A photoacoustic-raman combined full-range multi-component coal mine gas high-sensitivity detection system and method

Abstract

The application discloses a photoacoustic-Raman combined full-range multi-component coal mine gas high-sensitivity detection system and method, belongs to the technical field of laser spectrum gas detection, and comprises a host computer, a first excitation light source module, a second excitation light source module, a photoacoustic-Raman high-performance coupling gas cell module and a combined signal acquisition and processing module comprising a spectrometer and an acoustic-electric signal processing link; the host computer controls the first excitation light source module and the second excitation light source module to irradiate the photoacoustic-Raman high-performance coupling gas cell module and processes signals from the combined signal acquisition and processing module; the second excitation light source module and the spectrometer are used for wide-range concentration detection of background gas; the first excitation light source module, the photoacoustic-Raman high-performance coupling gas cell module and the acoustic-electric signal processing link are used for high-sensitivity detection of trace self-heating gas; and the host computer performs dynamic re-calibration of a resonance frequency. The application realizes stable capture of photoacoustic detection signals under extreme working conditions.

Description

Technical Field

[0001] This invention belongs to the field of laser spectroscopy gas detection technology, specifically relating to a photoacoustic-Raman combined full-range multi-component coal mine gas high-sensitivity detection system and method. Background Technology

[0002] The coal mining industry is a core pillar industry for my country's energy security. Coal mine gas explosions and spontaneous combustion of coal seams are the main causes of major underground safety accidents. Accurate detection of high concentrations of methane (CH4) released during gas outbursts and trace amounts of characteristic gases (such as acetylene C2H2 and ethylene C2H4) generated during coal seam self-heating processes are crucial prerequisites for early warning of mine disasters and ensuring the safety of underground workers. Coal mine gas detection instruments must possess two core capabilities: first, the ability to detect CH4 gas across a wide dynamic range of 0-100%, adapting to extreme gas outburst conditions; and second, high-sensitivity detection of trace amounts of self-heating characteristic gases such as C2H2 and C2H4 at the ppm (parts per million) level, meeting the needs for early warning of spontaneous combustion in coal seams.

[0003] Currently, commonly used gas detection technologies in coal mines are mainly divided into two categories: contact sensing and chemical analysis technologies, and optical detection technologies.

[0004] In contact sensing and chemical analysis technologies, sensor-based catalytic combustion methods can only detect low concentrations of CH4 (0-4%), exhibiting binary detection at high concentrations, which easily leads to false positives. Electrochemical sensors have short lifespans, poor long-term stability, and are severely affected by downhole temperature, humidity, and cross-interference. While thermal conductivity and optical interferometry methods based on gas physical properties can achieve wide-range CH4 detection, they suffer from low accuracy and weak anti-interference capabilities, failing to achieve ppm-level trace detection. Furthermore, gas chromatography based on chemical separation suffers from bottlenecks such as detection lag, cumbersome pipeline maintenance, and inability to meet real-time online monitoring requirements.

[0005] In infrared absorption spectroscopy, although nondispersive infrared (NDIR) technology is highly stable, it has poor adaptability to the detection of multi-component alkane gases, making it difficult to achieve simultaneous detection of multiple gas components. Although tunable semiconductor laser absorption spectroscopy (TDLAS) technology has high sensitivity, it suffers from severe absorption saturation and nonlinear response in the detection of high-concentration gases, and cannot avoid the cross-interference problem of broadband spectra. Similarly, it cannot simultaneously cover the detection of full-range CH4 and trace self-heating gases.

[0006] Photoacoustic spectroscopy (PAS), as an indirect absorption spectroscopy technique, holds great promise for trace gas detection due to its extremely high sensitivity and zero background detection advantages. However, when faced with the extreme and complex conditions of underground coal mine gas outbursts, where the composition of mixed gases undergoes drastic changes, directly applying traditional photoacoustic spectroscopy to full-range multi-component detection presents the following technical bottlenecks:

[0007] First, drastic changes in background gas can cause shifts in photoacoustic resonant frequencies and signal loss. Resonant photoacoustic spectroscopy is highly dependent on a strict match between the excitation light modulation frequency and the acoustic resonant frequency of the gas pool. During a gas outburst, the composition of the underground gas mixture changes dramatically (e.g., a sudden surge in CH4 concentration), altering the equivalent sound velocity of the mixture and causing a severe shift in the inherent photoacoustic resonant frequency of the gas pool. If the system continues to operate at the originally calibrated modulation frequency, the operating point will deviate from the acoustic resonance enhancement region, leading to a sharp attenuation or even complete loss of the photoacoustic signal from trace target gases, resulting in missed detections. Existing non-resonant photoacoustic technologies have too low sensitivity, and resonance tracking methods based on acoustic scanning are too time-consuming, failing to meet the rapid early warning requirements for mine disasters.

[0008] Second, there is cross-interference in absorption spectra against a high-concentration broadband background. In the detection of multi-component mixed gases, when the concentration of the background gas (such as CH4) is extremely high, its absorption coefficient in the weak absorption band will be amplified many times over. This broadband absorption background of high-concentration gas will cause strong cross-interference on the characteristic absorption peaks of the target trace autothermal gas, making the acquired photoacoustic signal an unidentifiable mixed superimposed signal. Traditional electrical filtering or simple multivariate equation solving methods cannot fundamentally decouple this nonlinear cross-interference at the physical level, resulting in serious distortion of the concentration inversion results of trace gases.

