Post-Combustion Capture Part 1

Why Post-Combustion Capture?

As of July 24th, 2024, our atmosphere has 425.55 parts per million of CO2. Renewable energy such as solar energy, windmills, and water turbines are a positive, necessary step towards fewer carbon emissions in the future, however, post-combustion carbon capture is vital to decrease the CO2 footprint of factories that persist to use fossil fuels for upsides of price and reliability. 

Post-combustion capture (PCC) is capturing CO2 from flue gas released after a fuel source is burned. PCC methods are employed in industrial factories and exhaust gases of nuclear power plants but can be useful for atmospheric air too. This post explains the different CO2 separation methods that are in the works, or in usage at a small scale today. 

CO2 Separation Through Chemical Absorption

Chemical absorption is using a liquid solvent to capture carbon dioxide. Amine solutions are most commonly used as solvents. When in contact with flue gas the CO2 undergoes a chemical reaction with amines forming a soluble compound. This method is the most widespread and utilized in natural gas and food production industries.

CO2 Separation Through Physical Adsorption 

Physical adsorption involves isolating CO2 from flue gas or atmospheric air using a solid filter made of zeolites or activated carbon. This process is facilitated by temperature swing adsorption (TRA), pressure swing adsorption (PSA), and vacuum regenerative adsorption (VRA). TRA involves the adsorption of CO2 molecules to the filter upon a decrease in temperature (40-60° C) and the desorption of CO2 upon increasing the temperature (120-160° C). PSA involves adjusting the pressure for the adsorption or desorption of carbon dioxide. High pressure results in the adsorption of CO2 and high pressure leads to its release. VRA utilizes a vacuum and pressure to adsorb and release CO2. A slightly elevated pressure is applied for adsorption and a vacuum is used for desorption.

CO2 Separation With Membranes

Semi-permeable membranes can be used in CO2 separation, acting as a filter that allows carbon dioxide to pass through and blocks bigger molecules in flue gas such as nitrogen. These membranes can be organic, inorganic, or mixed matrix membranes. Organic membranes would be made of polymers and inorganic membranes would be made of ceramics, zeolites, or metals. Mixed matrix membranes are a hybrid between organic and inorganic materials. Membrane separation usually requires high initial pressure.

CO2 Separation Through Cryogenic Methods

Cryogenic separation involves cooling flue gas to below -18° C. As a result, CO2 becomes a liquid or solid while the rest of the molecules in flue gas remain in a gaseous state. Therefore, the CO2 is isolated.