US938987A - Process of burning lime and apparatus therefor. - Google Patents

Description

H. L. DOHERTY.

APPLICATION FILED SEPT. 14, 1908.

Patented Nov. 2, 1909.

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HENRY L. DOHERTY, OF NEW YORK, N. Y.

PROCESS OF BURNING LIME AND APPARATUS THEREFOR.

Speciflcatien of Letters Patent.

7 Patented Nov. 2, 190a.

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To all whom it may concern:

Be it known that I, HENRY L. DoHER'rY, a citizen of the United States, and resident of New York city, in the county of New York andState of New York, have invented certain new and useful Improvements in Processes of Burning Lime and Apparatus Therefor, of which the following is a specification.

My invention relates to aprocess for burning lime and to apparatus therefor.

It relates, particularly, to that kind of process and apparatus wherein the heat for burning the lime is derived from the com-' bustion of producer gas. 7

The objects of my invention are, the furnishing of a method and means whereby more of the heat of the kiln can be recuper-.

ated than is at present possible in apparatus of the class mentioned, the avoidance of the danger of over-burning the lime by the regulation of the temperature in the combustion zone of the kiln, and the expansion of this combustion or high temperature zone to occup; a much greater space in the kiln.

- y invention consists, briefly, of a limekiln in which the lower part or cooler of the'kiln is deeper than is, at present, customary,-a gas producer appurtenant to the kiln, a second and outer s ell surrounding the main shell of the kiln and forming with said shell an open annular air space, said annular space having air inlets around its periphery at the top and connections from the bottom through a fan with the ash pit of the producer, air inlets at the lower part of the cooler, a fan for the purpose of introducing kiln gas into the ash it of the producer together with the air be ore mentioned and another fan for the purpose of Introducing another portion of the kiln gas mto the producer gas for the fpurpose of so moderating the temperature the flame of the burn1 ng gas that the danger of over-burning 1s avoided, and the process of burnin lime by means of a producer gas so modifi by the aforementioned apparatus. 1

In the drawing I have shown across-section of the apparatus in a moreor-less-diagrammatic form, the two blowers being in elevation.

Figure 1 is a section through the kiln and producer on a vertical plane throu h their axis, the blowers being shown in e ovation. Fig. 2 is a front elevation of the lower part of the cooler showing the doors for the disoperation in the manner wellknown to thosev charge of the lime and the ports for the inlet;

of the air for the combustion of the pre ducer gas.

1,'refers to the lime-kiln proper; 2 is the gas fproducer; 3 is the blower supplying the dra current to the producer; and 1 is the blower supplying the kiln gas to the pro ducer gas.

5 is the lower portion of the kiln, commonly called the cooler; 6 designates the zone of the kiln in which the stone is dissociated or burned; and 7 designates the upper portion of the kiln Whose function is simply to insure suflicient time of contact between the stone and hot gases from the dissociation zone to enable the stone to take up from the gases all the heat which it is capable of absorbing.

8 is t e charging opening of the kiln, closed by a door or cover, 9.

10 is an annular air space formed between the shell proper of the kiln and the outer shell, 12. Aroundthe upper part of this jacket are arranged ports, 13, controlled by the gates or dampers, 14.

is the waste gas stack of the kiln, havinga gate, 16; 17 is a pipe or conduit conveying a portion of the kiln gas to the suction pipes 18 and 19 of the fans or blowers 3 and 4, respectively; 20 and 21 are gates on the respective suction pipes 18 and 19; 22 is a pipe connecting the lower part of the air jacket 10 with the suction pipe 18 of blower 3, having a gate, 23.

24 is the discharge pipe of blower 3, and 25 that of blower 4.

26 is the producer gas conduit leading off from the producerand connected with a bustle-pipe, 27, encircling the kiln. This bustle-pipe, 27, has connections, 28, into the shaft, 6, of'the kiln.

