Study On The Characteristics Of Transportation Of CO₂ Pipeline And Propagation Of Decompression Wave During Leakage

As a key link of carbon capture and storage（CCS）,pipeline transportation is an important choice for large-scale delivery of CO₂.Due to various complex factors in the transportation process,CO₂ pipeline may leak during operation,and even have major pipeline safety accidents such as crack extension.At present,the research on the transient characteristics of the tube in the process of CO₂ pipeline leakage is still in its infancy,especially for the gas-liquid phase transition which may be caused by large temperature drop.In addition,in the actual pipeline transportation process,limited by capture technology and cost,pipeline CO₂ may contain a variety of impurities,forming CO₂ flow.The type and content of impurities significantly affect the thermophysical properties of CO₂ flow,and further affect the hydraulic and thermodynamic calculation in the process of CO₂ pipeline transportation.In order to ensure the safe and efficient operation of the pipeline,and improve the theory of controlling the pipeline crack extension and pipeline transportation characteristics,it is crucial to study the efficient transmission process characteristics of gas CO₂ pipeline,as well as the transient characteristics in the pipeline,phase change law,impurity influence law and medium pressure reduction wave propagation characteristics of the pipeline leakage process.In view of the gas-liquid phase transition which may be caused by large temperature drop in the leakage decompression of gaseous CO₂ pipeline,a"quasi-three-dimensional"computational fluid dynamics（CFD）model of CO₂ pipeline leakage is established,and verified with industrial scale pipeline experimental data.The response curve of pressure and temperature in the pipeline during the leakage process of CO₂ in the gaseous state as well as the possible phase change rule were studied,and the influence of gas-liquid phase change on the propagation characteristics of decompression wave in the pipeline was deeply analyzed.It is found that in a very short time after the full opening of the gaseous CO₂ pipeline blasting,the pressure and temperature of the medium in the pipeline will drop rapidly.In some working conditions,the medium in the pipeline will change in the gas-liquid phase during the process of decompression wave propagation.The initial value of sound velocity of gaseous CO₂ is about 240 m/s,and then it gradually decreases with the decrease of pressure.When the initial temperature is higher and the initial pressure is lower,the pressure will be lower at the intersection of the isentropic line corresponding to the medium decompression process in gaseous CO₂ pipeline and the gas-liquid saturation line of pure CO₂,that is,the lower the platform pressure corresponding to the decompression wave curve,and the smaller the potential risk of pipeline fracture.Accurate prediction of thermophysical properties is the basis for accurate establishment of flow models in tubes when CO₂ leaks.It is a complicated technical problem how to express the exact equation of state mathematically and embed it into the flow model in a tube through the real gas model.Based on the constraint equation of phase fugacity,component and phase material balance,a program for calculating the physical properties of CO₂ containing impurities was developed and embedded in the CFD model to analyze the influence of impurity type and content on the decompression wave propagation characteristics in tubes.Studies show that compared with pure CO₂pipelines,polar impurities（such as SO₂ and H₂S）improve the decompression platform of decompression wave curve in gaseous CO₂ pipelines with full opening leakage,while non-polar impurities（such as CH₄ and N₂）reduce the decompression platform of decompression wave curve in gaseous CO₂ pipelines with full opening leakage,making it easier to crack arrest after pipeline fracture leakage.For gaseous CO₂pipelines,the increase in the content of polar impurity SO₂ aggravates the risk of crack propagation,while the increase in the content of non-polar impurity N₂ reduces the risk of crack propagation.Establishing the transportation process model of the CO₂ long pipeline under stable working conditions is the basis for determining the key nodes（such as pressurized station）in the CO₂ pipeline transportation system and its process configuration.ASPEN-Hysys software is used to establish a one-dimensional CO₂transportation pipeline hydraulic and thermal model,and to study the transportation process of CO₂ long-haul pipeline in Jilin oilfield under complex terrain conditions.The change law of the physical properties of CO₂ containing impurities was deeply analyzed,and the influence of the impurity type,the dip Angle of the pipeline and the ambient temperature on the CO₂ flow transport process was studied.The research shows that the pressure drop in the pipeline is larger when the CO₂ flow containing impurity CH₄ or N₂ is transported than when the pure CO₂ is transported.The greater the proportion of CH₄ or N₂ is mixed,the greater the difference in pressure drop,and the influence of impurity N₂ on pipeline transport is more obvious.Under the same height difference,the pressure and temperature change of the down pipeline is the smallest,while that of the ascending pipeline is the largest,which indicates that the topographic difference along the line will affect the conveying capacity and process of the pipeline.In addition,the environmental temperature will affect the heat transfer between the pipeline and the outside environment in the process of pipeline transportation.Under the same inlet temperature,the greater the temperature difference between the temperature in the pipeline and the environment temperature is,the greater the heat loss to the outside is,and the lower the temperature of the medium outlet in the pipeline is.