US3612497A

US3612497A - Center feed rotary hearth calciner

Info

Publication number: US3612497A
Application number: US887450A
Authority: US
Inventors: Victor D Allred
Original assignee: Marathon Oil Co
Current assignee: Marathon Oil Co
Priority date: 1969-12-22
Filing date: 1969-12-22
Publication date: 1971-10-12
Anticipated expiration: 1988-10-12
Also published as: CA934696A

Abstract

A rotary hearth calciner having a hearth somewhat like a phonograph turntable rotating within a covered chamber. Granular material to be calcined is fed to the center of the rotating hearth and is gradually moved outward by rabbles so that it discharges near the outer edge of the rotary hearth. Thus, the coke bed is thinnest at the outermost portion of the hearth, permitting highest temperature exposure when the last volatiles are to be driven from the coke.

Description

United States Patent Inventor Victor D. Allred Littleton, Colo.

Appl. No. 887,450

Filed Dec. 22, 1969 Patented Oct. 12, 1971 Assignee Marathon Oil Company Findlay, Ohio CENTER FEED ROTARY HEARTH CALCINER 8 Claims, 4 Drawing Figs.

US. Cl 263/22, 201/17 Int. Cl F271) 9/18 Field of Search 263/7, 22, 25; 201/17 References Cited UNITED STATES PATENTS 625,336 5/1899 Keiper 263/22 2,973,568 3/1961 Greger 3,448,012 6/1969 Allred Primary Examiner-John J. Camby Att0meys-Joseph C. Herring, Richard C. Willson, Jr. and

Jack L. Hummel ABSTRACT: A rotary hearth calciner having a hearth somewhat like a phonograph turntable rotating within a covered chamber. Granular material to be calcined is fed to the center of the rotating hearth and is gradually moved outward by rabbles so that it discharges near the outer edge of the rotary hearth. Thus, the coke bed is thinnest at the outermost portion of the hearth, permitting highest temperature exposure when the last volatiles are to be driven from the coke.

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INVE N TOR VICTOR D. ALLRED ATTORNEY CENTER FEED ROTARY HEART CALCINER BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the devolatilization of materials which yield combustible volatile matter under the action of heat and particularly to furnaces for this purpose which have rotary hearths and are useful for calcining coke.

2. Prior Art Calcining in rotary hearth furnaces is taught in British Pat. No. 1,055,857 and in U.S. Pat. No. 3,448,012. Somewhat related furnaces are shown in U.S. Pat. No. 3,227,627. Various drying apparatus is taught by U.S. Pat. No. 1,997,937 and 1,673,464.

SUMMARY General Statement of the Invention The preferred ovens of the invention utilize a substantially horizontal circular hearth, preferably consisting of a roof and sidewalls, which reflect radiant heat down onto calcining material lying in a bed on the hearth. The material is deposited substantially continuously at a point near the center of the hearth and is gradually moved outward through the action of a series of rabbles which are plowlike devices which move circu larly in relation to the hearth. (The hearth may rotate about its center point or a series of rabbles may be arranged along one or more radii and caused to rotate about the center point of the hearth or other relative motion between the hearth and rabbles may be provided.) The rabbles arrange the bed of material into windrows and furrows with each rabble moving the material contacting it into the next windrow.

Air (or other oxygen-containing gas) is injected at points preferably within the inner one-third of the hearth and mixes with volatiles released by the calcining material lying in the bed. It is desirable that the air be injected at relatively slow velocities so that it mixes with the volatiles at a point substantially above the bed. Combustion then occurs away from the calcining material and losses, e.g. of fixed carbon in calcining coke, are reduced to a minimum. Heat from the combustion zone is reflected back onto the hearth by the walls and roof of the oven. One or more stacks, preferably located near the sidewalls, remove exhaust gases.

