JPH07225214A - Nox measuring apparatus - Google Patents

Abstract

PURPOSE:To transform the entire nitrogen oxides other than NO2 quickly without requirement for consumption reagent such as oxidizer. CONSTITUTION:An NOx sensor 1 and a filter 40 are housed in a tube body 42. The tip part of the tube body 42 on the side of the filter 40 is opened so as to become a sampling probe 44. The probe 44 is inserted into an exhaust gas flue 46. A suction pump 52 is connected to the base end side of the tube body 42 through a flow control valve 50. At the halfway of the tube body 42, an ozone feeding port is provided at the upstream side of the filter 40. Air containing ozone is introduced at the flow rate, which is 1% or less of the flow rate of a sample, with an ozone generator 54. The ozone generator 54 is the unvoiced-discharge type ozone generator. Instrumentation air or atmospheric air is supplied into the ozone generator 54 through a flow control valve 56. About 10% of the oxygen in the supplied air is transformed into the ozone by the unvoiced discharge. The nitrogen oxide other than NO2 in the sample gas is oxidized with the ozone and becomes NO2. The sample gas, whose dust is removed by passing through the filter 40, comes into contact with the sensor 1. The electromotive force or the current caused by the contact is detected and amplified with an amplifier 32, and the result is outputted.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a NOx measuring device for measuring nitrogen oxide NOx. Nitrogen oxide measurement is used for monitoring of combustion exhaust gas such as flue gas and automobile exhaust gas, combustion control, and environmental monitoring of atmospheric air, tunnel air, indoor air, high-rise air, and the like.

[0002]

2. Description of the Related Art As a method for measuring nitrogen oxides, several methods are adopted according to the purpose of measurement. As a method for measuring with a nitrogen oxide generation source, a chemiluminescence method, a non-dispersive infrared absorption method, an infrared absorption method and a potentiostatic electrolysis method have been adopted as JIS as an NOx automatic measuring instrument and are widely used. For emissions sources, it is necessary to measure all nitrogen oxides NOx, including NO and NO ₂ . The composition ratio of nitrogen oxides in the gas in the emissions source NO ₂ / NO = 10~30 / 90~
70.

As a NOx measuring method used for environmental monitoring, the Salzman absorption spectrophotometric method is adopted as an official method,
The chemiluminescence method is also adopted in JIS as an automatic measuring instrument. The environmental standard is defined by the NO ₂ concentration, and the daily average value of 1-hour value is specified to be 0.04 to 0.06 ppm or less.

For the measurement of NOx by the ultraviolet absorptiometry, a method of air-oxidizing NO at room temperature and high pressure has been put into practical use. NO + (1/2) O ₂ → NO _{2 In the} NOx measurement by the infrared absorptiometry or the chemiluminescence method, NO ₂ is converted into NO by a reducing agent, and the total NO measurement method is put into practical use. For reduction, for example, with carbon, 150 to
It is reduced at 400 ° C. 2NO ₂ + C → 2NO + CO ₂

Although no practical NOx sensor is available yet, research reports have been increasing rapidly in recent years. No already reported
There are roughly two types of x sensors. One is an oxide semiconductor type, and a sensor mainly made of WO ₃ is the mainstream, but there are still unsolved problems in stability, reproducibility, and moisture interference, and it is far from practical use. The other is a sensor that uses a sodium ion conductor (NASICON) as the base material of the solid electrolyte, which is remarkable and has a sensitivity ratio of NO ₂ / N.
O = 2 to 10/1, little interference by CO ₂ , O _2, etc., good linearity and almost Nernst slope (“CHEMICALSENSOES” 1993 Vol.9 Supplement A,
See pp.81-84). The working temperature of the sensor using NASICON is 150 ° C.

[0006]

The solid electrolyte type NOx sensor based on NASICON has a sensitivity ratio of NO ₂ /
Set NO = 1/1 to detect NO ₂ and NO with the same sensitivity,
Efforts have been made to detect total nitrogen oxide NOx, but at present it will be difficult. Therefore, NASIC
In order to use the solid electrolyte type NOx sensor whose main material is ON as the NOx sensor for the emission source, it is necessary to convert NO into NO ₂ .

