AU706961B2 - Electrochemical sensor for SOx or SOx and O2 measurements - Google Patents
Electrochemical sensor for SOx or SOx and O2 measurements Download PDFInfo
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- AU706961B2 AU706961B2 AU70221/96A AU7022196A AU706961B2 AU 706961 B2 AU706961 B2 AU 706961B2 AU 70221/96 A AU70221/96 A AU 70221/96A AU 7022196 A AU7022196 A AU 7022196A AU 706961 B2 AU706961 B2 AU 706961B2
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- 238000005259 measurement Methods 0.000 title claims description 12
- MCMNRKCIXSYSNV-UHFFFAOYSA-N Zirconium dioxide Chemical compound O=[Zr]=O MCMNRKCIXSYSNV-UHFFFAOYSA-N 0.000 claims description 51
- 239000007789 gas Substances 0.000 claims description 50
- 239000007784 solid electrolyte Substances 0.000 claims description 49
- 229910052751 metal Inorganic materials 0.000 claims description 44
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- 229910052760 oxygen Inorganic materials 0.000 claims description 44
- 239000001301 oxygen Substances 0.000 claims description 44
- QVGXLLKOCUKJST-UHFFFAOYSA-N atomic oxygen Chemical compound [O] QVGXLLKOCUKJST-UHFFFAOYSA-N 0.000 claims description 41
- 150000003467 sulfuric acid derivatives Chemical class 0.000 claims description 37
- 239000000567 combustion gas Substances 0.000 claims description 36
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- PNEYBMLMFCGWSK-UHFFFAOYSA-N aluminium oxide Inorganic materials [O-2].[O-2].[O-2].[Al+3].[Al+3] PNEYBMLMFCGWSK-UHFFFAOYSA-N 0.000 claims description 16
- 239000000463 material Substances 0.000 claims description 13
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- 238000000034 method Methods 0.000 claims description 8
- BASFCYQUMIYNBI-UHFFFAOYSA-N platinum Chemical compound [Pt] BASFCYQUMIYNBI-UHFFFAOYSA-N 0.000 claims description 8
- VYPSYNLAJGMNEJ-UHFFFAOYSA-N silicon dioxide Inorganic materials O=[Si]=O VYPSYNLAJGMNEJ-UHFFFAOYSA-N 0.000 claims description 6
- 229910052681 coesite Inorganic materials 0.000 claims description 5
- 229910052906 cristobalite Inorganic materials 0.000 claims description 5
- 239000000377 silicon dioxide Substances 0.000 claims description 5
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- 230000007704 transition Effects 0.000 claims description 5
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- 239000000919 ceramic Substances 0.000 claims description 4
- RUDFQVOCFDJEEF-UHFFFAOYSA-N yttrium(III) oxide Inorganic materials [O-2].[O-2].[O-2].[Y+3].[Y+3] RUDFQVOCFDJEEF-UHFFFAOYSA-N 0.000 claims description 4
- PCHJSUWPFVWCPO-UHFFFAOYSA-N gold Chemical compound [Au] PCHJSUWPFVWCPO-UHFFFAOYSA-N 0.000 claims description 3
- 229910052737 gold Inorganic materials 0.000 claims description 3
- 229910052697 platinum Inorganic materials 0.000 claims description 3
- 239000000470 constituent Substances 0.000 claims 4
- 239000002131 composite material Substances 0.000 claims 3
- -1 oxygen ion Chemical class 0.000 claims 3
- CPLXHLVBOLITMK-UHFFFAOYSA-N Magnesium oxide Chemical compound [Mg]=O CPLXHLVBOLITMK-UHFFFAOYSA-N 0.000 claims 2
- 238000005266 casting Methods 0.000 claims 2
- ODINCKMPIJJUCX-UHFFFAOYSA-N Calcium oxide Chemical compound [Ca]=O ODINCKMPIJJUCX-UHFFFAOYSA-N 0.000 claims 1
- 241000968352 Scandia <hydrozoan> Species 0.000 claims 1
- 239000000292 calcium oxide Substances 0.000 claims 1
- 235000012255 calcium oxide Nutrition 0.000 claims 1
- 238000010304 firing Methods 0.000 claims 1
- 239000011872 intimate mixture Substances 0.000 claims 1
- 239000010416 ion conductor Substances 0.000 claims 1
- 239000000395 magnesium oxide Substances 0.000 claims 1
- HJGMWXTVGKLUAQ-UHFFFAOYSA-N oxygen(2-);scandium(3+) Chemical compound [O-2].[O-2].[O-2].[Sc+3].[Sc+3] HJGMWXTVGKLUAQ-UHFFFAOYSA-N 0.000 claims 1
- 239000000843 powder Substances 0.000 claims 1
- 241000894007 species Species 0.000 claims 1
- ZCUFMDLYAMJYST-UHFFFAOYSA-N thorium dioxide Chemical compound O=[Th]=O ZCUFMDLYAMJYST-UHFFFAOYSA-N 0.000 claims 1
- 229910052815 sulfur oxide Inorganic materials 0.000 description 33
- AKEJUJNQAAGONA-UHFFFAOYSA-N sulfur trioxide Inorganic materials O=S(=O)=O AKEJUJNQAAGONA-UHFFFAOYSA-N 0.000 description 28
- RAHZWNYVWXNFOC-UHFFFAOYSA-N Sulphur dioxide Chemical compound O=S=O RAHZWNYVWXNFOC-UHFFFAOYSA-N 0.000 description 24
- 238000006243 chemical reaction Methods 0.000 description 17
