Costs for geological sequestration are currently quite high, more than $30 a ton of carbon dioxide, according to the DOE.86 The technical challenges for reducing those costs are significant. A February 2003 workshop on carbon management by the National Academy of Sciences concluded, "At the present time, technology exists for the separation of carbon dioxide and hydrogen, but the capital and 20 operating costs are very high, particularly when existing technologies is considered for fossil fuel combustion or gasification streams."87 Significant R&D is being invested to bring the costs down.
The key question is where to put the carbon dioxide. Recent attention has focused on pumping highly compressed liquid CO2, so-called supercritical CO2, into geological formations, such as deep underground aquifers. As the National Academy workshop noted, "Less dense than water, CO2 will float under the top seal atop the water in an aquifer and could migrate upward if the top seal is not completely impermeable."
The problem here is that even very tiny leakage rates can undermine the environmental value of such sequestration. If we are trying to stabilize CO2 concentrations at twice preindustrial levels, a 1% leakage rate could add $850 billion per year to overall costs by 2095, according to an analysis by Pacific Northwest National Laboratory. That study concluded, "Leakage of CO2 from engineered CO2 disposal practices on the order of 1% or less per year are likely intolerable as they represent an unacceptably costly financial burden that is moved from present generations to future generations."
If we cannot be certain that leakage rates are below 1%, "the private sector will find it increasingly difficult to convince regulators that CO2 injected into geological formations should be accorded the same accounting as CO2 that is avoided," avoided, that is, directly through technologies such as wind power. The authors note that, "there is no solid experimental evidence or theoretical framework," for determining likely leakage rates from different geological formations.
How long will it take before carbon capture and storage emerges as a major solution to global warming? That remains uncertain. As Princeton's Bob Williams wrote in 2003, "One cannot yet say with high confidence that the CO2 storage option is viable."89 The technology itself is very challenging, and just as commercializing fuel cells has taken much longer and has proven far more difficult than was expected, so, too, may building large commercial coal gasification combined cycle units.
Through its FutureGen program, DOE is aiming to design and build a prototype coal plant that would cogenerate electricity and hydrogen and sequester 90% of the carbon dioxide. The goal is to "validate the engineering, economic, and environmental viability" of a system by 2020.