Laser gas detection method and system for leakage inspection of hydrogen-doped natural gas pipeline

Abstract

The invention discloses a laser gas detection method and system for leakage inspection of a hydrogen-doped natural gas pipeline. Comprising a master control MCU, a signal generation module, a first laser driving temperature control module, a second laser driving temperature control module, a first semiconductor laser, a second semiconductor laser, a wavelength division multiplexer, a standard gas cell, a multi-reflection gas cell, a first photoelectric detection amplifier, a second photoelectric detection amplifier and a digital gas pressure intensity temperature sensor, the device comprises an orthogonal phase-locked amplification demodulation module, an OLED display screen, a vacuum pump, a drying tube, a handheld sampling tube and a porous gas diffusion probe. A laser absorption spectrum technology of time division multiplexing and wavelength modulation is adopted, and a specific implementation mode is that a semiconductor laser with narrow linewidth and adjustable wavelength is utilized, and non-interference accurate detection of methane and hydrogen in leakage inspection of the hydrogen-doped natural gas pipeline is realized through a high-precision time division multiplexing wavelength modulation spectrum absorption detection technology.

Description

Technical Field

[0001] The present invention relates to the technical field of pipeline leakage detection, and specifically to a laser gas detection method and system for leakage inspection of hydrogen-blended natural gas pipelines. Background Art

[0002] Hydrogen is a clean energy source that does not produce carbon dioxide when burned. Hydrogen blending can effectively reduce greenhouse gas emissions generated by natural gas combustion, meeting the global carbon reduction goal. Hydrogen-blended natural gas can utilize existing natural gas infrastructure, including pipelines, storage tanks, terminals, etc., avoiding the huge costs of building new infrastructure and reducing the environmental impact. Hydrogen-blended natural gas can be used as a storage and transportation method for hydrogen, facilitating the long-distance transportation and application of hydrogen and alleviating the bottleneck of hydrogen storage and transportation. Therefore, pipeline hydrogen transportation is an important way to reduce the cost of hydrogen energy storage and transportation and promote the large-scale application of hydrogen energy. However, hydrogen is more flammable and explosive than natural gas, and its explosion limit range is between 4% - 75% (volume percentage). Hydrogen-blended natural gas pipelines require stricter safety management and technical guarantees. It is necessary to monitor the leakage of hydrogen-blended natural gas pipelines. Once a leakage is detected, a quick response and alarm are required to prevent the leakage or explosion accident of hydrogen-blended natural gas and ensure the safety of facilities and personnel.

[0003] Existing gas sensors mainly include: semiconductor sensors, catalytic combustion sensors, electrochemical sensors, quartz crystal microbalance sensors, laser sensors, etc. Among them, semiconductor sensors are inexpensive, small in size, and easy to integrate, but have low sensitivity, are vulnerable to environmental interference, and have a limited lifespan. Catalytic combustion sensors have high sensitivity, fast response speed, and strong anti-interference ability, but have high power consumption and are vulnerable to poisoning. Electrochemical sensors have relatively high sensitivity, but have a limited lifespan and require special maintenance. Quartz crystal microbalance sensors have high sensitivity, fast response speed, and can be used for low-concentration methane detection, but are expensive and vulnerable to the environment. Laser sensors have the advantages of high sensitivity, high selectivity, non-contact measurement, and fast response, and have been applied to fields such as environmental monitoring, industrial safety, medical diagnosis, and agriculture.

[0004] In terms of leakage monitoring of hydrogen-blended natural gas, there are not many sensors for simultaneous monitoring of methane and hydrogen that are industrialized as mature products at home and abroad. Most of them cannot meet the actual use requirements in terms of indicators. Therefore, there is a need for a laser gas detection method and system for leakage inspection of hydrogen-blended natural gas pipelines. Summary of the Invention

[0005] The purpose of the present invention is to provide a laser gas detection method and system for leakage inspection of hydrogen-blended natural gas pipelines, and through the narrow line width and wavelength tunable characteristics of a tunable semiconductor laser, achieve precise monitoring of methane and hydrogen leakage in hydrogen-blended natural gas pipelines, so as to solve the technical problems mentioned in the background art.

