Analysis of the state of the art of vulnerability and impairment thresholds associated to physical effects of a CO2 release. Application to the Allam cycle case study.

Carbon Capture Utilization and Storage (CCUS) technology is considered a mitigation measure to control the increasing CO2 emissions in atmosphere during energy transition. Even if CO2 infrastructures already exist in remote areas of USA and Europe, the CCUS facilities have to be deeply analyzed as safer systems, considering new issues strictly connected to their presence in more densely populated areas. As part of the Quantitative Risk Assessment, the critical transportation conditions and the released stream characteristics have been analyzed to assess the effects of an unexpected CO2 release event. The aim of this thesis is to fill the gaps of knowledge in the Quantitative Risk Assessment for dense phase CO2, for which consolidated and shared methods do not exist, due to limited use of this stream in traditional plants. The evaluation of vulnerability of people and facilities has been done, collecting information about effects of dangerous doses of inhaled CO2 and consequences of exposure to critical release conditions. Additional considerations for the Escape, Evacuation and Rescue Analysis have been done, collecting data about impairment caused by lack of visibility, low temperatures, physical blast and inhalation of toxic substances. Additional engineering hazards have been analyzed in order to identify the prevalent phenomena able to affect the structures integrity within a reduced exposure time. The vulnerability analysis results in definition of threshold values for each dangerous effect of a CO2 release. Analyzing the state of the art about CCUS plant risk assessment, existing mortality predictive models have been considered. For those aspects of the analysis not yet discussed in literature, some assumptions have been done to associate to some physical effects the expected consequences on exposed individuals and materials. Dose-based criteria have been analyzed to consider the toxicity of CO2. Also the released plume cryogenic effects have been investigated. Due to absence of references about lethality of low temperatures, correlations between ambient temperature, duration of exposure and probability of death have been extracted from medical papers. In conclusion, overpressure values able to cause 0% and 100% mortality for outdoor or indoor populations have been obtained from Energy Institute’s evaluations about CO2 releases from pipelines. Vulnerability of facilities exposed to the CO2 cold plume may be referred to conducted tests on different types of metallic and non-metallic materials, considering their embrittlement and hardening points and their thermo-dynamic interaction with CO2. In addition, some limit operating temperatures for emergency equipment have been collected for QRA and EERA evaluations. The obtained vulnerability results have been applied to the Allam cycle case study. Through the DNV software Phast 8.23, the analysis of consequences of liquid, vapour and supercritical CO2 releases has been conducted, starting from process data extracted from Eni previous works. Following a sensitivity analysis, all required parameters have been set to obtain downwind concentration and temperature values. Collecting distance results, the 1% and the 50% fatality regions have been determined, adopting models chosen in the first part of the analysis. Finally, safety distances from CO2 releases have been compared with that defined to consider a CH4 discharge from equipment of the same Allam cycle plant.