JPH1073561A - Oxygen concentration measuring apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an oxygen concentration measuring apparatus which can measure the oxygen concentration accurately even after long term use. SOLUTION: The oxygen concentration measuring apparatus comprises a voltage supply section 20 incorporating a voltage varying mechanism 21 for applying a voltage between the opposite electrodes of a gas detection element 1 having first and second electrodes made of a material for ionizing oxygen arranged on a solid electrolytic substrate and provided with means for regulating the gas contact efficiency of respective electrodes differently and means for heating respective electrodes and the substrate. The oxygen concentration measuring apparatus is constituted such that the oxygen concentration of a gas touching the electrode can be measured based the output current from the gas detection element 1 when a voltage is applied thereto while heating up the gas detection element 1 to a first temperature of 350 deg.C-500 deg.C. A purge mechanism 60 performs electrode recovery processing where the gas detection element 1 is heated up to a second temperature higher than the first temperature and the sulfide gas desorption temperature for the first electrode for a specified time of 5 sec or longer.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention provides a first electrode and a second electrode made of a noble metal material on a substrate made of a solid electrolyte, and a regulating means for making the gas contact efficiency with each electrode different. A gas supply element for applying a voltage between both electrodes of the gas detection element; and a heating means for heating the gas detection element, wherein the gas detection element is provided at 350 ° C. to 500 ° C. The present invention relates to an oxygen concentration measuring device configured to be capable of measuring the oxygen concentration in a gas contacting an electrode based on an output current flowing when a voltage is supplied to a gas detection element while being heated to a first temperature of ° C.

[0002]

2. Description of the Related Art When an oxygen gas is ionized at a first electrode when the gas detecting element is arranged in a gas to be detected, the solid electrolyte transports the ionized oxygen gas to a second electrode. Further, at this time, between the two electrodes, since the gas contact ratio is different, oxygen ions emit electrons at the second electrode, and when the oxygen gas is regenerated, the effect of the concentration cell is exhibited. Output current (limit current) that shows an almost constant value in a predetermined pressure range corresponding to the oxygen concentration
Is obtained, and the oxygen concentration can be known based on the output current. Further, such a gas detection element may not be able to accurately detect the oxygen concentration because the function of the above concentration cell cannot be obtained at a low temperature and the electrode deteriorates at a high temperature. From the viewpoint, it is usually used in a temperature range of 400 ° C. to 550 ° C., and as the oxygen concentration measuring device, a device configured to perform a continuous concentration measurement while maintaining the gas detection element at the predetermined temperature is used. Are known.

[0003]

However, for example, when measuring the concentration of oxygen contained in flue gas or the like, the gas detection element gradually deteriorates with long-term use, and the output current decreases to reduce the output current. In some cases, it was impossible to know the oxygen gas concentration.

Accordingly, it is an object of the present invention to provide an oxygen concentration measuring device capable of accurately measuring the oxygen concentration even during long-term use in view of the above-mentioned circumstances.

[0005]

Means for Solving the Problems The present inventors have determined that the deterioration of the gas detection element is caused by sulfur oxides (SOx) contained in flue gas and the like adsorbed on the electrodes. SOx adsorbed on the electrode
Has obtained a new finding that the electrode can be desorbed from the electrode by heating the electrode to a second temperature not lower than the SOx desorption temperature for a predetermined time of at least 5 seconds or more. Further, it has been found that the above deterioration gradually progresses from a low voltage side to a high voltage side of the voltage at which the limit current is obtained, and the output current value gradually decreases. The present invention is based on the above new findings,

[Structure 1] A first electrode and a second electrode made of a material capable of ionizing oxygen are provided on a substrate made of a solid electrolyte, and a restricting means for making gas contact efficiency to each of the electrodes different is provided. With the provided gas detection element,
A state in which a voltage supply unit for applying a voltage between both electrodes of the gas detection element is provided, and heating means for heating the gas detection element is provided, and the gas detection element is heated to a first temperature of 350 ° C to 500 ° C. In an oxygen concentration measurement device configured to be able to measure the oxygen concentration in the gas that has come into contact with the electrode based on an output current flowing when a voltage is supplied to the gas detection element, the gas detection element includes the first temperature. And a purge mechanism for performing an electrode deterioration recovery process of heating for a predetermined time of at least 5 seconds or more to a second temperature equal to or higher than the sulfur oxide gas desorption temperature for the first electrode, The purge mechanism may be provided with a control mechanism for starting the electrode deterioration recovery processing at the start of the voltage supply between the two electrodes, and the purge mechanism may be provided with the electrode deterioration recovery processing. There may be provided a timer mechanism for performing the processing at each set time, a storage means for storing the set time and the predetermined time is provided, and a control device for operating the purge mechanism based on the storage means is provided. The advantageous effects are as follows.

