JP2011191246A - Laser-type gas analyzer - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a laser-type gas analyzer which allows accurate computation of concentration, since influence of interference is removed by adding a simple analysis logic. <P>SOLUTION: In the laser-type gas analyzer, when the gas concentration of a measured gas containing an interference gas is measured, a current control section of an emission section side and a temperature stabilization means of an emission side are controlled for a laser driving current and a laser operating temperature to emit intermediate-infrared laser light of two or more differing wavelengths, and absorbed components due to the measured gas and an absorbed components due to the interfering gas are distinguished, on the basis of two or more gas absorbed waveforms which are each obtained by the intermediate-infrared laser light of two or more differing wavelengths within a certain period, thereby computing the concentration of the measured gas. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a laser gas analyzer that analyzes the presence and concentration of various gases to be measured in a flue.

A conventional laser gas analyzer will be described. First, the principle of gas concentration measurement of the laser gas analyzer will be described. FIG. 10 is a characteristic diagram showing an example of an absorption spectrum of NH ₃ (ammonia) gas. It is known that each gaseous gas molecule has its own light absorption spectrum. For example, as shown in the characteristic diagram of the absorption spectrum of NH ₃ gas in FIG. 10, the vertical axis represents the absorption amount. The amount of absorption varies depending on the wavelength.

The laser gas analyzer irradiates such gas with laser light, absorbs laser light of a specific wavelength in proportion to the gas concentration, and measures the gas concentration based on the amount of absorption. Measurement methods of such a laser gas analyzer are broadly divided into a two-wavelength difference method and a frequency modulation method.
Among these, as a conventional technique of a laser type gas analyzer relating to a frequency modulation method, for example, an invention described in Patent Document 1 (International Publication No. WO2008 / 096524, the title of the invention “Laser Gas Analyzer”) is known. Yes.

FIG. 11 shows a detection signal when measuring the concentration of SO ₂ gas with a frequency modulation type laser gas analyzer. When SO ₂ gas is absorbed, the laser gas analyzer detects a gas absorption waveform as shown in FIG. Since this peak value becomes the gas concentration, the concentration of SO ₂ gas can be measured by measuring the peak amplitude of this output. Further, the signal change may be integrated.

International Publication No. WO2008 / 096524 (paragraph numbers 0082 to 0087, FIGS. 15A and 15B)

In recent years, a quantum cascade laser capable of continuous oscillation at room temperature (hereinafter referred to as QCL, which is an abbreviation for quantum cascade laser), which is a kind of semiconductor laser, has been put into practical use. QCL can emit light in a wide range of wavelengths in the mid-infrared region (4 to 10 μm), which could not be realized with conventional semiconductor lasers. By using this QCL, other than NH ₃ (ammonia) gas, it is possible to measure a gas component whose absorption wavelength is included in the mid-infrared laser beam, such as SO ₂ , NO, NO _2, etc. Thus, it is possible to realize a laser type gas analyzer that was impossible.

  As the light receiving element, it is preferable to use an infrared detecting element having sensitivity in the mid-infrared region, for example, an MCT (Mercury_Cadmium_Tellurium) photoconductive element (hereinafter referred to as MCT).

  However, there are various problems in making it possible to analyze a laser gas analyzer equipped with QCL or MCT with high accuracy. One of the problems is that in the mid-infrared region, other than the measurement target gas, there are other interference gases. There was a problem of absorption. This point will be described.

First, FIG. 12 shows an example of an absorption spectrum of SO ₂ gas. In the case where SO ₂ concentration measurement is performed using the same apparatus configuration as that of the frequency modulation method, the QCL wavelength is preferably set to 7.2 to 7.4 μm. In addition, this SO ₂ gas may have an offset absorption (hereinafter referred to as offset absorption) that does not clearly depend on the wavelength, in addition to an absorption peak observed at a predetermined wavelength in a wavelength range suitable for concentration measurement. It is a feature.

Now, examples of the interference gas for the SO ₂ gas include CH ₄ gas and water (steam).
FIG. 13 shows an example of an absorption spectrum of CH ₄ gas. CH ₄ has a number of significant absorption peaks in the measurement wavelength range of SO ₂ (7.2~7.4μm), which is one of the interference gas can cause a measurement error in the SO ₂ concentration measurement.
FIG. 14 shows an example of an absorption spectrum of H ₂ O (water vapor). Similar to CH ₄ , H ₂ O has many large absorption peaks in the SO ₂ measurement wavelength region (7.2 to 7.4 μm), and causes interference errors in measuring SO ₂ concentration. one of.

Next, measurement errors due to the presence of interference gas will be described.
For example, when CH ₄ that is an interference gas is present in the measurement environment at a concentration of 500 ppm · m, the detection signal from the laser gas analyzer includes SO ₂ and CH ₄ at the same detection wavelength as shown in FIG. Absorption of appears. Therefore, as shown in FIG. 16, in the 50 ppm · m full-scale SO ₂ analyzer, a measurement error of 6%, which is three times the target specification, occurs.

