JP4588520B2 - Gas medium analyzing apparatus and gas medium analyzing method for simultaneously detecting various kinds of gas medium - Google Patents

Description

  The present invention relates to a gas medium analyzing apparatus and a gas medium analyzing method for optically measuring the concentration of a gas medium contained in a sample gas. More specifically, the present invention relates to a gas medium analyzer and a gas medium analysis method for simultaneously measuring the concentrations of various types of gas media contained in a sample gas by infrared spectroscopic analysis.

  In thermal power plants, etc., exhaust gas after combustion is discharged from generator turbines, boilers, etc., and usually exhausted to the atmosphere after recovering waste heat, but in order to reduce the concentration of nitrogen oxides in the emitted gas Then, exhaust gas is introduced into a denitration apparatus having a catalyst and reacted with ammonia by a catalytic reduction method, nitrogen oxides are decomposed into nitrogen gas and water, and then the treated gas is released to the atmosphere.

  In addition, nitrogen oxides are also contained in combustion exhaust gas from diesel engines such as automobiles. In order to reduce the concentration of nitrogen oxides, the combustion exhaust gas is introduced into a denitration device having a catalyst and reacted with ammonia by a catalytic reduction method. The nitrogen oxides are decomposed into nitrogen gas and water, and then treated in the atmosphere. The gas is released.

In a denitration apparatus (Patent Documents 1 to 3) used for such applications, it is necessary to adjust the supply amount of ammonia in accordance with the generation amount of nitrogen oxides. A system to detect and monitor with high accuracy in real time is required.
JP 2000-266724 A JP 2000-234723 A Japanese Patent Laid-Open No. 2003-80026

  Means for optically detecting a gas medium such as nitrogen oxide or ammonia include laser absorption, laser-induced fluorescence, and Raman scattering. In gas detection using laser absorption, the output of the light source may be relatively low, but a pair of light sources is required for one type of measured medium. For this reason, when detecting many types of gas media simultaneously, since a plurality of light sources are required, the apparatus becomes large and expensive.

Gas detection using Raman scattering requires a high-power laser light source of several W or more, and the apparatus is large and expensive.
On the other hand, a technique for generating difference frequency light by near infrared light from two near infrared laser light sources and measuring intrinsic absorption lines of molecules in the middle infrared region by this difference frequency light is known. Then, near-infrared light used for generation of difference frequency light is removed, and only one kind of gas to be measured is measured using only mid-infrared difference frequency light.

  In the present invention, the absolute concentrations of two or more kinds of gas media contained in a sample gas, in particular, a gas medium having an absorption band in the near infrared region and a gas medium having an absorption band in the mid infrared region are simultaneously increased in real time. An object of the present invention is to provide a gas medium analyzing apparatus and a gas medium analyzing method capable of accurately detecting.

The gas medium analyzer of the present invention is a gas medium analyzer that optically measures the concentration of a plurality of types of gas media contained in a sample gas,
A first near-infrared light source that generates pump light of a specific wavelength;
A second near-infrared light source that generates signal light of a specific wavelength corresponding to an absorption band of at least one gas medium contained in the sample gas;
A combined light generating means for combining the pump light and the signal light to generate combined light;
A difference frequency that generates a difference frequency light between the pump light and the signal light corresponding to a near-infrared or mid-infrared absorption band in at least one other gas medium included in the sample gas from the combined light. Light generating means;
A cell in which the sample gas is sealed and the combined light of the signal light, difference frequency light and pump light is incident;
A separation filter that separates the near-infrared signal light and pump light that has passed through the cell, and the near-infrared or mid-infrared difference frequency light;
A pump light removal filter that removes the pump light from the combined light of the signal light and the pump light separated by the filter;
Optical axis coupling means for aligning optical axes of the signal light that has passed through the pump light removal filter and the difference frequency light from the separation filter on the same optical axis;
An infrared detector that simultaneously detects and converts the signal light and the difference frequency light whose optical axes are matched by the optical axis coupling means into an electrical signal;
An analyzer for analyzing the concentration of a plurality of types of the gas medium based on an electrical signal from the infrared detector;
It is characterized by providing.

