(346e) Design and Evaluation of Thermodynamic Conditions for an Off-Shore Topside CO2 Injection System

Carbon capture and storage (CCS) is one of the dominant technologies to tackle the global warming issue. The transport of CO₂ for geological storage is economically feasible by ship when the storage site location is off-shore and installment of an off-shore pipeline requires a huge capital cost. Ship transportation requires the captured CO₂ to be in liquid phase under pressurized thermodynamic conditions. The injection of liquid CO₂ into the geological reservoir involves pressurization and heating in order to maintain the safe well head operating conditions. Most of the recent studies focusing on ship transportation of CO₂ set their research boundary upstream of an injection system. On the other hand, studies investigating the storage mechanism in the EOR field or other geological locations assumed the fixed CO₂ conditions from the literature. The purpose of this study is to evaluate the design of CO₂ injection system for an off-shore storage site where CO₂ is available in the liquid phase at the vessel. Many previous studies considered the liquid CO₂ at the thermodynamic conditions of 6-7 bara and -52 ^oC assuming large CO₂ transport volumes. However, currently there are no commercial ships which transport liquid CO₂ at such cryogenic conditions and only small size ships up to 1,500 m³ are being used for CO₂ transport at -20 °C and 20 bar. This study evaluates and compares the two thermodynamic conditions (7 bara and 20 bara) considering all the design and operational constraints of a liquid CO₂ injection system. This study also aims to fill the study gaps from the previous researches. For example, all the previous studies assumed the CO₂ stream as a pure liquid CO₂, however, practically this may not be possible since some amount of water is left in the CO₂ stream posing a threat of hydrate formation. In this study, we incorporate the CO₂ vapor return line to the pressure vessel which is an important aspect from safety point of view. This paper also investigates the application of two-stage rankine cycle to utilize the cold energy available from liquid CO₂ which can help to improve the overall efficiency of the injection system compared to that of the base case design. The working fluid employed in both the stages of rankine cycle is ammonia because of its highest power generation performance, easy availability and environment friendly characteristics compared to other working fluids. A well head injection pressure and temperature of 65 bar and 10 ^oC is set as a base case value for a storage depth of 1000 m to avoid any hydrate formation in the injection well or at the well outlet. The results show that the net specific power consumption for the two conditions (7 bara and 20 bara) is 4.52 and 1.90 kWh per ton of CO₂ injected respectively. Economic comparison has been performed to investigate the impact of operating conditions on the CO₂ injection process. The results show that the CAPEX for both the conditions is almost same since equipment involved in both the processes is the same. However, the OPEX for 7 bara design condition is approximately 2.4 times high compared to that of the 20 bara conditions. The high OPEX is attributed from the high operational energy requirement of the process. For a project life of 10 years, the specific cost per unit ton of CO₂ injected for the two design conditions (7 bara and 20 bara) came out to be 1.20 $ and 1.03 $ respectively. Finally, a sensitivity analysis has been done in order to investigate the effect of some important variables in the study.