Imaging of hot infrared emitting surfaces obscured by particulate fume and hot gases

Abstract

An apparatus is described which is capable of producing an image of a smelt bed of inorganic chemicals collected at the bottom of a kraft pulp recovery boiler. The image produced is free of interferences of fume particles and gaseous radiation which have obscured prior attempts to view hot surfaces under such environmental conditions. The apparatus includes an industrial closed circuit video camera fitted with an infrared imaging detector or vidicon tube. An objective lens obtains the image. An optical filter interposed between the lens and the vidicon is a key element of the invention and is selected to reject radiation less than about a micrometer to avoid fume interference. The filter is further selected to reject all but limited ranges of radiation to avoid gaseous species overlying the smelt bed which are strongly emitting and absorbing. As an example, a spectral filter centered at 1.68 micrometers with a band width of 0.07 micrometer is suitable for imaging a kraft recovery smelt bed.

I claim: 
     
       1. An imaging apparatus for observing infrared emitting surfaces .Iadd.of a kraft waste liquor recovery boiler smelt bed .Iaddend.where the surfaces are .Iadd.at temperatures of at least 1000° C. and the overlying gases and fumes are at substantially higher temperatures with the smelt bed surfaces being .Iaddend.substantially obscured by fume particles which strongly scatter visible light and by hot gas species which are themselves strong infrared emitters and which selectively absorb radiation from the infrared emitting surface which comprises: a. image detection means sensitive in the infrared region for sensing an image of the emitting surfaces;   b. lens means for projecting and focusing the infrared image of the surfaces on the detection means;   c. optical filter means limiting the spectral sensitivity of the imaging apparatus to wavelengths longer than 1 micrometer and rejecting wavelengths strongly scattered by suspended fume particles and strongly radiated by or absorbed by said gaseous species overlying the surfaces to be imaged; and   d. display means responsive to the detection means for producing a substantially interference free visual image of the surfaces being observed.   
     
     
       2. The imaging apparatus of claim 1 wherein the optical filter means limits spectral sensitivity of the apparatus to bands lying in the infrared regions between 1.57-1.73, 2.23-2.43, 3.25-4.05, 4.80-5.30, 6.90-7.20, 7.60-7.80, and 7.90-13.30 micrometers, said bands substantially avoiding interference caused by fume particles, water vapor, carbon dioxide, and carbon monoxide. 
     
     
       3. The imaging apparatus of claim 2 wherein the spectral sensitivity lies in a narrow band centered substantially at 1.68 micrometers. 
     
     
       4. The imaging apparatus of claim 1 wherein the detection means is a television camera. 
     
     
       5. The imaging apparatus of claim 4 wherein said detection means includes an infrared sensitive vidicon tube. 
     
     
       6. Apparatus for observing the smelt bed surface and adjacent furnace surfaces in a pulp liquor chemical recovery boiler, wherein the surfaces are substantially obscured by fume particles which strongly scatter visible light and by hot gas species which are themselves strong infrared emitters and which selectively absorb radiation from the infrared emitting surface which comprises: a. image detection means sensitive in the infrared region for sensing an image of the emitting surfaces;   b. lens means for projecting and focusing the infrared image of the surfaces on the detection means;   c. optical filter means limiting the spectral sensitivity of the imaging apparatus to wavelengths longer than 1 micrometer and rejecting wavelengths strongly scattered by suspended fume particles and strongly radiated by or absorbed by said gaseous species overlying the surfaces to be imaged, wherein the optical filter means limits spectral sensitivity of the apparatus to bands lying in the infrared regions between 1.57-1.73, 2.23-2.43, 3.25-4.05, 4.80-5.30, 6.90-7.20, 7.60-7.80, and 7.90-13.30 micrometers, said bands substantially avoiding interference caused by fume particles, water vapor, carbon dioxide, carbon monoxide, and sulfur compounds; and   d. display means responsive to the detection means for producing a substantially interference free visual image of the surfaces being observed.   
     
     
       7. The imaging apparatus of claim 6 wherein the spectral sensitivity lies in a narrow band centered substantially at 1.68 micrometers. 
     
     
       8. The imaging apparatus of claim 6 in which the optical filter means adapts the apparatus for use in an environment in which the fume particles and hot gases above the surfaces being observed are hotter than the surfaces. 
     
