Mercury emissions from coal-fired power plants are a concern to both the state and federalgovernments in the U.S. US EPA had proposed to regulate mercury emissions from coal-firedpower plants using the Clean Air Mercury Rule (CAMR). This two-phase program allowedtrading of mercury emissions, similar to the SO2 trading program already in place. As of now,all aspects of CAMR have been vacated by a court ruling, and the EPA has committed toproposing draft mercury emissions regulations for coal-fired power plants by March, 2011. Asof October 2009, nineteen states had passed their own mercury emissions regulations.Mercury exists as the elemental form (Hg~0) in the high-temperature regions of coal-fired boilers.As the flue gas is cooled, a series of complex reactions begin to convert the Hg~0 to gaseousoxidized forms (Hg~(2+)) and particulate-bound mercury (Hg_p). The extent of conversion of Hg~0to Hg~(2+) and Hg_p depends on the flue gas composition, the amount and properties of fly ash and theflue gas temperature and quench rate. The speciation of mercury in coal combustion flue gasaffects the performance of activated carbon (a dedicated mercury control technology) and theremoval of mercury by wet FGD scrubbers (a “co-benefit” approach to mercury control).In order to prepare for the existing and impending mercury emission regulations, utilities musthave useful tools for compliance planning. REI’s MerSim~(TM) mercury simulation tool includeshomogeneous and heterogeneous oxidation kinetics, adsorption on fly ash, oxidation acrossSCRs, and removal and re-emission across wet FGD scrubbers. This model can be used byutilities, given inputs that are generally available to them. On the surface, this requirement mightseem obvious and inconsequential; however, matching models for the complex chemistry ofmercury in practical combustion systems with the information that is generally available to plantengineers is a difficult undertaking.We have endeavored to create fundamentally based submodels for mercury behavior, but thesemodels must be based on input data that a plant engineer can reasonably supply. An integratedmodel requiring inputs that are not available or difficult to obtain will not be a useful compliancetool. Therefore, a balance must be struck between the complexity of the submodels and thecomplexity of the input parameters, in order to provide utilities with a useful and accurate tool.Detailed homogeneous and heterogeneous kinetic pathways for mercury oxidation are includedin the MerSim integrated power plant model. Overall results of the model have been previouslyreported.However, details of some of the key submodels have not always been reported indetail. Innovation and improvement continue in the integrated model. Recently, a more detailedsubmodel has been implemented to predict both mercury oxidation and adsorption acrossregenerative air preheaters.Using full-scale mercury speciation data assembled from previous DOE- and EPRI-fundedprograms, one can see that regenerative air preheaters promote formation of oxidized and, to a lesser extent, particulate-bound mercury. Figure 1 illustrates these points with observed valuesof oxidized and particulate-bound mercury at air preheater outlets.
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