[0009] Third, single-system detection methods struggle to simultaneously achieve a wide dynamic range, high-sensitivity optical excitation, and acoustic noise reduction. While traditional gas Raman scattering possesses excellent linear anti-saturation capabilities across a wide concentration range, its scattered signal is extremely weak, making it difficult to achieve high signal-to-noise ratio trace detection. Furthermore, the long-path, multi-reflection techniques employed to improve the lower limit of photoacoustic detection are highly prone to generating photothermal coherent noise on the gas chamber walls. In addition, existing photoacoustic or Raman detection devices are often independent and bulky, lacking a compact core sensing unit that can simultaneously enhance the signal intensity of both physical mechanisms while completely suppressing background noise.

[0010] It is evident that existing single-spectral technologies cannot simultaneously solve the problems of resonance frequency shift and spectral cross-interference caused by high-concentration background. There is an urgent need for a new joint detection system to achieve high signal-to-noise ratio, interference-free, and accurate full-range multi-component gas sensing under extreme conditions with large-scale concentration changes. Summary of the Invention

[0011] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0012] A photoacoustic-Raman combined full-range multi-component coal mine gas high-sensitivity detection system includes: a host computer, a first excitation source module, a second excitation source module, a photoacoustic-Raman high-performance coupled gas cell module, and a combined signal acquisition and processing module including a spectrometer and an acoustic-electric signal processing link;

[0013] The host computer is used to control the first excitation light source module and the second excitation light source module to irradiate the photoacoustic-Raman high-performance coupled gas cell module, and to process the signals from the joint signal acquisition and processing module;

[0014] The second excitation source module and the spectrometer constitute a Raman detection unit, which is used to perform wide-range concentration detection of background gas.

[0015] The first excitation light source module, the photoacoustic-Raman high-performance coupled gas cell module, and the acoustic-electric signal processing link constitute the photoacoustic detection unit, which is used to perform highly sensitive detection of trace self-heating gases.

[0016] The host computer performs dynamic recalibration of the resonant frequency: based on the background gas concentration inverted in real time by the Raman detection unit, it calculates the change in the equivalent sound velocity of the mixed gas caused by the change in gas composition; according to the preset mapping relationship between the change in equivalent sound velocity and the offset of photoacoustic resonant frequency, it dynamically recalibrates and controls the modulation frequency of the first excitation light source module to match the changed acoustic resonant frequency in order to stably capture the photoacoustic signal.

[0017] A photoacoustic-Raman combined high-sensitivity detection method for full-range multi-component coal mine gas, used in the aforementioned photoacoustic-Raman combined high-sensitivity detection system for full-range multi-component coal mine gas, comprising:

[0018] Step 1, System Initialization and Gas Introduction: Under constant operating temperature conditions, the system is powered on and the coal mine mixed gas to be tested is introduced into the first and second chambers of the photoacoustic-Raman high-performance coupled gas pool module through the inlet at a stable flow rate, and discharged through the outlet. After the gas replacement is completed, the detection is started.

[0019] Step 2, Raman wide-range detection and threshold determination: The second excitation source module outputs highly stable continuous light, which is incident on the second cavity through the second convex lens and the second optical window; the Raman scattering signal is enhanced by self-convergence using the second spherical reflection ring, and is efficiently collected and transmitted to the spectrometer through the second spherical reflection mirror cavity, Fresnel collimating lens, third convex lens and optical fiber; the host computer inverts the concentration of the background gas based on the wide concentration linear detection capability of the Raman spectrum, and quickly determines whether the concentration of the background gas exceeds the gas outburst detection threshold; if it does not exceed the gas outburst detection threshold, the initially calibrated resonance frequency is used as the modulation frequency of the first excitation source module, and step 4 is executed; if it exceeds the gas outburst detection threshold, step 3 is executed;

[0020] Step 3, dynamic recalibration of photoacoustic resonance frequency: The host computer calculates the equivalent sound velocity of the current mixed gas based on the background gas concentration obtained from Step 2.

[0021] Based on a pre-established linear proportional mapping model of photoacoustic resonant frequency offset and gas sound velocity change under constant temperature and pressure, the photoacoustic resonant frequency is calculated and recalibrated, thereby locking the differential resonant mode of the system. At the same time, based on the pre-established approximate linear relationship between photoacoustic resonant frequency offset and trace gas sound pressure amplitude, the preliminary sound pressure amplitude after synchronous acquisition by the first and second microphones and differential preprocessing is corrected.

[0022] Step 4, Cross-interference decoupling and precise photoacoustic detection: The first excitation light source module emits a photoacoustic excitation beam modulated with the current recalibrated photoacoustic resonance frequency, which enters the first cavity through the first convex lens and the first optical window;

[0023] The first and second microphones simultaneously pick up the sound pressure signals in the first and second cavities. The two sound pressure signals pass through the differential module, the filtering module and the amplification module in sequence. The acoustic symmetry of the structure is used to maximize the suppression of incoherent common-mode noise and amplify the differential-mode signal. Finally, the data acquisition card inputs the signal to the host computer for phase-locked demodulation.

[0024] Step 5, Concentration Inversion and Graded Early Warning: The host computer uses the actual trace gas photoacoustic signal amplitude S obtained after decoupling in steps 3 and 4. trace It accurately reflects the true concentration of trace self-heating gases;

[0025] Step 6: Combine the background gas concentration retrieved in Step 2 with the actual concentration of trace self-heating gas retrieved in Step 5 to determine whether the gas concentration exceeds the limit and trigger the corresponding level of coal mine gas outburst or spontaneous combustion safety warning; if the gas concentration does not exceed the limit, the system returns to Step 2 and continues to perform continuous online monitoring.