     
       9. The imaging apparatus of claim 6 wherein the detection means is a television camera. 
     
     
       10. The imaging apparatus of claim 9 wherein said detection means includes an infrared sensitive vidicon tube. .Iadd. 
     
     
       11.  An imaging apparatus for observing an infrared emitting surface in a kraft pulp liquor recovery boiler where the smelt bed surface is substantially obscured by fume particles which strongly scatter visible light and by hot gas species which overlie the smelt bed surface and are themselves strong infrared emitters and which selectively absorb radiation from the infrared emitting surface, which apparatus comprises smelt bed surface observation means including: (a) infrared radiation sensing and detection means including optical filter means, said radiation sensing and detection means being responsive only to radiation in a limited band avoiding absorptions and emissions of radiation from the hot gas species overlying the smelt bed surface and greater than about 1 micrometer to avoid fume interference; and   (b) display means responsive to the limited band radiation detection means providing a substantially interference free visual image of the smelt bed surface. .Iaddend. .Iadd.   
     
     
       12.  The imaging apparatus of claim 11 wherein the image detection means includes an infrared sensitive vidicon tube the spectral sensitivity of which is in the region up to about 2 micrometers wavelength. .Iaddend. .Iadd.13. An apparatus according to claim 11, wherein the spectral sensitivity of the sensing and detection means lies in a narrow band 
     
     
        centered substantially at 1.68 micrometers. .Iaddend. .Iadd.14.  The imaging apparatus of claim 11, comprising filter means limiting the spectral sensitivity of the apparatus to the infrared region between 1.57 and 1.73 micrometers. .Iaddend. .Iadd.15. The imaging apparatus of claim 11 in combination with a kraft pulp recovery boiler. .Iaddend. .Iadd.16. In an apparatus for recovering inorganic chemicals from black liquor, said apparatus including a pulp liquor chemical recovery boiler for receiving said black liquor in regulated amounts, said black liquor entering said recovery boiler and forming a smelt bed with a smelt bed surface within said recovery boiler; ignition means within said recovery boiler for burning said black liquor and forming fume particles and hot gas species which together overlie said smelt bed surface at temperatures of about 1100°-1300° C.; and smelt bed surface observation means, said observation means comprising, in combination: image detection means mounted on said recovery boiler for sensing an image of said smelt bed surface and adjacent interior surfaces of said boiler in a limited band of radiation out to about 2 micrometers wavelength and responsively providing a radiation image thereof;   lens means for projecting and focusing the infrared image of the surfaces on the image detection means;   optical filter means limiting the spectral sensitivity of the imaging apparatus to wavelengths longer than 1 micrometer to avoid fume interference and rejecting all but a limited band of radiation to avoid emissions and absorptions from hot gaseous species overlying the smelt bed surface; and   visual display means, responsive to the limited band radiation detection means providing a substantially interference free visual image of said smelt bed surface and adjacent interior surfaces of the boiler. .Iaddend.   
     
     
        .Iadd.17.  An apparatus according to claim 16, wherein the image detection means includes an infrared sensitive vidicon tube with a spectral sensitivity limited to the region up to about 2 micrometers wavelength. .Iaddend. .Iadd.18. An apparatus according to claim 17, wherein the spectral sensitivity of the apparatus lies in a narrow band centered 
     
     
        substantially at 1.68 micrometers wavelength. .Iaddend. .Iadd.19.  In an apparatus for recovering inorganic chemicals from black liquor, said apparatus including a pulp liquor chemical recovery boiler for receiving said black liquor in regulated amounts, said black liquor entering said recovery boiler and forming a smelt bed with a smelt bed surface within said recovery boiler; ignition means within said recovery boiler for burning said black liquor and forming fume particles and hot gas species which together overlie said smelt bed surface at temperatures of about 1100°-1300° C.; and smelt bed surface observation means viewing the smelt bed surface through a port of the boiler, the observation means comprising infrared radiation sensing and detection means including optical filter means, said radiation sensing and detection means being responsive only to radiation in a limited band avoiding absorptions and emissions of radiation from the hot gas species overlying the smelt bed surface and greater than about 1 micrometer to avoid fume interference; and display means responsive to the limited band radiation detection means providing a substantially interference free visual image of the smelt bed   
     
     
        surface. .Iaddend. .Iadd.20.  An apparatus according to claim 19, an image sensing and detection means including an infrared sensitive vidicon tube with a spectral sensitivity limited to the region up to about 2 micrometers wavelength. .Iaddend. .Iadd.21. An apparatus according to claim 19, wherein the spectral sensitivity of the sensing and detection means lies in a narrow band centered substantially at 1.68 micrometers. .Iaddend. .Iadd.22. An apparatus according to claim 19, an optical filter means limiting the spectral sensitivity of the apparatus to a band lying in the infrared radiation region substantially between 1.57-1.73 micrometers wavelength. .Iaddend.