Mechanism of steam-declined sulfation and steam-enhanced carbonation by DFT calculations

Sulfur dioxide (SO2) and carbon dioxide (CO2) removals are of great significance for fossil fuel combustion, where they can be simultaneously captured by calcium-based absorbents. Nevertheless, the CO2 uptake capacity declines with SO2 partial pressures. This paper aims at explaining the mechanisms of steam-declined sulfation and steam-enhanced carbonation by density functional theory calculations. CaO(001) surface is chosen as the absorbent, and the transition state is calculated to obtain the desorption barrier energy of the adsorbates. By analyzing the desorption of the adsorbate on pristine CaO(001) surface and the CaO(001) surface that has adsorbed other adsorbate, it can be concluded that SO2 adsorption inhibits CO2 adsorption since the barrier energy of CO2 desorption on SO2-CaO(001) surface (24.15 kJ mol(-1)) is less than CO2 desorption on CaO(001) surface (129.52 kJ mol(-1)). By comparing the coadsorption energy of the two adsorbates with the sum of the adsorption energy of each adsorbate, it is practical that the H2O adsorption inhibits SO2 adsorption because the calculated coadsorption energy (-221.27 kJ mol(-1)) is larger than the sum of H2O adsorption energy (-100.00 kJ mol(-1)) and SO2 adsorption energy (-194.37 kJ mol(-1)). However, the calculated coadsorption energy of H2O and CO2 adsorption (-254.89 kJ mol(-1)) is less than the sum of CO2 adsorption energy (-144.23 kJ mol(-1)) and H2O adsorption energy (-100.00 kJ mol(-1)), indicating the promotion of CO2 adsorption. Steam in the adsorption process plays the roles of sulfation suppression and carbonation enhancement. (c) 2019 Society of Chemical Industry and John Wiley & Sons, Ltd.