FI82167C - AVBINDNING AV HETA, INFRAROETT LJUS EMITTERANDE YTOR FOERDUNKLADE AV PARTIKELDIMMA OCH HERA GASER. - Google Patents

Description

1 82167

Imaging infrared light-emitting surfaces obscured by hot, particulate mist, and hot gases

BACKGROUND OF THE INVENTION The present invention relates to an apparatus for producing a continuous visual image of physical conditions within a hot vessel. The technology of interest describes hot interior surfaces when the surfaces are obscured by fog and radiation emitting / absorbing gases 10 associated with the process in the vessel. In particular, the invention relates to the chemical description of the interior of a chemical pulp recovery boiler.

In the production of wood pulp, raw wood is boiled in the presence of inorganic chemicals. In sulphate or kraft-15 pulp processes, the active cooking chemicals are sodium hydroxide and sodium sulphide. An important feature of kraft soup is the recovery of these inorganic cooking chemicals from the liquid soup effluent. This waste liquor, more commonly referred to as black liquor, is an aqueous mixture of lignin separated from wood and reacted inorganic cooking chemicals, which are basically sodium carbonate and sulfate. The economics of the entire cooking process require the recovery of relatively expensive cooking chemicals from waste black liquor.

25 Conventionally, black liquor is burned in a boiler designed for the recovery of cooking chemicals and the separated thermal energy from the organic matter in the black liquor is used to produce steam. The chemical recovery boiler receives the black liquor after it has been concentrated in the evaporators. The lye is burned in an oven and the chemicals are recovered as a melt at the bottom of the boiler, from where it is discharged for recycling back into the fresh cooking solution system.

The basic operation of the recovery process is the conversion of sodium sulphate back to active sodium sulphide by means of carbon residues in the boiler.

2 82167 Temperature and airflows are adjusted to maintain a reduction zone in the boiler to convert as much sodium sulfate as possible to sodium sulfide. The degree of reduction of sulfate to sulfide in the melt reflects the efficiency of the chemical recovery function and determines the quality of the chemical product obtained from the recovery space.

The properties of the molten solids layer and how they affect heat and chemical recovery are not known with certainty. This is due in large part to the lack of means to accurately monitor or measure the structure of the layer. Mist gas emissions from the molten bed typically prevent significant continuous visual observation.

The layer itself is generally a very porous lattice containing 15.5% by weight of carbon. The typical temperature of the molten bed is about 1,000 ° C with the combustion gases above it at temperatures of 1,100-1,300 ° C.

It is known to suggest that the height of the molten layer on the furnace floor affects the efficiency of the primary goal in converting sodium sulfate to sulfide. In general, the average layer height represents a good chemical reduction environment. On the other hand, however, a low bed height is safest depending on the geometry of the boiler, providing a short cooling time after an emergency stop. In general, unstable bed size is also associated with less stable boiler operation, leading to potentially hazardous shutdowns.

Current knowledge of the effect of bed composition and distribution on the operation of the recovery system as a whole is sufficient to suggest that a constant bed composition is essential to maintain stable boiler operation. To achieve this goal, a clear continuous visual image of the layer would provide a significant portion of the information needed to allow better control of the layer and thus improved chemical recovery.

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Technical background

One prior method for measuring the height of a layer involves installing pyrometers, for example, on the levels of the air vents in the furnace. Temperature measurements are used to infer the height of the 5 layers. This simple technique associates a relatively low and high temperature with the presence or absence of a melt. The problem with this technique is that gas temperatures near the bed can vary greatly with variations in air distribution and cause false readings independent of bed height. Such a monitoring system also tells little about the distribution of the layer material in the central areas of the layer, which is important information because previous observations suggest that the distribution of the material is uneven.

15 Many chemical recovery boiler applications today use closed circuit television cameras to produce molten bed images. These cameras should generate all the necessary information about the structure and height of the layer in a format that should be easy for the user of the capture-20 to interpret and use. However, the produced image is intermittent with individual areas of the layer visible only about 10-20% of the time. The impression of the layer structure is obtained only by observing the images over a period of minutes during which several areas disappear or become visible. Most of the time, the image is blurred by swirling clouds of dust or white glowing gas. Bright flashes illuminate small areas of the layer and occasionally outline the parts between the layers, giving a momentary clear impression of the structure of the layer. However, the entire floor is never visible at once and the oven walls are not separable. Experience with these video recording systems has been that, due to the periodic nature of the image, the information provided is generally insufficient to allow for functional control decisions.

