
Y. Kato et al. / Applied Thermal Engineering 19 (1999) 239±254
Although these high temperature experiments, which were performed above 1.0 atm, were not examined with respect to restrictions of experimental apparatus, higher temperature reactions and output would be possible, because the reaction rates will be accelerated with the rise in the reaction temperature. On the other hand, lower temperature heat input [process (1)] is dicult because of the low reactivity of PbO carbonation [process (2)]. Lower temperature heat input is realized by lower temperature decarbonation of CaCO3, which is induced by CO2 consumption during carbonation of PbO at lower pressure. However, the current PbO material has limited reactivity under low pressure and temperature [7]. Thus, performing process (2) under lower pressure and process (1) at lower temperature heat input are dicult to realize at present. 5.2. Application of the heat pump to high-temperature processes 5.2.1. Principle of the applied system The heat consumed by the CaCO3 decarbonation (Qd1 at Td1) corresponds to the heat output of a heat source plant. From the derived optimal temperature of Td1 of the heat pump, the output of a high temperature gas reactor (HTGR) is an appropriate heat source for Qd1 of the heat pump. The possibility of the application of the heat pump to a HTGR for a heat utilization system is discussed below based on the experimental results. A HTGR output temperature of 8608C, which corresponds to Td1, is assumed in this discussion. Fig. 9(A) shows a common system that utilizes the heat generated from a HTGR, denoted Qa, for the standard load of the power plant. The heat is used as the heat source for a multi-stage gas-turbine and steam turbine of a generator and is transformed into electric power. The gray arrows in the ®gure and following Fig. 9(B) indicate heat ¯ows between these units. The heat ¯ows are realized by the introduction of heat transport components, that is, blowers and heat exchangers. For heat transports between HTGR, the CaO reactor and the gas turbine, helium gas would be one of the candidates for the heat transport media because of its stability and high speci®c heat. For heat transports related to the PbO reactor and the steam turbine, steam would be the ®rst candidate of the media because of its high-reliability in general use. An applied system that combines the heat pump with a HTGR is shown in Fig. 9(B). This system can contribute to load leveling of the power plant. Fig. 9(B) process (1) shows the heat storage mode during low demand for electricity. The heat output generated from a HTGR at a Td1 (=Tr) of 8608C is stored in the CaO reactor as reactant by the decarbonation of CaCO3 [process (1)], and the exothermic heat of PbO carbonation at a Tc2 of about 3008C is used as the heat source of the steam-turbine. Fig. 9(B) process (2) shows the heat output mode during high demand. The decarbonation of PbCO3 proceeds using a low temperature heat source (Qd2 at a Td2 of about 4508C) which is introduced from down stream of the gas-turbine ¯ow, and then the formed CO2 reacts with CaO in the CaO reactor. The exothermic heat generated by the carbonation (Qc1) at a Tc1 of above 8808C is injected into the top stage of the turbine and is transformed into electricity with the Qb2 of a HTGR standard output at a peak load. The dierence in reaction rate between the processes would be overcome by changing the reactant amount in both reactors as mentioned in the following Section 2(b). The extraction of Qd2 for the decarbonation of PbCO3 is eective for thermal ampli®cation at peak demand, because the amount of Qc1 is larger than that of Qd2 since DH81>DH82, and
Y. Kato et al. / Applied Thermal Engineering 19 (1999) 239±254
251
Fig. 9. Applied system that combines the heat pump with a high temperature gas reactor for a turbine generator.
252
Y. Kato et al. / Applied Thermal Engineering 19 (1999) 239±254
the thermal quality of Qc1 is higher than that of Qd2 since Tc1>Td2. The applied system will be available for load leveling of the electricity supply and will contribute to the stable operation of the HTGR. As the heat pump can be operated under thermal driving conditions without mechanical work, the system design will be simple and cheap, and heat pump operation will be safe and highly reliable.
5.2.2. Estimation of heat pump performance To estimate the scale and feasibility of the heat pump combined with the HTGR in Fig. 9(b), the speci®c reaction amount to output and the reaction amount ratio were calculated using the experimental results. 5.2.2.1. Speci®c reaction amount to output. The amount of reactant CaO to obtain 1 MW output is de®ned as the speci®c reaction amount to output. The speci®c amount is inversely proportional to the heat density and is calculated from experimental results in order to estimate the reactor size. Fig. 10 shows the dependency of the amount on output temperature (Tc1) and number of repetitions. For an output of 1 MW for 60 min, that is 1 MWh, 1.25 and 1.76 t/ MW are required for output temperatures of 800 and 8808C, respectively. The seventh cycle of 8808C output has a 3.42 t/MW. Since the speci®c amount is appreciably aected by the repetition cycle, enhancement of the reactivity upon repetition is important for the further development of heat pumps. Although considerations of heat and material transfer resistance are required for scaling up the reactor bed size for practical purposes as well as enhancement of repetitive reactivity, the speci®c amount values are appropriate for practical use.
