GAO-12-545R Air Emissions and Electricity Generation at U.S. Power Plants

April 18, 2012

The Honorable Sheldon Whitehouse

Chairman 

Subcommittee on Oversight

Committee on Environment and Public Works

United States Senate

Subject: Air Emissions and Electricity Generation at U.S. Power Plants

Dear Mr. Chairman: 

The United States depends on a variety of fuels to generate electricity, including fossil fuels 

(coal, natural gas, and oil), nuclear power, and renewable sources. Power plants that burn fossil 

fuels provide about 70 percent of U.S. electricity, but they also produce substantial amounts of 

harmful air emissions.1 In particular, electricity generating units at fossil fuel power plants are 

among the largest emitters of sulfur dioxide and nitrogen oxides, which have been linked to 

respiratory illnesses and acid rain, as well as of carbon dioxide, the primary greenhouse gas 

contributing to climate change.2 Of the three fossil fuels, coal is the most widely used fuel in the 

United States, providing about 45 percent of electricity in 2010, followed by natural gas, which 

provided about 24 percent.3 Coal plays a critical role in the reliability of the electricity grid, 

especially in certain geographic areas, but coal-fired units also generally emit more air pollution 

than units burning natural gas or oil.

Under the Clean Air Act, the Environmental Protection Agency (EPA) establishes national 

ambient air quality standards for six pollutants that states are primarily responsible for attaining.4

 

1

Fossil fuels are responsible for nearly all emissions of carbon dioxide, sulfur dioxide, and nitrogen oxides 

from power plants.

States attain these standards, in part, by regulating emissions of these pollutants from certain 

stationary sources, such as electricity generating units. Numerous Clean Air Act requirements 

apply to electricity generating units, including New Source Review (NSR), a permitting process 

2

An electricity generating unit consists of any combination of an electricity generator, reactor, boiler, 

combustion turbine, or other equipment operated together to produce electrical power. A power plant is a 

facility with one or more generating units, together with other equipment used to produce electric power.

3

Oil is used to a very limited extent, providing less than 1 percent of electricity in 2010.

4

EPA has set national ambient air quality standards for six pollutants, termed "criteria" pollutants: carbon 

monoxide, lead, nitrogen oxides, ozone, particulate matter, and sulfur oxides.

United States Government Accountability Office

Washington, DC 20548
Page 2 GAO-12-545R Air Emissions and Electricity Generation at U.S. Power Plants

established in 1977.5 Under NSR, owners of generating units must obtain a preconstruction 

permit that establishes emission limits and requires the use of certain pollution control 

technologies. NSR applies to (1) generating units built after August 7, 1977, and (2) to existing 

generating units—regardless of the date built—that seek to undertake a “major modification,” a 

physical or operational change that would result in a significant net increase in emissions of a 

regulated pollutant. Units built before August 7, 1977, are not required to undergo NSR unless 

they undertake a major modification. For the purposes of this report, we refer to units that began 

operation in or before 1978—the first full year after NSR was established—as “older units” and 

those that began operating after 1978 as “newer units.”6

In limiting NSR’s requirements to facilities built or undertaking major modifications after August 

7, 1977, Congress allowed existing facilities to defer installation of pollution controls until they 

made a major modification, with the expectation that over time all facilities would either install 

such equipment or shut down, thereby lowering overall emissions. According to EPA data, 

1,485 older units (43 percent of fossil fuel units) were still in operation in 2010. Some research 

suggests that many of these older units continue to operate without emissions controls, and in 

June 2002, we reported that older fossil fuel electricity generating units emitted air pollution at 

higher rates than newer units.7

This report responds in part to your request for information on electricity generation and 

emissions at U.S. electricity generating units and the implementation of NSR. Our objective is to 

provide information on how older fossil fuel electricity generating units compare with newer units 

in terms of their air emissions and electricity generation. To respond to this objective, we 

reviewed selected data elements in the Ventyx Velocity Suite EV Market-Ops database. This 

proprietary database contains consolidated energy and emissions data from EPA, the Energy 

Information Administration (EIA), and other sources. Specifically, we analyzed how older plants 

compare with newer plants in their emissions, energy production, location, and fuel type. We 

reviewed energy and emissions data from calendar year 2010. We reviewed data from units that 

(1) listed a fossil fuel (coal, natural gas, or oil) as a primary fuel; (2) generated electricity in 

2010; and (3) had a net summer capacity greater than 25 megawatts, making them subject to 

EPA emissions monitoring and reporting requirements. In all, we examined the characteristics of 

3,443 electricity generating units—1,485 older units and 1,958 newer units. We focused our 

analysis on power plant emissions of three regulated pollutants: sulfur dioxide (as a proxy for 

sulfur oxides), nitrogen oxides, and carbon dioxide (a greenhouse gas).8

 

5

This report focuses solely on fossil fuel electricity generating units, although NSR also applies to certain 

other major stationary sources of air pollution, such as other industrial facilities.

To assess the 

reliability of the Ventyx data, we reviewed existing documentation about the data and the 

system that produced them, interviewed Ventyx staff who were knowledgeable about the data, 

6

We used 1978 as the cutoff date for our analysis—instead of August 7, 1977—because 1978 was the 

first full year after NSR’s establishment, and our analysis reports only annual data.

7

GAO, Air Pollution: Emissions from Older Electricity Generating Units, GAO-02-709 (Washington, D.C.: 

June 12, 2002). Note: This report defines “older units” as those fossil fuel generating units that began 

operation before 1972.

8

Regulated pollutants are sulfur oxides; nitrogen oxides; other criteria pollutants; and, since January 2, 

2011, greenhouse gases. 
Page 3 GAO-12-545R Air Emissions and Electricity Generation at U.S. Power Plants

and consulted with EPA and EIA agency officials knowledgeable in energy issues. We 

determined the Ventyx data to be sufficiently reliable for the purpose of this report.

We conducted this performance audit from February 2012 to April 2012 in accordance with 

generally accepted government auditing standards. Those standards require that we plan and 

perform the audit to obtain sufficient, appropriate evidence to provide a reasonable basis for our 

findings and conclusions based on our audit objectives. We believe that the evidence obtained 

provides a reasonable basis for our findings and conclusions based on our audit objectives.

