GB1596456A - Oxygen sensor for industrial air/fuel control and method of making same - Google Patents

Description

(54) OXYGEN SENSOR FOR INDUSTRIAL AIR/FUEL CONTROL AND METHOD OF MAKING SAME (71) We, UOP INC. a corporation organized under the laws of the State of Delaware United States of America, of Ten UOP Plaza, Algonquin & Mt. Prospect Roads, Des Plaines, Illinois, 60016, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us. and the method by which it is to be performed. to be particularly described in and bq the following statement: This invention relates to an oxygen sensor and a method of making an oxygen sensor.

In particular the invention relates to oxygen sensors for use in industrial air/fuel control systems.

Sensors included as parts of industrial air/fuel control systems must meet requirements peculiar to industrial air/fuel control.

Airifuel ratio control in an industrial system is done in an excess air region. This ensures that complete combustion of the fuel occurs and that no visible smoke stack emissions are produced. .R second beneficial reason for controlling in this region has been a demonstrated improvement in combustion effici encv and a resultant lowering of fuel consumption.

For a sensor to meet these requirements and still be commercially feasible, sensor performance must meet a number of criteria not normally significant for other sensor applications. For example. sensor-to-sensor reproducibility is important otherwise, control system recalibration would be required at the time of each sensor replacement.

Stability of the sensor signal over long term usage is also an important feature. so that frequent system recalibration is not neces sark. Another feature which is important in an industrial application is simplicity of installation of the sensor Finally. low sensor cost is important. particularly when replacement of sensors may be necessary.

The application of' oxygen measuring sensors as either simple monitors of oxygen content or as sensors in control systems has received considerable interest in the patent literature. The earliest commercial application of oxygen sensing devices was as integral components in systems designed for measuring dissolved oxygen in molten metal baths.

This area is still of considerable commercial importance as indicated by U.S. Patents 3,359,188; 3,378,478; 3,403,090; and 3,791,954. The devices covered in these patents are all similar in that they are solid electrolytic sensors which are immersed in a liquid metal and which generate an electrical signal which is a function of the oxygen content of the metal. The first three devices incorporate closed ended tubes of the electrolytic material while the fourth sensor uses a disc of solid electrolyte sealed into a retaining tube.

Most of the sensing devices utilized for measuring the oxygen content in molten metals are destructable sensors, in that they react with the molten metal being measured and are consumed in the course of the measurement. In U.S. Patent 3,297,551 a liquid metal sensor is presented which is used in the liquid metal heat exchanger of a nuclear reactor on a long term continuing basis.

Other sensors have been designed for measuring oxygen content specifically in gaseous mixtures rather than in liquid metals. For example, U.S. Patent No. 3,974,054 covers a disc electrolyte bonded to an aluminum oxide tube which is used for determining oxygen concentrations in gaseous mixtures. Another gas sensor is presented in U.S.

Patent 3,989,614 which specifiesa sensor composed of a tubular solid electrolyte.

Recently, there has been a considerable amount of interest in the measurement of the components which make up automotive exhaust gases. A number of devices have been patented for the measurement of the stoichiometric air/fuel ratio going into internal combustion engines. These are proposed to be used with control systems to hold engine operating conditions at the stoichiometric value. Proposed devices include both those that work as electrical resistance sensing elements as well as electrolytic sensors.

Typical resistance sensors are covered in U.S. Patents 3.911.386. 3,936.794 and 3.959.765. These devices sense a change in the electrical resistance of ceramic elements which show a change in electrical resistance that can be correlated with ambient oxygen pressure.

Of greater interest are those automotive oxygen sensors which operate on electrolytic principles. A number of patents have been issued in this area on sensors which have the electrolyte in the form of tubes or discs.

Tube sensor patents include British Patent 1,3X5.464 and U.S. Patents 3.841.987; 3,935,089; 3.960693 and 3.978.006. These patents specify automotive oxygen sensors where the solid electrolyte is in the form of an open-ended tube, the open end being in communication with the ambient air environment.

The disadvantages inherent in such tubular electrolytic sensors have led to the development of simplified automotive oxygen sensors incorporating electrolyte discs rather than tubes. The disadvantages of the tubular sensors include complexity and the resultant cost in producing the electrolyte tube, and the expense and difficulty involved in applying large amounts of costly electrode materials over the surface of the tube.

