Study On Thermochemical Storage Performance And Kinetic Analysis Of Ca（OH）₂ With Expanded Graphite Doping

A series of environmental problems caused by the continuous consumption of fossil energy have caused an impact on human society.In this context,it is extremely necessary to develop renewable energy and corresponding energy storage systems.Harvesting solar energy with thermal storage systems can overcome its intermittency and instability.Thermal energy storage systems include sensible energy storage,latent energy storage and thermochemical energy storage.Thermochemical energy storage is highly suitable for solar energy storage,industrial waste heat recovery and other fields,because it has the advantages of high energy storage density,long-term storage and long distance transportation.Compared with other thermochemical energy storage systems,the Ca（OH）₂-Ca O energy storage system stands out with the advantages of high energy storage density,convenient operation,safety and non-toxicity,which makes it become a promising thermochemical energy storage system.Ca（OH）₂/EG was prepared by doping expanded graphite in Ca（OH）₂,Then the difference of heat storage and release performance and cycle stability of the doping materials react with atomized water and water vapor relatively were investigated.Results of reaction are shown as follow.During the reaction between Ca（OH）₂/EG and water vapor,the bed center temperature reaches the maximum 450～490℃at 9～13 min,and the heat storage and release capacity is1200～1250 k J/kg,which is 6～10 times than heat release by the reaction between Ca（OH）₂/EG and atomized water,and the material is evenly scattered.The reaction between Ca（OH）₂/EG and atomized water get a lower bed temperature and lower heat storage and release capacity.This difference from the fact that the atomized water absorb energy to vaporize,which can reduce heat release capacity.Furthermore,Ca（OH）₂/EGI was prepared by doping expanded graphite into industrial Ca（OH）₂.During the hydration,maximum bed center temperature is about 450～470℃.During circulation,the heat storage and release capacity of Ca（OH）₂/EGI is about 1288.65～1332.51k J/kg,can reach more than 92%of the maximum capacity.As the reaction proceeds,the bed center temperature and heat storage and release capacity decrease.The particle size of reactants will increase due to the bond of water vapor and reactants,and the CO₂ in the atmosphere can not be ignored.Utilizing N₂ or inert gas in reactor can reduce CO₂,which can maintain heat storage and release capacity and cycle stability of Ca（OH）₂/EGI.Thermal kinetic analysis was utilized in this paper to investigate the thermodynamic parameter of Ca（OH）₂/EG which doping ratio of expanded graphite isω=0.2 in Nitrogen atmosphere.Ca（OH）₂/EG decompose at 356～366℃,and optimum heating rate was 10 K/min.The average results of E_a by iso-conversional method is 203 k J/mol.The E_a obtained by Kissinger method is 157.88 k J/mol,and the pre-exponential factor ln A is 19.70.Under the experimental conditions,the thermal decomposition mechanism function of Ca（OH）₂/EG inω=0.2 is NO.29 function.The corresponding integral mechanism function is G（α）=1-（1-α）^1/3,the differential mechanism function is f（α）=3（1-α）^2/3,the decomposition kinetic model of compound material conforms to shrinking spherical model with the phase boundary reaction.A kinetic compensation effect exists between activation energy E_a and ln A at different heating rates.The results can be a reference for design and optimization of thermochemical energy storage reactor.