Under-expanded jets and dispersion in supercritical CO2 releases from a large-scale pipeline

Abstract : Long-distance CO2 pipelines will be widely applied to transport captured CO2 from fossil fuel fired power plants for subsequent sequestration. In the event of pipeline failure a large mass of the inventory may be discharged within a short time, this represents a significant hazard if leaks continue undetected. An important result of the risk assessment for a CO2 pipeline is the safety distance. At present the lack of knowledge concerning near-field source terms and the far-field dispersion behavior of CO2 leaking from pipelines can make the calculation of safety distances imprecise. Study of near-field source terms and dispersion behavior is therefore necessary and of paramount importance for assessing safety distances and the impact of CO2 pipeline releases on the surrounding environment. In order to study CO2 pipeline leakage, a large-scale pipeline set-up with a total length of 258 m and an internal diameter of 233 mm was constructed to study the near-field characteristics and dispersion behavior of supercritical CO2 during sudden releases. The dynamic pressure near the orifice and CO2 concentrations and temperatures within the downstream dispersion region were measured together with the pressures inside the pipeline. The under-expanded jet flow structure and phase transitions in the near-field were studied for supercritical CO2 released though different orifice diameters (15 mm, 50 mm and Full Bore Rupture). The formation of the visible cloud, the distribution of cloud temperatures and CO2 concentrations in the far-field were analyzed using the measured data, photographs and video recordings. The safety distances along the horizontal direction for 5% CO2 concentration for each of the three orifice diameters were determined from the lower limit for adverse human effects.