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Several imaging systems have been developed to monitor the interiors of non-recovery boilers in closed, hot vessels to determine some internal property or condition. Most of these systems 5 are designed for use in blast furnaces, coke ovens and the like.

In a recovery boiler, typical melt bed temperatures are about 1,000 ° C with the above combustion gases at temperatures of 1,100-1,300 ° C. In blast furnaces, the surface temperatures of the charge or ore / coke are in the order of 150-300 ° C and the above gases are in the range of 90-130 ° C. Due to these relatively low temperatures, very little visible radiation is emitted from the charge about 12 orders of magnitude less than the intensity of visible radiation from the molten layer of the recovery-15 boiler. Several prior systems, such as U.S. Patent No. 3,718,758 to Ponghis, suggest the use of a light source in conjunction with a television camera, both mounted on a probe and placed in a container. The absence of visible light coupled to the charge-20 to measure the surface temperatures of the charge resulted in

Ponghis to use optical illumination in the infrared range.

Where the temperatures of the surfaces to be photographed are high enough, an infrared scanner can be used if insufficient visible light is available.

U.S. Patent No. 3,588,067 to Shimotsuma describes just such a system in which an industrial television camera using an infrared widget produces an image corresponding to the height and shape of the materials in the container. The infrared signal also produces a temperature distribution. Shimotsuma does not mention the ability of the system to separate the charge and the above gases. In light of the conditions observed in the vicinity of the recovery boiler, the lack of mention by Shimotsuma of any need to prevent interference from turbid gases or particles gives the impression that such conditions are not present in the blast furnace environment.

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In the vicinity of a chemical recovery boiler, it is not sufficient to use a camera with an infrared vidicon alone to monitor the bed, as the vidicon is also sensitive to visible radiation. In contrast to the system of interest in Shimotsuma 5, the size and load of the mist particles in the recovery boiler is such that visibility is substantially blurred in the visible area, in fact. The sodium element and possibly some sodium hydroxides have evaporated on the surface of the layer and react with carbon and oxygen. Sodium carbonate is formed, which condenses into liquid droplets with a diameter of about 0.1 to 0.5. These form a mist present at the bottom of the oven. As these droplets sweep upward with the rising gas flow, they encounter areas of high sulfur dioxide content and the carbonate is replaced by sulfate so that the composition of the mist at the top of the furnace is almost entirely sodium sulfate.

The environment of the chemical recovery boiler also differs from the blast furnace environment in terms of radiation absorption / emission of the gases present above the layers of material. In the recovery boiler, the gases above the molten layer are significantly hotter than those in layer V; surface. As a result, the radiation emitted by the gases in the absorption and emission bands of the spectrum associated with the major gaseous substances present are considerably stronger than what is emitted from the bed. Even at temperatures present in the recovery boiler, these gaseous absorption / emission bands are greatly broadened.

30 In a blast furnace, on the other hand, the gases above the charge are somewhat cooler than the surface of the charge, so that the total radiation from the gases is small compared to the radiation from the surface. The absorption / emission bands are much narrower compared to the corresponding bands in the recovery-35 boiler. The primary effect of the gases in the blast furnace situation would be to absorb only that part of the radiation emitted from the surface 6 82167 which is in relatively narrow absorption bands.

Thus, in a blast furnace situation where there is apparently no significant fog or dust interference and limited absorption / emission interference, a broadband system that is sensitive to a wide range of visible light and infrared light would likely work well. However, suspended light scattering particles or mist and gaseous absorptions and emissions at high temperatures are present in the chemical recovery furnace, both of which together obscure the surface to be imaged. The state-of-the-art broadband infrared system does not perform much better than current visible range systems.

15 Summary of the Invention

What has been found is that an infrared vidicon camera arrangement for imaging produces images suitable for controlling boiler operation when the spectral sensitivity of the system to certain infrared wavelengths 20 is firstly limited to greater than about 1 μm by appropriate filtering. Second, the spectral sensitivity of the system must also be limited to avoid radiation absorptions and emissions from the major constituents of the gases just above the melt.

The imaging device of the present invention for monitoring infrared light emitting surfaces, where the surfaces are substantially obscured by mist and dust particles that strongly scatter visible light and the absorptions and emissions of the above major gases, includes the following elements. An industrial closed-circuit video camera, including an infrared image detector or a video tube, is placed close to the surfaces under consideration, but thermally insulated therefrom. A holding device mounted on the camera places an objective lens for the camera to view the pin-35 pins. The optical filter means limits the spectral sensitivity of the system to wavelengths greater than about 1 μm. The optical filter means also selectively rejects wavelengths of strong gas absorptions and emissions. Finally, the television monitor receives the image detected by the video camera and produces 5 continuous images of the surface of the layer.

