Abstract:
An imaging system with the ability to produce clear images of deposits inside operating high temperature process equipment such as kraft recovery boilers and power utility boilers uses a terahertz/mm-wave imaging system. This system allows direct inspection capability and the ability to directly measure deposit thickness on tubes and other interior surfaces at all locations within the boiler and precipitator. Terahertz and mm-wave imaging systems employ active imaging in which a beam of terahertz or mm-wave radiation generated within the imaging system is used to illuminate a region of the scene under investigation. The reflected radiation is collected by a lens or mirror system and focused onto a detector that converts the collected radiation into an electrical signal. Both the illuminating beam and the receiving optics are scanned across the scene in a raster fashion to produce a time-varying signal that is converted into an image of the scene.

Description:
[0001]    This invention pertains to imaging in high temperature process equipment such as Kraft recovery and power utility boilers and in cement kilns, and also to quantitative measurement of the thickness of deposits on boiler tubes and other interior surfaces in places where it has not been possible to see clearly before owing to obscuration by particle laden gases with strong molecular emission and absorption lines. Such particle-laden gases include gases resulting from the combustion of coal, black liquor, or any other “dirty” fuel and places include the interior of furnaces such as coal-fired boilers, oil-fired boilers, chemical recovery boilers and cement clinker coolers. 
       BACKGROUND OF THE INVENTION 
       [0002]    Boilers are a key component of coal-fired power plants and all kraft pulp mills, being the production bottleneck in most such mills. For financial reasons, it is therefore critical to avoid unscheduled boiler outages and to maximize the run times of these boilers. In order to prevent forced outages due to plugging of the convective passages around boiler tubes, sootblowers are used to remove saltcake deposits from the boiler tubes, but as currently employed, sootblowers are often not very effective and the current use of sootblowing steam is inefficient. Sootblowers are often operated on a fixed schedule with little or no feedback based on the actual degree of fouling in any given region of the convective section. Currently the principal means for collecting direct information on the degree of fouling is through the use of infrared inspection cameras such as those currently marketed to the pulp and paper, cement, and power generation industries. While these systems provide relatively clear images of the front of the superheater and in the sootblower lanes in the generator section, their ability to provide images at the back of the generator bank, in the economizer, and in the precipitator) or to provide images of deposits in the gas passages between tube banks is severely limited. Imaging in the economizer is limited by the relatively low temperatures and correspondingly low levels of thermal radiation, low contrast in that part of the boiler, and by the limited sensitivity of current infrared imaging sensors. 
         [0003]    The limitation on imaging in the gas passages is the result of the geometry of the boiler and the fact that deposits in these regions cannot be seen from the inspection ports located on the side walls of the boiler. It is possible, with a suitable periscopic lens system that can be inserted into the boiler, to obtain images of deposits in the gas passages between tube banks. However, this periscopic lens tube is extremely expensive and suffers from relatively poor optical quality. Therefore, there is a need for an imaging system that can produce clear images of deposit formation throughout the interior of the process equipment such as a boiler—even in areas of low temperature such as the economizer or precipitator. There is also a need for an imaging system capable of producing images in areas that are currently inaccessible due to geometric constraints imposed by the construction of the boiler. 
         [0004]    Another key unmet need is to be able to quantitatively measure the thickness of deposits on boiler tubes and other internal surfaces of the process equipment to enable the operator to assess the degree of fouling of these interior surfaces, such as convective pass tubing. Deposits on such tubes impede heat transfer and, if left unchecked, can eventually block the gas passages between the tubes causing a forced outage which, as discussed above, can be extremely costly. A much better approach would be to quantitatively measure the thickness of deposits on boiler tubes. This capability would provide real-time information on deposit buildup and sootbiower effectiveness and would facilitate implementation of feedback-controlled sootblowing allowing substantial reductions in sootblowing steam usage. 
       SUMMARY OF THE INVENTION 
       [0005]    Accordingly, this invention provides an imaging system with the ability to produce clear images of deposits inside operating high temperature process equipment such as kraft recovery boilers and utility boilers using a terahertz/mm-wave imaging system. Such a system allows direct inspection capability and the ability to directly measure deposit thickness on tubes and other interior surfaces at all locations within the boiler and precipitator. 
         [0006]    The terahertz region of the electromagnetic spectrum is variously taken to extend from 0.1 THz (100 GHz) or 0.3 THz (300 GHz) to about 3 THz (at the upper end of the infrared portion of the spectrum) and is sometimes referred to as the far infrared. Millimeter-wave frequencies are commonly defined to cover the range from about 30 GHz to about 300 GHz but sometimes this range is taken to extend to as much as 2,000 GHz. Thus, there is a substantial overlap between what is referred to as THz and what is called millimeter-wave (mm-wave) radiation. In the rest of this disclosure, the region from 100 GHz to 3 THz will be referred to alternately as the THz/mm-wave region, the THz region or the mm-wave region interchangeably. 
         [0007]    The operation of terahertz and mm-wave imaging systems is completely different from current infrared imaging systems. Modern infrared imagers generally rely on the emission of thermal radiation from the scene itself. A lens system collects and focuses this thermal radiation onto a passive detector array that converts the infrared radiation in each portion of the scene into electrical signals that can be displayed as a visible image. Terahertz and mm-wave imaging systems would instead employ an active imaging approach in which a beam of terahertz or mm-wave radiation generated within the imaging system is used to illuminate a region of the scene under investigation. The reflected radiation is collected by a lens or mirror system and focused onto a single element detector that converts the collected radiation into an electrical signal. Both the illuminating beam and the receiving optics are scanned across the scene in a raster fashion to produce a time-varying signal that can be converted into an image of the scene. Because these imaging approaches employ active illumination, they are not limited by the inherent brightness of the scene and can be used to image regions such as the economizer or precipitator which do not emit sufficient radiation to produce clear infrared images. 
