Mine return airway methane emission continuous monitoring method

Abstract

A continuous monitoring method for methane emission of a mine return airway is characterized in that a methane concentration detection method is realized by combining DAS with a WMS technology, the WMS technology has good linearity when the concentration is low, when the absorbance is close to an absorbance threshold value A0, a nonlinear effect begins to appear, and the gas concentration cannot be accurately inverted, at the moment, the DAS technology shows good linearity, and the WMS technology shows high linearity when the gas concentration cannot be accurately inverted. And the detection range can be greatly improved. Therefore, the concentration detection of the methane gas in the whole range can be realized by combining the DAS with the WMS technology, and by setting a proper absorbance threshold value A0, when the absorbance of the detected gas to be detected is lower than the set threshold value, the WMS technology is adopted to ensure the measurement accuracy of the detection system under the low-concentration condition; when the absorbance of the detected to-be-detected gas is higher than a set threshold value, the DAS technology is adopted for detection, so that the nonlinear problem caused by adopting the WMS technology is avoided. The method has high sensitivity to the detected change of the volume concentration of the methane, and the change of the volume concentration of the methane can be detected in time.

Description

Technical Field

[0001] The present invention relates to a method for monitoring methane emissions, specifically a continuous monitoring method for methane emissions in the return air roadway of a mine, belonging to the technical field of methane emission monitoring in the return air roadway. Background Technique

[0002] Methane is the second largest greenhouse gas globally, and it is of great significance to comprehensively control methane emissions.

[0003] In coal mines, the methane concentration level in the air is usually several hundred parts per million. When the methane concentration exceeds 1.2%, the monitoring system usually issues an alarm. When it is higher than 5% (the explosion limit is 5% - 15%), the concentration may quickly reach several tens of percent in an emergency. Therefore, it is necessary to dynamically monitor in real time whether the methane concentration reaches the lower explosion limit.

[0004] Currently, in the coal mine environment, thermal conductivity and carrier catalytic methane sensors are often combined to monitor high and low methane concentrations. In addition, infrared methane sensors based on molecular absorption spectroscopy have also been widely used in coal mines. Using DAS (Dual-Action Spectroscopy) technology to detect high and low concentrations of methane gas, the change in light intensity will be affected by factors such as the light source and optical path when the methane concentration is low, and the inherent noise of related devices in the circuit will also cause interference, resulting in a large error in baseline fitting and making it difficult to ensure the measurement accuracy; although using WMS (Wavelength Modulation Spectroscopy) technology can effectively overcome 1 / f noise and improve the measurement accuracy, according to the principle of WMS technology, it is based on the premise that the gas absorbance is less than 0.05. To achieve full-range measurement, it is at the cost of shortening the optical path and reducing the measurement accuracy based on this technical principle, and the problem of non-linearity in harmonic detection that occurs during high-concentration measurement has not been effectively solved, and only the detection system can be frequently calibrated. However, the detection accuracy of the system at high concentrations is affected by the calibration step size; two absorption lines of methane are selected. The wavelengths of the two lines are close but the intensities are quite different. The stronger absorption line is used to detect low-concentration methane, and the weaker line with the laser output adjusted by a thermostat is used to detect high-concentration methane. This method requires a high stability in the temperature control of the laser. Usually, it is required that the temperature of the laser fluctuates within ±0.01°C of the set temperature. Therefore, the time constant of the temperature control circuit of the laser will increase significantly, the response time of the system will increase, and when detecting a certain methane concentration, the wavelength output by the laser will surely oscillate due to the laser vibrating between the two set temperature points, resulting in inaccurate detection results during the temperature adjustment period of the detection system.

[0005] In summary, for the existing detection systems based on laser absorption spectroscopy technology, there is a problem that the detection range and detection accuracy restrict each other. Only by balancing the detection accuracy and detection range can a full-range laser methane sensor that meets the requirements be designed. Summary of the Invention

[0006] The object of the present invention is to provide a method for continuous monitoring of methane emissions in the return air roadway of a mine. By adopting the DAS technology combined with the WMS technology to realize the methane concentration detection method, it has a very high sensitivity to the change of the measured methane volume concentration and can detect the change of the methane volume concentration in a timely manner.

[0007] To achieve the above object, the present invention provides a method for continuous monitoring of methane emissions in the return air roadway of a mine, including the following steps:

[0008] ① Collect signals through the detection system corresponding to this method, and set the judgment and selection of the absorbance threshold A0 according to the actually measured gas and application requirements collected.