Although it is desirable to select a detection wavelength and a wavelength modulation condition in which such an interference phenomenon does not occur, as shown in FIGS. 12, 13, and 14, the wavelength region where SO ₂ absorption exists, CH ₄ and H ₄ There is a problem that the wavelength region where the absorption of ₂ O exists largely overlaps, and a great deal of labor is required to select the wavelength region where there is no overlap. Further, in the QCL design specification, since the likelihood with respect to the reference wavelength and the wavelength tolerance is reduced, there is a problem that the cost due to the yield reduction at the time of manufacturing the QCL is high.

In addition, since the laser gas analyzer of the prior art described in Patent Document 1 does not employ QCL, it does not measure SO ₂ due to the difference in wavelength, and such a problem of interference does not occur. The above-mentioned problems were not considered.

  Therefore, the present invention has been made in view of the above-described problems, and its purpose is to add a simple analysis logic to eliminate the influence of interference and to accurately calculate the concentration. To provide an analyzer.

A laser type gas analyzer according to claim 1 of the present invention comprises:
A light source unit that emits a frequency-modulated mid-infrared laser beam, a light source side optical system that collimates the light emitted from the light source unit, and a light source side optical system that propagates through the space where the measurement target gas exists A light receiving side optical system for collecting the transmitted light, a light receiving unit for receiving the light collected by the light receiving side optical system, a signal processing circuit for processing an output signal of the light receiving unit, and a processed signal In the laser type gas analyzer having a central processing unit for measuring the concentration of the gas to be measured based on
The light source unit is
A laser element emitting mid-infrared laser light;
A light emission side temperature stabilizing means for stabilizing the temperature of the laser element;
A variable drive signal for changing the emission wavelength of the laser element so as to scan the absorption wavelength of the gas to be measured, and an offset signal for stopping emission of the laser element so as to reduce the amount of heat generated by the laser element. A laser drive signal generating unit that synthesizes a high frequency modulation signal for modulating the emission wavelength and outputs it as a laser drive signal to a wavelength scanning drive signal including;
A current controller that converts the laser drive signal output from the laser drive signal generator into a current and supplies the current to the laser element;
With
The light receiving unit is
A light-receiving element having sensitivity in the mid-infrared region;
A light receiving side temperature stabilizing means for stabilizing the temperature of the light receiving element;
With
The signal processing circuit includes:
A synchronous detection circuit that detects the amplitude of a signal having a double frequency component of the modulation signal in the light source unit from the output signal of the light receiving unit and outputs a detection signal;
With
The central processing unit is
When measuring the gas concentration of measurement target gas including interference gas,
Control means for controlling the current control section on the light emitting section side and the light emitting side temperature stabilizing means to control a laser driving current and a laser operating temperature so as to emit a plurality of mid-infrared laser beams having different wavelengths; ,
Based on a plurality of gas absorption waveforms obtained by mid-infrared laser beams of different wavelengths at a certain time, the absorption component due to the measurement target gas and the absorption component due to the interference gas are separated, and the concentration of the measurement target gas is determined. A calculating means for calculating;
It functions as.

A laser gas analyzer according to claim 2 of the present invention is
The laser gas analyzer according to claim 1, wherein
The central processing unit is
The interference gas measurement error ratio a between the different wavelengths X and Y is registered in advance,
A generating means for generating the concentration S _X [%] of the measurement target gas at the wavelength X and the concentration S _y [%] of the measurement target gas at the wavelength Y;
Measuring object gas concentration calculating means for calculating the concentration of the measuring object gas by S ₀ = S _Y / (1-a) −aS _X / (1-a);
It functions as.

A laser gas analyzer according to claim 3 of the present invention is
In the laser type gas analyzer according to claim 1 or 2,
The central processing unit is
The measurement error x of the absorption at the wavelength X of the interference gas having the concentration B and the measurement error y of the absorption at the wavelength Y of the interference gas having the concentration B are registered in advance.
A generating means for generating the concentration S _X [%] of the measurement target gas at the wavelength X and the concentration S _y [%] of the measurement target gas at the wavelength Y;
Interference gas concentration calculating means for calculating the concentration of the interference gas by B ₀ = B (S _X −S _Y ) / (xy);
It functions as.

  According to the present invention, it is possible to provide a laser type gas analyzer that can eliminate the influence of interference and accurately calculate the concentration by adding simple analysis logic.

