Aqueous ammonia (NH3) stands out as a promising alternative to conventional amines for post-combustion CO2 capture, offering high capacity and low solvent cost. Yet, NH3-based processes face two major hurdles: (i) ammonium bicarbonate (NH4HCO3) precipitation under certain operating conditions and (ii) NH3 slip, which demands additional abatement efforts. In this study, we present new vapor–liquid equilibrium (VLE) experiments for 4–8 wt% NH3 solutions at 40–80 °C and pressures up to 700 kPa, together with an extended Non-Random Two-Liquid (eNRTL) model for accurately capturing ionic speciation (NH4+, NH2COO-, HCO3-) and solid formation. The model shows that solutions below ~6.8 wt% NH3 at 40 °C avoid solid precipitation up to a CO2 loading of unity, whereas moderate concentrations remain feasible if CO2 loading is constrained. Process-level simulations reveal that raising the stripper temperature to ~129 °C in a 10 wt% NH₃ solution reduces regeneration energy to ~2.9 GJ/ton CO2 by decreasing solvent circulation. However, increased stripper pressure raises lean CO2 loading and negates efficiency gains unless compensated by higher temperature. These findings highlight the delicate balance between maximizing CO2 capture, preventing solid formation, and minimizing NH3 slip. By combining rigorous thermodynamic modeling with energy analyses, this work identifies operating windows and design strategies for stable, low-energy ammonia-based CO2 capture systems. The results underscore the potential for advanced process configurations—such as vapor compression or blended solvents—to address volatility challenges and sustain economical large-scale deployment of NH3 capture technology.
