DFT study on NO reduction with CO catalyzed by CaO in carbonation stage of calcium looping process

CHU Leizhe; CHAI Shoubing; LI Yingjie

doi:10.13225/j.cnki.jccs.2022.1754

Abstract：Global CO2 concentration rapidly increases over the past few centuries,particularly in recent decades,and the greenhouse effect is sharply accelerated by the increase of CO2 emissions.CO2 emissions are mainly produced from fossil fuel combustion,in particular coal-fired power plants.Calcium looping,i.e.the carbonation/calcination cycles of CaO,is one of the most promising CO2 capture technologies for coal-fired power plant.At the same time,coal-fired power plants are also one of the main sources of NOxemissions.CO can be used as a reducing agent for NO due to its low cost and low toxicity.CaO is a suitable catalyst for NO reduction by CO.The combustion of biomass is used to provide the required en-ergy for the decomposition of CaCO3 in the calciner.Few unburned char and CaO flow into the carbonator,where the un-burned char reacts with O2 in the flue gas to generate CO.Thus,the simultaneous NO removal and CO2 capture in the car-bonator of the calcium looping process can be realized.However,the reaction mechanism of the effect of CaO on NO re-moval by CO in the carbonation stage is unclear.The mechanism of NO reduction with CO catalyzed by CaO in the car-bonation stage is investigated by the density functional theory(DFT)calculations.The structural,adsorption,and reaction path parameters,including atomic layout,bond length,adsorption energy,and energy barrier of CO and NO molecules on the CaO(100)surface,are determined.The optimal adsorption sites of CO and NO molecules on the CaO(100)surface are O-top sites,and the adsorption energies are-0.35 and-0.79 eV,respectively.The CO and NO adsorption energies on the CaO(100)surface in the presence of CO2 are-0.36 and-0.20 eV,respectively.The co-adsorption of CO and NO mo-lecules appears feasible on the CaO(100)surface,while CO2 exhibits obvious inhibition for the co-adsorption.The reac-tion path of NO reduction by CO on the CaO(100)surface undergoes two elementary reaction stages:CO2 formation and adsorption,and N2 generation.Finally,one N2 molecule and three CO32-ions are formed,with a total energy barrier of 11.08 eV.CO2 formation and adsorption(R→IM1→IM2)are the rate-determining steps of the whole reaction process.