Patent Publication Number: US-2012045728-A1

Title: Apparatus and method for cooling kiln exhaust gases in a kiln bypass

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is the United States national phase under 35 U.S.C. §371 of International Application No. PCT/EP2008/065744, filed on 18 Nov. 2008. The entirety of International Application No. PCT/EP2008/065744 is incorporated by reference herein. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to an apparatus for cooling kiln exhaust gases in a kiln bypass, which apparatus comprises a mixing chamber for extracting and cooling a portion of the kiln exhaust gases from a kiln system, said mixing chamber comprising a tubular housing being provided at one end with an exhaust gas inlet for kiln exhaust gases and provided at its other end with an outlet for cooled exhaust gases, said mixing chamber further comprising a tangential inlet for cooling gases, where the apparatus also comprises a first fan for supplying cooling gases to the mixing chamber and a second fan for drawing the kiln exhaust gases through the kiln bypass. The invention also relates to a method for cooling exhaust gases in a kiln bypass. 
     BACKGROUND OF THE ART 
     An apparatus of the aforementioned kind is known for example from EP 927 707 and used for reducing the quantity of volatile components such as chloride, alkali and sulphur which have been introduced to a cement manufacturing plant together with the cement raw materials and the fuel and circulating in the kiln system of the plant and potentially causing clogging and unstable kiln operation. Briefly described, the apparatus operates according to a method where a portion of the kiln exhaust gases via a bypass is extracted and cooled allowing the volatile components in solid form to be separated from the exhaust gases and subsequently disposed of or possibly used in the finished cement or for other purposes. 
     The apparatus according to EP 927 707 is designed as a double tube construction, consisting of an outer tube and an inner tube forming between them an annular channel, and having a mixing zone immediately in front of the inner tube. The kiln exhaust gases are introduced into the apparatus via the outer tube which is connected to the kiln system, and subsequently mixed and cooled in the mixing zone by means of cooling gases which in the form of a rotating flow following a spiral-shaped flow path are directed to the mixing zone via the annular channel provided between the outer tube and the inner tube. The mixed and cooled exhaust gases are subsequently discharged via the inner tube for further treatment in a subsequent process stage. 
     During the operation of a kiln bypass of the aforementioned kind the operator will determine the necessary quantity of kiln exhaust gases to be drawn through the kiln bypass in order to maintain a constant level of the volatile components circulating in the kiln system. Typically the quantity of kiln exhaust gases being drawn through the kiln bypass will constitute between 2 and 10 per cent of the total exhaust gas volume, depending on the quantity and composition of the volatile components. During operation regulation of the kiln bypass is traditionally based on the maintenance of a predetermined value for the temperature of the mixed exhaust gases being discharged from the mixing chamber. The regulation per se is carried out on the basis of continuous measurements of the temperature of the mixed exhaust gases and subsequent regulation of the quantities of respectively the cooling gases and kiln exhaust gases as a function of the measured temperature according to a predetermined procedure through an adjustment of one or both fans of the apparatus. The inherent disadvantage of this mode of regulation is that, for example, variations in the temperature of the extracted kiln exhaust gases or varying quantities of dust in the extracted kiln exhaust gases may cause major variations in the quantity of kiln exhaust gases being drawn through the kiln bypass. This is undesirable given that the quantity of combustion air/kiln exhaust gases being drawn through the kiln will also exhibit variations, hence making it difficult for the operator to maintain a specific temperature in the burning zone and a specific air surplus in the kiln. This may affect not only the product quality but may also influence the evaporation of the sulphur and alkali compounds. This involves increased risk of coatings or cloggings occurring in the kiln system due to increased concentration of volatile components or the risk of coatings being formed in the mixing chamber due to insufficient cooling of the kiln exhaust gases. Also, there is a risk of cooling gases entering the kiln system as false air in cases where the swirl number of the cooling gases introduced to the mixing chamber will be so high that the apex of the formed eddy protrudes into the kiln system. 
     Therefore, it is desirable to have the capability to regulate a kiln bypass so that the quantity of kiln exhaust gases being drawn through the kiln bypass will be substantially constant, while at the same time the cooling in the mixing chamber will be sufficient to prevent coatings from being formed and will take place without entry of cooling gases into the kiln system in the form of false air. 
     BRIEF SUMMARY OF THE INVENTION 
     It is the object of the present invention to provide an apparatus as well as a method for cooling kiln exhaust gases in a kiln bypass by means of which the above mentioned desirable objectives can be achieved. 