29 is the metallic hopper in which the cooler, 5, terminates. This hopper is provided with doors, 30, operated in any con venient manner, and air inlets, 31. The air inlets are so designed that their free area may be regulated at will.

32, 32, etc., are ports passing through the kiln walls, and permitting .the ignition of the gas when the kiln is started in operation and the barring down of the lime. 33 are plugs closing these ports; 34 are suitable liallges or dampers on the gas inlets into the The method of operation is as follows: The gas producer having been brought into skilled in the art of producer operation, the producer gas which, at first, is allowed to escape into the atmosphere until the producer is in normal operating condition, is-

immediately upon its entrance into the shaft,

of the kiln and burns. The air for its combustion (which I. will hereafter call the secondary air) is admitted through the damper 31 at thebottom of the cooler. When the gas flame has been maintained for a suf-- cient len h of time to thoroughly heat the interior 0 the kiln for, say, ten or fifteen feet above the ports 28 to full redness, limestone is graduallyi charged into the kiln. After the level of the stone has been brought up to within a short distance of the gas ports it should be charged in small portions so as to avoid reducing the temperature of the gas flame below the point of ignition. When a body of highly heated stone (it should be, at least, at a full red heat) has been built up. for a distance of six to ten .feet above the gas ports, the remainder of the shaft can be filled as rapidly as may be desired. As the calcination of the stone in the vicinity of the gas ports proceeds, its temperature becomes hlgher and higher since the heat developed by the gas flame goes more andmore to raising the temperature of the calcined stone (lime) and less and less is rendered latent in the dissociation of the stone. If this action proceeds unchecked the temperature of the lime finally reaches a point at which the impurities, silica, alumina, etc., which are present in greater or less amount in all limestones, will unite chemically with part of the lime (CaO) present. This causes a partial fluxing of the lime with the roduction of a roduct which will slake wit difliculty, and, 1n large part, not at all.

In practical lime burning it is the aim to draw the lime from the bottom of the kiln at such a rate that the lime is removed from the influence of the high temperature flame just before the calcination is complete. As mentioned above, during the operation of calcination the tem erature of the material is'maintained consi erabl below the tem erature of the flame with w ich it is bathe on account of the fact that a large portion of the heat taken up by the stone is transformed into latent heat through the dissociating reaction incident to llme burning. This reaction-takes lace according to either one or both of the ollowing equations Reaction 1 is the one which takes place limit.

when calcite, marble or any carbonate rock in which the base is calcium is subjected to heating at or above redness. Reaction-2&5

heated. Usually the so-called limestone" is neither .a pure calcium carbonate rock nor a the one which occurs when dolomite is pure dolomite but of a composition between the two.

By reaction 1, reducing the datatostand- A ard conditions of temperature and pressure,

about 778 B. T. U. are absorbed erpound'.

of stone calcined. If we take t e specific heat of limestone as .22, this heat rendered latent would have been sufficient without dissociation to have 'raised the stone to a temperature of .22

cut, therefore, that if the limecould be withdrawn from the influence of theflame immediatelyon the completion of the above re- E=3536 F. It isappar action there would be no danger of overburning. In ractice, however, with the customary met Ode of burning, it is, for

various reasons, very diflicult'toso regulate the operation of the kiln'so as to even approximately secure the desired end. The

lime produced is therefore, usually, of an. I

irregular uality. Besides,.grea t skill is required of t e men operating the kiln. Now, by t-he method of operation which 1 herein describe and claim all danger-of over-burning is done away with, and, at the same time, a great saving effected in fuel consumption owing to the recu eration of the heat ordinarily carried out 1n the lime and the saving in radlatlon losses which I effect. I secure my temperature control in the kiln by mixing with the producer gas generated in the producer 2, more or less gas withdrawn from the upper portion of the kiln. The heat liberated in thecombustion of the gas must therefore go to raising the temperature of this portion of kiln as so introduced as well as to the calcinationof the stone and the raising of the temperature of the stone, air and calcination products. By regulating theproporti'on of kiln gases to producer as it is manifest, that I can reduce the initial temperature of the producer gas to any point-within the ignition It is my object, however, to add simply-a sufficient quantity of kiln gases to reduce the temperature of the flame to a point at which there' is no danger ofoverburning the lime. The actual proportion of kiln gas added to the producer gas depends upon the conditions in the kiln at any given time and should be regulated from time to time accordin to the indications given by the actual con tions prevailing in the kiln.

