Impact of impurities on pipeline specification and hydraulics

The purity of the CO2 stream emitted from carbon capture plants is extremely important for the design and operation of CO2 pipelines, affecting, amongst other things, the hydraulic efficiency and potential operating range as well as having implications for the safety and integrity of the pipeline system. However, to date, there is still uncertainty regarding the range of impurities that could enter the transport and storage systems for Carbon Capture and Storage (CCS) schemes. The main difficulty in being able to specify a CO2 pipeline composition is that the type and levels of potential impurities in the CO2 stream will differ between power plants and industrial sources and also between the capture technologies installed at the sources. The problem is further compounded by the fact that, not only does each impurity cause different effects on the transportation system, these effects, for any individual component, can vary with conditions and can, for example, alter gaseous and dense phase pipeline operation in different ways. Additionally, the mixtures of impurities in CO2 streams leaving different capture processes could vary substantially, leading to dramatically different effects on compression, transport and storage operations. This paper investigates the effects of impurities on pipeline sizing for dense phase and gaseous phase pipeline transportation using a series of twelve CO2 impurity scenario compositions. The scenarios have been selected as worst-case compositions that are representative of plausible CO2 streams from different capture technologies and industry sources. Two analyses are presented: i) an initial hydraulic analysis, conducted for a single point-to-point pipeline transporting a fixed flow rate of CO2 in either the dense phase or gaseous phase and ii) a sensitivity analysis to evaluate the effects of inlet pressure, ambient temperature and mass flow rate and pipeline size and identify an optimum pipeline size to handle each scenario in a hydraulically efficient manner. The inlet and outlet temperature and pressure ranges for each pipeline were selected based on a detailed analysis of the thermodynamic properties of the different streams, which is also discussed. As a result of the study, conclusions are drawn regarding the compositions that present the most challenge with respect to hydraulic efficiency and pipeline costs. In addition, guidance is provided on the specification of inlet conditions to improve the hydraulic performance of the pipeline. This work forms part of a study supported by IEAGHG on the “Impact of CO2 Impurity on CO2 Compression, Liquefaction and Transportation”. The study was commissioned to identify potential impurities and address the consequences of their impact on CO2 transportation