The imaging device of the invention produces a substantially clear image of a molten layer of inorganic chemicals recovered from the combustion of kraft black liquor derived from the cooking of wood. An optical filter that excludes all radiation except the narrow band, thus avoiding interference from mist particles and gases above the surface of the molten layer and obscuring the visibility of these surfaces, is a key element of the invention. One practical filter is centered at a wavelength of 1.68 μm and has a bandwidth of 0.07 μm. All other narrowband filters that avoid the above particulate and gas interference are considered within the scope of the invention, including filters for other hot surface systems.

20 Brief explanation of the drawings

Figure 1 is a schematic diagram of an imaging apparatus of the invention together with a molten layer of a chemical recovery boiler.

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Figure 2 predicts extinction coefficients (m) as a function of wavelength-25 (μm) for gas emissions and light scattering particles above the molten layer.

Description of the preferred embodiment

Referring to Figure 1, the device of the invention is illustrated in a schematic manner. A closed-circuit television camera 10 including an infrared video tube component (not shown in detail) is located adjacent to a boiler 20, the inside of which is to be imaged. The lens tube assembly 11 fitted to the camera 10 extends into the boiler 20 through an opening 21 in the wall 22 of the cat-35 space. The lens tube includes such objective, collection and collimation lenses 8 82167 (not shown in detail) as are conventionally required to transmit a remotely reproduced image of the object being monitored to the infrared widget 10. The camera 10 is mounted on a bracket 23 which allows horizontal and vertical adjustment of the boiler floor 30 viewing a substantial portion and any molten layer 31 accumulated thereon.

The optical filter 12 is a key element of the present invention. It is selected to limit the wavelength of the light transmitted to the vidicon from the subject to be recorded at wavelengths greater than 1 μm. This restriction is intended to avoid image interference caused by particles and fog covering the subject.

The optical filter 12 further restricts the light transmitted from the imaged surfaces to a narrow band, which prevents light emissions from the main constituents of the hot gases covering the imaged surface. Careful selection of transmitted wavelengths to avoid these gas interferences is critical to achieving dramatically improved images of reproducible surfaces.

20 Both the camera and the lens holder are equipped with a screen 13 designed to protect them from a dirty foundry environment. The cooling and deaerating system 14 ensures that the temperature of the lenses and the camera does not exceed safe operating levels. The cooling air-25 system also generates high-speed deaeration at the end of the lenses to prevent flue gases, carbon, or melt from contaminating the optics. It has been found that by placing the end of the lens tube 11 about 5-10 cm back from the wall tubes 22, contamination of the lens tube is substantially eliminated without sacrificing the field of view.

The cable 17 transmits the image obtained through the lens and the vidicon for playback on the television monitor 19.

As mentioned above, the most critical element of this imaging system and its ability to produce blur-free images is based on the selection of an optical filter 12 that avoids the effects above the surfaces to be imaged.

II

9 82167 Interference from mist particles and hot gases. The selection process requires an analysis of the transfer of radiation through the particles charged with the particles and the identification of the properties of the gases and particles that affect this transfer process. In its simplest form, the solution of the radiation-displacement equation can be expressed as: Ι (λ) = ls (\) e ~ yl + Bx (Tg) (le'YA) 10 where Ι (λ) is the radiation intensity at light length λ, which reaches the observer at a distance £ from the surface s , Xs (λ) is the intensity of the radiation at wavelength λ flowing out of the surface s in the direction of the observer, (T ^) is the intensity of the black body at gas temperature, T, and γ is the ex-15 tincture factor for particulate gases between surface s and the observer. This requires that the extinction coefficient and the temperature of the gases are constant along the path £.

As can be seen from this equation, when γ is large, the radiation emanating from the surface is substantially attenuated 20 and only the radiation emitted by the gases reaches the observer, when γ is very small, the radiation from the surface reaches the observer with low attenuation and the gases cause little interference. These low γ regions, if any, are areas where clear images of the surface can be obtained, such as the molten layer of interest herein.

