Method and system for detecting concentration of trace methane by locking position of absorption peak

Abstract

The invention relates to a method and a system for detecting the concentration of trace methane by locking the position of an absorption peak, and belongs to the technical field of laser gas detection. Modulated laser output by a laser is divided into two paths, one path enters a reference light path, the other path firstly passes through a gas absorption cell and then enters an actual detection light path, and methane gas with known concentration is sealed in the reference light path; demodulating a modulation signal output by the actual detection light path, and calculating to obtain a trace methane gas concentration value; and demodulating the modulation signal output by the reference light path to obtain a second harmonic signal, comparing the amplitude of the second harmonic signal with the amplitude of a harmonic signal corresponding to a preset methane absorption peak to generate a wavelength shift error signal, and adjusting the driving parameter of the laser through a feedback mechanism to generate a wavelength shift error signal. The output power and wavelength drift of the laser are monitored in real time, and it is ensured that the laser wavelength is always locked within the methane absorption peak range. According to the invention, high-precision and high-stability trace methane concentration detection can be realized.

Description

Technical Field

[0001] The invention belongs to the technical field of laser gas detection, and relates to a method and a system for detecting trace methane concentration by locking an absorption peak position. Background Art

[0002] Methane is one of the gases that cause the greenhouse effect. Its global warming potential (GWP) is 28 to 36 times that of carbon dioxide on a 100-year time scale, and its impact on climate change is particularly prominent. According to data from the United Nations Framework Convention on Climate Change, coal mine methane emissions account for about 10% to 15% of global methane emissions, and mine wind exhaust gas accounts for more than 70% of coal mine methane emissions. Accurate monitoring of trace methane concentrations (ppm to ppb level) is not only a core requirement for coal mine safety production, but also a key technical support for achieving methane emission reduction targets. However, existing detection technologies still face major challenges in terms of high sensitivity, long-term stability and adaptability to complex environments.

[0003] At present, methane detection technology is mainly divided into two categories: chemical sensor method and spectroscopy method. Chemical sensor method includes catalytic combustion sensor and electrochemical sensor. Catalytic combustion sensor detects concentration through the heat change generated by methane combustion, but its sensitivity is low (usually >100ppm), it is easily interfered by gases such as hydrogen sulfide, and the catalyst is easily poisoned and fails. Although electrochemical sensors have a lower detection limit (about 10ppm), they have a long response time (>30 seconds) and a short life (1 to 2 years), which makes it difficult to meet the real-time monitoring needs of coal mines. Spectroscopic methods are represented by tunable diode laser absorption spectroscopy (TDLAS) and Fourier transform infrared spectroscopy (FTIR). TDLAS technology uses the absorption characteristics of methane molecules at specific wavelengths (such as 1653nm), combined with wavelength modulation technology (WMS) and second harmonic detection, to achieve ppb-level detection sensitivity, becoming the mainstream solution. Although FTIR can detect multiple gases at the same time, the equipment is large and the cost is high.

[0004] Among them, TDLAS technology has received much attention due to its low cost and high detection sensitivity, and has achieved great technological development, but it still has the following key defects in practical applications:

[0005] Laser wavelength drift problem: The center wavelength of the laser is affected by temperature fluctuations (±1°C can cause a drift of 0.08nm) and device aging, which causes the absorption peak position to shift. Experiments show that a wavelength shift of 0.02nm can increase the detection error by 30%, seriously restricting the reliability of long-term monitoring.

[0006] Insufficient suppression of environmental interference: The underground environment of coal mines is complex. Changes in temperature (-20℃ to 50℃) and pressure (80~110kPa) will change the gas absorption cross-section. The existing system lacks a dynamic compensation mechanism, resulting in distorted measurement results.

[0007] Long-term stability defects: Existing solutions rely on fixed reference optical paths or offline calibration, and cannot correct laser wavelength drift in real time. Studies have shown that after 6 months of continuous operation, the detection error can accumulate to ±5%, which is difficult to meet the ±2% accuracy requirement in the Coal Mine Safety Regulations.

[0008] In summary, there is an urgent need to improve the detection technology for the amount of exhaust gas, so as to meet the needs of high-sensitivity and long-term stability monitoring of exhaust gas in coal mines. Summary of the invention

[0009] In view of this, the purpose of the present invention is to provide a method and system for detecting trace methane concentration by locking the absorption peak position, so as to solve the problem of reduced methane concentration detection sensitivity and low detection stability caused by laser central wavelength drift when using TDLAS technology to detect methane concentration.