[0006] To achieve the above object, the present invention provides the following technical solutions: A laser gas detection method for leakage inspection of hydrogen-doped natural gas pipelines, comprising the following steps:

[0007] S1. According to the absorption characteristics of methane and hydrogen, set the central wavelengths and wavelength scanning ranges of the light emitted by the first semiconductor laser and the second semiconductor laser, so that the absorption peak of the gas to be measured is at the center of the wavelength scanning range.

[0008] S2. According to the central wavelengths and wavelength scanning ranges of the first semiconductor laser and the second semiconductor laser set in step one, calculate the laser driving and temperature control parameters, signal modulation waveform parameters (frequency, amplitude, phase, offset, etc.), and set the above parameters through the main control MCU.

[0009] S3. Fill nitrogen in both the multi-reflection gas cell and the standard gas cell, and collect two-way spectral signals at the same time. Take the spectral signal passing through the standard gas cell as the reference spectral background, and the spectral signal passing through the multi-reflection gas cell as the baseline spectral background.

[0010] S4. Fill the multi-reflection gas cell with a mixed gas of methane, hydrogen, and nitrogen with the same known concentration as in the standard gas cell, and collect two-way spectral signals at the same time. Subtract the reference spectral background from the spectral signal passing through the standard gas cell as the reference spectral reference, and subtract the baseline spectral background from the spectral signal passing through the multi-reflection gas cell as the baseline spectrum, and then determine the data acquisition and processing parameters of the optical path to be measured. Set the data acquisition and processing parameters through the main control MCU.

[0011] S5. Fill the multi-reflection gas cell with the gas to be measured, collect the spectral signal to be measured, perform non-linear fitting on the spectral signal to be measured after deducting the background and the baseline spectrum, and then correct according to the gas pressure and temperature data to obtain the accurate concentration of the gas to be measured.

[0012] S6. To overcome the error caused by the wavelength shift of the laser after long-term operation of the system, measure the reference optical path once every hour. Compare the measured reference spectrum after deducting the reference spectral background with the reference spectral reference to obtain the wavelength shift amount. If the shift amount is greater than the threshold, convert the wavelength shift amount into a temperature adjustment amount, and fine-tune the laser temperature control parameters by the main control MCU to ensure the stability of the laser central wavelength, and thus ensure the accuracy of gas concentration measurement.

[0013] A laser gas detection system for leakage inspection of hydrogen-doped natural gas pipelines, comprising a main control MCU, a signal generation module, a first laser driving and temperature control module, a second laser driving and temperature control module, a first semiconductor laser and a second semiconductor laser, a wavelength division multiplexer, a standard gas cell, a multi-reflection gas cell, a first photoelectric detection amplifier and a second photoelectric detection amplifier, a digital gas pressure and temperature sensor, a quadrature lock-in amplification and demodulation module, an OLED display screen, a vacuum pump, a drying tube, a hand-held sampling tube and a porous gas diffusion probe.

[0014] Preferably, the main control MCU is connected to the signal generation module, the first laser driving and temperature control module, the second laser driving and temperature control module, the quadrature lock-in amplification and demodulation module and the OLED display screen through serial communication. The first laser driving and temperature control module and the second laser driving and temperature control module are correspondingly connected to the first semiconductor laser and the second semiconductor laser, and the first semiconductor laser and the second semiconductor laser are synchronously connected to the wavelength division multiplexer;

[0015] Two output ends of the wavelength division multiplexer are respectively connected to the standard gas cell and the multi-reflection gas cell. The output side of the standard gas cell is connected to the quadrature lock-in amplification and demodulation module through the first photoelectric detection amplifier. The output side of the multi-reflection gas cell is connected to the quadrature lock-in amplification and demodulation module through the second photoelectric detection amplifier. One side of the multi-reflection gas cell is connected to the quadrature lock-in amplification and demodulation module through the digital gas pressure and temperature sensor. One side of the multi-reflection gas cell is hermetically connected to the vacuum pump. The other side of the multi-reflection gas cell is connected to the drying tube. The input end of the drying tube is connected to the hand-held sampling tube, and a porous gas diffusion probe is installed at the outer end of the hand-held sampling tube.

