Research On Cyclic CO₂ Capture Performance Of Ca/Al Sorbents And Reaction Mechanism By Experiment And DFT Calculations

CO2 capture,utilization and storage(CCUS)is a feasible way for the large-scale greenhouse gases reduction.The cyclic calcination/carbonation of calcium-based sorbents,i.e.,calcium looping,is one of the feasible technologies to capture CO2 for the current coal-fired power station and hydrogen production by coal/biomass gasification.The decay of CO2 capture activity of calcium-based sorbents is one of the main problems to hold back the development of this technology.Taking carbide slag as a main raw material and alumina-based materials as support,various types of highly active Ca/Al CO2 sorbents were prepared in this work.The CO2 capture performance and the reaction mechanism of the highly active Ca/Al CO2 sorbents was studied by combing experimental,microscopic analysis and density functional theory(DFT)simulation method.The study was aimed at providing theoretical guidance for coal-fired power stations and hydrogen production stations to realize efficient CO2 capture using Ca/Al sorbents.A method of preparing porous Ca/Al sorbents using carbide slag as the calcium precursor,high alumina cement as the aluminum precursor and byproduct of biodiesel as the pore-forming agent by combustion synthesis was proposed.The effects of preparation conditions and CO2 capture conditions on the CO2 capture characteristics of Ca/Al sorbents were studied.The cyclic CO2 capture performance of Ca/Al sorbents under harsh calcination conditions was explored.The kinetic characteristics of calcination and carbonation reactions were analyzed.The pore-forming mechanism of byproduct of biodiesel was clarified.The Ca/Al sorbent with the high alumina cement content of 5%possesses the best CO2 capture performance.Under harsh calcination conditions,the CO2 capture capacity of Ca/Al sorbent after 30 cycles is 0.27 g/g,which is 1.7 and 2.6 times as high as those of carbide slag and limestone,respectively.Ca12Al14O33 is formed by the solid-phase reaction of high alumina cement and carbide slag at high temperature.Therefore,under the combined effects of the pore forming during combustion and the support of Ca12Al14O33,the Ca/Al sorbent possesses high cyclic CO2 capture performance.A new process for the synthesis of a hollow microspheric Ca/Al sorbents with controllable size and structure was proposed,in which carbide slag and high alumina cement were used as raw materials,and carbon microspheres formed by the hydrothermal carbonization of glucose were used as templates.In order to overcome the problem of uneven distribution of CaO and support in the microspheric Ca/Al sorbent because of insoluble Ca/Al sources,soluble calcium and aluminum precursors instead of carbide slag and high alumina cement were used.The cyclic CO2 capture performance of the hollow microspheric Ca/Al sorbent was further improved.The effects of preparation conditions and reaction conditions on the cyclic CO2 capture performance of the hollow microspheric Ca/Al sorbent were studied.The mechanism of multiple calcium and aluminum sources affecting the formation of Cal2Al14O33 as a supporting material was elucidated.When the Al2O3 content is 5%,the hollow microspheric Ca/Al sorbent achieves the highest cyclic CO2 capture performance.Under harsh calcination conditions,the CO2 capture capacity of the hollow microspheric Ca/Al sorbent after 10 cycles is 80.9%higher than that of carbide slag.The special structure of the hollow microspheric Ca/Al sorbent reduces CO2 diffusion resistance,which leads to the better CO2 capture performance and reaction rate.Compared with carbide slag,the utilization of the hollow microspheric Ca/Al sorbent can greatly reduce the energy consumption for each unit of CO2 captured in the calciner and save energy.In order to reduce the man-made template cost,a hollow microtubular Ca/Al sorbent by impregnation with acetic-treated carbide slag and aluminum nitrate as raw materials and paper fiber as a biotemplate was prepared.The effects of various reaction conditions on the cyclic CO2 capture characteristics of the hollow microtubular Ca/Al sorbent were studied.The structural evolution characteristics of the hollow microtubular Ca/Al sorbent were analyzed.The CO2 capture mechanism of the hollow microtubular Ca/Al sorbent was clarified.The high CO2 capture performance of the hollow microtubular Ca/Al sorbent is due to the special microstructure and good support of Ca12Al14O33 which provides more CO2 active sites.After 10 cycles,the reaction rate constant of the hollow microtubular Ca/Al sorbent is more than 3 times higher than that of carbide slag and 2 times higher than that of the hollow microspheric Ca/Al sorbent.Under harsh calcination conditions,the CO2 capture capacity of the hollow microtubular Ca/Al sorbent with the Al2O3 content of 7.5%is 0.33 g/g after 30 cycles,which is 16%and 112%higher than those of the hollow microspheric Ca/Al sorbent and carbide slag,respectively.The effect of Ca12Al14O33 in the Ca/Al sorbent on the sintering resistance of CaO was revealed at the microscopic atomic level by DFT.The crystal structures and surface properties of CaO and Ca12Al14O33,the bonding rule between CaO and Ca12Al14O33.the adsorption energy and the electronic structure were systematically analyzed.The effect of steam in the carbonation atmosphere on the adsorption of CO2 by the Ca/Al sorbent was studied at the atomic level.The simulation results show that the reason why Ca12Al14O33 can retard the sintering of CaO at high temperature lies in the strong interaction between the A1 sites in Ca12Al14O33 and CaO,which effectively restrains CaO movement and inhibits CaO structural deformation.The adsorption of CO2 and H2O belongs to the chemisorption.CO2 adsorption by CaO is stronger than H2O adsorption by CaO.H2O adsorption of CaO leads to the activation of adjacent 0 atoms of CaO and thus stronger CO2 adsorption on the 0 atom with H2O adsorbed.The combined effect of Ca12Al14O33 and H2O promotes the adsorption of CO2 and is conducive to the CO2 capture by Ca/Al sorbents.