Fig. 10. Relationship between the speci®c reaction amount to output and output temperature (=Tc1).
Y. Kato et al. / Applied Thermal Engineering 19 (1999) 239±254
253
Fig. 11. Eect of reaction operation time of CaO carbonation on the reaction amount ratio.
5.2.2.2. Reaction amount ratio. The reaction rate and reaction amount are quite dierent between the carbonation of CaO and PbO (Figs. 4 and 7). PbO carbonation will control the whole operation rate because of its low reactivity. Consequently, a larger amount of PbO than CaO is required for practical heat pump operation. The eect of the reactivity dierence on the heat pump scale was estimated. The reaction amount ratio Z [mol%/mol%], de®ned as the ratio of the carbonation amount of PbO (xc2 [mol%]) to that of CaO (Xc1 [mol%]) is calculated by: Z Xc1 =xc2
7
The ratio Z indicates the amount of PbO required to complete a speci®c amount of CaO carbonation. Fig. 11 shows the result of the calculation under CaO carbonation conditions of 8808C and 1.0 atm, and PbO carbonation conditions of 3008C and 0.4 atm. The sixth cycle results of both PbO and CaO carbonation are used as stable reaction conditions in the calculation. Carbonation times of 5±120 min for CaO were used as the variable parameter. The horizontal axis in Fig. 11 shows the carbonation time of PbO, that is, the time required for the decarbonation process of CaCO3. When the carbonation time of CaO is 1 h, the value of Z implies that 19.4 times PbO compared with CaO is required if PbO carbonation is to be completed within the same time frame. Even if 120 min are allowed for the carbonation of PbO, 7.32 times PbO is required. The decrease in the amount of PbO required allows for a more compact system and simple operation, and would thus be eective for the design of a heat pump and the enhancement of operability.
254
Y. Kato et al. / Applied Thermal Engineering 19 (1999) 239±254
6. Conclusions The feasibility of a CaO/PbO/CO2 chemical heat pump was examined using kinetic studies of CaO/CO2 and PbO/CO2 reaction systems. The following results were obtained: 1. Optimal reaction conditions for practical thermal drive operation were proposed. 2. Heat output by carbonation of CaO was con®rmed up to 8808C under sub-atmospheric pressure. Higher output temperature is expected by a rise in the carbonation pressure of CaO. 3. Enhancement of durability of reaction materials were required in repetitive reactions. 4. An applied system that combined the heat pump with a high temperature gas reactor was proposed as a high-temperature heat utilization system. 5. The scale of the heat pump is expected to have practical value. A decrease in reaction amount ratio would be eective for heat pump design and enhancement of operability.
References [1] R. Barker, The reactivity of calcium oxide towards carbon dioxide and its use of energy storage, J. Appl. Chem. Biotechnol. 24 (1974) 221±227. [2] Kyaw Kyaw, H. Matsuda, M. Hasatani, Applicability of carbonation/decarbonation reactions to high-temperature thermal energy storage and temperature upgrading, J. Chem. Eng. Japan 29 (1996a) 119±125. [3] Kyaw Kyaw, M. Kanamori, H. Matsuda, M. Hasatani, Study of carbonation reactions of Ca±Mg oxides for high temperature energy storage and heat transformation, J. Chem. Eng. Japan 29 (1996b) 112±118. [4] Y. Kato, Y. Watanabe, Y. Yoshizawa, Application of inorganic/carbon dioxide reaction system to a chemical heat pump. Proceedings of the 31st Intersociety Energy Conversion. Washington, D.C., USA. 2. 1996, pp. 763± 768. [5] JANAF thermochemical database, NSRDS-NBS-37, 2nd ed., 1971. [6] E.A. Peretti, Thermal decomposition of lead carbonate, J. Am. Ceramic Soc. 40 (1957) 171±173. [7] Y. Kato, D. Saku, N. Harada, Y. Yoshizawa, Utilization of high temperature heat using a calcium oxide/lead oxide/carbon dioxide chemical heat pump, J. Chem. Eng. Japan 30 (1997) 1013±1019. [8] Y. Kato, K. Kobayashi, Y. Yoshizawa, Durability to repetitive reaction of magnesium oxide/water reaction system for a heat pump, Applied Thermal Engineering 18(3-4) (1998) 85±92.
