Bench-scale testing of elemental mercury (Hg0) sorption on selected activated carbon sorbents was conducted to develop a better understanding of the interaction among the sorbent, flue gas constituents, and Hg0. The results of the fixed-bed testing under simulated lignite combustion flue gas composition for activated carbons showed some initial breakthrough followed by increased mercury (Hg) capture for up to approximately 4.8 hr. After breakthrough, the Hg in the effluent stream was primarily in an oxidized form (>90%). Aliquots of selected activated carbons were exposed to simulated flue gas containing Hg0 vapor for varying time intervals to explore surface chemistry changes as the initial breakthrough, Hg capture, and oxidation occurred. The samples were analyzed by X-ray photoelectron spectroscopy to determine changes in the abundance and forms of sulfur, chlorine, oxygen, and nitrogen moieties as a result of interactions of flue gas components on the activated carbon surface during the sorption process. The data are best explained by a competition between the bound hydrogen chloride (HCl) and increasing sulfur [S(VI)] for a basic carbon binding site. Because loss of HCl is also coincident with Hg breakthrough or loss of the divalent Hg ion (Hg2+), the competition of Hg2+ with S(VI) on the basic carbon site is also implied. Thus, the role of the acid gases in Hg capture and release can be explained.

A bench-scale entrained-flow reactor was used to extract flue gas produced by burning a subbituminous Belle Ayr coal in a 580-MJ/h combustion system. The reactor was operated at 400, 275, and 150 °C and a flow rate corresponding to residence times of 0–7 s. Elemental mercury (Hg0) and total gas mercury (Hggas) concentrations in the reactor were measured using an on-line Hg analyzer. At 400 and 275 °C, approximately 30% of the Hg0 released from the Belle Ayr coal was converted to Hg2+ in the 580-MJ/h combustion system. This conversion occurred rapidly at >400 °C as evidenced by similar Hg0 concentrations at 275 and 400 °C and the lack of Hg0 to Hg2+ conversion measured in the reactor at residence times of 0.5–7 s. Conversions of Hggas to particle-associated mercury Hg(p), and Hg0 to Hg2+ and/or Hg(p) were detected in the reactor at 150 °C. The reaction order and rate constant for Hggas and Hg0 conversions at 150 °C were 4.62 and 1.73 and 9.8×10−5 μg−3.62/(m−10.86 s) and 5.1×10−2 μg−0.73/(m−2.19 s), respectively.

The emission of elemental mercury in the flue gas from coal-burning power plants is a major environmental concern. Control technologies utilizing activated carbon show promise and are currently under intense review. Oxidation and capture of elemental mercury on activated carbon was extensively investigated in a variety of flue gas atmospheres. Extensive parametric testing with individual and a variety of combinations and concentrations of reactive flue gas components and spectroscopic examination of the sulfur and chlorine forms present before and after breakthrough have led to an improved model to explain the kinetic and capacity results. The improved model delineates the independent Lewis acid oxidation site as well as a zig-zag carbene site on the carbon edge that performs as a Lewis base in reacting with both the oxidized mercury formed at the oxidation site and with the acidic flue gas components in competing reactions to form organochlorine, sulfinate, and sulfate ester moieties on the carbon edge.