For example, where it is the practice to draw the lime at comparatively long intervals,- (say-6 hours) during the drawing and'for some time afterward the kiln gas should be cut to a minimum or omitted altogether.

This is on account of the fact, that, the withdrawal of the lime and the consequent settling-of the column of material in the kiln carries'into the combustion or high temperature zone a large mass of material which is at a temperature very much below the normal temperature in that zone. The heat absorbed from the flame by the stone is therefore much more rapid than that which normally takes place at this zone. Asthe temperature of the stone increases to the normal temperatore of the calcining stone in this zone of the kiln, I gradually increase the proportion of kiln gas until normal conditions have been reestablished. \Vith gas of normal composition I find that the volume of kiln gas added to the producer gas as diluent should be about equal to double that of the producer gas. As I have previously stated, however, the proportion must be varied to meet the local conditions, which are seldom exactly identical at any two plants. For example, one plantmay be calcining a stone which is a comparatively pure calcium carbonate, where another may be working on a dolomite rock. In such case a hi h-flame temperature would be dc mand ed in the first plant while only a moderate one would be necessary in thesecond. I would therefore use less kiln gas with the producer gas in the first case than in the second.

The kiln gas withdrawn from the upper part of the kiln and added again at the combustion zone acts simply as a carrier of heat. It takes up in thecombustion zone the heat which would otherwise go to raising the temperature of the calcining and calcined stone to a dangerous degree and yieldsit up again to the cooler material above the calcining zone. Its action is therefore to keep a large column of the material at a safe calcining temperature while preventing a dangerously high temperature developing in the combustion zone. In other words,

while limiting the maximum temperature.

in the. kiln, I greatly increase the mean effective temperature in the calcining portion. Owing to this expansion of the high temperature (11. c. calcining) zone, I find it advisable to considerably increase the height of my kiln above the dimensions that usually obtain. For example, if a. height of 40 feet above the gas inlet ports gives the best results in a kiln working with the ordinary method, to give the best results by my method of burning the height should be increased to say 55 feet above. the gas inlets. The secondary air enters through the dainpers 3] in the bottom of the cooler 5, and

rises through the interstices of the column of lime occupying the cooler. This lime passes out of the combustion zoneat very nearly the temperature, therein. Partjof the heatwhich it carries is absorbed in the .utilize the heat of the lime.

elimination of the last portions of carbon dioXid, which is usually not completely eliminated in the calcining zone proper. A large portion of the heat which the lime carries out of the calcining zone is, however, carried down into the cooler 5 as sensible heat, and yielded up by the lime to the secondary air passing up through the cooler.

