Methane and ethane dual-gas detection method based on intermediate infrared laser

Abstract

The invention relates to the technical field of TDLAS (Tunable Diode Laser Absorption Spectroscopy) gas monitoring, in particular to a methane and ethane dual-gas detection method based on an intermediate infrared laser. A DAC pin of a single-chip microcomputer is adopted to output sawtooth scanning signals modulated by superposition sine waves, by adjusting the amplitude of the sawtooth scanning signals, the wavelength of light emitted by a laser is controlled to be 3344.4 nm and 3345.8 nm, meanwhile, the positions of absorption peaks of ethane and methane are covered, simultaneous detection of methane and methane is achieved, and the detection precision is high. The methane scanning signal and the ethane scanning signal can be independently modulated by changing the amplitude of sine waves of different wave bands of the sawtooth scanning signal, so that the signal-to-noise ratio of single gas detection is increased, the sensitivity is higher, and the ppb level can be easily reached. According to the method, the single chip microcomputer is adopted to output scanning signals, sine waves with different modulation amplitudes can be output in a quick response mode, use is more flexible, cost reduction is facilitated, and the detection precision of single gas is greatly improved by independently modulating different gas scanning signals.

Description

Technical Field

[0001] The invention relates to the technical field of TDLAS gas monitoring, and in particular to a methyl and ethyl ethane dual gas detection method based on mid-infrared laser. Background Art

[0002] In recent years, with the advancement of urbanization, natural gas, as a high-quality, efficient, green, clean, low-carbon energy source that can form a benign complement to the development of renewable energy, has been in increasing demand. However, the ensuing natural gas leakage accidents have also attracted more and more attention. In particular, in urban natural gas pipeline networks, some areas have caused natural gas leakage or micro-leakage due to long-term contact with corrosive substances, peeling of pipeline anti-corrosion paint, aging of internal components, external force damage, etc. In order to prevent major safety accidents caused by natural gas leakage, the inspection of natural gas pipelines has been increasingly strengthened, and there are also higher requirements for the identification of air components in the atmosphere.

[0003] Tunable semiconductor laser absorption spectroscopy (TDLAS) mainly uses the narrow line width and wavelength of tunable semiconductor lasers that change with the injection current to measure a single or several absorption lines of molecules that are very close and difficult to distinguish. Due to its high selectivity, high resolution, and no interference from other gases, it has developed into a commonly used atmospheric trace gas monitoring technology.

[0004] Since the spectral line of a tunable semiconductor laser is relatively narrow, a laser can usually only detect one gas component. If it is necessary to accurately identify natural gas leaks, it is necessary to detect both methane and ethane gases at the same time. In the Chinese invention patent: "CN114965357A A TDLAS-based Methane and Ethane Dual Gas Detection Method and Device", a technical solution for detecting methyl and ethane dual gases by a laser is disclosed. In this technical solution, a near-infrared laser is used. However, in the near-infrared band, the absorption intensity of methyl and ethane gases is relatively low, which leads to low detection accuracy of this technical solution. At the same time, in this technical solution, a unified modulation parameter technical solution is used for methyl and ethane, and the laser output beam can simultaneously scan the absorption peaks of methane and ethane. However, the use of unified modulation parameters will cause the problem of early saturation of individual gases, affecting the signal-to-noise ratio of a single gas measurement, and further affecting the detection accuracy.

[0005] Therefore, the present invention application proposes a methane and ethane dual-gas detection method based on mid-infrared laser. In the method of the present invention, since the absorption intensity of methane and ethane gases in the mid-infrared band is 2 to 3 orders of magnitude higher than that in the near-infrared band, the detection sensitivity of the method of the present invention can easily reach the ppb level; in addition, the method of the present invention adopts dual-gas independent modulation and scanning, which improves the signal-to-noise ratio of single gas measurement and realizes accurate, fast and highly sensitive natural gas leak detection. Summary of the invention

[0006] The purpose of the present invention is to provide a methane and ethane dual-gas detection method based on a mid-infrared laser, so as to solve the problem that the scanning signals of the methane and ethane dual gases are uniformly modulated in the existing detection method, thereby limiting the detection accuracy.

[0007] To achieve the above object, the present invention adopts the following technical solution: a method for detecting methyl and ethyl ethane dual gases based on mid-infrared laser, comprising the following steps:

[0008] S1. According to the absorption characteristics of methane and ethane in the mid-infrared band, the scanning signals of methane and ethane are modulated independently;

[0009] S2, using the independently modulated scanning signal of S1 to drive the mid-infrared tunable semiconductor laser, and the laser emits a laser signal according to the scanning signal;

[0010] S3. Detect the laser signal after being absorbed by the gas to obtain the concentration values ​​of methane and ethane in the gas to be tested.

