Impact of CO2 impurity on CO2 compression, liquefaction and transportation

The impurities present in CO₂ streams are important for CO₂ pipeline and ship transport, as they can affect various aspects, such as the range of operation, safety considerations, fracture control, cracking, corrosion control, dispersion in the event of a release, fluid density, operating pressure, temperature and the quantity of CO₂ that can be transported. The range and levels of potential impurities emitted from CO₂ capture facilities will differ between different power plant and industrial sources and between the capture technologies installed at the source. It is essential to improve the understanding of the effect of these potential impurities on CO₂ compression, liquefaction and transportation under relevant conditions. Therefore, IEAGHG commissioned a study on this topic to a consortium comprised of Newcastle University and the University of Edinburgh.The study identified twelve worst-case but plausible impurities scenarios that are representative of the main CO₂ capture processes.

- Impurities greatly affect the thermodynamic and transport properties of CO₂ streams.
- Apart from pure CO₂, the CH₄-rich scenario exhibited the most desirable qualities for dense phase pipeline transport.
- Only one case (i.e. adsorption with high N₂ content) showed significantly higher compression energy requirement than the other scenarios, being 7% more than the base case.
- For dense phase transport, the worst-case scenarios (i.e. adsorption with high N₂ content and oxyfuel combustion with high O₂ content) would lead to an increase in pipeline sizes compared to the base case, which would raise their capital cost.
- Temperature has a more pronounced effect on density than pressure, meaning that in order to increase the pipeline’s capacity the inlet temperature should be as low as possible.
- Saturation pressure of CO₂ is a critical variable for fracture control. Especially H₂ has a strong elevating effect on saturation pressure. High levels of O₂ and N₂ also lead to an increase.
- The water specification for some scenarios (i.e. adsorption with high N₂ content and oxyfuel combustion with high O₂ content) will require careful consideration, due to the influence of impurities on water solubility, which in turn can affect corrosion and stress corrosion cracking. CO and H₂S may also increase the risk of stress corrosion cracking.
- Ship transport under most investigated scenarios would be uneconomical, as it requires high pressures and very low, i.e. cryogenic, temperatures. Thus, it will only be economically viable in case of very high CO₂ purity.