In the application to lime burning of producer gas firing by methods heretofore used it has been found impossible to secure an output per unit weight of fuel consumed of more than about 55% of that secured by the earlier methods of burning the fuel in direct contact with the stone. This is due to the fact that the early lime burners, more as a matter of convenience than for any intelligent appreciation of the benefits to be secured thereby, entered air which was to burn the fuel mixed with the stone, at the bottom of the kiln. The entering cold air took up the heat of the hot lime and thus restored it to the calcining or combustion space, Now, when producer gas firing was applied to lime burning the importance of this heat .recupcration in the old style kilns was apparently overlooked. At any rate, it has been the universal custom, where producer firing has been used, to introduce the air for the combustion of the gas at the same section of the kiln as that at which the gas was introduced. Indeed, by ordinary methods of producer operation, it is not possible to fully In this case it is necessary to pass about one-half of the theoretical air required for the combustion of the fuel through the gas producer itself. Now the weight specific heat of air and lime is very nearly the same (about .22 in each case). The best results heretofore obtained' in lime burning have been about 8 lb. .of lime to 1 lb. fuel. One pound of carbon requires, theoretically, eleven and one-half pounds of air for its combustion. The full theoretical heat capacity of the air, there? fore, in the old style of lime burning was only about one and'one-third times that of the lime. If now we would try to take up the heat of the quantity of lime mentioned above in a producer gas fired kiln of the type at present used by the air for the combustion of the gas we would immediately perceive that the heat'capacity of the portion of the air which we could enter at the bottom of the kiln would be only about twothirds of that of the lime. By thepresent methods, therefore, it would not be possible to return to the kiln all of the heat of the lime and even the improvement on present methods of entering the air for the combustion of the gas at the bottom of the kiln would not permit/of as high an output per to combustion, but this would result in no saving since the excess of air would cause to be carried out at the top rather more than the heat it would take up in the lower part. Now, one of the principal objects of .my

present invention is to make possible such a N ow, this reaction is a highly endothermic or heat absorbing reaction, the net heat absorption being about 6700 B. T. U. per lb. Y

of carbon consumed. I have utilized this reaction in the process revealed in my Letters Patent 829,105, dated Aug. 21, 1906, for the purpose of regulating the temperature of my producer, In this present invention this object is only corollary to my main object, which is to burn as large a proportion of the fuel in the producer as possible by the available oxygen of carbon dioxid. For this reason, I introduce the air and the kiln. gas bearing the carbon dioxid into the producer at the highest practicable temperature. In

other words, I aim to maintain the temperature of 'my fuel bed, so far as possible, by the sensible heat of the gaseous current introduced, rather than by combustion of the fuel by air.

It is evident that for every 3.67 lb. of carbon dioxid which I can dissociate in my producer I can dispense with a weight'of air carrying 1.33 lb. of oxygen or 5.77 lb.-of

air. At the same time Ihave increased the demand for air in the secondary combustion by an equal amount due to the extra volume of carbon monoxid roduced from the carbon ofthecarbondioxid issociated. 'Asaresultof my improved method of operation,therefore, I transfer a portion of the volume of rimary air-ordinarily passed into the pro ucer to the secondary air which I enter at the botton of the kiln. Since I am continuously drawing off from the kiln a volume of gas corresponding to that which I introduce into the producer, I do not in any way increase the loss of heat in the gases passing out of the kiln.

, In order to introduce the greatest possible quantity of heat in the draft current supplied to the producer, I heat the primary air-as shown in the drawing by drawing it from an annular air jacket enveloping the 7 shell of the kiln. The air is admitted l through dampers at the top of the jacket so as to insure a proper circulation of the air through the annular space. The ,circula tion of the air through the jacket being comparatively slow, owing to the relatively small quantity of primary air used in my method of the air comes into play. That 1s, the air space acts in a measure as it would if it were a strictly closed air jacket. The heat not beingwithdrawn as rapidly from the shell of the kiln as when the latter is freely exposed to the atmosphere accumulates, so to s eak, in the metal shell, raising the latter an theair in contact with it to a much higher temperature than the normal temperature of the shell 'when the'latter is exposed to the atmosphere. In the ordinary type of steelshell-kiln the heat transmitted to the atmosphere through the shell amounts to at least twelve to fifteen per cent. of the total calorific value of the fuel burned in the kiln.

heat, it does take up and return to the kiln via the producer the larger part of it.

The kiln gas which I introduce into the producer, I withdraw from the kiln at quite a high temperature. By mixing the air, which is at a much lower temperature than the gas, with the latter, before it reaches the fan or blower, I am able to introduce the draft current into the producer at the highest tem erature at which it is practicable to handle it with blowers of the ordinary type.

