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 Δ G pure * 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.