[0026] The present invention has the following beneficial effects:

[0027] (1) This invention constructs a dynamic recalibration mechanism for the resonant frequency of photoacoustic-Raman combined, introduces Raman spectrum, which is not affected by absorption saturation, as a reference, and uses Raman scattering signal to invert large-scale concentration changes of background gas (such as CH4) in real time and over a wide range. Combined with the mapping model of gas sound velocity and concentration change, the resonant frequency shift caused by the equivalent sound velocity change of the mixed medium is derived, thereby dynamically relocking the modulation frequency of photoacoustic excitation light and correcting the sound pressure amplitude. It effectively solves the technical problem that existing photoacoustic spectroscopy technology suffers from severe resonant frequency shift and photoacoustic signal de-extraction from the resonant region due to drastic changes in background gas concentration when facing extreme working conditions such as gas outbursts, and achieves stable capture of photoacoustic detection signal under extreme working conditions.

[0028] (2) This invention establishes a physical decoupling model for spectral cross-interference based on a wide-range Raman reference. Based on the volume fraction (concentration) of high-concentration background gas accurately inverted by Raman spectroscopy, it calculates the interference of its absorption coefficient in the target trace gas detection band by combining a gas absorption database. Through numerical mapping, the photoacoustic contribution component of the background gas is accurately separated from the total differential photoacoustic signal, thus achieving physical decoupling. This effectively solves the technical problem that the weak absorption band of high-concentration broadband background gas causes serious cross-interference to multi-component trace gases (such as C2H2 and C2H4), resulting in superposition and confusion of photoacoustic signals, serious distortion of concentration inversion results and poor linearity in traditional detection methods. This ensures high fidelity in the concentration inversion of trace self-heating gases under complex high-concentration backgrounds.

[0029] (3) This invention constructs a Helmholtz differentially coupled gas cell integrating a spherical reflection ring and a fully symmetrical spherical reflection mirror cavity. This gas cell is a variant of the Helmholtz photoacoustic cell and adopts a fully symmetrical dual-cavity differential structure. On the one hand, in the first cavity, the spherical reflection ring on the side wall is used to realize multiple internal reflections and central self-convergence of the photoacoustic excitation beam, which greatly increases the absorption optical path of the gas to be tested to enhance photoacoustic excitation. In the second cavity, the spherical reflection ring of the same structure is used to realize multiple reflections and central self-convergence of the Raman excitation beam, which multiplies the Raman focus energy density and, combined with the spherical reflection mirror cavity, greatly improves the scattering collection rate. On the other hand, By utilizing a differential resonance structure composed of perfectly symmetrical upper and lower cavities and connecting tubes, the non-coherent common-mode noise caused by gas flow is suppressed to the maximum extent. A Raman excitation beam modulated with the same frequency and phase is introduced as an anti-phase source to form acoustic interference cancellation in the Helmholtz structure, thereby physically canceling the photothermal coherent noise generated by the absorption of light energy by the pool wall. This effectively solves the problems of weak gas signal and low excitation efficiency of single Raman technology, as well as the strong background thermal noise and airflow interference caused by wall absorption in traditional high-power photoacoustic detection. It provides an extremely compact, high-gain core sensing unit with extremely low background noise for multi-system joint detection.

[0030] (4) This invention constructs a photoacoustic-Raman joint full-range precision sensing architecture, adopts a photoacoustic and Raman spatiotemporal synchronous acquisition strategy and an adaptive threshold determination process, and organically combines the trace high sensitivity of photoacoustic spectroscopy with the wide concentration linear unsaturation characteristics of Raman spectroscopy. Under normal conditions, differential photoacoustic is used to monitor self-heating gas with high precision. When the background concentration determined by Raman spectroscopy changes drastically, the system automatically performs recalibration and decoupling processes, and the two complement each other. It effectively solves the bottleneck that a single-system detection system cannot take into account both large dynamic range and high sensitivity detection. In a single compact system, it realizes full-coverage online monitoring and precise graded early warning from ppm-level trace self-heating ignition gas to 100% full-range gas outburst gas. Attached Figure Description

[0031] Figure 1 A schematic diagram of the structure of the photoacoustic-Raman combined full-range multi-component coal mine gas high-sensitivity detection system of the present invention: 1-Host computer, 2-First excitation light source module, 3-Second excitation light source module, 4-First convex lens, 5-Second convex lens, 6-First optical window, 7-Second optical window, 8-Connecting tube, 9-First cavity, 10-Second cavity, 11-First microphone, 12-Second microphone, 13-Air inlet, 14-Air outlet, 15-First spherical reflection ring, 16-Second spherical reflection ring, 17-First spherical reflection mirror cavity, 18-Second spherical reflection mirror cavity, 19-Fresnel collimating lens, 20-Third convex lens, 21-Fiber optic cable, 22-Spectrometer, 23-Differential module, 24-Filtering module, 25-Amplification module, 26-Data acquisition card;

[0032] Figure 2 This is a schematic diagram of the photoacoustic-Raman high-performance coupled gas cell module of the present invention;

[0033] Figure 3 This is a flowchart of the photoacoustic-Raman combined full-range multi-component coal mine gas high-sensitivity detection method of the present invention. Detailed Implementation

[0034] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Furthermore, the technical features involved in the various embodiments of this invention described below can be combined with each other as long as they do not conflict with each other.