It is a whole lineblock diagram showing the laser type gas analyzer of an embodiment of the invention. It is a block diagram of the light source part of the laser type gas analyzer of embodiment of this invention. It is an output signal figure from a wavelength scanning drive signal generation part. Wavelength scanning driving signal waveform of the laser gas analyzer of a laser device according to the embodiment of the present invention, the absorption waveform of the SO ₂ gas is a diagram showing the gas absorption waveform of the synchronous detection circuit. It is a block diagram of the light-receiving part of the laser type gas analyzer of embodiment of this invention, a signal processing circuit, and a central processing part. It is a principle diagram of a frequency modulation system. For explaining a measurement value correction in the embodiment of the present invention, it is a diagram showing the relationship between the measurement error due to detection wavelength interference gas (H 2 _O). For explaining a measurement value correction in the embodiment of the present invention, it is a diagram showing the relationship between the measurement error due to detection wavelength interference gas (H 2 _O). It is a figure which shows the absorption waveform by measurement object gas and interference gas for demonstrating the measurement correction | amendment in embodiment of this invention. NH ₃ is a characteristic diagram showing an absorption spectrum of the gas. It illustrates a detection signal at the time concentration measurement of SO ₂ gas by laser gas analyzer of the frequency modulation method. Is a characteristic diagram showing an absorption spectrum example of SO ₂ gas. It shows the absorption spectrum example of CH ₄ gas. It shows the absorption spectrum example of H 2 _O. CH ₄ is a diagram showing the gas absorption waveform signal when the concentration measured interfering SO _2. CH ₄ is a diagram showing the concentration measurement example of interfering SO _2.

  Then, the form for implementing this invention is demonstrated below, referring a figure. FIG. 1 is a structural diagram showing a laser type gas analyzer of this embodiment, and shows the overall configuration. This laser type gas analyzer is a frequency modulation type laser type gas analyzer. As shown in FIG. 1, the laser gas analyzer is fixed by welding or the like to flanges 101a and 101b, for example, walls 201 and 202 such as pipes through which the measurement target gas flows like a flue. Yes. One flange 101a is provided with a transparent exit window 101c. Further, a bottomed cylindrical cover 103a is attached to the flange 101a via a mounting seat 102a.

  A light source unit 104 is disposed inside the cover 103a, and laser light emitted from the light source unit 104 is collimated into parallel light by a light source side optical system including a collimator lens 105, passes through the center of the flange 101a, and exits through an exit window. The light enters the inside of the walls 201 and 202 (inside the flue) through 101c. The parallel light is absorbed when it passes through the measurement target gas inside the walls 201 and 202.

  A bottomed cylindrical cover 103b is attached to the other flange 101b via a mounting seat 102b. The flange 101b is provided with a transparent incident window 101d. The parallel light that has passed through the inside of the flue passes through the entrance window 101d, is collected by the condensing lens 106, which is the light receiving side optical system inside the cover 103b, is received by the light receiving unit 107, is converted into an electrical signal, and is converted into a subsequent stage. The signal is input to the signal processing circuit 108. The light source unit 104 and the signal processing circuit 108 are connected to the central processing unit 109.

  Next, the light source unit will be described. FIG. 2 shows the configuration of the light source unit 104. This light source unit 104 includes a wavelength scanning drive signal generation unit 104a that changes the emission wavelength of the laser element so as to scan the absorption wavelength of the measurement target gas, and, in order to detect the absorption wavelength of the measurement target gas, for example, 6. A laser drive signal generation unit 104s including a harmonic modulation signal generation unit 104b for frequency-modulating the wavelength with a sine wave of about 5 kHz is provided, and the output signals of these signal generation units 104a and 104b are synthesized. A laser drive signal is generated. The laser drive signal is converted into a current by the current control unit 104c and supplied to the laser element 104e by QCL. The QCL laser element 104e emits mid-infrared laser light. Note that the wavelength scanning drive signal generation unit 104a is connected to the central processing unit 109, and in particular, the amplitude and emission timing are input to control the laser drive current.

  The laser element 104e is provided with a light emission side temperature stabilizing means. The light emission side temperature stabilization means includes a temperature control unit 104d, a thermistor 104f, and a Peltier element 104g. A thermistor 104f as a temperature detection element is disposed in the vicinity of the laser element 104e, and a Peltier element 104g is disposed in the vicinity of the thermistor 104f. The Peltier element 104g is controlled by the temperature control unit 104d so that the resistance value of the thermistor 104f becomes a constant value. As a result, the Peltier element 104g operates to stabilize the temperature of the laser element 104e. The temperature control unit 104d is connected to the central processing unit 109 and controls the laser operating temperature.

