Safe transportation of CO2 for carbon capture and storage requires strict control of contaminants in the captured CO2, so that corrosion and particulate matter problems are avoided. The present work tested two almost similar CO2 specifications for chemical reactions and liquid or solid drop-out. The tests were carried out with CO2 in liquid and gaseous state, at field-relevant pressure and temperatures. It was shown that the NO2 content had a strong effect on chemical reactions and tendency to drop-out, but the threshold was different depending on the physical state of the CO2. Presence of NO2 resulted in chemical reactions. Drop-outs were observed for 3 ppm-mol NO2 in the liquid state (+4°C, 100 bar) and for 8 ppm-mol in the gaseous state (−3°C, 25 bar).
Several full-scale carbon capture and storage (CCS) projects are under planning in Europe. These CCS projects will transport large quantities of CO2 with ships and pipelines. Previous work has shown that certain components that can be present at trace levels (ppm-range) in the captured CO2 can react and create products that are corrosive to carbon steel. For example, it was shown that sulfuric and nitric acid can be formed for certain undesirable impurity combinations and concentrations,1 see reactions 1 through 5. Elemental sulphur may also form in some situations (reaction 6).2
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The first published CO2 specifications were mostly focused on health, safety, and environmental issues in case of accidental release of CO2 and had therefore quite relaxed impurity limits (Table 1). However, as more and more experiments showed that undesirable reaction products could be formed, the impurity limits were significantly reduced.
Even if some impurities can react and create strong acids, it has also been shown that other impurity combinations can be nonreactive (at least within the experimental timeframe). In practice, a trade-off must therefore be made between keeping the impurity combinations and concentrations low enough to avoid operational problems, while at the same time not too strict so unnecessary cleaning is avoided.
