Abstract:
A method for protecting a tuyere assembly and a refractory lining of a furnace, and in particular a blast furnace, against damage caused by expansion of the refractory lining. This method includes providing a clearance between the tuyere assembly and a refractory lining portion below the tuyere assembly and monitoring this clearance by means of a displacement sensor.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a method for protecting a tuyere assembly and a refractory lining of a furnace. 
     The interior of a shaft furnace, such as a blast furnace, is generally lined with a refractory material. The latter usually consists of items such as bricks or blocks, e.g. made from carbon, aluminium silicate or ceramic material, which are cemented for imperviousness and stability. Usually, different types of bricks or blocks are used in different zones, according to the predominant type of stress in the respective zone. 
     It is well known in the art that the refractory lining is subject to expansion. Basically two different effects can cause refractory lining expansion. A first effect is thermal expansion caused by the temperature increase of the refractory lining during start-up of the blast furnace. Thermal expansion is generally reversible. A second effect is referred to as “chemical expansion”. This effect is due to chemical reactions that take place in the refractory material during its lifetime. Such chemical reactions cause an irreversible expansion of the refractory lining. 
     It will be noted that the refractory lining can find external bodies on the way of its expansion displacement. Such a situation occurs with the plurality of circumferentially arranged tuyere assemblies, which penetrate through the refractory lining into the blast furnace. As the refractory lining surrounds each of these tuyere assemblies, the latter can be on the way of the expansion of the wall lining. This can result in deformation of the tuyere assemblies and/or in a crushing of the expanding refractory lining under the tuyere assemblies. 
     To prevent unnecessary downtime and damage, it is important to take preventive measures. A known approach is to provide softening layers between refractory items, which compensate for dilatation of the refractory lining. They generally consist of thin, compressible and isolating joint plates. U.S. Pat. No. 3,805,466 describes such an approach. However, for stability and other reasons, the height of such known softening layers is limited. Thus, the summed vertical dimension of such layers is generally in the order of tenths of a percent of the summed vertical refractory lining dimension from furnace foundation to the tuyere assembly. Such layers can, at least partly, compensate for thermal expansion or dilatation of the refractory lining. However, they can normally not compensate for chemical expansion of the refractory lining. Indeed, chemical expansion is variable, generally irreversible and difficult, if not impossible, to predict. Moreover, chemical expansion is progressing over refractory lining service-life. With increasing extent of chemical expansion, the capability of the abovementioned layers to compensate for dilatation is reduced. Consequently, damage to the tuyere assemblies and/or the refractory lining cannot be efficiently prevented by known softening layers. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention provides a method for protecting a tuyere assembly and a refractory lining of a furnace against damage caused by expansion of a refractory lining. This method comprises the steps of providing a clearance between the tuyere assembly and a refractory lining portion below the tuyere assembly and monitoring this clearance by means of a displacement sensor. The clearance is a space deprived of refractory lining, usually consisting of an air gap or a gap filled with a compressible material. Advantageously, the clearance is provided immediately adjacent and underneath, preferably at the lower half of every tuyere assembly. Monitoring of the clearance warrants detection of critical expansion of the refractory lining during operation. More specifically, it warrants that the combined effect of thermal and chemical expansion is taken into account in preventive manner. Furthermore, the monitoring allows acquisition of information regarding the condition of the refractory lining, thereby contributing to preventive maintenance. It will be appreciated that monitoring of the clearance by means of a displacement sensor is not absolutely necessary on every tuyere assembly. By using additional information and mathematical methods, e.g. rotational symmetry of the furnace and interpolation, it is possible to estimate the expansion status of the lining below each tuyere assembly while having installed sensors only at some of the tuyere assemblies. However, it is also possible to provide multiple sensors to monitor the same clearance, thereby providing more detail and redundancy of measurements. In summary, the method according to the present invention provides a simple and reliable method of protecting tuyere assemblies and refractory lining in a furnace such as a shaft furnace and in particular a blast furnace. More specifically, the combined effect of thermal dilatation and chemical expansion is taken into account. Thus the method in accordance with the present invention increases service-life of tuyere assemblies as well as service-life of refractory lining. 
