The 1977 amendments to the Clean Air Act discouraged  the use of dispersion techniques to help attain NAAQS. Specifically,  section 123 prohibits states from counting the dispersion effects of stack  heights in excess of a stack’s GEP height when determining a source’s  emissions limitation. The Clean Air Act defines GEP as “the height  necessary to insure that emissions from the stack do not result in  excessive concentrations of any air pollutant in the immediate vicinity of  the source as a result of atmospheric downwash, eddies, or wakes which  may be created by the source itself, nearby structures, or nearby terrain  obstacles.” According to federal regulations, a stack’s GEP height is the  higher of  65 meters, measured from the ground-level elevation at the base of the  stack; a formula based on the height and width of nearby structure(s) (height  plus 1.5 times the width or height, whichever is lesser); or the height demonstrated by a fluid model or field study that ensures  the emissions from a stack do not result in excessive concentrations of  any air pollutant as a result of atmospheric downwash created by the  source itself, nearby structures, or nearby terrain features. 1). At coal power plants, these controls are generally installed in either the  boiler, where coal is burned, or the duct work that connects a boiler to the  stack. While about half of the  tall stacks began operating more than 30 years ago, there has been an  increase in the number of tall stacks that have begun operating in the last 4  years, which several stakeholders attributed to the need for new stacks  when retrofitting existing plants with pollution control equipment. 284 Tall Stacks Were Operating at about 170 Coal Power Plants, with Approximately One-Third Located in the Ohio River Valley

As of December 31, 2010, we found a total of 284 tall stacks were operating  at 172 coal power plants in the United States. We found that 207 tall stacks (73 percent) are between 500 and 699 feet tall  and that 63 stacks (22 percent) are between 700 and 999 feet tall. Stack Height Contributes to Interstate Transport of Air Pollution, and the Emissions from Several Tall Stacks Remain Uncontrolled for Certain Pollutants

Stack height is one of several factors that contribute to the interstate  transport of air pollution. According to reports and stakeholders with  expertise on this topic, tall stacks generally disperse pollutants over longer  distances than shorter stacks and provide pollutants with more time to  react in the atmosphere to form ozone or particulate matter. However, the  interstate transport of air pollution is a complex process that involves  several variables—such as total emissions from a stack, the temperature  and velocity of the emissions, and weather—in addition to stack height. As  a result, stakeholders had difficulty isolating the exact contribution of  stack height to the interstate transport of air pollution, and we found  limited research on this specific topic. Use of Pollution Controls at Coal Power Plants Has Increased in Recent Years, but Emissions from Some Plants, Including Several with Tall Stacks, Remain Uncontrolled for Certain Pollutants

The use of pollution control equipment, particularly for SO and NO  emissions, has increased over time, largely in response to various changes  in air regulations, according to stakeholders and reports we reviewed. Collectively, we found  that these boilers without post-combustion controls accounted for 54  percent of the total generating capacity of boilers attached to tall stacks. Based on Available Information, 17 of 48 Tall Smokestacks Built Since 1988 Exceed Their GEP Height, and A Variety of Factors Can Influence Height Decisions

We identified 48 tall stacks built since 1988 that states reported are subject  to the GEP provisions of the Clean Air Act and for which states could  provide GEP height information. Of these 48 stacks, we found that 17  exceed their GEP height, 19 are at their GEP height, and 12 are below their  GEP height. When we followed up with utility officials regarding why these stacks  were built above GEP, they reported that a variety of factors can influence  stack height decisions. These factors included helping a plant’s emissions  clear local geographic features, such as valley walls. We were unable to obtain GEP height information for an additional 25  stacks that were built since 1988 for two reasons. First, some of these  stacks replaced stacks that were exempt from the GEP regulations,  according to state officials. Second, states did not have GEP information readily available for some  stacks. Both EPA and DOE stated they had no comments. Appendix I: Scope and Methodology

