Patent Publication Number: US-2012028203-A1

Title: High capacity shaft kiln

Description:
FIELD OF INVENTION 
     The present invention relates to kilns useful for manufacturing cement and other allied products and, more particularly, to a high capacity shaft kiln having a plurality of vertical shaft kilns, which are parallel with one another. 
     BACKGROUND OF THE INVENTION 
     Lime, or quicklime, is the oxide of calcium, CaO, and is commonly obtained by calcining limestone. Limestone is calcined in two main types of kilns, vertical or shaft kilns, and horizontal, rotary kilns. Lime is a very important basic material used in a variety of different industries. These include construction, agriculture, chemicals and several processing industries. There are two forms of lime: quicklime and hydrated lime. Quicklime is produced by heating rock or stone containing calcium carbonate (limestone, marble, chalk, geologically stratified seashells, etc.) to a temperature of around 1000° C. for several hours. In this process, known as “calcining” or simply “burning”, the carbon dioxide in the calcium carbonate is driven off leaving calcium oxide plus any impurities. 
     Quicklime can be hydrated, becoming more stable, and easier and safer to handle. Hydrated lime is produced by adding water to quicklime in a process called “hydration” or “slaking”, where the calcium oxide and water combine chemically to form calcium hydroxide. 
     During slaking the quicklime lumps will disintegrate to a fine powder. If high quality limes are required some form of screening and/or classification will be required at this stage to grade the lime. Hydrated lime is normally supplied and sold in bagged form. 
     If quicklime is hydrated with a large amount of water and well agitated, it forms a milky suspension known as milk of lime. Allowing the solids to settle and drawing off the excess water forms a paste-like residue known as lime putty. Methods of burning lime range from traditional and simple to highly sophisticated and automated. The traditional methods are labor-intensive and energy inefficient, and tend to produce unevenly burned lime, with a proportion that is underburnt and/or overburnt, while the highly sophisticated and automated methods produce lime of a very consistent quality. 
     One type of method used for producing lime and other related products is with vertical shaft kilns or shaft kilns. The raw material is fed in at the top of the kiln and the product is withdrawn from the bottom, causing the material to move slowly downwards through the kiln. Heat to calcine the material is introduced roughly in the middle of the kiln and therefore any material above is preheated by rising hot exhaust gases, and any material below the middle of the kiln is cooled by incoming air. In this way, material entering the kiln at the top is first preheated, then calcined and finally cooled during its passage through the kiln. 
     Currently, shaft kilns are limited in capacity to approximately 600 metric tons per day (MTPD). This limitation arises from maldistibution of gas and material in a single shaft. Specifically, as the capacity of the shaft kiln increases the active cross section must increase proportionally. However, at approximately 600 metric tons per day (MTPD), the cross section is so large that the gases cannot be evenly distributed across the bed of the shaft kiln and material flow is not plug flow. 
     Accordingly, it would be desirable to overcome these limitations by using multiple shafts in the same unit, wherein each shaft has a material feeding device and gas flow regulating device, such that the stone flow and gas distribution can be regulated to suit each shaft kiln. 
     SUMMARY OF THE INVENTION 
     In accordance with an exemplary embodiment, a high capacity shaft kiln comprises: a plurality of vertical kiln shafts, which are sized and configured to receive a source of material, which descends from an upper end thereof to a lower end thereof, and wherein each of the plurality of vertical kiln shafts has an inlet, which is sized and configured to receive a fuel source from a distribution device; a common heating unit which is sized and configured to receive the source of material from the lower end of each of the plurality of vertical kiln shafts; and a cooling unit attached to the common heating unit. 
     In accordance with another exemplary embodiment, a high capacity shaft kiln comprises: a plurality of parallel vertical kiln shafts, which are sized and configured to receive a source of material, which descends from an upper end thereof to a lower end thereof, and wherein each of the plurality of vertical kiln shafts has an inlet, which is sized and configured to receive a fuel source from a distribution device and an exhaust duct; a common heating unit which is sized and configured to receive the source of material from the lower end of each of the plurality of vertical kiln shafts; and a cooling unit attached to the common heating unit. 