[0016] Preferably, the signal generation module generates signals A and B which are the superposition of a modulation driving signal sine wave and a triangular wave (or sawtooth wave) required by the system, a scanning fundamental wave synchronous signal, a sine wave synchronous second harmonic signal I, a sine wave synchronous second harmonic signal Q or a sine wave synchronous fundamental wave signal. The modulation driving signals A and B have the same frequency, and the second harmonic signals I and Q have a phase difference of 90°.

[0017] Preferably, the first laser driving and temperature control module and the second laser driving and temperature control module control the output current through internally digitally set voltages. The output current proportionality coefficients of the first laser driving and temperature control module and the second laser driving and temperature control module are 50 mA / 1V, and the 500 kHz sine wave constant current bandwidth (100 kHz square wave constant current bandwidth). A group of power-off protection, slow-start laser diode protection and compatible TTL shutdown pins are respectively installed in the first laser driving and temperature control module and the second laser driving and temperature control module.

[0018] Preferably, the central wavelengths of the first semiconductor laser and the second semiconductor laser are respectively located at the centers of the strong absorption lines of methane and hydrogen, and stably output a laser signal with a periodically varying wavelength within a specified range under the action of the laser driving temperature control module;

[0019] The wavelength division multiplexer combines the optical signals of the first semiconductor laser and the second semiconductor laser and then divides them into two paths. One main beam accounts for 90% and is connected to the multi-reflection gas cell, and the other reference beam accounts for 10% and is connected to the standard gas cell. The standard gas cell is filled with a mixed gas of methane, hydrogen, and nitrogen with a known concentration. An adjustment knob for adjusting the gas cell is provided outside the multi-reflection gas cell, and the optical path of the multi-reflection gas cell is adjustable in the range of 25 meters to 50 meters.

[0020] Preferably, the first photoelectric detection amplifier and the second photoelectric detection amplifier convert the optical signal into an electrical signal, convert the current signal into a voltage signal, and perform filtering and amplification processing on the voltage signal;

[0021] The digital gas pressure and temperature sensor is used to detect the gas pressure and gas temperature in the gas path, and transmits the pressure and temperature data results to the quadrature lock-in amplification and demodulation module by using the 3.3V-TTL serial communication method.

[0022] Preferably, the quadrature lock-in amplification and demodulation module performs digital quadrature lock-in amplification extraction and calculation on the second harmonic signal. The measured data results of the quadrature lock-in amplification and demodulation module are displayed on the OLED display screen. The quadrature lock-in amplification and demodulation module has two groups of analog signal inputs. One group of the analog signals is the signal that has passed through the multi-reflection gas cell by high-speed AD acquisition, and the other group of the analog signals is the signal that has passed through the standard gas cell by low-speed AD acquisition.

[0023] Preferably, the OLED display screen is connected to the quadrature lock-in amplification and demodulation module, receives the quadrature second harmonic signal and displays it on the OLED display screen, and receives the methane and hydrogen concentration information output by the main control MCU and displays it on the OLED display screen.

[0024] Preferably, the vacuum pump is used to extract the gas in the multi-reflection gas cell. A hydrogenated natural gas pipeline is hermetically installed on one side of the multi-reflection gas cell. The hydrogenated natural gas pipeline can continuously pump the nearby gas to be measured into the multi-reflection gas cell. The drying tube is used to dry the gas to be measured entering the multi-reflection gas cell. A porous gas diffusion probe for collecting the gas to be measured is installed at the end of the hand-held sampling tube.

[0025] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0026] By adopting the time-division multiplexing and wavelength modulation laser absorption spectroscopy technology, the specific implementation method is to use a semiconductor laser with a narrow linewidth and wavelength tunability. Through the high-precision time-division multiplexing wavelength modulation spectroscopy absorption detection technology, the non-interfering and accurate detection of methane and hydrogen in the inspection of hydrogen-doped natural gas pipeline leaks is realized. By tuning the laser wavelength to accurately scan the characteristic absorption peaks of the gas, the interference of other gases and environmental factors is avoided, and the anti-interference ability of the system is improved. The orthogonal lock-in amplification signal processing algorithm is adopted to eliminate low-frequency interference and improve the signal-to-noise ratio, ensuring the accuracy of the detection results and ensuring that the gas concentration changes can be detected and reflected in a timely manner. By adopting the fast tuning and real-time signal processing technology, the real-time monitoring of the methane and hydrogen concentrations is realized, improving the inspection efficiency. The laser detection system has high stability, reduces the frequency of calibration and maintenance, reduces the use cost and workload, has a long service life and high stability, reduces the problems of sensor contamination and aging, and reduces the maintenance requirements. In summary, through technological innovation and optimization, the present invention not only solves the key problems in the prior art, but also significantly improves the gas detection accuracy, response speed and anti-interference ability, while simplifying the maintenance operation, and has significant technical advantages and application value. Brief Description of the Drawings

