Capturing carbon – the basics

In order to store the carbon, you first need to capture it.  There appear to be three main ways to separate the CO2.  This post is not intended to be a thorough technical discussion; I am neither qualified nor inclined to go into the detail, however, it’s interesting to have a basic understanding of what happens where.

Generating gases with CO2 concentration:

CO2 is typically captured in one of three stages of fuel combustion: flue-gas separation, oxy-fuel combustion or during pre-combustion.

1. During flue-gas separation, or post-combustion capture, the gases resulting from burning fuels includes various gases, including N, CO2, H20, SO2 etc.  The assumption is that the fuel combusts by reacting with air.  This means that there’s a high concentration of nitrogen component in the flue gas.  In order to separate the CO2, the Nitrogen will need to be removed => energy required.
2. In oxy-fuel combustion, the fuel is burnt with oxygen (or as near to pure O2 as possible).  This removes the nitrogen prior to combustion, meaning that the concentration of CO2 is much higher in the exhaust gases, making it easier to separate.  Naturally, there would have to have been effort involved prior to combustion to separate the oxygen from the air => energy required.
3. Pre-combustion capture_ “involves reacting a fuel with oxygen or air and/or steam to give mainly a ‘synthesis gas (syngas)’ or ‘fuel gas’ composed of carbon monoxide and hydrogen. The carbon monoxide is reacted with steam in a catalytic reactor, called a shift converter, to give CO2 and more hydrogen._” [source: IEA Clean Coal]  What results is a mixture of gases with a much higher concentration of CO2 than in post-combustion processes.  This can be cheaper to operate than options 1 and 2, however the capital costs can be higher.

[Source: hindawi]

Separating the CO2:

Once there is a gas with a high enough CO2 concentration, there are a few ways to separate the CO2.  This short document, produced by the CO2 Capture project, outlines three methods of removing CO2 from mixed gases; namely separation with

• sorbents/solvents;
• membranes; and/or
• cryogenics.