The Effect of Impurities (H₂O, O₂, SO₂, NO, and NO₂) on Supercritical CO₂ Structures in Relation to CO₂ Pipeline Transport

Supercritical CO2 (s-CO2) pipeline transport is a critical component of the carbon capture and storage system. One primary safety concern of the pipeline structural integrity is corrosion and stress corrosion cracking induced by the presence of aggressive impurities in the transported high-pressure s-CO2 streams. Although a considerable number of studies have been conducted to address s-CO2 corrosion, fundamental knowledge gaps, particularly the influence of these corrosive impurities on s-CO2, remain to be addressed. This study employs molecular dynamics simulations to investigate the effects of representative impurities (H2O, O2, SO2, NO, and NO2) on s-CO2 structures under the designed s-CO2 pipeline transportation conditions. The results indicate that the self-interactions among s-CO2 molecules shall be enhanced with the introduction of trace amounts of impurities, reaching a plateau value, and then weaken with further increases in impurity concentrations. For the impurities investigated, s-CO2 exhibits an affinity in the order of NO > NO2> SO2 > O2> H2O. In the s-CO2, H2O molecules tend to aggregate locally, while other impurity molecules are uniformly distributed. Similar to the pure s-CO2 scenario, s-CO2 molecules can still form T-shapes with neighboring s-CO2 molecules in the presence of impurities. Besides, s-CO2 molecules show the tendency to form T-shape structures with all the examined impurities except H2O. There is no preferential structure presented between CO2 and H2O due to the H2O aggregation. These findings advance the understanding of how the impurities affect s-CO2 structures and consequently lead to different corrosion damage to s-CO2 pipeline steels.