From: Andy Soos, ENN
Published December 6, 2011 01:09 PM

CO2 from the Air

What seems to be directly correlated to global warming is CO2 in the air. So why not take it directly out of the air? 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 power plants that burn fossil fuels.

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Carbon dioxide in earth's atmosphere is considered a trace gas currently occurring at an average concentration of about 390 parts per million by volume or 591 parts per million by mass. The total mass of atmospheric carbon dioxide is about 3,000 gigatonnes. Its concentration varies seasonally and also considerably on a regional basis, especially near the ground. In urban areas concentrations are generally higher and indoors they can reach 10 times background levels.

Herzog, one of the authors and a peer reviewer of earlier studies, says that the earlier reports 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 power plant.

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 low balling 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 power plant — 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 power plant scrubbers.

Jennifer Wilcox, Assistant Professor of Energy Resources Engineering at Stanford University and a co-author of the study, says, "direct air capture sounds great in theory. In reality, though, using fossil-based energy sources to capture and regenerate the CO2 would result in net-positive CO2 emissions."

"If you look at the ideal equations," Herzog says, it’s possible to come up with air-scrubbing systems that appear feasible, "but if you look at empirical data — how engineers look at this, with real-world efficiencies — you don’t find many reasons" to be hopeful. The burden is on the inventors to show that their proposed systems really could work, he says: “It can be done, that’s not the question. The question is what is the cost.”

While the study found that such technologies are unlikely to have a place over the next few decades, it did see one possible area where a particular variation of such systems might make sense, at least to a limited extent: planting trees or other plants to extract carbon dioxide from air, then burning them to produce electricity while scrubbing the carbon dioxide at that power plant. That type of process would take advantage of plants natural ability to carry out the initial extraction from the air, and would be renewable because the plants could be harvested and then replaced. The total cost of such a system could be a few hundred dollars per ton of carbon removed — which is not competitive at today’s prices, but might be in the future.

For further information: http://web.mit.edu/newsoffice/2011/carbon-dioxide-capture-1206.html#1851

Photo: Graphic: Christine Daniloff

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