Large-scale Carbon Capture and Storage (CCS) hubs have emerged as a cost-effective means for industries to reduce carbon dioxide (CO2) emissions to atmosphere. Designing a system that connects multiple emitters into a common dense phase transportation pipeline and storage site introduces significant challenges, not least of which is the management of non-CO2 components (impurities). A unique aspect of CO2 pipelines is that they typically operate close to the fluid phase envelope. The presence of simple impurities at low volumes can therefore lead to major impacts. The impurities will have an impact on fluid behaviour (thermodynamics), transport (thermo-hydraulic), integrity (chemical reaction, corrosion, material selection, reservoir, etc.), safety, sequestration (injectivity, long term storage) etc. Impurities therefore can ultimately impact the capacity, integrity and operability of the pipeline network.
The primary control mechanism is to set a tight specification on impurity levels. A substantial amount of work has been carried out and published on impurity level recommendations/ specifications. These references give various thresholds for the impurities. However, the reasons behind the recommended tolerances are sometimes vague or non-existent. Some thresholds are set based on potential upset conditions that may not actually occur, and for some of them the presence and/or lack of other impurities are not considered. These limitations can make it very difficult to adapt the thresholds for a specific system or project. Furthermore, overly onerous requirements will discourage users (emitters) from connecting into the network.
A case study will be presented demonstrating the influence of impurities on a dense phase CO2 pipeline design, and then showing the development of a practical and implementable CCS hub fluid specification to protect the pipeline integrity, based on the CO2 sources and expected system operating conditions.