Is the presence of impurities in the CO2 stream destined to be stored underground a problem? The possible impacts of impurities are reservoir-specific and depend on the mineralogical composition of the rocks and of course the type of impurity and its concentration. Impacts can vary from slight dissolution creating micro-voids, to mineralisation which fills-up the pore-space.

Although the potential mechanisms through which certain impurities could affect storage capacity or integrity are well understood, simulating the exact conditions of a storage complex and the gradual accumulation of impurities in the laboratory pose significant problems.  

Introduction  

CO2-rock interactions are investigated with models and laboratory experiments using pure CO2. However, as a consequence of the capture process, the CO2 stream is likely to contain impurities, which may alter the behaviour of the stream through the compression, transport and storage elements of the CCS chain. Research on the effects of these impurities on the integrity of geological storage formations is limited.

The ‘other components’ or ‘impurities’ in CO2 streams1 can include nitrogen (N2), oxygen (O2) and water (H2O), but also air pollutants such as sulphur and nitrogen oxides (SOx and NOx), particulates, hydrochloric acid (HCl), hydrogen fluoride (HF), mercury, other metals and trace organic and inorganic contaminants. In addition small amounts of chemical solvents used in post-combustion capture may be present in the CO2 stream. The removal of certain contaminants may be required for health, safety and environmental protection reasons, but also to ensure the effective transport and storage of the CO2 stream.

However, without overlooking the importance of health, safety and the protection of the environment, the extent to which impurities must be removed from the CO2 stream is also an economic issue. Reaching higher levels of CO2 purity will involve a number of incremental gas treatment processes each incurring capital and operation costs, potentially increasing the energy penalty and reducing CO2 avoidance. It is important that if deemed necessary, legal provisions regulating the maximum levels of impurities permitted in a captured CO2 stream strikes a balance between the economics of the entire CCS chain and protecting people and the environment. In order to do this, policymakers need access to reliable scientific research, review existing regulation and consult a range of stakeholders.

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Source: Carbon Capture Journal newsletter (27 Apr 2012)