(632c) Mercuric Chloride Vapor Adsorption On Cupric Chloride-Impregnated Activated Carbon Sorbents

Carbon sorbent injection is the most promising control technology in reducing mercury emissions from coal-fired power plants.  CuCl₂-impregnated activated carbon sorbent has demonstrated excellent mercury oxidation and adsorption performance in our previous studies.  Our previous study using XAFS shows that mercuric chloride (HgCl₂) is a major resultant oxidized mercury compound generated over the CuCl₂-impregnated activated carbon sorbent as a result of the reaction between elemental mercury and cupric chloride.  This study focused on the kinetic adsorption behavior of mercuric chloride vapor onto the CuCl₂-impregnated activated carbon sorbents.

The experiments were performed in our lab-scale fixed-bed system at 140 °C by varying inlet HgCl₂ concentrations in the range of 5~20 ppbv.  Three different sorbents were tested, i.e. raw commercial activated carbon (Norit’s DARCO FGD), 4%(wt) CuCl₂-impregnated activated carbon, and 8%(wt) CuCl₂-impregnated activated carbon.  The adsorption capacities of HgCl₂ onto raw, 4%, and 8% sorbents were 30 mg, 17 mg, 9 mg HgCl₂ per g sorbent, respectively, at 10 ppbv HgCl₂.  The adsorption isotherms of HgCl₂ on DARCO-HG activated carbon and CuCl₂-impregnated activated carbon were found to be of the Langmuir type, and therefore the maximum of HgCl₂ adsorption concentration and the equilibrium constant were correlated based on the Langmuir equation.  The Langmuir adsorption equilibrium constants of three sorbents were calculated to be 2.1×10⁴ m³/g for raw AC, 5.0×10⁴ m³/g for 4% CuCl₂-AC, 26.0×10⁴ m³/g for 10% CuCl₂-AC.  The kinetic adsorption constants were estimated by fitting the model simulation with experimental data.  The breakthrough data from experiments are in good agreement with the calculation results from the modified kinetic model.  The simulation results indicate that pore diffusion resistance significantly increases with an increase in sorbent particle size.  For large particles (> 30 µm), pore diffusion controls the total adsorption rate.  For the sorbents used in this work (dp=15 µm), both pore diffusion and local surface adsorption was found to be a rate-limiting step.