EP0236441A1

EP0236441A1 - In-stream gas sensor

Info

Publication number: EP0236441A1
Application number: EP86905622A
Authority: EP
Inventors: Ralph A. Dalla Betta; David R. Sheridan
Original assignee: SONOXCO Inc
Current assignee: SONOXCO Inc
Priority date: 1985-09-09
Filing date: 1986-09-04
Publication date: 1987-09-16
Also published as: JPS63500743A; KR880700263A; EP0236441A4; MX168184B; WO1987001453A1; CN86106172A

Abstract

Le dispositif ci-décrit permettant d'analyser des gaz dans un courant gazeux en mouvement comprend des moyens d'analyse (1, 2) placés dans le courant (41) et capables d'analyser les gaz qui le compose. Un corps sensiblement poreux (4) et des moyens chauffants (5) formant une combinaison entourent les moyens d'analyse. La combinaison permet la pénétration des gaz à analyser mais empêche les fluctuations de mouvement du courant gazeux qui affectent sensiblement et rendent imprécis l'analyseur, la combinaison permettant de maintenir l'analyseur à une température sensiblement constante indépendamment des fluctuations de température dans le courant gazeux.The device described below for analyzing gases in a moving gas stream comprises analysis means (1, 2) placed in the stream (41) and capable of analyzing the gases which compose it. A substantially porous body (4) and heating means (5) forming a combination surround the analysis means. The combination allows the penetration of the gases to be analyzed but prevents fluctuations in the movement of the gas stream which substantially affect and make the analyzer imprecise, the combination making it possible to maintain the analyzer at a substantially constant temperature independently of the temperature fluctuations in the gas stream .

Description

IN-STREAM GAS SENSOR

Technical Field of Invention

Broadly, the present invention relates to the manufacture of gas sensors generally and in particular to the manufacture of accurate combustible gas sensors which can be located in situ within a moving gaseous stream.

Background of the Invention

Combustible gas sensors for the measurement of a high level of gases such as carbon monoxide and hydrogen have been prepared in the past. These sensors operate by measuring the temperature differential developed when the combustible gas is catalytically reacted with oxygen at the sensor. Two opposing temperature measuring devices are generally provided such that the output of the device is not a function of an individual sensor temperature but is instead a function of the temperature differential between the catalytic and reference portions of the sensor. Thermocouples, resistance thermometers, thermistors and other temperature sensitive devices can be used as the individual sensors.

It has traditionally been the approach in prior art systems to "sample" combustion flue gases by withdrawing a sample from a flue stream and measuring its concentration. Certain problems have been recognized in performing such a sampling as exhaust gases may be noxious and sometimes even toxic so that the process of analysis can be potentially dangerous to the equipment operator. Furthermore, since the ambient conditions outside of the exhaust duct in many instances may be at a temperature below the dew point of the sample gas, condensation may occur in the gas sample line. The condensation together with dust and soot particles in the sample can deposit in the sample line, initially restricting and eventually plugging the sample line and making the system inoperable.

U.S. Patent No. 3,960,500, recognizing these problems and discloses and claims one "solution" to the above-recited difficulties by providing a closed loop sampling and analyzing system which returns the sampled and analyzed gas back to the same area from which the sample was drawn. The reference accomplishes its function by sealably mounting a sample probe and a return line to a duct containing exhaust gases. The sample probe draws a sample of the gases from the duct and conveys the sample to an analyzer assembly which establishes a control signal indicative of the concentration of a component of the gas. The analyzer assembly must then be connected to an aspirator which draws the analyzed sample from the analyzer assembly and conveys it back to the return line which exhausts the sample gas back into the duct where it may be exhausted along with the duct exhaust gases.

It is quite evident that although the invention disclosed and claimed in U.S. Patent No. 3,960,500 represents an advancement in the art over prior sampling devices, the invention disclosed therein is cumbersome, complicated and expensive to fabricate.