Applied Thermal Engineering 19 (1999) 239±254
Kinetic feasibility of a chemical heat pump for heat utilization of high-temperature processes Yukitaka Kato *, Naozumi Harada, Yoshio Yoshizawa Research Laboratory for Nuclear Reactors, Tokyo Institute of Technology, O-okayama, Meguro-ku, Tokyo 152-8550, Japan Received 20 October 1997
Abstract To utilize heat generated from high-temperature processes, the kinetic feasibility of a calcium oxide/ lead oxide/carbon dioxide chemical heat pump was examined experimentally by kinetic studies of CaO/ CO2 and PbO/CO2 reaction systems, which constitute the heat pump's reaction. In order to determine the optimal reaction conditions that still allow practical operation of the heat pump, both reaction systems were examined with respect to thermal drivability and reaction material durability. The heat pump was able to store heat of about 8608C and transform it to a heat of above 8808C under subatmospheric pressure without mechanical work. An applied system that combined the heat pump with a high-temperature process was proposed for high-eciency heat utilization. The scale of the heat pump in the combined system was estimated from the experimental results. # 1998 Elsevier Science Ltd. All rights reserved. Keywords: Heat pump; Calcium oxide; Lead oxide; Carbonation; Decarbonation
Nomenclature mCaO P Q r T t X
speci®c reaction amount to output, t/MW reaction pressure of CO2, atm heat amount, J temperature rise ratio, 8C/min reaction temperature, 8C reaction time, h ®xed reacted fraction, mol %
* Corresponding author. Fax: 00-81-3-5734-2959.
1359-4311/99/$ - see front matter # 1998 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 9 - 4 3 1 1 ( 9 8 ) 0 0 0 4 9 - 0
240
x DH8 Z
Y. Kato et al. / Applied Thermal Engineering 19 (1999) 239±254
reacted fraction, mol % standard reaction enthalpy, J/mol reaction amount ratio, mol%/mol% = [ÿ]
Subscripts 1 CaO/CO2 reaction system 2 PbO/CO2 reaction system a output from high temperature gas reactor for a common system b output from high temperature gas reactor for an applied system c carbonation d decarbonation r output of a high temperature gas reactor
1. Introduction High-temperature processes such as those involved in high temperature gas reactors (HTGR), iron smelting and some chemical processing, produce a large amount of hightemperature heat above 8008C. This heat may be used as the heat source for various heat utilization systems because of the high thermal quality of such heat. In order to increase the number of ways this heat can be used, heat storage and temperature transformation technology need to be improved. A chemical heat pump is one candidate for such improvements, because it can be applied to a wide temperature range by choosing the appropriate reaction system, can store heat as reactants via its chemical reaction with little heat loss for long periods of time, and under the appropriate reaction conditions, can transform the stored heat into another form over a wide temperature range. However, the use of chemical heat pumps for utilization of heat at such high temperatures has not yet been clari®ed in sucient detail. The characteristics of a particular chemical heat pump are determined by the reaction system employed. A calcium oxide/carbon dioxide reaction system (CaO/CO2) appears to be an appropriate reaction for a high-temperature heat pump above 7008C [1, 2]. The key consideration in the development of a heat pump system that employs this reaction is how to handle and store the carbon dioxide produced as the gas phase reactant. Employing a compressor, which would entail mechanical work, and using zeolites or Ca±Mg oxides as the absorbent of carbon dioxide for gas storage has been proposed [2, 3]. A practical solution for carbon dioxide storage that does not entail signi®cant mechanical work has yet to be established. In our previous study, a lead oxide/carbon dioxide reaction system (PbO/CO2) was demonstrated to be eective for storage of carbon dioxide [4]. Based on this ®nding, a heat pump that combines this reaction with the calcium oxide/carbon dioxide reaction system to form a calcium oxide/lead oxide/carbon dioxide (CaO/PbO/CO2) chemical heat pump was proposed. The heat pump uses a CaO/CO2 reaction system and a PbO/CO2 system. The equilibrium relationship of the heat pump cycle was determined from equilibrium and basic kinetic studies of both reaction systems. The heat pump eectiveness above about 8308C and under sub-atmospheric pressure was derived from the results. Next, the heat pump operability
Y. Kato et al. / Applied Thermal Engineering 19 (1999) 239±254
241
based on comprehensive kinetic studies of both reaction systems needed to be determined for heat pump realization. Thus, in the present study, the kinetics were studied with respect to thermal drivability and reaction material durability. Thermal drivability is preferable for a practical heat pump because the lack of mechanical work associated with the drivability allows simple, safe and low-cost development. This point is particularly important at such hightemperatures because of the diculty of designing a mechanical work system in that temperature range. Both reaction systems are repeated cyclic systems under heat pump operation. Reaction material durability in the repetitive cycle reaction was also examined. Based on the results of the kinetic studies, an applied system that combines a heat pump with a high-temperature process was proposed, and the scale of the applied system was discussed.
