Carbon Capture

Post-combustion Capture

Post-combustion capture refers to separation of CO₂ from flue gas (the exhaust from combustion) after fossil fuels are oxidised (burnt) in boilers, furnaces or other industrial apparatus.

The established technique at present is to scrub the flue with a suitable solvent such as an amine solution. The amine-CO₂ complex formed is then decomposed by heat to release high purity CO₂ and the regenerated amine is recycled to be reused in the capture process.

Post-combustion capture is applicable to coal-fired power stations but additional measures, such as desulphurisation of the gas stream, are needed to prevent the impurities in the flue gas from contaminating the CO₂ capture solvent.

Two challenges for post-combustion capture are:

• the large volumes of gas which must be handled, requiring large-scale equipment and high capital costs, and
• the amount of additional energy needed to operate the process.

Pre-combustion Capture

Pre-combustion capture increases the CO₂ concentration of the flue stream, requiring smaller equipment size and different solvents with lower regeneration energy requirements.

The process involves:

1. the fuel being partially reacted at high pressure with oxygen or air and, in some cases, steam, to produce carbon monoxide (CO) and hydrogen (H2)
2. the CO is reacted with steam in a catalytic shift reactor to produce CO₂ and additional H2, and
3. the CO₂ is then separated and, for electricity generation, the H2 is used as fuel in a combined cycle plant.

Although pre-combustion capture involves a more radical change to power station design, most elements of the technology are already well proven in other industrial processes.

Oxyfuel Combustion

The concentration of CO₂ in flue gas can be increased by using pure or enriched oxygen (O2) instead of air for combustion, either in a boiler or gas turbine.

The O2 would be produced by cryogenic air separation (already used on a large scale industrially), and the CO₂-rich flue gas would be recycled to avoid the excessively high flame temperature associated with combustion in pure O2.

The advantage of oxyfuel combustion is that the flue gas contains a high concentration of CO₂, so the CO₂ separation stage is simplified.

The primary disadvantage of oxyfuel combustion is that cryogenic O2 is expensive, and that oxyfuel combustion for power generation has so far only been demonstrated on a small scale.