Compressor selection methods for multi-stage re-liquefaction system of liquefied CO2 transport ship for CCS

Abstract

Recently, CCS (carbon dioxide capture and sequestration) has been receiving considerable attention as a possible means of dealing with emissions of CO2, a greenhouse gas. To this end, EOR (enhanced oil recovery) and EGR (enhanced gas recovery) are regarded as being viable options for economically sequestrating large amounts of CO2. A feasible approach would involve capturing CO2 from large-scale CO2 emission sources such as power plants, and then transporting that captured gas to near-depleted oil and gas wells to maintain the reservoir pressure and enhance the oil/gas recovery rate. In the future, CO2 will be transported large distances from emission sources in developed countries to oil/gas producing regions by pipelines or ships. The long-distance ship-based transport of CO2 would require that the gas be compressed or liquefied (LCO2). Furthermore, an LCO2 transport ship would have to be capable of processing boil-off gas while at sea.In this study, the selection method of compressors for re-liquefaction system of dedicated LCO2 transport ship was investigated by computational method. The performance of same compression ratio (SCR) method 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 these three methods, 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 was found to be better in terms of reliability, with a decrease in performance of only 5% relative to that obtained by IPO. Consequently, we recommend 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.