US 3597345 A
Description (OCR text may contain errors)
/MW ZVTORNEY w. M. HlcKAM ETAL Filed Nov. 18, 1968 Aug. 3, 1971 OXYGEN DETEGTION APPARATUS FURNACE United States Patent O 3,597,345 D OXYGEN DETECTION APPARATUS Wllllaln M. Hicham, Pittsburgh, and .lohn F. Zamaria, Monroeville, lPa., assignors to Westinghouse Electric Corporation, Pittsburgh, Pa.
Filed Nov. 18, 1968, Ser. No. 776,637 Int. Cl. Gtlln 27/46 U.S. Cl. 204--195 12 Claims ABSTRACT F THE DISCLOSURE An oxygen detection system is shown including an oxygen sensor having a differential oxygen pressure responsive electrolyte cell and a catalytic agent for catalytically burning a residual fuel in sample gases before the gases reach the effective detection zone of the cell. The effective detecting zone occurs where two electrodes contact directly opposite sides of an electrolyte wall. A heater arrangement heats the regions of the detection zone and the catalytic agent to an effective operating temperature. The cell includes a tubular electrolyte member having respective open and closed opposite ends. A gas entry tube extends into the bore of the electrolyte member thereby forming a concentric space therebetween. The inner surface of the entry tube and the inner surface of the closed end of the electrolyte member are comprised of a catalytic agent, so that sample gas entering the entry tube and doubling back through the space between the entry tube and the electrolyte member is exposed to an extensive area of heated catalytic agent before it reaches the effective detection zone. The detection system further includes a differential pressure pump connected to the sensor outlet through a capillary restriction, the inlet of the sensor being connected to a subatmospheric source of gas being sampled. The capillary restriction provides substantially constant gas flow rate.
BACKGROUND OF THE INVENTION In fuel combustion systems utilizing the combustion reaction of a fossil fuel and oxygen, the fuel is usually uid for example oil, gasoline, gas or powdered coal, and the combustion is produced by continuous supply of the fuel and air into a combustion chamber where the reaction between the fuel and the oxygen in the air takes place. Sufficient air is supplied to provide an excess of oxygen. The efficiency of the reaction varies as a function of the relationship between the quantities of fuel and oxygen that take part in the reaction, and data is available from which relationships that lead to optimum efliciencies may be derived.
The oxygen-to-fuel ratio in the feed to the combustion apparatus may be set by determining the oxygen content of the gases flowing through the output and by adjusting the ratio in accordance with this determination. In U.S. patent application Ser. No. 514,871 led on Dec, 20, 1965 now U.S. Pat. No. 3,404,836, in the name of William M. Hickam, one of the applicants herein, apparatus is disclosed in which the relationship of the flow of oxygen and fuel into a combustion chamber is continuously controlled in dependence upon continuous determination of the oxygen content of the gases involved in the combustion reaction. In accordance with that disclosure, the oxygen involved in the combustion reaction is continuously sensed by an oxygen solid electrolyte cell which is interposed in the gas involved in the reaction. A typical such cell is disclosed in U.S. Pat. No. 3,347,767 to William M. Hickam for Device for Monitoring Oxygen Content Gases. Briefly the oxygen solid electrolyte cell disclosed therein includes a tube of an electrolyte material including zirconia and thoria and containing such oxides as calcium oxide, yttrium oxide and lanthanum oxide. The tube is coated on the inside and outside with porous platinum which is electrically conducting. The inside and outside coatings are insulated from each other and serve as electrodes. When the electrolyte is heated to a high ternerature, between about 400 C. and 1000 C. and the electrodes are connected in an electrical circuit a difference of potential is produced in this circuit. The magnitude of this potential is dependent on the partial pressures of the oxygen on one side of the cell to the partial pressure of the oxygen on the other side; specifically the partial pressures of the oxygen flowing through the tube to the partial pressure of the oxygen outside of the tube.
In accordance with the aforesaid application Ser. No. 514,871, the cell is connected so that gases derived either from the input to the system or from the output through which the products of combustion are exhausted, are transmitted through the tube, which produces a potential depending on the partial pressure of the oxygen in the gases transmitted through the tube and a partial pressure of the oxygen outside of the tube. Typically, the latter is air and the comparison partial pressure is the partial pressure of oxygen in the air. The potential produced by the cell is impressed on control apparatus to control the relationship or ratio between the fuel and the oxygen supplied to the system. This control is instantaneously dependent on the gases being supplied or the gases being exhausted and continuously adjusts the relationship between the oxygen and the fuel.