The development of sensors incorporating very simple electrolytic discs has circumvented these problem areas. Examples of such automotive oxygen sensors include U.S.

Patents 3,819.500: 3.909.385; 3,768,259 and 3.940.327.

There are a number of disadvantages in applying either currently available industrial or automotive sensors to feedback control systems designed for use in industrial combustion control. Sensors specifically designed for industrial use are generally of a large size, are costly to replace, operate at high temperatures which limits useful life, or are designed to be used only once.In addition, those industrial sensors constructed with tubular electrolytes suffer the same disadvantage as tubular automotive sensors. that is high electrolyte cost and the necessity for relatively large electrodes made of expensive materials. Those industrial sensors commer civilly available generally operate at about S C. ontinuoUK operation at this temperature causes deterioration of the electrode which seI-iously affects performance as well as thermal fatigue in the electrolyte when it is subjected to heating cycles.

Likewise, the application of sensors designed for automotive use to industrial air/ fuel control has not generally been successful. Sensor-to-sensor reproducibility of sut'fi- cient accuracy has not been demonstrated with this type of sensor to be useful in controlling a specific oxygen content far from the stoichiometric value. In general. these automotive sensors have been designed as indicators only of the stoichiometric air/fuel ratio. rather than sensors which measure actual oxygen content quantitatively. They have also been designed to survive in the severe automotive environment. This design is not necessary for industrial air/fuel applications and results in a considerable unnecessary increase in cost and design complexity.

One problem with automotive sensors is the introduction of undesirable electrical signals due to dissimilar material junctions.

While this is not a problem in sensors used for stoichiometric air/fuel ratio definition.

such signals can have extremely deleterious effects on sensors which are being used to quantitatively measure oxygen content, such as is necessary in off-stoichiometric air/fuel control in industrial application.

The present invention seeks to provide an oxygen sensor for measuring the concentration of oxygen in industrial flue gases. which sensor eliminates the aforementioned problems.

According to one aspect of the present invention, an oxygen sensor for measuring the concentration of oxygen in industrial flue gases comprises a housing and means on the housing for attaching the sensor to the wall of a flue gas containing conduit so that a sensing end of the housing is exposed to gases in the conduit and a reference end, remote from the sensing end, is exposed to the atmosphere, a non-electrically conductive tube, e.g. a fabricated forsterite tube.

mounted within the housing and having a sensing end and a reference end remote therefrom, the tube having such a length that the sensing end and reference end of the housing terminate between the sensing end and the reference end of the tube. retaining means on said tube intermediate its ends cooperating with retaining means in said housing to position fixedly and to seal said tube in gas-tight relation with said housing, an oxygen ion conducting solid electrolyte disc, typically made of ZrO2-8 mole % Y2O1, sealed, for example using glass, such as Corning 1415, as the sealant, in a recess in the sensing end of said tube, sensing and reference platinum family electrode coatings respectively on the sensing and reference surfaces of said electrolyte disc, conductive stripes of said platinum family electrode coatings extending continuously along substantially the entire length of said tube for electrically connecting said sensing and reference electrode coatings on said electrolyte disc to spaced contact portions adjacent the reference end of said tube, a pair of felted ceramic fibre filter members one of which is positioned in bonded contact with the sensing platinum family electrode coating and the other of which is positioned in bonded contact with the reference platinum family electrode coating, and particles of a platinum family metal uniformly dispersed throughout the filter members, some of which particles are in contact with said electrode coatings.

The proposed design incorporates features common to most of the sensors in the patents cited hereinabove, in that it is a solid electrolytic oxygen ion conducting cell with platinum family electrodes. The electrolyte is in the form of a disc and shares the advantages of the cited industrial and automotive disc sensors when compared to the tubular varieties of these sensors. It is much simpler in design, however, than other sensors and, most importantly, the novel features of this design have been incorporated so that it can be used in industrial air/fuel control systems without the disadvantages of other designs.

Suitably the tube. e.g. the forsterite tube, is machined prior to firing to produce the recess into which the electrolyte disc is sealed and also. preferably. to produce two axially spaced apart circumferential grooves adjacent the reference end of the tube to constitute said spaced contact portions. The recess is preferably machined to a depth greater than the combined thickness of the electro lyte disc and one of the filter members.

