JP2014206541A - Laser gas analyzer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a laser gas analyzer capable of measuring a concentration of a multicomponent mixed gas by using an area method not requiring a statistical technique.SOLUTION: The laser gas analyzer for analyzing a measurement target gas in a multicomponent mixed gas using wavelength variable semiconductor laser absorption spectroscopy comprises: a wavelength calibration gas cell that is used for wavelength calibration of an absorption spectrum, and includes a wavelength calibration gas made of multiple kinds of gases encapsulated therein.

Description

  The present invention relates to a laser gas analyzer, and more particularly to a laser gas analyzer that can efficiently measure a hydrocarbon multicomponent mixed gas.

  Laser gas analyzers using the TDLAS (Tunable Diode Laser Absorption Spectroscopy) method only irradiate light from a wavelength tunable semiconductor laser to the measurement target components such as high temperature and corrosive gas. Even in the case of a concentration of 1, there is an advantage that the component selectivity is high without being interfered by other components, and measurement can be performed in real time at high speed without contact.

  FIG. 19 is a block diagram showing an example of a conventional laser gas analyzer using the TDLAS method, in which a light source unit including a semiconductor laser that irradiates a measurement laser beam toward a measurement gas atmosphere, and a measurement space in the measurement gas atmosphere And a detection unit including an arithmetic processing unit for processing an output signal of the light receiving element.

The laser gas analyzer shown in FIG. 19 uses a semiconductor laser having an extremely narrow oscillation wavelength spectrum line width for a light absorption spectrum unique to a molecule caused by vibration / rotational energy transition of a measurement target molecule existing in the infrared to near-infrared region. To measure. The absorption spectra peculiar to most molecules such as O ₂ , NH ₃ , H ₂ O, CO, CO ₂ are in the infrared to near-infrared region, and are measured by measuring the light absorption (absorbance) at a specific wavelength. The concentration of the component can be calculated.

  In FIG. 19, the semiconductor laser 11 provided in the light source unit 10 irradiates and outputs measurement laser light into the atmosphere of the measurement gas 20. The laser light output from the semiconductor laser 11 has a very narrow oscillation wavelength spectrum line width, and the oscillation wavelength can be changed by changing the laser temperature or drive current. Therefore, only one of the absorption peaks of the absorption spectrum can be measured.

  Therefore, the absorption peak that is not affected by the interference gas can be selected, the wavelength selectivity is high, and it is not affected by other interference components. Therefore, the process gas is removed without removing the interference gas in the previous stage of measurement. Can be measured directly.

  By scanning the oscillation wavelength of the semiconductor laser 11 in the vicinity of one absorption line of the measurement component, an accurate spectrum that does not overlap with the interference component can be measured. The spectrum shape includes the measurement gas temperature, the measurement gas pressure, It changes due to the broadening phenomenon of spectrum due to coexisting gas components. For this reason, the actual process measurement accompanied by these environmental variations requires correction.

  Therefore, the apparatus of FIG. 19 uses a spectrum area method in which the spectrum area is obtained by scanning the oscillation wavelength of the semiconductor laser 11 and measuring the absorption spectrum, and converting the spectrum area into the component concentration.

  In other laser gas analyzers, the peak height method for obtaining the measurement component from the peak height of the absorption spectrum or the wavelength scan signal is modulated and the P-P (peak-to-peak) value of the frequency-modulated waveform is double that frequency. The 2f method for obtaining the concentration of the measured component from the above is used, but these are easily affected by variations in temperature, pressure, coexisting gas components, and the like.

  On the other hand, the spectrum area is not affected by changes due to the difference in the coexisting gas components in principle (the spectrum area is almost constant regardless of the coexisting gas components), and the spectrum area is also in principle against pressure fluctuations. Shows a linear change.

  In the peak height method and the 2f method, the above three fluctuation factors (temperature, pressure, coexisting gas components) all affect nonlinearly, and when these fluctuation factors coexist, correction is difficult. The linear correction for the gas pressure fluctuation and the non-linear correction for the gas temperature fluctuation can be performed, and an accurate correction can be realized.

  The measurement laser light that has passed through the atmosphere of the measurement gas 20 is received by the light receiving element 31 provided in the detection unit 30 and converted into an electrical signal.

  The output signal of the light receiving element 31 is adjusted to an appropriate amplitude level via a variable gain amplifier 32, input to the A / D converter 33, and converted into a digital signal.

  With respect to the output data of the A / D converter 33, in synchronization with the wavelength scan of the semiconductor laser 11, the integration is performed a predetermined number of times (for example, several hundred to several thousand times) between the integrator 34 and the memory 35 and the memory 35 Is repeated, noise included in the measurement signal is removed and the data is smoothed, and then input to the CPU 36.

  The CPU 36 performs arithmetic processing such as measurement gas concentration analysis based on the measurement signal from which noise has been removed, and an amplifier when the amplitude level of the output signal of the light receiving element 31 is not appropriate as the input level of the A / D converter 33. 32 gain adjustment is performed.

