Liquid CO2 Transportation and Storage; A Comparison of Low Pressure and Medium Pressure Solutions

Abstract

Abstract Transportation of CO2 from a carbon capture unit to the final sequestration destination poses several technical challenges. For short / medium distances, dense phase transport via pipeline is usually the preferred solution. However, for longer distances, a more practical solution is liquid transportation (marine or rail), followed by offloading, intermediate storage and transfer to a dense phase pipeline for sequestration. The intent of this study is to perform a comparative assessment between "Low Pressure / Low Temperature" and "Medium Pressure / Medium Temperature" solutions for liquid CO2 transportation, considering overall process energy consumption, CAPEX and operational safety. For CCS chains involving refrigerated CO2 transport, the full transportation chain from the carbon capture unit to sequestration point includes compression, dehydration, liquefaction, buffer storage, marine (or rail) transfer, and conditioning. A low-pressure solution requires a low temperature to be able to liquify CO2 (7 Barg, -46°C); while a medium pressure solution requires a medium temperature to be able to liquify CO2 (13 Barg, -30°C), with each option having its own advantages and disadvantages. To perform a techno-economic assessment, both options were simulated, equipment lists produced and a cost estimate and lifecycle cost including OPEX, developed for the entire transportation chain between the low pressure capture units and the dense phase sequestration / injection point. The assessment was performed for a study by Wood, which had a CO2 production rate of 0.6 MTPA, however the result remains valid for higher capacity as levelized cost of CO2 handling is used. It was observed that the low-pressure option required a higher energy demand to liquify and heated the CO2, in terms of kwh/tonne of CO2. It was also noted that the low-pressure option required a material compatible with the low temperature (e.g. Stainless Steel), while Low Temperature Carbon Steel (LTCS) can be utilized for the medium pressure option. Although a higher material thickness was required for medium pressure buffer storage and marine tanks, this is compensated by the material cost difference between SS and LTCS, leading to a higher CAPEX for a low-pressure solution. Additionally, the CO2 triple point of -56°C is fairly close to the low-pressure storage conditions of -46 °C, hence any fluctuation in process conditions risks solidification of CO2. Based on the above, it is concluded that medium pressure CO2 storage (at circa 13 Barg, -30°C) is more desirable to a low pressure / low temperature solution. This study seeks to provide a holistic view for liquid CO2 transportation, considering both the marine and traditional onshore processing and buffer storage elements, paving the way towards a standard set of guidelines for future developments.