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Preface to the AQVI special issue of fuel processing technologies entitled: Air quality VI: Mercury, trace elements, SO3, particulate matter, and greenhouse gases
Fuel Processing Technology 90 (2009) 1327–1332
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Preface
Preface to the AQVI special issue of fuel processing technologies entitled: Air quality VI: Mercury, trace elements, SO3, particulate matter, and greenhouse gases John H. Pavlish
The Air Quality Conference held September 24–27, 2007, in Arlington, Virginia (AQVI) reviewed the state of environmental science and policy on mercury, particulate matter, and greenhouse gases. The proceedings addressed impacts on health and ecosystems, emission prevention and control, and atmospheric and geochemical modeling. This special edition is focused on the status of science and technology for the control of mercury emissions from coal-burning electric generating plants, drawing principally on papers presented at AQVI and contained herein.
1. Health impacts of low-level mercury exposure Low-level exposure to mercury remains a leading public health concern in the United States, especially for children and women of childbearing age. In 2001, the Environmental Protection Agency (EPA), supported by a review by the National Academy of Science, established a mercury reference dose of 0.1 µg/kg body weight/day as a level without recognized adverse effects, with a 10-fold margin of safety to account for biological variability and database insufficiencies. Essentially all mercury exposure in the United States is from consuming fish containing high levels of mercury that have bioaccumulated in the food chain. Health effects statistically linked to mercury exposure include cognitive development in infants and toddlers, the IQ of adolescents and young adults, and possible cardiovascular disease in adult males. None of these effects has been shown at exposure levels below the regulatory threshold established by EPA [1]. Mercury exposure in the United States has, to date, been reduced through the elimination of most industrial, medical, and household uses of mercury and the issuance of EPA guidelines and state advisories limiting the consumption of certain kinds or amounts of fish. Major differences in the neurological impacts of dietary mercury exposure observed in various studies in the Faroe Islands, New Zealand, the Seychelles, and the United Kingdom have been linked to the beneficial effect of an excess of dietary selenium over mercury as a cofactor protecting against the adverse biological effect of mercury to bind with selenium and impair the synthesis of selenium-dependent enzymes required for neurological functions [2]. Research suggests that future guidelines would be advanced by considering the ratio of selenium to mercury in seafood.
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2. Regulation of mercury emissions from coal-fired power plants Mercury emissions from U.S. coal-fired power plants were estimated to be 51.3 tons in 2005 [3] or about 40% of the nation's total anthropogenic emissions. In March 2005, EPA issued the Clean Air Mercury Rule (CAMR) providing a two-phase cap-and-trade approach that would have reduced power plant emissions to 38 tons in 2010 and to 15 tons in 2018. The first phase was intended to be largely accomplished as a cobenefit of the 2005 Clean Air Interstate Rule (CAIR), which required utilities in the eastern United States to install wet flue gas desulfurization (FGD) for SO2 control and selective catalytic reduction (SCR) for NOx control. The U.S. Court of Appeals for the DC Circuit vacated CAMR in February 2008 and CAIR in July 2008, which has had the effect of requiring all coal-fired power plants to meet a more stringent Maximum Achievable Control Technology (MACT) standard for mercury. A pending Senate bill, S 2643, would require emission reductions of not less than 90% for all new and existing plants. Also, at the end of 2007, some 16 states had established programs that require reductions of 80% to 90% when fully implemented [4]. Canada-Wide Standards for coal-fired power plants call for a first-phase 60% reduction in emissions from existing plants by 2010 under individual provincial caps that reflect regional differences, a second-phase target of 80% in 2018, and immediate implementation of best available control technology for all new plants [5]. The Province of Alberta, which accounts for 46% of the total emissions from coal plants across Canada, will be required to reduce emissions by 70% by 2010 and to submit proposals for targeting 80% under a regulatory review in 2013 [6].
3. The international context on mercury emission and control Benefits from more stringent mercury control requirements will be constrained in their timing and ultimate effect by the readiness of the control technologies to meet a 90% control standard and by uncertainties surrounding the effectiveness of U.S. controls in the context of much larger world emissions from natural and anthropogenic sources. Mercury is a global pollutant with a half-life on the order of 6–12 months in the atmosphere, meaning that a major fraction of U.S. emissions are deposited outside the United States, and a major portion of U.S. deposition originates outside the country. While estimates of the effect of overseas emissions vary, EPA has estimated that 83% of mercury deposition in the United States originates from international sources outside the auspices of U.S.
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policy, including natural emissions, rerelease of historic mercury previously deposited, and anthropogenic sources [7]. Total global emissions of approximately 7000 tons of mercury are roughly estimated to be one-third natural, one-third reemitted, and one-third anthropogenic [8]. The United Nations Environment Programme (UNEP) estimates that anthropogenic emissions in the United States totaled 130 short tons in 2005 or approximately 6% of a world total of 1930 tons (U.S. coal-fired power plants contribute about 2.4% to the world's anthropogenic emissions) — compared to an anthropogenic total of 635 tons (33%) for China and 1281 tons (67%) for all of Asia [9]. Since the issuance of this report, a global treaty to reduce mercury emissions has been signed by over 140 countries [10]. The data on global emissions and current understanding of the atmospheric transport, transformation, and deposition of mercury suggest that the global mercury balance is more important than utility emissions in determining local deposition patterns. However, hot spots of deposition in the vicinity of power plants and other sources remain a concern depending on the form of the emissions. Most mercury is either emitted in elemental form or is partially and rapidly reduced to elemental form in the atmosphere, causing it to be widely dispersed [11, 12, 13, 14]. Oxidized and particulate forms of mercury have a lifetime of only a few days and are more readily deposited locally. Mercury concentrations in rainfall close to two power plants in New South Wales, Australia, were found to be twice as high on average as those measured 100 miles away, but mercury deposition was also shown to be influenced by seasonal variations in local climate conditions at the two sites [15]. The significance of hot spots as an issue affecting the need for more stringent controls has not been fully resolved. 4. Overview of the state of development for mercury control technology Mercury control technologies needed to achieve 90% mercury emission control throughout the coal-fired power industry have been tested and show promise but still require additional development and demonstration. While progress has been substantial, technical issues remain in matching technologies to coal properties and plant designs and in demonstrating long-term reliability without adverse impacts on balance of plant. Coal properties are critically important in determining the forms of mercury produced in the combustion process and their capture in pollution control devices. Higher levels of chlorine and iron, typically found in Appalachian and Interior Basin bituminous coals, promote conversion of elemental mercury to the oxidized form that is more easily captured for example in a wet FGD scrubber. Western subbituminous coals and lignites from the northern Great Plains contain lower levels of chlorine and generate primarily elemental mercury that is not captured in a scrubber and is more difficult to capture by activated carbon injection (ACI). Gulf Coast lignites are more variable and generate between 50% and 90% elemental mercury. Higher levels of sulfur trioxide (SO3) in flue gas reduce the effectiveness of ACI control technology, whether the SO3 is generated by burning high-sulfur bituminous coal, by adding SO3 for flue gas condition ahead of an electrostatic precipitator (ESP), or by catalytic conversion of SO2 to SO3 in a SCR unit used for NOx control. The alkali and alkaline-earth elements in subbituminous coals and lignites react with HCl and SO3 in flue gas, thereby reducing both the chlorine available for mercury oxidation and the SO3 impeding mercury capture on activated carbon. Starting in 2000, the U.S. Department of Energy (DOE) National Energy Technology Laboratory, under the department's Innovations for Existing Plants Program, initiated a three-phase field testing program to bring Hg control technologies to the point of commercial deployment readiness [16]. The program incorporates lessons learned to advance Hg control technologies for specific areas of need. To date,
