CA1143174A - Electro-optical flue gas analyzer - Google Patents

Abstract

IN THE UNITED STATES PATENT AND TRADEMARK OFFICE
APPLICATION FOR UNITED STATES LETTERS PATENT

TITLE: ELECTRO-OPTICAL FLUE GAS ANALYZER
INVENTOR: EDWIN D. NELSON
Abstract of the Disclosure Apparatus is disclosed for the simultaneous measurement of opacity, carbon monoxide content and hydrocarbon content of flue gases with such measurements taken along a single measure-ment path across a flue by visible light energy and infrared energy traversing the flue along such path.

Description

14 . Background of the Invention This invention re1ates to apparatus for the measure-16 ment of certain properties of furnace gases which are expelled 17 through a flue. Such measurements are used to control the air-18 to-fuel ratio of the fuel and air mixture being fed to the 19 furnace, with the object being to munimize wastage of fuel due to the heating and subsequent discharge to the atmosphere of 21 heated excess air.
22 Modern combustion control practice involves the 23 simultaneous measurement and control of a numher o~ properties 24 of exhaust gases from furnaces utilizing fossil fuels. In the : 25 usual case the carbon monoxide level of the gases is measured 26 and controlled to a predetermined level or target. At high 27 air-to-fuel ratios the carbon monoxide level is usually at a 28 low level of about one hundred parts per million. ~s the oxygen 29 abundance just falls below the level necessary to completely oxidize all of the ~arbon and hydrogen in the fuel, the carbon _32 .

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3~4 1 monoxide level begins to rise rapidly. It is at the beginning

2 ~ of this rise in carbon monoxide level that the carbon monoxide

3 , control point is generally set.

4 Under certain conditions, other limitations to

5 I reduction of the air-to-fuel ratios are reached before the

6 carbon monoxide level begins its rapid rise. In some cases

7 elemental carbon will precipitate before burning, causing higher

8 smokestack opacity. In other cases, poor burner adjustment or ~ burner tip fouling can lead to undesirably higher levels of hydrocarbons. Accordingly, in order to control the operation 11 of the furnace in a most desirable manner it is important to ~ measure simultaneously the levels of carbon monoxide and hydro-13 carbons and the degree of opacity of the gases being exhausted 14 up the flue.
There are a number of well known methods of measuring 16 the levels of carbon monoxide and hydrocarbons in combustion 17 gases. The preferred method i5 by infrared absor~tion spec-18 trometry. In this method infrared light energy at selected 19 wavelengths is transmitted through a sample of the gas to be analyzed. The le~el of the infrared energy passing through the 21 gases is then compared with the infrared energy level in the 22 absence of such ~ases. The resulting measurement of the 23 absorption of the energy at these wavelengths provides a 24 qualitative and quantitative measurement of the gas constituent ~ levels, in this case carbon monoxide and hydrocarbons.
26 There are also well known methods for measuring the 27 opacity of the combustion gases. ~ preferred and well known 28 method is to transmit a source of photopic light energy 2~ (equivalent in wavelength distribution to the response of the human eye) through the gases to be analyzed and then to measure 31 ~/

3~

1 the amount of light transmutted, the opacity being defined as 21 unity minus the transmission.
31 Measurement of the combustion gas composition is best 4 done directly in the flue or smokestack to eliminate effects of ~¦ stratification of the gas constituents. To protect the measuring 61 instrument from the hot, corrosive combustion gases, special 7¦ windows isolate the device from the gases. Over a period of 81 time, dirt tends to build up on the windows and to cause errors ¦ in the flue gas analysis. Also, changes in the source output 101 and detector sensitivity can cause errors. One way of reducing 11¦ such errors is by the provision of a sample chamber extending 12¦ across the flue and which can periodically be purged with clean 13 air for purposes of calibrating the apparatus.
14¦ It is particularly desirable to measure the levels of lS¦ carbon monoxide and hydrocarbons and the opacity of the flue 16 gases simultaneously, and preferably in a single sampling 17¦ volume passing through the sample chamoer. Thus, while two 18¦ separate beams of energy, one infrared and one visible, could 19¦ be transmitted through a single sample cha~ber, the resultant 20¦ size of the sample chamber would tend to be so large for the 21¦ two beams that the sample chamber itself could cause undesirable 22¦ turbulence which could interfere with the periodic purging 231 thereof. Heretofore, it has thus been impractical to integrate 241 these control and measuring devices with a desirable sample 2~1 chamber.
2~ Summary of the Invention ~7 To overcome the disadvantages of the prior art, it is 2B an object of the present inVentiQn to provide apparatus for ~9 simultaneous measurement of opacity, carbon monoxide content 31 and hydrocarbon aontent of flue gases which is suitable for use ~ 3~'7~ ~

