Abstract:
A shield is disclosed that is particularly useful for protecting exposed optical elements at the end of optical probes used in the analysis of hazardous emissions in and around an industrial environment from the contaminating effects of those emissions. The instant invention provides a hood or cowl in the shape of a right circular cylinder that can be fitted over the end of such optical probes. The hood provides a clear apertures through which the probe can perform unobstructed analysis. The probe optical elements are protected form the external environment by passing a dry gas through the interior of the hood and out through the hood aperture in sufficient quantity and velocity to prevent any significant mixing between the internal and external environments. Additionally, the hood is provided with a cooling jacket to lessen the potential for damaging the probe due to temperature excursions.

Description:
STATEMENT OF GOVERNMENT INTEREST  
       [0001] This invention was made with U.S. Government support under contract no. DE-AC04-94AL85000 awarded by the U. S. Department of Energy to Sandia Corporation for the management and operation of Sandia National Laboratories. The Government has certain rights in the invention, including a paid-up license and the right, in limited circumstances, to require the owner of any patent issuing in this invention to license others on reasonable terms. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    The present invention relates to a device for transmitting and receiving light from a remote sample. More particularly, the present invention relates to a system for cooling a manifold containing a high energy optical probe used for measuring the optical response of a remote sample to a high energy stimulus.  
           [0003]    The ability to monitor particulate matter in process streams and emissions to the air from industrial operations, and particularly the ability to do so in-situ and in real-time, is becoming increasingly important in many industrial processes. This is the case not only because of the desire to control and modify various processes in real-time to improve their efficiency but also to comply with various environmental regulations governing the composition and quantity of various industrial emissions.  
           [0004]    In particular, atmospheric emissions of hazardous and/or regulated materials is now strictly controlled by certain state and federal regulatory authorities. Not only are industrial operations required to closely monitor their air emissions but also they are required to do so on a continuous basis. Measurement of concentrations of hazardous materials in stack emissions, however, is a difficult task. Currently, air emissions from industrial operations are measured using extractive sampling followed by off-line chemical analysis; a procedure that is costly and typically having analysis times ranging from days to weeks. Furthermore, certification tests require that more than one sample be taken for a given operating condition and that at least one sample be taken for each operating condition and the many manual operations involved in extractive sampling introduce a significant potential for sampling error. The long turnaround times inherent in extractive sampling prevent the use of manual air emissions measurements as a realistic method for controlling operating parameters in real-time.  
           [0005]    A wide variety of instruments are currently available for on-line analysis of flow streams. However, the optical probes that these instruments use are typically designed for analysis of the concentration of constituents in fluid streams. Furthermore, many of these instruments employ beam dividers or splitters, an arrangement which causes more than 75% of the available light to be lost. Because of the requirement for a second probe that receives light transmitted through the sample, instruments that operate in the transmittance mode are generally unsuited for use in the harsh environments of stack emissions from boilers, incinerators, furnaces and the like.  
           [0006]    A method for circumventing many of the problems associated with analysis delays and sampling errors was disclosed in commonly owned U.S. Pat. No. 5,953,120 (herein incorporated by reference). Here, a compact optical probe is provided that is useful for analysis of stack emissions in industrial environments by means of laser spark spectroscopy. The geometry of the prior art optical probe provides a means for making optical measurements in environments where it is difficult and/or expensive to gain access to the vicinity of a flow stream to be measured. However, since this probe is exposed to the environment of the flow stream the optical elements at the probe end inserted into the flow stream suffers from ash fouling. Furthermore, this probe cannot be placed into flow streams where temperatures may exceed more than about a few hundred degrees Fahrenheit.  
           [0007]    For the reasons set forth above, it is highly desirable to have an optical probe that permits measurements to be made at a plurality of locations within a flow stream, is rugged enough to be used for monitoring emissions from industrial boilers, incinerators and furnaces and can introduce an optical input of sufficient intensity into the flow stream to heat an entrained particle to a plasma and thus induce an optical response. It is further desired that the probe should be reliable, easy to handle and operate, and robust enough for continuous unmanned operation within a dirty, hostile environment.  
           [0008]    The present invention provides an optical probe whereby all of its optical components (source, detector, relay optics, etc.) are either located external to the flow stream being monitored or protected by isolating them from the external stack environment thereby permitting a rugged and robust system. The geometry of the optical probe disclosed herein thus provides a means for making optical measurements in environments where it is difficult and/or expensive to gain access due to the hostile natures of the flow stream, making it particularly useful for remote sampling operations in industrial environments.  
