Concentrating CO₂ prior to storage is much more efficient

The Netherlands is committed to a substantial reduction in CO₂ emissions. To achieve this target, the government needs to pull out all the stops. This includes the introduction of CO₂ capture and storage. Daan Jansen of ECN has looked at all the possible technologies for capturing CO₂, and he still has plenty to keep him busy. “There is still much to discover and develop,” says the programme manager of the Hydrogen and Clean Fossil Fuels unit.

Jansen always starts his explanations with the same diagram: a distribution of the costs involved in reducing CO₂ emissions. This shows that CO₂ capture is responsible for about 70 -80 percent of these costs. The remaining 20-30 percent is divided evenly between transport and storage, usually underground. “These days everyone is talking about the problems of storing CO₂, as if this was the only major bottleneck,” says Jansen. “These problems must be solved, of course, but financially speaking, CO₂ capture is also a major obstacle. It needs to be much cheaper!”
CO₂ capture is already technically possible. And it is also clear where the CO₂ should be captured: in Europe, coal-fired and gas-fired power stations between them are responsible for two-thirds of all emissions. It is tempting to declare that all these power stations should just fit a CO₂ filter in their chimneys, like the soot filter in the exhaust of old diesel cars. “If only that were possible,” Jansen sighs. “Doing this retrospectively is an immense operation. The owner of the power station would never recover the investment. There is only one solution: the government – European or national – must make it compulsory for all power station building permits issued from now on to specify that measures must be taken to ensure that CO₂ capture can be incorporated at a later stage.

In addition to policy measures, CO₂ capture also has many technical aspects. Jansen shows that there are three main options for CO₂-free emissions.
• Post-combustion, in which the CO2 is captured before it disappears into the atmosphere via the chimney;
• Pre-combustion CO₂ capture, where the fuel is first converted into CO₂ and hydrogen. The CO₂ is separated out and the hydrogen is burned in a gas turbine. Another option is to purify the hydrogen even further and use it in a fuel cell;
• Nitrogen-free combustion, in which the fuel (hydrocarbon) is burned with pure oxygen, leaving pure CO₂ and water. This is known as the oxyfuel process.

For the latter two options, ECN is developing new, cheap ‘capture’ technologies. This preference is based on the fact that the higher the CO₂ concentration, the easier (and therefore cheaper) the capture of CO₂.

Reducing costs
The production of CO₂-free electricity is technically feasible, but at a price. The yield of the power stations would decrease, and it would be necessary to invest in air separation or gas separation. Whatever happens, the price of electricity would have to rise in order to recover the investments and the extra energy consumption. The research at ECN therefore serves two purposes: reducing the energy consumption of CO₂ capture by 40 to 50 percent and also reducing the extra investment costs.
One of the technologies being examined by ECN is a membrane reactor that converts natural gas and steam into H2 and CO₂. The hydrogen is separated off and sent to the power station burners. The CO₂ that remains is suitable for transport and storage. The researchers are also hoping for a breakthrough with the membrane technology for the oxyfuel process. The traditional method of separating air into nitrogen and oxygen is to cool it to nearly -200 degrees Celsius. The liquid air can then easily be separated into oxygen and nitrogen. However this process requires large amounts of energy and cannot be considered a sustainable option. ECN researchers are therefore trying to separate the gases using oxygen-conducting membranes.
Finally, ECN is working on the development of a technology in which CO₂ is absorbed by an absorbent. In this pre-combustion technology, CO₂ is removed from the mixture instead of hydrogen. This research is part of the EU project CACHET. More information about this will be given in a subsequent Newsletter.