JP2024009600A - gas sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a COe-gas gas sensor with which it is possible to obtain high Coe sensitivity without being affected by sulfur dioxide gas concentrations.

SOLUTION: Provided are a COe gas sensor 1 that includes at least a pair of detection electrodes 12 and a counter electrode 13 that are ion conductively connected via a solid electrolyte substrate 11, the film thickness of the detection electrodes 12 being 10-35 μm, the film thickness of the counter electrode 11 being 30-50 μm, and a method for detecting the COe gas using the gas sensor. The method includes a step in which the gas to be measured is brought into contact with the detection electrodes and the counter electrode under an atmosphere where the temperatures of the detection electrodes are 580-650°C, and a step in which a potential difference between the detection electrodes and the counter electrode is measured.

SELECTED DRAWING: Figure 1

COPYRIGHT: (C)2024,JPO&INPIT

Description

The present invention relates to a gas sensor. In particular, the present invention relates to a COe gas sensor that eliminates false detection due to sulfur dioxide and has excellent COe sensitivity.

Conventionally, various gas sensors have been used to detect gas components or determine gas concentrations in gas mixtures to be measured, such as sensors constructed based on solid electrolytes and operated on the mixed potential principle, and catalytic combustion type gas sensors. It has been known.

Carbon monoxide gas sensors are known that include at least one pair of electrodes that are ionically conductively connected via a solid electrolyte substrate (see, for example, Patent Documents 1 and 2). These gas sensors are required to accurately detect the gas to be detected without being affected by the miscellaneous gas even if the gas to be detected is mixed with the gas to be detected.

A gas sensor for detecting combustible gases including carbon monoxide is known, in which a gas-selective permeable body made of porous ceramic is provided near the electrodes in order to reduce poisoning of the electrodes by miscellaneous gases including sulfur dioxide. (For example, see Patent Documents 3 and 4). Furthermore, in order to eliminate the influence of sulfur dioxide and oxygen on carbon monoxide measurements, gas sensors are known in which electrodes for detecting sulfur dioxide are provided on the surface of a solid electrolyte (for example, see Patent Document 5). ).

Japanese Patent Application Publication No. 2020-95014 Japanese Patent Application Publication No. 2021-135235 Japanese Patent Application Publication No. 2002-71630 Japanese Patent Application Publication No. 2001-41924 Japanese Patent Application Publication No. 2000-65789

The composition of miscellaneous gases that may be included in the gas to be measured varies greatly depending on the environment in which the gas sensor is placed, and there are also various miscellaneous gases that cause false detection. For example, when the detection target gas is COe (CO equivalent) gas, sulfur dioxide (SO ₂ ) gas may cause false detection as a miscellaneous gas. When the gas to be measured is exhaust gas fueled by natural gas, etc., the concentration of _SO2 gas in the exhaust gas is usually 30 ppm or less; however, when the gas to be measured is exhaust gas fueled by coal, etc. As much as 2000 ppm of SO ₂ gas may be included. In such cases, SO ₂ gas has a large influence as a miscellaneous gas on COe gas detection, and even if the COe gas concentration is 0, SO ₂ gas may be mistakenly detected as COe gas, making accurate measurement difficult. there were.

When detecting COe gas using a mixed potential type gas sensor, it is required to eliminate the influence of SO ₂ gas, eliminate false detections, and accurately obtain the COe gas concentration.

As a result of intensive studies, the present inventors focused on an electrode structure and measurement conditions that can detect COe gas and do not falsely detect SO ₂ gas. By using a sensor in which the thickness of the detection electrode and counter electrode are set within a predetermined range, and by setting the electrode temperature during measurement within a predetermined range, it is possible to reduce false detection of COe gas due to SO ₂ gas. They discovered something and completed the present invention.

According to one embodiment, the present invention is a COe gas sensor, which includes at least one pair of sensing electrodes and a counter electrode that are ionically conductively connected via a solid electrolyte substrate, and the sensing electrode has a film thickness of 10 to 10 ml. 35 μm, and the counter electrode has a film thickness of 30 to 50 μm.

In the COe gas sensor, it is preferable that the detection electrode and the counter electrode are electrodes to which sulfur atoms are adsorbed.

In the COe gas sensor, the sensing electrode is preferably a sintered body containing alloy particles containing platinum and solid electrolyte particles.

The COe gas sensor preferably includes a heater capable of heating the detection electrode and the counter electrode to 580 to 650°C.

According to another embodiment, the present invention relates to a gas detection meter in which the above-mentioned COe gas sensor is housed in a tubular casing having an open end.