The extinction coefficient can be expressed as the sum of the proportions of gases and particles; 30 Y V V + YP »where is the gas absorption coefficient, Y ^ a is the particle absorption coefficient and γ ^ 5 is the particle binding coefficient. In order to obtain meaningful estimates of these large values, it is necessary to study the conditions prevailing in the environment of interest, in this case the bottom of the furnace.

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Certain information / required is required for gas temperature levels, gasifier concentrations and optical properties, particle sizes and mass loads as well as particle optical properties. In fact, no ko-5 linguistic data exists for any of these quantities at the bottom of the furnace. However, satisfactory estimates of these quantities can be made that allow the selection of a suitable filter.

Lack of reliable measurements of gas species 10 concentrations in different zones of the recovery furnace, a computer model was designed using thermochemical equilibrium calculations and heat and mass balances to obtain at least rough expressions of the probable levels and gas temperatures of these quantities in the different zones of the furnace. See. Merriam, R.L., "Computer Model of a Kraft Recovery Furnace - Version 1.0," Vol. II, pp. 3-7, Arthur D. Little, Inc. (August 1979). Computer model concentrations were combined with data on the spectral properties of alcohol combustion products of CO, CO 2, H 2 O, N 2 and O 2 and data on the low temperature absorption spectrum of sulfur compounds expected to be developed to provide rough estimates of the difference between the probable proportions . Estimates of extinction-25 coefficients calculated using a computer model and related data based on typical concentrations at the bottom of the furnace are shown in Figure 2 as a function of wavelength. Figure 2 shows that the main proportions or disturbances of the gaseous substances are due to the substances CO2, H2O, SO2 and Na.

A somewhat different analysis is needed to estimate extinction coefficients for mist particle disturbance. Reported measurements of particulate emissions in recovery boilers, performed both before and after the particulate removal devices, suggest the presence of three distinct particle classes in the vicinity of the chemical recovery boiler 11,816,167. It has submicron mist particles (0.2-0.5 in diameter) consisting mainly of sodium sulphate and to some extent sodium carbonate. It has large spheres (10-100 μm in diameter) of sodium sulfate and carbonate formed when the suspension burns from liquid droplets. The third class of particles contains very large (up to 2,000 μm in diameter) pieces of vegetable carbon with a high carbon content.

Literature studies show that the average particle size of the mass is about 1 μm. These studies typically involve sampling after the boiler and it is very likely that the majority of the large particles in the furnace have adhered to the boiler tubes. Thus, the average pulp particle diameter in the furnace is pal-15 ion greater than 1 μm. One model indicates that approximately 35% of the mass of the particulate load is mist from the bed and 65% is charcoal and the remainder from the combustion of the liquid suspension. This would suggest that the average particle diameter of the pulp in the furnace would be strongly weighted towards 20 larger residual particles. Recent measurements of supercharger deposition show that the deposition level is consistent with an aerodynamic mean diameter of 50 μm, supported by scanning electron micrographs of spherical particles collected from the top of the furnace. Another model shows that about 9% of the sodium in the black liquor ends up as particles, which requires a typical kraft pulp recovery boiler to have a total particle load of about 2.3 g / m 3.

Literature analysis and models allow estimation of particle size, mass loads, and compositions, as shown in the following table.

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Kraft chemical recovery boiler

Estimated particle sizes, mass loads and compositions

Particle Mass Mass Volume Number Number Composition Type 5 Average Load Concentration Diameter Composition Composition (μ m) (g / m3) (m3 / m3) (cm-3) Mist 0.3 0.8 3.0 x 10 "7 2.0 x 10 7 Na 2 SO 4 + Na 2 CO 3

residue 75.0 1.5 5.5x10 2.5 Na2SO4 + Na2CO2 + O

10+ vegetable carbon

In order to estimate the levels of the absorption and scattering coefficients of the particles, it is necessary to consider both their size and composition. The absorption and scattering coefficients are related to the "efficiency factors" for absorption and scattering described by Mie's theory for the interaction of light with small particles. See H.C. Van De Hulst, "Ligt Scattering by Small Particles," John Wiley and Sons, New York (1957), and M. Kerker, "The Scattering of Light and 20 Other Electromagnetic Radiation," Academic Press, New York (1969). The particle absorption and scattering factors can be expressed by these efficiency factors as follows: 25 Ypa = 3/2? A (m, »Cv / D32

Ypj = 3/2 5S (m, X) Cv / D32 where Q_ is the average absorption efficiency for a given set of 30 particles, Q is the average scattering efficiency, «9 m is the complex particle reflectance, λ is the wavelength of light, Cv is the volume concentration-gas concentration per) and is the average diameter of the Sauter particle collection equal to the total particle volume of 35 divided by the total surface area of the particles.

