Scrubbing the skies

Environment: Removing carbon dioxide directly from the atmosphere could help combat climate change. Will it really work?

Preventing catastrophic climate change, most people agree, will mean reducing the level of man-made carbon dioxide (CO₂) in the atmosphere. That, in turn, will require the widespread use of “low carbon” technologies such as solar and wind power, more energy-efficient buildings, and so on. Some countries have pledged to reduce their greenhouse-gas emissions by 80% by 2050, and campaigners are calling for cuts of 90% or even 100%. New Zealand, Costa Rica and Norway are racing to become the world's first “carbon neutral” country. But some researchers think there might be a simpler way to reduce the level of CO₂ in the atmosphere: to build “air capture” machines that, as their name suggests, grab it from the air.

This is not as mad as it sounds. After all, such machines already exist. they are used to “scrub” carbon dioxide from the air on board submarines and spacecraft. “It has been around for decades, but the only people who cared were at NASA, because too much CO₂ in a space shuttle means you die,” says Matthew Eisaman, a researcher at the Palo Alto Research Centre (PARC) in California. Proponents of air capture propose scaling up such machinery so that it can process the atmosphere directly, extracting the CO₂ so that it can be sold for industrial use or stored underground.

In some respects this is a more ambitious version of the “carbon capture and sequestration” (CCS) technology that is being developed to strip carbon dioxide from the exhaust gases produced by coal- and gas-fired power stations. The exhaust from a coal-fired power station is around 10% carbon dioxide, however, whereas the level in the atmosphere is a mere 0.04%. But scientists working on air capture suggest that this difference is not as significant as it may seem, and that the kinds of industrial methods needed to strip CO₂ from the air have already been proven in industrial processes such as papermaking.

Air capture has the further advantage that it can be done anywhere, not just in places where carbon dioxide is being emitted, such as power stations. An air-capture plant could, for example, be set up at a site where CO₂ can be easily stored, such as an empty oilfield, and air capture would open the way to capturing emissions produced by millions of cars and aircraft.

If air capture is to get anywhere, however, it must overcome three sets of objections: technical, financial and political. The process is no good if it produces more carbon-dioxide emissions than it removes from the atmosphere. Nor is it of any practical value unless the cost of removing each ton of CO₂ is lower than the alternatives. And whether or not it can be made to work efficiently and cheaply, air capture will be politically controversial because the mere possibility of its deployment could be used as an excuse to put off other action to reduce emissions.

Sucking air

How does air capture actually work? Several designs are being developed, but they involve variations on the same theme. In each case air is brought into contact with a “sorbent” material, which binds chemically with the carbon dioxide. The efficiency of this process depends on the surface area of the sorbent, and an easy way to increase the surface area is to spray a liquid sorbent into the air as a fine mist. At PARC, researchers propose building towers several meters high, through which the air would be wafted, coming into contact with a sorbent mist. Having absorbed CO₂ from the air, the liquid would drain into a chamber where the gas would be extracted from the sorbent by a series of chemical reactions, or by applying an electric current, depending on the system's design. The sorbent can then be recycled, and the CO₂ compressed into liquid form for removal. A group at the University of Calgary, led by David Keith, has already demonstrated an air-capture prototype based on a spray tower.

Klaus Lackner, professor of geophysics at Columbia University and a pioneer in the field, has devised another approach that uses a solid sorbent, consisting of thin sheets of material coated with proprietary chemicals. Carbon dioxide is trapped as the air wafts over these sheets, and is then absorbed by liquid chemicals that are washed over the sheets. The CO₂ is extracted from the liquid by applying heat. A cupboard-sized prototype (pictured) has already shown that the concept will work, and Dr. Lackner is a member of a company, Global Research Technologies (GRT), that hopes to commercialize the technology. A machine the size of a standard shipping container, he estimates, could capture one ton of CO₂ a day.