To identify the number and location of smokestacks at coal power plants  that were 500 feet or higher as of December 31, 2010, we analyzed data on  power plants from the Department of Energy’s (DOE) Energy Information  Administration (EIA). We also used these data to determine when these  stacks began operating. Other stakeholders reported that they considered a stack built above GEP  to be “tall.”
To determine what is known about tall stacks’ contribution to the  interstate transport of air pollution, we reviewed reports from the  Environmental Protection Agency (EPA) and academics and spoke with  stakeholders with expertise on this topic.
Why GAO Did This Study

Tall smokestacks--stacks of 500 feet or higher, which are primarily used at coal power plants--release air pollutants such as sulfur dioxide (SO2) and nitrogen oxides (NOx) high into the atmosphere to help limit the impact of these emissions on local air quality. Tall stacks can also increase the distance these pollutants travel in the atmosphere and harm air quality and the environment in downwind communities. The 1977 amendments to the Clean Air Act encourage the use of pollution control equipment over dispersion techniques, such as tall stacks, to meet national air standards. Section 123 of the Act does not limit stack height, but prohibits sources of emissions from using the dispersion effects of stack heights in excess of a stack's good engineering practice (GEP) height to meet emissions limitations. GAO was asked to report on (1) the number and location of tall stacks of 500 feet or higher at coal power plants and when they began operating; (2) what is known about such stacks' contribution to the interstate transport of air pollution and the pollution controls installed at plants with these stacks; and (3) the number of stacks that were built above GEP height since 1988 and the reasons for this. GAO analyzed Energy Information Administration (EIA) data on power plants, surveyed states with tall stacks, and interviewed experts on the transport of air pollution. GAO is not making recommendations in this report. The Environmental Protection Agency and the Department of Energy stated they had no comments on this report

What GAO Found

According to analysis of EIA data, which were updated with GAO's survey results, a total of 284 tall smokestacks were operating at 172 coal power plants in 34 states, as of December 31, 2010. Of these stacks, 207 are 500 to 699 feet tall, 63 are 700 to 999 feet tall, and the remaining 14 are 1,000 feet tall or higher. About one-third of these stacks are concentrated in 5 states along the Ohio River Valley. While about half of tall stacks began operating more than 30 years ago, there has been an increase in the number of tall stacks that began operating in the last 4 years, which air and utility officials attributed to the need for new stacks when plants installed pollution control equipment. Stack height is one of several factors that contribute to the interstate transport of air pollution. According to reports and stakeholders with expertise on this topic, tall stacks generally disperse pollutants over greater distances than shorter stacks and provide pollutants greater time to react in the atmosphere to form ozone and particulate matter. However, stakeholders had difficulty isolating the exact contribution of stack height to the transport of air pollution because this is a complex process that involves several other variables, including total emissions from a stack, the temperature and velocity of the emissions, and weather. The use of pollution controls, which are generally installed in boilers or the duct work that connects a boiler to a stack, has increased in recent years at coal power plants. However, GAO found that many boilers remain uncontrolled for certain pollutants, including several connected to tall stacks. For example, GAO found that 56 percent of boilers attached to tall stacks lacked scrubbers to control SO2 and 63 percent lacked post-combustion controls to capture NOx emissions. In general, GAO found that boilers without these controls tended to be older, with in-service dates prior to 1980. GAO identified 48 tall stacks built since 1988--when GEP regulations were largely affirmed in court--that states reported are subject to the GEP provisions of the Clean Air Act and for which states could provide GEP height information. Of these 48 stacks, 17 exceed their GEP height, 19 are at their GEP height, and 12 are below their GEP height. Section 123 of the Clean Air Act defines GEP as the height needed to prevent excessive downwash, a phenomenon that occurs when nearby structures disrupt airflow and produce high local concentrations of pollutants. Officials reported that a variety of factors can influence stack height decisions. For example, some utility officials reported that stacks were built above GEP to provide greater protection against downwash or to help a plant's emissions clear local geographic features, such as valley walls. GAO was unable to obtain GEP height information for an additional 25 stacks that were built since 1988 for two reasons: (1) some of these stacks were exempt from GEP regulations, and (2) states did not have GEP information readily available for some replacement stacks because the GEP calculation was sometimes made decades earlier and a recalculation was not required at the time the replacement stack was built.