Air Pollution Emission Control: Existing Technologies and Mercury Cobenefits

Order Code RS21936

September 15, 2004

CRS Report for Congress

Received through the CRS Web

Air Pollution Emission Control: Existing

Technologies and Mercury Cobenefits

name redacted

Analyst in Science and Technology Policy

Resources, Science, and Industry Division

Summary

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.

Introduction

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

1

69 Fed. Reg. 4652-4752, January 30, 2004, and 70 Fed. Reg. 12398-12472, March 16, 2004.

Congressional Research Service ˜ The Library of Congress

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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.

Background

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

interdependence.

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.

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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

2

Stationary Source Control Techniques Document for Fine Particulate Matter, U.S.

Environmental Protection Agency, October 1998, Section 5.1.

3

Stationary Source Control Techniques Document for Fine Particulate Matter, U.S.

(continued...)

CRS-4

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.

Policy Issues

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.

3

(...continued)

Environmental Protection Agency, October 1998, Section 5.2.

4

Stationary Source Control Techniques Document for Fine Particulate Matter, U.S.

Environmental Protection Agency, October 1998, Section 5.3.

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Table 1. Average Mercury Capture by Existing Post-combustion

Control Configurations Used for Pulverized Coal Fired Boilers

Post-combustion

Control Strategy

Average Total Mercury Capture by

Control Configuration

Post-combustion

Emission Control

Device

Configuration

Bituminous

Coal

Subbituminous

Coal

Lignite

CS-ESP

36 %

3%

-4 %

HS-ESP

9%

6%

not tested

FF

90 %

72 %

not tested

PS

not tested

9%

not tested

Dry FGD + ESP

not tested

35 %

not tested

Dry FGD + FF

98 %

24 %

0%

Dry FGD +FF + SCR

98 %

not tested

not tested

Wet FGD + PS

12 %

-8 %

33 %

Wet FGD + CS-ESP

74 %

29 %

44 %

Wet FGD + HS-ESP

50 %

29 %

not tested

Wet FGD + FF

98 %

not tested

not tested

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.

5

69 Fed. Reg. 4652-4752, January 30, 2004, at p. 4698.

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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

6

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.

7

69 Fed. Reg. 4652-4752, January 30, 2004.

8

Letter from David Foerter, Executive Director, Institute of Clean Air Companies, to Michael

Levitt, Administrator, EPA, June 29, 2004, available online at

[http://www.icac.com/hgmonitoring62904.pdf].

9

Memorandum from L.D. Carter, Department of Energy, to B. Maxwell, EPA, regarding

Mercury Control Technologies, January 8, 2004, available online at

[http://www.epa.gov/mercury/control_emissions/mercurytechnologiesjan04.pdf].

10

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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