Analysis of the state of art on modelling of the consequences of CO2 releases into the atmosphere. Application to the Allam-cycle case study.

As part of the large-scale development of Carbon Capture and Sequestration (CCS) infrastructures, this thesis focuses on the analysis of the state of art on modelling of the consequences related to accidental or controlled releases of CO2 into the atmosphere, with the aim of understanding whether the techniques available are able to simulate this phenomenon. After a description of release characteristics, such as the formation of solid particles of CO2 and the possible formation of the dry ice bank, specific models were presented with their capability to predict the thermo-fluid dynamic behaviours of CO2 in a broken pipeline, to define the source term and to predict the near- and far-field dense dispersion of CO2. In the analysis, the integral and CFD models were studied. It was found that among the integral gas dispersion models analyzed, the PHAST and EFFECTS models are the only ones able to simulate the pipeline depressurization, considering the formation of solid CO2 particles, and the consequent sublimation, with the following discharge and dense dispersion. For CFD models instead, there is not a general model applicable to a wide range of scenarios, but the entire simulation has to be constructed. With them it is possible to represent complex terrain, space, and time-variable meteorological conditions, paying in more computational effort and time compared with integral models. Finally, all the validation work on dense CO2 models has been summarized, considering the few data available for CO2 with respect to other dense gases. Particular attention is given to the effects of topography and meteorological conditions on the dispersion phenomenon and to the ability of models to handle them. The second part of this study focused on the application of one of the models analyzed to a real case study. Phast version 8.23 software was used to study the analysis of the consequences and areas of damage resulting from a possible release of CO2 in the supercritical, dense, or vapor state, in the plant of an Allam-Fetvedt cycle. That is an oxy-combustion thermodynamic cycle that produces electricity and captures CO2 for sequestration, with supercritical CO2 as the working fluid. Among the objectives of this study, there was the analysis of how the solid fraction of CO2 influences the cloud temperature and dispersion, the comparison between the damage areas at certain concentrations of CO2 and low temperatures, that can be reached during the release and the following dispersion, and the determination of the most critical thermodynamic point within the cycle corresponding to the inlet or outlet of one of the components. At the end of the study, it was confirmed that the chosen Phast software is suitable for modelling and simulating such phenomena.