Abstract:
An apparatus for preheating particulate material in which the particulate material is transferred from one or more upper storage bins to a circular lower chamber that has an outer, essentially annular, portion which serves as a gas flow passage. The particulate material is directed from the feed bin or bins into a plurality of essentially vertical cylindrical feed cassettes via intermediate feed ducts. The lower chamber has a flat roof which is in contact with the bottom portion of the vertical feed cassettes. The vertical feed cassettes are approximately evenly spaced on top of the outer perimeter of the flat roof. The particulate material is preheated in the annular flow passage by hot kiln gases flowing in countercurrent heat exchange relationship with the particulate material. Each feed cassette is completely segregated from its adjacent cassettes, and the bottom of each cassette is positioned over a hole in the flat roof of the lower chamber to thereby enable the particulate material to fall from each cassette into the annular flow passage section of the lower chamber. A plurality of particulate discharge mechanisms, the number of which correspond to the number of cassettes, discharges particulate material that has fallen into the annular flow chamber from the overhanging cassettes into a material outlet located in the floor located at the center of the lower chamber.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates to a method and apparatus for preheating material with the hot gas being exhausted from a heater or kiln.  
         [0002]     Preheaters are commonly used for preheating particulate material, including preheating limestone. Limestone is generally preheated by directing hot exhaust gases from a rotary calcining kiln through the limestone particulate material in counter-current flow prior to the limestone entering the calcining kiln. The gases heat the limestone particles prior to their introduction to the rotary kiln, thus requiring less heating in the rotary kiln to complete the calcining process, thus making the calcining process more energy efficient. Preheating apparatuses of this general type are known in the art.  
       SUMMARY OF THE INVENTION  
       [0003]     The present invention is an improved method and apparatus for preheating particulate material.  
         [0004]     According to the present invention, there is an apparatus for preheating particulate material in which the particulate material is transferred from one or more upper storage feed bins to a basically circular lower chamber that has an outer, essentially annular, area which serves as an annular gas/material preheating passage.  
         [0005]     It is an essential feature of the present invention that the particulate material is directed from the feed bin or bins via intermediate feed ducts into at least one, and preferably, a plurality of vertical and essentially cylindrical feed and initial preheating cassettes. The lower chamber has a roof, preferably a flat roof, which is in contact with the bottom portion of the vertical feed cassettes. The vertical feed cassettes are preferably approximately evenly spaced around the top of the outer perimeter of the roof, and, further, are preferably evenly spaced from the perimeter of the roof. The particulate material is preheated in first the vertical feed cassettes and then the annular flow passage by hot kiln gases flowing in countercurrent heat exchange relationship with the particulate material. The roof has a plurality of holes therethrough, with each hole being positioned above the annular flow passage. The holes serve the dual function of providing the inlet through which particulate material enters the lower chamber and the outlet via which preheating gas exits the lower chamber. Each feed cassette is positioned over at least one hole. Each feed cassette is completely segregated from its adjacent cassettes. The particulate material will fall from each cassette into the annular flow passage section of the lower chamber. A plurality of particulate discharge mechanisms discharge particulate material that has fallen into the annular flow chamber from the overhanging cassettes into a material outlet located in the center of the lower chamber&#39;s floor. Preferably, the discharge mechanisms are reciprocal rams, and their number will equal the number of cassettes. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]     For a complete understanding of the present invention, reference can be made to the detailed description which follows and to the accompanying drawings, in which:  
         [0007]      FIG. 1  is an elevational view of a preheater incorporating the present invention shown partly in cross section and with portions of the exterior wall broken away.  
         [0008]      FIG. 2  is an over head perspective, shown partially in cross section, of three feed cassettes utilized in the present invention.  
         [0009]      FIG. 3  is a top-plan view, partially in relief and partially in cross section, of the preheater shown in  FIG. 1 .  
         [0010]      FIG. 4  is a sectional view taken along the line  4 - 4  of  FIG. 3  looking in the direction of the arrows.  
         [0011]      FIG. 5  is a broken-away fragmentary plan view in cross-section of a portion of a preheater of the present invention.  
