Preliminary study on energy release characteristics of eggshell derived organic CaO combined materials

CaCO₃/CaO is rich in resources, low in price, and has large energy storage/release capacity. It is a potential thermochemical energy storage material, but its energy storage capacity decays rapidly after multiple cycles. Based on the separation and purification treatment of egg shell waste, three organic acids （including acetic acid, citric acid and gluconic acid） and purified egg shells were used to prepare the corresponding calcium acetate （DCA）, calcium citrate （DCCi） and calcium gluconate （DCG）. A detailed study was conducted on its energy storage and heat release efficiency as well as the evolution of its structural and morphological characteristics during multiple cycles. It was found that DCG-CaO has the highest energy storage capacity and heat release efficiency, but its cycle stability is poor. In order to improve the sintering resistance and cyclic exothermic performance of DCG-CaO, the sol-gel combustion synthesis method （SGCS） was used to dope Al₂O₃ with different mass ratios. Research on its cyclic energy release showed that the optimized DCG-CaO/Al₂O₃ mass ratio was 9∶1, the cyclic energy storage capacity and heat release efficiency are significantly improved compared to DCG-CaO without Al₂O₃ load, but as the number of cycles increases, they decrease slightly and the interaction between Al₂O₃ and CaO will also lose part of the energy storage capacity of CaO. Therefore, based on the mass ratio of DCG-CaO/ Al₂O₃ as 9∶1 and further introduction of MgO with higher heat capacity, the DCG-CaO/MgO/Al₂O₃ composite was prepared by the wet mixing method of DCG and the three-step SGCS method with the corresponding nitrates. It was found that the energy storage capacity and heat release efficiency of the CaO composite absorbent prepared by the three-step method were stable at 2.59 kJ/g and 90.55% respectively. It not only has more outstanding energy storage capacity and cycle stability but also avoids the interaction between CaO and Al₂O_3. Therefore, it has great application potential and development prospect.