JPH10142147A - Method for measuring concentration of nitrogen oxides - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method of measuring the concentration of nitrogen oxides, in which the concentration of the nitrogen oxides in a sample gas can be measured simply. SOLUTION: Light from a light source 10 is incident on a Michelson interferometer 20. The Michelson interferometer 20 is constituted of a beam splitter 21, of a moving mirror 22 and of a fixed mirror 23. Light which is radiated from the Michelson interferometer 20 is incident on an absorption cell 31 via a mirror 30. Light which is transmitted through the absorption cell 31 is incident on a phtodetector 32 which detects an optical intensity. A detected signal is input to a computing unit 33.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a nitrogen oxide concentration measuring device for measuring a nitrogen oxide concentration.

[0002]

2. Description of the Related Art In order to control a combustion engine such as a boiler, a refuse incinerator, a gas turbine, a diesel engine, or a gasoline engine and to evaluate the properties of exhaust gas, it is important to measure the concentration of nitrogen oxides in the exhaust gas. It is. In addition, devices that may generate nitrogen oxides, such as devices that use combustion or chemical plants, and their surroundings, or places where nitrogen oxides are likely to accumulate, such as underground parking lots, tunnels, and tollgates on expressways However, the measurement of the nitrogen oxide concentration is performed.

[0003] Nitrogen oxides are NO, NO ₂ , N ₂ O, N
It is a general term for O ₃ , N ₂ O _5, etc. Conventionally, the concentration of NO in the emission source of various devices and the concentration of NO ₂ mainly in the atmosphere have been measured by the following methods. (1) Chemical analysis method A sample gas containing nitrogen oxides is passed through a coloring reagent solution,
This is a method of measuring the color development at a certain wavelength by an absorptiometry method and calculating the concentration of nitrogen oxide NO. The method is based on a zinc reduced naphthylethylenediamine absorptiometry (Zn-NEDA).
Method), phenorodisulfonic acid absorption photometry, Salzman absorption photometry, and the like. (2) Automatic measurement method Chemiluminescence method: By measuring the intensity of emission of nitrogen dioxide (NO ₂ ) in an excited state generated by a chemical reaction between NO and ozone (O ₃ ) when returning to a ground state. N
Obtain the O concentration. Further, NO ₂ in the sample gas is reduced to NO through a converter, and the same measurement is performed.
₂ Find the concentration.

[0004] Infrared absorption method: NO near wavelength 5.3 μm
The NO concentration is measured by measuring the absorbance in this wavelength range using the infrared absorption of the above. In addition, the infrared absorption of NO ₂ near the wavelength of 6.2 μm is used, and the absorbance in this wavelength range is measured to measure the NO ₂ concentration.

[0005] Ultraviolet absorption method: utilizing the ultraviolet absorption of NO having a wavelength of 195 to 230 nm, the absorbance in this wavelength range is measured to determine the NO concentration. In addition, wavelength 350-450
Utilizing the ultraviolet absorption of NO ₂ in nm, the absorbance in this wavelength range is measured to measure the NO ₂ concentration.

[0006] Potentiostatic electrolysis method: sulfuric acid is used as an electrolytic solution,
An electrolytic layer using a working electrode such as gold or platinum and a counter electrode such as manganese or lead is used as a detector, and NO diffused into an electrolyte through a gas-permeable diaphragm is electrolyzed on a working electrode to which a potential of about 1 V is applied. The electrolytic current generated when undergoing oxidation is taken out, quantified, and the concentration is measured.

[0007]

However, the above-mentioned chemical analysis method has a problem that it is not suitable for continuous measurement because it takes a long time until the color development is completed because of the skill in collecting the sample gas. In addition, since the above method cannot directly measure NO, there is a problem that a pretreatment such as conversion to NO ₂ is required.

Further, both the chemical analysis method and the automatic measurement method are subject to interference due to the coexistence of other gases, moisture, dust, etc., so that it is necessary to remove or correct the coexisting gas, moisture, dust, etc. was there. Further, in order to guarantee a certain measurement accuracy, there is a problem that the temperature and the flow rate of the sample gas need to be constant.

Furthermore, in the conventional method, since it is necessary to sample a fixed amount of the sample gas, it is impossible to directly measure in a boiler furnace, a flue, a road, or the like, and it is impossible to measure the distribution. There was a problem.

An object of the present invention is to provide a nitrogen oxide capable of rapidly measuring the nitrogen oxide concentration in a sample gas without the need for sampling or pretreatment of the sample gas and without interference from a coexisting gas. An object of the present invention is to provide a concentration measuring device.

