Understanding the Impacts of Different Impurities on Elemental Mercury Removal by CaS in Chemical Looping Combustion...

CaSO₄ has the advantages of abundant yield, high oxygen-carrying capacity, low cost, and no heavy metal pollution, making it promising as an oxygen carrier for chemical looping combustion (CLC). In comparison with other oxygen carriers, CaS as the reduced product of CaSO₄ exhibits superior adsorption efficiency for Hg⁰ in the flue gas. In this paper, density functional theory (DFT) was used to investigate the adsorption mechanism of Hg⁰ on the adsorbent surface of CaS(001). The adsorption energies of different oxidized mercury species such as HgS, HgCl, and HgBr over the CaS surface were summarized. Furthermore, the effects of various flue gas components including SO₂, H₂S, S, HCl, Cl₂, CO, H₂, H₂O, and C on Hg⁰ adsorption over the CaS(001) surface were evaluated. The results show that Hg⁰ can be adsorbed on the CaS(001) surface in a chemisorption manner with a reaction energy of -65.1 kJ/mol. The adsorption energy of different forms of mercury on the CaS(001) surface varies greatly, and mercury in the oxidized state is more easily captured by CaS. SO₂ inhibits while other flue gas components promote Hg⁰ adsorption over the CaS surface. Overall, CaS tends to adsorb mercury in the reduction reactor and release mercury when CaS is re-oxidized to CaSO₄ in the oxidation reactor. This is detrimental to mercury removal in the CLC of coal. This study sheds light on the migration and transformation of mercury in the CLC of coal with CaSO₄ as the oxygen carrier.