  Non-Patent Document 1 describes the measurement principle, features, and specific measurement examples of a laser gas analyzer that applies wavelength-tunable semiconductor laser spectroscopy.

Tamura, one and three others, “Laser gas analyzer TDLS200 and its application to industrial processes”, Yokogawa Technical Report, Yokogawa Electric Corporation, 2010, Vol. 53 No. 2 (2010) p. 51-54

  However, the laser gas analyzer configured as shown in FIG. 19 is limited to the measurement of a single component because the wavelength variable range of the semiconductor laser 11 is narrow.

For example, when measuring a hydrocarbon multi-component gas mixture, there is no absorption because hydrocarbons other than CH ₄ have a complicated structure and a large number of absorption lines overlap, so that broad absorption exists in the base other than sharp absorption lines. There is no wavelength. For this reason, it is impossible to correct the baseline fluctuation due to the change in the transmittance of the gas cell itself.

Moreover, when measuring a hydrocarbon multi-component mixed gas, a technique for obtaining the concentration (gas partial pressure) of each hydrocarbon from an absorption spectrum in which broad absorptions of hydrocarbons other than CH ₄ overlap is necessary.

  As such a method, a statistical method (chemometrics) has been conventionally known. However, since a calibration curve needs to be obtained individually according to each application, a large engineering man-hour is generated.

  On the other hand, in the measurement of a single component, an area method not based on a statistical method as described in Non-Patent Document 1 is used.

  In order to measure multi-component mixed gas with broad absorption overlap using the area method not based on this statistical method, it is necessary to separate the absorption spectrum of each gas from the overlapped absorption spectrum. It has not been put into practical use as an analyzer.

  The present invention solves these problems, and its purpose is to use a wavelength tunable laser having a wide wavelength tunable width as a laser light source, and to use an area method in which the concentration of a multi-component mixed gas is not based on a statistical technique. The object is to realize a laser gas analyzer which can be measured relatively easily.

In order to achieve such a problem, the invention according to claim 1 of the present invention is:
In a laser gas analyzer for analyzing a gas to be measured in a multi-component mixed gas using wavelength tunable semiconductor laser absorption spectroscopy,
It has a gas cell for wavelength calibration used for wavelength calibration of absorption spectrum,
The wavelength calibration gas cell is characterized in that a wavelength calibration gas composed of a plurality of types of gas components is sealed therein.

The invention described in claim 2 is the laser gas analyzer according to claim 1,
The wavelength calibration gas contains the same type of gas as the gas to be measured.

The invention according to claim 3 is the laser gas analyzer according to claim 1,
The wavelength calibration gas contains the isotope of the gas to be measured.

A fourth aspect of the present invention is the laser gas analyzer according to the first aspect,
The wavelength calibration gas is methane and ethylene.

The invention according to claim 5 is the laser gas analyzer according to any one of claims 1 to 4,
The wavelength calibration gas is depressurized.

The invention described in claim 6 is a laser gas analyzer,
A tunable laser having a wide tunable width;
Light branching means for branching the output light of the wavelength tunable laser into measurement light and reference light;
A gas cell for measurement into which a multi-component gas mixture is introduced and the measurement light is incident;
Based on a reference signal related to the reference light and an absorption signal related to the output light of the measurement gas cell, an absorption spectrum of each component of the gas to be measured contained in the multi-component gas mixture is obtained, and the concentration of each component is determined. The desired data processing unit;
A wavelength calibration gas cell that is used for wavelength calibration of the absorption spectrum and encloses a wavelength calibration gas composed of a plurality of types of gas components;
The wavelength calibration gas cell is arranged in front of the optical branching means.

  By these, the density | concentration of each gas contained in multicomponent mixed gas can be measured comparatively easily using the area method which does not depend on a statistical method.

It is a block diagram which shows one Example of this invention. It is a block diagram which shows the specific example of the laser gas analyzer based on this invention. It is a flowchart explaining the flow of the whole measurement operation | movement of the apparatus of FIG. It is a spectrum figure of methane. FIG. 5 is a spectrum diagram of methane, ethylene, ethane, propylene, and propane in the vicinity of 1674.5 nm of peak P1 in FIG. 4. It is a spectrum diagram of ethylene. FIG. 7 is a spectrum diagram of methane, ethylene, ethane, propylene, and propane in the vicinity of 1675.9 nm of peak P1 in FIG. 6. It is a spectrum diagram of ethane. FIG. 9 is a spectrum diagram of methane, ethylene, ethane, propylene, and propane in the vicinity of 1683.1 nm of peak P2 in FIG. 8. It is a spectrum figure of propylene. FIG. 11 is a spectrum diagram of methane, ethylene, ethane, propylene, and propane in the vicinity of the propylene peak shown in FIG. 10. It is a spectrum figure of propane. FIG. 13 is a spectrum diagram of methane, ethylene, ethane, propylene, and propane in the vicinity of the propane peak shown in FIG. 12. It is an example figure of the change of the peak width by the difference in constituent gas. It is a flowchart explaining the procedure which selects an appropriate spectrum from a database. It is the spectrum figure of the extracted ethane. It is explanatory drawing which determines the spectrum area area | region of ethylene. It is an area calculation area | region figure. It is a block diagram which shows an example of the conventional laser gas analyzer.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1 is a block diagram showing an embodiment of a laser gas analyzer according to the present invention. In FIG. 1, a wavelength tunable laser 101 generates measurement light of an absorption spectrum of a gas to be measured, and is connected to an oscillation wavelength control circuit 102 that controls the oscillation wavelength.

