Authors:
David Sandalow
Center on Global Energy Policy,
Columbia University
Chair, ICEF Innovation Roadmap
Project
Julio Friedmann
Center on Global Energy Policy,
Columbia University
Energy Futures Initiative
Colin McCormick
Walsh School of Foreign Service,
Georgetown University
Valence Strategic
Sean McCoy
Department of Chemical andPetroleum Engineering,
University of Calgary
This roadmap was prepared tofacilitate dialogue at the Fifth
Innovation for Cool Earth Forum(Tokyo October 2018), for final
release at COP-24(Katowice December 2018).

Contents

iv Introduction
1

Chapter 1
Carbon Dioxide Removal Technologies

9

Chapter 2
Direct Air Capture Technologies

15

Chapter 3
Advantages of Direct Air Capture

4
19 Chapter
Challenges Facing Direct Air Capture
5
21 Chapter
Direct Air Capture Technology Pathways
631 Chapter
Policy Considerations
7
37 Chapter
Findings and Recommendations

December 2018iii

Concentrations of carbon dioxide in theatmosphere have reached their highest
levels in roughly three million years.
Those concentrations continue to climb, year after year.
The Intergovernmental Panel on Climate Change (IPCC)
warns of extraordinary risks (including from storms,
floods, droughts, heat waves and sea level rise) unlessthe buildup of carbon dioxide in the atmosphere slows
and then reverses in the decades ahead.1
More than 175 nations have now ratified the Paris
Agreement, which calls for countries to achieve net
zero emissions of greenhouse gases in the second halfof this century. However, progress towards that goal isslow. Few major economies are now meeting their initial
reduction targets under the Paris Agreement. Strategies
for deep decarbonization of some sectors of the global
economy remain unclear.
Against this backdrop, carbon dioxide removal (CDR)
has grown in prominence as a component of global
climate strategies. CDR refers to a range of approaches
for drawing carbon dioxide (CO2) from the atmosphereusing biological, engineered or hybrid means. Examplesof CDR include afforestation, bioenergy with carbon
capture and storage (BECCS), and direct air capture ofcarbon dioxide (DAC). Removal of carbon dioxide fromthe atmosphere has the potential to play a significant
role in climate mitigation, supplementing the many
vitally important strategies for reducing and preventing
emissions of carbon dioxide in the first place. Indeed
the IPCC’s Global Warming of 1.5°C report (2018) statesthat 100-1000 gigatons (GT) of carbon dioxide removal
will be required this century to prevent global averagetemperatures from climbing 1.5°C (2.7°F) above preindustrial levels.2
Carbon dioxide can be captured directly from airusing chemicals, refrigeration or membranes. Some ofthese techniques date to at least the 1930s, when air
separation units designed to produce oxygen produced
CO2 as a byproduct. (This CO2 was often sold to the foodand beverage industry.) Atmospheric CO2 scrubbers haveiv

This roadmap explores the potential for direct air
capture of carbon dioxide to contribute to climate
mitigation (and provide feedstock for commercial
processes). Chapter 1 of the roadmap examines theneed for carbon dioxide removal broadly and surveys therange of CDR approaches available. Chapter 2 describes
direct air capture technologies. Chapter 3 discusses theadvantages of direct air capture. Chapter 4 explores
challenges facing direct air capture, and Chapter 5
considers research pathways for addressing them.
Chapter 6 discusses policy options. Chapter 7 offers
findings and recommendations.
1

See IPCC, Global Warming of 1.5°C: Summary for Policy
Makers (2018) at p.13, http://report.ipcc.ch/sr15/pdf/
sr15_spm_final.pdf, IPCC, Fifth Assessment Report
Working Group 1 – Information from Paleoclimate
Records (2013) at p. 394, https://www.ipcc.ch/site/
assets/uploads/2018/02/WG1AR5_Chapter05_FINAL.
pdf; NASA, Carbon Dioxide Hits New High, https://climate.nasa.gov/climate_resources/7/graphic-carbondioxide-hits-new-high.

2

IPCC, Global Warming of 1.5°C: Summary for Policy
Makers (2018) at p.19, http://report.ipcc.ch/sr15/pdf/
sr15_spm_final.pdf.

source: nasa.gov

Introduction

been used in submarines since the 1940s and spaceships
since the 1950s. The movie Apollo 13 recounts a
dramatic effort to design a CO2 scrubber aboard adamaged spacecraft en route to the Moon.

Figure 1-1. The improvised carbon dioxide scrubber onboard the Apollo 13 lunar module.

December 2018

Chapter 1
Carbon Dioxide
Removal Technologies
A. Need for Carbon Dioxide Removal
Global average temperatures are increasing due toemissions of greenhouse gases, the most important of
which is CO2.1 In the Paris Agreement, more than 180
countries agreed to:“hold the increase in the global averagetemperature to well below 2°C above pre-industrial
levels and pursue efforts to limit the temperature
increase to 1.5°C above pre-industrial levels.” 2
The IPCC estimates that, in order to have a 67% chanceof preventing global average temperatures fromincreasing 2°C (3.6°F) above pre-industrial levels (the“below 2°C carbon budget”), emissions of CO2 after the
beginning of 2018 must be kept below 1170 GtCO2 (319GtC).3 The IPCC also estimates that, in order to have a
67% chance of preventing global average temperaturesfrom increasing 1.5°C (2.7°F) above pre-industrial levels(the “below 1.5°C carbon budget"), emissions of CO2
after the beginning of 2018 must be kept below 420
GtCO2 (114 GtC).4
To put this in perspective, annual CO2 emissions fromhuman activities are roughly 42 Gt (11 GtC).5 Thus, theremaining amount of the below 2°C carbon budget is
equivalent to roughly 28 years of global CO2 emissions atcurrent rates. The remaining amount in the below 1.5°C
carbon budget is equivalent to roughly 10 years of global
CO2 emissions at current rates.6
Both these estimates are subject to uncertainties due
to factors including differences in radiative forcing
estimates, pathways for emissions of non-CO2 gases(e.g., methane, nitrous oxide and aerosols) and otherfactors. (See Box 1.)
Although there are uncertainties with respect to theprecise values of the carbon budget for any stabilization
target,11 there is no uncertainty with respect to threeimportant conclusions:
1. to keep global average temperatures from increasing
2°C (3.6°F) or 1.5°C (2.7°F) above pre-industrial
levels , the remaining carbon budgets are small in
relation to past human emissions of greenhouse gas,
2. with each passing year those budgets grow smaller,and3. the time remaining before we exhaust those carbon
budgets is very small compared to the time required
to transition to a zero-emissions society.12
The primary motivation for CDR is the expectation that
CO2 emissions will not be reduced rapidly enough or to alevel low enough to keep within the carbon budget. Thisimplies that withdrawing CO2 from the atmosphere will
be necessary in the future.13 Future climate scenariosthat seek to limit temperature increases to 2°C (3.6°F)

BOX 1 The impact of non-CO2 greenhousegases on the carbon budget
Non-CO2 GHGs and aerosols have a strong impact on climate but also tend to be short-lived.
For example, methane (CH4) and nitrous oxide (N2O) have lifetimes in the atmosphereof around 12 years and 121 years, respectively.7 In contrast, a substantial fraction of
CO2 emitted today will persist for millennia, contributing to warming over that entire
time8—a factor in determining the carbon budget. If all non-CO2 GHG and aerosol
emissions were halted today, their climate impact would diminish within decades,
while CO2 emissions would continue to warm the planet for centuries.9 Smalldifferences in the future emissions trajectories of non-CO2 GHGs and aerosols canhave a strong impact on the carbon budget. Both CO2 and non-CO2 greenhouse gas
emissions must be addressed to tackle climate change.10

December 20181

(e.g., IPCC RCP2.6) reflect this expectation, as mostrequire CDR technologies to deliver “net negative”
emissions after 2050.14 (See Box 2.)
Even if emissions were aggressively reduced—far
more than implied by current nationally determined
contributions (NDCs)—CDR would be required to achieve
warming limited to 1.5°C (2.7°F).15 Indeed, in its Global
Warming of 1.5°C report (2018), the IPCC found that all
pathways to a 1.5°C (2.7°F) stabilization target requiresignificant CDR—between 100 and 1000 GtCO2 through2100.16
While the primary motivation for CDR is the expectationthat we will be unable to meet the constraints imposedby the carbon budget, some CDR approaches could
also serve to reduce the impacts of CO2 emissions.17 For

Cost

example, ocean alkalinity modification techniques could
help offset ocean acidification resulting from the mixingof CO2 from the atmosphere into the ocean. Similarly,
approaches to increase soil carbon could also increase theability of soil to retain water, improve plant growth andlimit runoff during extreme precipitation events, which
are expected to increase as a result of climate change.
As this section highlights, much of the carbon budget
available for limiting warming to 2°C (3.6°F) has alreadybeen exhausted and the remaining space in the budget
is shrinking rapidly. Ambitious mitigation efforts are
required to stretch this remaining budget for as longas possible. CDR pathways are not a substitute forconventional mitigation approaches but will be needed
as a supplement to deep emissions reductions.18

EnergyRequirements

Land
Use

Water
Risk ofConsumption Reversal

Veriﬁability

Implement
Readiness

NATURAL

Reforestation & Enhanced
Forest Management
Wetland & Coastal
Restoration

2

HYBRIDCOTECHNOLOGICAL

Soil Carbon
Restoration

LEGEND

DACS
Terrestrial Enhanced
Weathering
Ocean Alkalinity
Modification
Hybrid Bioenergy
with CCS (BECCS)
Bioenergy with Biochar
Sequestration (BEBCS)
Generally Acceptable/ Available

Exercise Caution

Potentially Unacceptable/ Unavailable

Figure 1-2. Summary table of pathways in the CDR portfolio, highlighting strengths, weaknesses and indicativetechnical potential.

2

December 2018

BOX 2 What does “net-negative” mean?
The term “net-negative” appears in both the discussion of carbon dioxide removal (CDR) technologies and longterm emissions trajectories.
CDR technologies are often viewed as simply a mechanism for removing CO2 from the atmosphere. Unfortunately,
they will also be responsible for emitting some amount of CO2 themselves. For example, direct air capture (DAC)
plants will be built from plastics, steel and cement, the production of which emits CO2 and may consume energy thatis not from zero-carbon sources. Therefore, the correct way to consider the climate impact of CDR technologies is interms of “net” CO2 removal, which is the amount of CO2 removed minus any corresponding
emissions resulting from removal or as a consequence of removal. This highlights theimportance of examining the removal process from a full lifecycle perspective.
If enough CDR is deployed, global emissions could also become “net-negative.”
This means that emissions from all human activities—such as power generation,industry, transport and CDR—are smaller than the amount of CO2 removed fromthe atmosphere. The result would be a decrease in atmospheric concentrations ofCO2. For the same “net-negative” emissions target, the lower the emissions fromhuman activities, the lower the required CO2 removal rate.

B. Carbon Dioxide Removal Portfolio
Direct air capture (DAC) is one of several CDR
pathways.19 (See Figure 1-2.) No single CDR pathway will
likely be able to deliver all the carbon dioxide removal
needed to stay within the below 2°C budget.20
These pathways were the subject of a major study
by the U.S. National Academies to assess the state ofcurrent CDR approaches and to develop a research,development and demonstration (RD&D) agenda formany key CDR pathways.21 The study concluded that all
approaches have merits and challenges. Importantly,
the study concluded that neither the U.S. nor the world
can achieve the required rate and volume of CDR using
pathways available today below a $100/tCO2 cost. For
all 2°C pathways, additional deployment of CDR was
needed at costs above $100/tCO2 today. The reportconcluded that innovation, research, development andinvestment are needed to reduce cost, increase potential
volume and rates, and manage the trade-offs between
land, cost, energy requirements and other important
constraints.
CDR approaches can be grouped into natural,technological and hybrid pathways. Natural pathways
are those in which ecosystems are conserved,
restored or managed to remove carbon dioxide fromthe atmosphere and increase carbon sequestration.
December 2018

Examples of natural pathways include reforestation,
improved forest management, use of cover crops
in farming, changes to animal grazing practices andrestoration of wetlands, peatlands and coastlines (often
referred to as “blue carbon”).22
The cumulative potential of natural pathways to remove
CO2 is large relative to current annual emissions.
Moreover, in addition to removing CO2, many natural
pathways enhance other ecosystem services, such as
regulating water flow or reducing erosion. The potentialof a wide range of natural pathways was recently
estimated at 14 GtCO2/y in 2030 (after accounting
for “safeguards”)23, of which around 5 GtCO2/y wasestimated to cost less than $100/tCO224. However,these pathways are limited by the availability of land,
water and nutrients.25 Moreover, this type of approach
may have drawbacks such as reductions in crop or
grazing land, biodiversity (e.g., replacing virgin forest
with managed plantations) or albedo (i.e., increased
absorption of solar radiation and, thus, local heating).
Many of these pathways are also subject to saturation(CO2 removal from trees slows over time) and may bereversible (e.g., forest fires releasing previously removedCO2), meaning that their aggregate removal rates
will decline over time and could release CO2 under achanging climate.
3

Ocean fertilization, where nutrients are spread overnutrient-limited regions of the ocean—enhancing algal
growth and CO2 uptake26—can also be considered anatural pathway. However, the efficiency of ocean
fertilization appears low, and the activity may havesubstantial negative impacts on the ocean ecosystem.27
Because of the uncertainties about its effectiveness,status in international law and concerns about
unintended consequences, we do not consider ocean
fertilization in this document.
Technological pathways depend fundamentally on
deployment of engineered technological systems toremove CO2. These pathways include DAC28, indirect
ocean capture29, terrestrial-enhanced weathering30 andocean alkalinity modification.31 The cumulative potentialof some of these pathways has been individually
evaluated and is very large, but their practical potential ispoorly understood. For example, accelerated weathering
using mined, crushed silicate rocks spread on suitable
croplands could remove billions of tons of CO2 annuallyat costs well below $100/tCO2.32 At very large scales,
alkaline runoff from such enhanced weathering would
partially mitigate ocean acidification.33 In contrast, ocean
alkalinity modification schemes directly increase thealkalinity of the oceans—mitigating ocean acidification—
while sequestering atmospheric CO2. Some of thesepathways (e.g., electrochemical weathering) also havethe benefit of generating hydrogen.34 However, theprincipal challenge with large-scale (i.e., multiple GtCO2
per year) use of enhanced weathering or ocean alkalinity
modification is the need to mine and process enormous
amounts of virgin rock.35 Additionally, where large-scaleenhanced weathering impacts the oceans, it could be
limited by marine dumping treaties (e.g., the London
Convention and Protocol).
The cumulative removal potential of DAC is constrainedby the available capacity for geological (or other)sequestration, while the rate is limited by power
requirements of DAC and the rate at which CO2 can besequestered. The global geological storage capacity
for CO2 is equivalent to hundreds of years of emissions(though this large theoretical capacity must be resolved
on a case-by-case basis through further exploration andcharacterization).36 Sustainable rates of CO2 injection
into the subsurface are, likewise, controlled by local
factors. However geological storage projects currently
in operation have demonstrated sustained injection

