Neil McBride, new members Lynn Evans and Joe Ritch and new TVA CEO Bill Johnson, from left, listen as the TVA board of directors hold a public meeting Thursday at TVA's Chattanooga Office Complex.

The Tennessee Valley Authority has shut down or announced plans to soon shutter more than a dozen of its oldest and smallest coal-fired generation units to meet federal clean air standards.

But environmental leaders appealed to TVA's new directors Thursday to move away more aggressively from the fossil fuels that have powered most of the Tennessee Valley for the past half century.

"The age of coal has passed," Louise Gorenflo, a member of the Tennessee Chapter of the Sierra Club, told TVA board members during their quarterly meeting in Chattanooga. "Now is the time for TVA to retreat from these fossil fuels that pump out more carbon into the atmosphere. We need to move to other renewable sources and do more to promote energy efficiency."

Gorenflo and a half-dozen other activists from the Tennessee Environmental Council, the Sierra Club and Tennessee Climate Action Campaign targeted the Gallatin Fossil Plant in Middle Tennessee. Gallatin is TVA's third biggest coal plant and TVA directors decided two years ago to spend more than $1 billion to add scrubbers and selective catalytic reduction systems to all four of the plant units by the end of 2017.

TVA's new CEO, Bill Johnson, said the retrofit of the 54-year-old plant is needed because Gallatin is still a relatively low-cost source of electricity and key to reliable power around Nashville.

"(Gallatin) continues to be a cost-effective, important and big asset for us," Johnson said. "We heard a lot of input today and we'll certainly go back and look at what was said. It still looks like a pretty good project and from what we know today we're going to continue on with that project."

Overall, TVA still gets more than 40 percent of its energy from coal-fired power plants and TVA Chief Financial Officer John Thomas said coal generation is often among TVA's cheapest sources of power.

But economic and environmental changes are prompting TVA to again reassess its future power plans. Joe Hoagland, senior vice president of policy and oversight, said TVA will begin a new Integrated Resource Planning process this summer to reexamine future power plans over the next year and a half. The previous Integrated Resource Plan completed two years ago was the first in 15 years, and Hoagland acknowledged "things have changed significantly" from what guided that plan. TVA power sales have declined and natural gas prices are cheaper than most forecasters expected.

Megan Spooner, a UTC professor who volunteers for the Chattanooga Local Climate Action Team, presented TVA directors petitions from 70 area residents urging the shut down Gallatin, or at least a re-examination of the move to spend up to $1.2 billion on pollution controls to extend it life.

"Other electric utilities have proven that it is possible to use energy efficiency at much higher levels," Spooner said.

The Sierra Club commissioned a study by Synapse Energy Economics that concluded TVA's plan to put scrubbers on its Gallatin and Allen fossil plants in Tennessee, Colbert plant in Alabama and Shawnee plant in Kentucky will likely cost at least $11 billion. That expense, the consultants suggested, would raise rates for decades.

TVA Chairman Bill Sansom said the federal utility will benefit from "a balanced portfolio" of nuclear, coal, hydro and renewables.

"We shouldn't put our eggs in any one basket," he said.