The Leakage And Dispersion Characteristics And Quantitative Risk Assessment Of Supercritical CO₂ Released From Pipelines

Global warming caused by greenhouse gas emissions especially carbon dioxide(CO₂)is threatening the common home of mankind,the Earth.Reducing greenhouse gas emissions is already a matter of human destiny.Carbon capture and storage(CCS)technology is recognized as one of the key technologies that can effectively reduce greenhouse gas emissions.CO₂ transportation is a necessary part of CCS technology and pipeline transportation under supercritical condition can ensure transport efficiency and good economy.However,little is known about the safety of supercritical CO₂ pipeline leakage.Due to the special physical properties of CO₂ and high pressure transport mode,the supercritical CO₂ pipeline leakage dispersion is accompanied by complex physical changes such as complex phase transition,transonic flow,particle evolution and heavy gas dispersion.The research on this topic is still in the exploration stage.At present,there is no clear risk assessment method for CO₂ pipelines,and there is no consensus on the risks arising from CO₂pipeline transportation.Therefore,this study combined theories of fluid mechanics,phase transition kinetics,phase equilibrium and probability and statistics methods,and combined experimental,theoretical and numerical simulation methods to study the leakage and dispersion characteristics of supercritical CO₂pipelines.For the characteristics of CO₂ leakage and dispersion,the leakage dispersion process is divided into two stages:near-field jet and far-field low-temperature heavy gas dispersion.On the scientific level,from the macroscopic and microscopic perspectives,the mechanism of jet and dispersion processes is analyzed,and the phase transition model under transonic low-temperature flow is coupled to establish a dispersion model at different stages,focusing on the effect of low temperature and impurities on dispersion.At the same time,the CO₂ leakage and dispersion characteristics under complex terrain were also studied.At the application level,based on the improved Gaussian dispersion model,combined with the failure probability of supercritical CO₂pipeline,CO₂ exposure threshold and profit function,a quantitative risk assessment model for supercritical CO₂ pipeline was established and the risk equivalent curve was drawn.The main results include:(1)A set of supercritical CO₂ leakage and dispersion experimental device was developed independently,which can realize the safe and controllable releases of high pressure CO₂ under indoor conditions.The experimental device was used to study(1)The hydro-thermal characteristics during supercritical CO₂ leakage process.(2)The mechanism of near-field jet during leakage process.(3)The size distribution and evolution characteristics of dry ice particles.The physical laws and mechanisms during the supercritical CO₂ leakage process were fully grasped from different aspects.The results show that the shape of the leakage orifice had an important impact on the leakage characteristics.Compared with the typical under-expanded jet formed during the leakage from a circular orifice,the fan-shaped shock wave system was formed when leaked from a rectangular orifice;and then the jet angle and the motion of the shock wave were affected,which was finally reflected in the difference of the dispersion range.According to the change of pressure drop rate,the depressurization process of supercritical CO₂ can be divided into three stages:supercritical stage,saturated gas-liquid stage and gas stage.The superheat of the saturated gas-liquid stage increased with orifice size.When leaked from a circular orifice,the larger the leakage orifice,the lower the near-field jet temperature,the faster the leakage rate.For the rectangular orifice,the orifice area and aspect ratio of orifice could affect the near-field jet temperature.In contrast to the circular orifice of the same area,the rectangular orifice caused the increasement of the leakage rate.During the leakage process,supercritical CO₂ first transforms into a quasi-steady state(supersaturated state)in the pipeline,then a rapid droplet nucleation and growth occur,and then coalescence occurs,and finally multiphase flow is formed with different gas-liquid fraction according to the type of the orifice.The particle size distribution of solid CO₂ in the near-field centerline of the jet is in the range of 1–10?m,and obeys the law of logarithmic distribution.Evaporation and coalescence work together on the particles,so that the particle size of the dry ice particles changes during the movement.(2)The influence of delayed phase transition on the strength of supercritical CO₂ dispersion source was found for the first time.A mathematical model based on the homogeneous relaxation model can explain the phenomenon of non-equilibrium phase transition during the leakage process.The theory of CO₂ phase transition is enriched by the“smoothing treatment”of physical properties,which solves the problem that the cubic state equation cannot accurately predict the physical property change near the triple point of CO₂.A mathematical model of compressible multiphase flow considering supercritical-liquid-gas-solid phase transition was established.The effects of relaxation time,hole orifice and initial pressure on the strength of the dispersion source were studied.The experimental verification results showed that the model has good accuracy and good reproducibility for the jet structure.The results also showed that the maximum vortex strength located behind the orifice edge and Mach disk.The phase fraction of condensed CO₂(liquid and solid)decreases rapidly outside the pipeline.However,at the outlet of the orifice,the liquid phase fraction is at its highest value during the jet.The choice of relaxation time has a great impact on the carbon dioxide jet temperature and the condensed phase fraction,while the effect on jet velocity and outside pressure is negligible.(3)The influence mechanism of vapor phase transition(frosting condensation)on the dispersion of low temperature CO₂ was revealed for the first time.It was proved that the simulation results considering the vapor phase transition were better than that without considering water vapor.Based on Eulerian-Eulerian two-fluid model and phase transiton dynamics theory,a three-dimensional model of low-temperature CO₂ heavy gas dispersion considering phase transition of water was established.The model considered the effects of surface roughness,atmospheric stability and turbulence on the dispersion,and k-?model is preferred for supercritical CO₂ leakage dispersion simulation.Studies have shown that the frost condensation caused by the near-field low temperature releases heat,which causes the air mass temperature to rise and the thermal motion of the gas molecules to accelerate,and then leads to a lower CO₂ concentration than that without considering the phase change.The urban building affects the dispersion of CO₂,which tends to make CO₂ spread along the surface of the building(especially the windward side),while forming a low concentration zone on the leeward side.Based on the Euler-Lagrange method,the influence of dry ice formation on dispersion was studied.The movement and evolution law of dry ice particles was analyzed.The result showed that the dispersion simulation considering dry ice formation was more conservative than pure gas dispersion.The study also found that the risk of dangerous impurities leakage was equally large,and the evaluation criteria for determining uniform impurity hazards were critical for the assessment of the consequences of CO₂ pipeline leakage.(4)For the first time,the concept of“fatal length”was introduced into the quantitative risk assessment of CO₂ pipelines,providing a new method for scientifically evaluating the risk of CO₂pipelines.The leakage hazards of supercritical CO₂ pipelines containing impurities were comprehensively analyzed.The specific failure probability of CO₂ pipelines was summarized,corrected and determined.Based on the basic steps of pipeline risk assessment,a quantitative risk assessment model suitable for supercritical CO₂ pipelines was established.Studies have shown that,the hazards to the population are limited when CO₂ is leaked from small and medium holes,while it may cause casualties when full bore rupture occurs.Therefore,it is recommended to regard the CO₂pipeline quantitative risk assessment as a prerequisite for determining the safe distance of the pipeline.