Abstract:
A method of cooling a substance heat treated in a material processing plant in which the substance is cooled in a cooling apparatus to a temperature that is about 25% to about 55% of the temperature at which the partially cooled substance was when it entered the cooling apparatus. Heated gas is recycled from the cooling apparatus to at least one other process within the plant and the partially cooled substance is delivered from the cooler to a cogeneration system in which the substance is further cooled and the heat removed from the substance is used to generate power.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates broadly to material processing plants in which particulate material is heat treated in a kiln or similar equipment and is thereafter passed to a material cooler. This invention is particularly applicable to cement and lime manufacturing facilities. 
       BACKGROUND OF THE INVENTION 
       [0002]    In a cement manufacturing plant cement raw meal is precalcined and then calcined to cement clinker in the sintering zone of a rotary kiln. The cement clinker is thereafter directed to a clinker cooler wherein it is cooler prior to further processing. The clinker cooler&#39;s primary function, therefore, is to quench the hot clinker as it discharges from the kiln. Additionally, the hot gases exiting the clinker cooler are recovered by the kiln hood and tertiary air duct and utilized as “super heated” secondary air for combustion in the kiln and tertiary air for use in the precalciner that is upstream, based on material flow, from the kiln. A portion of the hot gases can also be used to dry the raw materials entering a cement plant&#39;s raw mill. This system of recovering cooler exhaust air forms a very important heat recovery system contributing greatly to the overall energy efficiency and productivity of the modern cement plant. 
         [0003]    The temperature of the clinker leaving the rotary kiln is about 1400° C. and between about 80-140° C. when discharged from cooler. The cooling air directed to the upper or front, i.e. recuperative, end of cooler is heated to a temperature from 750-1300° C., and this hot air is recycled back into the production line, normally into the kiln and calciner of the pyroprocessing tower via the so-called tertiary air duct. Typically, the heat lost by the clinker in the lower, or non-recuprative, end of the cooler, when it cools from about 400-700° C. to below about 80-140° C. is not recovered or is recovered via comparatively inefficient co-gen systems. 
         [0004]    It would therefore be advantageous to have a method and an apparatus to recover such heat to thereby make the cement manufacturing facility, or any plant that utilizes a kiln and material cooler, more energy efficient. 
       SUMMARY OF THE INVENTION 
       [0005]    This invention integrates one or more waste heat recovery heat exchangers in a cogeneration system with a material cooler whose primary purpose is to cool particulate material that was previously heat treated in a high temperature oven. The preferred embodiment of this invention is to incorporate the waste heat recovery economizer, boiler, and superheater of a steam Rankine cycle, Kalina cycle, Organic Rankine, or Brayton cycle after the recuperative section of a material cooler at a cement or lime plant. By placing the heat recovery heat exchanger in the clinker cooler, in the case of a cement plant, to absorb the energy in the clinker instead of, or example, in the cooler vent duct, more energy at a higher quality (i.e. at a higher temperature) is available which can be converted into electricity more efficiently and thereby also increase the electric power generated by 25% to 300%. This invention also improves the economics of a cogeneration system at cement or lime plants without significantly affecting plant layout. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]      FIG. 1  is a schematic flow diagram of a portion of a prior art cement plant. 
           [0007]      FIG. 2  is a schematic flow diagram of another prior art cement plant having an internal cogeneration system. 
           [0008]      FIG. 3  is a schematic flow diagram of one embodiment of this invention. 
           [0009]      FIG. 4  is a schematic flow diagram of another embodiment of this invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0010]    It should be understood at the outset that identical reference numbers on the various drawing sheets, either of the prior art or of the invention, refer to identical elements. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating principles of the invention. With reference to  FIGS. 1-4 , the movement of clinker through the apparatus of the present invention is depicted by shaded arrows  1 , and the movement of cooling air, which subsequently becomes heated during the process, is depicted by non-shaded arrows  2  or  2   a.    
