JPH07306176A - Incomplete combustion detecting element for exhaust gas - Google Patents

Abstract

PURPOSE:To obtain a high sensitivity incomplete combustion detecting element by forming a pair of cermet electrodes, composed of a platinum based metal and an oxide, on the surface of an oxygen ion conductive solid electrolyte and covering one of them with an oxidation catalyst. CONSTITUTION:A pair of cermet electrodes 2, 3 are formed by applying a mixture paste of Pt powder and ZrO2 powder stabilized by Y2O3 to the opposite sides of a ZrO2 substrate 1 stabilized by an oxygen ion conductive solid electrolyte, i.e., Y2O3. A paste of alumina powder, impregnated with Pt, is then applied while covering the electrode 2 to form a flammable gas oxidation catalyst layer 4. The three-phase interface, where reaction takes place between a flammable gas and the electrode 2, increases between the electrode 2 and the border of the substrate 1 thus preventing growth of Pt particles. When the flammable gas touches the detecting element, chemical reaction takes place between the flammable gas and the electrode 2 on the three-phase interface formed at the border between the electrode 2 and the substrate 1 to generate electromotive force between the electrodes 2, 3. Presence of flammable gas is detected, along with the concentration, by detecting the electromotive force.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an incomplete combustion detecting element for detecting incomplete combustion of a combustor from exhaust gas of the combustor.

[0002]

2. Description of the Related Art Conventionally, when a combustor such as a gas appliance causes incomplete combustion, the size of the flame changes, the oxygen concentration decreases, and carbon monoxide (CO) is generated.
Focusing on this phenomenon, a method of detecting incomplete combustion using a thermocouple, an oxygen sensor, a flame rod, a CO sensor, etc. is known.

Although detection of incomplete combustion by a thermocouple, an oxygen sensor, or a flame rod is effective for a combustor such as a small water heater having a small fluctuation in combustion load, a combustor having a large fluctuation in load such as a large water heater. For, the shape of the flame and the oxygen concentration in the flame change significantly with load changes.Therefore, this method that uses a sensor that detects changes in the shape of the flame and the oxygen concentration in the flame may cause false recognition of incomplete combustion. There is. On the other hand, the method of directly detecting carbon monoxide using a CO sensor is effective without such a problem.

[0004]

A CO sensor for measuring carbon monoxide in the exhaust gas of a combustor is of a semiconductor type,
Catalytic combustion type and solid electrolyte type are known.

The semiconductor type CO sensor is a semiconductor, Sn.
The principle that oxygen adsorbed on the surface of O ₂ reacts with combustible gas to change the amount of adsorbed oxygen on the semiconductor surface, and the number of electrons in the semiconductor changes accordingly, thereby changing the resistance value of the semiconductor itself. It is used to detect the concentration of combustible gas, that is, carbon monoxide by detecting this resistance value. However, when this CO sensor is used in exhaust gas, the combustion load changes even during normal combustion. Since the oxygen concentration in the exhaust gas changes, the resistance value of the sensor changes, which may result in erroneous recognition of incomplete combustion. Therefore, it cannot be used in principle for a combustor with large load fluctuations.

In the catalytic combustion CO sensor, a combustible gas oxidation catalyst is attached to a platinum (Pt) wire, and when a combustible gas such as carbon monoxide reaches the catalyst, catalytic combustion occurs and the temperature of the platinum wire rises. It utilizes the principle that the resistance value of a platinum wire changes, and the concentration of carbon monoxide is detected by detecting this resistance value. When this sensor is used, the combustion load fluctuates during normal combustion, and even if the oxygen concentration in the exhaust gas changes, no oxidation reaction occurs on the catalyst without carbon monoxide, so the resistance of the platinum wire does not change. ,
Therefore, only incomplete combustion can be reliably detected.