         [0008]    There are numerous techniques that could be used for producing terahertz radiation. One method would employ a femto-second laser to produce a pulse of visible or near infrared radiation. This light pulse would be focused onto a non-linear crystal or photoconductive antenna which, through optical rectification, would convert the visible or near infrared radiation into a femto-second terahertz pulse. This terahertz pulse is then collimated and used to illuminate the scene. The reflected terahertz radiation is detected by a gated photoconductive antenna which produces an electrical output proportional to the electric field intensity of the terahertz radiation. The gating is produced by illuminating the receiving antenna with a portion of the femto-second laser pulse used to generate the terahertz illumination beam. By varying the delay between the generated terahertz pulse and the gating of the receiving antenna, a time domain signal can be produced representing the time dependent intensity of the reflected terahertz pulse. A Fourier transform of this signal yields the spectrum of the reflected terahertz energy which is indicative of the wavelength-dependent absorption of the deposits at the interrogation point, and of the intervening gas along the line of sight within the frequency range of the terahertz pulse. This method is considered suboptimal because the alignment and timing accuracy required to implement this method successfully would be difficult to achieve in a simple and reliable device in an industrial environment. 
         [0009]    The visibility inside boilers, in the mid-infrared, various according to particle loading. It is relatively clear in kraft recovery boilers, but greatly degraded in coal-fired boilers burning lower grades such as lignite by scattering of the infrared radiation by suspended particulates in the gases. In utility power boilers and cement kilns, the suspended particulates in the gases are also much larger and produce a nearly opaque atmosphere, making imaging very difficult. Some of these particles have diameters on the same order as the wavelength of the infrared radiation which results in high scattering efficiency and poor visibility. The wavelengths of Terahertz and millimeter wave radiation, on the other hand, are much longer than that of infrared radiation (1 THz corresponds to a wavelength of 0.3 mm); therefore, in this frequency range (because the particles are much smaller than the wavelength of the probing radiation) scattering by particles is many orders of magnitudes weaker than in the infrared. Nevertheless, in the mm-wave/THz spectral region, the wavelength is still short enough to provide required image resolution. For these reasons, mm-wave/THz imaging provide significantly better visibility than infrared imaging inside boilers and other furnaces. 
         [0010]    A very important additional capability offered by this type of imaging system is the ability to quantify the thickness and shape of deposits on boiler tubes in real time. THz imaging employs extremely short duration pulses of radiation to illuminate the scene, making it possible to measure the distance to the illuminated spot with high precision by measuring the time delay of the reflected pulse. Resolutions of a few picoseconds are thus possible which translates to sub-millimeter distance resolution. Measurement of the distance to each point within the scene provides a map of the surface position of the deposits throughout the scene which can be translated into a tube deposit map using a model of the boiler tube locations from construction drawings or from measurements performed after a water-wash when the tubes are clean. The thickness of the deposits can also be determined by measuring the time delay between the THz pulse reflected from the surface of deposits and the pulse reflected from the metal tube surface beneath the deposits. Millimeter wave radar systems accomplish the same thing by using very broad band continuous, wave illumination. By performing a Fourier transformation of the reflected data, time domain information that provides time resolution of about ten picoseconds can be obtained. This type of mm-wave radar system in a kraft recovery boiler is capable of producing clear images of boiler tubes at the back of the generating bank and of measuring the thickness of deposits on the tubes. 
         [0011]    This invention provides an improved imaging system based on millimeter wave imaging technology to produce clear images of surface deposits throughout the convective sections of the process equipment, such as boilers, etc., including in regions where there is very little ambient illumination, such as at the bottom of the economizer. In addition, this system provides quantitative measurements of the thickness and size of the deposits allowing for accurate feedback on the degree of fouling on each surface, such as boiler tubes. By employing time-domain spectroscopy (TDS) to measure the chemical composition of the deposits, this invention enables on-line detection of corrosion-related chemistries—in real-time. This technology lends itself to the implementation of a robust, side-looking imaging system that can be incorporated into sootblowers to provide imaging capability in the gas passages between tube banks which otherwise cannot be viewed from existing sidewall inspection ports due to geometrical constraints of the boiler geometry. This would allow for complete, continuous monitoring throughout the convective sections of the boiler. Such a system could be used not only to provide information on tube deposits, but also to detect excessive tube deflection and to allow inspection of the generator tube attachments at the steam drums—a common point of failure. 
         [0012]    There is also a great need for these capabilities in kraft recovery boilers, in coal-fired boilers, in biomass boilers, and in spray drying applications such as skim milk dryers (where deposits on the dryer surfaces can present an ignition hazard). 
         [0013]    The rapid development of new technology for generating and detecting mm-wave radiation—driven by the current interest within the security and military communities—offers a very bright future for the application of this technology in the process industries and promises to provide increasingly effective, robust, and economical components which can be used in products that address the needs of these industries. 
         [0014]    The use of the invention to reduce sootblowing steam usage without increasing the rate of boiler tube fouling has substantial benefits in terms of energy, economics and the impact on the environment. As an example, recovery boilers usually operate two-to-four sootblowers at all times, each one consuming approximately 20,000 lbs of steam per hour. Assuming that, on average, boiler operates 24/7 for 350 days per year, the sootblowing steam usage amounts to about 5.04×10 8  pounds per year per boiler, or approximately 8.57×10 10  pounds per year for the 170 recovery boilers in operation in the US. 