[0011] Furthermore, in the above S1, the scanning signal is a sawtooth scanning signal modulated by a superimposed sine wave outputted by a DAC pin of a single-chip microcomputer.

[0012] Furthermore, the frequency of the sine wave is 40 kHz, and the frequency of the sawtooth scanning signal is 1-100 Hz.

[0013] Furthermore, in S2, the central wavelength of the mid-infrared tunable semiconductor laser is 3.345 μm.

[0014] Furthermore, in the above S1-S2, by adjusting the amplitude of the sawtooth scanning signal, the wavelength of the laser light emission is controlled at 3344.4nm and 3345.8nm, while covering the absorption peak positions of ethane and methane.

[0015] Furthermore, the voltage amplitude of the sawtooth scanning signal is 50-250 mV.

[0016] Furthermore, a certain section of the sawtooth scanning signal adopts one sine wave amplitude, and another certain section adopts another sine wave amplitude, so as to realize independent modulation of the methane and ethane scanning signals respectively.

[0017] Furthermore, the voltage amplitude of the sine wave in the first half of the sawtooth scanning signal is 40-60mV, which is used for independent modulation of methane; the voltage amplitude of the sine wave in the second half of the sawtooth scanning signal is 60-80mV, which is used for independent modulation of ethane.

[0018] Furthermore, in the S3, the gas absorbs the laser signal in the absorption cell, and the laser signal after being absorbed by the gas is converted into an electrical signal through a photodiode. The electrical signal output by the photodiode passes through a bandpass filter, a phase-locked amplifier chip and a low-pass filter, and then outputs a second harmonic signal to the ADC of the microcontroller; the IO pin of the microcontroller synchronously outputs a sine wave twice the frequency signal as a reference signal of the phase-locked amplifier chip.

[0019] Furthermore, the single chip microcomputer obtains the concentration values ​​of methane and ethane in the gas to be tested through a component analysis algorithm and a concentration calculation algorithm.

[0020] Beneficial effects of the present invention:

[0021] 1. The method of the present invention adopts a mid-infrared tunable semiconductor laser. In the mid-infrared band, the absorption intensity of methane and ethane is 2 to 3 orders of magnitude higher than that in the near-infrared band, thereby greatly improving the detection sensitivity during the detection process;

[0022] 2. The method of the present invention performs separate modulation on methane and ethane in the scanning signal, thereby solving the problem of early saturation of a single gas in traditional unified modulation, improving the signal-to-noise ratio of a single detection, and further improving the sensitivity of the detection, so that the detection sensitivity of the method of the present invention can easily reach the ppb level. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 It is a schematic diagram of the system architecture of the method of the present invention;

[0024] Figure 2 This is the absorption intensity diagram of methane and ethane in the near infrared band;

[0025] Figure 3 This is the absorption intensity diagram of methane and ethane in the mid-infrared band;

[0026] Figure 4 Schematic diagram of sawtooth scanning signal of the method of the present invention;

[0027] Figure 5 It is a microscopic schematic diagram of a sawtooth scanning signal of the method of the present invention;

[0028] Figure 6 It is a schematic diagram of the change of the amplitude of the sine wave in the sawtooth scanning signal of the method of the present invention. DETAILED DESCRIPTION

[0029] The technical solution of the present invention will be clearly and completely described below in conjunction with the accompanying drawings in the embodiments of the present invention.

[0030] like Figure 2-3As shown, by querying the Hitran database, it can be known that: the absorption intensity of methane in the mid-infrared band is 107 times that of the near-infrared band; the absorption intensity of ethane in the mid-infrared band is 612 times that of the near-infrared band; the absorption peak of methane is 3345.8nm, and the absorption peak of ethane is 3344.4nm.

[0031] Since the absorption intensity of methane and ethane in the mid-infrared band is significantly stronger than that in the near-infrared band, and the absorption peaks of methane and ethane are relatively close, the method of the present invention adopts a mid-infrared tunable semiconductor laser, and by adjusting the amplitude of the scanning signal, the wavelength of the laser light can simultaneously cover the absorption peaks of methane and ethane in the mid-infrared band, thereby realizing the simultaneous detection of methane and ethane dual gases.

