The Synergistic Removal Performance Of NO/CO₂ By Modified Ca-Based Materials During Carbonation Stage Of Calcium Looping

In carbonation stage of calcium looping process,NO could be reduced by CO using CaO as a catalyst,allowing simultaneous CO2 and NO removal.However,after CaO absorbs CO2 to generate CaCO3,the catalytic performance of CaO for NOx reduction by CO decreases drastically.Besides,to achieve efficient NO removal,the required CO/NO molar ratio in the carbonator is very high,which leads to lots of unreacted CO in the exhaust gas.To improve simultaneous NO/CO2/CO removal performance of CaO,CaO doped by Mn/Ce/Cu.The simultaneous NO/CO2/CO removal performance of modified CaO in the carbonation stage of the calcium looping was investigated.The effect mechanisms of modified CaO were revealed by microstructure characteristic analyses and density functional theory calculations.Mn-doped CaO was used to improve NO reduction by CO in carbonation of calcium looping and the mechanism of Mn-doped CaO on NO reduction by CO was determined by the density functional theory.The experimental results show that Mn doping not only improves the CO2 capture capacity of CaO,but also increases NO reduction by CO.Density functional theory calculation results show that the Mn atom provides new adsorption sites for CO and NO on CaO surface,respectively.After addition of Mn,the values of co-adsorption energy for CO and NO on CaO surface increases by 1.5 and 6.9 times in the carbonation stage,respectively.In addition,the total energy barrier for the reaction of NO reduction by CO under the catalysis of CaO in carbonation stage decreases from 11.08 to 8.66 eV due to the addition of Mn.The negative effect of the carbonation of CaO on NO removal by CO is greatly mitigated by Mn.To maintain high efficiency of NO removal at low CO concentration in carbonator,Ce-doped CaO was employed to simultaneously enhance NO and CO2 removal performance.The effects of Cedoped CaO on simultaneous CO2 and NO removal in bubbling fluidized bed reactor were investigated during the carbonation stage of calcium looping process.When the molar ratio of CO/NO is about 3.57:1,Ce-doped CaO obtains an 83%CO2 capture efficiency and a 97%NO removal efficiency in 20 cycles in chemical reaction controlling stage.The average CO concentration in exhaust gas of Cedoped CaO is reduced by 79%at the same NO removal efficiency as Mn-doped CaO.The presence of Ce3+makes the charge imbalance of the material,resulting in more charge transfer from the Cedoped CaO surface to the molecules.The density functional theory reveals that Ce doping reduces the energy barriers of the conversion of NO to N2 and the CO2 adsorption,promoting the simultaneous NO/CO2 removal in carbonation stage.Synergistic NO,CO,and CO2 removal performance of Cu/Ce co-doped CaO in the carbonation stage of the calcium looping process was tested in a bubbling fluidized bed reactor.At the optimal Cu/Ce/Ca molar ratio of 1:5:100,the NO removal efficiency,CO removal efficiency,and CO2 capture efficiency in the chemical reaction controlling stage of the 20th cycle using the Cu/Ce-CaO are 99%,99%,and 80%,respectively.The synergistic effect of Cu/Ce in the Cu/Ce co-doped CaO promotes simultaneous NO,CO,and CO2 removal,resulting in excellent cyclic stability during the repeated calcination/carbonation cycles.On the one hand,Cu doping provides new active sites,easing the competition between CaO carbonation and catalytic NO-CO reaction,enabling Cu/Ce-CaO to stably and efficiently catalyze NO reduction by CO while carbonation;On the other hand,combining with CeO2 lattice can overflow oxygen in the deep lattice of the material,increasing oxygen vacancies and catalytic sites,which is conducive to the diffusion of O2-,thereby enhancing the catalytic activity of Cu/CeCaO in the carbolation stage of the calcium looping.In addition,the diffusion of O2-in the material surface and bulk phase is conducive to the formation of the intermediate product CO32-and to the absorption of CO2 by CaO.At the same time,the low energy barrier allows for electron transfer between Ce3+/Ce4+and Cu2+/Cu+,promoting the improvement of the catalytic ability of Cu/CeCaO.Therefore,Cu/Ce-CaO exhibits excellent synergistic removal performance of NO,CO,and CO2 during the carbonation stage of the calcium looping.