However, since the catalytic combustion type CO sensor has a low operating temperature, if the sensor is used for the exhaust gas of the combustor, the temperature of which changes in a relatively wide range of 50 to 200 ° C., the temperature of the exhaust gas becomes high. As the temperature changes, the temperature of the sensor itself also changes. At that time, since the sensor output also changes, temperature correction is necessary. Further, this sensor has low detection sensitivity for low concentrations of carbon monoxide. Furthermore, since the temperature-resistance change of the platinum wire is very small, it is necessary to prepare a reference sensor in addition to the CO sensor and form a bridge circuit with both sensors, which complicates the circuit configuration. If the platinum wire is made thin (for example, the wire diameter is several tens of μm) in order to maximize the temperature-resistance change of the platinum wire, there is a problem that the strength becomes very weak.

A flammable gas detection element utilizing an oxygen ion conductive solid electrolyte is known as a solid electrolyte type CO sensor (for example, Japanese Patent Publication No. 58-4985).

This type of sensor does not generate electromotive force when there is no combustible gas such as carbon monoxide because there is no difference in the adsorbed substances on the electrodes provided on both sides of the solid electrolyte. This is based on the principle that in the presence of gas, an electromotive force is generated by causing an electrochemical reaction between the flammable gas and oxygen on the electrode without the oxidation catalyst. If this sensor is installed in the exhaust gas of the combustor, even if the combustion load changes, no electromotive force is generated because there is no difference in the adsorbed substances on both electrodes unless combustible gas is generated. When a combustible gas such as carbon monoxide is generated in the exhaust gas due to incomplete combustion, an electromotive force, that is, a sensor output is generated due to a difference in adsorbed substances on both electrodes. Therefore, it can be used as an incomplete combustion detection sensor for exhaust gas.

However, since this type of sensor also has a low operating temperature (about 300 ° C. is optimum), when it is used in a combustor in which the temperature of exhaust gas changes in the range of 50 ° C. to 200 ° C. due to fluctuations in combustion load, the sensor Since the operating temperature changes depending on the temperature of exhaust gas, the sensor characteristics change. Therefore, temperature correction is still required, which makes the system complicated and causes cost increase. Regarding the gas responsiveness, the time until the output voltage after introducing the gas reaches a constant value is extremely slow and requires a time of 5 minutes or more, and this type of sensor cannot detect incomplete combustion promptly. . Furthermore, although platinum or gold alone is used as the material of the electrode, if these metals are used for a long time in exhaust gas, there is a problem that the sensor characteristics change due to grain growth.

By the way, it is known that, in addition to carbon monoxide, substantially the same amount of hydrogen gas is generated during incomplete combustion. Therefore, the present inventor pays attention to this point, and
In order to overcome the drawbacks of the incomplete combustion detection element represented by the O sensor, the incomplete combustion is not easily affected by the temperature of the exhaust gas due to changes in the combustion load, the gas responsiveness is fast, and the structure is simple and excellent in durability. It is intended to provide a sensing element.

[0012]

In order to achieve the above object, the present invention forms a pair of cermet electrodes composed of a platinum group metal and an oxide on the surface of an oxygen ion conductive solid electrolyte, and One of the cermet electrodes was covered with an oxidation catalyst to form an incomplete combustion detection element with high sensitivity to hydrogen gas. The mixing ratio of platinum in the cermet electrode is preferably 30% by volume to 80% by volume.

[0013]

[Function] Many three-phase interfaces are formed at the boundary between the oxygen ion-conducting solid electrolyte and the cermet electrode and at the cermet electrode itself, which facilitates the chemical reaction between the hydrogen gas and the electrode. The sensitivity to is improved and grain growth of platinum, which is a constituent material of the electrode, is suppressed.

[0014]

The present invention will be described in detail below.

FIG. 1A shows the structure of an incomplete combustion detecting element according to the present invention, and FIG. 1B is an enlarged view of an electrode portion of the same detecting element.

The sensing element is a yttria (Y ₂ O ₃ ) stabilized zirconia (Zr) which is an oxygen ion conductive solid electrolyte.
A paste prepared by mixing platinum (Pt) powder and yttria-stabilized zirconia (YSZ) powder is applied to both sides of the O ₂ ) substrate 1 to form cermet electrodes 2 and 3, and the upper electrode 2 Is coated with an alumina powder paste impregnated with platinum to form a combustible gas oxidation catalyst layer 4. FIG. 1B is an enlarged view of the boundary between the YSZ substrate 1 and the cermet electrode 2. As can be seen from this figure, there are three phases in which the combustible gas reacts with the electrode at the boundary and the electrode. The interface is significantly increased and platinum grain growth is suppressed.

