CN117214133A - Methane detection method with ranging function based on TDLAS - Google Patents
Methane detection method with ranging function based on TDLAS Download PDFInfo
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- VNWKTOKETHGBQD-UHFFFAOYSA-N methane Chemical compound C VNWKTOKETHGBQD-UHFFFAOYSA-N 0.000 title claims abstract description 178
- 238000001514 detection method Methods 0.000 title claims abstract description 28
- 238000000041 tunable diode laser absorption spectroscopy Methods 0.000 title claims abstract description 20
- 238000005259 measurement Methods 0.000 claims abstract description 21
- 238000000034 method Methods 0.000 claims abstract description 14
- 230000003287 optical effect Effects 0.000 claims abstract description 8
- 238000001228 spectrum Methods 0.000 claims description 13
- 238000000862 absorption spectrum Methods 0.000 claims description 6
- 238000010521 absorption reaction Methods 0.000 claims description 5
- 238000002156 mixing Methods 0.000 claims description 5
- 238000005070 sampling Methods 0.000 claims description 3
- 239000007789 gas Substances 0.000 description 54
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- 238000005516 engineering process Methods 0.000 description 3
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- CURLTUGMZLYLDI-UHFFFAOYSA-N Carbon dioxide Chemical compound O=C=O CURLTUGMZLYLDI-UHFFFAOYSA-N 0.000 description 2
- 239000003245 coal Substances 0.000 description 2
- 238000000354 decomposition reaction Methods 0.000 description 2
- 239000003345 natural gas Substances 0.000 description 2
- 238000009825 accumulation Methods 0.000 description 1
- 238000012271 agricultural production Methods 0.000 description 1
- 230000009286 beneficial effect Effects 0.000 description 1
- 229910002092 carbon dioxide Inorganic materials 0.000 description 1
- 239000001569 carbon dioxide Substances 0.000 description 1
- 238000002485 combustion reaction Methods 0.000 description 1
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Abstract
The application provides a methane detection method with a distance measurement function based on TDLAS, which comprises the following steps: changing the working current of a laser, transmitting a laser signal superimposed by a low-frequency sawtooth wave and a high-frequency sine wave to a target, absorbing the laser signal by methane gas, receiving an echo signal reflected by the target, selectively extracting a second harmonic amplitude by a phase-locked amplifier, and calculating the path integral concentration of the primary methane gas; obtaining a target distance by a linear frequency modulation continuous wave method, and obtaining a methane background gas concentration interference value in the atmosphere according to the product of the target distance and the methane background gas concentration in the atmosphere; subtracting the interference value to obtain the final methane gas path integral concentration. The application uses the infrared laser, the optical path structure and the photoelectric detector for methane detection and distance measurement, reduces the cost of the methane telemetering instrument and reduces the complexity of the structure.
Description
Technical Field
The application relates to the field of gas detection, in particular to a methane detection method with a distance measurement function based on TDLAS.
Background
In the twentieth century, people's living standard has entered a new era under the continuous promotion of science and technology, and social activities have been involved in various fields. But the concomitant environmental protection and industrial safety production have become problems facing countries around the world. During human activities and industrial production, certain gases are emitted into the air, and the components of the atmosphere may be changed and the greenhouse effect may be generated through long-term accumulation. However, methane CH 4 As a second important gas to influence greenhouse effect after carbon dioxide, although CH in the atmosphere 4 Is generally much lower than CO 2 Content of but CH 4 The greenhouse effect of (2) is CO 2 Is one fifth of the total amount of greenhouse gases that cause global temperature change, and CO 2 The concentration increases at a rate of 1% per year [1-2 ]]. In addition, CH 4 Is a main component of natural gas, marsh gas, oilfield gas and the like, and is used as a very important fuel in industrial production. CH (CH) 4 The main sources are as follows: decomposition of organic waste, extraction of fossil fuel, combustion of substances, microbial decomposition process, etc. As a colorless, odorless gas, CH in a normal atmospheric pressure room temperature environment 4 The content is very inflammable within the range of 5-15% so as to cause explosion, and particularly in the exploitation of coal mines, the method brings serious threat to the safety production of the mines. CH (CH) 4 Has wider distribution in the nature and has close relation with the life production of people. In many industries, such as coal mining, natural gas production and transportation, agricultural production, etc., where different concentrations of CH are required 4 Monitoring is performed.
Thus, for CH 4 Quick concentration detectionMeasurement has been the focus of attention. Not only need to monitor CH in the atmosphere 4 The concentration of the gas is used for determining the index of energy conservation and emission reduction, and CH in industrial production is required to be monitored 4 Concentration to ensure the safety of lives and properties of workers. Compared with the traditional gas detection technology, the gas molecular absorption spectrum technology developed in recent years becomes one of effective means for gas species identification and concentration measurement, and can obtain good gas selectivity and high detection sensitivity.
