Fact sheet - CO₂ capture

A fact sheet telling you the basics of CO2 capture is given below. You can also download this fact sheet as a pdf file

Introduction

CO2 emissions can be reduced by capturing CO₂ from large point sources and storing it at a safe underground location. CO₂ capture means separating the CO₂ from flue gas emitted from large point sources, such as fossil fuelled power plants or industrial installations.

Production of energy is responsible for about 50 percent of global CO₂ emissions, while transportation accounts for about 20 percent (IEA, 2007). The remaining emissions arise from industry and other sources. It is practical to capture CO₂ from large point sources, such as fossil fuelled power plants and major industrial installations, but it is not practical to capture CO₂ from small emission sources like vehicles. However, future energy carriers in the transportation sector can be electricity and hydrogen, which can be produced in large plants with CO₂ capture.

Technologies for CO₂ capture

There are multiple technologies for CO₂-capture available; most of them can be classified into three main groups:

• Post combustion CO₂ capture:  CO₂ capture from the flue gas after combustion of the fossil fuel.
• Pre combustion CO₂ capture: Removal of CO₂ from the fossil fuel prior to combustion.
• Oxyfuel with CO₂ capture: Combustion of fossil fuel with pure oxygen rather than air and removal of CO₂ after the combustion.

Post combustion CO₂ capture

To rinse existing emissions, post-combustion CO₂ capture can be used. The process is most commonly based on chemical absorption, where the flue gas is brought into contact with a chemical absorbent with an ability to attach the CO₂, see figure 1. Typical absorbents are amines and carbonates.

The scrubber column is designed to ensure that the exhaust gas and the absorbent are brought into close contact with each other. The CO₂ is then transferred from the flue gas to the absorbent, and there are two out-going flows from the scrubber column; a cleaned gas-stream with low CO₂ content and a liquid-stream containing water, absorbent and CO₂.

After the absorption process, the absorbent and the CO₂ are separated in a regeneration column. When heated, the absorbents ability to retain CO₂ is reduced, resulting in regeneration of the absorbent, which can then be re-used. The CO₂ leaves the regeneration column as a gas stream of high CO₂ purity. This gas can be transported to a CO₂ storage site.

80 to 90 percent of the CO₂ from a power plant can typically be removed by post-combustion CO₂ capture.

Figure 1. Post combustion CO2 capture.

An advantage of post-combustion capture technology is that it can be added to existing power plants without modifying the original power plant.

There are currently no large scale CO₂ capture plants. Smaller plants exist, but it is still a large technical challenge to build a capture plant at the size required for a coal power plant.

Pre combustion CO₂ capture

CO₂ can be separated from the fossil fuel before combustion, the so-called pre-combustion CO₂ capture method.

The principle of this process is first to convert the fossil fuel into CO₂ and hydrogen gas (H₂). Then, the H₂ and the CO₂ is separated in the same way as under post-combustion, however a smaller installation can be used. This results in a hydrogen-rich gas which can be used in power plants or as fuel in vehicles. The combustion of hydrogen does not lead to any creation of CO₂. The process is illustrated in figure 2.

The pre-combustion CO₂ capture is applicable to new coal power plants and there is a lot of focus on the IGCC technology (Integrated coal Gasification Combined Cycle), where the power is produced from combined hydrogen combustion and from a steam turbine. Pre combustion CO₂ capture is also applicable for natural gas power.

By pre-combustion CO₂ capture about 90 percent of the CO₂ from a power plant can be removed. As the technology requires significant modifications of the power plant, it is only viable for new power plants, not for existing plants.

Figure 2. Pre-combustion CO2 capture.

Using today’s technologies, the investment costs for a gas power plant with pre-combustion CO₂ capture will be higher than for a similar plant using post combustion of flue gas. The separation of CO₂ from fossil fuel prior to combustion will become far more interesting as technological development will bring down investment- and operating costs.

Oxyfuel

In traditional fossil fuelled power plants, combustion is carried out using air, where the nitrogen (N₂) in the air follows the flue gas. An alternative is to use pure oxygen (O₂) instead of air in the combustion. The advantage this so-called oxyfuel technique is that the flue gas only contains steam and CO₂, which can easily separated through cooling. The water then condenses, and a CO₂ rich gas-stream is formed. Up to 100 percent CO₂ can be captured in this process which is illustrated in Figure 3.

Figur 3. CO2 capture using oxyfuel combustion.

The combustion of natural gas and pure oxygen gives high material stress, hence the development of new materials is a prerequisite for deployment of this technology.

Another challenge is to develop a cost effective process for production of pure oxygen.

Current status of CO₂ capture

As of today, no power plants with CO₂ capture have been realized. The reasons being the significant financial risk associated with technological investments and lack of infrastructure for capture, transportation and storage.  

There is not expected any paradigm shift in CO₂ capture technology in the near future, and the short-term development is thus expected to be simply a further development of existing technologies.

Optimising current technology will lower the capture costs; raise the efficiency in power plants with CO₂ capture; and give greater flexibility in terms of fuel quality.

CO₂ capture is receiving an increasing amount of attention and is continuously advancing on the list of political and corporate priorities. Several initiatives is launched worldwide. One example is the EU where there are plans to build 10 to 12 demonstration plants for CO₂ capture and storage by 2015 and thereby commercialize the technology by 2020.

New novel technologies that could be viable post 2020, like membranes, adsorption and chemical looping are also being developed.

References

International Energy Agency (IEA), World Energy Outlook 2007, OECD and International Energy Agency report, Paris, 2007.

International Panel on Climate Change (IPCC), “Carbon Dioxide Capture and Storage”, http://www.ipcc.ch/activity/ccsspm.pdf

External links

Bellona CCS Web, http://www.bellona.org/ccs

The European Technology Platform for Zero Emission Fossil Fuel Power Plants (ZEP), http://www.zero-emissionplatform.eu/

CO2 Capture Project (CCP), http://www.co2captureproject.com/index.htm

FutureGen, http://www.fossil.energy.gov/programs/powersystems/futuregen/

All external links are valid per 29 January 2009. Changes in external links after this date are beyond the control of the Bellona Foundation.