Study on Re-liquefaction Cycle of Liquefied CO2 Transport Ship for CCS

Abstract

In the present study, the selection method of compressors for re-liquefaction cycle of dedicated LCO2 transport ship was investigated by computational method. The performance of the same compression ratio (SCR) method which was generally used exhibited low levels of efficiency and reliability as for coefficient of performance (COP) and compressor discharge temperature in terms of oil degradation compared to that of Intermediate pressure optimization (IPO). Of the three methods to be investigated in the present study, IPO produced the highest level of performance of all, but could not guarantee compressor reliability as like SCR, too. Intermediate pressure optimization with same discharge temperature (IPODT) method which was suggested in the present study was found to be better in terms of reliability with a decrease in performance of only 5% relative to that obtained by IPO. Consequently, it can be recommended the use of the IPODT method for the design of a multi-stage compression system for the re-liquefaction cycle of an LCO2 transport ship. In the stored LCO2, there exist other components like N2, O2, Ar, H2O, SOx and NOx depending on the separation method, conversion of fuel, stoichiometric combustion, etc. Dependent on transportation and storage specifications, some of these components should be removed before being loaded to the ship. However, after conditioning process, small amounts of them still remain in the liquefied CO2, and it will influence the characteristics of the re-liquefaction cycle in terms of performance and reliability. In the present study, the effects of representative impurities of N2 and O2 on the re-liquefaction cycle of LCO2 transport ship were investigated by simulation. Cycle performance in terms of COP reduced as the cooling capacity decreased and the work of compression increased compared to the pure CO2 condition. In the same composition of impurities with CO2, the mixture with nitrogen showed worse performance than that with oxygen. Moreover, impurities raised the discharge pressures considerably. However, impurities didnt tend to raise the risk of reliability of compressors caused by high discharge temperatures. Lastly, case study was conducted on the application of the re-liquefaction cycle for the 10MW power generation plant. For designing the LCO2 transport ship, exact calculation of BOR which means heat generation is most important. In the present study, quantitative study was conducted under the conditions of required cooling capacity and BOR inferred from commercialized LNG ships. It was assumed that total emitted CO2 is liquefied, stored temporarily in the storage tank on land, transported by LCO2 transport ship periodically between South Korea and Middle East for EOR and EGR