The gas detection meter further includes an oxygen gas sensor in the tubular casing having the open end, the oxygen gas sensor comprising a solid electrolyte substrate and at least a pair of electrodes connected to each other in an ion conductive manner via the solid electrolyte substrate. The pair of electrodes includes an oxygen sensing electrode made of a sintered body containing metal particles containing platinum and solid electrolyte particles, and an oxygen detection electrode made of a sintered body containing metal particles containing platinum and solid electrolyte particles. a counter electrode for detection, the oxygen detection electrode is configured to be able to come into contact with the gas to be measured flowing in from the open end, and the counter electrode for oxygen detection is shielded from the atmosphere of the gas to be measured. It is preferable.

According to another embodiment, the present invention provides a method for manufacturing the gas sensor described above, including the steps of forming at least one pair of sensing electrodes and a counter electrode on a solid electrolyte substrate; and a step of contacting with sulfur gas and aging.

According to yet another embodiment, the present invention provides a method for detecting COe gas using the above-mentioned gas sensor, in which an object to be measured is The present invention relates to a method including the steps of: bringing a gas into contact with the sensing electrode and the counter electrode; and measuring a potential difference between the sensing electrode and the counter electrode.

According to the present invention, it is possible to provide a mixed potential type COe gas sensor that can selectively detect COe gas with high sensitivity without being affected by SO ₂ gas. By using this COe gas sensor, the COe gas concentration can be accurately measured even in environments where the SO ₂ concentration in the exhaust gas is high, such as in coal-fired boilers. This makes it possible to monitor the combustion state of the boiler and achieve optimal combustion control, thereby improving energy efficiency and reducing CO ₂ emissions.

FIG. 1 is a conceptual diagram showing a cross-sectional structure of a COe gas sensor according to a first embodiment of the present invention. FIG. 2 is a conceptual diagram showing an example of a gas detector according to a second embodiment of the present invention. FIG. 3 is a conceptual cross-sectional view of a COe gas sensor located inside the casing of the gas detector shown in FIG. FIG. 4 is a graph showing the SO ₂ selectivity of the COe gas sensor according to the first embodiment of the present invention, where the horizontal axis is the film thickness (μm) of the sensing electrode, and the vertical axis is the sensitivity that is the same as the sensitivity at an SO ₂ concentration of 1000 ppm. It represents the resulting COe concentration (ppm). FIG. 5 is a graph showing the COe sensitivity of the COe gas sensor according to the first embodiment of the present invention at a COe concentration of 300 ppm, where the horizontal axis represents the film thickness (μm) of the sensing electrode and the vertical axis represents the sensitivity. FIG. 6 is a graph showing the COe sensitivity variation of the COe gas sensor according to the first embodiment of the present invention, where the horizontal axis is the operating temperature (°C) and the vertical axis is the graph showing [(maximum COe sensitivity) - ( The value (%) is expressed as [COe sensitivity minimum value)]/COe sensitivity average value x 100. FIG. 7 shows the relationship between the elapsed time and the SO ₂ gas sensitivity ratio when the COe gas sensor according to the first embodiment of the present invention is brought into contact with SO ₂ gas at a predetermined concentration and subjected to aging.

Embodiments of the present invention will be described below with reference to the drawings. However, the present invention is not limited to the embodiments described below.

[First embodiment: COe gas sensor]
According to a first embodiment, the present invention relates to a COe gas sensor. The COe gas sensor according to the present embodiment includes at least one pair of sensing electrodes and a counter electrode that are ionically conductively connected via a solid electrolyte substrate, the sensing electrode has a film thickness of 10 to 35 μm, and the counter electrode has a film thickness of 10 to 35 μm. This is a COe gas sensor with a diameter of 30 to 50 μm.

The COe gas sensor according to the present embodiment includes a COe gas as a detection target gas, and also includes a non-detection target gas as a measurement target gas. Here, COe gas is a gas generated during incomplete combustion of fuel, and is a gas containing carbon monoxide (CO) and hydrogen (H ₂ ). Therefore, in this specification, the COe gas concentration refers to the total concentration of CO gas and H ₂ gas. The COe gas sensor according to the present invention is a gas sensor that can detect COe gas concentration without erroneously detecting sulfur dioxide (SO ₂ ) gas.

FIG. 1 is a schematic cross-sectional view of a COe gas sensor according to a first embodiment. The COe gas sensor 1 includes a solid electrolyte substrate 11 , a detection electrode 12 , and a counter electrode 13 .

The solid electrolyte substrate 11 is a member that forms a three-phase interface between the detection electrode 12 or the counter electrode 13 and a gas phase containing the gas to be detected, and enables ion conduction. The shape of the solid electrolyte substrate 11 is not particularly limited, as long as it can connect the sensing electrode 2 and the counter electrode 13 such that ion conduction is possible. Therefore, for example, in addition to the flat solid electrolyte substrate 11 shown in FIG. 1, a cylindrical substrate or a cylindrical substrate with one end closed may be used.

The solid electrolyte substrate 11 is preferably made of stabilized zirconia, and examples thereof include, but are not limited to, zirconia stabilized with rare earth metal oxides such as yttria and ceria, calcia-stabilized zirconia, and magnesia-stabilized zirconia. From the viewpoint of ionic conductivity, it is particularly preferable to use yttria-stabilized zirconia.

