A multi-component greenhouse gas detection method based on laser absorption spectroscopy

Abstract

The present invention relates to the field of greenhouse gas monitoring technology, in particular to a multi-component greenhouse gas detection method based on laser absorption spectroscopy, comprising the following steps: acquiring a sawtooth signal A of each wavelength band laser after conversion by a photoelectric detector and signal conditioning; n [i], and calculate each array A n The difference between the maximum and minimum values ​​of [i] is L n ; Fill the multiple reflection absorption gas pool with high-purity nitrogen, and obtain the zero-point spectrum signal SG of each band laser absorption through the algorithm n [i], where SG n [i]=(A n [i]‑A n [i‑1]) / L n ; In the actual measurement process, the original absorption spectrum signal S of each band laser is obtained by the algorithm n [i], where S n [i]=(A n [i]‑A n [i‑1]) / L n , and then calculate the absorption spectrum signal SH after deducting the zero-point spectrum n [i] = S n [i]‑SG n [i], intercept the absorption spectrum signal SH n [i] Several sampling points in the non-absorption part, calculate their noise peak value N n ; if N n If it is greater than the set threshold, the zero-point spectrum signal SG is acquired again. n [i] The present invention greatly improves the sensitivity, accuracy and environmental adaptability of greenhouse gas detection equipment.

Description

Technical Field

[0001] The present invention relates to the technical field of greenhouse gas monitoring, and in particular to a multi-component greenhouse gas detection method based on laser absorption spectroscopy. Background Art

[0002] The existing technology has the disadvantages of low accuracy of greenhouse gas detection equipment and susceptibility to environmental interference. Summary of the invention

[0003] The purpose of the present invention is to solve the shortcomings of the existing greenhouse gas detection equipment in the prior art, that is, low precision and susceptibility to environmental interference, and to propose a multi-component greenhouse gas detection method based on laser absorption spectroscopy.

[0004] In order to achieve the above object, the present invention adopts the following technical solutions:

[0005] A multi-component greenhouse gas detection method based on laser absorption spectroscopy is designed, including the following steps:

[0006] S1: Couple multiple laser beams into the same multi-reflection absorption gas cell to generate differential laser current modulation waveforms in each band, so that the spectral signals received by the photodetector do not interfere with each other;

[0007] S2: Collect and obtain the sawtooth signal An[i] of each band laser after conversion by photoelectric detector and signal conditioning, where n is the number of components to be measured, i is the number of spectral sampling points, and calculate the difference Ln between the maximum and minimum values ​​of each array An[i];

[0008] S3: High-purity nitrogen is filled into the multiple reflection absorption gas cell, and the zero-point spectrum signal SGn[i] of laser absorption in each band is obtained through the algorithm, where SGn[i] = (An[i]-An[i-1]) / Ln;

[0009] S4: In the actual measurement process, the original absorption spectrum signal Sn[i] of each band laser is obtained by the algorithm, where Sn[i] = (An'[i]-An'[i-1]) / Ln, and then the absorption spectrum signal SHn[i] = Sn[i]-SGn[i] after deducting the zero-point spectrum is calculated, and several sampling points of the non-absorption part of the absorption spectrum signal SHn[i] are intercepted, and its noise peak value Nn is calculated, and Nn is used as the criterion for the system background noise;

[0010] S5: If Nn is greater than the set threshold, the zero-point spectrum signal SGn[i] is re-obtained; if Nn is less than the set threshold, several sampling points in the absorption interval of the absorption spectrum signal SHn[i] are intercepted, and the peak value is calculated as the absorption intensity In of each component gas. At the same time, the absorption spectrum signal SHn[i] in this interval is correlated with its standard spectrum absorption signal STn[i] to obtain the correlation coefficient Rn between the two. If Rn is greater than the set threshold correlation, it is a valid absorption signal, otherwise it is regarded as an interference signal;

[0011] S6: Obtain the temperature T and pressure data P inside the multiple reflection gas pool, and substitute them into the calibration curve function y(In), temperature function y(Tn), and pressure function y(Pn) respectively, and finally obtain the gas concentration of each component Yn=y(In)*y(Tn)*y(Pn).

