Climate change concerns have motivated several carbon dioxide capture and sequestration studies in recent years. A few of these have been implemented with other major schemes planned globally. Current work by the UK Health and Safety Executive has identified some gaps in knowledge on behaviour of CO2 releases from supercritical pressures. This is particularly relevant at the high pressures and large volumes envisaged for transport of and storage within depleted reservoirs in the North Sea. The combination of high pressures and large quantities have major accident hazard implications for large scale CO2 processing. Three primary areas of concern are discussed: source terms for dispersion modelling, strong cooling effect from pressure reduction and erosive effect of two phase solid/gas flows. To manage risks to persons involved with large quantities of supercritical it is necessary to be able to predict accurately its properties, both in containment and also during releases to atmosphere; both intended and accidental. At the high pressures being considered, CO2 is above its supercritical pressure and in certain parts of the process will also be above its critical temperature. In the supercritical state CO2 exhibits properties that need to be fully understood for accurate dispersion modelling. Some dispersion modelling of supercritical carbon dioxide releases has been carried out, but these have involved assumptions about the source terms and material properties at the point of release that are not considered reliable with present data. None of the available models account for the thermodynamics of solid formation and subsequent sublimation; the thermodynamic path followed during release is uncertain. As the release source terms largely define the subsequent dispersion characteristics, they are critical factors in modelling accurately carbon dioxide release behaviour. Carbon dioxide has a high Joule-Thomson effect that implies severe cooling effects in the immediate region of any pressure reduction event. In view of the large quantities to be handled, this will have significant cooling envelope on the surrounding plant. This applies to piping throttle downs, operational blowdown events and accidental releases. High cooling rates may lead to local solid carbon dioxide formation and/or low temperature enbrittlement of process plant or structural members. Supercritical CO2 is used commercially for erosion jet cleaning and cutting, and it is also used to enhance drilling penetration rates. Accidental releases of supercritical CO2, e.g. from flange failures, might lead to serious effects on adjacent piping, plant or structures. However, as with the other areas of concern, experimental evaluation of its release characteristics will be necessary to enable plant to be designed to a minimum risk configuration. Results of preliminary small-scale experimental tests are also presented to help quantify the above issues.
Hazards from high pressure carbon dioxide releases during carbon dioxide sequestration processes
2007
5 Seiten, 12 Quellen
Conference paper
Storage medium
English
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