Condensation and nucleation of water vapor in H₂–CO–CH₄–CO₂ multicomponent mixtures: molecular insight into gas–liquid separation from supercritical water gasification products

Understanding the phase behavior of water in supercritical water gasification (SCWG) products is crucial for efficient water recovery. However, the mechanisms by which gaseous species influence the condensation and nucleation of water remain unclear, as the experimental observation of nanoscale clusters is challenging. Molecular dynamics (MD) simulations were employed to investigate the condensation and nucleation processes of water vapor in the presence of representative SCWG gas products, including H₂, CO, CH₄ and CO₂. The results demonstrate that both CH₄ and CO suppress water condensation and nucleation. Within the temperature range of 298–358 K, 5–30 % CH₄ lead to an average decrease of 21.66 % in nucleation rate J, accompanied by an average increase of 8.49 % in nucleation energy barrier ΔG∗; for CO, the corresponding values are 33.49 % and 14.58 %, respectively. These gaseous impurities disturb the formation of hydrogen-bond (HB) networks by reducing the frequency of HBs formed within the optimal donor–acceptor distance and angle ranges. Moreover, multicomponent mixtures (3.57–8.82 % unit concentration of H₂–CO–CH₄–CO₂) exhibit a stronger inhibitory effect on water nucleation than binary-component systems, with an average decrease of 51.72 % in J and a 26.50 % increase in ΔG∗ relative to the pure water system. The suppression of nucleation and condensation is mainly attributed to the inhibition of early-stage critical nucleus formation and its subsequent growth. This study provides molecular-level insights into the nucleation behavior of H₂O in SCWG product gas environments and offers guidance for optimizing gas–liquid separation strategies.