| The goal of carbon peaking and carbon neutrality is an important guarantee for China to achieve sustainable development.Carbon Capture,Utilization and Storage(CCUS)is an important way to deeply reduce CO2 emissions.Chemical-Looping Combustion(CLC)is a CCUS technology with superior performance and cost effectiveness.According to China’s "coal-rich,oil-poor,gas-poor" energy structure,coal chemical looping combustion technology is of great significance to the clean utilization of coal and environmental protection.Most of the elemental mercury in coal is released in the gaseous form of Hg0 and Hg2+ during the chemical looping combustion.Oxygen carriers are widely studied as carriers of oxygen and heat transfer in chemical looping combustion,and also have the ability to adsorb and oxidize Hg0.The spinel configuration of MnFe2O4 oxygenate carrier is typical.By studying the adsorption and oxidation of Hg0 by MnFe2O4 oxygen carriers in the presence of oxygen vacancies during the reduction process,the competitive adsorption of SO2 with Hg0 and the migration mechanism of oxygen in the lattice of MnFe2O4 oxygen carriers,we can provide a theoretical basis for improving the performance of manganese-iron-based oxygen carriers for mercury removal.(1)Periodic flat plate models of intact and defective MnFe2O4(100)surfaces were constructed for Hg0 adsorption on different MnFe2O4(100)surfaces.The results indicate that the presence of oxygen vacancies promotes the adsorption of Hg0,and Hg0 receives more charge transferred from oxygen carriers;The results for the transition state show that the oxidation mechanism of Hg0 on the surface of MnFe2O4(100)is a MvK mechanism and the presence of oxygen vacancies reduces the reaction energy barrier for Hg0 oxidation.(2)A reasonable SO2 structure model was established,multiple adsorption configurations were considered,and the structure optimization revealed that SO2 molecules would gradually move to nearby metal sites to form a stable adsorption structure.The results of the differential charge density show that there is an electron transfer between Mn-O,the highest adsorption energy sites for both SO2 and Hg0 are at the Obltop site,and the adsorption energy of SO2 is greater than that of Hg0 on the surface of MnFe2O4(100).SO2 may adversely affect the adsorption of Hg0 on MnFe2O4.(3)Structural models of the surface lattice oxygen-oxygen vacancies and deep lattice oxygen vacancies of MnFe2O4 oxygen carriers were constructed.The lattice oxygen migration process starts with the target oxygen atoms in the fourth layer gradually entering the lattice between the atoms in the fourth and third layers,causing the Mn-O and Fe-O bonds of the Mn and Fe atoms adjacent to the fourth layer to elongate and eventually break with the O atoms,leaving an oxygen vacancy in the fourth layer,after which the O atoms continue to move up and bond with the Fe atoms in the second layer and gradually migrate to the oxygen vacancy in the second layer. |
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