What is Carbon Capture and Storage

With the effects of climate change and ocean acidification caused by greenhouse gases worsening, government bodies worldwide are pressed to find a way to curb with carbon dioxide emissions. But with fossil fuel still proving to be the dominant source of fuel and with the recent G8 agreement to target a 50% reduction in carbon dioxide emission by the year 2050, it may seem to be a daunting task. Therefore, there is a need to find a technology that allows us to continue the use of fossil fuels but with minimal or zero carbon dioxide emission.

One such technology is Carbon Capture and Storage or CCS. The basic principle behind CCS is this - Capture and isolate the carbon dioxide produced during the combustion of fossil fuel and then store it. A very simple idea but it may not be as simple as it looks. To facilitate the understanding of CCS, I will be breaking it down into 2 parts : Part 1 - Capturing the carbon dioxide and Part 2 - Storage of carbon dioxide.

Capturing the Carbon Dioxide

There are several methods of capturing carbon dioxide. Theses methods can either be human induced or it can happen naturally. Lets take a look at them.

1. Pre-combustion

Carbon dioxide is captured from a gas mixture produced by partial oxidation of natural gas or biomass. As the mixture contains predominantly hydrogen and carbon dioxide, physical absorption is used to capture the carbon dioxide.

2. Post-combustion Capture

Carbon dioxide is captured at low pressure and low carbon dioxide content from flue gas by separation from nitrogen and oxygen gas. This is achieved through the use of chemical absorption by monoethanolamine.

3. Oxyfuel combustion

For this process, oxygen is separated and is then used to burn fossil fuels. A part of the flue gas, which consists mainly of carbon dioxide and water is recycled to the combustion chamber to enhance the carbon dioxide concentration for subsequent removal.

4. Direct Capture from the atmosphere

This method can be achieved by building "carbon scrubbers" or towers that can capture carbon dioxide from the atmosphere.

5. Biological Capture

CCS may seem to be a new technology to us but in fact it has been occurring naturally throughout history. The ocean has the natural ability to absorb atmospheric carbon dioxide. According to a report by RSC(Royal Society of Chemistry), about 90 gigatonnes of carbon are exchanged between the ocean and the atmosphere each year with a net uptake by the ocean of 2.2 gigatonnes. It is because of this ability and also with the increase in carbon dioxide emission that is causing ocean acidification.

Now with a better understanding of how the carbon dioxide is captured, in the next part, I will present how the carbon dioxide is stored.

After being captured, the carbon dioxide can be either stored undergound ( geological sequestration ) or in the ocean ( ocean sequestration ).

Geological Sequestration

1. Oil fields and Aquifier Storage

The carbon dioxide that is captured can be stored in depleted or depleting oil and gas fields, deep saline aquifers and unmineable coal seams. The benefits of pumping carbon dioxide in existing oil fields is that it actually improves oil yield and the carbon dioxide is stored in the fields. This greatly help in the development of enhanced oil recovery ( EOR ) projects. Storing carbon dioxide in saline aquifiers is also possible as deep saline aquifiers has the potential to store huge quantities of carbon dioxide.

One example of carbon storage in saline aquifiers is at the Sleipner West field in the Norwegian North Sea . It is operated by Statoil.

2. Carbon dioxide Sinks

Carbon dioxide sink is actually a European project where they aim to store carbon dioxide in subterranean rocks. Recently, Germany has inaugurate Europe’s first underground carbon dioxide storage site. 60,000 tonnes of carbon dioxide will be pumped into the porous, salt- water filled rocks at depths of more than 600m over the next 2 years.

Pros and Cons of Geological Sequestration

Pros

1. We can store the carbon dioxide with our current infrastructure like oil fields
2. Can help to improve oil yield

Cons

1. May not have enough geological reservoirs to store the carbon dioxide
2. The carbon dioxide may leak due to the corrosion of the cement walls that lines the well

Ocean Sequestration

1. Use of marine phytoplankton

Marine phytoplankton assimilate carbon from seawater and when they die, they carry the carbon eith them to the deep ocean. However, phytoplankton growth is limited by the lack of the nutrient, iron. Thus if iron were to be introduced into the ocean to promote the growth of these phytoplankton, it could help in the absorption of carbon dioxide. But these method has many downsides. Firstly, very little of the carbon dioxide is taken in by the phytoplankton makes it to the deep ocean. Instead, as the phytoplankton decays, it releases the carbon back into the water. Secondly, if the growth of these phytoplankton were to be left unattended, it could cause the region of the ocean to become a dead zone because as the phytoplankton dies and decomposes, it actually requires oxygen.

2. Direct Injection into the Deep Ocean

By injecting carbon dioxide into the ocean at a depth of 3000m, the carbon dioxide can be kept away from being in contact with the atmosphere for 200 years. At ocean levels between 800 and 3000m a stream of liquid carbon dioxide is less dense than seawater and tends to rise to the surface, slowly mixing and dissolving with the surrounding water. At below 3000m liquid carbon dioxide reacts with the seawater to form clathrate, a solid ice-like substance that is denser than the surrounding water.

Pros and Cons of Ocean Sequestration

Pros

1. The ocean has an enormous natural capacity to absorb and store carbon

Cons

1. Environmental Issues - storage of carbon dioxide in the deep sea may lead to the pH of the seawater to decrease and become acidic.
2. Social Issues - Some considers carbon dioxide to an industrial waste and thus dumping it in the ocean is similar to contributing to water pollution.

The setback of CCS is that it requires energy and with this we need to use more fossil fuel. The intergovernmental panel on climate Change (IPCC) estimates that the cost of CCS technology would increase the energy needs of power plants by 10-40% and increase the cost of energy from plants by 30-60%. One solution would be to use renewable energy to provide for the energy required.

Because of the global impact of this issue, it cannot be addressed by any one nation or by special-interest sector. It must be addressed at an international and intergovernmental level. But first, we need to promote public awareness so as to let the public understand and accept CCS as a safe and technologically sound option for carbon dioxide abatement.

References

Physorg.com, 2008, Germany to start storing carbon dioxide underground
, 17th July 2008

RSC.org, 2008, Can we bury our carbon dioxide problem
, 17th July 2008

Ioc.unesco.org, Ocean Brief: Ocean sequestration
, 17th July 2008

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