1 ¦ with relatively small and efficient sample chambers extending 2 ¦ transversely of a flue. It is a further object of the invention ¦
3 j to provide such apparatus in which all of the measurements are 4 taken along a single measurement path extending across a flue.
3 It is yet another object of the invention to provide such 6 apparatus in which the source of visible light energy is 7 separate and independent from the source of infrared energy used 8 for such measurements.
91 To achieve the foregoing as well as other objects, 10¦ this invention provides apparatus for the simultaneous measuremen4 11¦ of opacity, carbon monoxide content and hydrocarbon content of 12¦ flue gases along a single measurement path across a flue. The 13¦ apoaratus includes a sample chamber extending transversely of the 14¦ flue from one side to the other and through which the flue gases 1~¦ may flow, means defining a single axis optical path extending 16 longitudinally through the sample chamber and including focusing 17¦ means adjacent one side of the flue and an optically reflective 18¦ member adjacent the other side of the flue, a source for 19¦ producing a predetermined level of visible light energy, a 20¦ source for producing a predetermined level of infrared energy 21¦ independen~ of the visible light energy source, means for 22¦ sensing the level of the visible light energy which has traversed 231 the sample chamber along the optical path to the reflective 241 member and back and means for sensing the level of the infrared 2~1 energy which has traversed the sample chamber along the optical 26 path to the reflective member and back. The infrared energy 2~1 source provides such infrared energy in predetermined spectral 28¦ bands between about 3 microns and 5 microns and directs this 291 infrared energy along the single axis optical path toward the 301 reflective member.

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1 BrieE Description of the Drawings A particularly preferred embodiment o~ the apparatus of this invention will be described in detail below wi-th respect to the drawings in which:
FIGURE l is an exploded view of the general components of the optical flue gas analyzer apparatus of this invention, and FIGURE 2 is a side elevation, partially in section, of the apparatus of this invention as installed in a flue.
- Detailed Description of a Preferred Embodiment In the exploded view of FIGURE l and the schematic side elevation, partially in section, of FIGURE 2 is illustrated a particularly preferred embodiment of the apparatus of this invention. This apparatus includes a sample chamber assembly 2 extending transversely oE the flue 4 from one side of the flue to the other side. When this sample ~hamber is in the position illustrated in FIGURE l, the gases of combustion, indicated generally by the large arrow 6, may flow through the sample chamber. This sample chamber 2 suitably is of the type illustrated in the applicant's United States Patent ~,206,63Q
which issued June lO, 1980, entitled "Sample Chamber for Gas Analy~er". Since this sample chamber apparatus is disclosed in great detail in that only a portion of one end of that sample chamber assembly is illustrated in FIGURE l.
At one end of the sample chamber 2 is provided a window 8 (shown in phantom) of a suitable material transparent to both visible and infrared energy, forming a window through the side of the flue 4. Outside and directly adjacent that side of the flue is mounted a corner cube retro-reflector, which :

,~

~ ~3~4 1 comprises an opticallv reflective member reflectiv= of both 2 visible and infrared enexgy.
3 ¦ A second window 8', likewise transparent to both ~ ¦ infrared and visible energy~ is also provided through the side of the flue opPosite window 8. Adjacent that second window 8' 6 ¦ is provided a suitahle focusing means, illustrated schematically 71 in FIGURE 1 by a lens 12. If desired, the lens 12 may be used B¦ in place of the window 8'. Suitably the windows 8 and 8' and the

9 ¦ focusing means 12 may be formed of calcium fluoride or other

10¦ material transmissive to both infrared and visible light. The

11¦ optical axis of the focusing means 12 and the retro-reflector 10,

12¦ extending longitudinally through the sample chamber 2, define

13¦ an optical path for ener~y to traverse the sam~le chamber from

14¦ the focusing means 12 to the retro-reflector 10 and back through

15¦ the chamber 2 to the focusing means 12. This single axis

16 optical path is conveniently denoted by reference number 14.

