Coal plant smokestack. Charlie Riedel/AP
Reading Time: 3 minutes

Coal power plants use tall smokestacks to release air pollutants like sulfur dioxide and nitrogen oxides high into the atmosphere, in an effort to disperse pollution and decrease the impact on the local community. But wind currents are faster at higher altitudes, causing pollution to travel hundreds of miles to other areas or states. While the EPA has been attempting to decrease interstate air pollution, there has been an increase in smokestacks taller than 500 feet in the last four years.

A Government Accountability Office review found that there are 284 tall smokestacks operating at 172 coal power plants. About a third are located in the Ohio River Valley. About half of the tall stacks are over 30 years old. But 51 of the 284 stacks were built since 2000, with the vast majority going into service in the last four years. About 5 percent of the tall stacks are 1,000 feet or higher, with the tallest peaking at 1,038 feet at the Rockport Power Plant in Indiana—almost as tall as the Eiffel Tower.

“Stack height is one of several factors that contribute to the interstate transport of air pollution. While the use of pollution controls has increased in recent years at coal power plants, several boilers connected to tall stacks remain uncontrolled for certain pollutants,” the GAO said.

In the early 1970s, power plants installed tall stacks to disperse pollutants and meet EPA’s air quality standards. While a tall stack may reduce pollution concentrations locally, it does not actually reduce total emissions, which travel downwind and can hurt air quality elsewhere. At the same time, stacks must be built high enough to prevent “downwash,” which occurs when large buildings alter wind patterns and cause emissions to reach ground-level more quickly.

The 1977 amendments to the Clean Air Act encouraged the use of pollution control equipment, like scrubbers and control measures, in place of dispersion techniques. A provision was added to the Clean Air Act to prohibit states from counting the dispersion effects of stack heights in excess of a stack’s good engineering practice (GEP) height when determining a power plant’s emissions limit. The good engineering practice is defined as the necessary height to prevent excessive downwash. If a stack’s GEP height is 600 feet, and the stack is 800 feet, the power plant cannot count the dispersion effects resulting from the excess 200 feet toward its emissions limit.

EPA reported that measured levels of SO2 and NOx, along with ozone and particulate matter, decreased between 1990 and 2008. But about 127 million people live in counties where pollution exceeds one or more air quality standards. Power plants emitted over 65 percent of SO2 emissions and almost 20 percent of NOx emissions. These pollutants contribute to acid rain, ozone and particulate matter, which can exacerbate emphysema, bronchitis and asthma, and acid rain, which can damage crops, vegetation and aquatic ecosystems.

EPA has taken steps to reduce SO2 and NOx emissions that contribute to the interstate transport of air pollution through recent rule makings. EPA is proposing the Transport Rule, which would limit the interstate transport of SO2 and NOx emissions to avoid violations of excess levels of particulate matter and ozone. EPA has found that emissions of SO2 and NOx from 31 eastern states and the District of Columbia prevent downwind states from meeting their NAAQS for ozone and particulate matter. The Transport Rule would replace the Clean Air Interstate Rule, cover more states and require emissions of SO2 to decrease by 71 percent and emissions of NOx to decrease by 52 percent over 2005 levels by 2014.

The GAO report found that 56 percent of the boilers attached to tall stacks do not have scrubbers to control SO2 emissions and 63 percent of boilers connected to tall stacks do not have post-combustion controls for NOx which help reduce emissions more than combustion controls alone.

FAST FACT: The installation of pollution control equipment is expensive. A study by Massachusetts Institute of Technology found that it may cost coal power plants $215,000 to $330,000 per megawatt to install controls for particulate matter, SO2 and NOx. For a typical coal power plant producing 500 megawatts, it could cost more than $107 million to install controls at a newly built plant and $165 million to retrofit an existing facility.

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