[0011]

The concentration measuring apparatus of the present invention is configured as follows to solve the above-mentioned problems. (1) A concentration measuring apparatus according to the present invention is configured such that a light source that irradiates a sample gas with light in a near-infrared region and a light transmitted through the sample gas are narrower than an absorption line of a nitrogen oxide. It is characterized by comprising means for calculating the amount of light absorbed by nitrogen oxide of a specific wavelength at a resolution, and concentration calculating means for calculating the concentration of nitrogen oxide in the sample gas from the obtained amount of absorption.

It is desirable that the light source emits a wavelength including a wavelength in the near infrared region of 1.28 to 1.86 μm.
Further, for example, it is desirable to obtain the absorption amount for each wavelength using a Fourier transform spectroscope. (2) The concentration measuring device of the present invention (claim 2) irradiates the sample gas with light having a wavelength in the near infrared region that is absorbed by the nitrogen oxide and having a spectrum width narrower than the absorption line of the nitrogen oxide. Laser, means for sweeping the oscillation wavelength of the laser, a photodetector for detecting the intensity of light transmitted through the sample gas, and a laser corresponding to the oscillation wavelength of the laser based on a detection signal of the photodetector. A phase-sensitive detector for obtaining the amount of light absorbed by the nitrogen oxides in the sample gas for each specific wavelength, and a calculator for calculating the nitrogen oxide concentration of the sample gas from the obtained light absorption for each specific wavelength It is characterized by comprising.

It is desirable that the light source can oscillate a wavelength in the near infrared region of 1.28 to 1.86 μm. Also,
The light source is desirably a wavelength-tunable semiconductor laser, for example, an InGaAs strained MQW / DFB semiconductor laser.

The light from the light source is modulated.
The modulation signal is extracted from the light transmitted through the sample gas,
Detect absorption by nitrogen oxides. The concentration measuring device of the present invention has the following operations and effects by the above configuration.

In general, the intramolecular vibration of a molecule has a wavelength of 2.5 to
When light having a wavelength in the infrared region, which is in the infrared region of 25 μm (wave number 400 to 4000 cm ⁻¹ ), is irradiated to the molecule, the light is absorbed by the molecule and the molecule is excited from the vibrational ground state to the vibrationally excited state. Since the amount of light absorbed at this time is proportional to the number of molecules, it can be used for measuring the concentration of molecules. In fact, the infrared absorption method utilizes the amount of infrared absorption. Table 1 shows the fundamental frequencies of nitric oxide, nitrogen dioxide and nitrous oxide.

[0016]

[Table 1]

The near-infrared region having a wavelength of 0.8 to 2.5 μm (wave number 4000 to 12500 cm ⁻¹ ) falls between the fundamental sound spectrum of molecular vibration and the electronic spectrum of atoms and molecules, and has an originally transparent wavelength. Although it is in the region, absorption based on the overtones and coupled sounds of intramolecular vibration and absorption based on electronic transition of some atoms and molecules appear. Thus, for frequencies in Table _{1, Σn i ν i (n} i is an integer) It is expected theoretically with absorption in the vicinity of wave number represented by, in the near infrared region of the observation absorption May be possible.

Strictly speaking, the wavelength is separated from the harmonic oscillator by a shift (anharmonic oscillation) by the amount corresponding to the shift (anharmonic oscillation). Further, since the molecule is rotating, it is split into many absorption lines and observed. Should be.

Therefore, if the absorption wavelength of nitrogen oxide is specified, the absorption in the near infrared region can be used for the concentration measurement. However, the absorption in the near-infrared region is expected to be much weaker than the absorption by the fundamental sound, and in many cases, the absorption is so weak that the absorption cannot be observed.

Further, in the concentration measurement using absorption in the near infrared region, it is hard to receive interference from a coexisting gas such as CO, CO ₂ , hydrocarbon or moisture. Furthermore, if the measurement is performed at a resolution higher than the absorption line width of the absorption spectrum, the absorption line of the target nitrogen oxide can be completely distinguished from the absorption line of another gas, and removal or correction of coexisting gas can be performed. It becomes unnecessary. In addition, since light absorption is proportional to the number of NO _x molecules existing in the optical path, skill is not required to collect the sample gas. In addition, since the light absorption occurs instantaneously, the measurement time is short and continuous measurement is possible.

