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It's expensive to upgrade coal-burning power plants to control smokestack emissions of mercury and other pollutants. The new technology doesn't reduce greenhouse gases.

PlantOwnerLocationOutput*StartupRetrofit CostCompletion

Big StoneOtter Tail Power†Big Stone City, S.D.475MW1975$405M2015

Boswell 4Minnesota Power†Cohasset, Minn.585MW1980$431M2015

Boswell 3Minnesota PowerCohasset, Minn.350MW1973$240M2009

Genoa 3Dairyland Power‡Genoa, Wis.379MW1969$136M2015

MadgettDairyland Power‡Alma, Wis.400MW1979$179M2016

Sherco 1-2Xcel Energy§Becker, Minn.1,500MW1976-77$45M2014

* one megawatt (MW) equals 1 million watts. † majority owner and sole operator; Big Stone owned by three utilities; Boswell 4 owned by two utilities.

‡ La Crosse, Wis., cooperative supplies power to Minnesota municipal utilities and co-ops, including Great River Energy, which contracts for 50% of Genoa plant's output.

§ No decision on other upgrades estimated to cost $340 million Source: Individual utilities