The Effect Of Etching Modification On The Sorption Performance Of Nano CaO-based CO₂ Sorbents

As a new generation of CO2 capture technology,calcium looping technology can not only be applied to flue gas decarbonization and solar storage,but also has important applications in reactive sorption-enhanced reforming(ReSER)of methane for hydrogen production.The CaO-based CO2 sorbent is a key part of the calcium looping technology.Among them,the carbonation reaction of CaO is a gas-solid reaction,and the microstructure of the sorbent has a direct effect on the mass transfer coefficient and reaction activity.Therefore,studying the influence of the microstructure of nano-CaO-based sorbents on the CO2 sorption performance has important theoretical and practical significance for guiding the preparation of sorbents and the industrial application of calcium looping technology.Firstly,the wet etching technology was introduced into the preparation of nano CaO-based sorbents for the first time in this paper.The inorganic material Al2O3 was selected as the hard template,and the pore volume of the sorbent was expanded by wet etching Al2O3.The effects of etching conditions such as etching temperature,alkali concentration and etching duration on the microstructure were studied respectively.The results showed that the specific surface area of the sorbent after etching was between 20-30 m2/g,and the pore volume was between 0.2-0.3 cm3/g.Compared with the unetched sorbent,the specific surface area of the sorbent was increased by 2.5-3.8 times,and the pore volume was increased by 3.4-5.1 times.Therefore,the pore volume expanding of the nano CaO-based sorbent could be achieved by the wet etching technology.Secondly,the peak pore diameter was used to describe the pore size distribution of the nano CaO-based sorbent in this paper.By changing the particle size of the hard template Al2O3,a series of nano CaO-based sorbents with different peak pore diameters were prepared.The results showed that the peak pore diameter of the sorbent obtained after etching of Al2O3 with particle size of 10 nm was between 10-30 nm,and the peak pore diameter of the sorbent obtained after etching of Al2O3 with particle size of 30 nm was between 20-50 nm.And the peak pore diameter of the sorbent obtained by etching Al2O3 with particle size of 50 nm was between 50-70 nm.Therefore,the particle size of the inorganic hard template Al2O3 had a corresponding relationship with the peak pore diameter of the sorbent after etching.According to the mass transfer theory,the influence of the peak pore diameter on the sorption performance of the sorbent was explored.The results showed that when the peak pore diameter range increased from 10-30 nm to 50-70 nm,the diffusion coefficient of CO2 gradually increases from 2.05×10-3cm2/s to 1.44×10-2 cm2/s,the maximum sorption rate of the sorbent in the fast stage increased from 7.6 mol/(kg·min)to 8.8 mol/(kg·min),and the sorption capacity also increased from 9.0 mol/kg to 10.9mol/kg.The analysis showed that larger peak pore diameter can not only increase the diffusion coefficient of CO2,but also provide larger growth space for CaCO3,leading to a better sorption performance.However,when the carbonation temperature was 600℃ and the calcination temperature was 800℃,the stable sorption capacity of the sorbents with different peak pore diameters were at about 4.0 mol/kg after 15 calcium looping cycles.It showed that the peak pore diameter did not have a good cycle continuity for the improvement effect of sorption performance.In addition,based on nano CaO-based sorbents with different peak pore diameters,the effect of calcium looping temperature on the stability of sorption performance was studied.The results showed that when the carbonation temperature was 700℃ and the calcination temperature was 800℃,the cyclic stability of the sorbent with three peak pore diameters was the best.Among them,the sorption capacity of the nano CaO-based sorbent with peak pore diameter range of 50-70 nm could reach 8.1 mol/kg after 15 calcium looping cycles,which is higher than the sorption capacity of those sorbents with peak pore diameters of 10-30 nm and 20-50 nm.Finally,aiming at the problem of rapid deactivation of nano CaO-based sorbents in practical applications,wet etching technology was applied for the deactivated sorbent to explore the activation effect of the etching modification in this paper.Studies had shown that the sorption capacity of the deactivated sorbent after 5 calcium looping cycles was increased by 59.2%with etching modification;the sorption capacity of the deactivated sorbent after 10 calcium looping cycles was increased by 70.6%with etching modification.Therefore,wet etching had the activation effect on deactivated sorbents.The more serious the sintering deactivation of the sorbent,the more obvious the activation effect of etching modification.In addition,adding MgO as a pore stabilizer can delay the sintering of the sorbent,which can improve the cyclic stability of the reactivated sorbent.