  The light emitted from the wavelength tunable laser 101 is converted into parallel light by the lens 103, passes through the isolator 104, and is divided into two parallel lights of measurement light and reference light by the beam splitter 105.

  One of the parallel lights divided into two by the beam splitter 105 is incident on a gas cell 106 into which a gas to be measured is introduced as measurement light, collected by a lens 107, incident on a photodiode 108, and converted into an electrical signal. And input to one input terminal of the wavelength calibration means 111.

  The other parallel light is condensed by the lens 109 and incident on the photodiode 110 as reference light, converted into an electric signal, and input to the other input terminal of the wavelength calibration means 111.

  The wavelength calibration unit 111 is connected to an absorption line wavelength data storage unit 112 for storing absorption line wavelength data of the gas to be measured. The wavelength calibration unit 111 calibrates the wavelength of the spectrum data obtained from the photodiodes 108 and 110 using the absorption wavelength data stored in the absorption line wavelength data storage unit 112 and the absorption spectrum obtained from the photodiode 108. .

  Further, the wavelength calibration unit 111 calculates the absorbance by dividing the spectrum data of the wavelength-calibrated photodiodes 108 and 110 to obtain the absorption spectrum of the absorbance, and inputs the obtained absorbance spectrum to the spectrum separation unit 113. To do.

  The spectrum separation unit 113 is connected to an absorption spectrum data storage unit 114 that stores absorption spectrum data of each gas to be measured. The spectrum separation means 113 obtained by, for example, separating the absorbance spectrum of each measured gas from the absorbance spectrum input from the wavelength calibration means 111 by least square fitting using the absorption spectrum of the measured gas. The absorption spectrum of each gas to be measured is input to the concentration detection means 115.

  The concentration detecting means 115 is connected to wavelength range data for calculating the area of the absorption line of each gas to be measured and an area-to-concentration ratio data storage unit 116 for storing proportional constant data of the area and gas partial pressure. . The concentration detection means 115 obtains the area of the designated wavelength range from the absorption spectrum of the absorbance of each gas to be measured input from the spectrum separation means 113, and further multiplies the designated proportionality constant to separate the measurement gas. Calculate the pressure.

  FIG. 2 is a block diagram showing a specific example of a laser gas analyzer based on the present invention for measuring hydrocarbon multicomponents, and the same reference numerals are given to the parts common to FIG. The wavelength tunable laser 101 includes a first MEMS-VCSEL (MEMS-Vertical Cavity Surface Emitting Laser) emitting a measurement light of a hydrocarbon absorption spectrum and a wavelength 1670. And a second MEMS-VCSEL that emits light of up to 1700 nm. Each MEMS-VCSEL is connected to the oscillation wavelength control circuit 102, and the oscillation wavelength control circuit 102 alternately oscillates by applying current to each MEMS-VCSEL and also applies a voltage to be applied to the oscillating MEMS-VCSEL. Control each oscillation wavelength.

The light emitted from each MEMS-VCSEL is converted into parallel light by the lens 103, passes through the isolator 104, and further passes through the wavelength calibration gas cell 117 in which CH ₄ and C ₂ H ₄ are sealed. Divided into two parallel lights.

  One parallel light divided into two by the beam splitter 105 is incident on the gas cell 106 into which the gas to be measured is introduced as the measurement light, and becomes an absorption signal of the gas to be measured. The output light of the gas cell 106 is collected by the lens 107, enters the photodiode 108, is converted into an electric signal, and is input to one input terminal of the wavelength calibration unit 111.

  The other parallel light is condensed by the lens 109 and directly incident on the photodiode 110 as reference light, converted into an electric signal, and input to the other input terminal of the wavelength calibration means 111. Thereby, it becomes a reference signal of the output light intensity of each MEMS-VCSEL.

  The gas cell 106 is controlled to a predetermined temperature by the gas cell temperature control device 118, and the gas to be measured is controlled to the same temperature as the gas cell 106 by the gas temperature control device 119.

The absorption line wavelength data storage unit 112 connected to the wavelength calibration unit 111 stores the absorption line wavelength data of CH ₄ and C ₂ H ₄ enclosed in the wavelength calibration gas cell 117 among the gas to be measured. Yes. The wavelength calibration unit 111 uses the CH ₄ and C ₂ H ₄ absorption wavelength data stored in the absorption line wavelength data storage unit 112 and the CH ₄ and C ₂ H ₄ absorption spectra obtained from the photodiode 110. The wavelength of the spectral data obtained from the photodiodes 108 and 110 is calibrated.

