The highly sensitive laser gas analyzer is designed to analyze the content of impurity gases in air samples. The main elements of the gas analyzer: a waveguide CO 2 -laser, a resonant optical-acoustic cell, and a computer, the library of which contains information about the absorption lines of 37 gases. Information on the limits of gas detection by the developed gas analyzer is presented. The detection limit for ammonia with an error of 15% is 0.015 ppb.

The need for constant monitoring of the content in the air of a large number of contaminants in large areas at a reasonable cost of funds and labor sets the task of equipping the environmental control service with gas analyzers that meet the following requirements: 1) the detection threshold at the level of maximum permissible concentrations of analyzed substances; 2) high selectivity in relation to foreign substances; 3) multi-component analysis; 4) high speed (short measurement cycle time when taking one sample), which provides the ability to work in motion and a relatively quick response to exceeding a given concentration level; 5) continuous measurements for 2-4 hours to determine the size of the contaminated area.

The existing methods for detecting gases can be conditionally divided into traditional (non-spectroscopic) and optical (spectroscopic) ones. The paper lists the advantages and disadvantages of the main traditional methods from the point of view of their application for the analysis of gas impurities of a complex composition in air.

Spectroscopic methods, the rapid development of which is determined by the unique characteristics of lasers, make it possible to eliminate the main disadvantages of traditional devices and provide the necessary speed, sensitivity, selectivity, and continuity of analysis. In most cases, to detect air pollution by spectroscopic methods, the middle IR spectral region is used, where the main vibrational bands of the overwhelming majority of molecules are concentrated. The visible and UV regions are less informative in this respect.

A special place in the family of IR laser gas analyzers is occupied by devices with CO 2 lasers. These lasers are durable, reliable and easy to use and can detect over 100 gases.

A gas analyzer (prototype) that meets the above requirements is described below. A waveguide CO 2 laser is used as a radiation source, and a resonant optoacoustic cell (RSA) is a sensitive element. The optical-acoustic method is based on the registration of a sound wave excited in a gas upon absorption of an amplitude-modulated laser radiation in a ROA. The sound pressure, which is proportional to the specific absorbed power, is recorded by the microphone. The block diagram of the gas analyzer is shown in Fig. 3.1. The modulated radiation of the CO 2 laser hits the wavelength tuning unit. This unit is a diffraction grating that allows you to tune the radiation wavelength in the range of 9.22-10.76 microns and obtain 84 laser lines. Further, the radiation is directed through the system of mirrors into the sensitive volume of the ROA, where the gases that absorb the radiation entering it are recorded. The energy of the absorbed radiation increases the temperature of the gas. The heat released on the cell axis is transferred mainly by convection to the cell walls. The modulated radiation causes a corresponding change in gas temperature and pressure. The change in pressure is perceived by the membrane of the capacitive microphone, which leads to the appearance of a periodic electrical signal, the frequency of which is equal to the modulation frequency of the radiation.

Figure 3.1. Gas analyzer block diagram

Fig. 3, 2 shows a sketch of the internal cavity of the r.o.a.y. It is formed by three cylindrical active volumes: symmetrically located volumes 1 and 2 with a diameter of 20 mm and an internal volume 3 with a diameter of 10 mm. The inlet 4 and outlet 5 windows are made of BaF 2 material. The microphone is installed at the bottom of the cell and is connected to the active volume by a hole 6 with a diameter of 24 mm.

Figure 3.2 The inner cavity of the resonant optical-acoustic cell. 1, 2 - external volumes, 3 - internal volume. 4, 5 - input and output windows, 6 - microphone hole

Optical resonance "caused by the absorption of laser radiation by a gas, under normal conditions arises at a modulation frequency of 3.4 kHz, and the background signal due to absorption of radiation by the ROA windows is maximum at a frequency of 3.0 kHz. The Q factor in both cases is> 20 Such a design of the ROA provides a high sensitivity of the gas analyzer and makes it possible to suppress the contribution of the background signal using a frequency- and phase-selective amplifier. At the same time, the ROA is insensitive to external acoustic noise. electrical signal when measuring concentration is determined by the formula

where K is the cell constant, is the laser radiation power, b is the absorption coefficient of radiation by the gas, and C is the gas concentration.