         [0015]    The most common current practice is to operate sootblowers on a timed schedule developed from past experience of boiler plugging behavior. Sootblowers are operated in all areas at least some of the time just to be safe, but there is currently no means to determine and localize the instantaneous state of boiler tube fouling throughout the convective sections in order to determine where deposits are forming or to assess the effectiveness of individual sootblowers in removing deposits. This invention will enable direct observation and measurement of deposits on the tubes and allow sootblowers to be operated only when and where they are needed. Because a complete feedback-control system for adjusting the operation of sootblowers in real-time has never been implemented in a kraft recovery boiler, no operational data exists on the potential benefits that could be realized through the implementation of such a system; there is no precise data to show the amount of sootblowing steam that can be saved by not blowing in areas where there are no deposits and concentrating on areas where fouling is occurring. Prior attempts to reduce sootblowing steam usage have not employed any feedback mechanism (other than visual inspection at the end of a run) to dictate where and when to reduce sootblowing. Direct observation with infrared imaging systems in operating kraft recovery boilers has shown that sootblowers often are operated in areas where boiler tubes are relatively clean and deposit buildup is proceeding slowly. Sootblowing could easily be reduced in these areas with little negative impact on overall boiler cleanliness. Therefore, a conservative estimate would be that, at a minimum, a 10% reduction in sootblowing steam usage can be achieved without increasing boiler plugging by employing the proposed technology. 
         [0016]    Whether the additional steam made available by reduced sootblowing is used to offset purchased power or to offset other sources of steam in the pulp mill, it will be possible to reduce the amount of coal, fuel oil, or natural gas used by the electric utility or the fuel oil being burned in other boilers in the pulp mill to produce the same process steam. In either case, the emissions from the combustion of those fuels would be eliminated. 
         [0017]    Approximately half of the electricity in the US is generated from the combustion of coal. According to a 1995 Environmental Protection Agency publication, the standard (AP42) for calculating emission factors for coal combustion dictates levels of 1.75 lb of SO x /MMBTU, 1.19 lb of NOx/MMBTU, and 240 lb of CO 2 /MMBTU. If the postulated 10% reduction in sootblowing steam frees up electricity derived from coal combustion, this would translate into reductions of 8,177 tons/year of SO x , 5,560 tons/year of NO x , and 1.12 MM tons/year of CO 2  nationwide. 
         [0018]    Fuel oil has a typical heat content of about 19,000 BTU/lb, so the anticipated reduction in sootblowing steam usage would make it possible to forego burning approximately 500 MM lb of fuel oil per year nation-wide. Combustion of fuel oil with a typical carbon content of 89% by weight produces approximately 3.263 lb of CO 2  per pound of fuel oil. Therefore, the reduction in CO 2  released to the atmosphere is expected to be on the order of 1.6 billion pounds (0.8 MM tons) per year nation-wide. 
         [0019]    A recent EPA report indicates that typical emissions from boilers burning No. 6 fuel oil are  ˜ 2 lb of SO 2  and  ˜ 0.4 lb of NOx per MM BTU. For the anticipated reduction in oil usage, this translates into a reduction in SO 2  emissions of about 19 million lb/yr (9,345 tons/yr) and a reduction in NO emissions of 3.74 million lb/yr (1,869 tons/yr) nation-wide. 
         [0020]    The benefits outlined above have been calculated solely on the basis of reduced sootblowing steam usage, but additional benefits may accrue from avoiding unplanned outages by detecting problems early enough to schedule an outage at a convenient time or take corrective action during a planned outage, and from avoiding damage to equipment by detecting and correcting problems before they lead to catastrophic failures. These benefits may actually dwarf the dollar value of the savings from sootblowing steam reduction—especially if they involve avoiding long periods of lost production. A recent presentation at the St. Petersburg Recovery Cycle meeting put the cost of unscheduled downtime at between $100,000 and $300,000 per day. Avoiding even one unscheduled outage due to the ability to better monitor internal processes in the boiler could pay for the deposit monitoring system of this invention. 
         [0021]    Two models have been developed to predict the transparency of the combustion environment inside an operating kraft recovery boiler at mm-wave and terahertz frequencies: 1) a Mie scattering model that predicts the extinction of light due to scattering and absorption by particles suspended in the combustion product gases, and 2) a molecular absorption model that predicts the absorption of radiation due to vibrational and rotational transitions of the combustion gas molecules. 
         [0022]    Even in cases where the particles are nonabsorbing, light scattered by particles is randomized and loses its ability to convey image information. Therefore, the parameter that is of importance in predicting the impact of particles on visibility is the extinction coefficient which is the sum of the scattering and absorption coefficients. 
         [0023]    In order to predict the extinction of light due to scattering it is necessary to specify the size distribution and concentration of particles in the recovery boiler environment. The particles present inside the boiler derive from a variety of combustion-related processes that take place inside the boiler. Among these are in-flight drying, pyrolysis, and burnout of black liquor droplets to form char (carryover) particles; ejecta and fragments from these char particles are believed to be the source of intermediate size particles. Vaporization of sodium or sodium compounds from the smelt bed or from burning liquor droplets and char particles and the reaction of these sodium species with SO 2  and CO 2  form sodium sulfate and sodium carbonate fume particles. It has long been accepted within the recovery boiler community that fume particles are present in high concentrations and that the mean size of these particles is in the range of 200 to 500 nm. The particulates in a utility power boiler are primarily ash, minerals and other particles from the burning of coal powder in the boiler burner. 
