The Case for Carbon Capture From Air

By Klaus Lackner
Professor,
School of Sustainable Engineering and the Built Environment
Director,
Center for Negative Carbon Emissions, Arizona State University

Stabilizing the atmospheric carbon dioxide (CO2) concentration requires the nearly complete elimination of all anthropogenic CO2 emissions.1 In the popular bathtub analogy that equates atmospheric CO2 concentration with the water level in a tub, the water level is held constant by matching the input from the faucet with the outflow through the drain. Unfortunately—and contrary to the usual explanation—in the case of the atmosphere, the drain clogs as the faucet is turned down. Once the atmospheric concentration stops rising, the surface ocean and the biosphere find their balance with the new CO2 level, and transport into the deep ocean will slow as the top layer of the ocean, which is close to equilibrium with the air, grows in size. Therefore, to meet international climate goals annual global emissions must approach zero. This is a tremendous challenge considering population growth and the existing energy infrastructure. If emissions cannot be fully eliminated or the atmospheric CO2 concentration overshoots the limit considered safe by climate scientists, negative emissions will be required as an additional drain so that the tub does not overflow. In fact, in its latest report, the Intergovernmental Panel on Climate Change (IPCC) suggests that negative emissions (i.e., an anthropogenic drain) will be necessary to meet the international goal of limiting climate change to 2˚C.2

Removing CO2 from the air would likely be required in a zero-emissions future.

Removing CO2 from the air would likely be required in a zero-emissions future.

This poses a challenge for today’s energy engineers: Provide affordable energy for a rapidly growing world economy while eliminating all CO2 emissions. This must also be accomplished without other environmental impacts, without creating energy shortages, and in the most economical way possible. Moreover, the time available for this radical transition is short. Thus, there is an urgency to introduce zero-emissions technologies across all energy sectors. This would include bold negative-emissions technologies for recovering and storing atmospheric CO2 to cancel out residual emissions and, if necessary, actually reduce the CO2 concentration in the atmosphere. Action is necessary, because learning by doing requires doing, and any delay will further increase the difficulty of stabilizing concentrations at a safe level.

NO SMALL FEAT

It is important to consider the scale of the challenge. Fossil fuel consumption could easily quadruple over the course of this century without global per capita energy consumption exceeding that of the U.S. today. With more than 80% of today’s energy derived from fossil fuels, virtually the entire energy system must change. In addition, the world may need to correct an overshoot that could easily be as large as 100 ppm, or 400 Pg of excess carbon. This is more than all emissions of the 20th century. In any overshoot scenario carbon storage becomes unavoidable, and potentially a very large component of carbon management.

LARGE-SCALE LOW-EMISSIONS ENERGY OPTIONS

Solar, nuclear, and fossil energy are the three truly large-scale energy options for the future.3 Sunshine exceeds human energy consumption by four orders of magnitude, nuclear resources could supply thousands of years of power, and fossil carbon, even at increased consumption, could support global energy demand for several hundred years. However, none of the three large options are ready for a future carbon-neutral world. From a business perspective it is entirely rational to back a profitable technology, even if it cannot operate at global scale. From a policy perspective, however, the risk mitigation resulting from developing a global solution is very worthwhile, even if cobbling together a global energy system from many small sources may prove feasible. All energy options need to be pursued, but advances in these three fields are particularly important as each could result in an energy infrastructure that is sufficient to satisfy human needs.

While all large-scale low-emission energy options are critical, each faces unique challenges.

While all large-scale low-emission energy options are critical, each faces unique challenges.

The available options for stabilizing CO2 can broadly be classified into three categories: improved efficiency, increased deployment of renewables and nuclear, and carbon capture and storage.

