Carbon Capture and Storage (CCS)
Process to concentrate carbon dioxide e.g. produced during combustion and secure storage of the carbon dioxide in corresponding deposits.
Carbon Capture Strategies
There are different strategies to get the carbon dioxide from the flue gas:
- Air as oxidant: Chemical/physical extraction: The carbon dioxide is washed out by liquid chemicals (adsorbent) which adsorb the carbon dioxide. The adsorbed carbon dioxide can be released from the adsorbent by heat and stored. The adsorbent will be reused in a cycle process. This process avoids the storage of nitrogen which is the major fraction (ca. 80 %) of the flue gas stream.
- Oxigen as oxidant: Direct storage of flue gas: The nitrogen is removed from the air e.g. by air liquefaction and distillation and oxygen is used to burn the coal. This technique is called oxyfuel and avoids the after-processing of flue gas which consists in this case mainly of carbon dioxide.
Both strategies demand 1. additional processing systems for power plants and 2. additional energy demand inside the power plant. Net efficiencies are reduced by 10-20 % - a coal fired powerplant (current generation) with an efficiency of 45 % delivers, equipped with a CCS system, just 35...40 % net efficiency.
Carbon Storage Strategies
Secure carbon dioxide storage of gaseous carbon dioxide for long periods - several tenths of thousands of years - is a non-proven technology. Carbon dioxide as gas can permeate stony layers much easier compared to liquid or solid compounds. Geological properties of CO2 deposits have to be known exactly which is not always possible - especially in the case of long term predictions of the stability of geological formations. The requirements in terms for CO2 deposits are similar to nuclear waste deposits.
There exist other proposals for the mineralization of carbon dioxide to geologically stable compounds. This method is secure but requires additional material flow of solid educts and products. To minimize transportation input mineral deposit, coal mine/power plant and output mineral deposit should be located in the same region.
Further proposals use biomass or chemical processes to regain fuels from carbon dioxide. In that case the carbon (dioxide) is not stored but produced one times for two phases of energetic use:
Coal ? ENERGY (1) ? + CO2 ? Fuel ? ENERGY (2) ? + CO2 ?
Up arrows mean that the product is used and NOT regained. In that case one can see a reduction of the specific carbon dioxide emissions by a factor of two.