Before measurements, the gas analyzer is calibrated using a span gas (CO2) with a known concentration.

The amplitude is measured using an ADC board included in the Advantech computer. The same computer is used to control the wavelength tuning unit and calculate the concentrations of the measured gases.

The developed information processing program is intended for the qualitative and quantitative analysis of the gas mixture by the absorption spectrum of the laser radiation of the CO 2 laser. The initial information for the program is the measured absorption spectrum of the analyzed gas mixture. An example of an absorption spectrum of nitrogen, plotted in units of optical thickness, is shown in Fig. 3.3a, and Fig. 3.3b shows an example of an absorption spectrum with a small addition of ammonia.

Figure 3.3 Absorption spectra: a - nitrogen at normal atmospheric pressure, b - nitrogen-ammonia mixture.

Optical thickness, where

Cm -1 atm -1 - absorption coefficient of the j-th gas on the i-th laser line, С i, atm - concentration of the j-th gas, i

The library of possible components contains the values ​​of the absorption coefficients and is a matrix of dimensions (N x m). The number of gases presented in the library is m = 37, the maximum number of analyzed laser lines is N - 84 (21 lines in each branch of the CO 2 laser).

In the process of analyzing the spectrum of a gas mixture formed by overlapping absorption lines of gases included in the mixture, the program selects from the library those components that allow the best description of the mixture spectrum. One of the main criteria for searching for the best set of components is the value of the root-mean-square deviation between the experimental and the absorption spectrum found as a result of iterations:

The algorithm for solving the inverse problem - searching for concentrations from the known absorption spectrum - was constructed using the Gaussian elimination method and the Tikhonov regularization method, and the main difficulties in its implementation are associated with the estimation of the stability of the solution (the elements of the absorption coefficient matrix, as well as the free terms, are known only approximately ), choosing the regularization parameter and finding criteria for terminating the iterative process.

The table contains calculated information about the detection limits of some gases described by the gas analyzer:

Main operating characteristics of the gas analyzer: the number of simultaneously measured gases - up to 6; measurement time 2 min; detection limit for carbon dioxide 0.3 ppm: detection limit for ammonia 0.015 ppb: measurement range for carbon dioxide 1 ppm -10%; measurement range for ammonia 0.05 ppb-5 ppm; measurement error 15%; supply voltage 220V ± 10%. [ 1]

Characteristic

The device is designed for operational gas analysis of atmospheric air by the method of optical-acoustic laser spectroscopy

The principle of operation of the gas analyzer is based on the generation of acoustic waves in air when a modulated laser beam interacts with molecules of a gas impurity that absorbs laser radiation at a given wavelength. Acoustic waves are converted by the microphone into electrical signals proportional to the concentration of the absorbing gas. By tuning the laser wavelength and using the known spectral data on the absorption coefficients of various gases, it is possible to determine the composition of the detected gas impurity.

A distinctive feature of this gas analyzer is the combination in a single design of a tunable waveguide CO2 laser and a pumped-through optical-acoustic detector (OAD) of a differential type. The OAD is located inside the laser cavity and forms a single structure with the laser. Due to this, the losses on the optical elements are reduced, the power inside the working channel of the OAM and the rigidity of the entire structure increase. The gas analyzer uses an automatically line-tunable waveguide CO2 laser with high-frequency (HF) excitation, in which a repetitively pulsed generation mode is set by modulating the power of the RF generator, which makes it possible to optimize power consumption by adjusting the duty cycle of the excitation pulses. In the design of the used differential type OAM, there are two resonant acoustic channels, in

which form antiphase acoustic waves, which allows, with the introduction of appropriate treatment, to minimize noise when air flows through the channels.

