Order Code RS21936
September 15, 2004
CRS Report for Congress
Received through the CRS Web
Air Pollution Emission Control: Existing
Technologies and Mercury Cobenefits
Analyst in Science and Technology Policy
Resources, Science, and Industry Division
The Environmental Protection Agency (EPA) regulates the amount of pollution
emitted into the atmosphere by stationary combustion sources. To meet these
regulations, stationary sources use various techniques to reduce air pollutant emissions,
including installing post-combustion emission control technologies. Some postcombustion technologies reduce the emissions of other pollutants besides the one for
which they are designed. These concomitant reductions are called cobenefits. The EPA
has proposed regulating mercury emissions from coal-fired electric power plants by
relying on the results that these post-combustion emission control technologies achieve
through cobenefits. The appropriateness of using cobenefits to set emission limits, the
reproducibility and reliability of cobenefits, and the likelihood that new technologies
specifically designed to reduce mercury emission will be commercially available in the
near future are issues of congressional interest. This report will not be updated.
The Environmental Protection Agency (EPA) has proposed regulations to reduce
atmospheric mercury emission from coal-fired electric power plants,1 as a plausible link
exists between methylmercury concentrations in fish, a source of mercury-related health
effects in humans when consumed, and electric utility mercury emissions. The final EPA
regulation will provide either a mercury emission rate per generating unit or alternatively
a national mercury emission cap with tradeable emission credits. For more on the EPA
Mercury Rule, see CRS Report RL31881, Mercury Emissions to the Air: Regulatory and
Legislative Proposals, by (name redacted) and CRS Report RL32273, Air Quality:
EPA's Proposed Interstate Air Quality Rule, by (name redacted) and (name redacted).
These proposals have led to renewed congressional interest in current postcombustion emission control technologies. Under the proposed regulatory frameworks,
the required mercury emission reductions would be realized through cobenefits,
reductions in mercury which arise as a side effect from current emission control
69 Fed. Reg. 4652-4752, January 30, 2004, and 70 Fed. Reg. 12398-12472, March 16, 2004.
Congressional Research Service ˜ The Library of Congress
technologies designed for capture of other pollutants. This report will discuss these and
other current emission control technologies and their application to reducing mercury
emissions from electric utilities.
The EPA, under the provisions of the Clean Air Act (42 U.S.C. 7401-7671), has set
National Ambient Air Quality Standards for six pollutants. These six pollutants, often
referred to as criteria pollutants, are lead, ozone, nitrogen oxides (NOx), sulfur dioxide
(SO2), carbon monoxide, and particulate matter. The degree to which stationary
combustion sources emit criteria pollutants often depends on the fuel used. For example,
combustion of fuels containing larger amounts of sulfur-containing compounds may lead
to greater emissions of sulfur dioxide. Approaches to reduce these emissions include
pretreatment of fuel, optimization of combustion, and post-combustion control
technologies. This report focuses solely on post-combustion control technologies.
The performance of post-combustion emission control equipment depends on many
factors, including fuel composition, the design of the emission system, the flow speed of
the flue gases (exhaust), the completeness of the combustion process, and the presence
of other complementary emission control technologies. Thus, determining the control
efficiencies for different technologies without empirical testing is very difficult. Control
efficiency models for a given emission control technology usually contain experimentally
determined values specific to a particular combustion process.
Companies that own and operate stationary combustion sources may choose to
implement different control technologies, so long as pollutant emissions meet the limits
established in EPA regulations. A company’s choice of post-combustion emission control
technology may not be based solely on pollutant control efficiency. Other factors include
relative costs, both capital and operating; production of additional waste streams, such as
from technologies using liquid solutions to remove pollutants; energy use; and technology
Post-combustion Emission Control Technologies
Post-combustion emission control technologies are generally designed to remove a
particular pollutant, and may be operated in different combinations. An overview of
select post-combustion control technologies is provided below.
Selective Reduction for Nitrogen Oxides. Selective reduction techniques
reduce the amount of nitrogen oxides (NOx) present in the flue gas through injection of
ammonia gas, which reacts with the nitrogen oxides to form nitrogen and water vapor.
There are two types of selective reduction technologies, catalytic and non-catalytic.
Selective catalytic reduction (SCR) employs a bed of materials, such as specially prepared
ceramics, which enhances the reaction of ammonia with the nitrogen oxides. Selective
non-catalytic reduction (SNCR) involves the same process without the assisting bed of
materials, instead operating at higher temperatures.
Scrubbers for Sulfur Dioxide. Flue gas desulfurization (FGD) technologies,
also known as scrubbers, are primarily designed to remove sulfur dioxide from flue gas.
