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The primary aim of this document is to describe the main compositional characteristics of the CO₂ stream downstream of the capture unit, taking into account common purification options. Accordingly, this document will characterize the different types of impurities and present examples of concentrations determined in recent capture pilot projects as well as through literature review. It identifies ranges of concentrations, giving priority to in situ measurements when available.

The second aim of this document is to identify potential impacts of impurities on all components of the CCS chain, from surface installations (including transport) to the storage complex. For example, impurities can have a significant effect on the phase behaviour of CO₂ streams in relation to their concentration. Chemical effects also include the corrosion of metals. The composition of the CO₂ stream can also influence the injectivity and the storage capacity, due to physical effects (such as density or viscosity changes) and geochemical reactions in the reservoir. In case of a leakage, toxic and ecotoxic effects of impurities contained in the leaking CO₂ stream could also impact the environment surrounding the storage complex.

In order to ensure energy efficiency, proper operation of the whole CCS chain and not to affect its surrounding environment, operators usually limit the concentrations of some impurities, which can, in-turn, influence the design of the capture equipment and purification steps. Such limits are case specific and cannot be described in this report; however, some examples of CO₂ stream specifications discussed in the literature are presented in Annex A.

The required purity of the CO₂ stream delivered from the capture plant will to a large degree depend on the impurity levels that can be accepted and managed by the transport, injection and storage operations. The capture plant operators will therefore most probably need to purify the CO₂ stream to comply with the required transport, injection, storage specifications or with legal requirements.

Monitoring of the CO₂ stream composition plays an important role in the management of the entire CCS process. Methods of measuring the composition of the CO₂ stream and in particular the concentrations of impurities are described and other parameters relevant for monitoring at the various steps of the CCS chain are described.

The interplay between the set CO₂ stream specifications and the efficiency of the entire CCS process is also explained. Finally, the mixing of CO₂ streams coming from different sources before transport or storage is addressed, and the main benefits, risks and operational constraints are presented.