South Carolina Electric & Gas Co. will shut down two coal-fired electric generating units at its Canadys Station power plant near Walterboro by year’s end, 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 reevaluation the company decided to close them by the end of December, it announced today.

The utility shut down 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 units will help the company meet increasingly stringent environmental regulations and achieve a more balanced 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 company will assist them in looking for other positions within the company.

SCE&G announced in 2012 it planned to snuff out six aging coal-fired units to meet EPA’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.

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.

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