JP3669788B2 - Oxygen concentration measuring device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention provides a gas detection element in which a first electrode made of a noble metal material and a second electrode are provided on a base material made of a solid electrolyte, and a restricting means for making gas contact efficiency different from each electrode is provided. A voltage supply unit that applies a voltage between both electrodes of the gas detection element, and includes a heating unit that heats the gas detection element to heat the gas detection element to a first temperature of 350 ° C. to 500 ° C. In this state, the present invention relates to an oxygen concentration measuring apparatus configured to be able to measure the oxygen concentration in the gas in contact with the electrode based on an output current that flows when a voltage is supplied to the gas detection element.
[0002]
[Prior art]
When the gas detection element is arranged in the gas to be detected, when the oxygen gas is ionized by the first electrode, the solid electrolyte transports the ionized oxygen gas to the second electrode side. At this time, since the gas contact rate is different between the two electrodes, oxygen ions emit electrons at the second electrode, and regeneration of the oxygen gas has the effect of a concentration cell. Corresponding to the oxygen concentration, an output current (limit current) showing a substantially constant value within a predetermined pressure range is obtained, and the oxygen concentration can be known based on the output current. In addition, such a gas detection element may not be able to accurately detect the oxygen concentration because the operation of the previous concentration cell cannot be obtained at low temperatures and the electrode deteriorates at high temperatures. From the viewpoint, it is normally used in a temperature range of 400 ° C. to 550 ° C., and the oxygen concentration measuring device is configured to perform continuous concentration measurement while maintaining the gas detection element at the predetermined temperature. Are known.
[0003]
[Problems to be solved by the invention]
However, for example, when measuring the oxygen concentration contained in combustion exhaust gas, etc., the gas sensing element gradually deteriorates with long-term use, and the output current is reduced, so that the accurate oxygen gas concentration can be known. Sometimes it disappeared.
[0004]
Accordingly, an object of the present invention is to provide an oxygen concentration measuring apparatus capable of measuring an accurate oxygen concentration even during long-term use in view of the above circumstances.
[0005]
[Means for Solving the Problems]
The present inventors have found that the deterioration of the gas detection element is caused by adsorption of sulfur oxide (SOx) contained in the combustion exhaust gas or the like on the electrode, and thus adsorbed on the electrode. The present inventors have obtained a new finding that SOx can be desorbed from the electrode by heating the electrode to a second temperature equal to or higher than the SOx desorption temperature for a predetermined time of at least 5 seconds. Further, it has been found that the above-mentioned deterioration gradually proceeds from the low voltage side to the 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 new knowledge,
[0006]
[Configuration 1]
A gas detection element provided with a first electrode made of a material capable of ionizing oxygen and a second electrode on a substrate made of a solid electrolyte, and provided with regulating means for making the gas contact efficiency different from each electrode. A voltage supply unit that applies a voltage between both electrodes of the gas detection element, and includes a heating unit that heats the gas detection element to heat the gas detection element to a first temperature of 350 ° C. to 500 ° C. In the oxygen concentration measuring device configured to be able to measure the oxygen concentration in the gas in contact with the electrode based on the output current that flows when a voltage is supplied to the gas detection element in the state,
Purge for performing electrode deterioration recovery processing in which the gas detection element is heated to a second temperature higher than the first temperature and higher than the sulfur oxide gas desorption temperature with respect to the first electrode for a predetermined time of at least 5 seconds. There is a mechanism,
With the start of voltage supply between the two electrodes, the purge mechanism may be provided with a control mechanism for starting the electrode deterioration recovery process,
The purge mechanism may be provided with a timer mechanism that performs the electrode deterioration recovery process at each set time,
The operation and effect of providing a storage device for storing the set time and the predetermined time and providing a control device for operating the purge mechanism based on the storage device is as follows.
[0007]
[Operation effect 1]
That is, an electrode deterioration recovery process is performed in which the gas detection element is heated to a second temperature higher than the first temperature and higher than the sulfur oxide gas desorption temperature with respect to the first electrode for a predetermined time of at least 5 seconds. Since the purge mechanism is provided, even if the electrode adsorbs SOx contained in the gas to be detected with long-term use, the purge mechanism only performs the electrode deterioration recovery process, so that the SOx adsorbed on the electrode is removed. It is possible to desorb and recover the activity for measuring an accurate oxygen concentration, and to continue to measure the oxygen concentration accurately over a long period of time.