Further, since it is not necessary to keep the temperature and flow rate of the sample gas constant, a laser is applied to a space where nitrogen oxides are considered to exist without sampling the sample gas.
By measuring the light absorption in the space, "in-situ observation" of the nitrogen oxide concentration is possible, and it is possible to directly measure in the boiler furnace, flue, chimney outlet, road, etc. Measurement is also possible.

[0022]

Embodiments of the present invention will be described below with reference to the drawings. (First Embodiment) FIG. 1 is a schematic diagram showing a configuration of a nitrogen oxide concentration measuring apparatus according to a first embodiment of the present invention. In this apparatus, the amount of absorption at each wavelength is observed by a Fourier transform spectrometer using a Michelson interferometer.

A Michelson interferometer 20 is arranged on the optical path of the light from the light source 10. Michelson interferometer 20
Is composed of a beam splitter 21, a movable mirror 22, and a fixed mirror 23. A mirror 30 and an absorption cell 31 are arranged on an optical path of light emitted from the Michelson interferometer 20. On the optical path of the light transmitted through the absorption cell 31, a photodetector 32 for detecting light intensity is arranged. Photodetector 32
The signal detected at is input to the arithmetic unit 33.

A sample gas is introduced into the absorption cell 31 at a predetermined pressure by a pressure gauge 34 and exhausted by a vacuum pump 35. In a general Fourier transform spectroscope, a global light source or a tungsten lamp is used as a light source. However, the device of the present embodiment is not limited to these, and a semiconductor laser having a wide spectral width can also be used. When observing the absorption of molecules, it is advantageous that the brightness of the light source is higher. Therefore, when a specific nitrogen oxide is analyzed using a semiconductor laser, the measurement can be performed with high sensitivity.

The operation will be described. Light emitted from the light source 10 enters a beam splitter 21 in the Michelson interferometer 20. The beam splitter 21 divides light into transmitted light and reflected light. The reflecting mirror is reflected by the fixed mirror 23 and passes through the beam splitter 21. The transmitted light is reflected by a movable mirror 22 that moves sequentially, and is reflected by a beam splitter 21.
Incident on. The two lights returning to the beam splitter 21 overlap and interfere with each other and exit from the Michelson interferometer 20. As a result of the interference, the intensity of the interference light emitted from the Michelson interferometer 20 is a function of the optical path difference and the wavelength.

The interference light is transmitted through a mirror 30 to an absorption cell 31.
, And penetrates the sample gas in the absorption cell 31 and enters the photodetector 32. The light detector 32 calculates the light intensity,
The result is output to the calculator 33. The calculator 33 obtains an intensity spectrum by performing a Fourier transform on a fluctuation component (interferogram) of an interference pattern obtained when the position of the movable mirror 22 in the Michelson interferometer 20 is sequentially changed. Then, the absorption spectrum is determined from the intensity spectrum, and the nitrogen oxide concentration is determined from the absorption at the absorption wavelength of the specific nitrogen oxide.

Next, an absorption spectrum when the gas containing each nitrogen oxide is filled in the absorption cell of this apparatus is shown. (1) Carbon monoxide: NO The standard vibration of intramolecular vibration (stretching vibration) of carbon monoxide is 1
904 cm ^-1 and a wavelength of 0.8 to 2.5 μm (wave number 4
In the near-infrared region of 000 to 12500 cm ^-1 ), it is expected that there will be absorption by the third to sixth harmonics of this reference vibration. Therefore, in the above configuration, a wavelength resolution of 6 pm (wave number resolution of 0.019 cm) is used with a tungsten lamp as a light source.
^Under the conditions of ^-1 ), an absorption cell having an optical path length of 15 m was filled with a standard gas (concentration: 100%) at 3 Torr, and the spectrum was measured.

FIG. 2A shows a part of the absorption spectrum of NO in this wavelength region. Wavelength 1.787-1.852μ
Only the third harmonic was observed in the region of m, and the fourth to sixth harmonics were not observed. Therefore, the concentration of NO in the sample gas can be measured by utilizing part or all of the absorption of NO in this wavelength region. (2) Nitrogen dioxide: NO ₂ The fundamental vibration of intramolecular vibration of NO ₂ is as shown in Table 1, but the wavelength is 0.8 to 2.5 μm (wave number 4000 to 125
It is expected that these harmonics or combined sounds will also absorb the near infrared region of 00 cm ^-1 ).

