Theoretical and experimental study of NH3 suppression by addition of Me(II) ions (Ni, Cu and Zn) in an ammonia-based CO2 capture process

Abstract

Aqueous ammonia is a promising solvent for CO2 capture, but its application is limited by the high volatility of ammonia. In the present study, we theoretically and experimentally investigate the potential of three metal ions – Ni(II), Cu(II) and Zn(II) – as additives to reduce ammonia volatilisation in ammonia-based CO2 capture technology. A thermodynamic equilibrium model of the Me(II)–NH3–CO2–H2O system was established to provide theoretical guidance for the study and analysis of experimental results. The effect of factors such as ammonia concentration, absorption time, CO2 loading, regeneration on ammonia loss and/or CO2 absorption rate was investigated experimentally. The experimental results were in good agreement with the theoretical simulation. Ni(II), Cu(II) and Zn(II) were highly soluble in ammonia solution at a pH range of 9–11, due to the complexation of metal ions with free ammonia. Among the three metal species, Cu(II) had the best adaptability to the variation of pH and CO2 loading, and Ni(II) suppressed ammonia volatilisation most effectively. Our theoretical and experimental results showed that the order of ammonia suppression efficiency was Ni(II) > Cu(II) > Zn(II). The addition of Me(II) ions (Ni, Cu and Zn) significantly reduced ammonia loss in absorption and regeneration processes, and only slightly decreased the rate of CO2 absorption. The regeneration result showed that metal additives can accelerate the CO2 desorption rate. This study provided a novel approach to effective NH3 suppression in an aqueous ammonia-based CO2 capture process, which holds the potential of an industrial application.