  Here, as shown in FIG. 3, the wavelength scanning drive signal output from the wavelength scanning drive signal generation unit 104a becomes one unit waveform by the variable drive signal S1 and the offset signal S2, and such unit waveform has a fixed period. It is a substantially trapezoidal signal repeated in

The variable drive signal S1 of the wavelength scanning drive signal is a signal for scanning the absorption wavelength, and by linearly changing the magnitude of the current supplied to the laser element 104e via the current control unit 104c, the laser element 104e This is a signal for scanning the absorption wavelength by gradually shifting the emission wavelength. The emission wavelength can be scanned in the sub-nm to several nm range according to the slope of the signal S1, that is, the amount of change in the supply current. For example, in the case of SO ₂ gas, it is a portion that can scan a line width of about 0.2 nm.

  The offset signal S2 of the wavelength scanning drive signal is a signal having a value that supplies a current less than the threshold current value emitted by the laser element 104e to the laser element, and is an offset portion that does not cause the laser element 104e to emit light. . At the timing when the wavelength scanning drive signal generation unit 104a outputs the offset signal S2, the QCL does not emit light. The signal S1 and the signal S2 are inserted so as to be switched alternately.

Thus, the offset signal S2 is less than the threshold current value at which the light emission of the laser element 104e is stabilized, and the time of the offset signal S2 is significantly longer than the time of the variable drive signal S1.
Such intermittent light emission conditions, that is, the ratio of the time between the signal S1 and the signal S2, may be determined in consideration of the amount of heat generated by the laser element 104e as the QCL and the performance of the temperature stabilizing means such as the Peltier element. For example, by setting S1: S2 = 1: 4, the calorific value can be reduced to 1/5 compared to the case of continuous light emission.

  If the light source unit using QCL is made to emit light continuously, or if it stops for a while but emits light almost continuously, it is expected that heat generation will be excessive and temperature control by the Peltier element will be difficult. As described above, by causing QCL to emit light intermittently so that the extinction time is longer than the light emission time, the amount of heat generated by QCL can be reduced, and the QCL can be used with the same configuration and cost as a conventional laser gas analyzer. I did it. The ratio between the light emission time and the quenching time is determined from the limit temperature when a limit temperature that can be stabilized by the light emission side temperature stabilization means (temperature control unit 104d, thermistor 104f, and Peltier element 104g in FIG. 2) is assumed. The ratio between the light emission time and the quenching time is determined so that the temperature is lower. In this case, the temperature is lowered by making the extinction time longer than at least the light emission time. Such a driving method contributes to elimination of instability of QCL and MCT.

Now, the wavelength scanning drive signal output from the wavelength scanning drive signal generation unit 104a is subjected to frequency modulation by synthesizing the high frequency modulation signal from the high frequency modulation signal generation unit 104b, and the laser as shown in FIG. A drive signal is generated. In the case of SO ₂ gas, the laser drive signal has a high frequency modulation signal frequency of 6.5 kHz and a wavelength scanning drive signal frequency of 20 Hz, and λ ₁ and λ ₂ are scanning ranges corresponding to the absorption wavelength of the SO ₂ gas. The upper and lower limit values are shown.

Note that although λ ₁ and λ ₂ of the wavelength scanning drive signal are described as scanning ranges corresponding to the absorption wavelength of SO ₂ gas, in addition to SO ₂ , NO gas components are measured or NO ₂ is measured. The gas component can be measured. However, the characteristics of QCL and (current and wavelength scanning range due to temperature) _SO 2, NO, In consideration of the absorption spectra of NO _2, SO 2, NO, individually as a single component meter for any one of NO ₂ It becomes a laser type gas analyzer to measure. In this case, in the laser type gas analyzer, a QCL having an emission wavelength corresponding to the absorption wavelength of the measurement target selected from SO ₂ , NO, NO _2, etc. is selected, and depending on the gas component of the measurement target Then, λ ₁ and λ _{2 in} the mid-infrared region are set. Such a laser drive signal is output.

  Next, FIG. 5 shows the configuration of the light receiving unit 107 and the signal processing circuit 108. Since the MCT photoconductive element having sensitivity in the mid-infrared region cannot obtain sufficient sensitivity unless the temperature is low, a light receiving side temperature stabilizing means is provided in the light receiving unit 107 for the light receiving element 107a which is an MCT photoconductive element. ing.

  The light receiving side temperature stabilizing means further includes a thermistor 107b, a Peltier element 107c, and a temperature control unit 107d, and cools the light receiving element 107a. Specifically, the thermistor 107b and the Peltier element 107c are built in the MCT photoconductive element. As described above, the thermistor 107b as a temperature detecting element is arranged close to the laser element 107a, and the Peltier element 107c is arranged close to the thermistor 107b.

  The Peltier element 107c is controlled by the temperature control unit 107d so that the resistance value of the thermistor 107b becomes a constant value, and as a result, operates so as to stabilize the temperature of the light receiving element 107a. By such light receiving side temperature stabilizing means, for example, the operating temperature of the MCT photoconductive element is kept constant at −3 ° C.