[0027] Figure 1 It is a schematic structural diagram of the present invention.

[0028] In the figure: 1, main control MCU; 2, signal generation module; 3, first laser drive temperature control module; 4, second laser drive temperature control module; 5, first semiconductor laser; 6, second semiconductor laser; 7, wavelength division multiplexer; 8, standard gas cell; 9, multi-reflection gas cell; 10, first photoelectric detection amplifier; 11, second photoelectric detection amplifier; 12, digital gas pressure and temperature sensor; 13, orthogonal lock-in amplification demodulation module; 14, OLED display screen; 15, vacuum pump; 16, drying tube; 17, hand-held sampling tube; 18, porous gas diffusion probe. Detailed Embodiments

[0029] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0030] The present invention provides a laser gas detection method for the inspection of hydrogen-doped natural gas pipeline leaks, as Figure 1 shown, including the following steps:

[0031] S1. According to the absorption characteristics of methane and hydrogen, set the central wavelengths and wavelength scanning ranges of the light emitted by the first semiconductor laser 5 and the second semiconductor laser 6, so that the absorption peak of the gas to be measured is at the center of the wavelength scanning range.

[0032] S2. According to the central wavelengths and wavelength scanning ranges of the first semiconductor laser 5 and the second semiconductor laser 6 set in step one, calculate the laser drive and temperature control parameters, signal modulation waveform parameters such as frequency, amplitude, phase, offset, etc., and set the above parameters through the main control MCU1.

[0033] S3. Both the multi-reflection gas cell 9 and the standard gas cell 8 are filled with nitrogen, and two-way spectral signals are collected simultaneously. The spectral signal passing through the standard gas cell 8 is used as the reference spectral background, and the spectral signal passing through the multi-reflection gas cell 9 is used as the baseline spectral background.

[0034] S4. The multi-reflection gas cell 9 is filled with a mixed gas of methane, hydrogen, and nitrogen with a known concentration same as that in the standard gas cell 8. Two-way spectral signals are collected simultaneously. The spectral signal passing through the standard gas cell 8 minus the reference spectral background is used as the reference spectral benchmark, and the spectral signal passing through the multi-reflection gas cell 9 minus the baseline spectral background is used as the baseline spectrum, and then determine the data acquisition and processing parameters of the optical path to be measured, and set the data acquisition and processing parameters through the main control MCU1.

[0035] S5. The multi-reflection gas cell 9 is filled with the gas to be measured, and the spectral signal to be measured is collected. The spectral signal to be measured is non-linearly fitted with the baseline spectrum after deducting the background, and then corrected according to the gas pressure and temperature data to obtain the accurate concentration of the gas to be measured.

[0036] S6. To overcome the error caused by the wavelength shift of the laser after long-term operation of the system, measure the reference optical path once every hour. The measured reference spectrum minus the reference spectral background is compared with the reference spectral benchmark to obtain the wavelength shift amount. If the shift amount is greater than the threshold, convert the wavelength shift amount into a temperature adjustment amount, and finely adjust the laser temperature control parameters by the main control MCU1 to ensure the stability of the laser central wavelength, and then ensure the accuracy of the gas concentration measurement.

[0037] In addition, the present invention also provides a laser gas detection system for leakage inspection of hydrogen-doped natural gas pipelines, such as Figure 1As shown in the figure, it includes a main control MCU 1, a signal generation module 2, a first laser drive temperature control module 3, a second laser drive temperature control module 4, a first semiconductor laser 5 and a second semiconductor laser 6, a wavelength division multiplexer 7, a standard gas cell 8, a multi-reflection gas cell 9, a first photoelectric detection amplifier 10 and a second photoelectric detection amplifier 11, a digital gas pressure and temperature sensor 12, a quadrature lock-in amplification and demodulation module 13, an OLED display screen 14, a vacuum pump 15, a drying tube 16, a hand-held sampling tube 17 and a porous gas diffusion probe 18.

