> Carbon Dioxide Capture:

CO₂ can be captured from large point sources, such as large fossil fuel or biomass electricity power plants, industries with major CO₂ emissions, natural gas processing, synthetic fuel plants and fossil fuel-based hydrogen production plants. The main pathways of CO₂ capture are:
> pre-combustion capture
> post-combustion capture
> oxy-fuel combustion

Post-combustion capture

In post-combustion capture the CO₂ is removed after fossil fuel burning. This is the method that would be applied to most conventional power plants. Here, carbon dioxide is captured (‘scrubbed') from the exhaust (or ‘flue') gases using amine scrubbers.

The technology is well understood and is currently used in other industrial applications, although not at the same scale as might be required in a commercial scale CCS power station.

Pre-combustion capture

In pre-combustion capture, the fuel is decarbonised through gasification with oxygen or through steam reforming under pressure (around 30-70atm) to produce ‘synthesis gas’, which contains CO and H₂O. Syngas is then passed through catalyst beds to produce CO₂ and H₂. Finally, the CO₂ (at 30-35%) is separated from H₂ by physical or chemical solvents. CO₂/H₂ separation occurs at very high pressures (pressure difference up to 80bar) and high temperature (300-700°C), which makes the process more complex and costly.

Pre-combustion capture is costly but it produces a more concentrated CO₂ gas stream. Pre-combustion capture is widely applied in fertilizer manufacturing and in hydrogen production.

Oxyfuel

In oxy-fuel combustion, fuel is burnt with nearly pure oxygen (up to 97%) instead of air, resulting in flue gas mixture of mainly (75% on wet basis) CO₂ and water vapour. Due to the high concentration of CO₂ and condensable water vapour, the CO₂ separation stage is easier compared to the same stage in other two capture options and involves simple condensation of water vapour, followed by extraction of CO₂. CO₂ is captured by direct physical compression and cooling techniques.

CO₂ Separation

Various physical and chemical methods are applied to separate CO₂ from the gas mixture. Capture technologies are classified into the following broad categories:
- Chemical, physical or hybrid absorption (solvent scrubbing)
- Adsorption (physical and chemical)
- Separation (using Membranes or Cryogenically)
- Bio reactors

Technologies for Carbon Capture and Time to Commercialisation

Reference: Figueroa et al 2008. Int. J. Greenhouse Gas Control 2;9-20.

Source: The Nottingham Centre for Carbon Capture and Storage

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