Development of a methane concentration monitoring sensor in underground boreholes of coal mines

At present, during underground drilling operations in coal mines, the methane concentration monitoring system can only reflect the total drainage volume of methane at the borehole, and cannot observe the effect of methane drainage of a certain section of the borehole. With the increase of the borehole depth for underground methane drainage, the effect of methane drainage changes greatly along the length of the borehole. There is a big difference in the effective extraction radius at different hole depths, leading to greater difficulty in arranging the boreholes for methane extraction, and increased uncertainty. In response to this problem, a methane concentration monitoring sensor in underground coal mine boreholes is designed in this paper. On the basis of the principle of Tunable Diode Laser Absorption Spectroscopy(TDLAS), the sensor allows simultaneous online monitoring of multiple points in the borehole and ensures that the borehole is passive, which is intrinsically safe. First, the basic principle of TDLAS methane measurement is analyzed. On the basis of the principle of methane molecule absorption spectroscopy, the selection of laser light source is introduced, and the methane concentration calculation formula is derived according to Beer-Lambert law. Then, the methane concentration monitoring sensor design is carried out, including optical path design, structure design, protection process design and operation process in the hole. Finally, in terms of both performance and reliability, the relative error test, stability test, response time test, performance comparison with the non-dispersive infrared sensor, waterproof test and dustproof test are conducted. The designed methane concentration monitoring sensor has a diameter of 40 mm and a length of 80 mm. It is intrinsically safe and is structurally applicable to underground coal mines. In the performance test, the maximum relative error of the full range of the sensor is 2.8%, which is less than the monitoring standard of the methane concentration of ±6% in the hole. In the stability test, the fluctuation range of sensor data is 0.015%, and the stability is 0.28%, which meets the requirement of stability less than 1%. The response time of the sensor is about 8 s, which meets the requirement of response time less than 10 s. In the comparison test with the non-dispersive infrared sensor, the relative error and response time of the TDLAS concentration monitoring sensor are significantly better than those of the non-dispersive infrared sensor. In the reliability test, its measurement accuracy is not affected when the sensor is in a high humidity environment for a long time, due to the effective waterproof protection process. The results of the performance test and reliability test show that the sensor can meet well the methane concentration monitoring requirements in the hole, and has a good application prospect in the monitoring of underground coal mines.