         [0012]      FIG. 6  is a side view of an embodiment of a ram assembly, which can be utilized in the present invention. 
     
    
       [0013]     Like numerals in different drawings refer to similar elements.  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0014]     Referring first to  FIGS. 1, 2  and  4 , the present invention is a preheater  10  which consists of a lower preheating area  11  and an upper feed delivery and initial preheating area  12 . The preheater  10  can be used with a large variety of particulate materials, but is particularly designed and intended to preheat and precalcine limestone. The preheater  10  can also be used with a variety of heating fluids, but is particularly designed and intended to heat with exhaust gases received from a calcining kiln.  
         [0015]     Lower preheating area  11  is an upright, essentially circular, area. Lower preheating area  11  is separated from initial preheating area  12  by a flat roof  13  having upper surface  14 , lower surface  15  and perimeter  22 . Lower preheating area  11  has a sloped floor  16  and vertical inner and outer side walls  17 ,  18 . At the center of floor  16  there is an initial central discharge area  19  through which material passes to the preheater discharge  20  after which it is delivered to rotary kiln  21 . Unlike certain prior art preheaters that have a number of compartments in their preheating chamber, in the present preheater lower preheating chamber  11  has essentially no internal dividers so that if the chamber were empty of material there would be an unimpeded passage completely around its inner perimeter.  
         [0016]     Upper preheating area  12  is comprised of a plurality of vertical feed and initial preheating cassettes  30 , having outer wall  31 , inner wall  32  and upper side  38 , which are fed feed from feed bins  71  via material inlet and feed duct  70 . Particulate material is initially preheated in cassette  30  after which it is delivered to lower preheating area  11 . Upper preheating area  12  is enclosed by side walls  85  and roof  86 , which in the depicted embodiment is conical, but can have other configurations, depending on the shape of the feed bins and/or the space requirements of the end user. Roof  86  is supported in part by roof supports  87 .  
         [0017]     Flat roof  13  of the lower preheater area  11  has a number of holes  23  through which particulate material enters and preheating gas exits lower preheating area  11 . Preferably, above each hole  23  there is positioned a separate vertical feed cassette  30 . In the preferred embodiment, holes  23  are preferable arranged in a ring or a semicircle near the perimeter  22  of roof  13 . In operation, particulate material  33  is discharged into lower preheater area  11  by gravity, traveling down through each vertical feed cassette  30 , through roof  13  to land on or above sloped floor  16 , down sloped floor  16  to central discharge  19  and then eventually out preheater discharge  20  through which it is delivered to kiln  21 . In flowing downwardly through the feed cassette  30  and lower preheater area  11 , the particulate material  36  is preheated and precalcined by the countercurrent flow of the hot kiln gases which flow upwardly from the kiln  21 , into lower preheating area  11 , through holes  23  into cassette  30 , in which said kiln gas will rise and pass through the particulate material  33  in said feed cassette  30 . Preheating air will exit each cassette  30  through an air takeoff  80  located in the interior and in the upper region of each cassette  30  and, preferably, in the center of the interior of cassette  30 . From air takeoff  80  exiting air travels to duct  81  in which it exits cassette  30  and thereafter travels to common air duct  82  which is located on the outside of the preheater and then to air outlet  83 , from which it will be directed to a dust collector (not shown).  
         [0018]     As indicated, lower preheating area  11  is an essentially circular compartment. As such, with reference to  FIG. 3 , in one embodiment the distance from center point  34  of upper roof  12  to each point on the outer perimeter  22  of upper roof  13  will be equal, which profile will extend down through each horizontal plane of lower preheating chamber  11 . Alternatively and preferably, in a unique feature of this invention, the outer perimeter  22  of upper roof  13  and, correspondingly, the outer surface of lower preheating area  11  will be “knuckled-shaped”, as illustrated in  FIGS. 3 and 5 . In such a configuration, the longest distance from center point  34  of upper roof  13  to the outer perimeter  22  of the roof is when measured through center point  35  of each hole  23  in upper roof  13 . The shortest distance from the center point  34  of upper roof  13  to the perimeter  22  of the roof is the distance when measured through a point  37  on perimeter  22  equidistant from center point  35  of adjacent holes  33 , assuming holes  33  are equally sized and spaced, from each other and perimeter  22 . This profile will extend downward through each horizontal plane of lower preheating chamber  11 . This “knuckle-shaped” configuration serves to eliminate “dead-zones” in the lower preheating chamber, that is, areas in the chamber where the material would not be in a state of movement.  
