Flue gases produced from industrial processes, thermal power stations and combustion or gasification processes contain a range of particulate and gaseous contaminants the release of which is regulated. CO2 is rapidly being added to the list of regulated emissions in many jurisdictions worldwide.
For more than a century one of the main technologies used for contaminant removal has been wet scrubbers.
Flue gas is brought into contact with water or chemical solvents in spray or contactor towers. The effectiveness of a contactor derives from the amount of fluid surface area, the length of exposure time and solvent chemistry. Mixing of the solvent ensures that all of the liquid is exposed to the gases being cleaned.
The liquid is sprayed into the top of a tower filled with specially shaped 'structured packing'. The packing slows the solvent flow and creates a large contact surface area. While the solvent flows down over the packing, flue gas flows up through the contactor tower from the bottom. This is called a 'counter-flow' arrangement.
The height of the tower determines the contact time. The gas flow velocity is moderated by the diameter of the tower. As a consequence, in all CO2 capture plants and in many other industrial situations, absorption towers are the largest structures in the plant., the equipment, engineering, transport and erection of this important component can be up to one-third of the capital cost of a CO2 capture plant.
In an effort to reduce the cost of absorption columns, recently some projects have chosen to use a membrane-lined concrete shell instead of the traditional steel columns. While the material cost is reduced, the required foundations, erection cost and time are not.