         [0024]    In the late 1990&#39;s, Dr. Ariessohn and Dr. Donald Holve performed laser light scattering measurements inside two different recovery boilers. They established the presence of significant numbers of intermediate-size particles (ISPs) with diameters in the 5 to 100 micron diameter range, which are believed to be formed by fracturing of char particles during in-flight combustion and also by ejection of material as the organics in the liquor droplet vaporize explosively during volatilization. Virtually no carryover particles of a size greater than 100 micron diameter were observed in the upper furnace. The size distribution measured in this study is shown in  FIG. 1   
         [0025]    From these data, one can calculate the extinction of light due to these two size classes of particles (fume and ISPs) separately and add their contributions to obtain an overall estimate of light extinction due to particle scattering. A Mie scattering model has been constructed which calculates the extinction coefficient as a function of size parameter (=πD/λ) assuming spherical particles. For each particle diameter and radiation wavelength, the extinction coefficient can then be determined from a look-up table. These extinction coefficients are then weighted by the concentrations of particles of that particular diameter. The contributions from all size classes are then summed to obtain the overall extinction coefficient for a particular wavelength of radiation due to the size distribution of particles under consideration. 
         [0026]    While the extinction due to fume is dominant in the visible and near infrared part of the spectrum, the extinction due to fume in the mm-wave and THz regions of the spectrum is several orders of magnitude smaller than that due to ISPs because of the much larger ratio of wavelength to particle size in that spectral region. The maximum extinction coefficient for ISPs (at a wavelength of 10 microns) is only about 0.037 m −4  and decreases rapidly with increasing wavelength. 
         [0027]    Experience with infrared imaging in recovery boilers has shown that attenuation of as much as 20% still allows for reasonably clear images. Therefore, we can safely conclude that extinction by suspended particles will have little or no effect on visibility in the mm-wave and THz spectral range. Furthermore, it is quite apparent that attenuation due to particle scattering and absorption will be much smaller in the mm-wave region (frequencies below 0.3 THz) than at higher frequencies. 
         [0028]    Using the molecular absorption mode, predictions of molecular absorption have been generated using the HITRAN spectroscopic database of molecular parameters. This database contains parameters for more than 2.7 million molecular transitions. These parameters can be used to calculate the intensity and width of absorption features resulting from mixtures of 39 molecular species. Harvard University maintains the HITRAN database and makes it available to researchers; they also provide a software program (JAVA-HAWKS) which facilitates translating the molecular parameters into spectroscopic parameters specific to particular species or mixtures and particular gas temperatures. We have created models based on these data that permit the calculation of molecular absorption spectra for arbitrary mixtures of gases over specified path lengths at a number of temperatures. 
         [0029]    This model allows the user to specify an arbitrary molecular gas composition, the total spectral range to cover, and the spectral resolution (bandwidth) to use in generating the spectrum. The gas mixture shown above is typical of the combustion gases inside a kraft recovery boiler. The three dominant species are nitrogen, water vapor and carbon dioxide. 
         [0030]    A sample spectrum is shown in  FIG. 2 . This figure shows that there are numerous strong absorption features at frequencies greater than 1 THz. However, there are windows where the transmission is predicted to be near 100% over a 50 foot path in the region of 1.05, 0.96, 0.83 and 0.71 THz. At frequencies less than 0.4 THz (400 GHz), this gas mixture is seen to be relatively transparent with only a few isolated, relatively weak absorption features. 
         [0031]    This same spectrum is shown in  FIG. 3 , plotted against wavelength; this gas mixture is highly transparent at wavelengths longer than about 700 microns. Spectra were also generated for temperatures of 2000, 1500, 1000 and 500 degrees F., which showed only very slight differences in the absorption spectra as a function of temperature. 
         [0032]    The model has also been run for wavelengths between 20 and 100 microns and shows that there are virtually no spectral windows in that region due to numerous water vapor absorption bands (see  FIG. 4 ). 
         [0033]    In the near and mid-infrared (1 to 20 microns), absorption by many of the species present in the recovery boiler atmosphere are significant and offer only a few narrow spectral windows. The 8 to 12 micron “atmospheric” window that is quite clear in the earth&#39;s atmosphere is less clear in the combustion atmosphere due to the presence of CO 2  hot bands (see  FIG. 5 ). 
         [0034]    From this study, it is clear that the combustion product gases inside a recovery boiler are much clearer in the mm-wave region than they are in the near-, mid-, or long-wave infrared and that there are several transparent windows in the THz region that are comparable in transparency to the windows in the near- to mid-infrared. The frequency of operation for best results is selected from among the following specific transparent “windows” in which imaging and measurement may best be performed: 499.0 to 503.5 GHz, 505.5 to 524.0 GHz, 525.0 to 530.0 GHz, 532 to 540 GHz, 542 to 546 GHz, 550 to 554 GHz, 558 to 571 GHz, 573 to 578 GHz, 580 to 594 GHz, 596 to 620 GHz, 622 to 645 GHz, 647 to 658 GHz, 659 to 724 GHz, 726 to 734 GHz, 736 to 751 GHz, 754 to 766 GHz, 768 to 775 GHz, 777 to 793 GHz, 795 to 840 GHz, 843 to 853 GHz, 855 to 863 GHz, 866 to 898 GHz, 908 to 916 GHz, 918 to 922 GHz, 928 to 967 GHz, 972 to 978 GHz, 980 to 987 GHz, 989 GHz to 1.000 THz, 1.006 to 1.068 THz, 1.070 to 1.077 THz, 1.088 to 1.095 THz, 1.100 to 1.108 THz, 1.122 to 1.130 THz, 1.230 to 1.271 THz, 1.299 to 1.322 THz, 1.444 to 1.472 THz, 1.476 to 1.493 THz, 1.497 to 1.516 THz, 1.545 to 1.563 THz. 