[0035] To address the problems of severe resonant frequency shift, high-concentration background spectral cross-interference, and high background noise in single-system applications of existing technologies for full-range multi-component gas detection in coal mines, this invention provides a photoacoustic-Raman combined high-sensitivity detection system and method for full-range multi-component coal mine gases. The aim is to achieve high signal-to-noise ratio, interference-free, and accurate full-range multi-component gas sensing under extreme operating conditions with large concentration variations by constructing a Helmholtz differentially coupled gas cell with an integrated spherical reflector ring, combined with a photoacoustic resonant frequency recalibration mechanism based on a wide-range Raman reference and a cross-interference decoupling method.

[0036] like Figure 1 As shown, the photoacoustic-Raman combined full-range multi-component coal mine gas high-sensitivity detection system (hereinafter referred to as the system) of the present invention includes: a host computer 1, a first excitation light source module 2, a second excitation light source module 3, a photoacoustic-Raman high-performance coupled gas cell module, and a combined signal acquisition and processing module including a spectrometer 22 and an acoustic-electric signal processing link.

[0037] The host computer 1 is used to control the first excitation light source module 2 and the second excitation light source module 3 to irradiate the photoacoustic-Raman high-performance coupled gas cell module, and to process the signals from the joint signal acquisition and processing module; the second excitation light source module 3 and the spectrometer 22 constitute a Raman detection unit, which is used to perform wide-range concentration detection of background gas; the first excitation light source module 2, the photoacoustic-Raman high-performance coupled gas cell module and the acoustic-electric signal processing link constitute a photoacoustic detection unit, which is used to perform highly sensitive detection of trace self-heating gas.

[0038] The host computer 1 performs dynamic recalibration of the resonant frequency: based on the background gas concentration in real time inverted by the Raman detection unit, it calculates the change in the equivalent sound velocity of the mixed gas caused by the change in gas composition; according to the preset mapping relationship between the change in equivalent sound velocity and the offset of photoacoustic resonant frequency, it dynamically recalibrates and controls the modulation frequency of the first excitation light source module 2 to match the changed acoustic resonant frequency in order to stably capture the photoacoustic signal.

[0039] The first excitation source module 2 is a photoacoustic excitation source for trace self-heating gases, preferably a tunable semiconductor laser (such as a distributed feedback DFB laser). Wavelength modulation scanning of the trace gas (such as C2H2 with a center wavelength around 1532nm) is achieved through current and temperature control. The output photoacoustic excitation beam is focused by the first convex lens 4 and then enters the first cavity 9 of the photoacoustic-Raman high-performance coupled gas cell module through the first optical window 6. The first excitation source module 2 preferably uses a near-infrared distributed feedback (DFB) laser or a mid-infrared quantum cascade (QCL) laser with a center wavelength covering the strong absorption peak of the self-heating gas being measured. The output power is preferably 5-50mW, the spectral linewidth is ≤2MHz, the wavelength tuning range covers the characteristic absorption peak of the target gas, and the bidirectional tuning margin based on the center wavelength of the absorption peak is not less than ±0.5nm. The modulation frequency matches the differential resonant frequency of the gas cell.

[0040] The second excitation source module 3 is a Raman excitation source for a methane background gas, preferably a high-power continuous laser (e.g., 532nm). Under normal conditions, it outputs continuous light to obtain a highly stable spontaneous Raman scattering signal. It can synchronously output a modulated beam with the same frequency and phase as the first excitation source module 2, canceling the photothermal noise caused by the first excitation source module 2. After being focused by the second convex lens 5, the output beam passes through the second optical window 7 and enters the second cavity 10 of the photoacoustic-Raman high-performance coupled gas cell module. The Raman excitation source is preferably a 532nm narrow-linewidth single-mode continuous laser, with an output power preferably of 200-1000mW, a spectral linewidth ≤0.005nm, a beam divergence angle ≤0.5mrad, and an output power stability ≤±1% / h.

[0041] The photoacoustic-Raman high-performance coupled gas cell module is a variant of the Helmholtz photoacoustic cell, forming a Helmholtz differential acoustic resonant structure. The main body is made of 304 / 316 stainless steel, and its overall sealing performance meets IP65 (an international protection rating standard consisting of two digits: the first digit 6 indicates dustproof level, and the second digit 5 ​​indicates waterproof level). The electrical and optical structures are intrinsically safe for use in coal mines, adapting to the application requirements of high humidity, high dust, and explosive gas environments in underground coal mines. Figure 1 , Figure 2As shown, the photoacoustic-Raman high-performance coupled gas cell module includes a first optical window 6 and a second optical window 7 for beam entry and exit. The first optical window 6 and the second optical window 7 are preferably quartz windows with double-sided antireflective coatings (AR films) or barium fluoride (BaF2) windows to reduce light energy loss and end-window thermal noise caused by window absorption. The main body of the photoacoustic-Raman high-performance coupled gas cell module includes a first cavity 9 and a second cavity 10 arranged in parallel and symmetrically, and a connecting pipe 8 connecting the two cavities; and is equipped with an inlet 13 and an outlet 14 for gas flow. The first cavity 9 and the second cavity 10 adopt a completely symmetrical cylindrical structure, with an inner diameter preferably of 15-30 mm and a cavity length preferably of 20-40 mm. The connecting pipe 8 connecting the two cavities has an inner diameter preferably of 3-6 mm and a length preferably of 10-20 mm. The size matching design of the first cavity 9 and the second cavity 10 with the connecting pipe 8 ensures that the first-order differential resonant frequency of the photoacoustic-Raman high-performance coupled gas cell module falls within the acoustic low-frequency noise reduction range of 1 kHz-4 kHz, and the acoustic quality factor Q value is ≥50. The side walls of the first cavity 9 and the second cavity 10 are respectively provided with a first spherical reflection ring 15 and a second spherical reflection ring 16, and the interiors of the first cavity 9 and the second cavity 10 are respectively provided with a first spherical reflection mirror cavity 17 and a second spherical reflection mirror cavity 18. The inner surfaces of the first spherical reflecting ring 15, the second spherical reflecting ring 16, the first spherical reflecting mirror cavity 17, and the second spherical reflecting mirror cavity 18 are all coated with high-reflectivity optical thin films (such as gold-plated films or multilayer dielectric high-reflectivity films), which can maximize the internal reflection efficiency of photoacoustic excitation light and Raman excitation light, respectively. The radius of curvature of the inner surfaces of the first spherical reflecting ring 15 and the second spherical reflecting ring 16 is preferably 20-50 mm, and the reflectivity of photoacoustic excitation light and Raman excitation light is ≥98%; the incident angle of the excitation light is preferably 5°-15°, so that the beam forms more than 10 reflections and self-convergences within the first spherical reflecting ring 15 and the second spherical reflecting ring 16, and the optical path magnification is ≥10 times.