FIG. 2B shows a part of the result obtained by introducing a gas containing NO ₂ into the absorption cell of the apparatus shown in FIG. 1 and accurately measuring the absorption spectrum. Wavelength 1.667 to 1.684
Only the absorption spectrum belonging to the coupling vibration 2ν ₁ + 2ν ₃ is observed in the region of μm.

Therefore, the concentration of NO _{2 in} the sample gas can be measured by using part or all of the NO ₂ absorption line in the wavelength range of 1.667 to 1.684 μm. (3) Dinitrogen monoxide: N ₂ O The basic vibration of N ₂ O is as shown in Table 1, but the wavelength is 0.8 to 2.5 μm (wave number 4000 to 12500 c
In the near-infrared region of m ^-1 ), it is theoretically expected that there will be absorption by harmonics or combined sounds of the fundamental vibration.

FIG. 2C shows a part of the result obtained by introducing a gas containing N ₂ O into the absorption cell of the apparatus shown in FIG. 1 and accurately measuring the absorption spectrum. As a result of the near-infrared light absorption measurement of N ₂ O, light absorption was observed in a number of wavelength ranges below the light absorption based on the combined sound of vibration.

1.282 to 1.294 μm, 1.515
1.541 μm, 1.562 to 1.580 μm,
582-1.601 μm, 1.665-1.689 μ
m, 1.689-1.712 μm, 1.763-1.7
92 μm The concentration of N ₂ O in the sample gas can be measured by using part or all of the absorption lines in these wavelength regions.

Therefore, the concentration of nitrogen oxide can be measured by specifying the absorption wavelength of each nitrogen oxide and measuring the amount of absorption at that wavelength. (Second Embodiment) FIG. 3 is a schematic diagram showing a configuration of a nitrogen oxide concentration measuring apparatus according to a third embodiment of the present invention. A ramp wave (0 .0) for wavelength sweep generated by the first oscillator 50.
01 to 10 Hz) and a sine wave for modulation (1 k to 10 MHz) generated by the second oscillator 51 are input to the adder 52. The adder 52 superimposes the input ramp wave and sine wave, and the superimposed signal is input to the LD power supply 53. A current corresponding to an input signal is injected from a LD power supply 53 into a wavelength tunable semiconductor laser (light source) 54. A temperature controller 55 is provided for the wavelength tunable semiconductor laser 54.
A Peltier element 56 to which the semiconductor laser 54 is connected is provided, and the temperature of the semiconductor laser 54 is adjusted.

On the optical path of the laser light emitted from the semiconductor laser 54, a collimating lens 60 for converting the light into parallel light is provided.
An optical isolator 61, two irises 62, two mirrors 63 and 64, and a half mirror 65 are arranged. The laser light incident on the half mirror 65 is divided into transmitted light and reflected light, and the reflected light is incident on a wavelength meter 66 and the wavelength is measured. An absorption cell 31 is arranged on the optical path of the transmitted light, and a photodiode (photodetector) 70 is arranged on the optical path of the laser light transmitted through the absorption cell 31.

The output of the photodiode 70 is input to a preamplifier 71, amplified, and output to a phase sensitive detector 72. Further, the second oscillator 51 outputs the phase-sensitive detector 7.
The sine wave is output to 2. The phase-sensitive detector 72 converts the signal input from the preamplifier 71 into an LD
Only the component in which the phase and frequency of the modulation sine wave are synchronized is extracted, and only the signal dependent on the wavelength of the LD, that is, only the absorption of molecules is detected. Then, the absorption signal is output to the digital oscilloscope 73, the recorder 74, and the personal computer (calculator) 75.

The operation will be described. First oscillator 50
Generates a ramp wave for wavelength sweeping and outputs it to the adder 52. The second oscillator 51 generates a sine wave for modulation, and outputs the generated sine wave to the adder 52 and the phase-sensitive detector 72. The adder 52 superimposes the input ramp wave and sine wave,
Output to D power supply 53. The LD power supply 53 injects a current corresponding to the input signal into the semiconductor laser 54, and the semiconductor laser 54 oscillates a laser beam. After the laser light is collimated by the collimating lens 60, the optical isolator 6
The light enters the absorption cell 31 through the iris 62 and the mirrors 63 and 64. A part of the laser light is incident on the wavelength meter 66 by the half mirror 65 provided between the mirror 64 and the absorption cell 31, and the wavelength of the laser light is measured.

Then, the laser beam transmitted through the absorption cell 31 enters the photodiode 70,
0 is converted into an electric signal and output to the preamplifier 71. The preamplifier 71 amplifies the signal and outputs it to the phase sensitive detector 72.

