A literature review of
CO2 capture directly from ambient air
Joshua Fabian
Massachusetts Institute of Technology
Department of Civil and Environmental Engineering
19 October 2012
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Abstract
The concentration of atmospheric carbon dioxide has risen substantially since the beginning of
the Industrial Revolution by the burning of fossil fuels. Because of the negative effects that global climate
changes threatens to cause, finding ways to reduce CO2 levels has become very important in both the
scientific and policy worlds. Capturing carbon directly from ambient air is a proposal that can reduce CO2
from both industrial and non-stationary emissions sources because the capture devices, known as
“artificial trees,” would not be integrated into any one source. A variety of sorbents are being considered
for use in filters entrapping CO2 molecules: these include a solid resin, liquid hydroxides, and a variety of
amine compounds. Each sorbent carries its pros and cons with respect to cost, efficiency, and applicable
climates, though the more recent reports favor using an amine because of its low temperature
requirements when extracting the captured CO2 for storage purposes. There is also much disagreement as
to the costs and efficiencies of these methods, as many of the estimates rely on projected advances in the
relevant technologies. Further analysis on the specific sorbents is needed to determine how to best build
artificial trees, and a method of storing the carbon after it has been collected must also be analyzed.
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Introduction
Geoengineering is most often described as planetary-scale environmental engineering to respond
to climate change and the effects of the greenhouse phenomenon. What we know today as “climate
change” or “global warming” refers to the relatively dramatic increases in global temperatures and carbon
dioxide concentrations since the start of the Industrial Revolution (and especially in the past fifty years)
(Karl, 2009). Geoengineering is seen by some in the science community and the general public as a
necessary way to offset the negative effects of climate change, and by others as an extreme measure that
should only be used if and when it becomes immediately obvious that taking no such action would cause
harmful societal effects.
There are two primary branches of geoengineering techniques that have been and are still
currently of research value: solar radiation management (SRM) and carbon capture and storage (CCS)
(Lenton, 2009). The former, Lenton goes on to say, involves reflecting sunlight from reaching the Earth’s
surface, thus cooling it down or at least stabilizing its temperature. This could be facilitated by sending up
millions of small, reflecting disks into the atmosphere to counteract the greenhouse effect, although other
ideas include spraying aerosols with low mean residence times into the stratosphere or painting all
rooftops white to increase the planet’s albedo—a measure of how much radiation is being reflected off the
surface. Carbon capture and storage, on the other hand, deals not directly with sunlight, but rather
capturing gaseous carbon dioxide and moving it to a location of storage. A prominent proposal involving
CCS would have CO2 be separated from the flue gas that is directly output from emissions-producing
factories and power plants by use of specially-produced filters. After captured successfully, the carbon
dioxide would travel in a supercritical liquid state by vehicle or pipeline to storage sites, most often
natural carbon sinks in old oil fields or aquifers where it will become trapped (as its residence time will be
longer by many orders of magnitude) (Herzog, 2010). This review will provide an overview of a
relatively new proposal involving CCS.
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Design of artificial trees
One recent variant of the flue gas capturing scheme proposes to entrap atmospheric carbon
dioxide not at the source of emissions-causing energy producers, but rather out in the open, by way of socalled “artificial trees.” This process, known as direct air capture, works by erecting a series of capturing
devices—resembling small buildings or even football goalposts (or any other feasible and economic
scale) with a collecting screen-like material held in between the posts (IMechE, 2009). Air makes contact
with and flows through the physical screen, which is made of a sorbent that is readily able to extract the
passing CO2 (Lackner, 2009). After the collection, the sorbent is at some time recovered and the carbon
dioxide is transported elsewhere so that it can be either deposited at a carbon storage site or recycled for
use in feedstock or fuels (such as for synthesizing oil) (Goeppert et al., 2012).
These carbon-capturing devices could be placed anywhere it may be deemed convenient, whether
it be alongside urban highways or on open, rural fields, effectively creating an artificial forest. The
freedom of choosing the collection point comes from the fact that air is quick to mix into a state of
equilibrium; this means that the concentration of carbon dioxide is nearly consistent across the globe at
about 390ppm (Goeppert et al., 2012). A report by the Institution of Mechanical Engineers (2009) is
quick to note, however, that placing these “trees” immediately adjacent to heavily-traveled roadways may
still be slightly advantageous because CO2 concentrations will be higher due to emissions from vehicular
tailpipes (also see Figure 1). Additionally, a lack of prevalent winds in a particular region need not
exclude it from being considered for artificial reforestation, as the size of the air collector largely
determines the amount of CO2 that can be absorbed (Lackner, 2009). Direct air capture is also preferred to
flue gas capture when looking at the composition of the captured CO2: emissions coming directly from
the smokestacks of power stations and industrial buildings carry high concentrations of NOx and SOx in
addition to CO2 (Goeppert et al., 2012). These other compounds must be separated from the carbon
dioxide before storage, as there is no present method for storing all of the compounds in concentrated
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form. The comparatively low concentrations of the pollutants in the atmosphere mean that the collected
CO2 would be easier to purify than if it had come from flue point-sources.
