Reaction Characteristics And Mechanism Of CO Chemical Looping Combustion Based On CuMn₂O₄ And NiMn₂O₄ Composite Oxygen Carriers

Chemical looping combustion has been regarded as a promising technolygy to realize low-cost CO₂ separation.Satisfactory performance of oxygen carrier is the key of its application.This paper investigates the chemical looping combustion using Cu Mn₂O₄ and Ni Mn₂O₄ as oxygen carrier and CO as fuel.Based on the thermodynamic calculation,thermogravimetric experiments and density functional calculations,the reaction characteristics,microscopic reaction mechanism and synergistic effect of metal atoms have been revealed.The Cu Mn₂O₄ and Ni Mn₂O₄ oxygen carriers prepared by the sol-gel combustion method maintain uniform particle size and abundant pore structure.Thermodynamic calculations show that the reduction of Mn³⁺is prior to Cu²⁺and Mn²⁺,and the reduction of Mn³⁺to Mn is limited.Thermogravimetic analysis show that the the temperature of Cu Mn₂O₄and CO reaction is 265-540^oC.The reaction temperature of Ni Mn₂O₄ and CO are 355-536^oC and 536-804^oC,corresponding to the reduction of Mn₂O₃ and Ni O,respectively.Compared with Mn₂O₃,the oxygen carrying capacity of Cu Mn₂O₄ and Ni Mn₂O₄increased by 53.12%and 46.52%,respectively.The reduction rate of oxygen carriers is significantly improved.The addition of Cu and Ni increase improve the reaction activity.Cu Mn₂O₄ can be reduced to Cu and Mn O by CO,and Ni Mn₂O₄ can be reduced to Ni and Mn O.The reduced oxygen carrier still obtains good pore structure.The effects of reaction temperature,CO concentration and heating rate on the reaction characteristics of Cu Mn₂O₄ and Ni Mn₂O₄ with CO were investigated.The reduction rate of oxygen carrier is improved with the increasing of temperature.Cu Mn₂O₄ oxygen carrier can sufficient reacts with CO even at 500^oC.The reaction temperature of Ni Mn₂O₄ and CO is recommended to be no less than 800^oC.The reaction temperature range of oxygen carrier reduction shifts to lower temperature with CO concentration increase,and the reduction rate significantly improved.For the Ni Mn₂O₄,there is no obvious effect on the reduction of Ni O with CO concentration increase.The increase of heating rate makes the reduction rate of the oxygen carrier improved and the reduction temperature shifts to higher temperature.Density function calculation is used to investigate the oxidation mechanism of CO on oxygen carriers and reveal the mechanism of synergetic effect.The CO oxidation on the Cu Mn₂O₄（100）surface includes CO adsorption,CO₂ formation and CO₂desorption.The rate-limiting-step of the CO oxidation is the CO₂formation.Due to the high reactivity of Cu,the CO oxidation on Cu-terminated surface has lower activation energy of 58.83k J/mol than that of Mn-terminated surface（64.59k J/mol）.The oxidation of CO on the Ni Mn₂O₄（100）surface is divided into two steps:the formation of CO₂ and the desorption of CO₂.The activation energies corresponding to the three reaction paths are 18.2,36.53,and 67.71k J/mol,respectively.The oxygen that bonds with two Ni atoms have the highest reactivity,indicating that the presence of Ni enhances the reactivity of Mn-based oxygen carriers.The calculated results illustrate the synergistic effect between Cu,Ni and Mn-based oxygen carriers at the microscopic level,and are in good agreement with the experiment results.