         [0035]    Thus, the two models developed to study attenuation of electromagnetic radiation due to particle scattering and absorption, and due to molecular absorption by combustion product gas species have confirmed that the recovery boiler atmosphere would be much more transparent at mm-wave and THz frequencies than in the near and mid infrared. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0036]    The invention and its attendant features and benefits will become better understood upon reading the following description of the preferred embodiments in conjunction with the following drawings, wherein: 
           [0037]      FIG. 1  is a graph showing size distribution of intermediate size particles in a recovery boiler; 
           [0038]      FIG. 2  is a plot showing the THz/mm-wave transmission spectrum of recovery boiler gases 
           [0039]      FIG. 3  is a plot of the THz/mm-wave spectrum of recovery boiler gases 
           [0040]      FIG. 4  is a plot of the long-wave infrared spectrum of recovery boiler gases 
           [0041]      FIG. 5  is a plot of the near- and mid-infrared spectrum of recovery boiler gases 
           [0042]      FIG. 6  is a block diagram of W-band (75-110 GHz) frontend; 
           [0043]      FIG. 7  are two contour plots of metal pipes in rectangular coordinates after distance correction and level adjustment using radar frequency of 200-205 GHz; tubes with deposits on the left; clean tubes following the water wash on the right; 
           [0044]      FIG. 8  is a schematic plan view of an arrangement for fixed mount scanning MMW radar deposit monitor; 
           [0045]      FIG. 9  is a sootblower providing thermal protection of integrated mm-wave imager; 
           [0046]      FIG. 10  is a block diagram of 200 GHz imaging radar 
           [0047]      FIG. 11  is a schematic of a dielectric constant measurement at high temperature setup (up to 1200 C) 
           [0048]      FIG. 12  is a simplified block diagram of the 300 GHz radar frontend; and 
           [0049]      FIG. 13  is a schematic of a single antenna system on the left, and a schematic of up- and down-convertors configuration on the right. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0050]    A portable, THz measurement system for performing tests in an industrial plant environment starts with microwave frequency radiation and uses non-linear mixers to up-convert the radiation to higher frequencies. The equipment required for this latter process is much more robust and less expensive than femto-second laser equipment, but can produce radiation in roughly the same frequency band. A field hardened system operating at frequencies of 95 to 225 GHz (0.095 to 0.225 THz) has been built and tested, and another system that will operate at up to 0.3 or possibly 0.5 THz has been designed. 
         [0051]    Both 100 and 200 GHz systems use an Agilent vector network analyzer (VNWA). The microwave frequency (12.5-18.3 GHz) is multiplied by 6 to create 75-110 GHz and by 12 to create 140-220 GHz. The received mm-wave signal is down-converted into an intermediate frequency in the radar frontend ( FIG. 6 ) and processed by the VNWA. The radar front-ends are portable and can be mounted on a tripod to scan the region of interest; all the measurements are conducted in the broadband frequency-domain. Time-domain information is obtained by taking the Fourier transform of the band-limited frequency-domain signal; the time resolution is approximately given by δt=1/bandwidth. The tripod is equipped with a stepping motor controlled rotational stage. Our scanning mm-wave radars, therefore, can obtain 2-D images which consist of the down-range (distance) using the time-domain data and cross-range using the angle scan. Addition of a vertical scan capability allows us to generate 3-D images. The current system has separate transmitting and receiving antennas with collimating lenses. With this system, the 3-dB antenna beam width is approximately 0.5 deg at 100 and 200 GHz. 
         [0052]    To test the performance of the system, we performed laboratory measurements of two 76-mm diameter metal pipes with and without  ˜ 7 mm thick plastic covers. The pipes were mounted vertically 6″ apart at a distance of about 15 feet from the mm-wave radar apparatus. The time delay between the reflection from the front of the plastic covering and the reflection from the pipe was about 65 picoseconds, which corresponds to a the round trip distance through the plastic and back again, and was close to the actual thickness (plus a small air gap). This demonstrates the ability to accurately measure the thickness of a “deposit” on a metal tube in the laboratory using the time difference between the multiple reflections to determine the spatial separation between the partially reflecting surfaces. 
         [0053]    Although a better down-range (time) resolution can be obtained with a wider bandwidth, a broadband mm-wave system is also much more expensive; therefore, it would be desirable to identify the optimum combination of center frequency and bandwidth to produce a system with adequate spatial and temporal resolution to produce clear images of the boiler tubes and to measure deposit thickness accurately at reasonable cost. Better horizontal (and vertical) spatial resolution can be obtained by increasing the frequency and increasing the antenna diameter, however, the size of typical inspection ports limits the size of the antenna that can be used unless the value of the system can justify installing larger openings. 
         [0054]    We have experimentally demonstrated improved visibility relative to IR imaging in a combustion environment using THz/mm-wave radiation, showing not only that THz waves can propagate through the recovery boiler environment with minimal attenuation, but also experimentally demonstrating the ability to use THz/mm-wave radiation to produce clear images of boiler tubes in an area of the boiler where infrared imaging can, at best, produce images of marginal quality, and demonstrating the ability to distinguish reflections from the surface of boiler tubes and from the surface of saltcake deposits on the outside of the boiler tubes. The time delay between these reflections can readily be measured and this information can be used to make real-time, quantitative measurements of saltcake deposit thickness—a capability that has never before been demonstrated. 
         [0055]    There are three factors which can compromise visibility of infrared imaging in recovery boilers: 1) Interference from Molecular Absorption and Emission, and from Scattering and Absorption by Suspended Particles. 