Knowledge of the amount of excess air being admitted to a fossil fuel fired furnace is very important. Excess air reduces the furnace temperature, which is turn reduces a rate of heat transfer. Furthermore, since all of the air that passes through the system must leave at the flue temperature, the excess carries out additional heat in the flue gases. An insuflicient quantity of air results in incomplete combustion with the appearance of CO and sometimes smoke in the flue gases which contribute to air pollution. Similar problems occur with internal combustion engines. In application Ser. No. 514,871 the desirability of Ineasuring excess oxygen is recognized and to that end provides platinum gauze (at 163 in FIG. 2) at the entrance to an oxygen sensor.
SUMMARY OF THE INVENTION In accordance with one embodiment of the invention an oxygen sensor is provided wherein substantial areas of catalytic agent is disposed along the gas path at a location between the inlet to the sensor and the effective detection zone, so that complete combustion of residua-l fuels that may be present in the gas samples is effected before the gas sample reaches the detection zone. The exemplary embodiment of the invention is directed to a unique compact configuration wherein a gas path having a reentrant bend is defined by a closed end electrolyte tube and a gas entry tube extending axially into the electrolyte tube. Electrodes are provided on opposite sides of the electrolyte tube wall to provide a detection zone. The inner surface of the gas entry tube and the inner surface of the closed end of the electrolyte tube are coated with a catalytic agent such as platinum so that the gas sample must be exposed to a large area of catalytic agent before it reaches the detection zone.
A further aspect of the invention is directed to a system wherein a differential pressure pump is connected to the outlet of the oxygen sensor through a capillary restriction thus to provide constant flow rate to the gas `being sampled.
It is therefore an object of the present invention to provide improved oxygen detection apparatus.
Y It is another object of the invention to provide oxygen detection apparatus which will measure the excess oxygen in a gas sample having residual or unburned fuel.
It is a further object of the invention to provide a compact and accurate oxygen detection apparatus.
Still another object of the invention is to provide oxygen detection apparatus employing a differential pressure pump and wherein the flow rate is controlled without requiring a throttle valve or a flow meter.
Other and further objects and advantages of the invention will become apparent from the following detailed specification taken in connection with the single figure drawing illustrating a preferred embodiment of the invention. The drawing is part diagrammatic and part in cross-section.
DESCRIPTION OF THE PREFERRED EMBODIMENT In the illustrated apparatus, an oxygen sensor is connected to detect the oxygen in gas samples drawn from a furnace duct 12, and to applysignals to utilization apparatus 14 in accordance with the quantity of oxygen detected. The flue 12 is connected to the tirebox of a fossil fuel (for example gas) fired furnace 16 in order to receive the gases involved in and resulting from the combustion reaction of fuel with the oxygen in air occurring in the frebox. The utilization apparatus 14 may be a meter indicating oxygen concentration, and/or control apparatus for controlling the oxygen or the fuel supply or `both to the iirebox of the furnace 16.
The sensor 10 comprises a closed end tubular electrolyte mem-ber 18 made of a solid material which conducts oxygen ions with negligible electronic conductivity. Suitable materials for the electrolyte member 18 and the electrodes in contact therewith, are disclosed and discussed in U.S. Patent application Ser. No. 534,322 led Dec. 2, 1966, now US. Pat. No. 3,400,054 in the names of R. J. Ruka and J. Weissbart, and in U.S. Pat. No. 3,347,767 to W. M. Hickam, and for disclosures of such materials the application Ser. No. 534,322 and Pat. No. 3,347,767 are incorporated herein by reference.
