Carbon Capture and Transport

The thermodynamic and hazardous properties of CO₂ present safety challenges for risk management in the capture and transportation phases. CO₂ is usually handled in dense or supercritical phase. At such conditions, and in the quantities involved in full-scale CCUS operations, a loss of containment with rapid depressurisation has Major Accident Hazard (MAH) potential through asphyxiation and/or cryogenic exposure of people in close proximity to infrastructure.

Risktec has found that, with suitable adaptation, many of the technical safety techniques from the oil & gas sector are equally applicable to CO₂ hazards. For example, the broad methodology of a Quantitative Risk Assessment (QRA) will remain the same, but careful consideration needs to be given to the CO₂ equation of state, so that the thermodynamics and phase behaviour of a rapid depressurisation are appropriately modelled.

CO₂ characteristics affecting risk analysis

The logical comparison is with natural gas which is typically stored and transported in a single phase at medium pressure, is lighter than air, and where fires and explosions are the principal concern. CO₂ on the other hand is processed and transported in the dense/supercritical phase at higher pressure, is heavier than air, has a higher vapour pressure, and poses asphyxiation and/or cryogenic risks.

When modelling or assessing the risks of CO₂ releases, the following must therefore be considered:

• Releases tend to slump and gather in low lying areas
• Small amounts of impurities have large effects on phase behaviour of CO₂, affecting pipeline hydraulics (and routing), fracture dynamics, and corrosion protection required
• Accurate calculation of thermodynamic properties in the critical region is non-trivial, requiring specific equations of state
• CO₂ undergoes a violent expansion as it depressurises
• Solid particles (dry ice) form in the cold depressurised CO₂ jet (with potential for dry ice “grit blasting” effects), and subsequently sublimate
• CO₂ blowdown must be controlled over long time periods to avoid excessively low temperatures
• Ambient temperatures can have a significant effect on the density and compressibility of CO₂
• Long running ductile fracture propagation needs to be controlled through design of pipe toughness and/or crack arrestors