What can be used as an effective sorbent?
Much research has been done on different types of sorbents that can be used in direct air capture,
and many of the proposed substances carry both pros and cons. In research by Klaus Lackner (2009),
sodium hydroxide was initially considered as a liquid sorbent, but it was quickly realized that extracting
carbon dioxide from such filters would require overcoming a very large bonding energy, which ultimately
defeats the purpose of CO2 capture and sequestration. In order to overcome the bonding energy, very high
temperatures would have to be created—not a problem with carbon collection directly from factory
smokestacks, but something completely different in the open atmosphere. Also, the researchers expressed
concerns that the strong-base surfaces would easily dry and corrode, both of which make the solution
impractical.
The research team went on to find that solid sorbents would dramatically increase the efficiency
of the system and that a solid resin of composite material would work best (Lackner, 2009). The resin
material, already made as a proprietary, electrochemical membrane, still relies on strong-base reactions
for the advantage of faster kinetics and on the very low bonding energy associated with carbonates.
Through experimentation, Lackner (2009) found that only water—even if it be only water vapor at
45°C—is required to separate the captured carbon dioxide from the filter. In this particular case, using
water vapor at such a temperature would release the CO2 as a gas, which would then be funneled into
pipelines for further transport. Further analysis by MIT’s Manya Ranjan (2010) does note a major pitfall
to this resin: regions of cold temperatures and high humidity would not at all be conducive to successful
operation. The places where this sorbent would be the most practical are in open deserts.
Other sorbents proposed for direct air capture include tertiary amines as well as weak-base
amines that require electric currents to maintain carbon dioxide concentrations around the collection
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device. An advantage of using amines (especially when compared to strong bases) is its relatively low
temperature requirement for extraction (Goeppert et al., 2012). Another scheme would create an
electrolysis system that would produce an acid and a base on two electrodes to form carbonates from
reacting CO2, although this would require excessive energy to run continually (Lackner et al., 2012). A
material’s bonding energy is one factor that must be closely considered when choosing an appropriate
sorbent, as any system that requires a high input of energy will dramatically drive up maintenance costs
and potentially shorten its expected lifetime. Lackner et al. (2012) suggest that other factors can include
the rate at which a particular material can intake CO2, at what thicknesses this is best achievable, in which
climates the sorbents can work best (it may be that different materials will be best suited for different
regions) and whether or not the optimal parameters are even structurally possible (which also affects
costs, to be discussed in detail later). While the research literature suggests different implementations for
the sorbent filter, the most recent papers do converge to using some sort of amine for the reasons
previously listed.
Efficiency and effectiveness of operating synthetic trees
Carbon capture by way of artificial trees and forests can in several ways be more efficient and
effective than capturing gases directly from emissions sources. Anthropogenic CO2 emissions can be
divided between stationary, point sources (these are mostly from industrial output) and non-stationary
sources, as shown in Figure 1 (Goeppert et al., 2012). It can be seen that entrapping carbon dioxide
directly from point sources could never completely negate all human output; capturing carbon in ambient
air, on the other hand, has a far greater potential because the emission source no longer matters.
Current numbers and research as to the efficiency of air capture devices is limited, and current
figures are not very high. Goeppert et al. (2012) spell out a 45% efficiency as a ballpark figure. Lackner’s
(2009) proposal to use a resin comes with a 50% thermodynamic efficiency, but all other methods of air
capture have efficiencies of around 10%, as summarized in Figure 2 (House et al., 2011). Whilst these
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7.90%
2.80%
25.90%
Energy supply
13.10%
Industry
Forestry
Agriculture
Transportation
13.50%
Non-industrial buildings
Waste and wastewater
19.40%
17.40%
Figure 1. Anthropogenic sources of carbon dioxide emissions in 2004.
(IPCC, 2007).
