Simultaneous NO/CO2 removal performance using Ce-doped CaO in calcium looping process: Experimental and DFT studies

Abstract

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, to achieve efficient NO removal, the required CO/NO molar ratio in the carbonator is very high (up to 30:1), which leads to lots of unreacted CO in the exhaust gas. In this work, 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 Ce-doped 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 Ce-CaO was 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 Ce-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. Therefore, under a low molar ratio of CO/NO in carbonation stage of calcium looping, Ce considerably improves the simultaneous CO2 and NO removal action of CO and CaO.