Further, having configured a rather elaborate maze of input and output lines, it is believed that the rather elaborate gas path would be prone to clogging with high particulate-containing gases unless a separate filter was provided. In addition, if the sampling assembly was to employ a catalytic combustion sensor, it would have, to be heated by a resistance heater to prevent condensation of the sample from occurring anywhere within the sampling assembly and to maintain the gas sample at a temperature at which it can catalytically oxidize on a differential catalytic surface. Prior to the present invention, the art has invariably taught away from placing a combustible gas sensor directly within a moving gaseous stream. Such in situ measurement has been rejected out of hand because fluctuations in gas velocity as well as rather dramatic temperature fluctuations such as found in flue gases from a combustion process would render differential temperature gas sensors of prior art design virtually useless. It is thus an object of the present invention to provide a gas sensing device which is exceedingly accurate and sensitive to the presence of certain combustible products found within a moving gaseous stream. It is yet a further object of the present invention to provide a gas sensing device which can be placed within a flue gas chimney or other hostile environment without compromise to the accuracy of the sensing measurements. These and further objects will be more readily perceived when considering the following disclosure and appended drawings wherein:

Fig. 1 depicts an overall representation of the present invention configured for the in situ measurement of combustible components within a stream of flue gas effluent.

Fig. 2 is a cut-away, partially sectioned detailed representation of the sensor device shown in Fig. 1. Fig. 3 represents a prospective, partially cutaway view of a further embodiment of the present invention.

Summary of the Invention The present invention deals with a device useful for analyzing gases within a moving gaseous stream. The device comprises an analyzer means located within the stream which is capable of analyzing for said gases contained therein. Substantially surrounding the analyzer means is the combination of a substantially porous body and heater means. The combination allows the penetration of the gases to be analyzed but prevents fluctuations in the movement of the gaseous stream to substantially affect and render inaccurate the analyzer while maintaining the analyzer at a substantially constant temperature independent of temperature fluctuations within the gaseous stream.

The sensing elements of the present invention are shown as elements 1 and 2 of Fig. 2 and 16 and 17 of Fig. 3. As previously noted, the sensing elements operate by measuring a temperature differential developed when the combustible gas is catalytically reacted with oxygen at the sensor. Elements 1 and 16 are characteristically coated with a high surface area oxidation catalyst while opposing elements 2 and 17 are coated with an inactive material. The active catalyst coating acts to catalyze the exothermic oxidation of combustible gases such as CO and H₂ while the inactive device remains insensitive thereto. The heat liberated in the catalytically promoted oxidation reactions results in a temperature differential between catalytic and refereπce temperature measuring devices. The particular method of applying the catalytic coating, as well as the catalytic coating itself, such as Group VIII metals-platinum, palladium, iridium and rhodium, and combinations thereof, is taught by U.S. Patent No. 4,355,056, the disclosure of which is incorporated herein by reference. The temperature-sensitive elements can consist of thermocouple junctions as described in U.S. Patent No. 4,355,056 or, alternatively, the elements can consist of thin film platinum resistors forrαed on an alumina plate which is available commercially from a variety of sources including Degussa Corp. of South Plainfield, New Jersey and Heraeus-Volkert of Queens Village, New York. Such resistance elements preferrably have a physical dimension of approximately 3 mm by 10 mm by 0.5 mm in thickness and exhibit a resistance at room temperature of approximately 1,000 ohms.

As previously noted, the catalytic and noncatalytic elements used for gas analysis are placed within stack 40 directly in a flowing gas stream 41 such as a flue gas stack from a boiler. Such a stack has a large fluctuating velocity and the gas temperature also fluctuates depending upon ambient temperature and boiler load. Only through the practice of the present invention is one able to achieve a stable signal independent of stack gas velocity and temperature. This is accomplished by not only providing a heater means capable of maintairiing the analyzer at a substantially constant temperature independent of the temperature of the gaseous stream, but also by providing a substantially porous body located proximate the analyzer means. The porous body is characterized as (1) being capable of allowing the penetration of the gaseous stream therethrough so that gases can contact and be analyzed by the analyzer means, (2) preventing fluctuations in the movement of the gas stream to substantially affect and render inaccurate the analyzer and (3) maintaining the analyzer means at a constant temperature.

Ideally, the heater and porous body can be configured as a unitary element, and this preferred embodiment is depicted in Fig. 2. More specifically, porous body 4, can be comprised of any material which is not catalytically active for the measurement reaction and which is sufficiently porous to perform the intended function. For example, if the present invention is intended to measure the concentration of CO in a gaseous stream by catalytically oxidizing CO to CO₂ at the surface of catalytic element 1, porous body 4 must not catalyze any reactions to remove CO or react with CO or O₂, thus interfering with the measurement. Porous body 4 should also be resistant to degradation at those temperatures which are anticipated being encountered within gas stream 41. Suitable materials for construction are ceramics such as alumina, silica, zirconia (ZrO₂), mullite, cordierite, magnesia, berylium oxide, as well as a wide variety of other ceramics, ideally which would be thermally conductive, the identity of which would clearly be obvious to a person skilled in the art upon review of this disclosure. In addition, metals such as steel, stainless steel or aluminum can be used if made porous. The porous body should broadly exhibit an average pore diameter of approximately 0.1 to 1,000 μ, and more preferably 1 to 500 u, and most ideally 10 to 200 u. Elements 4 should also exhibit a void volume broadly between 5 to 80%, and more preferably 20 to 70%, and most preferably 40 to 60%.

