Properties And Kinetics Of High-temperature CO₂ Adsorption Of Doped Lithium Orthorsilicate Sorbents

High-temperature CO₂ adsorption technology is a very promissing route for CO₂capture.Among various high-temperature CO₂ adsorbents,Li₄SiO₄ is one of the most promissing adsorbents because its large theoretical adsorption capacity,fast adsorption rate,and low cost.On the other side,it also has some drawbacks such as low surface area and easily sintering.In order to solve these issuse,the effect of Li sources on CO₂adsorption property was analyzed and the physico-chemical properties of Li₄SiO₄ were controlled by doping.A series of modified Li₄SiO₄ adsorbents were synthesized and the relationship between doping and physico-chemical properties and CO₂ adsorption were investigated.We hoped that these studies can provide theoretical basis and technical supports for the development of high temperature CO₂ adsorption technology over Li₄SiO₄.Three Li₄SiO₄ adsorbents derived from LiNO₃,LiOH,and Li₂CO₃ were synthesized,respectively.The effect of Li precursors on the physico-chemical and CO₂ adsorption properties of Li₄SiO₄ were analyzed.The results show that the temperature of synthesis can be lower by using LiNO₃ and LiOH as Li precursors.And meanwhile,large amount of gas would be released during synthesis process,which could relief the sintering the synthesis.The reaction condition greatly affected the high-temperature CO₂ adsorption over Li₄SiO₄ and relatively high reaction temperature and CO₂ partial pressure promoted CO₂ adsorption.Among three Li₄SiO₄ adsorbents derived from different Li precursors,LN700 exhibited the largest CO₂ adsorption capacity and best cycle performance.This is because LN700 had the largest initial specific surface area and many pores were formed between the partices during the cycle reaction.As a result,the surface area and pore volume of LN700 enhanced after cycle experiments,which is helpful for the diffusion and asdosrtpion of CO₂ in/on the Li₄SiO₄ particles.However,LH800 adsorbent was sintered after the cycle experiments and its surface area remarkably reduced.Therefore,CO₂adsorption capacity decreased during cycle experiment.The change of the morphology and surface area of LC800 after cycle reaction is not obvious,thus its CO₂ capacity only decreased slightly.K doped Li₄SiO₄ adsorbents with different K concentrations were synthesized through solid-state method and their high-temperature CO₂ adsorption property was studied.The results shows that K doping remarkably enhanced CO₂ adsorption.When 3%K was doped,the CO₂ adsorption capacity of the adsorbent could reach 34.3%after 20min in CO₂ atmosphere at 700?,which was the highest one among all the samples.And after 15 CO₂ adsorption/desorption cycles,the CO₂ adsorption of 3%K/Li₄SiO₄ only slightly decreased,and still remained 65%of the theoretical values.However,when much more amount of K was doped,the CO₂ adsorption would decrease.This is because appropriate amount of K doping can promote the formation of K₂CO₃/Li₂CO₃ eutectoid,which could lower the melting temperature of Li₂CO₃ and accelerate the diffusion of CO₂,enhancing CO₂ adsorption over Li₄SiO₄.However,when much more amount of K was doped,the concentration of active component?Li₄SiO₄?in the K doped Li₄SiO₄ would greatly decrease,leading to the decrease of CO₂ adsorption capacity.Appropriate size of Li₄SiO₄ partices is also important to the CO₂ adsorption capacity of Li₄SiO₄ adsorbents.Ca-doped Li₄SiO₄?LiCa?adsorbents were synthesized using a solid-state reaction method and their CO₂ sorption capacities were investigated.The results indicate that Ca doping decreased the particle size of the Li₄SiO₄ particles and increased the surface area of the LiCa sorbents.LiCa32?with Ca/Si molar ratio 0.32?had the largest surface area(i.e.,0.314 m² g^-1).Ca doping can enhance the chemisorption of the Li₄SiO₄ sorbents.Isothermal analyses showed that LiCa6 sorbent had the highest CO₂ sorption of 35.1 wt%at 700?for 1 h.LiCa6?with Ca/Si molar ratio 0.06?sorbent also exhibited excellent cycle performance for CO₂ sorption/desorption.After 15 cycles,the CO₂ chemisorption remained approximately 26 wt%.The Ca species covered on the surface of Li₄SiO₄inhibited the agglomeration of the Li₄SiO₄ particles and decreased the particle size of Li₄SiO₄,which enabled the sufficient contact between CO₂ and Li₄SiO₄,and enhanced the CO₂ sorption.The transformation of the Ca species from Ca₂SiO₄ to Li₂CaSiO₄,which occurred during the CO₂ absorption process,is beneficial for the transfer of CO₂ to Li₄SiO₄,and the transformation of Li₂Ca₂SiO₄ to CaSiO₄ favors CO₂ desorption.Therefore,Ca-doped Li₄SiO₄ exhibited excellent CO₂ chemisorption performance.Double exponential model and Jander model were used to analyze the experimental data of isothermal CO₂ adsorption over Li₄SiO₄ derived from different Li sources,K doped Li₄SiO₄,and Ca doped Li₄SiO₄ adsorbents.The effect of different modification aproaches on the reaction kinetics of Li₄SiO₄ adsorbents were investigated.The results show that,for Li₄SiO₄ derived from different Li sources,the double exponential model could well fitted the experimental data and the adsorption of CO₂ on the Li₄SiO₄ was kinetically controlled by Li diffusion.For K doped Li₄SiO₄ adsorbents,the kinetic parameter k₁ of the double exponential model increased first and then dereased with the increase of K doping content,which indicates that appropriate doping of K could accelerate CO₂ chemisorption reaction.Jader model could well fit the CO₂ adsorption reaction except for the initial stage because the Jander model ignored the influence of surface chemcial reaction.For Ca doped Li₄SiO₄ adsorbents,the double exponential model shows that CO₂ adsorption over them was mainly controlled by the Li ion diffusion.And the Jander model could well fitted the experimental data at low temerature but did not at high temerature becuase it ignored the influence of CO₂ adsorption reaction rate and Li ion diffusion rate.