Known suitable materials for the electrolyte member are solid solutions of oxides whose compositions may be represented by the formula (MO2)1 X(RyOZ)X where M represents at least one tetravalent element from the group consisting of zirconium, thorium and hafnium, R represents at least one element from the group consisting of elements which form cations with stable +2 and +3 valences in the oxide such as calcium, barium, strontium, lanthanum, yttrium, scandium, ytterbium, and samarium, X represents a number having a value of from about 0.0.5 to about 0.3 and y and z represent numbers having values sufficient to make Ry, OZ electrically neutral. A readily available material of this group is a solid solution of zirconium oxide and calcium oxide and referred to as calcium stabilized zirconia, for which the formula is (ZrO2)1 X(CaO)X, Where x is a number having a value from about .05 to about 0.3. One end 20 of the tubular member 18 is closed, while the opposite end 22 is open. Thus the member 18 consists of longitudinally extending peripheral wall and an end wall.
The electrolyte member 18 should be thin to decrease its internal resistance, and as thin as it can be fabricated consistent with necessary mechanical strength and ability to permit only oxygen ions to pass through it. To prevent diffusion of molecular gases through the electrolyte, it is also necessary that the material be relatively dense or at least have no interconnecting pores.
Disposed on the outer surface of the peripheral wall of the tubular electrolyte member 18 in conductive contact therewith is an electrically conductive electrode 24. Another electrically conductive electrode 26 is disposed on and in conductive contact with the inner surface of the peripheral wall of the electrolyte member 18. Although a substantial portion of the inner electrode 26 is directly opposite the outer electrode 24, the inner electrode 26 also extends around the end 22 of the electrolyte member 18 so that a portion 28 is disposed on a portion of the outer surface of the electrolyte tube 18.
The electrodes 24 and 26 are of layers disposed in intimate contact with the electrolyte surface and have electrical continuity, but are suiiiciently porous to permit gas reactants to reach-the electrolyte. It is necessary that the electrode material be suitable for operation at the high temperatures to which the device is subjected, and members of the platinum group of metals are suitable examples for this purpose. The electrodes may for example be formed by applying to the electrolyte surface a slurry of small particles of platinum in an organic binder and heated to remove the binder and fuse the platinum to the electrolyte. For this purpose, commercially available platinum paste may lbe used after thinning to suit with xylene.
Electrical leads 30 and 32 are connected to the respective electrodes 24 and 26, for example by tightly wrapping the end of each lead around its associated electrode as shown to provide good electrical contact. Alternatively, the leads may be connected to the electrodes through conductive brushes in pressure engagement with the electrodes.
The coextensive (directly opposite) portions of electrodes 24 and 26 and the electrolyte wall therebetween form an oxygen differential partial pressure detection zone which is rendered effective for such detection when sufiiciently heated for that purpose, for example from about 650 C. to about 1000 C. Under such conditions a "sandwich cell arrangement of this type produces an EMF across the electrodes when there is a difference between the partial pressures of oxygen on opposite sides of the cell. At the operating temperature, oxygen molecules on the high oxygen pressure side of the cell (anode) gain electrons to become ions which enter the electrolyte. Simultaneously, at the other electrode (cathode), oxygen molecules are formed by the reverse action. The EMF developed across the cell is a logarithmic function of the oxygen concentration in an unknown gas at one side of the cell as compared to a gas with known oxygen concentration on the other side of the cell. Oxygen on either side of the cell may exist in a gas mixture without the operation being materially affected since only the oxygen partial pressure ratio determines the EMF established between the electrodes.
Except for a part of a base fitting 34 supporting the tubular member 18 and later described elements in particular association, cross-sectioned items in the drawing are symmetrical volumes of revolution around the longitudinal central axis of the tubular member 18. Thus electrodes 24 and 26 are of cylindrical configuration conforming to the cylindrical peripheral wall of the member 18. It will be observed that the outer and inner electrodes 24 and 26 are coextensive (directly opposite each other) between the index marks X and Y. Thus the maximum possible detection zone extends longitudinally from X to Y, the mark Y defining the end of the detection zone nearest the closed end 20 of tube 18. However as described later herein, only a part of this zone is rendered effective by heating it to a proper operating temperature.
The open end 22 of the tubular member 18 is supported in the main bore 36 of the base fixture 34, which base includes a passage 38 communicating with the bore 36. In turn the passage 38 is connected with vacuum tight connections to an output line 40. The tubular member 18 is sealed vacuum tight to the bore 36 by means of an arrangement including a resilient O-ring 42 and a gland nut 44. An insulative collar 46 encircles the end 22 of the tubular member 18 to provide additional insulation and support relative to the base lixture 34.