The sensing electrode 12 functions as a working electrode and is configured to be able to measure the difference in electromotive force between it and the counter electrode 13. In FIG. 1, the sensing electrode 12 and the counter electrode 13 are each formed in contact with the solid electrolyte substrate 11, and the sensing electrode 12 and the counter electrode 13 are provided apart from each other. However, it is sufficient that the sensing electrode 12 and the counter electrode 13 are ion-conductively connected via the solid electrolyte substrate 11. For example, if the sensing electrode 12 and the solid electrolyte substrate 11 are It may be through a member. In addition, in FIG. 1, the sensing electrode 12 and the counter electrode 13 are provided on one surface of the flat solid electrolyte substrate 11 at a distance from each other. A sensing electrode can also be placed on the other surface with a counter electrode. However, it is necessary to configure the sensing electrode 12 and the counter electrode 13 so that they are in contact with the same gaseous atmosphere, and it is necessary to configure the sensing electrode 12 and the counter electrode 13 in such a manner that the atmosphere between the sensing electrode 12 and the counter electrode 13 is not blocked by the solid electrolyte substrate. An electrode 12 and a counter electrode 13 are arranged.

The sensing electrode 12 may be a sintered body containing metal particles made of a metal alloy containing platinum (Pt) and solid electrolyte particles. The metal alloy containing Pt may include an alloy of Pt and gold (Au), an alloy of Pt and rhodium (Rh), and the like. The mass ratio of Pt to other metals in the alloy containing Pt may be, for example, about 98:2 to 85:15, but is not particularly limited. The average particle diameter of the metal particles may be about 0.5 to 2.5 μm, but is not particularly limited. The solid electrolyte particles may be stabilized zirconia particles, and may be one or more types selected from any stabilized zirconia listed as the material for the solid electrolyte substrate 11. Further, the stabilized zirconia may have the same composition as the stabilized zirconia that is the main component of the solid electrolyte substrate 11, or may have a different composition. The solid electrolyte particles are particularly preferably yttria-stabilized zirconia particles. The average particle diameter of the solid electrolyte particles may be about 0.1 to 1 μm, but is not particularly limited.

Such a sintered body is made by dispersing a paste obtained by dispersing a mixture containing metal particles made of a Pt alloy and solid electrolyte particles in an appropriate solvent such as an organic solvent dissolved in a binder, and adding a paste to the solid electrolyte. It can be obtained by coating and forming, for example, a thin layer onto the substrate 11 and baking it at 1200 to 1400° C. in the atmosphere.

The film thickness of the sensing electrode 12 is configured to be 10 to 35 μm. More preferably, the thickness is 15 to 30 μm. The film thickness refers to the film thickness after sintering. By setting the film thickness of the sensing electrode 12 within the above-mentioned predetermined range, the SO ₂ sensitivity can be kept low, the COe sensitivity can be sufficiently obtained, and the variation in the COe sensitivity can be reduced.

The material of the counter electrode 13 may be a sintered body containing metal particles containing Pt or metal particles consisting of Pt and solid electrolyte particles. The particle size and preferred composition range of the metal particles and solid electrolyte particles may be the same as those for the sensing electrode 12, and the method for manufacturing the sintered body may also be the same as the method for manufacturing the sensing electrode 12.

The thickness of the counter electrode 13 is set to 20 to 60 μm. More preferably, the thickness is 30 to 50 μm. The film thickness refers to the film thickness after sintering. By setting the film thickness of the counter electrode 13 within the above predetermined range, a film without defects can be obtained, and the SO ₂ sensitivity can be kept low, sufficient COe sensitivity can be obtained, and variations in COe sensitivity can be reduced. can do.

The relationship between the thickness of the sensing electrode 12 and the thickness of the counter electrode 13 is not particularly limited, as long as the thickness of the sensing electrode 12 and the counter electrode 13 are each within the above-described predetermined thickness range.

Both the detection electrode 12 and the counter electrode 13 are preferably electrodes that have been aged with SO ₂ gas, and preferably contain a sulfur component derived from SO ₂ gas. Specifically, the sulfur component is adsorbed in the form of sulfur atoms on the surfaces and gaps of particles constituting the sintered body. The adsorption of sulfur atoms can be detected by analyzing the electrode components, and specific analysis methods include X-ray photoelectron spectroscopy (XPS) and time-of-flight mass spectrometry (TOF-SIMS). can be mentioned.

By bringing the detection electrode 12 and the counter electrode 13 into contact with SO ₂ gas under predetermined conditions and aging them, the sensitivity of the COe gas sensor 1 according to the present embodiment to SO ₂ gas can be reduced, and false detection of SO ₂ gas can be reduced. It can be prevented.

