Research Of Modification Of Li₄SiO₄-based CO₂ Adsorbent Through Molten Doping And Its Modification Mechanisms

Since mankind entered the industrialized society,a large amount of emission of greenhouse gases,mainly containing CO₂,has seriously affected the human living environment,causing a series of problems.Post-combustion capture based on Li₄Si O₄-based adsorbent has attracted much attention due to its excellent cycle stability for high-temperature CO₂ capture.Eutectic doping is the modification method that widely used and most effective to improve the performance of Li₄Si O₄-based adsorbent.However,the current researches on eutectic doping in China and abroad mainly focus on the determination and mechanism study of modification performance of single molten salt.In addition,the preparation methods and test conditions are different,and the number of cycles is very different.It is impossible to directly compare the modification performance of different molten salt doped adsorbents.Therefore,this work will directly compare the modification performance of common molten salts to solve the screening problem of molten salt that has not been studied.First of all,in this work,a total of twelve molten salts such as K₂CO₃,KCl,KNO₃,K₂SO₄,KF,KBr,Na₂CO₃,Na Cl,NaNO₃,Na₂SO₄,NaF and NaBr were doped into Li₄Si O₄-based adsorbent.Besides,the phase composition,micromorphology,and porosity of each group of samples were tested.KF,KCl,KBr,K₂SO₄ and Na₂SO₄ were doped for the first time.Moreover,the temperature-programmed,isothermal adsorption and cyclic adsorption and desorption tests were carried out.Their adsorption performance,adsorption rate,cycle stability and other important adsorption factors were also compared.The temperature-programmed experimental results showed that the modification performance could be ranked as follows:fluoride salt>carbonate>chloride salt>sulfate salt>bromide salt>nitrate salt.Isothermal adsorption experimental results showed that alkali fluoride,carbonate,and nitrate could significantly improve the adsorption capacity and adsorption rate.Na F displayed the best adsorption capacity modification performance and Na NO₃exhibited the best adsorption rate modification performance.The results of 10 cycle of adsorption-desorption tests indicated that in addition to KCl,Na Cl,KBr,Na Br and KF,the overall cycle stability was well,and potassium salt doped adsorbent was better than that of the sodium salt.Secondly,the thermal analysis of the molten salt was carried out,and it was found that the unstable thermochemical properties of Na NO₃ and KNO₃ are not suitable as molten salts.In addition,according to the foregoing cycle test results,KCl,Na Cl,KBr,Na Br and KF should also be excluded for further research.It was found that KCl,Na Cl,KBr and Na Br were doped and their co-vaporization temperatures with Li₂CO₃ were too low,lower than the desorption temperature（700℃）,which led to the vaporization of the eutectic melt and the cyclic performance gradually declined.Further,the available molten salts were quantitatively evaluated from two aspects of performance and economy.From the perspective of improving performance,Na F had the best unit improvement performance,up to 0.141 g CO₂/g adsorbent.From an economic perspective,Na F also owned the best unit cost modification effect,with a minimum of 0.44 yuan/g CO₂/g Li₄Si O₄.Furthermore,the difference in the modification performance of each doped salt was demonstrated from the modification mechanism via TG-DSC technology and software calculation.The modification performance had a close negatively relationship with the co-melting temperature of the doped salt and Li₂CO₃ mixture.In general,whether from the perspective of improving performance or an economic perspective,Na F is the best optional molten salt in eutectic doping modification.