Enhancement Of MgO Reaction Reactivity And Thermal Storage Performance At Elevated Temperature And Pressure And Simulation Study

Greenhouse gas emission is aggravated with the increasing usage of fossil fuels.To control CO₂ emissions,renewable energy power generation technology and carbon capture and storage have received extensive attention.MgO is promising for pre-combustion CO₂ capture and the thermochemical energy storage in direct contact with the working fluid of the supercritical carbon dioxide Brayton cycle due to its abundance,low cost,environmental friendliness,high theoretical capture capacity and energy storage density.However,the application of MgO is limited by its slow carbonation kinetic,which is mainly caused by the poor mass transfer performance.Therefore,in this work,a series of doping experiments and theoretical investigation were performed,aiming at the enhancement of the MgO reactivity and cyclic stability under various reaction conditions ranging from medium temperature and high pressure to supercritical carbon dioxide.This work can lay a foundation for the development of advanced MgO-based CO₂ capture and thermal energy storage technologies.Alkali metal carbonate doping is an effective strategy to enhance the reactivity of MgO through increasing basic sites.In this work,the effects of preparation methods,doping amounts,sorption conditions and ankali metal carbonate species on the MgO conversion and cyclic stability of samples were studied.The highest and stable MgO conversion（0.32）was obtained during the 30-cycle tests at 400℃,2MPa,where the material exhibited a strong skeleton structure after repeated cycles by the microstructure analysis when prepared by the ball-milling method.With the increase of Na₂CO₃ content in the mixture,a higher MgO conversion can be achieved,but the cyclic stability will be impaired.The density functional theory（DFT）calculation indicated that an appropriate amount of Na₂CO₃ doping can increase active sites.The oxygen vacancy defect generated by Na doping was found to help induce electrons from the Na and Mg atoms to the CO₂ molecule.On the other hand,the electrostatic interaction between Na₂CO₃and CO₂ also enhanced the adsorption ability of O_MgO atoms to CO₂.In addition,the cyclic stability of different carbonate doped samples was tested with increasing sorption conditions.The results indicated that the MgO-Na₂CO₃ showed a stable MgO conversion of 0.3 after 20 cycles under the severe sorption condition of 500℃,3 MPa,but the MgO conversion of MgO-Li₂CO₃ decreased from 0.46 to 0.25 after 20 cycles.Alkali nitrates doping can improve the reactivity of MgO by preventing the formation of the rigid product layer.However,it suffered from an slow sorption rate at the beginning stage.Herein,Na₂CO₃ was further added after nitrate doping.The results indicated that its reaction rate was significantly improved by doping Na₂CO₃,and the MgO conversion was up to 0.62 after 4 min of reaction at 400℃,2 MPa by doping 10mol%Na₂CO₃ into MgO-20wt%KNO₃.DFT calculations revealed that the CO₃^2-not only provided chemisorption sites but also served as the carrier to promote CO₂ diffusion in the melted ion liquid through the formation of C（CO₂）-O(CO₃^2-)bonds during the reaction process,thus increasing the reaction rate.In addition,the effect of different nitrate doping on the cyclic performance was compared at 400°C,2 MPa.All samples exhibited excellent cyclic stablity.The MgO conversion of the sample with KNO₃,Na NO₃ and Li NO₃ doping were corresponding to 0.86,0.63 and0.45 after 30 cycles,respectively.The excellent performance of the sample with KNO₃ doping was attributed to the lattice distortion of Na₂CO₃ according to the crystal structure analysis,which leaded to lattice defects of the product layer thus improving the MgO conversion.With the increase of the sorption parameter to540°C,5 MPa,the sorbent exhibited a significant decay in the MgO conversion during the cyclic test due to the sintering problem.The nitrate doped sample suffered from decreased reaction activity under the severe condition due to the sintering problem.Herein,we selected the（Li-Na-K）CO₃with melting point of 397℃as promoter for MgO application under severe operating conditions,as it can enhance the mass transfer and cyclic stability perforamnce through maintaning the porous structure.The effects of doping amounts and reaction conditions on the cyclic stability were investigated.The sample presented a low MgO conversion at the beginning cycle,whose reactivity needs to be activated through repeated adsoption/desorption process.The MgO converison of the sample with10wt%,20wt%and 33wt%（Li-Na-K）CO₃ doping were corresponding to 0.6,0.83 and0.92 after 30 cycles at 400℃,2 MPa,respectively.Moreover,the activation duration decreased with increased doping amounts.The DFT calculations revealed that the Li Na K doping lowered the formation energy of surface oxygen vacancy and the oxygen ion migration energy barrier.Therefore,it can be inferred that large doping amount benefits the penetration of Li Na K into MgO,thus increasing the activation rate of MgO.With the further elevation of operating parameters to 540°C and 5 MPa,the sample still showed a stable MgO conversion of 0.69 after 30 cycles.For the application of MgO in the thermochemical energy storage in direct contact with the working fluid of the supercritical carbon dioxide Brayton cycle,we made a screening of molten salts with good thermal stability as dopants,including the eutectic chloride,sulfate and carbonate molten salts.The cyclic stability and thermal storage performance of the samples were tested in the high-pressure reactor.The results indicated that the samples with（Li-K）CO₃ and（Li-Na-K）CO₃ doping showed a good cyclic stability under the sorption condition of 610℃and 14 MPa,whose MgO conversion remained at 0.39 and 0.42 after 30 cycles,respectively,and the corresponding energy storage density was 889.53 and 954.48 k J/kg,respectively.However,the samples with Na Cl-Ca Cl₂,（Li-K）SO₄ and（Li-Na-K）SO₄ doping exhibited poor cyclic stability after 20 cycles with the MgO conversion decreasing from 0.44,0.32,0.34 to 0.11,0.14,0.12,respectively.The phase analysis revealed that the Ca Cl₂ hydrate was decomposed during the cyclic test,leading to a significant decay in MgO conversion.The eutectic sulfate and carbonate molten salts possessed a good thermal stability during the cyclic test.Additionally,the effect of the magnesite with high mechanical strength used as precursor on the cyclic performance was investigated,and the MgO conversion as was only about 0.1 after 10 cycles at610℃,14 MPa despite its strong sintering-resistance property.