The base xture 34 is provided with another smaller bore 48, in which is supported the inlet end 49 of a gas entry tube 50 that extends axially into the tubular member 18 and in concentric relation therewith. The other end 51 of tube 50 may or may not abut the closed end 20 of the tubular member 18, and in the event that it does so abut, it is desirable that this end of tube 50 be cut at a diagonal to prevent blockage thereof. The tube 50 is made of material that is chemically inert for the use environment so as not to contaminate the electrolyte. It may for example be made of a high temperature ceramic such as magnesia, alumina or zirconia. The inlet end 49 of the tube 58 is sealed in vacuum tight relation with the bore 48 by an arrangement including an O-ring 52 and a gland nut 54. The inlet end 49 of tube 50 communicates through vacuum tight connections with a gas sampling line or pipe 56 connected to the ue 12. The entire length of line 56 is heated, for example by an electrical heating element wrapped around the line and symbolically shown at 58 in order to prevent condensation of Awater in the system.
It will be observed from the drawing that the tube 50', by itself and in cooperation with the tubular member 18, forms sections of a conduit defining a reentrant gas dow path. One section of this path is the bore of tube 50 extending from its inlet end 49 to its inner end 51 adjacent the closed end of tubular member 18. A second reentrant section of this path extends from the inner end 51 of tube 50 to that end (Y) of the detection zone nearest the closed end of member 18 by way of a back bend defined by the closed end 20 of the tubular member 18. A third section of the path extends from the end Y of the detection zone through the space 76 defined between tubes 18 and 50 to the open end 22 of the electrolyte member 18.
The output line 40 is connected to the inlet 60 of a differential pressure pump 62 for pumping gas but preferably also capable of handling water and gas mixtures so as not to be incapacitated by water condensation or precipitate. The outlet of the pump is indicated at 64. The pump 62 is necessary for continuously sampling gas from the flue 12 if the flue gases are at sub-atmospheric pressures. A capillary restriction 63 having sufficient impedance is inserted in the line 40 to insure a constant flow rate, thereby eliminating the need of a throttle valve to control the flow rate and also the need for a ow meter.
Disposed around the tubular member 18 and in concentric relation therewith is a tube 64 spaced from the tube 18 to provide a passageway 65 for exposing the outer surface of the member 18 to air as an oxygen reference.
Suitable means is provided for heating at least a portion of the possible detection zone between X and Y to render that portion effective for detecting the difference between the partial oxygen pressures on opposite sides of the electrolyte member 18. The heating means may for example be an electric heating element 66 wrapped around the tube 64 and connected to a suitable power source. Although the tube 64 may be of ceramic material, perferably it is made of metal for example Inconel, in which case the heating wire 16 must be insulated. The heating coils 66 may be for example of the sheathed or clad electric resistance type in which the resistance wire core is insulated from an outside metal sheath. This type can be wound around tube 64 with the sheath in contact with the metal tube without danger of short circuit. To increase the efficiency of the heating unit 66, it may be surrounded by a metal shell 68 packed with heat insulating material 70. By way of example, power is supplied to the heater coils 66 from a power supply source which is contained in a regulator 72, and which is controlled in response to a temperature responsive transducer 74 located within the tube 64 and adjacent the end 20 of the tubular member 18, thus to regulate the temperature.
The heat concentration will be in a zone encircled by the heating coil 66 and extending the axial length of the coil, opposite ends of this zone being marked by the characters Z and W. As seen in the drawing, part of the possible detection zone extending from X to Y falls within the heat concentration zone that extends from Z to W. The portion of the potential detection zone within the heat concentration zone extends from Z to Y, thus rendering this part of the apparatus effective for detecting partial pressures of oxygen on opposite sides of the electrolyte wall. Thus the end of the effective detection Zone nearest the closed end 20 of the member 18 is at Y. The other end of the effective detection Zone is at Z.
