JP2006112900A - Infrared gas analyzing method and infrared gas analyzer - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To readily correct errors with respect to the reference concentration of CO<SB>2</SB>. <P>SOLUTION: A pneumatic CO<SB>2</SB>infrared detector 1 is installed in the vicinity of the cell 8, on the side of the gas discharge port 13 of the cell 8, and a pneumatic CO<SB>2</SB>infrared detector 2 is installed on an optical rear stage. The light transmitted through the cell 8 is measured by the detector 1, while the light transmitted through the cell 8 and the first infrared detector is measured by the detector 2; and a correction factor (k) for linearizing a calibration curve of the reference gas concentration is calculated, from the ratio of the measuring results due to both the detectors and the sensitivity of a reference gas is corrected. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention is used for process monitors relating to gas concentrations in chemical factories and steelworks, combustion gas analysis of boilers and combustion furnaces, air pollution monitoring, automobile exhaust gas measurement, etc., utilizing the infrared absorption effect inherent to gas molecules, The present invention relates to a non-dispersive infrared gas analyzer that continuously measures the concentration of a specific component in gas and vapor, and more particularly to a so-called differential method infrared gas analyzer that is used for measuring VOC (gaseous organic matter) concentration. .

  Many heteronuclear molecules composed of two or more different atoms undergo a transition of vibrational and rotational kinetic energy levels specific to the chemical species when irradiated with infrared light having a wavelength of 1 to 20 μm. This causes thermodynamic changes such as an increase in internal energy, volume or pressure. A non-dispersive infrared gas analyzer is a device that measures the concentration using such characteristics of gas components (see Patent Document 1).

All VOC concentration measurement of the sample gas, after conversion to CO ₂ by burning VOC in combustion furnace, there is a method of measuring in a CO ₂ meter. At this time, if CO ₂ originally exists in the sample gas before combustion, the measurement is performed using a so-called differential quantity infrared gas analyzer.
The difference method is a method in which a reference gas and a sample gas are periodically switched by a solenoid valve or the like and introduced into a cell, and a difference in CO ₂ concentration between the reference gas and the sample gas is measured. Here, the gas before combustion is a reference gas, and the gas after combustion is a sample gas. Measuring the difference in CO ₂ concentration between the _two gases is equivalent to the total VOC concentration contained in the sample gas, since it is a measure of the CO ₂ concentration that has increased as a result of combustion of all VOCs.

However, normally, VOC and CO ₂ (referred to as reference CO ₂ ) coexist in the sample gas. If the reference CO ₂ concentration fluctuates during the measurement, a measurement error is caused by the nonlinearity of the relationship between the CO ₂ concentration and the infrared absorption amount in the cell in accordance with the Lambert-Beer law of infrared absorption. An example of this measurement error is shown in FIG.
FIG. 4 shows the change in detector output when the VOC concentration is constant and the reference CO ₂ concentration is changed. The VOC concentration where the reference CO ₂ concentration is low is (1b-1a), and the detector output at that time is (1c-1d). The VOC concentration where the reference CO ₂ concentration is high is (2b-2a), and the detector output at that time is (2c-2d). At this time, although the VOC concentrations (1a-1b) and (2a-2b) at the two points are equal, the detector output (1c-1d) on the side with the lower reference CO ₂ concentration is ( 2a-2b). Thus, a measurement error occurs due to the reference CO ₂ concentration.

When measuring low-concentration CO ₂ with high accuracy, it is necessary to increase the length of the cell through which the sample gas flows in order to increase the amount of infrared absorption of the sample gas. In this case, the non-linear degree of the detector output is further increased, and a large error is generated with respect to the change in the reference CO ₂ concentration. Therefore, a reference CO ₂ concentration measurement CO ₂ meter with a short cell length is separately installed, the reference CO ₂ concentration is measured, and the CO ₂ concentration value measured by the difference method is corrected using this measured value. Yes.
Japanese Patent Laid-Open No. 9-49797

In conventional infrared gas analyzer, in order to correct an error due to reference the CO ₂ concentration changes, since CO ₂ meter for measuring the reference CO ₂ concentration is required separately, apparatus are large complex.
An object of the present invention is to easily perform error correction.

  The infrared gas analysis method of the present invention is a gas analysis method using a differential method infrared gas analyzer that measures the concentration difference between a reference gas and a sample gas. Alternatively, a second infrared detector in which a first infrared detector filled with a gas whose absorption region overlaps with the same component is installed, the light transmitted through the cell is measured, and the output is proportional or nearly proportional to the component to be measured. And measuring the light transmitted through the first infrared detector, calculating a correction coefficient for linearizing the calibration curve of the reference gas concentration from the ratio of the measurement results by both detectors, This is a method for correcting gas sensitivity.

