Xcel Energy wants to boost capacity and reduce emissions at its coal-fired Sherco power plant in Becker, Minn., with a $900 million expansion. The proposal would add 140 megawatts of electricity to Sherco’s current 2,400-megawatt capacity and use state-of-the-art technology to reduce emissions of mercury, nitrogen oxides, sulfur dioxide and soot, Xcel officials said Thursday. [...]