Abstract:
A method of monitoring combustion properties in an interior of a boiler of the type having walls comprising a plurality of parallel steam tubes separated by a metal membrane. First and second penetrations are provided in the metal membrane between adjacent tubes on opposite sides of the boiler. A beam of light is projected through a pitch optic comprising a pitch collimating lens and a pitch relay lens, both residing outside the boiler interior. The pitch relay lens projects the beam through a penetration into the boiler interior. The beam of light is received with a catch optic substantially identical to the pitch optic residing outside the boiler interior. The strength of the collimated received beam of light is determined. At least one of the pitch collimating lens and the catch collimating lens may then be aligned to maximize the strength of the collimated received beam.

Description:
TECHNICAL FIELD 
       [0001]    The present disclosure is directed toward a method and apparatus for measuring combustion properties in an interior of a boiler, and more particularly toward a method and apparatus for measuring combustion properties in a boiler of the type having walls comprising a plurality of parallel steam tubes separated by a metal membrane without reconfiguring the steam tubes. 
       BACKGROUND 
       [0002]    U.S. Pat. No. 7,469,092, entitled “Method and Apparatus For The Monitoring And Control Of A Process,” describes a method and apparatus for the monitoring and control of a process using tunable diode laser absorption spectroscopy (TDLAS). Briefly stated, the TDLAS method and apparatus involves directing a beam of light, which may be a multiplexed beam of a number of distinct wavelengths, into a boiler combustion chamber to measure boiler combustion properties such as temperature and the concentration of various combustion species including CO, CO 2 , O 2  and H 2 O. The technique requires a line of sight through the boiler. In fact, many lines of sight are typically required as it is often desirable to measure combustion properties in multiple boiler locations. Typically a wavelength multiplexed laser beam is transmitted from a pitch optic to a catch optic on the opposite side of the boiler. Certain applications require up to 15 measurement paths, thus requiring 15 pitch/catch optic pairs and 30 boiler penetrations. 
         [0003]    Typical coal-fire boilers comprise walls made of a series of parallel steam tubes spaced by a metal membrane. The steam tubes are typically about 2 inches (5.08 cm) in diameter and occur on about 2.5 inch (6.35 cm) centers. The metal membrane between the tubes is typically about 0.5 inch (1.27 cm) wide and 0.375 inch (0.9525 cm) thick. To gain optical access for measurements using a wavelength-multiplexed laser beam optical access must be provided through the wall of the boiler. Known TDLAS apparatus require an approximately 2 inch (5.08 cm) diameter hole in the boiler wall to provide adequate optical access. 
         [0004]      FIG. 1  illustrates the current state of the art for providing optical access to a boiler interior. Referring to  FIG. 1 , the boiler wall  10  comprises a series of parallel steam tubes  12  separated by a metal membrane  14 . To provide the required 2″ hole for optical access, the tubes must be rerouted using tube bends as illustrated in  FIG. 1 . Once completed, the use of tube bends to provide the optical access works well. However, it is difficult and expensive to provide the required number of tube bends for satisfactory combustion monitoring. The problem stems primarily from the fact that in order to install even a single tube bend, the boiler must be shut down for a significant period of time. As a result, the tube bends and thus the TDLAS monitor can only be installed during a long planned outage. Planned outages occur only every one or two years. Thus, unfortunate timing may result in having to wait up to two years before a particular power plant will be in a position to purchase and install a TDLAS monitor. Thus, an apparatus for monitoring combustion properties within a boiler that eliminates the need for tube bends is highly desirable. 
         [0005]    The present invention is directed toward overcoming one or more of the problems discussed above. 