[Effect 1] That is, the temperature of the gas detecting element is raised to a second temperature higher than the first temperature and higher than the sulfur oxide gas desorption temperature with respect to the first electrode by at least 5%.
Since a purge mechanism for performing an electrode deterioration recovery process for heating for a predetermined time of not less than seconds is provided, even if the electrode adsorbs SOx contained in the gas to be detected due to long-term use, the purge mechanism performs the electrode deterioration recovery process. , The SOx adsorbed on the electrode can be desorbed, and the activity for accurately measuring the oxygen concentration can be recovered, so that the oxygen concentration can be accurately measured for a long time. I can do it. In addition, if the purge mechanism is provided with a control mechanism for starting the electrode deterioration recovery process at the start of the voltage supply between the two electrodes, at least the oxygen gas concentration measurement start timing is accurate. It is preferable because the concentration can be measured, and for example, abnormality during operation of the combustion equipment can be accurately detected. In addition, if the purge mechanism is provided with a timer mechanism that performs the electrode deterioration recovery process at each set time, even if the electrode is deteriorated based on, for example, a change in combustion conditions due to long-term use of the combustion equipment, a periodic operation is performed. The electrode is reactivated, and in order to measure an accurate oxygen concentration, the combustion condition of the combustion equipment is estimated from the oxygen concentration to be useful for combustion control such as optimization of combustion conditions. Can be done. In particular, when performing the above-described combustion control, particularly by providing a storage unit that stores the set time and the predetermined time, and by providing a control device that operates the purge mechanism based on the storage unit, The purge mechanism can be automatically operated, and safer use can be ensured without causing inadequate management due to long-term use of the combustion equipment and the like.

[Structure 2] In addition to the above structure, a limit current characteristic in which a substantially constant current (this current is referred to as a limit current) flows regardless of the voltage when the gas detection element is not deteriorated. Within a range where is observed, a voltage variable mechanism that allows the supply voltage by the voltage supply unit to be reduced is provided,
The purge mechanism may perform the electrode deterioration recovery process when a difference between a flowing current and a limit current is equal to or more than a predetermined value when a supply voltage applied to the gas detection element is reduced. Good.

[Effect 2] Since the limiting current characteristic gradually deteriorates from the low voltage side (see FIG. 7), the measurement of the oxygen concentration is usually performed at a relatively high voltage side of the voltage value at which the limiting current characteristic is observed. It is set to try to minimize the effects of deterioration. With this configuration, when the electrode starts to deteriorate, before the output current decreases due to the deterioration of the electrode while the voltage for measuring the oxygen concentration (measurement voltage) is being supplied, the limit current characteristic is reduced. A voltage that is equal to or lower than the measured voltage within the voltage that can be seen is set as the detection voltage, and a decrease in the output current can be observed from the difference between the current flowing at the detection voltage (detection current) and the limit current. In the initial stage when the electrodes start to deteriorate, the output current at the measurement voltage can be regarded as the limit current, so that when the difference between the detection current and the output current at the measurement voltage starts to increase, the measurement voltage gradually increases. Can be expected to start to decrease under the influence of the deterioration of the electrode. Therefore, when the difference between the output current at the measurement voltage and the detection voltage within the range in which the limit current characteristic is observed is equal to or greater than a predetermined value, when the electrode deterioration recovery processing is performed on the gas detection element, The output reduction can be effectively prevented, and the gas detection element accurately measures the limiting current, so that the oxygen concentration measuring device can measure the accurate oxygen concentration as a whole.

