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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.

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.”

Some of the air-capture proposals are based on small-scale laboratory experiments, but don’t address key questions of how they would process the huge volumes of air required. “They don’t have answers, yet they give cost estimates,” Herzog says. “It’s irresponsible of them to give cost estimates and hold out hope that this could be a good alternative to addressing climate change.”

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 powerplant. 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.

Even then, it would be more expensive than cleaning up fossil-fuel emissions right at the powerplants, Herzog says: “What makes us think, if we’re not willing to do these cheaper alternatives today, that future generations will do these much more expensive things?”

Gary Rochelle, a professor of chemical engineering at the University of Texas at Austin, who was not involved in this study, says he agrees with the conclusion that “air-capture technology is economically infeasible, especially compared to near- and intermediate-term technologies for climate-change mitigation.” He adds that the new research “provides a good argument to support the position that research on air-capture technology is a distraction from pursuing post-combustion technology and other important technologies for climate-change mitigation.”

Comments

if the cellulose fixation rate of tree or plants is ultered than co2 fixation will be economical and environment friendly?

sorry to suggest.

Mike Childs

December 7, 2011

Our analysis suggestes that we will need to use air-capture if we want to prevent climate chaos (www.foe.co.uk/resource/reports.... It will be expensive, and is much more expensive than mitigation. I don't think that given the failure to reduce emissions over last 20 years we now have the option of saying don't bother researching these technologies.

Friends of the Earth (England, WAles & Northern Ireland)

Alberto

December 8, 2011

according to my humble knowledge of math one would need to process about 3000 times the volume or 2000 times the mass of air to remove 1 part of CO2. these numbers are based on an atmospheric CO2 content of 390 ppm v/v and 591 ppm w/w (http://www.enn.com/top_stories...

Alberto

December 8, 2011

Capturing CO2 will cost energy. I'm still wondering how much CO2 (or CO2-equivalent)

will be produced (or shall I write released..) per ton of CO2 captured?

Michael Desmond

December 8, 2011

As a member of the APS committee that prepared the report cited above, and responsible for being the lead on the economic assessments, the MIT work provides a realistic balance to the numbers put in the scientific press by the proponents of air capture. In the APS report we mentioned we were did our economic analysis using assumptions that were on the optimistic end of the realistic range. Hence it is not surprising the MIT work came up with a higher number.

The bottom line is that air capture will only make sense from an energy efficiency and cost point of view once the world's electric grid is completely decarbonized.

The weathering products from the granitic uplift which "made" the Great Basin are highly alkaline, and have accumulated in "alkaline lake deposits".

For the price of a power plant and approx 600 miles of branched piping, spraying the supernatant water in the air over the formed shallow lakes (or bubbling ambient air thru holes in pipes beneath the surface) will immediately begin addressing the problem.

A 100 foot deep slice of the

Black Rock Desert will sequester ~3 GTonnes. There are thousands of square miles of this stuff. A human investement similar in scope to that given to central point irrigation will eliminate 30 Gtonnes/year.

Regards,

David

DougCortez

December 28, 2011

Howard Herzog is correct that capturing anthropogenic CO2 from ambient air using process technology is prohibitively expensive. What also needs to be understood is that capturing CO2 from manmade sources such as flue gas and refining/petrochemical and other industrial sources is also very expensive.

Even if the CO2 is sold for beneficial use (i.e. EOR), the net cost is still very high.

At this time our society has placed a very low value on avoidance of carbon emissions. In parts of the US (i.e. California) the state has forced the added cost of carbon reduction on the electric ratepayers. This obviously has unintended consequences to the California economy. But elsewhere, carbon values are very low of no zero.

If society decides to force low carbon power on itself, the cost of power will increase several fold. This will drive down consumption of power by conservation and transfer of industrial production offshore.

But will this really reduce global warming?

David Newell

December 28, 2011

Although there is a huge concern expressed about the cost of implementing strategies which would reduce current CO2 releases,

the "net present value" of that which is guaranteed to be lost in the future

if it is NOT effectively addressed

makes the decision obvious, from a business perspective, (As well as many others.)

Regards,

California’s greenhouse gas (GHG) cap-and-trade program is at least working.

China has ??"committed"?? to binding CO2 emission controls to be in place by 2020, if the USA does likewise.

Ultimately, we are all in this together.

David Newell

July 23, 2012

I do not have a PhD in chemistry, and therefore then, (apparently) no standing sufficient to elicit a comment on the premise I'm suggesting, which is to use the finely ground cationic alkaline deposits resulting from the weathering of granitic rocks, (which have been conveniently accumulated in "playas" in the Great Basin area,) to effect sequestration of CO2, merely through spraying water exposed to these playas in the air above the artificial lagoons: said droplets of spray adsorbing air gasses and dropping back into the pH 8.1 or higher supernatent liquid, and thus forming carbonates and/or bicarbonates, which will effect geologic-period sequestration.

No offense to the APS or to Howard Herzog: this is NOT "snake oil", and is the cheapest technology proposed to address the CO2 problem:

neither entertained this DAC technology when formulating their respective positions.

Personally, I strongly support Mike Child's comment above...

thank you.

David Newell

July 26, 2012

I have sent information in regard to the above proposal to many of the signers of the APS white paper regarding their having overlooked this source of alkali metal cations for carbonate formation. NOT ONE has responded.

Direct Air Capture is totally feasible and absolutely the least expensive methodology proposed to date.

It is similar to the Calera chemistry, but orders of magnitude less expensive to implement, and produces NO waste side streams.

Will someone who disagrees with my asessment please assert their rebuttal?

I have also sent this to Dr. Herzog. ~~~~~~~~~~~~~~~~~~ ???Hello??~~~~~~

David Newell

July 26, 2012

Passed $20/tonne yesterday...

Bryan

January 7, 2013

from the report your friend Mike Child sent:

"Many of the possible technologies

would be far more expensive than the

cost of cutting carbon. Economically

it is much more sensible therefore to

prioritise cutting emissions than to pay

far more at a later date to remove carbon

from the atmosphere."

And that is that. It is feasible (kinda) technologically. But you still haven't resolved the root problem (CO2 emissions), only balanced it with another expensive temporary fix.