Minnesota regulators on Thursday endorsed a utility's plan to shutter two older coal-fired power generators in Fergus Falls in 2020.

Otter Tail Power Co. will invest $10 million to comply with a mercury-control deadline, allowing the two-unit Hoot Lake plant to burn coal another five years.

The generators, which went into service in 1958 and 1964, supply about 20 percent of the electricity for Otter Tail's 129,000 customers in western Minnesota, North Dakota and South Dakota.

The state Public Utilities Commission voted 4-0 to approve the 2020 shutdown plan. That means the Fergus Falls-based utility must now propose how to replace the coal generators. The options include new natural gas generators, renewable energy or more conservation.

With these two planned retirements, a dozen aging coal generators in Minnesota are now slated to be shut down or converted to natural gas, though all of the others will act by 2016. That's when utilities must comply with long-delayed federal regulations to reduce mercury pollution.

Environmental groups had urged the commission to require an earlier closure of the plant, but the utility, business groups and a state agency that tracks utility policies all endorsed keeping the coal burner running until 2020.

The plant employs 40 people and is served by a short-line railroad that said it might go out of business if the plant ended coal deliveries.

David Shaffer • 612-673-7090 @ShafferStrib