4

rates of millions of tons per year,37 and modelingstudies suggest these projects are not outliers.38 In
addition to geological storage, conversion of CO2 into
aggregates and durable products could also be a meansof sequestering CO2, albeit at a much smaller scale.39
This suggests that DAC with sequestration (“DACS”)
could play an important role as a “backstop” technology
in an overall climate strategy (see Chapter 3 for further
discussion).
Hybrid pathways harness both ecosystems andtechnological systems to remove CO2. This category
encompasses bioenergy with carbon capture and storage(BECCS),40 in which biomass is converted to electricity or
fuels and the resulting CO2 is captured and geologicallystored; and bioenergy with biochar carbon sequestration(BEBCS), where agricultural soils are amended withcharcoal derived from bioenergy processes.41 The captureand subsequent sequestration of CO2 released frombioenergy and small-scale production of biochar (foruse as a soil amendment) are undertaken today. A wide
range of studies suggests that both BECCS and BEBCS
could be deployed at levels sufficient to remove billions
of tons of CO2 annually.42 In both of these cases, factors
limiting large-scale deployment will likely be competition
for land and water and nutrient demand for biomass
production.43 Biomass is used widely as a fuel today,
supplying approximately 10% of primary energy demand
globally (55 EJ/y) in 2015.44 Creutzig et al.45 report that
there is a “medium level of agreement” that this could
be increased up to 300 EJ/y in a sustainable manner.
Reversibility of carbon removal is a question that isfrequently raised in the context of DACS, BECCS and
BEBCS, as well as in natural pathways, as discussed
above. While carbon contained in biochar tends to berelatively stable in the soil, the factors affecting thepermanence of carbon sequestration, the relationshipbetween soil types and productivity benefits of biochar
application, and the relationship between feedstocks andproduction conditions needs to be better understood.46
Conversely, the permanence of geological storage andthe associated risks are relatively well understood.47
Research and development, demonstration projects,
and analogues support the expectation that CO2 stored
in well-characterized geological formations using bestpractices will be retained for geological time scales withrelatively low (and manageable) risks.48

December 2018

In summary, while it is not possible to sum up thecurrent estimates of potential for each of thesepathways, it appears that a relatively low-cost portfolioof CDR activities could be created to remove billions oftons of CO2 per year from the atmosphere. Given thelarge removal potential associated with DAC coupled
to geological CO2 storage relative to the other CDR
pathways, DAC does set an upper limit on the costof climate mitigation.49 However, DAC does not offer
the co-benefits that are associated with some otherpathways, ranging from energy production (e.g.,
BECCS, electrochemical weathering) to enhancementof ecosystem services (e.g., forest conservation andreforestation). The ultimate composition of the CDR
technology portfolio depends on future developments intechnology, policy and behavior.
1

Matthew Collins et al., “Long-Term Climate Change:
Projections, Commitments and Irreversibility,” Climate
Change 2013 - The Physical Science Basis, by IPCC
(Cambridge, United Kingdom: Cambridge University
Press, 2013) at p.1029–1136, https://doi.org/10.1017/
CBO9781107415324.024.

2

Paris Agreement, Article 2.1(a), https://unfccc.int/sites/
default/files/english_paris_agreement.pdf.

3

IPCC, Global Warming of 1.5°C, Chapter 2: Mitigation
pathways compatible with 1.5°C in the context
of sustainable development (2018), Table 2.2 at
p.108, https://www.ipcc.ch/site/assets/uploads/
sites/2/2018/11/SR15_Chapter2_Low_Res.pdf.

4IPPC, op. cit. (Global Warming of 1.5°C, Chapter 2).

5

IPCC, Global Warming of 1.5°C: Summary for Policy
Makers (2018) at p.14, http://report.ipcc.ch/sr15/pdf/
sr15_spm_final.pdf.

6

IPCC, Global Warming of 1.5°C: Summary for Policy
Makers (2018) at p.14, http://report.ipcc.ch/sr15/
pdf/sr15_spm_final.pdf. See also Richard J. Millar
et al., “Emission Budgets and Pathways Consistent
with Limiting Warming to 1.5 °C,” Nature Geoscience(October 2017) at p.741–47, https://doi.org/10.1038/
ngeo3031; Katarzyna B. Tokarska and Nathan P. Gillett,“Cumulative Carbon Emissions Budgets Consistent with
1.5 °C Global Warming,” Nature Climate Change (April2018) at p.296–99, https://doi.org/10.1038/s41558018-0118-9.

7Gunnar Myhre et al., “Anthropogenic and Natural
Radiative Forcing,” in Climate Change 2013: The
Physical Science Basis. Contribution of Working
Group I to the Fifth Assessment Report of the

December 2018

Intergovernmental Panel on Climate Change, ed. T. F.
Stocker et al. (Cambridge, U.K.: Cambridge University
Press, 2013), http://www.climatechange2013.org/
images/report/WG1AR5_Chapter08_FINAL.pdf.
8

F. Joos et al., “Carbon Dioxide and Climate Impulse
Response Functions for the Computation ofGreenhouse Gas Metrics: A Multi-Model Analysis,”Atmospheric Chemistry and Physics (March 2013) at
p.2793–2825, https://doi.org/10.5194/acp-13-27932013.

9

Collins et al., op. cit.

10

Joeri Rogelj et al., “Disentangling the Effects of CO2 andShort-Lived Climate Forcer Mitigation,” Proceedingsof the National Academy of Sciences (October 2014),
201415631, https://doi.org/10.1073/pnas.1415631111.

11

Glen P. Peters, “The ‘Best Available Science’ toInform 1.5 °C Policy Choices,” Nature Climate Change(July 2016) at p.646–49, https://doi.org/10.1038/
nclimate3000.

12

Vaclav Smil, “Examining Energy Transitions: A Dozen
Insights Based on Performance,” Energy Research &Social Science (December 2016) at p.194–97, https://doi.org/10.1016/j.erss.2016.08.017.

13

Sabine Fuss et al., “Betting on Negative Emissions,”
Nature Climate Change (October 2014) at p.850–53,
https://doi.org/10.1038/nclimate2392; T. Gasser etal., “Negative Emissions Physically Needed to Keep
Global Warming below 2 °C,” Nature Communications(August 2015): at p.7958, https://doi.org/10.1038/
ncomms8958.

14

Sabine Fuss et al., “Negative Emissions—Part 2:
Costs, Potentials and Side Effects,” Environmental
Research Letters (May 2018): 063002, https://doi.
org/10.1088/1748-9326/aabf9f.

15

Elmar Kriegler et al., “Pathways Limiting Warming
to 1.5°C: A Tale of Turning around in No Time?,”
Philosophical Transactions of the Royal Society A:
Mathematical, Physical and Engineering Sciences,(May 2018): 20160457, https://doi.org/10.1098/
rsta.2016.0457.

16

IPCC, Global Warming of 1.5°C: Summary for Policy
Makers (2018) at pp.14, 19, http://report.ipcc.ch/sr15/
pdf/sr15_spm_final.pdf.

17

National Academies, 2018, Negative Emissions
Technologies and Reliable Sequestration: A Research
Agenda: https://www.nap.edu/catalog/25259/
negative-emissions-technologies-and-reliablesequestration-a-research-agenda.

18

National Academies 2018, op. cit.

5

19

Jan C. Minx et al., “Negative Emissions—Part 1:
Research Landscape and Synthesis,” Environmental
Research Letters (May 2018): 063001, https://doi.
org/10.1088/1748-9326/aabf9b; National ResearchCouncil, Climate Intervention: Carbon Dioxide
Removal and Reliable Sequestration (Washington,
D.C.: National Academies Press, 2015), https://doi.
org/10.17226/18805.

20

Fuss et al., op. cit. (“Negative Emissions—Part 2”);James Mulligan, Gretchen Ellison and Kelly Levin,“Foundational Questions on Carbon Removal in the
United States” (Washington, D.C.: World Resources
Institute, September 2018), https://www.wri.org/
publication/foundational-questions-on-carbonremoval-usa.

21

National Academies 2018, op. cit.

22

Bronson W. Griscom et al., “Natural Climate
Solutions,” Proceedings of the National Academy of
Sciences (October 2017) at p.11645–50, https://doi.
org/10.1073/pnas.1710465114.

23

Griscom et al. include both approaches that avoid
emissions as well as remove CO2 from the atmospherein their definition of natural climate solutions and also
consider biochar amendment as a natural CDR pathway(which is characterized here as a hybrid pathway). They
report a total of 24 GtCO2 per year of avoided andremoved emissions, of which 14 GtCO2 are removal and
10 GtCO2 are avoided.

a Negative Emissions Technology,” International
Journal of Greenhouse Gas Control (March 2018) at
p.243–53, https://doi.org/10.1016/j.ijggc.2017.10.007;
Matthew D. Eisaman et al., “Indirect Ocean Captureof Atmospheric CO2: Part II. Understanding the Costof Negative Emissions,” International Journal ofGreenhouse Gas Control (March 2018) at p.254–61,
https://doi.org/10.1016/j.ijggc.2018.02.020.
30

R.D. Schuiling and P. Krijgsman, “Enhanced Weathering:
An Effective and Cheap Tool to Sequester CO2,” Climatic
Change (January 2006) at p.349–54, https://doi.
org/10.1007/s10584-005-3485-y.

31Phil Renforth and Gideon Henderson, “Assessing
Ocean Alkalinity for Carbon Sequestration,” Reviewsof Geophysics (September 2017): 2016RG000533,
https://doi.org/10.1002/2016RG000533; Haroon S.
Kheshgi, “Sequestering Atmospheric Carbon Dioxide
by Increasing Ocean Alkalinity,” Energy (September
1995) at p.915–22, https://doi.org/10.1016/03605442(95)00035-F; Kurt Zenz House et al.,“Electrochemical Acceleration of Chemical Weathering
as an Energetically Feasible Approach to Mitigating
Anthropogenic Climate Change,” Environmental Science& Technology (December 2007) at p.8464–70, https://doi.org/10.1021/es0701816.

32

Jessica Strefler et al., “Potential and Costs of Carbon
Dioxide Removal by Enhanced Weathering of Rocks,”
Environmental Research Letters (March 2018): 034010,
https://doi.org/10.1088/1748-9326/aaa9c4.

24Griscom et al., op. cit.

33

25

Pete Smith et al., “Biophysical and Economic Limits
to Negative CO2 Emissions,” Nature Climate Change(January 2016) at p.42–50, https://doi.org/10.1038/
nclimate2870.

Lyla L. Taylor et al., “Enhanced Weathering
Strategies for Stabilizing Climate and Averting Ocean
Acidification,” Nature Climate Change (April 2016) at
p.402–6, https://doi.org/10.1038/nclimate2882.

34

Greg H. Rau, Heather D. Willauer and Zhiyong Jason
Ren, “The Global Potential for Converting Renewable
Electricity to Negative-CO2-Emissions Hydrogen,”Nature Climate Change (July 2018) at p.621–25, https://doi.org/10.1038/s41558-018-0203-0.

35

Fuss et al., op. cit. (“Negative Emissions—Part 2.”)36

IEAGHG, “CCS Deployment in the Context of Regional
Developments in Meeting Long-Term Climate Change
Objectives” (Cheltenham, UK: IEAGHG Programme,2015), https://ieaghg.org/publications/technicalreports.

37

IEA, “20 Years of Carbon Capture and Storage:
Accelerating Future Deployment” (Paris, France:
International Energy Agency, November 15, 2016),
https://webstore.iea.org/20-years-of-carbon-captureand-storage.

26

Daniel P. Harrison, “Global Negative Emissions Capacityof Ocean Macronutrient Fertilization,” Environmental
Research Letters (February 2017): 035001, https://doi.
org/10.1088/1748-9326/aa5ef5.

27

Fuss et al., op. cit. (“Negative Emissions—Part 2”);
Aaron Strong, John Cullen and Sallie Chisholm,“Ocean Fertilization: Science, Policy, and Commerce,”
Oceanography (September 2009) at p.236–61, https://doi.org/10.5670/oceanog.2009.83.

28

29

6

Robert Socolow et al., “Direct Air Capture of CO2 with
Chemicals,” American Physical Society (June 2011),
https://www.aps.org/policy/reports/assessments/
index.cfm.
Charles-Francois de Lannoy et al., “Indirect Ocean
Capture of Atmospheric CO2: Part I. Prototype ofDecember 2018

38

39

Simon A. Mathias et al., “Impact of Maximum Allowable
Cost on CO2 Storage Capacity in Saline Formations,”
Environmental Science & Technology (November2015) at p.13510–18, https://doi.org/10.1021/acs.
est.5b02836; Júlio Carneiro et al., “Injection Rates
and Cost Estimates for CO2 Storage in the West
Mediterranean Region,” Environmental Earth Sciences(March 2015) at p.2951-2962, https://doi.org/10.1007/
s12665-015-4029-z.
David Sandalow et al., “Carbon Dioxide Utilization(CO2U): ICEF Roadmap 2.0” (Innovation for Cool Earth
Forum, November 2017), http://www.icef-forum.org/
platform/upload/CO2U_Roadmap_ICEF2017.pdf; Issam
Dairanieh et al., “Carbon Dioxide Utilization (CO2U)–ICEF Roadmap 1.0” (Tokyo, Japan: ICEF Innovation
Roadmap Project, 2016), http://www.icef-forum.org/
platform/upload/CO2U_Roadmap_ICEF2016.pdf.

40

M. Obersteiner et al., “Managing Climate Risk”(Laxenburg, Austria: International Institute for Applied
Systems Analysis, 2001).

41

Dominic Woolf et al., “Sustainable Biochar to Mitigate
Global Climate Change,” Nature Communications(August 2010) at p.56, https://doi.org/10.1038/
ncomms1053.

42

Fuss et al., op. cit. (“Negative Emissions—Part 2”);
Dominic Woolf, Johannes Lehmann and David R.
Lee, “Optimal Bioenergy Power Generation for
Climate Change Mitigation with or without Carbon
Sequestration,” Nature Communications (October2016): 13160.

43

Smith et al., “Biophysical and Economic Limits to
Negative CO2 Emissions,” Nature Climate Change(December 2015) at p.42-50, https://doi.org/10.1038/
nclimate2870; Felix Creutzig et al., “Bioenergy andClimate Change Mitigation: An Assessment,” GCB

December 2018

Bioenergy (September 2015) at p.916–44, https://doi.
org/10.1111/gcbb.12205.
44

IEA, “IEA World Energy Balance,” accessed July 28,
2017, https://www.iea.org/Sankey/.

45“Bioenergy and Climate Change Mitigation,” op cit.

46

Patrick Brassard, Stéphane Godbout and Vijaya
Raghavan, “Soil Biochar Amendment as a Climate
Change Mitigation Tool: Key Parameters andMechanisms Involved,” Journal of Environmental
Management (October 2016) at p.484–97, https://doi.org/10.1016/j.jenvman.2016.06.063; Lori A.
Biederman and W. Stanley Harpole, “Biochar and Its
Effects on Plant Productivity and Nutrient Cycling: A
Meta-Analysis,” GCB Bioenergy 5, no. 2 (2013): 202–14,
https://doi.org/10.1111/gcbb.12037.

47

Mai Bui et al., “Carbon Capture and Storage (CCS):
The Way Forward,” Energy & Environmental Science(March 2018) at p.1062-1176, https://doi.org/10.1039/
C7EE02342A.

48

Rajesh J. Pawar et al., “Recent Advances in Risk
Assessment and Risk Management of Geologic CO2
Storage,” International Journal of Greenhouse Gas
Control (September 2015) at p.292–311, https://doi.
org/10.1016/j.ijggc.2015.06.014; D. G. Jones et al.,“Developments since 2005 in Understanding Potential
Environmental Impacts of CO2 Leakage from Geological
Storage,” International Journal of Greenhouse Gas
Control (September 2015) at p.350–77, https://doi.
org/10.1016/j.ijggc.2015.05.032.

49

D. W. Keith, M. Ha-Duong and J. K. Stolaroff, “Climate
Strategy with CO2 Capture from the Air,” Climatic
Change (January 2006) at p.17–45, https://doi.
org/10.1007/s10584-005-9026-x.