But given that air-capture machines are electrically powered, and generating electricity usually produces carbon-dioxide emissions, do the sums add up? Dr Keith's prototype captured a ton of CO₂ using 100 kilowatt-hours of electricity. To generate this much power, a coal-fired power station would add around 35kg of carbon dioxide to the atmosphere, or 3.5% of the amount removed by the air-capture machine. Using a cleaner source of power, and anything is cleaner than coal,would make the machine come out even further ahead. Similarly, GRT estimates that when its technology is scaled up, the emissions associated with operating each machine will be less than 5% of the CO₂ captured over its lifetime. So there seems little doubt that air capture would indeed be carbon-negative overall.

At the moment, air capture is merely experimental. A breakthrough could pose some tricky questions, but that is no good if the process is prohibitively expensive. One big cost, at least for some designs, is the sorbent material, which cannot be recycled indefinitely. Dr. Lackner believes, however, that air-capture technology could have near-term, small-scale commercial uses that will enable it to pay its way as it is perfected and scaled up. He thinks GRT could start off by selling small air-capture devices to produce CO₂ in places that need the stuff, but currently have to pay high prices to have it trucked in. By mass, carbon dioxide is in fact the 19th most important commodity chemical in America, according to the Department of Energy. It can be piped into greenhouses to improve plant yields and is used in food processing, water treatment and fire extinguishers, among other things. Forcing CO₂ into oil fields can also increase the amount of oil recovered. Air capture could, says Dr. Lackner, be a viable way to supply carbon dioxide for industrial uses, even at a cost of $200 per ton, the current cost of the technology.

That is far higher than the cost per ton of carbon dioxide on emissions-trading markets, where the price of permits that entitle their holders to emit a ton of carbon dioxide recently fell below $10. Only if the cost of air capture falls below the cost of an emissions permit will it be economically attractive; otherwise emitters will find it cheaper to buy the right to pollute. But, environmentalists expect emissions-trading markets eventually to price the gas at about $50 a ton, and Dr. Lackner hopes to get the cost of his process down to $30 per ton in the long run.

Fuel from the air?

Building huge air-capture plants to reabsorb the carbon dioxide produced by burning fossil fuels would be a perverse outcome. For one thing, the fossil fuels will run out eventually. But air capture could provide an ingenious alternative. If the CO₂ extracted from the atmosphere were combined with hydrogen (extracted from water using electrolysis), it could make synthetic hydrocarbon fuels. This would allow the use of existing fossil-fuel vehicles and fuel-distribution systems, but with much-reduced environmental impact. Dr. Keith calls this approach “carbon- neutral hydrocarbons.”

Unfortunately, making synthetic hydrocarbons is very energy intensive, and it would make sense only if they were powered by renewable energy sources. “Fundamentally the biggest cost of making fuel from CO₂ is the cost of getting the energy for it,” says Dr. Eisaman. He estimates that synthetic fuel would cost $4-5 per gallon once all is said and done. So the oil price would have to be much higher than it is today for this to make sense. And using renewable energy to make fuel might seem like an odd thing to do, given that it could power electric cars directly.

One way in which air capture and renewable energy might fit together, however, would be to divert excess power from wind farms to air-capture systems. Wind farms sometimes produce more power than utilities can use, particularly if it gets windy at night, when electricity demand is low. Some utilities turn off their wind turbines to avoid overloading their grids. Dr. Lackner suggests diverting it instead to air-capture systems. In theory, air-capture systems could then operate at a profit by generating carbon offsets that could be sold on carbon-trading markets. Dr. Lackner even suggests selling such offsets at petrol stations, along with fuel.

Air capture appears to be technically feasible, but the economics are still unclear, and the politics murkiest of all. Will developing countries build enormous air-capture plants, powered by coal, to offset the emissions associated with industrialization? Will the technology discourage efforts to improve energy efficiency, or might it be a valuable tool in the fight against climate change? At the moment, air capture is merely experimental, but a sudden breakthrough could pose some tricky questions.