Resonance type photoacoustic cell and multi-component trace gas simultaneous detection device based on integrating sphere photoacoustic cell

Abstract

The utility model provides a resonance type photoacoustic cell and based on integrating sphere photoacoustic cell multi-component trace gas simultaneous detection device, wherein the resonance type photoacoustic cell comprises integrating sphere gas chamber and acoustic tube, the sphere wall of integrating sphere gas chamber is provided with gas inlet, gas outlet, light inlet and photoacoustic signal detection port, one ends of the acoustic tubes are connected to the photoacoustic signal detection ports, the other ends of the acoustic tubes extend in the direction away from the integrating sphere air chamber, the acoustic tubes are different in length, and one acoustic tube corresponds to one photoacoustic signal detection port. According to the utility model, a plurality of acoustic tubes with different lengths are designed, and different resonant frequencies can be generated by the acoustic tubes with different lengths, so that various gases can be detected at the same time on the premise of ensuring the detection sensitivity.

Description

Technical Field

[0001] The utility model relates to the technical field of trace gas detection, in particular to a resonance photoacoustic cell and a multi-component trace gas simultaneous detection device based on an integrating sphere photoacoustic cell. Background Art

[0002] As global climate change becomes increasingly serious, countries around the world have begun to pay attention to ecological and environmental protection. As one of the world's largest greenhouse gas emitters, China's responsibility and role in addressing climate change are becoming increasingly important. To this end, the Chinese government has proposed a "national dual carbon" strategy, aiming to achieve carbon neutrality and comprehensively build a modern economic system with green and low-carbon development by 2060. Therefore, under the background of the "dual carbon" goal, a highly accurate and reliable gas monitoring system will serve as an important monitoring method to conduct online monitoring of the carbon emissions of emission-controlled enterprises and the content of mixed gases of CO2, CH4, and CO in the air, providing effective data support and scientific basis for low-carbon projects.

[0003] At present, gas detection technology mainly consists of chemical technologies such as catalytic combustion, electrochemistry, gas chromatography, and spectral technologies such as non-dispersive infrared spectroscopy (NDIR), Fourier transform infrared spectroscopy (FTIR), photoacoustic spectroscopy (PAS), etc. Compared with chemical technology, spectral technology has the advantages of flexible sampling, non-destructive measurement, non-contact, and multi-component detection.

[0004] Photoacoustic spectroscopy has the advantages of high sensitivity, good selectivity, and zero background detection, and has attracted widespread attention in the field of mixed gas detection. The performance of the photoacoustic spectroscopy gas detection system is mainly affected by the excitation light source, acoustic detector, and photoacoustic cell. At present, the development of the excitation light source is relatively slow. The research on improving the performance of photoacoustic gas detection at home and abroad mainly focuses on acoustic detectors, and there is less systematic research on photoacoustic cells. Compared with non-resonant photoacoustic cells, resonant photoacoustic cells can amplify photoacoustic signals and suppress low-frequency noise. Therefore, the design and development of high-performance resonant photoacoustic cells has become an important way to improve the gas detection capabilities of photoacoustic sensors, which is of great significance to the rapid development of various industries.

[0005] The relevant technologies are as follows:

[0006] Symmetrical multi-resonance photoacoustic cell designed by TailinLi. The entire length of the photoacoustic cell is 110mm, and it consists of two buffer cavities and a photoacoustic chamber. Two microphones are symmetrically located on both sides of the photoacoustic chamber to achieve two-point detection. The photoacoustic chamber consists of three cylindrical acoustic tubes, which provide different acoustic resonance frequencies. The lengths of acoustic tube 1 (AR1), acoustic tube 2 (AR2), and acoustic tube 3 (AR3) are 60mm, 50mm, and 46mm, respectively, and the radius is 3mm. A channel with a radius of 1.4mm penetrates each acoustic tube as a channel connecting the acoustic tube and the microphones (microphone 1 and microphone 2). In the finite element model, the material used is air, and the temperature and pressure are set to 298.15K and 1atm, respectively, and the resonance frequencies obtained are approximately 3069Hz (AR1), 3496Hz (AR2), and 3703Hz (AR3). By using a near-infrared diode laser and two mid-infrared quantum cascade lasers for simultaneous detection, the lowest detection limits of C2H2, NO and CF4 were 480ppb, 260ppb and 0.57ppb, respectively. The disadvantage is that the effective path for the gas to absorb light energy to generate a sound signal is equal to the length of the cavity, and the cavity length is limited, so the detection sensitivity is low.