7

■ Cryogenic: Because CO2 has a relatively high freezing

Chapter 2
Direct Air CaptureTechnologies
A. What is Direct Air Capture?
Direct air capture (DAC) is the physical or chemical
separation and concentration of CO2 from ambient air.
After capture, this CO2 can be stored underground, used
for enhanced oil recovery, or used to make products
such as chemicals, fuels and cement. (See Figure 2-1.)
DAC is distinct from “point-source” carbon capture andsequestration (CCS) because it removes CO2 from ambient
air, not flue gas. Today, the community of investigatorsand practitioners has focused on three different classes ofapproaches to separate CO2 from the air:
■ Chemical: In this class of separations, CO2 in the

air reacts with liquid solvents or solid sorbents,
temporarily binding it. The solvent or sorbent is thenheated or subjected to a vacuum, releasing the CO2 forconcentration. This approach is similar to point-source
carbon capture systems that remove CO2 from flue gas.

temperature for a gas, it can be frozen out of the air.
Currently, CO2 is recovered from air by freezing, as a
byproduct of cryogenic oxygen separation.
■ Membranes: CO2 can be separated from air andseawater using membranes, including ionic exchange
and reverse osmosis membranes. This mimics theway plants and animals separate CO2. Today, CO2
is separated from seawater during conventional
desalination.
The companies most actively engaged in DAC mostly
favor chemical approaches, using either liquid solvents
or solid sorbents. Since heat and power are required
to regenerate the key chemical agents, the goal ofmany companies and researchers is to improve CO2
loadings, reduce input energy requirements and costs,
and improve concentrations of CO2 in the produced gas
mixture.
Currently, mainstream DAC technologies are based onreversible chemical sorbents that can be cycled manytimes to capture and release CO2. The choice of sorbent
material is an extremely important part of DAC systemdesign since it determines most other aspects of theoverall system. Chemical sorbents are of great interestin flue-gas carbon capture, and there are significant
research, development and demonstration (RD&D)

Low CO2AI R

AMBIENT AIR
ENERGYAir
Contactor
CO2 Utilization

CO2 Geological Storage

Figure 2-1. Direct Air Capture of Carbon Dioxide.

December 20189

efforts to develop new variants of these materials. Whilesome of this is helpful for DAC design, research on newmaterials specifically for DAC is extremely limited.1
The primary categories of chemical sorbents that areused in DAC designs are aqueous hydroxides, solidsupported amines and solid alkali carbonates (bulk andsupported).2 Research on flue-gas capture technology
has also included physiosorbent materials, such aszeolites and metal-organic frameworks (MOFs), which
typically bind CO2 much more weakly than chemicalsorbents. However, the limited amount of research on
using these for DAC has produced discouraging results
since they appear to perform poorly at the very low CO2
concentrations of ambient air and are also inhibited by
the presence of atmospheric moisture.3 No current DAC
designs use primarily physiosorbent materials.
For economic and technical reasons, sorbent materials
must be re-used many times, through many cycles
of capturing and releasing CO2. This process tends todegrade the material, reducing its ability to capture CO2
and making it necessary to replenish it over time. An
important design goal in DAC systems is to minimize thisexhaustion of material.
The term used to describe the stage of releasing
captured CO2 from the sorbent material is regeneration.
The most common way to do this is by heating thesorbent material, whether solid or liquid. Where heat is
primarily used to regenerate a solid sorbent the process
is known as temperature-swing adsorption (TSA). Somesorbents can be regenerated by changing the amount of
ambient moisture/humidity, and systems that primarily
use this effect are known as moisture-swing adsorption(MSA). A third way to regenerate sorbents is throughchanging the ambient pressure of the air, which isreferred to as pressure-swing adsorption (PSA).4 Thesemethods can be used in combination, depending on theproperties of the sorbent material.
A crucial component of DAC designs is how the sorbents
are brought into contact with ambient air. This is doneusing an air contactor, designed to handle both large
volumes of air throughput and either the flow of a liquidsorbent or the structural support of a solid sorbent. The
structural materials, geometric design, pressure dropand other features of the air contactor are important
challenges for DAC designs and dominate capitalcosts. A further important design consideration is to10

minimize any loss of material to the passing air before
it is released, which could pose environmental hazards
and increases the amount of sorbent that must be
replenished.

B. DAC Technologies Today
This section describes the most prominent DAC designs
to date, including what is known about their sorbent
materials, regeneration strategy, air contactor and otheraspects.
Carbon Engineering. Founded in 2009 with the goalof large-scale DAC deployment, Carbon Engineering is
the only solvent-based cycle currently in the market.
Although the Carbon Engineering design is suited forlarge-scale DACS systems, the company is currently
focused on combining DAC-derived CO2 with fuelsynthesis (air-to-fuels). Based in Squamish, British
Columbia, the company has an operating facility that
produces both synthetic gasoline and diesel and partners
with GreyRock for fuel synthesis (a modified FischerTropsch process).
Much of the system is described in a recent technicalpaper.5 This is the most detailed description of a full
DAC system available in the public literature and is
supported by data from Carbon Engineering’s operating
pilot. The paper reports the levelized cost of thedesign will be between $94 and $232/tCO2 captured
for a mature facility, depending on assumptions about
financing, utilizing captured CO2 to make liquid fuel, andcomponent technology costs.

Figure 2-2. Direct Air Capture system—Carbon Engineering.

December 2018

The sorbent is an aqueous solution of potassiumhydroxide (KOH), which interacts with solid pellets in aslurry reactor that includes a separate aqueous solutionof calcium hydroxide [Ca(OH)2]. A final stage recuperates
the CO2 in a separate high-temperature calcination
regeneration process. The air contactor uses a plastic(low-cost) packing material that is designed to allow
the sorbent solution to flow downwards under gravity
while air is blown across it at right angles (cross-flow
configuration), which is uncommon.
What’s distinctive about this company: Carbon
Engineering is currently the only company pursuinga liquid-solvent-based approach to DAC. This
enables a continuous process which can operate
at steady state (in contrast to solid-sorbent-based
systems, which are inherently a batch process).
Most components of the design are commerciallyavailable, meaning that their cost and performanceare relatively well understood. The proprietary air
contactor is made from low-cost, earth-abundantmaterials, helping reduce capital costs.
The design currently uses a combination of renewableelectricity and natural gas (burnt in an oxyfired calciner)
to provide heat. In this configuration, the requirement
for heat energy is four times greater than for electricalenergy, on a raw energy content basis. The system
can operate in an alternative configuration in which
natural gas also provides power, and the CO2 in the fluegas from the turbine is captured by the system andincluded in the levelized cost of capture (between 30%
and 48% of the CO2 delivered by the overall systemcomes from combusting natural gas, depending on theconfiguration). The company is working to develop apurely electrical process in which high-temperatureelectrical heating provides thermal energy for the
calcination step.
Climeworks. This company has three pilot plants
currently in operation (one in Switzerland, one in Icelandand one in Italy). The first plant captures approximately
900 tCO2/y and sells the CO2 to a nearby greenhouse.6,7The second plant is associated with the CarbFix project,8
capturing approximately 50 tCO2/y and sequestering it.9
The third plant supplies approximately 150 tCO2/y for
production of renewable methane in the framework of
the EU-funded STORE&GO project.10 No cost estimates
are publicly available.

December 2018

Figure 2-3. Direct Air Capture system—Climeworks.

The sorbent is amine supported on solid porous granules
arranged in a proprietary filter. The regeneration isbased on a combined temperature- and pressure-swing
process. The air contactor consists of a set of fans thatmove air horizontally across the sorbent filters. The
system requires 1,800-2,500 kWh of thermal energy (at100°C) and 350-450 kWh of electrical energy per ton of
CO2 (a ratio of four to seven times more thermal energy
than electrical energy).11 The thermal energy is currently
provided by free waste heat from a local incinerator andgeothermal energy in the Swiss and Icelandic projects,
respectively.
What’s distinctive about this company: Climeworks
was the first company to deliver CO2 from DAC as
a commercial product and the first to offer CO2
removal services as a commercial product. Whilethe initial design included sub-optimal components,
there are straightforward paths to improving these(e.g., replacing shell-and-tube heat exchangers withhigher performing units). They have an intrinsicallymodular design and an active factory floor andproduction facility.
The Climeworks Switzerland plant is currently the only
DAC facility in the world operating at near-ktCO2/y scale.
The free waste heat and the revenue from CO2 sales
to the nearby greenhouse suggest that it has favorable
economics. However, it is difficult to extrapolate this toa larger installed base of DAC facilities because niche
opportunities for free waste heat and nearby CO2 offtakesuch as this are limited.

11

Climeworks is focused on food-and-beverage andcarbon-removal services at present and is partneringwith the Gebrüder Meier greenhouse in Switzerland
and the CarbFix project in Iceland. In addition, they haveseveral partnerships developing synthetic fuels, notably
with Audi and Sunfire (an electrocatalytic fuel synthesis
company based in Germany).
Global Thermostat. This company has a demonstration
plant operating in California and is completing apilot plant in Huntsville, Alabama that will capture
approximately 4,000 tCO2/y.12 The plant will produce CO2
for a global food and beverage company. The company
claims that the cost of capture at scale (1 million tCO2/y)would be $50/tCO2, although it has not made details of
its cost estimates public.13
The sorbent is amine supported on a porous ceramic“monolith” structure that is normally used forautomobile catalytic converters. The regeneration isbased on temperature-vacuum swing, using steam at85-100°C to strip CO2 from the amine.14 The energy
requirement is 4.4 GJ of thermal energy and 160 kWh ofelectrical energy per ton of CO2 (a ratio of approximately
eight times more thermal energy than electricalenergy).15
What’s distinctive about this company: Global
Thermostat’s use of mass-produced honeycomb
monoliths for the air contactor enables cheap
manufacturing and provides a high surface area perpressure drop. This reduces the energy requirement
for moving air through the contactor. The company
has a large set of U.S. and international patents anda technology with an intrinsically modular design.

Once commissioned, the facility in Huntsville will be thelargest DAC plant in the world. The energy source usedwill take advantage of the availability of low-temperature
heat at the specific site where it will be installed.
Center for Negative Emissions (Arizona State
University). This research group is developing a DAC
process based on an anionic exchange resin, which isregenerated using moisture swing.16 The group is not
currently affiliated with a for-profit company developing
the technology for commercial distribution. The
estimated costs of the current design are unclear, but
Center leader Klaus Lackner has previously stated they
are $200/tCO2 for initial prototypes, and would fall to$30/tCO2 for Nth-plant through learning-by-doing.17 The
Center has not made engineering details public.
The energy requirement is low, at 50 kJ/mol CO2, or
1.1 GJ/tCO2.18 No estimate is available for the amountof electrical energy required. However, this systemproduces CO2 at low concentration (5%) and does notcompress it. It also consumes water—approximately
5-15 tons/tCO2.19 If a water supply can be secured (atlow cost and energy consumption), then this type ofsystem is particularly suited to arid environments where
the necessary evaporation to drive the moisture-swing
process is favored.
Additional systems under development. Several
additional systems are being developed that are lessmature than the ones discussed previously. These
include the following:■ The VTT Technical Research Centre of Finland has

demonstrated a direct air capture system basedon an amine-functionalized polymer resin sorbent.
The regeneration is based on both temperature andpressure swing, with a day/night capture cycle. This is
not a commercial project but is continuously operatingat 1-2 kg/day. The mean specific energy requirement
for CO2 capture is 89 GJ/tCO2.20,21■ Oak Ridge National Laboratory has demonstrated a

proof-of-concept direct air capture system based on an
aqueous amino acid solution. The CO2-loaded aminoacids are reacted with a guanidine compound, whichleads to the precipitation of a CO2-containing salt. CO2
is released from the salt with low-grade heat (80160°C), making this a liquid-sorbent system with much
lower temperature requirements than hydroxidebased systems.22
Figure 2-4. Direct Air Capture system—Global Thermostat.
12

December 2018

■ The U.S. Naval Research Lab has developed anelectrolysis/ion-exchange system for extracting
both CO2 and hydrogen from seawater. Based on anelectrolytic cation exchange module, a prototype
device has been tested on seawater in Key West,
Florida. The current demonstration device is capableof making one gallon of jet fuel from the CO2 per day,with a CO2 removal rate of 5 tons per year.23

6“World-first Climeworks plant: Capturing CO2 from air
to boost growing vegetables”, Climeworks press release(May 2017): http://www.climeworks.com/wp-content/
uploads/2017/05/02_PR-Climeworks-DAC-Plant-CaseStudy.pdf .

7

C. Marshall, “In Switzerland, a giant new machine issucking carbon directly from the air,” Science (June2017): https://www.sciencemag.org/news/2017/06/
switzerland-giant-new-machine-sucking-carbondirectly-air.

8

T. Nguyen, “Climeworks joins project to trap CO2”,
C&EN (October 2017) at p.15, https://pubs.acs.org/doi/full/10.1021/cen-09541-notw12.

9

J. Tollefson, “Sucking carbon dioxide from air is cheaper
than scientists thought,” Nature (June 2018): https://www.nature.com/articles/d41586-018-05357-w.

10

Climeworks website, accessed November 18, 2018:
http://www.climeworks.com/case-studies/store-andgo/.

11

Y. Ishimoto et al, “Putting Costs of Direct Air Capturein Context,” FCEA Working Paper (June 2017):
https://papers.ssrn.com/sol3/papers.cfm?abstract_
id=2982422.

12

Presentation by Dr. Graciela Chichilniski, CEO, Global
Thermostat, August 7, 2018: https://www.usea.
org/event/global-thermostats-flexible-co2-capturetechnology.

13

Ishimoto, op. cit.

14

W. Li et al., “Steam-Stripping for Regenerationof Supported Amine-Based CO2 Adsorbents,”
ChemSusChem (June 2010) at p.899-903, https://doi.
org/10.1002/cssc.201000131.

15

Ishimoto, op. cit.

16

T. Wang et al., “Moisture-swing sorption for carbondioxide capture from ambient air: a thermodynamic
analysis,” Physical Chemistry Chemical Physics (January2013) at p.504-14, https://doi.org/10.1039/c2cp43124f

17

K. Lackner, “Capture of carbon dioxide from ambient
air,” European Physical Journal (September 2009) at
p.93-106, https://doi.org/10.1140/epjst/e2009-01150-3.

18Ishimoto, op. cit.

19

Wang, op. cit.

20

C. Bajamundi et al., “Direct air capture of CO2:
Opportunities and Challenges,” Presentation (February2017): http://www.neocarbonenergy.fi/wp-content/
uploads/2015/03/Bajamundi.pdf.

■ X (formerly known as Google X) and PARC havedeveloped an electrolytic process for extracting CO2
from seawater.24 Known as Project Foghorn, the system
was focused on utilizing extracted CO2 to make liquid
fuels. The project was ended in 2016 after concludingthat the process was not economically viable.25■ Skytree is a spin-out of the European Space Agency(ESA). Their air capture technology was originallydeveloped to make longer space missions possible
by extracting the CO2 exhaled by astronauts on board
spacecraft. They are focused on indoor air cleanup but
produce a high-CO2 concentration product.26,27■ Infinitree offers a DAC product based on MSA

technology (derived from technology developed at
the Center for Negative Emissions at Arizona State
University) and is focused on the greenhouse market.
They do not appear to have any currently operational
installations.
1

2

3

4

5

D. Sanchez et al., “Federal research, development, anddemonstration priorities for carbon dioxide removal
in the United States,” Environmental Research Letters(January 2018), https://doi.org/10.1088/1748-9326/
aaa08fE. Sanz-Pérez et al., “Direct Capture of CO2 from
Ambient Air,” Chemical Reviews (August 2016)
at p.11840-76, https://doi.org/10.1021/acs.
chemrev.6b00173
A. Kumar et al., “Direct Air Capture of CO2 by
Physisorbent Materials,” Angewandte ChemieInternational Edition in English (October 2015) at
p.14372-7, https://doi.org/10.1002/anie.201506952
L. Ribaldi et al., “Overview on Pressure Swing
Adsorption (PSA) as CO2 Capture Technology: Stateof-the-Art, Limits and Potentials,” Energy Procedia(July 2017) at p.2390-400, https://doi.org/10.1016/j.
egypro.2017.03.1385
D. Keith et al., “A Process for Capturing CO2 from theAtmosphere,” Joule (June 2018) at p.1573-94, https://doi.org/10.1016/j.joule.2018.05.006

December 2018

13

21

P. Simell etl al., ‘Fuels and Chemicals from the Sun
and Air,” Presentation (November 2017): https://
ertc.wraconferences.com/wp-content/uploads/
sites/59/2017/11/Pekka-Simell-VTT-Technical-ResearchCentre-of-Finland.pdf.

