The Minnesota Commerce Department recommended earlier this year that the Allete subsidiary retire Larkin units 1 and 2 in Hoyt Lakes, Minn., and Taconite Harbor unit 3 in Schroeder, Minn., by 2016. Unless circumstances change, the agency also wants the utility to shutter Boswell unit 1 in Cohasset, Minn., by 2020.

But the Duluth-based company, which serves 144,000 customers, is not convinced the baseload plants are ready for the scrap heap. For one thing, Minnesota Power said in June, the state’s iron ore mines depend heavily on electricity generated by the coal units, and replacing that power may not be cheap or easy.

That is why Minnesota Power told the state that a “deeper analysis” is needed on a baseload diversification study it conducted as part of its long-range planning process. That process is expected to culminate in 2013 with the filing of a new integrated resource plan with the state.

Over the past six years, Minnesota Power has invested more than $355 million to reduce air emissions such as sulfur dioxide, nitrogen oxides and mercury by more than 70% at Laskin, Taconite Harbor and Boswell.

“Taking early action on emissions reductions has put us in a better position to meet federal standards,” said David McMillan, the company’s executive vice president. “We know that environmental regulations will increase costs to our customers, and that knowledge has driven us to make the proactive investments we’ve made to date. For now, we believe we’ve taken the best steps we can to keep our energy affordable into the future.”