TDLAS (Tunable Diode Laser Absorption Spectroscopy) detection method and system for inhibiting cross spectrum interference of methane and ethane

Abstract

The invention relates to the technical field of gas detection, and particularly discloses a TDLAS (Tunable Diode Laser Absorption Spectroscopy) detection method for inhibiting methane and ethane cross spectrum interference, which comprises the following steps: S1, providing a first TDLAS laser and a second TDLAS laser; s2, controlling the first TDLAS laser to enable the central wavelength of output light of the first TDLAS laser to be stabilized at a first target wavelength; a second TDLAS laser is controlled, so that the central wavelength of output light of the second TDLAS laser is stabilized at a second target wavelength; wherein the sum corresponds to different absorption lines of methane and ethane, respectively; by effectively inhibiting cross interference: selecting a specific wavelength combination of 1654nm and 1683nm, utilizing the absorption cross section difference of methane and ethane at the two wavelengths and combining a linear equation set for resolving, respective concentrations can be accurately separated from superposed absorption signals, and the cross interference is fundamentally inhibited.

Description

Technical Field

[0001] This invention relates to the field of laser gas detection technology, specifically a TDLAS detection method and system for suppressing cross-spectral interference between methane and ethane. Background Technology

[0002] Tunable semiconductor laser absorption spectroscopy is a highly sensitive and selective gas detection technology. Its basic principle is: by controlling the current and temperature of the laser, its output wavelength scans through the specific absorption lines of the gas to be measured. According to Beer-Lambert's law, the concentration of the gas can be deduced by detecting the degree of attenuation of the laser intensity.

[0003] In the analysis of major components of natural gas, process monitoring, and leak detection, methane and ethane are two key gases to be measured. However, the absorption lines of methane and ethane in the near-infrared band are adjacent or even overlapping, especially under atmospheric or near-atmospheric pressure conditions, where pressure broadening of the spectral lines exacerbates this cross-interference. When using TDLAS technology to detect a mixture of methane and ethane, the absorption signal measured at a single laser wavelength is usually a superposition of the absorptions of both gases. This cross-spectral interference significantly reduces measurement accuracy and can even lead to erroneous concentration readings.

[0004] In existing technologies, two methods are commonly used to solve the cross-interference problem: one is to find completely independent and interference-free absorption lines, but for methane and ethane, it is difficult to find such ideal spectral lines with sufficient absorption intensity near common communication bands; the other is to use complex spectral fitting algorithms, but this requires wide-range wavelength scanning and high-performance computing units, which is costly and has poor real-time performance.

[0005] Therefore, there is an urgent need in this field for a TDLAS detection solution that can effectively suppress cross-interference between methane and ethane, while also possessing high accuracy, high real-time performance, and moderate cost. Summary of the Invention

[0006] The purpose of this invention is to solve the above-mentioned technical problems and provide a TDLAS detection method and system for suppressing cross-spectral interference between methane and ethane.

[0007] To achieve the above objectives, the present invention employs the following technical solutions;

[0008] The technical solution adopted by this invention to solve its technical problem is: a TDLAS detection method for suppressing cross-spectral interference between methane and ethane, comprising the following steps:

[0009] S1: Provides the first TDLAS laser and the second TDLAS laser;

[0010] S2: Control the first TDLAS laser to stabilize the center wavelength of its output light at the first target wavelength. Control the second TDLAS laser to stabilize the center wavelength of its output light at the second target wavelength. ;in, and These correspond to different absorption lines for methane and ethane, respectively.

[0011] S3: Superimpose the scanning signal and the modulation signal onto the drive currents of the first TDLAS laser and the second TDLAS laser, respectively, so that the output wavelengths of the first TDLAS laser and the second TDLAS laser are respectively at... and Scanning and modulation are performed nearby;

[0012] S4: Pass the output light from the first TDLAS laser and the second TDLAS laser through the methane and ethane mixture to be tested, and detect the light signals after absorption by the gas to obtain the first detection signal. Second detection signal ;

[0013] S5: Based on the first detection signal Second detection signal The concentration of methane was separated by the solution model. Concentrations of methane and ethane Ethane; where the solution model is based on methane and ethane at wavelengths and The differences in absorption characteristics at different locations were established.

[0014] A TDLAS gas detection system for implementing the TDLAS detection method for suppressing cross-spectral interference between methane and ethane as described in any one of the above-mentioned methods, comprising:

[0015] First TDLAS laser and second TDLAS laser;

[0016] A laser control module is configured to control the temperature and drive current of the first TDLAS laser and the second TDLAS laser, and to generate the scanning signal and modulation signal.

[0017] A gas chamber is used to contain the methane and ethane mixture.

