CASPER, Wyo. (AP) — Black Hills Corp. plans to close two coal-fired power units in Wyoming by early 2014 because the company says it would be too expensive to bring them in line with new Environmental Protection Agency pollution rules.

The Associated Press  reported Wednesday that the units slated for closure are the Osage plant and one of six power stations at a complex in Gillette.

The Osage plant has been idle since October 2010 but Black Hills has continued to maintain it.

As many as 13 workers could be affected.

Black Hills also plans to close power stations in Colorado and near Rapid City, S.D. The Gillette complex general manager, Greg Hager, says senior employees at the South Dakota facility have the option of transferring to Gillette, which would displace some workers there.