Trace amounts of mercury are found in all coals. During combustion this mercury is vaporized and can be released to the atmosphere. This has been a cause for concern for a number of years, and has resulted in a determination by the EPA to regulate and control these emissions. Present technology does not, however, provide inexpensive ways to capture or remove mercury from flue gases.The mercury that exits the furnace in the oxidized form (HgCl2) is known to much more easily captured in existing wet pollution control equipment (e.g., wet FGD for SO2), principally due to its high solubility in water. Until recently, however, nobody knew what caused this oxidation, or how to promote it. Recent DOE-funded research in our group, along with work by others, has identified the gas phase mechanism responsible for this oxidation. The scenario is as follows. In the flame the mercury is quantitatively vaporized as elemental mercury. Also, the chlorine in the fuel is released as HCl. The direct reaction Hg+HCl is, however, far too slow to be of practical consequence in oxidation. The high temperature region does supports a small concentration of atomic chlorine due to disassociation of HCl. As the gases cool (either in the furnace convective passes, in the quench prior to cold gas cleanup, or within a sample probe), the decay in Cl atom is constrained by the slowness of the principal recombination reaction, Cl+Cl+M  Cl2+M. This allows chlorine atom to hold a temporary, local superequilibrium concentration. Once the gases drop below about 550°C, the mercury equilibrium shifts to favor HgCl2 over Hg, and this superequilibrium chlorine atom promotes oxidation via the fast reactions Hg+Cl+M  HgCl+M, HgCl+Cl+M  HgCl2+M, and HgCl+Cl 2  HgCl2+Cl. Thus, the high temperature region provides the Cl needed for the reaction, while the quench region allows the Cl to persist and oxidize the mercury in the absence of decomposition reactions that would destroy the HgCl2.Promoting mercury oxidation is one means of getting high-efficiency, "free" mercury capture when wet gas cleanup systems are already in place. The chemical kinetic model we developed to describe the oxidation process suggests that oxidation can be promoted by introducing trace amounts of H2 and/or CO within the quench region. The reaction of these fuels leads to free radicals that promote the selective conversion of HCl to Cl, which can then subsequently react with Hg.The work reported here from our Phase I Innovative Concept grant demonstrated this phenomenon, but it also showed that the process must be applied carefully to avoid promoting the recombination of Cl back to HCl. For example, addition of H2 at too high a temperature is predicted to actually decrease Cl concentrations via Cl+H2HCl+H. At lower temperatures this reaction is slowed due to its activation energy. Thus, within the correct window, the process becomes selective for Cl promotion. Key parameters are the injection temperature of the promoter, the amount of the fuel added.A successful process ba...

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Oxidized mercury formed in combustors (e.g., HgCl2) is much more easily captured in existing pollution control equipment (e.g., wet scrubbers for SO2) 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H 2 O+Cl. The Cl (and Cl 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 2 , O 2 , and H 2 O. The experiments systematically explore the influence of reaction temperature, HCl concentration, and H 2 O concentration. These baseline conditions are then perturbed by the addition of varying amounts of H 2 , CO, and H 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 2 , CO, and H 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.

Oxidized mercury formed in combustors (e.g., HgCl2) is much more easily captured in existing pollution control equipment (e.g., wet scrubbers for SO2) 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H 2 O+Cl. The Cl (and Cl 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 2 , O 2 , and H 2 O. The experiments systematically explore the influence of reaction temperature, HCl concentration, and H 2 O concentration. These baseline conditions are then perturbed by the addition of varying amounts of H 2 , CO, and H 2 /CO added jointly. The following report describes the experimental facilities and the results of the baseline tests completed in the first year. These results show that temperature and HCl promote oxidation, which is consistent with our earlier work and work reported elsewhere. In addition, we systematically explored the influence of H 2 O concentration on oxidation. The inhibition of oxidation by H 2 O had been previously predicted from chemical kinetic modeling, but the present data represent the first experimental evidence of the role of H 2 O in reducing oxidation.