Heat recovery from a thermal energy storage bad on
the Ca(OH)2/CaO cycle
M.N.Azpiazu a ,J.M.Morquillas
b,*,A.Vazquez c a Departamento de Ingenier ıa Qu ımica y del Medio Ambiente,E.T.S.Ingenieros (UPV/EHU),Alda.Urquijo s/n,
48013Bilbao,Spain
b Departamento de M a
quinas y Motores T e rmicos,E.T.S.Ingenieros (UPV/EHU),Alda.Urquijo s/n,48013Bilbao,Spain
c Departamento de Energ ıa y Propulsi o n Mar ıtima.E.S.da Mari ~n
a Civil,Campus de Riazor,La Coru ~n a,Spain Received 8July 2002;accepted 10January 2003
Abstract
Thermal energy storage is very important in many applications related to the u of waste heat from industrial process,renewable energies or from other sources.Thermochemical storage is very interesting for long-term storage as it can be carried out at room temperature with no energy loss.
Dehydration/hydration cycle of Ca(OH)2/CaO has been applied for thermal energy storage in two types of reactors.One of them was a prototype designed by the authors,and in the other type conventional laboratory glassware was ud.Parameters such as specific heats,reaction rate and enthalpy,mass loss and heat relea were monitored during cycles.Although in the hydration step water is normally added in vapour pha,liquid water,at 0°C has been ud in the experiences.
Results indicated that the energy storage system performance showed no significant differences,when we compared veral hydration/dehydration cycles.The lected chemical reaction did not exhibit a complete reversibility becau complete Ca(OH)2dehydration,was not achieved.However the system could be ud satisfactorily along 20cycles at least.Heat recovery experiments showed general system behaviour during the hydration step in both types of reactors.The designed prototype was more efficient in this step.
Main conclusions suggested carrying out one complete cycle at a higher dehydration temperature to
recover total system reversibility.A modification of the prototype design trying to enhance heat transfer from the Ca(OH)2bed could also be propod.
Ó2003Elvier Science Ltd.All rights rerved.
Keywords:Thermal energy storage;Chemical heat pump;Hydration/dehydration cycle of lime;Combustion engine preheating
*Corresponding author.Tel.:+34-946-014075.宝宝多大会说话
E-mail address:iapazalm@bi.ehu.es (J.M.Morquillas).
1359-4311/03/$-e front matter Ó2003Elvier Science Ltd.All rights rerved.
doi:10.1016/S1359-4311(03)00015-2
Applied Thermal Engineering 23(2003)733–741
/locate/apthermeng
大青衣
734M.N.Azpiazu et al./Applied Thermal Engineering23(2003)733–741
1.Introduction
Rearch and development in high efficiency heat storage systems is very important for the effective utilisation of thermal energy.Besides,increasing energy consumption is leading to higher atmospheric pollution and making it necessary tofind new ways to u successfully all kinds of energy,including natural sources(solar and geothermal)and industrial waste heat.
Thermal energy produced nowadays by combustion of fossil fuels is energy chemically stored in the past with a high storage density.Therefore thermal energy could be converted to chemical energy and thus stored for a long period.For this reason the u of a reversible chemical reaction with absorption of thermal energy endothermically,and its reversible relea of stored heat exothermically has been suggested.The repetition of endo/exothermic reactions in a clod cycle is also attractive becau we could employ the chemicals indefinitely,at least theoretically. Thermochemical storage systems are more complex than others bad,for example,on latent heat becau all the individual system components should be adequately asmbled due to pos-sible interactions between them.In
the cas the reversibility of the lected reaction could be disturbed by reactions involving chemical impurities[1].
Many reactions have been studied for this purpo such as the decomposition of magnesium nickel hydride[2],the depolymerization of paraldehyde[3],the dehydrogenation of2-propanol[4] or carbon dioxide systems[5].Fig.1shows the cycle for a thermal storage using the reaction Ca(OH)2fiCaO+H2O that has also been reported[6,7].
