The most widespread technology for post-combustion CO2 separation is chemical adsorption with solvents. This technology is now widely established and can be easily implemented in existing plants (both in the power generation sector and in many industrial sectors). The operating principle of the capture section considered is essentially based on the purification of gases by chemical absorption of carbon dioxide by the solvent, regeneration of the latter by heating with steam, and compression of the high-purity carbon dioxide stream so that it can be liquefied and stored. Among the chemical solvents used to capture carbon dioxide, amines play a fundamental (and almost exclusive) role. In particular, the most widely used is monoethanolamide (MEA) in aqueous solution.

The most critical aspect related to these processes is the high energy consumption required for solvent regeneration (3-4 MJ/kgCO2), which represents an important fraction of the operating costs and leads to a reduction in the overall plant efficiency of 12-13%. It is not of secondary importance that amines have a strong environmental impact and release carcinogenic compounds during their regeneration. Therefore, research activities are mainly aimed at developing alternative solvents that can reduce energy consumption during regeneration and have a limited environmental impact.

CO2 separation processes with liquid solvents based on amino acid salts are nowadays a very promising option for CO2 capture in post-combustion processes for various applications (such as conventional power plants, combined cycles and various industrial sectors). Despite the high cost of solvents, these processes have considerable advantages from both a plant and environmental point of view.

The study conducted by Sotacarbo is currently focused on the characterisation of these solvents in the absorption phase. The experimental activity, conducted at the GAIA pilot plant available at Sotacarbo's laboratories, is flanked by process modelling performed by LEAP, Laboratorio Energia e Ambiente Piacenza, and the Politecnico di Milano.

The GAIA plant (Greenhouse gas Absorption in Amine-based solvents pilot plant) is a bench-top structure consisting of an absorption and stripping column divided into two sections: an upper section filled with SS 316 Raschig rings and a lower section equipped with a shell and tube type heat exchanger (three coaxial coils).

In the absorption phase, the gas stream to be treated consists of CO2 and N2, stored in separate cylinders. The two gases pass through the column and rise upwards in countercurrent to the solvent, which is pushed at the head of the column by a pump. The system is fully automated and managed by a dedicated computer using special software that allows the control, acquisition and recording of flow rate and mixture composition data. The concentration of the latter is measured by means of an online analyser and a micro-gas chromatograph.

The experiments carried out on the GAIA plant made it possible to evaluate the performance of solvents based on amino acid salts, in particular potassium prolinate, potassium sarcosinate and potassium lysinate. The result of this activity provides information on the reaction kinetics and capacity (expressed as the ratio of captured CO2 moles to solvent moles) of the absorbent solution.

The tests performed showed the good capacity of the solvents tested, which can be regarded as promising alternatives to amines. In particular, the tests performed were compared to similar tests conducted on 5 M MEA aqueous solution.