- 239000008188 pellet Substances 0.000 description 13
- XTQHKBHJIVJGKJ-UHFFFAOYSA-N sulfur monoxide Chemical class S=O XTQHKBHJIVJGKJ-UHFFFAOYSA-N 0.000 description 12
- 239000004020 conductor Substances 0.000 description 11
- 229910021653 sulphate ion Inorganic materials 0.000 description 8
- 238000001514 detection method Methods 0.000 description 7
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- 239000010935 stainless steel Substances 0.000 description 6
- 229910001220 stainless steel Inorganic materials 0.000 description 6
- 235000010269 sulphur dioxide Nutrition 0.000 description 6
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- 238000009792 diffusion process Methods 0.000 description 5
- 239000007787 solid Substances 0.000 description 5
- 230000000694 effects Effects 0.000 description 4
- INHCSSUBVCNVSK-UHFFFAOYSA-L lithium sulfate Chemical compound [Li+].[Li+].[O-]S([O-])(=O)=O INHCSSUBVCNVSK-UHFFFAOYSA-L 0.000 description 4
- HHVIBTZHLRERCL-UHFFFAOYSA-N methylsulphonylmethane Natural products CS(C)(=O)=O HHVIBTZHLRERCL-UHFFFAOYSA-N 0.000 description 4
- 102100033697 DNA cross-link repair 1A protein Human genes 0.000 description 3
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- VAYGXNSJCAHWJZ-UHFFFAOYSA-N dimethyl sulfate Chemical compound COS(=O)(=O)OC VAYGXNSJCAHWJZ-UHFFFAOYSA-N 0.000 description 3
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- 239000007832 Na2SO4 Substances 0.000 description 2
- KDLHZDBZIXYQEI-UHFFFAOYSA-N Palladium Chemical compound [Pd] KDLHZDBZIXYQEI-UHFFFAOYSA-N 0.000 description 2
- PMZURENOXWZQFD-UHFFFAOYSA-L Sodium Sulfate Chemical compound [Na+].[Na+].[O-]S([O-])(=O)=O PMZURENOXWZQFD-UHFFFAOYSA-L 0.000 description 2
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- BHPQYMZQTOCNFJ-UHFFFAOYSA-N Calcium cation Chemical compound [Ca+2] BHPQYMZQTOCNFJ-UHFFFAOYSA-N 0.000 description 1
- UGFAIRIUMAVXCW-UHFFFAOYSA-N Carbon monoxide Chemical compound [O+]#[C-] UGFAIRIUMAVXCW-UHFFFAOYSA-N 0.000 description 1
- RWSOTUBLDIXVET-UHFFFAOYSA-N Dihydrogen sulfide Chemical compound S RWSOTUBLDIXVET-UHFFFAOYSA-N 0.000 description 1
- DGAQECJNVWCQMB-PUAWFVPOSA-M Ilexoside XXIX Chemical compound C[C@@H]1CC[C@@]2(CC[C@@]3(C(=CC[C@H]4[C@]3(CC[C@@H]5[C@@]4(CC[C@@H](C5(C)C)OS(=O)(=O)[O-])C)C)[C@@H]2[C@]1(C)O)C)C(=O)O[C@H]6[C@@H]([C@H]([C@@H]([C@H](O6)CO)O)O)O.[Na+] DGAQECJNVWCQMB-PUAWFVPOSA-M 0.000 description 1
- 239000002228 NASICON Substances 0.000 description 1
- ZLMJMSJWJFRBEC-UHFFFAOYSA-N Potassium Chemical compound [K] ZLMJMSJWJFRBEC-UHFFFAOYSA-N 0.000 description 1
- 229910004298 SiO 2 Inorganic materials 0.000 description 1
- BQCADISMDOOEFD-UHFFFAOYSA-N Silver Chemical compound [Ag] BQCADISMDOOEFD-UHFFFAOYSA-N 0.000 description 1
- NINIDFKCEFEMDL-UHFFFAOYSA-N Sulfur Chemical compound [S] NINIDFKCEFEMDL-UHFFFAOYSA-N 0.000 description 1
- 239000005864 Sulphur Substances 0.000 description 1
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- 239000003209 petroleum derivative Substances 0.000 description 1
- 229910052700 potassium Inorganic materials 0.000 description 1
- 239000011591 potassium Substances 0.000 description 1
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- 239000004332 silver Substances 0.000 description 1
- 229910000367 silver sulfate Inorganic materials 0.000 description 1
- 239000011734 sodium Substances 0.000 description 1
- 229910052708 sodium Inorganic materials 0.000 description 1
- KKCBUQHMOMHUOY-UHFFFAOYSA-N sodium oxide Chemical compound [O-2].[Na+].[Na+] KKCBUQHMOMHUOY-UHFFFAOYSA-N 0.000 description 1
- 229910001948 sodium oxide Inorganic materials 0.000 description 1
- 239000004449 solid propellant Substances 0.000 description 1
- 229910002076 stabilized zirconia Inorganic materials 0.000 description 1
- 238000006467 substitution reaction Methods 0.000 description 1
Landscapes
- Measuring Oxygen Concentration In Cells (AREA)
Description
P/00/0i 128/5/91 Regulation 3.2
AUSTRALIA
Patent Act 1990
ORIGINAL
COMPLETE SPECIFICATION STANDARD PATENT INVENTION TITLE: 9 *6 9.
9".
9 99* 9* 09 I 999*99 9* 9 9 *0*0*9
I
"Electrochemical Sensor for SOx or SOx and 02 measurements" The following statement is a full description of this invention, including the best method of performing it know to us: This invention relates to an improved gas sensor for measurement of sulphur oxides in a combustion gas stream.