Results in Brief

Older electricity generating units—those that began operating in or before 1978—provided 

45 percent of electricity from fossil fuel units in 2010 but produced a disproportionate share of 

emissions, both in aggregate and per unit of electricity generated. Overall, in 2010 older units 

contributed 75 percent of sulfur dioxide emissions, 64 percent of nitrogen oxides emissions, and 

54 percent of carbon dioxide emissions from fossil fuel units. For each unit of electricity 

generated, older units collectively emitted about 3.6 times as much sulfur dioxide, 2.1 times as 

much nitrogen oxides, and 1.3 times as much carbon dioxide as newer units. The difference in 

emissions between older units and their newer counterparts may be attributed to a number of 

factors. First, 93 percent of the electricity produced by older fossil fuel units in 2010 was 

generated by coal-fired units. Compared with natural gas units, coal-fired units produced over 

90 times as much sulfur dioxide, twice as much carbon dioxide and over five times as much 

nitrogen oxides per unit of electricity, largely because coal contains more sulfur and carbon than 

natural gas. Second, fewer older units have installed emissions controls, which reduce 

emissions by limiting their formation or capturing them after they are formed. Among coal-fired 

units—which produce nearly all sulfur dioxide emissions from electric power generation—

approximately 26 percent of older units used controls for sulfur dioxide, compared with 63 

percent of newer units. Controls for nitrogen oxide emissions were more common among all 

types of fossil fuel units, but these controls vary widely in their effectiveness. Among older units, 

14 percent had installed selective catalytic reduction (SCR) equipment, the type of control 

capable of reducing the greatest amount of nitrogen oxides emissions, compared with 33 

percent of newer units. In addition, approximately 38 percent of older units did not have any 

controls for nitrogen oxides, compared with 6 percent of newer units. Third, lower emissions 

among newer units may be attributable in part to improvements in the efficiency with which 

newer units convert fuel into electricity. Nonetheless, older units remain an important part of the 

electricity generating sector, particularly in certain regions of the United States.

Background

The efficient and reliable operation of the electricity industry is critical to the health of the U.S. 

economy. Residential consumers rely on electricity to power their households, and electricity is 

a key input for businesses that produce trillions of dollars in products and services. Domestic 

energy production also provides U.S. jobs and supports economic growth. Plentiful, reliable, and 

relatively inexpensive fossil fuels have been a mainstay of the U.S. electricity mix.

The electricity generating sector is also a major contributor to air emissions in the United States. 

Based on emissions data published by EPA, in 2010, fuel combustion by electric utilities was 

responsible for about 65 percent of sulfur dioxide emissions and 16 percent of nitrogen oxides 
Page 4 GAO-12-545R Air Emissions and Electricity Generation at U.S. Power Plants

emissions from all sources nationwide.9 Electricity generation was also responsible for 39 

percent of carbon dioxide emissions from all sources in 2009.10

As we have previously reported, sulfur dioxide and nitrogen oxides have been linked to a variety 

of health and environmental concerns, and carbon dioxide has been linked to climate change.

Units in the Southeast, South 

Central, and Great Lakes regions produced most of the electricity and emissions from fossil fuel 

units. (See encls. I through IV for additional information on the location of generating units, their 

electricity production, and their emissions.) 

11

For example, sulfur dioxide and nitrogen oxides contribute to the formation of fine particles, and 

nitrogen oxides contribute to the formation of ozone.12 Fine particles have been linked to 

premature death, aggravated asthma, and chronic bronchitis, and ozone can inflame lung tissue 

and increase susceptibility to bronchitis and pneumonia. In addition to affecting health, sulfur 

dioxide and nitrogen oxides reduce visibility and contribute to acid rain, which harms aquatic life 

and degrades forests. These emissions affect local air quality, and they can also travel 

hundreds of miles to affect the air quality of downwind states. In addition, the accumulation of 

carbon dioxide in the atmosphere is linked to increases in air and ocean temperatures, which 

could threaten coastal areas with rising sea levels, alter agricultural productivity, and increase 

the intensity and frequency of floods and tropical storms.

Emissions controls can help reduce the emissions from generating units by either limiting their 

formation or capturing them after they are formed.13 At coal-fired units, emissions controls are 

generally installed in a boiler, where coal is burned, or the duct work that connects the boiler to 

a smokestack. Figure 1 shows some of the pollution controls that may be used at coal power 

plants, including flue gas desulfurization units—known as scrubbers—to control sulfur dioxide 

emissions, fabric filters or electrostatic precipitators to control particulate matter, and selective 

catalytic reduction (SCR) or selective noncatalytic reduction (SNCR) units to control nitrogen 

oxides emissions. According to a 2010 EPA report, the development of effective and 

commercially viable carbon dioxide controls for coal-fired electricity generating units has

received significant attention, but some of these technologies are still in the research and 

development phase, and most are not yet commercially viable.14

 

9

EPA, National Emissions Inventory Air Pollutant Emissions Trends Data, “Current Emissions Trends 

Summaries,” accessed October 2011, http://www.epa.gov/ttn/chief/trends/index.html#tables. 

10EPA, Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2009, EPA-430-R-11-005 

(Washington, D.C.: Apr. 15, 2011).

11GAO-02-709 and GAO, Air Quality: Information on Tall Smokestacks and Their Contribution to 

Interstate Transport of Air Pollution, GAO-11-473 (Washington, D.C.: May 11, 2011).

12Sulfur dioxide and nitrogen oxides can transform into fine particles in the atmosphere. Fine particles are 

a subset of particulate matter, a regulated pollutant. Ozone, also a regulated pollutant, forms when 

nitrogen oxides react with volatile organic compounds (chemicals that are emitted as gases from natural 

sources, such as trees, as well as from anthropogenic sources, such as motor vehicles) in the presence 

of heat and sunlight. 

13GAO-11-473.

14EPA, Office of Air and Radiation, Available and Emerging Technologies for Reducing Greenhouse Gas 

Emissions from Coal-Fired Electric Generating Units (Research Triangle Park, NC: October 2010).
Page 5 GAO-12-545R Air Emissions and Electricity Generation at U.S. Power Plants

Figure 1: Sample Layout of Emissions Controls at a Coal Power Plant

The reduction in emissions from the use of emissions controls can be substantial, as shown in 

table 1. Used commercially since the early 1970s, scrubbers are the most common technology 

for reducing sulfur dioxide emissions, and are capable of removing up to 99 percent of sulfur 

dioxide emissions. Commercially available low-nitrogen-oxide burners and SCRs are among the 

more common nitrogen oxide control technologies, with SCRs able to reduce these emissions 

by more than 90 percent. The installation of emissions controls can also be expensive. 