[0007] If the NO ₂ sensor and sensitivity ratio NO ₂ / NO with selective sensitivity to NO ₂ measures the total nitrogen oxides NOx using any NOx sensor, quickly nitrogen oxides other than NO ₂ Needs to be converted to NO ₂ . The conventional method by air oxidation has a slow reaction rate and requires a long time under normal pressure, so that oxidation must be performed under high pressure in order to accelerate the reaction. Therefore, the NOx measuring device becomes large and expensive. Further, use of a consumable reagent such as an oxidizing agent not only increases maintenance costs, but also increases the work for replacing the consumable reagent.

An object of the present invention, a measuring device capable of measuring the rapidly converted to NO ₂ total nitrogen oxides total nitrogen oxides other than NO ₂ without the need for consumable reagent such as oxidizing agents Is to provide.

[0009]

A NOx measuring device of the present invention comprises a NOx sensor made of a solid electrolyte or an oxide semiconductor, a sample gas channel for guiding a sample gas to the NOx sensor, and a sample gas channel other than NO _2. And an oxidizing means for converting the nitrogen oxide component of the above into NO ₂ by ozone oxidation.

[0010]

[Function] Oxidation of NO to NO ₂ by the ozone oxidation method proceeds extremely rapidly and is suitable for measurement requiring a quick response. In addition, consumable reagents such as oxidizing agents and reducing agents are not required.

[0011]

FIG. 1 shows an embodiment in which the present invention is applied to an exhaust gas measuring device. A dust removing filter 40 is mounted in contact with the NOx sensor 1 to prevent dust from adhering to the surface of the sensor 1. The dust removing filter 40 is made of, for example, glass fiber, and the exhaust gas passes through the filter 40 to remove dust.

The sensor 1 and the filter 40 are housed in a tubular body 42, the tip of the tubular body 42 on the filter 40 side extends as a thin tube, and the tip thereof is opened to serve as a sampling probe 44 to form an exhaust gas flue 46. Has been inserted into. The tubular body 42 is used by being fixed to the exhaust gas flue 46 with a flange 48.

A suction pump 52 is connected to the base end of the cylindrical body 42 via a flow control valve 50. An ozone supply port is provided on the upstream side of the filter 40 in the middle of the cylindrical body 42, and ozone-containing air is introduced by an ozone generator 54 at a flow rate of 1% or less of the sample flow rate. . The ozone generator 54 is a silent discharge ozone generator, and instrument air or atmospheric air is supplied to the ozone generator 54 via a flow rate control valve 56, and the oxygen in the supplied air is about 10 due to silent discharge. % Is converted to ozone. 5
Reference numeral 8 is a high voltage transformer of the ozone generator 54.

In the measuring apparatus shown in FIG. 1, the air containing about 10% ozone from the ozone generator 54 is 1% of the sample gas flow rate.
The exhaust gas is sucked from the sampling probe 44 from the suction pump 52 at a flow rate of, for example, about 10 l / min while being supplied at a flow rate of about the following. NO _{2 in} sample gas
Oxides other than the above are oxidized by ozone to NO ₂ and the sample gas from which dust has been removed passes through the filter 40 and contacts the sensor 1, and the electromotive force or current is generated by the amplifier 32.
Is detected, amplified, and output.

FIG. 2 shows that the present invention is suitable for N measurement.
It shows an embodiment applied to an Ox measuring device. The ozone generator 60 is an ultraviolet ozone generator and is provided with a low pressure mercury lamp 61. Low pressure mercury lamp 61 is 185n
It is necessary to emit m light. A suction pump 68 is provided through a dust filter 66 in a flow path connected to a probe 64 that introduces the atmosphere, and the atmosphere sucked by the suction pump 68 is guided to the ozone generator 60 and the ozone generator 60.
The sample gas that has passed through contacts the NOx sensor 1. Ozone is generated by the following formula by ultraviolet rays from a low-pressure mercury lamp,
O is oxidized to NO ₂ . O ₂ + UV (185 nm) → 2O O + O ₂ → O ₃ NO + O ₃ → NO ₂ + O ₂ It is convenient to use if the part surrounded by the chain line is assembled as a measuring unit.

The ultraviolet ozone generator is suitable for environmental measurement because it produces less ozone than the silent discharge type. In the embodiment of FIGS. 1 and 2, it is a solid electrolyte or an oxide semiconductor that constitutes the NOx sensor 1. Preferred examples of solid electrolytes are those based on silver halide, NASICON, or based on stabilized zirconia.