         [0019]     Feed and preheating cassettes  30  are a unique feature of the present invention. Referring to  FIG. 2 , the cassettes are arrayed in a ring or a semi-circle on top of upper side  14  of roof  13 . Cassettes  30  are preferably identical in size and shape and, further, are preferably evenly spaced and separate and distinct from each other. In addition, cassettes  30  are preferably uniformly spaced from outer perimeter  22 . As a result of the placement of cassettes  30 , material discharged from each cassette  30  will, through its natural angle of repose, form piles  40  on the sloped floor which are spaced from both (a) the material piles discharged from each immediately adjacent feed cassette  30  and (b) side walls  17  of lower preheater area  11 . As a result, it is an unique feature of this invention that preheating air will have an easy passage between each of the material piles  40  and also through an annular passageway formed by the space between each material pile  40  and side walls  17  of lower preheating chamber  11 , thus ensuring substantial and uniform material/air contact throughout lower preheating area  11 . With reference to  FIGS. 1 and 2 , preheating air will travel radially from discharge  19  between material piles  40  via passageways  50  (only one of which is depicted in  FIG. 2 ), which are formed between adjacent material piles  40  by the natural angle of repose of each material pile  40 . The air will travel through passageways  50  to the inner side wall  17  of the preheater. Once there, the air will travel along rear annular air passageway  51  which, when cassettes  30  form a ring on upper side  14  of roof  13 , extends completely around the inner perimeter of the lower preheating chamber and which is the result of the placement of cassettes  30  away from perimeter  22  and the natural angle of repose of the material piles from each cassette  30 .  
         [0020]     The bottom portion  30   a  of each feed cassette  30  can be flush against the upper surface  14  of flat roof  13 . Preferably, however, bottom portion  30   a  will extend slightly below the lower surface  15  of roof  13  and into (by no more than approximately 6 inches) lower chamber  11 . This feature is advantageous because by varying the extent by which the bottom portion  30   a  of feed cassettes  30  extent into lower chamber  11  the size and shape of air passageway  51  will be varied and thereby the air distributorship through air passageway  51  can be optimized based on the characteristics of the particulate material being processed.  
         [0021]     Whether cassettes  30  are flush against surface  15  of roof  13  or extend into lower chamber  11 , the position and sizing of cassettes  30  relative to holes  33  will be such that all of the preheating air that exits lower chamber  11  through holes  33  will go into cassettes  30 . Therefore, if cassettes  30  are placed flush against roof  12  the size and shape of the inside diameter of cassette  30  will be matched with the size and shape of hole  33  with which it is mated. In a less preferred embodiment, cassette  30  can be larger than and overlap its respective hole  33 .  
         [0022]     Cassettes  30  are preferably made from fabricated steel and are lined with suitable refractory materials. As such, this gives the operator the option over the lifetime of the preheater to vary the cross section of cassettes  30  and/or replace feed cassette  30  to thereby vary the resultant gas velocity through the preheater as needed in a cost effective manner.  
         [0023]     Preferably, each heating air duct  81  will exit its respective cassette  30  at no more than a 45-degree angle from the vertical. The vertical take off of heating air contributes to both the duct&#39;s possessing self-cleaning properties and reduced pressure drop.  
         [0024]     In another feature of the invention, material feed enters each cassette essentially vertically by gravity from a centrally positioned material inlet and feed duct  70 . In a preferred embodiment, inlet duct  70  enters through a location essentially in the center of upper face  38  of cassette  30 . In this preferred embodiment, air intake  80  is positioned in the interior of cassette  30  directly below inlet duct  70  so that a substantial amount of feed  33  entering cassette  30  from inlet duct  70  will fall on the top of air intake  80 . Coarser material  33   b  will roll around and down the side of air take off  80  air take off  80  and from there travel down through the center of feed cassette  30 . Fine material  33   a  will tend to migrate toward the outer wall  32  of each cassette  30 . This design will, therefore, lead to a natural segregation of fine material  33   a  from coarse material  33   b.  As coarse material  33   b  falls down cassette  30  it will form a natural angle of repose  85  underneath air intake  80 . This segregation of coarse and fine materials promotes uniform gas distribution over the full cross-section of the cassette.  
