Carbon Capture and Storage

Carbon Capture and Storage (CCS) is another possible technique for mitigating climate change. By 2015, 20 commercial sized plants will start trials of CCS to see whether this is a viable method (Haszeldine and Scott, 2011). By capturing the CO₂ produced during combustion and storing it in a suitable place, there is the opportunity to reduce CO₂ emissions by 20% (Haszeldine, 2009). Currently 3 megatons of CO₂ are being captured and stored. There are three methods to capture CO₂.

Post-Combustion

During post-combustion the CO₂ is captured from the exhaust gas. The exhaust gas is bubbled through chemicals which selectively react with CO_2. Heating the solvent releases the concentrated CO₂ (Haszeldine, 2009).

Disadvantages:

•          Equipment is large and therefore larger power stations will be required.
•         Large volumes of the solvent will be needed and can produce toxic by-products.
•          Toxic by-products need to be dealt with appropriately.

Advantages:

•          It can be applied to already constructed plants.

Pre-Combustion

Pre-combustion involves removing the CO₂ before combustion commences. To remove the CO₂ at this stage the fossil fuel is converted to syngas (carbon monoxide and hydrogen), which is then reacted with water vapour to form a mixture of CO₂ and hydrogen. The hydrogen is then burned (Haszeldine, 2009).

Disadvantages:

•          High construction costs.
•          Decreased short term flexibility.

Advantages:

•          Multiple fuels can be used, with multiple products produced.
•          Efficient technology.

Oxyfuel Combustion

This is when the fuel is burnt in pure oxygen rather than air which results in CO₂ and water. However, production of pure oxygen is a costly and energy-intensive process (Haszeldine, 2009).

Disadvantages:

•          High energy cost of producing O_2.
•         Materials that can withstand higher temperatures would be required.

Advantages:

•          Easier separation of CO₂.
•          No solvent.
•          Potential to convert existing plants.
•         Smaller physical size.

After the CO₂ has been captured it is pressurized so that it forms a liquid. It is then stored 800m below the surface within rock pores (Haszeldine, 2009).  Good storage sites will be able to store CO₂ without seepage for thousands of years (Haszeldine, 2009). However, monitoring of the sites will have to take place to ensure that there are no adverse impacts. The step which consumes the most energy is the capturing of CO₂ but as CCS becomes more widely used improvements to the efficiency of the capturing processes will surely happen.

It is extremely important that we understand as much as possible about CCS because of the lack of action taken to reduce our CO₂ emissions. The world is dependent on fossil fuels for energy and it is unrealistic to believe that we can rapidly reduce this dependency. Therefore CCS can be a viable method of assisting a transition to a carbon neutral world. 

Haszeldine, S (2009) 'Carbon Capture and Storage: How Green Can Black Be?' Science, 325, 5948, pp.1647-1652

DOI: 10.1126/science.1172246

Haszeldine, S and V. Scott (2011) 'Carbon capture: An unprecedented challenge' New Scientist, 2806