Study On Modification Of High-temperature Lithium-based Adsorbents And Adsorption Properties Of Low Concentration Carbon Dioxide

Coal-fired power plants are the main source of CO₂ emissions in the atmosphere.CO₂ capture and storage of coal-fired boiler flue gas is one of the most direct and effective measures to mitigate CO₂ emissions.At the same time,a large amount of CO₂ is produced during the process of reforming and cooperating hydrogen production.For this process,CO₂ capture and storage not only can effectively reduce CO₂ emissions,but also can improve the hydrogen production efficiency.lithium orthosilicate（Li₄SiO₄）have been considered as one of the most promising sorbent for capturing CO₂ at high temperature since its large absorption capacity,stable cyclability and reasonable material costs.The synthesis of lithium silicate adsorbent prepared by conventional solid state method has the disadvantages of low CO₂adsorption capacity and slow reaction rates,which are difficult to meet the needs of practical application.To overcome the above disadvantages,this paper mainly adopts different methods to improve its adsorption performance under low CO₂ concentration conditions and make corresponding researches for practical promotion and application in the future.Various characterization methods,such as X-ray powder diffraction（XRD）,scanning electron microscopy（SEM）,N₂ adsorption analyzer and thermogravimetric analyzer（TG）and so on,were employed to characterize the phase composition,structural characteristics and adsorption properties of the synthesized samples.The main research contents are as follows:（1）A unique gluconic acid treatment coupled with a carbon coating process was used to synthesize Li₄SiO₄ ceramics（GAC-Li₄SiO₄）with superior performances for high-temperature CO₂ capture.The effects of different synthetic methods,carbonization temperatures,acid sources and acid contents on the performance of the sorbent were studied and characterized by X-ray diffraction（XRD）,scanning electron microscopy（SEM）,nitrogen absorption and thermogravimetric analyses.After the gluconic acid treatment and carbon coating,the GAC-Li₄SiO₄ had a small crystal size,hollow morphology and enhanced pore features.Significant improvements in CO₂sorption performance（i.e.,low absorption temperature,fast absorption rate,and high capacity）were thus obtained.Moreover,the carbonization temperature,acid content and acid type greatly affected the morphology and chemisorption properties of the Li₄SiO₄ sorbents.Under the optimized synthetic conditions,the maximum CO₂absorption capacity was 34.7wt.%after isothermal absorption at 665℃ for 10 min.This sorbent also maintained good cyclic properties.（2）Highly efficient K₂CO₃-doped lithium orthosilicate（Li₄SiO₄）sorbents were synthesized using acid treatment to overcome the typical kinetic limitations of sorbents at low CO₂ concentrations.The effects of K₂CO₃ on the structure and absorption properties were characterized using various morphological characterization techniques,differential scanning calorimetry（DSC）and thermogravimetric analysis（dynamic and isothermal）.The results showed that different amounts of K₂CO₃greatly affected the physical-chemical properties and absorption kinetics of the sorbents.On one hand,K₂CO₃ formed a solid solution with Li₄SiO₄ and generated favorable characteristics（a smaller crystal size,corrugated morphology,larger surface area and higher surface concentration of Li₂O）,significantly facilitating the chemisorption processes.On the other hand,a higher amount of doped K₂CO₃ was located on the surface,which induced molten phases during CO₂ absorption,remarkably decreasing the diffusion processes.Therefore,0.1 M K₂CO₃ doped sorbent obtained noticeable absorption capacity and good regenerability in 15 vol.%CO₂.（3）The doping of sodium salt has a positive effect on the improvement of the effect of lithium silicate sorbent.On the basis of carbon encapsulation,the effect of doping with NaBr and NaF dopant on the performance of the sorbent is emphatically studied.On the one hand,the NaBr co-doped Li₄SiO₄ adsorbent was synthesized by the carbon-coating method to overcome the kinetic limitation of the lithium silicate adsorbent in a low carbon dioxide atmosphere.On the other hand,NaF-doped Li₄SiO₄adsorbent was also discussed and compared with commonly used sodium salt dopants,possible adsorption process diagrams were drawn,and the dynamic characteristics of lithium silicate adsorbent at low CO₂ concentration were further explored.amples were analyzed by XRD,SEM,N₂ adsorption,XPS,differential scanning calorimetry（DSC）and thermogravimetric analysis（dynamic and isothermal）.The results showed that the adsorbents modified by NaBr doping,Na and Br,respectively,went into the Li₄SiO₄ structure and replaced the Li and oxygen sites,respectively.The doped samples exhibited small particle size,uniform particles,large pore features,and Li₂O enriched on the surface of Li₄SiO₄.Simultaneously,during the absorption of CO₂,the production of the surface melt phase is promoted,and this favorable structural property promotes the surface chemical adsorption process and accelerates the diffusion of lithium.Doping with 0.1 mole of NaBr-doped sorbent reached a maximum absorption capacity（>30.0 wt.%）in a 15 vol.%CO₂ atmosphere,indicating that the NaBr-doped Li₄SiO₄ ceramic has a large CO₂ capture potential.On the other hand,co-doped sodium and fluorine also replace the lithium and oxygen sites,respectively,and this doping method produces a high concentration of Li on the surface of the particles.This favorable characteristic greatly promotes the surface chemisorption process,accelerates the transport of Li⁺and O^2-,and reduces the diffusion resistance of CO₂.At the same time,only 3wt.%of Na F need to be doped per gram of Li₄SiO₄ adsorbent,and the maximum adsorption capacity can always be higher than 33.0wt.%in a wide range of temperature（475-575℃）under a 15 vol.%CO₂ atmosphere.In addition,the adsorbent still maintained a more stable adsorption after 10 adsorption/desorption cycles,indicating that the NaF-doped Li₄SiO₄adsorbent has high CO₂ capture potential.（4）Based on the data from previous studies,the effect of doped lithium salts on the adsorption of lithium silicate sorbents was investigated.Several types of lithium halide-doped Li₄SiO₄ sorbents were prepared via sacrificial carbon template technology to overcome the typical kinetic limits under low CO₂ partial pressure.The synthesized samples were characterized by XRD,SEM,XPS and thermogravimetric analysis.The results reveal that the highest uptakes were obtained by doping with Li F among three doped sorbents.Different amount of LiF doping also significantly affected the intrinsic properties and absorption characteristics of Li₄SiO₄ sorbents.When the doping amount was 10 mol%,Li and F were incorporated into Li₄SiO₄structure and substituted for Li and oxygen sites,respectively.Such doped features generated the smaller crystallite size and particle size as well as Li₂O-riched Li₄SiO₄surface,which is responsible for the maximum uptake capacity of 36.2 wt.%.This value is the largest uptake reported in the literatures so far.Moreover,the high capacity was maintained over 8 sorption/desorption cycles.