Accelerating its shutdown schedule for aging coal-fired power units, South Carolina Electric & Gas Co. announced Tuesday it will snuff out two units at its Canadys Station power plant near Walterboro by Dec. 31, four years ahead of schedule.

The Cayce-based utility had planned to convert the two units to natural gas as an interim measure before retiring them in 2017, but after a re-evaluation the company decided to close them by the end of December.

The utility closed another coal-fired unit at the plant in 2012.

The units are among SCE&G’s oldest and smallest coal-fired operations, ranging in age from 45 to 57 years.

Retiring the older coal plants will help the company meet increasingly stringent environmental regulations and balance its generation portfolio, said Steve Byrne, SCE&G president of generation and transmission and chief operating officer.

“By the end of 2018, we anticipate that roughly one-third of our electric generation will be fueled by nuclear power, one-third by natural gas and one-third by scrubbed coal-fired plants,” he said.

Canadys Station employs 69 workers. Byrne said the utility will help them find other positions within the company.

SCE&G, a subsidiary of Scana Corp., serves about 673,000 electric customers in South Carolina. About half of those are in the Charleston metro area.

It announced in 2012 it planned to decommission six aging coal-fired units to meet the Environmental Protection Agency’s mercury air toxic standards. Three of those were at Canadys. Two at its McMeekin Plant near Irmo will be converted to natural gas by 2015 before being shut down completely by the end of 2018. Another unit at SCE&G’s Urquhart Plant near Aiken was converted to natural gas last year and also will close by 2018.

The six units make up 730 megawatts of generating capacity.

Santee Cooper also is reducing its reliance on coal. For the first time in its history, the state-owned utility shut down four units that burn the fossil fuel, including two at the Jefferies Generating Station in Moncks Corner, on Dec. 31.

Reach Warren L. Wise at 937-5524 or twitter.com/warrenlancewise.