Ciarán Reid, Crondall Energy Subsea Intern
Carbon Capture, Usage and Storage (CCUS) is quickly becoming one of the most glamourous waste transport industries in the world. Politicians and engineers alike consider CCUS to be crucial in the world’s energy transition to combat climate change. It will not be possible to stop climate change before the world stops using carbon based fuels without CCUS. There is a demand to increase the capacity for cost effective CO2 storage. This will require a full system approach to CCUS considering the needs of capture, transportation, and storage as one.
Over the last 5 months I have been carrying out a research project with Crondall Energy Subsea in which I set out to investigate the impact of impurities in CO2 rich mixtures on the pipelines system. This has been interesting, stimulating, challenging and worthwhile work. This included the opportunity to contribute to potential new standards for CO2 transportation, improve the understanding within Crondall Energy of CO2 transportation and engage in discussions across the industry on challenges ahead.
Crondall Energy Subsea are experts in pipeline transportation of traditional hydrocarbons and in recent years have utilised this expertise in the furthering the development of anthropogenic CO2 transportation and storage. One key area they have identified as a gap in knowledge across the industry is detailed understanding the impact of impurities on the pipeline transportation of CO2. As a result, it was identified that there was a need for some fundamental research to be carried out in this area and so I joined Crondall as a placement student to study the implications of impurities on the phase behaviour and flow assurance of CO2 mixtures.
Currently the transportation and storage requirements are assumed to set specifications for CO2 rich mixtures. To reach these specifications CO2 streams require a high level of expensive purification in the capture plant. Thus, if there is scope to relax the transport specifications there are potentially large savings to be made in the capture part of the system.
Thirty potential impurities were considered. It was found that it is most feasible to increase the allowed concentrations of non-condensable impurities N2, O2, Ar and H2, as they have limited impact on corrosion and safety of the pipeline. They also occur at high concentrations in the capture plant requiring intensive purification steps, hence large potential cost savings.
Increasing the impurity concentrations brings with it considerable challenges in the fluid behaviour. For CO2 transport it is desirable to operate in the dense phase and avoid entering the multiphase region. This is particularly difficult due to the phase envelope occurring within the pressures and temperatures of normal pipeline operation. Increasing N2, O2, Ar and H2 impurities increases the pressure multiphase flow occurs at, consequently increasing the pressure the pipeline must operate in to avoid it.
It was thus important to analyse the hydraulics of different fluids within a realistic pipeline system to observe the pressure and temperature drops of varying compositions in different scenarios. Pipeline length and flow rates in later life were found to lead to large pressure drops. This leads to design and operational challenges for maintaining pressures to keep CO2 mixtures in the dense phase. These challenges could be addressed by increasing input pressures with more compression and by careful pipeline sizing. Each increase brings with it further challenges and increased transportation costs. Increased costs in transporting higher concentrations of impurities can be offset by savings in the capture plants.
This work showed that while impurities will impact the phase behaviour and operation of the pipeline considerably, there is potential to overcome the challenges they cause. Each system has individual requirements, and the pipeline gas specification is the transport operator’s decision. If more relaxed specification is chosen for impurities, there is an opportunity to enable more capture plants to become financially viable and tie into the system.
The overall experience of carrying out this research within industry was brilliant and allowed me to apply skills learned in university to multiple engineering disciplines. It proved the practical application and usefulness of my skillset in the workplace. It also gave me a great insight into the interesting field of subsea pipelines. I am delighted to now be joining the Crondall Energy team as a Subsea Engineer.
To discuss this work or any other subsea engineering requirements in more detail please contact Crondall Energy enquiries@crondall-energy.com.
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