Kentucky regulators have approved a plan to let a utility company convert part of the coal-fired Big Sandy power plant in Eastern Kentucky to natural gas.

Big Sandy is more than 50 years old. Because of federal regulations requiring power plants to reduce the amount of mercury and other air toxins their plants emit, Kentucky Power had a choice: upgrade the plant’s pollution controls to the tune of a billion dollars or stop burning coal at the plant.

This latest PSC decision paves the way for the company to convert the smaller of two units at Big Sandy to natural gas. Last year, commissioners also approved a plan to replace the larger unit with electricity generated in West Virginia.

Converting one of Big Sandy’s units to natural gas will cost about $50 million. Regulators determined that was the least-cost option, and would have the smallest effect on ratepayers.