[Structure 3] An atmospheric oxygen detecting mechanism for exposing the gas detecting element to the atmosphere is provided, and a standard output current based on the atmospheric oxygen detection of the gas detecting element is obtained in advance. An output current of the gas detection element based on the calibration current is obtained as a calibration current, and a calibration means for calibrating an output current is provided based on a relationship between the standard output current and the calibration current, and the purge mechanism adjusts the output current. The electrode deterioration recovery processing may be performed when the amount becomes equal to or more than a predetermined value.

[Effect 3] Oxygen is a component of the atmosphere, and its concentration is usually kept at a constant value of about 21%. Therefore, the standard output current for the oxygen gas in the atmosphere is obtained using the gas detection element, and when the output current (calibration current) for the oxygen gas in the atmosphere is detected after the use of the gas detection element, the output difference is calculated from the output difference. The degree of deterioration of the gas detection element can be estimated, and the calibration means can calibrate the output current for the gas to be detected based on the calibration current. At this time,
It is estimated that the larger the output difference, the larger the amount of calibration of the output current to be calibrated by the calibration means, and the greater the degree of deterioration of the gas detection element. Therefore, when the electrode deterioration recovery process is performed by the purge mechanism when the calibration amount of the output current is equal to or more than a predetermined value, the calibration current can be obtained at any time, and thus the gas detection element can be obtained. Degradation can be efficiently recovered.

Therefore, the oxygen concentration can be measured accurately over a long period of time, so that the life of the entire apparatus can be set to be long. In the case where such an apparatus is used for combustion control of a combustion apparatus, etc. , High reliability and accurate utilization.

[0013]

Embodiments of the present invention will be described below with reference to the drawings. As shown in FIG. 2, a first electrode 11 made of a material capable of ionizing oxygen and a second electrode 12 are provided on a base material 10 made of a solid electrolyte.
A regulating means 13 for varying the gas contact efficiency with the electrodes 11 and 12 is provided, and the electrodes 11 and 12 and the base material 1 are provided.
Gas detecting element 1 provided with a heating means 14 for heating 0
As shown in FIG. 1, a voltage supply unit 20 having a built-in voltage variable mechanism 21 for applying a voltage between both electrodes 11 and 12 of the gas detection element 1 is provided. The combustion control mechanism 3
Exhaust gas path 32 from gas-fired equipment 30 incorporating 1
, A flow switching device 3 for switching the flow of exhaust gas and the atmosphere
A bypass 34 provided with an atmospheric oxygen detection mechanism consisting of
Is provided, and the gas detection element 1 is provided to constitute an oxygen concentration measuring device. Further, an output device 40 for outputting an output current based on a voltage supply to the gas detection element 1 is provided, and an output current Iw from the output device 40 is determined based on a relationship between the standard output current Iw ₀ and the calibration current Iw _1. Calibration means 50 for calibrating is provided, and the output current calibrated by the calibration means is input to the combustion control mechanism of the gas combustion device as combustion control information indicating the oxygen concentration, and is used for stable combustion of the gas combustion device. Can be

As the solid electrolyte, a solid electrolyte is used which is usually stabilized by adding about 10 mol% of magnesium oxide, calcium oxide, yttrium oxide or the like to zirconium oxide, and the first electrode and the second electrode are made of oxygen. Among the ionizable materials, noble metals are preferably used, and for example, platinum, palladium, rhodium and the like are used. The regulating means is provided with a cover member 13a that covers the first electrode 11 side of the base material 10, and is provided with a gas flow hole 13b in the cover member 13a, so that the gas contact rate with the first electrode 11 is reduced. It is set so as to be lower than the gas contact ratio with respect to the second electrode 12. Instead of such a configuration, a porous filter is used as the first electrode 11.
May be provided in a state of being covered. The heating means 14 is provided with a heating coil 14a in the vicinity of the gas detection element 1 and is configured to receive a voltage supply from the voltage supply unit 20 and generate Joule heat. The variable mechanism 21 converts the voltage supplied to the gas detection element 1 to a measurement voltage (V
w) and a detection voltage (Vw−
Vd), and furthermore, the supply and stop of the voltage to the heating means 14 at the time of performing the electrode deterioration recovery process can be switched. The purge mechanism 60 is provided with a switch 65 to the control mechanism 61 for starting the electrode deterioration recovery processing.
The heater mechanism 14 is operated to energize the heating means 14, and a timer mechanism 62 for performing the electrode deterioration recovery processing at each set time, and a storage means 63 for storing the set time and a predetermined time are provided. The control mechanism 61 starts the electrode deterioration recovery process based on the set time and stops the electrode deterioration recovery process based on the predetermined time based on the storage unit 63. Usually, the set time is every 10 minutes to 60 minutes, and the predetermined time is It is set to 5 seconds or more.