In one embodiment, the sensor may be used to determine the sulphur oxides concentration in the combustion gas stream. In another embodiment, the sensor may be used for simultaneous determination of oxygen and sulphur oxides concentrations in the combustion gas stream.
Combustion of fossil fuels is a widely used method for obtaining energy in a large number of industries. Sulphur dioxide (SO2) and trioxide (S03) are present in exit or stack gases of combustion processes which utilise sulphur-containing fuels. Sulphur oxides S02 and S03, hereafter referred to as SOx are released from the burning of any fossil-fuel including the burning of solid fuels, such as coal, for example as used in furnaces, kilns, boilers, and including the burning of liquid fuels, such as fuel oil and other petroleum products.
Sensors for measuring SOx content of combustion gases have been described in the prior art. A SOx solid electrolyte sensor consists of two major components: a solid electrolyte and a reference electrode. Such a sensor generally utilise S04 2- ion-conducting solid electrolytes. Sulphate electrolytes are attractive electrolytes for SOx sensors because the S04 2- ion can equilibrate with either S02 or S03 in the combustion gas stream and establish the cell reaction. Thus, the potentiometric response of a sulphate-electrolyte sensor is directly related to the partial pressure or concentration of SOx in the gas. The Na+conductive sulphate-electrolytes formed on Na-3"-alumina and NASICON Na2Zr2Si3PO12 in connection with Na2S04 can also establish a reversible cell reaction to SOx. However, they e• appear to suffer from the chemical instability of the solid electrolytes in SOx-containing atmospheres and also provide rather sluggish responses.
25 The second major component of a SOx solid electrolyte sensor is the reference electrode. In principle, any mixture (gas, liquid or solid) which establishes a content SOx pressure or concentration is a suitable reference electrode. However, gas-reference 4 electrodes S03-N2; S03-Ar etc) whilst reliable in short-term laboratory tests, but they are cumbersome and impractical in commercial applications. The reason for this is that they require a reliable maintenance a constant reference-gas composition and an external calibration technique to monitor the reference-gas composition. Therefore, solid-reference I electrodes have many advantages with respect to chemical and mechanical stability. Q.G.Liu and W.L.Worrell in U.S. Pat. No.4,622,105 describe a silver-silversulphate mixture as one of the most successful solid-reference electrodes for a SOx sensor. However, this solid electrolyte SOx sensor has a narrow work temperature range from 515 to 560 0
C.
A sulphate-electrolyte sensor with solid reference electrode can be expressed as cell Mex, MexSO4 sulphate electrolyte Pt, S02, SO3, air. 1 The half-cell reaction at the electrode-gas interface is the equilibration of the S04 2- ion in the sulphate electrolyte when the S03 is present in the gas. In this sensor, the Pt measuring electrode also works as a catalyst for the oxidation of SO2: SO2 1/202 S03 (2) and the equilibrium constant, K, of reaction 2 is expressed as K Pso3 Ps2 P02 1/2 3 With a catalyst, the cell potential is established by the equilibrium pressure of S03, which can be related to the inlet S02 pressures using equation 4 where K is the equilibrium constant for reaction 2 PSO2(in) PSO2 PSO3 PS02 1 1/KPO2 1/2 (4 It is important that the oxygen pressure of the gas is known. In some cases in air with a small amount of SOx, the oxygen pressure is known and is essentially constant. In other cases, especially in combustion gases with low and unknown oxygen pressure, it must be measured using an additional zirconia sensor.
If an electrical current flows through a sulphate-electrolyte cell, the half-cell reaction at the gas electrode-electrolyte interface is the equilibration of the S03 in the gas with the S4 2ion in the electrolyte: S03 1/202 2e- S04 2 25 The other half-cell reaction at the metal-metal sulphate electrode-electrolyte interface is: 2Me S042- Me2SO4 2e-. 6 The cell reaction for cell 1 is obtained by combining the half-cell reactions 5 and 6 for the gas electrode and the reference electrode, respectively: S2Me SO3 1/202 Me2SO4. (7 The cell potential for reaction 7 is given by the Nernst equation: S° E Eo RT/2F In PSO3. PO2 m AMe2SO4), (8 *o° where: T is temperature (Kelvin); R is the gas constant; F is Faraday's constant; PO2 m is the partial oxygen pressure at the measuring electrode; PSO3 is the partial sulphur trioxide pressure at the measuring electrode, AMe2S04 is the activity of metal-sulphate reference electrode, Eo is the standard cell potential calculated for reaction 7 using the standard free energies of formation for the Me2SO4 and SO3.
The S03 concentration in the inlet gas can be directly determined from the cell potential of reaction 8 if the partial oxygen pressure and the Me2SO4 activity in the reference electrode are known. The S02 concentration in the inlet gas can be determined from the cell potential of reaction 8 using an equilibrated gas mixture and equation 4 One sensor of known design includes a hollow tube made of a refractory material, such as alumina, and having a disk plug or pellet of solid electrolyte sealed in one end of the tube.
Other sensors utilise a tube 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. Another Australian Patent, No.575,551 in the name of Corning Glass Works, shows a different configuration for an "in-situ" oxygen detector apparatus incorporating a •solid state electrochemical sensing cell physically located within a boiler exhaust stream with an integral heater/electrode and a disk or test-tube shaped solid electrolyte.
Many solid electrolyte materials are known to be suitable for use in SOx sensors.
Examples include the combination of K2S04, Na2SO4 and Li2SO4, Na-3"-alumina U.S. Pat.
25 No. 4,855,034 Na2Zr2Si3PO12, Li2SO4-Ag2SO4 Pat. No. 4,622,105 Other U.S.
Patents No. 5,288,675 and No. 5,399,327 describe (MgO-La203-A203 composition with a catalytically active amount of promoters for S02 oxidation. 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.
