Clean air suit targets largest Minn. power plant MINNEAPOLIS (AP) — Clean air advocates sued the Environmental Protection Agency on Wednesday, seeking an order to require the EPA to mandate that Xcel Energy install the best available technology to reduce emissions at Minnesota's biggest power plant. [...] he added that Xcel is investing $50 million in improvements at Sherco that will cut sulfur dioxide and nitrogen oxide emissions by about half, which he said will be adequate.