         SUMMARY OF THE INVENTION  
         [0009]    The prior art optical probe modified by the present invention comprises a laser spark spectroscopy system for directing a focused incident light beam onto a plurality of analysis locations within a flow stream, creating a spark discharge in the flow stream to ionized any entrained particle or particles, and collecting a return light beam from each of the plurality of analysis locations. The improvement to the optical probe include a means for shrouding and purging the end of the probe inserted into the flow stream. The optical probe also includes an optical fiber means for receiving light from the return light beam in the vicinity of the final focusing lens.  
           [0010]    In one aspect of the present invention, the optical probe includes a cowl or shield at the insertion end of the probe that further includes an orifice, or aperture, through which the incident beam may interact with an effluent flow stream and through which some of the emitted light created by the interaction may be collected. This embodiment is particularly advantageous, because it completely protects sensitive exposed optical surfaces from the contaminating effects of the flow stream ash particulate matter. This embodiment also provides conditions around the insertion end of the probe that do not significantly disturb the surrounding local flow stream environment.  
           [0011]    In another aspect of the present invention the shield includes an interior wall that terminates in a conical funnel-section adjacent to the probe aperture.  
           [0012]    In still another aspect of the present invention the shield is purged with a dry gas such a dry air or nitrogen.  
           [0013]    In another aspect of the present invention the probe aperture is sized to provide only so much clearance as is necessary to permit focusing the laser optics at a point up to several feet beyond the laser source and to permit acquisition of the return light signal by an optical fiber situated at the periphery of the final focusing lens.  
           [0014]    In another aspect of the present invention the shield includes a water cooled jacket .  
           [0015]    In yet another aspect of the present invention the cooling jacket extends the full length of the probe. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    [0016]FIGS. 1A and 1B shows schematic illustrations of embodiments of two possible a prior art optical probes.  
         [0017]    [0017]FIG. 2 shows a schematic illustration of an embodiment of the present invention showing the location of the gas plenum and gas distribution means as well as the angled conduit for holding the optical fiber.  
         [0018]    [0018]FIGS. 3A and 3B show cutaway illustrations of two possible configurations of the end of the probe shield of the present invention illustrating the cooling jacket. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0019]    The present invention provides an improved optical probe useful for measuring the optical response of a particle that is heated and ionized by a high power energy source. For the purposes of the present discussion it is understood that the term “particle” is meant to include a single particle, or an ensemble of particles that may comprise any or all of solid matter such as dust, smoke, or fly ash; liquid droplets such as an aerosol; and “gaseous” complexes of organic and/or inorganic species such as VOC&#39;s (volatile organic compounds), organometallic compounds, and compounds of heavy metals.  
         [0020]    The improvement of the present invention generally comprises an optical probe that includes a cowling or shield covering the end of the probe that is placed in the flow stream during operation; the shield being designed to protect the optical surfaces of the probe from the contaminating effects of the matter entrained in the flow stream.  
         [0021]    The principal of operation of the optical probe is well known in the art. In particular, a short discussion is presented in commonly owned U.S. patent Ser. Nos. 5,777,734, issued Jul. 7, 1998 and 5,953,120 issued Sep. 14, 1999 and herein incorporated by reference. The prior art technique uses a pulsed laser to rapidly heat either a single particle, or an ensemble of particles, to produce a plasma (or laser “spark”) that dissociates and ionizes some of the atoms comprising the particle. As the plasma cools, the excited species relax and decay through a specific sequence of energy states emitting optical energy at frequencies that are characteristic of the emitting elements at each energy state. Spectrally resolved measurements of these emissions can be used to identity and quantify the elements comprising the particle.  
         [0022]    Regardless of the design of the probe, however, all need to project a high intensity high power, focused beam of light into the monitored flow stream in order to heat and ionize some portion of the atoms comprising one or more particles entrained in the stream and thereafter detect the light response emitted by the ionized atoms as each decay to successively lower energy states. To accomplish this an optical window must be provided allowing the light beam to enter the flow stream while isolating the analysis equipment from the stream. Generally, one end of the probe itself is placed some distance into the flow stream and final focusing lens provides the clear aperture as well as the isolation protection.  
         [0023]    Unfortunately, most, if not all, monitored sites comprise an effluent stack delivering a sizable quantity of hot particulate ash that has a strong tendency to quench, and deposit onto any obstructing surface within its path. It has been observed that prior art optical probes are quickly fouled in use and require frequent removal and cleaning in order to maintain the aperture. However, even with frequent cleaning, operation between cleanings is none-the-less subject to the continuous degrading effects of surface deposits making it difficult to obtain reproducible measurements over time since the quantity of light passing back through the aperture to the probe photodetectors is heavily influenced by the degree to which the probe lens is coated with particulate ash.  
