Unit 4, part of the 1,025-megawatt Boswell Energy Center in Cohasset, Minnesota, is undergoing a $350 million environmental upgrade to significantly reduce emissions of mercury, particulates, sulfur dioxide and other pollutants. The project is expected to be completed before the end of 2015.

In a September integrated resource plan filing with the Minnesota Public Utilities Commission, Minnesota Power stressed Boswell’s importance in serving its 143,000 customers in a 26,000-square-mile area of northeastern Minnesota that includes some of the largest industrial customers in the United States.

While efficient, the smaller Units 1 and 2 at Boswell, totaling 130 megawatts, could be the focus of an in-depth analysis next decade about whether they should be retired. Unit 3, rated at 365 megawatts, is expected to remain in operation.

Boswell burns low-sulfur coal from the Powder River Basin.

Longer term, Minnesota Power is aiming for a balanced generation portfolio of about one-third coal, one-third natural gas and one-third renewables. Coal still fuels about 70% of the utility’s generation, down from 90% a decade ago.

Dave McMillan, Minnesota Power executive vice president, said the new resource plan, dubbed EnergyForward, “offers a detailed explanation of how we will balance our resource mix to preserve environmental quality without sacrificing affordability and reliability.”

To that end, Minnesota Power expects to have removed about 560 megawatts of coal-burning generation from its power supply by 2020, compared with 2005 levels.

Later this fall, the 225-megawatt Taconite Harbor coal plant on the north shore of Lake Superior will be “economically idled,” not permanently closed. That means the plant could be restarted during the next few years if needed. However, Minnesota Power has decided to halt all coal operations at the plant in 2020, according to company spokeswoman Amy Rutledge.

Earlier this year, Minnesota Power completed a $15 million conversion of its 110-megawatt Laskin coal plant at Hoyt Lakes, Minnesota, to natural gas.

Between 2006 and 2015, Minnesota Power signed power purchase agreements and constructed more than 600 megawatts of wind facilities. It also entered into a deal with Canada’s Manitoba Hydro to purchase 383 megawatts of hydropower starting later this decade.

As a result, Minnesota Power is a decade ahead of schedule in meeting Minnesota’s renewable energy standard that requires 25% of all its retail electricity sales to be generated by eligible energy technologies by 2025.

The company is forecasting modest load growth for its customers over the next 15 years with more than 200 megawatts of additional load by 2029. Julie Pierce, director of strategy and planning for the utility, said Minnesota Power’s annual load growth over the 15-year period is expected to average about 0.7%.

In the 2020s, the utility plans to add 200-300 megawatts of gas-fired generation and 33 megawatts of solar energy.