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How: details

29/01-2009

The greatest source of CO₂ emissions are all the big coal power plants that emit large volumes of CO₂ directly into the atmosphere. Altogether, coal power plants, gas power plants, steel and cement plants and other large industrial plants emit more than half of global man-made CO₂ emissions.

The emissions from these plants is not only CO₂, but also harmless steam and nitrogen. CCS is a technology whereby the CO₂ is separated from these components. After the CO₂ is captured, it can be transported in a ship or by pipeline to a safe storage location.

Storage of CO₂ takes place deep underground in special geological formations where the CO₂ remains safely deposited.

« CO₂ sources	« CCS in brief
« Capture	« Transport
« Storage	« See also

CO₂ sources

Today coal power plants like the one below emit large amounts of CO₂ into the atmosphere. There are more than thousand coal power plants like this worldwide, and they represent about 30 % of global man-made CO₂ emissions.

Coal power plant

worldpress.com

In addition to coal power plants, we also get a lot of CO₂ emissions from gas power plants, refineries and industrial plants that produce steel, cement and ammonia. Worldwide, there are 8000 large plants emitting about half of global man-made CO₂ emissions. The other half of man-made CO₂ emissions stem mainly from transportation and buildings.

Check out a map of the large CO₂ sources.

CCS in brief

The purpose of CCS is to stop CO₂ emissions from large point sources like coal power plants, without closing down the plant. In order to illustrate how CCS works, you can compare the two illustrations below. The first one is a coal power plant today, without CCS, where the CO₂ is emitted directly into the atmosphere. The second illustration is a coal power plant equipped with CCS technology.

Coal plant without CCS

The concept of CCS is portrayed in the illustration below where a coal power plant is equipped with CCS. Instead of emitting the CO₂ to the atmosphere, the flue gas from the power plant goes directly to large tanks where the CO₂ is separated from other components in the flue gas, mainly nitrogen. The harmless components like nitrogen are then released to the atmosphere, while the CO₂ is captured. The CO₂ is now ready to be transported in a pipeline to a safe storage site.

Coal power plant with CCS

CO₂ can be stored underground at special storage sites. In most cases the storage sites are located some distance away from the power plants, which means that CO₂ has to be transported to a proper storage location.

Capture

From a technical and economic point of view there is no point in storing all of the flue gas coming out of a coal power plant. The flue gas contains only 10 percent CO₂. It is better to separate the CO₂ from the other components (mainly steam and nitrogen, but also some particles, sulfur, etc.) to get the smallest possible volume for transport and storage. Furthermore, the other components in the flue gas would cause corrosion problems and would contaminate the storage site. Therefore, the only feasible solution is to separate CO₂ from the other components in the flue gas. This separation process is often referred to as CO₂ capture.

There are many different technologies for CO₂ capture, one of which is described here to illustrate the concept. A detailed description of the different technologies available are given in a separate site on CO2 capture.

Capturing CO₂ from the flue gas of a power plant is often referred to as post-combustion CO₂ capture, because CO₂ is removed after (i.e. post) the combustion in the power plant. This process is illustrated below.

post combustion CO₂ capture with solvents.

SINTEF

In the figure above the flue gas from a power plant, or an industrial plant, enters the left tank. A liquid solvent is also added to this tank, and this solvent reacts with CO₂, but not with the other components of the flue gas, like nitrogen. The nitrogen gas flows through the tank and leaves at the top of the tank. The liquid solvent, which has reacted with the CO₂, leaves at the bottom of the tank.

Typical solvents are chemical compounds like amines or ammonia dissolved in water. These solvents cannot be transported or stored together with the CO₂ because they are hazardous and expensive. The solvents are therefore regenerated so that they can be reused.

In the right tank the solvent-CO₂ mixture is heated, and the bonds between solvent and CO₂ are broken. The CO₂ is then released and leaves the top of the second tank as gas. The pure amine leaves the bottom of the right tank and is recirculated to the first tank.

The amount of energy needed for heating the mixture of solvent and CO₂ is substantial. In fact, approximately one-third of the energy produced by the power plant is needed for this heating process. This means that the power plant must use a large part of its power production to run the CO₂ capture plant. This represents a substantial cost, which is one of the reasons why CCS has not been widely implemented yet.

Transport

The CO₂ collected by the CO₂ capture plant is transported to a suitable storage location. Prior to transport, however, the CO₂ must be dried and compressed. Drying is necessary to remove all water which can otherwise cause serious corrosion problems. Compression is necessary to increase the pressure and reduce the volume of CO₂ to be transported.

There are two transport options; ship or pipeline.

CO₂ Transport options: ship, onshore pipelines and offshore pipelines.

Prosjektlab

In general, ship transport is the cheapest solution for small volumes and large distances, while pipelines are cheapest for large volumes and longer distances.

Storage

CO₂ is stored more than 800 meters below ground. It cannot be stored everywhere, but only in locations where we are sure it will not leak out again.

CO₂ storage more than 800 meters under ground. The CO₂ is trapped in suitable geological formations.

StatoilHydro

There are several storage options. One of the most obvious is storage in oil and gas fields. The CO₂ can either be injected into depleted oil and gas fields for pure storage, or it can be injected into producing oil and gas fields in order to push out more oil and gas. This is referred to as enhanced oil recovery (EOR) and enhanced gas recovery (EGR).

The storage capacity in oil and gas fields is limited. Another storage option which is even more promising are aquifers. An aquifer is porous sandstone where water is present naturally in the pores in the sand. If CO₂ is injected it can occupy the pores just like water does, or it can even be dissolved into the water. Such aquifers are found all over the world and have the greatest capacity for CO₂ storage.

Only aquifers containing salt water are suitable for CO₂ storage. Aquifers containing fresh water will not be used for CO₂ storage, because such aquifers can be used for the production of drinking water.

Another limitation is the quality of the bedrock. CO₂ injected into an aquifer is buoyant, meaning that it will try to move upwards. This necessitates a lid or cover on top of the aquifer to stop the CO₂ from moving upwards. Such a lid can be a thick layer of impermeable mud or clay, as in the illustration above.

Storage safety
Aquifers suitable for CO₂ storage are found all over the world, but careful selection of storage sites is required to ensure CO₂ storage without leakage. One should not use aquifers in areas with a high risk of earthquakes. Furthermore, as stated above, there must be an impermeable layer on top of the aquifer, also called a caprock. Therefore, before CO₂ storage can take place it is necessary to conduct geological surveys to ensure that the caprock really is impermeable to the CO₂.

If a storage site is carefully selected after comprehensive characterisation of the geology, the risk of leakage is very low. According to scientific studies, more than 99 percent of the CO₂ injected will remain stored after thousands of years.

Actually, the safety of the storage increases as times goes by. Just after the CO₂ has been injected, the CO₂ will not leak due to the caprock.

CO₂ is injected into pores in the sand in the aquifer. The CO₂ will try to move upwards, but it is stopped by the impermeable caprock on top of the aquifer.

ProsjektLab

As times goes by the CO₂ will start to dissolve in the water. This naturally occuring process lowers the risk of leakage because the water has already been trapped in the aquifer for millions of years. After several thousands of years, the CO₂ will react with other minerals and form solid limestone, which is the safest form of CO₂ storage.

CO₂ is initially trapped by the caprock (left), but as times goes by the CO₂ is dissolved into water (middle) and finally, after thousands of years, the CO₂ reacts with other minerals to form limestone which is the safest way to store the CO₂(right).