Referring again to the preferred embodiment shown in Fig. 2, the heater means can consist of a helically wound, resistance wire 5 embedded within porous body 4. The resistance wire can consist of, for example, nichrome which will generate heat energy upon the application of an electric current. Alternatively, the heating wire can be mounted around the outside surface of the porous body and, in fact, means for heating other than a wire can be selected. For example, a thin film resistance heater can be deposited on the inside surface of the porous ceramic or on its outside surface. Yet another alternative is to use a porous ceramic that has embedded within its structure electrically conductive material such as metal particles or carbon which will cause the generation of heat upon the application of a current. The only truly meaningful limitation regarding the composition of the heating elements, be they embedded in porous body 4 or not, is that they not exhibit any catalytic activity for the measurement reaction. As an example, in the measurement of CO by oxidation to CO₂ using a platinum catalyst, a heater winding composed of platinum, pallidium, rhodium or a variety of other catalytically active metals may just react and remove all or a portion of the CO prior to reaching the catalytic resistance element. If such heating means is contemplated, the wire, for instance, can be coated with an inactive material prior to use in the present environment.

Any convenient means can be used to maintain the uniformity of temperature proximate the analyzer means to ensure a uniform catalytic reaction of the oxidizable material. As shown in the preferred embodiment as illustrated in Fig. 2, lead 7 can be embedded within porous body 4 as a thermocouple whose leads pass through base 10 and pipe 11, emanating in electrical connection box 8 which is placed outside of stack 40 via flange 12. The thermocouple can be connected to a temperature controller, well-known in the art, to regulate power to the heater and maintain a constant temperature. The heater leads 6 can similarly be made to parallel the leads to the thermocouple 7. In turn, connector box 8 can be electrically connected via line 23 to the analyzer electronics. As indicated previously, the electronics can consist of a wide variety of heat change-responsive devices. By means of illustration, reference is made to U.S. Patent Nos. 4,325,912 and 4,305,724 which disclose typical networks for rendering a signal in response to a differential temperature change.

The temperature which is intended to be maintained by the heater depends, for the most part, upon the nature of the gas being analyzed. It is safe to say that for monitoring flue gas from a combustion process, one would not want to go below 60ºC or risk water and acid mist condensation appearing on the surface of the analyzer. When employing a platinum catalyst for the analysis of CO concentration, one would generally operate at temperatures between approximately 150°C to 500°C or above with approximately 300°C being preferred.

Although the porous body is ideally configured in a basically cylindrical shape shown by element 4 of Fig. 2, it can also be fabricated in a number of varying shapes. The only important design criteria is that the porous body be capable of allowing the penetration of the gaseous stream therethrough so that the gases can be contacted with and analyzed by the analyzer means while preventing fluctuations in the movement of the gaseous stream to substantially affect and render inaccurate the analyzer means. As being illustrative of a further embodiment, reference is made to Fig. 3 wherein the substantially porous body comprises a box-like structure which fully encloses the analyzer means 16 and 17 wherein at least one wall 14 or 14A is porous. The analyzer means is fully enclosed by the box-like structure by providing side walls 15 which need not be porous. As in the previous embodiment, porous members 14 can be composed of any nonreactive porous material such as, ideally, an alpha-alumina ceramic having an average pore diameter of approximately 10 to 200 u and a void volume of approximately 40 to 60%. air can be controlled by a variety of means well-known to those skilled in the art, including the application of a constant pressure of air to a fixed orifice. In addition, Tube 13B can also be employed to calibrate the device while installed in a gas stream. For example, analyzer zero can be adjusted while flowing a gas containing no combustibles while the full scale or span of the device can be adjusted when a gas containing combustibles such as CO is passed through tube 13B. Tube 13B can also be used to feed combustion air in an oxygen starved environment.

Certain modifications and improvements will occur to those skilled in the art upon reading the specification. It will be understood that all such improvements and modifications have been deleted herein for the sake of conciseness and readability but are properly covered within the scope of the following claims.