[0009] ② In the first half cycle T1 of the current drive signal within one cycle, only a low-frequency sawtooth signal is included. The microprocessor can use the DAS technology to calculate the absorbance A of the gas to be measured in the gas absorption cell, and compare the absorbance A with the set absorbance threshold A0.

[0010] ③ If the absorbance A is greater than the absorbance threshold A0, then calculate the concentration C1 of the gas to be measured in the absorption cell according to formula (1). At this time, take the concentration C1 of the gas to be measured as the original value Cm of the measurement result of the concentration of the gas to be measured in the absorption cell; if the absorbance A is not greater than the absorbance threshold A0, then process the second half cycle T2 of the current drive signal within one cycle according to the demodulation method of the WMS technology, demodulate to obtain the second harmonic signal, and calculate the concentration C2 of the gas to be measured in the absorption cell according to formula (2). At this time, take the concentration C2 of the gas to be measured as the original value Cm of the measurement result of the concentration of the gas to be measured in the absorption cell.

[0011] ④ Combine the temperature value T measured by the temperature sensor integrated in the absorption cell, and use the temperature compensation function for concentration to perform temperature correction on the original value Cm of the measurement result of the concentration of the gas to be measured.

[0012] The selection of the absorbance threshold A0 in step ① of the present invention is as follows:

[0013] To select an appropriate absorbance threshold A0, the DAS and WMS methods were used to demodulate the gas volume concentration respectively. The detection system of this method was tested with gases of different standard volume concentrations at room temperature (25 °C) and normal pressure (1 standard atmosphere). After each experiment was ventilated until the measurement results were stable, 10 measurement values were averaged. The results of demodulating the gas concentration by the DAS and WMS methods showed that the absorbance threshold A0 of methane gas was set between 0.0236 and 0.035 cm-1.

[0014] 3. A continuous monitoring method for methane emissions in a mine return airway according to claim 2, characterized in that the formula (1) in step ③ is:

[0015]

[0016] In the formula: L is the optical path;

[0017] P is the pressure exerted by the gas;

[0018] A is the absorbance;

[0019] The formula (2) is:

[0020]

[0021] In the formula: α is the fitting coefficient of the second harmonic signals of the measured gas and the standard concentration gas;

[0022] C Ref is the standard concentration value of the known reference gas;

[0023] I 01 、L 01 are respectively the light intensity before absorption of the reference signal and the measurement optical path;

[0024] I 02 、L 02 are respectively the light intensity after absorption of the measured object and the measurement optical path of the measured gas.

[0025] The formula of the temperature compensation function F(T) for the concentration in step ④ of the present invention is:

[0026]

[0027] In the formula: Cm is the original value of the measurement result of the concentration of the gas to be measured;

[0028] ΔT = T - T0, where T is the actual temperature and T0 is the assumed temperature value;

[0029] a, b, c are the polynomial coefficients fitted according to the concentration values detected by the standard concentration gas at different temperatures.

[0030] The detection system in step ① further includes a microprocessor, a laser, a current control circuit, a temperature control circuit, an absorption cell, and a preamplifier. The beam emitted by the laser reaches the beam splitter and then enters the absorption cell through the air inlet. After multiple reflections in the absorption cell, the beam is converted into a current signal by a photodetector. The current signal passes through the preamplifier and is then converted into a digital signal by an A / D converter and input into the microprocessor. In the microprocessor, the digital signal is demodulated and analyzed and compared through DAS and WMS technologies to calculate the gas concentration, and the calculated gas concentration is temperature-compensated according to the temperature collected by the temperature sensor in the absorption cell. Finally, the gas concentration obtained after temperature compensation is output as the concentration of the gas to be detected by the system.

[0031] Compared with the prior art, the present invention realizes a methane concentration detection method by adopting DAS technology combined with WMS technology in the detection system. When the WMS technology is at a low concentration, that is, when the absorbance is low, it has good linearity. When the absorbance reaches near the absorbance threshold A0, a non-linear effect begins to appear, and the accurate inversion of the gas concentration is no longer possible. At this time, the DAS technology shows good linearity and can greatly improve the detection range. Therefore, by combining DAS technology with WMS technology, the concentration detection of methane gas can be realized in the full range, that is, by setting a suitable absorbance threshold A0. When the absorbance of the gas to be detected is lower than the set threshold, the WMS technology is used to ensure the measurement accuracy of the detection system in the low-concentration case; when the absorbance of the gas to be detected is higher than the set threshold, the DAS technology is used for detection to avoid the non-linear problem that appears when using the WMS technology. The present invention has a high sensitivity to the change in the detected methane volume concentration and can detect the change in the methane volume concentration in a timely manner. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 is the gas detection flow chart of the present invention;