The sensors described above can be used to assist in the measurement of the SOx content in the combustion processes. However, the EMF responses of these sensors are often unstable or vary with time, possibly because of the leakage of gases through the porous sulphate electrolytes or the formation of pyrosulphate at the interfaces.
Moreover, Yan et al. in the articles "Characteristics and sensing mechanism of SOx sensor using stabilised zirconia and metal sulphate", Sensors and Actuators B, 12 (1993) 77- 81 and "High-performance solid-electrolyte SOx sensor using MgO-stabilized zirconia tube and Li2SO4-CaSO4-SiO2 auxiliary phase", Sensors and Actuators B, 20 (1994) 81-87 found that a stabilised zirconia tube coated with Li2SO4, Na2SO4, Li2SO4-CaSO4 and Li2SO4- CaSO4.SiO2 can be utilised for fabricating an SOx sensor. However, the mechanical toughness, the design, the narrow range of measuring SOx concentration (10 200ppm) and the narrow range of measuring temperature (600-750 0 C) do not yet meet the demands of industry. Furthermore, Skeaff et al. in review paper entitled "Electrochemical measurement of S03 S02 in process gas stream", Sensors and Actuators B, 10 (1993) 161-168 described a variety of SOx solid electrolyte gas sensors and concluded with a summary of the requirements for a successful inexpensive commercial solid-state sensor for S03 and S02 measurement in a process gas stream. It is apparent from this evidence that no work has been done on a practical solid-state sensor to measure concentration of SOx under industrial conditions.
It is an object of the present invention to provide the sensors and detecting systems 20 which are suitable for the detection of gaseous SOx or SOx and oxygen in a combustion gas.
More particularly, it is an object of this invention to provide sensors which are compact, assembled into the probes, which may be fabricated easily and which have improved stability over time. Another object is to provide such sensors that have voltage outputs that are very close to the predicted theoretical values for the sensor.
In a first aspect, the present invention provides a sensor for detecting SOx in a combustion gas. It comprises a solid electrolyte having oxygen-ion conductivity which is tightly connected to an auxiliary phase of the combination of metal sulphates. This combination of metal sulphates have a first electrode in electrical contact with metal sulphates and a second electrode which is in electrical contact with a solid electrolyte, the second part being on the opposite side of the solid electrolyte to the combination of metal sulphates. The gas pathway represents a reference gas, which must pass oig the tube to The 1~11111 reach the second electrode.
Sensors in this invention comprise of ion-conductive materials which are sensitive to the species to be detected. In this regard, those of ordinary skill in the art appreciate that electrochemical sensors are required to have the greatest possible ionic conductivity, while at the same time, having the smallest possible electronic conductivity. Sensing devices having relatively large ionic conductivities coupled with relatively small electronic conductivities are able to demonstrate relatively high reliability and reproducibility in detecting concentration of the species. It is necessary to select ion-conductive materials which are suitable for detection of the appropriate species to be detected.
Zirconia based materials are preferable as these materials are known to generate adequate EMF in most environments and to be generally suitable as a base material for sensing devices for such employment.
A solid electrolyte with oxygen-ion conductivity may be any zirconia-based electrolyte stabilised by MgO, Y203, Sc203 or Th203 that is capable of measuring oxygen potential in the electrochemical junction between metal sulphates and the solid electrolyte. The combination of metal sulphates represents an auxiliary phase of BaSO4-K2SO4-SiO 2 3:5:2 in molar ratio The electrochemical junction between zirconia and metal sulphates requires the presence of an interfacial phase containing K' and O 2- at the interface. The role of the sulphate as an K' conductor is very important. In the present sensor, a small amount of K2ZrO3 is formed: 2K O 2- ZrO2 K2ZrO3. (9) In a combination ofBaSO4-K2SO4-Si 2 the potassium and the calcium ions are the predominant mobile species. The electrodes should be suitable for use with solid electrolyte sensors. The electrodes are preferably porous gold or platinum electrodes. The skilled person 25 will recognise that there are many electrodes that will perform satisfactorily and the present invention extends to cover all such electrodes. Electrodes used in solid electrolyte sensors are well known to the skilled addressee and will not be described in greater detail.
SSe S" In one preferred embodiment of the first aspect of the present invention, the solid S- electrolyte is described as being located at and closing one end of the elongate tube. This may be achieved by providing a hollow tube, for example of a ceramic or refractory material, .•.and placing a disk, plug or pellet in the end thereof The present sensor is considered to be composed of the following electrochemical cell: SO2, SO3, combustion gas Au//BaSO4-K2SO4-SiO2//ZrO 2 +Y203//Au reference air.( first electrode conductor) (02- conductor) (second electrode) The combination of metal sulphates may be considered as an auxiliary sensing material.
In use of the sensor of the first aspect of the present invention, the sensor is placed in a combustion gas stream "in-situ" measurement and combustion gas makes contact with the first electrode. The first electrode reaction as an example for K+ mobile species is 2K+ SO3 1/202 2e- K2S04 (11) while the second electrode reaction on the stabilised zirconia electrolyte is 02- 1/202+ 2e-. 12) That is, the sensor can be considered as combining two half cells in series with an SOx electrode and an 02 electrode. Both of these electrodes are referenced to K2ZrO3. The electromotive force (EMF of this sensor should be the sum of the contributions from the two cells.
From reactions 11 12), and 9 the following expressions can be derived: E Eo (RT/4F)ln(PO2 m/P0 RT/2F In PS03, (13) where PSO3 and P02 are the partial pressure of S03 and 02, R, T and F have the usual meanings.