[0012] Preferably, in S1, the current modulation waveform of each band laser is generated differentially as a sawtooth signal of 1 cycle and a DC level signal of n-1 cycles, and it is ensured that at the same time only the current modulation signal of one light source is a sawtooth signal, and the current modulations of other light sources are all DC level signals. When conditioning the photodetector signal, the cutoff frequency of the high-pass filter is designed according to the frequency of the sawtooth signal to filter out the DC level signals of other light sources to avoid signal crosstalk.

[0013] Preferably, the standard spectrum absorption signal of each band is obtained by filling the target standard gas into the multiple reflection absorption gas pool, intercepting several sampling points in the absorption interval of the absorption spectrum signal SHn[i] as the standard spectrum STn[i] and saving them.

[0014] Preferably, in S6, the temperature function y(Tn) and the pressure function y(Pn) are obtained by filling a mixed standard gas inside the gas pool, keeping the concentration of the mixed standard gas filled inside the gas pool unchanged, and obtaining data on the changes in concentrations of different component gases with temperature and pressure by changing the pressure and temperature inside the multiple reflection absorption pool, and then obtaining the temperature and pressure functions of the different component gas concentrations based on quadratic polynomial fitting.

[0015] The invention proposes a multi-component greenhouse gas detection method based on laser absorption spectroscopy, which has the beneficial effect that: the invention determines in real time whether the spectral signal of each band has noise in the absorption spectrum band through differentiated scanning of light sources in different bands and combining the absorption zero-point spectrum of each band, and updates the zero-point spectrum according to the noise situation.

[0016] At the same time, the signal absorption intensity of each spectral absorption band is calculated, and the effectiveness of the absorption is judged by combining the correlation between the absorption waveform and the standard spectrum line. Finally, the final concentration value is obtained by combining the calibration function, temperature function and pressure function.

[0017] It can realize the detection of multi-component greenhouse gases in a single gas cell, reducing the system volume. Compared with existing traditional equipment, it greatly improves the scalability and portability of the equipment.

[0018] At the same time, due to the narrowband absorption characteristics of the laser spectrum and the long life of the laser light source, the sensitivity, accuracy and environmental adaptability of the greenhouse gas detection equipment are greatly improved, and the difficulty of subsequent maintenance is greatly reduced. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 This is a light source differential scanning diagram of a multi-component greenhouse gas detection method based on laser absorption spectroscopy proposed in the present invention.

[0020] Figure 2 This is a flow chart of a multi-component greenhouse gas detection method based on laser absorption spectroscopy proposed by the present invention. DETAILED DESCRIPTION

[0021] The technical solutions in the embodiments of the present invention will be described clearly and completely below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all the embodiments.

[0022] The present invention uses a multi-path coupling method in optics to realize the detection of multi-component greenhouse gases in a single gas cell, reducing the system volume. Compared with existing traditional equipment, it greatly improves the scalability and portability of the equipment. At the same time, due to the narrow-band absorption characteristics of the laser spectrum and the long life characteristics of the laser light source, the sensitivity, accuracy and environmental adaptability of the greenhouse gas detection equipment are greatly improved, and the difficulty of later maintenance is greatly reduced.

[0023] Multi-path coupling can be achieved by fiber beam combining or discrete optical lens spatial coupling according to the selected light source wavelength and packaging form. In order to reduce the difficulty of equipment integration, fiber beam combining is preferred. Multiple reflection absorption cells can use gold-plated reflectors to take into account light sources of different wavelengths.

