Combating climate change requires a combination of solutions, of which one is CO2 capture, transportation, storage and utilisation (CCS).
CCS consists of the following phases:
CO2 capture, involving the separation of the CO2 contained in the flue gases produced at thermal power plants and in industrial processes.
Transportation of the CO2 captured to the geological site selected for storage.
Storage of the CO2 in the ground in geologically suitable locations.
Utilisation of the CO2, wherever possible.
CO2 capture involves separating carbon dioxide from the other gases produced in combustion chiefly at thermal power plants, oil refineries and cement plants and in the steel industry. Once separated it is transported and injected into deep geological formations where it will remain securely confined for an indefinite period. Thus the volume of CO2 emitted to the atmosphere is reduced.
In CO2 capture is essential to be able to compress the gas until it reaches a state similar to that of liquid water. This ensures the technical and economic viability of the subsequent phases of transportation, storage and use.
ELCOGAS´ pilot CO2 capture plant
There are currently three technologies which will in the near future allow CO2 to be captured on a large scale before it is emitted to the atmosphere:
Post-combustion: the CO2 is separated from the gases generated by the conventional combustion of fossil fuels. These gases contain mostly nitrogen from the air used in combustion. The option closest to commercial application in this category is chemical absorption, consisting of sending the gas flow from combustion through a column containing the liquid sorbent which captures CO2. Then the liquid flow goes through a regeneration stage in which the CO2 absorbed is released for subsequent transportation and storage and the sorbent, free of CO2, is re-circulated to the process.
Pre-combustion: this technology is generally used in conjunction with coal/waste gasification processes. As the fuel is gasified with oxygen a syngas is produced, containing mainly CO and H2. The CO is then made to react with water so as to generate more H2 (and more CO2). The hydrogen is used for energy recovery in an electricity-generating turbine or is stored for later use. Meanwhile the CO2 is separated for transportation and storage. The main advantage of this technique is that the CO2 is concentrated before separation and is moreover at a high pressure, so more efficient separation methods can be used.
Oxy-fuel combustion: in this case the combustion is with oxygen instead of air, which means that the flue gases consist almost entirely of CO2 (and water) and are separated relatively easily. This process, like the previous one, involves oxygen production and special arrangements in order to conduct the combustion in suitable conditions, but it is a highly promising technology.
Schematic diagram of the main CO2 capture processes and systems