Carbon Dioxide Capture: Part I

I am not ignoring the solar engineering forms of geo-engineering entirely in this blog but at the moment I’m focusing on carbon dioxide removal (CDR) techniques as I feel these will have fewer unknown and unintended consequences because they aim to reverse the fossil fuel trend of adding carbon to the atmosphere and do not try to reduce solar radiation reaching Earth. Atmospheric Carbon Dioxide Capture is a form of CDR geo-engineering that I feel has great potential. This involves taking carbon dioxide out of the atmosphere through chemical reactions and storing it deep underground or using it in industry. Many authors do not consider the capture of carbon dioxide at power plants to be a form of geo-engineering because it is not dealing with carbon dioxide that has already been released to the atmosphere. I personally disagree with this and see the removal of carbon dioxide from power plants as a key way to reduce the effects of climate change into the future on a large scale which is the current objective of all geo-engineering techniques. However, if you hold the view that it is not a form of geo-engineering, the procedures of carbon dioxide removal in power plants mentioned in this article are still applicable to global atmospheric carbon dioxide removal (which is a form of geo-engineering). This is why I feel it is particularly important to write a post on these techniques. This topic will be divided into two posts, the first on the different methods being used and the second on the stores, benefits, drawbacks and feasibility of these techniques.

How does it work?

According to Metz et al. (2005) there are four basic systems for capturing carbon dioxide from fossil fuels and biomass. These are as follows:

1. Capture from industrial process streams
2. Post-combustion capture
3. Oxy-fuel combustion capture
4. Pre-combustion capture

The first of these has been operational for 80 years but the CO₂ captured is still added to the atmosphere as until now there was no reason to store it. This is very unfortunate as, if the incentive to store this carbon had started 80 years earlier, the human race would not be in such a dangerous and unpredictable climate change situation. Implementation of this method therefore seems very straightforward, with few changes needed. Post-combustion capture involves the capture of carbon dioxide from flue gases produced during the burning of fossil fuels and biomass. The CO₂ is then pumped back underground. Ox-fuel combustion capture is very similar to Post-combustion but pure oxygen is used in the combustion and therefore the flue gas is mainly CO₂ and H₂O which are easier to recycle. Finally, pre-combustion capture involves ‘reacting a fuel with oxygen or air and/or steam to give mainly a ‘synthetic gas (syngas)’ or ‘fuel gas’ composed of carbon monoxide and hydrogen.’ A catalytic converter, such as those used on car exhausts, can then be used to produce carbon dioxide and more hydrogen and this can then be separated by physical or chemical absorption leaving hydrogen-rich fuel which can be used in many systems such as boilers and gas turbines. The latter three methods all revolve around the idea of making combustion less polluting to the atmosphere.

The techniques for separating carbon dioxide are varied and each have their merits. The three main processes for separation are shown in Figure 1. The first technology I wish to discuss is called ‘Separation with sorbents/solvents’ (Diagram A in Figure 1). This involves bringing the gas into contact with a liquid absorbent or a solid sorbent that separates the carbon dioxide from the rest of the gas. The sorbent that is loaded with CO₂ is then transported to another vessel where it releases the carbon dioxide following a change in the environment (e.g. heating or increased pressure). This sorbent is then recycled to collect more CO₂ in a cyclical process.

Separation with membranes is a method whereby only certain gases are able to permeate through the membrane. The flow through the membrane is driven by pressure differences. There is currently research and development ongoing to manufacture a membrane suitable for the large scale capture of carbon dioxide. This means this method is less widely used at present.

Cryogenic distillation (Diagram C in Figure 1), describes the process by which gases are converted to liquid through compression, cooling and expansion and then are distilled to capture the CO₂. This form of CDR is currently being carried out at a large scale commercially which suggests that it has great potential.

Figure 1: Main separation process for carbon dioxide capture.

These three capture technologies highlight the different ways carbon dioxide is removed in power plants and during biomass burning. The methods here deal with the carbon dioxide at the source and therefore prevent the carbon from reaching the atmosphere and influencing climate change. Achieving this reduces the need for other extreme methods of geo-engineering that could lead to unintended negative consequences for climate. These systems can also be applied to the removal of carbon dioxide already in the atmosphere. However, due to the lower atmospheric concentrations of carbon dioxide in the atmosphere than in power plants suggests that the efficacy will be less for atmospheric carbon dioxide capture.

This concludes the first part on the topic of how carbon dioxide removal is carried out in power plants and from the atmosphere. The next post will continue on this topic but turn the focus to where the carbon would be stored once it has been captured. In addition to this, the post will look into the benefits and drawbacks to this form of geo-engineering and whether it is feasible as a method to reduce the effects of climate change at a large enough scale to be considered by governments worldwide.