The phase-sensitive detector 72 extracts a signal synchronized with the phase of the sine wave for modulation from the input signal, thereby detecting only the signal dependent on the wavelength of the laser light from the semiconductor laser 54, that is, only the absorption of molecules. It is output to the digital oscilloscope 73, the recorder 74 and the personal computer 75. The digital oscilloscope 73 and the recorder 74 display an absorption signal. Then, the personal computer 75 specifies the type of molecule from the absorption wavelength and calculates the concentration of nitrogen oxide from the absorption intensity.

An example of the measurement with this apparatus is shown below. N ₂ O gas having a concentration of 3% is supplied to an absorption cell having an optical path length of 25 cm.
Sealed at Torr pressure and having a spectral width of 3 pm In
GaAs strained MQW (quantum well type) DFB (distributed feedback type)
When the semiconductor laser was swept in the wavelength range of 1.7657 to 1.7669 μm, as shown in FIG.
Strong absorption could be observed at 9197, 1.765810 μm.

Since the oscillation wavelength of the semiconductor laser is modulated with a sine wave of 10 kHz and the phase is detected at twice the frequency (20 kHz), the absorption is a second derivative.
The peak where the signal intensity is high is the center of absorption.

The present invention is not limited to the above embodiment. For example, in the first embodiment, an absorption spectrum is obtained by using a Fourier transform spectroscope, but the light transmitted through the absorption cell is passed through a pass filter to light of a specific wavelength, and the amount of absorption is measured. May be determined.

In the above embodiment, the sample gas is sealed in the absorption cell. However, it is not necessary to seal the sample gas in the absorption cell, and the nitrogen oxide concentration can be measured directly. In addition, various modifications can be made without departing from the spirit of the present invention.

[0043]

As described above, according to the nitrogen oxide measuring apparatus of the present invention, by using the light absorption intensity in the near infrared region, there is no interference of the coexisting gas. Further, since the sampling and the pretreatment are not required and the measurement time is short, the nitrogen oxide concentration can be continuously measured.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing a configuration of a nitrogen oxide concentration measuring device according to a first embodiment.

FIG. 2 is a characteristic diagram showing a light absorption spectrum of a nitrogen oxide in a near infrared region.

FIG. 3 is a schematic diagram showing a configuration of a nitrogen oxide concentration measuring device according to a second embodiment.

FIG. 4 is a characteristic diagram showing a light absorption spectrum of N2O measured by the apparatus of FIG.

[Explanation of symbols]

 Reference Signs List 10 light source 20 Michelson interferometer 21 beam splitter 22 movable mirror 23 fixed mirror 30 mirror 31 absorption cell 32 photodetector 33 arithmetic unit 34 pressure gauge 35 vacuum pump 50 first oscillator 51 second oscillator 52 adder 53 LD power supply 54 Variable wavelength semiconductor laser 55 Temperature controller 56 Peltier element 60 Collimating lens 61 Optical isolator 62 Iris 63, 64 Mirror 65 Half mirror 66 Wavelength meter 70 Photodiode (photodetector, PD) 71 Preamplifier 72 Phase sensitive detector 73 Digital oscilloscope 74 Recorder 75 Personal computer (calculator)

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Kotaro Fujimura 5-717-1 Fukahoricho, Nagasaki City, Nagasaki Prefecture

Claims (2)

[Claims] 1. A light source for irradiating a sample gas with light in a near-infrared region; and a light transmitted by the sample gas, the light being emitted by the nitrogen oxide having a specific wavelength at a resolution narrower than the absorption line of the nitrogen oxide. A nitrogen oxide concentration measuring device comprising: means for calculating an absorption amount; and concentration calculating means for calculating a nitrogen oxide concentration of the sample gas from the obtained absorption amount. 2. A laser for irradiating a sample gas with light having a wavelength in the near-infrared region absorbed by the nitrogen oxide and having a narrower spectral width than the absorption line of the nitrogen oxide, and sweeping the oscillation wavelength of the laser. Means for detecting the intensity of light transmitted through the sample gas; and a nitrogen oxide in the sample gas for each specific wavelength corresponding to the oscillation wavelength of the laser based on a detection signal of the photodetector. Nitrogen, comprising: a phase-sensitive detector for calculating the amount of light absorbed by the detector; and a calculator for calculating the nitrogen oxide concentration of the sample gas from the obtained amount of light absorption for each specific wavelength. Oxide concentration measurement device.