\Vith this invention I can do, what has never, heretofore, been accomplished, secure with a. producer fired kiln about as high an output of. lime per unit of combustible fuel as is secured by the older methods of burning lime in which the fuel is charged with the limestone. The advantage of my method is that where, in the type of kiln referred to, the use of the most expensive fuels, coke of anthracite, is an absolute necessity, in the process herein revealed I can use the cheaper grades of fuel, such as slack, lignites, etc. Aside from the saving in fuel effected thereby, the recuperation of the heat of the lime, and the consequent withdrawal of it in a erable practical value, in that it permits of its immediate loading for transport as well as lessens the labor and annoyance of handling it.

already described twodifferent methods are open .to me. In the first, as alreadyv de scribed, I introduce the portion of kiln gas usedfor modifying the producer gas by means of a separate blower 4 into the roi ducer above the fuel bed or into the con uit between the producerand kiln. In the second method, I introduce into the fuel bed of the producer, in admixture with the pri- VVhile my device does not save all of this comparatively cool condition, has a consid- In modifying my producer gas in the wayof operating, the non-conductingproperty mary air, the entire quantity of kiln gas bustible gas so produced, another portion of which is required in both the producer and kiln.

The extent to which the carbon dioxid introduced into the fuel bed is dissociated depends upon the temperature of the latter, the thickness of the incandescent fuel, and the completeness .and duration of contact between the gas and fuel. The latter condition may be said to vary inversely with the velocity of the gaseous current through the fuel b'edl From the above it follows that by regulating the thickness of the fuel, and the velocity of the gaseous current it will be possible to secure the dissociation of any proportion of the carbon dioxid that maybe desired; or, to put it another way, it will be possible to pass through the producer any quantity of carbon dioxid that we may desire and at the same time secure the dissociation of the pro ortion of the dioxid which I'desire. By ta ing advantage of this fact, I am enabled to greatly simplify the apparatus required by my process and the operation of the same. By properly proportioning my producer andregulating the thickness of the fuel bed, I am enabled to effect the temperature control of the producer and, at the same time, secure the proper dilution of the producer gas, by means of the single'blower, 3. In normal running, I find it feasible, by maintaining the proper thickness of fuel bed to operate in the manner just described.

I consists in contacting with 1 To meet abnormal fluctuations in the operating conditions I have found it desirable to retain the auxiliary blower, 4. I can thus more quickly vary the quality of the producer gas to meet any abnormal fluctuations without, at the same time, disturbing the normal working of the producer, than is possible when depending solely upon the one blower, 3. v

I .am aware of the process, patented to Eldred, (Letters Patent 795,257, dated July 18, 1905) in which he uses the gases from the calcination of limestone for the endothermic control of the temperature of the fuel bed in a gas producer, and I do not claim that particular invention.

As hereinbefore pointed out, the primary object ofmy invention is, in a producer-gas fired-limekiln, the recuperation of the heatof the hot lime. By my present invention I accomplish this by so arranging my apparatus and the circulation of the air and gas currents that the maximum possible ropor tion of the total air required by the kiln and gas-producer is passed in contact with the hot lime.

Having described my invention, what I claim is p l v 1. The process of calcining limestone which ited carbon, a portion of the gases produce in the calcina- .tion of the limestone, mixing with the comthe gases from the calcination of the stone, heating air by contacting the same with the hot lime, and burning said gas mixture with said heated air in contact with said limestone, the portion of calcination gases added to said combustible gas, being such that the temperature developed in the burning of said gas is below that at which the impurities in {he stone will combine chemically with the ime.

2. In the calcination of limestone the process which consists in mixing a portion of the gaseous products of the calcination with air, in contacting such aseous mixture with incandescent carbon orming producer gas, in mixing such producer gas with another ortion of the gaseous products of the calcination of said limestone, whereby a modified producer as is formed, in contacting the calcined limestone with a current of air whereby said air is heated, in mixing such heated air and said modified producer gas in contact with the incandescent lime, whereby a calcining flame of regulated intensity is secured, and in subjecting the limestone to calcination by said flame, all substantially as described.

