Effects of impurities on re-liquefaction system of liquefied CO2 transport ship for CCS

Abstract

For EOR and EGR in the CCS, CO2 must be captured from large-scale CO2 emission sources like power plants, and transported to depleting oil and gas wells to maintain the reservoir pressure and enhance the recovery rate. To achieve this, CO2 should be transported over long distances from the emission sources of developed countries to oil and gas producing regions by pipeline or ship. Compressed or liquefied CO2 transport ship can be used when pipeline is not recommendable. In case of liquefied CO2 transport ship, the BOG should be treated during sailing using methods such as re-liquefaction and adsorption.In the present study, the effects of representative impurities of N2 and O2 on the re-liquefaction cycle of liquefied CO2 transport ship were investigated by simulation. The concentration of impurities with boiling points lower than that of CO2 increased much more in the boil-off gas phase than in the liquid phase. This phenomenon led to low cycle performance compared to that of pure CO2 condition, and the decrease rate according to the concentration of the impurities was significant. In the mixture, the discharge pressure of the highest stage in the cycle increased much more than that of the pure CO2 condition. Therefore, when designing re-liquefaction cycle for treating the BOG of liquefied CO2 transport ship, consideration should be given to the effects of impurities on the cycle performance and safety problem in terms of the design pressure. However, impurities did not significantly increase the discharge temperatures of the compressors to such an extent to cause reliability problem of oil degradation in the compressors. With the same composition in the mixture of the liquid phase, nitrogen had a more negative influence than oxygen.