[0010] To achieve the above object, one aspect of the present invention provides a method for detecting trace methane concentration by locking the absorption peak position, which comprises:

[0011] The modulated laser output by the laser is divided into two paths through an optical coupler; one path of light enters the reference light path, and the other path of light first passes through a gas absorption cell and then enters the actual detection light path; wherein the reference light path is sealed with methane gas of known concentration;

[0012] Demodulating the modulation signal output by the actual detection optical path to obtain a second second harmonic signal corresponding to the methane laser absorption spectrum, and then integrating the second second harmonic signal to obtain a trace methane gas concentration value;

[0013] The modulation signal output by the reference optical path is demodulated to obtain a first second harmonic signal, and the amplitude of the first second harmonic signal is compared with the amplitude of the second harmonic signal corresponding to the preset methane absorption peak to generate a wavelength shift error signal, and the driving parameters of the laser are adjusted through a feedback mechanism, and the output power and wavelength drift of the laser are monitored in real time to ensure that the laser wavelength is always locked within the methane absorption peak range.

[0014] Furthermore, the method for modulating the wavelength of the laser output light is specifically as follows: collecting and processing the absorption peak characteristic information of the modulation signal output by the reference optical path, using the correlation method to detect the methane absorption peak in real time, quantitatively characterizing the drift of the methane absorption peak position by the correlation coefficient, and converting the drift into a current signal, thereby calibrating the central wavelength of the laser output laser.

[0015] Furthermore, the reference optical path and the actual detection optical path both include a photodetector and a phase-locked amplifier, the photodetector receives a transmitted light signal absorbed by methane gas, and the phase-locked amplifier filters an electrical signal output by the photodetector.

[0016] The photodetector of the reference optical path is sealed with methane gas of known concentration.

[0017] Another aspect of the present invention provides a system for detecting trace methane concentration by locking the absorption peak position, which includes a laser, an optical coupler, a first light collimator, a second light collimator, a gas absorption cell, a first light path, a second light path, a single chip microcomputer, a signal generator and a temperature control drive module.

[0018] The laser is used to provide the modulated laser required for methane concentration detection; the optical coupler is used to divide the laser output by the laser into two beams, one beam enters the first optical path after being collimated by the first optical collimator, and the other beam enters the gas absorption cell after being collimated by the second optical collimator, and then enters the second optical path after being absorbed by the methane gas in the gas absorption cell; wherein the first optical path is sealed with methane gas of known concentration. The single-chip microcomputer receives the modulation signal output by the first optical path and the second optical path and demodulates it to obtain the second harmonic signal; wherein the second harmonic signal corresponding to the first optical path is compared with the amplitude of the second harmonic signal corresponding to the preset methane absorption peak to generate a wavelength offset error signal, and the temperature control driving module adjusts the central wavelength of the laser output according to the offset error signal, so that the laser wavelength is always locked within the range of the methane absorption peak; the second harmonic signal corresponding to the second optical path is integrated to obtain the concentration of trace methane gas in the gas absorption cell. The signal generator generates a modulation signal and outputs it to the temperature control driving module, thereby modulating the laser output by the laser.

[0019] Furthermore, the first optical path and the second optical path both include a photodetector and a phase-locked amplifier. The photodetector receives the transmitted light signal absorbed by the methane gas, and the phase-locked amplifier filters the electrical signal output by the photodetector.

[0020] The beneficial effects of the present invention are:

[0021] (1) The present invention monitors the position of the methane absorption peak in real time through a reference optical path, combines the correlation coefficient method and the temperature control drive module, and dynamically adjusts the laser output wavelength to ensure detection accuracy and stability. Compared with traditional methods, the present invention can effectively suppress wavelength drift and ensure the high reliability of detection results, which is particularly suitable for long-term monitoring scenarios.

[0022] (2) The present invention has a built-in self-calibration algorithm that can correct laser wavelength drift in real time without the need for frequent calibration with standard gas. This design greatly reduces the maintenance frequency of the system, extends the operating cycle of the equipment, and reduces maintenance costs, making it particularly suitable for industrial scenarios that require long-term stable operation.

[0023] (3) Through the integrated optical path design and domestic temperature control module, the present invention significantly reduces the system cost while ensuring high performance. In addition, the system is small in size and low in power consumption, suitable for portable or embedded applications, and can meet the detection needs in a variety of scenarios.

[0024] (4) The design of the present invention is not only applicable to methane concentration detection, but can also be extended to the detection of other gases. By changing the laser wavelength and adjusting the absorption peak locking parameters, the system can adapt to the needs of various gas detection and has a wide range of application prospects.

[0025] The present invention not only solves the core problems of traditional TDLAS technology, but also provides reliable technical support for the fields of coal mine methane emission reduction, smart mine construction, etc., with significant economic and social benefits.