3. In the calcination of limestone the process which consists in mixin a portion of the gaseous products of such ca cination with air preheated by a portion of the sensible heat of said gaseous products, in contacting such aseous mixture with ignited carbonaceous uel, thereby forming producer gas, in mixing such producer gas with another portion of the gaseous products of the calcination of said limestone, whereby a modified producer gas is formed, in contacting the calcined lnnestone with a current of air whereby said air is heated, and in mixing such heated air and said modified producer gas in contact with the highly-heated lime, whereby a calcining flame of regulated intensity is secured, all substantially, as described.

4. In the calcination of limestone, the process which consists in mixing a portion of the gaseous roducts resultin from such calcination with air reheated y a portion of the sensible heat 0 said gaseous products, in contacting such gaseous mixture with incandescent carbonaceous fuel, thereby, formin roducer gas, in contacting the hot calcine imestone with a current of air whereby the said air-current is heated and the said lime cooled, in modifying the said producer gas by mixing therewith sufiicient of the gaseous products of the calcination to reduce the temperature of the flame formed on the subsequent combustion of said producer gas, be-

low the temperature at which the im urities tact with incandescent material, whereby said gas is burned, and in subjecting the limestone to the flame of such burning gas, substantially as described.

'5. In the calclnation of limestone the rocess which consists, in withdrawing at a igh temperature a ortion of the gaseous products resulting rom such calcination, in contacting such portion of the gaseous products of the calcination with incandescent carbonaceous fuel, suflicient air preheated by a part of the sensible heat of the-gaseous products being admixed with said portion of the said products to maintain said fuel in an ignited condition, whereby a producer gas of regulated calorific value is formed, in contacting the lime from the calcina-tion of the limestone with a current of air, whereby said lime is cooled and said air current heated, in mixing said heated air-current and said producer gas in contact with incandescent material, thereby burning said gas, and in subjecting the said limestone to calcination by the flame of said burning gas, all, substantially, as described.

6. In the operation of a producer-gas-firedlimekiln, the process, which consists in generating the major portion of the producer gas by withdrawing a portion of the kiln gas, while the same is at a high temperature, and contacting it with ignited carbonaceous fuel, in contacting the hot lime with a current of air, whereby said lime is cooled and said air-current heated, and in mixing the said heated air-current, and the producer gas in contact with the hot lime, whereby said.

gas is burned and the limestone subjected to a calcinin limestone, the combination of a furnace for burning said stone, a gas producer, means flame of moderate intensity, all substantia 1y as described.

the material in said lime-.

for heating air by contact with the shell of said furnace, means for introducing the soheated air and kiln gas into the fuel bed of said producer, means for introducing another ortion of kiln gas into the gas enerated in said producer, means for conucting said gas mixture to.;said furnace, and means for burning said gas in contact with the material in said furnace.

9. In an apparatus for the calcination of limestone, the combination of a furnace for burning said stone, comprising ,a vertical shaft, having means for charging the limestone and dischargim the kiln gas at the top thereof, means for withdrawing the finished lime from the bottom thereof, meansfor introducing a combustible gas thereinto and burning the same in contact with the charge in said furnace, and means for heating air by contacting the same with the shell of said v furnace; a gas producer;- means for passing into said producer a draft current comprised of the said heated air and kiln gas, and

means for conducting the gas from said gas therefrom, means for withdrawing the fin ished lime from the bottom thereof, means for heating a portion of air by-contacting the same with the shell of said kiln, means for introducing into the bottom of said'shaft another portion of air and heating the same by contact with the finished lime in the lower part of said kiln, and means for introducing a. combustible gas into said kiln, means for igniting said gas and means for burning the-same; a gas producer; means for introducing the first portion of heated air and kiln gas into said gas producer; and means for conducting the combustible gas generated in said sproducer to said kiln.

Signed at New ork city in the county of New York and State of hew York.

HENRY L. 'DOHERTY.

Witnesses: Y I i F. D. :TAYLOR, JOHN MCGUIRE.

for the calcination of

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