[0024] Differential scanning of light source Figure 1 As described, at time T1, the current modulation waveform of light source 1 is a sawtooth signal, and the current modulation of light sources 2 to 5 is a DC signal. At time T2, the current modulation waveform of light source 2 is a sawtooth signal, and the current modulation of other light sources is a DC signal. In this way, the current modulation signal of each light source is periodically changed. In the process of periodically changing the scanning current of the light source, the temperature control of the laser remains unchanged.

[0025] The photoelectric detector receives the superimposed signal of 5 light sources. In order to avoid signal crosstalk, after the signal is collected and the analog-to-digital conversion is completed, the algorithm generates a high-pass filter. The cutoff frequency of the high-pass filter needs to be combined with the frequency of the sawtooth wave. The high-pass filter filters out the DC component of the spectrum and retains the high-frequency sawtooth wave component. The high-pass filter must ensure that the falling edge of the sawtooth wave is steep and not deformed.

[0026] Example

[0027] In this embodiment, the number of components to be tested is selected as one, and after obtaining the above sawtooth wave component, according to Figure 2 The steps shown are used to further process the signal. The specific steps are as follows:

[0028] S100: Calculate the maximum and minimum values ​​of the entire cycle of the collected sawtooth signal to obtain the light intensity data L, and then execute the algorithm (A[i]-A[i-1]) / L for the sampling points of the entire cycle to obtain the light intensity normalized derivative spectrum signal. The derivation interval in the algorithm needs to be adjusted according to the absorption width of the light source in different bands.

[0029] S101: Fill the multiple reflection absorption cell with high-purity nitrogen gas, obtain the zero-point spectrum array SG[i] of each band according to the algorithm of S100, and save it.

[0030] S102: Switch the measurement gas path channel. The gas path switching can be performed by a solenoid valve to switch between high-purity nitrogen, the gas to be measured, and the calibration gas. According to the algorithm of S100, the measurement spectrum array S[i] of each band is obtained and saved.

[0031] S103: Calculate the differential absorption spectrum signal S[i]-SG[i] of each channel. This spectrum only retains the effective absorption spectrum information and removes fixed-phase noise such as gas cell interference and circuit noise, thereby improving detection accuracy.

[0032] S104: intercept the spectrum segment without characteristic absorption in the differential absorption spectrum signal in step S103 as the basis for noise characterization. Usually, the first half of the differential absorption spectrum data can be intercepted, and the peak value N of the intercepted spectrum segment data is calculated. N is compared with the set threshold. If N is greater than the threshold, the solenoid valve is switched to the high-purity nitrogen channel, and the steps in S100 are repeated to re-acquire the zero-point spectrum array SG[i], and update and save the zero-point spectrum. If N is less than the set threshold, the subsequent steps are executed sequentially.

[0033] S105: intercepting the spectrum segment with characteristic absorption of the differential absorption spectrum signal in step S103, and calculating the absorption intensity I of the signal;

[0034] There are two calculation algorithms: 1) calculate the signal peak of the spectrum; 2) select an interval for numerical integration. For data quality control, select several sampling points in the absorption spectrum to perform correlation calculation with the spectral standard spectrum to obtain the correlation coefficient R. If R is greater than the threshold, the measurement is valid. If R is less than the threshold, the measurement result is discarded. The spectral standard spectrum can be obtained experimentally. The acquisition method is: fill the multiple reflection absorption cell with a standard gas of known concentration to obtain the differential absorption spectrum in step S103, and then intercept several sampling points of the spectral segment with characteristic absorption of the differential absorption spectrum signal as the standard spectrum.

[0035] S106: Obtain the temperature and pressure data inside the multiple reflection gas cell, and substitute the absorption intensity I obtained in step S105 into the calibration function y[i], the temperature function y(T), and the pressure function y(P) to obtain the final concentration value. The temperature and pressure functions can be obtained experimentally, and the acquisition method is:

[0036] A mixed standard gas is filled into the gas cell, and the concentration of the mixed standard gas filled into the gas cell is kept unchanged. By changing the pressure and temperature inside the multiple reflection absorption cell, the data of the concentration of different components of gas changing with temperature and pressure are obtained, and then the temperature and pressure functions of the different components of gas concentration are obtained based on quadratic polynomial fitting.