[0026] Other advantages, objectives and features of the present invention will be described in the following description to some extent, and to some extent, will be obvious to those skilled in the art based on the following examination and study, or can be taught from the practice of the present invention. The objectives and other advantages of the present invention can be realized and obtained through the following description. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] In order to make the purpose, technical solutions and advantages of the present invention more clear, the present invention will be described in detail below in conjunction with the accompanying drawings, wherein:

[0028] Figure 1 A system for detecting trace methane concentration provided by an embodiment of the present invention;

[0029] Figure 2 This is a schematic diagram of the process of detecting trace methane concentration;

[0030] Figure 3 is the laser methane absorption signal after absorption;

[0031] Figure 4 is the second harmonic signal.

[0032] Figure numerals: 1-laser; 2-optical coupler; 3-optical collimator I; 4-optical collimator II; 5-gas absorption cell; 6-photodetector I; 7-photodetector II; 8-phase-locked amplifier I; 9-phase-locked amplifier II; 10-single chip microcomputer; 11-temperature control drive module; 12-signal generator. DETAILED DESCRIPTION

[0033] The following describes the embodiments of the present invention by specific examples, and those skilled in the art can easily understand other advantages and effects of the present invention from the contents disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and the details in this specification can also be modified or changed in various ways based on different viewpoints and applications without departing from the spirit of the present invention. It should be noted that the illustrations provided in the following embodiments only illustrate the basic concept of the present invention in a schematic manner, and the following embodiments and features in the embodiments can be combined with each other without conflict.

[0034] Among them, the drawings are only used for illustrative explanations, and they only represent schematic diagrams rather than actual pictures, and should not be understood as limitations on the present invention. In order to better illustrate the embodiments of the present invention, some parts of the drawings may be omitted, enlarged or reduced, and do not represent the size of actual products. For those skilled in the art, it is understandable that some well-known structures and their descriptions in the drawings may be omitted.

[0035] The same or similar numbers in the drawings of the embodiments of the present invention correspond to the same or similar parts; in the description of the present invention, it should be understood that if the terms "upper", "lower", "left", "right", "front", "rear", etc. indicate the orientation or position relationship, they are based on the orientation or position relationship shown in the drawings, which is only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operate in a specific orientation. Therefore, the terms describing the position relationship in the drawings are only used for illustrative purposes and cannot be understood as limiting the present invention. For ordinary technicians in this field, the specific meanings of the above terms can be understood according to specific circumstances.

[0036] The purpose of the present invention is to provide a device and method for detecting trace methane concentration, so as to solve the problems existing in the trace methane detection technology.

[0037] like Figure 1 As shown, a system for detecting trace methane concentration provided by an embodiment of the present invention includes a laser 1, a temperature control drive module 11, an optical coupler 2, a collimator, a spectral absorption cell 5, a photodetector, a signal acquisition and processor, and the signal acquisition and processor can adopt a single-chip microcomputer to realize its corresponding functions.

[0038] Among them, for laser 1, the continuous light semiconductor laser is controlled to continuously output the laser signal in the methane absorption peak band, and the wavelength modulation technology (WMS) is used to modulate the output laser signal. The added modulation signals are triangular wave signal and sine wave signal. The modulated optical signal is divided into two by optical coupler 2, and the splitting ratio of optical coupler 2 is 50:50.

[0039] The two beams of light separated by the optical coupler 2 are collimated and then enter the spectral absorption cell 5 and the reference optical path. The reference optical path is composed of an optical collimator Ⅰ3, a photodetector Ⅰ6 and a phase-locked amplifier Ⅰ8. The reference photodetector Ⅰ6 is filled with methane gas of known concentration.

[0040] Photodetector Ⅰ6 and photodetector Ⅱ7 are both used to collect laser methane signals with methane concentration information after absorption. Then, the laser methane signal collected by photodetector Ⅰ6 is filtered by phase-locked amplifier Ⅰ8, and the laser methane signal collected by photodetector Ⅱ7 is filtered by phase-locked amplifier Ⅱ9, thereby filtering out the low-frequency scanning signal and retaining the high-frequency modulation signal. The two-way modulation signal is input into the single-chip microcomputer 10, and the second harmonic signals corresponding to the two absorption lines are demodulated respectively. Then, the second harmonic signal obtained at the measuring end is integrated to demodulate and obtain the methane concentration value.

