Why should CO2 be captured and stored?

If deep reductions in greenhouse gas emissions are required, (to meet the UNFCC goal of stabilisation of anthropogenic greenhouse gas emissions), then one method that could be used is CO2 capture and storage (CCS). CO2 capture and storage technology would be used in combination with the other mitigation measures (e.g. fuel switching, energy efficiency and renewable energy) to achieve the necessary deep reductions in greenhouse gas emissions.

Approximately one third of all CO2 emissions due to human activity come from fossil fuels used for generating electricity, with each power plant capable of emitting several million tonnes of CO2 annually. A variety of other industrial processes also emit large amounts of CO2 from each plant, for example oil refineries, cement works, and iron and steel production. These emissions could be reduced substantially, without major changes to the basic process, by capturing and storing the CO2. Other sources of emissions, such as transport and domestic buildings, cannot be tackled in the same way because of the large number of small sources of CO2.

Methods to capture and store CO2

There are many ways in which CO2 emissions can be reduced, such as increasing the efficiency of power plant or by switching from coal to natural gas. However, most scenarios suggest that these steps alone will not achieve the required reductions in CO2 emissions. The capture and storage of CO2 from fossil fuel combustion could play an important part in solving this problem. Widespread use of this technique could be achieved without the need for rapid change in the energy supply infrastructure.

In the long-term the world's energy system may have to be based on non-fossil energy sources. Decarbonising the use of fossil fuels, by capture and storage of CO2, would help the transition to a future carbon-free energy system.

What is the status of CO2 capture?

CO2 is already being captured in the oil and gas and chemical industries. Indeed several plants capture CO2 from power station flue gases for use in the food industry (see figure over page). However, only a fraction of the CO2 in the flue gas stream is captured - to reduce emissions from a typical power plant by 75% the equipment would need to be 10 times larger.

If capture is used to minimise CO2 emissions from power plant it would add at least 1.5 US cents/kWh to the cost of electricity generation. In addition, the generating efficiency would be reduced by 10 to 15 percentage points (e.g. from 55% to 45%) based on current technology. It is expected that wide-spread application of this technology would result in developments leading to a considerable improvement in its performance. The cost of avoiding CO2 emissions is 40-60 US$/tonne of CO2 (depending on the type of plant and where the CO2 is stored), which is comparable to other means of achieving large reductions in emissions.

What is the status of CO2 storage?

Having captured the CO2 it would need to be stored securely for hundreds or even thousands of years, in order to avoid it reaching the atmosphere.

There are two potential storage options, which are;

• Storage in the oceans
• Storage in geological formations

When describing CO2 in geological formations and oceans, the term “CO2 storage” is used. It is now commonly accepted that sequestration refers only to the terrestrial storage of CO2.

There are currently considerable uncertainties about the science of ocean storage. In addition, there are attendant legal issues that need to be addressed. Hence, ocean storage is less likely to be promoted as a mitigation option in the current situation.

Underground storage of CO2 in geological formations has taken place for many years as a consequence of injecting CO2 into oil fields to enhance recovery.

There are a number of potential geological formations that can be used to store captured CO2. These include;

• Depleted and disused oil and gas fields
• Deep saline aquifers
• Deep unminable coal seams

Many of these geological structures have already held hydrocarbons or liquids for many millions of years.

There are currently a number of large geological storage projects underway around the world e.g. Sleipner, Weyburn, In Salah, and several new projects in development e.g. Snohvit, Gorgon. There are also a number of smaller research scale projects e.g. Frio, CO2SINK. For further details of worldwide CO2 Capture and Storage projects see: IEA GHG Practical CO2 R, D & D Projects Database.

IEA GHG has produced a world map of large CO2 Capture and Storage Projects (0.1 MB Adobe Acrobat PDF).

The next steps

The main priority for the development of CO2 capture technology is to reduce its cost.

For CO2 storage the priority is to establish its credibility and acceptability as a safe, reliable, long-term store. Proof that any losses will be insignificant is a major issue for storage. The fact that CO2 has been naturally stored for geological time-scales enhances the credibility of many of the storage options. The number of practical Research, Development and Demonstration (R, D & D) projects on CO2 Capture and Storage is continually increasing.