WO1992009877A2

WO1992009877A2 - Ftir remote sensor apparatus and method

Info

Publication number: WO1992009877A2
Application number: PCT/US1991/009574
Authority: WO
Inventors: Orman A. Simpson; Robert H. Kagann
Original assignee: Mda Scientific, Inc.
Priority date: 1990-07-16
Filing date: 1991-07-15
Publication date: 1992-06-11
Also published as: EP0541734A1; AU1567392A; JPH07503532A; EP0541734A4; WO1992009877A3; CA2087439C

Abstract

An apparatus (16) and method for analyzing one or more gases in a target area (10). A modulated infrared light source (40) provides an infrared beam (20) that is capable of being absorbed at different wavelengths by the gases being analyzed. An optical arrangement (30) transmits the infrared beam (20) from the light source (40) in a first path (22) across the target area (10) where a reflective element (26) is positioned to return the infrared beam back along the first path to the optical arrangement (30). Based upon the absorption spectrum of the received beam (28), a determination can be made of the gases present and the concentrations thereof in the target area (10).

Description

FTIR REMOTE SENSORS-APPARATUS AND METHOD

BACKGROUND OF THE INVENTION

The present invention relates to an apparatus and method for environmental monitoring of one or more gases in a specific target area. In particular, the present invention is an apparatus and method for determining the identity and quantity of airborne emissions at plants or other geographically defined locations, for example in the vicinity of chemical or wastewater plants or nuclear facilities.

Present environmental regulations by Federal, state and local governmental agencies impose certain environmental monitoring requirements on operators of plants that produce airborne emissions regarded as pollutants. It^' is anticipated that in the future regulations of this type will require even more stringent monitoring. Therefore, improved methods to conduct environmental monitoring of plants for airborne emissions can result in decreasing the burden of such an effort on the plant operator. In addition, improved monitoring capability benefits the regulatory agencies involved and ultimately the public in terms of better data on actual emissions so that appropriate emission control systems and programs can be implemented.

Present methods for monitoring for gaseous airborne components include both canister and open long path techniques. With canister techniques, the identity and the concentration of airborne pollutants in a specific geographical area can be determined by obtaining one or more samples of air from the area and performing an analysis of the samples. This technique consists of collecting an air sample in an evacuated canister at a location where the identities of the gases are required to be determined (i.e. the target area). The air sample may be subsequently taken to a laboratory where an analysis is performed.

Spectroscopic analysis is one method that may be used to identify gases present (and the concentration thereof) in a gaseous sample. Spectroscopic analysis is based on the determination that matter absorbs light at characteristic wavelengths. Accordingly, to identify components of a gaseous sample, a beam of light is transmitted through the sample and then the light beam is collected after it has passed through the sample. An absorption spectrum is then generated from the collected light beam. By comparing the sample's absorption spectrum with known reference spectra, the identities (and concentrations) of the components, such as pollutants, present in the gaseous sample can be determined.

As applied to the canister method described above, spectrographic analysis may be used to identify the gases present in a sample as well as the concentration. While being a reliable technique, the canister method is essentially only a "point monitor" of gases and the data obtained are not necessarily representative of the concentrations of gases over large distances. In order to overcome this disadvantage, multitudes of canisters can be placed in the target area and the results averaged in order to determine the overall compositions of the gases in the target area. Thus, even where automated, this method can be unduly burdensome. Moreover, the canister method is essentially historical in the sense that it provides a determination of the identity and concentration of gaseous components at only the point of time at which the sample is taken and thus is not necessarily a "real-time" reading.

In contrast to the canister technique, an analysis of the components in the atmosphere can be made directly in the environment across a portion of the target area, (i.e. the "open long path" technique). Known open long path techniques essentially are optical systems that transmit an optical beam across the target area and treat the volume of atmosphere through which the beam is transmitted as the "sample". The system is "open" in that the sampling is not done on a portion of the atmosphere contained in a canister or other closed sampling medium as such, but rather is open in the environment. Systems of this type can be utilized to identify and measure the concentrations of gases over large distances (such as over several hundred meters or more and even as far as 1 kilometer). Thus, with an open long path system, which may comprise a relatively small number of components, an analysis can be performed that otherwise would require numerous canisters and a separate laboratory facility. The open long path technique has the potential to be more readily manageable than the canister technique. Known open long path techniques are not without their own shortcomings however.

