Fabrication Of Efficient Li₄SiO₄-based CO₂ Sorbents For Sorption-enhanced Reforming And Its Environmental Applicability Study

As an ideal energy source of high efficiency and clean,hydrogen has important value in the fields of energy,metallurgy and chemical industry.Gas reforming and biomass gasification based on sorption-enhance can realize large-scale and low-cost production of hydrogen,and in-situ adsorption of CO₂ is one of the key processes of this technology.As an efficient CO₂ adsorption material,lithium orthosilicate can adsorb CO₂ in-situ to enhance the reforming reaction,thus greatly improving the yield and purity of hydrogen.However,the development of lithium orthosilicate-based CO₂adsorption technology still faces two challenges.On the one hand,the realization of CO₂ adsorption and desorption process depends on the reaction system of circulating fluidized bed,which requires the lithium orthosilicate sorbent to exist in pellet form and have good abrasion resistance,so as to reduce the agent loss caused by elutriation of powder sorbent.On the other hand,the presence of gaseous impurities,especially hydrogen sulfide,in the process of reforming reaction has a potential impact on the process of CO₂ adsorption and desorption.Under this background,a lot of experimental work and characterization analysis have been carried out in this work,aiming to construct abrasion-resistant lithium orthosilicate adsorption pellets with great adsorption performance and to clarify the influence law and mechanism of gas phase impurity hydrogen sulfide on the CO₂ adsorption process of lithium orthosilicate.In this work,a variety of lithium orthosilicate powder sorbents were synthesized by the solid phase method with the precursor of different silicon sources,based on the extrusion-spheronization technology,lithium orthosilicate sorbent pellets with excellent adsorption and mechanical properties were successfully prepared by adding the porous auxiliary polyethylene.The results show that the optimal adsorption temperature of the lithium orthosilicate sorbent F-powder synthesized using fumed silica as a silicon source precursor is 625℃,and it has an excellent adsorption capacity of 0.31g-CO₂/g-sorbent at 15 vol.%CO₂,reaching 81.7%of the theoretical adsorption maximum,and shows good cycle stability in 20 cycle tests.In addition,20 wt.%is the optimal loading ratio of the polyethylene,the modified pellets can reach a CO₂adsorption capacity of 0.31 g-CO₂/g-sorbent,and maintain good stability in 40 cycle tests.Meanwhile,the compressive strength of the modified pellets reaches 27.5 Mpa,and the fluidized abrasion rate after 10 hours is only 0.06 wt.%/h.It is worth mentioning that the abrasion of the pellets continues to slow down as the cycle time increases,which is extremely beneficial for the efficient recycling of sorbents.Secondly,this work studies the potential impact of gas phase impurities in the reforming reaction.The effect of H₂S concentration and adsorption temperature on the adsorption of CO₂ by lithium orthosilicate was reported for the first time,combined with a series of characterization tests for in-depth analysis,the mechanism by which H₂S impurities affect the adsorption of CO₂ by lithium orthosilicate is obtained.The results show that the high concentration of H₂S has a serious negative effect on the CO₂adsorption process of lithium orthosilicate.Under the condition of 500 ppm H₂S,the adsorption capacity of the sorbent quickly decreases to 0.089 g-CO₂/g-sorbent after 20cycles,when the concentration of H₂S reaches 4000 ppm,the adsorption capacity drops sharply to only 0.007 g-CO₂/g-sorbent,the drop is more than 97%.The study also found that the mechanism of action of H₂S is greatly affected by the adsorption reaction temperature,different action mechanisms under different adsorption reaction temperatures.In the range of 325～475℃,Li₄SiO₄ reacts with H₂S to form Li₂S and a small amount of sulfate.In the range of 625～775℃,K-doped Li₄SiO₄ reacts with H₂S to form KLiSO₄ and covers the surface of the sorbent,which greatly increases the contact resistance between CO₂ and internal Li₄SiO₄,resulting in a rapid decline in the CO₂ adsorption capacity.When the temperature reaches 925℃ or above,Li₄SiO₄undergoes severe sintering,leading to significant changes in the microscopic pore structure of the sorbent,resulting in a sharp decline in the CO₂ adsorption capacity.This work has enriched the research on the pelletization of lithium orthosilicate-based CO₂ sorbent and expanded the influence of gas phase impurities on the CO₂ adsorption process of lithium orthilicate,providing reference for the application of industrial lithium orthilicate-based adsorption materials.