Carbon capture and storage (CCS) technology has been in the
spotlight recently. Last month, the UK government announced a
policy paper on “Delivering Energy Security and Net
Zero”, which places a focus on making the UK a leader in CCS
technology. In January, Germany and Norway published a joint
declaration on climate and renewable energy that includes an
agreement to explore carbon storage on the Norwegian continental
shelf. The USA’s Inflation Reduction Act included increases in
the tax credits available for projects that capture and store
carbon. But what exactly is this technology, how does it work, and
what role might it play in the ongoing fight against climate
change?
CCS refers to a range of technologies that are designed to
prevent carbon dioxide from entering the atmosphere, or remove
carbon dioxide that is already in the atmosphere, thereby
preventing it from contributing to climate change through global
warming. While the sector is often referred to with the umbrella
term CCS, the capture and storage aspects are quite separate and
can be geographically distant from one another.
Capture
Carbon capture facilities are often located within or nearby
large producers of carbon dioxide, such as fossil-fuel power plants
or concrete manufacturing. This enables carbon dioxide to be more
efficiently captured at the time of production, where it is at
higher concentrations. There are two main methods by which carbon
dioxide can be captured at this type of facility.
The first method is to modify the production process so that its
output gas stream is composed of pure or nearly pure carbon
dioxide. After processing to remove impurities, the carbon dioxide
gas can then be directly compressed or liquefied for transport.
Modifications can include burning fossil fuels in pure oxygen at
power plants so that the output gas does not include the unreactive
nitrogen that makes up approximately 78% of normal air.
The second method is to remove the carbon dioxide from the
output gas using a filtration medium. Common filtration media
include quicklime (calcium oxide), which reacts with carbon dioxide
to form limestone (calcium carbonate), or certain amine solutions
that also react with carbon dioxide. Once the medium is used, it is
removed from the filter and regenerated by heating. Heating
releases the carbon dioxide again at high purity, at which point it
can be collected similarly as for the first method.
Carbon capture at industrial plants has been used commercially
in certain locations for some time. However, to combat climate
change, “direct-capture” plants have also been proposed
that would remove carbon dioxide directly from the atmosphere.
Direct-capture plants would use a filtration method, but are
hindered by the relatively low concentration of carbon dioxide in
the atmosphere as a whole, which is currently only around 0.04%.
This makes it harder to efficiently and quickly remove carbon
dioxide from ambient air than from industrial output gas.
Transport
Once carbon dioxide has been captured, it must be transported to
where it is to be stored. This part of the process is quite
well-developed, because technology and logistics for transportation
of gas have existed for a long time and are easily adapted to
transporting carbon dioxide. Carbon dioxide can be transported via
pipelines, typically in gas form, or in sealed containers in liquid
or gas form by road, rail or sea.
Storage
In order to achieve the desired effect of removing carbon
dioxide from the atmosphere, it must be stored in a location where
it cannot escape back into the atmosphere over time. Typically,
this means underground storage.
The most common location proposed for storage of carbon dioxide
is depleted oil and gas fields, where carbon dioxide is pumped back
in to fill the spaces in porous rock formations left behind after
fossil fuels are removed. This process is in fact already used in
some declining oil fields to extract the remaining oil more
efficiently. Other rock formations are also potential candidates
for carbon dioxide storage, such as porous sedimentary rock or
unused coal seams where carbon dioxide can be adsorbed onto the
coal.
Injecting carbon dioxide into porous rock means it remains in
gaseous form or is dissolved into deep, saline water deposits.
Another possibility is to inject carbon dioxide into rock
formations where chemical reactions between the carbon dioxide and
the surrounding rock cause mineralisation of the carbon into a
solid form such as calcite. While mineralisation can take a longer
time, it has a reduced risk of carbon dioxide escaping back into
the atmosphere.
A recent study in Nature Sustainability has also brought into
the spotlight the possibility of storing carbon dioxide in crushed
rock. It has long been known that crushing rocks in an atmosphere
of carbon dioxide can lead to trapping of the carbon dioxide in the
resulting rock powder. This is of interest because rock crushing is
widely used to produce aggregate for construction purposes. Using
the process to trap carbon would help to offset carbon emissions
from the construction industry, such as from concrete and steel
production. The study shows that polymineralic rocks such as
granite and basalt are more effective at trapping carbon dioxide in
a stable state, and estimates that up to 0.5% of global carbon
emissions could be captured in this way.
Conclusions
Carbon capture and storage has been demonstrated on small scales
for some time. However, its large-scale feasibility and deployment
are still uncertain. Many environmental groups are opposed to
large-scale CCS because they are concerned it could divert
attention from other decarbonisation priorities such as the switch
to renewable power generation.
It seems unlikely that large-scale deployment of carbon capture
at fossil-fuel power plants will be a realistic solution. The costs
of retrofitting power plants or modifying their processes are high,
and power for downstream activity such as regenerating filtration
media and storing carbon would need to come from renewable sources
anyway in order to achieve a net reduction in carbon output. With
the rapidly dropping cost of wind and solar power, it is likely to
be cheaper and easier to simply increase renewable power generation
and storage capacity than to use CCS for fossil-fuel power
plants.
Nonetheless, there are some important processes such as concrete
production or production of ammonia for fertiliser where full
decarbonisation is unlikely in the short to medium term. For these,
CCS would allow the processes to continue while mitigating their
climate impact. In addition, many climate scientists now consider
that direct-capture of atmospheric carbon dioxide will be necessary
this century to control global temperatures, especially since
current policies make it likely that some overshoot of temperature
rise targets will occur. While some carbon capture can be achieved
by activities such as tree planting, CCS is also likely to play a
role. Therefore, despite its somewhat controversial status,
development of and investment in CCS technologies seems likely to
continue to grow in the near future.
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