Investigation On The CO₂ Capture Characteristics Of Alkali Salt Modified CaO-based Adsorbent

CO₂ is the major culprit of global warming;hence,it is urgent to take efforts to mitigate the CO₂ emissions.CCS is well considered to be one of the most effective carbon emission reduction technologies.As a typical post-combustion CO₂ capture process,Calcium looping process(Ca LP)is regarded as one of the most promising CO₂ capture technologies.Ca LP has a series of technical advantages,such as large theoretical adsorption capacity,superior reaction kinetics and rich source materials,etc.,but its development still faces several technical problems,such as the drastic drop of CO₂ capture capacity of the Ca O sorbents over the repeated cycles in high temperature reactions,and the significant attrition and pulverization during the cycles,etc.In this work,we developed an economic and environmentally friendly Ca O based sorbent with fascinating CO₂ capture capacity and attrition resistance performance by means of modelling study and experimental research.Firstly,DFT method was used to study the effect of K⁺on the CO₂ adsorption behavior on the surface of Ca O(100).The results showed that the O-top of Ca O(100)was the most stable adsorption site of CO₂,and after K⁺doping,the most stable adsorption site of K⁺on the surface of Ca O(100)is also the O-top.After K-doping,the modified Ca O showed stronger adsorption energy with CO₂.In this paper,the cyclic CO₂ capture capacity of K⁺modified Ca O was tested by TGA,and the results illustrated that the CO₂ cyclic capture capacity is 2-3 times of that of unmodified Ca O.Besides,the enhancing mechanism was discussed by in situ/ex situ XRD,TEM,SEM,BET and so on.Under mild Ca LP(calcination under 100%N₂),K⁺was homogeneously doped into the Ca O lattice,and K-Ca solid solution was accordingly formed;while under harsh Ca LP(calcination under 80%CO₂/N₂),affected by K⁺,the carbonated Ca O would form a new crystal structure of Ca CO₃.Secondly,the effects of various alkali salts on the cyclic CO₂ capture performance of the modified Ca O-based sorbent were compared by TGA,BET,TPA/TPD and so on.In addition,the synergistic effect of halogen ions and alkali metal ions on the carbonation performance of Ca O was illustrated.KCl and Na Cl boosted the CO₂ capture capacity of the modified Ca O,and the carbonation conversion of modified Ca O was twice as high as that of unpromoted Ca O.Cl^-inhibited the sintering of the Ca O sorbent,but decayed the cyclic carbonation activity of the Ca O.K⁺and Na⁺enhanced the carbonation activity of the Ca O,while they displayed a great negative effect on the thermal stability of the Ca O sorbent.It is the synergistic effect of K⁺,Na⁺and Cl^-that boosted the cyclic CO₂ capture behavior of the modified Ca O sorbent.Thirdly,based on the synergistic effect of halogen ion and alkali metal ion on CO₂capture capacity of modified Ca O absorbents,this paper further investigated and verified the well CO₂ capture performance of low-cost sea salt modified limestone by a fixed bed reactor.The results displayed that when incorporated 2-3wt.%of sea salt in the modified sorbent,the modified limestone held a CO₂ capture capacity of 0.32g/g after 40 cycles,which was 2.3times of the natural limestone.In addition,the absorbent,which modified by sea salt,showed better anti-attrition performance,and the mechanical strength of the absorbent particles increased with the increase of sea salt doping ratio.Fourthly,to further improve the attrition resistance and CO₂ capture capacity of the sea salt modified limestone sorbent,this work adopted extrusion-spheronization method to shape the modified powder absorbent into particle absorbent;besides,biomass powder such as bamboo was taken as pore forming agent and binder to improve the shaped absorbent.Furthermore,the effects of cellulose,hemicellulose and lignin on the CO₂ capture capacity and attrition resistance of the pellet sorbents were studied by TGA,FT,SEM,BET,XRD and so on,and then a screening method for pore forming agent and binder materials was proposed.Combining the cyclic CO₂ capture capacity and attrition resistance performance of the modeled absorbent,cellulose would be the optimum templating agent.Through further granulation of the powder limestone sorbent,the sea salt modified limestone behaved better cyclic CO₂ capture capacity than that of unmodified limestone.Finally,sorption enhanced steam reforming of ethanol(SESRE)based on the sea salt modified limestone pellets were examined and discussed.Through the Gibbs free energy minimization method,the effect of Ca O on the H₂ production from steam reforming of ethanol was simulated,and the simulation prediction was well verified in the fixed bed reactor.The optimal reaction temperature of steam ethanol reforming is 600?and above,and the water ethanol ratio has a significant effect on the H₂ production.Under the enhancing effect of sea salt modified limestone sorbent,during the 10^th SESRE process,H₂can be constanctly produced with purity above 90%within at least 15mintes of ethanol reforming.