  The light receiving element 107a, which is an MCT photoconductive element, is a light receiving element having sensitivity to the emission wavelength of the mid-infrared laser beam of the laser element 104e, which is QCL. The output current of the light receiving unit 107 is input to the I / V converter 108a. The I / V converter 108a receives the 2f signal (second harmonic signal) from the oscillator 108c, modulates the output current with the 2f signal (second harmonic signal), converts it to a voltage, and converts the voltage signal to a voltage signal. Is output. This voltage signal is input to the synchronous detection circuit 108b. The synchronous detection circuit 108b detects this voltage signal and extracts only the amplitude of the double frequency component of the modulation signal.

Here, the measurement principle of the frequency modulation type laser gas analyzer will be described. FIG. 6 shows a principle diagram of the frequency modulation method. The laser gas analyzer of the frequency modulation method, the center frequency f _c, the output light of the semiconductor laser is frequency-modulated at a modulation frequency f _m, is irradiated to the measurement target gas. Here, the frequency modulation is to make the waveform of the drive current supplied to the laser element 104e sinusoidal.
To cancel the effect of the distance in the frequency modulation method is at the same time the frequency f _m if the frequency modulated output of the semiconductor laser device is the may be performed amplitude modulation and multiplied by frequency modulation to the output of the semiconductor laser element amplitude This can also be used since it is also modulated.

As shown in FIG. 6, since the absorption lines of the gas is almost quadratic function with respect to the modulation frequency, the absorption line plays the role of a discriminator, the light receiving portion of the double modulation frequency f _m A frequency signal (double frequency signal) is obtained. Since the modulation frequency f _m good at any frequency, for example, choose the modulation frequency f _m to several kHz, a digital signal processor (DSP) or using a general-purpose processor, extraction of the double frequency signal It is possible to perform advanced signal processing such as.

  In addition, if envelope detection is performed by the light receiving unit, the fundamental wave by amplitude modulation can be estimated, and the ratio between the amplitude of the fundamental wave and the amplitude of the double frequency signal can be detected in phase synchronization to measure regardless of the distance. A signal proportional to the target gas concentration can be obtained.

Based on such a principle, when the synchronous detection circuit 108b does not absorb the laser beam by the measurement target gas, the double detection signal is not detected by the synchronous detection circuit 108b. Therefore, the output of the synchronous detection circuit 108b is almost linear. It becomes.
On the other hand, when there is absorption of laser light by the measurement target gas, a double wave signal, which is a signal obtained by extracting only the amplitude of the double frequency component of the modulation signal of the emitted light, is detected by the synchronous detection circuit 108b. The output waveform is as shown in the output waveform of the synchronous detection circuit 108b shown in the rectangular frame of FIG. This waveform is noise-removed by the filter 108d, amplified as appropriate, and output to the central processing unit 109, which is a subsequent CPU, DSP, or the like.
The output signal from the I / V converter 108a is also input to the extraction means (filter) 108e, and the extracted wavelength scanning drive signal component is sent to the central processing unit 109. This wavelength scanning drive signal component is abnormal. It will be used for judgment.

  Since the peak value of the output waveform of the synchronous detection circuit 108b shown in FIG. 4 corresponds to the concentration of the gas to be measured, the peak value is measured, or a part or all of the waveform is integrated and measured from the integrated value. What is necessary is just to detect the density | concentration of object gas.

  Next, a method for measuring the concentration of the measurement target gas will be described. First, the analysis principle in the laser gas analyzer of the present invention will be described. Here, a gas including an interference gas and a measurement target gas is assumed.

  First, in advance, when there is an absorption with respect to an interference gas having a concentration of B [ppm · m] at a wavelength X [nm], a measurement error associated with the absorption is x [%], and a wavelength Y [ It is assumed in advance that the measurement error associated with the absorption is y [%] when there is absorption with respect to the interference gas having a concentration of B [ppm · m] at [nm].

Subsequently, laser gas analysis is performed. First, laser analysis is performed using a laser having a wavelength of X [nm] as a gas. As a result of laser analysis, it is assumed that the gas concentration S _X [%] is measured at the wavelength X [nm].
Subsequently, laser analysis is performed using a laser having a wavelength of Y [nm] as a gas. As a result of the laser analysis, it is assumed that the gas concentration S _Y [%] is measured at the wavelength Y [nm]. These S _X [%] and S _Y [%] are the concentrations of the measurement target gas including the measurement error due to the interference gas.

Subsequently, the true values of the concentrations of the measurement target gas and the interference gas are calculated.
When the actual concentration of the measurement target gas (SO ₂ ) is S ₀ [%] and the actual concentration of the interference gas (H ₂ O) is B ₀ [ppm · m], the gas concentrations S _X and S _Y Is expressed as follows:

[Equation 1]
S _X = S ₀ + (B ₀ / B) · x

[Equation 2]
S _Y = S ₀ + (B ₀ / B) · y

  Here, the measurement error ratio a is expressed by the following Equation 3.

