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The following are project highlights from research performed through CATM during 2005.
Mercury Transformations in Coal Combustion Flue Gas
CATM continues to seek cost-effective ways to transform elemental mercury to the oxidized form in order to promote mercury capture. This has previously proven most challenging in low-chlorine coals, but recent tests with coal additives have been very successful.
Research has provided additional insights into the mechanisms by which both heterogeneous and homogeneous transformations occur in coal combustion systems to affect mercury, and the role of other flue gas components in these complex reactions is becoming clearer.
Reactions between gaseous mercury and halogen species in a high-temperature flue gas regime are very fast. By adding halogens into the combustion zone, the mercury-flue gas chemistry is quite different from the normal low-rank, halogen-lean coal flue gas. Ongoing mercury kinetics studies have indicated that kinetic effects limiting the transformation of Hg0 at temperatures >400°C are greatly reduced by the addition of halogens in the combustion zone.
CATM continues to evaluate unburned carbon for its benefits as a mercury sorbent, both with and without additives.
The specific mechanisms by which active sites sorb mercury continue to be studied, but research this year shows that sulfur likely plays a prominent role.
HCl is the critical factor in determining the level of mercury oxidation with and without an SCR. However, an SCR was seen to improve mercury oxidation with increased HCl. SO2/SO3 was found to have minimal effect, with SO2 slightly reducing mercury and SO3 slightly improving mercury oxidation, but both effects were statistically insignificant.
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Measurement of Halogens
The overall goal of this ongoing project is to use existing methods and develop improved methods for evaluating the effects of halogens on the conversion of Hg0 to inorganic and organic Hg compounds with coal combustion flue gas. Two specific objectives to accomplish this goal are as follows:
- Perform Hg halogen analyses on coal, fly ash, and Hg sorbent samples and statistically evaluate the results for interelemental correlations.
- Modify and simplify EPA Method 26A for determining the concentrations and speciation of Cl and Br in coal combustion flue gas.
The samples and chemical supplies required for achieving the project goals and objectives are being acquired. Project research will begin in early 2006.
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Development of a Laser-Based Mercury Continuous Emission Monitor
This work applied fundamental research toward developing a laser-based method for measuring elemental mercury. Experiments were conducted using two different excitation schemes as well as argon and nitrogen carrier gases. Research will continue toward improving the detection limit of this method.
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Methods to Improve Measurement of Mercury and Chlorine in Combustion Flue Gases
The EERC is striving to improve the mercury measurement results obtained with impinger-based methods, such as ASTM International Method D6784-02 (Ontario Hydro), and CMM (e.g., Semtech Hg 2000, PS Analytical [PSA] Sir Galahad, Tekran) by investigating a potential source of analytical bias: the mercury-fly ash interactions that occur on filter medium (i.e., glass fibers) may promote the formation of Hg1+, 2+ and/or particle-associated mercury forms (Hg[p]), thus negatively biasing Hg0 measurements.
A second-generation, small ESP was constructed, tested, and successfully showed that it could be used to effectively remove most of the bias that is introduced by the fly ash on accurate measurement of Hg, Cl, and HCl in flue gas.
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Development of an Oxidized Mercury-Spiking System
This project focused on design of an oxidized mercury-spiking system based on the catalytic effects gold films can exhibit on mercury and chlorine. Testing of gold thickness, operating temperature and flow rate, as well as procedures for conditioning and operating such a spiking system were completed
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Development of Mercury Control Technologies
- Bench-scale tests were performed to evaluate and aid in the development of Hg sorbents and additives.
- The effect of SO3 on sorbents was explored by completing a full factorial matrix of tests with four variables at two levels. The results showed that SO3 in the flue gas mixture can have varying effects on the reactivity and capacity of the activated carbon (AC) sorbents.
- NO2, NO, and O2 are the contributors to mercury oxidation at the carbon surface, while HCl has a small effect on the oxidation rate.
- A novel mercury sorbent enhancement technology has been developed to reduce the amount of AC required for mercury control in coal-fired utilities.
- Particle-size distributions of AC injected into flue gas are not generally dependent on method of injection. However, using a piston/wire brush dry powder dispenser seemed to offer better performance than the standard volumetric screw feeder, which tended to agglomerate the AC.
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Modeling Mercury Speciation in Coal Combustion Systems and Interactions on Activated Carbon
A quantum mechanical approach has been employed in the study of mercury interactions with flue gas components on AC surface. Initial results based on free energy and enthalpy data for the elementary reactions on the AC surface seem to indicate that predominantly aromatic compounds will suffer a larger energy penalty in oxidizing elemental mercury at room temperature as compared to aliphatic counterparts. For example, the insertion step of Hg0 on the graphene edge surface is not thermodynamically favorable at 298.15 K. On the other hand, surface activation by acidic flue gas components and the capture of Cl ions by the chemisorbed mercury adducts is feasible and may yield organomercury chlorine compounds, which eventually release the captured Hg as HgCl2 at breakthrough.