[0038] The main control MCU 1 is connected to the signal generation module 2, the first laser drive temperature control module 3, the second laser drive temperature control module 4, the quadrature lock-in amplification and demodulation module 13 and the OLED display screen 14 through serial communication. The first laser drive temperature control module 3 and the second laser drive temperature control module 4 are correspondingly connected to the first semiconductor laser 5 and the second semiconductor laser 6. The first semiconductor laser 5 and the second semiconductor laser 6 are synchronously connected to the wavelength division multiplexer 7. The main control MCU 1 is used to control the signal generation module 2, the first laser drive temperature control module 3 and the second laser drive temperature control module 4, so that the laser can pass through the quadrature lock-in amplification and demodulation module 13 and be shunted through the standard gas cell 8 and the multi-reflection gas cell 9, thereby ensuring the detection accuracy of the leakage gas.

[0039] The two output ends of the wavelength division multiplexer 7 are respectively connected to the standard gas cell 8 and the multi-reflection gas cell 9. The output side of the standard gas cell 8 is connected to the quadrature lock-in amplification and demodulation module 13 through the first photoelectric detection amplifier 10. The output side of the multi-reflection gas cell 9 is connected to the quadrature lock-in amplification and demodulation module 13 through the second photoelectric detection amplifier 11. One side of the multi-reflection gas cell 9 is connected to the quadrature lock-in amplification and demodulation module 13 through the digital gas pressure and temperature sensor 12. One side of the multi-reflection gas cell 9 is hermetically connected to the vacuum pump 15. The other side of the multi-reflection gas cell 9 is connected to the drying tube 16. The input end of the drying tube 16 is connected to the hand-held sampling tube 17. The outer end of the hand-held sampling tube 17 is equipped with a porous gas diffusion probe 18. The porous gas diffusion probe 18 is used to draw external gas into the device through the hand-held sampling tube 17, and the set drying tube 16 is used to dry the gas, so that the gas entering the standard gas cell 8 and the multi-reflection gas cell 9 will not be wet with excess water vapor to affect the laser detection accuracy.

[0040] The signal generation module generates signal A and signal B which are the superposition of a modulation drive signal sine wave and a triangular wave (or sawtooth wave) required by the system, as well as a scanning fundamental wave synchronous signal, a sine wave synchronous second harmonic signal I, a sine wave synchronous second harmonic signal Q or a sine wave synchronous fundamental wave signal. The modulation drive signals A and B have the same frequency, and the second harmonic signals I and Q have a phase difference of 90°.

[0041] The first laser-driven temperature control module 3 and the second laser-driven temperature control module 4 control the output current through the internally digitally set voltage. The output current proportionality coefficients of the first laser-driven temperature control module 3 and the second laser-driven temperature control module 4 are 50 mA / 1V. The 500 kHz sine wave constant current bandwidth is greater than the 100 kHz square wave constant current bandwidth. A group of power-off protection, slow-start laser diode protection, and compatible TTL shutdown pins are respectively installed in the first laser-driven temperature control module 3 and the second laser-driven temperature control module 4, which can perform high-precision digital temperature control for the laser temperature control system composed of TEC + RTD thermistors. The temperature control accuracy is 0.001 °C, and the maximum output current of the TEC is ±2A.

[0042] It should be noted that the central wavelengths of the first semiconductor laser 5 and the second semiconductor laser 6 are respectively located at the centers of the strong absorption lines of methane and hydrogen, and stably output laser signals with a periodically varying wavelength within a specified range under the action of the laser-driven temperature control module;

[0043] The wavelength division multiplexer 7 combines the optical signals of the first semiconductor laser 5 and the second semiconductor laser 6 and then divides them into two paths. One main light beam accounts for 90% and is connected to the multi-reflection gas cell 9. The other reference light beam accounts for 10% and is connected to the standard gas cell 8. The standard gas cell 8 is filled with a mixed gas of methane, hydrogen, and nitrogen with a known concentration. An adjustment knob for adjusting the gas cell is provided outside the multi-reflection gas cell 9. The adjustable range of the optical path of the multi-reflection gas cell 9 is 25 meters - 50 meters. After the laser enters the multi-reflection gas cell, it is reflected multiple times between the mirrors at both ends of the gas cell to increase the optical path, by adjusting the adjustment knob of the gas cell.