The COe gas sensor 1 includes a detection section (not shown) connected to a detection electrode 12 and a counter electrode 13, respectively. The detection section includes a detection circuit and wiring. The wiring includes a wiring whose one end is connected to the detection electrode 12 and the other end to the detection circuit, and a wiring whose one end is connected to the counter electrode 13 and the other end to the detection circuit. The detection circuit may be a device capable of measuring the electromotive force (potential difference) between the detection electrode 12 and the counter electrode 13, and may be a general electrometer. Further, the wiring may be a wiring made of a conductive material, and may be a wiring made of a Pt wire or a sintered body having the same composition as the electrode material to which the wiring is connected.

The COe gas sensor 1 according to this embodiment may include a heater (not shown) as a further optional element. The heater may be a device capable of heating the solid electrolyte substrate 11 and the sensing electrode 12 to a predetermined temperature as necessary, and may be a thin layer type heater made of a tungsten (W) thin film or a platinum (Pt) thin film. It may be a ceramic heater, or it may be any other heater. When the COe gas sensor 1 shown in FIG. 1 further includes a heater, the heater is placed, for example, on one surface of the solid electrolyte substrate 11, on the surface opposite to the surface on which the detection electrode 12 and the counter electrode 13 are provided. , an insulating film may be provided, and the sensing electrode 12 and counter electrode 13 and the heater may be formed in a positional relationship facing each other with the solid electrolyte substrate 11 and the insulating film interposed therebetween. Alternatively, the heater may be provided in the vicinity of the solid electrolyte substrate, for example around the solid electrolyte substrate, without contacting the solid electrolyte substrate, but the heater is not limited to a specific embodiment. A heater that can set and maintain the temperature of the sensing electrode 12 in the range of 580°C to 635°C can be used.

The COe gas sensor 1 according to the present embodiment may further include one or more sensing electrodes in addition to the pair of sensing electrodes and the counter electrode as an optional element. Further, two or more pairs of COe gas detection electrodes and counter electrodes may be provided in some cases. However, it is not necessary to provide an electrode or the like for detecting SO ₂ gas. Furthermore, there is no need to provide an adsorbent or catalyst for adsorbing or removing SO ₂ gas. The COe gas sensor 1 according to the present embodiment eliminates the influence of SO ₂ gas without providing a detection system or components including additional electrodes by having a detection electrode and a counter electrode that detect COe gas in a predetermined configuration. This is extremely advantageous in that COe gas can be detected.

Next, a method for manufacturing a gas sensor having such a configuration will be described. The method for manufacturing the COe gas sensor 1 according to this embodiment includes a step of forming a sensing electrode 12 and a counter electrode 13 on a solid electrolyte substrate 11.

In the electrode forming step, a sensing electrode 12 and a counter electrode 13 are formed on the solid electrolyte substrate 11 . The method for forming each electrode is as described above. Both the paste made of the material of the sensing electrode 12 and the paste made of the material of the counter electrode 13 are formed on the solid electrolyte substrate 11, and the wiring connecting between each of the sensing electrode 12 and the counter electrode 13 and the detection circuit is made of a solid material. It is arranged on the electrolyte substrate 11. After that, it is preferable to bake these. A commercially available product can be used for the solid electrolyte substrate 11, or a step of manufacturing the solid electrolyte material by molding it into a desired shape can be performed prior to the electrode forming step. In addition, in a gas sensor that includes a heater, which is an optional component, on a solid electrolyte substrate, an electrically insulating layer such as alumina is formed in advance, and a heater electrode pattern made of Pt paste or the like is formed on the layer by a printing method or the like. By forming and firing the heater, a heater can be formed on the solid electrolyte substrate 11. Through the electrode forming process, a COe gas sensor 1 including a solid electrolyte substrate 11, a sensing electrode 12, a counter electrode 13, and wiring can be obtained.

When the sensing electrode 12 and the counter electrode 13 are electrodes to which sulfur atoms are adsorbed, the method includes a step of contacting the sensing electrode 12 and the counter electrode 13 with SO ₂ gas at the operating temperature of the COe gas sensor and under normal pressure after the electrodes are formed. It is preferable. This process can also be called an aging process. The SO ₂ gas to be contacted may be a gas mixture, for example, comprising 1.2% by volume of SO ₂ gas and 3% by volume of O ₂ gas, with the balance being N ₂ gas, to carry out the aging step. The time can be at least about 100 hours. Further, the aging temperature can be the operating temperature of the COe gas sensor, and can be 580 to 650°C. As a specific operation, the solid electrolyte substrate on which the sensing electrode and the counter electrode are formed can be heated in the SO ₂ atmosphere for a predetermined period of time in a furnace that can be heated to the predetermined temperature.

