Ozone

Naturally occurring ozone in the upper layer of the atmosphere forms a protective shield around the earth, filtering out harmful solar radiation. But the ozone that’s found at ground level is only partly natural, and its effects are thought to be negative. It can cause respiratory problems in people, reduce crop productivity, and even damage some building materials. And it’s a major component of smog, the chemical soup that forms a brownish-yellow haze over many urban areas.

Ground-level ozone is produced by a series of reactions involving other chemicals, mostly volatile organic compounds and nitrogen oxides (NOx). Especially high levels of ozone develop in the summer, when the sunlight is intense and the air is hot and stagnant.

Most volatile organic compounds in the Tennessee Valley’s air come from natural sources, and the remaining ones are generated by motor vehicles and some industries (though not power plants).

Fossil-fuel power plants and motor vehicles produce about a third of the total NOx emissions each; the rest come from other sources, such as decaying vegetation, industrial processes, and nitrogen fertilizers.

What TVA is doing about it

TVA is reducing NOx emissions by installing pollution-control equipment on units where the greatest environmental and public-health benefits can be realized. Its first selective catalytic reduction (SCR) system, for example, began operation at Kentucky’s Paradise Fossil Plant in 2000. The system was the first of 21 SCRs installed in TVA’s coal plants. Also, TVA has installed SCRs on its five natural gas fired combined cycle plants.

The SCR technology controls NOx emissions by transforming them into harmless nitrogen and water vapor. Once in place, the 26 systems, along with existing boiler-optimization controls and the operation of low-NOx burners, will reduce TVA’s NOx emissions by 70 to 75 percent during the “ozone season,” which lasts from May to September. Reductions of this magnitude should help bring ozone production down.