Carbon capture
Concerns about rising global temperatures that are believed to be caused in part by an increase in greenhouse gas emissions, such as carbon dioxide from the combustion of fossil fuels, have prompted the development of technologies to capture CO2.  There are two main categories of capture technologies, namely post-combustion and pre-combustion technology.  Post-combustion technology involves scrubbing CO2 from flue gas using chemical solvents such as amines or chilled ammonia.  Pre-combustion technology removes CO2 from a synthetic gas (syngas) prior to combustion using a physical solvent.

Post-combustion
Flue gas cooler
Prior to removing CO2, the flue gas is cooled to enhance the absorption reaction.  This is done in a quench tower using cooling water.  This tower also removes various impurities such as limestone slurry, dust, and halogens.  The low pressure drop characteristics of FLEXIPAC® structured packing used in this tower reduces the load on the flue gas blower.  A larger crimp size provides enough contact area with the cooling water to sufficiently reduce the flue gas temperature for effective absorption efficiency in the CO2 absorber.
FLEXIPAC® structured packing

CO2 Absorber
The CO2 absorber usually contains a wash section above an absorption section. 

The absorber operates at atmospheric pressure, resulting in low gas densities and very large towers.  Both sections are packed with FLEXIPAC® structured packing to minimize the tower diameter, which can be quite large based on the huge amount of flue gas to be treated. 

The wash section removes any entrained amine from the treated flue gas, reducing expensive solvent make-up costs.  In the lower section, CO2 is absorbed by chemical reaction with the solvent, and the packing provides surface area for this absorption to take place.  The aggressive surface treatment on the structured packing helps spread the liquid and increase the mass transfer efficiency.

The solvent must be regenerated for reuse by heating to break the chemical bonds. The solvent recirculation rate is minimized to reduce the required regeneration heat load. This is also referred to as the parasitic heat load because it reduces the overall power plant efficiency. With a minimized solvent recirculation rate, any maldistribution of liquid or vapor can cause an equilibrium pinch and will result in higher CO2 slip than desired.  The use of well-designed high performance INTALOX® liquid and vapor distributors will allow the use of lower solvent recirculation rates and result in less energy used for regeneration.

 
Solvent Regenerator
The rich solvent is fed to the top of the solvent regenerator where heat is applied at the base to drive off the CO2, creating lean solvent for recycling back to the absorber.
  • Heat from the reboiler turns some of the water in the solvent to steam, a portion of which condenses to supply latent heat to break the chemical bonds between the CO2 and solvent.
  • The remainder of the steam promotes stripping by reducing the CO2 partial pressure. 

The solvent regenerator tower diameter can be smaller than the absorber because the only vapors present are steam and the stripped CO2.  As a result, the liquid fluxes are higher and structured packing may not be the preferred choice.

INTALOX® ULTRA random packing

A high performance random packing, such as INTALOX® ULTRA random packing, is used when the liquid rates are high enough that structured packing efficiency is degraded. INTALOX® ULTRA random packing allows reduction in tower diameter and height compared to other random packing.

FLEXITRAY® valve trays combine high capacity and excellent efficiency with a wide operating range. Reflux from the overhead is returned to the tower above several wash trays, e.g. FLEXITRAY® valve trays to remove any entrained liquid.


Pre-combustion

Absorber and Regenerator
Physical solvents are used at high pressure to dissolve (absorb) CO2 from a syngas.  The rich solvent can then be let down in pressure and/or stream stripped to remove the CO2.  This results in lower energy penalties compared to post-combustion chemical solvent regeneration. 
  • The higher operating pressures result in higher gas densities allowing the use of smaller diameter towers. 
  • The reduced tower diameters and low CO2 loading compared to chemical solvents result in higher liquid fluxes. 

These towers generally use random packing, such as INTALOX® ULTRA random packing, to cope with the high liquid rates.