CCS (Carbon Capture and Storage) involves separating carbon dioxide, particularly from combustion plants, and injecting and storing it in deep layers of rock underground. Currently, three processes are being tested for CO2 separation in the power-plant sector:
During the separation process, the CO2 is segregated from the other exhaust components, dried, and compressed for transport. Research is still needed in order to make CO2 separation more efficient and thereby more cost-effective.
There are two main options for transporting the CO2 from the separation site to the appropriate storage sites: by ship and by pipeline. Carbon dioxide is transported as liquid on ships, and as supercritical fluids in pipelines (phase state above certain pressures and temperatures; supercritical CO2 behaves like a gas, but has the density of a liquid). Both transport types are currently used for supplying propane and butane (LPG, Liquid Petrol Gas) and for natural gas.
In principle, gases can be stored deep underground for very long periods of time. Most of the natural gas reserves that are now used worldwide developed millions of years ago; since that time, they have securely trapped the gas underground. In addition, natural CO2 gas reserves (for instance in the Rhön region of Germany and in the south of France) show, that it is possible to store CO2 efficiently in the long term in suitable layers, and to seal them off using cap rocks. If future CO2 storage sites are chosen carefully and investigated according to the latest science and technology, no leakage is to be expected.
The best options for storing CO2 are depleted oil and natural gas storage sites as well as deep rock layers containing salt water (saline aquifers).
The geological storage of CO2 is an important issue both nationally and worldwide. Since 1992, pure CO2 has been stored on an industrial scale in the Sleipner gas field off the coast of Norway and in the Algerian natural gas field In Salah (since 2004). At Weyburn in Canada, CO2 has been used since 2000 to increase natural oil production. Worldwide, there are currently more than 20 ongoing projects.
Germany can draw on decades of experience in related technology fields, such as the underground storage of natural gas and saline injection. Based on this experience, we assume that the injection and geological storage of CO2 can fundamentally be implemented with little risk to people or the environment. However, the geological storage of CO2 does raise several technical and scientific questions and requirements for injection operations and for the evaluation and monitoring of storage systems. In addition, because of the long observation periods involved, the process of evaluating a CO2 storage site and evaluating its long-term safety relies heavily on reservoir simulations. These simulations must be calibrated and validated using geophysical and geochemical monitoring methods in the course of active storage projects.