The gas medium analysis method of the present invention is a gas medium analysis method for optically measuring the concentrations of a plurality of types of gas media contained in a sample gas,
Combining a pump light having a specific near-infrared wavelength with a signal light having a near-infrared wavelength corresponding to an absorption band of at least one gas medium contained in the sample gas to generate a combined light;
From the combined light, a difference frequency light between the pump light and the signal light corresponding to a near-infrared or mid-infrared absorption band in at least one other gas medium included in the sample gas is generated,
The combined light of the signal light, the difference frequency light and the pump light is incident on a cell in which the sample gas is sealed,
From the signal light that has passed through the cell, the difference frequency light and the combined light of the pump light, only the signal light and the difference frequency light are extracted on the same optical axis,
The signal light and difference frequency light taken out on the same optical axis are incident on an infrared detector and simultaneously detected,
The concentration of the gas medium of a plurality of types is analyzed based on an electrical signal from the infrared detector.

  According to the present invention, the absolute concentration of various types of gas medium contained in the sample gas, for example, the gas medium having an absorption band in the near infrared region and the absolute concentration of the gas medium having an absorption band in the mid infrared region, At the same time, it can be detected in real time with high accuracy.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the present specification, the “near infrared region” represents an infrared wavelength region of less than 2.5 μm, and the “middle infrared region” represents a wavelength region of 2.5 μm to 25 μm.

FIG. 1 is a diagram showing a configuration of a gas medium analyzer in one embodiment of the present invention. This gas medium analyzer 31 is a sample of NH ₃ in a sample gas containing NH ₃ and NO ₂ as a gas medium.
And the absolute concentration of NO ₂ are measured simultaneously in real time.

The gas medium analyzer 31 includes a Yb fiber laser 1 that generates pump light having a wavelength of 1064 nm as a near-infrared light source that generates pump light.
As a near-infrared light source that generates signal light for detecting NH ₃ , a wavelength 1536
A DFB (Distributed Feedback) semiconductor laser 2 that generates a signal light of nm is provided.

  The pump light emitted from the Yb fiber laser 1 and the signal light emitted from the DFB semiconductor laser 2 are linearly polarized by passing through the polarizers 3a and 3b, respectively, and then WDM (Wavelength Division Multiplexing). ) Coupled by a multiplexer 4.

The combined multiplexed light is condensed on a PPLN (Periodically Poled Lithium Niobate) crystal 5 and passes through the PPLN crystal 5 to be a difference frequency light between pump light having a wavelength of 1064 nm and signal light having a wavelength of 1536 nm. Mid-infrared light having a wavelength of 3.46 μm is generated. NO ₂ in the sample gas is detected by this mid-infrared difference frequency light.

These signal light, difference frequency light, and pump light are guided to the multipass cell 7 in which the sample gas to be measured is sealed, and NH ₃ contained in the sample gas while reciprocating the multipass cell 7 a plurality of times. The signal light is absorbed by, and the difference frequency light is absorbed by NO ₂ contained in the sample gas.

  The light that has passed through the multipass cell 7 is guided to the Ge filter 9. The Ge filter 9 transmits difference frequency light that is mid-infrared light and reflects signal light and pump light that are near-infrared light. Thereby, the difference frequency light, the signal light and the pump light are separated.

  The signal light and the pump light reflected from the Ge filter 9 are guided to the long pass filter 10, and the pump light unnecessary for measurement is removed by the long pass filter 10, and only the signal light is transmitted through the long pass filter 10.

The output of the signal light is attenuated by an ND (Neutral Density) filter and then guided to the wedge-shaped CaF ₂ substrate 11 so that the signal light and the difference frequency light are aligned on the same optical axis by the CaF ₂ substrate 11.

The combined light of the signal light and the difference frequency light matched to the same optical axis is condensed on the light receiving surface of the MCT detector 13 by the parabolic mirror 12.
The electrical signal photoelectrically converted by the MCT detector 13 is amplified by the DC amplifier 14, then A / D converted, and input to the computer 15, where the absolute concentrations of NH ₃ and NO ₂ are analyzed.