According to EPA, a typical coal unit with a capacity of 700 megawatts could incur from

$269 million to $329 million to install a scrubber and from $108 million to $129 million to install 

an SCR, plus operating and maintenance costs.15 Additionally, emissions controls can require 

additional electricity from the unit to operate, which reduces the amount of electricity available to 

be used by customers.

 

15EPA, Integrated Planning Model, “TR Base Case v.4.10,” accessed April 2, 2012, 

http://www.epa.gov/airmarkets/progsregs/epa-ipm/BaseCasev410.html#documentation. 
Page 6 GAO-12-545R Air Emissions and Electricity Generation at U.S. Power Plants

Table 1: Summary of Emissions Control Equipment Used at Electricity Generating Units

Pollutant Control equipment How it works Removal efficiency

Sulfur dioxidea Flue gas 

desulfurization unit 

(commonly referred to 

as a “scrubber”)

Wet flue gas desulfurization units inject a 

liquid sorbent, such as limestone, into the 

flue gas to form a wet solid that can be 

disposed of or sold.

Dry flue gas desulfurization units inject a 

dry sorbent, such as lime, into the flue 

gas to form a solid by-product that is 

collected and removed. 

Wet flue gas desulfurization units can 

remove 80-99 percent of sulfur dioxide.

Dry flue gas desulfurization units can 

remove 70-95 percent of sulfur dioxide.

Nitrogen oxides Combustion control 

technologies, such as 

low-nitrogen-oxides 

burnersb

Coal combustion conditions are adjusted 

to inhibit the formation of nitrogen oxides.

These technologies can reduce 

formation of nitrogen oxides by 40-

45 percent.

Postcombustion 

controls, such as 

selective catalytic 

reduction and selective 

noncatalytic reduction 

units

Selective catalytic reduction units inject 

ammonia into flue gas to form nitrogen 

and water and use a catalyst to enhance 

the reaction.

Selective noncatalytic reduction units

also inject ammonia but do not use a 

catalyst.

Selective catalytic reduction units can 

remove 70-95 percent of nitrogen 

oxides.

Selective noncatalytic reduction units 

can remove 30-75 percent of nitrogen 

oxides.

Source: GAO.

a

Another approach to reducing sulfur dioxide emissions from a coal-fired electricity generating unit is for a plant to 

switch from using coal with a higher sulfur content to coal with a lower sulfur content or to blend higher-sulfur coal 

with lower-sulfur coal.

b

Low-nitrogen-oxides burners can be used in conjunction with postcombustion controls for nitrogen oxides.

Older Fossil Fuel Units Produce More Emissions and Less Electricity Than Newer Units

In general, older electricity generating units, which run primarily on coal, produce more 

emissions than newer units, and at higher rates per unit of electricity. Much of this difference 

can likely be attributed to a greater reliance on coal among older units. In addition, fewer older 

units have installed emissions controls, and older units are generally less efficient at converting 

fuel to electricity. Nonetheless, some older units play an important role in providing reliable 

electricity to certain U.S. regions.

Older Units Emit More Than Newer Units

In 2010, older fossil fuel electricity generating units—those that began operating in or by 1978—

produced a disproportionate share of emissions for the electricity they produced. Specifically, 

older units generated 45 percent of the electricity from fossil fuel units in 2010 but emitted 

75 percent of sulfur dioxide emissions, 64 percent of nitrogen oxides emissions, and 54 percent 

of carbon dioxide emissions (see fig. 2). Units that began operating after 1978 were responsible 

for the remainder of the emissions and electricity production.16

 

16Of the 3,443 fossil fuel electricity generating units we assessed, 1,485 were older units, and 1,958 were 

newer units.
Page 7 GAO-12-545R Air Emissions and Electricity Generation at U.S. Power Plants

Figure 2: Share of Total Electricity and Emissions per Unit of Electricity Generated by Fossil Generating 

Units in 2010

Note: Chart reflects older and newer generating units that use coal, natural gas, and oil. Older generating units are mostly coal fired, 

whereas newer generating units rely more heavily on natural gas. 

Older units produced emissions at a higher rate than newer units in 2010; that is, they produced 

more emissions per unit of electricity they generated. As shown in figure 3, for each megawatthour of electricity generated, older units, on average, emitted about 3.6 times as much sulfur 

dioxide as newer units, about 2.1 times as much nitrogen oxides, and about 1.3 times as much

carbon dioxide.

Figure 3: Emissions per Unit of Electricity Generated in 2010

Note: Chart reflects older and newer generating units that use coal, natural gas, and oil. Older generating units are mostly coal-fired, 

whereas newer generating units rely more heavily on natural gas. 
Page 8 GAO-12-545R Air Emissions and Electricity Generation at U.S. Power Plants

Emissions from fossil fuel units in 2010 varied widely, depending on the decade in which the 

unit began operating. In aggregate, the oldest units produced the most emissions, and more 

recently built units produced the least emissions (see fig. 4). For example, units that began 

operating in the 1960s generated 12 percent of electricity in 2010, but also produced 26 percent 

of sulfur dioxide emissions, 21 percent of nitrogen oxides emissions, and 15 percent of carbon 

dioxide emissions in that year. In comparison, units that began operating in the 2000s produced 

over twice as much electricity (26 percent) in 2010, but a much lower share of emissions—

1 percent of sulfur dioxide, 3 percent of nitrogen oxides, and 15 percent of carbon dioxide.

Figure 4: Share of Total Electricity and Emissions Produced by Fossil Fuel Generating Units in 2010, by 

Decade Unit Began Operating

Note: Chart reflects generating units that use coal, natural gas, and oil. Older generating units are mostly coal-fired, whereas newer 

generating units rely more heavily on natural gas. 