When the oxygen concentration measuring device is used, <1> the gas detecting element is brought to a second temperature (580 ° C. when the first electrode is made of platinum) at the same time as the start of combustion of the gas combustion device. The electrode deterioration recovery process of heating for 5 seconds (predetermined time) is performed, and after the predetermined time has elapsed, the gas detection element is heated to the first temperature (400 ° C.) (actually, the gas detection element is energized. 2V (= Vw)
It is in the state controlled to the extent. In general, the voltage at which the limiting current of the gas detection element is affected by water vapor on the high voltage side, making it difficult to obtain measurement accuracy, but the Vw is a voltage that is not easily affected by water vapor as the gas detection element. I can say. ), The exhaust gas is brought into contact with the gas detection element, and the oxygen concentration is measured. During the measurement of the oxygen concentration, the gas detection device is brought to a second temperature (580 ° C. when the first electrode is made of platinum) at the same time as the combustion of the gas combustion device is started at regular intervals (set time). A purge mechanism for performing an electrode deterioration recovery process of heating for 5 seconds (predetermined time) is operated. <2> Further, when the supply voltage Vw applied to the gas detection element is reduced by Vd, the purge mechanism determines that the difference between the detection current Id flowing and the limit current Iw measured by the supply voltage Vw is smaller. It is set so that the electrode deterioration recovery processing is performed when the value becomes equal to or more than the predetermined value ΔI, and purging corresponding to a substantial degree of deterioration of the electrode is also possible. <3> Further, when a highly accurate gas concentration is required, a standard output current Iw ₀ based on atmospheric oxygen detection of the gas detection element is obtained in advance, and the standard output current Iw ₀ based on the atmospheric oxygen detection mechanism is obtained. the output current of the gas sensing element, calculated as a calibration current Iw _1, the calibration current to the original, it is possible to obtain an accurate oxygen concentration. That is, the oxygen concentration C, based on the obtained limiting current Iw, since obtained by Cw = k * Iw (k is a constant), the state in which no deterioration of the electrode, obtained from the standard output current Iw ₀ Constant Assuming that k ₀ = C ₀ / Iw ₀ (where C ₀ is the oxygen concentration in the atmosphere) is k, Cw is obtained, and under the condition that the calibration current Iw ₁ is given, a constant k ₁ = C ₀ / Iw obtained from the calibration current Iw _{1 An} accurate oxygen concentration can be obtained by providing a calibration means for obtaining Cw, where ₁ (C ₀ is the oxygen concentration in the atmosphere) as k. Here, the amount of calibration of the output current (Iw ₀ / Iw ₁ ) is large. If so, it can be said that the first electrode is deteriorated,
When the calibration amount of the output current (Iw ₀ / Iw ₁ ) becomes equal to or more than a predetermined value (A), the electrode deterioration recovery process is performed, whereby an accurate oxygen concentration can be continuously obtained. . The oxygen concentration obtained in this manner is input to the combustion control mechanism as combustion control information of the gas combustion equipment, and is used for combustion control such as adjustment of a gas supply amount and an air supply amount.

[0016]