Scrubbers fall into two general types: wet and dry. In both cases a material, typically
powdered or dissolved limestone, reacts with sulfur oxides present in the flue gas,
forming compounds which are later removed.
Wet scrubbers spray a liquid mixture into the flue gas. There are many different
configurations of wet scrubbing technology, but they operate on the same principles of
gas/liquid interaction. Their energy costs, control efficiencies, and other factors vary by
particular configuration. Because of the use of liquids in wet scrubbers, handling and
potential treatment of waste liquids is often a design concern.
In contrast, dry scrubbers, also known as spray dryer adsorbers (SDA), react
powdered, dry material with the flue gas. Dry scrubbers usually create significant
particulate matter during their operation, and are often coupled with a particulate matter
control technology to remove the additional powdered material. The sulfur dioxide
emissions reduction in a dry scrubber are generally less than in a wet scrubber, but dry
scrubber use avoids generating a liquid waste stream.
Particulate Matter Controls. Three common technologies specifically for
particulate matter (PM) control are mechanical collectors, electrostatic precipitators, and
fabric filters. The choice between mechanical collectors, electrostatic precipitators, and
fabric filters depends on the amount and size of PM generated. PM control is also a
cobenefit of wet scrubbing.
Mechanical Collectors. Mechanical collectors, also known as particle scrubbers
(PS), use gravity or inertia to extract particulate matter from the flue gas. Baffles and
chambers that allow large particles to settle from the flue gas are examples of mechanical
collectors. Also in this category are cyclones, in which the flue gas is induced to travel
in a spiral pattern so that centrifugal force pushes the particulate matter out of the gas onto
the walls of the cyclone, where it is collected and disposed of. Mechanical collectors are
most efficient for removal of large particulate matter.2 In situations where multiple
emission control technologies are being used, cyclones are sometimes used to remove the
majority of large particulate matter, with finer particulate matter being reduced
subsequently by another technology.
Electrostatic Precipitators. Electrostatic precipitators (ESP) use a strong
electric field between electrodes to draw particulate matter out of the flue gas. The
efficiency of this process hinges on several factors, including the length of the precipitator
(longer precipitators have higher efficiencies), the strength of the electric field (stronger
electric fields have higher efficiencies), and the electrical resistivity of the particles in the
flue gas (only particles within a range of electrical resistivity are efficiently removed).
Electrostatic precipitators are referred to as cold-side (CS-ESP) or hot-side (HS-ESP)
depending on their location in the flue exhaust stream. Properly designed electrostatic
precipitators are very effective at reducing particulate matter. Particles removed from the
flue gas and collected at the electrodes are later removed.3
Stationary Source Control Techniques Document for Fine Particulate Matter, U.S.
Environmental Protection Agency, October 1998, Section 5.1.
Stationary Source Control Techniques Document for Fine Particulate Matter, U.S.
Fabric Filters. Fabric filters (FF), also known as baghouses, are large filters of
fabric, often sewn into long bags, which physically sieve particles from the flue gas.
Particulate matter collects on the surface of the fabric and is periodically removed by
shaking the filters, pulsing gas through them, or reversing the air flow through the filter.
The performance of the fabric filter depends on the type and amount of fabric used and
the regularity with which the fabric is cleaned.4 High fabric filter control efficiencies have
been reported, and they can be effective in controlling small particulate matter.
Wet Scrubbers. Wet scrubbing technology, a form of flue gas desulfurization
(FGD), discussed above, has also been used to reduce particulate matter concentrations.
The impact of the liquid droplets on the particles in the flue gas draws the particles into
the droplets, which are collected and removed.
Removal of Mercury Using Existing Control Technologies
Existing post-combustion control technologies are being assessed for their ability to
remove mercury from the flue gas. Because of the many equipment configuration and
coal types already found in electric utilities, both the limitations and advantages of these
technologies are important in considering EPA’s proposed cobenefits-focused regulation.
Elemental mercury is a liquid at room temperature, but it evaporates easily, and is
considered difficult to remove from the flue gas stream. In contrast, mercury compounds
which form ions, charged atomic or molecular species, present in flue gas are more easily
removed. In combustion emissions, mercury can react with or attach to particles in the
flue gas, so particulate control can provide mercury removal as a cobenefit.
Partly because of the difficulties in predicting mercury capture by existing control
technologies, the EPA in 1998 issued an information collection request to electric utilities
to gather information regarding the effectiveness of current control technologies with
respect to mercury removal. Analysis of this data showed a wide variation of mercury
emission control as a cobenefit, with significant variation occurring between different coal
types for a given emission control technology. See Table 1. The control efficiencies
estimated by the EPA analysis were based on the technologies currently in use by the
electric utility industry. These control technologies were not reoptimized for mercury
capture, but rather operated under normal conditions.