In addition, if the purge mechanism is provided with a control mechanism for starting the electrode deterioration recovery process at the start of voltage supply between the electrodes, an accurate oxygen gas can be obtained at least at the start time of the oxygen gas concentration measurement. It is preferable because the concentration can be measured and, for example, abnormalities during operation of the combustion device can be accurately detected.
Further, 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 deteriorates due to, for example, a change in combustion conditions due to long-term use of the combustion equipment, In addition, the electrode is reactivated, and in order to accurately measure the oxygen concentration, it is useful for combustion control such as estimating the combustion state of the combustion device from the oxygen concentration and optimizing the combustion conditions. I can do it.
In particular, when performing the above-described combustion control, a storage unit that stores the set time and a predetermined time is provided, and a control device that operates the purge mechanism based on the storage unit is provided. The purge mechanism can be automatically operated, and safer use can be ensured without incurring inadequate management associated with long-term use of the combustion equipment or the like.
[0008]
[Configuration 2]
Further, in addition to the above-described configuration, within a range in which 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 is observed. Providing a voltage variable mechanism capable of lowering the supply voltage by the voltage supply unit;
The purge mechanism may be configured to perform the electrode deterioration recovery process when a difference between a flowing current and a limit current becomes a predetermined value or more when a supply voltage applied to the gas detection element is lowered. Good.
[0009]
[Operation 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 set on the relatively high voltage side of the voltage value at which the limiting current characteristic can be seen, and the influence of the deterioration. Try to minimize. In this case, when the electrode starts to deteriorate, the limiting current characteristic is reduced before the output current drop due to the deterioration of the electrode is observed in the state where the voltage (measurement voltage) for measuring the oxygen concentration is supplied. Within the voltage that can be seen, a voltage equal to or lower than the measurement voltage is set as the detection voltage, and a decrease in the output current can be observed from the difference between the current (detection current) flowing at the detection voltage and the limit current. Since the output current at the measurement voltage can be regarded as a limit current at the initial stage when the electrode starts to deteriorate, when the difference between the detection current and the output current at the measurement voltage starts to increase, the measurement voltage is gradually increased. It can be expected that the output current at 1 begins to decrease due to the deterioration of the electrode.
Therefore, when the electrode deterioration recovery process is performed on the gas detection element when the difference between the output current at the measurement voltage and the detection voltage within the range where the limit current characteristic is seen is equal to or greater than a predetermined value, A reduction in output can be effectively prevented, and the gas sensing element accurately measures the limit current, whereby the oxygen concentration measuring device can measure the accurate oxygen concentration as a whole.
[0010]
[Configuration 3]
An atmospheric oxygen detection mechanism for exposing the gas detection element to the atmosphere is provided,
The standard output current based on the atmospheric oxygen detection of the gas detection element is obtained in advance, the output current of the gas detection element based on the atmospheric oxygen detection mechanism is obtained as a calibration current, and the standard output current and the From the relationship with the calibration current, a calibration means for calibrating the output current is provided,
The purge mechanism may perform the electrode deterioration recovery process when the calibration amount of the output current becomes a predetermined value or more.
[0011]
[Operation effect 3]
Oxygen is a component of the atmosphere, and its concentration is usually about 21% and maintains a constant value. Therefore, when the standard output current for the oxygen gas in the atmosphere is obtained using the gas sensing element and the output current (calibration current) for the oxygen gas in the atmosphere is detected after the gas sensing element is used, The degree of deterioration of the gas detection element can be estimated, and the output current for the gas to be detected can be calibrated based on the calibration current by the calibration means. At this time, it is estimated that the larger the output difference, the larger the calibration amount of the output current calibrated by the calibration means, and the greater the degree of deterioration of the gas sensing element.
For this reason, when the electrode deterioration recovery process is performed by the purge mechanism when the calibration amount of the output current exceeds a predetermined value, the calibration current can be obtained at an arbitrary time. Can be efficiently recovered.
[0012]
Therefore, it is possible to continue to measure oxygen concentration accurately over a long period of time, so that the lifetime of the entire apparatus can be set long, and reliability can be improved when such an apparatus is used for combustion control of combustion equipment. It is expensive and can be used for accurate use.
[0013]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described below with reference to the drawings.
As shown in FIG. 2, a base electrode 10 made of a solid electrolyte is provided with a first electrode 11 and a second electrode 12 made of a material capable of ionizing oxygen, and gas contact efficiency with each of the electrodes 11 and 12. 1 is provided, and the gas detection element 1 provided with the heating means 14 for heating the electrodes 11 and 12 and the base material 10 is provided. As shown in FIG. A voltage supply unit 20 having a built-in voltage variable mechanism 21 for applying a voltage between the electrodes 11 and 12 is provided, a purge mechanism for performing electrode deterioration recovery processing is provided in the gas detection element 1, and a combustion control mechanism 31 is built in. A bypass path 34 provided with an atmospheric oxygen detection mechanism including a flow path switching device 33 for switching the flow of exhaust gas and the atmosphere is provided in the exhaust gas path 32 from the gas combustion device 30, and the gas detection element 1 is disposed. It has configured the oxygen concentration measuring device.
