CN107941276B - Gas flow and methane content laser measuring instrument and measuring method - Google Patents
Gas flow and methane content laser measuring instrument and measuring method Download PDFInfo
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- VNWKTOKETHGBQD-UHFFFAOYSA-N methane Chemical compound C VNWKTOKETHGBQD-UHFFFAOYSA-N 0.000 title claims abstract description 90
- 238000000034 method Methods 0.000 title claims abstract description 20
- 238000005259 measurement Methods 0.000 claims abstract description 30
- 238000012545 processing Methods 0.000 claims abstract description 15
- 238000005070 sampling Methods 0.000 claims abstract description 13
- 238000005516 engineering process Methods 0.000 claims abstract description 12
- 239000004065 semiconductor Substances 0.000 claims abstract description 10
- 239000007789 gas Substances 0.000 claims description 89
- 238000010521 absorption reaction Methods 0.000 claims description 18
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- 238000000862 absorption spectrum Methods 0.000 abstract description 15
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- 239000003345 natural gas Substances 0.000 description 15
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01D—MEASURING NOT SPECIALLY ADAPTED FOR A SPECIFIC VARIABLE; ARRANGEMENTS FOR MEASURING TWO OR MORE VARIABLES NOT COVERED IN A SINGLE OTHER SUBCLASS; TARIFF METERING APPARATUS; MEASURING OR TESTING NOT OTHERWISE PROVIDED FOR
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Abstract
The invention discloses a gas flow and methane content laser measuring instrument and a measuring method, wherein the laser measuring instrument comprises a laser, a semi-transparent semi-reflective lens, a total reflection mirror, a first photoelectric detector and a second photoelectric detector, wherein the area between the laser and the two photoelectric detectors is the gas flow to be measured, light emitted by the laser is divided into two beams after passing through the semi-transparent semi-reflective lens, one beam of light is perpendicular to the direction of the gas flow to be measured, and the other beam of light forms a certain included angle with the direction of the gas flow to be measured; the sampling circuit is connected with two photoelectric detectors, the two photoelectric detectors receive two beams of light passing through the airflow area to be detected, the received light signals are converted into electric signals, the electric signals are input into the sampling circuit, and sampled data are processed by the data processing system and the gas flow and the methane content are calculated. The invention combines the tunable semiconductor laser absorption spectrum technology through the laser Doppler effect, takes the absorption spectrum of the gas to be measured as the basis, and simultaneously realizes the measurement of the gas flow rate and the methane concentration.
Description
Technical Field
The invention relates to a gas laser measuring instrument, in particular to a gas flow and methane content laser measuring instrument and a measuring method.
Background
Natural gas is a clean fuel and a high-quality chemical raw material, and is also one of main energy sources in China. The main component of natural gas is methane, the content of methane determines the gas quality of the natural gas, and detection of the natural gas content of a pipeline is very necessary. Currently, the most common methods for detecting methane are chemical methods, gas chromatography and spectral absorption methods. The chemical method has simple structure and quick reaction, but has poor selectivity and stability to gas; the gas chromatography has wide application range and high sensitivity, but has slow reaction speed and poor real-time performance; the spectrum absorption method is stable, high in precision and quick in response.
The natural gas trade metering in China is carried out by a volume or energy method under the quality index required by legal requirements, and the volume metering is basically the main at present. With the rapid development of the natural gas industry, the working pressure of the natural gas is continuously improved, the flow range is gradually increased, and the requirements on the flow metering instrument are also higher and higher. The traditional natural gas flow meter is provided with an orifice plate flow meter, a vortex street flow meter, an ultrasonic flow meter and the like. Orifice plate flowmeters, while well accepted by the international standards organization, have narrow measurement ranges, large pressure losses and require straight pipe segment lengths, which are complicated due to numerous factors, and are difficult to improve in accuracy. The vortex shedding flowmeter has wide measuring range, small pressure loss and high accuracy, but has poor vibration resistance and temperature resistance, poor adaptability to measuring dirty media and high requirement on a straight pipeline. The ultrasonic flowmeter is a non-contact laser measuring instrument, which does not change the flowing state of fluid, has no pressure loss, is convenient to install, but has poor anti-interference capability, low reliability and precision level, short service life and the like. Traditional measurement methods have hardly met the requirements of the measurement development of modern industrial natural gas pipelines.