[Equation 3]
a = y / x

  This measurement error ratio a is constant even if the concentration of the interference gas varies.

Subsequently, the actual concentration S ₀ of the measurement target gas is expressed using the measured gas concentrations S _X and S _Y. When Formula 1 is transformed, B ₀ / B is expressed as the following formula.

[Equation 4]
B ₀ / B = (S _X −S ₀ ) / x

By substituting Equations 3 and 4 into Equation 2 and transforming it into an equation for obtaining S ₀ , the following equation is obtained.

[Equation 5]
_{S 0 = S Y / (1} -a) -aS X / (1-a)

Here, the measured concentrations S _X and S _Y can be obtained because they are actually measured. As for a, when there is an absorption with respect to an interference gas having a concentration of B [ppm · m] at a wavelength X [nm], a measurement error x accompanying the absorption is calculated in advance. When there is absorption with respect to an interference gas having a concentration of B [ppm · m] at the wavelength Y [nm], a measurement error y associated with the absorption is calculated, and the measurement error ratio a is calculated from the above [Equation 3]. Calculate it. The measurement error ratio a and the measurement errors x and y in the case of the wavelength X and the wavelength Y are registered, and the measurement error ratio a can be calculated. The measurement error ratio a is registered in a number of combinations with different wavelengths.

In this way, the measured value S _X of the concentration of SO ₂ at the wavelength X, the measured value S _Y of the concentration of SO ₂ at the wavelength Y, the ratio between a plurality of wavelengths of the measurement error due to interference gas previously obtained a From the above, it can be seen that the concentration of SO ₂ that is the measurement target gas contained in the gas along with the interference gas can be obtained.

In addition, the concentration B ₀ of the interference gas in a certain measurement environment is expressed by the following equations using Equations 3, 4, and 5.

[Equation 6]
B ₀ = B (S _X -S _Y ) / (xy)

Concentration B [ppm · m] here the interfering gas are known in advance, the measured value _{S X} of the concentration of SO ₂ at the wavelength X, the measured value _{S Y} of the concentration of SO ₂ at the wavelength Y, the wavelength X [ measurement error x [%] due to absorption of interference gas at a concentration B [ppm · m] in nm] and measurement error y [%] due to absorption of interference gas at a concentration B [ppm · m] at wavelength Y [nm]. ], It is understood that only the concentration B ₀ of H ₂ O, which is an interference gas contained in the gas, can be obtained together with the measurement target gas. The measurement principle is like this.

Next, analysis processing by a laser gas analyzer will be described. Here, it is assumed that the gas including the interference gas and the measurement target gas flows in the flue. For the sake of concrete explanation, it is assumed that a gas in which H ₂ O (moisture) is mixed as an interference gas and SO ₂ as a measurement target gas flows and this gas is subjected to laser analysis. When performing SO ₂ concentration measurement, the wavelength of the QCL is 7.2 to 7.4 μm, but in this embodiment, the detection wavelength is a wavelength X and a wavelength Y slightly deviating from 7358 nm (7.358 μm) as a reference. Hereinafter, the detection wavelength is expressed as wavelength X ₇₃₅₈ [nm], wavelength Y ₇₃₅₈ [nm].

Then, a measurement interference test is performed in advance to acquire data.
FIG. 7 shows the result of a measurement interference test of moisture having a detection wavelength of X ₇₃₅₈ [nm]. This test is performed in advance. The measurement error due to moisture in this case is x = + 5%.
Moreover, the measurement interference test result of the water _| moisture content whose detection wavelength is _Y7358 [nm] is shown in FIG. This test is performed in advance. It can be seen that the measurement error due to moisture in this case is y = + 2.5%.
The results of these tests are registered in advance. The central processing unit 109, the measurement error x = + 5% at a wavelength _{X 7358} [nm] Wavelength _{Y 7358} measurement error y = + 2.5% in [nm], the measurement error ratio a = 0 represented by y / x. 5 is remembered.

  That is, in the memory (not shown) of the central processing unit 109, the measurement error ratios x and y that are measurement errors in the case of the wavelengths X and Y, the wavelengths X and Y, and the measurement errors x and y are measured. It is assumed that a is registered in advance. In the memory, not only the case of wavelengths X and Y but also a number of combinations are registered by changing the wavelengths. Such registration is performed in advance at the time of factory shipment, for example.

The analysis is then started.
First, the central processing unit 109 controls the light source unit 104 to emit a laser beam having a wavelength range so that the operation range corresponding to the absorption wavelength is centered on the wavelength X ₇₃₅₈ [nm]. Specifically, the central processing unit 109 instructs the wavelength operation drive signal generation unit 104a to adjust the laser drive current, and also instructs the temperature control unit 104d to adjust the laser operating temperature, and the wavelength X ₇₃₅₈ from the laser element 104e. Emits mid-infrared laser light having a wavelength range around [nm].