         [0056]    In the infrared—within the 3.9 micron spectral window—atmospheric interference due to molecular absorption is minimal over path lengths typically encountered in a recovery boiler, however extinction due to particle scattering and absorption can have a significant effect on visibility. 
       2) Lack of Contrast in the Scene 
       [0057]    In some regions of the boiler, objects within the scene may offer little contrast. This is especially true if all surfaces are coated with deposits at nearly the same temperature. In this case, the radiation emitted or reflected from all surfaces is of nearly the same intensity and it becomes difficult to discern any detail. This phenomenon is commonly observed in ceramic kilns. When the objects being fired reach the temperature of the kiln, they become virtually invisible. 
       3) Insufficient Illumination 
       [0058]    The third obstacle to good visibility is the lack of sufficient illumination or emission to produce a useful image. In this case, the intensity of the light delivered to the imaging system is insufficient to produce a signal substantially greater than the noise level of the imaging device which, again, results in very low contrast. 
         [0059]    The water inside the boiler tubes as it enters the generating bank is close to saturation at the pressure of the steam being produced by the boiler. Typically this means the tube temperature at that point is 490-600 deg F. The temperature of the gas exiting the generating bank is typically between 600 and 700 deg F., so surfaces coated with saltcake will have surface temperatures only slightly higher than the bare tubes. Therefore, if all surfaces have a uniform coating of saltcake, they may be at very nearly the same temperatures and there may be only slight differences in brightness. 
         [0060]    A portable mm-wave imaging system was tested in a kraft recovery boiler. The system was transported to the mill and set up at the back of the boiler where there was a 2 foot square opening with a hinged metal door. The economizer section had been removed from this boiler, so there was a clear view of the back of the generator bank from this location. 
         [0061]    A heat shield transparent to mm-waves, consisting of an alumina/silica mixture was used to cover the boiler opening. In this configuration, separate transmitter and receiver lenses and transducers were used. In future systems, a single transmitter/receiver unit may be employed to reduce the size and complexity of the system. 
         [0062]    The mm-wave imaging system was scanned in the horizontal plane to produce a 2-dimensional (angle and time) image of the boiler tubes. Measurements were made at several different frequencies and using a range of bandwidths. 
       Field Experiments in Recovery Boilers 
       [0063]    Field measurements were conducted using both 94 GHz and 200 GHz systems at a kraft pulp mill. Mm-wave radar reflection data were obtained prior to and following a water wash which allowed us to obtain images of fouled and clean boiler tubes from the same measurement location; the radar data obtained in the post-washing period provided images of bare pipes. 
         [0064]      FIG. 7  shows contour plots of the same scene in the horizontal plane (x-y axis in units of meters). The presence of saltcake is clearly visible. On the left, the deposits on the tubes are seen as protrusions towards the left (towards the mm-wave radar system), whereas on the right, the front surface of the tubes (the back of the generator bank) is seen to be free of those protrusions. In these figures, the transformation from time to distance was made without taking into account the refractive index of the saltcake. Therefore, the thickness of the deposit shown may be overestimated by a factor of between 1.4 and 1.85. Applying this correction the measured deposit thickness are consistent with visual observation of the tubes made after the boiler was shut down, but before they did the water wash. 
         [0065]    The time-domain data with saltcake clearly shows pulse broadening due to reflections from air-saltcake and saltcake-metal interfaces. The time-domain results for the case with a bandwidth of 20 GHz can clearly separate the air-saltcake interface from that of saltcake-pipe. In other words, this system can accurately estimate the thickness of the saltcake deposit. These results also show the importance of identifying optimum frequency and bandwidth for this application. 
         [0066]    We have demonstrated that, even in an area of the boiler where infrared imaging produces a marginal image, the mm-wave radar system can obtain clear reflection images of boiler tubes. We have also demonstrated that the mm-wave system can distinguish reflections from the tubes and from the overlying deposits and can provide quantitative data that can be used to infer deposit thickness. 
       Fixed Location System 
       [0067]    The experimental prototype that was tested in the recovery boiler is already fairly close to what is required for an industrial sensor. Improvements incorporated into a commercial sensor replace the 2 antenna/lens system with one that employs a single antenna/lens. By developing a system that operates at higher frequency, the size of the antenna/lens can be reduced. This system does not require a large opening since the transmitter/receiver head is only 2 to 3 inches in diameter and the scanning mirror can be placed very close to the port opening. This system can therefore function using a port that involves only a single tube bend, greatly improving cost-effective access to the process. A special port covering that prevents hot gases from escaping from the boiler and damaging the equipment is employed. 
         [0068]    A scanning system for propagating the transmitted and received electromagnetic radiation through a small inspection port in the side of a large enclosure containing the high temperature combustion environment is shown in  FIG. 8 . A virtual pivot can be used conveniently to provide scanning motion with a center of rotation flush with the inside surface of the boiler wall. This allows for scanning over a very wide field of view, while allowing all the components to be located behind the port covering, as shown schematically in  FIG. 8 . Enertechnix manufactures virtual pivot systems for use with infrared imaging technology and their adaptation for use with the mm-wave imaging system is straight-forward (see  FIG. 8 ). Nevertheless, in some situations it may be necessary to employ a port that is wider than the standard inspection port in order to achieve wide angle coverage of the interior of the boiler. For the mm-wave imaging system, it may be necessary to include an encoder to measure the angular position. In addition, to maximize the deposit area probed by the system, it may be necessary to include vertical scanning. 
         [0069]    An alternative approach would be to utilize a scanning mirror as shown in  FIG. 10 . This would allow for rapid scanning in both the horizontal and vertical directions with fine resolution. 