As shown in figure 5, units that began operating more recently are more likely to have 

comparatively lower emissions rates. In 2010, units that began operating in the 1970s or later 

generally emitted substantially less sulfur dioxide and nitrogen oxides for each unit of electricity 

they generated. For example, units built in the 1960s emitted an average of of 7.8 pounds of 

sulfur dioxide and 2.5 pounds of nitrogen oxides per megawatt-hour of electricity generated, 

whereas units built in the 2000s emitted an average of 0.1 pounds of sulfur dioxide and 0.2 

pounds of nitrogen oxides per megawatt-hour. 
Page 9 GAO-12-545R Air Emissions and Electricity Generation at U.S. Power Plants

Figure 5: Emissions of Sulfur Dioxide and Nitrogen Oxides per Unit of Electricity Generated by Fossil Fuel 

Generating Units in 2010, by Decade Unit Began Operating

Note: Chart reflects generating units that use coal, natural gas, and oil. Older generating units are mostly coal-fired, whereas newer 

generating units rely more heavily on natural gas. 

Carbon dioxide emissions rates have also declined among newer units in the last several 

decades, although to a smaller extent. As shown in figure 6, units that began operating in the 

1980s emitted an average of 2,160 pounds of carbon dioxide per unit of electricity in 2010, a 

decline of around one percent compared to units that began operating in the 1950s. In contrast, 

units that began operating after the 1980s had substantially lower carbon dioxide emissions 

rates, on average. For example, units that began operating in the 2000s emitted carbon dioxide 

at a rate of 1,016 pounds per megawatt-hour, about 53 percent less than units that began 

operating in the 1980s.
Page 10 GAO-12-545R Air Emissions and Electricity Generation at U.S. Power Plants

Figure 6: Carbon Dioxide Emissions per Unit of Electricity Generated by Fossil Fuel Generating Units in 2010, 

by Decade Unit Began Operating

Note: Chart reflects generating units that use coal, natural gas, and oil. Older generating units are mostly coal-fired, whereas newer 

generating units rely more heavily on natural gas. 

As discussed below, we identified three reasons why older units may emit more than newer 

units: (1) older units are more likely to use coal, (2) older units are less likely to have installed 

emissions controls, and (3) older units are generally less efficient.

Older Units Are Mostly Fueled by Coal, and Newer Units Rely More on Natural Gas

The difference in emissions between older and newer units is likely due, in part, to significant 

changes in the fuels used to generate electricity—in particular, a shift from coal to natural gas. 

Compared with coal, natural gas produces substantially lower emissions per unit of electricity 

generated, largely because natural gas contains less sulfur and carbon. This disparity is most 

apparent with sulfur dioxide emissions, as shown in figure 7. On average, coal-fired units—both 

older and newer—produced over 90 times as much sulfur dioxide emissions per unit of 

electricity as natural gas-fired units in 2010. Compared to natural gas-fired units, coal-fired units 

also produced over twice as much carbon dioxide and over five times as much nitrogen oxides 

per unit of electricity in 2010. 
Page 11 GAO-12-545R Air Emissions and Electricity Generation at U.S. Power Plants

Figure 7: Emissions per Unit of Electricity Generated by Fossil Fuel Generating Units in 2010, by Type of Fuel

Older generating units are much more likely than newer units to use coal as a primary fuel. In 

2010, coal-fired units accounted for about 93 percent of electricity from older units and 47 

percent of electricity from newer units. While other fossil fuels, particularly oil, played an 

important role in electricity generation in the late 1960s and 1970s, many of these units have 

since been retired, a development likely attributable to rising oil prices.

17 In addition, between 

1978 and 1987, the Powerplant and Industrial Fuel Use Act of 1978 (Fuel Use Act) restricted 

construction of new power plants using oil or natural gas as a primary energy source. 

Since the 1990s, natural gas has played a much larger role in electricity generation (see fig. 8).

After the 1987 repeal of provisions in the Fuel Use Act, use of natural gas by the electric power 

sector increased by more than 240 percent.18 As we have previously reported, factors 

contributing to the rise of natural gas include the comparatively low levels of emissions 

produced by natural gas units and a long period of low natural gas prices in the 1990s.19

 

17In 2010, oil-fired units (both older and newer) accounted for less than one percent of electricity from 

fossil fuel units.

After 

rising in the early 2000s, natural gas prices have again fallen in recent years, a trend that may

18The 240 percent increase refers to 1987 through 2009. See Energy Information Administration, Annual 

Energy Review 2010, DOE/EIA-0384(2010) (Washington, D.C.: October 2011).

19GAO, Natural Gas: Factors Affecting Prices and Potential Impacts on Consumers, GAO-06-420T

(Washington, D.C.: Feb. 13, 2006).
Page 12 GAO-12-545R Air Emissions and Electricity Generation at U.S. Power Plants

influence decisions about the future composition of the fleet of generating units, with the 

potential for retirements of older coal-fired units and the construction of new natural gas units.

In 2010, natural gas units accounted for approximately one-third of the electricity generated by 

fossil fuel units. Most of this natural-gas-based generation—about 78 percent—came from units 

that began operating in 2000 or later. Conversely, most coal-based generation—about 

66 percent—came from units that have been operating more than 30 years. 

Figure 8: Electricity Generation from Fossil Fuel Generating Units in 2010, by Decade Unit Began Operating 

and Fuel Type

Nonetheless, coal-fired units, both older and newer, remain both a key source of electricity and 

a significant source of emissions. As shown in figure 9, coal-fired units generated 67 percent of 

electricity from fossil fuel units in 2010, but also produced 99 percent of sulfur dioxide 

emissions, 92 percent of nitrogen oxides emissions, and 82 percent of carbon dioxide emissions

from fossil fuel units in that year. 
Page 13 GAO-12-545R Air Emissions and Electricity Generation at U.S. Power Plants

Figure 9: Electricity Generation and Emissions from Coal-Fired Units in 2010, as a Percentage of Total 

Generation and Emissions from Fossil Fuel Generating Units 

Older Units Are Less Likely to Use Emissions Controls

Another likely reason that older units have higher aggregate emissions is that many older units 

have not installed emissions controls. Over the last several decades, the electricity generating

industry has employed a number of technologies to comply with NSR and other state and 

federal air quality regulations. Some of the most common and effective technologies include flue 

gas desulfurization unit systems, or “scrubbers,” to control sulfur dioxide and SCRs to control 

nitrogen oxides.

