Fire prevention and extinguishing and CO2 sequestration in coal mine gob require continuous transportation of liquid CO2 in pipelines with large height difference (from ground to underground). However, the temperature and pressure variation of liquid CO2 in pipelines with large height difference is still unclear, which hinders the design of a liquid CO2 pipeline transportation system. The influence of pipe diameter and inlet parameters (temperature and pressure) on the variation of temperature and pressure of liquid CO2 along the 1000 m vertical pipeline was studied in this paper. The study found that for each pipeline diameter considered there existed a range of flowrates where safe flow conditions could be ensured, at which no phase transition occurs throughout the length of the pipeline. When the transporting flow is larger than the maximum limit flow, phase transition occurs dramatically, which will lead to a sudden drop in temperature and pressure. When the transporting flow rate is lower than the minimum limit flow rate, phase transition of CO2 occurs slowly along the pipeline. According to the requirement of underground fire prevention and extinguishing for transporting flow rate and the economic cost of the pipeline system, the optimum diameter is 32 mm, and the corresponding safe transporting flow range is 507–13,826 kg/h. In addition, when the inlet pressure is constant, if the inlet temperature is too high, phase transition of CO2 occurs dramatically at the entrance. For a 1000 m vertical pipe with diameter of 32 mm, when the inlet pressure is 14 bar, 16 bar, 18 bar, 20 bar, 22 bar, 24 bar, the corresponding maximum allowable inlet temperatures are −30 °C, −26 °C, −23 °C, −19 °C, −16 °C and −13 °C, respectively. This research has significant guidance for safety transportation scheme of liquid CO2 from coal mine surface to underground.
To prevent coal spontaneous combustion and store CO2 in the coal mine, it is necessary to establish a fire-prevention pipeline transport system which continuously injects a large amount of liquid CO2 from the ground to the underground area directly. At present, few studies are focused on the law of liquid CO2 transport with great altitude difference. Moreover, the complex transport environment in the coal mine affects the design and application of the pipeline transport system for ground direct injection of liquid CO2. This study explores the influence of environmental factors at different depths in the coal mine on the liquid CO2 transport. Excessive altitude difference, ambient temperature and airflow velocity may lead to the boiling of liquid CO2 during pipeline transport and a sudden drop in CO2 temperature and pressure, which may cause danger in the pipeline transport system. The critical insulation thickness is determined based on the occurrence of the boiling of CO2. In addition, the influence law of adding an insulating layer of different thicknesses to the CO2 pipeline system is obtained. This study is of great significance to the establishment of a pipeline system that safely transports liquid CO2 from the ground to the underground mine.
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