Controlling Power Plant Emissions: Mercury-Specific

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Controlling Power Plant Emissions

Activated carbon injection
Advanced sorbents
Additional information

Activated Carbon Injection (ACI)

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 (FF). 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. This approach is shown schematically in Figure 1.

Schematic showing sorbent being injected into flue gas upstream of a particulate matter control device

Figure 1. Schematic of Activated Carbon Injection for Mercury Control

The performance of activated carbon is related to its physical and chemical characteristics. Generally, the physical properties of interest are surface area, pore size distribution, and particle size distribution. The capacity for Hg capture generally increases with increasing surface area and pore volume. The ability of Hg and other sorbents to penetrate into the interior of a particle is related to pore size distribution. The pores of the carbon sorbent must be large enough to provide free access to internal surface area by Hg⁰ and Hg²⁺ while avoiding excessive blockage by previously adsorbed reactants. As particle sizes decrease, access to the internal surface area of the particle increases along with potential adsorption rates.

Carbon sorbent capture is also dependent upon temperature, the concentration of Hg in the flue gas, the flue gas composition, and other factors. The selection of a carbon for a given application should take into consideration the total concentration of Hg, the flue gas composition, and the method of capture (i.e., ESP, FF or dry scrubber).

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 coal-fired boilers will not, in general, be equivalent to that on MWCs and MWIs.
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/technologies/coalpower/cctc/

Advanced Sorbents

Several promising advanced sorbents have been developed with the support of funding from EPA's Small Business Innovative Research (SBIR) Program. These and other promising advanced sorbents are being tested. If successful, these sorbents may significantly reduce the costs of mercury control by obtaining high levels of mercury capture with lower amounts of injected sorbent. Development and demonstration of these innovative sorbents will continue, especially for sub-bituminous and lignite coals and for use in high temperature environments (e.g., hot-side ESP).

Additional Information

In addition to the mercury control approaches discussed above, other approaches are also under development. Some of these approaches are described in two EPA reports that are available at:

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

"Performance and Cost of Mercury and Multipollutant Emission Control Technology Applications on Electric Utility Boilers," EPA-600/R-03-110, October 2003.

Report (PDF) (108 pp., 1.8 MB, About PDF)
URL: http://www.epa.gov/appcdwww/aptb/EPA600R03110.pdf
Appendices (PDF) (75 pp., 517 KB, About PDF)
URL: http://www.epa.gov/appcdwww/aptb/EPA600R03110Appendices.pdf