The unit at the facility near Mooresboro returned performances well under permit limits for pollutants such as carbon monoxide (0.065 pounds per million British thermal units versus the legal threshold of 0.12lb/MMBtu), nitrogen oxide (0.04lb/MMBtu, legal threshold 0.07lb/MMBtu), sulphur dioxide (0.02 to 0.04lb/MMBtu, legal threshold (0.12lb/MMBtu); and mercury (0.001 to 0.008 pounds per gigawatt-hour, threshold 0.019lb/GWh), among others.

Plant manager Rick Roper said the unit had the most effective emissions controls possible using already proven technologies.

“During the time the company was going through the air permitting process to build Cliffside unit 6, there was significant debate about whether a new coal unit could achieve this level of emissions reduction,” he said.

“We can close that chapter and put those concerns to rest. This unit's air permit is one of the most restrictive in the country and it's performing very well.”

Duke has been following the unit’s performance closely – it performed annual emissions tests for hazardous air pollutants in July and is conducting quarterly hydrogen chloride emission reviews as well as continuous evaluations of other pollutants.

Roper said that the state’s regulators determined permit limits and to be considered a minor source of HAPs a unit must emit less than 10 tonnes a year of any one HAP or 25 tonnes a year of all HAPs combined.

“Hydrogen chloride is the HAP emitted in greatest volume from a coal plant, and Cliffside unit 6 is emitting less than a quarter of its permitted amount,” Roper said.

“This is equivalent to less than 2.5 tonnes of HCl emitted annually or greater than a 99.913% removal rate.”

Cliffside’s unit 6 removes 99% of sulphur dioxide, 90% of nitrogen oxides and 90% of mercury.

“The performance of unit 6 shows it is well positioned to demonstrate compliance with the federal mercury rule that will be in effect by April 2015, including the mercury standard of 0.013 pounds per gigawatt-hour,” he said.

“It also handles all coal ash in a dry form and manages it in an engineered, lined landfill.”

Unit 6 began commercial operation in December 2012.

Duke’s nationwide plan is to bring about 6300MW of coal capacity offline in the coming years, or about 25% of its coal fleet.

By the end of this year, it will have retired about 3800MW of that total.