By performing the aging process, sulfur atoms derived from SO ₂ gas can be adsorbed to the electrode, and the SO ₂ gas sensitivity of the COe gas sensor can be reduced. In order to adsorb sulfur atoms derived from SO ₂ gas onto the electrode, the integrated value of the SO ₂ gas concentration (volume %) and exposure time (hour) is not limited to the above SO ₂ gas concentration and exposure time. The gas concentration and/or exposure time can be changed as long as the desired value is 120 or more. The upper limit of the integrated value is not particularly limited, but may be, for example, about 1.2% by volume x 300 hours (360). The aging step of contacting the sensing electrode 12 and the counter electrode 13 with SO ₂ gas may be performed after the formation and sintering of the electrodes, or even before combining other optional components such as a casing. You can do it later. However, from the viewpoint of using a highly concentrated corrosive gas, it is preferable to do this before combining other structural members.

A COe gas detection method using the COe gas sensor according to this embodiment will be described. The COe gas detection method can also be called a COe gas sensor operating method. The COe gas detection method includes the following steps.
A step of bringing the gas to be measured into contact with the sensing electrode and the counter electrode in an atmosphere where the temperature of the sensing electrode and the counter electrode is 580 to 650°C, and a step of measuring the potential difference between the sensing electrode and the counter electrode.

The gas to be measured may generally be a gas that may include COe gas. Typically, the gas may be gas generated in equipment such as a garbage incinerator or a boiler, but is not limited thereto. The temperature of the gas generated in the equipment is not particularly limited, but may be, for example, -10°C to 600°C. When these gases come into contact with the sensing electrode, the temperature of the sensing electrode can be controlled to a certain constant temperature within the range of 580 to 650°C. Preferably, the temperature of the sensing electrode is controlled to be a constant temperature within the range of 600 to 620°C. Here, the temperature of the sensing electrode is the temperature at the intersection of diagonals when the electrode pattern of the sensing electrode excluding the lead part is rectangular, and is the temperature at the intersection of the diagonal lines, using an electrically insulated thermocouple or resistance temperature detector. temperature measured by a sensor. In detecting gas, a COe gas sensor can be installed in a flue or the like through which such a gas to be measured flows. In this case, the COe gas sensor is installed in such a manner that both the detection electrode and the counter electrode are in contact with the gas to be measured.

The temperature holding mechanism for holding the sensing electrode and the counter electrode at a predetermined temperature is not particularly limited. An example of the temperature holding mechanism is a heater that can control the temperature of the sensing electrode. It may be a mechanism that includes the above-mentioned heater, a temperature sensor, and a device that enables temperature control. Another example of the temperature maintenance mechanism is the positional relationship between the sensing electrode and the counter electrode on the solid electrolyte substrate. The COe gas sensor according to this embodiment can be used by being placed in a pipe or the like through which a high temperature gas to be measured flows through plant equipment or the like. Placed in piping, etc., in a state where the high temperature gas to be measured can constantly contact the sensing electrode and the counter electrode, the solid electrolyte substrate is placed so that the sensing electrode and the counter electrode are within a predetermined temperature range. The positions of the sensing electrode and the counter electrode can be determined. In this case, a temperature sensor can be provided to detect the temperature of the sensing electrode and the counter electrode.

In this way, the temperature of the sensing electrode is maintained at a constant temperature within a predetermined temperature range, the gas to be measured is brought into contact with the sensing electrode and the counter electrode, and the electromotive force between the sensing electrode and the counter electrode is measured as a potential difference. Based on this potential difference, the COe gas concentration can be obtained.

According to the COe gas sensor according to the first embodiment of the present invention, by configuring the detection electrode and the counter electrode with a predetermined material and a predetermined film thickness, the COe gas concentration can be accurately determined without being affected by SO ₂ gas. Obtainable.

[Second embodiment: gas detection meter]
According to a second embodiment, the present invention relates to a gas detection meter. The gas detection meter according to the present embodiment is a gas detection meter in which the COe gas sensor of the first embodiment is built into a tubular casing having an open end, and the sensing electrode and the counter electrode flow into the gas sensor from the open end. This is a gas detection meter configured to be able to come into contact with the gas to be measured.

The gas detection meter includes a COe gas sensor and may optionally include an oxygen gas sensor. Each aspect will be described with reference to drawings illustrating embodiments of the gas detector.

2 and 3 are conceptual diagrams showing an example of a gas detector according to the second embodiment, and FIG. 2 is a side view of a test tube-shaped solid electrolyte substrate in a casing that constitutes the gas detector. 3 is a cross-sectional view of the solid electrolyte substrate passing through its central axis. The illustrated gas detector 2 is a direct insertion type gas detector in which a structure in which a COe gas sensor according to the first embodiment and an oxygen gas sensor are integrated is built into a casing. Referring to FIGS. 2 and 3, a casing 26 includes a structure including a test tube-shaped solid electrolyte substrate 21, a sensing electrode 22 provided thereon, a counter electrode 23, an oxygen sensing electrode 24, and an oxygen sensing counter electrode 25. . The gas detection meter 2 further includes a COe detection circuit 27 connected to the detection electrode 22 and the counter electrode 23, and an oxygen detection circuit 28 connected to the oxygen detection electrode 24 and the oxygen detection counter electrode 25.