[0042] The first cavity 9 is used to inject a photoacoustic excitation beam. The photoacoustic excitation beam undergoes multiple reflections within the first spherical reflection ring 15 of the first cavity 9, forming a centrally converging reflective plane light, which greatly increases the absorption optical path of the gas under test to enhance the excitation of the resonant photoacoustic signal.

[0043] The second cavity 10 is used to inject a Raman excitation beam. The Raman excitation beam is also reflected multiple times within the second spherical reflection ring 16 of the second cavity 10 to form a centrally converging reflective plane light to enhance Raman excitation. At the same time, the second excitation light source module 3 is controlled to emit a modulated beam with the same frequency and phase as the first excitation light source module 2, which is injected into the second cavity 10 as an anti-phase excitation source. Acoustic interference cancellation is formed in the Helmholtz differential structure to cancel the photothermal coherent noise generated by the absorption of the pool wall.

[0044] To construct a Helmholtz differential structure with completely symmetrical acoustic impedance, the cavity geometry and internal structure of the first spherical mirror cavity 17 and the second spherical mirror cavity 18 are completely identical to achieve acoustic geometric symmetry, thereby maximizing the suppression effect of the differential structure on common-mode noise. The first spherical mirror cavity 17 and the second spherical mirror cavity 18 have no optical gain effect on the resonant photoacoustic signal. A Raman signal collecting optical path is arranged below the second spherical mirror cavity 18 at the second cavity 10, and the Raman signal collecting optical path includes a Fresnel collimating lens 19. The Raman excitation beam is self-converged after multiple internal reflections by the second spherical reflection ring 16, forming a laser focus at the center of the second cavity 10. Among the omnidirectional spontaneous Raman scattered light generated at this focus, the upward scattered light towards the second spherical reflection mirror cavity 18 is reflected by the spherical reflection surface of the second spherical reflection mirror cavity 18 and returns downward along the original path. It merges with the downward divergent Raman scattered light propagating towards the Fresnel collimating lens 19, and together they are collimated into parallel light by the Fresnel collimating lens 19 and emitted. Then, it is focused and coupled into the optical fiber 21 by the third convex lens 20, and finally transmitted to the spectrometer 22 through the optical fiber 21.

[0045] The joint signal acquisition and processing module includes a spectrometer 22 at the rear end of the Raman scattering signal collection optical path (laser focus in the second cavity 10 → second spherical mirror cavity 18 → Fresnel collimating lens 19 → third convex lens 20 → optical fiber 21 → spectrometer 22), and a first microphone 11 and a second microphone 12 respectively sealed and symmetrically positioned on the upper and lower sides of the sidewalls of the first cavity 9 and the second cavity 10. The first microphone 11 and the second microphone 12 are preferably high-sensitivity electret condenser microphones, optical microphones, or microelectromechanical systems (MEMS) condenser microphones; the sound pressure signals acquired by the two microphones are sequentially input to the differential module 23, the filtering module 24, and the amplification module 25, and finally converted from analog to digital by the data acquisition card 26 and transmitted to the host computer 1. The host computer 1 uses digital phase-locked loop technology to perform phase-locked demodulation of the signal input from the data acquisition card 26, and performs photoacoustic resonance frequency recalibration, cross-interference decoupling, and full-range multi-component gas concentration inversion based on the Raman detection results. The first microphone 11, the second microphone 12, the differential module 23, the filter module 24, the amplification module 25, and the data acquisition card 26 constitute the acoustic-electric signal processing link.

[0046] like Figure 3 As shown, the photoacoustic-Raman combined full-range multi-component coal mine gas high-sensitivity detection method of the present invention includes:

[0047] Step 1, System initialization and gas introduction: This invention is implemented under a constant operating temperature of 25℃. The system is turned on and running. The coal mine mixed gas to be tested is introduced into the first chamber 9 and the second chamber 10 of the photoacoustic-Raman high-performance coupled gas pool module through the inlet 13 at a stable flow rate of preferably 500mL / min. It is discharged through the outlet 14. After the gas replacement is completed, the detection is started.

