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

Abstract

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.