[0018] A photodetector is used to receive light signals passing through the air chamber and convert them into electrical signals;

[0019] The signal processing module is configured to perform steps S4 and S5 to extract the first detection signal from the electrical signal. Second detection signal , and calculate the concentration Cmethane of the methane and the concentration Cethane of the ethane.

[0020] A computer-readable storage medium, on which a computer program is stored. When the program is executed by a processor, the steps S5 in any one of the above methods are implemented.

[0021] The TDLAS detection method and system for suppressing the cross-spectrum interference of methane and ethane provided by the present invention have the following beneficial effects:

[0022] 1. The present invention effectively suppresses cross interference: By selecting a specific wavelength combination of 1654 nm and 1683 nm, and using the absorption cross-section difference of methane and ethane at these two wavelengths, combined with the solution of a system of linear equations, the respective concentrations can be accurately separated from the superimposed absorption signals, fundamentally suppressing cross interference;

[0023] 2. The present invention has high precision and high sensitivity: By adopting wavelength modulation spectroscopy technology, by detecting the second harmonic signal and shifting the signal spectrum to the high-frequency region, the baseband 1 / f noise and environmental background interference are effectively suppressed, thereby achieving extremely high detection sensitivity and signal-to-noise ratio;

[0024] 3. Good real-time performance: Compared with complex spectral fitting algorithms, the linear equation solving model adopted by the present invention has a small computational amount and a fast response speed, and can meet the real-time requirements of industrial on-line monitoring;

[0025] 4. High cost-effectiveness: The solution of the present invention does not require a laser with a wide tuning range or an expensive computing unit. While achieving high performance, it controls the system cost and complexity, which is conducive to popularization and application. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] The following further describes in detail the specific embodiments of the present invention with reference to the drawings;

[0027] Figure 1 is a schematic structural diagram of the TDLAS gas detection system described in the present invention;

[0028] Figure 2 is a simulated absorption spectral line diagram of methane and ethane in the 1650 nm - 1690 nm band, showing the wavelength positions selected by the present invention;

[0029] Figure 3 is a flowchart of the method of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0030] It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0031] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0032] It should be noted that in the embodiments of the present invention, all directional indications (such as up-down-left-right-forward-backward...) are only used to explain the relative positional relationship and movement between the components in a certain specific posture (as shown in the figure). If the specific posture changes, the directional indication will also change accordingly. The connection can be a direct connection or an indirect connection.

[0033] Please see Figure 1-3 As shown, a TDLAS detection method for suppressing cross-spectral interference between methane and ethane includes the following steps:

[0034] S1: Provides the first TDLAS laser and the second TDLAS laser;

[0035] S2: Control the first TDLAS laser to stabilize the center wavelength of its output light at the first target wavelength. Control the second TDLAS laser to stabilize the center wavelength of its output light at the second target wavelength. ;in, and These correspond to different absorption lines for methane and ethane, respectively.

[0036] S3: Superimpose the scanning signal and the modulation signal onto the drive currents of the first TDLAS laser and the second TDLAS laser, respectively, so that the output wavelengths of the first TDLAS laser and the second TDLAS laser are respectively at... and Scanning and modulation are performed nearby;

[0037] S4: Pass the output light from the first TDLAS laser and the second TDLAS laser through the methane and ethane mixture to be tested, and detect the light signals after absorption by the gas to obtain the first detection signal. Second detection signal ;

[0038] S5: Based on the first detection signal Second detection signal The concentration of methane was separated by the solution model. Concentrations of methane and ethane Ethane; where the solution model is based on methane and ethane at wavelengths and establish the absorption characteristic difference at

[0039] The first target wavelength is 1654 nm, and the second target wavelength is 1683 nm.

[0040] In step S3, by controlling the amplitude of the scanning signal, the wavelength scanning ranges of the first TDLAS laser and the second TDLAS laser reach 300 - 500 pm.

[0041] The wavelength scanning range is 400 pm.

[0042] In step S3, by controlling the amplitude of the modulation signal, the wavelength modulation depth is set to match the pressure broadening widths of the absorption lines of methane and ethane in the mixed gas.

[0043] In step S4, the wavelength modulation spectroscopy technique is adopted, and the first detection signal and the second detection signal are the peaks of the second - harmonic signals.

[0044] The solution model in step S5 is a system of linear equations:

[0045] S1 = k 11 * Cmethane + k 12 * Cethane

[0046] S2 = k 21 * Cmethane + k 22 * Cethane

[0047] where are coefficients预先 determined through calibration experiments.

