Study On The Properties And Kinetics Of Sorption-Regeneration Of Nano Ca-based CO₂ Sorbents

CaO is a promising sorbent for the capture of CO₂,widely used in many industrial processes such as hydrogen production from CO₂ reactive sorption enhanced methane reforming(RSER),CO₂ separation from flue gas or syngas,chemical heat pump and energy storage system.In this work,nano Ca-based CO₂ sorbents were prepared with nano CaCO₃ precursor and additives.The properties and kinetics of sorption-regeneration was studied of nano Ca-based CO₂ sorbents.The sorption properties of nano CaO/Al₂O₃ sorbents were tested by a thermo gravimetric analysis(TGA)under the atmospheres of 50%CO₂ at 700℃.The results showed that the sorption ratio,sorption capacity and sorption rate of rapid reaction section of nano CaO/Al₂O₃ sorbents increased by 29.72%,3.98mol/kg and 25% respectively comparing to micro CaO/Al₂O₃ sorbent.For nano CaO/Al₂O₃ sorbent,the sorption ratio only dropped 6%after 40 cycles and its cycle sorption stability greatly improved.Compared with the published data of CO₂ sorbents,nano CaO/Al₂O₃ sorbents possess better cycle sorption ratio,cycle sorption capacity and cycle sorption stability.The sorption kinetics of the nano CaO/Al₂O₃ sorbent was investigated by TGA under the temperatures of 500～650℃and atmospheres of 0.015-0.025MP CO₂ partial pressure The sorption time point corresponding to two times of max sorption rate has been proposed as the criterion to divide the sorption curve into rapid reaction section and slow reaction section.Boltzmann and Avrami-Erofeeve equations were selected to simulate kinetics of both rapid reaction section and slow reaction section.The activation energies are 27.52 kJ/mol,70.25kJ/mol respectively for the rapid reaction section and slow reaction section,with the mean average relative deviation of 10.29%,4.17%respectively.The decomposition kinetics of the nano CaO/Al₂O₃ sorbent was investigated by TGA under the temperatures of 650--800"C and atmospheres of N2,20%CO2, 40%CO2 partial pressure respectively.The decomposition kinetic equations under the temperatures of 650⁸00"C and atmospheres of N2 and CO2 were correlated by shrinking core model with ignore the effects of mass and heat transfer.The activation energies are 141.9kJ/mol,34.7 kJ/mol and ll3.2kJ/mol in the atmospheres of N2, 20%CO2,40%CO2,with the average relative deviation less than 5.66%,7.82%, 5.01%respectively. SiO2 coating nano CaCO3 sorbents(nano SiO2/CaCO3)containing 1%-5%Si were prepared by CO2 as precipitant and NaSiO3 as silica source.TEM results showed that nano CaCO3 was coated by SiO2.TGA was used to investigate the decomposition temperature and sorption properties under the condition of 600"C,20%CO2 of sorbents. Compared to SiO2 uncoated nano CaCO3,the decomposition temperature of SiO2 coated nano CaCO3 decreased by 9--42^C o With the decreasing of Si content,the cycle sorption ratio,cycle sorption capacity and sorption rate of SiO2 coated nano CaCO3 sorbents increased.For the nano SiO2/CaCO3 sorbent containing 1.05%Si,the sorption rate of rapid reaction section,the 1 st arid 5th cycle sorption capacity improved by 10%, 11%and 50%comparing to nano CaC03 uncoated,respectively,and its capacity were 8.9,6.0mol/kg for the related cyclic runs.Si02 coated nano CaC03 possessed better durability sorption than SiO₂ uncoated nano CaCO₃.