Gov. Scott Walker scrapped plans Thursday to convert a power plant to run on natural fuels such as wood chips and paper pellets, a move that could save up to $100 million but drew stern criticism from at least one environmental group. The decision affects the Charter Street Heating Plant on the University of Wisconsin-Madison campus. Its coal-fired burners will be retired next year and were to be replaced with two boilers that run on natural gas and a third that would burn biomass, state officials said.