Role of CO2 on trace arsenic and selenium separation from coal-fired flue gas by CaO

Abstract

Coal-fired power plants have been recognized as a major source of arsenic and selenium emissions. Considering that the common temperature window exists, in which both As2O3, SeO2 and CO2 can be effectively captured by CaO-based adsorbents, new principles of CO2 presence affecting arsenic and selenium removal process were investigated by density functional theory (DFT) calculations. The results confirm the suppression role of CO2 on As2O3/SeO2 adsorption by CaO (100) surface. On the basis of single-molecule adsorption on CaO, the adsorption energies of As2O3 and SeO2 are −2.21 eV and −2.05 eV, which are stronger than that of CO2. Three molecules can be chemically adsorbed on CaO with the surface O atom as active sites, leading to the competition for adsorption sites on the CaO surface. Despite the higher preferential adsorption of As2O3 and SeO2 on CaO (100) than CO2, the presence of CO2 weakens the electron transfers from the surface O atoms to As and Se atoms in the co-adsorption configurations. Under the atmosphere of high-concentration CO2, the adsorption of CO2 on CaO surface primarily changes the original electronic field of CaO (100) surface, and abolishes the function of O sites for As2O3/SeO2 adsorption at the final stage of complete carbonation. As2O3 and SeO2 adsorb on the fully carbonated CaO surface by physisorption. The bond population and density of states verify that the presence of CO2 weakens the covalent bonds between the surface O atoms and As/Se atoms, and brings about the intermolecular repulsion in the following As2O3/SeO2 adsorption. In addition, the negative effect of CO2 on arsenic capture by CaO is more pronounced than that on selenium capture. Finally, four mechanisms including isolation mode, competition mode, low-degree carbonation mode and high-degree carbonation mode are determined to gain further insight into the structure-reaction correlation during arsenic and selenium removal from the microscopic aspect.