Let (PS02 )in stand for the S02 partial pressure of the inlet gas: (PSO2 )in PS02 PS3. (14) From equations 3 and 14 PSO3 is given as K (P02) 1/2 PS03 PS02 )in. 1+K(P02) 1 2 25 When P02 m and P02 r are constant, the substitution of equation 15 into equation 13 gives the following final expression for the output EMF of the sensor: E Eo RT/2F )In PS02)in (16) In a preferred embodiment of the first aspect of the invention, the sensor includes an elongate tube having the zirconia solid electrolyte located at the closing one end thereof The zirconia solid electrolyte also located at the closing one end of the layered tube having a smooth transition in composition through the thickness of the components from alumina at the outer surface to zirconia at the inner surface. The processing route chosen is often determined by the materials being processed, type of gradient required, and thickness and geometry of the parts being produced. The inner volume of the layered tube sealed by the combination of metal sulphates. Preferably, the first electrode is located on the outer face of the metal sulphates and the second electrode is located on the inner face of the solid electrolyte.
The sensor of the first aspect of the invention should also be provided with means for measuring an electrical potential difference between the first and second electrodes. For example, each electrode may be provided with electrical connections that lead from the respective electrodes and are adapted to be connected to a suitable measuring apparatus. The electrical connectors may comprise conductive paths formed by painting metallic pastes onto the elongate tube and/or the inner rod or tube. Alternatively, the electrical connectors may be one or more wires in electrical contact with each electrode. The skilled addressee will be aware of many arrangements for the electrical connectors that fall within the scope of the present invention and these will not need to be described in greater detail.
The present invention also provides a method for detecting SOx in a combustion gas comprising providing a sensor comprising a solid electrolyte material in electrochemical contact with the combination of metal sulphates having a first electrode in electrical contact with the combination of metal sulphates and a second electrode in electrical contact with a solid electrolyte, the second part being on an opposite side of the solid electrolyte to the combination of metal sulphates, and gas pathway means along which a reference gas must pass to reach the second electrode, placing said sensor in a combustion gas stream whereby combustion gas contacts said first electrode and reference gas passes along to contacting the second electrode and measuring an electrical potential difference between the first and S 25 second electrodes.
The problem of the material selection for corrosion protection is very important for any industrial application. Dry SO2 and S03 are slightly more corrosive than air, and even at temperatures of 400-500'C any stainless steel can be used to handle them. Sulphur oxides and hydrogen sulphide are highly corrosive at high temperatures. Furthermore, if traces of sodium oxide are also present, from combustibles or fuel oil, the eutectic with sodium will form, which has a melting point of 500'C, and sudden failures will occur when operating temperatures are above the melting point of the eutectic. Therefore, the MA 253 stainless steel should be selected for outer sensor's sheath. This stainless steel has been developed especially for use in the extraordinarily harsh conditions which attend some of the metallurgical processes, temperature reaction vessels or atmospheres, flue gasses, power boilers, pollution control equipment, and a whole host of oxidative, corrosive, high temperature, and other harsh environments. It would appear that sensing devices in accordance with this invention will also be useful in less harsh environments as well. Thus, the sensors and methods in accordance with this invention are particularly suitable to the determination of SOx in harsh environments.
In a second aspect, the present invention provides a sensor measuring SOx and oxygen content of a combustion gas, said sensor including a first electrode which, in use, comes into contact with the combustion gas, a second electrode, a third electrode, each of said first and second electrodes being separated by the combination of metal sulphates and by solid electrolyte, each of said second and third electrodes being separated by solid electrolyte, a first gas pathway means for passing a reference gas of known oxygen concentration to the second electrode, a second gas pathway means for passing a combustion gas to the first and to the third electrodes, the second gas pathway means a combustion gas must contact the first and the third electrodes, and means for measuring an electrical potential difference between two of the following: S 25 a) the first and second electrodes, and b) the second and third electrodes.
Preferably the means for measuring the electrical potential difference includes means for measuring the electrical potential difference between the first and the third electrodes and means for measuring the electrical potential difference between the second and the third electrodes.
In one embodiment of the second aspect of the invention, the sensor includes an *9 S elongate tube having the zirconia solid electrolyte located at the closing one end thereof The zirconia solid electrolyte also located at the closing one end of the layered tube having a smooth transition in composition through the thickness of the components from alumina at the outer surface to zirconia at the inner surface. The inner space of the layered tube sealed by the combination of metal sulphates. Preferably, the first electrode being located on the outer surface of the metal sulphates, the second electrode being located on the outer surface of the solid electrolyte and the third electrode being located on the inner surface of the solid electrolyte. The solid electrolyte extending between the second electrode and the third electrode.
In this embodiment, combustion gas can contacts the first and the second electrodes.
The means to measure the electrical potential differences may include one or more electrical wires separately connected to each of the first, the second and the third electrodes. These electrical connections may be similar to those described in respect of the first aspect of the invention and will not be described further.
In the use of the sensor of the second aspect of the invention, the sensor is placed in a combustion gas stream. The combustion gas including SOx and excess of oxygen contacts the first electrode and the second electrode. The third electrode is contacted by reference gas of known oxygen concentration. Due to the differences in partial oxygen pressure of the gas contacting the second and the third electrodes, electrical potential difference is established between the second and the third electrodes. Measurement of the electrical potential **"differences between the first and the third electrodes enables the sulphur oxides concentration of the combustion gas to be determined. Similarly, measurement of the electrical potential difference between the second and the third electrodes enables the oxygen concentration of combustion gas to be measured.
