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Life-cycle assessment of an industrial direct air capture process based on temperature–vacuum swing adsorption

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Current climate targets require negative carbon dioxide (CO2) emissions. Direct air capture is a promising negative emission technology, but energy and material demands lead to trade-offs with indirect emissions and other environmental impacts. Here, we show by life-cycle assessment that the commercial direct air capture plants in Hinwil and Hellisheiði operated by Climeworks can already achieve negative emissions today, with carbon capture efficiencies of 85.4% and 93.1%. The climate benefits of direct air capture, however, depend strongly on the energy source. When using low-carbon energy, as in Hellisheiði, adsorbent choice and plant construction become more important, inducing up to 45 and 15 gCO2e per kilogram CO2 captured, respectively. Large-scale deployment of direct air capture for 1% of the global annual CO2 emissions would not be limited by material and energy availability. However, the current small-scale production of amines for the adsorbent would need to be scaled up by more than an order of magnitude. Other environmental impacts would increase by less than 0.057% when using wind power and by up to 0.30% for the global electricity mix forecasted for 2050. Energy source and efficiency are essential for direct air capture to enable both negative emissions and low-carbon fuels.
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https://doi.org/10.1038/s41560-020-00771-9
1Institute for Technical Thermodynamics, RWTH Aachen University, Aachen, Germany. 2Institute of Energy and Climate Research, Energy Systems
Engineering (IEK-10), Forschungszentrum Jülich, Jülich, Germany. 3Energy and Process Systems Engineering, ETH Zurich, Zurich, Switzerland.
e-mail: abardow@ethz.ch
Fossil energy is still important to most societies, which led to
36.8 Gt yr1 of carbon dioxide (CO2) emissions in 2019 (refs. 1,2).
Moving from fossil energy to renewable energy will reduce
greenhouse gas (GHG) emissions. However, there is broad scientific
consensus that the target of the Paris Agreement of the 2015 Climate
Conference (COP 21)3 requires not only a massive reduction in
GHG emissions but even up to 30 Gt yr–1 of negative emissions46.
Negative emissions could be provided by direct air cap-
ture (DAC) of CO2 with subsequent storage for carbon dioxide
removal (CDR)7,8. Captured CO2 can be stored geologically or via
mineralization9,10. DAC not only allows us to remove GHG emis-
sions from our past use of fossil fuels but also enables future fuels
with a closed carbon cycle. The captured CO2 could serve as a
carbon feedstock for fuels11,12 and other value-added products
like chemicals13,14 and building materials15,16 via carbon capture
and utilization.
The most developed DAC concepts separate CO2 from the air
by either absorption or adsorption1719. DAC based on absorption
typically uses aqueous hydroxy sorbents like alkali and alkali-earth
hydroxides. By contrast, DAC based on adsorption can employ
a wide range of solid sorbents, for example, alkali carbonates20,21,
amines supported on oxides22,23, solid organic materials22,2426 and
metal–organic frameworks22,27. Absorption by aqueous sorbents
allows for low costs and continuous operation28 but leads to high
water loss29. Furthermore, sorbent regeneration requires high tem-
peratures19,30. By contrast, DAC by adsorption can operate at low
regeneration temperatures (<100 °C)19,28,31,32. The first commercial
DAC system employs solid adsorbents in cyclic temperature–vac-
uum swing adsorption3335.
While DAC removes CO2 directly from the atmosphere, the
potential climate benefits of DAC are partly offset by indirect
environmental impacts due to the supply of energy and materials.
So far, a detailed assessment of this trade-off is only available for
GHG emissions for a DAC process with aqueous hydroxy sorbents,
where high-temperature heat is usually obtained from natural
gas, and the resulting CO2 emissions are recaptured12,29. Available
assessments for adsorption-based DAC systems consider energy
requirements but use proxy data for plant construction and adsor-
bent36,37. Currently, requirements for water and land38 as well as
energy and materials for sorbent production39,40 are intensely
debated as key issues for the potential large-scale deployment of
DAC. Thus, a comprehensive environmental assessment is missing
for adsorption-based DAC but urgently needed to establish the role
of DAC in climate change mitigation41.
Herein, we comprehensively evaluate the environmental impacts
of adsorption-based DAC using the method of a life-cycle assess-
ment (LCA)42,43. Temperature–vacuum swing adsorption is stud-
ied based on data from the first commercial DAC plants. Climate
impact reductions depend strongly on the energy supply, while the
adsorbent and infrastructure become important when low-carbon
energy is used. Even large-scale deployment of DAC, capturing 1%
(ref. 44) of the global annual CO2 emissions, is not constrained by
material and energy supply for plant construction and operation,
nor would it lead to substantial trade-offs in other environmental
impact categories.
LCA goal and scope
LCA accounts for all flows of energy and materials exchanged with
the environment throughout the life cycle. The DAC system con-
sidered captures CO2 from the air by cyclic temperature–vacuum
swing adsorption. The climate benefit of removing CO2 from the
atmosphere is reduced by indirect emissions, for example, due to
the construction and operation of the DAC plant, for which the
company Climeworks provided industrial data.
Life-cycle assessment of an industrial direct air
capture process based on temperature–vacuum
swing adsorption
Sarah Deutz 1 and André Bardow 1,2,3 ✉
Current climate targets require negative carbon dioxide (CO2) emissions. Direct air capture is a promising negative emission
technology, but energy and material demands lead to trade-offs with indirect emissions and other environmental impacts.
