Technology

CO₂ capture and storage (CCS) is a 3 step technology to reduce CO₂ emissions from power and industrial plants (or production plants) that emit large volumes of CO₂:

1. Capture of CO₂ from large emission sources like coal power plants or iron and steel industrial plants.
2. Transport of CO₂ to a suitable storage location
3. Injection of CO₂ deep underground to ensure safe storage

A brief introduction to CCS

CO₂ capture and storage (CCS) is a technology that can enable a significant reduction in global CO₂ emissions.

About half of global man-made CO₂ emissions come from large plants like coal and gas power plants, and industrial plants for the production of iron and steel, cement and chemicals. Today, the CO₂ from these plants is emitted into the atmosphere. CCS makes it technically possible to eliminate most of these emissions.

For example, the flue gas from a coal power plant or from a industrial plant for production of chemicals contains large amounts of CO₂, which is captured, i.e. separated from other flue gas components. The CO₂ is then transported in a pipeline for deep underground storage. Safe storage sites are in most cases located some distance away from the CO₂ source, and transport for hundreds of kilometers might be required. Safe storage sites require the geology of porous rocks that can hold large amounts of CO₂. On top of that porous rock there must be a solid 'cap' rock which does not let CO₂ pass through. CO₂ storage occurs by the same mechanisms that have kept oil and gas underground for millions of years.

CO2 capture

: Figure 1. Post-combustion capture.

There are many ways to capture CO₂. The illustration above (Figure 1) shows one of the capture methods, applied to fossil fuel power generation plants, also applicable for industrial plants. The CO₂ is captured, or separated, from the flue gas after (or post) the combustion of coal in a coal power plant. This way of capturing CO₂ is therefore called post-combustion CO₂ capture.

Another method is to remove CO₂ before combustion. This is called pre-combustion CO₂ capture and in the process the fuel, i.e. coal or natural gas, is treated in a special process to transform it to hydrogen and CO₂, which can be separated. The hydrogen is then used as fuel in a power plant, and the CO₂ is transported to a storage site.

A third option is to combust the fuel (coal, gas) with pure oxygen, instead of air. This 'oxyfuel' process makes capture of CO₂ very simple for power generation and industrial plants, as virtually all of the flue gas is CO₂. The challenge is to efficiently separate pure oxygen from air.

A number of separation technologies are available for both post- and pre-combustion CO₂ capture. The most common is to absorb the CO₂ in a liquid containing amines (shown above). Numerous other technologies are also being developed, including new absorption liquids, gas separation membranes, thermal (cryogenic) separation methods, techniques for binding CO₂ in solid materials (mineralisation), and chemical-looping-combustion. These technologies can also be used to capture CO₂ from industrial processes, such as cement production or oil refining.

CO2 transport

: Figure 2. Options for CO2 transport: Ships and pipelines onshore and offshore.

There are two ways to transport CO₂: by pipelines or by ship (Figure 2).

When the storage site is located onshore, CO₂ is transported by pipelines from the power plant to the storage site. When the storage site is offshore, CO₂ can either be transported by ship or by pipelines on the bottom of the sea. Ship transport is often preferable for long distances, and is a good option when the amount of CO₂ to be transported is small or when flexibility is important. Pipeline transport is preferable when the distance to the storage site is shorter, or when the volume of CO₂ is large.

CO2 storage

: Figure 3. CO2 storage deep in a suitable geological formation deep underground

CO₂ is stored more than 800 meters below the ground (Figure 3). At such depths, CO₂ is no longer found in the form of a gas; it is in a liquid state.

CO₂ storage can only take place in locations where the geology ensures that there will not be any leakage. Suitable geological formations are found in layers of porous rock which have space available for CO₂ similar to the way a sponge has space available for water. To be sure that the CO₂ is contained in the porous rock layer, a solid, non-porous, layer of rock must lie on top of the porous layer, providing a 'cap' that does not allow CO₂ to permeate upwards.

Read more on CO₂ storage and storage safety.

Where can we capture CO2?

For economic and practical reasons, capture of CO₂ is expected to be viable only at large point sources, such as fossil-fuel based power plants and large industrial installations (refineries, production plants for steel, cement, chemicals).

A large part of global CO₂ emissions come from cars, trucks, ships and planes running on oil, and these emissions are growing fast as international trade increases and more and more people can afford to buy cars. To combat global warming, it is crucial that we build a truly low-emission transport system where at least cars and trucks run on emission-free fuels: electricity or hydrogen. In effect, this means that the emissions are centralised to large electricity and/or hydrogen production facilities. In this manner way, CCS can also play an important role in reducing emissions from transport.

What is the potential for CCS?

About 50 percent of global man-made CO₂ emissions come from large industrial plants like coal and gas power plants and production plants for steel, cement and chemicals. Today, the CO₂ from these plants is emitted into the atmosphere. However, if CCS is added to these plants, their CO₂ emission could be almost eliminated.

There are however barriers to CCS deployment, and a study conducted by Bellona shows that CCS can reduce global CO₂ emissions by 33 percent by 2050.

CCS can also pave the way for other green future solutions. One example is to combine energy production from biomass with CCS. This will in fact remove CO₂ from the atmosphere, as biomass consumes CO₂ as it grows and CCS ensures that the CO₂ is stored underground.

Read more on Future solutions including CCS.

Challenges

CCS technology has been proven on small-scale applications but not on full-scale plants such as a commercial coal power plant. There are several challenges to overcome before CCS can be classified as a full-scale commercial technology.

The main challenge for capture is to scale up the technology to the same level as that required by a coal power plant. The best way to solve this challenge is to build large-scale demonstration plants, for which there are many plans worldwide. However, they are expensive and the question of how to finance them has not yet been resolved.

There are also several new CO₂ capture technologies that are not fully developed yet and more research is needed to further this development.

Transportation of CO₂ represents a huge challenge in itself as there is a lack of infrastructure for CO₂ transport. Engineering studies to find the best way to link sources and sinks (i.e. storage sites) of CO₂ must be performed, and pipelines must be built.

The challenges related to storage are substantial. There are possible storage sites all over the world, but these must be carefully characterised before we can judge which ones are safe storage locations. In addition, more research is necessary to determine how the CO₂ will behave when it is injected underground.

Environmental issues

CCS represents several environmental challenges. Solvents (or chemicals) used in the CO₂ capture process are hazardous waste and should be treated afterwards. If small leaks of CO₂ should occur during transportation and storage, the local environment can be affected.

The risks mentioned above are managable, and if CCS projects are well operated leaks should not occur. In sum, the risks are minor compared to the positive environmental effects of CCS given the large potential it has to reduce CO₂ emissions.