At this time, the temperature control unit 104d first detects in advance the temperature of the laser element 104e in FIG. 2 by the thermistor 104f, and the measurement target gas (SO _{2) in} the central portion of S1 of the wavelength scanning drive signal shown in FIG. The temperature control unit 104d in FIG. 2 controls the energization of the Peltier element 104g to adjust the temperature of the laser element 104e so that (gas) can be measured (a predetermined absorption characteristic can be obtained). The Peltier element 104g is controlled by PID control or the like so that the resistance value of the thermistor 104f becomes a constant value. Under such setting conditions, the laser element 104e is driven to emit laser light into the space where the measurement target gas exists inside the walls 201 and 202, and the condensed light is incident on the light receiving unit 107. The light receiving unit 107 detects the signal, converts it into a current signal, and outputs it to the signal processing unit 108.

Subsequently, the central processing unit 109 performs control so that the light source unit 104 emits laser light having a wavelength range so that the operation range corresponding to the absorption wavelength is centered on the wavelength Y ₇₃₅₈ [nm]. The wavelength Y ₇₃₅₈ [nm] is slightly deviated from the wavelength X ₇₃₅₈ [nm]. Specifically, the central processing unit 109 instructs the wavelength operation drive signal generation unit 104a to adjust the laser drive current, and also instructs the temperature control unit 104d to adjust the laser operating temperature, and from the laser element 104e to the wavelength Y _7358. Emits mid-infrared laser light having a wavelength range around [nm]. Compared with X ₇₃₅₈ [nm], the laser drive current and the laser operating temperature are shifted and adjusted. The light receiving unit 107 detects the signal, converts it into a current signal, and outputs it to the signal processing unit 108.

FIG. 9 shows the output of the synchronous detection circuit 108b at the time of measuring the SO ₂ concentration when X ₇₃₅₈ [nm] and Y ₇₃₅₈ [nm] are continued. When the detection wavelength was continuous from _{X 7358} [nm] _{Y 7358} to [nm], and SO ₂ is measured gas, shows the gas absorption waveform moisture is the measured interference gas. It can be seen that both have absorption peaks in the same wavelength region. In the absorption waveform of SO ₂ , _two absorption peak amplitudes are obtained as AA ′ and BB ′ in the figure. Since this peak value becomes the gas concentration, the AA ′ peak amplitude of this output is measured as the gas concentration S _X, and the BB ′ peak amplitude is measured as the gas concentration S _y .

The central processing unit 109 generates the concentration S _X [%] of the measurement target gas at the wavelength X and the concentration S _Y [%] of the measurement target gas at the wavelength Y. It functions as means for generating the density S _X and the density S _Y from the output minimum value and the maximum value of the circuit 108b. AA ′ is registered as the density S _{X at the} wavelength X. Furthermore, B-B 'is registered as the concentration S _Y at a wavelength Y.

  Note that the emission wavelength of the QCL is scanned so that the peak amplitudes of AA ′ and BB ′ are maximized. In this embodiment, the wavelength scanning is mainly performed by the operating temperature of the QCL. The operating temperature difference ΔT of the detected wavelength was 1.5 ° C. The time interval of wavelength scanning can be set to, for example, 1 minute in consideration of the light emission stabilization time due to the change in the operating temperature of the QCL.

The central processing unit 109 calculates the concentration of the measurement target gas. Specifically, the calculation unit calculates the measurement target gas concentration S ₀ = S _Y / (1-a) −aS _X / (1-a). Function as.

Here, the actual _S X = 60 (%) measurements at the wavelength _{X 7358} [nm], indicating an _S Y = 55 [%] at a wavelength _{Y 7358} [nm], and the ratio a of the measurement error By substituting measurement errors S _X and S _Y due to the two types of detection wavelengths into Equation 5, the actual concentration of SO ₂ can be obtained as 50%.

The central processing unit 109 functions as a calculation unit that calculates the concentration of the interference gas by B ₀ = B (S _X −S _Y ) / (xy).
Wavelength _{X 7358 S X} = 60 [%] in [nm] Wavelength _{Y 7358 S Y} = 55 [%] in [nm], x = + 5% at a wavelength _{X 7358} [nm], y at a wavelength _{Y 7358} nm, Since + = 2.5% and B = 50 [ppm · m], it can be determined that B ₀ = 100 [ppm · m].

  The central processing unit 109 turns off the light source unit 104 and the signal processing circuit 108 and ends the laser analysis. A laser gas analyzer is like this.

  As described above, according to the present embodiment, the light source unit 104 can scan the emission wavelength of the laser element 104e over a predetermined range and measure the gas concentration with the measurement target gas.