As a further preferred embodiment, it is contemplated that the present invention include a substantially porous filter means 9 which substantially envelops substantially porous body 4. The porous filter is characterized as being capable of substantially filtering solid particulate matter which may be located in the gaseous stream from contacting substantially porous body 4, thus allowing the penetration of the gaseous stream therethrough so that the gases can contact and be analyzed by the analyzer means but preventing particulate matter from doing so. The outer filter also performs the function of mediating gas velocities in high velocity environments. As such, when gas velocities are relatively slow, the outer filter can be dispensed with.

The outer filter, which may be especially useful in coal-fired combustion applications where a large amount of ash is present in the flue gas, can be composed of the same composition as porous body 4. It can exhibit a porosity between 0 to 1,000 μ, and more preferably between 1 to 400 μ, and most ideally between 10 and 200 μ. The same limitations previously discussed regarding the nonreactivity of porous body 4 apply to outer filter 9. As a further preferred embodiment, it is contemplated that the present invention include inlet tube 13A for carrying air proximate to the analyzer means. As previously noted, the present analyzer is intended to detect and measure the concentration of a combustible material in a gaseous stream. As such, in the event of oxygen starvation, combustion will be limited which may give a false reading to the operator. To ensure sufficient oxygen, inlet tube 13A is provided which extends beyond flange 12 to the ambient. Tube 13A can feed air at a flow rate of 10 cc per minute to the space surrounding catalytic surface 1. The flow of

Claims

1. A device for analyzing gases within a moving gaseous stream comprising:

A. analyzer means located within said stream being capable of analyzing for said gases contained therein; and

B. the combination of a substantially porous body located proximate said analyzer means and heater means, said combination (i) substantially allowing the penetration of the gaseous stream therethrough so that said gases can contact and be analyzed by said analyzer means, (ii) substantially preventing fluctuations in the movement of said gaseous stream which substantially affect and render inaccurate said analyzer means while (iii) maintaining said analyzer means at a substantially constant temperature substantially independent of temperature fluctuations in said gaseous stream.

2. The device of claim 1 wherein said substantially porous body is configured in the shape of a cylinder fully enclosing said analyzer means.

3. The device of claim 1 wherein said substantially porous body comprises a box-like structure fully enclosing said analyzer means wherein at least one wall of said box-like structure is porous.

4. The device of claim 1 wherein said heater means comprises a resistance wire located substantially proximate said substantially porous body.

5. The device of claim 1 where said heater means comprises a resistance wire located within said substantially porous body.

6. The device of claim 1 wherein said heater means comprises a thin film located on a surface of said substantially porous body.

7. The device of claim 1 wherein said heater means comprises an electrically conductive material located within said substantially porous body such that said body will increase in temperature upon the application of an electric current thereto.

8. The device of claim 1 wherein said substantially porous body comprises an alpha-alumina ceramic.

9. The device of claim 1 wherein said porous body possesses an average pore diameter of approximately 10 to 200 μ and a void volume of approximately 40 to 60%.

10. The device of claim 1 further comprising means to monitor and control the temperature of said heater means.

11. The device of claim 10 wherein said temperature monitoring device comprises a thermocouple.

12. The device of claim 1 further comprising a substantially porous filter means substantially enveloping said substantially porous body and further being characterized as being capable of substantially filtering solid particulate matter located in said gaseous stream from contacting said substantially porous body but allowing the penetration of the gaseous stream therethrough so that the gases can contact and be analyzed by said analyzer means.

13. The device of claim 12 wherein said porous filter possesses an average pore diameter of approximately 10 to 200 μ.

14. The device of claim 1 wherein said analyzer means comprises a gas analyzer for providing an electrical signal in response to a specific concentration of gases contained in said gaseous stream.

15. The device of claim 14 wherein said analyzer includes two surfaces, a first surface being in contact with a catalyst for the oxidation of a gas, the concentration of which is to be measured within the gaseous stream and a second which is noncatalytic and nonreactive with the gases contained within the gaseous stream.

16. The device of claim 15 wherein said catalyst contains a Group VIII metal selected from the group consisting of platinum, palladium, iridium and rhodium.

17. The device of claim 1 further comprising an inlet tube for the introduction of gas from outside the moving gaseous stream to a location proximate said analyzer means.

18. The device of claim 1 wherein said substantially porous body is thermally conductive.