[0032] like Figure 4-5 As shown, in the method of the present invention, the scanning signal is a sawtooth scanning signal loaded with a 40kHz sine wave output by the DAC pin of the single-chip microcomputer, the amplitude of the scanning signal is 50-250mV, and the frequency is 1-100Hz; at the microscopic level, the scanning signal is a segmented sine wave and is distributed in a sawtooth shape under the macroscopic state.

[0033] like Figure 1 As shown, a mid-infrared tunable semiconductor laser is driven by a scanning signal, and the laser emits lasers of different wavelengths as the amplitude of the scanning signal changes. In the technical solution of the present invention, the wavelength of the laser emitted by the laser can completely cover the absorption peaks of methane and ethane; therefore, the gas to be tested is passed into a gas absorption cell, and methane and ethane in the gas to be tested absorb the laser, and the laser absorbed by the gas is reflected out of the gas absorption cell and converted into an electrical signal through a detector (photodiode), and the electrical signal is passed through a band-pass filter, a phase-locked amplifier chip (Lock-inAmplifier) ​​and a low-pass filter (Low-pass), and then outputs a second harmonic signal to the ADC of the single-chip microcomputer; the IO pin of the single-chip microcomputer synchronously outputs a sine wave doubled frequency signal as a reference signal of the phase-locked amplifier chip.

[0034] The second harmonic signal received by the single chip microcomputer is subjected to the component analysis algorithm and the concentration analysis algorithm to obtain the concentration values ​​of methane and ethane in the gas to be measured.

[0035] like Figure 5As shown, in the method of the present invention, the amplitude of the sine wave loaded in the scanning signal can be changed, so as to realize flexible modulation of the scanning signal. For example, in the first half of the scanning signal, the amplitude of the loaded sine wave is set at 40-60mV, and the scanning signal modulated by the sine wave with this amplitude has a better signal-to-noise ratio when detecting methane; in the second half of the scanning signal, the amplitude of the loaded sine wave can be set at 60-80mV, and the scanning signal modulated by the sine wave with this amplitude has a better signal-to-noise ratio when detecting ethane alkalinity; therefore, the method of the present invention further improves the detection sensitivity by separately modulating the scanning signals of methane and ethane gases.

[0036] In the prior art, a dedicated DDS (signal generator) chip is used to generate a sine wave to output a high-frequency signal. The disadvantage of using a DDS chip is that the amplitude of the sine wave cannot be easily changed within a short time range (25 to 50 ms). Therefore, the traditional DDS chip cannot realize the individual modulation of the methyl and ethyl gas scanning signals, and thus cannot improve the signal-to-noise ratio of a single gas detection. In the method of the present invention, a single-chip microcomputer is used to output a high-frequency signal. It only needs to change the output register, and there is no need to communicate with the chip or wait for the chip to respond. Compared with the DDS initialization-write register-communicate with the chip and wait for output process, the use of a single-chip microcomputer is faster and more flexible. This is also the fundamental reason why the present invention can perform individual modulation by changing the amplitude of the sine wave. At the same time, in practical applications, the method of the present invention does not use a DDS chip, so it has lower costs and is more convenient to use.

[0037] Example 1

[0038] The method of the present invention is used to detect the concentrations of methane and ethane in a gas sample.

[0039] During the process, the gas to be tested is introduced into the absorption cell, and the mid-infrared tunable semiconductor laser and the separate modulation method of the present invention are used. During the detection process, the frequency of the sawtooth scanning signal is 10 Hz, the frequency of the loaded high-frequency sine wave is 40 kHz, and the voltage amplitude of the sawtooth scanning signal is 150 mV. Under this voltage amplitude of the sawtooth scanning signal, the emission wavelength of the mid-infrared tunable semiconductor laser covers the absorption peaks of methane and ethane at the same time.

[0040] In this embodiment, the sawtooth scanning signal is segmented and individually modulated based on the absorption characteristics of methane and ethane. In the first half of the sawtooth scanning signal, the amplitude of the sine wave is adjusted to 40mV to achieve individual modulation of the methane scanning signal; in the second half of the sawtooth scanning signal, the amplitude of the sine wave is adjusted to 65mV for individual modulation of the ethane scanning signal.

