Enhanced Elemental Mercury Removal from Coal-Fired Flue Gas by Sulfur−Chlorine Compounds

Oxidation of Hg0 with any oxidant or converting it to a particle-bound form can facilitate its removal. Two sulfur−chlorine compounds, sulfur dichloride (SCl2) and sulfur monochloride (S2Cl2), were investigated as oxidants for Hg0 by gas-phase reaction and by surface-involved reactions in the presence of flyash or activated carbon. The gas-phase reaction between Hg0 and SCl2 is shown to be more rapid than the gas-phase reaction with chlorine, and the second order rate constant was 9.1(±0.5) × 10−18 mL-molecules−1·s−1 at 373 K. The presence of flyash or powdered activated carbon in flue gas can substantially accelerate the reaction. The predicted Hg0 removal is about 90% with 5 ppm SCl2 or S2Cl2 and 40 g/m3 of flyash in flue gas. The combination of activated carbon and sulfur−chlorine compounds is an effective alternative. We estimate that co-injection of 3−5 ppm of SCl2 (or S2Cl2) with 2−3 Lb/MMacf of untreated Darco-KB is comparable in efficiency to the injection of 2−3 Lb/MMacf Darco-Hg-LH. Extrapolation of kinetic results also indicates that 90% of Hg0 can be removed if 3 Lb/MMacf of Darco-KB pretreated with 3% of SCl2 or S2Cl2 is used. Mercuric sulfide was identified as one of the principal products of the Hg0/SCl2 or Hg0/S2Cl2 reactions. Additionally, about 8% of SCl2 or S2Cl2 in aqueous solutions is converted to sulfide ions, which would precipitate mercuric ion from FGD solution.