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
Source: Haszeldine 2009 

Thoughts:

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.