     Preferably at least one removable refractory layer is provided below the tuyere assembly. This removable refractory layer is then removed if, during operation of the furnace, monitoring of the clearance shows that the height of the clearance falls below a predetermined value. Proceeding this way circumvents the necessity of oversizing of the initial clearance for security reasons. Indeed, if necessary, clearance can be increased by simply removing at least one removable refractory layer. Preferably, the removable layer consists of solid refractory material being cemented to the adjacent refractory lining. Of course, it is also possible to replace the removed refractory layer by a new removable refractory layer of reduced thickness. It will be appreciated that the step of monitoring the clearance by means of the displacement sensor will provide necessary expansion information to decide when to remove the removable refractory layer. 
     Advantageously, the method further comprises sealing the clearance with a compressible sealing material. This sealing prevents dust accumulation within the clearance, which could reduce its effectiveness, and protects the sensor against a direct exposure to hot furnace gases. 
     Preferably, the method comprises continuously monitoring the clearance during operation of the furnace. This allows detection of critical expansion of the refractory lining, and possibly preventive shutdown of the furnace. Moreover continuous monitoring of the expansion allows for observation of the refractory condition during operation. For example, integrity of the refractory lining can be monitored. In this way, a shutdown can be initiated before further damage occurs. 
     Advantageously, the method further comprises monitoring the clearance during shutdown of the furnace. Thereby, contraction behaviour of the refractory lining portion below the tuyere assembly is determined. 
     Preferably, the method comprises monitoring the clearance during start-up of the furnace. Thereby, expansion behaviour of the refractory lining portion below the tuyere assembly is determined. This step allows for gathering further information on the refractory lining condition, for example verifying uniform circumferential expansion of the refractory lining. The data thus obtained can be used as additional feedback control information for controlled heating and controlled expansion during start-up of the furnace. This data can also contribute to process control, e.g. by giving information on build-up of skull and partition of the heat load. When combined to monitoring the clearance during operation of the furnace, this step contributes to the follow-up of the refractory lining behaviour during the furnace campaign. For instance, additional expansion monitored after the start-up period can be the sign of chemical expansion due to a chemical attack such as the alkali attack. In combination with monitoring the clearance during shutdown, opening of crevices in the refractory lining can be detected. Observation of reduced thermal contraction during the cooling of a shutdown, generally followed by an increased expansion of the refractory lining after the beginning of a subsequent start-up, can indicate the opening of crevices, which have then generally been infiltrated with metal. 
     Advantageously, the method further comprises providing a temperature sensor and monitoring temperature within the clearance between the tuyere assembly and the refractory lining portion to detect possible hot gas leakage. As mentioned above, the clearance should be sealed with suitable material. In case the sealing degrades, hot gases including dust particles from the furnace interior can penetrate the clearance. Such degradation can occur because of reduced wear resistance of the compressible sealing material, when compared to the refractory lining or the removable refractory layer. 
     The method according to the present invention preferably uses a linear electromechanical displacement sensor. A relatively simple induction type electromechanical displacement sensor is advantageously used, because of its robustness and reliability. Such a sensor preferably includes a sensor body mounted in a mounting hole of a tuyere cooler and a measuring pin slidingly supported by the sensor body, wherein the pin has a tip that is in contact with an upper surface of the refractory lining or the removable refractory layer. The sensor body is preferably mounted so as to engage the mounting hole in sealing manner. Mounting the sensor body into a mounting hole of a tuyere cooler provides cooling of the displacement sensor without extra expenditure. Advantageously, the tip of the pin consists of heat resistant material, such as ceramic, cermet or refractory steel. In another advantageous embodiment, at least part of the tip is breakable, which protects the sensor from possible damage. 
     The method according to the present invention can be applied to any type of shaft furnace, and in particular a blast furnace. 
     It will be appreciated that, although the above description mentions tuyere assemblies, the present invention can be applied to protect other stationary fixed elements penetrating a refractory lining of a furnace. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       The present invention will be more apparent from the following description of not limiting embodiments with reference to the attached drawings, wherein 
         FIG. 1 : is a vertical cross sectional view of a first embodiment of a blast furnace wall immediately below a tuyere assembly, with a first embodiment of a displacement sensor; 
         FIG. 2 : is a partially cut rear view of the tuyere assembly of the first embodiment; 
         FIG. 3 : is a vertical cross sectional view of a second embodiment of a blast furnace wall immediately below a tuyere assembly, with a second embodiment of a displacement sensor; 
     