It should now be apparent that the path of sample gas as indicated by the directional ow arrows is such that the sampled gas enters the inlet end 49 of the gas entry tube 50 and ows through that tube toward the closed end 20 of the tubular member 18 where the flow pattern takes a hackbend or reentrant configuration, the gas flowing through the annular space 74 between the outer surface to tube 58 and the inner surface of the tubular member 18. When the gas sample passes by the effective detection zone Z-Y, an EMF will appear across the electrodes 24 and 26 having a magnitude which is a function of the ratio of the partial pressure of the oxygen in the gas sample to the partial pressure of the oxygen of the air in the annular space between the outer surface of the tubular member 18 and the inner surface of the tube 64. This then provides a measure of the free oxygen in the gas sample. However, it may or may not be an accurate measure of the excess oxygen in the gas sample, depending on whether or not there is any residual or unburnt fuel in the sampled flue gas.
To insure that all the residual fuel is burned up before reaching the effective detection Zone, and thus to insure an effective and accurate measure of the excess oxygen in the gas sample, there is disposed a catalytic agent along at least a portion of the gas flow path within the heat concentration zone Z-W and between the point Z within tube 50 and the end Y of the effective detection zone so that the gas sample is exposed to an extensive area of catalytic agent in the heat concentration zone before it reaches the effective detection zone. The catalytic agent is of a character to aid in providing complete combustion of the unburned fuel in the gas sarnple. The catalytic agent may for example be catalytically active platinum or other metal of the platinum group such as palladium, rhodium and iridium, and it may be in the form of a coating on the inner surface of tube 50, and a coating 82 disposed on the inner end surface of the tubular member 18. The coatings 80 and 82, may be applied in the same manner as the electrodes. It should be appreciated that either of the coatings 80 or 82 may be lalone sufficient to provide the desired result. This depends on the area of the coating under consideration. For example if the tube 50 is long enough the area of the coating 80 would be sufficient to effect the desired purpose. On the other hand, the apparatus may be so proportioned as to provide sufficient area of the coating 82 to provide the desired result. However, we find that a more compact and efficient structure results, and a complete combustion of the unburned fuel is amply insured by the use of both coated surfaces 80 and 82. The catalytic agent should be disposed in a portion of the gas ow path that is at a high temperature in order to burn the unburned fuel in the sample gas. Thus the catalytic `agent is most effective in a zone extending from Z to the end 20 of the electrolyte: member 18.
In operation the action of the pump 62 continuously draws gas samples from the Hue 12 through the sensor 10 by way of a path including in the order named: line 56; gas entry tube 50; the space 76; between tubes S0 and 18; bore 36; passage 38; and output line 40. Before any unit of gas reaches the effective detection zone between Z and Y, it is subjected to the catalytic agents disposed in the path of a gas that are sufficiently heated to react with the unburned fuels in the gas sample in a way to aid the combustion thereof. Thus when the unit of gas reaches the effective detection zone, the free oxygen therein is the excess oxygen over that required to burn all the fuel in the original mixture supplied to the furnace iirebox. Thus the EMF produced yacross the electrodes 24 and 26 is an accurate measure of the excess oxygen in the gas.
The EMF developed across the electrodes 24 and 26 is applied through lines 30 and 32 to the utilization apparatus 14, and within the apparatus 14 through whatever ampliers might be necessary to a suitably calibrated volt-meter 90, and/or control apparatus connected to the furnace controls for controlling the fuel to air ratio of the air-fuel mixtures supplied to the furnace firebox.
By way of example, the following measurements and relations may be employed in practicing the invention.
Electrolyte member 18 Inches Length 7.4
Outside diameter .187
Inside diameter .125
Gas entry tube S Inches Outside diameter .625
Inside diameter .037
Dimension U-V .2
Dimension Y-U .25
Dimension Z-Y 3.4
Pipes 40 and 60 Inside diameter .25
Capillary 63 Inside diameter .025 Length .4
Gas flow rate through sensor 10, 2 s.c.f.h.
Temperature of heat concentration zone Z-W 850 C.
The invention disclosed herein provides an oxygen sensor of the electrolyte cell type having a unique compact configuration which provides a most efficient location and sufficient area for a catalytic agent in the sensor to insure complete combustion of residual fuel in gas being sampled.
Although the invention may be employed for detecting oxygen in many kinds of systems and environments, it is especially useful for detecting oxygen in fuel cornbustion systems, for example furnaces, internal combustion engines, boiler plants, etc.
It is understood that the herein described arrangements are simply illustrative of the principles of the invention, and that other embodiments and applications are within the spirit and scope of the invention.
