(68e) A Generic Analysis of Energy Use and Solvent Selection for CO2 Separation from Post-Combustion Flue Gases

Absorption-based processes are potentially the most attractive technologies for CO2 separation from post-combustion coal flue gas. The energy used in a typical absorption process has three major components: reaction heat (heat of absorption), stripping heat (water evaporation), and sensible heat (heating the solvent to the stripping temperature). The relative importance of these energy components depends on solvent type and operating conditions of the absorption and stripping columns. In a state-of-the-art mono-ethanol-amine (MEA) process, the reaction, stripping and sensible heat contribute 53%, 33% and 14% of the total heat use, respectively.

A thermodynamic calculation was performed to determine the theoretical minimum energy used to separate CO2 from a coal combustion flue gas in a typical adsorption-desorption system. The results showed that, under ideal conditions, the minimum energy required to separate CO2 from post-combustion flue gas and produce pure CO2 at 1 atmospheric pressure was only about 1,183 kJ/kg CO2. This amount could double with the inclusion of the driving forces of mass and heat transfer and the adverse impacts of absorption heat release on adsorption capacity.

Thermodynamic analyses also were performed for the aqueous amine-based absorption process. The heat requirement for hypothetical solvents covering a wide range of heat of absorption and the equilibrium constant of the CO2 reaction was examined. Two CO2 reaction mechanisms, the carbamate formation reaction with primary/secondary amines and the CO2 hydration reaction with tertiary amines, were included in the absorption reaction. The relation between the reaction equilibrium and the heat of reaction was examined for commercial amines. The results showed that the reaction heat, sensible heat and stripping heat are all important to the total heat requirement. The heat use of an ideal tertiary amine amounted to 2,786 kJ/kg, compared to 3,211 kJ/kg for an ideal primary amine. The heat usage of an ideal amine is about 20% lower than that of commercially available amines. Optimizing the absorption process configuration could further reduce energy use.