In CCS operations it is likely that CO2 will be handled close to, or above, its critical pressure (73.82 bar) where many of its properties are similar to that of a liquid. In this state it is often referred to as ‘dense phase’, whereas above critical temperature (31.04°C) and pressure it is referred to as ‘supercritical’. Significant hazards associated with dense phase or supercritical CO2 arise when pressure falls suddenly or is lost completely.
CO2 is not currently defined as a dangerous substance under the Control of Major Accident Hazards Regulations 1999 (COMAH) or as a dangerous fluid under the Pipelines Safety Regulations 1996 (PSR). In June 2011 HSE published Assessment of the major hazard potential of Carbon Dioxide. This report concluded that CO2, based on the evidence available at that time, has major accident hazard potential if released at, or above, its critical pressure. However, where the risks are properly controlled the likelihood of a major hazard incident is expected to be very low, as in other similar processes in the energy, chemical and pipeline industries. HSE is committed to keeping CCS risks under review and will consider extending existing major accident hazard legislation to cover CCS if this is justified by the evidence.
In order to achieve a similar safety record to other energy, chemical and pipeline industries the CCS sector needs to continue to address the limitations of current knowledge regarding the control of risks from dense phase and supercritical CO2.
The ability to foresee loss of containment scenarios and accurately predict the dispersion of CO2 and its consequences are a fundamental element of CCS risk assessment. Currently the behaviour of CO2, when released in its dense and supercritical phases, is not yet fully understood. The CCS industry is continuing research into the use of appropriate dispersion models and these are being validated through appropriate scale experimental work. This will provide HSE and duty holders with a thorough understanding of how dense phase and supercritical CO2 behaves during foreseeable large-scale releases.
Since CO2 is not currently defined as a dangerous substance under the Control of Major Accident Hazards Regulations 1999 (COMAH) or as a dangerous fluid under the Pipelines Safety Regulations 1996 (PSR) HSE does not provide Land Use Planning (LUP) advice for CO2 capture, transport or storage.
However, for LUP purposes HSE uses Dangerous Toxic Load (DTL) to describe a substance’s airborne concentration and duration of exposure which would produce a particular level of toxicity in the general population. Potential CCS operators should be informed by Assessment of the Dangerous Toxic Load (DTL) for Specified Level of Toxicity (SLOT) and Significant Likelihood of Death (SLOD) when designing CCS CO2 capture sites and offshore installations and deciding pipeline routes.
When designing, fabricating, maintaining and operating plant and pipelines for handling and transporting dense phase or supercritical CO2 it is important that the full significance its physical properties, at the temperatures, pressures and compositions required, are fully recognised and managed.
Detailed standards and codes of practice written specifically for the design and operation of dense phase or supercritical CO2 plant and pipelines are still being developed. However, although general process engineering and pipeline standards exist (such as those for natural gas) that may be applied to CCS these may not be sufficient to ensure adequate containment for CO2 under all conditions.