An input energy of148.6kJ(¼54.0+94.6)can decompo1mol of Ca(OH)2at25°C to CaO+H2O at510°C.The produced CaO can be easily stored as a solid at25°C loosing23.5kJ. If water is allowed to escape to the atmosphere,instead of storing it at510°C in vapour pha, 61.5kJ are additionally lost.Therefore,57.2%(85.0kJ)is the portion of input energy that results in nsible heat of the products,and the remaining42.8%(63.6kJ)is actually stored in the form of chemical energy.有耐心的英语
When the stored high temperature steam at510°C is ud in the hydration step125.1kJ could be recovered(84.2%of the initial input of energy),but the recovery of stored heat using liquid water at25°C can only provide the already mentioned63.6kJ.
This reaction cycle has been applied for veral purpos such as lf-heating food containers for h
ot drinks like coffee,tea or milk[8],or sulphur emissions control in coal combustion[9].The
M.N.Azpiazu et al./Applied Thermal Engineering23(2003)733–741735 authors wish to u it to preheat the combustion engine in motor vehicles[10],high pollutants emissions and other undesirable effects of cold starting at very low temperatures could thus be avoided,and the thermal energy of exhaust gas could be partially ud.
For this purpo we ud,as input energy,heat from the engine exhaust gas and water at0°C in the hydration step of CaO cooled down to0°C.In this ca,the exothermic reaction heat(63.6 kJ in Fig.1),is partially ud to heat CaO and water from0up to25°C,and60.6kJ are left for this particular application.
The previously described dehydration/hydration cycle of the Ca(OH)2/CaO system has been considered to analyze its heat relea characteristics in a very simple static reactor and in a prototype,designed to be ud as a thermal storage device for the above mentioned application. System performance during20cycles was followed studying its reversibility,chemicals degrada-tion,side reactions due to impurities,heat relea rate and mass loss.This limit of20cycles has been stabilid becau carbonation of CaO and Ca(OH)2becomes a trouble in system perfor-mance after20cycles of hydration–dehydration[11].
2.Experimental methods
The lected product for the experiments was a commercial Ca(OH)2,which has the chemical composition and characteristics shown on Table1.It provided a very reactive CaO after dehy-dration.
For the experimental static reactor conventional laboratory beakers of25,50,100and250ml were ud.Samples of5and8g of CaO were placed inside the25and50ml beakers respectively. After cooling them down to0°C,water at0°C was poured over them with a lected proportion CaO:H2O of1:1.5(7.5ml for5g samples and12ml for8g samples)[11].
The beakers were put into100ml beakersfilled with an aqueous antifreeze solution at0°C, ud to r
ecover stored heat in the hydration step.Volumes of this heat recovery solution were 15ml for5g samples and24ml for8g samples.The t was covered and surrounded by empty 250ml beakers to avoid heat loss due to air streams around it.
The calcium hydroxide obtained in the preceding step was dehydrated in a furnace at550°C during45min,repeating the two steps up to20cycles.
Table1
Chemical composition and characteristics of the commercial Ca(OH)2
Component%
SiO20.52
Al2O30.14
Fe2O30.08雷冬竹
P2O50.03
MnO0.03
MgO0.30
CaSO40.10
CaCO3 6.40
Ca(OH)292.40
Characteristics:Density:0.35g/cm3.Particle size distribution:<200l m:95%;<90l m:89%.Specific heat:1.66kJ/kg K.
During the hydration step,both temperatures of the reacting solid and of the surrounding heat recovery solution(T1and T2)were monitored with thermocouples.Samples from each cycle were kept for further thermal analysis in a METTLER TA4000equipment with a differential scanning calorimetry(DSC)cell and a TC11processor.To study the acquired data we ud a computer with the graph ware TA72,also provided by METTLER.Specific heat changes,enthalpies and kinetic parameters were obtained in this way.
Additionally,trying to recover more efficiently the stored thermal energy,the hydration step with5g samples was carried out using three100ml beakersfilled each one with15ml of the heat recovery solution,instead of only one.The beaker with the reacting CaO was successively in-troduced in them.After the experiment all the solutions were mixed and itsfinal temperature was measured.The procedure was repeated up to5cycles.
Finally a heat exchanger prototype designed by the authors,following some ideas taken from literature[12],was ud(Fig.2).
It consisted of a rectangular central body of192·118·85mm withfins of192·20·1.5mm to enhance heat transfer.It was placed inside a rectangular container of200·126·110mm with a lid.All was made of Cu,and covered with a60mm layer of polyurethane.
