A Case for Direct Air-Capture of CO2
By Wallace Broecker ’53, ’58 GSAS
During the multi-decade transition from a fossil-fuel-dominated energy economy to one dominated by renewables, the CO2 buildup in our atmosphere will continue. Were the current rate of rise (i.e., 2.5 parts per million per year) to continue, the burden of this greenhouse gas would reach twice its pre-industrial level just after 2075. Model simulations suggest that such an increase will warm the atmosphere by about 3C (i.e., 5F). However, the rate of increase is unlikely to remain at its current level. Increased fossil fuel use by developing nations currently out strips the reductions accomplished by the signators of the Kyoto convention. This gap is likely to grow.
Two bounding scenarios can be envisioned. One is “business as usual” (BAU) in which we allow the CO2 we produce to build up in the atmosphere. The other is that we hold down the rise of CO2 by capturing and burying it. We face a transition period, many of decades in duration. Were we to go the BAU route, by the time we rein in fossil-fuel use, the content of CO2 in the atmosphere will likely have more than doubled and the atmosphere’s temperature will likely have risen by more than 3C. Further, although most of this CO2 will eventually be taken in by the ocean, it will take hundreds of years. Impacts, such as the gradual loss of coastal property resulting from the melting of the polar ice caps will likely lead to a desire to bring the atmosphere’s CO2 content back down. This will require CO2 capture and storage. As CO2-belching smoke stacks will have been eliminated, it will be necessary to pull the CO2 directly out of the air.
The bottom line is that, if we don’t capture and store CO2 during the transition period, we will be forced to do so after it is over. As a bonus when petroleum runs short, it will be possible to combine the CO2 captured from the atmosphere with hydrogen to produce “artificial” gasoline.
CO2 capture could be done either by removing it from electrical power plant effluents or by direct capture from the atmosphere. As the CO2 concentration in stack gas is about 10 percent (i.e., 100,000 parts per million) and in air only 400 parts per million, at first thought one might conclude that stack capture would be the obvious way to go. However, as shown by Columbia University’s Klaus Lackner, the extra cost of air capture could be as little as 30 percent over that for stack capture. Furthermore, air capture could be done away from cities and close to the site of storage. Finally, as only 35 percent of the CO2 we produce is currently emitted from electrical power plants, air capture would be required to deal with the remaining 65 percent.
Lackner favors small modules over power plant-sized units. His reason is that they could be mass produced and, like automobiles, each generation would be a bit better than the previous one. He envisions a dissembled unit which would fit in a standard shipping container. These units would cost about as much as an automobile. Each would capture about one ton of CO2 a day, i.e., the amount generated by 20 automobiles. Operating these units would require a surcharge of about 50 cents per gallon on U.S. gasoline. As there are currently 70 million automobiles worldwide, 3.5 million “Lackner units” would be required to compensate for their CO2 emissions. About 10 million would be needed to compensate for all CO2 emissions.
Key to Lackner’s device is a commercial plastic embedded with tiny chemically active units which, when exposed to air, pick up CO2 in preference to H2O. When exposed to water or steam, the CO2 molecules are released and H2O molecules take their places. This CO2-H2O cycle can be repeated hundreds of times without any loss of capacity.
Although Lackner envisions a working prototype, he has not been able to garner the funds needed to create it let alone to build and test it. Until this has been accomplished, he cannot make firm estimates of the construction and operation costs. To date no government, no industry or no venture capitalist has stepped forward. But for the funds provided by the late Gary Comer, Lackner would not have been able to establish the concept. Together all those interested in air capture have spent only about 10 million dollars. Considering that this is the amount paid yearly to many professional athletes it is a sad commentary on our ability to prepare for what is considered to be the greatest environmental threat facing our planet.
In the current political climate it is unlikely that CO2 capture and storage will be implemented. But, as much time-consuming research must be done in order to establish the cheapest and safest methods of CO2 capture and storage, it would be prudent to conduct pilot projects. Then if, as naysayers claim, the consequences of this rise turn out to be trivial, capture and storage can be shelved. But if, as the models suggest, the impacts will be large enough to make it difficult to support the planet’s 9 to 10 billion inhabitants, then we will be prepared to take action.