24

C.-F. De Lannoy et al., “Indirect ocean capture of
atmospheric CO2: Part I. Prototype of a negativeemissions technology,” International Journal ofGreenhouse Gas Control (March 2018) at p.243-53,
https://doi.org/10.1016/j.ijggc.2017.10.007.

22

F. Brethomé et al., “Direct air capture of CO2 via
aqueous-phase absorption and crystalline-phaserelease using concentrated solar power,” Nature Energy(May 2018) at p.553-9, https://doi.org/10.1038/
s41560-018-0150-z.

25

Eisaman et al., “Indirect ocean capture of atmospheric
CO2: Part II. Understanding the cost of negativeemissions,” International Journal of Greenhouse
Gas Control (March 2018) at p.254-61, https://doi.
org/10.1016/j.ijggc.2018.02.020.

23

H. Willauer et al., “Extraction of Carbon Dioxide andHydrogen from Seawater by an Electrolytic Cation
Exchange Module (E-CEM) Part V: E-CEM Effluent
Discharge Composition as a Function of Electrode
Water Composition,” NRL/MR/6360—17-9743, Naval
Research Laboratory (August 2017): http://www.dtic.
mil/dtic/tr/fulltext/u2/1038769.pdf.

26

Skytree website, accessed November 13, 2018: https://www.skytree.eu/about/.

27“Skytree wins environment category at the Hello
Tomorrow Challenge,” Amsterdam Science Park
(November 2017): https://www.amsterdamsciencepark.
nl/news/skytree-wins-environment-category-at-thehello-tomorrow-challenge/.

14

December 2018

Chapter 3
Advantages of Direct
Air Capture
Because direct air capture (DAC) starts with CO2 capture
from the air in any location, it can play two fundamentalroles that other carbon dioxide removal technologies
cannot. It serves as a backstop technology for managing
climate change and provides CO2 as a feedstock for CO2
utilization (CO2U) applications. Relative to other CDR
approaches, DAC has notable strengths and weaknesses.
The strengths include:■ The cumulative removal potential of DAC is very large

in relation to other CDR pathways.1 These removals
can be largely permanent (where CO2 is geologicallystored or mineralized).
■ The land area requirements for DAC are very small
relative to those for other pathways (unless the energy
for DAC comes from land-intensive energy sources,
such as solar). For example, 12 GtCO2 per year of CDR
through BECCS would require between 380 and 700
million hectares of land for cultivation of dedicatedenergy crops2—an area equivalent to at least half theland area of Australia. Moreover, unlike the productionof bioenergy crops for BECCS (or biochar), DAC can besited on unproductive lands.
■ Water requirements for DAC are far lower thanpathways that harness bioenergy crops for carbon
removal. For example, BECCS requires around 600 m3
of water for each metric ton of CO2 removed—largely
due to biomass cultivation—while evaporative losses
from DAC are likely less than 25 m3/tCO2. Some DAC
approaches produce fresh water as a by-product ofoperation, as will be discussed later.
■ DAC has no direct impacts on nutrient cycling andrequires no application of additional nutrients (suchas nitrogen or phosphorous fertilizers) in contrast to
biomass crop-based pathways,3 ocean fertilization andenhanced weathering.
■ Large annual rates of CO2 removal by DAC could
be sustained for centuries at the global level, as
the geological reservoirs that serve as the sinks for
captured CO2 are very large.4 This contrasts withmany natural pathways, whose removal rates decline
December 2018

over time (even if the carbon that has been removed
remains permanently sequestered).
■ DAC can be sited in a very large number of locations.
The facilities could be sited near high energy resourcesand geologic storage potential (e.g., the Middle East,
U.S. Midwest or offshore) and need not be close topopulation centers or sources of emissions. Moreover,the small footprint of DAC facilities themselves andsimilarities to existing industrial processes meanthat they could often be constructed and operated
at industrial facilities under existing local land-use
regulations.
■ DAC has several technological advantages over fluegas capture. First, DAC does not require the ability tooperate in the presence of high levels of contaminants(particularly SO2, NOx and mercury), which are present
in some flue gas and can degrade sorbent materials.
Second, while flue gas capture systems are usuallydesigned for nearly complete capture of CO2 (because
the gas only passes once through the system before
being released to the atmosphere), DAC systems need
not necessarily achieve near-complete CO2 removal ina single pass of gas through the system. In fact, some
DAC systems are intentionally designed to capture
only a small fraction of CO2 with each pass of ambient

Figure 3-1. DAC facilities can be sited on unproductive
lands, avoiding competiton with food and energy crop
production.
15

A

all global emissions
100% mitigation

B

DACS COSTS TODAY

Total mitigation costs

MAX
TOTAL
COSTS
BY USING
DAC– costTotal mitigation costs

FUTURE DACS COSTS+ cost ($)

MAX
TOTAL
COSTS
BY USING
DAC– cost+ cost ($)

DACS COSTS TODAY

all global emissions
100% mitigation

tons of mitigation required

tons of mitigation required

Figure 3-2. Conceptual cost curves for total climate mitigation with DAC as a backstop technology option.

air through the system (known as “skimming”).
This reduces the energy requirement and may beparticularly appropriate for some CO2 utilization
applications that require low-purity CO2.
■ Measuring the CO2 benefits from DAC is relatively
simple, especially compared to many of thenatural pathways for CO2 removal. The mass of
CO2 removed from the atmosphere by DAC can bedirectly measured, as can any emissions or energy
consumption, at the facility level. Where the removed
CO2 is geologically stored, accounting for net removals
is also relatively simple, provided adequate monitoringis in place to assess any leakage.5 This stands incontrast to natural pathways where avoiding reversals
may require permanent changes to patterns of land
use and sustained effort to manage carbon stocks(e.g., forest management).
■ No international agreements are needed to scale up
DAC. (Such agreements could be essential for largescale implementation of ocean-based CDR strategies.)

save money or avoid costs. The costs grow as more of
the system is decarbonized. When that curve intersects
the present costs of DAC, it is then possible to fully
decarbonize at that price.

These facts create an important setting for DAC—solving
climate change should never be more expensive than
DAC plus sequestration costs (DACS).6 Figure 3-2ashows that the total cost for 100% decarbonization of
the economy is the area under the climate mitigation
cost curve. The mitigation curve begins with negative
costs (revenues) because many efficiency measures

DAC can also provide CO2 as a feedstock for carbon-to-products enterprises.8 Since the cost of capture
from many industrial CO2 sources (e.g., ethanol,ammonia and hydrogen production) is lower thanfor DAC,9 there is an obvious tradeoff: the high priceof DAC-sourced CO2 competes with the benefit itprovides of a reliable, continuous source that can be

16

The green “wedge” represents costs avoided by choosing
DAC above a different, more expensive option (e.g., decarbonizing aviation). The current estimate of the size ofthat wedge, based on the integrated assessment modelsof the IPCC, is in the tens of trillions of dollars to achieve
2°C (3.6°F) stabilization by 2100.7 The DACS price point
also sets the maximum cost for CO2 climate stabilization,with the rate at which stabilization can be achieved set
by the amount invested in DACS beyond mitigation of
total global emissions. Similarly, every increment of cost
reduction associated with innovation and deployment
of DACS drops the total costs for climate mitigation andthe total costs for climate restoration (Figure 3-2b).
This is a strong case for investment in DAC innovationand deployment today, creating optionality for climate
management.

December 2018

delivered anywhere (i.e., independent of pipelines andtrucks). While CO2 supplied from a stationary (point)source is almost certain to have a lower initial cost, thetransportation infrastructure to deliver it may be absent
or expensive (for example, using trucking). DAC could
supply any location anywhere, albeit at higher initial cost
per ton of CO2.
Use of CO2 drawn from the air also offers the potential
for truly carbon-neutral or carbon-negative products,
while CO2 captured from fossil fuel sources can—atbest—reduce emissions. It is also possible that CO2
drawn from the air will be favored by customers whowish to avoid direct use of fossil CO2. As carbon dioxide
utilization (CO2U) technology improves and demand
grows for CO2-based products, the market may place
additional value on products with a negative CO2
emission footprint. In that context, DAC might provide an
additional benefit by meeting customer demand for CO2
sourced from the air.
Some DAC pathways remove substantial amounts ofwater from the air, which is produced as fresh-water
vacuum distillate when CO2 is recovered. This is particularly true for current sorbent systems, which capture
between one and two tons of water for every ton of
CO2 removed. While this has an impact on system-wide
cost and performance (see Chapter 5), in water-scarceregions this may prove to be a very attractive co-benefit
of DAC.
Currently there are few if any viable approaches for deep
decarbonization of some parts of the transportation andagricultural sectors (e.g., airplanes and N2O emissions
from fertilizer). Low-cost viable pathways may emerge,
but that is uncertain.10 DAC provides a certain and securepathway for deep decarbonization of these parts of theeconomy. This underscores the potential role of DAC asa “backstop” technology option—it is not a license tocontinue emissions but rather a strategy to deal withdifficult or expensive emissions proactively and overtly.

December 2018

1

National Academies, 2018, Negative Emissions
Technologies and Reliable Sequestration: A Research
Agenda, https://www.nap.edu/catalog/25259/
negative-emissions-technologies-and-reliablesequestration-a-research-agenda

2Pete Smith et al., “Biophysical and Economic Limits
to Negative CO2 Emissions,” Nature Climate Change(January 2016) at p.42–50, https://doi.org/10.1038/
nclimate2870.

3

Smith et al., op. cit.

4

National Academies 2018, op. cit.

5

Tim Dixon, Sean T. McCoy and Ian Havercroft, “Legaland Regulatory Developments on CCS,” International
Journal of Greenhouse Gas Control, Special Issuecommemorating the 10th year anniversary of the
publication of the Intergovernmental Panel on
Climate Change Special Report on CO2 Capture andStorage (September 2015) at p.431–48, https://doi.
org/10.1016/j.ijggc.2015.05.024.

6

David Keith et al., “Climate Strategy with CO2 Capture
from the Air,” Climatic Change (December 2005) at
p.17-45, https://doi.org/10.1007/s10584-005-9026-x

7

Kruger et al., in review

8

D. Sandalow et al., “Carbon Dioxide Utilization (CO2U):ICEF Roadmap 2.0,” Innovation for a Cool Earth Forum(December 2017), https://e-reports-ext.llnl.gov/
pdf/892916.pdf; I. Dairanieh et al., “Carbon Dioxide
Utilization (CO2U): ICEF Roadmap 1.0,” Innovation for a
Cool Earth Forum (November 2016), https://www.icefforum.org/platform/article_detail.php?article__id=109

9

Leeson et al. 2017, A techno-economic analysis andsystematic review of carbon capture and storage(CCS) applied to the iron and steel, cement, oil
refining, and pulp and paper industries, as well asother high purity sources, Intl. Jo. GHG Control, v.
61, pp 71-84, https://reader.elsevier.com/reader/
sd/pii/ https://reader.elsevier.com/reader/sd/pii/
S175058361730289X?token=FC6C7C2C868A2F698E07
DA69CD8CE4F3FA94B88CF267F57E93087DB79B6909
E67AB8E19F75B3BD532750AD1C40C6815C

10Royal Society, “Greenhouse Gas Removal” (2018):
https://royalsociety.org/topics-policy/projects/
greenhouse-gas-removal/

17

Chapter 4
Challenges Facing
Direct Air Capture
DAC is currently expensive. Today, DAC costs range from$300-600/tCO2, with estimates for future Nth-of-a-kindcosts in the range of $60-250/tCO2.1 DAC is generally
considered to be among the more expensive CDR
pathways.2 Since these costs are well above today’s
carbon market prices, DAC will likely need policies formarket entry and deployment (see below, Chapter 6).
DAC’s high costs reflect the fact that CO2 is much
more dilute in the atmosphere than flue gas. The
concentration of CO2 in the atmosphere is roughly
0.04%, compared to 5% in natural gas-fired fluegas and 12% in coal-fired flue gas. As a result, thetheoretical minimum energy needed to separate CO2from other gases is approximately three times larger
for ambient air than coal-fired flue gas.3 This minimum
energy calculation and others based on real-world
performance4 have suggested to some that DAC could
never be a practical mitigation technology, particularly incomparison to flue-gas capture.
These challenges and possible strategies for addressing
them were recognized in the early literature on DAC.5,6
An important study conducted by the American PhysicalSociety7 attempted to parameterize DAC systems using
extant technology and energy costs. More recent studies
have explored strategies for cost reduction in greater
detail.8

system where it provides CO2 as a feedstock for CO2
utilization.9■ DAC is a relatively large consumer of energy per tonof CO2 removed. Thus, large-scale deployment of
DAC requires an external energy source, such as fossil
fuels (only when their use is integrated with DAC andtheir emissions captured and used or stored), nuclear
energy or renewable resources (e.g., solar, wind,
renewable heat, etc.). There is an opportunity costassociated with dedicating carbon-free energy to use
for DAC rather than to displacing use of fossil-energy(without CCS). Conversely, DAC could be coupled withbioenergy CCS systems to remove CO2 both directly
and indirectly.
The potential of DAC to contribute to limiting CO2
concentrations in the atmosphere depends on several
factors, the most important of which are the carbon
intensity of the energy source and the fate of theremoved CO2. An ideal DAC system, optimized forclimate impact, would use near zero-carbon energy
and sequester removed CO2 in a saline aquifer. In thiscase the net removal would be nearly the same as thegross removal (with minor adjustments for emissions
associated with constructing the system).
By contrast, a DAC system that maximizes economic
value could be substantially different. If it used low-cost
natural gas (with capture and storage) as the energy

Other notable drawbacks for DAC (relative to other CDR
pathways) include:■ Most CDR pathways offer benefits besides CO2

removal. Many natural pathways could help achieve
additional global sustainability goals, such as
improving biodiversity, reducing runoff, improving
agricultural practices and offering a means to adapt
to climate change. Other technological and hybrid
pathways, such as enhanced weathering, BECCS and
BEBCS, generate valuable products, such as electricity,hydrogen and fuels. Coupled with geological storage
in saline aquifers, DAC provides no co-benefits (otherthan some degree of water production as discussed
above), although it could be incorporated into a
December 2018

Figure 3-1. DAC requires more energy per ton than flue-gas
capture, because CO2 is more dilute in the atmospherethan in flue gas.
19source and produced liquid transportation fuel withthe removed CO2 as feedstock, the net removal could
be extremely small or even increase absolute CO2
emissions.
Given that various policies (e.g., the California Low
Carbon Fuel Standard) and consumer preferences may
create value for CO2 as a feedstock, the latter case may
be important in the near-term as a driver for innovationand technology scale-up. However, the climate need for
DAC is ultimately predicated on the permanent removalof CO2 from the atmosphere, implying that it must be
geologically sequestered or mineralized.
A related question concerns the energy requirements
for DAC. To serve a useful climate function, DAC must
operate with sources of heat and power that are
effectively zero-carbon. While this may be most easily
accomplished using renewable energy, there could bealternative approaches using fossil energy under certain
circumstances (see discussion in Chapter 5). The costs,
footprint and operating requirements vary substantiallybased on different assumptions about the energy
source. Since DAC is not well represented in today’s
general equilibrium models, there is little scholarly
economic analysis to inform such questions.
Separately, some scholars and environmentalists areconcerned that the availability of DAC and other CDR
approaches may disincentivize or displace mitigation—
sometimes referred to as “moral hazard.”10 Some haveargued that the availability of CDR might lead to reboundphenomena, reducing incentives to cut CO2 emissions.11
Potential technological pathways for reducing DAC costs
and addressing other challenges in scaling up DAC are
discussed in Chapter 5.