     According to the present invention this is achieved by means of an apparatus of the kind mentioned in the introduction, and being characterized in that the apparatus comprises means for measuring the mass flow m A  and the flow velocity v A  of the cooling gases which are introduced to the mixing chamber, and the mass flow m B  and the flow velocity v B  of the cooled exhaust gases being discharged from the mixing chamber, a calculating unit to determine on the basis of the measured values m A , v A , m B  and v B  the actual mass flow m C  and the flow velocity v C  for the kiln exhaust gases being drawn through the kiln bypass and to compare the actual mass flow m C  with a predetermined value for kiln exhaust gases targeted for being drawn through the kiln bypass, a calculating unit to determine on the basis of the values m A , v A , m C  and v C  the actual swirl number S of the gases in the mixing chamber and to compare this with a predetermined, desired value for the swirl number of the gases in the mixing chamber, and means for regulating respectively the fan for feeding cooling gases to the mixing chamber, the fan for drawing the kiln exhaust gases through the kiln bypass and the pressure loss across the apparatus when Δm C  or ΔS deviates from 0. 
     The method according to the invention for cooling kiln exhaust gases in a kiln bypass comprises the steps that a portion of the exhaust gases from a kiln system are extracted and cooled in a mixing chamber which comprises a tubular housing where kiln exhaust gases are introduced at one end via an exhaust gas inlet, cooled exhaust gases are discharged at the other end via an outlet and cooling gases are introduced to the mixing chamber via a tangential cooling gas inlet, and where cooling gases are supplied to the mixing chamber by means of a first fan and the kiln exhaust gases are drawn through the kiln bypass by means of a second fan, and being characterized in that respectively the mass flow m A  and the flow velocity v A  of the cooling gases being introduced to the mixing chamber, and the mass flow m B  and the flow velocity v B  of the cooled exhaust gases being discharged from the mixing chamber are measured, and in that the actual mass flow m C  and flow velocity v C  for the kiln exhaust gases being drawn through the kiln bypass are determined on the basis of the measured values m A , v A , m B  and v B  and compared with a predetermined value for kiln exhaust gases targeted for being drawn through the kiln bypass, and in that the actual swirl number S of the gases in the mixing chamber are determined on the basis of the values m A , v A , m C  and v C  and compared with a predetermined, desired value for the swirl number of the gases in the mixing chamber, and in that at least one of, respectively, the fan for introducing cooling gases to the mixing chamber, the fan for drawing the kiln exhaust gases through the kiln bypass and the pressure loss across the apparatus are regulated when Δm C  or ΔS deviates from 0. 
     The swirl number S is defined as the dimensionless quantity expressed by: 
         S =(m A  v A  R 1 )/(m C  v C  R 2 ), 
     where R 1  and R 2  are characteristic radii in the mixing chamber. Numerous tests have demonstrated that the magnitude of the quantity is descriptive of the propagation of the internal vortex in the mixing chamber. The higher the value of S is, the longer the extension of the vortex will be. 
     For the apparatus as well as the method according to the present invention for cooling kiln exhaust gases in a kiln bypass it is hereby obtained that, even subject to major variations in operating conditions, the quantity of kiln exhaust gases being drawn through the kiln bypass can be kept essentially constant while simultaneously ensuring sufficient cooling of the kiln exhaust gases in the mixing chamber, thereby preventing coatings from being formed in the mixing chamber per se as well as at its outlet and preventing entry of cooling gases into the kiln system as false air. This is due to the fact that the actual mass flow for the kiln exhaust gases being drawn through the kiln bypass and the actual swirl number S of the gases in the mixing chamber serve as control parameters. Formation of coatings on the walls of the mixing chamber will thus be prevented in that the vortex of cooling gases will act as an insulating layer between the latter and the hot kiln exhaust gases. 
     The apparatus according to the invention for cooling kiln exhaust gases in a kiln bypass preferably comprises a conical transition piece which is provided between the tubular housing of the mixing chamber and the kiln system. 
     The apparatus may further advantageously comprise a tubular transition piece which is provided between the conical transition piece and the kiln system in order to generate an increased mixing zone for extracted kiln exhaust gases and cooling gases, and an increased interval for regulating the swirl number S of the gases in the mixing chamber. The tubular transition piece thus makes it possible to increase the swirl number S without involving risk of cooling gases entering the kiln system, thereby improving the mixture and cooling of the extracted kiln exhaust gases. 