When a flammable gas such as carbon monoxide comes into contact with such a sensing element from the arrow side, the cermet electrode 2
A chemical reaction occurs between the combustible gas and the electrode 2 at the three-phase interface formed at the boundary between the and the YSZ substrate 1, and an electromotive force is generated between the cermet electrodes 2 and 3. By detecting this electromotive force, it is possible to measure the presence or absence and the concentration of flammable gas.

Example In the example described below, a pair of cermet electrodes are formed on the same surface of a YSZ substrate which is an oxygen ion conductive solid electrolyte, and the sensing element shown in FIG. It is the same as shown. Pt powder and 5 mol% Y ₂ O
₃ Stabilized ZrO ₂ powder, Pt powder 8
It is weighed so as to be 0, 70, 60, 55, 50, and 40% by volume, and mixed with an organic solvent, a binder, and a surfactant to obtain a cermet electrode paste. Y
On the same surface of a 5 mol% Y ₂ O ₃ -stabilized ZrO ₂ green sheet (thickness 0.5 mm), which will be an SZ substrate, a pair of the above-prepared pastes are separated by about 1 mm square for 0.3 mm. Form a cermet electrode. The thickness of the cermet electrode is 10 μm. After fully dried, it is baked at 1450 ° C. A lead wire for extracting the sensor output is formed by printing with Au paste and baked at 900 ° C. After that, alumina powder as a catalyst carrier impregnated with Pt as an oxidation catalyst in an amount of 1.5% by weight is mixed with an organic solvent, a binder, and a surfactant to prepare a paste, which is covered with one of a pair of cermet electrodes. Then, an oxidation catalyst layer was formed by printing as described above and baked at 900 ° C. In this way, 6 types of sensing elements having different mixing ratios of Pt powder were manufactured.

FIGS. 2 (a) and 2 (b) show a hydrogen gas 100 when the gas sensor is composed of the six kinds of sensing elements thus manufactured and the temperature of the sensor is 400.degree. C. and 500.degree.
The sensitivity of the sensor to 0 ppm is shown.

In each case, the sensor is heated by a heater attached to the YSZ substrate of the sensor, and the amount (volume%) of Pt in the cermet electrode is as shown in FIG. 2A (sensor temperature is 400 ° C.). ) About 38% by volume to about 5
If selected within the range of 8% by volume, an output of about 100 mV or more can be obtained. In the case of FIG. 2B (sensor temperature is 500 ° C.)
It is understood that an output of about 100 mV or more can be obtained by selecting within a range of 30% by volume or less to about 80% by volume. When the sensor temperature is in the range of 400 ° C to 500 ° C, the amount of Pt (volume%) in the electrode is 30 to 80% in order to obtain the sensitivity that allows sufficient signal processing with a relatively simple processing circuit.
Is preferable, and 40 to 60% is preferable.

Next, the present inventor conducted an experiment in which the gas sensor constituted by using the sensing element manufactured in the example was attached to a large-sized water heater and the sensor output was obtained by changing the load of the water heater.

FIG. 3 shows an outline of the apparatus used in this experiment, 10 is a large gas water heater having a burner 10a and a heat exchanger 10b, and a gas sensor 11 is an exhaust port of the heat exchanger 10b. The output terminal of the gas sensor 11 is connected to a voltmeter (digibor) 12.
A damper 13 for generating incomplete combustion is provided near the tip of the exhaust pipe 11c of the gas water heater 10, and when the damper 13 is rotated to block a part of the exhaust pipe 11c, incomplete combustion occurs. Can be made. Exhaust stack 11c
An infrared absorption type CO analyzer 14 for extracting the exhaust gas from near the tip of the CO 2 and analyzing its carbon monoxide component is prepared. The output of the voltmeter 12 and the output of the CO analyzer 14 are input to the recorder 15 and recorded. It

In the experiment, a normal combustion state and an incomplete combustion state were created by operating the damper 13, and the combustion load of the gas water heater 10 was set to the minimum load (5,000 Kcal).
/ H), medium load (20,000 Kcal / h), maximum load (30,000 Kcal / h), and gas sensor output (m) for CO concentration (ppm) in exhaust gas
V) was measured and the results are shown in FIG.

