Oxygen-Induced Elemental Mercury Oxidation in Chemical Looping Combustion of Coal

Mercury emission is an important issue during chemical looping combustion (CLC) of coal. The aim of this work is to explore the effects of different flue gas components (e.g., HCl, NO, SO2, and CO2) on mercury transformation in the flue gas cooling process. A two-stage simulation method is used to reveal the reaction mechanism of these gases affecting elemental mercury (Hg-0) oxidation. Furthermore, using this method, Hg-0 oxidation by eight oxygen carriers (Co3O4, CaSO4, CeO2, Fe2O3, Al2O3, Mn2O3, SiO2, and CuO) commonly used in CLC are investigated and their Hg-0 oxidation efficiencies were compared with the existing experimental results. The results show that HCl, NO, and CO2 promote Hg-0 oxidation during flue gas cooling, while SO2 inhibits Hg-0 oxidation. The stronger the oxygen release capacity of oxygen carriers, the higher the oxidation efficiency of Hg-0 becomes. The order of Hg-0 removal efficiency from high to low is Co3O4, CuO, Mn2O3, CaSO4, Fe2O3, CeO2, Al2O3, and SiO2, and this sequence is in good agreement with the existing experimental results. Different flue gas components directly or indirectly affect the O-2 content, thus affecting the content of gaseous oxidized mercury (Hg2+). Different oxygen carriers have different oxygen release capacities and different Hg-0 oxidation efficiencies. Therefore, O-2 is the core species affecting the mercury transformation in CLC.