field tests have been conducted at 50 coal-fired power plants. Notable success has been achieved in using treated activated carbons for capturing mercury when low-chlorine lignites and subbituminous coals are burned and in increasing mercury oxidation to improve capture in wet FGD scrubbers. The effects of high levels of SO3 in flue gas have been mitigated by introducing alkaline compounds, either on treated sorbents or as separate additives. The TOXECON™ and TOXECON II™ ACI configurations developed by EPRI have been shown to preserve the sale of fly ash for use as a substitute for portland cement in concrete by capturing fly ash upstream of the point of ACI. Overall, the DOE program has contributed significantly to commercial deployment, with more than 80 full-scale ACI systems ordered by United States coal-fired power generators as of February 2008. The cost of higher levels of control up to 90% has been reduced from over $30,000 to less than $10,000 per pound of mercury removed using treated AC. However, the cost associated with loss of fly ash use still remains a concern and is significant. Levels of control up to and exceeding 90% have been demonstrated for many coal types and plant configurations in short-term tests, but average removals in longer-term tests have generally been lower because of variability in coal properties and plant operating conditions. The long-term reliability and availability of mercury control technologies, and their effects on balance of plant, remain to be demonstrated. 5. Fundamental coal mercury science Advances in mercury control technology are supported by research on the fundamental chemical mechanisms controlling the oxidation and capture of mercury. Mercury in a coal flame is initially released as elemental vapor Hg0 that is partially converted to oxidized mercury Hg2+ as the combustion gases cool. Thermodynamic equilibrium calculations predict nearly complete conversion to Hg2+, but kinetic limitations relating to flue gas chemistry and plant design result in conversion ranging from <10% to >90% oxidation. Kinetic experiments and modeling of mercury oxidation reactions show important roles both for homogeneous reactions of atomic Cl and Br in the vapor phase and for heterogeneous reactions of halogen species absorbed on the surface of fly ash, unburned carbon, or injected AC [17, 18]. Bromine has been consistently observed to be more effective than chlorine in promoting mercury oxidation and/or capture, whether in the form of a halide-treated AC or as a halide salt used as a sorbent enhancement additive (SEA), but there is no wellestablished theoretical basis to explain this difference. The absorption and reaction of mercury on various surfaces are limited by competition with other gaseous species present in much higher concentrations. A detailed mechanistic model of the chemistry of mercury oxidation and capture on AC has evolved from a matrix of laboratory tests wherein simulated flue gas containing either Hg0 or HgCl2 was passed through a fixed (thin) bed of AC in the presence or absence of all combinations of SO2, HCl, NO, NO2, and water vapor [19, 20]. Results of these experiments and related surface analysis of spent sorbents indicate that Hg0 is oxidized by NO2 or other oxidizing gases on carbon oxidation sites formed by prior surface reactions of HCl with zigzag edge structures on the carbon. If the AC did not originally contain such oxidation sites, an induction period in the presence of HCl is required before surface oxidation of elemental mercury can begin to occur. Oxidized mercury, Hg2+, whether absorbed from the flue gas or oxidized on the surface, is bound to a basic carbene site on the zigzag edge structure unless the site is already occupied. Competition for capture sites occurs when SO2 is oxidized by NO2 to form nonvolatile H2SO4 in the presence of water vapor, which occupies the basic capture sites. The presence of SO3 in flue gas has a similar effect [21]. Omitting either NO2 or water vapor avoids the formation of H2SO4 by surface oxidation of SO2. Theoretical calculations on energies of association have shown that addition of HCl to a
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zigzag carbon edge structure is a feasible first step for generating the required sites for mercury oxidation on a carbon surface. The calculation performed for the subsequent reaction of Hg0 with surface sites indicates initial formation of an Hg complex followed by reaction with a halide anion to form a strong C–Hg–X covalent bond as the capture mechanism [22]. 6. Chemically treated carbons and SEAs The focus of development of mercury control technology for western Powder River Basin (PRB) and lignite coals has been on halogenated ACs and halogen-based SEAs. Native mercury capture for these coals is usually much lower than that for eastern bituminous coals owing to lower concentrations of halogens in the coal. The most effective way to achieve higher levels of mercury capture for these coals is to introduce chlorine, bromine, or iodine, either by treating the carbon or by adding a halogen compound as an SEA. Several tests were run at conditions minimizing carbon contamination in fly ash to preserve sale as a cement amendment. Pilot-scale tests were run to evaluate mercury removals for a variety of candidate sorbents and SEAs across both an ESP and a spray dryer/fabric filter (SD/FF) when low-chlorine lignite is burned [23]. Baseline mercury removals of less than 10% were increased to levels in the range of 70% to 95% at injection rates in the range of 1 to 5 lb/Macf using either treated carbons (treated with HCl or a proprietary halide additive) or a combination of a carbon-based sorbents and a proprietary halogen SEA (the EERC's SEA2). Removals were higher in the SD/FF than across an ESP at similar injection rates. NaCl was a more effective SEA than CaCl2, but neither was as good as SEA2. Slipstream tests run on a pilot-scale fabric filter downstream of the plant's ESP (representing a TOXECON™ configuration) at the SaskPower Poplar River Station burning Northern Great Plains lignite showed significant differences in mercury capture among nine AC sorbents, with four capturing over 90% at an injection rate of 2 lb/Macf [24]. Parametric tests showed no appreciable effect on capture for temperature variations between 200°F and 350 °F, air-to-cloth (A/C) ratios of 2 to 8 ft/min, or ash loadings of 3 to 60 lb/Macf. A preliminary economic assessment estimated costs in the range of $17,770 at an A/ C ratio of 8 up to $20,520 at an A/C of 4, corresponding to an increase of 3 to 4 mills/kWh in generating cost. Full-scale short-term tests at the Sundance Plant burning Canadian subbituminous coal demonstrated 70% or higher capture across the cold-side ESP injecting approximately 2 lb/Macf brominated AC DARCO Hg-LH and up to 95% removal at 6 lb/Macf [25]. In a 30-day test, an average removal of 80% was achieved at an average injection rate of 2.1 lb/Macf, with removals varying from 62% to over 95% at injection rates between 0.55 and 8 lb/Macf. GE's patented combustion modifications for capturing mercury on unburned carbon reduced injection rates by 20% to 30% while reducing NOx emissions from about 380 to 200 ppm and increasing loss on ignition (LOI) in fly ash from 0.8% to 2%. Tests on a full-scale TOXECON™ system at the We Energies Presque Isle plant burning PRB coal showed mercury capture levels of 60% to 95% at injection rates between 0.5 and 3 lb/Macf [26]. Removals using brominated carbon DARCO Hg-LH were typically 5% higher than those for the baseline carbon DARCO Hg and were less affected by an increase in temperature above the baseline of 330 °F. A targeted removal of 90% was maintained for 48 consecutive days in a long-term test designed to validate performance goals. Mercury removals at higher temperatures up to 380 °F were improved by more frequent online cleaning of filter bags, suggesting that captured mercury may have been reemitted when the spent AC was retained on the filter for a longer period of time. Full-scale tests were performed at the NRG's Limestone Electric Generating Station firing a 30/70 blend of Texas lignite and PRB coal to evaluate three mercury control technologies that would allow the
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plant to continue marketing fly ash: 1) injecting only a small amount of carbon upstream of the ESP, 2) injecting a non-carbon or a “passivated” carbon sorbent upstream of the ESP, or 3) injecting AC midstream in the ESP after most of the ash has been collected using a TOXECON™ II configuration [27]. Injecting 2 lb/Macf of the brominated carbon DARCO Hg-LH ahead of the ESP achieved 90% capture while allowing the fly ash to provisionally pass a test on the amount of air entrainment agent needed to stabilize concrete. A passivated and brominated test carbon, C-PAC, also passed the air entrainment test when injected at the higher rate of ~5 lb/Macf needed to achieved 90% removal. A nontreated carbon, DARCO Hg, captured nearly the same percentage of mercury as the two other brominated carbons tested, possibly because of the relatively high-chlorine content in Texas lignite. The non-carbon test sorbent, MS200, achieved only 50% capture at injection rates up to 12 lb/Macf. Finally, the TOXECON™ II configuration tested at this plant achieved only about 60% mercury capture at injection rates up 5 lb/Macf. Carbon injection either upstream of the ESP or midstream resulted in some carbon breakthrough at the ESP outlet. 7. Improving mercury capture on activated carbon at high levels of SO3 SO3 concentrations of 20 and 25 ppm were measured at the SCR inlets at two plants burning high-sulfur bituminous coal, and SO3 levels increased slightly across the SCRs owing to catalytic conversion of SO2 to SO3 [28]. SO3 levels at a third plant burning PRB subbituminous coal were much lower, ~ 1 to 2 ppm, owing to the low sulfur content and the relatively high levels of alkali and alkalineearth elements in the coal. However, many plants burning PRB coal (comprising 25 GW of capacity) inject SO3 into the flue gas to lower fly ash resistivity for the purpose of improving particulate collection in ESPs [29]. Higher levels of SO3 resulting from either high-sulfur coal or SO3 injection interfere with the capture of mercury on AC because of competition for capture sites on the carbon. At the Labadie Power Plant, which burns PRB coal, the injection of SO3 at rates resulting in 5.4 and 10.7 ppm in flue gas showed that mercury capture on brominated AC, DARCO Hg-LH, was significantly reduced as the concentration of SO3 was increased, from a range of 60% to 95% without SO3 injection to a range of 10% to 75% with injection [29]. Substitution of a brominated carbon that also contained alkaline material to capture and neutralize SO3, DARCO Hg-E26, raised removals when injecting SO3 to levels comparable to those observed without SO3 using DARCO Hg-LH. 8. Improving mercury oxidation and capture in FGD and SCR systems Wet FGD scrubbers capture most of the oxidized mercury Hg2+ formed when high-chlorine bituminous coals are burned but remove essentially none of the elemental mercury that is predominantly formed when PRB coal and other low-chlorine coals are burned. Some of the Hg2+ initially captured is reduced back to Hg0 in the scrubber and reemitted by reactions that are importantly influenced by concentrations of chloride and sulfite and the pH of the scrubber solution [30]. Methods for increasing the level of mercury oxidation and improving capture in FGD scrubber systems include 1) catalysts that promote high Hg0 oxidation with low SO2-to-SO3 conversion, 2) halogen additives alone or in combination with SCR, 3) alkaline compounds that reduce the adverse effect of SO3 on gas-solid reactions, and 4) wet FGD additives that reduce reemission in the scrubber. Babcock-Hitachi has developed a plate-type catalyst reported to achieve Hg oxidation 1.5 to 2.0 times higher and SO2 to SO3 conversion about half that of conventional catalysts [31]. Tests on a
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noble metal catalyst performed by URS at the outlet of the ESP and ahead of the wet FGD at a number of plants have shown oxidation levels of 90% to 95% with fresh catalyst, 60% to 70% after 20 months, and the restoration of initial oxidation levels using regenerated catalyst [32]. The addition of only 5 ppm Br as a solution of calcium bromide (CaBr2) to a PRB coal increased the level of mercury oxidation at the ESP outlet from 60% to 90% at a plant equipped with SCR, with most of the oxidation occurring across the SRC at this low Br addition rate [33]. Higher addition rates of 165 and 328 ppm Br promoted the oxidation of mercury upstream of the SCR to levels of up to 98% at the SCR inlet. Additives developed by a number of companies to reduce the reemission of mercury from wet FGDs have been tested at full scale by URS [30] and B&W [34]. Two candidate additives developed by Solucorp and B&W, respectively, have been reported to effectively inhibit reemission and promote additional capture without observable impacts on scrubber performance or gypsum quality [34]. 9. The impact of mercury on coal utilization by-products (CUBs) Whenever mercury is captured on a sorbent or in a wet FGD scrubber, there is the potential for creating new pathways of mercury release into the environment when the resulting by-products are utilized or placed in a landfill. Several questions that must be answered are 1) how stable is the captured mercury over time, 2) does mercury leach from CUBs and enter the groundwater, and 3) how does mercury control technology impact the processing and reuse of CUBs. Frontier GeoSciences has been contracted by DOE to assess the mobility pathways of mercury and other trace elements in CUBs under Phases II and III of DOE's Mercury Control Field Testing Program. Mercury control technologies may variously increase the amount of mercury present in fly ashes collected in an ESP or fabric filter and in the solid by-products from wet and dry FGD scrubbers. Results reported by Frontier GeoSciences at AQVI indicate that thermal release of mercury from fly ashes is statistically insignificant after 30 days at 40 °C but is significant in some cases after 1 h at 190 °C, representative of wallboard manufacture [35]. A substantial fraction of mercury is released in 5 min at 1200 °C, representing conditions in cement production, but the faction released from fly ash collected with mercury control was not consistently either higher or lower than that released from baseline ashes collected without mercury control. Substantially 100% of selenium was released at 1200 °C. Microbial activity resulted in some increase in methylmercury release, but a number of other trace metals were stabilized. The EPA's Office of Solid Waste (OSW) has incorporated the test framework and probabilistic assessment developed at Vanderbilt University into EPA's SW846 manual, “Test Methods for Evaluating Solid Waste, Physical Chemical Methods.” OSW findings presented at AQVI indicate that mercury is strongly retained in CUBs and is unlikely to be leached at levels of environmental concern [36]. Results of leaching and thermal tests on fly ashes collected by the Energy and Environmental Research Center (EERC) from field demonstrations of ACI for mercury control are in agreement with results reported by OSW and Frontier GeoSciences [37]. Research at the University of Nevada suggests that alternate wetting and drying can result in increased mercury release to the atmosphere from disposal sites containing FGD materials [38]. Long-term release to the environment under actual field conditions is difficult to measure and has not been adequately determined. The physical and engineering properties of fly ashes containing spent AC are similar to those of the baseline fly ash with the exception that even small amounts of carbon can preclude the direct use of the fly ash in concrete where air entrainment is required [37]. Also, limited tests on concrete prepared from fly ashes sampled over a range of bromide addition to coal suggested a possible reduction in compressive strength which warrants further study [33]. Technolo-
gies for minimizing the impacts of sorbent injection for mercury control on the sale of fly ash for use in concrete have been addressed previously in this review. 10. Balance of plant impacts of mercury control The balance of plant impacts of mercury control technology can affect the reliability of an entire power plant at a cost that potentially far exceeds the direct cost of the mercury control alone due to increased maintenance and lost power generation. Cost impacts will be influenced by the regulations that are finally adopted, where longer-term averaging might allow continued operation under circumstances that could force a plant shutdown under a nearcontinuous requirement. Since long-term impacts have not been demonstrated, further work is required on a variety of issues that are not yet fully defined. Future regulatory compliance will require mercury measurements using either near-continuous sorbent traps or continuous mercury monitors. The complexity of these requirements requires operators to become thoroughly familiar with the measurement technology and the reference methods used to certify and calibrate the measurements. The EPA's Method 30B which uses a sorbent trap reference for performing relative accuracy test audits (RATAs) is currently the method of choice for calibration, but still simpler instrumental reference methods are under development [39]. Sorbents, catalysts, and additives variously containing alkali and halides to enhance control mercury emissions can potentially cause boiler tube corrosion, fouling and plugging in SCR units and air preheaters, changes in scrubber chemistry, increased stack opacity and emissions (e.g., bromine), and impacts on the disposal and reuse properties of fly ash and scrubber solids. Most of these possible impacts remain to be studied. ESP performance can be negatively affected by ACI because of the low resistivity and fineness of the carbon. The Indigo Agglomerator technology [40] reduces the concentration of fine particulates entering an ESP by electrostatic agglomeration to address problems with ESP performance and stack opacity and to improve mercury capture. Pressure drop in an existing or add-on FF can, in some cases, be a limiting factor in applying ACI because of buildup of residual material that is not removed by online cleaning. A monthlong test at TXU's Big Brown Station burning a 70/30 blend of Texas lignite and PRB coal determined that the installation of a FF downstream of the existing ESP (TOXECON™) would likely not be possible without major modifications to the plant and its operation concerned with fan capacity and power [41]. The increase in pressure drop over time correlated directly with total particulate loading but not with carbon content, indicating that the AC behaved much like additional fly ash. Carbon injection did not appear to affect the underlying mechanism of residual dust buildup, which was observed to be influenced by formation of ammonium sulfate from injection of SO3 and NH3 for flue gas conditioning ahead of the ESP. However, offline tests did show a larger decrease in filter permeability with ACI than without. The constraint posed by pressure drop can be reduced by installing a larger baghouse at additional cost, where in tests at SaskPower, sustained operation at a typical dust loading could be maintained within the maximum allowable pressure drop of 10 inches W.C. at an A/C ratio of 6 ft/min, but not at an A/C of 8 ft/min [24]. Autoignition of carbon in baghouse hoppers was experienced in the full-scale demonstration of TOXECON™ technology at the Presque Isle Power Plant when the AC/ash mixture exceeded a critical temperature of about 430 °F [26]. The cause was traced to heaters used for temperature control. Modeling of spontaneous combustion was used to establish conditions under which heat would be generated faster than it could be liberated. Guidelines for preventing autoignition involve monitoring and control of temperature,
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flowability and fluidization in the hopper, and measurement of CO as an indicator of incipient ignition.
11. The path forward There is a continuing need for development and demonstration of mercury control technologies to address the critical issues remaining for implementation of a MACT standard across the United States for all plant designs and coal types. Balance of plant impacts is a particularly important part of that future work. For existing plants, a MACT standard will be far more stringent than the CAMR cap-and-trade approach that would have allowed utilities to exercise higher levels of control where mercury could be captured most effectively and at least cost, and lower levels where control would be more difficult and costly. Federal support for mercury under DOE's Innovations for Existing Plants Program lapsed in fiscal year 2009 whereas industry still feels that there are many unresolved issues that remain in bringing mercury control technologies for existing plants to the point of commercial deployment readiness. Technology options are even more limited for advanced power systems, such as integrated gasification combined cycle and oxycombustion, needed to address CO2 capture and sequestration for future coal-fired power plants. The current status of the science and technology clearly indicates that high levels of mercury control up to 90% for coal-burning power plants can be achieved in the future, but the costs and risks for implementation will remain high unless research, development, and demonstration efforts are continued. References [1] L. Levin, A. Schure, Update on mercury health effects findings, Proceedings of the International Conference on Air Quality VI: Mercury, Trace Elements, SO3, Particulate Matter, and Greenhouse Gases, 2007, Arlington, VA, Sept 24–27. [2] N.V.C. Ralston, L.J. Raymond, Selenium's importance in regulatory issues regarding mercury, Proceedings of the International Conference on Air Quality VI: Mercury, Trace Elements, SO3, Particulate Matter, and Greenhouse Gases, 2007, Arlington, VA, Sept 24–27. [3] EIA, Annual Energy Outlook 2007, www.eia.doe.gov/oiaf/aeo/index.html 2007 (accessed Aug 2008). [4] National Association of Clean Air Agencies, December 2007. www.4cleanair.org/ Documents/StateTable.pdf (accessed Aug 2008). [5] CCME. Canada-Wide Standards for Mercury Emissions from Coal-Fired Electric Power Generation Plants. October 2006 (accessed Aug 2008). [6] P. Valupasas, Alberta mercury regulation for coal-fired power plants, Proceedings of the International Conference on Air Quality VI: Mercury, Trace Elements, SO3, Particulate Matter, and Greenhouse Gases, 2007, Arlington, VA, Sept 24–27. [7] EPA. The EPA's Roadmap for Mercury EPA-HQ-OPPT-2005-0013, July 2006. www. epa.gov/mercury/roadmap.htm (accessed Aug 2008). [8] Seigneur, C.7 Levin, L. Presented at the Global Chemical Transport Model for Mercury, Joint International Conference, Rome, April 2008. [9] United Nations Environment Programme (UNEP) Report on the Global Mercury Assessment: Sources, Emissions, and Transport, Dec 2008. www.unep.org/ (accessed Dec 2008). [10] UNEP memo, Feb 2009. www.unep.org/cpi/briefs/2009Feb24.doc (accessed Feb 2009). [11] C. Seigneur, K. Vijayaraghavan, K. Lohman, L. Levin, Effect of atmospheric chemistry on mercury deposition in the United States, Proceedings of the International Conference on Air Quality V: Mercury, Trace Elements, SO3, and Particulate Matter, 2005, Arlington, VA, Sept 19–21. [12] E. Prestbo, P. Swartzendruber, L. Levin, D. Laudal, R. Schulz, G. Dunham, W. Aljoe, J. Jensen, L. Monroe, R. Valente, D. Michaud, Interconversion of emitted atmospheric mercury species in coal-fired power plant plumes, Proceedings of the International Conference on Air Quality V: Mercury, Trace Elements, SO3, and Particulate Matter, 2005, Arlington, VA, Sept 19–21. [13] A.L. Robinson, A.P. Grieshop, D. Laudal, M. McCoy, Investigation of mercury transformation in coal power plant plumes using a dilution sampler, Proceedings of the International Conference on Air Quality V: Mercury, Trace Elements, SO3, and Particulate Matter, 2005, Arlington, VA, Sept 19–21. [14] D.L. Laudal, G.E. Dunham, D. Smith, S. Pletcher, D. Rose, The fate of mercury in a pilot-scale amine CO2 Scrubber at SaskPower Boundary Dam Station, Proceedings of the International Conference on Air Quality V: Mercury, Trace Elements, SO3, and Particulate Matter, 2005, Arlington, VA, Sept 19–21. [15] U. Dutt, P.F. Nelson, A.L. Morrison, V. Strezov, Mercury wet deposition and coalfired power station contributions: an Australian study, Proceedings of the International Conference on Air Quality VI: Mercury, Trace Elements, SO3, Particulate Matter, and Greenhouse Gases, 2007, Arlington, VA, Sept 24–27.
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[16] M.L. Jones, B.M. Pavlish, S.A. Benson, Evaluation of CO2 capture technologies for lignite-fired boilers, Proceedings of the International Conference on Air Quality VI: Mercury, Trace Elements, SO3, Particulate Matter, and Greenhouse Gases, 2007, Arlington, VA, Sept 24–27. [17] S. Niksa, C.V. Naik, M.S. Berry, L. Monroe, Interpreting enhanced Hg oxidation with Br addition at Plant Miller, Proceedings of the International Conference on Air Quality VI: Mercury, Trace Elements, SO3, Particulate Matter, and Greenhouse Gases, 2007, Arlington, VA, Sept 24–27. [18] C. Senior, A. Fry, C. Montgomery, G. Silcox, B. Cauch, Impact of Halogen Injection on Mercury Speciation and Capture in Coal-Fired Power Plants, In Proceedings of the International Conference on Air Quality VI: Mercury, Trace Elements, SO3, Particulate Matter, and Greenhouse Gases, 2005, Arlington, VA, Sept 24-27, 2007. [19] E.S. Olson, B.A. Mibeck, Oxidation Kinetics and the Model for Mercury Capture on Carbon in Flue Gas, Proceedings of the Air Quality V: International Conference on Mercury, Trace Elements, SO3, and Particulate Matter, 2005, Arlington, VA, Sept 19–21, 2005. [20] B.A.F. Mibeck, E.S. Olson, S.J. Miller, HgCl2 sorption on lignite activated carbon: analysis of fixed-bed results, Proceedings of the International Conference on Air Quality VI: Mercury, Trace Elements, SO3, Particulate Matter, and Greenhouse Gases, 2007, Arlington, VA, Sept 24–27. [21] A.A. Presto, E.J. Granite, Impact of sulfur oxides on mercury capture by activated carbons, Proceedings of the International Conference on Air Quality VI: Mercury, Trace Elements, SO3, Particulate Matter, and Greenhouse Gases, 2007, Arlington, VA, Sept 24–27. [22] E.S. Olson, A. Azenkeng, J.D. Laumb, R.R. Jensen, S.A. Benson, New developments in the theory and modeling of mercury oxidation and binding on activated carbons in flue gas, Proceedings of the International Conference on Air Quality VI: Mercury, Trace Elements, SO3, Particulate Matter, and Greenhouse Gases, 2007, Arlington, VA, Sept 24–27. [23] S.A. Benson, J.H. Pavlish, M.J. Holmes, C.R. Crocker, K.C. Galbreath, Y. Zhaung, Mercury control testing in a pulverized lignite-fired system, Proceedings of the International Conference on Air Quality VI: Mercury, Trace Elements, SO3, Particulate Matter, and Greenhouse Gases, 2007, Arlington, VA, Sept 24–27. [24] Thompson, J.S.; Pavlish, J.H.; Hamre, L.L.; Jensen, M.D.; Smith, D.; Podwin, S.; Brickett, L.A. Sorbent Injection into a Slipstream Baghouse for Mercury Control: Project Summary, Proceedings of the International Conference on Air Quality VI: Mercury, Trace Elements, SO3, Particulate Matter, and Greenhouse Gases, 2007, Arlington, VA, Sept 24–27. [25] J. David, T. Silva, S. Brown, Full-Scale WFGD Hg removal additives, Proceedings of the International Conference on Air Quality VI: Mercury, Trace Elements, SO3, Particulate Matter, and Greenhouse Gases, 2007, Arlington, VA, Sept 24–27. [26] S. Derenne, P. Sartorelli, J. Bustard, R. Stewart, S. Sjostrom, P. Johnson, M. McMillian, F. Sudhoff, R. Chang, TOXECON™ clean coal demonstration for mercury and multipollutant control at the Presque Isle Power Plant, Proceedings of the International Conference on Air Quality VI: Mercury, Trace Elements, SO3, Particulate Matter, and Greenhouse Gases, 2007, Arlington, VA, Sept 24–27. [27] K. Dombrowski, C. Richardson, J. Padilla, K. Fischer, T. Campbell, R. Chang, C. Eckberg, J. Hudspeth, A. O'Palko, S. Pletcher, Evaluation of low ash impact sorbent injection technologies for mercury control at a Texas Lignite/PRBFired Power Plant, Proceedings of the International Conference on Air Quality VI: Mercury, Trace Elements, SO3, Particulate Matter, and Greenhouse Gases, 2007, Arlington, VA, Sept 24–27. [28] Y. Cao, H. Zhou, C. Cheng, C. Chen, W. Pan, Investigation of sulfur trioxide (SO3) transformation through air pollutant control devices in coal-fired utility boilers by the modified controlled condensation method, Proceedings of the International Conference on Air Quality VI: Mercury, Trace Elements, SO3, Particulate Matter, and Greenhouse Gases, 2007, Arlington, VA, Sept 24–27. [29] S. Sjostrom, M. Dillon, B. Donnelly, J. Bustard, G. Filippelli, R. Glesmann, T. Orscheln, S. Wahlert, R. Chang, A. O'Palko, Influence of SO3 on mercury removal with activated carbon: full-scale results, Proceedings of the International Conference on Air Quality VI: Mercury, Trace Elements, SO3, Particulate Matter, and Greenhouse Gases, 2007, Arlington, VA, Sept 24–27. [30] J.E. Currie, D.W. DeBerry, G.M. Blythe, Enhanced mercury control by wet FGD systems, Proceedings of the International Conference on Air Quality VI: Mercury, Trace Elements, SO3, Particulate Matter, and Greenhouse Gases, 2007, Arlington, VA, Sept 24–27. [31] K. Kai, Y. Kata, H. Kikkawa, N. Imada, New SCR catalyst with improved mercury oxidation activity for bituminous coal-fired boilers, Proceedings of the International Conference on Air Quality VI: Mercury, Trace Elements, SO3, Particulate Matter, and Greenhouse Gases, 2007, Arlington, VA, Sept 24-27. [32] G. Blythe, B. Freeman, B. Lani, C. Miller, Mercury oxidation catalysts for enhanced control by wet FGD, Proceedings of the International Conference on Air Quality VI: Mercury, Trace Elements, SO3, Particulate Matter, and Greenhouse Gases, 2007, Arlington, VA, Sept 24–27. [33] M. Berry, K. Dombrowski, C. Richardson, R. Chang, E. Borders, B. Vosteen, Mercury control evaluation of calcium bromide injection into a PRB-fired furnace with an SCR, Proceedings of the International Conference on Air Quality VI: Mercury, Trace Elements, SO3, Particulate Matter, and Greenhouse Gases, 2007, Arlington, VA, Sept 24–27. [34] J. David, T. Silva, S. Brown, C. Miller, Full-Scale WFGD Hg removal Additives, In Proceedings of the International Conference on Air Quality VI: Mercury, Trace Elements, SO3, Particulate Matter, and Greenhouse Gases, 2007, Arlington, VA, Sept 24–27, 2007. [35] C.E. Hensman, A.L. Dahl, B.D. McIntosh, L. Hawkins, P.I. Kilner, J. Hien, E.M. Prestbo, C. Gilmour, L. Brickett, The impact of Hg control technologies on mobility pathways of Hg, Ni, As, Se, Cd and Pb from coal utilization by-products, Proceedings of the
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International Conference on Air Quality VI: Mercury, Trace Elements, SO3,Particulate Matter, and Greenhouse Gases, 2007, Arlington, VA, Sept 24–27. [36] S. Thorneloe, G. Helms, D. Kosson, F. Sanchez, Evaluating the fate of metals from management of coal combustion residues from implementation of multipollutant controls at coal-fired electric utilities, Proceedings of the International Conference on Air Quality VI: Mercury, Trace Elements, SO3, Particulate Matter, and Greenhouse Gases, 2007, Arlington, VA, Sept 24–27. [37] D.F. Pflughoeft-Hassett, D.J. Hassett, T.D. Buckley, L.V. Heebink, J.H. Pavlish, Activated carbon for mercury control: implications for fly ash management, Proceedings of the International Conference on Air Quality VI: Mercury, Trace Elements, SO3, Particulate Matter, and Greenhouse Gases, 2007, Arlington, VA, Sept 24–27. [38] M.S. Gustin, J. Ericksen, M. Xin, K. Ladwig, Investigation of Hg Release from FGD Materials and Soils amended with Coal Combustion By-Products, In Proceedings of the International Conference on Air Quality VI: Mercury, Trace Elements, SO3, Particulate Matter, and Greenhouse Gases, 2007, Arlington, VA, Sept 24–27, 2007.
[39] D.L. Laudal, Conducting a RATA of continuous mercury monitors using EPA Method 30B, Proceedings of the International Conference on Air Quality VI: Mercury, Trace Elements, SO3, Particulate Matter, and Greenhouse Gases, 2007, Arlington, VA, Sept 24–27. [40] R.J. Truce, R. Crynack, J. Wilkins, Avoiding new source review when injecting activated carbon, Proceedings of the International Conference on Air Quality VI: Mercury, Trace Elements, SO3, Particulate Matter, and Greenhouse Gases, 2007, Arlington, VA, Sept 24–27. [41] J.H. Pavlish, J.S. Thompson, C.L. Martin, L.L. Hamre, R. Wiemuth, S. Pletcher, Fabric filter bag investigation following field testing of sorbent injection for mercury control at TXU's Big Brown Station, Proceedings of the International Conference on Air Quality VI: Mercury, Trace Elements, SO3, Particulate Matter, and Greenhouse Gases, 2007, Arlington, VA, Sept 24–27.
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Fuel Processing Technology j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / f u p r o c
Preface
Preface to the AQVI special issue of fuel processing technologies entitled: Air quality VI: Mercury, trace elements, SO3, particulate matter, and greenhouse gases John H. Pavlish
The Air Quality Conference held September 24–27, 2007, in Arlington, Virginia (AQVI) reviewed the state of environmental science and policy on mercury, particulate matter, and greenhouse gases. The proceedings addressed impacts on health and ecosystems, emission prevention and control, and atmospheric and geochemical modeling. This special edition is focused on the status of science and technology for the control of mercury emissions from coal-burning electric generating plants, drawing principally on papers presented at AQVI and contained herein.
1. Health impacts of low-level mercury exposure Low-level exposure to mercury remains a leading public health concern in the United States, especially for children and women of childbearing age. In 2001, the Environmental Protection Agency (EPA), supported by a review by the National Academy of Science, established a mercury reference dose of 0.1 µg/kg body weight/day as a level without recognized adverse effects, with a 10-fold margin of safety to account for biological variability and database insufficiencies. Essentially all mercury exposure in the United States is from consuming fish containing high levels of mercury that have bioaccumulated in the food chain. Health effects statistically linked to mercury exposure include cognitive development in infants and toddlers, the IQ of adolescents and young adults, and possible cardiovascular disease in adult males. None of these effects has been shown at exposure levels below the regulatory threshold established by EPA [1]. Mercury exposure in the United States has, to date, been reduced through the elimination of most industrial, medical, and household uses of mercury and the issuance of EPA guidelines and state advisories limiting the consumption of certain kinds or amounts of fish. Major differences in the neurological impacts of dietary mercury exposure observed in various studies in the Faroe Islands, New Zealand, the Seychelles, and the United Kingdom have been linked to the beneficial effect of an excess of dietary selenium over mercury as a cofactor protecting against the adverse biological effect of mercury to bind with selenium and impair the synthesis of selenium-dependent enzymes required for neurological functions [2]. Research suggests that future guidelines would be advanced by considering the ratio of selenium to mercury in seafood.
E-mail address: [email protected]. 0378-3820/$ – see front matter © 2009 Published by Elsevier B.V. doi:10.1016/j.fuproc.2009.09.006
2. Regulation of mercury emissions from coal-fired power plants Mercury emissions from U.S. coal-fired power plants were estimated to be 51.3 tons in 2005 [3] or about 40% of the nation's total anthropogenic emissions. In March 2005, EPA issued the Clean Air Mercury Rule (CAMR) providing a two-phase cap-and-trade approach that would have reduced power plant emissions to 38 tons in 2010 and to 15 tons in 2018. The first phase was intended to be largely accomplished as a cobenefit of the 2005 Clean Air Interstate Rule (CAIR), which required utilities in the eastern United States to install wet flue gas desulfurization (FGD) for SO2 control and selective catalytic reduction (SCR) for NOx control. The U.S. Court of Appeals for the DC Circuit vacated CAMR in February 2008 and CAIR in July 2008, which has had the effect of requiring all coal-fired power plants to meet a more stringent Maximum Achievable Control Technology (MACT) standard for mercury. A pending Senate bill, S 2643, would require emission reductions of not less than 90% for all new and existing plants. Also, at the end of 2007, some 16 states had established programs that require reductions of 80% to 90% when fully implemented [4]. Canada-Wide Standards for coal-fired power plants call for a first-phase 60% reduction in emissions from existing plants by 2010 under individual provincial caps that reflect regional differences, a second-phase target of 80% in 2018, and immediate implementation of best available control technology for all new plants [5]. The Province of Alberta, which accounts for 46% of the total emissions from coal plants across Canada, will be required to reduce emissions by 70% by 2010 and to submit proposals for targeting 80% under a regulatory review in 2013 [6].
3. The international context on mercury emission and control Benefits from more stringent mercury control requirements will be constrained in their timing and ultimate effect by the readiness of the control technologies to meet a 90% control standard and by uncertainties surrounding the effectiveness of U.S. controls in the context of much larger world emissions from natural and anthropogenic sources. Mercury is a global pollutant with a half-life on the order of 6–12 months in the atmosphere, meaning that a major fraction of U.S. emissions are deposited outside the United States, and a major portion of U.S. deposition originates outside the country. While estimates of the effect of overseas emissions vary, EPA has estimated that 83% of mercury deposition in the United States originates from international sources outside the auspices of U.S.
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policy, including natural emissions, rerelease of historic mercury previously deposited, and anthropogenic sources [7]. Total global emissions of approximately 7000 tons of mercury are roughly estimated to be one-third natural, one-third reemitted, and one-third anthropogenic [8]. The United Nations Environment Programme (UNEP) estimates that anthropogenic emissions in the United States totaled 130 short tons in 2005 or approximately 6% of a world total of 1930 tons (U.S. coal-fired power plants contribute about 2.4% to the world's anthropogenic emissions) — compared to an anthropogenic total of 635 tons (33%) for China and 1281 tons (67%) for all of Asia [9]. Since the issuance of this report, a global treaty to reduce mercury emissions has been signed by over 140 countries [10]. The data on global emissions and current understanding of the atmospheric transport, transformation, and deposition of mercury suggest that the global mercury balance is more important than utility emissions in determining local deposition patterns. However, hot spots of deposition in the vicinity of power plants and other sources remain a concern depending on the form of the emissions. Most mercury is either emitted in elemental form or is partially and rapidly reduced to elemental form in the atmosphere, causing it to be widely dispersed [11, 12, 13, 14]. Oxidized and particulate forms of mercury have a lifetime of only a few days and are more readily deposited locally. Mercury concentrations in rainfall close to two power plants in New South Wales, Australia, were found to be twice as high on average as those measured 100 miles away, but mercury deposition was also shown to be influenced by seasonal variations in local climate conditions at the two sites [15]. The significance of hot spots as an issue affecting the need for more stringent controls has not been fully resolved. 4. Overview of the state of development for mercury control technology Mercury control technologies needed to achieve 90% mercury emission control throughout the coal-fired power industry have been tested and show promise but still require additional development and demonstration. While progress has been substantial, technical issues remain in matching technologies to coal properties and plant designs and in demonstrating long-term reliability without adverse impacts on balance of plant. Coal properties are critically important in determining the forms of mercury produced in the combustion process and their capture in pollution control devices. Higher levels of chlorine and iron, typically found in Appalachian and Interior Basin bituminous coals, promote conversion of elemental mercury to the oxidized form that is more easily captured for example in a wet FGD scrubber. Western subbituminous coals and lignites from the northern Great Plains contain lower levels of chlorine and generate primarily elemental mercury that is not captured in a scrubber and is more difficult to capture by activated carbon injection (ACI). Gulf Coast lignites are more variable and generate between 50% and 90% elemental mercury. Higher levels of sulfur trioxide (SO3) in flue gas reduce the effectiveness of ACI control technology, whether the SO3 is generated by burning high-sulfur bituminous coal, by adding SO3 for flue gas condition ahead of an electrostatic precipitator (ESP), or by catalytic conversion of SO2 to SO3 in a SCR unit used for NOx control. The alkali and alkaline-earth elements in subbituminous coals and lignites react with HCl and SO3 in flue gas, thereby reducing both the chlorine available for mercury oxidation and the SO3 impeding mercury capture on activated carbon. Starting in 2000, the U.S. Department of Energy (DOE) National Energy Technology Laboratory, under the department's Innovations for Existing Plants Program, initiated a three-phase field testing program to bring Hg control technologies to the point of commercial deployment readiness [16]. The program incorporates lessons learned to advance Hg control technologies for specific areas of need. To date,
field tests have been conducted at 50 coal-fired power plants. Notable success has been achieved in using treated activated carbons for capturing mercury when low-chlorine lignites and subbituminous coals are burned and in increasing mercury oxidation to improve capture in wet FGD scrubbers. The effects of high levels of SO3 in flue gas have been mitigated by introducing alkaline compounds, either on treated sorbents or as separate additives. The TOXECON™ and TOXECON II™ ACI configurations developed by EPRI have been shown to preserve the sale of fly ash for use as a substitute for portland cement in concrete by capturing fly ash upstream of the point of ACI. Overall, the DOE program has contributed significantly to commercial deployment, with more than 80 full-scale ACI systems ordered by United States coal-fired power generators as of February 2008. The cost of higher levels of control up to 90% has been reduced from over $30,000 to less than $10,000 per pound of mercury removed using treated AC. However, the cost associated with loss of fly ash use still remains a concern and is significant. Levels of control up to and exceeding 90% have been demonstrated for many coal types and plant configurations in short-term tests, but average removals in longer-term tests have generally been lower because of variability in coal properties and plant operating conditions. The long-term reliability and availability of mercury control technologies, and their effects on balance of plant, remain to be demonstrated. 5. Fundamental coal mercury science Advances in mercury control technology are supported by research on the fundamental chemical mechanisms controlling the oxidation and capture of mercury. Mercury in a coal flame is initially released as elemental vapor Hg0 that is partially converted to oxidized mercury Hg2+ as the combustion gases cool. Thermodynamic equilibrium calculations predict nearly complete conversion to Hg2+, but kinetic limitations relating to flue gas chemistry and plant design result in conversion ranging from <10% to >90% oxidation. Kinetic experiments and modeling of mercury oxidation reactions show important roles both for homogeneous reactions of atomic Cl and Br in the vapor phase and for heterogeneous reactions of halogen species absorbed on the surface of fly ash, unburned carbon, or injected AC [17, 18]. Bromine has been consistently observed to be more effective than chlorine in promoting mercury oxidation and/or capture, whether in the form of a halide-treated AC or as a halide salt used as a sorbent enhancement additive (SEA), but there is no wellestablished theoretical basis to explain this difference. The absorption and reaction of mercury on various surfaces are limited by competition with other gaseous species present in much higher concentrations. A detailed mechanistic model of the chemistry of mercury oxidation and capture on AC has evolved from a matrix of laboratory tests wherein simulated flue gas containing either Hg0 or HgCl2 was passed through a fixed (thin) bed of AC in the presence or absence of all combinations of SO2, HCl, NO, NO2, and water vapor [19, 20]. Results of these experiments and related surface analysis of spent sorbents indicate that Hg0 is oxidized by NO2 or other oxidizing gases on carbon oxidation sites formed by prior surface reactions of HCl with zigzag edge structures on the carbon. If the AC did not originally contain such oxidation sites, an induction period in the presence of HCl is required before surface oxidation of elemental mercury can begin to occur. Oxidized mercury, Hg2+, whether absorbed from the flue gas or oxidized on the surface, is bound to a basic carbene site on the zigzag edge structure unless the site is already occupied. Competition for capture sites occurs when SO2 is oxidized by NO2 to form nonvolatile H2SO4 in the presence of water vapor, which occupies the basic capture sites. The presence of SO3 in flue gas has a similar effect [21]. Omitting either NO2 or water vapor avoids the formation of H2SO4 by surface oxidation of SO2. Theoretical calculations on energies of association have shown that addition of HCl to a
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zigzag carbon edge structure is a feasible first step for generating the required sites for mercury oxidation on a carbon surface. The calculation performed for the subsequent reaction of Hg0 with surface sites indicates initial formation of an Hg complex followed by reaction with a halide anion to form a strong C–Hg–X covalent bond as the capture mechanism [22]. 6. Chemically treated carbons and SEAs The focus of development of mercury control technology for western Powder River Basin (PRB) and lignite coals has been on halogenated ACs and halogen-based SEAs. Native mercury capture for these coals is usually much lower than that for eastern bituminous coals owing to lower concentrations of halogens in the coal. The most effective way to achieve higher levels of mercury capture for these coals is to introduce chlorine, bromine, or iodine, either by treating the carbon or by adding a halogen compound as an SEA. Several tests were run at conditions minimizing carbon contamination in fly ash to preserve sale as a cement amendment. Pilot-scale tests were run to evaluate mercury removals for a variety of candidate sorbents and SEAs across both an ESP and a spray dryer/fabric filter (SD/FF) when low-chlorine lignite is burned [23]. Baseline mercury removals of less than 10% were increased to levels in the range of 70% to 95% at injection rates in the range of 1 to 5 lb/Macf using either treated carbons (treated with HCl or a proprietary halide additive) or a combination of a carbon-based sorbents and a proprietary halogen SEA (the EERC's SEA2). Removals were higher in the SD/FF than across an ESP at similar injection rates. NaCl was a more effective SEA than CaCl2, but neither was as good as SEA2. Slipstream tests run on a pilot-scale fabric filter downstream of the plant's ESP (representing a TOXECON™ configuration) at the SaskPower Poplar River Station burning Northern Great Plains lignite showed significant differences in mercury capture among nine AC sorbents, with four capturing over 90% at an injection rate of 2 lb/Macf [24]. Parametric tests showed no appreciable effect on capture for temperature variations between 200°F and 350 °F, air-to-cloth (A/C) ratios of 2 to 8 ft/min, or ash loadings of 3 to 60 lb/Macf. A preliminary economic assessment estimated costs in the range of $17,770 at an A/ C ratio of 8 up to $20,520 at an A/C of 4, corresponding to an increase of 3 to 4 mills/kWh in generating cost. Full-scale short-term tests at the Sundance Plant burning Canadian subbituminous coal demonstrated 70% or higher capture across the cold-side ESP injecting approximately 2 lb/Macf brominated AC DARCO Hg-LH and up to 95% removal at 6 lb/Macf [25]. In a 30-day test, an average removal of 80% was achieved at an average injection rate of 2.1 lb/Macf, with removals varying from 62% to over 95% at injection rates between 0.55 and 8 lb/Macf. GE's patented combustion modifications for capturing mercury on unburned carbon reduced injection rates by 20% to 30% while reducing NOx emissions from about 380 to 200 ppm and increasing loss on ignition (LOI) in fly ash from 0.8% to 2%. Tests on a full-scale TOXECON™ system at the We Energies Presque Isle plant burning PRB coal showed mercury capture levels of 60% to 95% at injection rates between 0.5 and 3 lb/Macf [26]. Removals using brominated carbon DARCO Hg-LH were typically 5% higher than those for the baseline carbon DARCO Hg and were less affected by an increase in temperature above the baseline of 330 °F. A targeted removal of 90% was maintained for 48 consecutive days in a long-term test designed to validate performance goals. Mercury removals at higher temperatures up to 380 °F were improved by more frequent online cleaning of filter bags, suggesting that captured mercury may have been reemitted when the spent AC was retained on the filter for a longer period of time. Full-scale tests were performed at the NRG's Limestone Electric Generating Station firing a 30/70 blend of Texas lignite and PRB coal to evaluate three mercury control technologies that would allow the
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plant to continue marketing fly ash: 1) injecting only a small amount of carbon upstream of the ESP, 2) injecting a non-carbon or a “passivated” carbon sorbent upstream of the ESP, or 3) injecting AC midstream in the ESP after most of the ash has been collected using a TOXECON™ II configuration [27]. Injecting 2 lb/Macf of the brominated carbon DARCO Hg-LH ahead of the ESP achieved 90% capture while allowing the fly ash to provisionally pass a test on the amount of air entrainment agent needed to stabilize concrete. A passivated and brominated test carbon, C-PAC, also passed the air entrainment test when injected at the higher rate of ~5 lb/Macf needed to achieved 90% removal. A nontreated carbon, DARCO Hg, captured nearly the same percentage of mercury as the two other brominated carbons tested, possibly because of the relatively high-chlorine content in Texas lignite. The non-carbon test sorbent, MS200, achieved only 50% capture at injection rates up to 12 lb/Macf. Finally, the TOXECON™ II configuration tested at this plant achieved only about 60% mercury capture at injection rates up 5 lb/Macf. Carbon injection either upstream of the ESP or midstream resulted in some carbon breakthrough at the ESP outlet. 7. Improving mercury capture on activated carbon at high levels of SO3 SO3 concentrations of 20 and 25 ppm were measured at the SCR inlets at two plants burning high-sulfur bituminous coal, and SO3 levels increased slightly across the SCRs owing to catalytic conversion of SO2 to SO3 [28]. SO3 levels at a third plant burning PRB subbituminous coal were much lower, ~ 1 to 2 ppm, owing to the low sulfur content and the relatively high levels of alkali and alkalineearth elements in the coal. However, many plants burning PRB coal (comprising 25 GW of capacity) inject SO3 into the flue gas to lower fly ash resistivity for the purpose of improving particulate collection in ESPs [29]. Higher levels of SO3 resulting from either high-sulfur coal or SO3 injection interfere with the capture of mercury on AC because of competition for capture sites on the carbon. At the Labadie Power Plant, which burns PRB coal, the injection of SO3 at rates resulting in 5.4 and 10.7 ppm in flue gas showed that mercury capture on brominated AC, DARCO Hg-LH, was significantly reduced as the concentration of SO3 was increased, from a range of 60% to 95% without SO3 injection to a range of 10% to 75% with injection [29]. Substitution of a brominated carbon that also contained alkaline material to capture and neutralize SO3, DARCO Hg-E26, raised removals when injecting SO3 to levels comparable to those observed without SO3 using DARCO Hg-LH. 8. Improving mercury oxidation and capture in FGD and SCR systems Wet FGD scrubbers capture most of the oxidized mercury Hg2+ formed when high-chlorine bituminous coals are burned but remove essentially none of the elemental mercury that is predominantly formed when PRB coal and other low-chlorine coals are burned. Some of the Hg2+ initially captured is reduced back to Hg0 in the scrubber and reemitted by reactions that are importantly influenced by concentrations of chloride and sulfite and the pH of the scrubber solution [30]. Methods for increasing the level of mercury oxidation and improving capture in FGD scrubber systems include 1) catalysts that promote high Hg0 oxidation with low SO2-to-SO3 conversion, 2) halogen additives alone or in combination with SCR, 3) alkaline compounds that reduce the adverse effect of SO3 on gas-solid reactions, and 4) wet FGD additives that reduce reemission in the scrubber. Babcock-Hitachi has developed a plate-type catalyst reported to achieve Hg oxidation 1.5 to 2.0 times higher and SO2 to SO3 conversion about half that of conventional catalysts [31]. Tests on a
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noble metal catalyst performed by URS at the outlet of the ESP and ahead of the wet FGD at a number of plants have shown oxidation levels of 90% to 95% with fresh catalyst, 60% to 70% after 20 months, and the restoration of initial oxidation levels using regenerated catalyst [32]. The addition of only 5 ppm Br as a solution of calcium bromide (CaBr2) to a PRB coal increased the level of mercury oxidation at the ESP outlet from 60% to 90% at a plant equipped with SCR, with most of the oxidation occurring across the SRC at this low Br addition rate [33]. Higher addition rates of 165 and 328 ppm Br promoted the oxidation of mercury upstream of the SCR to levels of up to 98% at the SCR inlet. Additives developed by a number of companies to reduce the reemission of mercury from wet FGDs have been tested at full scale by URS [30] and B&W [34]. Two candidate additives developed by Solucorp and B&W, respectively, have been reported to effectively inhibit reemission and promote additional capture without observable impacts on scrubber performance or gypsum quality [34]. 9. The impact of mercury on coal utilization by-products (CUBs) Whenever mercury is captured on a sorbent or in a wet FGD scrubber, there is the potential for creating new pathways of mercury release into the environment when the resulting by-products are utilized or placed in a landfill. Several questions that must be answered are 1) how stable is the captured mercury over time, 2) does mercury leach from CUBs and enter the groundwater, and 3) how does mercury control technology impact the processing and reuse of CUBs. Frontier GeoSciences has been contracted by DOE to assess the mobility pathways of mercury and other trace elements in CUBs under Phases II and III of DOE's Mercury Control Field Testing Program. Mercury control technologies may variously increase the amount of mercury present in fly ashes collected in an ESP or fabric filter and in the solid by-products from wet and dry FGD scrubbers. Results reported by Frontier GeoSciences at AQVI indicate that thermal release of mercury from fly ashes is statistically insignificant after 30 days at 40 °C but is significant in some cases after 1 h at 190 °C, representative of wallboard manufacture [35]. A substantial fraction of mercury is released in 5 min at 1200 °C, representing conditions in cement production, but the faction released from fly ash collected with mercury control was not consistently either higher or lower than that released from baseline ashes collected without mercury control. Substantially 100% of selenium was released at 1200 °C. Microbial activity resulted in some increase in methylmercury release, but a number of other trace metals were stabilized. The EPA's Office of Solid Waste (OSW) has incorporated the test framework and probabilistic assessment developed at Vanderbilt University into EPA's SW846 manual, “Test Methods for Evaluating Solid Waste, Physical Chemical Methods.” OSW findings presented at AQVI indicate that mercury is strongly retained in CUBs and is unlikely to be leached at levels of environmental concern [36]. Results of leaching and thermal tests on fly ashes collected by the Energy and Environmental Research Center (EERC) from field demonstrations of ACI for mercury control are in agreement with results reported by OSW and Frontier GeoSciences [37]. Research at the University of Nevada suggests that alternate wetting and drying can result in increased mercury release to the atmosphere from disposal sites containing FGD materials [38]. Long-term release to the environment under actual field conditions is difficult to measure and has not been adequately determined. The physical and engineering properties of fly ashes containing spent AC are similar to those of the baseline fly ash with the exception that even small amounts of carbon can preclude the direct use of the fly ash in concrete where air entrainment is required [37]. Also, limited tests on concrete prepared from fly ashes sampled over a range of bromide addition to coal suggested a possible reduction in compressive strength which warrants further study [33]. Technolo-
gies for minimizing the impacts of sorbent injection for mercury control on the sale of fly ash for use in concrete have been addressed previously in this review. 10. Balance of plant impacts of mercury control The balance of plant impacts of mercury control technology can affect the reliability of an entire power plant at a cost that potentially far exceeds the direct cost of the mercury control alone due to increased maintenance and lost power generation. Cost impacts will be influenced by the regulations that are finally adopted, where longer-term averaging might allow continued operation under circumstances that could force a plant shutdown under a nearcontinuous requirement. Since long-term impacts have not been demonstrated, further work is required on a variety of issues that are not yet fully defined. Future regulatory compliance will require mercury measurements using either near-continuous sorbent traps or continuous mercury monitors. The complexity of these requirements requires operators to become thoroughly familiar with the measurement technology and the reference methods used to certify and calibrate the measurements. The EPA's Method 30B which uses a sorbent trap reference for performing relative accuracy test audits (RATAs) is currently the method of choice for calibration, but still simpler instrumental reference methods are under development [39]. Sorbents, catalysts, and additives variously containing alkali and halides to enhance control mercury emissions can potentially cause boiler tube corrosion, fouling and plugging in SCR units and air preheaters, changes in scrubber chemistry, increased stack opacity and emissions (e.g., bromine), and impacts on the disposal and reuse properties of fly ash and scrubber solids. Most of these possible impacts remain to be studied. ESP performance can be negatively affected by ACI because of the low resistivity and fineness of the carbon. The Indigo Agglomerator technology [40] reduces the concentration of fine particulates entering an ESP by electrostatic agglomeration to address problems with ESP performance and stack opacity and to improve mercury capture. Pressure drop in an existing or add-on FF can, in some cases, be a limiting factor in applying ACI because of buildup of residual material that is not removed by online cleaning. A monthlong test at TXU's Big Brown Station burning a 70/30 blend of Texas lignite and PRB coal determined that the installation of a FF downstream of the existing ESP (TOXECON™) would likely not be possible without major modifications to the plant and its operation concerned with fan capacity and power [41]. The increase in pressure drop over time correlated directly with total particulate loading but not with carbon content, indicating that the AC behaved much like additional fly ash. Carbon injection did not appear to affect the underlying mechanism of residual dust buildup, which was observed to be influenced by formation of ammonium sulfate from injection of SO3 and NH3 for flue gas conditioning ahead of the ESP. However, offline tests did show a larger decrease in filter permeability with ACI than without. The constraint posed by pressure drop can be reduced by installing a larger baghouse at additional cost, where in tests at SaskPower, sustained operation at a typical dust loading could be maintained within the maximum allowable pressure drop of 10 inches W.C. at an A/C ratio of 6 ft/min, but not at an A/C of 8 ft/min [24]. Autoignition of carbon in baghouse hoppers was experienced in the full-scale demonstration of TOXECON™ technology at the Presque Isle Power Plant when the AC/ash mixture exceeded a critical temperature of about 430 °F [26]. The cause was traced to heaters used for temperature control. Modeling of spontaneous combustion was used to establish conditions under which heat would be generated faster than it could be liberated. Guidelines for preventing autoignition involve monitoring and control of temperature,
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flowability and fluidization in the hopper, and measurement of CO as an indicator of incipient ignition.
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