  Further, the wavelength calibration means 111 calculates the absorbance by dividing the spectrum data of the wavelength-calibrated photodiodes 108 and 110 to obtain the absorbance spectrum of the measured gas, and the obtained absorbance spectrum of the measured gas. Is input to the spectral separation means 113.

The absorption spectrum data storage unit 114 connected to the spectrum separation means 113 stores a hydrocarbon component (CH ₄ , C ₂ H ₄ , C ₂ H ₆ , C ₃ H ₆ , C ₃ H ₈ ) as a gas to be measured. The absorption spectrum data of is stored. The spectrum separation means 113 performs each hydrocarbon component (CH ₄ , C ₂ H ₄ , C ₂ H ₆₎ from the absorption spectrum of the absorbance input from the wavelength calibration means 111 by least square fitting using the absorption spectrum of the gas to be measured. , C ₃ H ₆ , C ₃ H ₈ ) absorbance absorption spectra are separated, and the obtained absorption spectra of the measured gases are input to the concentration detection means 115.

In the area-to-concentration ratio data storage unit 116 connected to the concentration detecting means 115, hydrocarbon components (CH ₄ , C ₂ H ₄ , C ₂ H ₆ , C ₃ H ₆ , C ₃₎ that are gas to be measured are stored. Wavelength range data for calculating the area of the absorption line of H ₈ ) and proportional constant data of the area and gas partial pressure measured in advance are stored.

  The concentration detection means 115 obtains the area of the designated wavelength range from the absorption spectrum of the absorbance of each measured gas input from the spectrum separating means 113, and multiplies the designated proportionality constant to multiply the measured partial pressure of each measured gas. Ask for. Furthermore, the concentration of each gas to be measured can be obtained by dividing the obtained partial pressure of each gas to be measured by the total pressure measured by the pressure gauge 120 provided in the outlet path of the gas cell 106.

  FIG. 3 is a flowchart for explaining the overall flow of the measurement operation of the apparatus configured as shown in FIG. The signal processing starts with wavelength calibration using a reference signal that is not affected by the gas to be measured.

The wavelength calibration means 111 detects the peak data number of the sharp absorption spectrum of CH ₄ and C ₂ H ₄ based on the reference signal output from the photodiode 110, and accurately determines the data number from the absorption line and wavelength table. Determine the correct wavelength. From the relationship between the obtained data numbers and wavelengths, for example, each data number is associated with the wavelength 1685 nm range and 1630 nm range by polynomial approximation (steps S1 to S3).

After wavelength calibration, the absorbance in each wavelength band is calculated from the reference signal and absorption signal (step S4), the optimum pure spectrum is selected (step S5), and a rough gas concentration analysis is performed (step S6). Then, in order from components having a large influence degree (C ₂ H ₄ → CH ₄ → C ₂ H ₆ → C ₃ H ₈ → C ₃ H ₆ ), spectral separation and concentration analysis are repeated (steps S7 to S16).

When the incident intensity to the measurement gas is I ₁ , the transmitted light intensity from the measurement gas is I ₂ , and the absorption rate of the measurement gas is A, the absorbance of the gas at the wavelength λ is
Absorbance = log ₁₀ [I ₁ (λ) / I ₂ (λ)]
= Log ₁₀ [I ₁ (λ) / I ₁ (λ) × A (λ)]
= Log ₁₀ [1 / A (λ)] (1)
It becomes.

Further, the light intensity of the reference light is I _r , the light intensity immediately before the wavelength calibration gas cell 117 is I ₀ , the absorption rate of the wavelength calibration gas is A ₁ , the transmittance of the wavelength calibration gas cell 117 is T _r1 , and the beam splitter 105. Is the branching ratio R ₁ : R ₂ (R ₁ + R ₂ = 1) and the transmittance of the measuring gas cell 106 is T _r .
I _r (λ) = I ₀ (λ) × A ₁ (λ) × T _r1 × R ₁ (λ) (2)
I ₂ (λ) = I ₀ (λ) × A ₁ (λ) × T _r1 × R ₂ (λ) × A (λ) × T _r (λ) (3)
It can be expressed as.

Therefore,
log ₁₀ [I _r (λ) / I ₂ (λ)] = log ₁₀ [R ₁ (λ) / (R ₂ (λ) × A (λ) × T _r (λ))]
= Log ₁₀ [1 / A (λ)] + log ₁₀ [R ₁ (λ) / (R ₂ (λ) × T _r (λ))]
= Absorbance + Instrument function (4)
It becomes. Here, since the second term is a term that does not depend on the measurement gas, the second term can be removed if a spectrum is acquired in advance by enclosing a gas that can be ignored in the measurement gas cell. That is, the absorbance can be obtained by calculating the common logarithm by taking the ratio of the output signals of the photodiodes 108 and 110 as in the first term of the equation (4).