     Other details, objects, and advantages of the invention will become apparent as the following description of certain present preferred embodiments thereof and certain present preferred methods of practicing the same proceeds. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Present preferred embodiments of shaft kilns, and methods of making such devices are shown in the accompanying drawings in which: 
         FIG. 1  is a perspective view of a high capacity shaft kiln in accordance with an exemplary embodiment. 
     
    
    
     DETAILED DESCRIPTION OF PRESENT PREFERRED EMBODIMENTS 
     Reference will now be made in detail to the present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers are used in the drawings and the description to refer to the same or like parts. 
       FIG. 1  is a perspective view of a high capacity shaft kiln  100  in accordance with an exemplary embodiment. As shown in  FIG. 1 , the high capacity shaft kiln (or annular shaft kiln)  100  include a plurality of vertical shafts (or kiln shafts)  102 , a common firing unit  104  upon which the plurality of vertical kiln shafts  102  are attached thereto, and a cooling unit  106 . 
     In accordance with an exemplary embodiment, the high capacity vertical shaft kiln  100  as described herein can be used for the burning of limestone, dolomite, magnesite, cement, and ores or the like. In use, hot gases calcine or burn a source of material  116  (i.e., limestone). As the source of material  116  (i.e., limestone) descends from the vertical shafts  102 , the source of material  116  enters a calcining zone, which is formed by the common firing area  104 . The burned material is then permitted to descend through the calcining zone to the cooling unit (or cooler)  106  wherein cooling air is permitted to enter the kiln  100 , so as to cool the burned material  116  as it descends through the cooling unit (or cooler)  106 . 
     Each of the plurality of vertical shafts (or kiln shafts)  102  includes a vertical conduit (or column)  110  having an inlet  112  at an upper end  114  wherein a source of material  116  such as limestone, cement, ores and other materials are fed. An exhaust (or offgas) duct  118  is located on an upper portion  120  of each of the vertical conduits  110 , which removes the hot gases (or fuel) from the vertical conduit. Each of the exhaust (or offgas) ducts  118  includes a damper  122 , which controls the airflow (and release of heat and/or gases) for each of the vertical conduits  110 . It can be appreciated that the exhaust (or offgas) duct  118  in addition to removing or providing an outlet for spent combustion gas, the exhaust (or offgas) duct  118  can also provide a conduit for fresh air to go down the exhaust (or offgas) duct  118  and corresponding vertical conduit (or column)  110  to generate preheated combustion air. In accordance with another exemplary embodiment, the hot gases from the exhaust (or offgas) duct  118  can be recycled to form a preheated burner air source (not shown). 
     Each of the vertical conduits  110  also include a distribution device or gas flow regulating device (not shown) which feeds a gas or fuel source  126  to the vertical conduit  110  through a gas or fuel inlet  128  located on a lower portion of the vertical kiln shaft  102 . In accordance with an exemplary embodiment, the gas or fuel source  126  is preferably oil, coal, wood and/or other suitable material. The gas or fuel inlet  128  is preferably located approximately ⅔ to ¾ down the vertical conduit  110 . In accordance with an exemplary embodiment, the shaft kiln  100  preferably has at least two (2) vertical kiln shafts  102 , and more preferably at least three (3), and most preferably at least four (4) or more vertical kiln shafts  102 . However, it can be appreciated that any number of vertical kiln shafts  102  can be used to obtain a desired capacity for the high capacity shaft kiln  100 . In addition, each of the vertical kiln shafts  102  is preferably in parallel with one another. 
