Monday, 30 November 2015

Carbon Dioxide Capture- An overview (Part 1)

This week’s blog diverts to a new type of geoengineering process, carbon dioxide removal.  Carbon Dioxide Removal (CDR) focuses on artificial ways of reducing carbon dioxide levels from the atmosphere.  This blog aims to focus on one CDR process, known as Carbon Dioxide Capture (CDC).   

CDC absorbs carbon dioxide from the air, through an industrial process.  Large power plants with a high level of thermodynamics are essential to capture carbon dioxide (The Royal Society 2009).  Once this carbon dioxide is captured it can either be stored or used for energy.   It is generally hard to capture carbon dioxide from the atmosphere as it composes of 0.04% of the air (The Royal Society 2009). Furthermore, large amounts of energy may also be required to collect carbon dioxide from the air, leading to high costs.  

 In general, there are three main ways to capture carbon dioxide from the atmosphere. However, all three processes have only been analysed at a laboratory scale. 

Absorption of carbon dioxide on solids

Firstly, Lackner (2009) suggests the creation of large filters (sorbents).  These filters allow air to pass through, and capture CO2.  This process is also referred to as absorption on solids and is considered a fairly safe process with no risks on the environment or people.  However, as time passes the sorbent will hold higher amounts of CO2, decreasing the amount of absorption occurring and decreasing the filter efficiency.  Further information about Lackner’s proposal will be analysed in more detail in my next blog. 

Highly Alkaline Solutions absorbing CO2

Secondly, another way to absorb carbon dioxide is through highly alkaline solutions.  Solutions of high alkalinity cause high kinetic reactions, absorbing CO2.  This process has been suggested to be undertaken using Sodium Hydroxide spray.  However, it is suggested that a reduction of CO2 by 30% would also decrease the moisture in the atmosphere (Stolaroff et al 2008).  For every molecule of carbon dioxide being captured, 30 molecules of water vapour would also be captured (30mol-H2O per mol-CO2), hence decreasing the atmospheric moisture substantially (Stolaroff et al 2008).  Consequently, as the air moisture decreases, this may have negative impacts on the frequency and intensity of rainfall leading to negative effects on the hydrological cycle (Stolaroff et al 2008).  Therefore, this may increase the amount of floods or droughts occurring, leading to catastrophic impacts on agriculture, decreasing food and economic security.

Moderate Alkaline Solutions with a Catalyst to absorb CO2

Lastly, another way to capture carbon dioxide is through using a moderate alkaline solution with a catalyst.  This process is similar to Stolaroff et al’s suggestion.  Yet, the catalyst increases the reaction rate of the alkaline solution with the atmosphere, hence absorbing carbon dioxide (Bao and Trachtenber 2006).  However, it is essential to acknowledge that water vapour will also be absorbed with this process.  Thus, again affecting air moisture and the hydrological cycle.

CDC Evaluation

Although CDC may reduce carbon dioxide emissions at a fast and efficient rate, a lot of energy is essential. High levels of electricity will be required for CDC to take place (Haszeldine 2009).   Hence, providing electricity by burning fossil fuels should be avoided. Therefore, a renewable energy like wind power or solar power will be required for this process to take place without any CO2 emissions (Lackner 2009). The Royal Society (2009) suggests that this process may be relatively expensive.  Nonetheless, in the long run when accounting all the carbon dioxide emission reduction, it may be worthwhile. 

Additionally, another problem with CDC is the disposal.  It has been suggested to store it in a secured location, such as near oil or gas fields.   This may be problematic as transport costs will occur and a large storage space will be required in the long run (The Royal Society 2009).  However, a more productive alternative may be to re-use this carbon dioxide.  It has been suggested to combine CO2 with hydrogen and convert it into a transport fuel (Lackner 2009).  For this to be achieved, CDC costs will increase substantially (Table 1).   However in the future, this may be a viable process, as costs may decrease with increasing technological advancement (Figure 1).