These features of the device are unique and together provide an extremely high detection sensitivity for optoacoustic devices, a low level of hardware noise, and a relatively low total power consumption.

The gas analyzer is capable of registering the minimum absorption coefficients of gaseous impurities in the atmosphere in a gas flow at a level of ~ 5 × 10-10 cm-1 with a high response speed inherent in optical methods of gas analysis. Due to these qualities, as well as the possibility of tuning the wavelength of laser radiation in the range of 9.3 ÷ 10.9 μm, the gas analyzer allows real-time measurements of low concentrations of atmospheric and anthropogenic gases (at a level of 1 ppb or less), such as C2

Н4, NH3, O3, C6, SO2, SF6, N2

O, CH3, CH3, etc.,

including vapors of a series of explosives and toxic substances (about 100 substances in total).

These properties make it possible to use the device for monitoring the concentrations of chemical molecular compounds in the atmospheric air and technological processes, to analyze the exhaled air in order to detect various diseases, etc.

Applying an effect

The obvious advantages of the OA method in combination with the use of sufficiently powerful cw frequency-tunable lasers make it especially attractive for solving problems requiring measurements of weak absorption of radiation by molecular gases. First of all, this concerns the problems of gas analysis at low and ultra-low concentrations of molecules in the medium.

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The operation of the Yokogawa TDLS200 laser gas analyzer is based on the diode laser absorption spectroscopy method.

This device is characterized by high selectivity and long-term stability, provides fast "in-situ" (directly in the pipe) analysis of gases with corrosive components or high temperatures. What is the principle of operation of this device and where does it find its application?

The laser gas analyzer uses Tunable Diode Laser Absorption Spectroscopy (TDLAS) and has the ability to measure the concentration in a sample gas with high selectivity and without direct contact - only by irradiating the sample gas with radiation from a tunable laser diode. Thus, fast and accurate “in-situ” measurements can be performed in process flues under various conditions. For example, measurements can be carried out at temperatures up to 1500 ° C, as well as in environments with pulsating pressure. The Yokogawa TDLS200 laser gas analyzer can also measure in the presence of corrosive or toxic gases. The analyzer delivers accurate analytical signals with fast response times to maximize product yields, energy efficiency and safety in a variety of manufacturing processes. The simplicity of the design (no moving parts and no limited life components) ensures operation and operation with little or no maintenance.

The Yokogawa TDLS200 laser gas analyzer is a new type of laser gas analyzer used for industrial measurement. The use of the peak area integration method eliminates measurement errors caused by pressure changes and the presence of other gases in the sample. It also allows for accurate determination of the concentration of gas components, even when its temperature and other parameters change at the same time. This article provides an overview of the TDLS200 laser gas analyzer, its functions and measuring principle, and also considers examples of its application.

The gas analyzer has a radiation unit and a detection unit, which are usually placed opposite each other on opposite sides (across) the gas duct through which the process gas flow passes. A similar option is used for gas ducts up to 20 m wide.

Optical windows separate the inside of the analyzer from the measured medium. Semiconductor laser radiation passes through the optical window of the radiation unit, the measured gas, the optical window of the detecting unit and reaches the photodetector. A photo detector registers the laser beam and converts its energy into an electrical signal. The computing device of the radiation unit determines the absorption spectrum of the measured component, calculates the peak area of ​​the spectrum, converts it into the component concentration and outputs it as an analog signal 4 ... 20 mA.

The adjustment mechanism has a corrugated design, which makes it possible to simplify the adjustment of the optical axis angle, while maintaining the tightness of the pipeline, which is especially important for technological processes in industry. The connection of the radiation unit and the detecting unit using the optical axis adjustment device simplifies the adjustment of the optical axis not only for the standard configuration (two units are placed on both sides of the pipe, as shown in Figure 1), but also for other installation options. This technical solution allows you to choose the method of installation of the device that is best suited for the measured components and technological design of the process, and at the same time guarantees optimal measurement conditions.

