Leakage And Dispersion Characteristics Of Small Scale CO₂ Buried Pipeline

Carbon Capture and Storage (CCS) technique is considered to be the most effective way to alleviate the greenhouse effect. As an intermediate link in "CCS" chain, the safety of carbon dioxide (CO2) pipeline transport attracts much more attention. High concentration and low temperature region caused by CO2 pipeline leakage will be harm to surrounding person and equipment. Currently, there is no appropriate model which can fully describe the leakage and diffusion process, and also more experiment data are needed to support to build this model. For buried pipelines passing through the densely populated areas, no related researches are systematically carried out yet. Therefore, in the present study, the leakage and dispersion characteristics of CO2 buried pipeline were studied as following.(1) Whole section leakage apparatus of buried pipelines of gaseous CO2, and small orifice leakage apparatus of buried pipelines of gaseous and liquid CO2 are built. The measuring instruments used in the present study are calibrated. Sandbox is used to simulate the situation of buried pipelines, and the related properties of sand used in the present study are measured.(2) By comparison, the main differences of leakage and diffusion characteristics between buried pipeline and leakage in atmosphere. When the buried pipeline diffused, the concentration near the venting port is higher, and the temperature is lower. When the discharge pressure is 0.5 MPa and discharge aperture is 2 mm, the maximum temperature differences is 18℃. Therefore, once buried CO2 pipeline leakage occurs, the harm and the influence caused by the leakage will be more serious.(3) By changing the discharge flow, leakage and diffusion characteristics of whole section leakage buried pipelines are studied with small size experimental setup. After the gas discharges, the variation process of concentration can be separated into three phases which including in the initial stage, the rising stage and the stable stage. Bigger the discharge flow rate, faster the rate of concentration rises, and higher the peak of concentration as well. Temperature gradient appears in both horizontal and vertical direction. A certain distance from the vent, the temperature of layer soil is lower than that of upper because of the settlement of heavy gas. With the increasing of flow rate and distance, the concentration stratification will be more significance. Using a certain similarity principle on experimental amplification, the relationship between large-scale discharge flow rate and safety distance can be attained.(4) By changing the discharge aperture and pressure, the leakage and diffusion characteristics of small orifice leakage of buried CO2 pipelines are studied. Larger the discharge pressure and aperture, shorter the time CO2 diffuses to the position, higher the peak of concentration and lower the temperature drops in the discharge area. The direction of discharge impacts the distribution of surface concentration. The concentration of upwards discharge is slightly higher than that of horizontal discharge and significantly higher than that of downward discharge. But the impact of discharge direction on the temperature distribution can be ignored. A permafrost ball is formed near the venting area, and a large temperature difference is observed between the inside and outside of the permafrost ball. By fitting the experiment data, the relationship between permafrost diameter and discharge pressure under 2 mm aperture is obtained.(5) Through the experiments, the leakage and diffusion characteristics of buried pipeline of liquid CO2 are studied. The temperature of liquid CO2 discharge zone is much lower than that of the gaseous CO2 under the same experimental conditions, and the maximum temperature difference liquid CO2 discharge zone is 80℃. Buried depth has a significant influence on the discharge morphology. If the depth is large enough, liquid CO2 will form into dry ice near the orifice, and the discharge area shows a dry ice area and a soil area. Temperature of dry ice area is under -50℃. Twice of sharp temperature decreasing appear in dry ice area during the process of decline, and this is closely related to the formation of dry ice.