唐宋八大家顺口溜
For the experiments200g of CaO and360ml of the heat recovery solution were ud.The hydration water was added with the same indicated proportion of1:1.5(300ml).In all cas every system component was cooled down to0°C prior to starting the next trials.In some experiments the heat recovery solution was renewed90and240s after the addition of the hydration water,to e if heat recovery could reach higher efficiencies.Temperatures of the reacting solid(T3)and of the aqueous antifreeze solution as it warms up(T4)were also monitored with copper/constantan thermocouples.
3.Results and discussion
Fig.3shows changes of heatflux received by the sample in W/g versus temperature,when it is
heated with a10°C/min rate in the thermal analysis equipment.Data for analytical grade and for
the commercial Ca(OH)2,are plotted together.Dehydration temperatures (516and 519°C)as well as reaction enthalpies (1060and 1040J/g)were similar in both cas.Specific heats had medium values of 1.66kJ/kg K for Ca(OH)2and 1.80kJ/kg K for CaO.
Kinetic evaluations for this dehydration reaction were performed on DSC peaks with the thermal analysis equipment software.Parameters were obtained according to the simple n th order model eq
思维能力测试uation combined with the Arrhenius approach of the temperature dependency of the reaction rate constant (e Appendix A).Data derived from the calculations are prented in Table 2.
In both cas reaction order (n )is very clo to zero as it is reported by other authors [13]for heterogeneous reactions with solids.It ems that commercial Ca(OH)2has a slightly higher dehydration rate that does not em to disturb its performance as a heat storage system.Metals additions or pressure lowering will be studied in the future in order to reduce dehydration tem-perature.
Calcium hydroxide dehydration weight loss were of 18%for 5g samples and 20%in 8g samples,evaluated with data after at least four cycles.Stoichiometric loss is a 24%indicating that reaction is not complete with the adopted working method.Besides,heat recovery solution temperature was lower after 15cycles of operation as was reported in [11].养成良好习惯
Some samples after 20cycles were dehydrated at 1000°C and higher weight loss were ob-tained.Reaction with CO 2,either dissolved in hydration water or prent in air,could lead to CaCO 3formation and thus higher dehydration temperature is needed.This possibility has
also Table 2
Kinetic parameters for Ca(OH)2dehydration
Analytical grade Ca(OH)2
Commercial Ca(OH)2ln k 0
14.822.7E a (kJ mol À1)
133181n 0.20.3
M.N.Azpiazu et al./Applied Thermal Engineering 23(2003)733–741737
738M.N.Azpiazu et al./Applied Thermal Engineering23(2003)733–741
been mentioned by other authors[14],and made us consider that the number of effective cycles with this working method is about20.Additionally repetitive hydration of magnesium oxide has been studied by[15].
Hydration experiments suggested general system behaviour during this step that can be en in Fig.4.Results for two samples in different cycles,carried out with the same experimental con-ditions,are prented in it.Heat recovery solution temperature(T2)incread gradually with time, reaching a basically constant value4min after hydration water addition.There are no significant differences between samples and cycles reaching always an approximatefinal temperature of35°C,which remained basically constant for another couple of minutes(not shown graphically). This solution does not cool down becau it is still receiving heat delivered by the reacting CaO bed,with a lower heat transfer rate becau temperature gradient is also lower.苏式肉月饼
This fact was considered as a possibility to obtain a more efficient recuperation of heat changing periodically the heat recovery solution by a fresh one.Experiments using three100ml beakers to recover stored heat made it possible to heat45ml of solution(3·15ml in each beaker), from0°C up to a medium temperature of13.5°C instead of15ml from0up to35°C as in the preceding experiments.
A calculation,bad on thefirst principle of thermodynamics,shows that the40%of the5.43kJ relead by the reacting CaO(5g CaO and1.09kJ/g CaO)could be recovered by the heat recovery solution(15ml heated up to35°C),increasing this percentage with an additional7%when the heat recovery solution was renewed.Heat loss were associated to heat absorption by beakers (10%),nsible heat of the remaining Ca(OH)2bed(5%)and,most of all,to partial evaporation of the excess of the hydration water added.
Prototype performance was previously studied to establish the adequate position for thermo-couples and guarantee its thermal insulation.This last point is very important to avoid heat loss and,principally,to be sure that the heat recovery solution,initially at0°C,is heated by the stored heat and not by surrounding air at a medium ambient temperature of20°C.