Tunable semiconductor laser absorption spectrum Tunable Diode Laser Absorption Spectroscopy, TDLAS for short, is a technique that utilizes tuned laser light to pass through an absorption gas, and estimates gas properties from the absorption spectrum of the gas. Specifically, the working temperature and injection current of the laser diode are controlled, the laser wavelength is changed, the gas absorption spectrum line is scanned, and spectrum line information related to the concentration of the gas to be detected is extracted, so that the gas to be detected is accurately detected.
The laser methane telemetry can be interfered by the concentration of methane background gas in the atmosphere when methane detection is carried out, and the farther the target distance is, the stronger the interference of the concentration of methane background gas in the atmosphere is, so that real-time distance measurement is needed to eliminate the interference. In the prior art, a laser receiving and transmitting system of the laser methane telemetry instrument comprises two laser beams, one laser beam is infrared laser for methane gas detection, the other laser beam is visible red/green laser beam for indicating the direction and measuring the distance, and the interference of the concentration of methane background gas in the atmosphere is eliminated, so that an additional laser, an optical path structure and a photoelectric detector are needed, the cost of the methane telemetry instrument is increased, and the structure is more complex.
Disclosure of Invention
The application aims to provide a methane detection method with a distance measurement function based on TDLAS, which realizes distance measurement and eliminates the interference of methane background gas concentration in the atmosphere without installing an additional laser, an optical path structure and a photoelectric detector, and installs an inexpensive laser indicator to realize a target indication effect, so that the methane detection and the distance measurement share an infrared laser, the optical path structure and the photoelectric detector, thereby reducing the cost of a methane telemetering instrument and the complexity of the structure.
For this purpose, the above object of the present application is achieved by the following technical solutions:
s1, changing the working current of a laser, transmitting a laser signal with low-frequency sawtooth waves and high-frequency sine waves overlapped to a target, absorbing the laser signal by methane gas, reaching the target, and receiving an echo signal reflected by the target;
s2, detecting the echo signal in the step S1 by a photoelectric detector, selectively extracting the amplitude of a second harmonic by a lock-in amplifier, and calculating the path integral concentration of the primary methane gas;
s3, changing working current of a laser, transmitting a linear frequency modulation continuous signal to a target, receiving a target reflection signal, sampling a difference frequency signal obtained by mixing the target reflection signal and the transmitted linear frequency modulation continuous signal to obtain a discrete difference frequency signal, performing Fourier transform on the discrete difference frequency signal to obtain a Fourier transform spectrum of the discrete difference frequency signal, calculating the distance of the target based on the peak frequency of the Fourier transform spectrum, and obtaining a methane background gas concentration interference value in the atmosphere according to the product of the distance of the target and the methane background gas concentration in the atmosphere.
And S4, subtracting the interference value of the methane background gas concentration in the atmosphere in the S3 from the initial methane gas path integral concentration in the S2 to obtain the final methane gas path integral concentration.
Further, the distance measurement and the methane gas concentration measurement share a laser, an optical path structure and a photoelectric detector.
Further, removing the part affected by methane gas absorption in the discrete difference frequency signal, performing Fourier transform on the rest discrete difference frequency signal to obtain a Fourier transform spectrum of the discrete difference frequency signal, and calculating the distance of the target based on the peak frequency of the Fourier transform spectrum.
Further, the distance of the target is:
wherein R is the distance of a target, c is the speed of light, T is the sweep repetition period of the transmitted signal, f is the frequency of the discrete difference frequency signal, and B is the modulation bandwidth of the transmitted signal.
Further, the lasing signal is achieved by varying the laser operating current.
Further, the discrete difference frequency signal is obtained by mixing the transmitted linear frequency modulation continuous signal and the target reflection signal through a mixer.