According to another aspect of the present invention. a method of making an oxygen sensor comprises the steps of taking a prefired non-electrically conducting hollow ceramic tube and forming a recess in a sensing end thereof by increasing the diameter of the tube bore at said sensing end; placing a coating of glass sealing material. e.g. glass, such as Corning 1415, at the bottom of the recess; applying a coating of a platinum paste. e.g. a commercially available fluxed platinum paste, such as Plessey 4276. electrode to each side of a disc of an oxygen ion conducting solid electrolyte; placing said electrolyte disc in said recess with its outer reference surface in contact with said glass sealing material: applying a stripe of platinum paste to the outer reference electrode on said disc and along the internal surface of the tube to a reference end of the latter remote from said sensing end; applying a stripe of platinum paste to the inner sensing electrode on said disc. around the inner end of said ceramic tube and along the outer surface thereof to a location adjacent the reference end of said tube: pressing a ceramic fibre filter member into the platinum paste electrodes on each side of said electrolyte disc; firing said tube in air at a temperature of at least 954 C and impregnating said electrodes with platinum metal utilising a solution of chloroplatinic acid applied to said filters and then firing at conditions to effect the reduction of said platinum metal.

The felted ceramic fiber material can be obtained from Cotronics Corp. of New York City as No. 300 ceramic paper and serves a number of functions. During operation of the sensing device, it acts as a combination filter to remove particulate matter and gaseous poisons from the stack gas being measured and a barrier to protect the electrode from any gaseous erosion that might occur.

Another important function of the ceramic fiber disc is its role as a wick during application of the solution of chloroplatinic acid (hereinafter CPA) to be discussed below. The open fiber structure of the ceramic fiber disc allows uniform deposition of the CPA on the paste substrate. A fiber member or disc thickness of about 1.02 mm has proved to be quite satisfactory.

The openness of the felted material is important in the operation of the device for industrial applications due to the relatively slow gas flow rate often inherent in such applications. Experiments conducted substituting a more dense material such as gammaalumina for the felted ceramic disc were not successful due to the inability of the gas to penetrate through the layer and reach the electrode material where oxygen ionization occurs.

Our application technique involves pressing the felted fiber disc into the tacky Pt paste and then firing in a furnace at a temperature of at least about 954"C. in air for about 30 minutes to cure the platinum paste and firmly bond the two ceramic fiber discs to the platinum paste.

The solution of chloroplatinic acid is applied through the ceramic fiber members directly onto the surface of the platinum paste substrate. Sufficient CPA is applied to ensure adequate coverage of the platinum paste substrate. This requires about 0.5 mg of platinum or about 0.01 ml of CPA per sensor where the electrolyte disc has a diameter of about 9.86 mm and a thickness of about 1.52 mm. The amount required is low due to the small area of the electrolyte and the efficient wicking action of the felted disc. Ionization of the oxygen in the stack gas environment takes place at the surface of the electrolyte, in the presence of both the platinum deposited as the paste substrate and high surface Pt from the CPA.If sufficient platinum surface is not present on the faces of the electrolyte disc, as would be the case if only plantinum paste were present, ionization of residual oxygen would not be complete, and nonreproducible sensor performance would result. It is important that both surfaces of the electrolyte disc having sufficient platinum surface area present to complete the ioniza tion-deionization reactions which occur on these surfaces. Therefore. sufficient CPA must be added to both the gas and reference sides of the electrolyte.

The purpose in bringing both the front and back conducting stripes to separate grooves fabricated in the ceramic. e.g. forsterite, tube is to ensure the elimination of the dissimilar material junctions. Such junctions have been shown to introduce irreproducible and unwanted signals onto the true, theoretically predictable cell output. While not important in stoichiometric sensors, this feature is critical in importance for non-stoichiometric industrial control. By bringing all of these junctions out of the cell. these unwanted voltages can be eliminated and sensor repro ducibility and stability insured if the other sensor features discussed above are included.

The purpose of the extra deep recess in the electrolyte end of the ceramic, e.g. forsterite.

tube is to assist in eliminating the erosion effects of a flowing gas stream. In addition, this recess promotes turbulence in the stream at the sensor electrolyte location and allows continuous interaction of the stack gas with the electrolyte. assuring gaseous measurements are valid representations of the gas stream.