The solid electrolyte substrate 21 constituting the gas detector 2 is a tubular structure with one end closed. More specifically, the solid electrolyte substrate 21 is formed in an elongated cylindrical shape that extends for a predetermined length with a constant diameter, and is open at its base end in the longitudinal direction and closed at its distal end in the longitudinal direction. , has a test tube shape. Solid electrolyte substrate 21 is fixed such that its tip is located near the open end of casing 26 . The counter electrode 23 is provided on the outer wall of the solid electrolyte substrate 21 at the tip of the test tube shape, apart from the sensing electrode 22 . The sensing electrode 22 and the counter electrode 23 are ionically conductively connected via the solid electrolyte substrate 21. The COe detection circuit 27 detects the electromotive force between the detection electrode 22 and the counter electrode 23 as a potential difference.

Next, an oxygen gas sensor, which is an optional component of the gas detector according to this embodiment, will be explained. The oxygen gas sensor includes a solid electrolyte substrate 21 , an oxygen sensing electrode 24 , an oxygen sensing counter electrode 25 , and a detection circuit 28 . The solid electrolyte substrate 21 can share the solid electrolyte substrate of the COe gas sensor. The material and structure of the oxygen sensing electrode 24 and the oxygen sensing counter electrode 25 may be the same as those of the counter electrode of the COe gas sensor according to the first embodiment. The oxygen sensing counter electrode 25 is provided on the inner wall of the solid electrolyte substrate 21 at the tip of the test tube shape, and is located in a position generally facing the oxygen sensing electrode 24 . That is, the oxygen sensing counter electrode 25 is ionically conductively connected to the oxygen sensing electrode 24 via the solid electrolyte substrate 21, and functions as an electrode that comes into contact with the calibration gas for oxygen sensing. The oxygen sensing counter electrode 25 is configured to be isolated from the atmosphere with which the oxygen sensing electrode 24 comes into contact, that is, the atmosphere of the gas to be measured, by the solid electrolyte substrate 21 . The detection circuit 28 can detect the oxygen concentration by measuring the electromotive force caused by the difference between the oxygen concentration of the atmosphere with which the oxygen detection electrode 24 is in contact and the oxygen concentration with which the oxygen detection counter electrode 25 is in contact. The oxygen gas sensor is located in the same casing and on the same solid electrolyte substrate 21 as the COe gas sensor, and can function separately and independently. Therefore, the gas detection meter according to this embodiment may or may not include an oxygen gas sensor.

The inner wall of the casing 26 may optionally be provided with a heater (not shown). The heater can be provided around the solid electrolyte substrate 21 in a manner that can heat the sensing electrode 22 and the counter electrode 23, and can be connected to an external power source.

A gas detection method using a gas detector according to this embodiment will be explained. The gas detection meter according to this embodiment can be directly inserted into a flue or the like through which a high-temperature measurement target gas flows to measure COe, SO _2, and optionally oxygen concentrations. In this case, the tip of the solid electrolyte substrate 21, ie, the position where the oxygen sensing electrode 24 is provided, is generally located near the open end of the casing 26, which serves as the inlet for the gas to be measured, and has the highest temperature. The closer the solid electrolyte substrate 21 is to the proximal end, the lower the temperature becomes, and the temperature distribution generally depends on the distance from the distal end. The gas to be measured is introduced to the outer periphery of the solid electrolyte substrate 21 inside the casing 26, and the calibration gas, for example, air, is introduced to the inner periphery of the solid electrolyte substrate 21. These introduction paths are hermetically blocked so that the two atmospheres do not mix. Then, by heating the sensing electrode 22 to a predetermined temperature with a heater, the electromotive force between the sensing electrode 22 and the counter electrode 23 can be measured, and the COe concentration in the gas to be measured can be obtained. Further, an electromotive force is generated in the solid electrolyte substrate 21 due to the difference in oxygen partial pressure between the measurement target gas in contact with the oxygen detection electrode 24 and the calibration gas in contact with the oxygen detection counter electrode 25, and this electromotive force is measured. As a result, the oxygen concentration in the gas to be measured can be obtained. According to the gas detection meter according to the present embodiment, in addition to being able to measure COe gas concentration accurately and easily by directly inserting it into factory equipment, etc., it can also be optionally installed on the same solid electrolyte substrate as the COe gas sensor. This oxygen gas sensor has the advantage of being able to easily measure oxygen concentration.

The gas detection meter according to this embodiment can detect COe gas concentration and optionally oxygen gas concentration with high sensitivity with a single device, and can be advantageously used for industrial gas measurement. can.