Incidentally, it should be noted is that it contains the effects of I _r and the wavelength calibration gas cell 117 in any of the I _2. From this, as shown in the first term of equation (4), if the ratio between the two signals is determined, the signal of the wavelength calibration gas cell 117 is eliminated, so that the calculation can be performed easily.

  Next, the wavelength band used when detecting the concentration (gas partial pressure) of each hydrocarbon will be described. The main point of the present invention is to separate the spectrum of each gas to be measured (hydrocarbon) from the measured absorption spectrum of the mixed gas, and obtain the concentration (gas partial pressure) from the area of the obtained single absorption line. It is.

  In order to perform this spectral separation with high accuracy, a wavelength band in which a characteristic absorption line of the gas to be measured exists and a characteristic absorption line of another mixed gas does not exist may be selected. If such a wavelength band is selected, the main effect of other gases will be the same as that of baseline fluctuations. Therefore, if a concentration (gas partial pressure) detection method that is not affected by baseline fluctuations is adopted, other gases will be affected. The impact can be minimized. Therefore, in the present invention, concentration (gas partial pressure) is detected by a method that uses different wavelength bands to which the above conditions apply for each gas to be measured (hydrocarbon) and is not affected by baseline fluctuations. Each will be described below.

  4A and 4B are spectrum diagrams of methane, in which FIG. 4A shows the 1685 nm band, and FIG. 4B shows the 1630 nm band. As is apparent from FIG. 4, methane has a plurality of sharp peaks in each wavelength band. Among these sharp peaks, it is peaks P1 to P6 indicated by arrows that have small absorption components of other gases and are suitable for concentration (gas partial pressure) detection.

  FIG. 5 is a spectrum diagram of methane, ethylene, ethane, propylene, and propane in the vicinity of 1674.5 nm of the peak P1 in FIG. According to FIG. 5, it can be seen that the peak of methane is clearly sharper and larger than other gas spectra. Thereby, the influence of other gases can be regarded as a baseline fluctuation.

  FIG. 6 is a spectrum diagram of ethylene, where (A) shows the 1685 nm band and (B) shows the 1630 nm band. As is apparent from FIG. 6, ethylene also has a plurality of sharp peaks in each wavelength band, but it is difficult to extract a specific peak because the peak interval is narrow in the 1630 nm band. On the other hand, peaks P1 to P3 indicated by arrows in the 1685 nm band are suitable for concentration (gas partial pressure) detection.

  FIG. 7 is a spectrum diagram of methane, ethylene, ethane, propylene, and propane in the vicinity of 1675.9 nm of the peak P1 in FIG. According to FIG. 7, it can be seen that the ethylene peak is clearly sharper and larger than other gas spectra. Accordingly, it can be said that the peak P1 is suitable for detecting the concentration (gas partial pressure) because the absorption components of other gases are small among these sharp peaks.

  8A and 8B are spectrum diagrams of ethane. FIG. 8A shows the 1685 nm band, and FIG. 8B shows the 1630 nm band. As is apparent from FIG. 8, ethane has no peak in the 1630 nm band and several peaks in the 1685 nm band. Among these peaks, peaks P1 to P3 indicated by arrows are suitable for concentration (gas partial pressure) detection.

  FIG. 9 is a spectrum diagram of methane, ethylene, ethane, propylene, and propane in the vicinity of 1683.1 nm of peak P2 in FIG. According to FIG. 9, ethylene and methane have absorption at around 1683.1 nm, but ethane has the highest absorption.

  FIG. 10 is a spectrum diagram of propylene, (A) shows the 1685 nm band, and (B) shows the 1630 nm band. As is clear from FIG. 10, propylene has only one peak at 1628.7 nm, and this peak is used for concentration (gas partial pressure) detection.

  FIG. 11 is a spectrum diagram of methane, ethylene, ethane, propylene, and propane in the vicinity of the propylene peak shown in FIG. According to FIG. 11, since absorption of ethylene and methane is large, a method of extracting propylene by removing the influence of these gases is important.

  FIG. 12 is a spectrum diagram of propane, (A) shows the 1685 nm band, and (B) shows the 1630 nm band. As is clear from FIG. 12, propane has only one peak at 1686.4 nm, and this peak is used for concentration (gas partial pressure) detection.

  FIG. 13 is a spectrum diagram of methane, ethylene, ethane, propylene, and propane in the vicinity of the propane peak shown in FIG. According to FIG. 13, since the absorption of ethylene and methane is large, a method of extracting propylene by removing the influence of these gases becomes important.

  Next, the spectrum separation method will be described. It should be noted here that the shape of the absorption spectrum changes depending on the total pressure, gas concentration (gas partial pressure), and the type of mixed gas. Unless these changes are accommodated, accurate spectral separation cannot be achieved.

  Therefore, in the present invention, the absorption spectrum of each gas to be measured in consideration of the change of the spectrum shape is used for the spectrum separation.