[0041] It should be noted that in the present invention, the signal output by the reference optical path is used as the reference for locking the laser wavelength. After the photodetector Ⅰ6 in the reference optical path receives the transmitted light signal absorbed by methane, it is synchronously demodulated using the phase-locked amplifier Ⅰ8 to extract the second harmonic signal as the reference for wavelength locking. Then, the amplitude of the second harmonic signal is compared with the preset harmonic amplitude corresponding to the methane absorption peak to generate a wavelength shift error signal. The laser drive parameters are adjusted through a feedback mechanism to monitor the output power and wavelength drift of the laser in real time to ensure that the laser wavelength is always locked within the methane absorption peak range. Specifically, the methane absorption peak signal is continuously scanned in real time, and key information such as the position, intensity and spectral line shape of the strongest absorption peak is analyzed and recorded. Finally, the absorption peak position is detected in real time using the correlation method, and the correlation coefficient C is used. C The drift of the absorption peak position is quantitatively characterized, and the position information corresponding to the correlation coefficient is converted into a temperature control current signal, so as to calibrate the absorption peak position of the laser. The temperature control current signal is input into the temperature control drive module 11, and the control electrical signal of the laser 1 is output under the action of the modulation signal of the signal generator 12.

[0042] The expression of the correlation coefficient is:

[0043]

[0044] Where P A , P B Represent the optical power of the recorded absorption peak signal and the existing absorption peak signal, τ 0Indicates the relative delay of the two chaotic signals, and <·> indicates taking the average value. The value range of the correlation coefficient is 0 to 1. The closer the correlation coefficient is to 1, the closer the existing laser wavelength is to the wavelength corresponding to the strongest absorption peak; the closer the correlation coefficient is to 0, the farther the existing laser wavelength is from the wavelength corresponding to the strongest absorption peak.

[0045] Another embodiment of the present invention provides a method for detecting trace methane concentration, such as Figure 2 As shown, the specific implementation steps of this method are as follows:

[0046] 1) Build a device for determining the position of the trace methane laser absorption peak and measuring the trace methane concentration. The device mainly includes a laser source, a reference optical path, an actual detection optical path, optical-mechanical components, and signal acquisition and processors. At the same time, the stable operation of the device needs to be ensured.

[0047] 2) Set up the laser source, select a laser light source with a central wavelength of 1653 nm and a methane absorption line, and control the laser center wavelength through a high-precision temperature control drive module.

[0048] 3) Designing optical components in the actual detection optical path to focus the laser beam through the methane gas sample to be tested, and sealing a certain known concentration of methane gas in the photodetector of the reference optical path. The optical components include a light collimator, which is used to converge the emitted laser signal.

[0049] Likewise, an optical component is designed in the reference optical path to focus the laser beam, and the optical component includes a light collimator.

[0050] 4) A measurement spectrum absorption cell is designed before the actual detection optical path to absorb the laser signal with a wavelength of 1653nm. The gas flow is kept stable during the gas sampling process, and the path for the interaction between the gas sample and the laser beam is long enough to ensure that the detection accuracy and resolution can reach the ppm level.

[0051] 5) Select a detector with a central wavelength near 1653nm to measure the intensity of the laser signal after being absorbed by methane gas. Silicon photodiodes are a commonly used type of detector. Using silicon photodiodes can ensure that the sensitivity and response range of the detector meet the measurement requirements. The electrical signals output by the two photodetectors are filtered by phase-locked amplifiers to filter out the low-frequency scanning signal and retain the high-frequency modulation signal. Among them, the laser signal after being absorbed by methane gas is as follows Figure 3 shown.

[0052] 6) Data collection and processing: Use a suitable data collection device to record the signals output by the two phase-locked amplifiers. Use an analog-to-digital conversion module (ADC) to convert the analog signals into digital signals. Use a single-chip microcomputer to demodulate the modulation signals output by the two phase-locked amplifiers to obtain the second harmonic signals corresponding to the two absorption lines. Then integrate the second harmonic signals obtained at the actual detection optical path end to demodulate and obtain the methane concentration value. Among them, the second harmonic signal is as follows: Figure 4 shown.

[0053] 7) The second harmonic signal extracted from the reference optical path is used as the reference for wavelength locking. The amplitude of the second harmonic signal is then compared with the preset harmonic amplitude corresponding to the methane absorption peak to generate a wavelength shift error signal. The laser drive parameters are adjusted through a feedback mechanism, and the output power and wavelength drift of the laser are monitored in real time to ensure that the laser wavelength is always locked within the methane absorption peak range.

[0054] Specifically, a single-chip microcomputer is used to process the collected absorption peak characteristic information, including the position, intensity and spectral line shape of the strongest absorption peak for analysis and recording. The methane absorption peak is detected in real time using the correlation method, and the correlation coefficient C is used to calculate the methane absorption peak. C The drift of the absorption peak position is quantitatively characterized, and the drift information corresponding to the correlation coefficient is converted into a current signal to calibrate the central wavelength of the laser output.