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The mist particles consist primarily of sodium sulfate and sodium carbonate, which are in fact transparent except for the local spectral regions in the infrared region behind 5 μm. As a result, with the exception of these 5 regions, the imaginary portion of their reflection coefficient will be insignificant as well as the absorption efficiency made. In the region behind 5 yum, their small size over the wavelength results in extremely low absorption efficiencies even in the case where the imaginary part of the reflection coefficient is significant. Thus, in the case of mist particles, γ is insignificant. However, the scattering efficiency has a strong resonance at wavelengths comparable to the particle diameter. As a result, these particles are scattered over substantially the entire visible light region and to about 1.0 microns in the infrared region. This is shown by the large peak in the extinction coefficient curve for the particles at about 0.2 μm in Figure 2.

The large residual and carbon particles are likely to contain significant amounts of solid carbon, making them highly absorbent and providing them with high imaginary reflection coefficients. This, combined with their large size at the wavelengths of interest, results in the scattering and absorption efficiency ratios of these particles being both very close to the unit. However, because they are so large, their number density is 3 very low (2-3 / cm). Thus, the absorption and scattering coefficients caused by these particles are small (γ = -1 Pa γ 5 0.015 m) at wavelengths smaller than ps 30 than about 30 μm. This effectively sets a lower limit for the extinction of the gas particle system throughout the region of interest. Thus, it is apparent that, with the exception of the sodium emission / absorption phenomenon, it is scattering from the mist particles, which limits the visibility in the visible part of the spectrum. In this area, the extinction coefficient caused by the mist particles is of the order of 1 to 5 meters, which means that the radiation intensity of 14,816,167 emanating from the bed has been reduced by 98% in 1 to 4 meters. This explains why it is not possible to see the rear wall of a 10 m wide oven using visible light.

After a wavelength of about 4 μm, the extinction coefficient for the particles in the infrared range is mainly based on large residual and carbon particles and is approximately constant at 0.03 m. As a result, in the spectral windows, between the gas absorption / emission peaks, the radiation should be able to move a distance of about 130 m before 98% attenuation. In these areas, the visibility in the oven should be excellent.

The spectral windows generally suitable for imaging the molten layer identified by the above-15a analysis are as follows: 1.57 to 1.73 μm, 2.23 to 2.43 μm, 3.25 to 4, 05 μm, 4.80 - 5.30 μm, 6.90 - 7.20 μm, 7.60 - 7.80 μm, 7.90 - 13.90 μm. Other windows behind 13.90 μm are apparent when examining Figure 2.

By far the best visibility 20 predicted by these calculations is seen behind about 4 in the windows in the gas absorption / emission spectrum. However, infrared imaging devices capable of operating at these long wavelengths are quite expensive, have relatively low resolution, and often use sensor elements that require cooling close to absolute zero, making them unsuitable for permanent uncontrolled use in a factory environment. Furthermore, special infrared light transmitting optics are required for wavelengths greater than about 2.5 microns. In recent years, 30 infrared-sensitive video tubes that are compatible with conventional closed-circuit television cameras have become commercially available. These tubes are typically sensitive to radiation at wavelengths of about 2 .mu.m.

Examination of Figure 2 shows that the lowest predicted value of the extinction coefficient in the range less than 2 μm is found in the window with about 1.7 μm between the two water vapor strips. At this wavelength, the predicted extinction coefficient is about 0.2 m. This is an order of magnitude smaller than in the visible region of the spectrum, and based on this estimate, light from 5 layers should travel across a 10-meter-wide furnace with an attenuation of about 86%. This means that the back wall will be visible at this wavelength.

Example 10 The modified system of the present invention is similar to the commercial visible light system currently in use except for fitting an infrared video tube in place of a conventional visible light tube and adding a narrowband interference filter. Motorola's 15 high temperature CCTV system, including a sealed camera and furnace lens tube, as described above, was installed as shown in Figure 1 with the lens tube passed through the secondary air vent of the Babcock and Wilcox kraft chemical recovery boiler. The Hamamatsu Type N 214, 20 lead sulfide, s, 54 cm (1 inch) diameter infrared imaging detector tube manufactured by Hamamatsu Company of Hamamatsu City, Japan, was .. fitted to a standard Motorola video tube sealer.

