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Scrubbing Coal

In all the details and statistics about power plant emissions and pollution control, one fact jumps out from the clutter: burning coal to generate electricity is much cleaner today than it was in 1970 when the Clean Air Act went into effect.

Understanding which pollutants have been reduced, how they continue to decrease, and why more improvements can be expected takes a little more study.

Two of the most talked about emissions are sulfur dioxide and nitrogen oxide. All coal contains some sulfur. Burning coal releases that sulfur, which then combines with oxygen to form sulfur dioxide. Scrubbers are designed to remove as much sulfur dioxide (SO2) as possible from the material coming out of the stack at a coal-burning plant. Other devices, known as SCRs, target the emission of nitrogen oxide (NOx), but this month we’ll focus on scrubbers, one of the easiest aspects of pollution control to understand.

Scrubbers have been helping clean Kentucky’s air for more than two decades. An installed scrubber looks a bit like a four-story farm silo. Scrubbers are a big deal because of their importance in cleaning the environment, and because they are expensive: it can cost $70-$90 million to install a scrubber onto an existing power plant, plus additional money to run it.

But not all coal-fired generating plants burn the same kind of coal; that’s why scrubbers play different roles in different parts of the state.

Big Rivers, the electric generation and transmission cooperative based in Henderson, serves consumers in 22 counties in western Kentucky. Mike Thompson, technical advisor for power generation at Big Rivers, says, “Big Rivers was formed with the economic development of this part of the state in mind, so we’ve always burned the cheap coal locally available from the western Kentucky coal fields.”

Big Rivers’ first power plant at Reid began producing electricity commercially in 1966. Then three more units (at a plant near Coleman) were completed in 1968, 1969, and 1971, generating electricity from local coal.

Built before the Clean Air Act of 1970, those plants weren’t subject to special rules or regulations. But as awareness of potential problems increased, utilities such as Big Rivers made adjustments. Thompson explains, “Since our coal happens to be high in sulfur, we installed our first scrubbers when we built two new generating units at our third site at Green Station in 1979 and 1980.”

Big Rivers now owns nine units at four different sites. Scrubbers are in use at three of those sites, and construction is under way to retrofit the Coleman site with scrubbers that will be cleaning the air by the end of 2003.

“With the technology available today,” Thompson says, “a scrubber system will reduce sulfur dioxide emissions by 90-95 percent.”

Scrubbers do their work on gases as they go up the flue. Chemicals “scrub” the gases to remove sulfur dioxide. The leftover particles are then collected for disposal.

In a typical year, the Wilson Station scrubber removes more than 100 tons of sulfur dioxide. Instead of going into the air, this residue can be disposed of in an on-site landfill or, as is planned for the Coleman site, offered to other businesses to create useful products with it, such as a substitute for traditional gypsum wallboard.

East Kentucky Power Co-op, which generates electricity for 16 cooperatives in the eastern half of the state, operates coal-fired units in three counties–Mason, Clark, and Pulaski. Being so much closer to the Appalachian coal fields means East Kentucky Power has steady access to a different kind of coal.

Bob Hughes, environmental affairs manager for East Kentucky Power, says, “We can meet the proper air-quality standards by burning this low-sulfur coal instead of operating pollution control equipment.”

That’s been true at all eight of East Kentucky Power’s coal-fired units for the past 20 years–with one important exception. Economic changes in the coal markets sent the price of low-sulfur coal soaring in the late 1970s and the early 1980s. The price went so high that building and operating a scrubber on Unit Two at the Spurlock Station so it could burn high-sulfur coal was actually the cheapest way to meet the air-quality standards in effect then. That scrubber ran for only four years (1981 through 1985), and has not been used since the plant switched back to low-sulfur coal.

Air-quality standards do change over time, getting more strict each year, even when plants burn low-sulfur coal. That’s why Spurlock Station’s proposed third unit, whose construction is going through the approval process, will feature a new style of dry-flash scrubber that will operate continuously to remove sulfur dioxide, no matter what kind of coal the plant burns.

To find out more about how a scrubber cleans the air, check out the diagrams at powerful_facts/powerplant.asp

Next month: Nitrogen oxides, SCRs, and you: more air-quality improvements

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