  The signal processing and analysis for calculating the absolute density can be performed by applying various conventionally known methods, but as an example, it is performed as follows. The signal light is modulated by a driver so as to be a sawtooth periodic signal, and a sawtooth input signal is generated with the vertical axis representing laser output and the horizontal axis representing wavelength (time). In the MCT detector 13, a shutter is installed in each optical path of the signal light and the difference frequency light. When measuring the signal light, the shutter on the difference frequency light side is closed, and when measuring the difference frequency light, the signal light is closed. Close the side shutter. The signal detected by the MCT detector 13 after passing through the cell is divided by the input signal and normalized to obtain the area of the absorption line. The concentration is converted from the area of the absorption line. Concentration conversion may be performed by a calibration curve prepared from a sample having a known concentration, or may be calculated by substituting the area of the absorption line into a Lambert-Beer equation.

  As an analysis device for analyzing the concentration of the gas medium, a computer having a CPU, a memory, and the like is usually used, and the above signal processing and arithmetic processing are performed according to a program stored in advance.

FIG. 2 is a diagram showing the result of spectroscopy of the absorption lines of NH ₃ and NO ₂ using the gas medium analyzer of the present embodiment. Thus, it was confirmed that both NH ₃ and NO ₂ can be detected simultaneously and with high accuracy.

  The gas medium analyzer of the present embodiment uses near infrared light used for generating the difference frequency light as measurement light, so it is not expensive as a whole, and can be further miniaturized. Is easy.

As mentioned above, although embodiment of this invention was described, this invention is not limited to this embodiment, In the range which does not deviate from the summary, various deformation | transformation and change are possible.
For example, by changing the wavelength of the DFB semiconductor laser and the fiber laser FBG (Fiber Bragg Grating), it is possible to measure molecules other than NH ₃ and NO ₂ . Specifically, for example, if a pump light laser light source with a wavelength of 941 nm and a signal light laser light source with a wavelength of 1605 nm are used, a difference frequency of 2.27 μm is generated. Since the absorption band of CO ₂ and CO exists at a wavelength of 1605 nm and the absorption band of NH ₃ exists at a wavelength of 2.27 μm, these spectral detections can be performed simultaneously.

Further, if a pump light laser light source with a wavelength of 941 nm and a signal light laser light source with a wavelength of 1577 nm are used, a difference frequency of 2.33 μm is generated. CO _{2 at a} wavelength of 1577 nm
And a CO absorption band, and a CH ₄ absorption band exists at a wavelength of 2.33 μm. Therefore, these spectral detections can be performed simultaneously.

Further, if a wavelength region where absorption lines of a plurality of molecules exist is selected as the wavelength of the signal light, it is possible to simultaneously measure a large number of three or more types of molecules. For example, in the wavelength region of 1605 nm, there are CO ₂ and CO absorption bands, and there are absorption bands of these isotopes. By measuring ₃ , three or more types of molecules can be measured simultaneously.

  The fiber laser that generates pump light is a laser oscillator in which a laser medium and an optical resonator are formed in an excitation optical fiber. A laser diode or the like is used as the excitation light source, and laser light from the excitation light source is guided to the excitation optical fiber by an optical fiber. For example, FBGs are provided at both ends of a double clad fiber in which an inner clad such as silica and an outer clad such as a polymer coat are provided on the outside of a glass core, and a multilayer having high reflectivity at the light source side end of the fiber In some cases, a film coat is applied and a multi-layer film coat having a low reflectance is applied to the output side end portion, and the excitation light is condensed in the inner clad and propagates in the fiber. Examples of rare earth elements doped in the core include Er, Tm, and Nd in addition to Yb.

  As the DFB semiconductor laser that generates signal light, a CW type laser that is generally used for communication can be used. The DFB semiconductor laser has a periodic diffraction grating structure (distributed feedback structure) around the active layer, and can be tuned using a refractive index change by current injection, a temperature change by a Peltier element or a heater electrode, and the like. .

As a polarizer for polarizing laser light from a pump light source and a signal light source, a grid polarizer in which a metal lattice is formed on an infrared transmission substrate is used.
A WDM multiplexer includes two optical fibers fused with their cores close to each other, and one using the wavelength dependence of the transmittance (reflection) of a dielectric multilayer film coated on a glass substrate. There is.

The wavelength conversion element that generates the difference frequency light includes a nonlinear optical crystal having a periodically poled structure, for example, one formed of LiNbO ₃ , LiTaO ₃ , KTiOPO ₄ or the like at a wavelength of 5 μm or less. At wavelengths exceeding ₁ , there are those formed of AgGaS ₂ , AgGaSe ₂ or the like. In addition, these crystals are doped with magnesium oxide.

