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The following are project highlights from research performed through CATM during 2003.
Mercury Transformations in Coal Combustion Flue Gas
Although prior bench-scale studies using synthetic coal combustion flue gas showed significant elemental mercury (Hg0) oxidation effects due to nitrogen oxide (NOx), similar NO2 injections into a laboratory-scale combustion system firing a subbituminous coal did not show significant Hg speciation. The lack of heterogeneous Hg0-NOx reactions in the 42-MJ combustion system suggests that components of coal combustion flue gases and/or fly ashes inhibit heterogeneous Hg0-NOx reactions or the residence time-temperature conditions in this combustion system are much different relative to bench-scale flue gas simulators. Kinetic experiments designed to obtain quantitative data on the time-temperature oxidation of Hg0 were performed in an entrained-flow reactor (EFR) that received a slipstream of subbituminous coal flue gas. Hg transformations were not detected in the reactor at 400° and 275°C. At 150°C, however, Hg(gas) and Hg0 were transformed primarily into Hg(p) with increasing residence time. In this same system, significant mercury transformation from the bulk gas phase to the particulate phase was observed at high temperatures of 440°-878°C when 200 ppmv HCl was injected into the combustion zone, which might be the result of atomic chlorine production followed by a superfast flue gas quenching rate of 4833°C/s.
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Nickel, Chromium, and Arsenic Speciation of Ambient Particulate Matter in the Vicinity of an Oil-Fired Utility Boiler
Ambient particulate matter (PM), PM10, and PM2.5 were sampled continuously during August 26-31, 2002, from an urban State and Local Air Monitoring Stations site in the vicinity of two oil- and gas-burning power plants. Urban PM, PM10, and PM2.5 samples were analyzed using x-ray absorption fine structure spectroscopy (XAFS) during October 18-22, 2002, at the National Synchrotron Light Source at Brookhaven National Laboratory, New York. Preliminary XAFS data reduction procedures indicate that Ni and Cr were present in sufficient concentrations in all three samples to obtain good x-ray absorption spectra; however, As was below the detection limit (≈2 ppm). XAFS is being used to directly determine the speciation of Ni and Cr in ambient PM, PM10, and PM2.5 samples.
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Transition Metal Speciation of Fossil Fuel Combustion Flue Gases
XAFS spectroscopy measurements of five western U.S. Powder River Basin (PRB) subbituminous coal fly ashes sampled from power plants equipped with various pollution control equipment indicated that about 10%-30% of their total Cr contents (47- 79 ppm) exist as hexavalent chromium (Cr6+). These relatively high Cr6+ proportions support the hypothesis that western U.S. PRB subbituminous coal fly ashes contain greater proportions of Cr6+ relative to eastern U.S. bituminous coal fly ashes.
Thermodynamic modeling of Mg(OH)2 residual (No. 6 fuel) oil injection indicates that even though MgSO4 · 6H2O is formed, NiSO4 remains stable at <700°C. The addition of Mg(OH)2 is predicted to promote the formation of an oxide spinel compound, NiAl2O4, at the expense of a nickel silicate compound, Ni2SiO4.
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Fundamental Study of the Impact of SCR on Mercury Speciation
Previous testing conducted by the Energy & Environmental Research Center (EERC) to evaluate the impact of selective catalytic reduction (SCR) on mercury speciation indicated that the impact is coal-specific. This conclusion has been borne out on numerous projects. To investigate the role that SO2/ SO3 and HCl/Cl2 concentrations in the coal play, bench-scale tests using a fixed-bed system were conducted to help determine the effects of these gases. A full-factorial design was used to evaluate the independent variables, which include the reactor (none, SCR), presence of acid gases (HCl and SO2/ SO3), fly ash type, and residence time. The presence of ammonia depended on reactor mode.
The results show that the presence of acid gas is the critical factor in determining the level of mercury oxidation. Also, there is a clear correlation between the presence of acid gas and an SCR reactor on the level of mercury oxidation.