     The outlet of the mixing chamber for cooled exhaust gases may advantageously comprise a tube protruding axially into and having a maximum diameter which is smaller than the tubular housing. This will reduce the risk of the cooling gases just leaving the mixing chamber via the outlet without being mixed with the kiln exhaust gases. The inwardly protruding tube may be eccentrically located relative to the tubular housing, but should preferentially be coaxially located relative to the tubular housing. The inwardly protruding tube may furthermore advantageously be conically formed with its smallest diameter at its inner free end so as to reduce the pressure drop across the outlet. 
     The means for measuring respectively the mass flow m A  and the flow velocity v A  of the cooling gases being introduced to the mixing chamber and the mass flow m B  and the flow velocity v B  of the cooled exhaust gases being discharged from the mixing chamber may in principle be made up of any known and appropriate means and do not as such constitute a part of the invention. 
     Nor does the calculating unit per se for determination of Δm C  or ΔS constitute a part of the invention, and it may be made up of any appropriate calculating unit. 
     The means for regulating the fans for respectively the supply of cooling gases to the mixing chamber and for drawing the kiln exhaust gases through the kiln bypass may be constituted by generally known means, whereas the means for regulating the pressure loss across the apparatus may comprise means for varying the flow area for, respectively, the inlet of the cooling gases and the outlet. The means for varying the flow area of the cooling gas inlet may for example comprise a flap which is configured for rotation about an axis and being capable of regulation during operation by means of appropriate means. The means for varying the flow area of the outlet may for example comprise a throat or a damper which is located in the outlet just outside the mixing chamber. Alternatively a conical tube protruding axially into the tubular housing may be configured in a way which will permit variation of its conicity. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       The invention will now be explained in further details with reference to the drawings, being diagrammatical, and where 
         FIG. 1  shows a sectional view of a kiln system comprising an apparatus for cooling kiln exhaust gases in a kiln bypass according to the invention, and 
         FIGS. 2 and 3  show details of the apparatus shown in  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In  FIG. 1  is seen a sectional view of a kiln system for manufacturing cement clinker, said kiln system comprising a rotary kiln  1  in which cement raw materials in counter flow to hot kiln exhaust gases are burned into cement clinker, and a riser duct  3  for diverting the kiln exhaust gases from the rotary kiln. The kiln system shown in  FIG. 1  incorporates an apparatus  5  for cooling the kiln exhaust gases in a kiln bypass  7 . The apparatus  5  comprises a mixing chamber  9 , which is formed as a tubular housing with an exhaust gas inlet  11 , an outlet  13  for cooled exhaust gasses and a tangential inlet  15  for cooling gases. The apparatus  5  is used to extract and cool down some of the kiln exhaust gases from the kiln system  1 ,  3 . The apparatus  5  further comprises a first fan  17  for feeding cooling gases to the mixing chamber  9  and a second fan  19  for drawing the kiln exhaust gases through the kiln bypass  7 . The kiln bypass shown also is comprises a cyclone  21  for separating coarse solid particles from the cooled exhaust gas stream which is discharged from the mixing chamber  9 , with a possible return of said solid particles to the kiln  1 , a further cooling apparatus  23  for the mixed exhaust gases as well as a filter  25  for separating dust having a high content of chloride, alkide and/or sulphur. 
     According to the present invention the apparatus  5  comprises means  31  for measuring, respectively, the mass flow m A  and the flow velocity v A  of the cooling gases being introduced to the mixing chamber  9 , and means  33  for measuring respectively the mass flow m B  and the flow velocity v B  of the cooled exhaust gases being discharged from the mixing chamber  9 . The signals from the means  31  and  33  are transmitted to a calculating unit  35  to determine on the basis of the measured values m A , v A , m B  and v B  the actual mass flow m C  and the flow velocity v C  for the kiln exhaust gases being drawn through the kiln bypass and to relate the actual mass flow m C  to a predetermined value for kiln exhaust gases targeted for being drawn through the kiln bypass, and to determine on the basis of the values m A , v A , m C  and v C  the actual swirl number S of the gases in the mixing chamber and to relate it to a predetermined, desired value for the swirl number of the gases in the mixing chamber. The calculating unit  35  then transmits signals to means  37  for regulating the fan  17  for supply of cooling gases to the mixing chamber  9 , means  39  for regulation of the fan  19  for drawing the kiln exhaust gases through the kiln bypass  7  and for means  41  for regulating the pressure loss across the apparatus when Δm C  or ΔS deviates from 0. 