[0044] Specifically, the first photoelectric detection amplifier 10 and the second photoelectric detection amplifier 11 convert the optical signal into an electrical signal, convert the current signal into a voltage signal, and perform filtering and amplification processing on the voltage signal. The first photoelectric detection amplifier 10 and the second photoelectric detection amplifier 11 cooperate to process the voltage signal, so that the voltage signal can be better captured by the digital gas pressure and temperature sensor 12, thereby better detecting the gas leakage situation.

[0045] The digital gas pressure and temperature sensor 12 is used to detect the gas pressure and gas temperature in the gas path. The digital gas pressure and temperature sensor 12 uses a 3.3V - TTL serial communication method to transmit the pressure and temperature data results to the quadrature lock-in amplification and demodulation module 13. The set digital gas pressure and temperature sensor 12 is used to detect the gas pressure and gas temperature in the pipeline to ensure the safety of the overall use of the device.

[0046] In addition, the orthogonal lock-in amplifier demodulation module 13 performs digital orthogonal lock-in amplification extraction and calculation on the second harmonic signal. The data results measured by the orthogonal lock-in amplifier demodulation module 13 are displayed on the OLED display screen 14. The orthogonal lock-in amplifier demodulation module 13 has two groups of analog signal inputs. One group of analog signals is the signal that has passed through the multi-reflection gas cell 9 after high-speed AD acquisition, and the other group of analog signals is the signal that has passed through the standard gas cell 8 after low-speed AD acquisition. The orthogonal lock-in amplifier demodulation module 13 is used for analog signal input and measures the signal intensities of the two groups respectively, so as to better display the measured data on the OLED display screen 14.

[0047] Preferably, the OLED display screen 14 is connected to the orthogonal lock-in amplifier demodulation module 13, receives the orthogonalized second harmonic signal and displays it on the OLED display screen 14, receives the methane and hydrogen concentration information output by the main control MCU1 and displays it on the OLED display screen 14, and accurately displays the measured second harmonic signal through the OLED display screen 14, so as to more accurately reflect the signal measurement data of the standard gas cell 8 and the multi-reflection gas cell 9.

[0048] Specifically, the vacuum pump 15 is used to extract the gas in the multi-reflection gas cell 9. A hydrogen-doped natural gas pipeline is sealed and installed on one side of the multi-reflection gas cell 9. The hydrogen-doped natural gas pipeline can continuously pump the nearby gas to be measured into the multi-reflection gas cell 9. The drying tube 16 is used to dry the gas to be measured entering the multi-reflection gas cell 9. A porous gas diffusion probe 18 for collecting the gas to be measured is installed at the end of the hand-held sampling tube 17. By setting the vacuum pump 15, the extraction of the gas is ensured, so that the gas can quickly enter the interior of the multi-reflection gas cell 9 for use in cooperation with the first laser drive temperature control module 3 and the second laser drive temperature control module 4.