[0007] The ball-tube coupled photoacoustic cell designed by Zhengang Li consists of an integrating sphere air chamber and an acoustic tube. The diameter of the integrating sphere air chamber is 5.08 cm, and the inner wall is coated with a layer of high-reflectivity polytetrafluoroethylene material. There are four openings on the sphere wall, namely the light inlet, air inlet, air outlet and photoacoustic signal detection port. Excluding the photoacoustic signal detection port, the diameter of the other openings is 3 mm. An acoustic tube with a length of 5 cm and a radius of 2 mm is connected to the photoacoustic signal detection port. The incident light enters the integrating sphere air chamber, and the beam is superimposed after diffuse reflection, and the absorption path of the light is increased, which is conducive to the gas to fully absorb the light energy and generate a sufficiently large photoacoustic signal. By using a 450nm semiconductor laser (LD) as the light source, the minimum detection limit of NO2 is 0.7ppb. The disadvantage is that although this design is suitable for the detection of multiple trace gases, it can only detect one gas during the experiment, and cannot detect multiple gases at the same time.

[0008] Zhang Shuang proposed a time-division multiplexing scheme to achieve simultaneous measurement of multi-component gases. This scheme is based on tunable laser absorption spectroscopy technology and uses a time-division multiplexing scheme to achieve online detection of SF6 decomposition gas components in SF6 / N2 mixed gas. The 3-channel measurement control is achieved by switching the optical switch. During operation, the main dispatching board sets the optical path channel selection sequence of the 3 sets of lasers, allowing the lasers representing the 3 gases HF, H2S and CO to pass through the optical switch in turn. After passing through the optical switch, the laser enters the gas chamber and is received by the detector. The amplification circuit of the detector sets different amplification factors according to the different absorption coefficients of the 3 gases to be measured. In this way, the detector will receive the optical signals of the 3 modulated signals in turn. Since the optical signal is collected by 3 phase-locked boards at the same time, the main dispatching board is required to collect the serial port information of the channel that is opened synchronously with the optical switch, and the serial port data sent by the other 2 closed phase-locked boards will not be received. The concentration information of the 3 gases is received in turn through the 3 serial ports, stored and packaged, and sent to the host computer in a fixed format. The host computer receives the serial port data and demodulates the data according to the protocol to obtain the real-time concentration values ​​of the 3 gases. The refresh time for each channel is 3s. Its disadvantage is that the time division multiplexing technology interweaves different laser signals in different time periods and transmits them along the same optical path. Its core part is to measure different gases by switching the laser optical path. The real-time detection of each point in the system is not very high. Moreover, the performance of the light source switching component has always been difficult to guarantee, the large size is not conducive to integration, and sometimes there are problems of bounce jitter and poor repeatability.

[0009] In summary, for the existing photoacoustic pools, some photoacoustic pools are designed with three acoustic tubes of different lengths to detect multiple gases at the same time, but the sensitivity of detecting gases is low; another part of the photoacoustic pool uses the principle of integrating sphere to increase the optical path and improve the sensitivity, but it cannot detect multiple gases at the same time; and the time division multiplexing method can detect multiple gases, but the real-time detection of each point in the system is not very high. Based on this, a new solution is urgently needed to solve the above problems. Utility Model Content

[0010] The utility model aims to provide a resonant photoacoustic cell and a multi-component trace gas simultaneous detection device based on an integrating sphere photoacoustic cell to solve the problems existing in the above-mentioned prior art and to simultaneously detect multiple gases while ensuring detection sensitivity.

