HARRIMAN, Tenn. — The smokestacks that tower over the Kingston Fossil Plant showcase the evolution in TVA’s efforts to clean up one of its biggest coal plants.

By the spring of 2010, the federal utility plans to complete a $500 million addition of two scrubbers here to remove more than 90 percent of the plant’s smog-causing air emissions. Plant Manager Ronald Hall said the scrubbers, the third generation of air pollution controls, will replace the giant smokestacks that were erected in the 1970s to help limit localized toxic substances released when Kingston opened as the world’s biggest coal power plant in 1955.

But for all the investment and improvement the scrubbers represent, Mr. Hall and others in this riverfront community are focused on a more immediate and bigger environmental challenge. The rupture of a fly ash storage cell into nearby rivers and homes on Dec. 22 has shifted pollution concerns from air emissions to fly ash dumped on the land, in the river and now threatening to be blown around in the air.

“This may be the biggest environmental disaster ever, at least in its size and scope, and that is a very sad legacy TVA will have to live with,” said Stephen Smith, executive director for the Southern Alliance for Clean Energy, a Knoxville-based environmental group critical of coal plants. “We were pleased to see TVA’s decision to finally install scrubbers at Kingston. But the events of the past couple of weeks prove again that coal is still a very dirty source of power. I’m afraid Kingston will forever be known for this disaster.”

TVA has promised to clean up the 1.1 billion gallons of fly ash and muck that flowed out of an elevated ash storage pond.

Agency officials said they still don’t know how long or how expensive it will be to clean up the 300-plus acres covered with ash. But a much smaller 2005 spill of an ash pond at the Martins Creek Power Plant in Pennsylvania ended up costing the plant’s owner, PPL Corp., about $38.5 million in fines and cleanup costs.

TVA already has pledged to spend more than $1 billion to install scrubbers within the next four years at its three East Tennessee coal plants — Kingston, Bull Run near Oak Ridge and John Sevier near Rogersville — part of TVA’s overall $5.8 billion program to limit air pollution from its 11 coal plants.

“We continue to invest in clean air improvements to reduce our emissions, and these scrubbers are an important part of our ongoing efforts,” TVA spokesman Gil Francis said.

The upgrade is expected to cut more than 90 percent of smog-producing sulfur and nitrogen oxide emissions in the air.

Until two weeks ago, the upgrade also was expected to help shield the Kingston plant from persistent environmental complaints and pollution lawsuits. But the ash pond spill has put the Kingston plant in the global spotlight and at the epicenter of the debate over America’s coal future, according to Don Barger, senior director of the Southeast regional office of the National Parks Conservation Association.

“This spill shows us once again that there is no such thing as ‘clean coal,’ despite what the industry tries to tell us,” he said. “Coal is touted as a cheap source of power, but it’s not going to be cheap to clean up this mess.”

Despite the cleanup and pollution control costs for coal plants, however, TVA studies show that power generation from existing TVA coal plants is typically only about one-third as expensive as comparable generation from solar or wind sources and, at current fuel prices, about half the cost of natural gas-fired generation.

Last year, coal-fired plants generated nearly 60 percent of TVA’s electricity, Mr. Francis said.