[0048] A TDLAS gas detection system for implementing the TDLAS detection method for suppressing cross - spectral interference of methane and ethane described in any one of the above, comprising:

[0049] A first TDLAS laser and a second TDLAS laser;

[0050] A laser control module configured to control the temperature and drive current of the first TDLAS laser and the second TDLAS laser, and generate the scanning signal and the modulation signal;

[0051] A gas chamber for accommodating the methane and ethane mixed gas;

[0052] A photodetector for receiving the optical signal passing through the gas chamber and converting it into an electrical signal;

[0053] A signal processing module, configured to execute the steps S4 and S5, and extract the first detection signal from the electrical signal and the second detection signal , and calculate the concentration Cmethane of methane and the concentration Cethane of ethane.

[0054] A computer-readable storage medium, on which a computer program is stored, and when the program is executed by a processor, the step S5 in any one of the above methods is implemented.

[0055] 1. System hardware composition

[0056] As Figure 1 shown, the system includes:

[0057] Laser component: A first DFB laser with a central wavelength of 1654 nm; a second DFB laser with a central wavelength of 1683 nm. The lasers are installed in a TO-Can package with a thermoelectric cooler and a thermistor.

[0058] Laser control module: This module integrates:

[0059] Temperature control circuit: Based on the proportional-integral-derivative control algorithm, the temperature of the laser die is stabilized within the range of ±0.01 °C to ensure that the central wavelength drift is less than 1 pm.

[0060] Drive circuit: Provide a low-noise DC bias current with a range of 0 - 150 mA.

[0061] Waveform generation circuit: Generate a triangular wave with a frequency of 50 Hz and a peak-to-peak voltage adjustable from 0 - 100 mV as a scanning signal; and a sine wave with a frequency of 50 kHz and a peak-to-peak voltage adjustable from 0 - 20 mV as a modulation signal. An analog adder is used to superimpose the DC bias, the scanning signal, and the modulation signal.

[0062] Optical gas chamber: It is a multi-reflection White cell with an effective optical path of 10 meters and filled with the待测混合气体 (to-be-detected mixed gas) at about 1 atmospheric pressure.

[0063] Photodetector: An indium gallium arsenide photodiode is used, and its 3dB bandwidth is greater than 2 MHz.

[0064] Signal processing module: Includes a lock-in amplifier and an embedded microprocessor.

[0065] 2. Core principle and model construction

[0066] The ability of the present invention to suppress cross-interference stems from a profound understanding of the physical principle of absorption spectroscopy and the combination of a targeted mathematical calculation model. The core is as follows:

[0067] Physical Basis: The superposition of absorption signals. According to Beer-Lambert's law, the absorptivity of laser light passing through a gas mixture is the superposition of the absorptions of each component. In wavelength modulation spectroscopy, we use the second harmonic peak signal S as a characterization of concentration. Therefore, at the first laser wavelength... At (1654nm), the total signal It is a linear superposition of the absorption contributions of methane and ethane at this point, which can be preliminarily described as:

[0068]

[0069] Where α is the absorption coefficient

[0070] Similarly, at the second laser wavelength At (1683nm), we have:

[0071]

[0072] Mathematical Modeling: Establishing a System of Linear Equations. To transform the above physical relationships into a computable model, we introduce a set of sensitivity coefficients k, thereby constructing a system of two linear equations in two variables:

[0073]

[0074] in, This represents the sensitivity of methane in channel 1. This represents the sensitivity of ethane in channel 1 (i.e., cross-interference). This represents the sensitivity of methane in channel 2 (i.e., cross-interference). The sensitivity of ethane in channel 2 is represented. This model quantitatively describes the presence of cross-interference and provides a mathematical basis for concentration separation.

[0075] 3. Workflow and Parameter Settings

[0076] When the system is working, follow these steps and parameters:

[0077] Step S1: Wavelength stabilization. The temperature of the first laser is set to 25.0°C, corresponding to its emission wavelength of 1654nm, through the temperature control circuit; the temperature of the second laser is set to 30.5°C, corresponding to its emission wavelength of 1683nm.

[0078] Step S2: Current Modulation and Scanning. A DC bias current of 65mA is applied to the first laser, and a DC bias current of 70mA is applied to the second laser. Subsequently, by adjusting the output of the waveform generation circuit, a 50Hz triangular wave scanning signal with a peak-to-peak voltage of 80mV and a 50kHz sine wave modulation signal with a peak-to-peak voltage of 15mV are superimposed onto the drive currents of the two lasers, respectively. This configuration results in a scanning range of approximately 400pm and a modulation depth of approximately 50pm for the output wavelength of each laser, which is well matched to the broadening of the methane and ethane absorption lines at 1 atmosphere.

