| Summary: | Fire prevention and extinguishing and CO<sub>2</sub> sequestration in coal mine gob require continuous transportation of liquid CO<sub>2</sub> in pipelines with large height difference (from ground to underground). However, the temperature and pressure variation of liquid CO<sub>2</sub> in pipelines with large height difference is still unclear, which hinders the design of a liquid CO<sub>2</sub> pipeline transportation system. The influence of pipe diameter and inlet parameters (temperature and pressure) on the variation of temperature and pressure of liquid CO<sub>2</sub> 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 CO<sub>2</sub> 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 CO<sub>2</sub> 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 CO<sub>2</sub> from coal mine surface to underground.
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