The TDLS200 uses Diode Laser Absorption Spectroscopy (TDLAS). The method is based on measuring the absorption spectrum of radiation (infrared / near infrared region), inherent in the molecules of a substance due to the vibrational and rotational energy of the transition of molecules in the measured component. The radiation source for the formation of the spectrum is a semiconductor laser with an extremely narrow spectral line width. The optical absorption spectrum of basic molecules such as O2, NH3, H2O, CO and CO2 ranges from infrared to near infrared. Measuring the amount of absorbed radiation at a specific wavelength (spectral absorption capacity) makes it possible to calculate the concentration of the component to be measured.

Unlike conventional low-resolution spectrometers, the TDLS200 uses a laser beam with an extremely narrow spectral line width. The emitter is a tunable laser diode, the radiation wavelength of which can be changed by adjusting the laser temperature and excitation current. This allows measurements of a single absorption peak from several in the spectrum. Thus, as shown in Fig. 6, one absorption peak can be selected for measurement, which is not subject to interference from other gases.

Due to its high wavelength selectivity and the absence of interference from other components in the gas mixture, there is no need for additional sample preparation, which allows the TDLS200 to be used “in-situ” (directly in the process).

The TDLS200 measures the isolated absorption spectrum of a gas mixture component, free from interference from interfering components. The measurement is performed by sweeping the wavelength of a tunable laser diode along a single absorption peak of the measured component.

Although the absorption spectrum measured by the TDLS200 is isolated from interfering components, the shape of the spectrum can change (expansion effect) depending on the gas temperature, gas pressure, and other components present in the gas mixture. To make measurements under these conditions, compensation is required.

The TDLS200 gas analyzer sweeps the semiconductor laser radiation wavelength along the absorption line of the measured component and calculates its concentration from the absorption spectral region by integrating the peak area.

The Yokogawa TDLS200 gas analyzer, due to its fast in-situ measurement (directly in the pipeline), can be successfully used in existing technical processes both for their high-speed regulation, when the signals required for process control, containing readings of the component concentrations, are fed directly to the DCS, and for real-time process state management. In this way, the TDLS200 can help optimize the performance of various industrial processes. In this section, we will look at the measurement of the residual NH3 concentration in the flue gas. Note that the use of TDLS200 for combustion optimization has been described elsewhere by Yokogawa (3). Please refer to this report for details.

Ammonia (NH3) is injected into the flue gas in order to remove NOx (purify the flue gas from nitrogen oxides), improve the efficiency of dust collectors and prevent corrosion. Excessive NH3 increases operating costs and residual NH3, resulting in a putrid odor. Thus, the amount of NH3 in the flue gas must be measured, monitored and regulated. For example, the DeNOx ACR (selective catalytic reduction) process is used in the equipment for cleaning the off-gas of a combustion furnace from nitrogen oxides, in which NOx is reduced to N2 and H2O using NH3 injection and selective catalysis of the reduction process, and the residual NH3 concentration (on the order of ppm) in flue gas is measured in real time.

Traditional NH3 meters using indirect NOx measurement methods (chemiluminescence and ion electrode) have long response times, require a sampling line, including heated pipes to avoid NH3 adhesion, and consequently high maintenance costs such complex measuring systems. On the other hand, as shown in Figure 8, the TDLS200 laser gas analyzer installs directly into the process pipeline and measures NH3 directly, which greatly reduces response time and simplifies maintenance. In addition, a fast response analytical NH3 concentration signal can be used to control and optimize NH3 injection.

High selectivity, short response time, ease of maintenance, achieved due to the used measurement technology and design of the analyzer, provide the possibility of its use in a wide range of technological processes. Applications include not only the measurement of NH3 discussed in this article, but also the determination of CO and O2 in the optimization of combustion processes, the measurement of small quantities of water in electrolysis plants, etc. The use of such gas analyzers can make a significant contribution to preserving the environment and reducing operating costs , thanks to its use for process control, and not just for monitoring purposes.

