Can technology clear the air?

By Robert Kunzig and Wallace Broecker, Magazine issue 2690, 07 January 2009

THREE hundred and eighty-five parts per million: that's how much carbon dioxide there is in the atmosphere now. Just 100 parts per million more than before we started mucking things up, yet the Arctic ice cap is already melting, weather patterns are changing, and plants and animals are migrating towards the poles to find their comfort zones.

We can't go on like this. In fact, some climate scientists, notably James Hansen of NASA, say that 385 parts per million is too high and that we need not just to slow the increase in CO₂ but to clean up the mess we've already made. Three hundred and eighty-five invisible, colourless needles for every million stalks in the haystack. What are the chances that we can find and remove them?

Slim to none, according to many scientists and engineers. They say that removing CO₂ from the air is a complete non-starter because it takes far too much energy.

That hasn't stopped a handful of researchers from trying. They argue that air capture is not only theoretically feasible, it will soon be a practical weapon against global warming. Above all, they argue, we can't afford not to develop ways to scrub carbon from the atmosphere as a vital last line of defence. If Hansen is right and we've already gone past the point of no return, no amount of solar power or energy efficiency will save us. We need to take CO₂ directly out of the atmosphere, and fast.

Capturing CO₂ gas is not difficult. In fact, in the near future we are likely to be extracting quantities of the stuff from the flue gases of power plants and factories, and stashing it deep underground. In 2005 a special committee of the Intergovernmental Panel on Climate Change concluded that this strategy - known as carbon capture and storage or CCS - is likely to have a key role in tackling climate change. An entire industry, well supported by governments and energy companies, has since sprung up to make CCS a reality.

Once CO₂ gets out into the open air, however, capturing it gets trickier. Levels in the atmosphere are around 1/100th of its concentration in flue gases, and based on this the IPCC panel concluded that capturing CO₂ directly out of the air was impractical in terms of the energy that would be required.

Klaus Lackner of Columbia University in New York City begs to differ. A particle physicist by training, he has been pushing air capture for years. He has worked out the theoretical minimum energy required to remove CO₂ from air, and says that the IPCC panel - of which he was a member - got its numbers wrong. "It takes more energy to extract CO₂ from air than from flue gas, but the difference is quite small," he says.

Those calculations convinced Lackner that an air scrubber was a practical proposition, but they failed to convince his peers on the committee. To do that, he realised, he'd have to build one.

Which is what he has done. In mid-2008 he and his colleague Allen Wright were granted patents on a scrubber made of a plastic that spontaneously grabs CO₂ from the air. Their device collects just a few tens of kilograms of CO₂ a day from air blowing through the gaps between vertical sheets of the plastic. But with two years' work and, say, $20 million in venture capital - not easy to come by right now - Lackner says he could build a model that removes a tonne a day and would fit in a standard shipping container. Units like this might be of immediate interest to companies that have to buy in CO₂, and Lackner suggests that millions of them could eventually be deployed to suck CO₂ from the atmosphere and help save us from global warming.

"Millions of the devices could eventually be built to suck CO₂out of the air and reverse global warming"

Lackner isn't the only one working on air capture. Lab-scale units have also been built by teams at the University of Calgary in Alberta, Canada, and at the Swiss Federal Institute of Technology (ETH) in Zurich.

No pipelines attached

If interest in air capture is getting keener, it's because the potential advantages are so great. For one thing, air scrubbers would capture CO₂ from any source, big or small, including cars, planes and heating systems. These produce more than a third of global CO₂ emissions but it is impractical to capture the gas at source from tailpipes or flues. What's more, because CO₂ emissions mix into the atmosphere quickly and levels are the same pretty much everywhere, air scrubbers could be placed directly over sequestration sites. In contrast, CO₂captured at power plants will often have to be sent through hundreds of kilometres of pipeline. As David Keith of the University of Calgary puts it, air capture is "decoupled from the rest of the energy economy".

Keith and his student Joshuah Stolaroff built their first air-capture prototype three years ago, using a version of the tried-and-tested "spray towers" that remove sulphur dioxide from the smokestacks of coal-fired power plants. Like sulphur dioxide, CO₂ is an acid gas that can be soaked up by a solution of the alkali sodium hydroxide.

Keith and Stolaroff's prototype is a 4-metre-high cylinder of heavy-duty cardboard lined with PVC. The air to be treated is blown in at the top, where it is sprayed with a fine shower of sodium hydroxide solution. This reacts with CO₂ in the air to form droplets of sodium carbonate.

A full-scale scrubber might resemble an aircraft hangar, with fans at one end blowing air through a mist of sodium hydroxide from nozzles in the ceiling. Drains in the floor would collect the sodium carbonate solution.

The system works, but it takes a whopping amount of energy. At the end of the process, the sodium carbonate needs to be turned back into sodium hydroxide, which is cheap but not so cheap that you could afford to use it only once. This requires a kiln heated to 900 °C. Keith thinks he could halve the energy needed and cut the overall cost of capturing CO₂ and sequestering it to the vicinity of $100 per tonne (Environmental Science and Technology, vol 42, p 2728). That's far more than polluters pay under the European Union emissions control scheme, which currently prices a tonne of CO₂ at €15 (New Scientist, 19 April 2008, p 38). But it is perhaps not much more than the price will be once governments get serious about climate change.

Keith accepts that air scrubbers are unlikely ever to be the most economical way to tackle global warming, but reckons we might be forced into using them anyway. "You still might end up doing it," he says, "because you get into crisis mode."

