CO2 CAPTURE AND STORAGE

Zero emission target

According to all the scenarios, naturalgas and coal could supply the electricity and industry sectors with sufficient electricity for at least another 50 years. But hydrocarbons mean greenhouse gases, which in turn raises the question of carbon dioxide capture and storage, the energy cost of which remains much too high.

Julianna Franco, chercheuse à l’université de Melbourne (Australie) relevant les données d’expériences sur membranes d’extraction du CO2. © CO2CRC
Julianna Franco, researcher at Melbourne University (Australia) reading the results of experiments on membrane-based CO2 extraction. © CO2CRC
Options de stockage géologique du CO2. © CO2CRC
Options for geological storage of CO2. © CO2CRC

All of Europe’s oil- and coal-fired electrical power plants equipped with a carbon capture and storage system (CCS). Is this a realistic prospect or pure fantasy? For the past three years the Castor (CO2 from capture to storage) project has been looking at Europe’s energy sector to test the feasibility of postcombustion capture systems and the accompanying CO2 storage methods.

Postcombustion makes it possible to intercept the CO2 within the smoke that is usually emitted into the atmosphere. “It has the advantage of being easily adaptable to traditional electrical power plants and as such is the capture system most suitable for shortterm implementation,” explains Pierre Le Thiez, Castor coordinator for the French Oil Institute. “The principle is simple. The smoke that escapes is processed inside a contactor that contains a solvent that binds with carbon dioxide. Once ‘enriched’ in this way, the solvent passes to a regenerator where it is heated to break the chemical links that bind it to the CO2. The carbonic gas is then captured and the depleted solvent reinjected into the circuit.” The method has been tested in Esbjerg, Denmark, since March 2006 where a coal-fired plant fitted with a CCS system was introduced as part of the Castor project. This unique pilot project, the only one of its kind anywhere in the world, will make it possible to test and improve postcombustion. “Reduction of the capture costs, at present responsible for around two-thirds of the total cost of CCS, is crucially important. Because if the process of reducing CO2 emissions proves as energyhungry as the electricity generation itself, CCS clearly ceases to be of any interest.”

The competitors: pre- and oxycombustion

Since 2000, a major research effort has also focused on two other capture options: precombustion and oxycombustion. These both offer possible potential in the longer term.

Precombustion, which captures the CO2 upstream, adds steam or oxygen to the fuel so as to transform it into synthesis gas – syngas – made up of CO2 and hydrogen. Once isolated, the hydrogen is used to generate electricity while the CO2 is liquefied prior to storage.

This constitutes a first step towards the hydrogen society. In Europe the research is being carried out by the HypoGen project – the counterpart to the US FuturGen project – that aims to build Europe’s first electrical power plant equipped with precombustion CCS. The first phase of HypoGen is being implemented by the Dynamis (Towards Hydrogen and Electricity Production with Carbon Dioxide Capture and Storage) project which is looking at feasibility issues, seeking in particular to reduce capture costs by 50%. “Nothing is certain as yet, we still have to convince the creditors of the project’s viability if they are to finance the construction of pilot plants,” explains Nils Anders Røkke, Dynamis coordinator within Sintef, an independent Norwegian research institute. “Some technological problems continue to bar the route to this generation of clean energy production, in particular the fact that as yet there exists no turbine able to run on 100% hydrogen.” Much less advanced technologically than the two previous methods, oxycombustion makes it possible to generate an exhaust smoke with a very high CO2 concentration.

It is enough to burn the fuel in pure oxygen rather than air to obtain a smoke with more than 90% CO2 concentration and which can then be captured directly as such. Nevertheless, the process remains too costly at present as it requires large quantities of energy to produce pure oxygen.”

Burying the CO2

“No capture without storage,” is Nils Anders Røkke’s judicious reminder. Indeed, there is no point capturing CO2 if you don’t know how to store it. “The problem is that it is impossible to test the viability of a process over hundreds if not thousands of years. So studies are concentrating on the analysis of natural geological sinks in which CO2 has been imprisoned for millions of years, as well as on the observation and study of existing industrial storage systems.

The data obtained is then extrapolated in the framework of these projects with the help of predictive computer models,” explains Pierre Le Thiez.

Ocean sinks (1) and mineral sequestration (2) are today no longer regarded as viable storage solutions as they present too many disadvantages compared with geological storage.

The latter involves injecting CO2 into the intergranular space of porous and permeable rocks that are present in geological formations virtually all over the world. These deep sedimentary deposits sometimes extend over hundreds or even thousands of kilometres and are generally filled with salt water, which is why they are known as saline aquifers. They sometimes already contain CO2 in their natural state, which led to the idea of injecting them with the gas. Experiments in this field have been carried out since 1996 by the Norwegian company Statoil in particular, at its Sleipner (Norway) site in the North Sea, and they have proved very convincing to date.

A profitable storage?

Saline aquifers can also include “trapping” structures that contain methane or oil. Storage can thus be optimised by injecting CO2 into almost exhausted deposits, re-pressurising them and extracting the residual oil or natural gas. This method of “CO2 injection assisted recovery” has been practiced by the oil industry for decades now and could also be used for unexploited coal deposits that are also candidates for carbon storage.

However, Pierre Le Thiez explains that these assisted recovery methods, whether from depleted deposits or unexploitable coal seams, are losing some of their interest. “Many of these reservoirs are too small and, in the case of oil or gas deposits, were very often penetrated in the past by a number of wells, raising the problem of impermeability. This is why I believe that saline aquifers are the most viable geological storage method.” Time is short for introducing systems for reducing CO2 emissions, such as CCS. Electrical power plants today generate 40 % of global emissions and CCS could also be applied to industries that use coal or gas as the principal fuels. It is a question of resources, believes Nils Anders Røkke: “The political recognition of global warming has catalysed a growing interest in CO2 capture, resulting in increased financing for related research. But resources are still sadly lacking with which to perfect these technologies as quickly as is necessary.”

Julie Van Rossom

  1. The oceans are natural carbon sinks. But they seem to be saturated already by the atmospheric CO2 that is also increasing their acidity.
  2. We are accelerating mineral carbonation, a natural process for the formation of carbonated rocks. But the technology remains very costly.

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  • Castor
    30 partners – 11 countries
    (AT, DE, DK, ES, FR, EL, IT, NL, NO, SE, UK)
  • Dynamis
    32 partners – 12 countries (AT, BG, CH, DE, DK, ES, FR, IT, NL, NO, SE, UK)
  • www.dynamis- hypogen.com