[0033] Figure 2 is the schematic diagram of the detection system of the present invention;

[0034] Figure 3 is the comparison chart of the demodulation gas concentration results and relative errors of the DAS and WMS methods of the present invention;

[0035] Figure 4 is the curve of the CH4 absorption intensity changing with temperature of the present invention;

[0036] Figure 5 is the linear graph of the absorbance detection of the DAS and WMS detection technologies of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0037] The present invention will be further described below with reference to the accompanying drawings.

[0038] AsFigure 1 As shown in Figure 1 , a continuous monitoring method for methane emission in the return airway of a mine includes the following steps:

[0039] ① Collect signals through the detection system corresponding to this method, and set the judgment and selection absorbance threshold A0 according to the actually measured gas and application requirements. The selection of the absorbance threshold A0 in the present invention is specifically as follows:

[0040] In order to select a suitable absorbance threshold A0, the DAS and WMS methods are respectively used to demodulate the gas volume concentration. The detection system of this method is tested with gases of different standard volume concentrations at room temperature (25 °C) and normal pressure (1 standard atmosphere). After each experiment is ventilated until the measurement result is stable, 10 measurement values are taken for averaging. The results of demodulating the gas concentration by the DAS and WMS methods and the relative errors are as Figure 3 shown. Since the span of the detected gas concentration range is relatively large, in order to facilitate the observation of the detection results at low concentrations, the scale range of the coordinate axis in the figure is shown in logarithmic form;

[0041] From Figure 3It can be seen that when the standard volume concentration of methane is between 1.2% and 5.0%, the methane volume concentration values measured by the DAS and WMS methods both show good linearity, and the relative error values of the measurement results of the two methods are relatively small. However, when it is lower than 1.2%, the inversion of gas concentration by the DAS method begins to show non-linearity, while the results obtained by the WMS method show good linearity, and the relative error of the measurement results is generally lower than that of the DAS method and closer to the actual value. When detecting CH4 with a volume concentration of 0.025%, the average value of the results calculated by the DAS method is 0.007%, but in fact, the measurement results fluctuate greatly (-0.0052% to 0.02%), and the methane concentration data can no longer be measured. At this time, the measurement result of the WMS method is 0.03%, and although the relative error is as high as 20%, the absolute error is only 0.005%. Similarly, when the standard volume concentration of methane is above 5.0%, the measurement results of the WMS method begin to show non-linearity, and after reaching a certain value (35%), the measurement results do not increase but decrease, and the relative error increases rapidly with the increase of gas concentration. However, the measurement results of the DAS method show good linearity, and the relative error is generally low. The highest relative error occurs when detecting methane with a concentration of 25.0%, which is only 6.8% of the true value. The measurement results show that when the concentration of the methane gas to be measured is lower than 1.2%, the WMS method can improve the detection limit and accuracy of the detection system for methane gas; when it is between 1.2% and 5.0%, both methods can accurately measure the methane gas volume concentration; when it is higher than 5.0%, the DAS method can effectively improve the detection range and accuracy of the detection system for methane gas. Therefore, the absorbance corresponding to the detection of methane gas with a volume concentration of 5.0% by the detection system can be set as the absorbance threshold, and the absorbance at this time is 0.0236 cm-1. Because the WMS and DAS methods on which the full-range implementation method is based have a partial spectral overlap region, this ensures the flexibility in selecting the threshold and will not have a great impact on the measurement results. Figure 3 It can be seen that when detecting the standard methane gas with a volume concentration of 7.5%, the relative error (7.5%) of the measurement result by the WMS method is only slightly larger than the relative error (6.33%) of the measurement result by the DAS method, and the corresponding methane absorbance at this time is 0.0354 cm-1. Therefore, it is feasible to set the absorbance threshold of CH4 gas between 0.0236 cm-1 and 0.035 cm-1.

[0042] ② In the first half cycle T1 of the current drive signal within one cycle, only the low-frequency sawtooth signal is included. The microprocessor can use the DAS technology to calculate the absorbance A of the gas to be measured in the gas absorption cell, and compare the absorbance A with the set absorbance threshold A0.

