New Limit Pending on Greenhouse Gas Emissions After months of delay, the Obama administration is about to unveil the first federal standards to explicitly limit greenhouse-gas emissions from new electric power plants - one of the chief sources of carbon dioxide emissions linked to climate change. According to people briefed by the Environmental Protection Agency, all existing plants - including the 300 or so coal-fired power plants that now release the highest level of these emissions and yet-to-be-built plants that have already received E.P.A. permits - will be grandfathered in at current levels, meaning they are exempt from the new proposed rule. Under the new rule, expected to be announced this week, new power plants will have to emit no more than 1,000 tons of carbon dioxide per megawatt-hour of energy produced. That standard permits the level of emissions achieved by natural gas-fired plants of the type generally built in the last few years, but would be too strict for almost all coal-fired power plants if they were not exempted. A new natural gas plant produces a little less than 1,000 pounds of carbon dioxide per megawatt-hour of electricity generated. A coal plant produces about 1,800 pounds. A senior White House official who spoke on condition of anonymity because he was not authorized to disclose details ahead of the announcement said, "This standard provides a clear and certain path forward for industry and the important domestic energy sources they rely on, including natural gas, as well as clean coal technologies that lower carbon emissions. These sources will continue to be a part of our energy future, and what we have proposed is in line with the steps industry is already taking. The White House official explained that the goal of the rule is to hasten the introduction of carbon controls on new coal-fired power plants, while not causing immediate economic harm from the shutdown of existing plants. The rule is certain to face stiff challenges in Congress and the courts. Washington insiders, environmentalists and state regulators who have followed the course of the Obama administration's stutter-step approach to greenhouse gas regulations were both pleased that a rule was about to be proposed and disappointed by its apparent lack of teeth regarding existing polluters. Ian Bowles, the former secretary of energy and environmental affairs in Massachusetts, had a mixed reaction to the news of the rule. "I'm glad to see President Obama moving forward in the face of critics who seem to have forgotten that the Supreme Court has ruled E.P.A. must regulate greenhouse gas emissions," he said, but added, "I'm a bit surprised existing power plants are left harmless, but the reality is cheap natural gas is doing more to curtail coal use than any regulation would have in the near term." Some, like Paul Bledsoe, a senior adviser at the Bipartisan Policy Center, said that the regulation gave new impetus to the shift to natural gas as the electric power industry's fuel of choice. Coal plants are facing other new environmental mandates that could make them even less competitive economically, he said. "The impact in the next decade is expected to be relatively minor," he said. The coal industry, whose conservative Congressional allies have painted the E.P.A. as an overzealous regulator holding back the economy, was unhappy with the news of the proposal. They have repeatedly criticized the idea of setting limits by regulation when Congress refused to set similar limits by legislation. But the agency had little choice after it followed the Supreme Court's directive to make a decision on whether carbon dioxide was a pollutant. In 2009, the agency declared that it should be classified as a pollutant under the Clean Air Act. On Monday night, Luke Popovich, a spokesman for the National Mining Association, said, "The rule from our standpoint is a big mistake." He added, "It is virtually calculated to drive coal, a very, very affordable generator of electricity, out of the U.S. electricity market." The most promising technology for controlling greenhouse gas emissions from coal plants involves isolating the carbon dioxide before it leaves the smokestack and then storing it underground - a process known as "carbon capture and storage." Carbon capture and storage is not being done on a commercial scale anywhere in the world. Matthew L. Wald contributed reporting.