Regarding the use of controls for sulfur dioxide in 2010, 63 percent of newer coal-fired units—

those that began operating after 1978—had installed controls (see fig. 10). In contrast, 

26 percent of older, coal-fired units had installed sulfur dioxide controls.20

 

20We limited this analysis to coal-fired units because these units are responsible for nearly all (99 percent) 

of sulfur dioxide emissions from fossil fuel units.
Page 14 GAO-12-545R Air Emissions and Electricity Generation at U.S. Power Plants

Figure 10: Use of Sulfur Dioxide Emissions Controls among Older and Newer Coal-Fired Generating Units in 

2010

Older coal-fired units without sulfur dioxide controls were responsible for the majority of sulfur 

dioxide emissions from fossil fuel units in 2010. Specifically, older coal-fired units without sulfur 

dioxide control equipment generated about 18 percent of electricity while producing 55 percent 

of sulfur dioxide emissions. As shown in figure 11, older coal-fired units without sulfur dioxide 

controls accounted for 2.8 million tons of sulfur dioxide emissions, nearly three times as much 

as older units with sulfur dioxide controls. Among those coal-fired units without sulfur dioxide 

controls, the majority (88 percent) were older units.
Page 15 GAO-12-545R Air Emissions and Electricity Generation at U.S. Power Plants

Figure 11: Sulfur Dioxide Emissions from Controlled and Uncontrolled Coal-Fired Generating Units in 2010

The use of controls for nitrogen oxides is more widespread than for sulfur dioxide, although the 

effectiveness of these controls at removing nitrogen oxides varies widely. For purposes of our 

analysis, we classified controls for nitrogen oxides into three categories, as shown in table 2. 

Overall, older units were less likely than newer units to use controls for nitrogen oxides. Among 

older units, 38 percent had no controls for nitrogen oxides, compared with 6 percent of newer 

units. Older units were also less likely to have installed SCR controls, the most effective 

category of control for nitrogen oxides: about 14 percent of older units had such controls, 

compared with 33 percent of newer units.
Page 16 GAO-12-545R Air Emissions and Electricity Generation at U.S. Power Plants

Table 2: Controls for Nitrogen Oxides in Fossil Fuel Electricity Generating Units: Removal Efficiency and 

Extent of Use in 2010

Category Control type Number of older units Number of newer units 

1 None 564 116

2 Selective noncatalytic reduction, low 

nitrogen-oxides burners, and others 708 1,204

3 Selective catalytic reduction (SCR) 213 638

Source: GAO analysis of Ventyx data.

Note: Units in categories 2 and 3 may have installed multiple nitrogen oxides controls; no units in category 2, however, had installed 

an SCR.

As shown in figure 12, older units collectively emitted nitrogen oxides at higher rates than newer 

units. For example, older units without any nitrogen oxides controls emitted an average of 

3.5 pounds of nitrogen oxides per megawatt-hour of electricity generated in 2010, approximately 

twice the rate as newer units.

Figure 12: Emissions Rates of Nitrogen Oxides for Older and Newer Fossil Fuel Generating Units in 2010, by 

Type of Control for Nitrogen Oxides 

Note: “Non-SCR” refers to any other emissions control other than an SCR (see category 2 in table 2). Chart reflects generating units 

that use coal, natural gas, and oil. Older generating units are mostly coal fired, whereas newer generating units rely more heavily on 

natural gas. 

The increased use of emissions controls over time may be attributable, in part, to the Clean Air 

Act requirements such as NSR and the Acid Rain Program. The Acid Rain Program, created by 

the Clean Air Act Amendments of 1990, established a cap on the amount of sulfur dioxide that 
Page 17 GAO-12-545R Air Emissions and Electricity Generation at U.S. Power Plants

may be emitted by electricity generating units nationwide. According to EPA officials, both NSR 

and the Acid Rain Program have contributed to more widespread use of emissions controls. In 

addition, EPA has recently finalized regulations that may prompt generating unit owners to 

(1) retrofit older units with emissions controls or (2) retire units that are not cost-effective to 

retrofit.21 Conversely, some uncontrolled units may be limited in their ability to install certain 

types of emissions controls if these controls are not required.22

Older Units Are Generally Less Efficient Than Newer Units

The average efficiency with which electricity generating units produce electricity has improved 

over time, which has likely contributed to lower collective emissions among newer units. 

Generating efficiency is often measured by the total heat (fuel) required to generate each unit of 

electricity, known as a “heat rate.”23

As shown in figure 13, the operating efficiency of generating units built before 1950 is much 

lower than newer generating units. On average, in 2010, units that began operating in 2000 or 

later were approximately 46 percent more efficient than units that began operating before 1950. 

In addition to a greater reliance on coal, many older units only operate periodically, during times 

of peak electricity demand, which decreases their efficiency relative to units that operate on a 

more continuous basis.

More efficient units require less fuel to produce the same 

amount of electricity, generally resulting in lower emissions and fuel costs. A number of factors 

can affect a unit’s efficiency, including its design, its operation, the choice of fuel, the use of 

emission controls, and the environmental conditions in which the unit operates. For example, 

some types of coal are more efficient than others because they have lower ash and water 

content. Emissions controls can also reduce a unit’s efficiency because some electricity must be 

used to operate the controls. 

24

 

21These regulations include the Cross-State Air Pollution Rule—which limits sulfur dioxide and nitrogen 

oxides emissions from a number of states that contribute significantly to nonattainment of or interference 

with maintenance of certain national ambient air quality standards in downwind states—and National 

Emissions Standards for Hazardous Air Pollutants from Coal- and Oil-Fired Electric Utility Steam 

Generating Units, also known as the Mercury and Air Toxics Standards, which establish emissions 

limitations on mercury and other pollutants. Both regulations are being challenged in court.

While some energy efficiency technologies are available for 

application to existing units, the biggest efficiency gains result from the substitution of old units 

with new, more efficient units. Some natural gas units constructed in the last two decades, for 

example, are capable of achieving heat rates below 7,000 million British thermal units (Btu) per 

kilowatt-hour, which is substantially lower than the averages highlighted in figure 13.

22In many states, public utility commissions are responsible for approving the rates (or prices) electric 

utilities charge their customers and ensuring they are reasonable. As part of approving rates, these 

commissions approve such utility investments as emissions controls and, as a result, may consider 

whether specific technologies are reasonable. These commissions generally require some demonstration 

that the investment is prudent.