20

1David W. Keith et al., “A Process for Capturing CO2 fromthe Atmosphere,” Joule (August 2018) at p.1573–94,
https://doi.org/10.1016/j.joule.2018.05.006; Robert
Socolow et al., “Direct Air Capture of CO2 with
Chemicals,” American Physical Society (June 2011),
https://www.aps.org/policy/reports/assessments/
index.cfm; National Research Council, Climate
Intervention.

2

Sabine Fuss et al., “Negative Emissions—Part 2:
Costs, Potentials and Side Effects,” Environmental
Research Letters (May 2018): 063002, https://doi.
org/10.1088/1748-9326/aabf9f.

3

Socolow et al., op. cit.

4

Kurt Zenz House et al., “Electrochemical Accelerationof Chemical Weathering as an Energetically Feasible
Approach to Mitigating Anthropogenic Climate
Change,” Environmental Science & Technology(December 2007) at p.8464–70, https://doi.
org/10.1021/es0701816.

5K. S. Lackner et al., “Carbon Dioxide Extraction
from Air: Is It an Option?,” 1999, http://www.osti.
gov/energycitations/servlets/purl/770509-cJeS4J/
webviewable/.

6

K.S. Lackner et al., op. cit.

7

Socolow, op. cit.

8

Keith et al., op. cit.; Socolow et al., op. cit.; National
Research Council, op. cit.

9D. Sandalow et al., “Carbon Dioxide Utilization (CO2U):ICEF Roadmap 2.0,” Innovation for a Cool Earth Forum(December 2017), https://e-reports-ext.llnl.gov/
pdf/892916.pdf.

10

Jan C. Minx et al., “Negative Emissions—Part 1:
Research Landscape and Synthesis,” Environmental
Research Letters (May 2018): 063001, https://doi.
org/10.1088/1748-9326/aabf9b.

11Morrow DR. 2014 Ethical aspects of the mitigation
obstruction argument against climate engineeringresearch. Phil. Trans. R. Soc. A 372: 20140062. http://dx.doi.org/10.1098/rsta.2014.0062.

December 2018

Chapter 5This regeneration energy is directly related to how tightly
the sorbent/solvent binds CO2 (which is determined by
the enthalpy of the sorbent-CO2 reaction). This suggeststhat sorbents that bind CO2 only weakly are preferable
since they require less energy to reverse that reaction
and drive off the CO2 during regeneration.1 However,no sorbent is perfectly selective for the CO2 molecule.
Other molecules, particularly water, can also bind to
the sorbent material, reducing the total amount of
CO2 that can be removed per mass of sorbent. In thecase of ambient air, the very low concentration of CO2
makes this problem particularly severe. Unfortunately,
overcoming it generally requires using a sorbent thatbinds very tightly to CO2 and weakly to other molecules.
Therefore, there is an innate tradeoff between reducing
the energy required for regenerating the sorbent andpreferentially absorbing CO2 over other molecules.

Direct Air CaptureTechnology Pathways
In order to be successful, DAC technology will needto simultaneously achieve low total costs (including
both capital and operating costs) and high amounts ofnet (lifecycle) CO2 removal. These two goals interact
in complex ways and involve tradeoffs that must be
carefully examined. This chapter will explore the possible
technology pathways to achieve these goals and theconstraints they place on DAC design options.

A. Reducing Energy Consumption
As noted in the previous chapter, removing CO2 from theatmosphere takes more energy than removing it froma flue gas stream because it is more dilute. Most of the
DAC designs that have been proposed to date therefore
consume a significant amount of energy per ton of CO2
removed. This energy is the primary contribution tothe operating costs of DAC facilities. Unless this energy
comes from low-carbon sources, DAC cannot result in
net-negative emissions.

Improved sorbent materials are needed that canappropriately balance this tension based on a larger
system design. If energy is available at low cost (and with
low CO2 emissions), then sorbent materials with highselectivity for CO2 would likely be optimal, increasing theamount of CO2 removal per mass of sorbent. If this is not
the case, then sorbent materials with weaker binding to
CO2 would likely be necessary, at the cost of requiring
more sorbent per unit of CO2 removed.

Technology pathways to reduce the energy consumptionof DAC are therefore a high priority. It is important tonote that this energy is mostly in the form of heat; asoutlined in Table 1, most DAC designs require four toeight times more heat energy than electrical energy.
This heat is used to regenerate sorbents or solvents
after they are saturated with CO2. This means themost important goal for reducing energy consumption
is identifying or developing sorbent and/or solvent
materials that require less energy for regeneration.

A further consideration is the speed of reaction (kinetics)of the sorbent. This impacts the rate at which CO2 can beremoved from air passing through the system, and thus
the size of the air contactor required for a given removal
capacity. Sorbents with fast kinetics are therefore
desirable.
For liquid-solvent-based systems, another considerationis the heat capacity of the liquid. It may be possible tofurther reduce heat energy requirements by using a nonaqueous liquid with a lower heat capacity or improving
the heat recovery from the liquid during regeneration.

Table 1: Energy consumption for major DAC systems as reported. Heat:Power ratios are converted on a raw-energy basis.

Company

Thermal energy/ tCO2
(GJ)

Power/ tCO2
(kWh)

Heat: Power ratio

Climeworks

9.0

450

5.6

Ishimoto 2017

Carbon Engineering

5.3

366

4.0

Keith 2018

Global Thermostat

4.4

160

7.6

Ishimoto 2017

APS 2011 NaOH case

6.1

194

8.7

APS 2011

December 2018

Reference

21However, any negative impacts on viscosity, solubility orother factors would have to be considered in order to
identify the overall energy benefits of alternative liquids.

B. Providing Low-Carbon Heat Energy
Given the key role that heat plays in DAC systems, it will
be important to carefully examine options for providing
low-carbon heat energy. These options depend on thequality (temperature) that is required. Some DAC designs(primarily those using amine sorbents) require relatively
low-quality heat (90-120°C), while others (primarily
those using hydroxide-based solvents) require higherquality heat (500-1000°C). Although many government
research, development and demonstration (RD&D)
programs around the world include some attention to
aspects of renewable heat, these are limited and do not
focus on specific applications to DAC systems.
Nuclear fission: Conventional nuclear reactors generatehigh fluxes of zero-carbon heat at medium quality(approximately 320°C), which is used to produce steam
for a Rankine cycle to generate power. This heat ismostly too low quality to be useful for DAC systems
using hydroxide-based solvents and is of significantly
higher quality than the minimum needed for aminebased DAC systems. Ignoring any regulatory or economic
hurdles, conventional reactors therefore may be suitable
for supplying heat to sorbent-based DAC systems
but appear to be poorly suited for supplying heat forhydroxide solvent-based DAC systems.
However, new reactor designs, particularly those being
developed for use as small modular reactors (SMRs),
may be far more suitable. These reactors could bedesigned to deliver heat at the appropriate quality forhydroxide-based solvent DAC. They could also be built at
much smaller scale than conventional reactors, whose
average thermal capacity is approximately 3.5 GW,
likely much larger than a single DAC facility could utilize.
Adding to their appeal, small modular reactor designs
could potentially be deployed in many geographies,
providing local, scale-appropriate, high-capacity heat(and power) for DAC. While this class of reactor has not
yet been deployed widely, it appears to have clearedsome important regulatory hurdles in the United Statesand may begin entering service soon.2 Research on
system design, costs, integration needs and lifecycle
would help resolve key questions regarding DAC
applicability.

22

It is also worth noting that radioisotope thermoelectric
generators (RTGs) used by spacecraft operate at
approximately 1000°C, illustrating the engineering
viability of using nuclear fission to deliver zero-carbon
high-quality heat.3 Unfortunately, these systems appear
to be too small-scale to be material for DAC.
Hydrogen combustion: Because hydrogen burns with avery high temperature (1900-2600°C), renewably-sourcedhydrogen remains a potential source for low-carbon
heat that would be of high enough quality for all DAC
applications. This hydrogen could be derived fromconventional hydrolysis using renewable power (e.g., windand solar) or from conversion of renewable feedstocks(e.g., biomass gasification). Advances in electrolyzers andbiomass gasifiers would help reduce costs and improve
performance of renewable hydrogen, and substantial
RD&D programs exist today on these topics. Naturally,
the challenges that apply to hydrogen systems generally(e.g., pipeline infrastructure, storage systems) would likely
apply to DAC configurations that use hydrogen-derived
heat. Today, systems to produce renewable hydrogen are
expensive ($5-10/kg H2), although there are promising
approaches to cost reduction.4
An alternative to renewable hydrogen is production offossil hydrogen combined with CCS. Most commonly,
fossil-derived hydrogen comes from natural gas
reformation, although some countries (notably
China) produce industrial hydrogen supplies fromcoal gasification. These systems commonly produce
a highly concentrated stream of byproduct CO2. Foursites in the world today capture and store CO2 fromindustrial hydrogen production – Quest, Port Arthur, Al
Reyadah and Uthmaniyah.5 Current costs for hydrogenproduction with CCS provide delivered all-in costs of$1.6/kg H2 with 60% reduction of CO2 compared toconventional steam reforming systems. Importantly,
current estimates to achieve 90% H2 decarbonization
estimate delivery at ~$2.5/kg H2,6,7 substantially less thanthe cost of renewable hydrogen. While this appears to
be a cost-competitive approach to low-carbon hydrogenproduction for heat, lifecycle analyses, including the
upstream emissions associated with natural gas or coalproduction, must be incorporated to fully understandthe viability and limitations of this approach.
Fossil fuel combustion with CCS: Fossil combustion
provides the significant majority of process heat in theworld today.8 When combined with CCS, this can provide
December 2018

low-carbon heat that could potentially be used for
DAC. Notably, one DAC company (Carbon Engineering)uses this approach, combining oxygen-fired natural-gas
combustion with carbon capture as one of its central
design options (delivering CO2 that is a mixture of
the combustion product and gas removed from theatmosphere). The costs and viability of these systems
vary substantially from system to system, based on thefuel (e.g., coal, gas, pet-coke), combustion method,
combustion system design, geography, labor cost,capture system and other constraints. Full lifecycle
analyses and application-specific techno-economic
analysis and designs would help clarify the economicand emissions aspects of this method of providing heat
energy. It may be that this approach has special value
in particular locations, such as countries with stranded
fossil energy assets (e.g., Chad or Indonesia).
Solar thermal energy: Geographic regions withexcellent solar energy resources may be able to takeadvantage of concentrating solar energy to create
low-carbon heat. Today, commercial systems focus onconcentrated solar power (CSP), which uses the solar
heat resource to produce steam for a Rankine cycle togenerate power. These systems are relatively expensive
compared to solar PV systems, but they may have an
advantage in direct heat applications like DAC. Designs
based on parabolic troughs operate at temperaturesof 400°C and above, while those using power tower
technology operate in excess of 500°C.9 Even higher
temperatures are possible with dish-engine technology.
Heat of this high quality may be useful for some solventbased DAC systems. For amine-based DAC systems,
lower quality heat is sufficient, and it may be possible
to use emerging medium-temperature solar thermal
technologies, such as Scheffler reflectors and Fresnel
dishes.10 Conventional rooftop solar thermal systems,
such as those used to provide domestic hot water, aregenerally limited to temperatures that are significantly
lower than CSP but still relevant to amine-based DAC
systems. They may also be a source of pre-treatment to
reduce the overall heat requirements.
Geothermal heat: In locations with favorable geology,
geothermal systems can provide a significant amountof low-carbon heat. Most natural geothermal heat
resources are geographically restricted and are relatively
capital-intensive to exploit. However, where geothermalenergy is available, it is likely to prove a very robust,
December 2018

low-cost, high-capacity approach to providing heat.
Notably, the only full DACS (DAC with sequestration)system in the world (the Climeworks facility in Iceland)
uses both geothermal power and geothermal heatfor operation. The ability of DAC to be deployed inmany geographies could create a market for stranded
geothermal resources (e.g., the Afar triangle in Ethiopia
and La Réunion Island). Additionally, there may be
synergies between geothermal power production andCO2 sequestration, such as CO2 plume geothermal(CPG) power production.11 These options are largely
unexplored and unassessed.
Renewable electricity: Electricity-based heating
technologies are used in a variety of industrial process
heat applications. When supplied with renewableelectricity (such as from wind or solar), this is a form of
low-carbon heat and can leverage the rapid growth ofrenewable power generation, as well as scale rapidly.
The simplest of these (with the lowest capital cost)
is using renewable electricity for resistive heating.
Efficiencies can reach close to 100% for some materials,
and temperatures can exceed 600°C for convectionbased approaches, although it may be challenging todesign systems that heat complex structures, such as air
contactors, evenly.12
Heat pumps: Pumps based on the vapor-compression
cycle can provide process heat up to approximately
100°C, using heat sources at significantly lower
temperatures.13 These systems are commonly used incommercial industrial processes and can achieve highelectrical-to-thermal efficiencies (up to approximately300%) by using waste heat or ambient air heat. Notably,high-temperature air-source heat pumps that canreach 80°C are now commercially available, potentiallyproviding a source of low-carbon heat for amine-based
DAC systems using only air as the heat source. However,
it is important to note that the working fluids for heatpumps are often themselves highly potent greenhousegases (such as hydrofluorocarbons, or HFCs), and any
leakage of these fluids could significantly reduce theoverall climate benefit of the system. Alternatives suchas HFO- or ammonia-based working fluids may mitigate
this concern.
Microwave heating systems: Microwave systems
can provide industrial process heat for a number ofapplications, including drying grain and lumber in the23

agricultural sector and drying powered materials inpharmaceutical manufacturing.14 They are generally
most appropriate for non-electrically conducting
materials, which is the case for DAC sorbents. Since
microwave energy can penetrate many materials, it can
provide more even heating throughout a volume thanapplying heat only at a surface; this may be relevant for
DAC designs that use liquid sorbents. Related techniques
include radio-frequency heating and induction heating,both of which are also fully commercialized and usedindustrially.15
Waste heat: Heat that is rejected to the environment as
a byproduct of industrial activity is a good candidate foruse by DAC facilities. Generally, waste heat is considered
low-carbon, since it was not otherwise being utilized. Avery large amount of waste heat is theoretically available;
for example, there were approximately 25 quads ofwaste heat in the U.S. electricity generation sector in2017.16 A significant fraction of this is rejected at thesteam cycle condenser at a temperature of roughly 40°C,
which is not directly useful for DAC. However, this may
leave a substantial amount available at an appropriate
temperature for amine-based DAC. Industrial processes,
such as refining, cement production and aluminum
smelting, also generate large amounts of waste heat in
the United States—as much as 13 quads in 2017.