2. Principle and background of the heat pump 2.1. Principle of the heat pump The following CaO/CO2 and PbO/CO2 reaction systems were used in the CaO/PbO/CO2 chemical heat pump. CaO s CO2 g CaCO3 s; DH1 ÿ178:321kJ=mol;
1
PbO s CO2 g PbCO3 s; DH2 ÿ88:271kJ=mol:
2
The principle of a heat transformation type heat pump is shown in Fig. 1. A heat that has a higher temperature than the original heat source can be obtained using this operation. The system consists of CaO and PbO reactors. The operation settings of the heat pump include: (a) a heat storage mode and (b) a heat supply mode. Initially, CaCO3 and PbO are charged into each reactor. In the storage mode, the CaO reactor receives heat (Qd1) from a heat source at temperature Td1. Subsequently, CaO and CO2 are formed as a result of the decarbonation of CaCO3. The CO2 is reacted with PbO in the PbO reactor at some pressure (Pc2) and the exothermic heat of the carbonation is recovered at temperature Tc2, yielding PbCO3. In the heat supply mode, the decarbonation of PbCO3 proceeds in the PbO reactor using heat at temperature Td2, which is higher than Tc2. The CO2 formed at pressure Pd2, which is higher than Pc2, is introduced into the CaO reactor. Carbonation of CaO then proceeds and heat (Qc1) is generated exothermically in the reactor at a temperature of Tc1, which is higher than Td1, due to the higher reaction pressure. 2.2. Equilibrium relationship The equilibrium relationship of the heat pump operation is shown in Fig. 2. The solid and broken sloping lines indicate the reaction equilibrium of CaO/CO2 [5] and PbO/CO2. PbO/CO2 equilibrium consists of three equilibriums as follows [6]: Step A :
2PbCO3 Pb PbCO3 CO2 ;
3
242
Y. Kato et al. / Applied Thermal Engineering 19 (1999) 239±254
Fig. 1. Principle of CaO/PbO/CO2 chemical heat pump of heat transformation type operation; (a) heat storage mode, and (b) heat output mode.
Step B :
3 PbO PbCO3 2 2PbO PbCO3 CO2 ;
4
Step C :
2PbO PbCO3 3PbO CO2 :
5
The PbO/CO2 equilibrium line in Fig. 2 shows step C equilibrium. Step C is the equilibrium of heat pump operation, as determined in our previous study [7]. PbCO3 decarbonation and PbO carbonation are repeated reversibly during heat pump operation. Initial PbCO3 as a precursor is decarbonated through these three reaction equilibriums, and changes to PbO. The following carbonation is saturated at step C equilibrium, and the next repetitive reaction occurs under step C.
Y. Kato et al. / Applied Thermal Engineering 19 (1999) 239±254
243
Fig. 2. Heat pump cycle of reaction equilibrium.
2.3. Thermal drivability of the heat pump Preferably, as shown in Fig. 1 and mentioned in Section 1, the heat pump should be thermally driven without any mechanical work. To successfully realize the thermal drive operation, the choice of reaction pressure is important, as it ensures the reactivity of each reaction and the safety of the reactor design. In a previous study, the basic kinetics of each reaction was measured over a wide range of reaction pressures [7]. The four gray areas in Fig. 2 show the optimal reaction condition areas for thermal drive operation derived from that study. The number of each operation in Fig. 2 corresponds to the same numbered operations in Fig. 1. According to these operations, about 0.4 atm of reaction pressure is optimal for the heat storage mode [(1)±(2) in Figs. 1 and 2], and 1.0 atm or more reaction pressure is suitable for the heat output mode [(4)±(3) in Figs. 1 and 2]. However, the reactivity of each reaction is
244
Y. Kato et al. / Applied Thermal Engineering 19 (1999) 239±254
required to be high enough for practical operation of the heat pump and is aected by the reaction temperature. Thus, comprehensive kinetic studies of the reaction areas shown in Fig. 2 were required to determine the reaction conditions which would realize practical heat pump operation.