Several technology-related policy issues regarding regulation of mercury emissions
include the maximum feasible level of mercury reduction from cobenefits of current
emissions control technologies; the likely level of mercury emissions reduction using
alternative control technologies not currently used by the electric utility industry; and the
maturity and availability of these alternative technologies.
Environmental Protection Agency, October 1998, Section 5.2.
Stationary Source Control Techniques Document for Fine Particulate Matter, U.S.
Environmental Protection Agency, October 1998, Section 5.3.
Table 1. Average Mercury Capture by Existing Post-combustion
Control Configurations Used for Pulverized Coal Fired Boilers
Average Total Mercury Capture by
Dry FGD + ESP
Dry FGD + FF
Dry FGD +FF + SCR
Wet FGD + PS
Wet FGD + CS-ESP
Wet FGD + HS-ESP
Wet FGD + FF
PM Control Only
PM Control and
Dry FGD System
PM Control and
Wet FGD System
Source: Environmental Protection Agency, Control of Mercury Emissions From Coal-fired Electric Utility
Boilers: Interim Report Including Errata Dated 3-21-02, EPA-600/R-01-109, April 2002.
Note: See text for definition of acronyms.
In Use Technologies. Experts disagree over the degree of mercury reduction
resulting from cobenefits of existing emission control technologies. Table 1 shows that
average mercury reduction by existing control technologies is highly dependent both on
the type of emission control technology being used and the type of coal used as fuel in the
electric generating unit, varying from 0% to 98%. (The two negative results are
presumably artifacts of the testing procedure.) The EPA has stated that the maximum
amount of mercury collection for some existing control technologies has significant
uncertainty because of both small sample size and difficulties in measurement.5
Moreover, because these values were obtained without maximization of mercury capture,
it is unclear whether they should be considered representative of potential maximum
mercury capture. Further optimization of current emission control technologies to
enhance the capture of mercury might lead to higher collection of mercury emissions.
However, attempting to optimize mercury capture by already installed emission control
technologies might also degrade their ability to control other pollutant levels.
69 Fed. Reg. 4652-4752, January 30, 2004, at p. 4698.
New Technologies. Alternative technologies for mercury emissions reduction
are under development for use in the electric utility industry.6 No commercial electric
generating unit in the United States has installed a mercury-specific emissions control
technology. Mercury emission reduction technology, such as activated carbon injection
(ACI), has been used on other combustion systems, such as municipal incinerators, to
achieve substantial mercury emissions reduction. Performance of activated carbon
injection technologies are being assessed in field tests on commercial plants. These
results show mercury emissions reduction ranging from 60 to 90%, depending on coal
fuel type and carbon injection rates. The use of ACI in municipal incinerators, along with
initial test results with electric generating units and the serious health effects of mercury,
cause some to argue that the alternative technologies for mercury emissions reduction
should be considered when determining the regulatory emissions level for electric
generating units. However, the operating conditions of electric generating units are
different than those in municipal incinerators, and contention exists among stakeholders
over whether equivalent reduction in mercury emissions would be obtained in electric
generating units under normal operating conditions. Supporters of a cobenefits approach
to mercury regulation argue these technologies are not yet proven nor commercially
available for use in the electric generating units and thus their inclusion in a rulemaking
would be inappropriate.
Stakeholders disagree over how much and how fast mercury emissions reduction can
be achieved. The EPA estimates that a generating unit specific emission limit based on
cobenefits would reduce the mercury emissions 29% by 2008.7 Emissions control
technology manufacturers state that it is feasible to reduce mercury emissions 50 to 70%
by 2008 using cobenefits and near-term mercury specific technology, such as activated
carbon injection.8 Testing of activated carbon injection and other alternative control
technologies by the Department of Energy indicates that technologies achieving at least
a 50 to 70% reduction of mercury will likely be available for large scale commercial
deployment after 2011 to 2013.9 Other analysts disagree with this time frame, asserting
that transfer of mercury control technology from municipal waste incinerators to electric
generating units is straightforward, and, therefore, commercialization would occur quicker
than the Department of Energy projects.10
A brief overview of Department of Energy funded research and development in several
alternative technologies is found at 70 Fed. Reg. 12398-12472, March 16, 2004.
69 Fed. Reg. 4652-4752, January 30, 2004.
Letter from David Foerter, Executive Director, Institute of Clean Air Companies, to Michael
Levitt, Administrator, EPA, June 29, 2004, available online at
Memorandum from L.D. Carter, Department of Energy, to B. Maxwell, EPA, regarding
Mercury Control Technologies, January 8, 2004, available online at
NESCAUM, “Northeast States New Report Shows Over 90% Reduction In Power Plant
Mercury Emissions Is Achievable,” NESCAUM Press Release, November 4, 2003.
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