King Coal may soon be dethroned as the Tennessee Valley Authority's primary source of power.

After stoking most of its power plants with coal for more than a half-century, TVA appears ready to shut down many of its oldest and least-efficient coal plants in the face of stricter environmental rules and less demand for energy. The federal utility has shut down a half-dozen units in the past two years and is studying options to shutter at least a dozen more.

TVA expects to generate more power from nuclear plants than coal within the next three years.

"The average age of our coal fleet is over 51 years, so I like to joke that I would like to close the ones that are older than me," said Bill Johnson, TVA's 59-year-old CEO, who touts a vision of making TVA a national leader in low-cost, cleaner energy by 2020.

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