A far different approach to collecting and sequestering carbon dioxide is to absorb it straight from the ambient air. Rather than separating CO2 from flue gases or making hydrogen and CO2 from fossil fuels, filters containing an alkaline solution to react with CO2 could be placed on tall towers around the world, says Klaus Lackner, a Columbia University geophysicist. Eventually, the CO2 would be regenerated as a pure gas and stored in geologic formations, Lackner says.
“The first reaction almost everybody has to this concept is to say the concentration of CO2 in air is so low, it cannot be removed economically,” he says. But he believes this could be a practical way to lower the atmospheric concentration of CO2 while continuing to burn fossil fuels. One advantage of the system is that because CO2 is well mixed in the atmosphere, it does not have to be captured at the emission source. It can be captured almost anywhere.
Lackner explains that a cubic meter of air contains about 0.6 g of CO2. On average, each person in the U.S. is responsible for 22 metric tons of CO2 emissions a year from personal use of fossil fuels and the industrial production that goes into all the goods and services used. A collection system with a wire frame about the size of a 2-sq-ft television screen, placed in a location where wind speeds average 6 m per second—the kind of region where windmills could be located—would capture 22 metric tons of CO2 in a year, he says.
Lackner’s system, which would have a collector much larger than a TV screen, involves placing a wind-scrubbing unit on a tower. The unit would capture CO2 in an alkaline solution, such as limewater or sodium hydroxide. A prototype unit is being built in the Arizona desert near Tucson by a small company, Global Research Technologies (GRT). The project is funded by Gary Comer, the founder of Land’s End clothing.
So far, GRT has been working on efficient methods of capturing CO2. Several designs are being considered, Lackner says. The collector may have slats like a venetian blind that would allow the sorbent to flow through the structure. The prototype device, which will take a few years to design and build, will remove several hundred pounds of CO2 from the atmosphere each day, he says.
If sodium hydroxide is used as a sorbent, the whole process would involve the following reactions:
It is necessary to go through all of these phases in order to regenerate the Ca(OH)2 and the NaOH. The third reaction is very expensive, but the developers hope to find a way to lower its cost, perhaps by capturing some of the intense heat from the fourth reaction.
“Any energy system that truly cleans up after itself will require additional energy,” Lackner says. For example, to convert coal or oil into hydrogen, “you lose about 40% of the energy in the coal or oil, and by the time you construct an infrastructure for distributing the hydrogen, the energy penalty increases to about 50%,” he explains.
For this reason, Lackner believes his system will be competitive with those that produce hydrogen from fossil fuel and sequester the CO2 in geologic formations. The advantage of his system, he says, is that it does not require changing the existing infrastructure for distributing liquid fuels.
David W. Keith, Canada Research Chair of Energy & Environment at the University of Calgary, has estimated the cost of removing CO2 from the air with a system similar to Lackner’s. “I am cautiously optimistic about the idea,” Keith says. In the short term, over the next 10 or 15 years, it would probably be very expensive, he says. “If you put serious money into this, like billions, and built several large-scale test facilities, and some real plants, you might come in with a cost of a little under $500 per ton of CO2,” measured as carbon, he says. However, even that would beat the cost of fueling cars with hydrogen fuel cells. By 2050, with a lot of new engineering, the cost might be reduced to about $150 per ton of carbon, he calculates. Then, he says, the system would be economical.
However, the goal of R&D funded by the U.S. Department of Energy is to get CO2 collection and sequestration costs at coal-fired power plants down to around $37 per ton in carbon equivalents or $10 per ton of CO2. This level, DOE believes, would increase the price of electricity by about 10%. Using Lackner’s concept to meet the government’s current strategy—to be able to annually collect about 1.7 billion tons of CO2 (in carbon equivalents) by 2050—might cost $255 billion per year.