       Sootblower-Mounted System 
       [0070]    We have performed a thermal analysis of a special sootblower tube that can be used to provide a platform for an imaging/measurement system for propagating the transmitted and received electromagnetic radiation through a small inspection port in the side of a large enclosure containing the high temperature combustion environment device allowing it to be inserted into the furnace to perform inspections of regions that are not visible from ports on the outer walls of the boiler. This design is shown schematically in  FIG. 9 ; a triple wall pipe is used to provide a passage for steam flow and, at the same time, to provide adequate thermal insulation for the imaging system components. Steam flowing in the outermost annular passage ensures that the temperature at the outer surface of the middle annular passage is well below the furnace gas temperature. The middle annular passage is evacuated and has highly reflective inner surfaces (nickel or chrome plated, as in a Thermos bottle) to minimize heat transfer. In this case, the bore of the structure would carry the mm-wave components and also cooling air. Thermal modeling of this structure has shown that it will be possible to continuously maintain the mm-wave components below 100 F with only about 12 cfm of cooling air flow. In the design shown in  FIG. 9 , the mm-wave components inside the sootblower probe consist of a metal waveguide, a horn antenna (also metal), a lens that can be made of Teflon or high resistivity silicon, and a mirror. None of these components should be particularly sensitive to temperature and can certainly survive at temperatures well above 100 deg F. (Teflon can operate continuously above 400 deg F.). The cooling air flowing out through the mm-wave port in the side of the probe will prevent infiltration of saltcake particulate and combustion product gases. In this design, the sootblower provides both axial and rotational motion, eliminating the need for a scanning mirror. 
         [0071]    We have investigated the availability of commercial components that could be used to construct a fixed location THz/mm-wave imaging system suitable for continuous use in an operating recovery boiler.  FIG. 10  is a schematic diagram of these components together with a portable, low-cost network analyzer that can be incorporated into such a system; the network analyzer generates a 1-6 GHz signal that is up-converted to  ˜ 200 GHz by mixing it with a reference signal from a 199 GHz local oscillator. A horn antenna (labeled Tx in the figure) produces a mm-wave beam that passes through a wire mesh beam splitter and a quarter wave plate that converts the beam from linear to circular polarization. This beam passes through a Teflon lens that narrows the spread of the beam, allowing it to be projected over long distances. The beam is reflected from the surface of a scanning mirror that directs the beam towards the scene to be imaged and allows it to be scanned in both the horizontal and vertical directions in a raster pattern. The reflected radiation is collected and focused by the Teflon lens and directed through the same quarter wave plate which converts the circularly polarized radiation back to linear polarization with an orientation perpendicular to that of the transmitted beam. This allows the received beam to be reflected by the wire mesh beam-splitter and prevents crosstalk between the transmitted and received beams. The received beam is collected by a second horn antenna (labeled Re in the figure) which couples the radiation into a waveguide. This signal is down converted back to a 1 to 6 GHz signal by mixing it with the 199 GHz reference signal. The resulting signal is detected by the network analyzer. A computer is used to Fourier transform the output of the network analyzer from the frequency domain to the time domain allowing for precise measurements of the time delay between reflections from the tubes and from the surfaces of the deposits on the tubes. The computer is also used to control the scanning mirror and provide a user interface. 
         [0072]    A radar system operating at 300 GHz compared with that at 200 GHz radar offers some desirable benefits. Different front-end configurations and a single antenna system which uses circular polarization to separate the reflected and incident beams also provide attractive benefits. Mm-wave up- and down-convertors allow us to use a regular VNWA with limited bandwidth, and a single converter to perform both up-conversion and down-conversion by suitable time gating offers yet additional commercial advantages. 
         [0073]    Low-cost, ruggedized mm-wave/THz imaging and measurements in kraft recovery and power boilers offer the ability to obtain 3-dimensional boiler tube images and quantitative deposit thickness measurements in an operating recovery and power boiler and it makes possible to incorporate that functionality in an integrated, miniaturized, and ruggedized mm-wave sensor. 
         [0074]    The dielectric constants of deposits in different environmental conditions are important parameters for estimating the thickness of saltcake deposits. The mm-wave image results from reflections at the two surfaces of the saltcake deposit (the gas/saltcake interface and the saltcake/boiler tube interface) and from absorption of the mm-wave radiation within the saltcake deposit. The reflection coefficients at these two interfaces depend on the optical properties of the saltcake and the time delay between these reflections is determined by the propagation time of the mm-wave radiation in the saltcake deposit which depends on the deposit thickness and on the dielectric constant of the saltcake. The dielectric constant of many crystalline materials depends on temperature due to thermal expansion—increased temperature results in decreased density and a decrease in dielectric constant. However, saltcake deposits in a recovery boiler are subject to a much more complex, irreversible sintering process that depends on temperature, time and composition. At temperatures as low as  ˜ 500 C (932 F), low melting point eutectics may form due to the presence of impurities (primarily chlorides and potassium) and the deposits will undergo an irreversible consolidation. Over time, voids will disappear and the deposit will become much denser. It is best to take into account the impact of this process on dielectric constant to obtain the most accurate and consistent deposit thickness measurements. 
         [0075]    For refractive index measurements, saltcake is crushed into a powder and packed into a void created between two Styrofoam sheets to create a planar structure. It is important to note that the crushed saltcake sample has relatively low density due to voids and may be similar to some actual deposits in the cooler regions of the boiler where sintering doesn&#39;t occur, but these samples were clearly not representative of sintered saltcake deposits that would be found in the superheater or, possibly, in the front of the generator bank. The apparatus required to make these measurements is shown in  FIG. 11 . We conduct detailed saltcake characterizations from 75-325 GHz and at temperatures up to about 90° C. using this system. The experimental setup for 140-220 GHz and 220-300 GHz is similar. 