25 The sensor of the second aspect of the invention allows to measure the sulphur oxides concentration and the oxygen concentration simultaneously. The sensor provides a unitary V device that requires only a single access port in an exhaust-duct or other mechanisms for V 30 carrying exhaust gas. The value of the electrical potential difference across the solid electrolyte and the combination of metal sulphates is dependent upon the temperature of the electrolyte and the temperature of the combination of metal sulphates. Therefore, the sensor should incorporate the thermocouple or other temperature measuring means to measure the temperature of the solid electrolyte and the combination of metal sulphates. Any known thermocouple may be used. A preferred construction includes two wires of dissimilar materials located within the inner tube of the sensor and joined to each other adjacent to the solid electrolyte. The two dissimilar wires form the thermocouple, with the hot junction being the above-described junction of the two wires adjacent to the solid electrolyte and the cold junction being at a remote location maintained at known temperature.
The sensor of the second aspect of the invention requires only three electrodes. The first and the second electrodes are separated by the solid electrolyte and by the combination of metal sulphates, the second and the third electrodes are separated only by the solid electrolyte.
The preferred electrical potential differences measured in the sensor of the second aspect of the invention are the electrical potential differences between the first the third electrodes this allows the sulphur oxides concentration of the raw combustion gas to be determined) and the second and the third electrodes this allows the oxygen concentration of the combustion gas to be determined).
The sensor of the invention may also be constructed in such a way that two pairs of .electrodes are used, with each electrode of a pair being separated by the solid electrolyte which has an auxiliary combination of metal sulphates and solid electrolyte, respectively. In this construction, the solid electrolyte which has an auxiliary combination of metal sulphates S 20 of one pair of electrodes does not necessarily have to be in electrical contact with the solid electrolyte of the other pair of electrodes.
*Preferred embodiments of a sensor in accordance with the present invention will be a *described by way of reference to the accompanying drawings in which: Figure 1 is a cross-sectional view of one end portion of the sensor in accordance with a first embodiment of the invention; Figure 2 is a cross-sectional view taken along line A-A in Figure 1; Figure 3 is a perspective view of one end portion of an inner tube of one embodiment in accordance with the invention; In Figure 1 to 3, is shown an elongate cylindrical inner tube or rod 1 having four bores 2, 3, 4, 5 extending lengthwise through the rod. The rod 1 forms part of a first embodiment of the sensor. Three of the bores 2, 3, 4 are provided to house wires which, in use, extend through the bores. The an other bore 5 provides a passageway for a reference oxygen atmosphere to be described later). The rod 1 has typically a diameter of about 4.0 to mm with each of the four bores being typically 0.5 to 1.5 mm in diameter. Typically the rod is composed of 99.7% alumina. However, the composition of rod 1 may comprise any other suitable non-electrically conductive material.
A notch 6 which is located towards one end of rod 1 is of a depth sufficient to cut through bores 2, 3 whilst maintaining bores 4, 5 intact. Thus, notch 6 extends about halfway through the diameter of rod 1.
Wires 7, 8 extend through bores 2, 3, respectively and the free ends of these wires extend into notch 6 where they are joined together by clip 9. Wires 7, 8 are of dissimilar material and are in electrical connection to form the hot junction of a thermocouple which is used to measure the temperature of the atmosphere into which the sensor is inserted. End portion 10 of the rod is preferred as the cool or cold end and is contained in a sensor housing which is located outside the unit.
Wire 11 which extends through bore 4 projects outwardly from the end of rod 1 and is formed into a spiral spring arrangement 12 at the end of rod 1. The free end of wire 11 is folded back upon itself to be received in the end of bore 4 in order to securely locate and maintain spiral spring arrangement 12. Wire 11 forms one part of the electrical circuit of the sensor. This circuit may be referred to as the sulphur oxides detection circuit. Preferably, wire 11 should be made from MA 253 stainless steel.
Rod 1 is located in support internal tube 13 as shown in Figure 1. Typically, this tube 13 is composed of up to about 99.9% alumina, but may comprise any other suitable nonelectrically conductive material. Tube 13 is tight-leak sealed at one end by a pellet 14 of oxygen-conductive solid electrolyte. Preferably, pellet 14 comprises stabilised zirconia and is 25 coated with Au on the inner side of pellet 14 to form electrode 15. Electrode 15 is in contact with spring arrangement 12. The tube 16 has a graded transition in composition through the thickness of the components from 99.7% alumina at the outer surface to 99.7% zirconia at 30 the inner surface at one end and has only alumina content through the thickness at the other end. The end of tube 16, which has a zirconia inner surface, is tight-leak sealed by zirconia pellet 14. The slot 17 is made into another end of the tube 16, which has only alumina oo content. The inner volume of tube 16 from zirconia pellet 14 to slot 17 is also tight-leak S S sealed by the combination of metal sulphates 18. Preferably, the combination of metal sulphates 18 comprises an auxiliary phase of BaSO4-K2S4-SiO2 3:5:2 in molar ratio where the K+ ions are the predominant mobile species. The combination of metal sulphates 18 is coated with an Au coating on the outer side to form electrode 19 thereon and thiselectrode, in use, comes into contact with combustion gas. Tube 16, pellet 14 and the combination of metal sulphates 18 being compatible in their thermal expansion coefficients and being substantially non-porous. It is believed that the activities of K+ are fixed by the composition of the measuring gas in accordance with the reaction 11 at measuring electrode 19 and the activities of 02- are fixed by the composition of the reference gas in accordance with the reaction( 12 at reference electrode In the present sensor, the Au works not only as a measuring electrode, but also as a catalyst for oxidation of S02 to S03. The partial pressure of S02 can be substituted for the partial pressure of S03. The oxygen partial pressure in air is known and the measured partial pressure of S02 or S03 can be calculated from the EMF of the sensor.