[0011] To achieve the above purpose, the utility model provides the following solutions:

[0012] The utility model provides a resonant photoacoustic cell, comprising an integrating sphere air chamber and an acoustic tube, wherein an air inlet, an air outlet, a light inlet and a photoacoustic signal detection port are provided on the sphere wall of the integrating sphere air chamber, one end of the acoustic tube is connected to the photoacoustic signal detection port, and the other end extends in a direction away from the integrating sphere air chamber, a plurality of the acoustic tubes are provided, the lengths of the plurality of the acoustic tubes are different, and one of the acoustic tubes corresponds to one of the photoacoustic signal detection ports.

[0013] Preferably, three acoustic tubes are provided, and the lengths of the three acoustic tubes are different.

[0014] Preferably, the inner wall of the integrating sphere air chamber is coated with a layer of polytetrafluoroethylene material with high reflectivity.

[0015] Preferably, the diameter of the integrating sphere air chamber is 4.8 cm, the radius of the three acoustic tubes is 2 mm, and the lengths are 4 cm, 5 cm, and 6 cm respectively.

[0016] Preferably, it further comprises microphones, the number of which is the same as the number of the acoustic tubes and is arranged in one-to-one correspondence.

[0017] The utility model also provides a multi-component trace gas simultaneous detection device based on an integrating sphere photoacoustic cell, comprising a function generator, a laser controller, a laser, an optical fiber coupler, a phase-locked amplifier, a data acquisition device, a gas distribution system, a computer and the resonant photoacoustic cell as described above.

[0018] Compared with the prior art, the utility model has achieved the following technical effects:

[0019] The utility model designs a plurality of acoustic tubes of different lengths. Acoustic tubes of different lengths will produce different resonance frequencies. Three phase-locked amplifiers are used at the same time to set three lasers to match the first-order radial resonance frequencies of the three acoustic tubes. Although the microphone will still receive absorption signals of different modulation frequencies, the sound signal will only be amplified at the same frequency as the resonance of the acoustic tube, and the sound signals at other frequencies are very small and will hardly cause interference. Finally, the sound amplitudes of the three gases can be obtained at the same time, so that the concentrations of the three gases to be measured can be obtained at the same time.

[0020] In summary, the utility model can detect multiple gases simultaneously while ensuring detection sensitivity. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] In order to more clearly illustrate the embodiments of the utility model or the technical solutions in the prior art, the drawings required for use in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the utility model. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0022] Figure 1 A schematic diagram of the structure of a resonant photoacoustic cell provided in an embodiment of the utility model;

[0023] Figure 2 A structural block diagram of a device for simultaneous detection of multi-component trace gases provided by an embodiment of the utility model;

[0024] In the figure: 1-integrating sphere air chamber; 2-acoustic tube; 3-microphone; 4-air outlet; 5-air inlet; 6-light inlet; 7-air pump. DETAILED DESCRIPTION

[0025] The following will be combined with the drawings in the embodiments of the utility model to clearly and completely describe the technical solutions in the embodiments of the utility model. Obviously, the described embodiments are only part of the embodiments of the utility model, not all of the embodiments. Based on the embodiments in the utility model, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the utility model.

[0026] In order to make the above-mentioned purposes, features and advantages of the present invention more obvious and easy to understand, the present invention is further described in detail below in conjunction with the accompanying drawings and specific implementation methods.

[0027] The existing photoacoustic cell cannot be used to detect multiple gases at the same time. Based on this, the utility model provides a resonant photoacoustic cell, such as Figure 1 The device comprises an integrating sphere air chamber 1 and an acoustic tube 2. An air inlet 5, an air outlet 4, a light inlet 6 and a photoacoustic signal detection port are provided on the sphere wall of the integrating sphere air chamber 1. One end of the acoustic tube 2 is connected to the photoacoustic signal detection port, and the other end extends in a direction away from the integrating sphere air chamber 1. A plurality of acoustic tubes 2 are provided, and the lengths of the plurality of acoustic tubes 2 are different. One acoustic tube 2 corresponds to one photoacoustic signal detection port.

