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Controlling Power Plant Emissions: Control Technology

The ability to capture mercury from the flue gas of a coal-fired power plant is highly dependent upon the form (or species) of mercury in the gas stream. During combustion, the mercury in coal is volatilized and converted to elemental mercury (Hg0) vapor in the high temperature regions of coal-fired boilers. As the flue gas is cooled, a series of complex reactions begin to convert Hg0 to vapor phase ionic mercury (Hg2+) compounds and/or solid phase or particulate-bound mercury (Hgp). The specific partitioning of mercury into Hg0, Hg2+ and Hgp (known as mercury speciation) can have a considerable influence on the viability of specific capture technologies. The speciation of the mercury in the flue gas is influenced by a number of factors including the type of coal being burned, the time/temperature profile of the flue gas stream, the composition of the flue gas (especially the chorine content), the composition of the fly ash (e.g., unburned carbon, calcium, iron), the physical characteristics of the fly ash (e.g, surface area, porosity), and existing air pollution controls. In general, the majority of vapor phase mercury in bituminous coal-fired boilers tends to be Hg2+, while that in sub-bituminous and lignite coal-fired boilers tends to be Hg0.

Potential Mercury Control Technologies

1. Controlling Mercury with Existing Controls
Data from the EPA's Mercury Information Collection Request (ICR) revealed that many power plants have existing mercury reduction as a co-benefit of air pollution control technologies for NOx, SO2, and particulate matter (PM).These existing controls offer a significant opportunity for mercury capture.

2. Mercury-specific controls
Mercury-specific controls, most notably activated carbon injection (ACI), are used on municipal waste combustor (MWC) and medical waste incinerator (MWI) facilities in the U.S. and Europe. In ACI technology, powdered activated carbon (PAC) sorbent is injected into the flue gas at a location in the duct preceding the particulate matter (PM) control device, which usually is an electrostatic precipitator (ESP) or a fabric filter. The PAC sorbent binds with the mercury in the flue gas in the duct and in the PM control device. Subsequently, the mercury-containing PAC is captured in the PM control device. Greater mercury removal is obtained with a fabric filter compared to an ESP because of enhanced gas-particle contact in the filter cakes on the surface of the bags in a fabric filter. At present, ACI is the most widely studied of the mercury-specific control technologies for coal-fired power plants and shows the potential to achieve moderate-to-high levels of mercury control. However, the experience with ACI applications on MWC and MWI facilities is not directly transferable to applications on coal-fired boilers for several reasons:

In general, the concentration of mercury in the flue gas of MWCs and MWIs is an order of magnitude higher than for coal-fired boiler systems, for example, 200-1000 micrograms/dscm compared to 5-30 micrograms/dscm. It is well known that removal of mercury by ACI is limited by the mass transfer (i.e., the transfer of mercury from the bulk gas to the surface of the carbon particle) in the duct and/or the ESP. This mass transfer is greater for higher concentrations of mercury in the flue gases of MWC and MWI systems. Based on this difference in mass transfer, the amount of mercury captured per unit mass of AC injected will, in general, be higher in MWCs and MWIs compared to coal-fired boilers.

The flue gases of MWCs and MWIs have higher chlorine contents than those found in flue gases of coal-fired utility boilers, particulary boilers firing low-rank coals. Performance of ACI in situations with low levels of chlorine in the flue gas may be adversely affected as was evidenced at the Powder River Basin (PRB)-fired Pleasant Prairie plant. Consequently, ACI performance on MWCs and MWIs will, in general, not be equivalent to that on coal-fired boilers.

In general coal-fired power plants are much larger in size compared to MWCs or MWIs. For example a large MWC may be about the same size as a small, 40-50 MW, coal-fired plant. Accordingly, duct dimensions, generally, are much larger in coal-fired plants compared to those at MWCs and MWIs. Since mixing of injected AC and flue gas in the duct affects mercury capture performance, design of AC injection systems will, in general, be more involved for coal-fired boilers.

Taking the above into consideration, testing of activated carbon and other sorbents has been undertaken at coal-fired utility boilers. Research on ACI and other mercury-specific technologies has been underway at the Department of Energy (DOE) for several years. The first full-scale commercial demonstration project for ACI was initiated by DOE in April, 2004 and is projected to be completed in 2009. A number of similar projects will be necessary to establish this technology’s effectiveness on other coal types.

A description of first full-scale commercial demonstration project for ACI -Presque Isle Plant - is at: http://www.netl.doe.gov/publications/press/2004/tl_ccpi_weenergiesaward.html

General information on NETL demonstration projects, Projects under Round 1and proposals received under Round 2, and general strategic information on the program can be found at: http://www.netl.doe.gov/coalpower/ccpi/index.html

Additional Information

Control of Mercury Emissions from Coal-Fired Electric Utility Boilers (PDF, 279K, 15 pp, about PDF)
This paper presents the results of an assessment by EPA's Office of Research and Development of the state of mercury control technology as of January 1, 2004.
URL: http://www.epa.gov/ttn/atw/utility/hgwhitepaperfinal.pdf

Mercury Control Technology (PDF, 7 pp., 73KB)
This memo presents DOE’s views regarding the status of technologies for control of mercury emissions from coal-fired power plants as of January 8, 2004.
URL: http://www.epa.gov/mercury/control_emissions/mercurytechnologiesjan04.pdf

Environmental Technology Verification Program
Information about Mercury Continuous Emission Monitors (CEMs).
URL: http://www.epa.gov/etv/verifications/vcenter1-11.html

National Energy Technology Laboratory (NETL)
NETL currently manages the largest funded research program in the country for developing an understanding of fossil combustion-based mercury emissions, including their measurement, characterization, and the development of cost-effective control technologies for the U.S. coal-fired electric generating industry.
URL: http://www.netl.doe.gov/coal/E≀/mercury/index.html exit EPA

This index page links readers to both the basic mercury R&D projects, as well as work DOE is doing on evaluating the toxicity of power plant solid wastes and byproducts.
URL: http://www.netl.doe.gov/coalpower/environment/index.html exit EPA


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