Oxyfuel

Oxyfuel combustion with CO2 capture is very similar to post-combustion CO2 capture. The main difference is that the combustion is carried out with pure oxygen instead of air. As a result the flue gas contains mainly CO2 and water vapor, which can be easily separated. The challenge is that it is expensive to produce pure oxygen.

In traditional fossil fuelled power plants combustion is carried out by using air, with the nitrogen (N₂) in the air following the flue gas. An alternative is to use pure oxygen (O₂) instead of air in the combustion. The advantage of this so-called oxyfuel technique is that the flue gas contains only steam and CO₂. These two components are easily separated through cooling, by which the water then condenses and a CO₂ rich gas-stream is formed. Up to 100 percent CO₂ can be captured through this process.

Oxyfuel combustion with CO2 capture. Illustration: Prosjektlab

The currently available technologies for pure oxygen-production are based primarily on cryogenic separation of air. Here the air is cooled down below the boiling point before the liquefied oxygen, nitrogen and argon are separated. However, the high amount of energy involved in this process make it a very expensive process and much research is subsequently carried out in order to develop membranes that separate oxygen from air more efficiently.

CO2 capture by the oxyfuel combustion route

The oxyfuel CO₂ capture process is not as mature as the post-combustion capture technologies. But within a decade or two it is expected that already ongoing research activities will improve the technology and make it a very good alternative for CO₂ capture.

The main advantage of oxyfuel CO₂ capture is that it generates a flue gas of only water vapor and CO₂, which can easily be separated in a traditional condenser. By cooling down the gas mixture of CO₂ and steam, water will condense, and we will then have a stream of pure CO₂ gas. This is much simpler and cheaper than the complex and expensive absorption process that is needed in the post-combustion CO₂ capture process.

One might think that combusting with pure oxygen is the greatest advantage of oxyfuel, but the oxygen is also the biggest challenge. It is expensive to produce pure oxygen, and research activities are ongoing worldwide to find new ways to produce oxygen. The most common method is to separate oxygen from air by cryogenic distillation. This requires large amounts of energy. An alternative of the future that may prove more efficient is to produce pure oxygen in membrane modules

Another challenge is materials. The combustion of fossil fuels and pure oxygen creates high material stress, hence the development of new materials is a prerequisite for deployment of this technology.

• Post combustion CO2 capture
• Pre combustion CO2 capture
• Novel technologies

• Vattenfalls oxyfuel demo at Schwarze Pumpe
• ENCAP - R&D projects focusing on oxyfuel