EXAMPLE A platinum electrode was kept at 400 ° C.
After adsorbing x, the temperature of the electrode was raised to examine the temperature at which the SOx gas was desorbed. The results are as shown in Table 1 and FIG. As a result, the adsorbed SOx was formed at 580 ° C. for 5 seconds.
It can be seen that 90% or more of the electrodes are desorbed, and the deterioration of the electrode is effectively recovered. When the same test was performed for the other electrodes, the temperature was 500 ° C. when the palladium electrode was used and 600 ° C. when the rhodium electrode was used.
It was found that SOx was effectively desorbed from the electrode. In the present invention, these temperatures corresponding to the respective electrode materials are referred to as SOx gas desorption temperatures. Sensitivity is obtained between an oxygen concentration measuring apparatus provided with a purge mechanism for performing the above-described <1> and <2> electrode deterioration recovery processes (the present invention) and an oxygen concentration measuring apparatus not using any of the above-mentioned purge mechanisms (conventional). FIG. 9 shows a comparison of the long-term stability of the output. An oxygen concentration measurement device without a purge mechanism has almost no output in about one month, and is not practical. On the other hand, an oxygen concentration measurement device with a purge mechanism can be used for one year. It also showed high output stability and proved to be sufficiently durable for practical use assuming long-term use.

[Brief description of the drawings]

FIG. 1 is a schematic diagram of an oxygen concentration measuring device of the present invention.

FIG. 2 is a longitudinal perspective view of a main part of a gas detection element.

FIG. 3 is a diagram showing an oxygen concentration measurement state of the oxygen concentration measurement device.

FIG. 4 is a diagram showing an operation state of a purge mechanism of <1> of the embodiment.

FIG. 5 is a diagram showing an operation state of a purge mechanism of <2> of the embodiment.

FIG. 6 is a diagram showing an operation state of a purge mechanism of <3> of the embodiment.

FIG. 7 is a graph showing a limit current in a normal state and a limit current when the gas detection element is in a deteriorated state.

FIG. 8 is a graph showing the temperature dependence of the SOx gas desorption amount at a platinum electrode.

FIG. 9 is a graph showing the long-term stability of an electrode with and without a purge mechanism.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Gas detection element 20 Voltage supply part 21 Voltage variable mechanism 60 Purge mechanism

Claims (6)

[Claims] 1. A substrate comprising a solid electrolyte, a first electrode comprising a material capable of ionizing oxygen, and a second electrode are provided, and a regulating means for providing different gas contact efficiencies to the respective electrodes is provided. A gas detection element is provided, a voltage supply unit for applying a voltage between both electrodes of the gas detection element is provided, and a heating unit for heating the gas detection element is provided, and the gas detection element is heated to 350 ° C. to 500 ° C. An oxygen concentration measurement device configured to be capable of measuring the oxygen concentration in the gas in contact with the electrode based on an output current flowing when a voltage is supplied to the gas detection element while being heated to the first temperature, An electrode deterioration recovery process of heating the gas detection element to a second temperature higher than the first temperature and equal to or higher than a sulfur oxide gas desorption temperature for the first electrode for a predetermined time of at least 5 seconds is performed. Oxygen concentration measuring device is provided with a purge mechanism. 2. The oxygen concentration measuring apparatus according to claim 1, wherein a control mechanism for starting the electrode deterioration recovery process is provided in the purge mechanism when the voltage supply between the two electrodes is started. 3. A timer mechanism for performing the electrode deterioration recovery process at set times in the purge mechanism.
3. The oxygen concentration measurement device according to any one of 2. 4. The oxygen concentration measuring apparatus according to claim 3, further comprising storage means for storing the set time and the predetermined time, and a control device for operating the purge mechanism based on the storage means. 5. The voltage supply is performed within a range where a substantially constant current (this current is referred to as a limit current) flows regardless of a voltage in a state where the gas detection element is not deteriorated. Providing a voltage variable mechanism that allows the supply voltage by the unit to be reduced, when the purge mechanism reduces the supply voltage applied to the gas detection element, the difference between the flowing current and the limit current is equal to or greater than a predetermined value. The oxygen concentration measuring apparatus according to any one of claims 1 to 4, wherein the electrode deterioration recovery processing is performed when the electrode deterioration occurs. 6. An atmospheric oxygen detecting mechanism for exposing the gas detecting element to the atmosphere, wherein a standard output current based on the atmospheric oxygen detection of the gas detecting element is obtained in advance, and based on the atmospheric oxygen detecting mechanism. The output current of the gas detection element is obtained as a calibration current, and a calibration unit for calibrating the output current is provided based on a relationship between the standard output current and the calibration current. The oxygen concentration measuring device according to any one of claims 1 to 5, wherein the electrode deterioration recovery processing is performed when the electrode deterioration recovery value is equal to or more than a predetermined value.