[0041] The above-mentioned individually modulated scanning signal drives the mid-infrared tunable semiconductor laser, and the laser emits a laser signal and injects it into the absorption cell. The methane and ethane in the gas to be measured in the absorption cell absorb the laser. The laser signal absorbed by the gas refracts out of the absorption cell and is converted into an electrical signal through a photodiode. After passing through a bandpass filter, a phase-locked amplifier chip and a low-pass filter, the electrical signal outputs a second harmonic signal to the ADC of the microcontroller; the IO pin of the microcontroller synchronously outputs a sine wave twice the frequency signal as a reference signal for the phase-locked amplifier chip.

[0042] The second harmonic signal received by the single-chip microcomputer is subjected to a component analysis algorithm and a concentration analysis algorithm to obtain the concentration values ​​of methane and ethane in the gas to be tested. After detection, the concentration values ​​of methane and ethane in the gas to be tested are at the ppb level, which are 50ppb and 20ppb respectively. Compared with traditional near-infrared and unified modulation detection methods, the detection accuracy of the method of the present invention is significantly improved.

[0043] The present invention is not limited to the above-mentioned optimal implementation mode. Anyone can derive other various forms of products under the inspiration of the present invention. However, no matter what changes are made in the shape or structure, all technical solutions that are the same or similar to those of the present application fall within the protection scope of the present invention.

Claims

1. A method for detecting dual gases of methyl and ethyl based on mid-infrared laser, characterized in that: The following steps are involved: S1. According to the absorption characteristics of methane and ethane in the mid-infrared band, the scanning signals of methane and ethane are modulated independently; S2, using the independently modulated scanning signal of S1 to drive the mid-infrared tunable semiconductor laser, and the laser emits a laser signal according to the scanning signal; S3. Detect the laser signal after being absorbed by the gas to obtain the concentration values ​​of methane and ethane in the gas to be tested.

2. The method for detecting a dual gas of methyl and ethyl acetate based on a mid-infrared laser according to claim 1, characterized in that: In the above-mentioned S1, the scanning signal is a sawtooth scanning signal modulated by a superimposed sine wave outputted by a DAC pin of a single-chip microcomputer.

3. The method for detecting a dual gas of methyl and ethyl acetate based on a mid-infrared laser according to claim 2, characterized in that: The frequency of the sine wave is 40kHz, and the frequency of the sawtooth scanning signal is 1-100Hz.

4. The method for detecting a dual gas of methyl and ethyl acetate based on a mid-infrared laser according to claim 1, characterized in that: In the S2, the central wavelength of the mid-infrared tunable semiconductor laser is 3.345 μm.

5. A method for detecting methyl and ethyl ethane dual gases based on mid-infrared laser according to any one of claims 1 to 4, characterized in that: In the above-mentioned S1-S2, by adjusting the amplitude of the sawtooth scanning signal, the wavelength of the laser light emission is controlled at 3344.4nm and 3345.8nm, while covering the absorption peak positions of ethane and methane.

6. The method for detecting a dual gas of methyl and ethyl acetate based on a mid-infrared laser according to claim 5, characterized in that: The voltage amplitude of the sawtooth scanning signal is 50-250mV.

7. The method for detecting a dual gas of methyl and ethyl acetate based on a mid-infrared laser according to claim 5, characterized in that: A certain section of the sawtooth scanning signal adopts one sine wave amplitude, and another certain section adopts another sine wave amplitude, so as to realize independent modulation of the methane and ethane scanning signals respectively.

8. The method for detecting a dual gas of methyl and ethyl acetate based on a mid-infrared laser according to claim 7, characterized in that: The voltage amplitude of the sine wave in the first half of the sawtooth scanning signal is 40-60mV, which is used for independent modulation of methane; the voltage amplitude of the sine wave in the second half of the sawtooth scanning signal is 60-80mV, which is used for independent modulation of ethane.

9. The method for detecting a dual gas of methyl and ethyl acetate based on a mid-infrared laser according to claim 2, characterized in that: In the above-mentioned S3, the gas absorbs the laser signal in the absorption cell, and the laser signal after being absorbed by the gas is converted into an electrical signal through a photodiode. The electrical signal output by the photodiode passes through a bandpass filter, a phase-locked amplifier chip and a low-pass filter, and then outputs a second harmonic signal to the ADC of the single-chip microcomputer; the IO pin of the single-chip microcomputer synchronously outputs a sine wave signal with a frequency doubled as a reference signal of the phase-locked amplifier chip.

10. The method for detecting methyl and ethyl gas based on mid-infrared laser according to claim 9, characterized in that: The single chip microcomputer obtains the concentration values ​​of methane and ethane in the gas to be tested through a component analysis algorithm and a concentration calculation algorithm.

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