3. Experimental procedure The kinetics of the CaO/CO2 and PbO/CO2 reaction systems were studied using a thermobalance system under a closed system. Based on our previous study, 0.4 atm of reaction pressure for the heat storage mode [(1)±(2) in Fig. 2] and 1.0 atm for the heat output mode [(3)±(4) in Fig. 2] were appropriate reaction conditions. Reaction reactivity of both the CaO/ CO2 and PbO/CO2 systems under these conditions and under repetitive reaction were measured. Reaction samples were set in a sample cell placed on a balance. During each reaction, sample weight was monitored continuously and the reacted fraction was calculated based on the change in weight. Temperature was measured directly at the bottom of the cell using a thermocouple. This temperature was considered to be the reaction temperature and was maintained using an electric heater. After residual inert gas in the apparatus was removed using a vacuum pump at the beginning of each experiment, reaction pressure was controlled by introducing CO2 gas, and was measured using a manometer. The experiments were carried out at a reaction temperature between 300 and 9008C and under sub-atmospheric reaction pressure. The starting reactants were 45 mg of lead carbonate (PbCO3) and 18 mg of calcium carbonate (CaCO3) in 99.9% grade powder form (Kojundo Chemical Laboratory Co, Ltd). The dierent sample weights employed ensured identical sample volumes. Experimental analysis of the carbonation and decarbonation of PbO and the repetitive reaction was performed under the following process. PbCO3 ÿÿÿÿ4 PbO ÿÿÿÿ4 PbCO3 ÿÿÿÿ4 PbO . . . Carbonation Repetition Precursor Decarbonation Exp: Exp: Exp:
6
The reactivity of the carbonation and decarbonation of CaO was analyzed in a similar manner as the PbO/CO2 system.
4. Experimental results 4.1. PbO/CO2 reaction experiment 4.1.1. Decarbonation of PbCO3 The original PbCO3 sample was used for the PbCO3 decarbonation experiment. As mentioned in Section 2, the initial decarbonation reaction proceeds successively through the three steps of Eqs. (3)±(5) with a rise in temperature and loss of CO2, with 50, 16.7 and 33.3 mol% of reacted fraction amount, respectively. The step C reaction is only used as the heat pump cycle. The temperature dependence on decarbonation under a reaction pressure of
Y. Kato et al. / Applied Thermal Engineering 19 (1999) 239±254
245
Fig. 3. Temperature dependency of decarbonation of PbCO3.
1.0 atm is shown in Fig. 3. The sample was heated at 508C/min to the target decarbonation temperatures and was held at the temperature after 0 min in Fig. 3. Each reaction proceeds via step A and B during initial heating until about 4008C. This is con®rmed by the initial change in reacted fraction being about 67%, which is equal to the sum of 50 and 16.7 mol% for steps A and B, before 0 min in Fig. 3 and by our previous study. The reaction then attains step C. At 4208C, step C does not proceed because the reaction equilibrium temperature is higher than 4208C. The higher the reaction temperature was raised above 4308C, the faster the reaction rate increased. The reactions at 4408C and 4508C were complete at about 60 and 30 min, respectively, which indicates that decarbonation temperatures of 4408C or 4508C are optimal for the carbonation of a practical heat pump operation with respect to reaction rate.
4.1.2. Carbonation of PbO At the beginning of the carbonation experiment, the PbO sample for the carbonation was prepared by decarbonation of the original PbCO3 under ®xed conditions of 3508C and 0.01 atm. These conditions were employed to ensure completion of the decarbonation. The temperature dependence on carbonation reactivity under a reaction pressure of 0.4 atm is shown in Fig. 4. The carbonation proceeds in the reaction range of step C; the reaction is expected to be saturated at 33 mol% of the reacted fraction. However, a fairly long reaction time is required for saturation in Fig. 4 because of the low reactivity caused by the low reaction pressure. The reactivity is maximal around 3008C. Although the reaction rate increases along with reaction temperature rise, the exothermic carbonation approaches the reaction equilibrium of step C, and apparent reactivity decreases near the equilibrium. Therefore, the
246
Y. Kato et al. / Applied Thermal Engineering 19 (1999) 239±254
Fig. 4. Reactivity change in carbonation of PbO in response to change in reaction temperature.