[0048] Step 2, Raman wide-range detection and threshold determination: The second excitation light source module 3 outputs highly stable continuous light, which is incident on the second cavity 10 through the second convex lens 5 and the second optical window 7. The Raman scattering signal is enhanced by the self-converging of the second spherical reflection ring 16 and efficiently collected and transmitted to the spectrometer 22 via the second spherical reflection mirror cavity 18, Fresnel collimating lens 19, third convex lens 20, and optical fiber 21. The host computer 1 inverts the concentration of the background gas (e.g., CH4) based on the wide-range linear detection capability of the Raman spectrum and quickly determines whether the concentration of the background gas exceeds the gas outburst detection threshold. The gas outburst detection threshold is set to a CH4 concentration ≥ 1%, which can be flexibly adjusted according to the actual working conditions of the mine. If the gas outburst detection threshold is not exceeded, the initially calibrated resonance frequency is used as the modulation frequency of the first excitation light source module 2, and step 4 is executed without additional photoacoustic resonance frequency recalibration. If the gas outburst detection threshold is exceeded, step 3 is executed.

[0049] Step 3, Dynamic recalibration of photoacoustic resonance frequency: Based on the concentration of the background gas (e.g., CH4) obtained from Step 2, and combined with the concentrations of each component in the mixed gas synchronously acquired by Raman spectroscopy, the host computer 1 calculates the equivalent sound velocity of the current mixed gas based on the theory of sound velocity propagation in mixed gas media. The calculation formula is as follows:

[0050] ;

[0051] in, The equivalent speed of sound for the gas mixture. The thermodynamic temperature within the photoacoustic-Raman high-performance coupled gas cell module (this invention is implemented at 25°C, i.e., 298.15K). The number of components in the gas mixture. Let be the concentration of the i-th gas obtained from the Raman spectroscopy. Let be the adiabatic coefficient of the i-th gas. Let be the specific gas constant of the i-th gas. Let be the molar mass of the i-th gas; further, based on the pre-established linear proportional mapping model of photoacoustic resonance frequency offset and gas sound velocity change under constant temperature and pressure, the photoacoustic resonance frequency is calculated and recalibrated, thereby locking the differential resonance mode of the system; at the same time, based on the pre-established approximate linear relationship between photoacoustic resonance frequency offset and trace gas sound pressure amplitude, the preliminary sound pressure amplitude synchronously acquired by the first microphone 11 and the second microphone 12 and after differential preprocessing is corrected.

[0052] Step 4, Cross-interference Decoupling and Precise Photoacoustic Detection: The first excitation source module 2 emits a photoacoustic excitation beam modulated with the currently recalibrated photoacoustic resonance frequency, which enters the first cavity 9 through the first convex lens 4 and the first optical window 6. The photoacoustic excitation beam undergoes multiple reflections within the first spherical reflection ring 15, increasing the absorption optical path of the gas under test and significantly enhancing the resonant photoacoustic signal. Simultaneously, the second excitation source module 3 is controlled to emit a Raman excitation beam modulated with the same frequency and phase, which enters the second cavity 10 as an anti-phase excitation source. Acoustic interference cancellation is formed in the completely symmetrical Helmholtz differential structure, canceling the photothermal coherent noise of the pool wall. The first microphone 11 and the second microphone 12 synchronously pick up the sound pressure signals in the first cavity 9 and the second cavity 10. The two sound pressure signals pass through the differential module 23, the filtering module 24, and the amplification module 25 in sequence. The acoustic symmetry of the structure is used to maximize the suppression of incoherent common-mode noise and amplify the differential-mode signal. Finally, the data is input to the host computer 1 by the data acquisition card 26 for phase-locked demodulation.

[0053] For high-concentration background gases under gas outburst conditions, the host computer 1 calculates the actual absorption coefficient of the background gas in the trace target gas detection band based on the concentration of the background gas (such as CH4) obtained by inversion through the wide concentration linear detection capability of Raman spectroscopy in step 2, combined with the internationally recognized HITRAN (High Resolution Transmission Molecular Absorption Database) gas absorption database, based on the linear proportional relationship between the high-concentration gas concentration and its own absorption coefficient; the specific physical decoupling model is as follows:

[0054] Let the total amplitude of the differential optical-acoustic signal be S. total S contributed by trace target gas photoacoustic trace Cross-interference contribution S from high-concentration background gases bg It is formed by superposition, that is, S total =S trace +S bg Based on the concentration φ of the background gas (e.g., CH4) obtained in step 2... bg Combined with the pre-calibrated background gas absorption response coefficient K in the target detection band bg And the system constant C, to estimate the cross-interference contribution S of high-concentration background gases. bg =C×K bg ×φ bgThe system constant C is a fixed coefficient obtained beforehand through standard gas calibration, related to the photoacoustic cell structure, excitation light power, gain of the two microphones, and lock-in amplification parameters. The contribution S from the cross-interference of this high-concentration background gas is also considered. bg The sound pressure contribution of the background gas in the total differential photoacoustic signal was calculated, and the background gas was physically separated from the total signal to decouple the actual trace gas photoacoustic signal amplitude S. trace =S total -S bg .

[0055] Step 5, Concentration Inversion and Graded Early Warning: The host computer 1 uses the actual trace gas photoacoustic signal amplitude S obtained after decoupling in steps 3 and 4. trace (Pure sound pressure amplitude) accurately reflects the true concentration of trace self-heating gases (such as C2H2, C2H4, etc.);

[0056] Step 6: Combining the background gas (e.g., CH4) concentration retrieved in Step 2 with the actual concentration data of trace self-heating gases (e.g., C2H2, C2H4, etc.) obtained from the decoupled pure photoacoustic signal retrieval in Step 5, determine whether the gas concentration exceeds the limit and trigger the corresponding level of coal mine gas outburst or spontaneous combustion safety warning. If the gas concentration does not exceed the limit, the system returns to Step 2 and continues continuous online monitoring.

