Welcome back. For part 2 of this article, I will be talking about how 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 aquifiers 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 ( pic above ). 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.

I hope that these article has help you better understand what Carbon Capture and Storage is. You may like to refer to part 1 of this article : What is Carbon Capture and Storage : Pt 1- Capturing Carbon Dioxide

Source: RSC, Physorg, IOC