Safety risk and thermodynamic behavior during liquid CO2 injection into broken coal: An experimental study

Injecting liquid CO 2 into the goaf is the most promising prevention and control technology for alleviating the risk of spontaneous combustion of broken coal. However, the thermodynamic behavior of liquid CO 2 during injection due to phase change remains unclear, hindering prediction of pipeline ice-blockage risk and the effective range of coal spontaneous combustion. To address this issue, an experimental system of liquid CO 2 injection into broken coal was established, and the influence of nozzle aperture on the thermodynamic behavior of CO 2 in the injection tube was investigated. Meanwhile, the effects of nozzle diameter and coal particle size on the thermodynamic behavior of CO 2 in coal were analyzed. The results indicate that increasing the nozzle aperture intensifies the pressure drop and cooling process within the injection tube, driving the CO 2 state toward the triple point (0.518 MPa, −56.6°C) and promoting dry ice formation. In the broken coal medium, larger nozzle apertures enhance phase-change intensity and heat absorption, leading to accelerated temperature attenuation and a shorter time required to reach the dry ice formation temperature (−78.5°C). The pressure propagation behavior exhibits a non-monotonic dependence on particle size, with intermediate particle sizes achieving the most effective propagation due to the coupled effects of permeability enhancement and heat transfer resistance. In addition, an empirical correlation between dry ice-cooled propulsion speed and CO 2 pressure propagation model was established. The research results provide theoretical support for the design of safety parameters and the prediction of influence ranges for coal mine liquid CO 2 fire prevention technology.