[0057] To further illustrate the practical application effect of the present invention, a specific working scenario application example is provided below: The background methane (CH4) concentration in the coal mine is abruptly changed from 1% to 80% due to gas anomaly, while it is necessary to accurately detect 5 ppm (parts per million) of acetylene (C2H2) generated by early spontaneous combustion. During system operation, the system first uses a Raman wide-range detection link consisting of the second excitation light source module 3, spectrometer 22, and host computer 1 to measure in real time the CH4 concentration soaring to 80%. The host computer 1 immediately calculates the change in the equivalent sound velocity of the mixed gas based on this concentration change and automatically locks the modulation frequency of the photoacoustic excitation light from the normal photoacoustic resonance frequency to the new offset resonance frequency. Then, using the 80% CH4 concentration data, the host computer 1 calculates the contribution of CH4 to the cross-interference of high-concentration background gas in the 1532nm band of C2H2 using a physical decoupling model and subtracts it from the total amplitude of the differential photoacoustic signal. Finally, the system accurately inverts the true concentration of C2H2 of 5ppm, which is not affected by the high-concentration background, and triggers a dual early warning of high-concentration gas and spontaneous combustion.

[0058] The above description is merely an embodiment of the present invention and does not limit the scope of the invention. Any equivalent structural or procedural transformations made based on the description and drawings of this invention, or direct or indirect applications in other related system fields, are similarly included within the protection scope of this invention. Contents not described in detail in this specification are prior art known to those skilled in the art.

Claims

1. A photoacoustic-Raman combined full-range multi-component coal mine gas high-sensitivity detection system, characterized in that, include: The system includes a host computer, a first excitation source module, a second excitation source module, a photoacoustic-Raman high-performance coupled gas cell module, and a joint signal acquisition and processing module that includes a spectrometer and an acoustic-electric signal processing link. The host computer is used to control the first excitation light source module and the second excitation light source module to irradiate the photoacoustic-Raman high-performance coupled gas cell module, and to process the signals from the joint signal acquisition and processing module; The second excitation source module and the spectrometer constitute a Raman detection unit, which is used to perform wide-range concentration detection of background gas. The first excitation light source module, the photoacoustic-Raman high-performance coupled gas cell module, and the acoustic-electric signal processing link constitute the photoacoustic detection unit, which is used to perform highly sensitive detection of trace self-heating gases. The host computer performs dynamic recalibration of the resonant frequency: based on the background gas concentration inverted in real time by the Raman detection unit, it calculates the change in the equivalent sound velocity of the mixed gas caused by the change in gas composition; according to the preset mapping relationship between the change in equivalent sound velocity and the offset of photoacoustic resonant frequency, it dynamically recalibrates and controls the modulation frequency of the first excitation light source module to match the changed acoustic resonant frequency in order to stably capture the photoacoustic signal. The photoacoustic-Raman high-performance coupled gas cell module includes a first cavity and a second cavity arranged in parallel and symmetrically, with an inlet and an outlet for gas flow; the first cavity and the second cavity are respectively provided with a first spherical mirror cavity and a second spherical mirror cavity; the first cavity and the second cavity are connected by a connecting pipe, and together with the symmetrically arranged first spherical mirror cavity and the second spherical mirror cavity, they form a Helmholtz differential acoustic resonance structure; The gas to be tested enters through the inlet, flows through the first chamber and the second chamber, and finally exits through the outlet. The sidewalls of the first cavity and the second cavity are respectively provided with a first spherical reflection ring and a second spherical reflection ring; the photoacoustic excitation beam enters the first cavity through the first optical window and is reflected multiple times in the first spherical reflection ring; the Raman excitation beam enters the second cavity through the second optical window and is reflected multiple times in the second spherical reflection ring. Raman scattered light generated at the center of the second cavity is partially reflected by the second spherical mirror cavity, collimated by the Fresnel collimating lens along with the directly propagating light, and then focused by the third convex lens before being coupled into the optical fiber and transmitted to the spectrometer through the optical fiber. The first and second microphones respectively collect the sound pressure signals inside their respective cavities and output them to the external differential module.

2. The photoacoustic-Raman combined full-range multi-component coal mine gas high-sensitivity detection system according to claim 1, characterized in that, The first excitation source module is a photoacoustic excitation source for trace self-heating gases. It is a tunable semiconductor laser. The wavelength modulation scanning of the trace gas is achieved through current and temperature control. The output photoacoustic excitation beam is focused by the first convex lens and then enters the first cavity of the photoacoustic-Raman high-performance coupled gas cell module through the first optical window.

3. The photoacoustic-Raman combined full-range multi-component coal mine gas high-sensitivity detection system according to claim 2, characterized in that, The second excitation source module is a Raman excitation source for the background gas, which synchronously outputs a modulated beam with the same frequency and phase as the first excitation source module; the output beam is focused by the second convex lens and enters the second cavity of the photoacoustic-Raman high-performance coupled gas cell module through the second optical window.

4. The photoacoustic-Raman combined full-range multi-component coal mine gas high-sensitivity detection system according to claim 1, characterized in that, The first spherical mirror cavity and the second spherical mirror cavity have completely identical cavity geometry and internal structure to maximize the suppression effect of the differential structure on common-mode noise. The first spherical mirror cavity and the second spherical mirror cavity have no optical gain effect on the resonant photoacoustic signal. A Raman signal collection optical path is set below the second spherical mirror cavity in the second cavity.

