Capturing carbon dioxide directly from air not realistic in the forseeable future

Since most of the world's governments have not yet enacted regulations to curb emissions of greenhouse gases, some experts have advocated the development of technologies to remove carbon dioxide directly from the air. But a new MIT study shows that, at least for the foreseeable future, such proposals are not realistic because their costs would vastly exceed those of blocking emissions right at the source, such as at the powerplants that burn fossil fuels.

Some purveyors of various new technologies for scrubbing carbon dioxide out of the air are reminiscent of  ''snake-oil salesmen,'' says Howard Herzog, a senior research engineer at the MIT Energy Initiative and co-author of the new analysis published this week in the Proceedings of the National Academy of Sciences. The study was co-authored by MIT civil and environmental engineering postdoc Kurt Zenz House, along with researchers at C12 Energy in Berkeley, Calif., and at Stanford University.

Herzog and his co-authors are not alone in criticizing these proposals. An analysis earlier this year by the American Physical Society came to similar conclusions, although Herzog, a peer reviewer of that study, says that report ''didn't go far enough'' in its criticism of air-capture systems. In that analysis, the best open-air carbon-capture systems proposed were found to cost at least eight times as much, per ton of carbon avoided, as those installed at the powerplant.

It's not surprising that those promoting these concepts find an eager audience, Herzog says. ''It's so enticing - you don't have to change anything about your lifestyle'' to reduce greenhouse gases and slow the global climate change that virtually all the world's climate scientists agree is underway. ''It'd be such a great solution - if it were real.''

Unfortunately, when examined closely, it turns out that ''many of those advocating air-capture deployment and research are really lowballing the cost,'' Herzog says. When the underlying chemistry and mechanics are analyzed, their numbers don't hold up, he says. Compared with removing carbon dioxide from the emissions at a powerplant - technology that exists and can be measured - removing it from the outside air means processing about 300 times more air per ton of CO2 removed, because that's the difference in CO2 concentration.

Numerous studies have shown that the cost of removing one substance from a mixture depends on its initial concentration, so the much lower concentration of CO2 in outside air makes its removal from air much more costly than from exhaust gases. After a detailed comparison, the MIT-led team concluded that the cost of such removal is likely to be more than $1,000 per ton of CO2 avoided, compared to $50 to $100 per ton for current powerplant scrubbers.