Here, we show by life-cycle assessment that the commercial direct air capture plants in Hinwil and Hellisheiði operated by
Climeworks can already achieve negative emissions today, with carbon capture efficiencies of 85.4% and 93.1%. The climate
benefits of direct air capture, however, depend strongly on the energy source. When using low-carbon energy, as in Hellisheiði,
adsorbent choice and plant construction become more important, inducing up to 45 and 15 gCO2e per kilogram CO2 captured,
respectively. Large-scale deployment of direct air capture for 1% of the global annual CO2 emissions would not be limited by
material and energy availability. However, the current small-scale production of amines for the adsorbent would need to be
scaled up by more than an order of magnitude. Other environmental impacts would increase by less than 0.057% when using
wind power and by up to 0.30% for the global electricity mix forecasted for 2050. Energy source and efficiency are essential for
direct air capture to enable both negative emissions and low-carbon fuels.
NATURE ENERGY | VOL 6 | FEBRUARY 2021 | 203–213 | www.nature.com/natureenergy 203
Content courtesy of Springer Nature, terms of use apply. Rights reserved
... Evidently, for DACCS facilities connected to electric power grids, their environmental performance will depend on the electricity system context in which they will operate. Previous studies have shown that DACCS can achieve negative emissions, but capture efficiencies are sensitive to the operational efficiency and the energy source [22][23][24][25] . A recent life cycle assessment (LCA) of DACCS technologies also identified potential environmental trade-offs in increased land transformation if DACCS is operated by solar electricity (as compared to using grid electricity) 26 . ...
... Merely shifting to low-carbon energy sources for DACCS plant operation could lead to environmental trade-offs. These findings are in-line with other DACCS LCA studies 22,24,26 . We find that solvent-based DACCS generally has lower impacts than sorbentbased DACCS in five (climate change, human toxicity, freshwater CCI HTI FEI FTI TAI TTI MD WD 2020 2060 2100 2020 2060 2100 2020 2060 2100 2020 2060 2100 2020 2060 2100 2020 2060 2100 2020 2060 2100 2020 eutrophication, freshwater ecotoxicity, and metal depletion) out of eight impact categories studied herein. ...
... These differences appear to be linked to the study's optimistic electricity (180 kWh/ t CO 2 ) and heat (2.6 GJ/t CO 2 ) consumption assumptions for sorbent-based DACCS (under the reference case). These are less than half of those reported by several other studies 24,26,41 and the ones used herein (470-700 kWh/t CO 2 for electricity and 5.4-5.8 GJ/t CO 2 for heat). ...
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... However, its large-scale deployment may negatively impact ecosystems, create water stress, or drive biodiversity loss (mainly if energy crops are employed) [21]. These impacts could be alleviated with DACCS, but the chemical process requires large amounts of energy to operate, making it costly today and entailing associated environmental impacts [22]. What is clear is that there is no "silver bullet" (single technology or strategy) to solve the climate problem and deliver CDR at the scale and pace required [23]. ...
... To mention some prior studies, Minx et al. [5] and Fuss et al. [4] provided a thorough techno-economic assessment of several CDR alternatives. Other works investigated the CDR potentials and environmental impacts on the environment [29,30]; most of them focused only on BECCS [31][32][33][34][35] or DACCS [22,36,37], while other alternatives have received much less attention [9,11]. Another active research direction consists of assessing the role of the different CDR options in the climate change mitigation pathways and the broad implications of their large-scale deployment of the CDR [3,[38][39][40][41]. ...
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... The authors claim that the new plant will capture 4000 tons of CO 2 per yearmaking it the world's most extensive climate-positive facility to date [141]. Recently, Deutz and Bardow [142] analyzed the carbon footprint of Climeworks' DAC plant construction, considering the materials used, the energy required, the use of sorbents etc., to capture 1% of the global CO 2 emissions (see Fig. 5). It is clear that this approach requires a high amount of adsorbent and energy. ...
... The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. [142]. ...
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Since carbon dioxide (CO2) is the primary greenhouse gas emitted due to human activities into the atmosphere, strong research efforts have been developed towards capturing and decreasing its production. Unfortunately, specific processes and activities make it impossible to avoid CO2 emissions. Among the different strategies scientists propose for CO2 reduction, direct CO2 capture from the atmosphere, also known as direct air capture (DAC), represents a promising alternative in which sorbents have been mainly used. Recently, gas separation membranes have also been speculated to carry out such a separation, thanks to their smaller footprint and simpler setup and operation; however, their application remains a proposition in the field. This paper gives a perspective of the ongoing research and attempts of DAC applications via membrane separation and introduces the main membrane materials and types used for CO2 separation. Finally, the process considerations for DAC using membranes are stated to guide the new researchers in the field.
... In contrast, their side-effects and co-benefits beyond global warming have often been overlooked. Some studies have quantified the environmental impacts of DACCS and BECCS, but their results are hard to interpret from an absolute sustainability viewpoint [15][16][17][18][19] . Only recently, the impacts of BECCS were assessed against the Earth's biophysical limits 20,21 , i.e., the Planetary Boundaries (PBs) within which humanity could safely operate 22 . ...
... Nonetheless, previous studies suggest that the contribution of infrastructure to the total impacts of NETs might be minor. Notably, the impacts of constructing and decommissioning biomass power plants are negligible 62 , whereas the impacts related to the infrastructure of LTSS-DACCS are small relative to other life-cycle impacts 15 . ...
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