The Homogeneous Forcing of Mercury Oxidation to Provide Low-Cost Capture

Oxidized mercury formed in combustors (e.g., HgCl{sub 2}) is much more easily captured in existing pollution control equipment (e.g., wet scrubbers for SO{sub 2}) than elemental mercury. This is principally due to the high solubility of the oxidized form in water. Work over the last several years in our laboratory and elsewhere has identified the general outlines of the homogeneous chemistry of oxidation. The goal of the work reported here is to make use of this knowledge of the oxidation mechanism to devise simple and inexpensive ways to promote the oxidation. The hypothesis is that simple fuels such as hydrogen or CO can promote oxidation via the free radicals they generate during their decomposition. These free radicals then promote the formation of Cl from HCl via reactions such as OH+HCl {yields} H{sub 2}O+Cl. The Cl (and Cl{sub 2} derived from Cl recombination) are considered the principal oxidizing species. In our studies, mercury vapor is exposed to HCl under isothermal conditions in a gas containing N{sub 2}, O{sub 2}, and H{sub 2}O. The experiments systematically explore the influence of reaction temperature, HCl concentration, and H{sub 2}O concentration. These baseline conditions are then perturbed by the addition of varying amounts of H{sub 2}, CO, and H{sub 2}/CO added jointly. The following report presents the results of a literature review associated with the dissertation of the student supported by the program. This outlines the state-of-the-art in mercury behavior. It then describes the experimental facilities and the results of tests involving the promotion of the oxidation reaction by H{sub 2}, CO, and H{sub 2}/CO combinations. These results indicate a substantial enhancement of oxidation under isothermal conditions at 900-1000 K, while the additives inhibit oxidation at 1200 K. The next step is to determine whether the existing chemical kinetic models of mercury oxidation are capable of reproducing this behavior. These models can then be used to extrapolate the findings to nonisothermal conditions typical of boiler environments. This would provide guidance on where to inject the oxidation promoters in a practical boiler, and how much promoter is required