Gas sensor

Details Gas sensor Gas sensor

1997-09-10

2000-07-25

Tung, T.

Chemistry: electrical and wave energy

Apparatus

Electrolytic

204412, 204426, G01N 27407

Patent

active

060932953

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

1. Field of the Invention
The present invention relates to a sensor used in combustion gas streams. In one embodiment, the sensor may be used to determine if non-equilibrium conditions or incomplete combustion are present. In another embodiment, the sensor may be used to determine the concentration of oxygen and/or unburnt fuel in the combustion gas stream.
2. Description of the Related Technology
Combustion of fossil fuels and hydrocarbon fuels is a widely used method for obtaining energy in a large number of industries. Complete combustion of hydrocarbon fuels converts the hydrocarbons in the fuel into carbon dioxide and water, in accordance with the reaction given in equation 1; for example:
Incomplete combustion can occur if there is excess fuel. It can also occur with excess oxygen, if the fuel and air are not sufficiently well mixed.
Throughout this specification, the expression "combustion gas" is taken to mean off-gas, flue gas or exhaust gas from a combustion process. The combustion process may be the burning of any fossil-fuel or hydrocarbon fuel, and includes the burning of solid fuels, such as coal, for example as used in furnaces, kilns, boilers and the like, and the burning of liquid fuels, such as fuel oil and other petroleum products.
Sensors for measuring the oxygen content of combustion gases, such as engine exhaust gases and furnace off gases, have been described in the prior art. Such sensors generally utilise oxygen ion-conducting solid electrolytes. When opposed faces of the solid electrolyte are exposed to different oxygen partial pressures, an emf (E), or electrical potential difference, is developed which obeys the Nernst equation: ##EQU1## where: T is temperature (Kelvin) electrolyte, and the electrolyte.
Known sensors utilise electrodes formed at the surfaces of the solid electrolyte and measurement of the electrical potential difference (also commonly referred to as `emf` or `voltage`) between the electrodes enables a measurement of the difference between the oxygen partial pressures in the gas streams contacting the electrodes. If a reference gas of known oxygen concentration is used to contact one electrode, the oxygen concentration of a test gas or a combustion gas contacting the other electrode can be determined.
One sensor of known design includes a hollow tube made of a refractory material, such as alumina, and having a disc plug or pellet of solid electrolyte sealed in one end of the tube. Other sensors utilise a tube made entirely of the solid electrolyte. Australian Patent No. 466,251 in the name of Commonwealth Scientific and Industrial Research Organisation, describes various forms of oxygen sensors and the entire contents of this patent are incorporated herein by reference.
Many solid electrolyte materials are known to be suitable for use in oxygen sensors. Examples include zirconia or hafnia, both fully stabilised or partially stabilised by doping with calcia, magnesia, yttria, scandia or one or a number of rare earth oxides and thoria, also doped with calcia, yttria or a suitable rare earth oxide. Australian Patent No. 513,552 discloses the addition of alumina to these solid electrolyte materials to produce a composite solid electrolyte which is particularly suitable for sealing into the end of an alumina tube, thereby making a rugged and leak-tight sensor useful for demanding industrial applications. Australian Patent Application No. 47828/78 disclose the use of magnesium aluminate spinel as an alternative to alumina, either for the supporting tube or as the inert diluent in the composite solid electrolyte material.
The electrodes on solid electrolyte oxygen sensors generally consist of porous coatings of noble metals such as platinum, gold, palladium or silver, or alloys of these elements. For measurements in gases using a gaseous reference an electrode is required on each surface of the solid electrolyte; for measurements in molten metals an electrode is required only on the reference side of the solid electrolyte, and then only if a

REFERENCES:
patent: 3468780 (1969-09-01), Fischer
patent: 3597345 (1971-08-01), Hickam et al.
patent: 4132615 (1979-01-01), Linder et al.
patent: 4189355 (1980-02-01), Fujishiro et al.
patent: 4212273 (1980-07-01), Maruoka

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