reactivity has a maximal eect on the reaction temperature. This result shows that a reaction temperature of 3008C is optimal for the carbonation. 4.1.3. Repetitive reaction of the PbO/CO2 system The heat pump is operated by repetitive carbonation and decarbonation of PbO. The reactivity under the repetitive reaction was measured under the optimal reaction conditions described above. Fig. 5 shows the result of six repetitions under a decarbonation temperature, pressure and time of 4508C, 1.0 atm and 0.5 h, respectively, and a carbonation temperature, pressure and time of 3008C, 0.4 atm and 4 h, respectively. Although an initial decrease in reactivity was measured during the initial four cycles, reactivity was maintained over the next two cycles. The PbO sample is expected to maintain reactivity in further repetitive cycles after the initial reactivity decrease. 4.2. CaO/CO2 reaction experiment 4.2.1. Decarbonation of CaCO3 The original CaCO3 sample was used for the CaCO3 decarbonation experiment. The temperature dependency of the decarbonation under the target decarbonation pressure of 0.4 atm is shown in Fig. 6. The sample was heated at 508C/min to each objective decarbonation temperature and was held at the temperature after 0 min in Fig. 6. The higher the reaction temperature increased, the faster the reaction rate increased. The result indicates that over 8608C is the optimal temperature for the decarbonation of a practical heat pump operation, because the reaction is completed within 30 min of the reaction time.
Y. Kato et al. / Applied Thermal Engineering 19 (1999) 239±254
Fig. 5. Repetitive reaction of PbO/CO2 reaction system under derived optimal reaction conditions.
Fig. 6. Temperature dependency of decarbonation of CaCO3.
247
248
Y. Kato et al. / Applied Thermal Engineering 19 (1999) 239±254
4.2.2. Carbonation of CaO The temperature dependency of carbonation reactivity under a reaction pressure of 1.0 atm is shown in Fig. 7. For the carbonation, the precursor CaCO3 sample was decarbonated completely under a pressure of 0.01 atm during an increase in temperature before the reaction temperature attained the carbonation temperature. After the attainment of the carbonation temperature, CO2 gas was introduced into the reactor at 0 min, and the reactor maintained at the target pressure of 1.0 atm. The reactivity was observed up to 8808C. When the reaction temperature increases, the reaction rate decreases because the exothermic carbonation approaches the reaction equilibrium. As higher temperature output generated from the carbonation is preferred for the heat pump, a reaction temperature of 8808C is the optimal temperature for carbonation during practical heat pump operation. 4.2.3. Repetitive reaction of the CaO/CO2 system The reactivity under the repetitive reaction was measured under the optimal reaction conditions described above. Fig. 8 shows the result of seven repetitions under a decarbonation temperature, pressure and time of 8608C, 0.4 atm and 0.25 h, respectively, and a carbonation temperature, pressure and time of 8808C, 1.0 atm and 2 h, respectively. A continuous decrease
Fig. 7. Reactivity change in the carbonation of CaO in response to change in reaction temperature.
Y. Kato et al. / Applied Thermal Engineering 19 (1999) 239±254
249
Fig. 8. Repetitive reaction of CaO/CO2 reaction system under derived optimal reaction conditions.
in reaction amount during repetition was observed in contrast with the repetition result of the PbO/CO2 system (Fig. 5). The CaO sample is expected to lose reactivity in further repetitive cycles. A similar reduction in reactivity upon repetition was measured in a magnesium oxide/ water chemical heat pump, however, this reduction has already been overcome using a newly designed reaction material [8]. Thus, development of a highly-durable material for the CaO/ CO2 reaction is a realistic possibility. 5. Discussion 5.1. Optimal reaction conditions for heat pump operation The optimal reaction conditions for heat pump operation derived from the discussion in Section 4 are shown as four black dots in Fig. 2. The results show that the reaction systems can realize a heat transformation type operation, which means that the output temperature (Tc1) is higher than the input temperature (Td1), under thermal driving conditions without mechanical work. With respect to practical operation, the pressure gradient between reactors at (1) and (2), and between (3) and (4) in the ®gure are required to ensure the mobility of CO2 reactant between reactors. Thus, some adjustment of the reaction conditions would be required according to the heat pump design. Lower temperature input [process (1) in Figs. 1 and 2] and higher output [process (3)] are preferred for this type of heat pump. Higher temperature output is realized by higher temperature carbonation of CaO [process (3)] which is induced by the high pressure CO2 generated by the higher temperature decarbonation of PbCO3 [process (4)].