17 Preferably aligned with optical path 14, as shown in

18 FIGURE 1, is an incandescent lamp or other suitable source of

19 visible light energy having a~ least a substantial portion of

20¦ its energy in the spectral band between 500 and 600 nanometers.

21¦ This light energy source 16 directs its visible light energy out

22¦ to and along the optical path 14, as shown in FIGURE 1. The

23 ~ortion of ~he energy of this visible light source which is

24 directed along the optical path 14 is directed along a path extending through chopper and filter wheel 18. This chopper 26 and filter wheel 18 is mounted in a conventional manner for 27 rotation, driven by conventional means, at a predetermined 28 rate. This chopper and filter wheel 18 has, radially and 29 circumferentially spaced about its center of rotation a ~lurality of neutral density filters 22, 24, 26 and 28 31 ?ositioned to intersect the optical path aligned with ~ 31~ 1 1 ~ optica ath 14. ~he ch~pper wheel IB is als~ provided : 2 with a plurality of timing apertures cooperating with light 3 emitting diode 30 and phototransistor 32 in a conventional manner : ~ to provide timing signals to indicate when each filter and each space between filters is aligned with the optical path from 6 the light source 16 to the optical path 14. Similarly, one 7 additional hole is provided in the chopper wheel cooperating 8 with light emitting diode assembly 34 and phototransistor assembly 36 to provide an index signal to identify which of the filters or spaces between filters is aligned with the optical path .
11 Between the chopper wheel 18 and the focusing means 12 12 is then inserted a visible light beam splitter 38, suitably of 13 the type commonly referred to as a dielectric beam splitter, whic~
14 permits approximately half of the visible light energy from the source 16 to pass therethrough towards the focusing means 12.
16 Similarly aligned with the optical path 14 and between the beam 17 splitter 38 and the focusing means 12 is then positioned a beam 18 combiner/separator, which suitably is formed of glass with a 19 light metallic coating on the side facing focusing means 12.
~0 This beam combiner/separator 40 permits most of the visible 21 light energy to pass directly therethrough, thence to be focused 22 by the focusing means 12 and directed along the optical path 23 14 to the retro-reflector 10 and back through the focusing means 24 12 and the beam combinerJseparator 40 to the beam splitter 38.
The beam splitter 38 is angled such that approximately 26 half of the visible light energy received back from the retro-27 reflect~r and through the beam combiner/separator is then reflec-28 ted off the c~xis of optical path 14 through a suitcible focusing 2 lens 42 and a photopic filter which is selected such that about 90% ~f the energy passing therethrough will be in the 500 to 600 17~

1 ~ nanometer spectral range. From that photopic filter 44 the 2 I visible light energy is then directed to a conventi~nal silicon 31 detector which serves as a means for sensing the level of visible ; 4 ¦ light energy which has traversed the sample chamber along the optical path 14 and returned.
~ The beam combiner/separator 40 with its light metallic 7 coating on the side towards the focusing means 12 is highly 81 reflective of infrared energy. This beam combiner/separator 40 : 9 ¦ is also angled with respect to optical axis 14 such that infrared 10¦ energy received by it from the focusing means 12 along the 11¦ optical axis 14 will be directed off at an angle along another 121 optical axis 48. Aligned with this optical axis 48 is a suitable 13¦ source 50 of infrared energy covering at least the three to five 14¦ micron spectral range. Between this infrared source 50 and the 15¦ beam combiner/separator 40 is mounted for rotation in a suitable 16 manner and by conventional means a rotating filter wheel 52.
171 This filter wheel, which is illustrated containing five filters, 18 desirably contains at least three suitable interference filters 19 ¦ radially and circumferentially spaced about the center o~
rotation which will selectively transmit energy in the carbon 21 monoxide spectral band, in the hydrocarbon band and in a re~er-22¦ ence band in the region between 3.5 and 4.1 mucrons. The 23 reference band is chosen to avoid absorption by any gas component 24 in the flue; it allows a correction to be made for output changes that are unrelated to carbon monoxide and hydrocarbon concentra-26 tions in the flue. Suitably the filter wheel 52 may also have 27 coding holes cooperating with suitable light emitting diodes/
28 phototransistor pairs for purposes of indicating when each 29 filter and each space between filters is in position before the infrared source 50 for purposes of identifying samples in 3~1 the manner described above wi~h respect to chopper wheel 18.

32 Between the rotating filter wheel 52 and the beam ~ ~3~7~

1 combiner/separator 40 is interposed -the infrared beam splitter 54, comprising an uncoated germanium window. This infrared beam splitter 54 permits approxima-tely half of the energ~ from source 50 to pass therethrough and to be reflected by beam combiner/
separator 40 along optical path 14 through the sample chamber 2 to retro-reflector 10 and back to beam combiner/separa-tor 40.
There the energy is reflected down to the infrared beam splitter ~4 where about half of that energy received back from beam combiner/separator 40 is then reflected through infrared focusing lens 56 and onto infrared detector assembly 5~. This infrared detector assembly 58 may suitably comprise a lead selenide detector which is cooled by a thermo-electric device or other suitable means.
From the foregoing description of the apparatus it may be seen that chopped pulses of visible light may be directed along the optical path 14 through the sample chamber 2 and back to the detector 46 where the level of the visible light energy in the 500 to 600 nanometer spectral range may be measured by that detector and by suitable and conventional signal processing.
When the sample chamber is in the configuration illustrated in FIGURE 1 the flue gases, indicated by the large arrow 6, pass therethrough and the visible light signal will be attenuated by any carbon or soot content in those gases. When the sample chamber outer housing is rotated 90, in the manner described in detail in above mentioned U.S. patent 4,206,630, the sample chamber will be purged of the exhaust gases with only relatively clean air inside. During this purging condition the visible light source and detector system may be calibrated and standardized in order to provide absolute measurements of the opacity when flue gases are again permitted to flow through the sample chamber. Similarly, the absorption of the infrared energy from the source 50 in the different spectral bands selected by the g _ .~ .

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1¦ various filters of the filter wheel 52 may be determined by 21 measuring the level of infrared energy received by detector 58 31 both with the flue gases flowing through the sample chamber and 41 during the purged condition f~r ~standardization and calibration.
51 The particular structure of this invention i5 uniquely 61 advantageous in that both the visible light attenuation and the 71 infrared absorption are measured along a single axis optical 81 path and are measured simultaneously through the same position 91 in the flue. This is advantageous not only for purposes of 10¦ standarizing and controlling the sampling conditions but also 11¦ for permitting the use of a relatively small diameter sample 12¦ chamber which causes substantially less disturbance to the flow 13¦ of gases through the flue. While certain spectral bands of the 14¦ various filters have been noted above, it is of course understood 15¦ that numerous other combinations of spectral bands for the 1~¦ infrared filters and density l~vels for the neutral density 17 filters used with the visible light source may be incorporated.
18¦ Various combinations of such filters will be used in order to 19¦ meet the necessary control guidelines specified by the 20¦ Environmental Protection Agency as provided, for example, in 21 ¦ 40 C. F.R. Parts 60 through 99 relating to protection of the 22 ¦ environment. For different types of furnaces and different 231 fuels, different combinations will be required. It is also to ~41 be understood that the signals generated by the visible light 2~1 detector 46 and the infrared detector 58 may be processed in 26¦ any suitable manner, preferably digitally, to obtain the desired 27 data in the desired format.
28 While the foregoing describes a particularly preferred 29 embodiment of the apparatus of this invention, it is to be 301 understood tha~ it is illustrative only of the principles of the 31 invention and is not limitative thereof. Accordingly, since 32 numerous variations and modifications of the structure, all , -10-~3~7~

1¦ within th scope oi the inventlon, will readily occur to those 2 ¦ skilled in the art, this invention is to be limited solely by ' 3 the clai~ appended heret~.
1,~

'51 ~ 181 ~

29 .