A narrowband interference filter centered at a wavelength of 1.684 μm and having a bandwidth of 0.07 μm was used as the spectral filter.

When testing this arrangement, it was found that the focus was substantially improved by moving the video tube away from the lens tube by fitting a spacer between the camera body 30 and the lens tube. It is believed that this additional distance was due to an increase in the focal length of the lens system at longer wavelengths. Once the accuracy was adjusted, it was found that the visibility at the selected wavelength was significantly better than in conventional systems. The entire layer within the field of view could be clearly and continuously and the height and structure of the layer, ie 82167, could be easily observed. The surface properties of the layer could also be separated on the back wall of the furnace. The layer was very shallow during these experiments and the primary and secondary air openings in the rear wall could be seen quite clearly as dark 5 rectangular areas on generally bright wall surfaces.

It has been found that the resolution of the infrared widget system is such that individual primary air vents measuring approximately 5 x 25 cm could be separated. The figure indicates that under normal operating conditions, the system allows the separation of 10 features with dimensions of a few centimeters or less at a distance of 10 m.

The infrared widget inherently has a slower ai response than the visible light vidicon tube. This may be in part due to the fact that the larger floating particles 15 and liquid droplets were not visible in the image. However, the time response is short enough that material falling from the walls was observed as well as large "clumps" rolling across the layer from time to time. The dust raised by the primary air jets and the falling material hitting the layer was ha-20.

Because the system is relatively insensitive to radiation from gaseous substances that occur primarily at the wavelengths removed by the filter and because the system operates in the infrared range, the observed radiation is thermal radiation emitted by solid surfaces. As a result, the intensity observed is related to the temperature of these surfaces. However, the orientation of the surfaces as well as their emittance contribute to the observed intensity distribution. The fact that the visual assessment of the layer contours is very well in agreement with the direction of travel of the objects on the surface of the layer due to the gravity of the ground supports the view that surface orientation plays a significant role in determining the observed intensity distribution. At the same time, the fact that the material falling from the walls 35, which has cooled on contact with the water pipes, appears dark when it first

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17 821 67 hits the surface, but gradually becomes brighter and less separable from the material of the surrounding layer, supports the view that the temperature of the material also has a large effect on the intensity of the image.

5 The ability to look inside the furnace clearly provides an unprecedented opportunity to observe the processes inside the furnace and to increase knowledge of the operating phenomena of the boiler. The present invention is not limited in its usefulness to kraft pulp chemical recovery boiler systems. It is useful for viewing any dark surface obscured by the hot gases, vapors, and particles above, when suitable filters can be selected according to the strategy described above to avoid image interference.

Claims (4)

An imaging device for observing a surface (31) that emits infrared light in the molten layer of a sulfate cellulose effluent recovery boiler, wherein the surface of the molten layer is at a temperature of at least 1000 ° C and the gases above it and the smoke is at substantially higher temperatures, the surface of the molten layer being substantially obscured by smoke particles, which spread visible light, and by hot gas substances located above the surface of the molten layer and themselves being strong emitters of infrared light and which selectively absorbs radiation from the surface of the molten layer emitting infrared light and wherein such a device comprises sensor and detector means for an infrared image and a visual display device (19), characterized in that the image sensor and detector means comprise means an optical filter means (12) which provides an optical output for the sensor and detector means and in which case the radiations from said surface (31) having a path length below 1pm are attenuated to avoid smoke interference and in which the radiations from said surface (31), whose path lengths correspond to the radiation emissions of the hot gases and radiation absorptions, are also attenuated so that said infrared sensor and detector means are spectrally sensitive to a narrow band, centered substantially on the path length of 1.68 µm, and said display device (19) is arranged to be dependent on the output signal representing the signal from the sensor and detector means and to generate a substantially interference-free visible image from the surface of the molten layer. 2. Imaging device according to claim 1, characterized in that the infrared sensor and detector means include an infrared sensitive vidicon tube which is spectrally sensitive to infrared radiation up to the path length 2μπι. Imaging device according to claim 1 or 2, characterized in that the optical filter means has only one passband located in the path length range 1.57 - 1.73 µm. Imaging device according to any of the claims 1 to 3, characterized in that the infrared sensor and detector means include: an image detector means which is sensitive in the infrared range for sensing the image of the emitting surface and a lens means for projecting and focusing the infrared image of the surface of a detector means, the optical filter means being positioned between the lens means and the detector means, and the display means being arranged to depend on the output of the detector means for generating a substantially interference-free visual image while observing the surfaces.