Efforts to reduce smog and acid rain have electric utilities installing new equipment to clean up a group of pollutants known as nitrogen oxides–usually shortened to NOx and pronounced to rhyme with “socks.”
NOx refers to two separate chemicals, nitric oxide and nitrogen dioxide. Burning fossil fuels at very high temperatures (in car and truck engines and at coal-fired power plants, for example) releases NOx gases into the atmosphere.
While the earth’s atmosphere normally contains some nitrogen oxides (they occur naturally from decaying vegetation), too much NOx contributes to pollution–ground-level ozone and its companion smog, as well as acid rain.
During the hottest months of the year, NOx gases can combine with sunlight and other organic compounds to form ground-level ozone and smog–problems plaguing several cities and surrounding areas in North America. Particles in NOx gases can also combine in clouds with water droplets that fall to earth as acid rain, which several studies show seems to be harming plant and animal life in the northeastern United States.
Since many Kentucky power plants traditionally burn coal at extremely high temperatures, the first step to try to lower the emissions was to lower the temperature at which coal was burned. Bob Hughes, environmental affairs manager for East Kentucky Power Cooperative based in Winchester, explains, “During the early to mid-1990s, we changed over to ‘low-NOx burners.’ But you can only go so far with boiler technology.”
So utilities turned to cutting-edge equipment known as SCRs–short for selective catalytic reduction devices.
These huge box-like structures (up to 10 stories tall in some cases) add another step to the process of generating electricity. As flue gases emerge from the burning coal, they pass into the SCR. The SCR contains layers of a ceramic material with little holes one-eighth to one-quarter inch in diameter.
As the flue gases enter the SCR’s enormous chamber, ammonia separates water vapor and nitrogen from the flue gases. Since water vapor and nitrogen by themselves occur naturally in the atmosphere, they can be safely returned to the air.
Simple enough on paper. But the Tennessee Valley Authority found out it’s much more complicated in action.
“During the summer of 2000, we began operating our first SCR on Unit Two at our Paradise facility,” says Barbara Martocci, a spokesperson for TVA, which supplies electricity for five electric co-ops in western Kentucky. “We had some problems with the vaporizer in the ammonia system. As we went through the season we learned various things about the system, re-engineered part of the equipment, and found how to better manage the SCR.”
On May 1 East Kentucky Power began operating its first SCR at Spurlock Station near Maysville. “It still needs a lot of fine-tuning,” says Bob Hughes.
Before East Kentucky’s SCR began operating, NOx emissions measured about 0.25 pounds per each million Btu. With the SCR, NOx emissions will range from only 0.0225 to 0.005 pounds per each million Btu, a substantial reduction.
In western Kentucky, Big Rivers Electric Corporation, the third generator of electricity in Kentucky’s co-op system, recently began construction of its first SCR. This retrofitting of such a large pollution-control device at the Wilson unit, a 450-megawatt plant, will cost more than $50 million. Mike Thompson, technical advisor for power generation at Big Rivers, says, “Compared to sulfur dioxide, NOx is more expensive per ton removed than sulfur dioxide.” At East Kentucky Power, accountants figure the utility will spend nearly $200 million building three SCRs.
Aside from the money issues, an even bigger question looms: although SCRs can reduce NOx emissions, will that actually result in a noticeably cleaner environment?
While it’s relatively easy to measure the output of gases from a power plant, measuring ozone action days and the severity of acid rain in widely dispersed areas–and the relationship with NOx from power plants–will continue to keep scientists busy for decades.
Will SCRs reduce smog and acid rain? The answer will be very important for Kentucky’s electric co-op members–and for people living hundreds of miles away.
To find out more about nitrogen oxides and acid rain, visit these pages on the TVA Web site at www.tva.gov/environment/air/nox.
htm or www.tva.gov/environment/air/ozone.htm.
Next month: The debate over global warming and how it affects you