       SUMMARY OF THE EMBODIMENTS 
       [0006]    A first aspect of the disclosure is a method of monitoring combustion properties in an interior of a boiler of the type having walls comprising a plurality of parallel steam tubes separated by a metal membrane. The method comprises providing first and second penetrations in the metal membrane between adjacent tubes on opposite sides of the boiler without relocating the adjacent tubes. A beam of light is then projected through a pitch optic comprising a pitch collimating lens and a pitch relay lens, both residing outside the boiler interior. The pitch relay lens is optically coupled to the first penetration to project the beam into the boiler interior. The method further comprises receiving the beam of light with a catch optic residing outside the boiler interior. The catch optic comprises a catch relay lens optically coupled to the second penetration and a catch collimating lens optically coupled to the relay optic. The strength of the collimated received beam of light is determined. At least one of the pitch collimating lens and the catch collimating lens may then be aligned to maximize the strength of the collimated received beam. Embodiments may include both the pitch collimating lens and the catch collimating lens being aligned to maximize the strength of the received beam. The first and second penetrations may be elongated parallel to the steam tubes. The method may further comprise mounting the pitch optics in a pitch optics housing and the catch optics in a catch optics housing, with the pitch and catch relay lenses occupying an orifice in a leading wall of the pitch optics housing and the catch optics housing, respectively. In such an embodiment, the method may further comprise attaching proximal ends of first and second sight tubes to an exterior boiler wall with the first and second penetrations communicating with an interior of the first and second sight tubes, respectively. The pitch optics housing may be attached to the distal end of the first sight tube with the relay lens in optical communication with the interior of the first sight tube and the catch optics housing may be attached to the distal end of the second sight tube with the catch relay lens in optical communication with the interior of the second sight tube. 
         [0007]    Another aspect of the disclosure is an apparatus for sensing combustion properties in an interior of a boiler, the boiler comprising a plurality of parallel steam tubes separated by a metal membrane. The apparatus comprises a diode laser having a select lasing frequency. A pitch collimating lens is optically coupled to a beam generating diode laser. A pitch relay lens is optically couple to the pitch collimating lens, with the pitch relay lens being configured to project the beam from the laser into a first penetration in a first membrane between adjacent tubes. A catch relay lens is configured to receive a projected beam through a second penetration in a second membrane substantially opposite the first membrane. A catch collimating lens is optically coupled to the catch relay lens and an optical fiber is optically coupled to the catch collimating lens. A detector sensitive to the select lasing frequency in turn is optically coupled to the optical fiber. An alignment mechanism is operatively associated with at least one of the pitch and catch collimating lenses to provide for alignment of the collimating lenses with respect to the beam to maximize the quantity of light received by the detector. The pitch collimating lens and pitch relay lens and the catch collimating lens and the catch relay lens may be contained within a pitch housing and a catch housing, respectively, as described above with regard to the first aspect. Embodiments may further include first and second sight tubes attached at their proximal ends to the boiler exterior with the penetrations communicating with the interior of the sight tubes. In such an embodiment the pitch housings and catch housings can be attached to the distal ends of the first and second sight tubes, respectively, with the relay lenses in optical communication with the interior of the sight tubes. Embodiments may include alignment mechanisms operatively associated with each of the pitch and catch collimating lenses. The alignment mechanism may comprise means to tilt the collimating lens along first and second orthogonal axis with both the first and second orthogonal axes being substantially orthogonal to the projection beam. A data processing system may be operatively associated with the detector and the alignment mechanism. The data processing system receives data from the detector and causes the alignment mechanisms to align the operatively associated collimating lenses to maximize the strength of the beam. 
         [0008]    The method and apparatus for measuring combustion properties in an interior of a boiler described herein allows for detection of combustion properties without having to shut down the boiler to install tube bends to allow optical access. The method and apparatus therefore allow the many benefits of combustion monitoring to be enjoyed quickly and inexpensively as compared to systems requiring installation of tube bends. 
     
    
     DETAILED DESCRIPTION 
       [0009]    Unless otherwise indicated, all numbers expressing quantities of ingredients, dimensions reaction conditions and so forth used in the specification and claims are to be understood as being modified in all instances by the term “about”. 
         [0010]    In this application and the claims, the use of the singular includes the plural unless specifically stated otherwise. In addition, use of “or” means “and/or” unless stated otherwise. Moreover, the use of the term “including”, as well as other forms, such as “includes” and “included”, is not limiting. Also, terms such as “element” or “component” encompass both elements and components comprising one unit and elements and components that comprise more than one unit unless specifically stated otherwise. 
