INDIANAPOLIS - Indianapolis Power & Light Co. says it is continuing to update its energy mix with the retirement of four coal units at the Eagle Valley Generating Station in Martinsville. They will be replaced with natural gas and steam turbines as part of a $600 million upgrade.

IPL says, once the combined-cycle gas turbine power station is up and running, the plant will be among the most efficient thermal power operations to be built in Indiana. AES United States President Ken Zagzebski says the change is part of an "all-of-the-above approach of integrating clean and renewable resources with traditional energy resources."

In February, the utility ended coal burning at its Harding Street plant. That location had predominantly burned coal since it opened in 1931. The plant will convert to natural gas. In 2014, IPL set a goal of reducing the company's dependence on coal from 79 percent in 2007 to 44 percent in 2017.

The Eagle Valley plant went online in 1949. The new turbines are expected to be operational in 2017.