The final assembly operations include painting a gasket material such as graphite paste onto the ceramic, e.g. forsterite, tube at the locating shoulder or flange and compressing this gasket material between the metal body and the pressure nut. Alternatively, the forsterite tube can be produced without the locating ring and a compression gland can be used for sealing. This would simplify the design even further, thereby reducing costs.

The advantages of our improved sensor are the following: (1) it is simple; (2) uses very few parts: (3) the amount of platinum uses is very small; (4) it is accurate and reproducible so that it can be used in industrial applications to indicate true oxygen content: (5) all dissimilar material junctions have been eliminated; and (6) the sensor has demonstrated long term operation at 704"C., a temperature significantly lower than the 816"C. operating temperature of other industrial sensors. This relatively low operating temperature in an industrial environment is a marked improvement over currently available commercial sensors.In addition. it has been shown that the installation of this sensor directly into stack gas systems can be done very simply compared t() other available sensors.

The invention will now be described, by sus as oí' example. with reference to the accom- panxinr drawing. in which:- Fig. I is a side sectional view of the improved sensor taken on its axis; and Fig. 2 is an end sectional view taken on line 2 -2 of Fig. 1.

Our improved sensor indicated generally at 10 comprises a generally tubular metal body member 12 which is preferably made of a corrosion-resistant material such as stainless steel. The body 12 includes means such as integral threads 14 for attaching the sensor to the wall of a flue gas containing conduit such as a smoke stack. The interior of the body member 12 includes an inner wall portion 16 and a recessed shoulder portion 18. An electrical insulating ceramic tube 22 made of a material such as forsterite includes an integral flange or annular ring portion 24 which comprises a retaining means which cooperates with the recessed shoulder retain ing means 18 # in the body 12 to restrict upward movement of the tube 22 relative to the body.A pressure nut 26 which is threadably engaged with the body 12 prevents a downward movement of the tube 22 and cooperates with the graphite gasket members 28, 30 to mount the tube 22 in fixed gas-tight relation to the body 12. The upper or sensing end of the ceramic tube 22 is recessed at 36 to accommodate the oxygen ion conducting solid electrolyte disc 40 which is mounted in the recess by means of a glass seal 44. The front or sensing electrode 46 and a rear reference electrode 48 are applied to the electrolyte 40 in the form of a platinum paste.

Prior to firing, and while the paste electrodes 46. 48 are still wet, felted ceramic front disc member 52 and a felted ceramic rear disc member 54 are pressed into them. An outer conductive lead member 60. which is preferably formed of the same platinum paste material as the electrodes. is painted on the surface of tube 22 so as to extend from the sensing electrode 46, which it contacts, around the upper end of the tube and down the length of its outer side in a shallow groove 62 until it is terminated in an upper annular groove 64 which may be contacted by appropriate instrumentation (not shown).

The purpose of the groove 62 is to protect the very thin lead member 60 from damage or breakage as the tube 22 is handled or inserted into the housing 12. An inner platinum paste lead member 70 is painted into electrical contact with the reference electrode surface 48 and down the inside surface of the tube 22, around its lower end, and into a lower annular groove 72 which may be connected into an electrical circuit (not shown). The painted lead members 60, 70 are, of course, painted into contact with the electrodes 46, 48 before the ceramic filter discs 52, 54 are attached by pressing them into contact with the wet paste electrodes. Then, the entire tube 22 is fired in air at a temperature of at least about 954"C. for about 30 minutes.

Then, the electrodes 46, 48 are impregnated with platinum metal utilizing a solution of chloroplatinic acid which is applied to the filter discs 52, 54 and then fired at conditions to effect the reduction of the platinum metal.

The reduction is accomplished by heating the tube assembly in hydrogen for about 30 minutes to a temperature of about 454"C., holding the assembly at about 454"C. in hydrogen for about 60 minutes, and cooling the assembly down in an atmosphere of nitrogen. The reduced platinum particles 76 are uniformly dispersed over the electrode surfaces 46, 48.