Preferred examples of halogenation are silver silver chloride, silver bromide or silver iodide. When the NOx sensor is composed of silver halide, the silver halide is not limited to a simple substance, and may be a mixture of silver halide and nitrate, nitrite or another halogen compound, a mixed crystal or a solid solution. The NOx sensor using the solid electrolyte can be operated at room temperature to a temperature lower than the melting point of the solid electrolyte. Practically, the operation at room temperature which does not require a heating part is preferable.

When the NOx sensor is composed of a solid electrolyte using NASICON or stabilized zirconia, a heating unit is provided to keep the operating temperature at about 150 ° C., for example.
Preferred examples of the oxide semiconductor are TiO ₂ , InO ₂ , Sn
For example, O ₂ or WO ₃ . The NOx sensor 1 may be configured by using these oxide semiconductors alone or as a mixture, or the NOx sensor may be configured by using these oxide semiconductors as a base material and mixing a metal such as platinum, palladium or ruthenium. You can also do it. When the NOx sensor is composed of an oxide semiconductor, a heating unit for setting the operating temperature to about 300 ° C. is provided. A NOx sensor using a solid electrolyte body has a sensitivity ratio N at room temperature.
When O ₂ / NO = 1000/1 or more, it has overwhelmingly high sensitivity to NO ₂ . Further, the influence of interference on CO ₂ , SO ₂ , and O ₂ is small.

FIG. 3 shows an example in which a NOx sensor is composed of a solid electrolyte body. The NOx sensor 1 is the one shown in FIG. The solid electrolyte body 2 is formed into a disk shape having a diameter of 10 mm and a thickness of 5 mm, and the working electrode 4 is formed on the bottom surface.
As a lead wire 6, a gold wire having a diameter of 0.01 mm is fixed to the working electrode 4. A silver paste is provided as the reference electrode 10 on the upper surface of the solid electrolyte body 2, and a gold wire having a diameter of 0.01 mm is press-bonded and fixed as the lead wire 8 via the silver paste. Gold may be used in place of platinum for the working electrode 4, and platinum or gold may be used in place of silver for the reference electrode 10.

As shown in FIG. 3 (B), this NOx sensor 1 made of a solid electrolyte body has a glass tube 12 so that the reference electrode 10 side is inside the glass tube 12 and the working electrode 4 side is exposed. It is fixed to the tip end with an inorganic adhesive 14 while keeping airtightness. The glass tube 12 is a Pyrex glass tube having an outer diameter of 10 mm.

The solid electrolyte body 2 of the sensor 1 shown in FIG. 3 can be manufactured by any of the pressing method, the melting method and the plating method. An example of molding the AgCl solid electrolyte body by the pressing method is as follows. A platinum net body is laid on the bottom of a mold having concave portions of the solid electrolyte body to be molded, and A is formed thereon.
Add gCl powder and press at about 1 t / cm ² . Simultaneously with the pressing, the net body is fixed to the solid electrolyte body 2 to form the working electrode 4. The molded solid electrolyte body 2 is taken out of the mold, and a gold wire is bonded as a lead wire 8 to the other surface of the solid electrolyte body 2 by pressure bonding and silver paste to form a reference electrode 10, and the working electrode 4 is also formed. A gold wire is fixed as the lead wire 6.

A method for evaluating the sensor shown in FIG. 3B will be described. The evaluation device used was that shown in FIG. In FIG. 4, a glass tube 12 having a solid electrolyte NOx sensor 1 fixed at its tip is set on the upper part of a tube 16 of an evaluation device. In the tube 16 of the evaluation apparatus, a measurement section furnace heater 18 is provided in a portion where the sensor 1 is provided, and a residual heater furnace heater 20 is provided in a lower portion of the tube 16. The heaters 18 and 20 are temperature-controlled at the same temperature by a pair of temperature sensor and temperature controller 22, 24, respectively. From the bottom of the tube 16, NO ₂ or N entered the cylinder 26
An O measurement gas or an interference gas such as SO ₂ or CO ₂ and the air contained in the cylinder 28 are supplied through a gas mixer 30. The gas mixer 30 has a gas flow rate of about 0.5 liter / min, and the measurement gas mixing ratio is 0, 20, 40, 8
Adjust to 0 or 100%. The lead wire 6 from the working electrode of the sensor 1 and the lead wire 8 from the reference electrode are guided to the amplifier 32, and the electromotive force due to the sample gas is detected.
It is output to the recorder 34.