[0028] It also includes microphones 3, the number of which is the same as the number of the acoustic tubes 2 and is arranged in a one-to-one correspondence.

[0029] In the present invention, an acoustic tube 2 of a certain length is used to detect a gas, and the principle is as follows:

[0030] Photoacoustic spectroscopy is a method of detecting trace gases based on the photoacoustic effect. Under the action of light modulated at a specific wavelength, gas molecules in a closed gas chamber absorb light energy and are excited to a high energy state, and then release heat energy through intermolecular collisions. From a macroscopic point of view, the temperature of the gas will rise. If the volume of the gas remains unchanged at this time, the increase in temperature will lead to an increase in pressure. Because the incident light is periodically modulated, periodic pressure waves with the same frequency as the light source, namely photoacoustic signals, will be generated around the gas. A highly sensitive microphone is used to convert the detected acoustic signal into an electrical signal, and the amplified electrical signal is extracted by a phase-locked amplifier. Finally, the content of the gas to be measured can be calculated through data processing and concentration inversion.

[0031] When detecting multiple gases, if a sine wave modulation signal of the same frequency is superimposed on a sawtooth wave scanning signal to drive three lasers for detection, the three emitted lasers will carry sine waves of the same frequency, and the gas will absorb light energy and generate sound signals of the same frequency. The microphone 3 will receive the sound signals and demodulate them incorrectly, resulting in crosstalk between absorption signals of different gases. In order to solve this interference and realize simultaneous monitoring of multi-component gases at the same time, the utility model designs multiple acoustic tubes 2 of different lengths. Acoustic tubes 2 of different lengths will produce different resonance frequencies. At the same time, multiple phase-locked amplifiers are used to set multiple lasers to match the first-order radial resonance frequencies of multiple acoustic tubes 2. Although the microphone 3 will still receive absorption signals of different modulation frequencies, the sound signal will only be amplified at the same frequency as the resonance of the acoustic tube 2. The sound signals at other frequencies are very small and will hardly cause interference. Finally, the sound amplitudes of multiple gases can be obtained at the same time, so that the concentrations of multiple gases to be measured can be obtained at the same time.

[0032] The present invention does not limit the specific number of the acoustic tubes 2. It can be understood that the number can be any number above two, and the specific number is determined according to the number of gases that need to be detected simultaneously.

[0033] When the purpose of the detection device provided is to provide a gas monitoring system with high accuracy and high reliability, and to provide effective data support and scientific basis for low-carbon projects. CO2, CO and CH4 are all greenhouse gases, and their presence in the atmosphere leads to the greenhouse effect, which in turn causes climate change. CO2 is one of the most important greenhouse gases, and its emissions are directly related to global warming. Therefore, monitoring and controlling CO2 emissions are crucial to mitigating climate change and achieving low-carbon goals. At the same time, although the emissions of CO and CH4 are small, they also need to be monitored and controlled due to their high greenhouse effects. Therefore, when implementing low-carbon measures, it is necessary to comprehensively consider the emissions of CO2, CO and CH4, so in some embodiments, three acoustic tubes 2 are provided, and the lengths of the three acoustic tubes 2 are all different. In this way, the three-component gas of CO2, CO and CH4 is detected simultaneously.

[0034] In order to improve the gas detection sensitivity, the gas can be allowed to fully absorb light energy in the gas chamber to generate a sufficiently large photoacoustic signal. Based on this purpose, the embodiment of the utility model is coated with a layer of high reflectivity polytetrafluoroethylene material on the inner wall of the integrating sphere gas chamber 1. The laser will form diffuse reflection due to the polytetrafluoroethylene material on the inner wall of the integrating sphere gas chamber 1 to achieve beam superposition, thereby increasing the absorption path of the light.

