The silhouettes of emissions are seen rising from stacks of the Duke Energy Corp. Gibson Station power plant at dusk in Owensville, Indiana, U.S., on Thursday, July 23, 2015. The Center for Public Integrity ranked the Gibson generating station one of the top polluters in America. (Luke Sharrett/Bloomberg via Getty Images)
The silhouettes of emissions are seen rising from stacks of the Duke Energy Corp. Gibson Station power plant at dusk in Owensville, Indiana, U.S., on Thursday, July 23, 2015. Coal reclaimed its ranking as the top fuel for generating electricity at U.S. power plants in May, beating natural gas, which took the number one spot for the first time in April. Photographer: Luke Sharrett/Bloomberg via Getty Images