250
Y. Kato et al. / Applied Thermal Engineering 19 (1999) 239±254
Although these high temperature experiments, which were performed above 1.0 atm, were not examined with respect to restrictions of experimental apparatus, higher temperature reactions and output would be possible, because the reaction rates will be accelerated with the rise in the reaction temperature. On the other hand, lower temperature heat input [process (1)] is dicult because of the low reactivity of PbO carbonation [process (2)]. Lower temperature heat input is realized by lower temperature decarbonation of CaCO3, which is induced by CO2 consumption during carbonation of PbO at lower pressure. However, the current PbO material has limited reactivity under low pressure and temperature [7]. Thus, performing process (2) under lower pressure and process (1) at lower temperature heat input are dicult to realize at present. 5.2. Application of the heat pump to high-temperature processes 5.2.1. Principle of the applied system The heat consumed by the CaCO3 decarbonation (Qd1 at Td1) corresponds to the heat output of a heat source plant. From the derived optimal temperature of Td1 of the heat pump, the output of a high temperature gas reactor (HTGR) is an appropriate heat source for Qd1 of the heat pump. The possibility of the application of the heat pump to a HTGR for a heat utilization system is discussed below based on the experimental results. A HTGR output temperature of 8608C, which corresponds to Td1, is assumed in this discussion. Fig. 9(A) shows a common system that utilizes the heat generated from a HTGR, denoted Qa, for the standard load of the power plant. The heat is used as the heat source for a multi-stage gas-turbine and steam turbine of a generator and is transformed into electric power. The gray arrows in the ®gure and following Fig. 9(B) indicate heat ¯ows between these units. The heat ¯ows are realized by the introduction of heat transport components, that is, blowers and heat exchangers. For heat transports between HTGR, the CaO reactor and the gas turbine, helium gas would be one of the candidates for the heat transport media because of its stability and high speci®c heat. For heat transports related to the PbO reactor and the steam turbine, steam would be the ®rst candidate of the media because of its high-reliability in general use. An applied system that combines the heat pump with a HTGR is shown in Fig. 9(B). This system can contribute to load leveling of the power plant. Fig. 9(B) process (1) shows the heat storage mode during low demand for electricity. The heat output generated from a HTGR at a Td1 (=Tr) of 8608C is stored in the CaO reactor as reactant by the decarbonation of CaCO3 [process (1)], and the exothermic heat of PbO carbonation at a Tc2 of about 3008C is used as the heat source of the steam-turbine. Fig. 9(B) process (2) shows the heat output mode during high demand. The decarbonation of PbCO3 proceeds using a low temperature heat source (Qd2 at a Td2 of about 4508C) which is introduced from down stream of the gas-turbine ¯ow, and then the formed CO2 reacts with CaO in the CaO reactor. The exothermic heat generated by the carbonation (Qc1) at a Tc1 of above 8808C is injected into the top stage of the turbine and is transformed into electricity with the Qb2 of a HTGR standard output at a peak load. The dierence in reaction rate between the processes would be overcome by changing the reactant amount in both reactors as mentioned in the following Section 2(b). The extraction of Qd2 for the decarbonation of PbCO3 is eective for thermal ampli®cation at peak demand, because the amount of Qc1 is larger than that of Qd2 since DH81>DH82, and
Y. Kato et al. / Applied Thermal Engineering 19 (1999) 239±254
251
Fig. 9. Applied system that combines the heat pump with a high temperature gas reactor for a turbine generator.
252
Y. Kato et al. / Applied Thermal Engineering 19 (1999) 239±254
the thermal quality of Qc1 is higher than that of Qd2 since Tc1>Td2. The applied system will be available for load leveling of the electricity supply and will contribute to the stable operation of the HTGR. As the heat pump can be operated under thermal driving conditions without mechanical work, the system design will be simple and cheap, and heat pump operation will be safe and highly reliable.
5.2.2. Estimation of heat pump performance To estimate the scale and feasibility of the heat pump combined with the HTGR in Fig. 9(b), the speci®c reaction amount to output and the reaction amount ratio were calculated using the experimental results. 5.2.2.1. Speci®c reaction amount to output. The amount of reactant CaO to obtain 1 MW output is de®ned as the speci®c reaction amount to output. The speci®c amount is inversely proportional to the heat density and is calculated from experimental results in order to estimate the reactor size. Fig. 10 shows the dependency of the amount on output temperature (Tc1) and number of repetitions. For an output of 1 MW for 60 min, that is 1 MWh, 1.25 and 1.76 t/ MW are required for output temperatures of 800 and 8808C, respectively. The seventh cycle of 8808C output has a 3.42 t/MW. Since the speci®c amount is appreciably aected by the repetition cycle, enhancement of the reactivity upon repetition is important for the further development of heat pumps. Although considerations of heat and material transfer resistance are required for scaling up the reactor bed size for practical purposes as well as enhancement of repetitive reactivity, the speci®c amount values are appropriate for practical use.