The laser gas analyzer of the present invention has been described above.
In this embodiment, the SO ₂ concentration has been described as the measurement target gas. However, it is needless to say that it is not limited to SO _2. For example, the concentrations of NO and NO ₂ may be obtained.

  According to the present invention, in the light source unit of a laser gas analyzer using a QCL (quantum cascade laser), the laser driving current and the laser operating temperature are adjusted, and the mid-infrared region having a wavelength different from the laser element every certain time. A plurality of gas absorption waveforms can be obtained by emitting laser light. In the central processing unit, it is possible to calculate the concentration of the measurement target gas by separating the absorption component due to the measurement target gas and the absorption component due to the measurement interference gas included in the gas absorption waveform.

The laser type gas analyzer of the present invention can be applied to measurement of gas components such as SO ₂ , NO, NO ₂ having a unique light absorption spectrum in the mid-infrared region.

201, 202: walls 101a, 101b: flange 101c: exit window 101d: entrance window 102a, 102b: mounting seats 103a, 103b: cover 104: light source unit 104a: wavelength scanning drive signal generation unit 104b: high frequency modulation signal generation unit 104c: Current controller 104d: Temperature controller 104e: Laser element 104f: Thermistor 104g: Peltier element 104s: Laser drive signal generator 105: Collimator lens 106: Condensing lens 107: Light receiver 107a: Light receiver 107b: Thermistor 107c: Peltier element 107d: temperature control unit 108: signal processing circuit 108a: I / V conversion circuit 108b: synchronous detection circuit 108c: oscillator 108d: filter 108e: extraction means (filter)
109: Central processing unit

Claims (3)

A light source unit that emits a frequency-modulated mid-infrared laser beam, a light source side optical system that collimates the light emitted from the light source unit, and a light source side optical system that propagates through the space where the measurement target gas exists A light receiving side optical system for collecting the transmitted light, a light receiving unit for receiving the light collected by the light receiving side optical system, a signal processing circuit for processing an output signal of the light receiving unit, and a processed signal In the laser type gas analyzer having a central processing unit for measuring the concentration of the gas to be measured based on
The light source unit is
A laser element emitting mid-infrared laser light;
A light emission side temperature stabilizing means for stabilizing the temperature of the laser element;
A variable drive signal for changing the emission wavelength of the laser element so as to scan the absorption wavelength of the gas to be measured, and an offset signal for stopping emission of the laser element so as to reduce the amount of heat generated by the laser element. A laser drive signal generating unit that synthesizes a high frequency modulation signal for modulating the emission wavelength and outputs it as a laser drive signal to a wavelength scanning drive signal including;
A current controller that converts the laser drive signal output from the laser drive signal generator into a current and supplies the current to the laser element;
With
The light receiving unit is
A light-receiving element having sensitivity in the mid-infrared region;
A light receiving side temperature stabilizing means for stabilizing the temperature of the light receiving element;
With
The signal processing circuit includes:
A synchronous detection circuit that detects the amplitude of a signal having a double frequency component of the modulation signal in the light source unit from the output signal of the light receiving unit and outputs a detection signal;
With
The central processing unit is
When measuring the gas concentration of measurement target gas including interference gas,
Control means for controlling the current control section on the light emitting section side and the light emitting side temperature stabilizing means to control a laser driving current and a laser operating temperature so as to emit a plurality of mid-infrared laser beams having different wavelengths; ,
Based on a plurality of gas absorption waveforms obtained by mid-infrared laser beams of different wavelengths at a certain time, the absorption component due to the measurement target gas and the absorption component due to the interference gas are separated, and the concentration of the measurement target gas is determined. A calculating means for calculating;
It functions as a laser gas analyzer. The laser gas analyzer according to claim 1, wherein
The central processing unit is
The interference gas measurement error ratio a between the different wavelengths X and Y is registered in advance,
A generating means for generating the concentration S _X [%] of the measurement target gas at the wavelength X and the concentration S _y [%] of the measurement target gas at the wavelength Y;
Measuring object gas concentration calculating means for calculating the concentration of the measuring object gas by S ₀ = S _Y / (1-a) −aS _X / (1-a);
It functions as a laser gas analyzer. In the laser type gas analyzer according to claim 1 or 2,
The central processing unit is
The measurement error x of the absorption at the wavelength X of the interference gas having the concentration B and the measurement error y of the absorption at the wavelength Y of the interference gas having the concentration B are registered in advance.
A generating means for generating the concentration S _X [%] of the measurement target gas at the wavelength X and the concentration S _y [%] of the measurement target gas at the wavelength Y;
Interference gas concentration calculating means for calculating the concentration of the interference gas by B ₀ = B (S _X −S _Y ) / (xy);
It functions as a laser gas analyzer.

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