Investigation of metal ion additives on the suppression of ammonia loss and CO2 absorption kinetics of aqueous ammonia-based CO2 capture

Abstract

Aqueous ammonia is an attractive absorbent for the capture of CO2 from industrial flue gases. However evaporative ammonia loss limits the application of the ammonia-based CO2 capture process. The use of metal ion additives (where M(II)=Cu(II), Zn(II) and Ni(II)), has been reported previously for the effective suppression of ammonia loss in the post combustion capture of CO2. In this study, we have investigated the absorption of CO2 into various M(II)/NH3 solutions, the resulting loss of ammonia to evaporation, and have attempted to rationalise the observed effects using an extended chemical model that includes all chemical reactions in solution. The theoretical and experimental investigations suggest that complexation of ammonia by metal ions effectively reduces the concentration of free ammonia, resulting in reduced evaporative loss but also reduced reactivity towards CO2. Simple reduction of the total NH3 concentration to the same concentration of free NH3 concentration as that obtained by the addition of M(II) showed a similar reduction in the ammonia loss as well as the CO2 absorption rate. The effects on CO2 absorption capacity at 25 and 80°C, cyclic capacity and regeneration energy are also simulated and discussed.