MidAmerican Energy Co. said Tuesday that it will cease burning coal at its Riverside Generating Station near Bettendorf and convert to natural gas by 2016, part of an agreement with an environmental group that had claimed violations of the federal Clean Air Act.

The settlement agreement also includes commitments to stop burning coal at two other MidAmerican sites in Iowa.

The Riverside plant already is capable of burning natural gas, but officials could not say how soon the conversion would take place.

Environmentalists hailed the agreement as a milestone for improving air quality in Iowa, including the Quad-Cities.

The agreement comes six months after the Sierra Club notified MidAmerican that it intended to sue as a result of what it said were emissions that surpassed permitted amounts at facilities in the Quad-Cities, Council Bluffs and Sergeant Bluff, Iowa. The latter is near Sioux City.

In a statement Tuesday, the company flatly rejected the idea it violated the law. But it said it entered into confidential settlement negotiations to avoid costs to its customers, delays and the uncertainty of litigation.

The 20-page agreement, filed in U.S. District Court in Des Moines, says that by April 16, 2016, MidAmerican will stop burning coal at three boilers at the Riverside plant and at two boilers each at the Walter Scott Jr. Energy Center in Council Bluffs and the Neal Energy Center in Sergeant Bluff, Iowa.

MidAmerican also will complete installation of baghouses that contain emissions by the end of 2014 at two units at the Sergeant Bluff facility. The company said those projects are under way and were in progress before the intent to sue was filed.

The company also has agreed to fund a supplemental environmental project and is examining a 60-kilowatt solar power project at the Iowa State Fairgrounds.

MidAmerican indicated the agreement won’t cost customers.

“There will be no incremental costs to customers as a result of the settlement,” Tina Pothoff, a spokeswoman, said Tuesday.

Unlike at Riverside, the company said it is evaluating the use of other fuels, including natural gas, for the continued operation of the Sergeant Bluff and Council Bluffs facilities.

The Sierra Club hailed the agreement as a major step toward moving away from coal-fired plants and an improvement of air quality for Iowans.

It said the agreement will take about 20 percent of the state’s coal capacity offline.

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“Coal’s days are numbered here in Iowa,” said Pam Mackey Taylor, the energy chair of the Sierra Club’s Iowa chapter. “Retiring units at these coal plants and installing vital pollution controls at the remaining units will help Iowans breathe easier.”

Coal-fired plants emit dangerous amounts of mercury, causing thousands of asthma attacks, heart attacks and premature deaths, they say.

MidAmerican officials said the agreement merely builds upon its strong environmental record. The company is a leader in the development of wind energy, and it said it has been aggressively moving to reduce its carbon emissions, including completion of projects in 2007 and 2009 and others with completion dates of later this year and next.

“We already have had a number of these projects in the works,” Pothoff said.

Sierra Club officials said the agreement will ensure the changes at the three plants are made.

“With the consent decree, we have the assurance they will do it,” said Wally Taylor, legal chair of the Sierra Club’s Iowa chapter.

The Sierra Club says that the Iowa agreement pushes the amount of coal generation retired or announced to retire since 2010 nationwide to more than 50,000 megawatts.