Carbon dioxide makes up 20 percent of the gas resulting from hydrogen production, twice the concentration found in a gas stream from a typical coal plant. Recovering it from this stream rather than a coal plant smokestack would therefore be cheaper and simpler.

In the oil industry, drillers have for years tapped underground reservoirs of carbon dioxide, brought it to the surface and moved it by pipeline to oil fields. Then they inject it into the fields to help force oil to the surface in a process called “enhanced oil recovery.”

If the oil industry left the natural carbon dioxide where it was, and drew on carbon dioxide from industrial plants instead, far less manmade carbon dioxide would enter the atmosphere, experts say.

What oil drillers pay for carbon dioxide depends on the value of the oil it will help produce. When oil is at $70 a barrel, carbon dioxide goes for $10 or $11 a ton, said Tracy Evans, the chief executive of Denbury, the Texas company building the carbon dioxide pipeline.

Should the Congressional legislation mandate a cap-and-trade system, that modest price could be very important. “Wherever you can go to store a ton of carbon the most cheaply, you will go,” said Mr. Holmstead, the former E.P.A. administrator for air.

Another likely source of pure streams of carbon dioxide are plants that refine natural gas. The natural gas usually comes out of the ground mixed with carbon dioxide, which natural gas sellers routinely remove so the natural gas can be considered “pipeline quality.” That carbon dioxide is sometimes reinjected into the ground, but sometimes vented.

Then there are cement kilns, which produce a nearly pure stream of carbon dioxide.

For now, no one is sure what it will cost to capture and sequester carbon dioxide from coal plants because the first such project in the nation, at American Electric Power’s coal-fired plant in New Haven, W.Va., got under way only last month. At the moment, the process consumes 30 percent of the coal plant’s energy, but engineers are working to cut that in half.