Carbon Capture and Storage – The Impurities Conundrum

Carbon Capture Launch Boundary Dam carbon capture and storage in Estevan 2014

Is Collaboration on Impurity Standards for CO2 Pipeline Transport Feasible?

Carbon capture and storage on a large scale is crucial in the mitigation of climate change. The more effectively we can introduce suitable carbon capture technology to achieve this, the more effective the fossil fuel industry will be in its response to combat CO2 emissions.

In our efforts to implement carbon capture, utilisation, and storage (CCUS) projects, we cannot ignore the impact of impurities in the CO2 stream. These impurities arise from various combustion processes at industrial sources and include water, nitrogen oxides (NO­x), sulphur oxides (SOx) and hydrogen sulphide (H2S). If not properly controlled and monitored, they can lead to corrosion of CO2 pipelines. Yet there are no agreed international specifications for impurity limits and CO2 composition during pipeline transport.

As conscientious corrosion scientists, engineers, and industry leaders, shouldn’t we be pushing for defined impurities standards and limitations to ensure long-term feasibility, improve sustainability, and help to deliver positive environmental change to our planet by reducing greenhouse gas emissions?

Impurities in carbon capture and storage systems are costly

Impurities in carbon dioxide during transportation and storage can affect components in the system. Research has shown that even a small level of impurities can alter a CO2 stream (and affect geological formations deep underground in permanent storage locations). This results in physical changes, such as phase behaviour and density of CO2:

  • Higher-density carbon dioxide can reduce system capacity
  • A change in phase behaviour can result in sub-par performance of the system

Both effects increase the costs of operation of carbon capture and storage systems.

Impurities in carbon dioxide also cause chemical effects within carbon storage and transport systems. However, unlike physical effects, these can take some time to become apparent. One such effect is an increase in dissolution rate in the caprock of a storage system, affecting both the reservoir capacity and injectivity. Another is the damaging corrosive effect of impurities in CO2 pipelines. As Gareth Hinds of the National Physical Laboratory (NPL) and a previous President of the Institute of Corrosion explains:

“From a corrosion perspective, the most important impurity to consider is water. When the water concentration is below its solubility limit in dense phase CO2 (~ 2500 ppm under typical pipeline operating conditions in the absence of other impurities), no corrosion will occur. However, the presence of other impurities can increase the likelihood of corrosive phases forming, either by reducing the water solubility or via chemical reactions between different impurities.

“Acid dropout is the most significant concern for pipeline operators, whereby highly corrosive aqueous phases, such as nitric and sulphuric acid, can form as a result of reactions between water, NOx, SOx, O2 and H2S impurities.”

Removing impurities from carbon capture and storage systems is also costly

The more of these detrimental impurities we can remove after we capture the carbon dioxide, the more effective and sustainable the system will become.

Herein lies the conundrum facing us. The potential corrosion caused by the impurities contained within captured CO2 can be extremely costly. But to separate these impurities can also have a significant financial impact on the cost of a carbon capture project.

Consequently, the feasibility of a CCUS system requires a degree of balancing between functionality and commercial viability. It’s a question of balancing the cost of purification versus the impact of the remaining impurities on the pipeline lifetime.

Why do we need standards for setting acceptable impurity limits in carbon capture and storage systems?

It is generally not commercially viable to remove all impurities from CO2 streams. However, to ensure that projects remain feasible while complying with regulations, we require suitable standards of acceptable impurities.

Guidelines and best practices have been published by many international bodies. These include Det Norske Veritas (DNV) and the International Organization for Standardization (ISO). While such moves are to be welcomed, Gareth points out that further research is required:

“Under current regulations, the responsibility lies with the pipeline operator to carry out their own assessment and specify impurity limits during the design phase of a given CO2 pipeline project,” he says. “These limits can vary significantly depending on the composition of the CO2 stream, the economics of the purification technologies used and the operating conditions of the pipeline.

“For CO2 specifications, thresholds in relation to acid dropout are set based on limited available data (often not lower than 25oC) and are therefore likely not conservative enough. The development of reliable standard test methods that are more representative of service conditions will go a long way towards addressing the issues.”

The challenges of regulating impurity standards for carbon capture and storage

Writing in Corrosion Management, the leading journal for corrosion control and prevention, Gareth highlights some of the key challenges facing the regulation and standardisation of acceptable impurity levels in the CCUS as we seek to reduce carbon emissions from many industries, including oil and gas:

“Assessment of the risk of water and acid dropout in CO2 pipelines due to the presence of multiple impurities is a complex process, which requires an understanding of the thermodynamics of fluid composition, the impact of operating temperature and pressure variations (including potential upset conditions) and interactions between impurities.

“In addition, published corrosion rate data in the open literature should be treated with caution due to challenges in control of test parameters and the high degree of uncertainty around the correlation between laboratory test data and real-world application. Combined with the relative lack of service experience in transport of CO2 captured from a range of industrial sources, this often leads to a degree of over-conservatism in materials selection.”

The bottom line

Impurities within captured CO2 are a significant concern, affecting operation, safety, and environmental sustainability. The costs associated with the presence of these impurities can be high, as can the costs of removing them.

Some international bodies have established guidelines for acceptable and safe levels of impurities within captured CO2. However, in the most part the responsibility lies with pipeline operators to conduct assessment and specify impurity limits during the design phase of a CO2 pipeline project.

Isn’t the current carbon capture and storage landscape in need of a more highly focused, collaborative approach to this issue?

Shouldn’t we work more stringently towards internationally recognised standards for impurities in CO2 in carbon capture, storage, and transportation?

And shouldn’t such standards be regularly updated as our knowledge and understanding of CCUS technologies improves with more robust data?

And finally, because each carbon capture and storage project is so unique, is it feasible to create a single set of standards?

We’d love to hear what you think. Or, if you have any questions that you would like to ask an expert, please feel free to get in touch by emailing the Institute of Corrosion.

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