Study On Preparation Of Composite Potassium-based Adsorbent And Mechanism Of CO₂ Adsorption

CCUS technology is an effective way to reduce CO₂emissions,among which capture is the most expensive part,accounting for about 75%of the total cost.Therefore,it is imperative to study low-cost carbon capture technology.As a by-product of coal combustion,fly ash accounts for 60%-88%of the total solid waste from coal-fired power plants.The elemental compositions of fly ash are mainly Si and Al,which is expected to be used to prepare mesoporous silica-aluminum composite aerogel potassium based CO₂adsorbents.In CO₂capture link of CCUS,the use of cheap power plant solid waste fly ash can greatly cut down the cost of carbon capture,reduce the emissions of fly ash,decrease the demand for power plant waste landfill,and save natural resources.It is of great theoretical significance and practical value to develop efficient and cheap potassium based CO₂adsorbent.In this thesis,silica aerogel adsorbent was prepared by using tetraethyl orthosilicate as precursor;silica-alumina aerogel adsorbent was prepared by using aluminium chloride crystal as precursor and tetraethyl orthosilicate as surface modification reagent;fly ash composite aerogel adsorbent was prepared by alkali fusion and acid leaching of fly ash to extract Si and Al components.The effects of preparation conditions(such as particle size and designed loading)and reaction conditions(such as adsorption temperature,adsorption atmosphere and total gas flow rate)on the characteristics of adsorbent were studied.On this basis,the regeneration characteristics of composite aerogel adsorbent prepared from fly ash under different regeneration conditions(such as regeneration temperature and heating rate)were studied.Adsorption and regeneration experiments were carried out in a small fixed-bed reactor,and the adsorbents were characterized by various characterization analysis methods.Combined with the characterization results,the method of integrate experiment with model calculation was adopted.The adsorption kinetic model,reaction kinetic model,mass transfer kinetic model and deactivation model were established to explore CO₂adsorption performance and mechanism of adsorbents.Avrami fractional kinetic model together with intraparticle diffusion model,interparticle diffusion model and Boyd's membrane diffusion model were used to analyze the regeneration experimental results of silica-aluminum composite aerogel potassium-based CO₂adsorbent prepared from fly ash.The main results obtained are as follows:(1)Specific surface areas of silica aerogel adsorbent,silica-alumina aerogel adsorbent and composite aerogel adsorbent prepared from fly ash are respectively 110.46 m²/g,218.08m²/g and 191.80 m²/g.Cumulative pore volume of them are respectively 0.09 cm³/g,1.08cm³/g and 0.98 cm³/g.The maximum cumulative adsorption capacity of them are 1.68mmol/g,1.84 mmol/g and 2.02 mmol/g,respectively.(2)When sol-gel method is used to prepare adsorbent,the preparation conditions have a great influence on the structure of the adsorbents.The microstructure of the aerogel support is well-developed with a large number of pores,uniform and orderly pore size.The pore structure is cylindrical pores or regular polygon cylindrical pores open on both sides.After loading,the pore structure of adsorbent is ink bottle shape or capillary with narrow mouth and wide body.(3)The designed loading has an effect on structure of adsorbent,which makes the adsorption performance different.With the increase of designed loading,CO₂cumulative adsorption capacity and K₂CO₃utilization rate of adsorbent show the same trend,reaching the maximum value when the designed loading is 30%.(4)Avrami fractional kinetic model is suitable for describing adsorption kinetics of aerogel adsorbents.The adsorption of CO₂by aerogel adsorbents is heterogeneous multi-layer adsorption.The increase in temperature can increase the adsorption rate.Using water vapor to pretreat the adsorbent can increase the adsorption reaction rate and improve the adsorption performance.Increasing the total gas flow rate positively promotes the adsorption rate,and a lower total gas flow rate can make the contact time between CO₂and active sites longer.(5)Shrinking core model is suitable for describing reaction kinetics of the adsorbent.The carbonation reaction of adsorbent can be divided into two stages:surface chemical reaction-controlled region and internal diffusion-controlled region.In surface chemical reaction-controlled region,the conversion rate of carbonation reaches more than 80%of the total conversion rate.(6)Internal mass transfer is the rate controlling step of mass transfer when the adsorbent adsorbs CO₂.With the rise of temperature,both internal and external diffusion rates increase.Increasing CO₂concentration can enhance the external mass transfer,and increasing the total gas flow rate can greatly improve the external mass transfer.(7)With the process of adsorption,the activity of the adsorbent decreased.The increase in temperature speeds up the adsorption and desorption process at the same time.When CO₂concentration reaches the optimal value,the number and activity of adsorption sites on the surface of the adsorbent will not change.Increase of total gas flow rate significantly promoted the initial adsorption rate and deactivation rate.(8)When regeneration temperature is 150? and heating rate is 10?/min,the maximum desorption capacity of composite aerogel adsorbent prepared from fly ash is 1.75 mmol/g.Avrami fractional kinetic model is suitable for describing the regeneration kinetics.The early stage of regeneration is mainly controlled by intraparticle diffusion,the middle stage is controlled by both surface chemical reaction and intraparticle diffusion,and the final stage is controlled by membrane diffusion.(9)After ten adsorption-desorption cycles,the adsorption property of the adsorbent is stable at about 1.75 mmol/g.