         [0025]     The feed cassettes are tubular in the broadest sense of the word, that is, they are essentially elongated, hollow bodies, having a vertical axis longer than a horizontal axis with the exact ratio of the length of the cassette&#39;s vertical to horizontal axis being determined by the needs of the individual practitioner of the invention, based on factors such as the nature and size of the material being preheated, the preheating temperatures and the desired pass through rate of the material. The feed cassettes preferably will have a symmetrical horizontal cross-sectional profile at their bottom in the vicinity where the gas will enter the cassettes, which will contribute to the even preheating of the particulate material in the cassette. Because of the unique rear annular air passageway  51  and the preferred circular symmetry at the lower gas enter area  30   b  of each cassette  30 , gas will enter around the entire circumference of each cassette  30  leading to an optimal heat transfer/pressure drop trade-off.  
         [0026]     In one embodiment, the cassettes may be fabricated as being perfectly cylindrical. In another embodiment of the invention the cassettes are fabricated as a truncated inverted cone having a decreasing cross sectional area as gases move up the stone bed. Such cassettes will have a gradual slope, typically ranging up to about 5%, with the cross-sectional area at the top of each cassette ranging from about 80% to 100% of the cross-sectional area at the bottom of each cassette. This design provides for uniform fines carrying capacity throughout the cassettes. This is an improvement over conventional, uniform cross section stone beds, in which the carrying capacity at the bed bottom gives way to less carrying capacity at the bed top—thus generating a size range of trapped particles in between the top and bottom carrying capacity. The sloped design provides for more uniform feed distribution throughout the cassette and more uniform gas solid distribution. In another embodiment, the cassettes can have the shape of hollow multi-sided prisms, such as, for example, rectangular, hexagonal and octagonal prisms. Most preferably, the horizontal cross section of the cassettes will be circular. Preferably, all the cassettes in a given preheater will be uniformly shaped and sized. Typically there will be one cassette  30  for each hole  33 , although in certain embodiments, and depending on the type and characteristics of the material to be preheated, a single cassette can cover more than one hole.  
         [0027]     Material pushing rams  90  are located underneath each cassette  30  and push preheated and precalcined material down the sloped floor toward the material discharge  19 . The limestone is pushed uniformly by the reciprocating motion of the rams  90  actuated in a predetermined sequence. Rams  90  can be of the type conventionally utilized in the art—they typically have a rectangular boxed shape having a single-planed flat upper surface  93  and leading face  94 , which initially contacts and moves the particulate material when the ram moves inwardly—and are connected by rods  91  to actuator assemblies  92 , which provide reciprocal movement to rams  90 . The sequence of operation of each ram can be electronically controlled. When an actuator assembly  92  is activated the corresponding ram moves inwardly, that is, down the sloped floor, pushing the preheated and precalcined limestone toward material discharge  19 .  
         [0028]     Alternatively, as seen in  FIG. 6 , rams  90  can have a stepped design, i.e., with an upper surface  93  having two or more distinct steps or upper levels  93   a  and  93   b.  The step closest to leading face  94 , that is,  93   a,  is the shortest, with each succeeding step being progressively higher. This novel ram design is useful because preferential drawdown from the initial preheating cassettes  30  will correct any natural misdistribution from a uni-dimensional ram profile.  
         [0029]     It is understood that other types of material pushers can be used in conjunction with the present invention. The material pusher can involve any type of mechanism which causes the limestone to travel down sloped floor  16  when activated  
         [0030]     The invention has been shown in a single preferred form and by way of example only, and many variations and modifications can be made therein within the spirit of the invention. The invention, therefore, should not be limited to any specified form or embodiment except in so far as such limitations are expressly set forth in the claims.