[0043] ③ If the absorbance A is greater than the absorbance threshold A0, the concentration C1 of the gas to be measured in the absorption cell is calculated according to Equation (1). At this time, the concentration C1 of the gas to be measured is used as the original value Cm of the measurement result of the concentration of the gas to be measured in the absorption cell. If the absorbance A is not greater than the absorbance threshold A0, the second half cycle T2 of the current drive signal within one period is processed according to the demodulation method of the WMS technology, the second harmonic signal is demodulated, and the concentration C2 of the gas to be measured in the absorption cell is calculated according to Equation (2). At this time, the concentration C2 of the gas to be measured is used as the original value Cm of the measurement result of the concentration of the gas to be measured in the absorption cell.

[0044] In step ③, Equation (1) is:

[0045]

[0046] In the formula: L is the optical path;

[0047] P is the pressure exerted on the gas;

[0048] A is the absorbance (unit: cm -1 ), which represents the integral of the gas absorption signal in the frequency domain and can be obtained from the detected absorption spectrum

[0049] Equation (2) is:

[0050]

[0051] In the formula: α is the fitting coefficient of the second harmonic signals of the gas to be measured and the gas with a standard concentration, and can be obtained by fitting using the nonlinear LS method;

[0052] C Ref is the standard concentration value of the known reference gas;

[0053] I 01 , L 01 are respectively the light intensity before absorption of the reference signal and the measurement optical path;

[0054] I 02 , L 02 are respectively the light intensity after absorption of the object to be measured and the measurement optical path of the gas to be measured;

[0055] Usually, since the same absorption cell is used in the detection system, L 01 , L 02 are equal to each other, and I 01 , I 02 can be obtained through the triangular wave signal extraction circuit.

[0056] ④ Combine the temperature value T measured by the integrated temperature sensor in the absorption cell, and use the temperature-compensation function for concentration to perform temperature correction on the original value Cm of the measurement result of the concentration of the gas to be measured;

[0057] The measurement results of the detection system for gas concentration at different temperatures have a certain impact. The intensity of the gas absorption spectral line is also affected by temperature. Figure 4 As shown in the relationship between the absorption intensity of methane molecules at 1.653 μm and temperature, it can be seen that the absorption spectral intensity of methane molecules with a wavelength of 1.653 μm decreases with the increase of temperature. Compared with room temperature (25 °C), the spectral line intensity will change greatly at supercooled (-40 °C) or superheated (60 °C). In order to overcome the influence of environmental temperature changes on the system detection results, usually, a linear fitting of the absorption spectral line intensity and temperature can be performed, and the fitting degree reaches 0.99178. The measurement results of the detection system can be compensated in real time according to the obtained compensation function.

[0058] In step ④ of the present invention, the formula of the temperature-compensation function F(T) for concentration is:

[0059]

[0060] In the formula: Cm is the original value of the measurement result of the concentration of the gas to be measured;

[0061] ΔT = T - T0, where T is the actual temperature and T0 is the assumed temperature value;

[0062] a, b, c are the polynomial coefficients fitted according to the concentration values detected for the standard-concentration gas at different temperatures.

[0063] The detection system in step ① further includes a microprocessor, a laser, a current control circuit, a temperature control circuit, an absorption cell, and a preamplifier. The model of the microprocessor in the present invention is STM32. As for the current control circuit, the temperature control circuit, and the preamplifier, etc., they are all existing circuits and do not need to be described in detail here; the beam emitted by the laser is split by the beam splitter and then enters the absorption cell through the air inlet. After the beam is reflected multiple times in the absorption cell, it is converted into a current signal by the photodetector. The current signal passes through the preamplifier and is then converted into a digital signal by the A / D converter and input to the microprocessor. In the microprocessor, the digital signal is demodulated and analyzed by DAS and WMS technologies respectively to calculate the gas concentration, and the calculated gas concentration is temperature-compensated according to the temperature collected by the temperature sensor in the absorption cell. Finally, the gas concentration obtained after temperature compensation is output as the concentration of the gas to be measured detected by the system.

