Removing this pollution from power plants – called carbon capture and storage – is a useful way of preventing additional carbon dioxide from entering the atmosphere as we continue to burn fossil fuels. But what about the gas that is already out there?
The problem with removing carbon dioxide from the atmosphere is that it’s present at such a low concentration. In a power plant chimney, for instance, carbon dioxide is present at concentrations of 4-12% within a relatively small amount of exhaust air. Removing the gas takes a lot of energy, so it is expensive, but it’s feasible. To extract the 0.04% of carbon dioxide in the atmosphere would require enormous volumes of air to be processed. As a result, most scientists have baulked at the idea.
Fake plastic trees
Klaus Lackner, director of the Lenfest Center for Sustainable Energy at Columbia University, has come up with a technique that he thinks could solve the problem. Lackner has designed an artificial tree that passively soaks up carbon dioxide from the air using “leaves” that are 1,000 times more efficient than true leaves that use photosynthesis.
"We don't need to expose the leaves to sunlight for photosynthesis like a real tree does," Lackner explains. "So our leaves can be much more closely spaced and overlapped – even configured in a honeycomb formation to make them more efficient."
The leaves look like sheets of papery plastic and are coated in a resin that contains sodium carbonate, which pulls carbon dioxide out of the air and stores it as a bicarbonate (baking soda) on the leaf. To remove the carbon dioxide, the leaves are rinsed in water vapour and can dry naturally in the wind, soaking up more carbon dioxide.
Lackner calculates that his tree can remove one tonne of carbon dioxide a day. Ten million of these trees could remove 3.6 billion tonnes of carbon dioxide a year – equivalent to about 10% of our global annual carbon dioxide emissions. "Our total emissions could be removed with 100 million trees," he says, "whereas we would need 1,000 times that in real trees to have the same effect."
If the trees were mass produced they would each initially cost around $20,000 (then falling as production takes over), just below the price of the average family car in the United States, he says, pointing out that 70 million cars are produced each year. And each would fit on a truck to be positioned at sites around the world. "The great thing about the atmosphere is it's a good mixer, so carbon dioxide produced in an American city can be removed in Oman," he says.
The carbon dioxide from the process can be cooled and stored; however, many scientists are concerned that even if we did remove all our carbon dioxide, there isn't enough space to store it securely in saline aquifers or oil wells. But geologists are coming up with alternatives. For example, peridotite, which is a mixture of serpentine and olivine rock, is a great sucker of carbon dioxide, sealing the absorbed gas as stable magnesium carbonate mineral. In Oman alone, there is a mountain that contains some 30,000 cubic km of peridotite.
Another option could be the basalt rock cliffs, which contain holes – solidified gas bubbles from the basalt's formation from volcanic lava flows millions of years ago. Pumping carbon dioxide into these ancient bubbles causes it to react to form stable limestone – calcium carbonate.
These carbon dioxide absorption processes occur naturally, but on geological timescales. To speed up the reaction, scientists are experimenting with dissolving the gas in water first and then injecting it into the rocks under high pressures.