US2784956A - Lime kiln - Google Patents

Description

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March l2, 1957 Filed Dec. 16, 1953.

R. B. VOGEL LIME KILN 2 Sheets-Sheet. l

REURCULATED GAS PRO DUCER GAS INVENTOR.

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LIME xm 2 Sheets-Sheet 2 March 12, 1957 Filed Dec. 16, .1953

LEADS Tueur/mount 'ruenmocoumi LEADS ATTO R N YS LIME KILN Ralph B. Vogel, Carey, hio, assignor to The National Lime and Stone Company, Findlay, Ghia, a corporau tion of Ohio Application December 16, 1953, Serial N o. 398,468 4 Claims. (Cl. 263-29) This invention relates to improvements in the combustion control of vertical lime kilns.

The calcining of limestone in a vertical kiln is performed by burning substantial quantities of gas in the lower portion of the kiln with the heat thereby produced acting upon the limestone to drive off CO2 gas and to reduce the stone to as nearly pure lime as possible. In so burning the lime it is desirable to avoid vitriiication of the limestone and not to dead burn or over burn the stone during the disassociation process. Overburning, or overheating of the limestone deleteriously aiects the lime produced. Lime which has thus been overheated is called hard-burned lime, as contrasted to softburned lime-or lime having desirable qualities resulting from disassociation at a lower temperature.

One of the causes of hard-burned lime is the formation of local hot spots in the interior of the `mass of limestone:A resulting from the formation of chimneys through the mass of rock and the consequent excessive flow of heat through these open pathways.

The formation of hot spots has a second Very undesirable result in that it may burn away localized areas of the refractory kiln lining requiring that the kiln be taken out of production for the replacement of the lining.

A still further diculty with many vertical lime kilns arises from the necessity for the use of large size rock, say, in the order of l0 to l2 inches in average diameter, in order to provide sufiicient open space in the kiln to allow a draft to be maintained upwardly through the column of the kiln. When rock of smaller average diameter, say, 4 to 6 inches, is loaded into an ordinary vertical kiln it so reduces the interstices between the rock pieces as to make it almost impossible to maintain a satisfactory draft through the `column of the kiln. The objects of this invention are, therefore, to provide means for controlling the flow of h'ot gases through the kiln to evenly distribute the heat through `the mass of stone being calcined, thus to permit the use of smaller sized Vstones with the resulting advantage of relatively larger surfaces per unit of mass for heat absorption and a consequent higher rate of production and to eliminate the formation of local hot spots with the resulting deleterious effects not only on the limestone but on the refractory lining of the kiln itself.

More specifically, the objects of the invention include the provision of a simplified control for the operation of avertical kiln which enables the operator to determine the conditions existing within the kiln and to take steps to maintain the supply of heat to the kiln in a manner appropriate to maintain a uniform temperature throughout the calcining zone and thus to produce a soft-burned lime.

These and more specific'objects and advantages and the mode of their accomplishment will be better under- `stood from the specification which follows and from the drawings in which:

Fig. 1 is a vertical sectional view of a lime kiln equipped arent 2,784,956 Patented Mar. 12, 1957 with apparatus embodying the invention, being shown in elevation.

Fig. 2 is a fragmentary vertical sectional view, with parts in elevation and on an enlarged scale, taken substantially on the line 2-2 of Fig. 1.

Fig. 3 is a fragmentary, enlarged vertical sectional view taken on the line 3 3 of Fig. l.

Fig. 4 is a horizontal sectional view taken along the line 4 4 of Fig. 1.

A conventional vertical lime kiln comprises a main column generally indicated at 10 which usually consists of a steel casing lined with a thick refractory lining 11. The lining 11, for example, may vary from 2 to 3 feet in the hotter areas of the kiln to 6 or 8 inches in the cooler zones. The column 10 is erected upon a heavy frame 12 and its lower end terminates in a generally cone shaped hopper 13 below the level of the calcining zone and thus functioning both to furnish a cooling chamber and as an unloading hopper, its lower end having a conventional trap door 14 located at a height sufficient to permit cars, trucks, or conveyors to pass therebeneath.

At the upper end of the column 10 the top of the kiln is closed by a cover 15 provided with an openable door 16 through which untreated limestone is charged into the kiln. Flow of the limestone through the kiln is produced by gravity with the rate of flow being determined by the rate at which the lime is removed from the hopper 13.