While Keith is busy adapting existing technology, other researchers are shooting for something more novel. At ETH Zurich, Aldo Steinfeld, whose main interest is in solar energy, became interested in air scrubbers after Lackner visited the institute to talk on the subject. "We decided we could do that using solar technology," Steinfeld says.

Solar scrubber

The ETH concept is a modified version of an energy-generating technology called concentrated solar power that has been blossoming lately in deserts around the world (New Scientist, 10 April 2004, p 26). Such power plants consist of fields of sun-tracking mirrors that focus sunlight to generate steam that drives a generator. "We remove the boiler," explains Steinfeld. "We put our solar reactor there. In this we remove CO₂ from the air."

Steinfeld's reactor is a transparent tube filled with pellets of calcium oxide. In the table-top version the tube is a few centimetres high and an arc lamp replaces the sun. As the light heats the tube and its contents to 400 °C, air mixed with a small amount of steam is pumped in at the bottom and up through the pellets. At this temperature, the calcium oxide reacts with CO₂ to form calcium carbonate. "By the time the air leaves, there is no CO₂," says Steinfeld. "We go from 385 parts per million to practically zero."

In less than 15 minutes, the pellets are mostly converted to calcium carbonate. At that point, Steinfeld closes the intake valve and intensifies the light, raising the temperature in the reactor to 800 °C. This drives off the CO₂ as a stream of pure gas, which can be sent for sequestering, and converts the calcium carbonate back into calcium oxide. The researchers have run their reactor through five cycles of absorption and release with no decline in performance. Steinfeld believes his device could be scaled up to take significant amounts of CO₂ out of the atmosphere, though as yet he doesn't know how much it would cost per tonne.

Using sunlight to strip CO₂ out of the air clearly has advantages over a kiln. But Lackner suggests that if you're going to fill a desert with solar concentrators, it might make more environmental sense just to convert that sunlight into electricity.

Lackner's own strategy is to drastically reduce the amount of energy required to strip CO₂ out of the air. He and his colleagues have experimented with various designs, but the heart of each is an ion exchange resin, a polymer impregnated with sodium hydroxide. The sodium ions are firmly attached to the polymer but the hydroxide is loose and easily displaced by CO₂, which binds to the sodium to form sodium bicarbonate. The chemistry is essentially the same as in the Calgary device, says Lackner, but because of the enormous surface area of the resin sheets, the reaction goes much faster.

The resin has a second major advantage: it changes state when it gets wet, reducing its affinity for CO₂. "So we can drive the [absorbed] CO₂ off just by adding moisture," says Lackner.

The Calgary and Zurich air scrubbers need temperatures of up to 900 °C to regenerate their CO₂-absorbing material, but Lackner can do it at 40 °C. "We are dealing with extremely small amounts of energy," he says. As yet, though, he hasn't done a detailed analysis of how much it would cost per tonne of CO₂.

He does, however, think it will be cheap enough to open up commercial opportunities for his technology. Fruit and vegetable growers routinely enrich the air in their greenhouses with extra CO₂ and can pay as much as $300 a tonne for the stuff. Lackner reckons an air scrubber attached directly to the greenhouse could beat that price.

Miniature greenhouse

One of his demonstration devices is in fact a miniature greenhouse around a metre long. Attached to one end is a plastic tube containing the ion exchange resin, which absorbs CO₂ from the air. When most of the sodium hydroxide in the resin has been converted to sodium bicarbonate, Lackner evacuates the tube and then allows it to refill with moist air from the greenhouse. This releases the CO₂, which can then be flushed out into the greenhouse. The device generates around a kilogram of CO₂ per day, which is converted into biomass by tomato plants inside the greenhouse.

Oil companies also buy CO₂ by the tonne to flush oil out of ageing fields, and Lackner reckons that an air scrubber capable of capturing a tonne of CO₂ a day could become commercially viable both for this market and for horticulture, even in the absence of government caps on carbon emissions. "We'd learn how to drive the price down, and we wouldn't have to hold our breath for government regulations," he says. "If the government decides to help us, that's great. But we don't have to wait for it."

What he is having to wait for at the moment is the venture capital required to build a tonne-a-day prototype. For lack of this investment, the small company he had helped found to commercialise his idea recently closed its doors.

Using air capture to help tackle climate change faces an even bigger economic challenge. It will not take off until governments set a price on carbon that justifies the investment. But if and when that happens, a new and potentially vast market might open up: synthesising fuels out of thin air. "You capture CO₂and combine it with hydrogen to make fuel," says Keith. Fuels made from air-captured CO₂ would cause no net emissions, because the carbon they release would be have come from the air in the first place (New Scientist, 1 March 2008, p 32).

Of course there is still the hope that it's not too late and we can avert a climate crisis through a massive switch to solar, wind or nuclear power. But if not, air scrubbers could be the last-ditch lifeline.

It would be a monumental undertaking. At present we would need some 20 million tonne-a-day units to absorb emissions from the transportation sector alone. But if things get desperate it just might come to that, says Keith. "Our grandkids will clean up the mess we made by sucking it out of the air."

"At present emission rates we would need 20 million tonne-a-day units just to absorb CO₂ from transport"

Robert Kunzig is a writer based in Dijon, France, and Birmingham, Alabama. Wallace Broecker is a climate scientist at Columbia University in New York City. Their book, Fixing Climate, is published by Profile.

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