| Coal-Fired utility boilers are currently the largest single-known source of anthropogenic mercury emissions in the United States. Estimates of total mercury emissions from coal combustion processes are 40--52 tons. In this research, the potential of gas-phase chelating sorbents for the removal of mercury vapor from flue-gases emitted from coal-fired power plants has been investigated. Chelating adsorbents are currently limited to the removal of mercury from liquid (aqueous) phase.; The novel gas-phase chelating adsorbents that have been developed in this research uniquely combine a chelating ligand with an ionizing surface nano-layer on a mesoporous substrate. This enables selective, multidentate adsorption of mercury directly from the gas phase. Different chelating ligands including cysteine, dithizone bonded through 3-chloropropyltrimethoxysilane linker (CPTS-DZ), 3--mercaptopropyltrimethoxysilane (MPTS-Silica), and 2-mercaptobenzothiazole bonded through 3-aminopropyltriethoxysilane linker (APTS-MBT), were immobilized on silica substrates. The synthesis of the adsorbents is described, and detailed characterization data including FTIR, elemental analysis, BET surface area and pore size distribution, ThermoGravimetric Analysis (TGA), and mass spectroscopy are reported. The theoretical (equilibrium) capacity for mercury removal using these adsorbents is in the range of 17--117 mg Hg/g.; Evaluation of the thermal stability of the chelating adsorbents showed that cysteine activated adsorbent is stable at operating temperatures lower than 135°C. This somewhat limits its application in the flue gas stream downstream of the particulate collector. The adsorbents with higher operating temperature limits (CPTS-DZ, MPTS-Silica, and APTS-MBT) can operate up to 160°C. This enables their use immediately downstream of the particulate collectors without the need for gas cooling, as is required for the cysteine adsorbent.; Coating of the silica substrate with different ratios of the room-temperature molten salt, methylpolyoxyethylene(15)octadecanammonium chloride (MEC), was performed to determine the optimum coating conditions. It was found that MEC concentration in the range of 20--30 wt% maintain high accessible surface area for mercury chelation. The capability of the MEC ionic solvent to absorb mercury vapor and ionize it has also been studied. MEC solvent has high solubility for mercuric chloride (HgCl2), but limited solubility for elemental mercury (Hg0). (Abstract shortened by UMI.)... |
