Unless we figure out a technological way to capture CO2 into a solid very quickly and very effectively and on a massive scale,
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Dialogue
SOAKING UP CO2
Klaus Lackner is a geophysicist at the Earth Institute at Columbia University and codeveloper of the synthetic tree, a device designed to remove carbon dioxide from the air. By Lackner’s calculations, one synthetic tree could absorb 1,000 times more CO2 than a living tree.
How would the synthetic tree remove carbon dioxide from the air?
L: The device itself would look something like goalposts with venetian blinds. It would be equipped to use liquid sodium hydroxide, which converts to sodium carbonate as it pulls CO2 from the wind stream.
How much could one tree remove?
L: The unit, which has a collection area of 50 meters by 60 meters, could gather 90,000 tons of CO2 a year. That means one synthetic tree could handle an amount equivalent to the annual emissions of 15,000 cars.
How many of these synthetic trees worldwide would be needed to soak up the 22 billion tons of CO2 produced annually from fossil fuels?
L: About 250,000.
To make this process efficient, you need to recycle the sodium hydroxide, which means you need to take the absorbed carbon back out. How do you do that?
L: You percolate the liquid sodium carbonate over solid calcium hydroxide, and the calcium catches the carbon. So you have taken the carbon out of your sodium hydroxide, and you can use it again. But then you have to get the carbon out of the calcium so that you can repeat the process. You do this by heating the calcium carbonate to 900 degrees Celsius, and it lets loose the CO2. So now we have the CO2 back in hand as a concentrated stream, with which we can do whatever we want.
What do you suggest?
L: It can be sequestered underground. The question is, is there enough capacity? Short term, it will work, but for the long term we need to develop other alternatives. I have proposed mineral sequestration. There are entire mountain ranges made of magnesium silicates that over millions of years would naturally turn into magnesium carbonate. We could speed up that process in an industrial fashion. We could make a stable, harmless solid.
What percentage of the energy in, say, gasoline would be consumed in the process of cleaning it up?
L: About 40 percent. People say 40 percent is a big hit. But it’s not, compared with producing hydrogen from coal, which I think is the most likely way large quantities of hydrogen would be made. Those guys also have a 40 percent energy hit, if not larger. So in a sense, the cleanup will cost that much, whether it is converting hydrogen from coal or pulling carbon dioxide from the air. In one case, you pay for the energy upstream; in the other you pay for it downstream.
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