R&D For An Efficient Sodium-based CO₂ Solid Sorbent And Study On Its Bicarbonation/regeneration Mechanism

Global warming,extreme weather events and other problems,arising from greenhouse gas effects,are threatening the survival and development of human life.As a result,CO₂ reduction attracts extensive attention worldwide.Sodium-based solid sorbents are considered to be promising for CO₂capture in industry due to massive advantages such as low price of raw materials,high theoretical CO₂ capacity,low energy consumption from regeneration,no corrosion to facilities,no secondary pollution,easy retrofitting to current power plants and etc.However,bad kinetics and thermodynamics of bicarbonation reaction on sodium carbonate(Na₂CO₃)limit the application of this technology.To solve the problems above,several key issues were investigated in this paper.First,bicarbonation behaviors of pure Na₂CO₃ under various preparation conditions were studied.Dissolution/crystallization and calcination have no effects on crystaling form but these processes affect the exposure of crystal facets,crystal size and pore structure of particles,and then alter the bicarbonation performance.The bicarbonation on analytical reagent potassium carbonate(K₂CO₃)was studied as well.Its sorption rate and CO₂ uptake were both an order of magnitude higher than that of Na₂CO₃.Our experimental and computational results revealed the poor hygroscopicity of Na₂CO₃ results in the dominant gas-solid non-catalytic bicarbonation reaction while the prominent hygroscopicity of K₂CO₃ leads to the gas-liquid-solid ionic reaction during the CO₂ sorption.Second,the mechanism of bicarbonation reaction on Na₂CO₃ was investigated thoroughly at the atomic level.Our computational simulations showed that surface bicarbonation on Na₂CO₃ is facile,no matter on low-index or high-index facets and no matter under low humidity or high humidity atmosphere.Furthermore,the theoretical calculations indicated that bulk bicarbonation on Na₂CO₃prefers ion counter diffusion(ICD)mechanism than molecular interstitial diffusion(MID)mechanism.Three main steps in the ICD pathways were investigated,including H⁺/HCO₃^-generation at the NaHCO₃-gas interface,counter-diffusion of H⁺vacancy/interstitial Na⁺in the NaHCO₃ layer and interstitial H⁺/Na⁺vacancy generation in Na₂CO₃.The interstitial Na⁺diffusion in the NaHCO₃ layer and interstitial H⁺/Na⁺vacancy generation in Na₂CO₃ are considered to be the rate limiting steps in bulk bicarbonation reaction.Third,Li⁺/K⁺ doping and K₂CO₃/n-Ti O(OH)₂ blending are confirmed experimentally to benefit the kinetics and thermodynamics of bicarbonation on Na₂CO₃.In detail,Li⁺doping can reduce the defect formation energies in Na₂CO₃,promoting the bulk bicarbonation;K⁺doping can facilitate crystal rupture,increasing the surface exposure of Na₂CO₃;K₂CO₃/n-Ti O(OH)₂ blending strengthen the hygroscopicity of Na₂CO₃,enhancing the possibility and proportion of ionic reaction during the CO₂ sorption.Forth,the mechanism of sodium bicarbonate(NaHCO₃)heat regeneration with/without catalyst are both revealed.The intrinsic mechanism of NaHCO₃ heat regeneration follows the first-order reaction principle which is controlled by random C-O bond break in bulk phase.However,surface regeneration,abiding in shrinking core model(R2),may dominant considering the temperature gradient in real particles and defects abundance on surface.Catalyst,n-Ti O(OH)₂ in particular,alters the pathway of proton transfer from inter-layer transition to surficial migration,accelerating the surface regeneration.Moreover,the catalytic effect was confirmed by experiments.Fifth,the morphology changes of the supported Na-based sorbents manufactured by incipient-wetness method and their performance on CO₂ capture were investigated systematically.With the increase of Na₂CO₃ loading,the active component changes from monolayer dispersion to small grains.Thanks to the porous structure and abundant surface active sites of?-Al OOH,the small Na₂CO₃grains were still well-dispersed on support,especially on the inner surface,and they were not oversized and plenty of hydroxyl groups remained on surface.Na Al H29 showed the best capacity among this series of sorbents.The maximum CO₂ uptake was 2.30 mmol·g^-1 and the stable value was over 2.25 mmol·g^-1 after 43 sorption-regeneration cycles.Finally,the morphology changes of the supported Na-based sorbents manufactured by excess-wetness method and their performance on CO₂ capture were investigated systematically.When the concentration of the impregnating solution was low,the active component enriched outside the support,blocked the external pores.With the increase of the concentration or the times of impregnation,the active component can dispersed throughout the support,accompanied by crystal growth and particle agglomerate.42Na30Al H demonstrated the best performance due to its high loading proportion,small crystal size,and acceptable particle dispersion.Its CO₂ uptake tended to be stable after 10 cycles,around 3.0 mmol·g^-1 and steam pretreatment can promote CO₂ sorption in the first 450 s.Moreover,K₂CO₃/n-Ti O(OH)₂ blending effects were confirmed that K₂CO₃ can enhance both the sorption rate and capacity while n-Ti O(OH)₂ can make the regeneration happen in advance.