Effect of Mn-doped CaO on NO removal by CO in carbonation of calcium looping for CO2 capture in a fluidized bed reactor

Abstract

By utilizing CO as NO reductant and CaO as the catalyst, NO removal by CO can be realized in the carbonation stage of calcium looping for CO2 capture, but the carbonation of CaO decreases the catalytic effect on NO removal. To realize efficient and stable NO removal in the carbonation stage, CaO was modified by Mn in this work. NO removal performance of CO in the carbonation stage of Mn-doped CaO in the calcium looping process was investigated in a bubbling fluidized bed reactor. In the carbonation stage, NO removal efficiency sharply increases from 0% to 99% and CO2 capture efficiency increases from 73% to 88% due to the presence of Mn. Compared with CaO, Mn-doped CaO possesses a stronger catalysis on NO removal by CO and higher CO2 capture capacity. MnO in Mn-doped CaO is an excellent and stable catalyst for NO removal by CO, which greatly mitigates the negative effect of the carbonation and the sintering of CaO on NO removal. The molar ratio of Mn/Ca = 3.5:100 is recommended. To achieve efficient NO removal and CO2 capture in the carbonation stage of Mn-doped CaO, CO concentration in the range of 3000–4000 ppm and carbonation temperature of 650 °C are recommended. Under severe calcination conditions (950 °C and 80% CO2), CO2 capture and NO removal efficiencies reach 85% and 99% in 10 cycles, respectively. The periodic density functional theory calculations show that the catalytic active sites for NO removal by CO are changed from O-tops to Mn-tops in Mn-doped CaO, so the adsorptions of NO and CO2 are greatly enhanced, promoting the subsequent simultaneous NO removal and CO2 capture. Thus, Mn is a bi-functional additive to promote CO2 capture by CaO and catalyze NO removal by CO. Mn-doped CaO seems promising for efficient simultaneous NO removal and CO2 capture in the carbonation stage of the calcium looping process.