The evaluation results at room temperature (25 ° C.) are shown in FIG. When AgCl is used as the solid electrolyte body and AgI
Was evaluated for the case of using. Both are functional to NO ₂ at room temperature, and are nonfunctional to NO. Also CO ₂ ,
It was also unfunctional to SO ₂ .

When the NOx measuring device of FIGS. 1 and 2 is constructed by using the NOx sensor using AgCl or AgI as the solid electrolyte body, when the ozone generators 54 and 60 are turned on, the total nitrogen oxide NOx is removed. It can be measured, and if the ozone generator is turned off, only NO ₂ can be measured. Thus, AgCl or AgI as a solid electrolyte body
Since the NOx sensor using the NOx has a selective sensitivity to NO _2, it is possible to switch the NOx by turning the ozone generator on and off.
It becomes possible to switch between x measurement and NO ₂ measurement.

The NOx sensor can be detected by the potentiometric method (potential difference measuring method). The potentiometric method is a measuring method in which a potential difference E between the working electrode potential e ₁ and the comparison electrode potential e ₂ is measured, and almost no current flows in the solid electrolyte body. Here, e ₂ has a constant potential when it is in contact with a constant composition gas such as air. However, e ₁ is N
Since the potential becomes e ₁ ′ corresponding to the Ox concentration, theoretically the concentration can be obtained from the following Nernst equation by measuring ΔE (= e ₁ −e ₁ ′). ΔE = (2.303RT / nF) log (C ₁ / C ₂ ) where R is a gas constant, T is an absolute temperature, n is the number of charges, and F is
Is the Faraday constant, and C ₁ and C ₂ are the potentials e ₁ and e ₁ ', respectively.
Is the NOx concentration.

In the NOx measuring device of the embodiment, a solid electrolyte NOx sensor made of silver halide is used as a NOx sensor, a working electrode is provided on one surface of the pair of surfaces, and a reference electrode is provided on the other surface. If the working electrode side is brought into contact with the sample gas and the potential difference between the two electrodes is measured by the potentiometric method, NO at room temperature or low heating
Since it is possible to obtain a NOx sensor that is functional to ₂ and nonfunctional to NO, it is not necessary to heat it to a high temperature, and battery-operated NOx
It can also be realized as an x monitor, and can also be used as an unmanned monitor used in a tunnel, at sea, or in the sky.

The potentiometric method has the following advantages over the galvanic method. (1) Since the potential difference depends only on the interaction of the three-phase interface of the gas, the electrode, and the solid electrolyte, it can be measured at room temperature by the potentiometric method. On the other hand, in the galvanic method, the resistance is reduced by heating and the sensitivity is improved, and the resistance is large at room temperature, which makes measurement difficult. (2) With the potentiometric method, the temperature can be corrected according to the Nernst equation, so the measurement technique is easy.
However, in the galvanic method, it is difficult to correct the sensor temperature change because the resistance value changes depending on the material.

[0028]

In NOx measuring device of the present invention exhibits, since the nitrogen oxide components other than NO ₂ in the sample gas flow path is provided an oxidation means for converting to NO ₂ by ozone oxidation, requiring consumable reagent such as oxidizing agents Without this, all nitrogen oxides other than NO ₂ can be rapidly converted to NO ₂ and the total nitrogen oxide can be measured.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram showing a NOx measuring device according to an embodiment.

FIG. 2 is a schematic configuration diagram showing a NOx measuring device according to another embodiment.

3A and 3B are diagrams showing an example of a NOx sensor used in one embodiment, FIG. 3A is a front sectional view, and FIG. 3B is a front sectional view showing the sensor attached to a glass tube.

FIG. 4 is a schematic configuration diagram showing an evaluation device of the NOx sensor of FIG.

FIG. 5 is a diagram showing output characteristics of a NOx sensor using AgCl and a NOx sensor using AgI.

[Explanation of symbols]

 1 NOx sensor 44 Sampling probe 52,68 Suction pump 54,60 Ozone generator

Claims (1)

[Claims] 1. N comprising a solid electrolyte or an oxide semiconductor
And Ox sensor, and the sample gas flow path for guiding the sample gas to the NOx sensor, an oxidation means for converting the nitrogen oxide components other than NO ₂ in the sample gas flow path to NO ₂ by ozone oxidation, further comprising a Characteristic NOx measuring device.