Direct Measurement of Mercury Reactions In Coal Power Plant Plumes

[摘要] Recent field and pilot-scale results indicate that divalent mercury emitted from power plants may rapidly transform to elemental mercury within the power plant plumes. Simulations of mercury chemistry in plumes based on measured rates to date have improved regional model fits to Mercury Deposition Network wet deposition data for particular years, while not degrading model verification fits for remaining years of the ensemble. The years with improved fit are those with simulated deposition in grid cells in the State of Pennsylvania that have matching MDN station data significantly less than the model values. This project seeks to establish a full-scale data basis for whether or not significant reduction or oxidation reactions occur to mercury emitted from coal-fired power plants, and what numerical redox rate should apply for extension to other sources and for modeling of power plant mercury plumes locally, regionally, and nationally. Although in-stack mercury (Hg) speciation measurements are essential to the development of control technologies and to provide data for input into atmospheric fate and transport models, the determination of speciation in a cooling coal combustion plume is more relevant for use in estimating Hg fate and effects through the atmosphere. It is mercury transformations that may occur in the plume that determine the eventual rate and patterns of mercury deposited to the earth's surface. A necessary first step in developing a supportable approach to modeling any such transformations is to directly measure the forms and concentrations of mercury from the stack exit downwind to full dispersion in the atmosphere. As a result, a study was sponsored by EPRI and jointly funded by EPRI, the U.S Department of Energy (DOE), and the Wisconsin Department of Administration. The study was designed to further our understanding of plume chemistry. The study was carried out at the We Energies Pleasant Prairie Power Plant, Pleasant Prairie, Wisconsin, just west of Kenosha. Aircraft and ground measurements support the occurrence of a reduction in the fraction of reactive gaseous mercury (RGM) (with a corresponding increase in elemental mercury) as part of the Total Gaseous Mercury (TGM) emitted from the Pleasant Prairie stack. This occurrence is based on comparison of the RGM concentrations in the plume (at standard conditions) compared to the RGM in the stack. There was found to be a 44% drop in the fraction of RGM between the stack exit and the first sampling arc and a 66% reduction from the stack to the 5-mile sampling arc, with no additional drop between the 5- and 10-mile arcs. Smaller-scale experiments in both test chambers and pilot-scale coal combustor exhaust streams have indicated the presence of rapid and relatively complete reduction reactions converting divalent into elemental mercury within power plant plumes prior to full dispersion in the atmosphere. These measurements, however, have been unable to identify whether the reactions occur during plume rise from physical to virtual stack height (during positive thermal buoyancy). The presence, rate, completeness, ubiquity, and dependence on source characteristics of these reactions, however, must be demonstrated in plume environments associated with fully operational power plants. That requirement, to capture either the reactions or the reaction products of chemistry that may be occurring very close to stack exits in highly turbulent environments, constrains the precision and reproducibility with which such full-scale experiments can be carried out. The work described here is one of several initial steps required to test whether, and in what direction, such rapid mercury redox reactions might be occurring in such plumes.

[发布日期] 2005-12-31 [发布机构] Electric Power Research Institute

[关键词] Epri;Combustion;Redox Reactions;01 Coal, Lignite, And Peat;Oxidation