Further, an output device 40 that outputs an output current based on voltage supply to the gas detection element 1 is provided, and the output current Iw from the output device 40 is calculated from the 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 used for stable combustion of the gas combustion device. It is done.
[0014]
The solid electrolyte is usually a zirconium oxide that is stabilized by adding about 10 mol% of magnesium oxide, calcium oxide, yttrium oxide, etc., and the first electrode and the second electrode can ionize oxygen. Of the materials, noble metals are preferably used, for example, platinum, palladium, rhodium and the like.
The regulating means includes a cover member 13a that covers the first electrode 11 side of the base material 10, and a gas flow hole 13b is provided in the cover member 13a, and the gas contact rate with respect to the first electrode 11 is set. The gas contact rate with respect to the second electrode 12 is set to be lower, and instead of such a configuration, a porous filter is provided so as to cover the first electrode 11. May be.
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 voltage supply from the voltage supply unit 20 to generate Joule heat. The variable mechanism 21 switches the voltage supplied to the gas detection element 1 between a measurement voltage (Vw) for measuring the oxygen concentration and a detection voltage (Vw−Vd) for detecting the deterioration of the electrode. The voltage supply / stop for the heating means 14 during the deterioration recovery process can be switched.
The purge mechanism 60 controls a control mechanism 61 for starting the electrode deterioration recovery process by operating a switch 65 to energize the heating unit 14 and performing the electrode deterioration recovery process at a set time, The control mechanism 61 starts the electrode deterioration recovery process based on the set time based on the storage means 63, and stores the set time and a predetermined time at the predetermined time. Usually, the set time is set every 10 to 60 minutes, and the predetermined time is set to 5 seconds or more.
[0015]
When using the oxygen concentration measuring device,
<1> An electrode deterioration recovery process is performed in which the gas detection element is heated to a second temperature (580 ° C. when the first electrode is made of platinum) for 5 seconds (predetermined time) at the start of combustion of the gas combustion device. At the same time, after the predetermined time has elapsed, the gas detection element is heated to the first temperature (400 ° C.) (actually, the current supply to the gas detection element is controlled to about 2 V (= Vw)). The voltage that gives the limit 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 less susceptible to water vapor as the gas detection element. .), The exhaust gas is brought into contact with the gas sensing element, and the oxygen concentration is measured. While the measurement of the oxygen concentration continues, the gas detection device is set to the second temperature (580 ° C. when the first electrode is made of platinum) at the start of combustion of the gas combustion device at regular intervals (set time). A purge mechanism for performing an electrode deterioration recovery process of heating for 5 seconds (predetermined time) is activated.
<2> Further, the purge mechanism has a difference between the detection current Id flowing when the supply voltage Vw applied to the gas detection element is decreased by Vd and the limit current Iw measured by the supply voltage Vw. The electrode deterioration recovery process is set to be performed when the value exceeds a predetermined value ΔI, and purging corresponding to the substantial degree of electrode deterioration 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 above-described based on the atmospheric oxygen detection mechanism The output current of the gas detection element is obtained as a calibration current Iw ₁ , and an accurate oxygen concentration can be obtained based on the calibration current.
That is, the oxygen concentration C is based on the obtained limit current Iw.
Cw = k * Iw (k is a constant)
Because it is required in
In the state where no deterioration of the electrode, the standard output current Iw ₀ from the resulting constants _{k 0 = C 0 / Iw 0} (C 0 is the oxygen concentration in the atmosphere)
And Cw is determined with k as
Under conditions that provide a calibration current Iw _1, calibration current Iw ₁ from the resulting constant _{k 1 = C 0 / Iw 1} (C 0 is the oxygen concentration in the atmosphere)
It is possible to obtain an accurate oxygen concentration by providing a calibration means for obtaining Cw, where k is a value. Here, if the output current calibration amount (Iw ₀ / Iw ₁ ) increases, the first electrode deteriorates. Therefore, when the output current calibration amount (Iw ₀ / Iw ₁ ) exceeds a predetermined value (A), the electrode deterioration recovery process is performed to continuously obtain an accurate oxygen concentration. It will be possible to ask.