One known open long path system employs spectroscopic analysis in a bistatic configuration. A bistatic configuration is characterized by separate light transmission and reception units placed on opposite sides of the target area. The transmission and reception units must be aligned so that light transmitted from the transmission unit will enter the reception unit on the opposite side of the target area without any appreciable loss in light energy. This configuration presents difficulties associated with alignment especially when the analysis is required to be performed over long distances such as several hundred meters or more.

A bistatic open long path system, such as described above, can utilize quantitative Fourier transform infrared (FTIR) spectroscopic techniques for the analysis of the gaseous components therein. An FTIR system is described in "Remote and Cross-stack Measurement of Stack Gas Concentrations Using a Mobile FTIR System" Herget, Applied Optics, 2_1;635 (1982).

Another open long path method is described in U.S. Pat. No. 4,426,640 issued to Becconsall et al. The method described by this patent is not based upon spectroscopic analysis. The '640 patent discloses a laser scanning apparatus in which two laser beams (a detection beam and a reference beam) are used to determine the concentration of a selected gas in a specific area, such a plant site. The detection beam is tuned to the specific absorption wavelength of the gas to be monitored and the reference beam is adjusted to a wavelength that is significantly less strongly absorbed by the gas. The beams are combined and transmitted generally downwards from a tall tower overlooking the plant site. The combined beams are scattered by the ground, buildings, pipework, trees, etc. A portion of the combined laser light beams is scattered back in the direction of the scanning apparatus. Then, the device described in the *640 patent collects the portion of the scattered, reflected light and a detector produces electrical signals corresponding to the intensity of the collected light (electromagnetic radiation) . The concentration of the selected gas present in the monitored area is determined from the ratio of the electrical signals of the detection beam and the reference beam.

A limitation on the usefulness of the ~640 apparatus is that only one selected gas can be monitored at a time because the transmission wavelengths of the two lasers are specifically adapted for the one just the one selected gas. In order for the device described in the ^"640 to monitor additional gases, either the transmission wavelengths of the two lasers must be recalibrated or additional lasers must be incorporated. Essentially, the device described in the '640 patent requires an identification in advance of the specific gas to be monitored and then enables a concentration of that gas to be determined.

Accordingly, it is an object of the present invention to provide a monitoring system capable of readily measuring more than one gas in a target area.

It is a further object of the present invention to provide an open long path system that is readily alignable for the monitoring of one or more atmospheric components in a target area.

It is a further object of the present invention to provide for real-time, or near real time monitoring of gases in a target area.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention, there is provided an apparatus for analyzing gases in a target area that has at least one cooperative reflective element located on one side thereof. The apparatus comprises an infrared light source for producing a modulated infrared beam capable of being absorbed at wavelengths characteristic of the gases being analyzed and an optical arrangement for transmitting the infrared beam from the light source across the target area and receiving the infrared beam returned by the cooperative reflective element from across the target area. Based upon the absorption spectrum of the received beam, an determination can be made of the gases present and the concentrations thereof in the target area.

According to a second aspect of the present invention, there is provided a unistatic open long path system for analyzing gases in a target area. The system comprises an infrared light source located at one side of the target area for producing a modulated infrared beam capable of being absorbed at wavelengths characteristic of the gases being analyzed and an optical arrangement for transmitting the infrared beam from the light source across the target area. The system further includes at least one cooperative reflective element located on another side of the target area an positioned to receive the beam transmitted from the optical arrangement and to return it back along the path of incidence. The optical arrangement is capable of receiving the infrared beam returned by,the cooperative reflective element from across the target area. The identity and concentration of one or more gases in the target area can be determined by the absorption spectrum of the received beam.