         [0011]      FIG. 1  shows a portion of a conventional prior art cement plant. Cement raw material, which typically is pre-calcined in a preheating tower (not shown) is first directed to kiln  11  wherein it is calcined to cement clinker. Clinker enters clinker cooler  12  at temperatures between about 1300 to about 1500° C. and exits at approximately 100° C. The clinker is cooled via cooling air, the entry of which into the cooler is represented by arrows  2   a . In the recuperative section  14  of the cooler, which is the front end of a conventionally sized cooler, between 60 and 75%, depending upon the efficiency of the cooler, of the energy in the clinker is recovered as secondary air, directed to kiln  11 , and tertiary air, directed to the precalciner via duct  15 . Recuperative section  14  typically occupies the first ⅓ to ¼ of the length of the cooler. 
         [0012]    At the end of the recuperative section the clinker temperature ranges between 400 to 700° C. and therefore contains about 25 to about 55% of its initial thermal energy. The clinker in the non-recuperative section  16  is cooled by air to ˜100° C. The heat transferred to this air in the non-recuperative section exits through the cooler vent duct  17 . The cooler vent duct  17  air temperature ranges from ˜250 to 300° C. The air then is drawn by ID fan  21  to, respectively, heat exchanger  18  to cool the air before it enters dust collector  19 , from which clinker fines are removed via outlet  20 , and finally stack  22 , with the air exiting the stack ranges from ˜110 to 150° C. Clinker exiting the cooler can be first sent to crusher  50  prior being directed to a downstream cement mill (not shown). 
         [0013]      FIG. 2  shows a conventional cement plant with a typical cogeneration system that extracts heat from the cooler vent duct  17 . With this system, air is used to remove heat from the clinker in the non-recuperative section and transfer it to the working fluid in the heat recovery steam generator  25 , which works in concert with turbine  26 , generator  27 , condenser  28  and pump  29  as a cogeneration system. In effect, the co-gen system replaces the heat-exchanger in removing heat from the cooler vent duct air, and air exiting stack  22  is still at a temperature that ranges from ˜110 to 150° C. 
         [0014]    Although  FIGS. 3 and 4  are directed to a portion of a cement manufacturing facility, it is understood that the invention may be employed in any industrial process in which particulate material is first heat treated in a oven and thereafter directed to a material cooler. 
         [0015]    Calcined raw material is burned into cement clinker in rotary kiln  30 . The clinker is thereafter cooled in clinker cooler  31 . Super heated “secondary” air is thereafter directed from the cooler into the kiln via kiln hood  32  and heated “tertiary’ air is directed to the precalcining tower (not shown) via duct  33 . 
         [0016]    It is a feature of the present invention that a shortened clinker cooler  31  then is typically employed is utilized to cool the cement clinker. In a typical cement kilns the material is cooled to below about 80-140° C., which is about 4%-7% of the temperature at which the material enters the cooler, or less. The clinker cooler of the present invention is shortened to the point when the clinker has been cooled to about 25% to about 55%, of the temperature at which the material enters the cooler, or from about 400° C.-700° C. At such a point, the energy saving features of the cooler as exemplified by the transfer of the secondary and tertiary air, have been addressed, but the clinker still contains substantial heat energy that in prior art systems would have been wasted. 
         [0017]    In one embodiment of the invention the shortened cooler is approximately the same size as the recuperative section of a normal sized cooler so that secondary and tertiary air are still provided to other areas of the plant and therefore the energy efficiency of the plant remains the same as in prior art systems. In any event, the size of the clinker cooler can be varied to adjust the heat consumption of the cement plant versus the power produced from the downstream cogeneration system to meet the needs of the end user. The downstream cogeneration system is utilized recover much of the heat that would have been removed in the non-recuperative end of the clinker cooler and thereafter lost. 
         [0018]    In a less preferred embodiment of the invention, it is possible to reduce the size the conventional clinker cooler so that it contains only a portion of the recuperative section or in fact completely eliminate the conventional clinker cooler which would increase the quantity and quality of the heat going to the cogeneration system at the cost of increasing the heat consumption and decreasing the thermal efficiency of the cement plant. 
