Black Hills Power, a South Dakota utility with offices in Denver, filed papers Tuesday to shut down three aging coal-fired power plants in Wyoming and build a new natural gas-powered plant in Cheyenne – the first of its kind in the coal-dominated state. According to the Casper Star Tribune , Black Hills is making the $237 million move in anticipation of U.S. Environmental Protection Agency (EPA) rules regulating mercury and carbon dioxide (CO2) emissions from coal-fired power plants