According to a third aspect of the present invention, there is provided a method for analyzing gases in a target area comprising the steps of generating a modulated infrared beam that may be absorbed at different wavelengths by the gases, transmitting the infrared beam in a first path across the target area, reflecting the transmitted infrared beam at a location across the target area, receiving the reflected infrared beam, and comparing the absorption spectrum of the received beam to a library of absorption spectra characteristic of the gases to be analyzed in a storage library memory whereby one or more gases present in the target area can be identified.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is illustrated by reference to several embodiments thereof and a detailed description of the preferred embodiment, in which:

FIG. 1 shows a schematic of a first preferred embodiment of the present invention.

FIG. 2 shows a detailed schematic of the remote sensor portion of the embodiment of Figure 1.

FIG. 3 shows a detailed schematic of a second preferred embodiment of the present invention.

FIG. 4 shows a detailed schematic of another preferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS OF THE INVENTION

Figure 1 is a schematic of an arrangement utilizing a preferred embodiment of the present invention. As depicted in Figure 1, the preferred embodiment of the present invention is an open long path spectroscopic monitoring system having a unistatic configuration. A target area 10 may possess any shape and may be irregular, but is generally geographically contiguous. The target area 10 may have located upon it a facility that emits or has the potential for emitting components to the atmosphere the identity and concentration of which it an object of the present invention to determine. The facility may be a chemical plant, a manufacturing plant, a wastewater treatment site, a hazardous waste site, or a power generating plant. However, the present invention is understood not to be limited to these applications, and may be utilized for the monitoring of the gaseous components present in areas having other types of facilities located thereupon as well. When the target area 10 is a plant of the type mentioned above, the distance across the target area 10 may range up to a kilometer although distances even greater than a kilometer may be monitored as long as a path for optical transmission exists across the area. Furthermore, it is understood that the target area need not be contiguous with the entire plant property and it may be desired to define the target area to be a portion of the plant property or even to extend outside the plant property depending upon a determination as to where representative emissions from the plant may be monitored.

Figure 1 shows a remote sensor 16 located at one side 18 of the target area 10. The remote sensor 16 transmits an infrared light beam 20 that travels across the target area 10 in a first path 22. Located at another side 24 of the target area is a reflector element 26. The reflector element 26 is positioned and adapted to return the transmitted infrared beam 20 directly back along the first path 22 to the remote sensor 16 so that the reflected beam 28 returns to the remote sensor 16. The reflector element 26 is preferably a cooperative reflector, i.e. a component having high reflectivity back to the source. In the preferred embodiment, the reflector element is a cube corner retroreflector array. A light beam that impinges upon a cube corner retroreflee or array will be reflected back along its path of incidence if it strikes within the retroreflector's beam acceptance angle (typically up to 45 degrees) . Thus, the use of a cube corner retroreflector array facilitates alignment of the reflector element with respect to the remote sensor. However, other components may be used for the reflector element instead of a retroreflector array, such as a flat mirror or a Cassagrain telescope with a flat mirror at its focal plane.

Referring to Figure 2, there is depicted a schematic of the remote sensor 16 of Figure 1. The remote sensor 16 includes a optical arrangement portion 30 and an FTIR (Fourier Transform Infrared spectrometer) portion 32. The optical arrangement portion 30 includes an infrared light source 40 capable of producing a broad band infrared light beam 42. In a preferred embodiment, the infrared light source 40 is an incandescent filament, which produces an infrared beam encompassing wavelengths of 600 cm^"1 to 6000 cm^"1. The infrared light source 40 directs the infrared beam 42 to a beamsplitter 44 where the infrared beam 42 from the light source 40 is reflected along a path 50 through an aperture 54 and into a telescope portion 56 of the optical arrangement portion 30. The telescope can be of focal system design, such as Newtonian, Cassagrain, or Gregorian; or of off-axis design, such as off-axis Dall Kirkham, off-axis Newtonian, or off-axis focusing Dall Kirkham. (In the preferred embodiment, the telescope is a 14.5 inch Cassagrain telescope) . Within the Cassagrain focal system telescope shown 56, the infrared beam in the path 50 is reflected by a mirror arrangement 58 (preferably a parabolic mirror arrangement) . The mirror arrangement 58 includes mirrors 60 and 62. The infrared beam in the path 50 is reflected by the mirror 60 to the mirror 62 and then transmitted through a telescope opening 64 (as the beam 20) into the target area 10 (shown in Figure 1).