[0035] In some specific embodiments, the diameter of the integrating sphere air chamber 1 can be set to 4.8 cm, and the radius of the three acoustic tubes 2 can be set to 2 mm, and the lengths can be 4 cm, 5 cm, and 6 cm, respectively. Of course, the values ​​given in this embodiment only belong to one embodiment. In other words, in other embodiments, researchers can design the integrating sphere air chamber 1 and the acoustic tube 2 of any size according to actual conditions.

[0036] In order to prevent the diaphragm at the end of the microphone 3 from being worn during installation, a gap is reserved at the top of the acoustic tube 2. For ease of understanding, the following embodiment is proposed. The top radius of the microphone 3 used in this embodiment is 0.605 cm. Therefore, when the acoustic tube 2 is installed, a cylindrical tube with a radius of 0.615 cm and a length of 3 cm will be extended at the top of the acoustic tube 2. At 2 mm away from the end of the acoustic tube 2, the radius of the cylinder will be reduced to 0.605 cm, which is exactly the radius of the top of the microphone 3. The interface between the cylindrical tube and the acoustic tube 2 is sealed. During installation, the microphone 3 is directly placed in the cylindrical tube with a radius of 0.615 cm, and the gap between the diaphragm and the acoustic tube 2 is 2 mm.

[0037] like Figure 2 As shown, an embodiment of the utility model also provides a multi-component trace gas simultaneous detection device based on an integrating sphere photoacoustic cell, including a function generator, a laser controller, a laser, a fiber coupler, a phase-locked amplifier, a data acquisition device, a gas distribution system, a computer, and a resonant photoacoustic cell as described in any of the above embodiments.

[0038] In the utility model, acoustic tubes 2 of different lengths will produce different resonance frequencies. Three phase-locked amplifiers are used to set the three lasers to match the first-order radial resonance frequencies of the three acoustic tubes 2, thereby avoiding the crosstalk effect of the photoacoustic signal. When the phase-locked amplifier is used to demodulate the sound signals obtained by the three microphones 3, the sound amplitudes of the three gases can be obtained at the same time, thereby obtaining the concentrations of the three gases to be measured at the same time.

[0039] The specific detection process of the multi-component trace gas simultaneous detection device is as follows: (1) The computer software controls the solenoid valve to open the inlet and outlet, and at the same time commands the air pump 7 to start working, pumping the CO2, CH4, and CO mixed gas to be tested into the integrating sphere gas chamber 1; (2) After the gas replacement in the integrating sphere gas chamber 1 is completed, the air pump stops working and closes the solenoid valve to form a closed gas chamber; (3) Command the function generator to periodically modulate the wavelength of the laser; (4) The CO2, CH4, and CO mixed gas to be tested generates sound pressure after being excited by the periodic modulated light, and the microphone collects the acoustic signal at the closed end of the tube; (5) The phase-locked amplifier demodulates the sound pressure signal with the same modulation frequency, that is, the photoacoustic signal; (6) The data acquisition card collects the photoacoustic signal and performs analog-to-digital conversion; (7) The computer software records the corresponding digital signal, and inverts the corresponding concentration of the CO2, CH4, and CO samples according to the concentration calibration curve; (8) The computer turns off the function generator, and the single detection process ends.

[0040] The utility model also makes the following description:

[0041] 1. Relevant characteristics of integrating sphere gas chamber:

[0042] The inner wall of the integrating sphere air chamber is coated with a layer of high reflectivity polytetrafluoroethylene material, which has good corrosion resistance and temperature stability, and has high reflectivity in the wide band range from ultraviolet 250nm to near infrared 2500nm. Figure 1 As shown. When light enters the integrating sphere chamber from the light inlet, the light is evenly reflected inside the sphere due to the diffuse reflection characteristics of the inner wall material. The illuminance generated at any point on the sphere wall is the superposition of the illuminance generated by multiple reflected lights. It is a function of the geometric shape of the sphere, the diffuse reflection ratio of the coating, and the radiant flux, and has nothing to do with the position. The equivalent optical path of the integrating sphere can be obtained by formula (1.1).