[0055] 8) The current signal is input into the temperature control driving module, and the temperature control driving module outputs a control signal of the laser, so that the laser wavelength output by the laser can correspond to the absorption peak of methane gas.

[0056] The trace methane concentration detection method and system proposed in the present invention can achieve high-precision and high-stability measurement of methane concentration. Specifically, by detecting the methane absorption peak position in real time to keep the center of the absorption peak stable, high-precision methane detection can be achieved with the help of a harmonic demodulation algorithm and an accurate calibration curve; in addition, with the help of a stable high-precision temperature control drive module, the drift of the methane absorption peak position is detected in real time, and the center wavelength of the laser output laser is corrected according to the drift, so that the absorption peak position remains stable, thereby making the trace methane concentration detection have high stability and repeatability.

[0057] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solution of the present invention can be modified or replaced by equivalents without departing from the purpose and scope of the technical solution, which should be included in the scope of the claims of the present invention.

Claims

1. A method for detecting trace methane concentration by locking the absorption peak position, characterized in that: The method includes: The modulated laser output by the laser is divided into two paths through an optical coupler; one path of light enters the reference light path, and the other path of light first passes through a gas absorption cell and then enters the actual detection light path; wherein the reference light path is sealed with methane gas of known concentration; Demodulating the modulation signal output by the actual detection optical path to obtain a second second harmonic signal corresponding to the methane laser absorption spectrum, and then integrating the second second harmonic signal to obtain a trace methane concentration value; The modulation signal output by the reference optical path is demodulated to obtain a first second harmonic signal, and the amplitude of the first second harmonic signal is compared with the amplitude of the second harmonic signal corresponding to the preset methane absorption peak to generate a wavelength shift error signal, and the driving parameters of the laser are adjusted through a feedback mechanism, and the output power and wavelength drift of the laser are monitored in real time to ensure that the laser wavelength is always locked within the methane absorption peak range.

2. The method according to claim 1, characterized in that The absorption peak characteristic information of the modulated signal output by the reference optical path is collected and processed, the methane absorption peak is detected in real time by using a correlation method, the drift of the methane absorption peak position is quantitatively characterized by a correlation coefficient, and the drift is converted into a current signal, thereby calibrating the central wavelength of the laser output by the laser.

3. The method according to claim 1, characterized in that The reference optical path and the actual detection optical path both include a photodetector and a phase-locked amplifier. The photodetector receives a transmitted light signal absorbed by methane gas, and the phase-locked amplifier filters an electrical signal output by the photodetector.

4. The method according to claim 3, characterized in that: Methane gas with a known concentration is sealed in the photoelectric detector of the reference light path.

5. A system for detecting trace methane concentration by locking the absorption peak position, characterized in that: It includes a laser, an optical coupler, a gas absorption cell, a first optical path, a second optical path, a single chip microcomputer and a temperature control drive module; The laser is used to provide modulated laser required for methane concentration detection; the optical coupler is used to divide the laser output by the laser into two beams of light, one beam enters the first optical path, and the other beam enters the gas absorption cell, and then enters the second optical path after being absorbed by the methane gas in the gas absorption cell; the first optical path is sealed with methane gas of known concentration; the single-chip microcomputer receives the modulated signals output by the first optical path and the second optical path and demodulates them to obtain the second harmonic signal; wherein the second harmonic signal corresponding to the first optical path is compared with the amplitude of the second harmonic signal corresponding to the preset methane absorption peak to generate a wavelength offset error signal, and the temperature control driving module adjusts the central wavelength of the laser output by the laser according to the offset error signal, so that the laser wavelength is always locked within the methane absorption peak range; the second harmonic signal corresponding to the second optical path is integrated to obtain the concentration of trace methane gas in the gas absorption cell.

6. The system according to claim 5, characterized in that The first optical path and the second optical path both include a photodetector and a phase-locked amplifier. The photodetector receives a transmitted light signal absorbed by methane gas, and the phase-locked amplifier filters an electrical signal output by the photodetector.

7. The system according to claim 6, characterized in that Methane gas with a known concentration is sealed in the photodetector of the first optical path.

8. The system according to claim 5, characterized in that It also includes a first light collimator and a second light collimator, wherein the first light collimator collimates a beam of light separated by the optical coupler into the first light path, and the second light collimator collimates another beam of light separated by the optical coupler into the gas absorption cell.

9. The system according to claim 5, characterized in that It also includes a signal generator, which generates a modulation signal and outputs it to the temperature control driving module, so as to modulate the laser output by the laser.

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