Sulfur dioxide gas sensor

Details Sulfur dioxide gas sensor Sulfur dioxide gas sensor

1998-11-24

2001-03-13

Tung, T. (Department: 1743)

Chemistry: electrical and wave energy

Apparatus

Electrolytic

C204S291000, C204S292000, C204S293000, C204S412000, C204S425000, C204S426000, C204S427000, C205S784500, C205S786500

Reexamination Certificate

active

06200445

ABSTRACT:

BACKGROUND OF THE INVENTION AND RELATED ART STATEMENT
The present invention relates to an SO
2
gas sensor for measuring the sulfur dioxide (SO
2
) gas concentration in the exhaust gas of a combustion engine, or the like, or in the air. Particularly, the present invention relates to a SO
2
gas sensor which can reduce the influence of coexistent oxygen (O
2
) on a value obtained by a SO
2
gas measurement and which can operate even at such a high temperatures as 600° C. 900° C.
In boilers for thermal power generation or incineration facilities, there are emission standards on toxic gases that occur in the exhaust gas, such as NO
x
and SO
2
, which exist for environment protection. Each facility is required to monitor the concentration of these toxic gases in order to prove that the standards are being followed. In thermoelectric power plants or incineration facilities, there is used a measuring apparatus of a nondispersive infrared ray absorption type (NDIR type) to monitor these air-pollutive gases. Since the measuring apparatus is not directly inserted into the exhaust gas, the exhaust gas is sampled by an absorption pump and analyzed in a place separate from the passage for the exhaust gas.
However, in the NDIR type of measuring apparatus, a sampling apparatus is exposed to high temperatures. Therefore, it requires rather frequent maintenance checks, which because of various restrictions are difficult to carry out without stopping the operation of the boiler or the incineration facilities.
Further, the apparatus itself must have gas-pretreatment portions for removing dust and water contained in an exhaust gas, which in combination with the use of an absorption pump, inevitably enlarges the apparatus and raises its price.
Furthermore, because a measurement of the concentration of toxic gases such as SO
2
in an exhaust gas requires the absorption step by the use of an absorption pump, even when such a concentration in an exhaust gas reaches nearly a critical level by, for example, an unexpected extraordinariness of combustion facilities; it is difficult to avoid delay in a response time, and a certain time lag is necessary to cope with the extraordinariness, or the like, which increases the risk of an unexpected accident.
Additionally, since the sensor used in the apparatus sustains interference of CO
2
, hydrocarbons inevitably discharged into an exhaust gas, or the like, a precise measurement cannot be expected.
The other methods for measurement shown in JIS B7981 are (1) an electrolytic conductivity method, (2) an ultraviolet ray absorption method, and (3) controlled potential electrolysis. However, these methods have problems regarding sampling due to the aforementioned nondispersive infrared ray absorption, and each of these methods is influenced by peculiar interferential gases.
SUMMARY OF THE INVENTION
The present invention was made in view of the aforementioned problems and provides a SO
2
gas sensor for appropriately measuring a concentration of SO
2
gas contained in an exhaust gas from thermoelectric power plants or incineration facilities and further provides a SO
2
gas concentration measuring apparatus using the SO
2
gas sensor.
According to the present invention, there is provided a sulfur dioxide gas sensor comprising:
a solid electrolyte having oxygen ion conductivity;
a detecting electrode for measuring sulfur dioxide gas, electrically connected to at least a part of a surface of the solid electrolyte; and
a basic electrode for measuring sulfur dioxide gas, electrically connected to at least a part of a surface of said solid electrolyte;
wherein the detecting electrode contains glass and either gold or a gold alloy.
According to the present invention, there is further provided a sulfur dioxide gas sensor comprising:
a solid electrolyte having oxygen ion conductivity;
a detecting electrode for measuring sulfur dioxide gas, electrically connected to at least a part of a surface of the solid electrolyte;
a basic electrode for measuring sulfur dioxide gas, electrically connected to at least a part of a surface of the solid electrolyte; and
a detecting electrode for measuring oxygen and/or a basic electrode for measuring oxygen;
wherein the detecting electrode for measuring sulfur dioxide gas contains glass and either gold or a gold alloy.
According to the present invention, there is also provided an apparatus for measuring said SO
2
gas equipped with said sensor.
According to the present invention, there is furthermore provided a sulfur dioxide gas sensor comprising:
a solid electrolyte having oxygen ion conductivity;
a detecting electrode for measuring sulfur dioxide gas, electrically connected to at least a part of a surface of the solid electrolyte;
a basic electrode for measuring sulfur dioxide gas, electrically connected to at least a part of a surface of the solid electrolyte;
a detecting electrode for measuring oxygen and/or a basic electrode for measuring oxygen; and
an oxygen pump cell for controlling oxygen content in the atmosphere to be measured;
wherein the detecting electrode for measuring sulfur dioxide gas contains glass and either gold or a gold alloy.
It is preferable that the electrode used for the oxygen pump cell in said SO
2
gas sensor is made of a metal oxide, which does not oxidize SO
2
gas.
Preferably, the SO
2
gas sensor has a gas diffusion rate-determining layer on a surface of the detecting electrode. The sensor may have a structure in which both the detecting electrode for measuring SO
2
gas and the basic electrode for measuring SO
2
gas are disposed on the same surface of the solid electrolyte. Further, the sensor may have a three-electrode structure in which a reference electrode for measuring SO
2
gas is employed next to the detecting electrode for measuring SO
2
gas and the basic electrode for measuring SO
2
gas.
A SO
2
gas sensor of the present invention employs a method of measuring a change of electromotive force caused by adsorption/oxidation of sulfur dioxide gas in the detecting electrode for measuring SO
2
gas when a certain current is applied between the detecting electrode and the basic electrode for measuring SO
2
. This enables improvement of SO
2
detection sensitivity.
Alternatively, an SO
2
gas sensor of the present invention may employ a method in which a SO
2
gas concentration is measured by measuring the change of amperage caused by an oxidation reaction of SO
2
gas on the detecting electrode for measuring SO
2
gas when the voltage is kept constant between the detecting electrode for measuring sulfur dioxide and the basic electrode for measuring sulfur dioxide. This also enables improvement of SO
2
detection sensitivity and assists in conduction of excellent measurement of concentration.
A SO
2
gas sensor of the present invention having electrodes for measuring oxygen, which are separate from the electrodes for measuring SO
2
, employs a method in which a SO
2
gas concentration and an oxygen concentration are measured simultaneously, and the SO
2
gas concentration is amended according to the results of the measurement of the oxygen concentration.
In the SO
2
gas sensor of the present invention having a reference electrode for measuring oxygen, which are seperate from the electrodes for measuring SO
2
, a certain current is applied between the detecting electrode and the basic electrode for measuring SO
2
gas and the voltage between the reference electrode and the detecting electrode is measured, or a current between the detecting electrode and the reference electrode is measured by keeping the voltage constant between the detecting electrode and the reference electrode. This enables measurement with high precision by separating only the reaction of SO
2
gas on the detecting electrode. Therefore, in the present invention, it is possible to improve measurement precision by combining the aforementioned methods with the function of the structure of the SO
2
gas sensor.
It is preferable that a solid electrolyte, which is one of the members constituting the aforementioned SO
2
gas sensor, c

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