  A specific example in which the peak width changes will be described with reference to FIG. FIG. 14 is a diagram showing an example of changes in peak width due to differences in constituent gases, in which absorption other than methane is removed and only the absorption spectrum of methane is extracted. Characteristic A is a gas in which methane and hydrogen are mixed by 50% each. The characteristic B shows the spectrum of a gas in which methane and propylene are mixed by 50% each. As is clear from FIG. 14, the peak width is wider when mixed with propylene than when mixed with hydrogen.

  In order to cope with such a change in spectrum shape, the absorption spectrum of each gas to be measured that is closest to the actual spectrum shape is selected from the spectrum database acquired in advance by the processing procedure shown in the flowchart of FIG. To do. In order to select an absorption spectrum with a close spectral shape, the spectral database must contain absorption spectra of various shapes.

  The cause of the change in the spectral shape is thought to be a lot of collisions between molecules. That is, propylene or propane having a large molecular collision cross section has a large effect of changing the spectrum shape. On the other hand, molecules having a small effect of changing the spectrum shape include nitrogen molecules having a small collision cross-sectional area of the molecules.

Based on these reasons, a spectrum database is constructed by measuring a number of spectra with different mixing ratios for the combinations of the two types of mixed gas shown below, and a spectrum having the closest spectrum shape is selected from these databases and used. .
_{CH 4: CH 4 + N 2} , CH 4 + C 3 H 6, CH 4 + C 3 H 8
C ₂ H ₄ : C ₂ H ₆ + N ₂ , C ₂ H ₄ + C ₃ H ₆ , C ₂ H ₄ + C ₃ H ₈
C ₂ H ₆ : C ₂ H ₆ + N ₂ , C ₂ H ₆ + C ₃ H ₆
C ₂ H ₆ : C ₂ H ₆ + N ₂
C ₃ H ₈ : C ₃ H ₈ + N ₂

  A procedure for selecting an appropriate spectrum from the database will be described based on the flowchart of FIG. First, an absorption spectrum (hereinafter referred to as an initial pure spectrum) of each gas to be measured used for calculation is read from the database. Here, it is not necessary to consider the change of the spectrum shape, and any pure spectrum may be used (step S1).

  Next, a gas selection loop is started (step S2), and a gas type for determining the optimum pure spectrum is selected (step S3). Then, a preselected wavelength region portion where the selected gas has a characteristic peak is cut out from the mixed gas spectrum (step S4).

  Subsequently, a pure spectrum determination loop is started (step S5). For the selected gas, one pure spectrum is selected from the database one by one, and for the spectrum other than the selected gas, the pure spectrum of step S1 is selected (step S6).

Using these pure spectra, the spectrum residual is obtained as in the following equation.
A = CK + R (5)

  Here, A is a mixed gas spectrum, K is a list of pure spectra of each gas, C is a concentration (gas partial pressure) of each gas, and R is a spectral residual. If the pure spectrum of each gas is determined here, the concentration (gas partial pressure) can be obtained from the equation (5) as in the following equation (6).

C = AK ^T (KK ^T ) ⁻¹ (6)
The concentration (gas partial pressure) thus obtained is again substituted into the equation (5) to perform the square sum calculation of the spectrum residual, thereby obtaining the spectrum residual R (step S7).

  When the square sum of the spectral residuals is recorded (step S8), another pure spectrum is selected for the selected gas, and the square sum of the spectral residuals is recorded. This is performed for all the pure spectra of the selected gas (step S9). The pure spectrum when it is the smallest among the square sums of the spectral residuals thus obtained is set as the optimum pure spectrum (step S10).

  When the optimum pure spectrum is obtained, the same flow is repeated for the next gas to obtain the optimum pure spectrum for all the gases (step S11).

The formula (6) is recalculated near the peak of each gas using the optimum pure spectrum obtained in step S6 of FIG. 3, and approximate gas concentrations (gas partial pressures) are obtained. Extracting only the peak of the hydrocarbon by subtracting the optimum pure spectrum of the other gas by the concentration (gas partial pressure) obtained by equation (6) from the spectrum near the peak selected for the hydrocarbon. Can do. For example, when extracting only ethane from a mixed gas spectrum of five hydrocarbons (methane, ethylene, ethane, propylene, propane)
_{A (C 2 H 6) =} A-K (CH 4) C (CH 4) -K (C 2 H 4) C (C 2 H 4)
_{-K (C 3 H 6) C} (C 3 H 6) -K (C 3 H 8) C (C 3 H 8) (7)
And it is sufficient.

  As for the concentration (gas partial pressure) of each hydrocarbon gas used in the equation (7), the extraction accuracy increases if the value obtained by the equation (6) is used in the vicinity of each peak. The spectrum of ethane extracted in this way is shown in FIG. In FIG. 16, (A) shows the spectrum of the mixed gas, (B) shows the extracted ethane spectrum, and (C) shows the pure spectrum of ethane. As is clear from FIG. 16, ethane is contained only 15%, but an ethane spectrum having almost the same shape as the pure spectrum can be extracted.