         [0019]    If desired, a crusher  34  can be placed after the cooler and before (or in between heat recovery heat exchangers  37  when more than one heat exchanger is utilized), to reduce the material&#39;s particle size to thereby increase the heat transfer coefficient which will enable the end user to reduce the size of heat exchangers  37  without sacrificing the amount of electricity produced. 
         [0020]    The clinker, whether or not a crushing step is employed, is thereafter transferred to bucket elevator (or any other vertical material conveyor)  35  after which it is transferred to the top material inlet of 36 of vertical heat exchanger  37  that is typically, but not always, integral with the cooler, that is, in the same interior environment or within the same overall housing, as the cooler. As indicated, more than one heat exchanger  3 ′ 7  may be employed. Gravity moves the clinker vertically downward across the heat recovery heat exchanger(s) as the first step in a cogeneration process which, with regard to the system depicted in  FIG. 1 , can be either a steam Rankine, Organic Rankine, Kalina cycle, or Brayton cycle. As depicted, the cogeneration system employed is a Rankine cycle in which water is the working fluid and which consists of heat exchanger  37 , steam turbine  38 , electric generators  39  and condensor  40 . 
         [0021]    Clinker, at a temperature of approximate 65° C. above ambient, is discharged from the cooler and is directed to the next stage in the process, typically a finish grinding mill (not shown). 
         [0022]    The steam output of heat exchanger  37  is connected to an input of one or more steam turbines  38 , each of which is coupled by a shaft to one or more electric generators  39 . The outputs of steam turbines are connected to the gas inputs of a condensor  40 , condensed at constant temperature and pressure to liquid form and reinserted into heat exchanger  37  as part of a closed loop. 
         [0023]    In another embodiment shown in  FIG. 4 , the clinker exiting cooler  31  is crushed to a size that is fluidizable (typically about 0.1 to about 20 mm). The clinker enters a fluidized bed heat recovery heat exchanger  42  and moves essentially horizontally across heat exchanger  42 . If necessary a transportation mechanism can be used or the heat exchanger module can be slightly inclined to help move the crushed clinker across the tubes of the fluidized bed. The clinker velocity across the tubes can be adjusted to reduce wear. 
         [0024]    The steam generated in the heat exchanger  42  is directed to steam turbine  38  in the same manner as specified above. The main advantages of the configuration of  FIG. 4  compared to the configuration in  FIG. 3  are the elimination of the vertical conveyor and the higher heat transfer rates by using crushed clinker in the fluidized bed. 
         [0025]    The fluidization air, depicted by cross-hatched arrows  3 , can be vented through the clinker cooler to become part of the secondary air. 
         [0026]    The present invention has advantages over the prior art cement plant having a cogeneration system as depicted in  FIG. 2 . The first is that there is a lower temperature of the working fluid in the prior art system that is caused by using air to transfer heat from the clinker to the working fluid. In the prior art system the temperature of the working fluid is 100 to 400° C. lower than the temperature of the clinker exiting the recuperative section of the cooler. This is the result in part of having the heat bearing substance (in the depicted examples, cement clinker) in contact with the heat exchanger, rather than having the clinker transfer heat to cooling air which in turn contacts the heat exchanger, which is the case with the prior art. In the present invention, the working fluid (water in the case of a Rankine cogeneration cycle) is only between 30 to 200° C. lower than the temperature of the clinker exiting the recuperative section of the cooler. This increase in working fluid temperature increases the efficiency of the cogeneration between about 20-200%. Another disadvantage of the prior art system is that the air exiting the stack still has between about 5 and 20% of the energy contained in the clinker exiting the recuperative section of the cooler, whereas in the system of the present invention no air exists the stack so this energy can be transferred to the cogeneration system. Finally the present invention permits the elimination of the cooler vent duct, dust collector, ID fan and stack to thereby reduces the capital and operating cost of a cement plant. For example, by eliminating the ID fan the electric power consumption of a cement plant is reduce by ˜3 to 6%. 
         [0027]    Although this invention has been described in detail by reference to the drawings, this detail is for illustration only, and it is not to be construed as a limitation upon the invention as described in the appended claims.