WHAT WE CLAIM IS: 1. An oxygen sensor for measuring the concentration of oxygen in industrial flue gases comprising a housing and means on the housing for attaching the sensor to the wall of' a flue gas containing conduit so that a sensing end of the housing is exposed to gases in the conduit and a reference end, remote from the sensing end, is exposed to the atmosphere, a non-electrically conductive tube mounted within the housing and having a sensing end and a reference end remote therefrom, the tube having such a length that the sensing end and reference end of the housing terminate between the sensing end and the reference end of the tube, retaining means on said tube intermediate its ends cooperating with retaining means in said housing to fixedly position and seal said tube in gas-tight relation with said housing, an oxygen ion conducting solid electrolyte disc sealed in a recess in the sensing end of said tube. sensing and reference platinum family electrode coatings respectively on the sensing and reference surfaces of said electrolyte disc, conductive stripes of said platinum family electrode coatings extending continu ouslv along substantially the entire length of said tube for electrically connecting said sensing and reference electrode coatings on said electrolyte disc to spaced contact portions adjacent the reference end of said tube, a pair of felted ceramic fiber filter members one of which is positioned in bonded contact with the sensing platinum family electrode coating and the other of which is positioned in bonded contact with the reference platinum family electrode coating. and- particles of a platinum family metal uniformly dispersed throughout the filter members. some of which particles are in contact with said electrode coatings.

2. . The oxygen sensor of claim I wherein said recess is deeper than the combined thickness of the electrolyte disc and one of the filter members.

3. The oxygen sensor of Claim 1 or 2 wherein said filter members each have a thickness of about 1.02 mm.

4. The oxygen sensor of any of Claims I to 3 wherein said solid electrolyte is yttria stabilized zirconia.

5. The oxygen sensor of any of Claims I to 4 wherein said retaining means on said tube is a radially outwardly extending flange which is restrained in said housing against movement relative to said housing towards the sensing end of the housing by a shouldered recess in said housing and restrained against movement relative to said housing towards the reference end of the housing by a retaining member in said housing, said flange being sealed relative to said recess and said retaining member by gasket means.

6. The oxygen sensor of any of Claims I to 5 wherein one of said conductive stripes is recessed in a longitudinal groove formed in the outer surface of said tube.

7. The oxygen sensor of Claim 6 wherein said one conductive stripe is bonded to said sensing electrode coating and extends therefrom along the wall of said recess, over the sensing end of the tube and along the outer surface thereof to a termination point between the reference end of the housing and the reference end of the tube; the other conductive stripe being bonded to said reference electrode coating and extending along the inner surface of the tube, over the reference end of the tube to a termination point which is closer to the reference end of the tube than is the termination point of said one conductive stripe.

8. The oxygen sensor of Claim 7 wherein said conductive stripes terminate in a pair of grooves formed in the periphery of said tube adjacent the reference end thereof.

9. A method of making an oxygen sensor comprising the steps of taking a pre-fired non-electrically conducting hollow ceramic tube and forming a recess in a sensing end thereof by increasing the diameter of the tube bore at said sensing end; placing a coating of glass sealing material at the bottom of the recess; applying a coating of a platinum paste electrode to each side of a disc of an oxygen ion conducting solid electrolyte: placing said electrolyte disc in said recess with its outer reference surface in contact with said glass sealing material; applying a stripe of platinum paste to the outer reference electrode on said disc and along the internal surface of the tube to a reference end of the latter remote from said sensing end; applying a stripe of platinum paste to the inner sensing electrode on said disc, around the inner end of said ceramic tube and along the outer surface thereof to a location adjacent the reference end of said tube: pressing a ceramic fiber filter member into the plantinum paste electrodes on each side of said electrolyte disc; firing said tube in air at a temperature of at least 954"C. and impregnating said electrodes with platinum metal ulilizing a solution of chloroplatinic acid applied to said filters and then firing at conditions to effect the reduction of said platinum metal.