  When the influence of the baseline fluctuation remains, the baseline fluctuation can be eliminated by performing the least square fitting with the zero-order (simple offset), the first-order (tilted offset), and the pure spectrum of ethane. it can. In the present invention, the correction of the first-order term is performed to separate the absorption spectrum of each measured gas from which the baseline fluctuation has been removed.

  In order to calculate the concentration (gas partial pressure) using the extracted spectrum, the area of the spectrum is used. If the partial pressure of each gas is constant, there is a region where the proportional relationship between the area and the concentration (gas partial pressure) always holds even if the external environment such as total pressure or constituent gas changes. The partial pressure of each gas is calculated by using the area of such a region.

As a specific example, the case where the spectrum area region of ethylene is determined will be described with reference to FIG. First, in order to subtract the baseline, as shown in (A), in C ₂ H ₄ , a straight line connecting two points, the X point on the short wavelength side and the Y point on the long wavelength side, is sandwiched between the peak portions. Subtract. Thereby, as shown in (B), the influence of the baseline fluctuation can be completely eliminated. Then, the area between the two points is obtained.

  Such a process is similarly performed for all the spectra in the spectrum database whose concentration (gas partial pressure) is known. FIG. 18 is an area calculation area diagram, in which the values of X and Y are changed from 1 to 400, respectively, and the combination of X and Y when the relationship between the concentration (gas partial pressure) and the area is closest to the proportional relationship is obtained. The results are shown. The horizontal axis corresponds to X and the vertical axis corresponds to Y. The blue portion (B) indicates that the detection error is small, and the red portion (R) indicates that the detection error is large. For ethylene, the combination of X = 317 and Y = 373 is effective.

  Until now, analysis of hydrocarbon multi-component systems has mainly used gas chromatography because of its high component separation ability, but since measurement time is long, measured values could not be used for direct control. . On the other hand, laser gas analyzers capable of measuring in real time have a narrow wavelength variable width of the light source and are mainly limited to single component measurement.

  On the other hand, in the present invention, a multi-component mixed gas is obtained by using as a light source a MEMS-VCSEL that has no mechanical working part and has high reliability required as an industrial instrument and can be tunable in a wide wavelength range. An industrial laser gas analyzer capable of real-time measurement of the absorption spectrum was realized.

  Specifically, by using MEMS-VCSEL oscillating in the wavelength range of 1620 to 1750 nm as a light source, a laser gas analyzer capable of analyzing a hydrocarbon multicomponent gas mixture that can be measured in real time was realized.

By performing wavelength calibration using the absorption lines of CH ₄ and C ₂ H ₄ , an accurate absorption spectrum can be obtained in a wide wavelength range.

By using the wavelength calibration gas cell 117 in which CH ₄ and C ₂ H ₄ are enclosed, the gas concentration (gas partial pressure) used for wavelength calibration in the gas to be measured is lowered, and the absorption line used for wavelength calibration is measured. It can avoid being impossible.

  In the conventional apparatus, the wavelength calibration gas cell 117 is inserted into the optical path of the reference light divided into two by the beam splitter 105. However, in the present invention, the wavelength calibration gas cell 117 is inserted in the preceding stage of the beam splitter 105, so Optical output calibration can be performed with a single light receiving element. Then, the absorption signal of the calibration gas can be removed from the absorption signal of the measurement gas by simply dividing the absorption signal of the measurement gas obtained from the photodiode 108 and the reference signal obtained from the photodiode 110.

An accurate absorption spectrum can be obtained by performing wavelength calibration of the absorption spectrum of the hydrocarbon multi-component mixed gas obtained using the wavelength calibration gas CH ₄ and C ₂ H ₄ absorption lines and the wavelength table.

  By separating the absorption spectrum of each gas using the absorption spectrum database of each gas of the hydrocarbon multicomponent mixed gas, an absorption spectrum of each gas can be obtained.

  It is necessary to obtain a calibration curve for each application by developing and adopting an area method that does not use statistical methods (chemometrics) as a means of obtaining the concentration of each component (gas partial pressure) from the absorption spectrum of a multi-component system. This eliminates the need for significant engineering man-hours. Thereby, the absorption spectrum of each gas can be obtained, and the concentration (gas partial pressure) of each gas can be obtained without using a statistical method (chemometrics).

  In order to use the area method, it is necessary to separate the absorption spectrum of each gas under measurement from the overlapped absorption spectrum of the mixed gas. In the present invention, however, the absorption spectrum database includes changes in the shape of the absorption spectrum due to gas mixing. The least square fitting was performed in the wavelength band where the characteristic absorption line of each gas exists, and the absorption spectrum of each measured gas alone was separated from the absorption spectrum of the mixed gas. Thereby, the influence of absorption of other gases can be suppressed to the extent of baseline fluctuation.

  In the present invention, as the area method, the method of subtracting the value determined from the shape of the absorption line from the absorption spectrum is used to obtain the area, so that the influence of the baseline fluctuation can be eliminated.