Currently, few meters capable of simultaneously measuring gas concentration and velocity are available. With the development of an optical integrated system and a signal processing system, the laser Doppler velocimetry technology is rapidly developed, and the advantages of the laser Doppler velocimetry technology are obviously superior to those of the traditional method. Flow rate measurement using tunable semiconductor laser absorption spectroscopy (TDLAS) is a typical representation thereof, and combining TDLAS with doppler shift allows measurement of gas flow rate while measuring gas content.
Disclosure of Invention
The invention aims to provide a natural gas flow velocity and methane content laser measuring instrument based on a tunable semiconductor laser absorption spectrum technology and a Doppler frequency shift principle.
The technical scheme adopted by the invention is as follows:
The laser measuring instrument comprises a laser, a semi-transparent semi-reflective lens, a total reflection mirror, a first photoelectric detector and a second photoelectric detector, wherein the area between the laser and the two photoelectric detectors is an air flow to be measured, light emitted by the laser is divided into two beams after passing through the semi-transparent semi-reflective lens, one beam of light is perpendicular to the air flow direction to be measured, and the other beam of light forms a certain included angle with the air flow direction to be measured;
The laser measuring instrument also comprises a sampling circuit and a data processing system, wherein the sampling circuit is connected with the two photoelectric detectors, the two photoelectric detectors receive two beams of light passing through the air flow area to be measured, the received light signals are converted into electric signals, the electric signals are input into the sampling circuit, and the sampled data are processed by the data processing system and calculate the gas flow and the methane content.
The laser measuring instrument further comprises a modulation signal generator, a temperature control module and a current control module which are all connected with the laser, wherein the modulation signal generator generates specific modulation signals to carry out wavelength modulation on the laser, and the temperature control module and the current control module control the emission wavelength of the laser.
By adopting the technical scheme, the laser is an ultra-narrow linewidth semiconductor laser.
With the technical proposal, the included angle is 30-60 degrees.
The invention also provides a measuring method for simultaneously measuring the gas flow and the methane content, which comprises the following steps:
The light emitted by the laser is divided into two beams after passing through the semi-transparent semi-reflective lens, wherein one beam of light which is perpendicular to the direction of the airflow to be measured is used as a reference signal for calibrating frequency shift and a standard signal for calibrating gas concentration, and the other beam of light which forms a certain included angle with the direction of the airflow to be measured is used as a signal light for Doppler flow measurement;
The two beams of light are respectively received by two photoelectric detectors after passing through the airflow area to be detected, the two photoelectric detectors convert the received optical signals into electric signals, and the electric signals are input into a sampling circuit and then processed by a data processing system;
the data processing system calculates the frequency offset between the two paths of signals to invert the gas flow rate, and inverts the content of the actual gas by utilizing the different absorption intensities of the gases with different concentrations.
According to the technical scheme, the wavelength modulation is carried out on the laser by generating a specific modulation signal through the modulation signal generator, and the emission wavelength of the laser is controlled by the temperature control module and the current control module.
By adopting the technical scheme, the modulation signal generator generates a high-frequency modulation signal and a low-frequency sawtooth signal, the low-frequency sawtooth signal tunes the laser emission wavelength, and a single absorption spectrum line of a gas molecule is scanned at a fixed frequency so as to obtain a gas absorption spectrum line; the high-frequency sinusoidal signal is used as carrier wave to modulate the low-frequency sawtooth wave current, the wavelength modulation technology is adopted, and the generated laser signal is demodulated by a phase-locked amplifier after being absorbed by a gas medium, so as to obtain the second harmonic signal.
The invention has the beneficial effects that: the invention combines the laser Doppler effect with the tunable semiconductor laser absorption spectrum technology, and simultaneously realizes the measurement of the gas flow velocity and the methane concentration by taking the gas absorption spectrum to be measured as the basis. Compared with an all-fiber structure measurement system, the free space optical system has the advantages of low cost, simpler structure and fewer introduced optical devices, and can realize the light splitting effect only by two lenses.