10. A method as claimed in claim 9

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (12)

**WARNING** start of CLMS field may overlap end of DESC **. to effect the reduction of the platinum metal. The reduction is accomplished by heating the tube assembly in hydrogen for about 30 minutes to a temperature of about 454"C., holding the assembly at about 454"C. in hydrogen for about 60 minutes, and cooling the assembly down in an atmosphere of nitrogen. The reduced platinum particles 76 are uniformly dispersed over the electrode surfaces 46, 48. WHAT WE CLAIM IS:

1. An oxygen sensor for measuring the concentration of oxygen in industrial flue gases comprising a housing and means on the housing for attaching the sensor to the wall of' a flue gas containing conduit so that a sensing end of the housing is exposed to gases in the conduit and a reference end, remote from the sensing end, is exposed to the atmosphere, a non-electrically conductive tube mounted within the housing and having a sensing end and a reference end remote therefrom, the tube having such a length that the sensing end and reference end of the housing terminate between the sensing end and the reference end of the tube, retaining means on said tube intermediate its ends cooperating with retaining means in said housing to fixedly position and seal said tube in gas-tight relation with said housing, an oxygen ion conducting solid electrolyte disc sealed in a recess in the sensing end of said tube. sensing and reference platinum family electrode coatings respectively on the sensing and reference surfaces of said electrolyte disc, conductive stripes of said platinum family electrode coatings extending continu ouslv along substantially the entire length of said tube for electrically connecting said sensing and reference electrode coatings on said electrolyte disc to spaced contact portions adjacent the reference end of said tube, a pair of felted ceramic fiber filter members one of which is positioned in bonded contact with the sensing platinum family electrode coating and the other of which is positioned in bonded contact with the reference platinum family electrode coating. and- particles of a platinum family metal uniformly dispersed throughout the filter members. some of which particles are in contact with said electrode coatings.

2. . The oxygen sensor of claim I wherein said recess is deeper than the combined thickness of the electrolyte disc and one of the filter members.

3. The oxygen sensor of Claim 1 or 2 wherein said filter members each have a thickness of about 1.02 mm.

4. The oxygen sensor of any of Claims I to 3 wherein said solid electrolyte is yttria stabilized zirconia.

5. The oxygen sensor of any of Claims I to 4 wherein said retaining means on said tube is a radially outwardly extending flange which is restrained in said housing against movement relative to said housing towards the sensing end of the housing by a shouldered recess in said housing and restrained against movement relative to said housing towards the reference end of the housing by a retaining member in said housing, said flange being sealed relative to said recess and said retaining member by gasket means.

6. The oxygen sensor of any of Claims I to 5 wherein one of said conductive stripes is recessed in a longitudinal groove formed in the outer surface of said tube.

7. The oxygen sensor of Claim 6 wherein said one conductive stripe is bonded to said sensing electrode coating and extends therefrom along the wall of said recess, over the sensing end of the tube and along the outer surface thereof to a termination point between the reference end of the housing and the reference end of the tube; the other conductive stripe being bonded to said reference electrode coating and extending along the inner surface of the tube, over the reference end of the tube to a termination point which is closer to the reference end of the tube than is the termination point of said one conductive stripe.

8. The oxygen sensor of Claim 7 wherein said conductive stripes terminate in a pair of grooves formed in the periphery of said tube adjacent the reference end thereof.

9. A method of making an oxygen sensor comprising the steps of taking a pre-fired non-electrically conducting hollow ceramic tube and forming a recess in a sensing end thereof by increasing the diameter of the tube bore at said sensing end; placing a coating of glass sealing material at the bottom of the recess; applying a coating of a platinum paste electrode to each side of a disc of an oxygen ion conducting solid electrolyte: placing said electrolyte disc in said recess with its outer reference surface in contact with said glass sealing material; applying a stripe of platinum paste to the outer reference electrode on said disc and along the internal surface of the tube to a reference end of the latter remote from said sensing end; applying a stripe of platinum paste to the inner sensing electrode on said disc, around the inner end of said ceramic tube and along the outer surface thereof to a location adjacent the reference end of said tube: pressing a ceramic fiber filter member into the plantinum paste electrodes on each side of said electrolyte disc; firing said tube in air at a temperature of at least 954"C. and impregnating said electrodes with platinum metal ulilizing a solution of chloroplatinic acid applied to said filters and then firing at conditions to effect the reduction of said platinum metal.

10. A method as claimed in claim 9

carried out substantially as hereinbefore specifically described.

11. An oxygen sensor when made by a method as claimed in claim 9 or 10.

12. An oxygen sensor substantially as hereinhefore specifically described with reference to the accompanying drawing or as illustrated therein.