     Each of the plurality of vertical kiln shafts  102  are attached to and/or mounted to a common firing unit (or common firing portion)  104 . The common firing unit  104  includes an upper plate (or member)  105 , which is attached to a lower end  124  of the vertical conduit  110 , and housing  130  with one or more feeders  132  to control the flow rate of the source of material (or solids) on an upper portion thereof, and one or more gas or fuel inlets  134  located on a lower portion thereof. It can be appreciated that the one or more feeders  132 , which control the flow rate of the source of material  116  is an optional element. The one or more gas or fuel inlets  134  of the common firing unit  104  are preferably equally spaced around the upper portion thereof. In accordance with an exemplary embodiment, the housing  130  has annular upper section  136  and a conical (or tapered) lower section  138 . In accordance with an exemplary embodiment, the conical lower section  138  has a greater outer diameter adjacent to the annular upper section  136  and tapers downward to the cooling unit  106 . 
     In accordance with exemplary embodiment, the cooling unit  106  consists of an annular housing  140  having a plurality of air inlets  142  on a lower portion thereof. The air inlets  142  are preferably equally spaced around the annular housing  140 , an upper plate (or member)  144 , and a lower plate (or member)  146 . Attached to the lower plate  146  is one or more conical shaped outlets  148 , which deliver the treated source of material  116  (i.e., produced product  117 ) to a hopper or other storage system (not shown). 
     In accordance with an exemplary embodiment, each of the vertical conduits  110  are preferably approximately 4 to 10 meters in length depending on material  116  size, and having an inner diameter (ID) of approximately 2 to 4 meters, which also depends on material  116  size. In accordance with an exemplary embodiment, the vertical conduits  110  are steel shafts lined with a refractory material, such as alumina, magnesia or fireclay bricks. 
     The source of material  116  is preferably limestone or other mineral aggregate such as chalk and/or marble containing in excess of 90% calcium carbonate is charged (and/or placed) into an upper end  114  of each of the plurality of vertical kiln shafts  102  from a hopper or other source of material using a material feeding device (not shown) by way of inlets, and the limestone is calcined as the limestone descends slowly to a bottom portion of the kiln  100  where the limestone (or produced product  117 ) is discharged into a collection hopper via outlets within the cooling unit  106  from which it may be collected and transferred to a storage silo or other storage unit (not shown). 
     In accordance with an exemplary embodiment, the source of material  116  is preferably a limestone supply having a minimum of 97% calcium carbonate and has been processed to be clear and free of matter such as clay and/or dust. In addition, it is preferable that the source of material  116  has a minimum dimension of about 1 inch and a maximum dimension of about 6 inches, with a preferred dimension of about 2 inches by 4 inches. In accordance with an exemplary embodiment, the source of material  116  is preferably stored in a hopper having a material feeding device (not shown), which includes suitable metering and distribution mechanisms for controlling the feed of the source of material  116  into each of the plurality of kiln shafts  102 . 
     In operation, a source of material  116  is fed into the inlet  112  at the upper end  114  of each of the kiln shafts  102 , and the produced product  117  is withdrawn from the bottom of the high capacity shaft kiln  100 , causing the material to move slowly downwards through the plurality of kiln shafts  102 , the common heating unit  104 , and the cooling unit  106 . Heat to calcine the material is introduced roughly in the middle of the plurality of kiln shafts  102  and a second heating zone is located within the common heating unit  104 . Therefore, any source of material  116  above the heating zones is preheated by rising hot exhaust gases, and any source of material  116  below the heating zones is cooled by incoming air. The source of material  116  entering the plurality of kiln shafts  102  at the upper end  114  is first preheated, then calcined and finally cooled during its passage through the cooling unit  106  of the kiln  100 . 
     In accordance with an exemplary embodiment, by incorporating a plurality of kiln shafts  102  on single unit or kiln  100 , the capacity of the kiln  100  can be drastically increased above the approximately 600 metric tons per day (MTPD) that can be achieved with the current shaft kilns. For example, by incorporating a plurality of kiln shafts  102 , an increase in production of at least 2 to 5 times current capacity can be obtained within a single kiln  100 . 
     It is to be understood that the form of this invention as shown is merely a preferred embodiment. Various changes may be made in the function and arrangement of parts; equivalent means may be substituted for those illustrated and described; and certain features may be used independently from others without departing from the spirit and scope of the invention as defined in the following claims.