    
    
     DETAILED DESCRIPTION 
     In  FIG. 1 , reference number  10  globally identifies a blast furnace wall immediately below a tuyere assembly  12 , which is only shown in part. The blast furnace wall  10  comprises in a manner known per se an outer furnace shell  14  and an inner refractory lining  16 . The tuyere assembly comprises in a manner known per se: a blast tuyere  18 , a tuyere holder  20 , a tuyere arc cooler  22  and a tuyere block  24  with a tuyere cooler holder  26 . The tuyere block  24  is fixed, e.g. by welding, to a furnace shell  14 . The tuyere arc cooler  22  is press-fit into the tuyere cooler holder  26  of the tuyere block  24 , and the blast tuyere  18  is press-fit into the tuyere holder  20  of the tuyere arc cooler  22 . The tuyere assembly  12  has a rotational symmetry with a symmetry axis  30 . 
     Reference number  32  identifies a refractory block that is part of the refractory lining  16  below the tuyere assembly  12 . The upper surface  34  of the refractory block  32  is a curved surface delimiting the lower part of a through-hole  36  in the refractory lining  16 . The tuyere assembly  12  passes axially through the through-hole  36  in the refractory lining  16 . 
     Arrow  40  identifies a clearance or gap between the tuyere assembly  12  and the upper surface  38  of the refractory lining portion  16 , located below the tuyere assembly  12 . The clearance  40  surrounds the lower half of the tuyere assembly  12 . 
     According to an important aspect of the present invention, a displacement sensor  50  is provided to monitor the clearance  40 , and more specifically the height of the clearance  40 . This sensor  50  has a sensor body  52  mounted in sealed manner in a mounting hole  54  of the tuyere arc cooler  22 . In the embodiments shown on the figures, the sensor  50  is an electromechanical linear displacement sensor based on inductivity measurement. The sensor body  52  has a cylindrical cavity  56  with a sensor pin  58  slidingly fitted therein. The pin  58  comprises a soft iron core  60  and a ceramic tip  62 . The sensor body  52  includes a coil  64  with which the soft iron core  60  interacts as a plunger. Cast-in connectors  66  allow connection of measurement equipment. A spring  68  is associated with the sensor pin  58 , so as to bias the ceramic tip  62  of the sensor pin  58  into mechanical contact with the upper surface  38  of removable refractory layers  72 ,  74  resting on the upper surface  34  of the refractory block  32 . 
     As shown in  FIG. 2 , the removable layers  72 ,  74  are provided below the tuyere assembly  12 . At least one of the removable refractory layers  72 ,  74  is removed if the height of said clearance  40  is less than a predetermined value. The removable refractory layers  72 ,  74 , when piled, fit onto the upper surface  34  of refractory block  32 . They are preferably made of solid and durable material such as silicon carbide. Each of the removable refractory layers  72 ,  74  is, for ease of construction, composed of two arcuate elements. The latter elements define, when assembled a shell of U-shaped cross-section. The removable refractory layers  72 ,  74  allow to optimize the initial height of the clearance  40  to a minimum. 
     Returning to  FIG. 1 , reference number  80  identifies a compressible sealing material, which seals the clearance  40 . The compressible sealing material  80  is provided within the clearance  40  between tuyere assembly  12  and the upper surface  38  of the removable refractory layer  72 , or the refractory lining portion  16 . It seals the clearance, while taking up expansion of the refractory lining  16 . The compressible sealing material  80  is made of heat resistant, compressible material such as rock wool or preferably silica-alumina fibre. A free space  82  is provided within the compressible sealing material  80 , around the sensor pin  58  for unimpeded movement of the latter. 
     In a first phase, the clearance  40  filled with the compressible sealing material  80 , takes up or buffers expansion of the refractory lining  16  below the tuyere assembly  12 . The expansion evolution is monitored by means of displacement sensor  50  to decide when the expansion is considered as excessive. In a subsequent second phase, when excessive expansion, more specifically permanent chemical expansion, is detected by displacement sensor  50 , at least one removable layer  72 ,  74  is removed, for example pushed into the furnace. After removal of at least one removable layer  72 ,  74 , the aforementioned initial clearance  40  will be enlarged by the height of the removed removable layer  72 , 74 . 
