APS News

Assessment Casts Doubt on Utility of Direct Air Capture of CO2

APS Panel on Public Affairs (POPA) recently released an assessment that casts doubt on the feasibility of removing carbon dioxide from the atmosphere.

The study, titled “Direct Air Capture of CO2 with Chemicals: A Technology Assessment” found that using current technology, extracting carbon dioxide from the atmosphere would be significantly more difficult and expensive than reducing carbon emissions.

Direct air capture, or DAC, refers to technologies where air is circulated over a chemical, or a collection of chemicals, that absorbs carbon dioxide and prepares it for sequestration. Some experts claim that constructing large air capture facilities might be used to combat climate change by removing excess carbon dioxide from Earth’s atmosphere.

“We believed it was a timely issue with a lot of technical aspects to it and it deserved a closer look from physicists,” said Francis Slakey, APS associate director of public affairs.

The assessment put forth by POPA said that because of the tremendous engineering and technical obstacles “DAC is not currently an economically viable approach to mitigating climate change,” and that “it is entirely possible that no DAC concept under discussion today or yet to be invented will actually succeed in practice.” Carbon dioxide in ambient air is so sparse that the structures needed to make an appreciable difference would be so massive that their cost would be prohibitive. The assessment found that it is far more cost-effective to clean the exhaust of sources of carbon dioxide like power plants, factories and foundries.

“The report basically comes to the conclusion that because of the challenge of pulling such a dilute concentration of CO2 out of the air, isolating it and sequestrating it…. direct capture from air is much more expensive,” said co-chair Michael Desmond of BP. “You really should be going after all the big point sources first.”

The air capture facilities would need to be immense. One estimate is that in order to remove the six million metric tons of carbon dioxide emitted by an average 1000 megawatt power plant, the DAC system would have to have ten meter tall towers for 30 kilometers. The committee estimated that with existing technology it would cost at least $600 per ton of carbon dioxide removed from the air, as compared to the roughly $80 per ton when removed from major carbon dioxide producers like power plants.

Committee chair Robert Socolow of Princeton said that the prices were meant to be a way of comparing the costs rather than an absolute dollar value. “The ratio is more important than the numbers,” he said.

Desmond echoed this statement, and compared that to the cost per ton of completely decarbonizing transportation. He said that if all vehicles switched to electric and all fossil fuel power plants were replaced with renewable energy, the cost would be about $200 to $300 per ton. He added that the $600 was probably on the low end of estimates, as unexpected costs inevitably surface when a new technology is first implemented.

“You don’t know what you don’t know,” Desmond said. “When you try to integrate things, costs almost without exception go up.”

Air capture is difficult because carbon dioxide in air is very dilute, around 390 parts per million. The process needs a large surface area to absorb the carbon dioxide at an appreciable rate. Concentrations of carbon dioxide in smoke- stacks and exhaust flues are much greater, making it much more economical to deploy carbon capture technology there, rather than in ambient air.

The assessment did find that direct air capture technology has much in common with carbon capture inside of smoke-stacks and exhaust flues.

“It’s got similar principles…. [it’s] trying to pull CO2 selectively out of a gas stream,” Desmond said, “In most cases you should be able to apply that learning to the more difficult problem of direct air capture.”

The assessment concluded that further research into direct air capture would likely be of the most benefit to carbon capture in smokestacks and exhaust flues, and advances there could be transferred to direct air capture as well.


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