After being reflected by a reflector element, e.g. 26, as described above, the infrared beam 28 is returned along the first path 22 back into the opening 64 of the telescope portion 56. In the preferred embodiment, the telescope portion 56 is used for both transmitting and receiving the infrared beams 20 and 28, respectively. This is enabled because the reflector element, e.g. 26, is positioned and aligned to return a beam back along its path of incidence. Inside the telescope portion 56, the returned beam 28 travels along the same path as the transmitted beam but in the reverse direction back through the aperture 54 to the beamsplitter 44.

The beamsplitter 44 transmits a portion 66 of the reflected beam 28 through to a transfer optics group 68. The transfer optics group 68 includes mirrors 70 and 72 positioned to transmit the reflected beam, indicated by the numeral 80, to the FTIR portion 32. The FTIR portion 32 includes a mirror 82 that receives the beam 80 from the optical arrangement portion 38. The mirror 82 reflects the beam 80 into the Michelson design interferometer 90. In the preferred embodiment, the interferometer is a high speed, continuous-scan Michelson design interferometer. The interferometer 90 generates an interferometer output, e.g. interferogram signal 92, from the beam 80. The interferogram signal 92 propagates to another group of mirrors 94, 96, and 98 which focus the signal 92 from the interferometer 90 onto a detector 100. The detector 100 measures the power or intensity of the signal 92. In a preferred embodiment, the detector is a liquid nitrogen cooled photoconductive detector or a mercury cadmium telluride, an indium antimony, or a pyroelectric detector. The detector 100 and the Michelson interferometer 90 provide outputs 102 and 104 to a computer 106. The computer 106 identifies an absorption spectrum based upon these outputs. The computer 106 includes a memory 108 having a library of - 11 -

absorption spectra that have been developed under controlled conditions for specific gases in known concentrations. The computer 106 compares the absorption spectrum obtained from the remote sensor 16 to the known spectra in the memory library. In the preferred embodiment, a peak identification or a linear least squares fitting technique, or other quantitative data analysis techniques may be employed. By this comparison, the identity of the gases present in the target area 10 can be determined. In a preferred embodiment, the memory library includes identifying spectra for over 200 gases, however, the present invention may be adapted to analyze fewer than or more than 200 gases depending upon the needs of the user. Typical gases that may be monitored by the present invention include: SF₆, methylene chloride, methanol, ammonia, methane, chlorobenzene p- dichlorobenzene, CO, toluene, vinyl chloride. The computer preferably operates continuously and depending upon the speed of the computer, the analysis of the gases proceeds simultaneously or nearly so thereby providing real-time or near real-time monitoring.

The computer 106 can also be used to determine an analysis of the concentration of any of the gases identified as being present in the target area. The spectrum data stored in the computer memory 108 for each gas is based upon samples of each of the gases at a known concentration. By comparison of the observed spectrum with the library spectrum a determination of the concentration of the particular gas in the target area can be derived from Beer Lambert's law as is well known in the art. The light source of the present invention is readily capable of transmitting an infrared beam accurately over target area distances of up to 1 kilometer or more. Since the nominal sensitivity of the - 12 -

present invention at an optical path of 1 kilometer is approximately 1 part per billion, the identified gases can be quantified accurately.

Referring again to Figure 2, in a preferred embodiment of the present invention, a light source 110 such as a HeNe laser, is positioned inside in the FTIR portion 32. The light source 110 is associated with a mirror 112 for reflecting a beam of light from the light source 110 in the path of interferometer output beam 92 but in the reverse direction. The light source 110 is used for alignment of the internal optics as well as for wavelength calibration of the inter erometer.