The column itl may be roughly divided vertically into four zones. The uppermost indicated as the storage zone in Fig. 1 extends from the top of the column 10 to a level at which there is located an exhaust pipe 17 which extends across the column 10 and has draft openings 18 in its underside (see also Fig. 3). The exhaust pipe 1! may be provided with downwardly extending shields 19 which prevent the plugging of the draft openings 18. The pipe 1'7 is connected to a motor driven fan generaliy indicated at 2t) and the fan 20 has an exhaust stack 21. Operation of the fan 20 provides an induced draft through the column 16. The pipe 17 also has a damper 22 by means of which the negative pressure within the column iti may be controlled.

The second zonerof the kiln extends from the level of the induced draft exhaust pipe 17 downwardly to the level of a waste gas exhaust system generally indicated in Figs. l and 2 by the reference number 23. This zone is denominated the preheating zone in Fig.

The waste gas exhaust system 23 includes means providing for a recirculation of a ,substantial quantity of waste gases resulting from the burning of fuel gas, usually producer gas, introduced in the lower part of the kiln from gas mains 2d. (Fig. 2) which communicate with a producer gas duct 25 extending across the column 10 at the upper side of a bridge 26. The bridge 26 is constructed of refractory material and may be supported by an iron beam 27. The duct 25 has a plurality of laterally opening ports 28 through which the producer gas is fed into the interior of the column 1i).

The burning of the produ-cer gas as Iit is fed out of the ports 28 produces a flame yof large volume whichis drawn upwardly through the zone denominated calciuing zone in Fig. l and, most desirably, the heat from the producer gas flame should .be uniformly distributed throughout the mass of limestone in the calcining zone. The flow of hot gases upwardly takes place through the interstices between the lumps of limestone and unless the operator experiences good fortune, it is almost impossible kto prevent the formation of chimneys through the mass of stone which tend to `draw excessive quantities of hot gases and thus to overheat or hard burn the limestone adjacent the chimneys If such a chimney exists along the refractory lining of the kiln the heat may rapidly rise to such a temperature as to break down the certain portions refractory structure and. even, mit, away to, an. eX- tent `such that a hot spot appears on the steel exterior of the kiln. Of course, when this occurs it is necessary to Shutdown the kiln, and. replace the refractory material `Operation according to the invention; Provides a control-for more uniformly distributing the heat throughout the calciningl zone and over the lumps. of limestone therein. As a result the periodA of time for any particular stone within the calcining zone can be lessened since smaller stones can be usedproyiditng` for more eicient heat absorption and since the quantity of hard burned stone is minimized. As a result of this time reduction, and as a result of the moreeftlcient and uniform heating, the rate of lime production for a kiln formerly operating merely by induced. draft cany be increased many yfold without any resulting reduction, in refractory life.

. The. apparatusl of the invention includes an annular bustle pipey 29 extending horizontally around the exterior ofr the column and, located approximately at the plane of .demarcation between the preheating zone and the teal'cining zone. The bustle pipe 29 isvconnected to the interior of the column 10 at at leastk two points, approximately 180 from eac-h other, by waste gas exhaust ducts 30 which extend through the wall of the column 10 and the refractory l1 preferably being downwardly inclined toward the interior of the column 10. At their upper outer ends each of the waste gas ducts 30 opens into a dust separation hood 3l. The hood 31 extends over the bustle pipe 2 9 and has a flow damper 3,2 and an interior baille plate 33 which. extends downwardly below the level of the bustle pipe 2.9. As can best be seen in Fig. 4, the bustle pipe 29 terminates. at the side Walls of the dust hood' 31 and gas flowing out of the duct 30 must ow over the baille 33 and around and up under its downwardly tur-ned lip 34. The dust hood 31 has a downwardly `extending hopper shaped section 35 terminating in a dust removal pipe 36 which extends down the exterior of the column 10 and is provided `at its lower endV with a -dust removal trap 37.

Waste gases `drawn out through this duct 30 at relatively high velocity are pulled aroun-d the baffle 34 and the dust carried therewith separated from the gas by its own impetus, fall-ing downwardly into the pipe 36.

:A pair-of dow-ncomers generally indicated at 3d are located at the other two quarter pointsV of the bustle pipeV 29: YEach of the downcomers 33 consists of a short vertical pipe 39, a steam jet generally indicated att 40 and a lower section of vertical pipe 41 lwhich is' connected by a horizontal duct i2 to a recirculated gas duct 43 carried by :the bridge 26 and located immediately above the producer gas duct 2S. l The recirculated gas lduct 43 ltas'a plurality of laterally extending ports d4 through which the combination of interrnixed `recirculaterl waste gases and steam is fed into the interior of the kiln .along with the burning producer gas.