Materials discharge at a point near the outer edge of the hearth, preferably merely being plowed off the hearth by one of the rabbles. Because the rabbles divide the hearth into bands of increasing circumference, as the material moves outward the bed becomes thinner. This is especially important in the calcining of coal-derived coke or petroleum-derived coke. Cokes are frequently heat sensitive so that they tend to expand into undesirable low bulk densities or shatter into undesirable small particles under the action of rapid increases in temperature. Therefore, the present invention, by maintaining a thick bed when the material first encounters the heat of the oven, inherently provides for relatively slower heating of the cooled material. As the material increases in temperature, the bed automatically decreases in thickness, thus providing more exposure to the heat reflected from the arch of the oven. Since the density of calcined coke is substantially a function of the highest temperature to which the coke has been subjected, it is important that the coke be heated to the highest possible temperature just prior to being discharged from the hearth. This is also inherently provided by the present invention which arranges the coke in the thinnest bed just before it is discharged from the hearth. Thus the heat transfer is maximized at the point where the coke is at the hottest (and the temperature differential between the arch and the coke is at a minimum). This thinning out of the bed can produce coke having a commercially desirable higher density without the sacrifice in throughput which would be caused by spreading the bed thinly over the entire hearth.

It is a further feature of the invention that all of the heat is directed downward onto the bed and none comes upward through the hearth itself. At temperatures required for the calcining of coke, (preferably 1,000-3,200 F., more preferably i,800-3,000 F., most preferably 2,800" F.) heating through the hearth would involve great difiiculties in heat transfer through the hearth, and materials for the hearth, and in the design of nondistorting support structures for the hearth. The present invention tends to keep the hearth at a minimum temperature by virtue of the insulating value of the bed which covers the hearth at all points. Also, erosion of the interior of the oven is maintained at a minimum because no moving material touches the sidewalls or roof of the oven and movement across the hearth is kept at the minimum necessary to turn over the bed and to move the material from the inlet to the discharge point. This minimum movement also reduces the amount of undesirable fines" produced in the process.

Other advantages of the invention are that a center pivot can be used to stabilize the hearth and overcome undesired thrust against the drive mechanism, allowing much larger diameter hearths to be constructed. Also, the rabbles experiencing the greatest thrust of material, (due to the thickness of the bed) are located in the coldest zone and thus the maximum mechanical stress on the rabbles is experienced by relatively cool rabbles.

Utility of the Invention The present invention is useful for the calcining of a wide variety of materials including delayed petroleum coke, fluid petroleum coke, coal, coal-derived cokes containing volatile matter, and coat-tar-derived cokes containing volatile matter.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a plan view of a furnace according to the present invention with the roof removed.

FIG. 2 is an elevation taken through a section near the center of a furnace of the present invention according to FIG. 1.

FIG. 3 is a detailed view of an outer wall seal for use with the furnace of FIGS. 1 and 2.

FIG. 4 is a section view of the outer wall seal of FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Starting Materials As discussed above, a wide variety of cokes, e.g. delayed and fluid petroleum cokes, and other carbonaceous materials, e.g. anthracite coal, can be used with the present invention so long as they yield volatile combustible matter under the action of heat and provide a valuable product when calcined. The coke or other material will preferably be fed to the hearth at a rate of about 2 to about 50, more preferably 5 to about 30, and most preferably 10 to about 20 pounds per square foot of hearth area per hour. The material will preferably contain from about 2 to about 20, more preferably 6 to about 12 and most preferably 8 to about 10 percent by weight of volatile combustible matter as measured by heating to about 950 C. and analyzing the combustible volatile matter driven off.

The Oven FIGS. 1 and 2 show an oven having a rotary hearth 10, a center feed chute l1 fed by feed bin 12, an air manifold 13 having various inlets M which direct the air in a slightly downward fashion so as not to cause a high air velocity over the surface of the bed of material 15 which is deposited on rotary hearth l0. Rabbles l6 depend from a rabble support 17 which may be adjusted so as to vary the vertical penetration of the rabbles into the bed 15. In general, the rabbles will be adjusted so that there is a space of approximately 0.5 to 4 inches, more preferably 1-3 inches, and most preferably 2 inches between the bottoms of the rabbles and the hearth, providing a relatively stationary bed of coke which protects the rotating hearth from erosion. The rabbles are located in rabble pit 18 which is a radial depression in the roof 19 and sidewalls 19a (both of which are omitted from FIG. 1). A last rabble 20 which also depends from rabble support 17 plows the material off the rotary hearth l and into coke discharge chute 21 which is connected to waste heat boiler and cooler 22. A stack 23 removes exhaust gases from the furnace interior 25 and also from the interior of waste heat boiler 22. A recuperator (not shown) located within the stack, preheats the air fed to manifold 13.