Claims (17)

1. Apparatus for the simultaneous measurement of opacity, carbon monoxide content and hydrocarbon content of flue gases along a single measurement path across a flue, comprising a sample chamber extending transversely of the flue from one side of the flue to the other side and through which the flue gases may flow;
means defining a single axis optical path extending longitudinally through said sample chamber, said optical path defining means including focusing means adjacent said one side of said flue and an optically reflective member adjacent said other side of said flue, whereby the optically reflective member may serve to reflect a beam of radiant energy from said focusing means back along the optical path through said focusing means;
a source for producing visible light energy and for directing said visible light energy along said optical path toward said reflective member;
a source for producing infrared energy independent of said visible light energy source and for providing said infrared energy in predetermined spectral bands between about 3 microns and 5 microns and for directing said infrared energy along said optical path toward said reflective member;
means for sensing the level of said visible light energy which has traversed said sample chamber along said optical path through said focusing means to said reflective member and back through said focusing means; and means for sensing the level of said infrared energy which has traversed said sample chamber along said optical path through said focusing means to said reflective member and back through said focusing means, whereby the levels of both the visible energy and the infrared energy traversing the sample chamber may be measured along a single optical path through the chamber.

2. The apparatus of claim 1 further comprising unitary means for both dividing said visible light energy into pulses of predetermined duration and spacing and for periodically applying neutral density filters of predetermined density to said visible light energy produced by said source and directed along said optical path.

3. The apparatus of claim 2 wherein said unitary means comprises a rotating chopper wheel having neutral density filters radially and circumferentially spaced about its center of rotation.

4. The apparatus of claim 1 further comprising means for dividing said infrared energy into pulses of predetermined duration, spacing and wavelength.

5. The apparatus of claim 4 wherein said infrared energy dividing means comprises a rotating member having a plurality of filters radially and circumferentially spaced about the center of rotation.

6. The apparatus of claim 1 further comprising beam splitting means for passing said visible light energy from said visible light source to said sample chamber along said single axis optical path and for reflecting off said single axis optical path to said visible light level sensing means visible light energy received back from said sample chamber.

7. The apparatus of claim 1 wherein said infrared energy source is displaced from said single axis optical path and wherein said apparatus further comprises beam combiner/
separator means intersecting said single axis optical path for transmitting said visible light energy therethrough and for reflecting said infrared energy from said infrared source onto said single axis optical path.

8. The apparatus of claim 7 further comprising beam splitting means interposed between said infrared energy source and said beam combiner/separator means for passing said infrared energy from said source to said beam combiner/separator and for reflecting to said infrared level sensing means the infrared energy received back from said sample chamber.

9. Apparatus for simultaneously measuring at least two components of flue gases flowing through a flue, comprising:
disposed on one side of the flue, at least two independent sources for producing beams of radiant energy of different spectra and for directing said beams along a single-axis measure-ment path extending transversely of the flue from one side to the other side;
a reflective member disposed on the other side of the flue for reflecting the beams of radiant energy back to the one side; and disposed on the one side of the flue, means for sensing the levels of each of said beams which have both traversed the path.

10. Apparatus as in claim 9 wherein the single-axis measurement path extending transversely of a flue extends through a sample chamber disposed with the flue.

11. Apparatus as in claim 10 wherein the single-axis measurement path is defined by:
focusing means adjacent a first end of the sample chamber, and reflecting means disposed adjacent another end of the sample chamber.

12. Apparatus as in claim 9 wherein one of the sources produces a beam of radiant energy of visible light and another of the sources produces a beam of radiant energy of infrared light

13. Apparatus as in claim 12 wherein the infrared light has a wavelength of between about 3 microns and 5 microns.

14. Apparatus as in claim 12 wherein the means for sensing comprises an infrared detector means.

15. Apparatus as in claim 9 further comprising unitary means for both dividing at least one of the beams of radiant energy into pulses and for periodically applying filters to the at least one of the beams.

16. Apparatus as in claim 15 wherein the unitary means comprises a rotating object having a periphery and having the filters spaced about the periphery.

17. Apparatus as in claim 9 wherein at least one of the at least two independent sources is displaced from the single-axis measurement path and wherein the apparatus further comprises beam combining means intersecting the single-axis for transmitting the radiant energy from the at least one of the at least two independent sources along the single-axis measurement path.