The closest to opening is the Southern Company’s Kemper County plant in Mississippi, which will convert coal to gases and then filter out some of the carbon dioxide, reducing emissions by about 65 percent.

But the plant, at $5 billion, is $1 billion over budget. Southern Company said in a statement on Friday that the plant’s economics were peculiar to its location, and not a national model. Its captured carbon will be sold for use in the oil fields, where it helps force more oil to the surface.

But most power plants are not in areas where they can sell their carbon dioxide.

Revis W. James, director of the Energy Technology Assessment Center at the Electric Power Research Institute, said that before a technology could be considered commercially demonstrated, “you’d need Kemper to be operational and a couple more Kempers, and have the kinks worked out.”

Carbon capture and sequestration, he said, was unlikely to be competitive unless natural gas prices increased by 100 to 150 percent and the construction of nuclear plants was ruled out.

Ms. McCarthy also referred to three projects that would sell carbon dioxide to the oil industry: a coal plant in Saskatoon, Saskatchewan, the Boundary Dam project, where the provincial utility plans to rebuild a small 45-year-old unit to capture carbon dioxide; the Texas Clean Energy Project, 15 miles west of Odessa, where the builders hope to soon break ground; and Hydrogen Energy California, proposed for the oil fields of Kern County.

As the price of oil has risen, so has the value of carbon dioxide to oil drillers. The value as a carbon-reduction mechanism is unclear, though, because the result is to produce more oil, all of which will be burned, producing more carbon emissions.

E.P.A. officials say that a carbon-emissions rule would push improvements in the technology. Two decades ago when the agency required technology to reduce emissions of a different pollutant — nitrogen oxides, a smog precursor — that, too, was expensive and not thoroughly demonstrated, but today it is routine, they said.