In recent years, although the coal industry has developed steadily, safety accidents of coal spontaneous combustion have occurred from time to time, among which mine fires are one of the main spontaneous combustion disasters in coal mines. In fire-fighting technology, liquid CO2 is widely used in controlling spontaneous combustion of coal. In order to effectively control the flow and reduce the risk, it is necessary to gasify the liquid CO2 and then inject it into the goaf. At this time, the gaseous CO2 still has a low temperature, and the injection into the loose coal will play a dual role of gas concentration and temperature field. At present, some fire-extinguishing effects of low-temperature gaseous CO2 in fire-fighting applications are still unclear. For this reason, this paper starts with the spreading law of high temperature area in loose coal body, and studies the diffusion law and cooling range of low temperature gaseous CO2 after release in high temperature coal body by means of experiment and simulation. The main research results are as follows:
(1) The coal spontaneous combustion cooling test bench is used to carry out the low temperature gaseous CO2 injection experiment on the high temperature loose coal. The temperature spread characteristics of each plane measuring point during and after pressure injection are obtained. It is found that during the injection of low-temperature gaseous CO2 in the middle, the temperature of the loose coal body near the release port is rapidly reduced by the migration of low-temperature fluid, and the lowest temperature reaches -17℃. By studying the distribution characteristics of CO2 concentration injected into loose coal at different times, it is obtained that the diffusion rate of CO2 gradually tends to be stable after 0.55 m. The average diffusion rate of the diffusion paths at different distances from the central axis along the jet direction was calculated, and the average diffusion rate of CO2 in the loose coal was 0.051m/s.
(2) The cooling laws of low-temperature gaseous CO2 injected into loose coal under different experimental conditions were studied, and it was obtained: In the process of low-temperature gaseous CO2 injection, under certain quantitative conditions, as the injection position gradually approached the level of the high-temperature area and the injection time , the length of the pipeline increases, the cooling range of the high temperature coal area increases significantly, and the cooling efficiency is significantly improved. The length of the pressure injection pipeline plays a leading role in the cooling effect of the high temperature loose coal, followed by the injection time, and finally the injection position.
(3) Using the Fluent simulation software, a three-dimensional physical model that conforms to the heat and mass transfer characteristics of low-temperature gaseous CO2 in loose coal is established, and the simulation results are compared with the experimental results to verify the accuracy of the model. The cooling effect and diffusion process of low-temperature gaseous CO2 in the release process of loose coal were obtained, and it was found that the area occupied by the CO2 volume fraction greater than 90% with the injection time had a function of ExpDec1. The cooling effect of pressure injection of low-temperature gaseous CO2 in loose coal under different working conditions is simulated. The effects of the length of the pressure injection pipeline, the pressure injection flow rate, and the pressure injection position on the cooling range and cooling efficiency in different planes of the loose coal body are obtained. The calculation formula of the influence factors of pipeline length, pressure injection flow rate and pressure injection position on the cooling rate of the highest point in the loose coal body is obtained in the process of low temperature gaseous CO2 pressure injection.

In recent years, although the coal industry has developed steadily, safety accidents of coal spontaneous combustion have occurred from time to time, among which mine fires are one of the main spontaneous combustion disasters in coal mines. In fire-fighting technology, liquid CO2 is widely used in controlling spontaneous combustion of coal. In order to effectively control the flow and reduce the risk, it is necessary to gasify the liquid CO2 and then inject it into the goaf. At this time, the gaseous CO2 still has a low temperature, and the injection into the loose coal will play a dual role of gas concentration and temperature field. At present, some fire-extinguishing effects of low-temperature gaseous CO2 in fire-fighting applications are still unclear. For this reason, this paper starts with the spreading law of high temperature area in loose coal body, and studies the diffusion law and cooling range of low temperature gaseous CO2 after release in high temperature coal body by means of experiment and simulation. The main research results are as follows:
(1) The coal spontaneous combustion cooling test bench is used to carry out the low temperature gaseous CO2 injection experiment on the high temperature loose coal. The temperature spread characteristics of each plane measuring point during and after pressure injection are obtained. It is found that during the injection of low-temperature gaseous CO2 in the middle, the temperature of the loose coal body near the release port is rapidly reduced by the migration of low-temperature fluid, and the lowest temperature reaches -17℃. By studying the distribution characteristics of CO2 concentration injected into loose coal at different times, it is obtained that the diffusion rate of CO2 gradually tends to be stable after 0.55 m. The average diffusion rate of the diffusion paths at different distances from the central axis along the jet direction was calculated, and the average diffusion rate of CO2 in the loose coal was 0.051m/s.
(2) The cooling laws of low-temperature gaseous CO2 injected into loose coal under different experimental conditions were studied, and it was obtained: In the process of low-temperature gaseous CO2 injection, under certain quantitative conditions, as the injection position gradually approached the level of the high-temperature area and the injection time , the length of the pipeline increases, the cooling range of the high temperature coal area increases significantly, and the cooling efficiency is significantly improved. The length of the pressure injection pipeline plays a leading role in the cooling effect of the high temperature loose coal, followed by the injection time, and finally the injection position.
(3) Using the Fluent simulation software, a three-dimensional physical model that conforms to the heat and mass transfer characteristics of low-temperature gaseous CO2 in loose coal is established, and the simulation results are compared with the experimental results to verify the accuracy of the model. The cooling effect and diffusion process of low-temperature gaseous CO2 in the release process of loose coal were obtained, and it was found that the area occupied by the CO2 volume fraction greater than 90% with the injection time had a function of ExpDec1. The cooling effect of pressure injection of low-temperature gaseous CO2 in loose coal under different working conditions is simulated. The effects of the length of the pressure injection pipeline, the pressure injection flow rate, and the pressure injection position on the cooling range and cooling efficiency in different planes of the loose coal body are obtained. The calculation formula of the influence factors of pipeline length, pressure injection flow rate and pressure injection position on the cooling rate of the highest point in the loose coal body is obtained in the process of low temperature gaseous CO2 pressure injection.

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