The gases referred to herein as waste gases or recirculated` gases include not only the products of the combustion of the producer gas but also large volumes of COadisasscciated from the limestone in the lime producing processand large quantities of dust, consisting `of silica, alumina and other impurities' and ol some of the tdisassociated lime itself which isl carried away from the limestone by the blast of the `burning gas and the `draft through the column. Because thev gases carry with them this large volume of dust, the problem of recirculating thehc-avily laden gas is difficult since it tends to plug or interfere with the operation of most types of recirculation means.

The use of steam jet eductors to induce` the draft for the recirculation of the gas has several advantages. Among these are theeliminaition of movingparts, such as -fans orblowers, which are exposed to thehot gases being recirculated and which thus are quickly damaged and which require frequent replacement and expensive maintenance. Furthermore, by controlling the volume of steam introduced into the eductors it is possible to control the volume of' gas which is recirculated without requiring repeated adjustments of mechanical controllers such as dampers and ilow regulators. In addition, by introducing the steam into the midst of the recirculating gases the jet effect of `the steam nozzle thoroughly admixes the steam with the gases. Thus the steam jets accomplish the several purposes of causing the recirculation of the Waste gases, entering into the kiln itself for softening the flame or blast and. adding, heat', to the kiln.

in order to provide for controlled and accurate operation of the apparatus as described, a necessaryy element of the instant invention is the` provision and location of a number of temperature indicating means, for example, thermocouples. As is schematically indicated in Figs. l and 4, a thermocouple is located within each of the waste gas exhaust ducts 30 or at upper portions of the dust separation hoods 3l. These two thermocouples are generally indicated at 45'. These two thermo.- couples give the temperature of tthe recirculated waste gases at the time they are removed from the kiln. A second set` of thermocouples generally indicated at 46 in Fig. 2 are located at the upper ends ofthe downcomer-s 38 to indicate the temperature of the lrecirculated gases after having been `drawn out of the kiln and carriedv `around theexterior through the bustlepipe 29. A third set of thermocouples 47 arelocated one in each of the ydown-comer pipe sections 4l below the steam jets 40.

Knowing the temperature, volume, and specic heat ofthe steam fedv into the steam jets 33 it is possible by the use of the apparatus of the invention to determinel andV control the volume of recirculated waste gases by measuring the temperature of these gases in the bustle pipe above the steam jets andA in the downcomer pipes below the steam ljets. By comparing the temperatures ofthe gases in the two dust hoods and at the upper ends of the downcomers 3S the apparatus of the inventionA provides means for determining whether or not the gas llow and thus. thel temperature within the calcining zonev is uniform. Under ideal conditions, of course, the temperature in both dust hoods should be the samel and the temperatures at the upper ends of the two downcomers` should be the same. Similarly, if conditions are ideal, the temperatures of the combined steam and recirculated gas in the two pipe sections 41 of the downcomers 33' should also be the same. Variations between the temperatures of the gases in the two similar positions of any one ofthe three measurement points provide a certain and clear indication of variations in the heat within the arcas of' thecalcining zone. Such variations can be compensated for by changes in volume and temperature of the steam, increases and decreasesin the induced draft, changes in the volume of producer gas burned and by modifying the setting of the two dampers 32 in order to balance and control the quantity of waste gases drawn off and recirculated.

A typical operation according to the invention ispetformed with steam fed into the steam jets at pounds p. s. i. The average temperature of the waste gas drawn off through the ducts 30V is approximately 900 F. and the mixture of gas and steam at the bottom of the downcomers 38 is thus approximately 700 F. Maintaining the operating conditions at thisY or similar ranges determined particularly for a particular kiln, enables an apparatus according to the invention tooperate at'a very high production rate. Under the conditions mentioned it is possible-through the practice of the invention to main.- tain a desirable temperature, say, between 1800" and 2000" F., throughout the calcining zone and disassociation can be continuously accomplished with practically no spalling or vitriiication of the exterior surfaces of the lumps of lime. By enabling the reduction of stone size through the uniformity ofthe heat in the calcining zone and'elimination of chimneys and hotspots production on a kiln equipped with apparatus accordingA to the invention increased from approximately 18 tons per day before it was thus equipped to approximately 70 tons per day after it had been equipped, This four-fold increase in production was accomplished without increase in the operating temperature (it being limited by the nature of the operation being performed) and without any reduction in the life of the refractory. Operating under the temperature conditions mentioned above approximately 20,000 cubic feet of waste gases per hour were recirculated by the seam jets through the downcomers and approximately 200,000 cubic feet of waste gases per hour were exhausted from the exhaust stack 21 by action of induced draft fan 20.