[0037] The gas path switching as described above is performed by program control, and each independent gas path channel is connected to a solenoid valve. The gas path channel includes nitrogen, measuring gas, and calibration gas of different concentrations, which are finally collected into the measuring gas pool through a multi-way valve.

[0038] The above description is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any technician familiar with the technical field can make equivalent replacements or changes according to the technical scheme and inventive concept of the present invention within the technical scope disclosed by the present invention, which should be covered by the protection scope of the present invention.

Claims

1. A multi-component greenhouse gas detection method based on laser absorption spectroscopy, characterized in that: The following steps are involved: S1: Couple multiple laser beams into the same multiple reflection absorption gas pool, generate differential current modulation waveforms for lasers in each band, and periodically change the current modulation signal of each light source to ensure that only one light source has a sawtooth signal at the same time, and the current modulations of other light sources are DC level signals. When conditioning the photodetector signal, design the cutoff frequency of the high-pass filter according to the frequency of the sawtooth signal to filter out the DC level signals of other light sources to avoid signal crosstalk; S2: Collect and obtain the sawtooth signal An[i] of each band laser after conversion by photoelectric detector and signal conditioning, where n is the number of components to be measured, i is the number of spectral sampling points, and calculate the difference Ln between the maximum and minimum values ​​of each array An[i]; S3: High-purity nitrogen is filled into the multiple reflection absorption gas cell, and the zero-point spectrum signal SGn[i] of laser absorption in each band is obtained through the algorithm, where SGn[i] = (An[i]-An[i-1]) / Ln; S4: In the actual measurement process, the original absorption spectrum signal Sn[i] of each band laser is obtained by the algorithm, where Sn[i] = (An'[i]-An'[i-1]) / Ln, and then the absorption spectrum signal SHn[i] = Sn[i]-SGn[i] after deducting the zero-point spectrum is calculated, and several sampling points of the non-absorption part of the absorption spectrum signal SHn[i] are intercepted, and its noise peak value Nn is calculated, and Nn is used as the criterion for the system background noise; S5: If Nn is greater than the set threshold, the zero-point spectrum signal SGn[i] is re-obtained; if Nn is less than the set threshold, several sampling points in the absorption interval of the absorption spectrum signal SHn[i] are intercepted, and the peak value is calculated as the absorption intensity In of each component gas. At the same time, the absorption spectrum signal SHn[i] in this interval is correlated with its standard spectrum absorption signal STn[i] to obtain the correlation coefficient Rn between the two. If Rn is greater than the set threshold correlation, it is a valid absorption signal, otherwise it is regarded as an interference signal; S6: Obtain the temperature T and pressure data P inside the multiple reflection gas pool, and substitute them into the calibration curve function y(In), temperature function y(Tn), and pressure function y(Pn) respectively, and finally obtain the gas concentration of each component Yn=y(In)*y(Tn)*y(Pn).

2. The multi-component greenhouse gas detection method based on laser absorption spectroscopy according to claim 1, characterized in that: The standard spectrum absorption signal of each band is obtained by filling the target standard gas into the multiple reflection absorption gas pool, intercepting several sampling points in the absorption interval of the absorption spectrum signal SHn[i] as the standard spectrum STn[i] and saving them.

3. The multi-component greenhouse gas detection method based on laser absorption spectroscopy according to claim 1, characterized in that: In S6, the temperature function y(Tn) and the pressure function y(Pn) are obtained by filling the gas pool with a mixed standard gas and keeping the concentration of the mixed standard gas filled in the gas pool unchanged. By changing the pressure and temperature inside the multiple reflection absorption cell, the data of the concentration of different components of gas changing with temperature and pressure are obtained, and then the temperature and pressure functions of the different components of gas concentration are obtained based on quadratic polynomial fitting.

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