Kazuto Tamura,

Yukihiko Takamatsu,

Tomoyaki Nanko,

Laser gas analyzer "LGAU-02" is designed to measure the concentration of gaseous hydrocarbons in the air pumped through the gas cell of the device. The gas analyzer can be used both autonomously and as part of mobile auto and air laboratories. The complex includes:

• laser gas analyzer "LGAU-02";
• remote control unit with sources of sound signals;
• additionally: a personal computer with installed software.

Rice. 1

A diagram of the organization of an auto laboratory is presented for finding leaks from underground gas pipelines is presented in Fig. 1 In an air laboratory, you can do without a flow stimulator, providing an effective air intake by the outboard air pressure, and on a hand trolley, you can use an external sampler instead of a surface sampling device.

The advantages of the LGAU-02 gas analyzer are manifested when solving problems:

• detecting leaks from underground gas pipelines of city gas networks, as well as from main and distribution pipelines using an auto laboratory that performs measurements on the go;
• detecting leaks from underground, surface and air pipelines using a hand trolley that takes measurements on the go;
• detection of leaks from main gas pipelines using an aviation laboratory;
• measuring variations in the methane (hydrocarbon) background over large areas (hydrocarbon survey) using an air laboratory in order to search for oil and gas fields and environmental control of the atmosphere.

Rice. 2

• The software allows you to maintain archives. An event log is also kept.

Functionality of the complex

• The gas analyzer is made in the form of an optoelectronic measuring unit in a dust- and splash-proof IP54 case and is completed with a remote control panel equipped with an analog indicator, a single zero setting button and a two-stage sound and light signaling of increased concentrations with adjustable response thresholds. Ease of installation and maintenance of the device, high reliability, small dimensions and power consumption allow it to be used autonomously, on handcarts, cars and on board almost any aircraft carrier, including hang gliders and miniplanes. The gas analyzer can work completely autonomously, and instead of a remote control, any DC voltage measuring device from 0 to 5 V can be connected. Measurement data documentation and plotting in real time can be carried out on a regular personal computer with an RS 232C interface, including a portable one. When connected to the gas analyzer-computer satellite navigation system, it is possible to map the gas contamination field. The flow rate generator can be connected via a special button for switching the supply voltage on the front panel of the device.

Operating experience

• Operating experience. Since 1998, the Lengaz St. Petersburg City Gas Industry and since 2004 the Moscow State Unitary Enterprise Mosgaz have been operating auto labs to search for natural gas leaks from urban underground gas pipelines based on LGAU-02. The prototypes of the device were used as part of air laboratories during the atmogeochemical survey in the complex of gas and oil exploration in Tatarstan, Chuvashia and in the north of the Krasnoyarsk Territory and during the environmental examination of the atmosphere in the cities of Tula and Moscow. In addition, the instruments were used as part of auto laboratories for geoecological surveys of the territories of distribution of technogenic soils in a number of areas of mass development in Moscow, as well as autonomously - during ground geochemical surveys in Korea. On the basis of the gas analyzer, an onboard computerized complex for aviation hydrocarbon gas survey was created. In the 2001 field season, the flight time of the complex on board the An 2 aircraft without a single failure of the device exceeded 600 hours, and the total area covered was about 30 thousand square meters. km.

Prospects for the development of the complex

• Implementation of additional USB interfaces;
• Connecting a GPS satellite navigation device with an interactive map of the area;
• Implementation of additional features at the request of the user.

Journal "Instruments and Experiment Technique", 1999, No. 5

Laser gas analyzer for finding gas leaks from underground gas pipelines

Journal "Instruments and Control Systems", 1998, No. 9

Onboard laser absorption hydrocarbon gas analyzer

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