It can be seen from FIG. 4 that the sensor output for the same CO concentration is almost constant even if the hot water supply load changes, which means that the output of the gas sensor is not affected by the load change. . Therefore, there is no need to temperature-correct the sensor output as in the conventional gas sensor.

FIG. 5 is a graph showing the sensor output with respect to the maximum combustion load of the gas sensor according to the present invention at the time of normal combustion and at the time of incomplete combustion using the same apparatus shown in FIG.

In the figure, A indicates the output (mV) of the gas sensor, and B indicates the CO concentration (ppm) which is the output of the infrared absorption CO analyzer 14 of FIG. In FIG. 5, it is noticed that the output B has a slight time delay from the output A. This is because the gas sensor 11 is provided near the upstream end of the exhaust pipe 11c as shown in FIG.
It is considered that this is because the exhaust gas intake port of the analyzer 14 is provided near the downstream end of the exhaust pipe 11c, so that it takes a certain time for the exhaust gas to flow from the upstream end to the downstream end in the exhaust pipe 11c. Even so, it can be seen that the gas sensor according to the present invention has excellent responsiveness. In the above examples, ZrO ₂ stabilized with Y ₂ O ₃ was used as the oxygen ion conductive solid electrolyte, Pt-ZrO ₂ cermet electrode was used as the electrode, Pt was used as the oxidation catalyst, and Al was used as the catalyst carrier.
_{Although 2} O ₃ was used, the present invention is not limited to these substances, and similar effects can be expected even when the substances listed below are used.

(1) As the oxygen ion conductive solid electrolyte, one containing ZrO ₂ , CeO ₂ , ThO ₂ , or Bi ₂ O ₃ as a main component and an oxide of an alkaline earth element or a rare earth element is added. To use. For example, a mixture of CeO ₂ and ZrO ₂ CaO or M ₂ O ₃ (M is a rare earth element such as Gd and La) stabilized with CaO, MgO, and M ₂ O ₃ (M is a rare earth element such as Y and La). Mixture of ThO ₂ and M ₂ O ₃ (M is a rare earth element such as Y or La) Y ₂ O ₃ , Gd ₂ O ₃ , Nb ₂ O ₅ , WO ₃ , Sr
O, BaO, or a mixture of La ₂ O ₃ and Bi ₂ O ₃ (2) As the cermet electrode, P as a platinum group metal
One or more of t, Ru, Rh, Pd, Os and Ir, and Zr ₂ O, CeO ₂ , ThO ₂ and B as oxides.
A mixture with one of i ₂ O ₃ is used. For example, Pt-Z
_{rO 2, Pt-Bi 2 O} 3, Pt-CeO 2, Pt-T
As the hO ₂ (3) oxidation catalyst, oxides of Co, Cr, Mn, Cu, Ce, Fe, V and Ag, Pt, Ru, Rh, Pd, Au or LaCoO
A single compound oxide or a combination of two or more compound oxides such as ₃ , ZnCo ₂ O ₄ and FeCo ₂ O ₄ (4) As a catalyst carrier, SiO ₂ , TiO ₂ , SnO ₂ , MgO

[0028]

When a gas sensor for detecting incomplete combustion is formed by using the detection element according to the present invention, even a combustible gas has a large sensitivity to hydrogen gas and a high gas responsiveness.
A sensor with a relatively high operating temperature is obtained. It is considered that this is because the cermet electrode is used as the electrode of the sensor, and the number of three-phase interfaces involved in the reaction with hydrogen gas increases. Therefore, the gas sensor according to the present invention has about 5
When used for the exhaust gas of a combustor that changes in the temperature range of 0 ° C. to 200 ° C., the sensor output is not affected by the temperature of the exhaust gas, that is, the combustion load, so that the temperature correction of the sensor output is not necessary. This, coupled with the relatively large sensor output, makes the subsequent processing circuits and systems relatively simple, which is advantageous in terms of cost. Further, by using the cermet electrode, grain growth of platinum, which is a constituent component of the electrode, is suppressed, so that the durability of the electrode is also improved. Further, the gas responsiveness can be made extremely fast by the cermet electrode.