Drawings
The invention will be further described with reference to the accompanying drawings and examples, in which:
FIG. 1 is a block diagram of the overall architecture of the present invention;
FIG. 2 is a typical natural gas absorption line;
Fig. 3 is a typical second harmonic signal frequency shift diagram.
Detailed Description
The present invention will be described in further detail with reference to the drawings and examples, in order to make the objects, technical solutions and advantages of the present invention more apparent. It should be understood that the specific embodiments described herein are for purposes of illustration only and are not intended to limit the scope of the invention.
The invention utilizes tunable semiconductor laser absorption spectroscopy (TDLAS) to measure the flow velocity, and combines the TDLAS technology and Doppler frequency shift to measure the gas content and simultaneously realize the measurement of the gas flow velocity.
The invention relates to a laser measuring instrument for gas flow rate and methane content, which is shown in figure 1 and comprises a laser 7, a semi-transparent and semi-reflective lens 8, a total reflection mirror 9, a photoelectric detector 10 and a photoelectric detector 11. The photo detector 10 may be fixed on a pipeline perpendicular to the light emitted by the laser, the position of the photo detector 11 is related to the pipe diameter D and the distance L between the two lenses, specifically, l±d/tan θ, where θ is an included angle between the reflected light passing through the lens 9 and the air flow direction, the laser 7 and the photo detectors 10, 11 are respectively disposed on two sides of the air flow area to be measured, that is, the area between the laser 7 and the two photo detectors is the air flow to be measured, the light emitted by the laser 7 is split into two beams after passing through the semi-transparent lens 8, where a beam of light transmitted through the semi-transparent lens 8 at 45 ° is perpendicular to the air flow direction to be measured, and the reflected light is parallel to the air flow to be measured and reflected by the total reflection mirror 9 and forms an angle (e.g. 30-60 °) with the air flow direction to be incident into the air flow to be measured.
One beam of light perpendicular to the direction of the airflow to be measured is used as a reference signal for calibrating frequency shift and a standard signal for calibrating gas concentration, and the other beam of light forming a certain included angle with the direction of the airflow to be measured is used as signal light for Doppler flow velocity measurement.
The laser measuring instrument further comprises a sampling circuit 1 and a data processing system 2. The two beams of light are respectively received by the photoelectric detectors 10 and 11 after passing through the air flow area to be detected, the two photoelectric detectors convert the received light signals into electric signals, the electric signals are input into the sampling circuit 1 and then processed by the data processing system 2, and finally measured data are input into the central control room 3.
The data processing system 2 calculates the frequency offset between the two paths of signals to invert the gas flow rate, the actual gas content is inverted by utilizing the different absorption intensities of the gases with different concentrations, and the inverted result can be displayed on a display interface.
The laser measuring instrument also comprises a modulation signal generator 4, a temperature control module 5 and a current control module 6. The modulation signal generator 4 generates a specific modulation signal to perform wavelength modulation on the laser, and the temperature control module 5 and the current control module 6 precisely control the frequency of the emission center wavelength of the laser by using temperature and current respectively. Specifically, the temperature control module 6 is utilized to scan in a certain temperature range, and the temperature of the laser is locked after the optimal absorption peak is determined; and then the current control module 6 is utilized to automatically and accurately control the center frequency of the emergent laser.
The modulation signal generator generates a high frequency modulation signal and a low frequency saw tooth signal. The low-frequency sawtooth signal tunes the laser emission wavelength, and scans a single absorption spectrum line of the gas molecule at a fixed frequency so as to obtain a gas absorption spectrum line; the high frequency sinusoidal signal acts as a carrier to modulate the low frequency sawtooth current. After the generated laser signal is absorbed by a gas medium by adopting a wavelength modulation technology, the generated laser signal is demodulated by using a phase-locked amplifier to obtain a second harmonic signal, as shown in fig. 2, the second harmonic signal is typical of methane gas, and compared with a traditional direct absorption method, the interference of other gases is eliminated, and the measurement precision and the measurement range of the system can be greatly improved.