C. Utilizing Intermittent Renewable
Electricity
The increasing penetration of renewable electricity
on many grids has led to curtailment, which isconsidered “excess” or “unusable” electricity. Someanalysts consider this an opportunity for DAC, because
this otherwise-wasted zero-carbon electricity could
potentially be used to power DAC plants. This conclusion
appears to be strengthened by the fact that DAC facilities
have significant flexibility in their location and could
thus potentially be sited in areas with large amounts ofcurtailed renewable electricity (see below).18,19
However, curtailed electricity is intermittent and noteasy to predict on short timescales (seasonal curtailment
is generally more predictable).20 This implies that
DAC facilities using intermittent curtailed renewableelectricity would either have to ramp up and downin operation during the day or draw on other powersources—presumably grid-supplied—to operatecontinuously. From a capital-utilization viewpoint,intermittent operation reduces the overall capacityof a facility to remove CO2, thus increasing the capitalrequired to build a plant achieving a given average
removal rate. Also, ramping up and down the operationof DAC would likely introduce additional wear and tearon components that would increase O&M costs.

It is therefore no coincidence that two of the leading
DAC companies (Climeworks and Global Thermostat)use waste heat for regenerating their sorbents. Per Table
1 above, these processes consume approximately 7 GJof heat energy (plus a significantly smaller amount ofelectricity) per ton of CO2 removed, suggesting that 1
quad of waste heat (at appropriate quality) could supply
the thermal energy required to operate DAC facilities
with a capacity of 150 MtCO2/year. While electricalenergy would still be required and there would be
increased capital costs from heat-exchanger surfaces,
the scale of waste heat in terms of its potential for DAC
is clearly very large.

It may be possible to develop DAC plants with either
operational strategies or hardware additions to better
match an intermittent power supply. Operationally,
this could include strategies where the regeneration of
the sorbent is scheduled for times that best match theavailability of curtailed power, if that can be coarsely
predicted (such as day-night variation). Alternatively,
additional equipment to provide electrical energy
storage (such as batteries) or thermal energy storage(such as phase-change materials) could be used tosmooth the delivery of energy to the DAC facility andbuffer energy-supply variability, although this would
increase the capital cost of the facility.

Although the recent National Academies report
investigates some sources of low-carbon heat intheir cost assessment, this work represents earlyconsideration of current system designs, cost andapproaches. Substantial additional work is required aspart of a DAC development pathway that is both lowcost and low-carbon footprint.17

However, curtailment of renewable electricity appears
to be a phenomenon that decreases over time, astransmission lines are built to access stranded powerand grid operators become more adept at integrating
variable supplies.21 Therefore, this apparent “wasted”
resource may not be available to a DAC facility
permanently, undermining the engineering andeconomic logic of siting the facility near locations with24

December 2018

high amounts of curtailment. A better understanding of
the status and causes of curtailed renewables is needed
before pursuing any large-scale strategy of using thisenergy source for DAC.

D. Reducing Capital Costs Through Air
Contactor Design
In addition to the energy considerations for the sorbent/
solvent material discussed above, the rate of the CO2
binding reaction is another important constraint. If
this rate is slow, then building a system that achieves abenchmark capacity of a certain number of tons of CO2
removed per day would require a much larger area for
air-sorbent/solvent contact. Conversely, if the binding
reaction rate is fast, this area can be reduced. A larger
contact area results directly in increased capital costs
for a DAC plant, suggesting that a fast reaction is very
desirable.22 The design of an efficient air contactor must
therefore be based on the choice of sorbent/solvent, as
well as on its form (solid or liquid).
The geometry and blowing/pumping strategy will
determine the efficiency with which air contacts thesorbent/solvent material, the time over which thisoccurs and the energy required, and thus should be acentral focus of design attention. Additionally, while
liquid sorbents can be pumped away from the air
contactor to a separate location for regeneration, solid
sorbents must generally be regenerated in situ, which
implies certain design constraints for the air contactor
and may also reduce the fraction of time during which it
can remove CO2. The structural materials choice for the
air contactor is also an important determinant of capitalcost, since this is by far the largest physical componentof the overall facility; as noted earlier, some designs
appear to offer significant cost savings by using plastic
structural materials in place of steel.
Some loss of sorbent/solvent material to passing
air is inevitable in any real-world design, and thismust be taken into consideration for understanding
both operations and maintenance (O&M) costs (forreplacement) and environmental impacts (for downwind
areas). This issue is of particular concern for caustic
sorbents that can become entrained in passing air andare dangerous when inhaled.23 While initial reports
from one company (Carbon Engineering) indicate thisentrainment is low for their air contactor design, it will
require more testing to validate a variety of designs.

December 2018

E. Identifying Optimal Locations for DAC
Since DAC uses the atmosphere as the source gas forCO2 removal, it can in principle be located anywhere on
the Earth’s surface. However, there are many practicalconsiderations that limit this, and identifying the optimal
locations for DAC facilities will be important for reducing
their costs and increasing their lifecycle CO2 removal
total. These can be expressed as a series of tradeoffs or
as coefficients favoring or discounting sites. The primaryconsiderations are:
■ The availability of low-carbon heat and power—

particularly waste heat as discussed above—is ahigh-priority consideration for siting. Waste heatmust generally be immediately next to a DAC facility
to be used, since it is difficult to transport, so this is
the most restrictive factor. It also directly impacts theoperating costs, since it reduces the energy that mustbe purchased. The availability of low-carbon power
depends on the fuel mix of the local grid, while theavailability of curtailed renewable power may dependon the topology of grid transmission lines. These areboth looser constraints than waste heat. They also
primarily impact the lifecycle CO2 removal amountrather than the operating costs.
■ CO2 offtake, transport or storage opportunities will
be important in influencing the costs, or potential
revenues, related to the fate of the removed CO2.
If there is an opportunity to sell the CO2 (suchas to greenhouses) or otherwise utilize it, being
located near the point at which this happens will
reduce transport costs. Similarly, if the CO2 is to besequestered, being located at the point of injection
will keep these transport costs low. If the CO2 does
need to be transported, access to a pipeline network
will be important.
■ Favorable air conditions will help minimize operating
costs. These include (1) low levels of moisture, which
can lead to reduction of CO2 adsorbed because ofcompetition with water (unless there is a strong
business case to produce and sell fresh water); (2)favorable air temperature (to reduce the heat energy
required to regenerate sorbents); and (3) favorable
winds (to reduce fan power consumption). These
factors will have different impacts on the performanceand costs of a DAC facility depending on many aspectsof the system design, so these generalizations may nothold under all conditions.

25

There has been very little research on this issue, and it
would be valuable to examine the tradeoffs in greater
detail for different DAC technologies and related
assumptions.

F. Demonstrating Geological Carbon
Storage at Scale
To achieve large volumes of atmosphericdecarbonization, it is likely that most DAC facilities would
be paired with disposal and storage of CO2 in deep
geological formations, usually called geological carbon
storage or GCS. There is a substantial scholarship on
GCS, and today nearly 20 large-scale facilities capture,
store and monitor anthropogenic CO2.24
If DAC is deployed at the gigaton scale for CO2 removal,
it will likely require thousands of large GCS facilities,
and limits on the large-scale viability of GCS would
limit DAC.25 This raises a set of technical, social, legal
and policy questions. Some questions (e.g., public
acceptance) are ameliorated by DAC’s geographic
flexibility, and other concerns (e.g., environmental
impacts of leakage) have been addressed in existing
literature, regulation and experience. Several questions
remain in the DAC context and, if left unresolved, may
limit DAC volumes and deployment.
Seismic risk: Although no operating CO2 injection facility
has generated large earthquakes, some large-scale
injection projects have, mostly at geothermal or oil & gas
brine-disposal wells. While many scholars see this risk as
manageable, focus on avoiding potentially risky sites will
be needed.26
Long-term site care: To gain climate benefit, captured
and stored CO2 must stay isolated from the atmosphereindefinitely. Technologies exist to manage and avoid
operational risk, and liability stays with operators during
injection. However, questions remain about how best to
manage large GCS sites after operation. Some of thesequestions focus on technical questions of long-termmonitoring. Others focus on maintaining liability for an
appropriate length of time. Legal and policy scholarshipon the appropriate structure and mechanisms haveproposed many potential approaches that could serve
as viable solutions.27 Until conventional CCS scales upcommercially and key questions are resolved, DAC will
face these concerns.

26

G. Ensuring Net CO2 Removal from DAC
Removing CO2 from ambient air with DAC facilities may
not result in a net reduction in CO2 concentrations if
the energy and materials requirements for removal arethemselves very carbon-intensive. The most significant
instance of this is energy consumption, as discussed
previously. However, emissions associated withproducing materials, such as sorbents and structural
materials, and with supplying water could also play animportant role in the net CO2 balance of DAC systems.28
Similarly, if CO2 that is removed from ambient air
through DAC is used rather than sequestered, theutilization process may result in a lower net reductionof CO2 concentrations or possibly in a net increase.
Understanding this may be particularly challenging insituations in which the DAC facility producing CO2 as afeedstock and the CO2U utilization process consumingthat feedstock are operated separately or participate in alarger CO2 production-consumption market mediated bya pipeline infrastructure.29
These concerns indicate that it will be extremely
important to use Lifecycle Assessment (LCA) to assess
and account for all sources of emissions in the overall
DAC system (with associated utilization, if any).
This can identify the relative emissions implications
among alternative systems designs and help guide thedevelopment of targeted RD&D programs to improve
net CO2 reductions. A key consideration in this process
is understanding the longevity of CO2U products andCO2 storage locations since rapid return of CO2 to theatmosphere may negate most of the benefits of DACbased removal.

H. Roadmap: Key Focus Areas For RD&D
to Advance Direct Air Capture■ Materials: Research is needed on the discovery,

design, manufacture and functionalization of materials
that capture CO2 from the air. For sorbents, the key
performance metrics are low regeneration energy,
high CO2 selectivity, fast reaction times and low
degradation rates. For solvents, an additional key
performance need is liquid solutions that have lower
heat capacity than water and comparable or lower
viscosity. RD&D programs should consider approaches
that include high-throughput materials testing,computational materials discovery and a focus onearth-abundant, low-cost materials.
December 2018

improved operational concepts for DAC, including
optimizing heat integration for maximum heatrecovery. A related focus should be on sorbent/
solvent regeneration cycling concepts to maximize
equipment usage intensity and minimize equipment
degradation and material replacement requirements.
For batch processes, there may be opportunities totake advantage of diurnal temperature swings. Further
analysis is needed to understand the value of using
intermittent sources of renewable energy (particularly
those that are curtailed), and RD&D on process
designs that are matched to this energy source
may be needed. Process concepts beyond primarily
temperature-swing sorption should be considered.
■ Environmental impacts: Research is needed on
improving the understanding of downwind impacts(such as entrainment of caustics), water consumption
and other potential impacts.
■ System analysis: Research is needed on system
performance and economics, including better
understanding of optimal location/siting, tradeoffs
between water production and CO2 removal, and thevalue of CO2 production at different purity levels.

MEDIUM-TERM

LONG-TERMOPERATION AT SCALE(1 – 3 years)(2 – 8 years)(4 – 15 years)(8 – 20 years)

Develop improved sorbents
and working fluids

Demonstrate 2 – 4
integrated systems at
ktCO2 /yr scale

Demonstrate 1 – 2
integrated systems at
100 ktCO2 /yr scale

Identify sources ofrenewable heat at scale

Establish 2 – 4 reference
systems for cost andperformance comparisons

Validate cost / tCO2
with and without
CO2 offtake

Establish full portfolio
of DAC systems at
100 MtCO2/yr

Identify primary factors
and relative value for
DAC sites

Map optimal locations
for DAC facility siting; Link
to sequestration sites

Identify key earlymarkets for CO2

Continue developmentof CO2U and CO2
sequestration

Validate DAC LCA for
integrated system at
100 ktCO2 /yr scale

Provide ongoing
deployment support
for DACS

Provide RD&D support
of DACS

Support pilot-scaledemonstration projects

TECHNOLOGY

NEAR-TERMLOCATIONS

or improved technologies to generate renewable
and low-carbon heat. Both high-quality and lowquality heat can serve DAC systems. In many cases(e.g., existing nuclear fission plants, fossil plants with
CCS), the most valuable area of focus is recovery andintegration of low-carbon heat (potentially including
waste heat). In others (e.g., concentrating solar power
and renewable hydrogen), the focus should be on
low-cost, low-carbon energy production. In all cases,
research should focus on systems that can scale up
and should include full lifecycle accounting. Because of
the rapid cost reductions and increased deployment of
renewable power, electricity-based technologies meritadditional attention, including heat pumps with highlift and, potentially, microwave-based heating.
■ Air contactors: Research is needed on improved
materials and designs for large-scale air contact withsorbent- and solvent-type systems. These approaches
should focus on increasing contact area, while
minimizing pressure drop or the equivalent, to limit
energy requirements for air movement. These systems
should also minimize loss of sorbent/solvent material
and may need improved heat integration and highthermal conductivity.

CO2 FATE■ Process designs: Research is needed on new and

POLICY■ Low-carbon heat: Research is needed on new and/Develop advanced designs
for air contractors

Support large-scaledemonstration projects

Figure 5-1. Innovation Roadmap – Direct Air Capture.
December 201827

■ Synergistic CO2 utilization technologies: Researchis needed on CO2 utilization approaches that aresynergistic with DAC, particularly those that do not
require pipeline transport and/or high purity.
1

Secretary of Energy Advisory Board (SEAB) Task Forceon CO2 Utilization, Final Report (December 2016),
https://www.energy.gov/seab/downloads/final-reporttask-force-co2-utilization.

2

US Nuclear Regulatory Commission, Small Modular
Reactors website (accessed November 15, 2018),
https://www.nrc.gov/reactors/new-reactors/smr.html.

3

NASA Radioisotope Power Systems website (accessed
November 15, 2018), https://rps.nasa.gov/technology/.

4

US Dept. of Energy, “Chapter 7: Advancing Systems
and Technology to Produce Cleaner Fuels,”
Quadrennial Technology Review (2015), https://www.energy.gov/sites/prod/files/2017/03/f34/
qtr-2015-chapter7.pdf; Matthew R. Shaner et al., “A
comparative technoeconomic analysis of renewablehydrogen production using solar energy,” Energy &Environmental Science (May 2016) at p.2354-71,
https://doi.org/10.1039/c5ee02573g; Gunther Glenck
and Stefan Rechelstein, “The prospects for renewablehydrogen production,” (2017), https://www-cdn.law.
stanford.edu/wp-content/uploads/2018/04/171222_
RenewableHydrogen_final__1___1_.pdf.

5

Global CCS Institute, 2018, Global Status of CCS, 2018,https://www.globalccsinstitute.com/status.

6

Lyubovsky 2017, Shifting the paradigm: Synthetic
liquid fuels offer vehicle for monetizing wind andsolar energy, J. of Energy Security, http://www.ensec.
org/index.php?option=com_content&view=articl
e&id=604:shifting-the-paradigm-synthetic-liquidfuels-offer-vehicle-for-monetizing-wind-and-solarenergy&catid=131:esupdates&Itemid=414.

7

8

28

Summers W, 2014, Baseline Analysis of Crude Methanol
Production from Coal and Natural Gas, NETL Report
101514, https://www.netl.doe.gov/research/energyanalysis/search-publications/vuedetails?id=720.
US Department of Energy, “Chapter 6: Innovating Clean
Energy Technologies in Advanced Manufacturing,”
Quadrennial Technology Review (2015), https://www.
energy.gov/sites/prod/files/2016/06/f32/QTR2015-6IProcess-Heating.pdf.

9Concentrating solar power”, SEIA website (visited
October 5, 2018), https://www.seia.org/initiatives/
concentrating-solar-power.