FIG. 1 is a diagram showing a configuration of a gas medium analyzer in one embodiment of the present invention. FIG. 2 is a diagram showing the result of spectroscopy of NH ₃ and NO ₂ absorption lines using the gas medium analyzer of FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Fiber laser 2 DFB semiconductor laser 3a, 3b Polarizer 4 WDM multiplexer 5 PPLN crystal 6 CaF ₂ lens 7 Multipass cell 8 Pressure gauge 9 Ge filter 10 Long pass filter 11 Wedge type CaF ₂ substrate 12 Parabolic mirror 13 MCT Detector 14 DC amplifier 15 Computers 16a to 16f Reflector 31 Gas medium analyzer

Claims (9)

A gas medium analyzer for optically measuring the concentration of a plurality of types of gas media contained in a sample gas,
A first near-infrared light source that generates pump light of a specific wavelength;
A second near-infrared light source that generates signal light of a specific wavelength corresponding to an absorption band of at least one gas medium contained in the sample gas;
A combined light generating means for combining the pump light and the signal light to generate combined light;
A difference frequency that generates a difference frequency light between the pump light and the signal light corresponding to a near-infrared or mid-infrared absorption band in at least one other gas medium included in the sample gas from the combined light. Light generating means;
A cell in which the sample gas is sealed and the combined light of the signal light, difference frequency light and pump light is incident;
A separation filter that separates the near-infrared signal light and pump light that has passed through the cell, and the near-infrared or mid-infrared difference frequency light;
A pump light removal filter that removes the pump light from the combined light of the signal light and the pump light separated by the filter;
Optical axis coupling means for aligning optical axes of the signal light that has passed through the pump light removal filter and the difference frequency light from the separation filter on the same optical axis;
An infrared detector that sequentially detects the signal light and the difference frequency light whose optical axes are aligned by the optical axis coupling means and converts them into an electrical signal;
An analyzer for analyzing the concentration of a plurality of types of the gas medium based on an electrical signal from the infrared detector;
A gas medium analyzing apparatus comprising: The gas medium analyzer further includes a shutter in each optical path of the signal light and the difference frequency light,
The infrared detector is configured to sequentially detect and convert the signal light and the difference frequency light, whose optical axes are aligned by the optical axis coupling means, by switching between opening and closing of the shutter and converting them into electrical signals. The gas medium analyzer according to claim 1, wherein The first near-infrared light source, a gas medium analyzer according to claim 1 or 2, characterized in that a fiber laser. The gas medium analyzer according to any one of claims 1 to 3, wherein the second near-infrared light source is a DFB semiconductor laser. The combined beam generating means, the gas medium analyzer according to any one of claims 1 to 4, characterized in that a WDM multiplexer. It said difference frequency light generating means, a gas medium analyzer according to any one of claims 1 to 5, characterized in that a PPLN crystal. The optical axis coupling means, the gas medium analyzer according to any one of claims 1 to 6, characterized in that a CaF ₂ substrate wedge type. A gas medium analysis method for optically measuring the concentrations of a plurality of types of gas media contained in a sample gas,
Combining a pump light having a specific near-infrared wavelength with a signal light having a near-infrared wavelength corresponding to an absorption band of at least one gas medium contained in the sample gas to generate a combined light;
From the combined light, a difference frequency light between the pump light and the signal light corresponding to a near-infrared or mid-infrared absorption band in at least one other gas medium included in the sample gas is generated,
The combined light of the signal light, the difference frequency light and the pump light is incident on a cell in which the sample gas is sealed,
From the signal light that has passed through the cell, the difference frequency light and the combined light of the pump light, only the signal light and the difference frequency light are extracted on the same optical axis,
The signal light and difference frequency light taken out on the same optical axis are incident on an infrared detector and detected in order ,
A gas medium analysis method, comprising: analyzing concentrations of a plurality of types of the gas medium based on an electrical signal from the infrared detector. The signal light extracted on the same optical axis and the difference frequency light are sequentially detected by switching opening and closing of shutters provided in the optical paths of the signal light and the difference frequency light, respectively. The gas medium analysis method according to claim 8.

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