(1) Manufacture of test piece of COe gas sensor A COe gas sensor according to the first embodiment of the present invention was manufactured. An yttria-stabilized zirconia (YSZ) substrate was used as the solid electrolyte substrate. On the substrate, AuPt alloy particles made of 95% by mass platinum and 5% by mass gold, a detection electrode made of a sintered body of yttria-stabilized zirconia (YSZ) particles, Pt particles made of platinum, and YSZ particles. A counter electrode was formed from a sintered body of. Specifically, a paste in which a mixture of AuPt alloy particles and YSZ particles was dispersed in an organic solvent, and a paste in which a mixture of Pt particles and YSZ particles were dispersed in an organic solvent were screen printed to a predetermined thickness. It was formed on a YSZ substrate so as to have a similar shape, and was fired at 1300° C. in the air. In order to measure the electromotive force of the electrode, a Pt wire was placed on a YSZ substrate using a paste having the same composition as the electrode, and was fixed by firing at 1000° C. in the atmosphere to obtain a test piece of a COe gas sensor. The film thickness of the sensing electrode in the test piece was 12, 13, 20, 22, 24, 35, 36, 38, 47, or 48 μm. Regarding the test piece in which the thickness of the sensing electrode was 20 μm, three types of test pieces were prepared in which the thickness of the counter electrode was 30, 40, and 50 μm. For test pieces in which the thickness of the sensing electrode was other than 20 μm, the thickness of the counter electrode was 40 μm.

(2) Measurement of COe sensitivity and SO ₂ sensitivity The COe sensitivity and SO ₂ sensitivity of the manufactured test piece were measured. The measurement system consists of placing a test piece in the center of a quartz furnace tube that can be heated from 580°C to 665°C in an electric furnace, and running the test piece containing COe or SO ₂ from one end of the furnace tube to the other. Gas was caused to flow at a constant flow rate of 8.1 mm/s, and the electromotive force between the sensing electrode and the counter electrode was measured. The test COe gas was a base test air mixed with 100, 300, 1000, or 2000 ppm of COe. The test air had a composition of 3% by volume O ₂ and the balance N ₂ . Further, the COe gas was a mixture of CO gas and H ₂ gas at a volume ratio of 1:1. The test SO ₂ gas was a mixture of 1000 ppm SO ₂ gas in the test air.

The methods for measuring COe sensitivity and SO ₂ sensitivity in various test pieces were as follows. The operating temperature of the gas sensor was maintained at 580° C., and the concentration of the test COe gas was changed stepwise from a low concentration to a high concentration, and the potential difference therebetween was measured. The operating temperature refers to the temperature of the sensing electrode and the counter electrode. Similarly, the operating temperature was changed stepwise from low to high temperature at 600°C, 620°C, 635°C, 650°C, and 665°C, and the concentration of the test COe gas was changed from low to high concentration at each temperature. to obtain the electromotive force between the sensing electrode and the counter electrode. Next, after maintaining the temperature at 665°C and flowing test air, test SO ₂ gas was flowed, and the operating temperature was stepped from high to low at 665°C, 650°C, 635°C, 620°C, 600°C, and 580°C. to obtain the potential difference between the sensing electrode and the counter electrode.

(3) Evaluation Results Based on the above measurement results of COe sensitivity and SO ₂ sensitivity, the characteristics of the test piece of the COe gas sensor were evaluated when the detection electrode film thickness and operating temperature were changed. FIG. 4 is a graph showing SO ₂ selectivity, where the horizontal axis represents the film thickness of the sensing electrode (μm), and the vertical axis represents the COe concentration (ppm) at which the same sensitivity as the sensitivity at an SO ₂ concentration of 1000 ppm can be obtained. From FIG. 4, it was confirmed that the influence of SO ₂ sensitivity on COe sensitivity can be reduced when the thickness of the sensing electrode is in the range of 10 to 35 μm and the operating temperature is low, about 635° C. or less.

FIG. 5 is a graph showing the film thickness and operating temperature dependence of COe sensitivity, where the horizontal axis represents the film thickness (μm) of the sensing electrode and the vertical axis represents the COe sensitivity. From FIG. 5, it was confirmed that the dependence of sensitivity on film thickness was small when the film thickness of the sensing electrode was in the range of 10 to 35 μm. As the film thickness increased beyond 35 μm, there was a tendency for the sensitivity to decrease and the variation in sensitivity to increase. From FIGS. 4 and 5, the counter electrode film thickness had little effect on both SO ₂ sensitivity and COe sensitivity in the range of 30 to 50 μm.

Figure 6 is a graph showing COe sensitivity variations, where the horizontal axis is the operating temperature (°C) and the vertical axis is the value (%) expressed by (maximum value - minimum value)/average value x 100 for each COe concentration. shows. When the operating temperature exceeded 650° C., there was a tendency for variations in sensitivity to increase.