         [0000]    The results of this study allow us to determine the accuracy with which we can estimate deposit thickness from mm-wave radar measurements and also enable us to assess the feasibility of making measurements at higher frequencies where the absorption properties of the saltcake may preclude such measurements. Obtaining spectral absorption data from the saltcake that can be correlated with its composition provides the capability to identify the presence of species that are associated with accelerated corrosion. 
         [0076]    A new radar front-end operating at 300 GHz operates with the basic principle as with the 100 and 200 GHz front-ends, and the same VNWA can be used as the processing unit. A simplified diagram is shown in  FIG. 12 . This modular design is suited for both reflection (radar) and transmission (material characterization) measurements. It includes several WR3 (220-325 GHz) items including a multiplier, directional coupler, and harmonic mixer. A W-band source with a Spacek amplifier as the RF source complete the 300 GHz frontend and a sub-harmonic mixer are used to improve the signal-noise-ratio. Either 3″ or 2″ lens horn antennas will be used for collimating the beam. Two lens horn antennas, sub-harmonic mixer, W-band source, LO amplifier, and isolators are used. 
         [0077]    A dual antenna radar has many advantages including minimum coupling between transmitter and receiver channels. However, alignment can be simplified and the size of the system can be reduced by using a single antenna for both transmitting and receiving. This is particularly important for measurements made through a small inspection port. The antenna configuration of a single antenna system is shown in  FIG. 13 . To separate the received signal from the transmitted one, the transmitted signal is circularly polarized (eg. LHC). Since the reflected radiation usually has the opposite polarization (RHC), a λ/4 plate and wire mesh beam-splitter can be used for converting the polarization and separating the transmitted and received signals. Circularly polarized radar is less sensitive to scattering by small particles. This is an important advantage for a mm-wave radar system operating in a kraft recovery boiler. 
         [0078]    A 200 GHz single antenna system can be constructed with a λ/4 wave-plate and wire mesh beam-splitter. The up- and down conversion section incorporates several low-cost VNWAs operating at up to 6 GHz. If the thickness of the saltcake deposits can be estimated accurately using a 5 GHz bandwidth at 200 GHz in combination with models and a-priori data (discussed below), it is possible to use a low cost VNWA for processing the data. 
         [0079]    An alumina/silica material works effectively as a heat shield for providing protection from furnace gases and particles, and has sufficient structural rigidity and strength to act as a suitable seal over an open port boiler while being transparent to the millimeter wave frequencies. However, this material is somewhat soft and fragile and may offer only a short working life for continuous operation in a recovery boiler environment. The alumina/silica material is also available in the form of ceramic tiles. A material such as Transite™ which is used to line chimney flues would serve well as a heat shield material in this application due to its strength and refractory properties, depending on its transparency to THz/mm-wave radiation. Ceramics and other composite materials are also possibilities. 
         [0080]    It is preferable to prevent saltcake from building up on the inside of the heat shield. Mechanical rodding or scraping, high velocity air jets, steam cleaning, periodic rapping, or a combination of these methods can be used to prevent excessive saltcake buildup on the heat shield. 
         [0081]    It appears that a bandwidth of 20 GHz is sufficient to resolve deposit thicknesses of 2 mm. However, a saltcake layer of more than 10 mm may create significant attenuation and the reflection from the metal cylinder may not be detectable. The saltcake appears to be quite a bit more transparent at 100 GHz than at 200 GHz. Therefore, it may be possible to switch frequency to produce accurate measurements of saltcake deposits of various thicknesses (higher frequencies for thin layers and lower frequencies as the deposit thickness increase). 
         [0082]    Clear images of the interior of a recovery boiler and saltcake deposits measurements on tubes can be obtained using THz/mm-wave radiation. The images obtained with that system provided 2-dimensional data (horizontal scan angle plus distance measurements). In addition, with an enhanced THz/mm-wave imaging system it is possible to obtain 3-dimensional images of the interior of the recovery boiler (horizontal and vertical scan angles plus distance). The selection of center frequency and bandwidth provides high fidelity data for both the tubes and the deposits and allows the system to clearly distinguish reflections from the front surfaces of the deposits and reflections from the boiler or wall tubes. This data can be used to quantify the thickness of the deposits and obtain autonomous operation of the sensor at a fixed location (illumination of the target, collection of the reflected radiation, analysis of the data, and reporting of the results). The system can also have the ability to self-calibrate, eliminating the need for a calibration surface outside the boiler. 
         [0083]    A low-cost, integrated mm-wave deposit imaging radar system can be built around the recently introduced Agilent N9923 portable microwave network analyzer (NWA) shown in  FIG. 10 . The front-end employs a single lens horn antenna which uses circular polarization and a wire mesh polarizing beamsplitter to separate the received signal from the transmitted signal as above. The beam scan can be performed either by using a rotating reflector as shown in  FIG. 10 , a virtual pivot mount similar to that shown in  FIG. 8 , or a rotational stage. The up- and down-conversion of the 1-6 GHz IF signal from the VNWA uses two mixers and a 199 GHz LO (local oscillator) source. A receiver based on a low-cost FMCW for intermediate frequency signals can also be used, but, since typical FMCW radars have a bandwidth of less than 1 GHz, this approach may not be suited for the detection of a thin saltcake layer. 
         [0084]    Obviously, numerous modifications and variations of the described preferred embodiment are possible and will occur to those skilled in the art in light of this disclosure of the invention. Accordingly, we intend that these modifications and variations, and the equivalents thereof, be included within the spirit and scope of the invention as defined in the following claims, wherein we claim: