Controlling Mercury Emissions from Coal-Fired Power Plants

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1 em feature by Ramsay Chang Ramsay Chang is a technical executive with the Electric Power Research Institute and has more than 3 years experience in air pollution control. rchang@epri.com. Controlling Mercury Emissions from Coal-Fired Power Plants Increasingly stringent U.S. federal and state limits on mercury emissions from coal-fired power plants demand optimal mercury control technologies. This article summarizes the successful removal of mercury emissions achieved with activated carbon injection and boiler bromide addition technologies nearing commercial readiness as well as several novel control concepts currently under development. It also discusses some of the issues standing in the way of confident performance and cost predictions. awma.org Copyright 29 Air & Waste Management Association july 29 em 5

2 More than 2 states have already imposed stringent limits on mercury emissions from coal-fired power plants, while the U.S. Environmental Protection Agency (EPA) plans to develop Maximum Achievable Control Technology (MACT) regulations for these plants in response to a 28 U.S. court decision. 1 Inherent mercury removal can be achieved for some power plants, such as eastern bituminous coal-fired units equipped with selective catalytic reduction (SCR) and wet or dry sulfur dioxide (SO2) scrubbers. However, a significant number of plants will require additional controls to achieve regulatory compliance. Key options for controlling mercury emissions from power plants 2,3 are broadly classified as precombustion coal cleaning or combustion modifications (not discussed in this article); sorbent injection at various points along the flue gas path to adsorb mercury; coal, boiler, or flue gas chemical additives, corona discharge, or catalysts to oxidize mercury and facilitate its capture in a downstream particulate and/or SO2 control device; scrubber additives to prevent mercury re-emissions from a wet scrubber; and fixed structures such as honeycombs or plates placed in the flue gas stream to adsorb mercury. Activated Carbon Injection One of the most promising near-term, add-on approaches for achieving high mercury removal requirements is activated carbon injection (ACI) into the flue gas upstream of a particulate control device such as a fabric filter (FF) or an electrostatic precipitator (ESP). Typically, ACI would be used at plants that do not have SO2 controls or need to supplement the mercury removals achieved by their SO2 controls. Full-scale tests of ACI have been conducted at more than 5 units in the United States firing eastern bituminous coals and western coals such as Powder River Basin (PRB), subbituminous, and lignite. 2,3 Untreated or brominated sorbents made from coal are the most effective activated carbons (ACs) used in ACI. ACI data show significant variations in mercury removal from site to site, or even at a single site over time. Observed variations may reflect measurement errors, differences among ACs and coals, and differences in power plant operation. Achievable removal rates at each unit must be finetuned through actual, on-site testing. Finally, most ACI data represent less than three months of operation; the ongoing challenge is to maintain these observed performance levels during longterm operation. Units Equipped with ESPs Western Coals. At units firing western coals, high mercury removals (> 9%) across ESPs are achieved with brominated ACI at injection rates of 4 8 pounds per million actual cubic feet (lb/mmacf) (see Figure 1). Nonbrominated ACI is not as effective at units firing western coals and may not be able to achieve 9% removals. However, adding bromide to the coal can enhance ACI performance for these coals. Eastern Bituminous Coals. In contrast, at units firing eastern bituminous coals especially those with high-sulfur content ACI mercury removals across ESPs fall well below those seen for western coals (see Figure 2). The likely cause of poorer ACI performance is competition for the active adsorption sites on the carbon from sulfur trioxide (SO3) in the flue gas. Elevated flue gas SO3 levels may be due to combustion of high-sulfur (> 3%) coal, conversion of SO2 to SO3 across an SCR installed for nitrogen oxide control, or use of SO3 flue gas conditioning to improve ESP performance. Since the majority of coal-fired units in the United States that need to add mercury controls have ESPs, and many of these fire eastern bituminous coals, there is a need to understand and mitigate factors that degrade ACI performance. TOXECON II. In TOXECON II, AC is injected between the existing fields of an ESP. Installation of an additional control device and space are not required. Most of the fly ash is collected in the front ESP fields before contamination with ACs. This relatively new approach has seen only limited testing. Results to date show that high mercury removals may be achieved under some test conditions, but optimal carbon distribution between the ESP fields and potential increases in particulate matter (PM) emissions are issues whose resolution will require further development. 6 em july 29 Copyright 29 Air & Waste Management Association awma.org

3 Units Equipped with FFs Units firing either western or eastern coals and equipped with FFs (e.g., pulse-jet, shake-deflate, reverse-gas) or TOXECON (e.g., pulse-jet FF installed after the ESP with ACI between the ESP and FF) tested to date have been able to achieve high mercury removals (> 9%) with ACI at relatively low injection rates of 1 3 lb/mmacf. ACI mercury removal does not appear to differ significantly with type of coal or FF. Since no mercury removal data are available for FF units firing high-sulfur eastern bituminous coals, ACI performance under these conditions is unknown. With TOXECON FF units, the long-term impact of ACI on FF life and pressure drop is not well understood owing to limited data. High mercury removals (> 9%) across spray dryer-fabric filters (SD-FFs) are achieved without ACI at units firing eastern bituminous coals, and with brominated ACI at units firing western coals. Improving ACI Effectiveness in High SO3 Flue Gas High concentrations of SO3 in flue gas have a large, negative impact on ACI performance for both western and eastern bituminous coals. Tests show that trona, hydrated lime, and sodium bisulfite are effective in reducing flue gas SO3. However, injecting these sorbents can affect ash disposal and use or ESP performance. For cases where SO3 flue gas conditioning is used, operators can inject AC upstream of the air preheater, before the SO3 conditioning comes into play. In addition, if AC injected upstream can be de-agglomerated or reduced in particle size (e.g., through additional grinding), further improvements in mercury removal become possible. Controlling Fine PM Emissions ACI upstream of ESPs may increase fine PM emissions at the stack. This effect generally occurs at units with smaller ESPs (i.e., specific collection area ~ 3 ft 2 /kacfm or lower). At a number of ESP units, observations indicate that total PM mass at the stack appears to decrease with ACI, while AC mass increases. Even for FF units, PM emissions can increase slowly over time as filter bags age and fly ash penetrates across the filter fabric. ACI may exacerbate this problem. Measurements over extended periods of operation will reveal whether increased PM emissions are a problem requiring mitigation. % Vapor-Phase Hg Removal Brominated Carbons Figure 1. ACI mercury removal across ESPs for western coals. Notes: Shapes denote different ACs, with solids representing brominated and outlines representing nonbrominated carbons. Maintaining Ash Use with ACI Many power companies sell fly ash as a replacement for Portland cement and for use as an admixture in concrete. Adding modest amounts of AC, even at the lowest injection rate, may adversely affect the marketability of fly ash for these purposes. Ash marketers express concerns about the impact of the presence and concentration variability of AC on air entrainment additives used in concrete. Some approaches that can maintain the marketability of fly ash as a cement replacement/concrete admixture include injecting AC at a very constant carbon-to-ash ratio; chemically treating fly ash containing AC to block adsorption of air entrainment agents; Injecting specially treated ACs or noncarbon sorbents; and Installing TOXECON or TOXECON II, where sorbents are injected downstream of a primary particulate collector. In addition to these approaches, researchers are investigating the use of high-carbon ash in concrete production, as well as ash uses in other applications that do not require the specifications of a cement substitute. Boiler Bromide Addition Bromide addition to the coal or into the boiler 5 Sorbent (lb/mmacf) Non-Brominated Carbons 15 awma.org Copyright 29 Air & Waste Management Association july 29 em 7

4 % Vapor-Phase Hg Removal Coal S>3% Sorbent (lb/mmacf) Figure 2. ACI mercury removal across ESPs for eastern bituminous coals Notes: For low-sulfur coal, shapes denote different ACs, with solids representing brominated and outlines representing nonbrominated carbons. For high-sulfur coal represented by solid circles, symbol colors denote nonbrominated carbons used at two different sites. effectively increases the fraction of flue gas mercury that is oxidized and, thus, available for capture in downstream particulate control devices and wet or dry SO2 scrubbers. Full-scale tests of boiler bromide addition at units firing PRB and Texas lignite coals have shown that > 9% of flue gas mercury can be oxidized at boiler bromide additions equivalent to 25 3 parts per million (ppm) by weight in coal. The oxidizing effect of bromide is further enhanced at units equipped with SCRs. At these units, > 9% of flue gas mercury is oxidized at boiler bromide additions of < 2 5 ppm in coal. At units equipped with wet flue gas desulfurization (FGD) systems, bromine-oxidized mercury is readily removed by the scrubber. Adding bromide to the coal also enhances the mercury removal effectiveness of nonbrominated ACs. Bromide compounds enter the flue gas through boiler bromide addition and the evolution of bromide from brominated carbons. Their impact on mercury and other trace metal partitioning in power plant solid and liquid streams, or on potential balance-of-plant issues, is not well understood. To date, tests show little impact of bromide compounds on ash use in concrete. However, most of the bromide present in fly ash does leach from the ash. It is uncertain whether bromide leachates present any water discharge issues. With boiler bromide addition, concerns have been raised that the added bromide might accelerate boiler tube wastage and failure. Initial laboratory investigations 4 indicate little potential for boiler corrosion due to bromine, particularly at the low bromide levels (<.2%) added to the boiler. Previous laboratory studies showed that bromine in scrubber water can increase corrosion of some metals used in the scrubbers, especially at the elevated concentrations encountered in scrubbers where much of the water is recycled. Bromide effects on corrosion can vary significantly from alloy to alloy. 5 Novel Concepts Under Development Developers are targeting novel mercury control concepts that could be more efficient and costeffective with minimal balance-of-plant impacts and waste generation. Sorbents Various research groups and manufacturers are developing a large number of carbon and noncarbon sorbents. Novel sorbents include ACs modified for greater ash compatibility in cement replacement, ACs with improved performance in high- SO3 flue gas streams, and largely mineral-based noncarbon sorbents. On-Site AC Generation In the Sorbent Activation Process (SAP), 6 AC is produced on-site from facility coal in an entrained flow reactor and then injected directly into the flue gas upstream of a particulate control device. Initial proofof-concept testing with a small SAP reactor shows 8 em july 29 Copyright 29 Air & Waste Management Association awma.org

5 that AC produced using this process can approach commercial AC in mercury sorption capacity. Fixed Structures One approach to mercury control installs various fixed structures such as honeycombs, woven screens, or plates in the flue gas path (see Figure 3). Made of carbon or other mercury sorbents, these fixed structures can capture flue gas mercury efficiently, do not affect fly ash, and have regeneration potential. One example is MercScreen, a process currently being investigated by the Electric Power Research Institute (EPRI). It uses a bed of AC granules or pellets installed at the ESP outlet. 7 The bed acts as a filter to remove vapor-phase mercury exiting the ESP. Other structures serve as mercury oxidation catalysts to enhance mercury capture by a downstream SO2 control device; EPRI is collaborating with the U.S. Department of Energy and the Lower Colorado River Authority to demonstrate this concept at the -MW scale. Summary In testing conducted on western coal-fired units with FFs or TOXECON to date, ACI has generally achieved mercury removal rates > 9%. At units with ESPs, similar performance requires brominated ACI. Alternatively, units firing western coals can use boiler bromide addition to increase flue gas mercury oxidation (especially in the presence of an SCR) and downstream capture in a wet scrubber, or to enhance mercury removal by ACI. At eastern bituminous-fired units with ESPs, ACI is not as effective, largely due to SO3 resulting from the high sulfur content of the coal or the use of SO3 flue gas conditioning to improve ESP performance. Honeycomb Plates Carbon Cloth Figure 3. Example fixed structure concepts. stand in the way of confident performance and cost predictions. Resolution of these issues will require a fuller understanding of the factors that affect mercury removal performance, the fate of mercury and sorbents in plant discharge streams, and the unintended impacts of these control technologies on power plant operation in the form of ESP and FF performance, increased PM emissions, corrosion, and changes affecting fly ash use and water discharge. Also, the mercury removals were determined for a limited number of power plants representing a wide variety of coal types and plant configurations and the ability to maintain similar removals at all plants are not clear until more plants have mercury controls implemented and operating for extended periods of time. New mercury control approaches are under development. Novel carbon and noncarbon sorbents may have lower costs, better ash compatibility, higher tolerance for acid gases, less impact on power plant operation, and/or the potential to adsorb other trace hazardous air pollutants such as selenium and arsenic. em Although ACI and boiler bromide addition are nearing commercial readiness, significant issues References 1. See Massachusetts vs. EPA, 549 U.S. 497 (27) and Advanced Notice of Proposed Rulemaking, 73 Fed. Regist (July 3, 28). 2. Feeley, T.J.; Jones, A.P. An Update on DOE/NETL s Mercury Control Technology Field Testing Program: U.S. Department of Energy National Energy Technology Laboratory, Pittsburgh, PA, July 28. See 3. Mercury Control Technology Selection Guide; EPRI report 12672; EPRI, Palo Alto, CA, Investigation of the Effect of Bromine Derived Species on Waterwall Wastage, Analysis of a PRB Coal and a Texas Lignite; EPRI report 14724; EPRI, Palo Alto, CA, Steen, W.A.; Currie, J.E.; Dombrowski, K.D.; Blythe, G.M; Chang, R. Bromide in FGD Systems Electrochemical Corrosion Rate Tests. Presented at AIRPOL 29, Detroit, MI, June, Chang, R.; Rostam-Abadi, M.; Chen, S. Apparatus and Method for Removal of Vapor-Phase Contaminants from a Gas Stream by In-Situ Activation of Carbon-Based Sorbents, U.S. Patent 6,451,94 B1, September 17, Looney, B. Field Investigations of Fixed-Bed Sorbents for Mercury Capture from Coal-fired Flue Gas. Presented at the Power Plant Air Pollutant Control Mega Symposium, Baltimore, MD, August 28; Paper #6. Acknowledgments The author thanks the U.S. Department of Energy s National Energy Technology Laboratory, EPRI member companies, the Illinois State Geological Survey, URS Corp., ADA Environmental Solutions, Apogee Scientific Inc., Reaction Engineering International, and the various AC and materials suppliers for their participation in and support of key projects. awma.org Copyright 29 Air & Waste Management Association july 29 em 9