10

Hardik Naik et al., “Medium temperature applicationof concentrated solar thermal technology: Indian
perspective,” Renewable and Sustainable Energy
Reviews (September 2017) at p.369-78, https://doi.
org/10.1016/j.rser.2017.03.014.

11

Julie K. Langenfeld et al., “Assessment of sites forCO2 storage and CO2 capture, utilization, and storage
systems in geothermal reservoirs,” Energy Procedia(July 2017) at p.7009-17, https://doi.org/10.1016/j.
egypro.2017.03.1842.

12

US Department of Energy, “Improving process heating
system performance” (2015), https://www.energy.gov/
sites/prod/files/2016/04/f30/Improving%20Process%20
Heating%20System%20Performance%20A%20
Sourcebook%20for%20Industry%20Third%20Edition_0.
pdf.

13

IEA, “Application of Industrial Heat Pumps” (2014),
https://iea-industry.org/app/uploads/annex-xiii-part-a.
pdf; Global CCS Institute, “Heat pump applications”(website retrieved November 15, 2018), https://hub.
globalccsinstitute.com/publications/strategic-researchpriorities-cross-cutting-technology/44-heat-pumpapplications-industrial-processes.

14

Jack Browne, “Microwave energy powers manyindustrial applications,” Microwaves & RF (April 2017),
https://www.mwrf.com/systems/microwave-energypowers-many-industrial-applications.

15

US Department of Energy, “Chapter 6: Innovating Clean
Energy Technologies in Advanced Manufacturing,”
Quadrennial Technology Review (2015), https://www.
energy.gov/sites/prod/files/2016/06/f32/QTR2015-6IProcess-Heating.pdf.

16

Lawrence Livermore National Laboratory, “Estimated
U.S. Energy Consumption on 2017,” LLNL-MI-410527(April 2018), https://flowcharts.llnl.gov/content/assets/
docs/2017_United-States_Energy.pdf.

17

National Academies 2018, op. cit.

18

Jan Wohland et al., “Negative Emission Potential of
Direct Air Capture Powered by Renewable Excess
Electricity in Europe,” Earth’s Future (September 2018)at p. 1380-4, https://doi.org/10.1029/2018EF000954.

19

H.A. Daggash et al., “Closing the carbon cycle tomaximize climate change mitigation: power-to-

December 2018

methanol vs. power-to-direct air capture,” Sustainable
Energy Fuels (April 2018) at p.1153-69, https://doi.
org/10.1039/C8SE00061A.
20

Lori Bird et al., “Wind and Solar Energy Curtailment:
Experience and Practices in the United States,” NREL/
TP-6A20-60983 (March 2014), https://www.nrel.
gov/docs/fy14osti/60983.pdf; Michael Waite et al.,“Modeling wind power curtailment with increased
capacity in a regional electricity grid supplying a
dense urban demand,” Applied Energy (September2016) at p.299-317, https://doi.org/10.1016/j.
apenergy.2016.08.078.

21

Bird et al., op. cit.

22

Secretary of Energy Advisory Board (SEAB) Task Forceon CO2 Utilization, Final Report (December 2016),
https://www.energy.gov/seab/downloads/final-reporttask-force-co2-utilization.

23

NIOSH website, access November 15, 2018, https://www.cdc.gov/niosh/npg/npgd0523.html.

24

Global CCS Institute, 2017, Global Status of CCS Report,
https://www.globalccsinstitute.com/status.

December 201825

National Academies 2018, op. cit.

26

Ruben Juanes, Bradford H. Hager and Howard J.
Herzog, “No geologic evidence that seismicity causes
fault leakage that would render large-scale captureand storage unsuccessful,” Proceedings of the National
Academy of Sciences (December 2012) at E3623,
https://doi.org/10.1073/pnas.1215026109.

27

Michael Faure, Liability and Compensation for Damage
Resulting from CO2 Storage Sites, 40 Wm. & Mary Envtl.
L. & Pol'y Rev. 387 (2016), https://scholarship.law.
wm.edu/wmelpr/vol40/iss2/3.

28

Patricia Luis, “Use of monoethanolamine (MEA) forCO2 capture in a global scenario: Consequences andalternatives,” Desalination (February 2016) at p.93-9,
https://doi.org/10.1016/j.desal.2015.08.004.

29

David Sandalow et al., “Carbon Dioxide Utilization(CO2U): ICEF Roadmap 2.0,” Innovation for Cool Earth
Forum (November 2017), http://www.icef-forum.org/
platform/upload/CO2U_Roadmap_ICEF2017.pdf.

29

Chapter 6
Policy Considerations
Policy support will be central to development of DAC,
both in the short- and long-term. This chapter discusses
the rationale for policy support and range of policy tools
available.

A. Rationale
Carbon Dioxide Removal
The concentration of carbon dioxide in the atmosphereis now higher than at any time in human history.
Human activities including fossil fuel combustion anddeforestation continue to increase that concentration.
The impacts include devastating heat waves, more
severe and frequent storms, sea level rise, forest loss
and ocean acidification.1
Reflecting this situation, more than 175 countries have
ratified the Paris Agreement, which calls for “[h]olding
the increase in the global average temperature to well
below 2°C (3.6°F) above pre-industrial levels” andachieving net zero emissions in the second half of thiscentury.2
The problems that arise from high levels of CO2 in
the atmosphere are classic “externalities,” to use thelanguage of economics. Market forces alone will not

December 2018

control carbon dioxide emissions adequately, since CO2
emitters do not bear the full costs of their emissions.
Government policies are essential to provide solutions tothese problems.3
Meeting the goal of limiting temperature increases
requires that cumulative emissions from humanactivities remain within the “below 2°C budget.” Carbon
removal technologies are an important part of thetoolkit for achieving this. While reducing emissions ofcarbon dioxide and other greenhouse gases is a priority
in the short-term, the Intergovernmental Panel on
Climate Change and others have found that emissions
reductions alone are unlikely to be sufficient. Removing
carbon dioxide from the atmosphere is very likely tobe essential to achieving the goals set forth in the Paris
Agreement.4

Direct Air Capture
The set of policies that may be relevant to CDRtechnologies is quite broad, and the appropriate policy
mix is different for each CDR method. In DAC’s case, theearly stage of the technology means that government
policies should include support for RD&D. Privatecompanies do not usually invest in the socially optimallevel of early-stage RD&D (whether or not there areexternalities involved), in part because they are unlikely
to see returns within time frames important to most
corporate managers. This further emphasizes the needfor government support of DAC RD&D.5

31

Many climate-relevant technologies enter the market
as competitors with well-established conventional
technologies. Prominent examples of this include
electric vehicles (which compete with ICE vehicles),renewable electricity generation (which competes withfossil and nuclear generation), biofuel (which competes
with gasoline and diesel) and LEDs (which compete withincandescent, fluorescent and halogen lighting). In eachcase, the new technology provides an equivalent energy
service to the entrenched conventional technology.
This means that customers—individuals, companies,
and governments—will generally consider buying andinstalling the new technology only in the context of theexisting market for the conventional one.
This form of market entry implies clear cost targets thatthese technologies must achieve in order to deploy
widely without subsidies. While these technologies arealmost always more expensive than their conventionalcounterparts when they first enter the market,
RD&D and learning-by-doing can reduce their costs
significantly.6 If these costs fall to the point that they are
equal to or less than their conventional counterparts,
market forces will generally ensure that they will be
adopted. (There are additional considerations, suchas compatibility with technical standards, regulatory
barriers, customer awareness and interoperability, thatmust also be considered, but these are usually lessimportant than cost.)
DAC will follow a fundamentally different pattern, since itdoes not directly compete with established technologies:
there is no conventional market for technologies toremove CO2 from ambient air at scale. Therefore, it is
not clear what cost target would be relevant for DAC
to achieve market success. In limited instances, DAC
may compete with conventional sources of CO2, suchas geological deposits or co-production with ammonia.7
However, since the overall CO2 market is relatively small
and the end-point costs are often dominated by delivery
costs (as in the case of greenhouses), this does not serve
to establish a generally applicable cost target.
This situation implies that policy drivers will be required
to scale DAC in a way that is significantly different thanmany climate-relevant technologies developed to date.
These drivers may require a very different structure thanpolicy tools for other climate technologies and may need
to be sustained for a relatively long period of time.

32

Some have questioned whether scarce government
resources should be devoted to development of DAC
technologies, when those resources could be used forother purposes.8 The authors of this report believe nopotential pathway for reducing the threat of climatechange should be ignored. While recognizing that private
and governmental capital resources are finite, achieving
a 2°C (3.6°F) or 1.5°C (2.7°F) stabilization seems
impossible without a wider range of credible, actionableoptions. Investment and support for innovation isessential to create those options. DAC offers a potentially
important set of tools for mitigating and ultimately
reversing climate change. Current policy support for DAC
is exceedingly modest and could be increased by a factorof 10 or more without meaningfully competing withsupport for other technologies. Especially given the longlead times required to scale DAC technologies, devoting
resources to the development and deployment of thosetechnologies is strongly in the public interest.9

B. Policy Tools
Very few policies around the world today specificallytarget DAC for support. The most significant such policy
is probably in the United States, where a law enacted
in early 2018 provides a tax credit for direct air captureof carbon dioxide. Some governments invest small
amounts in RD&D for DAC. Low carbon fuel standards
in California, the European Union and several otherjurisdictions provide incentives for DAC.
Supportive policies will be essential for DAC to scale inthe decades ahead. Potential policy tools are discussed
below.

Government Support for RD&D
National governments spend roughly $15 billion annuallyon RD&D for clean energy technologies. These programs
have played important roles in the development ofcountless technologies in recent decades.10
Government spending on RD&D for DAC is exceedingly
modest. In 2015, the U.S. Department of Energy (DOE)’s
ARPA-E program offered $3 million for RD&D on DAC.
The Government of Alberta has provided grants to
support DAC.11 The U.K.’s Department of Business,
Energy and Industrial Strategy supports carbon dioxide
removal programs including DAC. Funds for these
programs are a tiny percentage of overall government
funding for clean energy RD&D.
December 2018

An increase in funding for RD&D on DAC could
speed deployment and yield important benefits. This
roadmap identifies a number of priority areas for RD&D
investment, including:■ improved sorbent materials,■ improved air contactor designs,■ improved sources of low carbon heat, and■ improved CO2 utilization technologies

In December 2015, heads of state from more than 20
countries announced Mission Innovation, a coalition
dedicated to accelerating clean energy innovation.
Member governments (including Japan, China, theUnited Kingdom, Germany and Saudi Arabia) pledged
to double RD&D on clean energy within five years.
The increase in RD&D budgets from these countries
in the next few years offers an opportunity to scale
up government RD&D funding for direct air capture,including in the areas above.
The United States helped launch Mission Innovation andremains a member. Although the United States is unlikely
to fulfill its overall doubling pledge under the Trumpadministration, in 2018 the U.S. Congress increased
funding for clean energy programs at DOE by roughly
15% over the prior year.12 DAC has won bipartisan
support in the U.S. Congress and could be an area inwhich the government’s RD&D spending increases in theyears ahead.

Tax Incentives
Tax incentives can play an important role in helping
spur development of clean energy products. In Norway,
for example, generous tax incentives helped plug-in
electric vehicles capture 39% of new car sales in 2017.
Such incentives could play a similar role in promoting
development and deployment of DAC.
Legislation providing a tax incentive for DAC was enacted
in the United States in early 2018. Known as the FUTURE
Act or “45Q” (for the provision of the U.S. Tax Code it
amends), the law provides a tax credit of $28-50/tCO2
captured from the air and stored in saline aquifers.
(The $28 credit is available in 2018 and increases $2-3
per year, reaching $50 by 2026.) The law also provides
a tax credit of $17-35/tCO2 captured from the air andused for enhanced oil recovery or converted into
useable products. (The $17 credit is available in 2018
and increases $2-3 per year, reaching $35 by 2026.) To

December 2018

qualify for the credit, facilities must capture a minimumof 100,000 tons of carbon dioxide per year.
This type of tax incentive could be a model for similar
provisions in many other countries. Alternative or
additional tax incentives could include tax-exempt debt
financing (e.g., private activity bonds), an investment
tax credit or bonus depreciation of capital assets, all ofwhich have been proposed in the U.S. Congress.

Carbon Price
A price on carbon dioxide emissions, whether throughan emissions trading program or tax mechanism,
provides emitters with an important incentive to cut
emissions. Carbon pricing programs are now in placein the European Union, Norway, California, nine Statesin the northeast United States, the Canadian provincesof British Columbia and Quebec, and seven Chinese
provinces. The Chinese government is in the process oflaunching a nationwide emissions trading program forthe power sector.
In the short-term, carbon prices are unlikely to approachthe cost of DAC. The lowest cost for DAC envisioned withcurrent technologies that has been publicly presented is$94/tCO2.13 Only three countries—Sweden, Switzerlandand Lichtenstein—currently have nominal carbon pricesthat exceed that amount. Most carbon prices are much
lower (below $25/tCO2).14 However, even a low carbon
price can create incentives for DAC by contributing toproject revenues. In addition, the prospect of a price on
carbon may help incentivize private sector investments inresearch and development on DAC, if market participants
expect the price to increase in the medium- to long-term.
A carbon price could provide significant incentives for
direct air capture, if emitters receive credit for tonsof CO2 removed with DAC. Indeed, a carbon priceis an important part of any long-term strategy for
DAC. Although DAC can provide carbon dioxide forcommercial purposes, it is unlikely to be the lowest-costsource for doing so, except perhaps in remote locations.
A price on carbon emissions is likely to be a central partof the rationale for many, if not most, DAC facilities.

Low-Carbon Fuel Standard
A low-carbon fuel standard (LCFS) sets a limit on the
carbon dioxide emissions of transportation fuels
throughout their lifecycle. California, Oregon, British
Columbia and the European Union (E.U.) have all33enacted low-carbon fuel standards and the Canadianfederal government is developing a national system.15
California’s LCFS requires producers of petroleum-based
fuels to reduce the carbon intensity of their fuels 10%
from 2010 levels by 2020 (state regulators are currently
considering a 20% by 2030 requirement.) The E.U.’s Fuel
Quality Directive requires reductions of 6% in the carbon
intensity of fuels from 2010 levels by 2020.
Fuels made with carbon dioxide from DAC, all else being
equal, have a lower carbon intensity than fuels madewith carbon from fossil fuels. An LCFS therefore incentivizes DAC, providing a potential market for the carbon
dioxide DAC produces. The more stringent the LCFS, thegreater the incentive.16 The California LCFS includes aprocess to gain credit through application of a lifecycle
analysis under a regulated protocol. Similarly, existing fuelstandards like the U.S. Renewable Fuel Standard or theEuropean Renewable Diesel Standard could be expanded
to account for fuels produced using DAC.
Finally, DAC can be interpreted to fall within the purview
of fuel standards as a pathway to reduce fuel emissions
post-combustion from the air and ocean. The recent
revisions to the California LCFS do this explicitly andoffer DAC crediting for projects without geographic
restriction. Under the revised rulemaking, DAC withgeological storage must comport with the other aspectsof the CCS protocol to be credited.

