SCIENCE:

Small-scale U.S. lab experiment removes CO2 from atmosphere at relatively low cost

Scientists say they have found a cheaper and more efficient way to capture carbon dioxide from the atmosphere.

The researchers from Lawrence Livermore National Laboratory also say that their tested process has added benefits -- it may be able to reduce ocean acidification and, at the same time, produce hydrogen for use as a fuel or an industrial feedstock.

"It is much less energy-intensive and much less costly than some of the other air capture ideas that have been proposed," said Greg Rau, a visiting scientist at the national lab and lead author of the research, which was published this week in the Proceedings of the National Academy of Sciences.

The lab team used saline water electrolysis, which involves running a current through slightly salty water to break it up into hydrogen and oxygen.

The test was at a small scale in a lab, but the mechanism at a commercial level could harvest a large amount of hydrogen for various purposes, said Rau. If renewable energy were used to power the electrolysis, the result would be "super green" hydrogen, he said. Large-scale electrolysis is something that is already available, he said.

"The process doesn't require high temperatures or exotic chemicals," said Rau. "It uses commonly available materials."

The process generated a pH divide in the saline water solution, so that one electrode in the solution was very acidic, and the other was very basic. The solution as a whole remained neutral initially.

The scientists then added silicate minerals, which neutralized the acidic component of the electrolyzed solution, allowing the overall solution to shift from neutral to basic, or alkaline.

This resulting solution proved highly absorbent of carbon dioxide from the air, sucking up the greenhouse gas at a rate approximately 45 times greater than a control test.

Silicate minerals hold the advantage of being very inexpensive and abundant, making up the majority of the Earth's crust, said Rau. The dynamic could make air capture with them relatively cheap -- the scientists estimated that a system at scale could cost roughly $100 a metric ton of mitigated carbon dioxide.

Cheaper than prior experiments

While this expense may not be feasible without a carbon price, it is vastly cheaper than some of the ideas proposed for capturing carbon dioxide from air, Rau said.

In 2011, for example, the American Physical Society said that the typical cost of an air capture system would be around $600 or more per metric ton of CO2, or six times the amount of Rau's procedure.

In addition to using easily found materials, the new procedure eliminates other expensive methods, such as the recycling of reagents, the scientists said.

Its "other beauty" is the creation of dissolved carbonates and bicarbonates that could alleviate ocean acidification in theory, said Rau.

If large amounts of the liquid solution from the study were put back in the ocean, it could offset acidification in the same way that Alka Seltzer neutralizes excess acid in the stomach, said Rau. It is an idea that needs significant additional research, he said, but should not be ruled out entirely, considering the dangers of acidification to coral reefs and shellfish.

The world's oceans are about 30 percent more acidic than they were at the beginning of the Industrial Revolution because of their absorption of human-generated carbon dioxide, according to the federal government.

Rau said the process raises numerous technological possibilities. He envisioned oil tankers converted into large electrolysis systems -- sucking up seawater, adding base minerals to it and generating hydrogen along the way.

In theory, the system could be used with seawater or with saline solutions sitting inland in tanks, although it is too early to know how large or how efficient such systems would be, he said.

"Our system is simply a proof of concept," he said.

Would planting more trees be cheaper still?

A common challenge to the idea of air capture is that it would be simpler to plant trees, which also ingest CO2. Rau said that air capture systems can have the advantage of allowing land to continue to be used for agriculture or other purposes. Forests are not always an option, he said.

The idea of direct air capture is not new. In 2007, billionaire Richard Branson announced a $25 million prize to pull carbon dioxide out of the air, a competition that currently has 11 finalists developing various technologies. Early pilot tests of synthetic, carbon-sucking trees resembled large telescopes (ClimateWire, Oct. 2, 2008).

There are many skeptics of the idea of tinkering with the world's oceans to control acidification as well as direct air capture, considering alternative solutions to control greenhouse gases.

In its analysis, the American Physical Society concluded that direct air capture is "not currently an economically viable approach to mitigating climate change." Additionally, several carbon capture experts published a paper in the Proceedings of the National Academy of Sciences in 2010 concluding that the cost of sucking carbon dioxide out of the air is vastly underestimated and could cost a prohibitive amount, at $1,000 per metric ton of CO2.

Some believers in the technology also are skeptical that the national lab's method is better than existing initiatives for air capture. They said the estimated electricity cost of a large-scale system in the paper -- $114 per ton of consumed CO2 -- is higher than the energy costs associated with other proposals.

Until there's a much larger proof of the concept, it's impossible to know whether the new method is an improvement.

"It's a unique and interesting early-stage idea worthy of consideration, but not a contender against the systems that are currently in early-stage commercialization," said Geoff Holmes, a research scientist at Carbon Engineering Ltd., which is financially backed by Microsoft founder Bill Gates and is one of the 11 finalists in Branson's competition.

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