[0043]

[0044] Where D is the diameter of the sphere and ρ is the reflectivity at the corresponding wavelength.

[0045] 2. Relevant characteristics of single free-end acoustic tube:

[0046] Assume there is an acoustic tube with an open end, a length of L, and a cross-sectional area of ​​S. The gas molecules in the tube move like a piston from the open end at x = 0 and end at the closed end at x = L. If this piston vibrates at a frequency at which only plane waves propagate, the sound wave in the tube can be expressed as:

[0047] P=Ae j[wt+k(L-x)] +Be j[wt-k(L-x)] (1.2)

[0048] In the above formula,

[0049] k=w / c (1.3)

[0050] Where k is the wave number, w is the angular frequency, c is the speed of sound, and A and B in equation (1.2) are determined by the boundary conditions of x=0 and x=L.

[0051] Due to the continuity of force and gas molecule velocity, the mechanical impedance of the sound wave at x = L is required to be equal to the mechanical impedance Z of the closed end of the tube. mL The force of the sound wave on the closed end of the tube is p(L,t)S, and the speed of the gas molecules is expressed as:

[0052]

[0053] Where ρ0 is the gas molecule density. Since impedance is equal to the quotient of force and volume velocity, according to equations (1.2), (1.4) and the force expression p(L, t)S, the mechanical impedance of the closed end of the tube is:

[0054]

[0055] And the input mechanical impedance Z at x = 0 m0 for:

[0056]

[0057] Finally, the resonant frequency of the single free-end acoustic tube is:

[0058]

[0059] Where n represents the order of resonance. Taking the first-order longitudinal mode as an example for analysis, the resonant frequency of the acoustic tube is:

[0060]

[0061] It can be seen from the above formula that the first-order longitudinal resonance frequency of the acoustic tube is inversely proportional to the length of the acoustic tube. The longer the tube, the smaller the resonance frequency.

[0062] The present invention uses specific examples to illustrate the principle and implementation of the present invention. The above examples are only used to help understand the method and core idea of ​​the present invention. At the same time, for those skilled in the art, according to the idea of ​​the present invention, there will be changes in the specific implementation and application scope. In summary, the content of this specification should not be understood as limiting the present invention.

Claims

1. A resonant photoacoustic cell, comprising an integrating sphere air chamber and an acoustic tube, wherein the sphere wall of the integrating sphere air chamber is provided with an air inlet, an air outlet, a light inlet and a photoacoustic signal detection port, one end of the acoustic tube is connected to the photoacoustic signal detection port, and the other end extends in a direction away from the integrating sphere air chamber, characterized in that: There are multiple acoustic tubes, the lengths of the multiple acoustic tubes are different, and one acoustic tube corresponds to one photoacoustic signal detection port.

2. The resonant photoacoustic cell according to claim 1, characterized in that: Three acoustic tubes are provided, and the lengths of the three acoustic tubes are different.

3. The resonant photoacoustic cell according to claim 1, characterized in that: The inner wall of the integrating sphere air chamber is coated with a layer of polytetrafluoroethylene material with high reflectivity.

4. The resonant photoacoustic cell according to claim 1, characterized in that: The diameter of the integrating sphere air chamber is 4.8 cm, the radius of the three acoustic tubes is 2 mm, and the lengths are 4 cm, 5 cm, and 6 cm respectively.

5. The resonant photoacoustic cell according to claim 1, characterized in that: It also includes microphones, the number of which is the same as the number of the acoustic tubes and is arranged in a one-to-one correspondence.

6. A multi-component trace gas simultaneous detection device based on an integrating sphere photoacoustic cell, characterized in that: The device comprises a function generator, a laser controller, a laser, an optical fiber coupler, a phase-locked amplifier, a data acquisition device, a gas distribution system, a computer and the resonant photoacoustic cell according to any one of claims 1 to 5.

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