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).
In the TVA region, transportation sources such as on-road and off-road vehicles, boats and airplanes account for about two-thirds of total NOx emissions. All point sources in the region such as power plants, refineries and mills account for about one-fourth of total NOx emissions, but TVA point sources only account for about one-tenth of the total NOx emissions in the region. The remaining amount comes from area sources such as decaying vegetation, industrial processes and nitrogen fertilizers.
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. The 35 systems, along with existing boiler-optimization controls, the operation of four SNCR systems and the operation of low-NOx burners, reduce TVA's NOx emissions by 70 to 75 percent. TVA has reduced its overall NOx emissions by 90 percent since 1995. Reductions of this magnitude help reduce ozone levels.