23Heat rate is a measurement used in the energy industry to calculate how efficiently an electricity 

generator uses heat energy. It is expressed as the number of British thermal units (Btu) of heat required 

to produce a kilowatt hour of energy. A lower heat rate signifies greater efficiency.

24Generating units that undergo frequent startups use comparatively large amounts of fuel and may be 

less efficient than units that operate continuously.
Page 18 GAO-12-545R Air Emissions and Electricity Generation at U.S. Power Plants

Figure 13: Average Efficiency (Heat Rate) of Fossil Fuel Units in 2010, by Decade Unit Began Operating

Note: A British thermal unit (Btu) is a standard unit of measurement used to denote the amount of heat energy in fuels. A lower heat 

rate indicates a higher level of efficiency. Chart reflects generating units that use coal, natural gas, and oil. Older generating units 

are mostly coal fired, whereas newer generating units rely more heavily on natural gas.

Nonetheless, older units remain an important part of the U.S. electricity generating sector. In 

2010, older units were responsible for 45 percent of total electricity production from fossil fuel 

units. In certain regions, older units played a more significant role in providing electricity. For 

example, in the Mid-Atlantic and Great Lakes regions, older units generated 64 percent and 

70 percent, respectively, of electricity coming from fossil fuel units.25

 

25These regions correspond to Environmental Protection Agency (EPA) Region 3 (Delaware, District of 

Columbia, Maryland, Pennsylvania, Virginia, and West Virginia) and Region 5 (Illinois, Indiana, Michigan, 

Minnesota, Ohio, and Wisconsin). 

Some older units also 

provide services that help ensure the reliable flow of electricity to certain regions; for example, 

some older units may be used to help restart the electricity system in the event of a blackout.
Page 19 GAO-12-545R Air Emissions and Electricity Generation at U.S. Power Plants

Agency Comments

We provided a draft of this report to EPA and the Department of Energy for their review and 

comment. EPA and the Department of Energy provided technical comments, which we have 

incorporated as appropriate.

- - - - - 

As agreed with your office, unless you publicly announce the contents of this report earlier, we 

plan no further distribution until 30 days from the report date. At that time, we will send copies to 

the appropriate congressional committees, the Administrator of EPA, the Secretary of Energy, 

and other interested parties. In addition, the report will be available at no charge on the GAO 

website at http://www.gao.gov. 

If you or your staff have any questions about this report, please contact David Trimble at (202) 

512-3841 or trimbled@gao.gov or Frank Rusco at (202) 512-3841 or ruscof@gao.gov. Contact 

points for our Offices of Congressional Relations and Public Affairs may be found on the last 

page of this report. GAO staff who made key contributions to this report were Michael Hix 

(Assistant Director), Ellen W. Chu, Philip Farah, Cindy Gilbert, Mitchell Karpman, Jessica 

Lemke, Jon Ludwigson, Nancy Meyer, Mick Ray, and Jeanette Soares.

Sincerely yours,

David C. Trimble

Director, Natural Resources and Environment

Frank Rusco

Director, Natural Resources and Environment

Enclosures - 4
Enclosure I

Page 20 GAO-12-545R Air Emissions and Electricity Generation at U.S. Power Plants

Electricity-generation and Aggregate Emissions from Fossil Fuel Electricity Generating 

Units in 2010, By State

Table 3 presents, by state, data on fossil fuel units’ electricity generation and aggregate 

emissions of sulfur dioxide, nitrogen oxides, and carbon dioxide in 2010.

Table 3: Electricity and Emissions from Fossil Fuel Electricity Generating Units in 2010, by State

State

Number of 

units

Electricity 

generation 

(megawatt-hours)

Carbon dioxide 

emissions (tons)

Nitrogen oxide 

emissions (tons)

Sulfur dioxide 

emissions (tons)

Alabama 88 100,961,300 84,828,993 63,336 204,195

Arizona 63 74,817,174 60,732,544 60,524 36,445

Arkansas 30 40,295,345 36,271,855 37,783 67,084

California 179 80,854,400 36,929,068 4,467 226

Colorado 58 45,728,202 44,826,939 54,088 45,391

Connecticut 39 12,497,043 8,526,856 2,783 1,955

Delaware 15 5,489,585 4,318,918 4,330 14,496

District of Columbia 2 207,916 220,765 373 874

Florida 246 188,566,114 135,313,627 79,759 144,829

Georgia 141 95,973,778 87,569,772 60,580 218,906

Idaho 7 1,566,178 693,069 113 3 

Illinois 196 97,862,368 107,058,198 77,063 220,077

Indiana 126 120,785,560 124,321,920 121,844 414,638

Iowa 48 41,249,495 45,296,689 44,796 104,389

Kansas 48 35,758,838 39,757,268 48,938 45,251

Kentucky 96 96,980,846 101,053,094 93,040 270,045

Louisiana 70 70,221,777 52,344,198 47,153 102,263

Maine 11 7,902,550 3,953,551 719 821

Maryland 46 26,381,736 27,999,389 19,434 29,946

Massachusetts 48 31,174,655 19,787,576 8,181 36,892

Michigan 96 76,833,143 74,272,141 79,566 242,409

Minnesota 45 31,496,681 32,900,506 29,911 41,076

Mississippi 57 40,802,413 30,624,811 29,773 54,696

Missouri 102 77,566,300 83,182,289 58,288 235,368

Montana 12 18,752,877 21,356,366 21,728 19,896

Nebraska 29 23,688,902 26,402,103 37,402 64,184

Nevada 37 28,136,479 17,222,084 10,812 7,889

New Hampshire 7 8,069,450 5,899,639 4,612 36,833

New Jersey 107 30,081,377 19,995,676 9,937 15,270

New Mexico 35 34,294,657 32,657,348 60,256 16,570

New York 165 63,663,802 41,687,271 28,311 47,488
Enclosure I

Page 21 GAO-12-545R Air Emissions and Electricity Generation at U.S. Power Plants

State

Number of 

units

Electricity 

generation 

(megawatt-hours)

Carbon dioxide 

emissions (tons)

Nitrogen oxide 

emissions (tons)

Sulfur dioxide 

emissions (tons)

