Duke Energy agreed in a legal settlement announced Tuesday to retire more than 1,600 megawatts of old coal-fired power plants, making enforceable the shutdown plans Duke had previously revealed.

The settlement removes the last legal challenges to Duke's newest N.C. coal-fired unit, at the Cliffside plant in Rutherford County about 60 miles west of Charlotte.

Four advocacy groups had challenged a state-issued permit for the 800-megawatt addition, which will begin operating this year.

The case was heard in state administrative court.

The terms of Tuesday's court settlement were included in a similar agreement Duke struck with the advocacy groups last month in South Carolina.

That agreement was linked to Duke's proposed merger with Progress Energy.

"We're pleased to have reached agreement with these groups because it removes any further legal issues with our Cliffside project," said Duke spokesman Tom Williams.

In planning documents filed with Carolinas regulators, Duke already had signaled that it planned to retire old coal units rather than upgrade them with new pollution controls. The plants will shut down by 2020.

"This is a concrete, enforceable plan, as opposed to a plan that could change next year," said John Suttles, the lead attorney in the case for the Southern Environmental Law Center. The center represented the Environmental Defense Fund, National Parks Conservation Association, the Sierra Club and the Southern Alliance for Clean Energy.

The new Cliffside unit will sport the most stringent acid gas controls in the United States, the advocacy groups said, with a 99.9 percent reduction.

Gases such as sulfur dioxide and hydrochloric acid can cause eye, nose and respiratory tract irritation, chest pain, impaired lung function, asthma attacks and chronic bronchitis.

The settlement "closes the door on operating energy generation that pollutes and looks forward to a clean-energy future," said Michael Regan of the Environmental Defense Fund.