         [0011]    U.S. Pat. No. 7,469,092, the contents of which are hereby incorporated herein in their entirety, discloses a method and apparatus for monitoring and control of a combustion process of the type requiring installation of tube bends in the wall of a boiler in order to provide optical access to the boiler. U.S. Pat. No. 7,469,092 describes a sensing system which incorporates an auto-alignment feature that allows the pitch and catch optics to maintain optical alignment even though they are bolted onto a boiler or hostile process chamber which is, itself, subject to movement from thermal effects or wind and vibration. 
         [0012]    The described system provides pitch and catch optics including pitch and catch collimating lenses that are mounted on feedback-control tilt stages. Multiplexed light is launched across the measurement region by a collimating pitch lens attached directly to an input fiber and the catch collimating lens optically couples transmitted light to an output fiber that is typically a multi-mode fiber. As a result, the catch optic must be oriented so that it is collinear with the beam emanating from the pitch optic. This is necessary so that the focused transmitted beam will arrive within the acceptance cone of the multi-mode fiber. The system described in U.S. Pat. No. 7,469,092 contemplates a penetration in the wall of the boiler on the order of 2 inch (5.08 cm) in diameter. The described system functions with a 1 cm tolerance over a typical transmission distance of 10 meters, or 1 milliradian. However, this tolerance is not suitable if the boiler penetration is to be provided in the metal membrane between adjacent steam tubes to eliminate the need for providing tube bends. Such a penetration is illustrated in  FIG. 2 . The penetration  16  has approximately a ½ inch (1.27 cm) width (equal to the width of the membrane) and is elongate in a direction parallel to the steam pipes. Elongating the penetration in this way helps somewhat in terms of light collection efficiency. However, alignment and maintenance of alignment is significantly more difficult than required with a 2 inch (5.08 cm) penetration supported by the tube bend approach. By way of example, the lateral alignment tolerance, assuming a 15 meter wide boiler, is approximately 1.25 cm over 14 meters, or approximately 0.8 milliradians. In order to provide required alignment resolution, an alignment increment at least a factor of 10 smaller (i.e., 0.08 milliradians) is required. These tolerances cannot be achieved with the method and apparatus described in U.S. Pat. No. 7,469,092. 
         [0013]    To meet the tighter alignment tolerance, a modified pitch optic and catch optic configuration are required. Such a configuration is illustrated in  FIGS. 3 and 4 . The collimating lens  18  is mounted to a tilt stage  19  allowing it to be tilted along orthogonal 90° axes as described in greater detail below and in U.S. Pat. No. 7,469,092. Instead of directly launching the beam into the boiler from a collimating lens, a relay lens  20  is provided in optical communication with the collimating lens  18 . The relay lens is aligned during construction on the axis of the slotted membrane penetration. As a result, the beam received by the relay lens must go through the slotted penetration  16  at what is the focal point of the relay lens. See  FIG. 4 . The angle that the beam goes through the slotted penetration can be adjusted in two dimensions by steering the beam from the collimating lens to different locations on the relay lens. This allows the beam to be steered through the slotted penetration at the pitch side to hit the slotted penetration at the catch side of the boiler. On the catch side of the boiler, the catch optics incorporate a relay optic  20  and tilt collimating lens  18  in the same manner depicted in  FIGS. 3 and 4 . Use of the tilt stage on the catch collimating lens ensures a maximum strength collimated received beam is conveyed to an optically coupled multi-mode fiber. To further provide effective optical coupling, the pitch beam is collimated to a diameter of about 5 mm, as opposed to on the order of 20 mm in prior art systems. 
         [0014]      FIG. 15  schematically illustrates an embodiment of alignable pitch and catch optics. The transmitter and receiver are similar in design: the transmitter generates a collimated beam of laser light emerging from an optical fiber, and the receiver captures a collimated beam of light and focuses it into a fiber. (It is possible to send the light backward through this optical system, and most of the elements of the transmitter and receiver are identical.) The following description applies to either the transmitter or receiver module. 
