2.2 Equipment
The gas emission test of coal particles under the constant temperature was carried out using a self-developed gas adsorption and desorption experimental device (Fig. 2). The device consists of an inflatable unit, a degassing unit, an adsorption-desorption unit, a temperature control unit and a data acquisition unit. The gas cylinder of the inflatable unit is divided into two types: methane and helium. The gas source can be selected according to the needs. The vacuum pump of the degassing unit can pump up to 4 L/s. The adsorption-desorption unit uses two coal sample tanks with a volume of 100 ml, one of which is coal sample tank and the other is reference tank for testing dead volume, both of which are made of stainless steel and can withstand pressure of 10 MPa. The coal sample tank is connected with temperature and pressure sensors, which can monitor the test environment parameters in real time. The temperature measurement accuracy is 0.1℃. The pressure measurement range is 6.0 MPa with the measurement accuracy of 0.1%. The constant temperature test box of the temperature control unit can control the range of -50℃~50℃, and the measurement error is not more than 1℃. The flowmeters in the data acquisition unit are 0 ~ 50 ml/min and 40 ~ 2000 ml/min each, and the accuracy is not greater than 1%FS, which can be automatically switched through the computer control. The data acquisition unit can realize the acquisition of temperature, pressure and flow parameters, and the acquisition frequency can achieve 1 time/s.
2.3 Test method and procedure
Using original coal and tectonic coal samples, coal particle gas emission tests under different adsorption equilibrium pressure conditions were carried out respectively. The steps of each coal particle gas emission test are as follows:
1) Pre-trial preparation stage. Open the constant temperature test box, set the temperature to 25°C, and keep the constant temperature for 1 h. Fill the coal sample tank and reference tank with helium until the pressure gauge reads 3 MPa, close the inlet and outlet valves, and wait for 30 minutes. If the pressure change is less than 0.01 MPa, it is judged that the air tightness is qualified, otherwise, the leak point needs to be checked, and perform the air tightness test again.
2) Free volume test stage. Weigh 50 g of dry coal sample and put it into the coal sample tank. After sealing, connect the test device to vacuumize to 10 Pa and continue for 2 h; close the valve of the coal sample tank, and fill the reference tank with helium gas to 3 MPa, after the pressure is stable, the coal sample tank is connected to the reference tank, and the free volume of the coal sample tank is calculated according to the gas state equation.
3) Constant pressure adsorption stage. The test device was evacuated to 10 Pa and continued for 2 h. The CH4 was charged into the reference tank to the target pressure, while the coal sample tank was connected to the reference tank. If the gas pressure change in the coal sample tank was less than 0.02 MPa after more than 6 h, it was judged as adsorption equilibrium.
4) Constant temperature emission stage. Keep the temperature of the constant temperature test chamber at 25°C, open the open pipeline, and use the desorption instrument to test the free gas volume for about 5 s; after the gas pressure in the coal sample tank drops to 0 MPa, switch to the flowmeter to collect gas desorption data.
The above steps were repeated under four conditions of adsorption equilibrium pressure of 0.5, 1.0, 1.5, and 2.0 MPa, respectively, to complete the gas emission test of primary coal and tectonic coal samples under the constant temperature.
The gas desorption amount recorded by the desorber needs to be converted into the volume under standard conditions according to the formula (1)[31].
Where Qt is the volume of the total gas desorption converted into the standard state, mL;Qt’ is the total gas desorption measured by the desorber, mL; tw is the room temperature during the experiment, ℃; Patm is the experiment indoor atmospheric pressure, Pa༛hw is the height of the liquid column in the desorber, mm༛P0 is the saturated water vapor pressure at temperature tw, Pa。
According to the variation law of gas desorption accumulation Qt with time, the gas emission process of coal particles includes three stages (Fig. 3). The period 0 ~ t1 is the initial stage of gas emission. In this period, the gas desorption speed is high, and the cumulative desorption amount increases rapidly. The time t1 is generally within 10 minutes. The period t1 ~ t2 is the middle stage of gas emission, in which the gas desorption rate decays significantly, and the cumulative desorption amount continues to increase. The period from t2 to t∞ is the late stage of gas emission, in which the gas desorption rate slowly decays to approach 0, and the cumulative desorption amount slowly increases and gradually approaches Q∞. At the third stage, the complete process of this stage cannot be carried out in the laboratory. Tracking tests can only be approximated based on the theoretical derivation and numerical analysis. The first stage is the most concerned stage for predicting the risk of coal and gas outburst. The gas desorption index K1 of drilling cuttings is calculated based on the measurement data of this stage. Considering that the measurement time of K1 is 5 minutes, and the exposure time t0 of coal samples before measurement is generally not more than 2 minutes, the measurement time of this test is determined to be 7 minutes to reflect the real situation of measurement of the gas desorption index K1 of drilling cuttings in coal mines as much as possible.