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April 6, 2020


Carbon Capture Versus Direct Air Capture


Carbon capture and direct air capture (DAC) have gained
prominence in recent years as options to address climate
change. The two technologies have similarities (beyond
their names), but they also have differences. Key
differences include how the technologies work, where the
technology can be used, how the technology can address
climate change, and levels of federal support.

Several major energy proposals in the 116th Congress would
increase federal support for carbon capture and DAC.
Examples include the Senate energy package debated on the
floor in March 2020 (S.Amdt. 1407 to S. 2657) and the
Climate Leadership and Environmental Action for our
Nation's Future Act (CLEAN Future Act) discussion draft
released by leaders of the House Energy and Commerce
Committee in January 2020.

This analysis explains key differences between the two
technologies to inform congressional deliberations.
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Carbon capture technologies prevent the release of carbon
dioxide (C02) to the atmosphere. In the most commonly
used arrangement today, a chemical that can grab CO2 is
placed in or near the stream of CO2 at a source. The
captured CO2 is then released and compressed so that it can
be transferred by pipeline. The CO2 can then be used, for
example, as a feedstock to an industrial process or
permanently stored (sequestered) underground. The
chemical that does the capturing can be reused in the
process many times. The full process is called carbon
capture, utilization, and storage (CCUS), or sometimes
carbon capture and storage (CCS).

Direct air capture technologies remove CO2 from the
atmosphere, even if that CO2 was released many years ago.
In many technological approaches, air is forced over a
chemical that can grab CO2. DAC and CCUS may use the
same chemicals, but some chemicals are better suited for
one application or the other. Regardless, the supporting
equipment must be optimized for the different CO2
concentrations involved in DAC and CCUS. After capture,
the process for DAC is very similar to that used for CCUS
and can use the same equipment for compression, transfer,
and storage. The chemical that does the capturing can be
reused for DAC many times.

Both technologies are in early stages of development, with
a few examples of operating projects worldwide. Of the
two, CCUS is more mature, though researchers expect
significant technology advancement can still be achieved.

Although the capture technologies are different for CCUS
and DAC, they face similar challenges. Both are typically


capital-intensive and energy-intensive. Also, the demand
for CO2 is small compared to its availability, resulting in
low CO2 revenues. The low value of CO2 presents a hurdle
to commercialization for both technologies.

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CCUS can be used at stationary sources of CO2 such as
power plants, ethanol production plants, or other industrial
facilities. Existing facilities can be retrofitted to add CCUS
equipment, or CCUS can be integrated into the design of
new facilities. The type of source can affect the cost of a
project because different sources emit CO2 in different
concentrations (purities). All else being equal, carbon
capture can be completed at lower cost per ton CO2
captured for sources with higher-purity CO2 emissions (e.g.,
ethanol production plants). Sources of captured CO2 are
often located far away from where CO2 may be used or
stored, creating logistical and cost challenges related to the
transport of CO2.

DAC can be used anywhere. Many proposals envision
building DAC projects close to either inexpensive
electricity sources or locations where CO2 can be used or
stored. Both options could serve to lower overall project
costs.

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CCUS would reduce CO2 emissions released to the
atmosphere. The extent of reduction is dependent upon the
end use of the CO2. Currently, the main use of captured
CO2 is for enhanced oil recovery (EOR). In EOR,
compressed CO2 is injected into aging oil wells. This
process increases oil production while also permanently
sequestering some CO2.

Many stakeholders see CCUS as a way to enable continued
use of fossil fuels even if CO2 emissions were restricted in
the United States and abroad. Fossil fuels have operational
advantages over alternative fuels in many economic sectors.
For example, cement, steel, and petrochemical
manufacturing all require very high temperatures, currently
provided almost exclusively by fossil fuel combustion.
CCUS may allow continued use of fossil fuels in these and
other sectors with lower CO2 emissions than today.

DAC would remove CO2 from the atmosphere. It is one
example of carbon removal, sometimes called negative
emissions technologies. Proponents see DAC and other
carbon removal options as a way to reduce atmospheric
CO2 concentrations to desired levels. Some studies estimate
DAC and other carbon removal options (e.g., afforestation)
would need to be deployed at large scales globally to
achieve climate targets investigated in those studies.


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