The emitted laser is split by the lens and then passes through the gas area to be detected to be received by the detector. The beam of light perpendicular to the air flow is used as a reference signal for calibrating frequency shift and a standard signal for calibrating gas concentration, the center frequency of the beam is used as the reference signal for Doppler frequency shift of the system, the second harmonic signal intensity of the beam is used as the standard signal for measuring the gas concentration, the gas concentration is directly proportional to the second harmonic signal intensity to calculate the actual concentration of the gas to be measured, as shown in fig. 2, the second harmonic signal of methane gas is shown, when the methane content is higher, the amplitude of the signal is higher, the methane content is lower, and the amplitude of the signal is smaller; the other beam of light with a certain angle with the airflow is used as signal light for Doppler flow velocity measurement, as shown in fig. 3, the waveforms of the two paths of signals are identical in amplitude, only the center frequency positions corresponding to the highest peaks are different, the larger the gas flow velocity is, the larger the change of the center frequency positions is, and conversely, the smaller the change of the center frequency positions is, and the gas flow velocity is calculated by calculating the frequency change between the two paths of signals.
The detector converts the optical signal into an electric signal and inputs the electric signal into the sampling circuit, the high-speed high-precision AD converter is utilized to greatly improve the resolution of the system, the frequency shift of 0.01cm -1 can be identified, the minimum flow rate of 0.01m/s can be measured, and the real-time gas flow rate can be obtained through the data processing system.
The second harmonic signal intensity of the methane gas is in direct proportion to the gas concentration, the actual concentration of the gas is inverted, the measurement of the methane content in any range can be realized through a calibration method, and the measurement concentration range is effectively enlarged while the measurement accuracy is improved.
The invention utilizes tunable semiconductor laser absorption spectrum technology and Doppler frequency shift principle to measure gas flow and methane content. An ultra-narrow linewidth semiconductor laser can be used as a light source, and the frequency modulation scanning range of the ultra-narrow linewidth semiconductor laser only comprises a single absorption spectrum line of the measured gas, and has extremely high resolution. The main component of the natural gas is methane, and the detection of the natural gas content in the region to be detected can be realized by scanning a single absorption spectrum line of methane molecules near 1650 nm. When a vibration source such as sound, light and radio waves and an observer relatively move at a relative velocity V, the frequency of vibration received by the observer is different from the frequency emitted by the vibration source, and this phenomenon is a doppler effect, and the difference between the transmitted and received frequencies due to the doppler effect is called doppler shift. According to the invention, the inversion of the gas flow rate is carried out according to the frequency shift between the gas absorption signals to be measured, and when the gas flow rate is larger, the frequency shift is larger, and the measurement is more accurate; the smaller the gas flow rate, the smaller the frequency shift, requiring a high resolution system to ensure the accuracy of the measurement, which can be used for both natural gas concentration measurements and for measuring a wide range of natural gas flow rates.
According to Lambert-Beer absorption law, after monochromatic light with intensity of I 0 and frequency of v passes through an absorption medium with length of L, the intensity measured at a receiving end is:
I(ν)＝I0e(-α(ν)CL) (1)
alpha (v) is the absorption cross section of the medium at frequency v and C is the concentration of the measured gas.
Direct absorption methods are susceptible to background interference and it is difficult to detect small absorption changes. The laser is modulated by adopting a harmonic detection technology, and a second harmonic signal of the detection line is obtained by demodulation of a phase-locked amplifier, so that noise and interference of a fitting base line on a measurement result can be effectively removed, and higher sensitivity is realized. The wavelength modulation technology uses a high-frequency sine wave as a carrier wave, and carries out current modulation on a low-frequency sawtooth wave signal to obtain the following emission frequency of the laser:
ν＝ν0+δνcosωt (2)
V 0 and delta ν are the laser emission center frequency and the frequency modulation amplitude, respectively, and ω is the modulation frequency. After light passes through the airflow region to be detected, the intensity can be changed, and Fourier expansion can be performed to obtain various subharmonic signals. The subharmonic of the detection signal has a better linear relation with the gas concentration, factors such as the anti-interference capability, the amplitude, the peak value positioning and the like of the system are comprehensively considered, and the second harmonic signal is generally selected to process and operate the detection signal. The expression of the second harmonic component is as follows:
The concentration of the gas to be measured can be calculated and obtained according to the second harmonic signal and the direct current signal intensity:
The concentration of the gas to be measured is in direct proportion to the amplitude of the second harmonic signal, and the K value can be obtained through calibration, so that the concentration value of the actual gas is obtained through calculation.
