Mercury emissions control is of great importance in environment protection as well as public health. Current mercury emissions control technologies are not well designed nor optimized, mainly due to the lack of fundamental understanding of adsorption and/or catalytic mechanisms and necessary kinetic modeling and reliable simulation data. This work aims to advance the fundamental mechanistic understanding of heterogeneous catalytic oxidation reaction and adsorption by using the reaction between Hg(0) vapor and CuCl2 and the subsequent adsorption of resultant oxidized mercury onto sorbents.
XANES and EXAFS were used to determine mercury compounds formed on AC sorbents. The XANES study on raw and CuCl2-impregnated AC sorbents suggests that little or no elemental mercury is formed onto any spent sorbents and the chemisorption of Hg(0) vapor is very likely to be the dominant mechanism. HgCl2 is found to be a major oxidation reaction product when CuCl2 and HCl were impregnated onto raw AC regardless of the type of the carrier gas (i.e. N2 or O2).
The adsorption isotherms of HgCl2 on DARCO-HG and CuCl2-impregnated AC were found to be of the Langmuir type. 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. HgCl2 adsorption removal performance was also predicted in an entrained flow system using a modified model.
The CuCl2/¿¿¿¿-Al2O3 catalyst possesses high activity for the oxidation of Hg(0) to Hg2+, with an excellent stability under the environment similar to the flue gas from coal-fired power plants. The CuCl2 crystallites formed onto ¿¿¿¿-Al2O3 were very stable up to 300oC, and undergo the thermal reduction process from Cu(II) to Cu(0) via Cu(I). In the absence of HCl and O2 gases, CuCl2 was found to follow a Mars-Maessen mechanism by consuming lattice chlorine of CuCl2 for Hg(0) oxidation and to be reduced to CuCl. In the presence of 10 ppmv HCl, 2,000 ppmv SO2, and 6% O2 gases, the CuCl2/¿¿¿¿-Al2O3 sample works as an Hg(0) oxidation catalyst exhibiting >90% conversion with good resistance to SO2 at 140oC. The reduced CuCl was able to be re-chlorinated to CuCl2 under HCl and O2 gases by following the Deacon reaction.
Multiple copper species were found to be formed when ¿¿¿¿-Al2O3 is used as a substrate as opposed to one Cu(II) species on ¿¿¿¿-Al2O3. The CuCl2/¿¿¿¿-Al2O3 catalysts with low CuCl2 loading (<3.5 wt%) showed low catalytic performances in mercury oxidation. In contrast, the high loading (i.e. 10 wt%) CuCl2/¿¿¿¿-Al2O3 catalyst showed almost complete Hg(0) oxidization in the presence of 10 ppmv HCl and 6%(v) O2 balanced with N2, regardless of the presence of 2,000 ppmv SO2 gas over 140 hrs of the performance evaluations. CuCl2 is expected to be used as a catalyst and a sorbent by impregnating onto non-carbonaceous and carbonaceous substrates in a temperature window after the air preheater.