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Developing SCR Technology Options for Mercury Oxidation in Western Fuels
The project evaluated the ability of SCR catalysts to oxidize mercury. Used SCR catalysts and new SCR catalysts formulated to enhance mercury oxidation, as well as the use of additives to enhance oxidation, were tested. The first catalyst tested was an existing formulation that Haldor Topsoe currently manufactures. A second set of tests was conducted on several new formulations developed in cooperation with Haldor Topsoe. The catalyst was tested in flue gas compositions similar to what is found in plants burning Powder River Basin (PRB) and lignite coals. The use of oxidation additives to promote the formation of oxidized mercury to levels of those seen for eastern coals was a primary emphasis.
The results of the baseline test indicated elemental mercury concentrations in the range of 85%-100%. The testing of three other catalysts, resulted in elemental mercury concentrations of 90%-100%, 70%-100%, and 60-100%.
A catalyst and an additive injected upstream was tested yielding promising oxidation. However, data were limited, and additional testing is needed.
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Investigation of Mercury and Carbon-Based Sorbent Reaction Mechanisms
Fort Union lignite fired power plants have shown a limited ability to control mercury emissions in currently installed ESPs, dry scrubbers, and wet scrubbers. This low level of control can be attributed to the high proportions of elemental mercury present in the flue gas, which occurs in low-acid flue gas environments. The overall goal of the project is to improve the mercury capture efficiency of carbon-based sorbents in flue gases typically resulting from firing lignite and other low-chlorine, low-sulfur fuels through a better understanding of mercury-sorbent reaction mechanisms.
Based on the results of experiments conducted thus far, the carbon surface collects SO2 from the flue gas and forms S(VI). At the same time, the chlorine content at the sorbent surface decreases. This continues until the sorbent capacity is reached.
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Mercury Metabolism and Selenium Physiology Studies
Research conducted under this project continued work initiated the previous year. Selenium has long been thought to be a mercury antagonist, but results of these studies support the hypothesis that mercury is a selenium antagonist. This distinction is important to understand since it defines a primary mechanism of mercury toxicity and is of pivotal importance in quantifying the risks associated with environmental mercury exposure. Results demonstrate that rats fed diets that contain deficient levels of dietary selenium were sensitive to mercury exposure. Meanwhile, rats that were fed selenium-rich diets showed little or no adverse effects from consumption of high levels of mercury. Because selenium is an essential nutrient that is particularly important in brain function, the signs and symptoms of mercury toxicity appear to occur as a consequence of mercury-dependent selenium sequestration and the resulting loss of physiological functions normally supported by selenium.
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Mercury-Selenium Interactions in Aquatic Ecosystems
CATM research conducted in this project investigated the affinity of selenium for mercury and selenium's influence on mercury retirement in aquatic ecosystems. The effect appears likely to occur intracellularly, resulting in an insoluble complex which limits the biological availability of both elements. Since selenium-dependent enzyme activities occur in all cells of all animal species, physiological experiments using rapid-growing, short-lived invertebrates were employed. Experiments are under way.
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Molecular Interactions of Toxic Metals
This project investigates the molecular interactions between mercury and selenium and explores a potentially novel molecular mechanism involving nickel subsulfide interaction with DNA. These studies compare and contrast direct, indirect, and consequent molecular mechanisms of toxicity. Distinctions between molecular mechanisms of various toxic substances determine how they inflict physiological damage. This provides a new perspective of the molecular mechanism of mercury toxicity, which appears to act through a consequent mechanism, rather than through inducing direct or indirect molecular damage within exposed cells.
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Mercury and Air Toxic Element Impacts of Coal Combustion By-Product Disposal and Utilization
The objective of this effort was to provide information on the stability of mercury and air toxic elements associated with CCBs under conditions relevant to typical CCB management practices. Controlled laboratory experiments were used to evaluate a wide variety of fly ash samples and FGD materials. Samples were obtained primarily from full-scale coal-fired power plants under both normal operating conditions and during mercury emission control demonstrations. The following are some preliminary observations:
| • | Analysis of the carbon forms data revealed that samples with anisotropic or isotropic coke as the dominant carbon form included samples with the higher mercury content. Those samples also generally contained activated carbon from mercury emission control. |
| • | Total mercury content and leachate concentrations of samples generated both with and without mercury emission controls present did not correlate and were independent of the short-term leaching procedures used. |
| • | Results obtained from experiments to evaluate long-term ambient-temperature release of mercury from CCBs ranged from a net release to a net sorption of mercury. Replicate tests frequently yield highly variable results, but the extremely low levels both of sorption and release of mercury indicate that this release mechanism has very low potential to impact the loading of mercury in the atmosphere. |
| • | Mercury is generally released at temperatures greater than 200°C, and in many samples, all the mercury is released when exposed to a temperature of 750°C. |
| • | Organomercury compounds are present in leachates and vapor generated in experiments performed to evaluate mercury release under microbiologically mediated conditions. |
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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 both domestic and international conferences, symposia, workshops, and other educational programs, and annual meetings; quarterly reports on topical issues related to mercury through a collaborative project funded by CATM Affiliates, DOE, and CEA; and the publication of a newsletter that is also available electronically. In addition, the CATM Director and staff provide input into various public forums during the year to assist in the development of venues of technology transfer that may not be directly funded by CATM.
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The CATM Web page has been maintained throughout the year and can be accessed at www.undeerc.org/catm. Copies of the CATM Newsletter and topical reports to the CEA are available and can be accessed via the CATM Web page for download and distribution.
2006 CATM Highlights
2004 CATM Highlights
2003 CATM Highlights
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