The relationship between the flow velocity and the frequency shift of the gas to be measured can be known according to the Doppler frequency shift principle:
V, which is the laser frequency received by the detector, C is the speed of light, V is the speed of the air flow in the light transmission direction, and the formula is processed to obtain:
v gas is the flow velocity of the gas to be measured, θ is the acute angle between the gas flow and the light transmission direction, as shown in fig. 1, and Δν is the difference between the frequencies of the signals received by the two detectors, i.e. the frequency shift.
The gas flow rate is proportional to the frequency shift, with a larger frequency shift resulting in a larger measurable gas flow rate. The gas flow velocity inversion is carried out according to the frequency shift between the gas absorption signals to be detected, so that the gas flow velocity can be accurately calculated and is not interfered by factors such as pipeline pressure, fluid state and the like.
In summary, the present invention uses an optical method in combination with doppler shift to measure flow velocity and concentration simultaneously, and compared with the conventional method:
(1) The invention has the advantages of non-contact measurement, high measurement precision, real-time quick response, wide measurement range, strong anti-interference performance and the like, and the laser Doppler flow rate measurement principle determines that the gas to-be-measured speed is only related to the laser center frequency, the light speed, the frequency shift of the absorption spectrum of the gas to be measured and the included angle between the gas transmission direction and the laser emission direction, and is irrelevant to the gas pressure, the temperature, the gas quality and the like;
(2) The invention adopts a space light structure, the lens is used for dividing the emitted laser into two beams, the signal to noise ratio and the measurement precision can be greatly improved through signal processing, and the optical system has simple and compact structure, thereby effectively reducing the influence of system vibration on the light path;
(3) The invention is based on the absorption spectrum of the gas to be measured, performs concentration measurement by utilizing the absorption intensity of the gas to be measured, inverts the gas flow velocity according to the frequency of the gas absorption spectrum, has multiple purposes, and realizes the measurement of the gas flow velocity and the methane concentration;
(4) The invention can realize real-time online measurement, the scanning interval is less than 0.3 seconds, the time delay is short, and the remote communication of the Internet of things is convenient for the remote signal transmission;
(5) The invention has the advantages of no vulnerable device, no maintenance, high automation degree and no field personnel on duty in the remote communication of the Internet of things.
It will be understood that modifications and variations will be apparent to those skilled in the art from the foregoing description, and it is intended that all such modifications and variations be included within the scope of the following claims.
Claims (6)
1. The laser measuring instrument for the gas flow and the methane content is characterized by comprising a laser, a semi-transparent semi-reflective lens, a total reflection mirror, a first photoelectric detector and a second photoelectric detector, wherein the area between the laser and the two photoelectric detectors is the gas flow to be measured, light emitted by the laser is divided into two beams after passing through the semi-transparent semi-reflective lens, one beam of light is perpendicular to the direction of the gas flow to be measured, and the other beam of light forms a certain included angle with the direction of the gas flow to be measured;
The laser measuring instrument also comprises a sampling circuit and a data processing system, wherein the sampling circuit is connected with the two photoelectric detectors, the two photoelectric detectors receive two beams of light passing through the air flow area to be measured, the received light signals are converted into electric signals, the electric signals are input into the sampling circuit, and the sampled data are processed by the data processing system and calculate the gas flow and the methane content;
The laser measuring instrument further comprises a modulation signal generator, a temperature control module and a current control module which are all connected with the laser, wherein the modulation signal generator generates specific modulation signals to carry out wavelength modulation on the laser, and the temperature control module and the current control module control the emission wavelength of the laser.
2. The gas flow and methane content laser measurement instrument of claim 1, wherein the laser is an ultra-narrow linewidth semiconductor laser.