Referring to Figure 3, in a preferred embodiment of the present invention, a scanning apparatus 116 is associated with the remote sensor 16. The scanning apparatus 116 is adapted to direct the transmitted beam 20 from the remote sensor 16 along one or more paths 22*, 22'' to one or more additional reflector elements 26', 26'', respectively, located at other positions on sides of the target area apart from reflector element 26. The scanning apparatus 116 may comprise an arrangement of mirrors that sequentially directs the light from the remote sensor 16 along the different paths. Alternatively, the scanning apparatus may include a motor or other driving apparatus attached to the remote sensor to turn it so that the telescope portion thereof is essentially aligned with each of the reflector elements. In the preferred embodiment in which the reflector elements are cube corner retroreflectors, a light beam transmitted to the retroreflector will be reflected back to the remote sensor as long as the angle of incidence is within 45 degrees of the perpendicular. This allows a 45 degree tolerance in the alignment of the retroreflector, thereby drastically reducing the time and effort required in conventional "open long path" systems to align the equipment. Accordingly, the scanning apparatus 116 allows a sampling of the atmosphere across different optical paths across different portions of the target area 10 to be made while utilizing a single remote sensor 16 and a plurality of reflector elements 26, 26', 26' ' .

In another preferred embodiment of the present invention, the remote sensor is adapted to transmit a modulated infrared beam across the target area. Referring to Figure 4, a remote sensor is depicted. The remote sensor includes a combined optical arrangement and an FTIR portion 120. The optical arrangement includes a light source 122 that may be similar or identical to the one disclosed in the previous embodiment. The optical arrangement directs the beam 124 from the light source to the interferometer module 126. In the interferometer module 126, the beam is modulated. The now modulated beam 128 is directed to the telescope portion 130 and is directed out (e.g. 132) to one or more reflector elements positioned across the target area, as described above. The reflected modulated beam is 134 returned back into the telescope portion 130 and through a beam splitter 136 which directs part 138 of the reflected modulated beam 132 into the detector module 140. The detector module 140 consists of a series of focusing mirrors and a detector which measures the power or intensity of the beam. As in the previous embodiment, the detector provides an output (not shown) to a computer that determines the presence and/or quantity (concentration) of one or more gases in the target area. Also, as in the previous embodiment, a laser 142 is provided for alignment purposes (comparable to laser 110). In this preferred embodiment, by transmitting a modulated - 14 -

infrared beam, a substantial improvement in the signal to noise ratio can be obtained.

As compared to prior bistatic open long path systems, the present invention overcomes the problems of alignment that are inherent in such systems. The ease of alignment afforded by the use of retroreflectors readily enables scans of the target area to be taken. Instead of analyzing a portion of the target area and then repositioning the apparatus at a different location, the preferred embodiments allow target area portions to be analyzed repetitively and/or sequentially without relocating the transmission/receiving unit. This is provided in part by the use of a single transmission/receiving unit and a plurality of retroreflector arrays. Since the preferred embodiment utilizes a single transmission/receiving unit, the light beam transmitted across the target area has only to be reflected back along its path of incidence to be collected and analyzed. The use of retroreflector arrays in a preferred embodiment facilitates alignment. The present invention possesses several advantages over the "640 device. The present invention, unlike the '640 device, is capable of detecting hundreds of gases simultaneously, or nearly so, that have absorption wavelengths within the infrared light region. Also, because the present invention relies on identification of an absorption spectrum for each gas, the present invention eliminates the need for the transmission of a separate reference beam required by the method of the '640 patent.

It is intended that the foregoing detailed description be regarded as illustrative rather than limiting, and that it be understood that it is the following claims, including all equivalents, which are intended to define the scope of the invention.

Claims

WE CLAIM:

1. An apparatus for analyzing one or more gases in a target area having a first cooperative reflective element on one side of the target area capable of returning an incident optical beam back along the path of incidence comprising: a. an infrared light source for producing a modulated infrared beam that is capable of being absorbed by the one or more gases being analyzed to produce a characteristic absorption spectrum; and b. an optical arrangement for transmitting the modulated infrared beam from said light source in a first path across the target area to the first cooperative reflective element and receiving the infrared beam returned by the cooperative reflective element back along the first path from across the target area whereby one or more gases present in the target area can be analyzed by the characteristic absorption spectrum associated with each of the one or more gases from the infrared beam received.

