Energy & Fuels, Vol.34, No.2, 2355-2365, 2020
Ranking Oxygen Carriers for Elemental Mercury Oxidation in Coal-Fired Chemical-Looping Combustion: A Thermodynamic Approach
Oxygen carriers in coal-fired chemical looping combustion can oxidize elemental mercury, but there are still no specific criteria to rank the efficiencies of Hg-0 oxidation by classification of different oxygen carriers. Both an experimental approach and thermodynamic modeling were adopted to evaluate the Hg-0 oxidation efficiencies in the case of eight commonly used oxygen carriers, applying a temperature range from 800 to 1000 degrees C. The efficiency of the Hg-0 oxidation process in experiments within this temperature range was found to correspond well with the amounts of Cl/Cl-2 in the calculated reaction products. Results have shown that the sequence of the amounts of Cl/Cl-2 for different oxygen carriers was as follows: CaSO4 > Co3O4 (Mn2O3) > Fe2O3 > CuO (CeO2) > SiO2 (Al2O3). This sequence was in agreement with the simulations of the experiments investigating the efficiency of Hg-0 oxidation. According to thermodynamic calculations, the oxidation mechanism of Hg(0 )can be classified into two categories: MxOy- based mechanisms (Fe2O3 , CuO, Co3O4, Mn2O3, CeO2, Al2O3, and SiO2) and CaSO4-based processes. Hg-0 oxidation by MxOy follows three reaction pathways. In the first instance, Hg-0 can be oxidized by Cl-2 to form HgCl2; in the second reaction pathway, Hg-0 can be oxidized by Cl to form HgCl, which is oxidized afterward by Cl/Cl-2 to HgCl2. In the third reaction pathway, Hg-0 is oxidized by oxygen atoms to HgO, which is then oxidized by Cl2O to form HgCl2. In comparison, the oxidation of Hg-0 by CaSO4 is different from that of other oxygen carriers with existing additional reaction pathways. Hg-0 is converted first to HgS before being oxidized to HgCl2. The approach in this study may be used for choosing the optimum oxygen carriers for Hg-0 oxidation in coal-fired chemical-looping combustion processes.