[0049] Although the embodiments of the present invention have been shown and described, those of ordinary skill in the art can understand that various changes, modifications, substitutions, and variations can be made to these embodiments without departing from the principles and spirit of the present invention. The scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A laser gas detection method for leak inspection of hydrogen-blended natural gas pipelines, characterized in that: The following steps are involved: S1. According to the absorption characteristics of methane and hydrogen, the central wavelength and wavelength scanning range of the light emitted by the first semiconductor laser (5) and the second semiconductor laser (6) are set so that the absorption peak of the gas to be measured is at the center of the wavelength scanning range; S2, according to the central wavelength and wavelength scanning range of the first semiconductor laser (5) and the second semiconductor laser (6) set in step 1, calculate the laser driving and temperature control parameters, signal modulation waveform parameters (frequency, amplitude, phase, offset, etc.), and set the above parameters through the main control MCU (1); S3, nitrogen is filled into the multiple reflection gas pool (9) and the standard gas pool (8), and two spectral signals are collected simultaneously, with the spectral signal passing through the standard gas pool (8) being used as the reference spectral background, and the spectral signal passing through the multiple reflection gas pool (9) being used as the baseline spectral background; S4, the multiple reflection gas pool (9) is filled with a mixed gas of methane, hydrogen and nitrogen of known concentrations, which is the same as that in the standard gas pool (8), and two spectral signals are collected simultaneously, and the spectral signal passing through the standard gas pool (8) minus the reference spectral background is used as the reference spectral benchmark, and the spectral signal passing through the multiple reflection gas pool (9) minus the baseline spectral background is used as the baseline spectrum, thereby determining the data acquisition processing parameters of the optical path to be measured, and setting the data acquisition processing parameters through the main control MCU (1); S5, filling the multiple reflection gas cell (9) with the gas to be measured, collecting the spectral signal to be measured, performing nonlinear fitting on the spectral signal to be measured with the baseline spectrum after deducting the background, and then performing correction according to the gas pressure and temperature data to obtain the accurate concentration of the gas to be measured; S6. In order to overcome the error caused by the wavelength offset of the laser after long-term operation of the system, the reference optical path is measured every hour. The measured reference spectrum is deducted from the reference spectrum background and compared with the reference spectrum benchmark to obtain the wavelength offset. If the offset is greater than the threshold, the wavelength offset is converted into a temperature adjustment amount. The main control MCU (1) fine-tunes the laser temperature control parameters to ensure the stability of the laser center wavelength, thereby ensuring the accuracy of the gas concentration measurement.

2. A laser gas detection system for leak inspection of hydrogen-blended natural gas pipelines, characterized by: The invention comprises a main control MCU (1), a signal generating module (2), a first laser driving temperature control module (3), a second laser driving temperature control module (4), a first semiconductor laser (5) and a second semiconductor laser (6), a wavelength division multiplexer (7), a standard gas pool (8), a multiple reflection gas pool (9), a first photoelectric detection amplifier (10) and a second photoelectric detection amplifier (11), a digital gas pressure and temperature sensor (12), an orthogonal phase-locked amplification and demodulation module (13), an OLED display screen (14), a vacuum pump (15), a drying tube (16), a handheld sampling tube (17) and a porous gas diffusion probe (18).

3. The laser gas detection system for leak inspection of hydrogen-blended natural gas pipeline according to claim 2 is characterized by: The main control MCU (1) is connected to a signal generating module (2), a first laser driven temperature control module (3), a second laser driven temperature control module (4), an orthogonal phase-locked amplification and demodulation module (13) and an OLED display screen (14) via a serial communication method; the first laser driven temperature control module (3) and the second laser driven temperature control module (4) are connected to a first semiconductor laser (5) and a second semiconductor laser (6) respectively; the first semiconductor laser (5) and the second semiconductor laser (6) are synchronously connected to a wavelength division multiplexer (7); The two groups of output ends of the wavelength division multiplexer (7) are respectively connected to a standard gas pool (8) and a multiple reflection gas pool (9); the output side of the standard gas pool (8) is connected to an orthogonal phase-locked amplification and demodulation module (13) via a first photoelectric detection amplifier (10); the output side of the multiple reflection gas pool (9) is connected to an orthogonal phase-locked amplification and demodulation module (13) via a second photoelectric detection amplifier (11); one side of the multiple reflection gas pool (9) is connected to the orthogonal phase-locked amplification and demodulation module (13) via a digital gas pressure and temperature sensor (12); one side of the multiple reflection gas pool (9) is sealedly connected to a vacuum pump (15); the other side of the multiple reflection gas pool (9) is connected to a drying tube (16); the input end of the drying tube (16) is connected to a handheld sampling tube (17); and a porous gas diffusion probe (18) is installed at the outer end of the handheld sampling tube (17).

4. The laser gas detection system for leak inspection of hydrogen-blended natural gas pipeline according to claim 3 is characterized by: The signal generating module (2) generates the modulation drive signal A and B which are the superposition signals of the sine wave and the triangle wave (or sawtooth wave) and the scanning fundamental wave synchronization signal, the sine wave synchronization double frequency signal I, the sine wave synchronization double frequency signal Q or the sine wave synchronization single frequency signal required by the system. The modulation drive signals A and B have the same frequency, and the double frequency signals I and Q have a phase difference of 90°.