Acceleration of Efficiency Improvements

The first option is to do more with less energy and increase efficiency throughout the entire energy value chain. There are abundant opportunities on the demand side, from LED lighting to more efficient cars. There are also many opportunities on the supply side, from higher-efficiency power plants to cogeneration of heat and power. Improving efficiency can greatly reduce emissions, but it cannot achieve zero or negative emissions. Since efficiency improvements stem from a myriad of different advances, progress is unlikely to come from a few large projects, but more likely from broad-based economic incentives. It is worth noting that improving efficiency is already built into the IPCC’s business-as-usual scenarios and maintains global energy consumption projections typically one percentage point below the world’s GDP growth. To go beyond business as usual, energy intensity improvements must go much further.4

Increased Deployment of Renewables and Nuclear

The second option is to expand the deployment of carbon-
neutral energy resources, such as renewable and nuclear energy. If all fossil carbon resources were to be replaced with non-fossil energy, anthropogenic CO2 emissions to the atmosphere would drop close to zero, but such a transition would be associated with huge costs, major infrastructure changes, and would likely take too much time to meet international climate goals. In addition, the intermittency issue of solar energy must be resolved in an affordable manner, and nuclear energy is saddled with the risk and legacy of Three Mile Island, Chernobyl, and Fukushima. While not comprehensive in the near term, renewable energy and nuclear energy, including fusion, are an important part of a low-emissions solution and need to be developed to create optionality in the energy sector.

Carbon Capture and Storage

The third major option is to capture and permanently dispose of CO2. For carbon capture and storage (CCS) to be compatible with a zero-emissions world, it must include CO2 capture from the atmosphere, since CCS does not capture 100% of emissions from fossil power plants.

Capture of carbon from the atmosphere, the surface ocean, or the biosphere makes it possible to create negative emissions that could recover past emissions or balance out remaining emissions that are difficult to capture by other means. This includes the fugitive emissions of a coal plant with CCS and its associated CO2 storage as well as the CO2 emitted from the transportation sector.

However, the path forward is challenging. Fossil energy can only contribute to a zero-emissions world if all CO2 and other greenhouse gas emissions can be eliminated. CCS is not yet widely applied and its costs must be reduced.

THE POTENTIAL ROLE OF AIR CAPTURE

If none of these three options for stabilizing CO2 emissions can be made to work, humanity would face the impossible choice between a climate disaster and a collapse of the world’s energy systems. Considering the risks, I believe current efforts and investment are far too lackadaisical.

My own focus has been finding ways of recovering CO2 from the atmosphere by technical means.5,6 Recovering CO2 from the environment by any means can help return the world to lower CO2 levels and it can close the anthropogenic carbon cycle regardless of the original carbon source. This will require the large-scale use of carbon storage technologies. Importantly, use of carbonaceous liquid fuels in the transportation sector can only be sustained if it is fully matched by CO2 recovery. For biofuels grown in open air, the recovery is automatic. For petroleum-based fuels, fuel from algae grown in closed bioreactors, or for synthetic fuels, the CO2 must be recaptured. Ultimately, as long as fossil fuels are used, for every ton of carbon taken from the ground another ton will have to be stored in a net-zero emission world. Even without fossil fuels, for every ton of CO2 injected into the air, another ton will have to be recaptured.

Direct capture of CO2 from air is already practiced today, albeit on a much smaller scale, to purify breathing air on submarines or spacecraft, or the removal of CO2 from air prior to its liquefaction. Thus, there is a foundation on which to build. Through innovation, researchers hope to reduce the costs and find new pathways to enable direct air capture to play a role in a low-emissions future. There are already several start-up companies demonstrating the feasibility of capture from air.

It is necessary to develop cost-effective technologies to store or use CO2 to realize the benefits of air capture.

It is necessary to develop cost-effective technologies to store or use CO2 to realize the benefits of air capture.