(4) Aging with SO ₂ gas The test piece manufactured in (1) above, with a sensing electrode of 20 μm and a counter electrode of 40 μm, was aged with SO ₂ gas. Specifically, in a gas atmosphere containing 1.2% by volume of SO ₂ gas and 3% by volume of O ₂ gas, with the remainder being N ₂ gas, and not containing H ₂ gas and CO gas. Then, the temperature was maintained at 650° C. for 300 hours, and the sensitivity was measured over time. FIG. 7 is a graph showing the relationship between aging time and SO ₂ sensitivity ratio. The sensitivity ratio is the ratio of the electromotive force measurement between the sensing electrode and the counter electrode at each time to the electromotive force measurement between the sensing electrode and the counter electrode before aging. From FIG. 7, it was confirmed that the SO ₂ sensitivity ratio decreased over time from the start of aging until about 110 hours, and after about 110 hours, there was almost no change in the SO ₂ sensitivity ratio.

Without intending to be bound by theory, reactions at a sensing electrode and a counter electrode provided on a solid electrolyte substrate will be described. In the mixed potential type gas sensor according to the present invention, a gas-phase CO oxidation reaction occurring within the electrode as shown in the following formula (1) and a CO oxidation reaction occurring at the three-phase interface as shown in the following formula (2) proceed. On the solid electrolyte substrate in a heated atmosphere of about 500° C. or higher, an electromotive force resulting from the reaction of formula (2) is obtained due to conduction of oxygen ions.
The potential at which the reaction currents for the oxygen reduction reaction and the CO oxidation reaction in (2) are the same is the hybrid potential, that is, the sensitivity. In the sensor after aging, the sensitivity changes because (1) the reaction decreases, and (2) the oxygen reduction reaction activity decreases and the CO oxidation reaction decreases, that is, the reaction activity at the three-phase interface decreases. It is thought that this will occur. Furthermore, even if the exposure to SO ₂ progresses as time passes during aging, the reduction in the oxygen reduction reaction and the CO oxidation reaction in (2) proceed to the same extent, so it is thought that the hybrid potential does not change significantly.

Although the preferred embodiments of the present invention have been described above, the present invention is not limited to such specific embodiments, and various modifications can be made within the scope of the gist of the present invention as described in the claims. - Can be changed.

The COe gas sensor according to the present invention can be inserted into the flue of a boiler or the like to accurately monitor the COe concentration in combustion exhaust gas. It is particularly suitable for systems using raw materials with high sulfur content, such as coal fuel. Furthermore, by combining it with existing oxygen concentration sensors, it becomes possible to construct combustion control systems such as boilers, contributing to energy savings.

1 COe gas sensor, 11 solid electrolyte substrate, 12 detection electrode, 13 counter electrode 2 gas detection meter, 21 solid electrolyte substrate, 22 detection electrode, 23 counter electrode 24 oxygen detection electrode, 25 oxygen detection counter electrode, 26 casing 27 COe detection circuit, 28 oxygen detection circuit

Claims (8)

at least one pair of sensing electrodes and a counter electrode connected in an ionically conductive manner via a solid electrolyte substrate;
The thickness of the sensing electrode is 10 to 35 μm, and the thickness of the counter electrode is 30 to 50 μm.
COe gas sensor. The COe gas sensor according to claim 1, wherein the sensing electrode and the counter electrode are electrodes to which sulfur atoms are adsorbed. The COe gas sensor according to claim 1, wherein the sensing electrode is a sintered body containing alloy particles containing platinum and solid electrolyte particles. The COe gas sensor according to claim 1, further comprising a heater capable of heating the sensing electrode and the counter electrode to 580 to 650°C. A gas detection meter incorporating the COe gas sensor according to claim 1 in a tubular casing having an open end. further comprising an oxygen gas sensor in the tubular casing having the open end;
The oxygen gas sensor includes a solid electrolyte substrate and at least a pair of electrodes ionically conductively connected via the solid electrolyte substrate,
The pair of electrodes is an oxygen sensing electrode made of a sintered body containing metal particles containing platinum and solid electrolyte particles, and an oxygen detection electrode made of a sintered body containing metal particles containing platinum and solid electrolyte particles. including the opposite pole,
The gas detection according to claim 4, wherein the oxygen detection electrode is configured to be able to contact the gas to be measured flowing in from the open end, and the counter electrode for oxygen detection is cut off from the atmosphere of the gas to be measured. Total. A method for manufacturing a gas sensor according to claim 1, comprising:
forming at least one pair of sensing electrodes and a counter electrode on the solid electrolyte substrate;
A method comprising the step of aging the sensing electrode and the counter electrode by contacting them with sulfur dioxide gas. A method for detecting COe gas using the gas sensor according to claim 1,
Bringing the gas to be measured into contact with the sensing electrode and the counter electrode in an atmosphere where the temperature of the sensing electrode and the counter electrode is 580 to 650°C;
measuring a potential difference between the sensing electrode and the counter electrode.

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