5. The laser gas detection system for leak inspection of hydrogen-blended natural gas pipeline according to claim 4 is characterized in that: The first laser driven temperature control module (3) and the second laser driven temperature control module (4) control the output current through an internal digitally set voltage; the output current proportionality coefficient of the first laser driven temperature control module (3) and the second laser driven temperature control module (4) is 50mA / 1V, and the sine wave constant current bandwidth is 500kHz (100kHz square wave constant current bandwidth); and the first laser driven temperature control module (3) and the second laser driven temperature control module (4) are respectively installed with a set of power-off protection, slow start laser diode protection, and TTL compatible shutdown pins.

6. The laser gas detection system for leak inspection of hydrogen-blended natural gas pipeline according to claim 5 is characterized by: The central wavelengths of the first semiconductor laser (5) and the second semiconductor laser (6) are located at the centers of the strong absorption lines of methane and hydrogen respectively, and under the action of the laser driving temperature control module, they stably output laser signals with wavelengths that periodically change within a specified range; The wavelength division multiplexer (7) combines the optical signals of the first semiconductor laser (5) and the second semiconductor laser (6) and then divides them into two paths, one of which is a main light beam accounting for 90% and connected to a multiple reflection gas pool (9), and the other is a reference light beam accounting for 10% and connected to a standard gas pool (8). The standard gas pool (8) is filled with a mixed gas of methane, hydrogen and nitrogen with known concentrations. An adjusting knob for adjusting the gas pool is arranged outside the multiple reflection gas pool (9). The optical path of the multiple reflection gas pool (9) can be adjusted in the range of 25 meters to 50 meters.

7. The laser gas detection system for leak inspection of hydrogen-blended natural gas pipeline according to claim 6 is characterized by: The first photodetection amplifier (10) and the second photodetection amplifier (11) convert the optical signal into an electrical signal, the first photodetection amplifier (10) and the second photodetection amplifier (11) convert the current signal into a voltage signal, and the first photodetection amplifier (10) and the second photodetection amplifier (11) perform filtering and amplification processing on the voltage signal; The digital gas pressure and temperature sensor (12) is used to detect the gas pressure and gas temperature in the gas path, and the digital gas pressure and temperature sensor (12) uses a 3.3V-TTL serial communication method to transmit the pressure and temperature data results to the orthogonal phase-locked amplification and demodulation module (13).

8. The laser gas detection system for leak inspection of hydrogen-blended natural gas pipeline according to claim 7 is characterized by: The orthogonal phase-locked amplification and demodulation module (13) extracts and calculates the digitalized orthogonal phase-locked amplification of the second harmonic signal, and the data results measured by the orthogonal phase-locked amplification and demodulation module (13) are displayed through an OLED display screen (14). The orthogonal phase-locked amplification and demodulation module (13) has two groups of analog signal inputs, one group of analog signals is a signal collected by high-speed AD and passed through a multi-reflection gas pool (9), and the other group of analog signals is a signal collected by low-speed AD and passed through a standard gas pool (8).

9. The laser gas detection system for leak inspection of hydrogen-blended natural gas pipeline according to claim 8 is characterized in that: The OLED display screen (14) is connected to the orthogonal phase-locked amplification and demodulation module (13), receives the orthogonal second harmonic signal and displays it on the OLED display screen (14), and receives the methane and hydrogen concentration information output by the main control MCU (1) and displays it on the OLED display screen (14).

10. The laser gas detection system for leak inspection of hydrogen-blended natural gas pipeline according to claim 9, characterized in that: The vacuum pump (15) is used to extract gas from the multiple reflection gas pool (9); a hydrogen-doped natural gas pipeline is sealed and installed on one side of the multiple reflection gas pool (9); the hydrogen-doped natural gas pipeline can continuously draw nearby gas to be tested into the multiple reflection gas pool (9); the drying tube (16) is used to dry the gas to be tested that enters the multiple reflection gas pool (9); and a porous gas diffusion probe (18) for collecting the gas to be tested is installed at the end of the handheld sampling tube (17).

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