North Carolina 104 78,576,396 76,242,017 53,390 120,254

North Dakota 11 28,269,255 33,609,781 54,744 124,096

Ohio 135 126,505,558 125,054,961 104,871 572,110

Oklahoma 66 60,394,162 49,876,813 71,434 85,135

Oregon 8 19,280,743 10,875,349 9,729 15,696

Pennsylvania 123 143,267,035 125,643,066 134,169 411,216

Rhode Island 8 7,094,295 3,504,392 578 18

South Carolina 71 48,054,008 44,815,737 28,123 94,641

South Dakota 8 3,255,521 3,765,854 12,433 12,589

Tennessee 85 45,166,515 46,449,221 31,239 118,723

Texas 284 318,045,099 255,708,493 145,798 461,754

Utah 26 39,211,138 37,005,962 61,415 21,598

Virginia 97 42,773,143 35,584,177 38,300 93,385

Washington 13 17,402,607 14,560,232 11,980 2,651

Wisconsin 84 46,382,410 49,753,419 33,082 109,158

West Virginia 50 79,543,178 79,135,925 53,757 109,028

Wyoming 21 42,829,664 49,382,465 60,902 64,849

Total 3,443 2,754,711,668 2,448,972,387 2,105,844 5,157,306

Source: GAO presentation of Ventyx data.

Note: As with most of the other data presented in this report, we limited our analysis to those units that (1) list a fossil fuel as a 

primary fuel, (2) have a net summer capacity greater than 25 megawatts, and (3) produced power in 2010. No reported electricity 

production or emissions were reported from fossil fuel units in Alaska, Hawaii, or Vermont.
Enclosure II 

Page 22 GAO-12-545R Air Emissions and Electricity Generation at U.S. Power Plants

Electricity Generation and Emissions Rates from Fossil Fuel Electricity Generating Units 

in 2010, by State

Table 4 presents, by state, data on fossil fuel units’ electricity generation and emissions rates in 

2010. 

Table 4: Electricity Generation and Emissions Rates from Fossil Fuel Electricity Generating Units in 2010, by 

State

State Number of units Electricity 

generation (MWh)

Carbon dioxide 

emissions rate 

(tons/MWh)

Nitrogen oxides 

emissions rate 

(lbs/MWh)

Sulfur dioxide 

emissions rate 

(lbs/MWh)

Alabama 88 100,961,300 0.84 1.25 4.05

Arizona 63 74,817,174 0.81 1.62 0.97

Arkansas 30 40,295,345 0.90 1.88 3.33

California 179 80,854,400 0.46 0.11 0.01

Colorado 58 45,728,202 0.98 2.37 1.99

Connecticut 39 12,497,043 0.68 0.45 0.31

Delaware 15 5,489,585 0.79 1.58 5.28

District of 

Columbia 2 207,916 1.06 3.59 8.41

Florida 246 188,566,114 0.72 0.85 1.54

Georgia 141 95,973,778 0.91 1.26 4.56

Idaho 7 1,566,178 0.44 0.14 0.00

Illinois 196 97,862,368 1.09 1.57 4.50

Indiana 126 120,785,560 1.03 2.02 6.87

Iowa 48 41,249,495 1.10 2.17 5.06

Kansas 48 35,758,838 1.11 2.74 2.53

Kentucky 96 96,980,846 1.04 1.92 5.57

Louisiana 70 70,221,777 0.75 1.34 2.91

Maine 11 7,902,550 0.50 0.18 0.21

Maryland 46 26,381,736 1.06 1.47 2.27

Massachusetts 48 31,174,655 0.63 0.52 2.37

Michigan 96 76,833,143 0.97 2.07 6.31

Minnesota 45 31,496,681 1.04 1.90 2.61

Mississippi 57 40,802,413 0.75 1.46 2.68

Missouri 102 77,566,300 1.07 1.50 6.07

Montana 12 18,752,877 1.14 2.32 2.12

Nebraska 29 23,688,902 1.11 3.16 5.42

Nevada 37 28,136,479 0.61 0.77 0.56

New Hampshire 7 8,069,450 0.73 1.14 9.13

New Jersey 107 30,081,377 0.66 0.66 1.02

New Mexico 35 34,294,657 0.95 3.51 0.97

New York 165 63,663,802 0.65 0.89 1.49
Enclosure II 

Page 23 GAO-12-545R Air Emissions and Electricity Generation at U.S. Power Plants

State Number of units Electricity 

generation (MWh)

Carbon dioxide 

emissions rate 

(tons/MWh)

Nitrogen oxides 

emissions rate 

(lbs/MWh)

Sulfur dioxide 

emissions rate 

(lbs/MWh)

North Carolina 104 78,576,396 0.97 1.36 3.06

North Dakota 11 28,269,255 1.19 3.87 8.78

Ohio 135 126,505,558 0.99 1.66 9.04

Oklahoma 66 60,394,162 0.83 2.37 2.82

Oregon 8 19,280,743 0.56 1.01 1.63

Pennsylvania 123 143,267,035 0.88 1.87 5.74

Rhode Island 8 7,094,295 0.49 0.16 0.01

South Carolina 71 48,054,008 0.93 1.17 3.94

South Dakota 8 3,255,521 1.16 7.64 7.73

Tennessee 85 45,166,515 1.03 1.38 5.26

Texas 284 318,045,099 0.80 0.92 2.90

Utah 26 39,211,138 0.94 3.13 1.10

Virginia 97 42,773,143 0.83 1.79 4.37

Washington 13 17,402,607 0.84 1.38 0.30

West Virginia 50 79,543,178 0.99 1.35 2.74

Wisconsin 84 46,382,410 1.07 1.43 4.71

Wyoming 21 42,829,664 1.15 2.84 3.03

Total 3,443 2,754,711,668 - - -

Source: GAO presentation of Ventyx data.

