Carbon Capture: How Does it Work?

How is CO₂ captured?

CO₂ capture systems fall into three categories: post-combustion, pre-combustion, and oxy-combustion. 

Post-Combustion CO₂ Capture 

Capturing CO₂ from the flue gas of a power plant is referred to as post-combustion CO₂ capture as CO₂ is removed after the combustion in the power plant. Post-combustion CO₂ capture is primarily used for power generation from natural gas and coal. In a coal or gas power plant, fuel is burned with air in a boiler to produce steam, driving a turbine to generate electricity. The boiler exhaust (flue gas) consists of mostly nitrogen and CO₂. 

A post-combustion CO₂ capture absorption process uses high-grade chemical solvents such as amines and is deployed commercially in the refinery and chemical industries. Amine-based (and other) solvents are used in an absorption-regeneration cycle. The solvent reacts safely and quickly with CO₂, and then the solvent is transferred to a regenerator with a higher temperature. This reverses the CO₂ absorption and produces pure CO₂ gas and solvent, which can be returned to the absorber to start the cycle again.

Pre-Combustion CO₂ Capture in Gasification Applications 

In pre-combustion CO₂ capture, the CO₂ is removed from the fuel before combustion. Simply put, in the gasification of a feedstock such as coal, partial oxidation occurs (when a molecule loses one or more electrons in a chemical reaction) to provide the high pressure and temperature to produce a synthetic gas composed of hydrogen and carbon monoxide. 

The synthesis gas is then transferred to a water-gas-shift reactor for CO₂ removal, allowing the hydrogen-rich gas to be converted to power. The resulting hydrogen has many potential applications and can be used to generate electricity in a combined cycle plant, or for power generation using fuel cells. Additionally, pre-combustion CO₂ capture also uses chemical solvents, or alternatively, chemical sorbents or membrane separation technology. 

Sorbent-based CO₂ capture involves the physical adsorption (the adhesion molecules from a gas, liquid or dissolved solid to a surface) of CO₂ using a solid sorbent. The chemical properties of CO₂ allow for adsorption into porous solid sorbents.  

Membrane-based technologies for separating CO₂ and H2 in synthetic gas produce concentrated CO₂ at a pressure near the synthetic gas feed pressure, resulting in reduced energy and cost for compression of CO₂. Membrane designs include metallic, polymeric, or ceramic materials capable of operating at high temperatures and with a variety of chemical and/or physical mechanisms for separation.

When compared to post-combustion technology whereby CO₂ is removed from flue gas at low pressure, the pre-combustion synthesis gas has a higher concentration of CO₂ and operates at a higher pressure, allowing for the removal of hydrogen before combustion. Due to the more concentrated CO₂, pre-combustion capture is more efficient but carries higher capital costs.‍

Oxy-Combustion CO₂ Capture ‍

Oxyfuel combustion with CO₂ capture is conducted with oxygen instead of air, thereby removing the nitrogen gas component from the process. Because Nitrogen is not present, fuel consumption is reduced, and higher flame temperatures are possible. The primary products of oxy-combustion are CO₂ and water (H2O), whereby the resulting CO₂ is captured by condensing the water in the exhaust stream. 

Both pre-combustion and oxy-combustion use air separation to combust the feedstock in an enriched oxygen environment. However, the amount of oxygen required in oxy-combustion is significantly greater than in pre-combustion applications, increasing CO₂ capture costs.

How is CO₂ transported?

Captured CO₂ can be transported in a variety of ways, mainly by pipeline, via truck, ship or rail. CO₂  may require further processing for purity, and to match pressure conditions for transport type. New and innovative transport storage systems allow for CO₂ export from industrial production facilities; import for purposes of industrial use; or distribution for purposes of sequestration.

Pipelines are the most economical method of transporting large quantities of CO₂  via land, as hydrocarbon and natural gas pipelines can be upgraded for CO₂  transfer. Captured CO₂ must be compressed to a pressure between 1,500 and 2,200 psi to transport it via pipeline for storage or utilisation. CO₂ is currently transported by pipelines – with a very well-established pipe specification informed from decades of research in oil and gas. 

More than some 6,500 kilometres of CO₂ pipelines are already distributed across Africa, Australia, the Middle East and North America. Recently, DNV published new procedures designed to set safety standards for the transport of CO₂ by pipelines and to strengthen the development of carbon capture and storage (CCS) projects.

Transport by ship in small volumes (less than 1500m3) is already widely practiced, but ship transport could make for more favourable and flexible economics for CO₂  transport (IEAGHG). In smaller quantities, CO₂  is transported either by tanker truck in cryogenic vessels or by rail in specialised rail cars. 

Today, large scale CCS projects such as the Northern Lights project use ships to transport CO₂  from industrial sites to a permanent storage facility.

How is CO₂ stored?

‍Geological CO₂ Storage

The most common CO₂ storage method is sequestration thousands of metres deep in offshore saline aquifers. CO₂ is compressed to a fluid state and injected deep underground into porous and permeable rock where it will remain contained and isolated.

Several projects in Norway have investigated this geological CO₂ storage. Examples such as Equinor’s pilot project at Sleipner injects 1 million tonnes of CO₂ annually into sandstone and clay almost 1,000 metres under the seabed. 

Our understanding of CO₂ injection is increasing; SINTEF’s Pre-ACT project monitors data from CO₂ storage demonstration plants. The data will be used to calibrate and demonstrate the value of the developed methods and to develop a “protocol” or recommendations. However, we do not know the effect of CO₂ injection over the long term i.e. thousands of years from now, and many critics warn that we do not know enough to safely deploy this sequestration strategy.

In addition, CO₂ can be mineralized and sequestered in solid form by various techniques. This strategy of underground sequestration increases the uptake of CO₂ in the reservoir through the interaction with rocks containing magnesium or calcium ions and because it is converted to a solid carbonate, there is no risk of CO₂ leakage. The research agenda published by National Academies of Sciences Engineering Medicine (2019) suggests spending USD 1 billion over the next 20 years to advance the deployment of CO₂ sequestration for geological reservoirs at the GtCO₂ /yr scale and develop CO₂ mineralisation at the MtCO₂ /yr scale. (Kelemen, Peter et al.)

Temporary CO₂ Storage

In areas where the geology of underground formations is not suitable for storage, transport infrastructure is required to transport the captured CO₂ for utilization or storage elsewhere. Smart and flexible infrastructure allows small scale carbon capture profitable without the need for CO₂ import via trucks or constructing time and resources intensive jetties for CO₂ import via ships.

For some potential projects, especially industrial sites looking towards capturing and temporarily storing CO₂ for export, space restriction can be a barrier for committing to carbon reduction measures. In such instances the use of a storage barge will be a key enabler. A floating storage system allows for a high degree of flexibility and can be deployed quickly and without the environmental impact of building conventional marine infrastructure.

‍