2. The apparatus of claim 1 wherein the optical arrangement for transmitting and receiving the infrared beam comprises: a telescope adapted and aligned with respect to said light source and the first reflective element so that a light beam from said light source is transmitted by said telescope in a first path across the target area to the reflective element and further so that the telescope is adapted and aligned with respect to the reflective element so that a beam of light returned by the reflective element along the first path is received by said telescope.

3. The apparatus of claim 1 further comprising: an FTIR interferometer connected to said optical arrangement for generating a signal representative of the absorption spectrum of the received beam.

4. The apparatus of Claim 1 further comprising: a scanning apparatus located with respect to said optical arrangement to transmit the infrared beam in a second path to a second reflective element located across the target area from said optical arrangement the second reflected element located apart from the first cooperative element.

5. The apparatus of Claim 1 further comprising: means for comparing the absorption spectrum of the received beam with known gas spectra in order to identify the gases present in the target area.

6. The apparatus of Claim 5 in which said means for comparing comprises a computer and said known gas spectra are stored in a computer memory connected to said computer.

7. The apparatus of Claim 1 further comprising: means for determining the quantity of the one or more gases identified in the target area.

8. A system for analyzing gases in a target area comprises: a. an infrared light source for producing a modulated infrared beam that is capable of being absorbed in absorption spectra characteristic of each the one or more different gases to be analyzed; b. an optical arrangement for transmitting the modulated infrared beam from the infrared light source in a first path across the target area and receiving the infrared beam from across the target area; and c. a first reflecting element located across the target area for directing the transmitted infrared beam from said optical arrangement back to said optical arrangement whereby the one or more gases in the target area can be analyzed from the absorption spectrum of the infrared beam received.

9. The system of Claim 8 further comprising: an FTIR interferometer connected to said optical arrangement for generating a signal representative of the absorption spectrum of the received beam.

10. The system of Claim 8 further comprising: means for comparing the absorption spectrum of the received beam with known gas spectra in order to analyze the gases present in the target area.

11. The system of Claim 8 further comprising means for determining the quantity of the gases identified in the target area.

12. The system of Claim 8 wherein the optical arrangement comprises a telescope.

13. The system of Claim 8 wherein the first reflecting element comprises: a cube corner retroreflector capable of reflecting light directly back along the path of incidence, thereby allowing measurements to be made over long pathlengths and facilitating alignment between the telescope and the retroreflector.

14. The system of Claim 13 wherein the reflecting means has a beam acceptance angle of 30 degrees.

15. The system of Claim 8 further comprising: a. a second reflecting means located across the target area displaced from the first reflecting element; and b. a scanning means for directing the infrared beam from said optical arrangement to said first and second reflecting elements.

16. The system of Claim 15 wherein the scanning means directs the infrared beam sequentially from said optical arrangement to said first and second reflecting elements.

17. A method for analyzing one or more gases in a target area comprising the steps of: a. generating a modulated infrared beam that may be absorbed at different characteristic spectra by the one ore more gases; b. transmitting the modulated infrared beam in a first path across the target area; c. reflecting the modulated infrared beam at a location across the target area back along its path of incidence; and d. receiving the reflected infrared beam from across the target area whereby one or more gases present in the target area can be analyzed by the characteristic absorption spectrum associated with each of the one or more gases from the infrared beam received.

18. The method of Claim 17 further comprising the step of: a. transmitting the modulated infrared beam in a second path; b. reflecting the infrared beam at a second location across the target area back along the second path; and c. receiving the reflected infrared beam from across the target area.

19. The method of Claim 17 further comprising the step of, generating an absorption spectrum from the infrared beam received from across the target area.

20. The method as claimed in Claim 19 further comprising the step of comparing the absorption spectrum of the reflected infrared beam with known gas spectra in order to identify the gases present in the target area.

21. The method as claimed in Claim 17 further comprising the step of determining the quantity of gases identified in the target area. - 21 -

22. A method as claimed in Claim 17 further comprising the step of modulating the infrared beam by the use of different modulation frequencies.

23. A method as claimed in Claim 17 further comprising the step of modulating the infrared beam by the use of different modulation phases.