  An infrared gas analyzer of the present invention includes a sample cell, a switching valve that selectively supplies a reference gas and a sample gas to the sample cell, a light source that irradiates the sample cell with infrared light, and a light source from the light source. An intermittent means for intermittently transmitting infrared light, a pneumatic first infrared detector filled with a measurement target component or a gas whose absorption region overlaps with the measurement target component, and a second whose output is proportional to or substantially proportional to the measurement target component And a calculation control unit for calculating a correction coefficient for linearizing the calibration curve of the reference gas concentration from the ratio of the measurement results of both detectors and correcting the sensitivity of the reference gas. .

  It is a preferable form for the apparatus of the present invention that the second infrared detector is a pneumatic type.

Conventionally, in order to perform error correction, a separate CO ₂ meter having a different cell length for measuring the reference CO ₂ concentration has been separately required, but in the present invention, a transmission type pneumatic type for measuring the difference CO ₂ concentration. the CO ₂ meter using CO ₂ detector, because the pneumatic type CO ₂ detector may only structural changes to add a single, entire apparatus is simplified small.

An embodiment of the analysis method according to the present invention will be described below.
FIG. 1 shows a schematic diagram of the apparatus configuration diagram.
The sample cell 8 has a gas inlet 12 and a gas outlet 13 at both ends of the cell 8, respectively. A light source 9 that emits infrared light is installed on the gas inlet 12 side of the cell 8, and the pneumatic CO ₂ detector 1 is optically located closer to the cell 8 on the gas outlet 13 side of the cell 8. A pneumatic CO ₂ detector 2 is installed in the far side.
The CO ₂ detector B may be any detector that can measure the infrared intensity of the CO ₂ absorption wavelength (eg, 4.25 μm). For example, it may be a pneumatic CO ₂ detector similar to the detector A or a pyroelectric type semiconductor infrared sensor such as a wavelength selected using an optical filter.

The flow path 3 and the flow path 4 for introducing the sample gas into the cell 8 have valves 6 and 7 in the middle of the flow path, and the flow rate of the sample introduced into the cell 8 can be controlled. A combustion furnace 5 is installed in the middle of the flow path 3, where VOC in the sample gas can be completely burned.
The CO ₂ detector 1 is filled with a predetermined concentration of a measurement target component CO ₂ or a gas in which the absorption region overlaps with the measurement target component CO ₂ (hereinafter simply referred to as CO ₂ gas), and detects infrared rays emitted from the light source 9 To do. Similar to the detector 1, the CO ₂ detector 2 is filled with CO ₂ gas at a predetermined concentration, and detects the absorption wavelength of infrared rays that have passed through the CO ₂ detector 1. For the detector 2, the detector 1 serves as a CO ₂ gas filter.

  The interiors of the detectors 1 and 2 are optically partitioned into two front and rear chambers, and gas components are placed in the two chambers. Since the amount of heat absorbed by the gas varies depending on the optical distance, a temperature difference occurs between the two chambers, and the diaphragm is displaced due to the contraction of the gas. The change in pressure due to the displacement of the diaphragm is taken out via a condenser microphone, a flow sensor, or the like.

In FIG. 1, the sample gas is introduced into the apparatus from either the flow path 3 or 4, supplied through the valve 6 or 7 from the gas inlet 12 into the sample cell 8, and discharged from the gas outlet 13 to the outside. Is done. At this time, a gas containing CO ₂ converted from VOC through the combustion furnace 5 is referred to as a sample gas 11, and a gas introduced into the cell 8 without being burned as a reference gas 10.
Both the reference gas 10 and the sample gas 11 contain the same amount of CO _{2 in} the sample gas, and the sample gas 11 further contains CO ₂ generated by burning VOC.

The detector 1 detects absorption due to the CO ₂ gas contained in the cell 8, and the detector ₂ detects absorption due to the CO ₂ gas contained in the cell 8 and the detector 1.
FIG. 2A shows an output obtained by detecting the reference gas 10 with the detector 1 and the detector ₂ while changing the reference CO ₂ concentration.
Since the detector 2 transmits more gas than the detector 1, the detection sensitivity is low, but the linearity approaches a linear linearity proportional to the reference CO ₂ concentration.
Focusing on the fact that the output of the detector 2 is nearly linear, FIG. 2B shows the detector output normalized with reference to the output when the reference CO ₂ concentration is 0.
FIG. 2B shows that the output Bn of the detector 2 is linearized substantially linearly, while the output An of the detector 1 is non-linear.

