Pulling vexing carbon emissions straight from the sky might become an important way to keep climate change in check. As pilot projects move forward, the prospect of capturing carbon dioxide from the air is growing increasingly plausible, though it may be some time before the technology, the demand and the costs align to make a dent in global emissions.
Earlier this year, instruments showed atmospheric carbon dioxide concentrations rising above 400 parts per million for the first time in 800,000 years (ClimateWire, April 24).
Energy consumption, and consequently carbon emissions, is poised to grow further even as cars, homes and aircraft become more efficient. Fossil fuels will continue to be the major energy source in the coming century as countries like China harness this energy to drive economic development.
As a result, some researchers argue that direct air capture is a necessary, though not sufficient, component of any climate change mitigation strategy.
"Our view is that air capture is a pathway that could be quite important," said David Keith, president of Carbon Engineering, a firm developing industrial air capture systems. He explained that controlling emissions at the source makes sense for large facilities like power plants and factories but scrubbing carbon dioxide from tailpipes or jet exhaust is too expensive.
The transportation sector accounts for 28 percent of greenhouse gas emissions in the United States, according to U.S. EPA, so there is still a critical need for a way to reduce the overall carbon dioxide produced from mobile sources.
Trapping CO2 in a liquid
Carbon Engineering is addressing this with a box fan-like air contactor that uses a liquid to sop up carbon dioxide from the air. The liquid, now enriched, circulates to a regeneration facility where it releases the carbon in a pure stream under high temperature.
This pure carbon stream is very useful, Keith observed. Carbon dioxide is a raw material for certain industrial processes, drillers use it to squeeze out more oil and gas from depleted wells and it serves as a building block for liquid fuels.
"We are trying to reduce the risk by using technologies that are proven," he said. The company is designing a large pilot plant that will capture 1 kiloton of carbon dioxide annually, due to come online next year in Alberta.
Putting carbon dioxide to work is an important step toward making direct capture economically feasible as well as environmentally sustainable. Unlike a stream from a carbon capture system attached to a coal-fired generator, an air capture system recirculates carbon that is already in the atmosphere.
Using this carbon instead of the stuff from the ground to make gasoline or jet fuel means it offsets humanity's carbon output so that there is no net change in CO2 concentrations in the atmosphere, provided the energy driving the process comes from a carbon-neutral generator.
"When you drive your car and emit some carbon and I capture that carbon and use it to make fuel for the car, then effectively I've created a closed loop," said Tim Fox, head of the Energy and Environment Division at the Institution of Mechanical Engineers speaking last month at the U.S. Energy Association in Washington, D.C.
Because carbon dioxide is fungible once it enters the atmosphere, Fox said it doesn't matter where developers build an air capture system. It doesn't have to be near a specific carbon source and can use cheap, marginal land, thereby reducing overhead.
Fox suggested that engineers could site air capture systems near stranded renewable energy sources, like sunny islands or windy, desolate cliffs.
Would zero-emission vehicles cost less?
However, others are skeptical. "Capture from the air is very expensive. Those ideas just make it more expensive," said Howard Herzog, senior research engineer at the Massachusetts Institute of Technology's Energy Initiative.
Herzog co-authored a study in 2011 that concluded direct air capture uses too much energy and costs too much to be an effective strategy to fight climate change (ClimateWire, Dec. 13, 2011). He said gradually switching to zero-emission vehicles would likely be cheaper and more effective at reducing emissions than offsetting tailpipe CO2 with air capture.
In order for air capture to make a difference in the climate, the technology and economics would have to change radically. "The first thing that's essential is you have to totally decarbonize your power system," Herzog said, referring to the energy needed to feed direct air capture devices. "The second thing, the electricity, the power you buy from this system, must become inexpensive. If that's the case, then it starts becoming more in the realm of possibility."
Still, researchers are pressing on and looking for better ways to collect carbon dioxide from the air. "I think people are too pessimistic," said Klaus Lackner, director of the Lenfest Center for Sustainable Energy at Columbia University. "If you look at the first-of-a-kind price of a new technology, it's not terribly informative."
Air capture systems would follow cost trajectories similar to those of wind and solar energy as the devices get better and companies build more of them, Lackner said. He noted that for carbon capture systems, the main energy sink isn't so much in collecting CO2 in the first place, but in regenerating the absorber and making a pure stream of the gas.
'Cleaning up' after ourselves
Scientists are therefore concentrating on improving the mechanisms used to snag and release carbon dioxide. Lackner developed a mechanism that passively collects CO2 on a membrane and releases it when wet, thereby avoiding energy-intensive fans and high temperatures. Kilimanjaro Energy is currently working to commercialize this system.
The process, however, still requires more energy than capturing emissions at the source. "The bottom line is, I pay a penalty, but it's a relatively small penalty, and that penalty is worth it for going after those emissions no one else is," he said.
In addition, air capture creates leverage that could aid international climate negotiations. "You could tell, for example, the coal producers or the coal consumers, 'You should clean up your own CO2, but if you don't, we will do it on your behalf and we will charge you for it,'" he said.
One criticism is that directly trying to change the atmosphere falls within the shadow of geoengineering, so air capture deserves extra scrutiny. Another issue is that some environmental groups, like Greenpeace, view carbon capture systems as a moral hazard and a way to enable complacency. With a way to pull carbon out of the air, people will not limit their emissions and will do less to address climate change, the argument goes.
Lackner said the motivation here is actually the opposite: Because the warming planet is such a threat and current systems aren't doing enough to solve the problem, researchers need to pursue every option and don't have the luxury of leaving direct air capture off the table.
"It's cleaning up after yourself, and it has very little to do with geoengineering," he said. "I view this as CO2 emissions control and closing the anthropogenic carbon cycle."