CCS explained > Carbon capture

CO2 accounts for a significant amount of the gases emitted from fossil fuel conversion for power generation, for example, making up 3-15% of the gas (known as flue gas) that results from fossil fuel combustion for power generation.

CO2 must be separated, or captured, from the flue gas before it can be geologically stored. There are three main methods for capturing CO2 from fossil fuel combustion. Post-combustion captures CO2 directly from flue gases exiting standard combustion processes, typically with the use of solvents, sorbents, membranes or cryogenics, pre-combustion capture produces a synthetic gas from fossil fuels which have been stripped of carbon prior to combustion, leaving hydrogen to burn, and oxyfuel combustion burns coal or gas in pure oxygen to yield only CO2 and water.

The captured CO2 must be as pure as possible – the presence of impurities, such as nitrogen and sulphur oxides, in the captured CO2 can increase the costs of transportation and compression and can lead to more uncertain behaviour in the geological storage reservoir.

Post-combustion capture

In post-combustion capture, fuel is burned as usual in a (more-or-less) unmodified power plant. Currently the commercialised technology for post combustion capture is chemical absorption. It is the most advanced method due to considerable industrial experience with similar processes. CO2 is absorbed from the flue gas in a separation tower using a solvent and regenerated by heating in a recovery column at temperatures over 100˚C. Post combustion is advantageous because it can be applied to already constructed plants (retrofit), where components can be replaced, developed and upgraded without fundamental impacts on the power plant.

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In post-combustion capture, fuel is burned as usual in a (more-or-less) unmodified power plant. Currently the commercialised technology for post combustion capture is chemical absorption. It is the most advanced method due to considerable industrial experience with similar processes. CO2 is absorbed from the flue gas in a separation tower using a solvent and regenerated by heating in a recovery column at temperatures over 100˚C. Post combustion is advantageous because it can be applied to already constructed plants (retrofit), where components can be replaced, developed and upgraded without fundamental impacts on the power plant. This also allows for staged introduction of carbon capture onto a plant, which reduces disruption to the plant as well as investment risk. The fact that post-combustion can be retrofitted to existing power plants means that such demonstration projects have thus far been more common than for other capture technologies. The major disadvantage of chemical absorption for post combustion capture is the large energy penalty associated with thermal solvent regeneration. Other research challenges include large equipment requirements due to high volumes of flue gas, corrosion of equipment in the presence of oxygen and other impurities, solvent degradation due to reaction with oxygenated impurities, potential releases of harmful solvent degradation products and disposal requirements of expired solvent. Future development of second and third generation capture technologies to improve efficiency and reduce cost may include better liquid solvents and novel membranes or microporus solids.

Pre-combustion capture

Pre-combustion capture, typically operated with Integrated Gasification Combined Cycles (IGCC), involves gasification and partial oxidisation of the fuel to produce CO2 and hydrogen which are then separated, commonly using physical absorption processes.

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Pre-combustion capture, typically operated with Integrated Gasification Combined Cycles (IGCC), involves gasification and partial oxidisation of the fuel to produce CO2 and hydrogen which are then separated, commonly using physical absorption processes. The hydrogen is then combusted in a modified gas turbine or fuel cell producing power and water. This method has been demonstrated at the megaton-per-year scale but it has not been fitted to an operational power plant. Advantages for pre-combustion are that multiple fuels can be used and the hydrogen produced can be utilised as a transportable fuel or product. There are potential increased efficiency gains from improved integration of the technology into power plants. Disadvantages include high construction costs, reliability of all components for efficient integration and decreased short-term flexibility. Further development of high-temperature membranes may allow syngas to be reformed into CO2 as hydrogen is separated. The FutureGen project is looking to build and demonstrate this capture technology in Illinois, USA, using a 275-MW capacity power plant.

Oxyfuel combustion

Oxyfuel combustion separates oxygen from air using established cryogenic methods and then burns the coal or gas fuel in a mixture of that oxygen, often combined with recycled flue gas to regulate the temperature of combustion.

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Oxyfuel combustion separates oxygen from air using established cryogenic methods and then burns the coal or gas fuel in a mixture of that oxygen, often combined with recycled flue gas to regulate the temperature of combustion.

Advantages of oxyfuel are the comparative ease with which CO2 can be separated (no solvent is required) which allows for very high capture levels, small physical size of the unit, and the possibility of retrofit to an existing plant with some alterations. Drawbacks include , inflexibility due to the use of multiple burners and air separation requirements, large energy penalty of the air separation unit, high combustion temperatures requiring expensive materials, very low SOx require on leaving burners, possible need for an extra purification stage for the CO2 and the challenges of operation at sub-atmospheric pressure to prevent leakage leading to ingress of air and reduced purity. Future gains may come from improved high temperature operation, reduction of energy costs of oxygen separation from the air and minimizing air leak.

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Carbon capture

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Carbon storage

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Systems and Policy