     The means  31 ,  33  for measuring respectively the mass flow m A  and the flow velocity v A  of the cooling gases being introduced to the mixing chamber  9 , and the mass flow m B  and the flow velocity v B  of the cooled exhaust gases being discharged from the mixing chamber  9  may for example be constituted by an aspiration trumpet, an aperture, a Venturi or a Pitot tube in which a pressure differential is measured, which, based on knowledge of the temperature of the gases, the geometric conditions, the barometer readings etc., can be used for calculating these values. For measuring m B  and v B , it will also be possible to use the pressure differential which is measured across the cyclone  21 . This solution is particularly advantageous since it does not require installation of additional equipment. The aspiration trumpet and the cyclone with associated temperature meters are illustrated in  FIG. 1  as means for measuring m A  and m B , respectively. Other means are sensors performing direct measurements of a velocity, e.g. by transmitting sounds through the flow stream or by perceiving changes in the electrical or magnetic characteristics of a dust-laden flow stream or by measuring the velocity of a turbine wheel. Finally, it will often be possible to derive from the motors of the fans  17 ,  19  electrical signals indicating the current or power consumption which can be used to estimate the mass flow being transported by the fans. If the operating principle of the cooling apparatus  23  involves injection of water, measurements of the inlet and outlet temperature of the apparatus and of the water consumption can be used to calculate the mass flow m B . 
     The calculating unit  35  for determination of Δm C  or ΔS and for transmitting signals to respectively the means  37 ,  39  and  41  may be constituted by a computer equipped with the appropriate software. 
     The means  37  and  39  for regulation of the fans  17 ,  19  for respectively the supply of cooling gases to the mixing chamber  9  and for drawing the kiln exhaust gases through the kiln bypass  7  may be constituted by frequency converters for the motors of fan or dampers at the aspiration point for or on the exhaust end of the latter. 
     The means  41  for regulating the pressure loss across the apparatus  5  may comprise means for varying respectively the flow area of the cooling gas inlet  15  and that of the outlet  13 . The means  41  for varying the flow area of the cooling gas inlet may, for example, as shown in  FIG. 2   a - 2   e , comprise a flap  43  which is configured for rotation about an axis  45 , and being capable of regulation during operation, for example with the help of a motor which receives signals from the calculating unit  35 . As indicated in  FIG. 2   e , the tangential cooling gas inlet may be divided into several channels and regulation can be achieved through the use of a flap in one of these channels. The means  41  for varying the flow area of the outlet may for example comprise a throat or a damper  47  which is located in the outlet immediately outside the mixing chamber  9 . A particular embodiment of a damper is indicated in  FIG. 3  in the form of a displaceable perforated plate having a number of holes of different sizes making it possible to apply a number of default values for the flow area. An alternative option would be to use a conical tube protruding axially into the tubular housing, said tube being configured in a way which permits variation of its conicity. 
     The apparatus  5 , shown in  FIG. 1 , comprises both a conical transition piece  8  and a tubular transition piece  10  which are located in extension of one another between the tubular housing of the mixing chamber  9 , and the kiln system  1 ,  3 . Hereby is provided an extended mixing zone for extracted kiln exhaust gases and cooling gases and a greater interval for regulating the swirl number S of the gases in the mixing chamber  9 . The tubular transition piece  10  thus makes it possible to increase the swirl number S without involving risk of cooling gases entering the kiln system, thereby improving the mixture and cooling of the extracted kiln exhaust gases. 
     In the embodiment shown in  FIG. 1  the outlet  13  of the mixing chamber  9  for cooled exhaust gases comprises a centrally fitted tube  12  which protrudes axially into and having a smaller maximum diameter than the tubular housing. This will reduce the risk of the cooling gases just leaving the mixing chamber via the outlet without being mixed with the kiln exhaust gases. The tube  12  is conically formed with the smallest diameter at its inner free end so as to reduce the pressure drop across the outlet  13 . 
     During the operation of the apparatus regulation can be carried out automatically and continuously using software which controls the regulating means  37 ,  39  and  41  according to a predetermined schedule. Alternatively the regulation can be carried out semi-automatically based on operator control of the regulating means  37 ,  39  and  41  based on the specific operating data for respectively ΔmC and ΔS.