Fig. 10. Relationship between the speci®c reaction amount to output and output temperature (=Tc1).
Y. Kato et al. / Applied Thermal Engineering 19 (1999) 239±254
253
Fig. 11. Eect of reaction operation time of CaO carbonation on the reaction amount ratio.
5.2.2.2. Reaction amount ratio. The reaction rate and reaction amount are quite dierent between the carbonation of CaO and PbO (Figs. 4 and 7). PbO carbonation will control the whole operation rate because of its low reactivity. Consequently, a larger amount of PbO than CaO is required for practical heat pump operation. The eect of the reactivity dierence on the heat pump scale was estimated. The reaction amount ratio Z [mol%/mol%], de®ned as the ratio of the carbonation amount of PbO (xc2 [mol%]) to that of CaO (Xc1 [mol%]) is calculated by: Z Xc1 =xc2
7
The ratio Z indicates the amount of PbO required to complete a speci®c amount of CaO carbonation. Fig. 11 shows the result of the calculation under CaO carbonation conditions of 8808C and 1.0 atm, and PbO carbonation conditions of 3008C and 0.4 atm. The sixth cycle results of both PbO and CaO carbonation are used as stable reaction conditions in the calculation. Carbonation times of 5±120 min for CaO were used as the variable parameter. The horizontal axis in Fig. 11 shows the carbonation time of PbO, that is, the time required for the decarbonation process of CaCO3. When the carbonation time of CaO is 1 h, the value of Z implies that 19.4 times PbO compared with CaO is required if PbO carbonation is to be completed within the same time frame. Even if 120 min are allowed for the carbonation of PbO, 7.32 times PbO is required. The decrease in the amount of PbO required allows for a more compact system and simple operation, and would thus be eective for the design of a heat pump and the enhancement of operability.
254
Y. Kato et al. / Applied Thermal Engineering 19 (1999) 239±254
6. Conclusions The feasibility of a CaO/PbO/CO2 chemical heat pump was examined using kinetic studies of CaO/CO2 and PbO/CO2 reaction systems. The following results were obtained: 1. Optimal reaction conditions for practical thermal drive operation were proposed. 2. Heat output by carbonation of CaO was con®rmed up to 8808C under sub-atmospheric pressure. Higher output temperature is expected by a rise in the carbonation pressure of CaO. 3. Enhancement of durability of reaction materials were required in repetitive reactions. 4. An applied system that combined the heat pump with a high temperature gas reactor was proposed as a high-temperature heat utilization system. 5. The scale of the heat pump is expected to have practical value. A decrease in reaction amount ratio would be eective for heat pump design and enhancement of operability.
References [1] R. Barker, The reactivity of calcium oxide towards carbon dioxide and its use of energy storage, J. Appl. Chem. Biotechnol. 24 (1974) 221±227. [2] Kyaw Kyaw, H. Matsuda, M. Hasatani, Applicability of carbonation/decarbonation reactions to high-temperature thermal energy storage and temperature upgrading, J. Chem. Eng. Japan 29 (1996a) 119±125. [3] Kyaw Kyaw, M. Kanamori, H. Matsuda, M. Hasatani, Study of carbonation reactions of Ca±Mg oxides for high temperature energy storage and heat transformation, J. Chem. Eng. Japan 29 (1996b) 112±118. [4] Y. Kato, Y. Watanabe, Y. Yoshizawa, Application of inorganic/carbon dioxide reaction system to a chemical heat pump. Proceedings of the 31st Intersociety Energy Conversion. Washington, D.C., USA. 2. 1996, pp. 763± 768. [5] JANAF thermochemical database, NSRDS-NBS-37, 2nd ed., 1971. [6] E.A. Peretti, Thermal decomposition of lead carbonate, J. Am. Ceramic Soc. 40 (1957) 171±173. [7] Y. Kato, D. Saku, N. Harada, Y. Yoshizawa, Utilization of high temperature heat using a calcium oxide/lead oxide/carbon dioxide chemical heat pump, J. Chem. Eng. Japan 30 (1997) 1013±1019. [8] Y. Kato, K. Kobayashi, Y. Yoshizawa, Durability to repetitive reaction of magnesium oxide/water reaction system for a heat pump, Applied Thermal Engineering 18(3-4) (1998) 85±92.