DENVER (AP) â¿¿ State regulators issued written orders Wednesday that would leave no coal-burning power plants in the Denver area after 2017. The Colorado Public Utilities Commission's orders formalized earlier decisions meant to help Colorado meet federal clean-air standards, including a plan for Xcel Energy to switch its coal-fired Cherokee 4 unit in Denver to run on natural gas before 2018. The commission also encouraged Xcel not to leave the aging unit running beyond 2022. It said Xcel should explore alternatives to running Cherokee 4 on natural gas, in case there are cheaper options that still would meet clean-air goals. A new state law required Xcel Energy and Black Hills Corp. to submit plans to significantly cut nitrogen oxide emissions by 2018 to help Colorado meet federal clean-air standards. The PUC had to approve the plans. One PUC order Wednesday approved plans for Xcel Energy to close five coal-fired units in Denver and Boulder, build a new plant in Denver that runs on natural gas, and convert two coal-fired units in Denver, including Cherokee 4, to run on natural gas. Xcel would put more emission controls at its Hayden and Pawnee plants. The other order approved Black Hills Corp.'s plan to retire two coal-fired units in Canon City. The $1.4 billion plan for Xcel Energy was expected to add about 2.5 percent to customers' electric rates by 2020, while Black Hills customers are expected to see a 5 percent rate increase once its plan is implemented. PUC Chairman Ron Binz said rate hikes would likely be higher if Colorado waited for the Environmental Protection Agency to impose a plan for cutting emissions. The PUC directed its staff to work on finding funding to help retrain coal mine workers if any lose their jobs because of the plans.