The oxygen concentration thus obtained is input to the combustion control mechanism as combustion control information of the gas combustion device, and is used for combustion control such as adjustment of the gas supply amount and the air supply amount.
[0016]
【Example】
When the platinum electrode was kept at 400 ° C. and SOx in the exhaust gas was adsorbed, the temperature of the electrode was raised and the temperature at which the SOx gas was desorbed was examined. The results were as shown in Table 1 and FIG. Thereby, it can be seen that 90% or more of the adsorbed SOx is desorbed at 580 ° C. for 5 seconds, and the deterioration of the electrode is effectively recovered. In addition, when the same test was performed on other electrodes, it was found that SOx was effectively detached from the electrode at 500 ° C. when a palladium electrode was used and at 600 ° C. when a rhodium electrode was used. It was. In the present invention, these temperatures corresponding to the electrode materials are referred to as SOx gas desorption temperatures.
Sensitivity between the oxygen concentration measuring device (present invention) provided with the purge mechanism for performing the electrode deterioration recovery processing of the above <1> and <2> and the oxygen concentration measuring device (conventional) that does not perform any of the above-described purge mechanisms. When the long-term stability of the output was compared, it was as shown in FIG.
An oxygen concentration measuring device that does not have a purge mechanism produces almost no output in about a month and cannot withstand practical use, whereas an oxygen concentration measuring device that has a purge mechanism can be used for one year. It also showed high output stability and was able to withstand practical use on the premise of long-term use.
[Brief description of the drawings]
FIG. 1 is a schematic diagram of an oxygen concentration measuring apparatus according to the present invention. FIG. 2 is a longitudinal perspective view of a main part of a gas detecting element. FIG. 3 is a diagram showing an oxygen concentration measuring state of the oxygen concentration measuring apparatus. FIG. 5 is a diagram showing the operating state of the purge mechanism of <1> in FIG. 5 is a diagram showing the operating state of the purge mechanism in <2> of the embodiment. FIG. 6 is an operating state of the purge mechanism of <3> in the embodiment. FIG. 7 is a graph showing the normal current limit current and the 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 the platinum electrode. Graph showing long-term electrode stability with and without mechanism [Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Gas detection element 20 Voltage supply part 21 Voltage variable mechanism 60 Purge mechanism

Claims (6)

A gas detection element provided with a first electrode made of a material capable of ionizing oxygen and a second electrode on a substrate made of a solid electrolyte, and provided with regulating means for making the gas contact efficiency different from each electrode. A voltage supply unit that applies a voltage between both electrodes of the gas detection element, and includes a heating unit that heats the gas detection element to heat the gas detection element to a first temperature of 350 ° C. to 500 ° C. An oxygen concentration measuring device configured to be able to measure the oxygen concentration in the gas in contact with the electrode based on an output current that flows when a voltage is supplied to the gas detection element,
Purge for performing electrode deterioration recovery processing in which the gas detection element is heated to a second temperature higher than the first temperature and higher than the sulfur oxide gas desorption temperature with respect to the first electrode for a predetermined time of at least 5 seconds. An oxygen concentration measuring device with a mechanism. 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 with the start of voltage supply between the electrodes. The oxygen concentration measuring apparatus according to claim 1, wherein the purge mechanism is provided with a timer mechanism that performs the electrode deterioration recovery process at each set time. The oxygen concentration measuring apparatus according to claim 3, further comprising a storage unit that stores the set time and a predetermined time, and a control device that operates the purge mechanism based on the storage unit. The supply voltage by the voltage supply unit is within a range where a limit current characteristic in which a substantially constant current (this current is referred to as a limit current) flows regardless of the voltage in a state where the gas detection element is not deteriorated. Provide a voltage variable mechanism that can be lowered,
The electrode deterioration recovery process is performed when the difference between a flowing current and a limit current becomes a predetermined value or more when the purge mechanism reduces a supply voltage applied to the gas detection element. The oxygen concentration measuring apparatus according to any one of 1 to 4. An atmospheric oxygen detection mechanism for exposing the gas detection element to the atmosphere is provided,
The standard output current based on the atmospheric oxygen detection of the gas detection element is obtained in advance, the output current of the gas detection element based on the atmospheric oxygen detection mechanism is obtained as a calibration current, and the standard output current and the From the relationship with the calibration current, a calibration means for calibrating the output current is provided,
The oxygen concentration measuring apparatus according to any one of claims 1 to 5, wherein the purge mechanism performs the electrode deterioration recovery process when a calibration amount of the output current becomes a predetermined value or more.

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