Duke Energy’s announcement last Tuesday that it will turn from coal to natural gas to fuel its Asheville power plant reduces its environmental footprint while saving the company money.

Duke will shut down the two coal-fired units by 2020, replacing them with a larger one fueled by natural gas. That will end an era of coal at the plant, whose smokestacks have towered over southern Buncombe County since 1964.

The $1.1 billion project will reduce air emissions from a power plant that has been the target of environmental advocates.

It will increase electricity generation, with cheaper fuel, in a region that has to import energy during peak periods. Duke has been forced to spend more to run the Asheville plant because of that dearth of power.

The move also lets Duke avoid the cost of converting the plant to handle coal ash in dry form by the end of 2019, as state law dictates. And Duke can scrap plans to build two oil-fired units to supply peak demand.

Duke has retired seven of its 14 North Carolina coal plants in the past five years. That happened as the price of natural gas plummeted and the heavy costs of meeting federal air standards loomed.

Duke had signaled in March 2014, a month after a coal ash spill into the Dan River, that it might retire Asheville’s coal units. Of four Duke plants that legislators singled out as high priorities for closing ash ponds, Asheville is the only one still operating.

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The plant’s two ponds have been linked to groundwater contamination, leading to charges by state regulators in February. Leaks from the ponds were among nine federal criminal charges that Duke settled last week for $102 million.

Spokesman Tom Williams said those pressures didn’t influence Duke’s decision to retool the plant. “There’s no one driver of this,” he said.

Williams cited the expanding availability of low-cost gas and the ability to take advantage of it through an upgraded gas line to the plant.

Duke Carolinas President Lloyd Yates credited state Sen. Tom Apodaca, a Hendersonville Republican, with prodding Duke to build the gas plant.

Apodaca, the powerful chairman of the Senate’s Rules Committee, knew that gas company PSNC already planned to replace a 26-mile pipeline in Polk, Henderson and Buncombe counties. It made sense, he told a Duke lobbyist, to tie the Asheville plant to the larger line.

“He just forced the question,” Yates said.

Apodaca said he will introduce legislation to extend the 2019 deadline for the Asheville plant to convert to dry ash handling.

As part of the project, Duke will build a transmission substation near Campobello, S.C., just across the state line, and connect it to the Asheville plant with a 40-mile transmission line.

The route of the new line has not been selected. It is expected to run through Buncombe, Henderson and Polk counties and South Carolina’s Greenville and Spartanburg counties.

Environmental advocates gave Duke’s announcement mixed reviews.

“It’s an exciting step forward to see that Duke Energy has heard the call from thousands of people in Asheville,” said Kelly Martin of the Sierra Club. “It’s exciting to know that the end of water pollution from coal ash and air pollution from carbon emissions from this plant is in sight.”

But the Asheville Beyond Coal Campaign, which includes the Sierra Club, called the plan to replace coal with natural gas a “half measure” because it is also a fossil fuel that pollutes.

The campaign says Duke should instead expand solar energy. Duke says a solar installation will be built at Asheville but does not yet know its size.

The Southern Alliance for Clean Energy applauded Duke “for doing the right thing for its customers, public health, and the environment by closing this facility and moving toward cleaner solutions.”

Asheville’s City Council voted in October 2013 to urge Duke to move away from coal to reduce carbon emissions.

The new gas plant will emit 60 percent less carbon dioxide for each megawatt hour of electricity it generates. Gas releases about half the carbon of coal. But because the gas unit will have nearly twice the generating capacity of the two coal units, it’s unclear whether the plant’s actual carbon emissions will be lower.

The gas plant will use combined-cycle technology that captures exhaust heat and turns it into additional energy. Low gas prices mean it will be about 35 percent less expensive to operate than the coal units.

The project will reduce the power plant’s emissions of sulfur dioxide, which contribute to haze and acid rain, by up to 95 percent, Duke said. Water withdrawals will go down 97 percent.

Charlotte Observer staff writer Jim Morrill contributed.