Mandates
Governments mandates can be effective in helping build
markets for clean energy products. In the United States,
many state governments require utilities to purchase aminimum percentage of their power from renewablesources. In India, a similar requirement is imposedby the Ministry of New and Renewable Energy. These
requirements have been important to the early growthof wind and solar power in both countries.17
Other experiences suggest caution, however. A U.S.
federal government mandate has required the use ofcellulosic ethanol in fuel supplies for almost a decade.
Nevertheless, the cellulosic ethanol industry remains
in its infancy and waivers to that requirement havebeen granted on a regular basis. Technology-forcing
requirements—in which governments require private
actors to meet standards that are not yet technicallyachievable—have been successful in some instances but
not in others.18
34

There may be instances in which government mandates
could help build DAC scale. As one option, governments
could require that a certain percentage of the CO2 usedin enhanced oil recovery (EOR) be provided by DAC.
This could create a substantial market for CO2 from DAC.
(EOR is one of the largest commercial markets for CO2
today.) It could also help reduce the carbon footprintof EOR.19 At present, DAC may be too expensive andunproven for such a requirement to be considered inmany jurisdictions. As DAC technologies mature and fallin price, however, such mandates might be more likely tobe adopted.

Government Procurement
In many countries, government procurement makes
up more than 10% of GDP.20 Government purchases
can play an important role in starting and building
new product markets. First, government purchase
contracts can provide developers and manufacturersof new products with an assured market, which can beespecially important in securing debt capital. Second,
government purchases can help establish standard
technical specifications for new products, which can help
catalyze efficient supply chains.
Governments could spur development of DAC
technologies using procurement authorities. For
example, governments could target CO2-based fuels for
procurement, with a preference for CO2 from DAC. (TheU.S. Navy has had a similar program for the purchaseof drop-in biofuels.21) Similar procurement authorities
could be applied to CO2-based plastics, concrete or othersynthetic building materials. As DAC technologies mature,
governments could play a powerful role in developing thetechnology with their procurement power.

Lifecycle Assessments.
Lifecycle assessments (LCAs) are essential for evaluating
the climate benefits of direct air capture. If the powerand heat used in a DAC process come from zero-carbon
sources, the climate benefits will be considerable. If thepower and heat used in a DAC process come from highcarbon sources, the climate benefits could be negative.
Governments can help develop and standardize LCA
methodologies for DAC, in part to facilitate GHG
accounting methodologies and protocols. Government
agencies, such as the U.S. National Institute of Standards

December 2018and Technology (NIST) or U.S. Department of Energy(through its national labs), can fund work by experts,
convene relevant stakeholders and issue guidelines
based on the inputs received. Governments can also
incorporate LCAs into their work on DAC, requiring that
DAC projects be carbon negative on a lifecycle basis
to meet any regulatory requirements or to qualify forgovernment support.
Related to this, CDR should be considered for inclusion
in international GHG accounting standards, which
would likely require additional work in quantifying andvalidating CDR volumes for different pathways andapproaches.22
A wide range of policy tools are available for supporting
DAC. Government policies—in particular one that puts
a price on carbon—are likely to be central to the growthand long-term role of the technology.
1

IPCC Working Group II, “Climate Change 2014:
Impacts, Adaptation and Vulnerability—Summaryfor Policymakers” at p.13, http://www.ipcc.ch/pdf/
assessment-report/ar5/wg2/ar5_wgII_spm_en.pdf. 3.

2

Paris Agreement Article 2(1)(a), https://unfccc.int/
sites/default/files/english_paris_agreement.pdf; Paris
Agreement Article 4(1), https://unfccc.int/sites/default/
files/english_paris_agreement.pdf.

3

See Sir Nicholas Stern, Stern Review on the Economicsof Climate Change (2006), http://webarchive.
nationalarchives.gov.uk/+/http://www.hm-treasury.gov.
uk/sternreview_index.htm. See also Alison Benjamin,“Stern: climate change a market failure” Guardian(November 29, 2007) (quoting Sir Nicholas Stern–"Climate change is a result of the greatest market
failure the world has seen”), https://www.theguardian.
com/environment/2007/nov/29/climatechange.
carbonemissions.

4

IPCC, op. cit.

5

See Robert Stavins, “Repairing the R&D Market Failure,”
Environmental Forum (Jan/Feb 2011), https://scholar.
harvard.edu/files/stavins/files/column_40.pdf; Kenneth
Gillingham and James Sweeney, “Market Failure andthe Structure of Externalities” (July 2008), http://
environment.yale.edu/gillingham/GillinghamSweeney_
MktFailureStructureExternalities_proof.pdf.

6Edward S. Rubin et al., “A review of learning rates
for electricity supply technologies,” Energy Policy(November 2015) at p.198-218, https://doi.
org/10.1016/j.enpol.2015.06.011.

December 20187

Hans A. Haugen et al., “Commercial Capture andTransport of CO2 from Production of Ammonia,”
Energy Procedia (July 2017) at p.6133-40, https://doi.
org/10.1016/j.egypro.2017.03.1750; Kinder Morgan
website (accessed November 15, 2018), https://www.
kindermorgan.com/pages/business/co2/supply/supply.
aspx.

8

See, e.g., Kevin Anderson and Glen Peters, “The
Trouble with Negative Emissions,” Science (October2016) at p.182-3, http://science.sciencemag.org/
content/354/6309/182.

9

See generally https://www.vox.com/
energy-and-environment/2018/6/14/17445622/directair-capture-air-to-fuels-carbon-dioxide-engineering

10

See Mission Innovation website at http://missioninnovation.net/our-work/baseline-and-doubling-plans/

11

See, e.g, “Opportunity: Capture CO2 from low
concentration-coal industry,” FedConnect (July 2015),

12

https://www.fedconnect.net/FedConnect/default.
aspx?ReturnUrl=%2fFedConnect%2f%3fdoc%3d
DE-FOA-0001342%26agency%3dDOE&doc=DEFOA-0001342&agency=DOE; Noah Deich, Direct
Air Capture Explained in 10 Questions (September2015), http://www.centerforcarbonremoval.org/
blog-posts/2015/9/20/direct-air-capture-explained-in10-questions.

13

Umair Ifran, “Trump wanted to slash fundingfor clean energy. Congress ignored him,” Vox(March 2018), https://www.vox.com/energyand-environment/2018/3/22/17151352/
omnibus-energy-environment-trump.

14

David Keith et al., “A Process for Capturing CO2 fromthe Atmosphere,” Joule (August 2018) at p.1573-94,https://www.sciencedirect.com/science/article/pii/
S2542435118302253?via%3Dihub.

15

See World Bank and Ecofys, “State and Trendsof Carbon Pricing” (May 2018) at p.11, https://
openknowledge.worldbank.org/bitstream/
handle/10986/29687/9781464812927.pdf; “Countries
with the highest carbon price,” Carbon Pulse(September 2016), https://www.carbonbrief.org/
mapped-countries-with-highest-carbon-price.

16

Gov. of Canada, 2018, Clean Fuel Standard(website), https://www.canada.ca/en/environmentclimate-change/services/managing-pollution/energy-production/fuel-regulations/clean-fuelstandard.html.

17

See Matt Lucas, “The $150+ Per Ton Incentive for
Carbon Capture Nobody is Talking About,”

35

18

(August 2018), https://www.centerforcarbonremoval.
org/blog-posts/2018/8/21/the-150-per-ton-incentivefor-carbon-capture-nobody-is-talking-about; Dave
Roberts, “Sucking carbon out of the air won’t solve
climate change,” Vox (July 2018), https://www.vox.com/
energy-and-environment/2018/6/14/17445622/directair-capture-air-to-fuels-carbon-dioxide-engineering.

19

See Tom Kenning, “Revision to ‘single most important
policy’ to drive solar ready for approval,” PV Tech
(November 2015), https://www.pv-tech.org/news/
intersolar-india-revision-to-single-most-importantpolicy-to-drive-solar-re.

20

36

See generally David Gerard and Lester Lave,“Implementing Technology-Forcing Regulations,”
Technological Forecasting and Social Change(September 2005) at p.761-78, http://faculty.lawrence.
edu/gerardd/wp-content/uploads/sites/9/2014/02/18TFSC-Gerard-Lave.pdf.

21

See generally Greg Cooney et al., “Evaluating the
Climate Benefits of CO2-Enhanced Oil Recovery Using
Life Cycle Analysis,” Environmental Science andTechnology (May 2015) at p.7491-500, https://pubs.
acs.org/doi/pdf/10.1021/acs.est.5b00700; Global
CCS Institute, “Enhanced Oil Recovery results insignificant net CO2 emissions reduction,” (September2016), https://www.globalccsinstitute.com/news/
institute-updates/iea-enhanced-oil-recovery-resultssignificant-net-co2-emissions-reduction.

22

Esteban Ortiz-Ospina and Max Roser, “Public Spending”(2017), https://ourworldindata.org/public-spending/.

23

U.S. Energy Information Administration, “Biofuels areincluded in latest U.S. Navy fuel procurement,” (July
2014)

24

Royal Academy of Engineering, Greenhouse Gas
Removal (2018), ISBN: 978-1-78252-349-9, https://www.raeng.org.uk/publications/reports/greenhousegas-removal.

December 2018

Chapter 7
Findings andRecommendations
Direct air capture (DAC) lies at one end of the carbon
dioxide removal (CDR) spectrum and could play animportant role in limiting the increase of atmospheric
CO2 concentrations. The potential scale of deployment
is very large. Today, DAC technologies exist, but theircosts, energy inputs and thermal requirements are veryhigh. These findings summarize key components ofthis roadmap and distill a set of recommendations forconsideration.

FINDING 1:
Large-scale CDR will likely be essential to achieve
the goal of preventing a global average temperature
increase of 2°C (3.6°F) above pre-industrial levels.
The overwhelming majority of integrated assessment
models, including 87% of the IPCC models, find that CDR
is required to achieve a 2°C (3.6°F) target. In addition, asubstantial number of current published assessments ofthe global carbon budget suggest that a CDR is required
today to avoid both overshoot and the associatedconsequences of anthropogenic global warming.

FINDING 2:
There are many engineered and natural pathways toachieve large-scale CDR, including DAC. These pathways
include reforestation and afforestation, increasing soil
carbon uptake, bio-energy with CCS (BECCS), enhanced
weathering and DAC. Each approach has challenges,
including technical readiness, cost, resource availability
and social acceptance. Many have additional economicor environmental benefits. Overall, a full portfolio of
options has the greatest chance of success and thelowest risk of failure. Unfortunately, most governments
have not yet formally recognized the likely necessityof CDR, and very few have committed to programs
supporting CDR.

December 2018

Recommendation 1:
Governments around the world should begin RD&D
on direct air capture today.

FINDING 3:
By its intrinsic nature, DAC can play a major and distinctrole in deployment of CDR. DAC is a concentrated
process requiring a small physical footprint. It uses
earth-abundant materials, and its removal operation isstraightforward to quantitatively measure. The capacityof DACS (DAC with sequestration) to remove CO2 isquite large and is limited primarily by cost, energy
requirements and geological storage capacity. RD&D
efforts can reduce costs and energy requirements, while
geological storage capacity is estimated to be many
trillions of tons. For these reasons, DAC plays a role as
the “backstop” technology on the cost curve for climate
strategies. Although DAC faces specific challenges (seebelow), including those related to geological CO2 storage,
it has important potential as a CDR technology and as ameans to produce CO2 as a feedstock.

FINDING 4:
The key limits to DAC are costs. Because CO2 is more
dilute in the atmosphere than in emissions sources suchas flue gas, more energy is required to separate andconcentrate it. On this basis, DAC will almost certainly
always cost more than conventional CCS on mostpower plant and heavy industrial sources. The costsof DAC include both the capital costs and operationalcosts, including very low-carbon or zero-carbon heatand power. Current costs are difficult to estimate andvary by location and technology; nonetheless, today’s
DAC technologies appear to operate at $300-600/tCO2
removed.

FINDING 5:
Innovation can play an immediate and substantial role
in reducing costs and improving performance of DAC
approaches. The field of DAC is very young. Today’s
devices are far from the limits of thermodynamics, andboth fundamental research and applied engineering37

are likely to produce significant cost reductions. It
appears likely that costs could be reduced to less than$200/tCO2 by 2025 and less than $100/tCO2 by 2030.
However, accomplishing this would require a sustained
investment in focused RD&D.

Recommendation 2:
Direct air capture programs should include
fundamental research, applied science and scale-up.
These programs should include research on new
CO2 solvents and sorbents, low-carbon heat andother topics. The program goals should focus on
reducing the cost of DAC, in part by deploying DAC
units to foster learning-by-doing. DAC should beincorporated into conventional CO2 storage projects
as part of an RD&D agenda where possible.

FINDING 6:
Enough companies and technologies exist today for
rapid progress in DAC cost-reduction and scale-up.
Today, three companies offer DAC devices and services
with a clear expectation of performance. Many morecompanies and innovators have begun work on
alternative designs and approaches. In addition, manyother new technologies (e.g., additive manufacturing,materials discovery and supercomputing) have thepotential to dramatically improve cost and performancefor DAC but have not yet been applied to that goal.

FINDING 8:
There are many policy options that could support
DAC deployment and CDR more broadly. As a firststep, governments could recognize DAC as part of aportfolio of CDR options, essential to important climate
outcomes. They could invest in innovation and research,
as well as procure both DAC services and materials madefrom atmospherically derived CO2. They could provide
incentives for deployment of DAC technology (e.g., therecent inclusion of DAC into the California Low Carbon
Fuel Standard). They could support lifecycle analyses
and industrial standards. Furthermore, for DAC to playa major role in a broader CDR portfolio, it will likely bepaired with geological CO2 storage, which faces policy
issues around permitting, long-term care, liability andsocial acceptance.
Private sector policies matter as well. Companies could
add DAC to their emissions reduction strategies, bothby deploying the technology and buying low-carbon andnegative-carbon materials made with DAC-derived CO2.

Recommendation 3:
Governments, industry and financial institutions
should work together to scale up direct air capture.
Policies to promote DAC should include investment
in RD&D, government procurement, targeted
incentives, standard-setting, work on lifecycle
analyses and recognition of DAC in carbon markets.

FINDING 7:
DAC provides some additional benefits beyond
atmospheric separation and concentration of CO2.
Some DAC approaches have a co-benefit of freshwater
separation. All approaches could help support the
manufacturing of goods from CO2 (e.g., fuels, plastics,
cement), and may offer a market differentiation of value
in that way.

38

December 2018

Final Thoughts
The movie Apollo 13 includes several scenes that feature
direct air capture. After a malfunction in the spacecapsule endangers the mission, NASA scientists on the
ground in Houston quickly realize that the CO2 scrubberin the capsule is damaged and requires modification
to function. Without the scrubber, CO2 concentrationsin the capsule of the spacecraft will exceed safe limits
and the crew will die. A team gathers around a large
worktable and dumps out a handful of gadgets andobjects, the few expendable and spare parts on board
the spacecraft that could be used to fashion a working
air-capture device. After 75 minutes, they find a solution
and coach the astronauts on how to modify their existing
technology to return the CO2 levels in the air to safe
levels.
These scenes serve as a metaphor as we face thethreat of climate change. In these scenes, a sudden
environmental challenge has become urgent, withtoo much CO2 in the space capsule, prompting areassessment of priorities and a change of mission.
Crucial to the success of that mission and to sustaining
the lives of the crew, the technology already exists toresolve the issue. Innovation is required, and teams ofscientists and decision makers learn rapidly. This allows
the team to survive with their limited resources during
their trip through space. The government had to assign
substantial resources on the ground and learn throughaction to find a solution.
The growing concentration of CO2 in the atmosphere isalso an urgent threat. With resolve and determination
like that showed by those who saved Apollo 13, we canmeet the threat. (To quote the movie’s most famous line:“Failure is not an option.”) Large-scale carbon dioxide
removal will likely be essential in the global response to
climate change. Direct air capture of CO2 could be an
important part of the solution.

We are deeply grateful to the Ministry of Economy, Trade and Industry (METI) and
New Energy and Industrial Technology Development Organization (NEDO), Japan,
for launching and supporting the ICEF Innovation Roadmap Project of which this is a part.

December 201839