Theoretical Studies On The Effect Of Active Component Structure,Supports And Additives On The Dry Absorption Of CO₂ By K₂CO₃

K₂CO₃ as absorbent for CO₂ capture used in flue gas from the power plant has many advantages.It is cheap and readily available as a raw materials,and the preparation process is simple and the recycling rate is high as a absorbent.In addition,the reaction requires lower temperature and energy consumption.However,the slow reaction rate of K₂CO₃ and CO₂,the low conversion rate of carbonation,and the poor removal effect of CO₂ have hindered the application of K₂CO₃ sorbent in CO₂ capture.In this work,the density functional theory method as well as micro-kinetic modeling are used to systematically investigate the effect of different active components,carriers and additives on the absorption mechanism of K₂CO₃ absorbents at the electron-atom level.The active components of absorbents were determined by calculating the rate of surface carbonation reaction of monoclinic and hexagonal K₂CO₃ crystals.The suitable absorbent supports also were determined by comparing nine kinds of carrier-loaded K₂CO₃ for absorption of CO_2.The effect of adding of V₂O₃ and TiO₂ to sorbents on the CO₂ capture was studied.Through the above research,the carbonation mechanism of CO₂ absorption by K₂CO₃ absorbents and the effect of different active components,carriers,and promoter on the performance of carbonation reactions were clarified,which providing valuable theoretical clues for the preparation of highly efficient and suitable potassium carbonate absorbents.The main conclusions based on the above work as follows:1.The hexagonal crystal K₂CO₃ is more suitable for the active components of CO₂ absorbents.There are two mechanisms for carbonation reaction on the surfaces of K₂CO₃ crystal.Mechanism I:The reactant moleculesH₂O and CO₂are firstly adsorbed on the surfaces,and then co-adsorbed H₂O and CO₂ react directly on the surface of K₂CO₃ to form KHCO₃?CO₂ and OH in H₂O combine to form HCO₃,while H in H₂O combines with surface CO₃ to form another HCO₃?.In this process,the activity of carbonation reactions are high,but the much low CO₂ coverage on the surface limits the rate of carbonation reaction;mechanism II:adsorbed H₂O firstly decomposes to form H and HO,and then H combines with surface CO₃ to form HCO₃,at the same time,HO reacts with gaseous CO₂ to obtain another HCO₃.H₂O dissociation is difficult,which is not conducive to carbonation reaction in the process.At the same time,the surface is mainly covered by H₂O,so it is concluded that the K₂CO₃?nH₂O is main product.The results show that the reaction rate of carbonation reaction on hexagonal crystal K₂CO₃ is higher than that on monoclinic crystal K₂CO₃.Therefore,it is more capable of capturing CO₂ for the hexagonal crystal than the monoclinic crystal K₂CO_3.As a result,hexagonal crystal is more suitable for the active component of the CO₂ absorbent.2.The carrier has a significant effect on the carbonation mechanism of CO₂absorption by K₂CO₃ and the performance of the carbonation reaction.After K₂CO₃ being loaded on the carrier,the anti-wear properties of the K₂CO₃absorbent can be enhanced,and the dispersibility of the active component also can be increased.When the active component K₂CO₃ is supported on the surface of the carrier,there are three areas for carbonation reaction to occur:the exposed surface of the support which is not covered with K₂CO₃,the interface between the active component and the carrier surface,and the potassium carbonate surface where the active component is deposited.the results show that he active region of the carbonation reaction is the interface between the active component and the carrier surface.The use of V₂O₃ as the carrier can increase the coverage of CO₂ at the interface between K₂CO₃ and the carrier surface,and thus improve the rate of carbonation reaction,and the use of CaO,Fe₂O₃,and TiO₂ as the carrier can promote the dissociation of H₂O at the interface between the K₂CO₃and the carrier surface,making the surface hold more HO that can react with gaseous CO₂,thereby increasing the rate of carbonation reactions.In addition,when CaO,MgO,and?-Al₂O₃ are used as carriers,stable mondendate or bidendate carbonate(CO₃^?-)species on the surface can be formed after absorbing CO₂?C in CO₂ directly reacts with O on the surface of the support to bond?.3.The carrier influences the regeneration performance of the absorbent during cyclic absorption of CO₂ by the K₂CO₃ absorbent.The addition of graphite,CaO,MgO,Fe₂O₃,V₂O₃,?-Al₂O₃,anatase,and rutile supports promotes the decomposition of KHCO₃ during the regeneration process of the absorbent——The loaded K₂CO₃ can regenerate quickly,and the regeneration performance of the absorbent is improved.However,on K₂CO₃/MgO?100?,K₂CO₃/?-Al₂O₃?110?,and K₂CO₃/CaO?100?,the by-product,carbonates can be formed during CO₂ absorption,and they are not completely decomposed at200?.4.Adding a small amount of promoter V₂O₃ and TiO_2,can improve the carbonation performance of K₂CO₃/?-Al₂O₃?0001?absorbent.The addition of small amounts of auxiliaries V₂O₃ and TiO₂ to K₂CO₃/?-Al₂O₃?0001?absorbents can increase the coverage of CO₂ on the surface of the absorbent,which in turn increases the rate of carbonation reactions and promotes K₂CO₃/?-Al₂O₃?0001?absorbents.