Sorbent proposed
Estimated cost, $/tCO2
Thermodynamic efficiency, %
NaOH and amine
136
7.9
NaOH
N/A
11.1
Ca(OH)2
about 200
2.4
Sorbent resin
220
50
Polyamine adsorbent
N/A
N/A
Figure 2. Estimated costs and thermodynamic efficiencies of several
sorbents proposed to be used in artificial trees. (Goeppert et al., 2011;
House et al., 2011).
figures may seem very low, House et al. assert that they still exceed the efficiency of photosynthesis,
which typically operates at 2% efficiency. Much of the literature on air capture pins hope on the
continuation of Moore’s law, which essentially predicts that technology will always continue advancing
exponentially. The assumption is that further research, development, and experimentation will all lead to
higher efficiencies and reduced costs. (Lackner et al., 2012). This should be prefaced, however, by an
earlier analysis by Keith et al. on atmospheric CO2 capture that suggests that an efficiency of 50% is
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optimal—much higher numbers would require too high an energy output, and much lower numbers would
require more widespread deployment of the capture devices, which in turn increases capital costs.
Costs involved in virtual afforestation
The cost that would be required in implementing direct air capture is a point of heavy
disagreement among the literature. Lackner’s (2009) early proposal for artificial trees estimates that each
capture unit would cost about $20,000 once fully in production, and that the bulk of the total cost would
come from the recovery process that occurs after CO2 has been captured, with the cost of recovering one
metric ton of CO2 having the potential to drop to around $30 (IMechE, 2009). Other proposals for direct
air capture, however, do not have as low cost estimates—most range $100-200/tCO2, with $1000/tCO2
not being outside the range of approximations (Lackner et al., 2012). A report by the American Physical
Society (2011) estimates that direct air capture will cost $600/tCO2, followed thereafter by a conclusion
that suggests that this method of carbon dioxide will never move beyond the prototypical stage.
Addressing the variability in cost estimates, there has been a split among the literature representing two
schools of thought. Some believe that it is impossible to extrapolate costs for future technologies and that
the provided estimates are useful as current benchmarks, whilst others believe that current price estimates
are inherently misleading as they can never be achieved using the technology of today (Ranjan & Herzog,
2011). Naturally, such opinions in the conclusions of past research is indicative of whether or not a
particular author or group of authors believe that direct air capture will be a viable option in the realm of
geoengineering.
Conclusion and recommendation
In short, there are many considerations that need to be made before proceeding on direct air
capture of carbon dioxide; there are many differing views and estimates on the best design, efficiency,
and optimal cost of the implementation of virtual trees. Regardless of these still-uncertain details, the
various authors do agree that air capture has certain inherent advantages over other methods of CO2
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capture, namely, as mentioned, the freedom to operate away from point-sources of emissions. What this
means, in application, is that air capture can take place relatively close to if not at the location of its
ultimate storage. Such placement would eliminate the cost, time, and legal and environmental challenges
involved in constructing an infrastructure of CO2 pipeline networks, as air capture would take place near
carbon sinks, which are located in rural, sparsely-populated areas of the world (versus anthropogenic
carbon sources, which are located in more densely-populated and often urban centers) (Lackner et al.,
2012). This all has the potential to make such geoengineering projects more practical for obtaining public
support, reducing the “‘NIMBY [not in my backyard]’ effects,” as Lackner et al. (2012) put it.
As a recommendation, the next step in investigating direct air capture as a possible
geoengineering strategy is to conduct a preliminary analysis of the specific workings of this carbon
capture method; this should include analyzing the various proposed sorbents to determine which is the
most optimal for both the needs and constraints (monetary, employment, or otherwise) of Global
Solutions. Focusing mainly on the method of capturing CO2 gas, this literature review has largely ignored
the specific methods and issues concerning the storage of the carbon. Any complete solution for reducing
atmospheric CO2 levels via carbon capture must of course include a storage method, and this will also
need to be investigated and analyzed in detail.
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References
American Physical Society. (2011). Direct air capture of CO2 with chemicals: A technology
assessment for the APS Panel on Public Affairs. Washington, DC.
Goeppert, A., Czaun, M., May, R. B., Prakash, S. G. K., & Olah, G. A. (2011). Carbon dioxide capture
from the air using a polyamine based regenerable solid adsorbent. Journal of the American
Chemical Society, 133, 20164-20167. doi:10.1021/ja2100005
Goeppert, A., Czaun, M., Prakash, S. G. K., & Olah, G. A. (2012). Air as the renewable carbon source of
the future: an overview of CO2 capture from the atmosphere. Energy & Environmental Science, 5,
7833-7853. doi:10.1039/C2EE21586A
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Lenton, T. (October, 2009). The climate cooling potential of different geoengineering options. Presented
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