Legend

Lb = pound

MWh = megawatt-hour

Note: As with most of the other data presented in this report, we limited our analysis to those units that (1) list a fossil fuel as a 

primary fuel, (2) have a net summer capacity greater than 25 megawatts, and (3) produced power in 2010. No reported electricity 

production or emissions were reported from fossil fuel units in Alaska, Hawaii, or Vermont.
Enclosure III

Page 24 GAO-12-545R Air Emissions and Electricity Generation at U.S. Power Plants

Energy and Emissions from U.S. Fossil Fuel Electricity Generating Units, by Location in 

2010

Figures 14,15, 16, and 17 show the location of electricity generating units and the amount of 

electricity, sulfur dioxide, nitrogen oxides, and carbon dioxide they produced in 2010. Units in 

the Great Lakes, South Central, and Southeast regions produced most of the emissions.26

Specifically, in 2010, units in these regions accounted for 69 percent of the sulfur dioxide, 59 

percent of nitrogen oxides, and 63 percent of the carbon dioxide emitted from older units 

nationwide in 2010 and generated 62 percent of the elecricity from fossil fuel units.

Figure 14: Geographic Distribution of Electricity Generation from Fossil Fuel Electricity Generating Units, 

2010

 

26These regions correspond to Environmental Protection Agency (EPA) Region 5 (Illinois, Indiana, 

Michigan, Minnesota, Ohio, and Wisconsin); Region 6 (Arkansas, Louisiana, New Mexico, Oklahoma, and 

Texas); and Region 4 (Alabama, Florida, Georgia, Kentucky, Mississippi, North Carolina, South Carolina, 

and Tennessee).
Enclosure III

Page 25 GAO-12-545R Air Emissions and Electricity Generation at U.S. Power Plants

Figure 15: Geographic Distribution of Sulfur Dioxide Emissions from Fossil Fuel Electricity Generating Units, 

2010
Enclosure III

Page 26 GAO-12-545R Air Emissions and Electricity Generation at U.S. Power Plants

Figure 16: Geographic Distribution of Nitrogen Oxides Emissions from Fossil Fuel Electricity Generating 

Units, 2010
Enclosure III

Page 27 GAO-12-545R Air Emissions and Electricity Generation at U.S. Power Plants

Figure 17: Geographic Distribution of Carbon Dioxide Emissions from Fossil Fuel Electricity Generating 

Units, 2010
Enclosure IV

Page 28 GAO-12-545R Air Emissions and Electricity Generation at U.S. Power Plants

Energy and Emissions from U.S. Fossil Fuel Electricity Generating Units in 2010, by 

Decade Unit Began Operating

Table 5 presents data on fossil fuel units’ electricity generation, aggregate emissions, capacity, 

and emissions per unit of electricity generated, based on the decade the unit began operating. 

Table 5: Electricity Generation and Emissions in 2010, by Decade Unit Began Operating

Decade unit began operating

1950s 1960s 1970s 1980s 1990s 2000s

Number of units 415 382 683 232 404 1,249

Electricity generation 

(MWh, in thousands) 195,561 353,497 770,451 513,904 163,507 728,165

Net summer capacity 

(MW) 52,130 93,570 177,014 83,160 52,967 241,798

Carbon dioxide 

emissions (tons, in 

thousands) 212,070 370,819 799,849 551,016 118,900 369,887

Carbon dioxide 

emissions rate 

(lbs/MWh)

 

2,169 

 

2,098 

 

2,076 

 

2,144 

 

1,454 

 

1,016 

Nitrogen oxides 

emissions (lbs) 273,290 446,312 697,687 524,769 85,282 65,186

Nitrogen oxides 

emissions rate 

(lbs/MWh)

 

2.79 

 

2.53 

 

1.81 

 

2.04 

 

1.04 

 

0.18 

Sulfur dioxide 

emissions (lbs) 1,135,001 1,372,766 1,506,432 987,113 74,822 31,368

Sulfur dioxide 

emissions (lbs/MWh)

 

11.61 

 

7.77 

 

3.91 

 

3.84 

 

0.92 

 

0.09 

Source: GAO analysis of Ventyx data.

Legend

lb = pound

MWh = megawatt-hour

(361383)
This is a work of the U.S. government and is not subject to copyright protection in the 

United States. The published product may be reproduced and distributed in its entirety 

without further permission from GAO. However, because this work may contain 

copyrighted images or other material, permission from the copyright holder may be 

necessary if you wish to reproduce this material separately.
The Government Accountability Office, the audit, evaluation, and 

investigative arm of Congress, exists to support Congress in meeting its 

constitutional responsibilities and to help improve the performance and 

accountability of the federal government for the American people. GAO 

examines the use of public funds; evaluates federal programs and 

policies; and provides analyses, recommendations, and other assistance 

to help Congress make informed oversight, policy, and funding decisions. 

GAO’s commitment to good government is reflected in its core values of 

accountability, integrity, and reliability.

The fastest and easiest way to obtain copies of GAO documents at no 

cost is through GAO’s website (www.gao.gov). Each weekday afternoon, 

GAO posts on its website newly released reports, testimony, and 

correspondence. To have GAO e-mail you a list of newly posted products, 

go to www.gao.gov and select “E-mail Updates.”

The price of each GAO publication reflects GAO’s actual cost of 

production and distribution and depends on the number of pages in the 

publication and whether the publication is printed in color or black and 

white. Pricing and ordering information is posted on GAO’s website, 

http://www.gao.gov/ordering.htm. 

Place orders by calling (202) 512-6000, toll free (866) 801-7077, or 

TDD (202) 512-2537. 

Orders may be paid for using American Express, Discover Card, 

MasterCard, Visa, check, or money order. Call for additional information.

Connect with GAO on Facebook, Flickr, Twitter, and YouTube. 

Subscribe to our RSS Feeds or E-mail Updates. Listen to our Podcasts. 

Visit GAO on the web at www.gao.gov. 

Contact:

Website: www.gao.gov/fraudnet/fraudnet.htm

E-mail: fraudnet@gao.gov

Automated answering system: (800) 424-5454 or (202) 512-7470

Katherine Siggerud, Managing Director, siggerudk@gao.gov, (202) 512-

4400, U.S. Government Accountability Office, 441 G Street NW, Room 

7125, Washington, DC 20548

Chuck Young, Managing Director, youngc1@gao.gov, (202) 512-4800

U.S. Government Accountability Office, 441 G Street NW, Room 7149 

Washington, DC 20548

GAO’s Mission

Obtaining Copies of 

GAO Reports and 

Testimony

Order by Phone

Connect with GAO

To Report Fraud, 

Waste, and Abuse in 

Federal Programs

Congressional 

Relations

Public Affairs

Please Print on Recycled Paper.