Compared with the prior art, the application has the beneficial effects that: according to the TDLAS-based methane detection method with the distance measurement function, working current of a laser is changed, a laser signal with low-frequency sawtooth waves and high-frequency sine waves overlapped is emitted to a target, and the laser signal reaches the target after being absorbed by methane gas and receives echo signals reflected by the target; detecting the echo signal by a photoelectric detector, selectively extracting the amplitude of a second harmonic by a lock-in amplifier, and calculating the path integral concentration of the primary methane gas; obtaining a target distance by a linear frequency modulation continuous wave method, and obtaining a methane background gas concentration interference value in the atmosphere according to the product of the target distance and the methane background gas concentration in the atmosphere; the method for detecting methane with the distance measurement function based on the TDLAS provided by the application calculates the interference value of the methane background gas concentration in the atmosphere according to the distance of the target, measures the distance by using a linear frequency modulation continuous wave method, only influences the light intensity and does not influence the frequency change, so that the detection result is more accurate, and simultaneously, the method for detecting methane with the distance measurement function based on the TDLAS provided by the application completes two functions of detecting the methane concentration and detecting the distance of the target through one laser without additional laser, optical path structure and photoelectric detector required by laser distance measurement, thereby reducing the cost required by methane telemetry equipment and the cost required by detection and having simpler structure.
Drawings
Fig. 1 is a flowchart of a methane detection method with a ranging function based on TDLAS provided by the application.
Fig. 2 is a schematic diagram of a methane detection method with a ranging function based on TDLAS according to the present application.
Fig. 3 is a graph showing frequency variation of an emission signal and a target reflection signal during distance measurement according to the present application.
Fig. 4 is a fourier transform spectrum diagram of a discrete difference signal provided by the present application.
Detailed Description
The application will be described in further detail with reference to the drawings and specific embodiments. It will be apparent that the drawings in the following description are only some examples or embodiments of the present specification and are not intended to limit the application.
It should be noted that although a logical order is illustrated in the flowchart, the steps illustrated or described are not necessarily performed in the order of the flowchart, and some operations may be removed or added.
In this embodiment, a method for detecting methane with a ranging function based on TDLAS is provided, and fig. 1 is a flowchart of a method for detecting methane with a ranging function based on TDLAS according to some examples of the present application, as shown in fig. 1, specifically the steps are as follows:
s1, changing the working current of a laser, transmitting a laser signal with low-frequency sawtooth waves and high-frequency sine waves overlapped to a target, absorbing the laser signal by methane gas, reaching the target, and receiving an echo signal reflected by the target;
s2, detecting the echo signal in the step S1 by a photoelectric detector, selectively extracting the amplitude of a second harmonic by a lock-in amplifier, and calculating the path integral concentration of the primary methane gas;
s3, changing working current of a laser, transmitting a linear frequency modulation continuous signal to a target, receiving a target reflection signal, sampling a difference frequency signal obtained by mixing the target reflection signal and the transmitted linear frequency modulation continuous signal to obtain a discrete difference frequency signal, performing Fourier transform on the discrete difference frequency signal to obtain a Fourier transform spectrum of the discrete difference frequency signal, calculating the distance of the target based on the peak frequency of the Fourier transform spectrum, and obtaining a methane background gas concentration interference value in the atmosphere according to the product of the distance of the target and the methane background gas concentration in the atmosphere.
And S4, subtracting the interference value of the methane background gas concentration in the atmosphere in the S3 from the initial methane gas path integral concentration in the S2 to obtain the final methane gas path integral concentration.
Fig. 2 is a schematic diagram of a TDLAS-based methane detection method with a distance measurement function, which is used for measuring a distance of a target and a path integral concentration of methane, and includes a control and acquisition system 1, a laser 2, a beam splitting prism 3, methane gas 4, a target reflecting surface 5, a lens 6, a photoelectric detector 7, a photoelectric detector 8, and a calculation and display system 9.
The distance measurement and methane gas concentration measurement share a laser 2 and a photodetector 8.
The control and acquisition system 1 supplies current to the laser 2, so that the laser 2 emits laser signals superimposed by low-frequency sawtooth waves and high-frequency sine waves to a target, the emitted laser reaches the target reflecting surface 5 after passing through the beam splitting prism 3 and the methane gas 4, echo signals reflected by the target reflecting surface 5 are converged to the photoelectric detector 8 by the lens 6, and the control and acquisition system 1 acquires signals 10 of the photoelectric detector 8.
The phase-locked amplifier in the control and acquisition system 1 extracts the second harmonic amplitude of the signal 10 according to the sine wave with the increased frequency to calculate the primary path integral concentration c of methane gas 0 ；
The control and acquisition system 1 supplies current to the laser 2, so that the laser 2 emits linearly transformed frequency modulated continuous laser, the repetition period of the sweep frequency is T, the modulation bandwidth is B,
the outgoing laser is divided into two beams of laser through the beam splitting prism 3, one beam of signal 11 received by the photoelectric detector 7 is collected by the control and collection system 1, the other beam of laser reaches the target reflecting surface 5 through the methane gas 4, the echo signals reflected by the target reflecting surface 5 are converged to the photoelectric detector 8 by the lens 6, the control and collection system 1 collects the signal 12 of the photoelectric detector 8, and the frequency change curves of the signal 11 and the signal 12 are shown in fig. 3;
the control and acquisition system 1 mixes the signal 11 and the signal 12, samples the mixed discrete difference frequency signal, and the calculation and display system 9 performs fourier transform on the discrete difference frequency signal to obtain a fourier transform spectrum of the discrete difference frequency signal, as shown in fig. 4. The computing and display system 9 computes the distance R of the object based on the peak frequency f of the fourier transform spectrum.