Wires 7, 8 forming the thermocouple are completely separated from wire 11 and measuring electrode 19, which forms second part of the electrical circuit for the SOx detection. With this arrangement, wires 7, 8 may be selected to provide the optimum performance for measurement the temperature by thermocouple.
Bore 5 provides a pathway for the reference air, which is typically 20,9% oxygen, to be admitted from outside the sensor to contact the electrode 15 of the pellet 14.
As a result, the sensor measures the SOx concentrations in the combustion gas.
The Figure 3 shows one end portion of the one embodiment of a sensor assembled into the probe in accordance with a second embodiment of the invention. In the sensor shown in :this Figure, the zirconia pellet 14 is coated with an Au coating on the outer side to form 25 electrode 20. The electrode 20 is connected to the current conductor 21, which provides an additional electrical path from pellet 14 to the cool end of the sensor. Typically, current conductor 21 has an identical composition to that of wire 11 as described above. The sensor for simultaneous measurement of SOx concentrations and 02 in combustion gas, requires o three electrodes. The measuring electrode 19 and the reference electrode 15 are separated by the solid electrolyte pellet 14 and by the combination of metal sulphates 18. These electrodes :are included in the circuit, referred to as the sulphur oxides detection circuit. The measuring a are included in the circuit, referred to as the sulphur oxides detection circuit. The measuring electrode 20 and the reference electrode 15 are separated only by the solid electrolyte pellet 14. These electrodes and current conductor 21 are included into the circuit, which may be referred to as the oxygen detection circuit. The electrode 15 is a common electrode for both circuits.
As a consequence of the particularly design of this sensor, oxygen and sulphur oxides concentrations in the combustion gases can be simultaneously measured.
Tube 13 is located inside the metal sheath with one closed end 22 as shown in Figure 3.
The gap between tube 13 and sheath 22 is sealed at the cool end of the sensor by ring of silicon material. Stainless steel spring 23 provides an electrical contact between the measuring electrode 19 and the metal sheath 22. Typically, the metal sheath 22 and the spring 23 are made of stainless steel MA 253, but may comprise any other suitable electrically conductive material.
The ceramic diffusion elements 24 were put into the apertures 25, which are made in the metal sheath 22. The diffusion element 24 protects the solid electrolyte with metal sulphates from harsh gas atmosphere. Wet, chemical-laden flue gases coat the diffusion element 24 eventually plugging it and preventing flue gas from diffusing to the sensor. In very corrosive atmospheres, for instance downstream of a wet scrubber, the corrosive chemicals may destroy the diffusion element 24. This condition eventually lead to sensor failure. Should problems occur, the gas sensor must be taken off line for maintenance and replacement of the diffusion element 24 allows quick replacement of the element should plugging occur.
.,:Apertures 25 in the metal sheath 22 and allow combustion gas to enter the annular space between tube 13 and sheath 22 and provide a pathway for supplying the gas to electrodes 19 S•and 9° 9 Dated: 16 October 1995
SO..
Applicant: ANALYT INSTRUMENTS N T a
Claims (13)
1. Electrochemical sensor for SOx or SOx and 02 measurements of a monitored gas environment by generating electrical signals on the basis of a difference in the partial pressure of the gas species between monitored gas environment and reference environment, comprising two elongate and short ceramic tubes having an oxygen-sensitive element present at one end thereof, elongate and short ceramic tubes being sealed and connected to each other by an oxygen-sensitive element, the inner space of short tube being non-porously sealed by the combination of metal sulphates with sulphate/metal atom ratio equal to or less than two, provided that at least one of said metal sulphates has a sulphate/metal atom ratio less than two, the first electrode being located on an outer surface of the combination of metal sulphates present at one end of the short tube and the second electrode being located on an inner surface of the oxygen-sensitive element present at one end of the elongate tube, the third electrode being located at an outer surface of the oxygen-sensitive element, the combination of metal sulphates and oxygen-sensitive element extending between the first electrode and the second electrode, oxygen-sensitive element extending between the second electrode and the third electrode, and a first gas pathway means for passing a reference gas of known oxygen concentration to the second electrode, a second gas pathway means for passing a combustion gas to the first and to the third electrodes, the second gas pathway means a combustion gas must contact the first and the third electrodes, and a means being available for measuring an electrical potential difference between two of the following: a) the first and second electrodes, and the second and third electrodes. :e
2. Electrochemical sensor according to claim 1 wherein the means for measuring the electrical potential difference include means for measuring the electrical potential difference between the first and second electrodes and means for measuring the electrical potential Uq*• difference between the second and third electrodes.
3. Electrochemical sensor according to claim 1 or 2 wherein the oxygen-sensitive element comprises a mixture of at least one non-electrolyte alumina constituent and at least one constituent of phase which is good oxygen ion conductor, the microstructure of the composite oxygen-sensitive material consisting of an intimate mixture of the oxygen ion conducting and the non-electrolyte alumina material, the oxygen ion constituent grains of which comprise from to 70 percent by volume of the mixture.
4. Electrochemical sensor according to claim 3 wherein the oxygen ion conducting constituent of the composite solid electrolyte material is selected from the group consisting of thoria doped with yttria, zirconia stabilised or partially stabilised with calcia, zirconia stabilised or partially stabilised with scandia, zirconia stabilised or partially stabilised with magnesia, and zirconia stabilised or partially stabilised with yttria. Electrochemical sensor according to claim 3 wherein the oxygen-sensitive material is produced by the admixture of stabilised or partially stabilised oxygen-conductive solid electrolyte in finely ground form with alumina powder, followed by consolidating the admixture and firing to a temperature below 1850°C which is sufficiently high to produce a high density impervious body.