     During operation of the blast furnace, the clearance  40 , and more specifically the height of the clearance  40 , is continuously monitored by displacement sensor  50 . To perform monitoring, the displacement sensor  50  is connected to an inductivity measurement device, known per se, by means of connectors  66 . An increase in temperature and/or chemical effect causes the refractory lining  16  below the tuyere assembly  12  to expand upwards such as to approach the lower half of the tuyere assembly  12 . The upper surface  34  of the refractory lining  16  and, if still present, the removable layers  72 ,  74  are displaced upwards. As a result, pin  58  of sensor  50  will be pushed into the cylindrical cavity  56 . As the soft iron core  60  further penetrates the coil  64 , it modifies inductivity of the coil  64 . Thus, the displacement sensor  50  serves to determine, when removal of, at least one of, the removable refractory layers  72 , 74 , becomes necessary. This step of monitoring the clearance  40  warrants detection of critical expansion of the refractory lining  16  during operation and provides a means to allow preventive intervention. More specifically, the combined effect of thermal and chemical expansion is taken into account in preventive manner. 
     According to another aspect, the clearance  40  is monitored during shutdown of the blast furnace. Thereby contraction behaviour of the refractory lining portion  16  below the tuyere assembly  12  is determined. This monitoring is carried out, mutatis mutandis, in similar manner to what is described above. Information regarding the condition of the refractory lining  16  is acquired in this step, thereby contributing to preventive maintenance. 
     According to a further aspect, the clearance  40  is measured during start-up of the blast furnace. Thereby expansion behaviour of the refractory lining portion  16  below the tuyere assembly  12  is determined. This monitoring is carried out, mutatis mutandis, in similar manner to what is described above. Determining expansion behaviour during start-up gives important feedback information about the refractory lining  16  and the process. 
       FIG. 3  shows a second, slightly different, embodiment. With regard to  FIG. 1 , like reference numbers identify like parts. In the embodiment of  FIG. 3 , only one removable refractory layer  72 ′ is provided. Less total expansion being predicted in the embodiment of  FIG. 3 , the upper surface  34  of refractory block  32  is located at a higher vertical position within the blast furnace wall  10 . 
     Reference number  90  identifies a temperature sensor with a probe tip  92 . The probe tip  92  protrudes into the clearance  40  and the compressible sealing material  80  therein, ending at approximately a quarter of the height thereof. The temperature sensor  90  is mounted in a sheath  94  associated with the sensor body  52  of the displacement sensor  50 . The temperature sensor  90  is connected to a measuring device by means of connector  96 . 
     According to the present invention, temperature sensor  90  is used to monitor temperature within the clearance  40  between tuyere assembly  12  and refractory lining portion  16  in order to detect possible hot gas leakage. Such hot gas leakage can occur after a degradation of either the compressible sealing material  80  or the removable refractory layer  72 ′. Monitoring temperature within the clearance  40  helps to monitor the condition of compressible sealing material  80  and to determine when the latter is to be serviced. 
     Reference number  100  identifies a bellows expansion sheath surrounding sensor pin  58 . Its upper end is sealingly connected to the sensor body  52 . Its lower end is closed and biased against the upper surface  38  of the removable refractory layer  72 ′. The bellows expansion sheath  100  prevents the compressible sealing material  80  from impeding the displacement sensor  50 , and more specifically the movement of sensor pin  58 . In case of hot furnace gas leakage, bellows joint  100  also prevents dust particles to impair displacement sensor  50 . 
     The following, not limiting, example illustrates improved protection: 
     EXAMPLE 
     
       
         
               
               
             
           
               
                   
               
             
             
               
                 Height of lower refractory lining (H rl ): 
                 10 m 
               
               
                 (from furnace foundation to tuyere centre line) 
               
               
                 Average buffering height 
                 125 mm 
               
               
                 (clearance + removable layer(s)) (h b ): 
               
               
                 Expansion buffering capacity in percent (H rl /h b ): 
                 1.25% 
               
               
                 (excluding compressible joint plates within refractory lining)