         [0024]    To solve this problem, an ingenious shrouding structure or member has been design to fit over the end of the prior art optical probe to prevent entrained ash from making contact with any of the optical surfaces of the probe.  
         [0025]    Two embodiments of the prior art optical probe are shown in FIGS. 1A and 1B. A descriptive account of the various components comprising these probes is provide in order help clarify the operation of the probe and the manner in which the present invention operates in cooperation with these devices. In the illustrates optical probe  100  is shown penetrating an industrial stack  102  wherein focusing lens  130  can be translated by mechanical means  160  or  161  such that new analysis locations can be acquired. Here, light beam  110 , issues from high energy light source  115 , typically a Nd-YAG Q-switched laser, and is collimated by collimating lens  105  or by beam expanding telescope  210  consisting of lenses  211  and  212 . The collimated beam  110  then passes through an aperture  120  in a mirror  125 , or re-directed by mirror  220 . Collimated beam  110  can then be focused by focusing lens  130 , onto any selected point A within the stack, for analysis of the particles instantaneously entrained within a volume around selected point A at any particular time at which light source  115  is operated. A portion of the optical response of the particles within the volume around point A re-enter through lens  130  to be re-formed and collimated into returning light beam  140 . Returning light beam  140  is reflected by mirrors  125  and  127  and directed onto lens  145 , which act to provide a means for focusing returning light beam  140  onto light transmitting sensor  150 , which can be the face of a optical fiber cable  250  that transmits light beam  140  to a detection means (not shown). New analysis points A i  (where i=1, 2, . . . , n) within the stack can be acquired by translating lens  130  such that the focal point of lens  130  is focused on a different analysis point A i  (not shown). No other changes in the optical probe are required.  
         [0026]    To protect focusing lens  130  from ash deposition the end of probe tube  101  is re-configured, as shown in FIG. 2, to include shield  300 . In it simplest form shield  300  comprises a metal shell opened at one end to cover the sensitive optical elements at the distal end of the probe inserted into the effluent stream. At the opposite end of this metal shell a smaller opening is provided as a clear aperture for the focused light beam exiting final focusing lens  130 . The shield therefore, provides several inches of separation between the optics of the probe and the external stack environment.  
         [0027]    However, by itself simply covering and separating the probe optics from the stack environment is not sufficient to prevent fouling of the lens surfaces. A second strategy is provided that includes a blanketing layer of flowing gas that is used to significantly lessen the extent to which the atmosphere in stack environment mixes with the atmosphere contained within the interior region enclosed by the shield. To accomplish this a gas plenum is hermetically sealed to a region of the shield exterior and connected with a gas line with which to feed gas into the plenum. Several small ports or orifices are cut through the wall of the metal shell of the shield such that each opens to the gas plenum. Gas can then be moved through the gas line, into the plenum and orifices into the interior region enclosed by the shield and out through the opening provided in the end of the shield for the light beam.  
         [0028]    Finally, to prevent damage due to high temperature excursions the shield is fitted with a cooling jacket within which to circulate a cooling fluid such as water.  
       EXAMPLE  
       [0029]    Shield  300 , therefore, is described as follows. Front and rear housing portions  310  and  320 , are provided wherein front portion  310  itself comprises coaxial inner and outer shells  303  and  302  joined together at a rearward end by a common base portion  302 ′ and at a forward end with cover  301  to provide a large enclosed interior region  400  surrounding the probe optical elements. In addition, inner and outer shells  303  and  302  are held apart from one another through a plurality of internal struts  306  to provide a first internal manifold  308  that, when connected with fluid input and output lines (not shown) through which a cooling media can be introduced, manifold  308  acts as a cooling jacket. Importantly, cover  301  includes clear opening  311  to allow a beam of light, focused by lens  130  along a central axis  350 , to exit shield  300  and also to allow passing of a flowing stream of dry gas from the interior region  400  of shield  300 .  