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Development of Mercury Sampling and Analytical Techniques
Because current methods of determining mercury speciation do not distinguish the inorganic forms of Hg2+ in a flue gas stream, there is a significant need for better sampling and analytical methods to be developed so that the data can be applied to accurate models of mercury behavior. This project focused on methods of sampling, analysis, and definitive identification and quantitation of Hg2+ species and compounds. Cryogenic and cold organic solvent trapping of volatile Hg2+ species from bench- and pilot-scale flue gas was used to isolate different mercury compounds and examine the transfer of mercury compounds containing nitrogen, sulfur, oxygen, and chlorine. Volatilization, separation, and transfer of the various mercuric compounds was refined to allow subsequent identification of these compounds. Mass spectrometry-gas chromatography analytical methods were developed and refined to allow identification of mercury compounds and resulting behaviors. In particular, the behavior of mercuric chloride (HgCl2) and mercuric nitrate are now better understood. These insights into the behavior of the Hg2+ species have benefited the development of a model for the chemisorption of Hg0 on a carbon sorbent surface, leading to a better understanding of the mechanics of possible mercury control technologies.
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Longer-Term Testing of Continuous Mercury Monitors
At the time this project was initiated, very little information existed concerning how mercury emissions varied over time and whether continuous mercury monitors, given their state of development, could be used long-term to assess variability. To answer this question and to evaluate how mercury semicontinuous emission monitors operated in the field for prolonged periods of time, longer-term monitoring (25-30 days) of mercury emissions and data evaluation was completed at four different power plants. The results indicate that mercury emissions can be quite variable over a 1-month period.
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Long-Term Mercury Monitoring at North Dakota Power Plants
This field sampling project was initiated to obtain mercury data concerning the variability and speciation of mercury emissions at two North Dakota power plants that combust Fort Union lignite and to further demonstrate use of continuous mercury monitors in the field. Mercury continuous emission monitors and the Ontario Hydro method were used to collect these data for use in developing mercury control strategies for North Dakota utilities. Along with analyses of coal, fly ash, and scrubber sludge, and operational emission data from the plant, these data show that although the speciation of emissions is highly variable with Fort Union lignites, plant configurations and operations may serve to reduce that variability.
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Development of a Laser Absorption Continuous Mercury Monitor
Experiments are being prepared in which two-photo frequency-modulation spectroscopy measurements are made of mercury in a closed cell. This has included the procurement of major pieces of equipment such as the diode laser system and an optical spectrum analyzer. Other equipment that has been acquired include a Tektronix oscilloscope, a LeCroy digital storage data acquisition system, Horiba imaging spectra-photometer, and various other hardware. Results using this instrument will become available over the next year.
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Development of Mercury Control Technologies
This project is intended to develop and test promising mercury control technologies such as sorbents that will provide rapid in-duct capture of mercury, sorbent regeneration, catalysts to oxidize Hg0 for capture in a wet scrubber, modification of wet scrubbing techniques to enhance mercury capture, or cleaning of the coal before combustion. A further goal is to determine the interaction between flue gas constituents, mercury species, and sorbents. This information has been used to develop and refine a dual functionality model for mercury- flue gas interactions with carbon sorbents. Protocol development for bench- and pilot scale testing is an ongoing task in this program, with a current focus on an EFR. To expand our understanding of mercury capture by activated carbon (AC) in flue gas, a full-factorial matrix of bench-scale fixed-bed tests was completed to determine the effects of the flue gas constituents on the capture of HgCl2. Under some conditions, significant reduction of Hg2+ to Hg0 occurs across the AC bed.
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Developing SCR Technology Options for Mercury Oxidation in Western Fuels
The project will evaluate the ability of SCR catalysts to oxidize mercury. The study will include both currently used SCR catalysts and new SCR catalysts formulated to enhance mercury oxidation, as well as the use of additives to enhance oxidation. The first catalyst to be tested will be an existing formulation that Haldor Topsoe currently manufactures. A second set of tests will be conducted on several new formulations developed in cooperation with Haldor Topsoe. The catalyst will be tested in flue gas compositions similar to what is found in plants burning PRB and lignite coals and will be varied accordingly. The use of oxidation additives to promote the formation of Hg2+ to levels seen for eastern coals will also be a primary emphasis.
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Modeling Mercury Speciation in Coal Combustion Systems
A preliminary mercury transformation model has been developed. This model integrates a general dynamic equation (GDE) model with a kinetic Hg interaction model. The CHEMKIN-based mercury speciation model has not been integrated to the model yet since it has not been made available. Both the GDE model and the kinetic model need to be further refined and validated. Both models need to incorporate test results that account for the Hg oxidation resulting from additives and sorbent injection. The GDE model also needs to take into account particle nucleation and coagulation in the postcombustion environment.