By using a wavelength region where the absorption of other hydrocarbon gases is small and the absorption of impurities is small as the peak of each hydrocarbon gas, it is not necessary to create a calibration curve.
By selecting the most suitable spectrum for the actual spectrum shape from the spectrum database, it became possible to cope with changes in the spectrum shape and improved robustness.
By using the area of the region where the concentration (gas partial pressure) error is small even if the spectrum shape changes, it is possible to measure the concentration (gas partial pressure) with high accuracy.

  Depending on the concentration (gas partial pressure) value, another gas spectrum in the vicinity of the selected peak may become large, and the extraction error may increase. In such a case, what is necessary is just to change the peak used with the density | concentration (gas partial pressure) value of mixed gas.

  The temperature of the gas to be measured input to the gas cell 106 is preferably made constant by the gas temperature control device 119. Thereby, the change of the spectrum shape accompanying a temperature change can be suppressed, and the measurement error by a temperature change can be reduced.

  By configuring the optical system using an optical fiber, design constraints can be relaxed and the size can be reduced, and the stability against mechanical vibration can be enhanced.

  By extending the optical path length by multipath, it is possible to apply to a gas having a small absorption and to detect a trace gas.

  The measurement gas may be measured under reduced pressure. Thereby, separation of the absorption line which has overlapped can be expected.

  A plurality of wavelength tunable light sources can be used to perform time-division measurement with a common light receiving element.

The wavelength calibration gas is not limited to CH ₄ and C ₂ H ₄ , and C ₂ H ₄ O or HCl may be used according to the wavelength band used for measurement. As the wavelength calibration gas, for example, a molecular gas having a sharp absorption line with a small number of atoms, or a gas having the absorption line in a near wavelength range, for example, the same isotope or composition as the gas to be measured is used. For example, when measuring the spectrum of a hydrocarbon, wavelength calibration is performed using a hydrocarbon having 1 or 2 carbon atoms.

  It is desirable to reduce the total pressure of the wavelength calibration gas in order to sharpen the absorption line used for calibration. In particular, by reducing the pressure until a plurality of absorption lines overlap and are separated, higher-precision measurement can be performed. For example, when methane is used for wavelength calibration, it is effective to reduce the overlap of a plurality of absorption lines to 0.1 atm or less.

  The absorption rate of the wavelength calibration gas cell 117 is desirably 1/10 or less of the absorption rate of the measurement gas cell 106 in order to reduce the influence of the measurement signal on the S / N ratio.

  As described above, according to the present invention, a wavelength tunable laser having a wide wavelength tunable width is used as a laser light source, and the concentration of a hydrocarbon multicomponent mixed gas is relatively easily determined by using an area method not based on a statistical method. A laser gas analyzer capable of measurement can be realized, and is effective for direct measurement of various process gases.

DESCRIPTION OF SYMBOLS 101 Wavelength variable laser 102 Oscillation wavelength control circuit 103, 107, 109 Lens 104 Isolator 105 Beam splitter 106 Gas cell 108, 110 Photodiode 111 Wavelength calibration means 112 Absorption line wavelength data storage part 113 Spectrum separation means 114 Absorption spectrum data storage part 115 Concentration Detection means 116 Area to concentration ratio data storage unit

The wavelength calibration unit 111 detects the peak of the sharp absorption spectrum of CH ₄ and C ₂ H ₄ based on the reference signal output from the photodiode 110, and determines the exact wavelength of the peak from the absorption line and wavelength table. To do. From the relationship between the obtained plurality of peaks and wavelengths, for example, each peak is associated with the wavelength 1685 nm range and 1630 nm range by polynomial approximation (steps S1 to S3).

Claims (6)

In a laser gas analyzer for analyzing a gas to be measured in a multi-component mixed gas using wavelength tunable semiconductor laser absorption spectroscopy,
It has a gas cell for wavelength calibration used for wavelength calibration of absorption spectrum,
The laser gas analyzer according to claim 1, wherein the wavelength calibration gas cell is filled with a wavelength calibration gas comprising a plurality of types of gas components.   The laser gas analyzer according to claim 1, wherein the wavelength calibration gas contains the same type of gas as the gas to be measured.   The laser gas analyzer according to claim 1, wherein the wavelength calibration gas contains an isotope of the gas to be measured.   The laser gas analyzer according to claim 1, wherein the wavelength calibration gas is methane and ethylene.   The laser gas analyzer according to claim 1, wherein the wavelength calibration gas is decompressed. A tunable laser having a wide tunable width;
Light branching means for branching the output light of the wavelength tunable laser into measurement light and reference light;
A gas cell for measurement into which a multi-component gas mixture is introduced and the measurement light is incident;
Based on a reference signal related to the reference light and an absorption signal related to the output light of the measurement gas cell, an absorption spectrum of each component of the gas to be measured contained in the multi-component gas mixture is obtained, and the concentration of each component is determined. The desired data processing unit;
A wavelength calibration gas cell that is used for wavelength calibration of the absorption spectrum and encloses a wavelength calibration gas composed of a plurality of types of gas components;
2. The laser gas analyzer according to claim 1, wherein the wavelength calibration gas cell is disposed in front of the optical branching unit.