FIGS. 3 and 4 show a sectional view of the outer wall seal 24 which consists of a band of silicon carbide bricks 30 set into rotary hearth near its periphery. A band of refractory seal blocks 31 are retained against the outside of sidewall 20 by retainer 32. The seal blocks are free to move vertically and therefore are held by gravity against the band of silicon carbide bricks 30 so as to form a seal. Leakage through this seal can also be controlled by varying the draft on stack 23 so as to provide a minimum differential across the seal.

Operation of the Oven Referring again to FIGS. 1 and 2, delayed petroleum coke having an average particle size of about 0.25 inch and a maximum particle size of about 2 inches is fed from feed bin 12 into feed chute ll. Feed chute 11 is so adjusted that its lower end is approximately 6 to 8 inches above the rotating hearth 10 so as to provide a level band of coke on the rotary hearth. The hearth rotates around a center varying post 40 from which the roof and sidewalls 19 and 20 are supported by structural members which are not shown. The innermost rabble l6 strikes the coke deposited on the hearth from feed chute 11 and disposes it into a windrow. After one revolution this same coke is contacted by the next rabble l6 and is moved into a second substantially concentric windrow. This process continues until the coke contacts the last rabble 20 which plows the coke oft the hearth into discharge chute 21. The coke then falls into vast heat boiler and cooler 22 though which water is circulated (air cooling could be used) to cool the coke and recover heat. The hot water from waste heat boiler 22 is used for other purposes.

As the coke moves from feed chute 11 over hearth 10 to discharge rabble 20 it is gradually heated, releasing volatile matter including volatile combustible matter. This volatile combustible matter contacts air from manifold 13 forming a combustion zone 50. Heat from the combustion zone 50 radiates directly onto the bed 15 and is also reflected from roof l9 and sidewalls onto bed 15 causing further heating of the material in the bed. (Auxiliary burners, e.g. of gas, fuel oil or other combustible material may be located within the chamber to add additional heat where heat from devolatilization is not sufficient. However, it is preferable that the volatile combustible matter from the material be sufficient to maintain the oven at the desired temperature.

Also as the material moves between feed chute l1 and discharge rabble 20, the bed 15 is constantly decreasing in thickness. Thus, as each individual particle nears discharge rabble 20 it is exposed for an increasing percentage of time to the heat radiated from roof l9 sidewalls 20 and combustion zone 50. This minimizes the temperature differential between the interior of the furnace and the temperature of the individual particle within the bed 15, thus tending to maximize the temperature to which each individual particle is heated. This maximizing of the temperature experienced by each particle tends to give a denser coke which is more desirable for the production of graphite electrodes and other purposes.

The rabbles 18 may be of high-temperature materials or, more preferably, may be water cooled by water circulated through the rabbles and then through a cooling tower. The chamber 25 may be divided into various zones according to the teachings of U.S. Pat. No. 3,448,012. Various other seal arrangements may be utilized including water seals, doubleor triple-seal arrangements, and the inner seals 51 may have these configurations or the configurations shown for the outer fields 24 in FIGS. 3 and 4. The hearth 10 may be stationary with a rabble support 17 being located within the chamber 25 and rotating about center point 40. The hearth 10 may be sloped somewhat from the inside to the outside or may be rippled so as to extend the surface of the hearth or to provide particular configurations of windrows.

