Mississippi Power Co., a unit of Atlanta-based Southern Co., wants to build the coal-fired plant, which would be powered by lignite mined nearby. The project was approved last year by the Mississippi Public Service Commission.

The Sierra Club argues in its appeal that the utility regulatory body didn’t have enough proof that the plant was needed or that Mississippi Power could build it while keeping rates reasonable for customers.

The 582-megawatt plant near the Liberty community will convert coal into a synthetic gas, aiming to generate electricity with fewer emissions than existing coal power plants. The plant is under construction and expected to start operation in 2014.