Carbon Capture and Storage: Burying our Problems?
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.
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.
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.
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.
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.
J A Kemp has extensive expertise in advising on inventions in this field and drafting, filing and prosecuting patent applications relating to such inventions worldwide. See our Green Energy and Climate-tech specialism to find out more.