The Environmental Protection Agency will unveil its new pollution rules for power plants this afternoon. The rules were required by court order to be finalized on Friday, but haven’t yet been released to the public.The new standard will reduce the amount of mercury and other heavy metals that are emitted from power plants, but it’s not known by how much. The proposed rule called for a 90 percent reduction in mercury over the next three years, but some speculate the final rule may allow power plants more time to comply.The EPA and environmental groups are touting the health benefits of the rules, while utility companies say installing the pollution controls will raise the costs of electricity and force the closure of some older plants.