The increase in production without the decrease in refractory life is accomplished without any appreciable decrease in the heat efficiency of the kiln, i. e., the quantity of producer gas and steam introduced for calcining per ton of lime from a heat standpoint remained approximately the same as the heat introduced for calcining a ton of lime entirely by producer gas in the kiln before it was equipped with apparatus according to the invention. This maintenance of heat eiciency while accomplishing the four-fold increase in production is believed to result from the better diffusion and uniform control of the hot gases, more uniform heating of the smallersize stones made possible thereby and assisted by the superior heat radiation ability of the steam and the elimination of hot spots and chimneys both within the mass of rock and alongside the refractory lining.

l claim:

1. In a kiln having a lowermost discharge outlet and an uppermost charging opening and an upper storage zone, in combination, a draft outlet from said kiln at the bottom of said storage zone, a preheating zone beneath said storage zone, a waste gas outlet at the bottom of said preheating zone at each side of said kiln, a damper for each of said outlets, a calcining zone located beneath said preheating zone, a ring bridge extending across said kiln at the bottom of said calcining zone and having a waste gas duct therein, conduit means including separate down pipes on opposite sides of said kiln and located exteriorly of said kiln for connecting said waste gas outlets to said waste gas duct, separately controllable steam jet eductors in each of said down pipes for inducing the controlled ow of waste gases therethrough and temperature responsive means in each of said waste gas outlets and in each of said down pipes below said steam jet eductors for measuring the flow of waste gases through each of said conduit means.

2. A method for controlling the operation of a vertical lime kiln, such method comprising, burning heating gas at the bottom of said kiln, feeding the limestone to be disassociated downwardly through said kiln, flowing the waste products of the combustion of said heating gas and the gas disassociated from the limestone upwardly through said kiln by maintaining a negative pressure in the upper part of said kiln, diverting a portion of such waste gases out each side of said kiln near the upper limit of the disassociation zone in volume inversely proportional to the temperature of the gases being diverted out of that side of said kiln, introducing a high pressure jet of steam into the midst of each separated portion of the diverted gases in volume proportional to the temperature thereof for inducing the flow thereof out of said kiln and re-introducing the diverted gases and steam into said kiln at about the level of the initial burning of the heating gas.

3. A method for controlling the operation of a vertical lime kiln, such method comprising, burning heating gas at the bottom of said kiln, feeding tne limestone to be disassociated downwardly through said kiln, flowing the waste products of the combustion of said heating gas and the gas disassociated from the limestone upwardly through said kiln by maintaining a negative pressure in the upper part of said kiln, diverting a portion of such waste gases out each side of said kiln near the upper limit of the disassociation zone in volume inversely proportional to the temperature of the gases being diverted out of that side of said kiln, measuring the temperature of each diverted portion of such waste gases at the point of diversion thereof out of said kiln, introducing a high pressure jet of steam into the midst of each diverted portion of such waste gases for inducing the flow thereof out of said kiln, measuring the temperature of each diverted portion of such waste gases at a point beyond the introduction of such steam therein, metering the ow of steam in relation to the change in temperature of each of such portions of such waste gases between such points of measurement and reintroducing such portions of waste gases and such steam into said kiln at about the level of the initial burning of the heating gas therein.

4. In a kiln having an uppermost charging opening, a storage zone therebeneath, a preheating zone beneath said storage zone, a calcining zone beneath said preheating zone, a cooling Zone beneath said calcining zone and a lowermost discharge opening, in combination, a draft outlet from said kiln at the bottom of said Storage zone, a waste gas outlet communicating with only one side of said kiln at the bottom of said. preheating zone, a separate waste gas outlet communicating with only the opposite side of said kiin at the bottom of said preheating zone, a separately controllable damper in each of said waste gas outlets, a tiring bridge extending across Said kiln at the bottom of said calcining zone and having a waste gas duct therein, down pipes connecting said waste gas outlets to said waste gas duct and separately controllable fluid pressure means in said down pipes for inducing flow of waste gases from opposite sides of said kiln and through said waste gas outlets and down pipes to said waste gas duct in said tiring bridge.

References Cited in the le of this patent UNITED STATES PATENTS 815,080 Enright Mar. 13, 1906 1,832,552 Haslam Nov. 17, 1931 Iv,2,199,384 Azbe May 7, 1940 2,409,527 Azbe Oct. 15, 1946 2,529,366 Bauer Nov. 7, 1950 2,532,077 Azbe Nov. 28, 1950 2,602,330 Kollsman July 8, 1952 2,684,840 Behme et al. July 27, 1954

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