Table 1: CDC Summary Evaluation
Source: The Royal Society 2009

Figure 1: CDC as time passes- the more time passes the more cost efficient it becomes
SourceHaszeldine 2009 

Provided that CDC becomes technologically efficient and relatively cheap, it could be successful.  Nonetheless, Stolaroffs and Bao & Trachentnber processes may not be as desirable due to the impacts on the atmospheric moisture.  Therefore, its success is questionable as it may also cause a lot of hydrological damage rather than reduce climate change impacts.  Controversially, Lackner’s process has no environmental or hydrological impacts and it seems to be a very safe process with less risks.  I believe this could be a viable solution to decreasing carbon dioxide levels.  Follow me next week where I will analyse two examples of CDC processes; artificial trees (Lackner) and a porous liquid (Zhang et al.).  But for now, please let me know your thoughts on the matter.


  1. Hi Maria, really interesting blog post!
    I think talking about the real possibility of CDR and specifically CDC methods is a progressive step that is becoming increasingly important. Do you think that the fact the methods are largely reliant on industry, such as large filters, is a negative element of the techniques? Do you think they should be completely decoupled from industry and that viable sustainable alternatives developed?
    I think they have a potential future in mitigating climate change, however are arguable too unstable and unpredictable at this precise moment in time?
    Looking forward to your "part 2" post! :)

    1. Hi Caitlin thank you for your comment. I think the fact that the methods are largely industrial is what creates insecurity for people, as it is 'big' and unknown, hence not knowing its implications. I am unsure if it is negative that they are reliant on industry. To some extent I guess it is bad as there is an issue of management and who will pay for it. However, with the correct technological advancement and correct management schemes, this may change, hence decreasing their reliance on industrial processes.

      Nonetheless, I believe, ideally, the best way to reduce carbon dioxide is through the 'traditional' approach. Unfortunately, it is not working, even though they have been trying for many years. Therefore, I do believe there is a large potential in CDC. However a lot more research is essential, especially in the reduction of costs and utilising the absorbed carbon dioxide. If that were to happen, I am sure it could be a viable process. However, the unfortunate thing with all geoengineering processes, is that they have never been tested, so nobody can tell us if this process will lead to any unpredictable negative implications, that have not been thought of. Another negative consequence is what will happen if one of these processes break or fail? There isn't an undo button.
      I hope I have answered your questions! If you disagree or have any other thoughts on the matter, by all means let me know!

  2. Great overview of some CDR methods I had not come across yet! You mention that one down-side of the CDR methods proposed here is that they require large amounts of electricity to work. Don't you therefore think that these methods, and therefore investment and research, should come second in line to trying to make our energy-mix more efficient and less carbon reliant ?

    1. Hi Loulou, Thank you for your comment! I completely agree, the fact that they require energy is a big downturn for CDR and yes it should come secondary in regards to making our own energy mix more efficient and less carbon reliant. I believe nuclear power would be very sufficient in terms of providing these kind of geoengineering schemes (although that has its own risks)! Nonetheless, I am unsure if other renewable energy would have the ability to provide these large amounts of energy for geoengineering (at least for now!).

  3. Hi Maria! I really enjoyed reading this post! The CDC requires large amounts of electricity - which you very rightly say shouldn't be provided through burning of fossil fuels. However do you think the energy produced from wind and solar energy could realistically reach the scale to power such a process? Furthermore, renewable forms of energy come with their own carbon dioxide emissions (e.g. in construction, transport of materials etc.) so would the process of CDC, particularly through it's required electricity provision, ever really be carbon neutral? I am sceptical. What do you think?

    1. Hi Shruti! Thank you for your comment!

      I am also questioning if wind or solar energy would have the ability to provide vast amounts of energy, I think a lot of research is required to provide large amounts of energy. However, I do believe that nuclear power would be more sufficient in this case.

      In regards to if CDC will ever be carbon neutral. I believe it has potential. Technology is constantly advancing, therefore, it is highly likely that more efficient methods of fossil fuel use can be managed, although this is very hard to achieve. However, I think for CDC to be truly carbon neutral it would need a renewable energy. Yes there would be some carbon costs in terms of construction and transport materials, however, I don't think it would be as much as all the absorption that could take place in the long run.

      I hope this answers your questions!