A report released on Friday by the Government Accountability Office (GAO) finds that power plant smokestacks of 500 feet or higher disperse pollutants over greater distances—and that stack height is one of several factors that contribute to the interstate transport of air pollution. The congressional investigative arm also finds that several boilers remain uncontrolled for certain pollutants, including several connected to tall stacks.
The report, “Air Quality: Information on Tall Smokestacks and Their Contribution to Interstate Transport of Air Pollution,” was requested by Sen. Sheldon Whitehouse (D-R.I.), chair of the Senate Environment and Public Works Committee’s oversight subpanel. The GAO says it sought to determine the number and location of tall stacks at coal plants, how those stacks contribute to interstate transport of air pollution, and the number of stacks built above good engineering practice (GEP) height since 1988.
The GAO conceded that the Clean Air Act does not limit stack height, but it said that the Act “prohibits sources of emissions from using the dispersion effects of stack heights in excess of a stack’s [GEP] height to meet emissions limitations.” Of a total of 284 tall smokestacks operating at 172 coal plants in 34 states, 207 are 500 feet to 699 feet tall, 63 are 700 feet to 999 feet tall, and the remaining 14 are 1,000 feet tall or higher, the report finds.
About half of all tall stacks began operating more than 30 years ago—but there was a marked increase in the number of tall stacks that began operating in the last four years, the GAO says, which air and utility officials attributed to the need for new stacks when plants installed pollution control equipment.
“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,” the GAO says. “These factors included helping a plant’s emissions clear local geographic features, such as valley walls.”
Smokestack height certainly contributes to the interstate transport of air pollution, but the GAO says it was difficult to isolate 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.”
One concrete finding was that the use of pollution controls—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, “many boilers remain uncontrolled for certain pollutants, including several connected to tall stacks,” it said.
About 56% of boilers attached to tall stacks lacked scrubbers to control SO2 and 63% lacked post-combustion controls to capture NOx emissions. In general, the GAO found that boilers without these controls tended to be older, with in-service dates prior to 1980. The GAO also 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.
Sources: POWERnews, GAO