This post is the second part of our four-part CCS Explainer series. In this series we are exploring what CCS is, why it is needed and the challenges it faces. As a way of introduction, today’s post this post will explain why CCS is an indispensable climate mitigation technology – even if it isn’t the sexiest solution on the shelf.
Earth has been going through cycles of higher and lower concentration of CO2 and other greenhouse gases in its atmosphere that span millennia. This is a natural result of increasing and reducing storage of these gases in the ground in what is known as the long-term carbon cycle. At a geological time scale, CO2 is released in the atmosphere, and then recaptured in geological formations over thousands of years. This is complemented by a short-term carbon cycle where CO2 is routinely captured in plants and animals and then released in a very short timeline back to the atmosphere. Anthropogenic emissions have completely de-regulated that system, as we have extracted millions of years of CO2 and released it in the atmosphere in the form of fossil fuels and other raw materials.
To a degree or another, the human contribution to excess carbon emissions can be overcome by switching to other sources of energy. However, there will remain a degree of unavoidable emissions, from the processing of raw materials into workable materials to sustain our modern day economy and society. This is where carbon capture has a role to play in addressing the climate crisis. But how does it work in practice? How can we capture CO2 out of thin air?
CLIMEWORKS CO2 Capture Plant in Iceland
For CCS to work, CO2 must be captured from industrial emissions before it reaches the atmosphere. This poses a technical challenge since the CO2 is mixed with complex flue gases and must be isolated before storage. While capturing CO2 itself is straightforward, doing so efficiently and cost-effectively can be more complex. There are three broad categories of CO2 that can be captured:
- Fossil CO2: Both CO2 that was emitted from the combustion of fossil fuels, and the CO2 that is released from process emissions on the basis of extracted material, such as lime or bauxite for aluminium production.
- Biogenic CO2: Biogenic CO2 captured whenever biomass, or bio-based materials release CO2 due to human activities.
- Atmospheric CO2: As its name implies, this refers to CO2 that is already in the atmosphere due to prior CO2 emissions, and that is being captured to reduce the current concentration of greenhouse gases in the atmosphere.
Both biogenic and atmospheric CO2 are considered to be permanent carbon removals, as they take carbon away from the short term carbon cycle and store it permanently underground. By contrast, capturing CO2 that we have extracted from the long-term carbon cycle in the form of fossil CO2 is only considered emissions reduction, since it does nothing to reduce the amount of CO2 in that short-term carbon cycle.
In most cases, CO2 is being captured in two stages: extraction, and then separation. The first stage is often neglected because carbon capture generally happens post-combustion, meaning the CO2 is already in a gaseous state and therefore free to be captured, but this is an essential step. The CO2 must be made available for capture. Similarly, once it is available it must be isolated from the complex gas in which it finds itself and purified into only CO2, which can then be liquefied and stored underground.
Extracting CO2
Capturing CO2 involves two main steps: first, extracting it from the feedstock into a gas, and second, isolating it within that gas for transport and storage. In most industrial processes, the first step happens naturally, as CO2 is released as a byproduct of fuel use or the production process itself. This method is known as post-combustion carbon capture, where CO2 is separated from the mix of gases emitted by the facility. While post-combustion capture is a well-established and efficient technology, it often results in a relatively low concentration of CO2, which can make capturing it more challenging.
To address this, alternative methods are being developed, including pre-combustion capture and oxy-fuel combustion:
- Pre-combustion capture: This approach involves extracting CO2 from the feedstock before it is used. Primarily applied to fossil fuels, this method produces a more concentrated stream of CO2 and generates hydrogen as a valuable byproduct. Although promising, pre-combustion technology is still in the early stages of development.
- Oxy-fuel combustion: Similar to post-combustion capture, this method involves burning the fuel, but in a pure oxygen environment instead of air. By minimising the production of other gases, it increases the concentration of CO2 in the emissions, making it easier to isolate and capture.
Each method offers unique advantages and challenges, and ongoing innovation is critical to improving their efficiency and scalability for industrial applications.
Separating the CO2
Carbon capture may seem like a uniform process, but there are multiple methods for isolating CO2 from surrounding gases. The most established technique is solvent-based carbon capture, which uses solvents to bind CO2. Typically, an amine (an ammonia derivative) reacts with the CO2 in flue gas, forming a compound that is then transferred to another chamber. Heat separates the CO2 from the solvent, allowing the solvent to be reused immediately. This approach is both efficient and effective, making it the most widely used method for post-combustion carbon capture.
However, solvent-based capture has a notable drawback: it requires significant heat and energy, as the gas must be repeatedly heated and cooled to allow the solvent to bind and release CO2.
Emerging methods, such as membrane filtration, aim to address this energy challenge. This technique uses high pressure and a CO2-selective membrane to trap CO2 molecules while letting other gases pass through. Membrane filtration could significantly lower the energy demands of carbon capture, but developing membranes with the necessary selectivity to ensure only CO2 is captured remains a key challenge.
Concluding thoughts
With these technologies, it is now possible to capture the CO2 from industrial processes before it reaches the atmosphere and to capture CO2 from the short-term carbon cycle to put it back underground where we extracted it over hundreds of years. Thanks to the technology, the EU can ensure the competitiveness of its hard-to-abate industries, while achieving climate neutrality by 2050. To do so, however, will require an estimated 280 Mt of CO2 being captured from industrial facilities by 2040, and about 75 Mt of industrial carbon removals. The scale of the challenge is daunting, but surmounting it is instrumental to the success of the green transition as a whole.
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