5. The photoacoustic-Raman combined full-range multi-component coal mine gas high-sensitivity detection system according to claim 4, characterized in that, The Raman signal collection optical path includes a Fresnel collimating lens. The Raman excitation beam undergoes multiple internal reflections through the second spherical reflecting ring and converges to form a laser focus at the center of the second cavity. Among the omnidirectional spontaneous Raman scattered light generated at the laser focus, the upward scattered light towards the second spherical reflecting mirror cavity is reflected by the spherical reflecting surface of the second spherical reflecting mirror cavity and returns downward along the original path. It merges with the downward diverging Raman scattered light propagating towards the Fresnel collimating lens and is collimated into parallel light by the Fresnel collimating lens before exiting. Subsequently, it is focused and coupled into the optical fiber by the third convex lens.

6. The photoacoustic-Raman combined full-range multi-component coal mine gas high-sensitivity detection system according to claim 1, characterized in that, The inner surfaces of the first spherical reflection ring, the second spherical reflection ring, the first spherical reflection mirror cavity, and the second spherical reflection mirror cavity are all coated with high-reflectivity optical thin films, which are used to maximize the internal reflection efficiency of photoacoustic excitation light and Raman excitation light, respectively.

7. The photoacoustic-Raman combined full-range multi-component coal mine gas high-sensitivity detection system according to claim 4, characterized in that, The joint signal acquisition and processing module includes a spectrometer at the rear end of the Raman scattering signal collection optical path, and a first microphone and a second microphone respectively sealed and symmetrically positioned on the upper and lower sides of the first and second cavities. The sound pressure signals acquired by the first microphone and the second microphone are sequentially input to the differential module, the filtering module, and the amplification module, and finally converted from analog to digital by the data acquisition card and transmitted to the host computer. The host computer performs phase-locked demodulation of the signal input from the data acquisition card using digital phase-locked technology, and performs photoacoustic resonance frequency recalibration, cross-interference decoupling, and full-range multi-component gas concentration inversion based on the Raman detection results.

8. A photoacoustic-Raman combined high-sensitivity detection method for full-range multi-component coal mine gas, employing the photoacoustic-Raman combined high-sensitivity detection system for full-range multi-component coal mine gas as described in any one of claims 1 to 7, characterized in that, include: Step 1, System Initialization and Gas Introduction: Under constant operating temperature conditions, the system is powered on and the coal mine mixed gas to be tested is introduced into the first and second chambers of the photoacoustic-Raman high-performance coupled gas pool module through the inlet at a stable flow rate, and discharged through the outlet. After the gas replacement is completed, the detection is started. Step 2, Raman wide-range detection and threshold determination: The second excitation light source module outputs highly stable continuous light, which is incident on the second cavity through the second convex lens and the second optical window; the Raman scattering signal is enhanced by the self-convergence of the second spherical reflection ring, and is collected and transmitted to the spectrometer with high efficiency through the second spherical reflection mirror cavity, Fresnel collimating lens, third convex lens and optical fiber; the host computer inverts the concentration of the background gas based on the wide concentration linear detection capability of Raman spectrum, and quickly determines whether the concentration of the background gas exceeds the gas outburst detection threshold; If the gas outburst detection threshold is not exceeded, the initially calibrated resonant frequency is used as the modulation frequency of the first excitation light source module, and step 4 is executed; if the gas outburst detection threshold is exceeded, step 3 is executed. Step 3, dynamic recalibration of photoacoustic resonance frequency: The host computer calculates the equivalent sound velocity of the current mixed gas based on the background gas concentration obtained from Step 2. Based on a pre-established linear proportional mapping model of photoacoustic resonant frequency offset and gas sound velocity change under constant temperature and pressure, the photoacoustic resonant frequency is calculated and recalibrated, thereby locking the differential resonant mode of the system. At the same time, based on the pre-established approximate linear relationship between photoacoustic resonant frequency offset and trace gas sound pressure amplitude, the preliminary sound pressure amplitude after synchronous acquisition by the first and second microphones and differential preprocessing is corrected. Step 4, Cross-interference decoupling and precise photoacoustic detection: The first excitation light source module emits a photoacoustic excitation beam modulated with the initially calibrated photoacoustic resonance frequency or the recalibrated photoacoustic resonance frequency, which enters the first cavity through the first convex lens and the first optical window; The first and second microphones simultaneously pick up the sound pressure signals in the first and second cavities. The two sound pressure signals pass through the differential module, the filtering module and the amplification module in sequence. The acoustic symmetry of the structure is used to maximize the suppression of incoherent common-mode noise and amplify the differential-mode signal. Finally, the data acquisition card inputs the signal to the host computer for phase-locked demodulation. Step 5, Concentration Inversion and Graded Early Warning: The host computer uses the actual trace gas photoacoustic signal amplitude S obtained after decoupling in Step 4. trace It accurately reflects the true concentration of trace self-heating gases; Step 6: Combine the background gas concentration retrieved in Step 2 with the actual concentration of trace self-heating gas retrieved in Step 5 to determine whether the gas concentration exceeds the limit and trigger the corresponding level of coal mine gas outburst or spontaneous combustion safety warning; if the gas concentration does not exceed the limit, the system returns to Step 2 and continues to perform continuous online monitoring.

9. The photoacoustic-Raman combined high-sensitivity detection method for full-range multi-component coal mine gas according to claim 8, characterized in that, The equivalent speed of sound for the current gas mixture is: ； in, The equivalent speed of sound for the gas mixture. The thermodynamic temperature within the photoacoustic-Raman high-performance coupled gas cell module. The number of components in the gas mixture. Let be the concentration of the i-th gas obtained from the Raman spectroscopy. Let be the adiabatic coefficient of the i-th gas. Let be the specific gas constant of the i-th gas. Let be the molar mass of the i-th gas.

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