What is claimed is:

1. A calcining oven for the production of high-density calcined coke comprising a heated chamber, a substantially horizontal and imperforate hearth located in the chamber, a material outlet at the outer periphery of the hearth, means for delivering material containing volatile combustibles to the hearth adjacent the center thereof to form a bed of material on the hearth, spaced rabbles disposed above the hearth and disposed to move material on the hearth progressively outward toward the material outlet upon relative motion between the hearth and the rabbles, said rabbles being so spaced that the annular areas of the hearth between rabbles lying closer to the periphery of said hearth is greater than the hearth area lying between rabbles nearer to the center of said hearth, means for injecting substantially only air laterally into an upper portion of said chamber so as to contact combustible materials being driven off by the action of heat on said material, thereby forming a combustion zone which radiates heat, wherein the chamber has a roof and sidewalls which reflect radiant heat from said combustion of volatile matter downward onto said bed of material exhaust means for withdrawing the products of said combustion from said chamber.

2. A process for producing high-density coke from material which yields oxidizable volatile matter under heat comprising the steps of:

a. continuously delivering the material to be treated onto a hearth in an inner zone of a heated oven chamber adjacent the center of said hearth to form bed thereon,

b. relatively moving the hearth and the material thereon with respect to a plurality of spaced rabbles disposed above the hearth,

c. progressively advancing the material outward toward a material outlet near the periphery of said hearth by disposing said rabbles to extend into the bed at an angle of attack that moves the bed progressively outwardly toward said material outlet,

d. turning the bed over and over during its progressive movement toward the material outlet,

e. feeding oxygen-containing gas laterally into an upper portion of said chamber so as to contact combustible materials substantially all of which have been driven oil by the action of heat on said material, thereby forming a combustion zone which radiates heat onto said material,

f. spacing said rabbles so that the hearth area between rabbles lying nearer to the periphery of said hearth is greater than that between rabbles lying closer to the center point of said hearth whereby said bed is thinner at said periphery than near said center point.

3. The process of claim 2 wherein the temperature in said chamber is above about 2,000 F.

4. The process of claim 2 wherein the chamber has a partition means and separating an inner zone from an outer zone and wherein the bed is moved under said partition means.

5. The process of claim 4 wherein the temperature in said inner chamber is sufiiciently low so as not to heat said bed to a temperature of above about 500 F. and wherein the temperature in said outer chamber is above about 2,000 F.

6. The oven of claim 1 wherein the hearth rotates about a center pivot and the rabbles are substantially stationary.

7. The oven of claim 6 wherein there is provided a sliding seal between the lower edge of said sidewalls and the upper surface of said rotating hearth.

8. The oven of claim 6 wherein the seal comprises blocks of a softer material sliding up against a substantially smooth harder material and wherein said harder and said softer materials are held together by he force of gravity.

Claims

2. A process for producing high-density coke from material which yields oxidizable volatile matter under heat comprising the steps of: a. continuously delivering the material to be treated onto a hearth in an inner zone of a heated oven chamber adjacent the center of said hearth to form bed thereon, b. relatively moving the hearth and the material thereon with respect to a plurality of spaced rabbles disposed above the hearth, c. progressively advancing the material outward toward a material outlet near the periphery of said hearth by disposing said rabbles to extend into the bed at an angle of attack that moves the bed progressively outwardly toward said material outlet, d. turning the bed over and over during its progressive movement toward the material outlet, e. feeding oxygen-containing gas laterally into an upper portion of said chamber so as to contact combustible materials substantially all of which have been driven off by the action of heat on said material, thereby forming a combustion zone which radiates heat onto said material, f. spacing said rabbles so that the hearth area between rabbles lying nearer to the periphery of said hearth is greater than that between rabbles lying closer to the center point of said hearth whereby said bed is thinner at said periphery than near said center point.

3. The process of claim 2 wherein the temperature in said chamber is above about 2,000* F.

5. The process of claim 4 wherein the temperature in said inner chamber is sufficiently low so as not to heat said bed to a temperature of above about 500* F. and wherein the temperature in said outer chamber is above about 2,000* F.