3. The gas flow and methane content laser measuring instrument of claim 1, wherein the included angle is 30-60 °.
4. A method of measuring gas flow and methane content simultaneously based on the gas flow and methane content laser gauge of claim 1, comprising the steps of:
The light emitted by the laser is divided into two beams after passing through the semi-transparent semi-reflective lens, wherein one beam of light which is perpendicular to the direction of the airflow to be measured is used as a reference signal for calibrating frequency shift and a standard signal for calibrating gas concentration, and the other beam of light which forms a certain included angle with the direction of the airflow to be measured is used as a signal light for Doppler flow measurement;
The two beams of light are respectively received by two photoelectric detectors after passing through the airflow area to be detected, the two photoelectric detectors convert the received optical signals into electric signals, and the electric signals are input into a sampling circuit and then processed by a data processing system;
the data processing system calculates the frequency offset between the two paths of signals to invert the gas flow rate, and inverts the content of the actual gas by utilizing the different absorption intensities of the gases with different concentrations.
5. The method of claim 4, wherein the wavelength modulation of the laser is performed by generating a specific modulation signal from a modulation signal generator, and the emission wavelength of the laser is controlled by a temperature control module and a current control module.
6. The method of measuring of claim 4, wherein the modulation signal generator generates a high frequency modulation signal and a low frequency sawtooth signal, the low frequency sawtooth signal tuning the laser emission wavelength to scan a single absorption line of the gas molecule at a fixed frequency to obtain the gas absorption line; the high-frequency sinusoidal signal is used as carrier wave to modulate the low-frequency sawtooth wave current, the wavelength modulation technology is adopted, and the generated laser signal is demodulated by a phase-locked amplifier after being absorbed by a gas medium, so as to obtain the second harmonic signal.
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| CN109612933A (en) * | 2018-10-11 | 2019-04-12 | 上海安杰环保科技股份有限公司 | A kind of gas phase molecular absorption spectrometer based on Internet of Things |
| CN109270008A (en) * | 2018-10-31 | 2019-01-25 | 润电能源科学技术有限公司 | A kind of burning gas temperature and multicomponent concentration measuring method and system based on TDLAS |
| CN110736720A (en) * | 2019-11-29 | 2020-01-31 | 阿拓米柯(北京)科技有限公司 | online detector for detecting HF gas content and detection method |
| CN112946232A (en) * | 2021-02-04 | 2021-06-11 | 成都秦川物联网科技股份有限公司 | Natural gas energy metering data acquisition method and system |
| CN113959955B (en) * | 2021-10-19 | 2024-08-13 | 武汉米字能源科技有限公司 | Method and device for online detection of natural gas relative density by laser |
| CN114814286B (en) * | 2022-02-25 | 2023-06-02 | 上海理工大学 | Online low-pressure system flow velocity testing device and method |
| CN117990655A (en) * | 2024-04-02 | 2024-05-07 | 清华大学合肥公共安全研究院 | Composite detector and detection method |
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| CN101149342A (en) * | 2007-10-25 | 2008-03-26 | 南京树声科技有限公司 | Laser gas remote measurement method and device |
| CN102478510A (en) * | 2010-11-24 | 2012-05-30 | 上海复莱信息技术有限公司 | Spectrum type sensor for methane detection |
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| CN2833577Y (en) * | 2005-04-30 | 2006-11-01 | 中国科学院安徽光学精密机械研究所 | Optical structure of instrument for online monitoring methane concentration in atmosphere by means of laser absorption spectroscopy |
| JP6430402B2 (en) * | 2013-01-23 | 2018-11-28 | カリフォルニア インスティチュート オブ テクノロジー | Small tunable laser spectrometer for trace gas detection |
| GB2524725B (en) * | 2014-03-24 | 2016-03-30 | Servomex Group Ltd | Method and system for correcting incident light fluctuations in absorption spectroscopy |
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| Publication number | Priority date | Publication date | Assignee | Title |
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| CN101149342A (en) * | 2007-10-25 | 2008-03-26 | 南京树声科技有限公司 | Laser gas remote measurement method and device |
| CN102478510A (en) * | 2010-11-24 | 2012-05-30 | 上海复莱信息技术有限公司 | Spectrum type sensor for methane detection |
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