Next, FIG. 2C shows the relationship between the output ratio An / Bn of the output An of the detector 1 and the output Bn of the detector 2 and the reference CO ₂ concentration. It can be seen that An / Bn has a one-to-one relationship with the reference CO ₂ concentration.
That is, by calculating An / Bn from the outputs of the detector 1 and the detector 2, the reference CO ₂ concentration in the sample gas can be known. By correcting the output of the detector 1 corresponding to the VOC concentration obtained by the difference method using the obtained reference CO ₂ concentration, the VOC concentration can be measured more accurately.
Actually, An / Bn is measured in advance when several kinds of CO ₂ standard gases having different concentrations are used as the reference gas in the apparatus. Using this value, the correction coefficient k is calculated for each standard gas so that the output of the detector 1 is linear, and the relationship between An / Bn and the correction coefficient k is shown in FIG. As shown in FIG.

FIG. 3 shows an output when the output of the detector 1 is multiplied by the correction coefficient k obtained in FIG. In this case, the detection output to the CO ₂ concentration for substantially linear, it is possible to measure the CO ₂ concentration of the sample gas without being affected by change in the reference CO ₂ concentration.
By obtaining the concentration difference between the sample gas 11 and the standard gas 10, the VOC concentration can be accurately measured.

The present embodiment relates to a CO ₂ concentration meter for measuring the VOC concentration, but can be used when measuring the concentration of the gas component increased with respect to the reference gas other than CO ₂ . For example, the NO ₂ is reduced to NO in the catalyst, increasing the NO concentration, i.e. NO ₂ meters and measuring the NO ₂ concentration was reduced in an equimolar ratio NH ₃ and NO, Decreased NO concentration, i.e. NH ₃ It can also be applied to an NH ₃ meter that measures the concentration (see Patent Document 1).

  It can be used for process monitoring of gas concentration in chemical factories and steelworks, combustion gas analysis of boilers and combustion furnaces, air pollution monitoring, and automobile exhaust gas measurement.

It is a schematic block diagram of the apparatus of one Example. (A) Relationship between output from detector and reference CO ₂ concentration, (B) Relationship based on reference CO ₂ concentration based on relationship between output from detector, (C) Output ratio An / Bn by detector and reference CO 2 It is a figure which shows the relationship between ₂ density | concentrations and the relationship between (D) correction coefficient k and An / Bn. It is the figure which showed what applied the correction coefficient k to the detector output. In the case of outputting only detector 1 is a diagram showing a relationship between the reference CO ₂ concentration.

Explanation of symbols

1 Detector 1
2 Detector 2
3, 4 Flow path 5 Combustion furnace 6, 7 Valve 8 Cell 9 Light source 10 Standard gas 11 Sample gas 12 Sample inlet 13 Sample outlet 14 Motor

Claims (3)

In a gas analysis method using a differential method infrared gas analyzer that measures the concentration difference between a reference gas and a sample gas,
A pneumatic first infrared detector in which a measurement target component or a gas in which an absorption region overlaps with the component to be measured is placed in an optical subsequent stage of the cell that has passed through the sample gas, and the light transmitted through the cell is measured. And
Installing a second infrared detector whose output is proportional or nearly proportional to the component to be measured, and measuring the light transmitted through the first infrared detector;
Calculating a correction coefficient for linearizing the calibration curve of the reference gas concentration from the ratio of the measurement results by both detectors;
An infrared gas analysis method for correcting sensitivity of the reference gas. A sample cell;
A switching valve for selectively supplying a reference gas and a sample gas to the sample cell;
A light source for irradiating the sample cell with infrared light;
Intermittent means for interrupting infrared light from the light source;
A first infrared detector of a pneumatic type enclosing a measurement target component or a gas in which an absorption region overlaps the component to be measured;
A second infrared detector whose output is proportional or nearly proportional to the component to be measured;
An arithmetic control unit for calculating a correction coefficient for linearizing a calibration curve of the reference gas concentration from a ratio of measurement results by both detectors, and performing sensitivity correction of the reference gas;
Infrared gas analyzer equipped with. The infrared gas analyzer according to claim 2, wherein the second infrared detector is a pneumatic type.

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