It is highly unlikely that the massive changes necessary to stabilize CO2 in the atmosphere could be cost-effectively achieved by deploying a single technology. It is even less likely that scientists, engineers, business people, or policy makers could successfully pick the winner today. With hindsight, it is clear that some technologies have come down in costs by orders of magnitude since their inception and operate at scales initially thought to be impossible. Eventually many processes approach a “frictionless” cost that is dominated by raw materials and energy. However, such reductions in cost are not predictable. Rather than making policies based on current cost, which is meaningless, or hypothetical cost, which is unknowable, a better strategy is to develop technology options and make decisions as advantages and disadvantages emerge. Thus, direct air capture is among the global options that need to be developed.7

Perhaps the strongest motivation for developing direct air capture technologies would be the resultant uniform cost for carbon emissions. Air capture, because it collects CO2 after it has been emitted, can balance out any emission, at any time, and from any location. If there is no better technological option (e.g., as is likely the case for emissions from widespread air travel) air capture combined with storage or fuel synthesis can become viable where no other option is feasible. In most cases, it will be easier or cheaper to collect the CO2 earlier in the energy chain (e.g., CCS at a coal-fired power plant), or it may be preferable to avoid making CO2 in the first place by raising energy efficiency or by deploying near-zero carbon energy sources.

By making emissions reversible, air capture puts a price on CO2 emissions as an option of last resort. If that price proves affordable, comprehensive carbon regulations become more palatable politically. With gradually tightening regulations that enforce a shrinking—and in the future perhaps even negative—annual carbon budget, air capture would provide an economic incentive for other low-emission technologies to reduce emissions at a lower cost. The policy value of air capture is not in the amount of CO2 that it cancels out, but in its ability to set the marginal cost of CO2 remediation. Even if the contribution of air capture to carbon mitigation remains small, its gradual cost reductions will likely spur advances in all other carbon-avoiding technologies.8

Researchers are already working on air capture technologies, such as carpet-like ion exchange resins, that could reduce costs.

Researchers are already working on air capture technologies, such as carpet-like ion exchange resins, that could reduce costs.

HEDGING OUR BETS

How a zero-emissions world will develop is difficult to predict. Different technologies, including air capture technologies, will compete for various sectors of the market. In a zero-emissions world, regulations placing a price on CO2 emissions directly or indirectly would ensure use of unabated fossil fuels is eliminated.

Rather than setting an artificial price for emissions, I propose that all CO2, whether released accidentally or intentionally, must be recovered. This will provide a niche for air capture and it will set the marginal cost of carbon emissions. Whether this niche is large or small will depend on all other technologies
that are under development and which will move forward if zero-emissions becomes an enforceable goal. Rather than prescribing an answer, I propose to set boundary conditions in which markets can find an optimal solution.

REFERENCES

  1. Archer, D. (2010). The global carbon cycle. Princeton: Princeton University Press.
  2. Qin, D., Plattner, G-K., et al. (2014). Climate change 2013: The physical science basis. Cambridge, UK, and New York: Cambridge University Press.
  3. Lackner, K. (2010). Comparative impacts of fossil fuels and alternative energy sources. In R.E. Hester, R.M. Harrison, et al. (Series Eds.), Issues in Environmental Science and Technology, No. 29 (pp. 1–40). Cambridge, UK: The Royal Society of Chemistry.
  4. Lackner, K.S., & Sachs, J. (2005). A robust strategy for sustainable energy. Brookings Papers on Economic Activity 2005, No. 2, 215–284.
  5. Lackner, K. (2010). Washing carbon out of the air. Scientific American, 302(6), 66–71.
  6. Lackner, K. (2014). The use of artificial trees. In R.E. Hester & R.M. Harrison (Eds.), Geoengineering of the climate system (pp. 38, 80). Cambridge, UK: Royal Society of Chemistry.
  7. Lackner, K., Brennan, S., Matter, J.M., Park, A.H.A., Wright, A., & Van Der Zwaan, B. (2012). The urgency of the development of CO2 capture from ambient air. Proceedings of the National Academy of Sciences, 109(33), 13156–13162.
  8. Lackner, K., & Brennan, S., (2009). Envisioning carbon capture and storage: Expanded possibilities due to air capture, leakage insurance, and C-14 monitoring. Climatic Change, 96, 357–378.

The author can be reached at Klaus.Lackner@asu.edu

 

The content in Cornerstone does not necessarily reflect the views of the World Coal Association or its members.
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