1. Oxygen pressure detection apparatus comprising:
,(A) a tubular electrolyte member of solid electrolyte material conductive of oxygen ions with negligible electronic conductivity, said member having a longitudinal axis and respective open and closed opposite ends;
(B) first and second electrode means in contact with directly opposite inner and outer surfaces, respectively, of the peripheral wall of the tubular electrolyte member, coextensive portions of the first and second electrode means and the electrolyte wall therebetween forming an oxygen differential partial pressure `detection zone which is rendered effective for such detection when suiiiciently heated for that purpose;
(C) first and second circuit connections means in contact with said first and second electrode means, respectively;
(D) conduit means including said tubular electrolyte member, said conduit means having an inlet and an outlet and defining a path for the flow of gas between the inlet and the outlet, said conduit means further comprising an open-ended gas entry tube extending axially into the bore of the tubular electrolyte member to a point beyond that end of said detection zone nearest said closed end of the tubular electrolyte member, whereby a portion of said gas entry tube is disposed along said detection zone, the outer surface of said tube and the inner surface of the tubular electrolyte member defining a space therebetween, whereby said tube individually and in conjunction with said tubular electrolyte member defines portions of said gas flow path, a first portion of said path extending from the outer end of the tube to the inner end of the tube, the inner end of the tube being that end nearest said closed end of the electrolyte member, a reentrant second portion of the path extending from the inner end of the tube to that end of said detection zone nearest said closed end by way of (a) a reverse bend defined by said closed end and (b) said space, a third portion of the path extending from said end of the detection zone through said space to the open end of said tubular electrolyte member, said inlet comprising the outer end of said tube, said outlet comprising the open end of the tubular electrolyte member;
(E) catalytic means for the combustion reaction of a fuel and oxygen associated with said conduit means and disposed along at least one of said first and second portions of said path, and.
(F) heating means operatively associated with said conduit means for heating the conduit means at the locations of said catalytic means and said detection zone.
2. The combination of claim 1 wherein said solid electrolyte material has the formula I(MO2)1 X(RyOZ)X where M represents at least one tetravalent element from the group consisting of zirconium, thorium, and hafnium, R represents at least one element from the group consisting of element of Groups II-A and III-B of the Periodic Table which forms cations with stable |2 and +3 valences in the oxide, x represents a number having a value of from about 0.05 to about 0.3 and y and z represent numbers having values suliicient to make RyOz electrically neutral.
3. The combination of claim 1 wherein said catalytic means comprises catalytically active platinum.
4. The combination of claim 1 wherein said catalytic means comprise an inner surface portion of said conduit means.
S. The combination as in claim 4 wherein said heating means is arranged to establish a heat concentration zone encompassing both said detection zone and said catalytic means.
6. The combination of claim 1 wherein said catalytic means comprises at least a portion of the inner surface of said tube.
7. The combination of claim 1 wherein said catalytic means comprises an inner surface portion of said tubular member between its closed end and that end of said detection zone nearest said closed end.
8. The combination as in claim 7, wherein said catalytic means further comprises at least a portion of the inner surface of said tube.
9. The combination as in claim 8 wherein said portion of said inner surface of said tube extends axially parallel to and beyond said detection zone in a direction toward said closed end of the electrolyte member.
10. The combination as in claim 9, wherein said heating means is arranged to establish a heat concentration zone encompassing both said detection zone and said catalytic means.
11. The combination as in claim 1 wherein said heating means is arranged to establish a heat concentration zone encompassing both said detection zone and said catalytic means.
12. The combination as in claim 1 which further includes (G) means defining a capillary restriction;
(H) a differential pressure pump whose inlet is connected through said capillary restriction to the outlet of said conduit means; and
(I) utilization means connected to said electrodes for responding in accordance with the oxygen pressure in said gas.
References Cited UNITED STATES PATENTS 3,347,767 10/1967 Hickman 204-195 3,400,054 9/1968 Ruka et al. 204-1T 3,347,767 10/1967 Hickam 20L-195 OTHER REFERENCES Weissbart et a1.: Review of Scientic Instruments, v01. 32, No. 5, May, 1961, pp. 593-595.
TA-HSUNG TUNG, Primary Examiner U.S. C1. X.R.