Pulling CO2 from the air: promising idea, big price tag

Pulling CO2 from the air is simple chemistry. After all, a bottle of sodium hydroxide - also known as lye and a primary constituent of everything from soap to pulp and paper - must be kept carefully sealed. That’s because the strong base - the opposite of acid - will be neutralised if exposed to air by rapidly sucking up the CO2 and then transforming the lye into sodium carbonate and, ultimately, baking soda. The captured CO2 can then be extracted via the industrial process of heating the compound above 900 degrees C in a kiln, releasing the CO2, and enabling the sodium hydroxide to regenerate its ability to suck up yet more CO2.

The process works, but as physicist Klaus Lackner at Columbia University’s Earth Institute - one of the scientists behind the GRT pinwheel - explains, “The energy to pry out the CO2 is very high”.

That’s why Lackner has moved in the direction of finding a strong base resin, such as Dow Chemical’s Marathon A, typically used to produce purified water. The synthetic resin in the pinwheel absorbs CO2 to form bicarbonates when dry, but then spits out the CO2 when exposed to water. “Basically, we can swing between being dry and wet,” Lackner says. “Let the resin sit in air, because air will dry it, and it will absorb CO2, taking an hour to load up. Make it wet, and it’s an hour to unload.”

This type of device could be housed in an “oversized furnace filter,” about three feet wide by eight feet long, loosely filled with sheets of the resin, constituting the leaves of this artificial tree. Such a device could capture CO2 for less than $300 per metric ton, though it wouldn’t function in cold climates or the humid tropics.

A number of experiments involving air capture technologies are underway, ranging from efforts to use solid amines - ammonia transformed into compounds capable of bonding with CO2 - to technologies now used to capture some flue gases from exhaust at fossil fuel-fired power plants. Scientists also are attempting to use algae - the workhorses of the Earth’s natural carbon cycle - to cleanse the air of excess CO2. That could have the benefit of creating a new source of fuel or power, since algae incorporate nearly as much energy per kilogram as coal. But as the UK’s Institution of Mechanical Engineers put it, algae bioreactors “are a fledgling technology and at the moment are too expensive to be commercially viable”.

Artificial trees, on the other hand, could be available as soon as next decade. The mechanical engineers believe a demonstration could occur as soon as 2014, followed by a full-scale “artificial forest” by 2018 and global deployment by 2040. In the long term, such air capture theoretically has the potential to cancel out human emissions of CO2, according to earth system scientist Tim Lenton of the University of East Anglia.

Assuming that CO2 can be pulled from ambient air, that still leaves the other half of the problem: storing it safely somewhere. Efforts to capture CO2 from coal-fired power plants have seized upon geologic sequestration as a potential solution. The US Department of Energy estimates that the continental US alone has room for 3.9 trillion tons of CO2 underground, more than enough room for the 3.2 billion tons emitted every year by large industrial sources. Still, major questions remain about underground sequestration, including its impact on groundwater supplies, subterranean pressure, and the potential for the CO2 to leak back into the atmosphere.

Certain geologic formations may offer a solution by mimicking the chemical transformation of air capture itself. Basalt formations - a residue of volcanic activity - can absorb CO2 and, over decades, transform it into minerals. An experiment by Reykjavik Energy to prove the concept by injecting the CO2 from a geothermal power plant into basalt beneath the surface is underway in Iceland, which is primarily composed of the igneous rock.

Even if technology and storage issues are resolved, CO2 air capture will require significant amounts of new electricity to power the devices. Lackner proposes a new fleet of nuclear reactors or widespread solar power.