Effect of CO2 on water condensation and homogeneous nucleation: Insights for supercritical water gasification syngas phase separation

Supercritical water gasification (SCWG) syngas products typically contain CO2 and unreacted H2O, necessitating gas–liquid separation for water recovery. However, molecular-level investigations into the influence of CO2 on water condensation and nucleation remain lacking. In this study, molecular dynamics simulations were employed to examine both cooling-induced and isothermal nucleation processes in pure water and CO2–H2O mixtures. The role of CO2 during water condensation was analyzed from multiple perspectives, including energy evolution, molecular spatial distribution, and hydrogen bond (HB) formation. CO2 was found to densify the core of water clusters while blurring their boundaries, resulting in structural instability. At 380 K, the average number of hydrogen bonds per H2O molecule decreased by 13.38%, reflecting a disruption of the HB network. Nucleation rates were quantified using classical nucleation theory (CNT), improved classical nucleation theory, and the Yasuoka–Matsumoto (Y–M) method, from which the average nucleation barriers were subsequently derived. At 340 K, the nucleation rate J was 3.93 × 1033 m−3·s−1, and the average nucleation barrier ΔGpure* was 3.68 × 10−20 J, in close agreement with the CNT prediction value of 3.16 × 10−20 J. Across all isothermal conditions, CO2 led to an average 86.89% reduction in nucleation rate and a 31.12% increase in the nucleation barrier, confirming its inhibitory effect as a non-condensable gas. This study provides molecular-level insights and theoretical guidance for optimizing gas–liquid separation and operational parameters in SCWG syngas.