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Stability of Mercury in Coal Combustion By-Products and Sorbents
This continuing project focused on three primary tasks with the goal of determining the mechanisms of mercury release from coal combustion by-products (CCBs). The major focus this past year was directed toward understanding releases of mercury from microbial activity in CCBs under a disposal setting. Three ash samples were evaluated in triplicate under aerobic versus anaerobic and fed versus starved conditions. Hg0 and organomercury releases were evaluated. Results to date have been confusing; however, general trends have emerged. The mercury released from the CCB slurry was generally higher in the samples fed with glucose versus starved samples and in aerobic versus anaerobic conditions.
Researchers worked with the U.S. Environmental Protection Agency (EPA) in an effort to determine a standard leaching protocol for environmental characterization of CCBs for environmentally sensitive elements in addition to mercury. A paper was presented at the 2003 International Ash Utilization Symposium with the intent to elevate awareness of the issues related to selection of leaching procedures.
The thermal effects on mercury vapor release were evaluated through long-term ambient release and thermal desorption at elevated temperatures. The method for the determination of blank values for the long-term ambient release of mercury from six ash samples previously studied under CATM was improved. This confirmed that the CCBs studied earlier acted as mercury sinks. Thermal desorption curves were determined for a large variety of samples, and attempts were made to determine if speciation of mercury forms was possible.
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Mercury in Alternative Fuels
This project was focused on expanding CATM research to additional alternative fuels. Thirteen candidate fuel sources were collected, air-dried, and analyzed for residual moisture and mercury concentration. Each sample was completely digested, and mercury content was determined with cold-vapor atomic absorption. A National Institute for Standards and Technology standard was also analyzed as a control. Biomass samples were collected from growers in rural areas of North Dakota, Minnesota, Wisconsin, and Iowa at least 50 miles from the nearest power plant. Common cattail had the highest mercury concentration of all samples analyzed, requiring further study.
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Studies of Mercury Metabolism and Selenium Physiology
Selenium's protective influence against mercury toxicity is well documented, but mercury toxicity's effects on selenium physiology are less well understood. Toxic concentrations of mercury are known to impair selenium-dependent physiological processes, possibly through direct sequestration of cellular selenium. It appears likely that formation of insoluble mercury selenides divert selenium from its normal role in synthesis of selenoproteins.
Studies are under way to examine the interactions of mercury and selenium at the molecular, cellular, animal, and human population level. Binding affinities between mercury and selenium are being examined through in vitro studies applying a quantitative chromatography system. The impact of mercury toxicity on selenium-dependent activities at the cellular level is being examined at the tissue, organ, and organism level through in vivo studies in animal models. The relationship between mercury neurodevelopmental toxicity and selenium status in relation to human mercury consumption is being quantified through a study of selenium status in maternal-fetal pairs selected from a population from the Seychelles Islands with known exposures to high quantities of methylmercury from fish in a collaboration with Dr. Thomas Clarkson's Mercury Research group from the University of Rochester, New York.
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Technology Commercialization, Education, and Publication
To facilitate the transfer of technical information produced by CATM, several communication vehicles are used, including participation in conferences, symposia, workshops, and other educational programs; annual meetings and peer review; and the publication of a semiannual newsletter.
The EERC, through CATM, EPA, and the U.S. Department of Energy, has organized and sponsored four conferences on Air Quality: Mercury, Trace Elements, and Particulate Matter, held in 1998, 2000, 2002, and 2003. The first two conferences were held in Tysons Corner, McLean, Virginia. In 2002 and 2003, the third and fourth conferences were held in Arlington, Virginia. Air Quality V: Mercury, Trace Elements, and Particulate Matter is scheduled to be held in September 2005 in Arlington, Virginia. The Air Quality conference is a forum for reviewing the current state of science and policy on the pollutants mercury, trace elements, and PM in the environment.
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This Web site has been maintained throughout the year and can be accessed at www.undeerc.org/catm. For copies of the CATM Newsletter visit the newsletters section.
2006 CATM Highlights
2005 CATM Highlights
2004 CATM Highlights
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