Why do we need as many as 10 large-scale demonstrations of carbon dioxide capture and storage technology, as provided for in the Boucher House legislation?

Because the technology is complicated. It consists of three steps -- CO2 capture, transport and storage -- that must be linked seamlessly. The capture step accounts for most of the cost. Power plants might use one of three different approaches, so all must be tested at full scale to be confident they would work reliably at large facilities.

Today's coal-burning plants would use "post-combustion" capture systems that can strip up to 90 percent of the CO2 from spent flue gases before they go up the chimney. "Pre-combustion" capture can do this at lower cost, but it requires a more expensive type of power plant that first converts coal into a gas, which is then cleaned and burned to generate electricity. A third approach, not yet as mature, uses near-pure oxygen instead of air to burn coal, producing a concentrated CO2 stream that is relatively easy to capture. However, the oxygen plant adds considerably to the cost. Ideally, all three approaches would be tested at both existing and new power plants.

All the capture systems would compress CO2 into a liquid, and CO2 in this form already is being transported by pipeline, so that technology is well established.

As for storage, there are different types of geological formations for sequestering CO2. The most plentiful are saline formations -- layers of salt water, sand and rock deep beneath the surface. These hold the most promise because they can absorb large amounts of CO2, which slowly dissolves in the brine and becomes part of the formation. Impermeable rock above the formation helps ensure the CO2 cannot escape to the surface. Depleted oil and gas fields also are potential storage sites.