The Southern Company on Wednesday effectively gave up on an ambitious “clean coal” project, announcing that a flawed coal-fired power plant in central Mississippi would now burn natural gas instead.

The Kemper County plant, built to take advantage of a strip coal mine next door, was three years behind schedule and, at a cost of about $7.5 billion, $4 billion over its projected budget. Equipment meant to turn the coal into gas and remove at least two-thirds of the carbon dioxide from it to keep it out of the atmosphere never worked as designed.