[0064] Figure 5The following shows the linear graphs of absorbance detection by the two detection technologies of DAS and WMS in the present invention. For the WMS technology, at low concentrations, that is, when the absorbance is low, it has good linearity. When the absorbance reaches near the absorbance threshold A0, non-linear effects begin to appear and it is no longer possible to accurately invert the gas concentration. At this time, the DAS technology shows better linearity and can greatly improve the detection range. Therefore, by combining the DAS technology with the WMS technology, the concentration detection of methane gas can be achieved over the entire range. That is, by setting a suitable absorbance threshold A0, when the absorbance of the gas to be detected is lower than the set threshold, the WMS technology is used to ensure the measurement accuracy of the detection system at low concentrations; when the absorbance of the gas to be detected is higher than the set threshold, the DAS technology is used for detection to avoid the non-linear problems that occur when using the WMS technology.

Claims

1. A method for continuous monitoring of methane emissions from a mine return airway, characterized in that: The following steps are involved: ① The signal is collected by the detection system corresponding to the method, and the absorbance threshold A0 is set and selected according to the actual measured gas collected and the application requirements; ② The first half cycle T1 of the current driving signal in one cycle only contains low-frequency sawtooth signals. The microprocessor uses DAS technology to calculate the absorbance A of the gas to be tested in the gas absorption cell and compares the absorbance A with the set absorbance threshold A0; ③ If the absorbance A is greater than the absorbance threshold A0, the concentration C1 of the gas to be measured in the absorption cell is calculated according to formula (1). At this time, the concentration C1 of the gas to be measured is used as the original value Cm of the measurement result of the concentration of the gas to be measured in the absorption cell; if the absorbance A is not greater than the absorbance threshold A0, the second half cycle T2 of the current driving signal within one cycle is processed according to the demodulation method of the WMS technology, and the second harmonic signal is demodulated to obtain the second harmonic signal, and the concentration C2 of the gas to be measured in the absorption cell is calculated according to formula (2). At this time, the concentration C2 of the gas to be measured is used as the original value Cm of the measurement result of the concentration of the gas to be measured in the absorption cell; ④ Combined with the temperature value T measured by the temperature sensor integrated in the absorption cell, a temperature-to-concentration compensation function is used to perform temperature correction on the original value Cm of the measurement result of the measured gas concentration.

2. A method for continuous monitoring of methane emissions from a mine return airway according to claim 1, characterized in that: The selection of absorbance threshold A0 in step ① is as follows: The gas volume concentration was demodulated using the DAS and WMS methods respectively. The detection system of this method was tested using gases of different standard volume concentrations at room temperature and normal pressure. After each experiment was ventilated until the measurement results were stable, 10 measurements were averaged. The results of gas concentration demodulation using the DAS and WMS methods showed that the absorbance threshold A0 of methane gas was set between 0.0236 and 0.035 cm-1.

3. A method for continuous monitoring of methane emissions from a mine return airway according to claim 2, characterized in that: Formula (1) in step ③ is: Where: L is the optical path; P is the pressure of the gas; A is absorbance; Formula (2) is: Where: α is the fitting coefficient of the second harmonic signal of the measured gas and the standard concentration gas; C Ref is the standard concentration value of the known reference gas; I 01 , L 01 are the light intensity of the reference signal before being absorbed and the measurement optical path length respectively; I 02 , L 02 They are respectively the intensity of the light after being absorbed and the measuring optical path of the gas being measured.

4. A method for continuous monitoring of methane emissions from a mine return airway according to claim 3, characterized in that: The formula of the temperature to concentration compensation function F(T) in step ④ is: F(T)=Cm[a(ΔT) 2 +bΔT+c] Where: Cm is the original value of the measured gas concentration; ΔT=T-T0, T is the actual temperature, T0 is the assumed temperature value; a, b, c are the polynomial coefficients fitted based on the concentration values ​​detected by standard concentration gas at different temperatures.

5. A method for continuous monitoring of methane emissions from a mine return airway according to claim 3, characterized in that: The detection system of step ① also includes a microprocessor, a laser, a current control circuit, a temperature control circuit, an absorption cell, and a preamplifier. The light beam emitted by the laser reaches the beam splitter and then passes through the air inlet to the absorption cell. The light beam is converted into a current signal by a photodetector after multiple reflections in the absorption cell. The current signal passes through the preamplifier and is converted into a digital signal by an A / D converter and then input into the microprocessor. In the microprocessor, the digital signal is demodulated and analyzed and compared by DAS and WMS technology to calculate the gas concentration, and the calculated gas concentration is temperature compensated according to the temperature collected by the temperature sensor in the absorption cell. Finally, the gas concentration obtained after temperature compensation is used as the concentration output of the system to detect the gas to be tested.

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