CO2 Capture

CO2 Capture and Storage is one of the main solutions in order to reduce CO2 emissions from fossil fuels, fighting climate change.

Before being stored, CO2 must be separated during the energy conversion process. At the moment, there are three main technology options for capturing CO2 as schematically shown in the diagram below.
 

Source: "CO2 Capture and Storage- VGB Report on the State of the Art"
 

Post-combustion capture

In Post-combustion separation technologies carbon dioxide is captured at the end of the fossil fuels and biomass combustion process, removing CO2 from flue gas produced.  There are many mature and widely applied separation technologies mainly based on absorption through liquid solvents, adsorption through solid sorbents and separation with membranes.

The selection of the technology is due to the flue gas characteristics (temperature, pressure, concentration and volume flow rate); CO2 concentration varies from 5% vol (dry) for a natural gas combined cycle plant to about 15% vol (dry) for a pulverized coal fired power plant.

Enel is currently testing absorption technology on a pilot plant in at the Federico II power plant in Brindisi, southern Italy. Absorption through liquid solvents is the technology chosen by Enel to develop its CCS demonstration Zero Emission Porto Tolle project.

Absorption through liquid solvents

In absorption process CO2 is captured from flue gas by a reactive liquid solvent. This technology is widely diffused in chemical process industries and represents the most mature technology now available.

The most experienced processes are based on the use of aqueous solutions of alkanolammines, such as monoethanolammina (MEA), diethanolamine(DEA), methyldiethanolamine(MDEA). The figure below shows the basic configuration of an amine based CO2 capture.

Before starting the absorbing phase, flue gas is treated in a cooling system (to reache the right temperature) and in a purifying section (in order to remove particulates and other possible impurities, mainly SOx and NOx, that could cause solvent losses for degradation). Once treated, the flue gas is sent to an absorber tower, in which CO2 reacts with the aqueous solution of amines, while the cleaned flue gas (essentially hot air) is released in atmosphere; the separation of CO2 is due to a chemical reaction that creates a chemical compound in which CO2 is loosely bound. The CO2-loaded solution is then sent to a stripper section where it is heated obtaining pure CO2 and regenerating the solvent. While CO2 is dried, compressed and transported in a dense phase to the storage site, the regenerated solvent is newly sent to the absorber tower, after having been cooled down  by the CO2-loaded solution exiting the absorber tower in a heat exchanger.
 

Figure 2: Schematic representation of CO2 absorbtion (source: "Program on Technology Innovation: Post-Combustion CO2 Capture Technology Development", EPRI 1016995)

This process can be designed to capture from 85% to 95% of the CO2 in the flue gas, with a >99.9% purity.

The main critical aspect for amine based processes is represented by the energy consumption needed for regeneration; research activities aim to optimize this issue.

Adsorption through solid sorbents

Solid sorbents can be used to adsorb CO2 from flue gas. Solid sorbents are typically made of materials with high surface areas, as zeolites and activated carbon, able to capture carbon dioxide, obtaining carbon free flue gas. The separation of CO2 and the regeneration of the adsorbent is based on the increase of temperature or pressure.

At the moment, this technology is not suitable for a large-scale applications, due to its low CO2 selectivity and to the high energy required for regeneration.

Membranes

Membranes are usually used to selectively transport gases. Particular membranes are able to separate CO2 from flue gas. This kind of CO2 removal is based on the difference in CO2 partial pressure across the membrane, allowing a selective transport of CO2 trough the membrane.

This technology is rather new and less common than absorption and adsorption.
 

Pre-combustion capture

In this process, coal is transformed into a hydrogen-rich fuel gas mixed with CO2; the CO2 is removed before combustion, through physical absorption, while the hydrogen is combusted in a gas turbine. This technology allows to remove CO2 at high pressure and at higher concentration.

Oxy-fuel combustion

The fuel is burnt using oxygen instead of air (nitrogen is separated from oxygen through an air separation unit), in an atmosphere of CO2; control temperature is realized by recycling CO2 and H2O. The flue gas stream is essentially composed by water and CO2; the CO2 can be easily captured by condensation.