         [0015]    The pitch and catch optics may be mounted in a housing  100  with the leading side  102  having an orifice  104  occupied by relay lens  20 . The housing may be an NEMA-4 enclosure to protect the pitch and catch optics from the environment. As shown in  FIG. 12 , a collimating lens  18  is attached to a kinematic tilt stage  106  positioned to tip and tilt the collimating lens  18  about orthogonal axes perpendicular to an optical axis of the pitch optics. Two direct drive stepper motors  108  accomplish the tip and tilt. These motors are controlled by a computer via an Ethernet or similar connection. This connection may be through an optical fiber in order to avoid electrical interference. The stepper motors  108  hold their positions when power is removed, so optical alignment is not effected by power outages. The stepper motors are driven by a motor drive  110 . 
         [0016]    During periodic or continuous system alignment, the control computer monitors the amount of laser light that is transmitted and detected. Preferably, a discrete alignment wavelength such as a visible or near-infrared light may be provided for continuous or periodic alignment proceedings. Any misalignment will reduce this detected signal. In auto-alignment mode, the computer measure the detected signal, directs one of the two stepper motors to move a small amount in one direction, then re-measures the detected signal. If the signal increases, the computer directs the stepper motor to move again in the same direction until the signal does not increase. The computer then directs the other stepper motor to move along the orthogonal axis to maximize the detected signal, then repeats the whole process for the other sensor head. As the detected signal increases, the detector amplifier gain automatically decreases so that the auto-alignment proceeds over several iterations of signal size. The auto-alignment system can function with detected powers from nanowatts to milliwatts. 
         [0017]    This “hill-climbing” algorithm is able to align the system after near-total loss of signal, in the presence of substantial noise, and is tolerant of beam blockages, power outages, mechanical shocks and other disturbances that could cause other alignment systems to misalign to the limits of the control electronics. All that is required for auto alignment is a finite signal with a global maximum in position space. Depending on the specific installation conditions, auto-alignment may occur periodically at set intervals such as every hour or as needed after an extended period, such as days of operation. The control computer may monitor the directed signal and auto-align only when the signal drops below a preset threshold. 
         [0018]    In one embodiment a sight tube  112  has a proximal and a distal end. The proximal end is attached to extend normally from an exterior wall  114  of the boiler with an elongate penetration  16  communicating with the interior of the sight tube  112 . A flange  116  is provided at a distal end of the sight tube  112 . The flange  116  allows the housing  110  to be attached with the leading end  102  abutting the boiler flange with the relay lens  20  in optical communication with the penetration  16 . In this manner a beam may be transmitted into the boiler interior  118  through the penetration  16  and across the boiler to a receiver containing catch optics substantially identical to those described above with regard to  FIG. 5 . 
         [0019]      FIG. 6  illustrates an alternative embodiment of alignable pitch and catch optics  200 .  FIG. 6  will be described as a transmitter and a receiver is of similar design. In the alternative embodiment  200  a lens  202  is optically coupled to an optical fiber  204 . The lens  202  is referred to herein as a “collimating” lens and may be a true collimating lens (that produces a beam of substantially constant diameter). Alternatively the collimating lens  202  may be a “near” collimating lens that provides a slight expansion of the beam  206 . The fiber  204  and the lens  202  are mechanically linked together in a fixed relationship and movable by “translation” along orthogonal X-Y axes  208  by a translation mechanism  210 . The emitted beam  206  is movable by translation to strike select portions of the relay lens  212  which directs the beam through the membrane slot and focuses the beam at about the receive or catch optic (corresponding to the lens  202  of the catch optic). Stepper motors, a computer controller and a “hill climbing” algorithm similar to that discussed above with respect to the embodiment of  FIG. 15  are operatively associated with the translation mechanism  210  to provide for substantially continuous alignment correction. 
         [0020]    Various embodiments of the disclosure could also include permutations of the various elements recited in the claims as if each dependent claim was multiple dependent claims incorporating the limitations of each of the preceding dependent claims as well as the independent claims. Such permutations are expressly within the scope of this disclosure. 
         [0021]    While the invention has been particularly shown and described with reference to a number of embodiments, it would be understood by those skilled in the art that changes in the form and details may be made to the various embodiments disclosed herein without departing from the spirit and scope of the invention and that the various embodiments disclosed herein are not intended to act as limitations on the scope of the claims. All references cited herein are incorporated in their entirety by reference.