Engineering

The Science Behind Carbon Capture

Can carbon capture save the planet, or is it just another costly distraction? Methods like post-combustion capture and direct air capture aim to reduce atmospheric CO2 levels. However, concerns about cost, energy use, and effectiveness raise questions about its role in fighting climate change.

Thomas Grieser

12 Mar 2025 — 6 min read

Figure 1: A carbon capture facility located near Leeds [A]

For thousands of years, the carbon cycle remained in balance, with carbon moving naturally around our planet. However, since the start of the industrial revolution in the 18^th century, human activities have disrupted this cycle, releasing CO₂at a faster rate than nature can absorb. However, what if there was a way to reduce the amount of CO₂ released every year and return atmospheric CO₂levels to their former state of balance?

Carbon capture and storage (CCS) is a process that involves capturing CO₂ from industrial facilities before it’s released into the atmosphere. As of November 2024, there were more than 50 CCS facilities operating worldwide [i], working to reduce emissions from power plants, factories, and other high-emission industries. This article will explore the different types of CCS, and how they work to reduce carbon emissions from industrial sources.

How does Carbon Capture Work?

The most common form of CCS is post-combustion capture (PCC) [ii]. This method captures emissions (known as flue gas) from smokestacks at industrial facilities, and then separates the CO₂from the other components of the gas. Once collected, the CO₂ can be stored underground or used for a variety of other industrial purposes.

The most common process for separating CO₂ uses amine, which chemically bonds to the CO₂, allowing it to be removed from the flue gas. The process is as follows:

1. Gas is captured from a smokestack.

2. The gas is put into a chamber (called an absorber) which contains amine solution, which is heated to 40-70°C.

3. The amine binds with the CO₂, separating it from the industrial gases.

4. The amine solution is transferred to a second chamber (called a regenerator),

which is heated to 100-150°C.

5. The amine releases the CO₂.

6. The separated CO₂ is then pressurised and transported to a storage facility.

Figure 1: a simplified flow diagram of an amine-based carbon capture process. [B]

The main advantage of PCC is that it can be installed on existing power plants and industrial facilities without the need for costly modifications to core infrastructure [iii]. However, PCC can only prevent the release of additional CO₂into the atmosphere and is unable to remove existing CO₂that’s accumulated over centuries of industrial activity.

To remove CO₂that’s already in the atmosphere, a process known as direct air capture (DAC) must be used [iv].

Figure 2: “Mammoth”, the world’s largest DAC facility, is located in Iceland. [C]

DAC uses large fans to draw air into a processing facility [v]. Once the air is in the facility, one of two methods may be used.

The first is known as solid DAC (S-DAC), which uses sorbent filters to remove CO₂from the air. These filters chemically bond with CO₂particles upon contact, and then release them when heated or placed under a vacuum. This creates a concentrated stream of CO₂which can then be transported or stored underground.

The second is known as liquid DAC (L-DAC). Air is passed through solvents (liquid chemicals) that bond with the CO₂. The solvents are then heated/placed under a vacuum, which causes the CO₂ to be released. The CO₂can then be collected and stored, while the solvents can be reused.

Once the air has been treated, it can be returned to the atmosphere, and new air can be processed. While this method plays an important role in removing CO₂from the atmosphere, it has a few notable drawbacks [vi]. Firstly, a significant amount of energy is required to power the fans and heat the sorbent materials used in DAC facilities. A 2024 study conducted by Ian Tinseo estimated that “removing one billion tonnes of carbon dioxide (GtCO₂) using Direct Air Capture (DAC) would require around 1,200 terawatt-hours [of energy].” This is approximately three times the total energy produced by the entire U.S. renewable sector in 2019 [vii]. For reference, there are currently 1,840 billion tonnes of excess CO₂in the atmosphere [viii], meaning that scaling DAC to remove a substantial portion of existing emissions would require a massive expansion of energy infrastructure that would likely result in further CO₂emissions.

In addition, cost is a significant drawback of DAC. At present, the price of DAC varies between $250 and $600 per tonne of CO₂ captured[ix]. In contrast, reforestation costs on average less than $50 per tonne, and does not carry any long-term costs. Lastly, DAC facilities take up large areas of land, which makes finding a suitable location difficult. For example, the largest DAC facility in the world, “Mammoth”, is located in a remote part of Iceland due to the cost and physical constraints of constructing it in a more urban setting [x].

Storing CO₂

CO₂must be stored deep underground to prevent it escaping into the atmosphere [xi]. The gas is pumped into rock formations like saline aquifers or depleted oil and gas reservoirs. These must be at a depth greater than 800 m [xii] and covered by a layer of dense rock to ensure the CO₂does not seep out into the air. In addition, the rock must be porous and permeable to allow the CO₂to be absorbed. Examples include sandstone and limestone.

Figure 3: A simplified diagram of the carbon cycle. [D]

Scepticism About Carbon Capture Adaptation

Studies from respected scientists and institutions cast doubt that carbon capture is the most effective (or even net positive) way of reducing CO₂ in the Earth's atmosphere. A study by Mark Z. Jacobson of Stanford University stated that "Even if you have 100 percent capture from the capture equipment, it is still worse, from a social cost perspective, than replacing a coal or gas plant with a wind farm because carbon capture never reduces air pollution and always has a capture equipment cost." [xiii] He continued, "There is a lot of reliance on carbon capture in theoretical modelling, and by focusing on that as even a possibility, that diverts resources away from real solutions. It gives people hope that you can keep fossil fuel power plants alive. It delays action. In fact, carbon capture and direct air capture are always opportunity costs."

However, others disagree. A consortium of leading research institutions including University of Oxford Smith School of Enterprise and the Environment, the University of Pennsylvania, and the European Research Council all make the case that faster innovation in carbon removal technologies is the answer, and that these must be used alongside low-tech strategies such as reforestation to remove enough carbon from the atmosphere in time to prevent irreversible climate change [xiv].

Conclusion:

The science behind carbon capture is ambitious and evolving. Carbon capture may be the key to removing suﬃcient CO₂ from Earth’s atmosphere to stop, or even reverse climate change. As scientists, energy providers, environmentalists and policymakers continue their eﬀorts, the question remains [xv]: is carbon capture a needed tool to get us to the clean energy future, or a costly distraction that will delay the fight against climate change?