6. Electrochemical sensor as claimed in any one of the claims 1 to 5 wherein the combination of metal sulphates comprises an auxiliary phase of BaS04-K2S04-SiO2 3:5:2 in molar ratio
7. Electrochemical sensor as claimed in any one of the claims 1 to 6 wherein the composition of the first electrode means is substantially the same as the composition of the second electrode means.
8. Electrochemical sensor as claimed in any of the claims 1 to 7 wherein the composition of the second electrode means is substantially the same as the composition of the third electrode means.
9. Electrochemical sensor as claimed in any of the claims 1 to 8 wherein the short tube comprises a graded casting composite materials at one end thereof having a smooth transition in composition through the thickness of the components from alumina at the outer surface of the tube to the stabilised zirconia at the inner surface of the tube present at one end thereof and casting alumina content through the thickness of the tube present at another end *i thereof. Electrochemical sensor as claimed in claim 9 wherein the end of the short tube, comprising a smooth transition in composition through the thickness of the components from alumina at the outer surface of the tube to the stabilised zirconia at the inner surface of the tube, sealed by an oxygen-sensitive element.
11. Electrochemical sensor as claimed in claim 9 wherein the inner space of the short tube sealed by the combination of metal sulphates.
12. Electrochemical sensor as claimed in any of the claims 1 to 11 wherein the combustion gas contacts one electrode of each pair of electrodes.
13. Electrochemical sensor according to any one of the preceding claims wherein the means to measure the electrical potential differences includes one or more electrical leads separately connected to each of the electrodes.
14. Electrochemical sensor according to any one of the preceding claims wherein the electrochemical sensor includes a further tube open to the reference atmosphere and mounted within the elongate tube and the composition of the further tube is substantially the same as the composition of the elongate tube. Electrochemical sensor according to claim 14 wherein the further tube has a passage therein and the reference gas is supplied through the passage therein and the reference gas is supplied through the passage whereby the reference gas contacts the electrode on the inner surface of the oxygen-sensitive element. Electrochemical sensor according to any one of the preceding claims wherein the 4 electrodes are porous gold or platinum electrodes. 0. to, 17. Electrochemical sensor according to any one of the preceding claims further including means to measure temperature of the combustion gas. Ma 18. Electrochemical sensor according to claim 17 wherein said means to measure temperature comprises a thermocouple.
19. Electrochemical sensor according to any one of preceding claims wherein said temperature is between about 650C and about 1100°C. o: 20. A method for detecting SOx and 02 concentrations in a combustion gas providing a sensor comprising the combination of metal sulphates connected to the oxygen- 4 *4 0 a sensitive element, the sensor having a first electrode in electrical contact with a first part of the So combination of metal sulphates and a second electrode in electrical contact with a second part of the oxygen-sensitive element, the second part being on an opposite side of the oxygen-sensitive element to the first part of the combination of metal sulphates, and a third electrode in electrical contact with a first part of the oxygen-sensitive element, the first part being on an opposite side of the oxygen-sensitive element to the second part, and gas pathway means being providing 18 along which a combustion gas must pass to reach the first and the third electrodes, and means being provided for measuring an electrical potential difference between the first and second electrodes and for measuring an electrical potential difference between the second and third electrodes. 9* 0 00 0 *0 o0 00** 00 0 0 0 O @0 L r -s^
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| AU70221/96A AU706961B2 (en) | 1995-10-16 | 1996-10-16 | Electrochemical sensor for SOx or SOx and O2 measurements |
Applications Claiming Priority (3)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| AUPN5991A AUPN599195A0 (en) | 1995-10-16 | 1995-10-16 | Electrochemical sensor for sox or sox and O2 measurements |
| AUPN5991 | 1995-10-16 | ||
| AU70221/96A AU706961B2 (en) | 1995-10-16 | 1996-10-16 | Electrochemical sensor for SOx or SOx and O2 measurements |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| AU7022196A AU7022196A (en) | 1997-04-24 |
| AU706961B2 true AU706961B2 (en) | 1999-07-01 |
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| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| AU70221/96A Ceased AU706961B2 (en) | 1995-10-16 | 1996-10-16 | Electrochemical sensor for SOx or SOx and O2 measurements |
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| Country | Link |
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| AU (1) | AU706961B2 (en) |
Citations (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO1985005681A1 (en) * | 1984-06-01 | 1985-12-19 | Kabushiki Kaisya Advance Kaihatsu Kenkyujo | Gas sensor |
| US4828672A (en) * | 1988-03-30 | 1989-05-09 | Westinghouse Electric Corp. | Unitary self-generating reference gas sensor |
| JPH10104197A (en) * | 1996-10-01 | 1998-04-24 | Shinko Electric Ind Co Ltd | SOX gas sensor and method of manufacturing the same |
-
1996
- 1996-10-16 AU AU70221/96A patent/AU706961B2/en not_active Ceased
Patent Citations (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO1985005681A1 (en) * | 1984-06-01 | 1985-12-19 | Kabushiki Kaisya Advance Kaihatsu Kenkyujo | Gas sensor |
| US4828672A (en) * | 1988-03-30 | 1989-05-09 | Westinghouse Electric Corp. | Unitary self-generating reference gas sensor |
| JPH10104197A (en) * | 1996-10-01 | 1998-04-24 | Shinko Electric Ind Co Ltd | SOX gas sensor and method of manufacturing the same |
Also Published As
| Publication number | Publication date |
|---|---|
| AU7022196A (en) | 1997-04-24 |
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