         [0030]    The critical feature of the present invention is the ability to isolate interior region  400  from the stack environment while maintaining a clear aperture for transmitting and receiving a light signal. To accomplish this the invention provides a means for passing a quantity of dry gas from the interior of shield  300  and out through opening  311 . To accomplish this, interior shell  303  is provided with forward and rear wall portions  304  and  305 , wherein forward portion  304  comprises a funnel-shaped surface extending from the inside edge of opening  311  to rear wall  305 . Furthermore, to provide a means for flowing gas through interior region  400 , gas plenum  340  is provided along the parameter of base portion  302 ′ and hermetically joined to a gas input line. Plenum  340  is formed by cutting a recessed channel  341  into base portion  302 ′ and covering and hermetically sealing recessed channel  341  by welding or brazing a covering band  342  along the two edges of channel  341 . Plenum  340  is then placed in fluid communication with interior region  400  by a single row of evenly spaced ports or orifices  312  disposed around the parameter of inner shell  303 , penetrating rear wall  305  and connecting interior region  400  with plenum  340 . Gas, typically dry nitrogen, dry air, or the like, can then be fed through plenum  340  into interior region  400  and out of opening  311 . Funnel-shaped forward wall  304  acts to concentrate and provide a generally streamline plug flow of gas passing through opening  311 . The cone angle of the funnel end of the wall is not particularly critical, but generally depends on the size of opening  311  and the mass flow rate of the gas exiting the opening. As noted, the intent is to provide generally laminar plug flow, i.e., flow having a low Reynolds number (generally below about N Re =1500). Flow in this range generally minimizes turbulence at the opening and thus minimizes mixing between the stack environment and the shield interior effectively isolating the optical surfaces of the probe from the stack environment without unduly constraining the probe aperture.  
         [0031]    Finally, shield  300  further comprises a further feature: an angled conduit  330  passing through outer shell  302  and interior shell  303  through which an optical fiber (not shown) may be passed in order to gain access to interior region  400 . Conduit  330  is positioned near the periphery of focusing lens  130  so as to the central axis  309  of conduit  330  passes directly through focal point A of lens  130 . The purpose of the optical fiber is to receive a portion of the light produced by the ionizing plasma created by the laser spark. This configuration eliminates many of the components described by the prior art optical probes and allows the user to configure a probe of nearly any useful length.  
         [0032]    The principal of operation of the shield configured probe is as follows. The optical probe configured with the protective shield is readied for insertion into an effluent flow stream by initiating a flow water through the manifold  308  (if the temperature of the environment requires cooling), and a flow of dry gas through a series of ports or orifices  312  penetrating interior shell  303  and issuing from an internal plenum (not shown) contained between inner and outer shells  303  and  302  and disposed along the parameter of the outer shell . Dry gas is thus directed into interior region  400  in sufficient quantity to provide a protective barrier against the migration of effluent particles in the flow stream into the interior region of the probe shield where they might deposit on optical surfaces. The amount and rate of gas flow is dependent upon the size of opening  311  which is itself dependent on the aperture of focusing lens  130  and the position and angle of conduit  330  in order that the focused laser beam can exit shield  300  unimpeded and so that the detecting optical fiber has an unobstructed view of the focus region. In the present invention the distance from lens  130  to the outside surface of cover  301  is about 2 inches while the diameter of opening  311  is about 0.94 inches. Other dimensions are of course possible, but the overall intent is to provide a reasonable volume within which to build a plug flow of gas out through opening  311  while maintaining a clear field of view for both focusing lens  130  and the optical fiber deployed in conduit  330  when focused on point A. The flow of gas is maintained below a flow rate that would interfere with the natural flow of matter within the effluent flow stream but high enough to provide isolation from any convective mass transfer through gas mixing. A rate of about 12 liters/minute was found to be effective in the present invention but fractionally higher or lower value are likely to be as effective.  
         [0033]    Once inserted into the effluent flow stream the primary laser is activated and low frequency pulse operation (˜10 Hz) begun. As effluent particles are captured within the plasma generated at the laser beam focal point and ionized the characteristic light emissions given off by the atom as it returns to a ground state are collected by the optical fiber inserted into conduit  330  and returned to a standard light photometer/spectrometer for analysis. Additionally, the cooling medium circulating through manifold  308  and the gas flowing out of interior region  400 , isolate and protect the optical elements within the end of the probe from the degrading effects of the hot gas and particles in the effluent stream. FIGS. 3A and 3B show cutaway illustrations of two possible configurations of the end of the probe shield of the present invention illustrating the cooling jacket. Other configurations are of course possible. The arrows shown in the illustrations show the path of the cooling medium as it enters and leaves the shield end.  
         [0034]    From the foregoing description and examples, therefore, one skilled in the art can readily ascertain the essential characteristics of the present invention. In particular, It will be appreciated that the shield of the present invention can be also be used with any of a variety of other optical techniques that rely on measuring an optical response in a hostile environment. Optical techniques for which the improvement to an optical probe provides an advantage comprise; laser spark spectroscopy, Raman scattering, laser-induced fluorescence, Rayleigh scattering, Mie scattering and laser-induced incandescence. The description and examples are intended to be illustrative of the present invention and are not to be construed as limitations or restrictions thereon, the invention being delineated in the following claims.