Further, the distance of the target is:
wherein R is the distance of a target, c is the speed of light, T is the sweep repetition period of the transmitted signal, f is the frequency of the discrete difference frequency signal, and B is the modulation bandwidth of the transmitted signal.
The methane concentration in the atmosphere reaches 1908ppb, 1.908ppm, the interference value of the methane background gas concentration in the atmosphere is 1.908R ppm m, and the path integral concentration c of the primary methane gas is calculated according to the distance R of the target 0 Subtracting the interference value of the methane background gas concentration in the atmosphere to obtain the final methane gas path integral concentration c=c 0 1.908. Multidot.R. The calculation and display system 9 performs calculation and display.
The application relates to a methane detection method with a distance measurement function based on TDLAS, which comprises the following steps:
1. and interference of methane gas absorption is eliminated during target distance detection, so that a detection result is more accurate.
2. The method can detect the target distance, eliminate the interference of the concentration of methane background gas in the atmosphere and enable the detection result of the concentration of methane gas to be more accurate.
3. The methane concentration detection and target distance detection are completed, an additional laser, an optical path structure and a photoelectric detector are not needed, the cost of the methane telemetering instrument is reduced, and the structure is simpler.
The foregoing description of the embodiments of the present application has been presented only by way of example and is not intended to limit the scope of the application, and any modifications, equivalents, improvements or the like, which fall within the spirit and principles of the application, are intended to be included within the scope of the application.
Claims (5)
1. The methane detection method with the ranging function based on the TDLAS is characterized by comprising the following steps of:
s1, changing the working current of a laser, transmitting a laser signal with low-frequency sawtooth waves and high-frequency sine waves overlapped to a target, absorbing the laser signal by methane gas, reaching the target, and receiving an echo signal reflected by the target;
s2, detecting the echo signal by a photoelectric detector, selectively extracting a second harmonic amplitude by a lock-in amplifier, and calculating the path integral concentration of the primary methane gas;
s3, changing working current of a laser, transmitting a linear frequency modulation continuous signal to a target, receiving a target reflection signal, sampling a difference frequency signal obtained by mixing the target reflection signal and the transmitted linear frequency modulation continuous signal to obtain a discrete difference frequency signal, performing Fourier transform on the discrete difference frequency signal to obtain a Fourier transform spectrum of the discrete difference frequency signal, calculating the distance of the target based on the peak frequency of the Fourier transform spectrum, and obtaining a methane background gas concentration interference value in the atmosphere according to the product of the distance of the target and the methane background gas concentration in the atmosphere;
and S4, subtracting the interference value of the methane background gas concentration in the atmosphere in the step S3 from the initial methane gas path integral concentration in the step S2 to obtain the final methane gas path integral concentration.
2. The TDLAS-based methane detection method with ranging function of claim 1, wherein the method comprises the steps of: the distance measurement and the methane gas concentration measurement share a laser, an optical path structure and a photoelectric detector.
3. The TDLAS-based methane detection method with ranging function of claim 1, wherein the step S1 comprises: and adjusting the frequency of the laser according to the absorption peak of the absorption spectrum signal corresponding to the methane gas, so that the absorption peak of the absorption spectrum signal of the methane gas is contained in each period of the signal.
4. The method of frequency extracting a discrete difference signal to calculate a distance of a target according to claim 1, wherein: the distance between the targets is as follows:
wherein R is the distance of a target, c is the speed of light, T is the sweep repetition period of the transmitted signal, f is the peak frequency of the Fourier transform spectrum, and B is the modulation bandwidth of the transmitted signal.
5. The TDLAS-based methane detection method with ranging function of claim 1, wherein the method comprises the steps of: the discrete difference frequency signal is obtained by mixing the transmitted linear frequency modulation continuous signal and the target reflection signal through a mixer.
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| CN117491314A (en) * | 2023-12-27 | 2024-02-02 | 金卡智能集团(杭州)有限公司 | Combustible gas detection device and detection method |
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| CN117491314A (en) * | 2023-12-27 | 2024-02-02 | 金卡智能集团(杭州)有限公司 | Combustible gas detection device and detection method |
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