Modeling and experimental studies for the control of mercury from coal combustion flue gas streams using cupric chloride-impregnated sorbents.

摘要

Coal-fired boilers account for 46% anthropogenic mercury emissions in the United States, and the dispersion modeling conducted in this study also suggested significant mercury depositions from coal-fired emissions. This study examined the Hg removal capability and investigated the Hg 0 oxidation and adsorption mechanisms of mercury sorbents. A brominated activated carbon (Darco Hg-LH) and cupric chloride-impregnated sorbents (CuCl 2 sorbents) were tested in fixed-bed, entrained-flow, and filter-added entrained-flow systems to evaluate the performance of those sorbents in Hg removal. The test results in the fixed-bed system showed that those sorbents have enough Hg0 removal capacity to demonstrate good performance in full-scale systems. The fixed-bed results also stressed the importance of testing those sorbents in an entrained-flow system to simulate inflight mercury capture in the full-scale ductwork. Therefore, an entrained-flow system was constructed and operated with typical conditions in the duct of coal-fired exhaust gas. In addition, the sorbent deposition on the reactor wall which can be found in the entrained-flow system rather than full-scale systems was significantly reduced, and additional mercury capture by the deposited sorbent was minimized. One of the CuCl2-impregnated activated carbons (CuCl 2-ACs) demonstrated very similar performance in mercury removal to Darco Hg-LH except for slightly higher Hg0 oxidation. Significant amounts of outlet oxidized mercury were found from the entrained-flow tests of both sorbents which indicate that Hg adsorption onto those sorbents is limited by the re-adsorption of the resultant oxidized mercury. In addition, the entrained-flow results of all tested sorbents also indicated that Hg adsorption may be limited by the external mass transfer resistance. While significant amounts of outlet oxidized mercury were found from all entrained-flow tests of Darco Hg-LH and CuCl2-ACs, very small amounts of oxidized mercury were found in the outlet of the filter-added entrained-flow system. Based on the test results, Hg0 oxidation and adsorption mechanisms of CuCl2-ACs were described, and the performance of mercury sorbents in full-scale systems was predicted. In addition, the mathematical model developed following the suggested Hg0 oxidation and adsorption mechanisms showed consistent results with the fixed-bed and entrained-flow test results.