[Brief description of drawings]

FIG. 1A shows a schematic structure of an example of an incomplete combustion detection element according to the present invention, and FIG. 1B is an enlarged view of an electrode portion of the detection element shown in FIG.

FIG. 2 (a) is a graph showing the relationship between the mixing ratio of platinum in the cermet electrode and the sensor output when the gas temperature is 400 ° C. when a gas sensor is composed of the incomplete combustion detection element according to the present invention, b) is a graph showing the relationship between the mixing ratio of platinum in the cermet electrode and the sensor output when the sensor temperature is 500 ° C.

FIG. 3 is a schematic diagram of an apparatus used in an experiment for examining the output of a gas sensor including an incomplete combustion detection element according to the present invention.

FIG. 4 shows a sensor output when a combustion load of a large-sized gas water heater equipped with a gas sensor including an incomplete combustion detection element according to the present invention is changed.

FIG. 5 is a graph showing the sensor output with respect to time as to the normal combustion and the incomplete combustion of the large-sized gas water heater equipped with the gas sensor constituted by the incomplete combustion detection element according to the present invention.

[Explanation of symbols]

 1 YSZ substrate 2, 3 Cermet electrode 4 Oxidation catalyst 10 Large gas water heater 11 Gas sensor 11c Exhaust pipe 12 Voltmeter 13 Incomplete combustion generation damper 14 Infrared absorption CO analyzer 15 Recorder

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[Procedure amendment]

[Submission date] July 7, 1994

[Procedure Amendment 1]

[Document name to be corrected] Drawing

[Name of item to be corrected] Figure 2

[Correction method] Change

[Correction content]

[Fig. 2]

Front page continued (72) Inventor Satoshi Tanaka 1-1 Yamashita-cho, Kokubun-shi, Kagoshima Prefecture Kyocera Kokubun Plant

Claims (7)

[Claims] 1. An exhaust gas characterized in that a pair of cermet electrodes composed of a platinum group metal and an oxide is formed on the surface of an oxygen ion conductive solid electrolyte, and one of the pair of cermet electrodes is covered with an oxidation catalyst. Incomplete combustion detection element. 2. The incomplete combustion detection element for exhaust gas according to claim 1, wherein a mixing ratio of platinum in the cermet electrode is 30% by volume to 80% by volume. 3. The exhaust gas incomplete combustion detection element according to claim 1, wherein the pair of cermet electrodes are formed on the same surface of the oxygen ion conductive solid electrolyte. 4. The pair of cermet electrodes are formed on different surfaces of the oxygen ion conductive solid electrolyte.
Alternatively, the incomplete combustion detection element for exhaust gas as described in 2. 5. The oxygen ion conductive solid electrolyte is C
The incomplete combustion detection element for exhaust gas according to claim 1 or 2, which is ZrO ₂ stabilized with aO, MgO or M ₂ O ₃ (M is a rare earth element such as Y or La). 6. The exhaust gas incomplete combustion detection according to claim 1 or 2, wherein the oxide constituting the cermet electrode is one or more selected from ZrO ₂ , Bi ₂ O ₃ , CeO ₂ and ThO _2. element. 7. The oxidation catalyst is Co, Cr, Mn, C
u, Ce, Fe, V, Ag oxides, Pt, Ru, R
h, Pd, Au or LaCoO ₃ , ZnCo ₂ O ₄ ,
A composite oxide such as FeCo ₂ O _{4 is} used alone or in combination of two or more, and the catalyst carrier is SiO ₂ , TiO ₂ .
The exhaust gas incomplete combustion detection element according to claim 1 or 2, which is composed of ₂ , SnO ₂ , MgO, and Al ₂ O ₃ .