[0079] Step S3: Signal Detection and Extraction. The transmitted light after passing through the gas chamber is received by the detector. The lock-in amplifier extracts and outputs the amplitude of the second harmonic (100kHz) of the 50kHz signal as a reference frequency, which serves as the first detection signal. Second detection signal Real-time data stream.

[0080] Step S4: Coefficient Calibration (performed before leaving the factory). Coefficients Predetermined through experimental calibration. For example, using a 1.0% vol concentration of pure methane standard gas, the measurement... Methane = 1050mV Methane = 45mV. Calculated... =1050, =45. The value was measured using 0.5% vol pure ethane standard gas. Ethane = 80mV Ethane = 980mV. Calculated... =160, =1960 (all units are mV / %).

[0081] Step S5: Concentration Calculation (Real-time Execution); The microprocessor captures [the concentration] in each scan cycle. The peak value is obtained and substituted into the following equation for solution:

[0082]

[0083] This solution process is mathematically equivalent to actively subtracting the interference of ethane on the methane channel from the mixed signal (Formula 1); Item B) and the interference of methane on the ethane channel ( (Item A), thus achieving precise concentration separation.

[0084] Through the above specific implementation scheme, the present invention achieves the following beneficial effects: In the test of a methane-ethane mixture with a total concentration of 2%, the method of the present invention reduces the cross-interference of ethane to methane measurement from about 15% in the traditional single-wavelength measurement to less than 1%, which significantly improves the detection accuracy.

[0085] In the description of this specification, references to terms such as "an embodiment," "example," "specific example," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the invention. In this specification, illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0086] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the claimed invention.

Claims

1. A TDLAS detection method for inhibiting cross-spectral interference of methane and ethane, characterized in that; The method comprises the following steps: S1: providing a first TDLAS laser and a second TDLAS laser; S2: controlling the first TDLAS laser such that the center wavelength of the output light is stabilized at a first target wavelength ; controlling the second TDLAS laser such that the center wavelength of the output light is stabilized at a second target wavelength ; wherein, and correspond to different absorption lines of methane and ethane, respectively; S3: superimpose the scanning signal and the modulation signal on the driving current of the first TDLAS laser and the second TDLAS laser respectively, so that the output wavelength of the first TDLAS laser and the second TDLAS laser respectively scans and modulates in the vicinity of and ​ S4: passing the output light of the first TDLAS laser and the second TDLAS laser through the methane and ethane mixed gas to be measured, and detecting the light signals after gas absorption respectively to obtain a first detection signal S1 and a second detection signal S2; S5: based on the first detection signal and the second detection signal to separate the concentration of methane by solving a model concentrations of methane and ethane ethane; wherein the model is solved based on differences in absorption characteristics of methane and ethane at wavelengths and ​ 2. The TDLAS detection method of claim 1, wherein: the first target wavelength is 1654 nm, the second target wavelength is 1683 nm.

3. The TDLAS detection method for inhibiting cross-spectral interference of methane and ethane according to claim 1 or 2, characterized in that: In the step S3, the wavelength scanning range of the first TDLAS laser and the second TDLAS laser reaches 300-500 pm by controlling the amplitude of the scanning signal.

4. The TDLAS detection method of claim 3, wherein: The wavelength scanning range is 400 pm.

5. The TDLAS detection method of claim 1, wherein: In the step S3, the wavelength modulation depth is set to match the pressure broadening width of the methane and ethane absorption spectrum lines in the mixed gas by controlling the amplitude of the modulation signal.

6. The TDLAS detection method of claim 1, wherein: In the step S4, the first detection signal and the second detection signal are peak values of second harmonic signals.

7. The TDLAS detection method of claim 1, wherein: The solving model in the step S5 is a linear equation group: S1 = k 11 *C methyl + k 12 *C ethyl S2 = k 21 *C methyl + k 22 *C ethyl wherein , , , are coefficients predetermined by calibration experiments.

8. A TDLAS gas detection system for implementing the TDLAS detection method of suppressing the cross-spectral interference of methane and ethane according to any one of claims 1-7, characterized in that, It comprises: a first TDLAS laser and a second TDLAS laser; a laser control module configured to control the temperature and driving current of the first TDLAS laser and the second TDLAS laser, and to generate the scanning signal and the modulation signal; a gas chamber for accommodating the methane and ethane mixed gas; a photodetector for receiving the light signal passing through the gas chamber and converting it into an electrical signal; A signal processing module, configured to perform the steps S4 and S5, extract the first detection signal from the electrical signal and the second detection signal , and calculate the concentration Cmethane of methane and the concentration Cethane of ethane.

9. A computer readable storage medium having stored thereon a computer program which, when executed by a processor, implements the step S5 in the method of any one of claims 1-7.

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