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F io . I— Gr a p h ic Il l u s t r a t io n o pt h e Re a c t io n si n Lim e Bu r n in g r a d i a t i o n— Another sourcc of loss of hcat is th a t lost by radiation from th e kiln shell.
h e a t c a r r i e d o f f b y h o t l i m e— The lim e itself carries ofl a certain q u antity of heat, in the ordinary flame kiln. This quantity am ounts to 0.22 B . t. u. per lb. of lim e per degree of temperature, or about 450 B . t. u. per lb. of lime.
One pound of good coal will liberate approxim ately 13,000 to 14,000 B . t . u. If we could transfer all of th is heat in the coal directly and w ithou t loss to the lim estone, one pound of coal would b u m approxim ately 8.95 to 9.65 lbs. of lim e. U nfortu
nately, however, w e cannot do this and there are appreciable losses, as explained above.
The actual results obtained vary considerably a t different plants, bu t the following m ay be said to represent good practice in lime burning, or abou t 3.5 lbs. of lime per pound of coal.
P e r c e n t H e a t c a rr ie d off b y p r o d u c ts of c o m b u s tio n 22 H e a t c a rr ie d ofT b y lim e ... 11 ’ H e a t lo s t b y r a d i a t i o n ... 17 H e a t lo s t b y in c o m p le te c o m b u s t io n ... 11 T o t a l h e a t l o s t ... 61 H e a t u ti liz e d ... 39 T o t a l h c a t s u p p lie d ... 100
In Fig. 1 is given diagram m atically the distribution of heat in lime burning.
To effect econom ies in lim e burning w e m ust elim inate these sources of loss.
T h e F l a m e K i l n
Let us take up first the flame kiln and the question of the general design of the sh aft. T he object here is to transfer the heat from the gases to th e stone. In order to do this, w e m ust present to the gases as much surface of stone as possible and bring the gases in contact w ith this surface as uniformly as possible.
s i z e o f s t o n e— U nquestionably, in theory, th e best' results would be obtained by having th e stone in small pieces, because this would present a greater surface of stone to the gases. In practice, however, the sm all stone presents too great a resistance
to the flow of gases, and hence it is probably n ot possible to improve present practice greatly in the m atter of stone sizes.
However, it should be remembered th at b y the use of induced draft (fans) we cau overcom e to som e exten t the resistance offered b y small stone to the flow of gas. W e should also bear in mind th at a stone of uniform size, even if sm all, will offer less ob
struction to the flow of gases than will a mixture of large pieces and
spalls-T iiE l i n e s O F spalls-T H E k i l n —T he diam eter of the kiln is controlled very largely by the question of distance to which the h ot gases from the fire box will penetrate into the kiln, and this depends to a considerable exten t on the fuel and draft used. Our own preference has been for a kiln w ith an oval cross section a t and above the fire box, with the lesser diam eter along the center line of the fire boxes. Tw o sides of the kiln are straight down to the cooling cone, while the other tw o sides are drawn in slightly, as shown in Fig. 1. If drawn in too m uch—and w ith some stone if drawn in at all— the charge will stick and hang up in the k iln .f^
All are, of course, familiar with th e fact th at if a flat substance
724 T H E J O U R N A L OF I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y Vol. 13, N o. S
Fig. 3— Q u ic k ActingD r a w i n g S h e a r s a n d C o n c r e t e P e d e s t a l is brought in contact w ith the top of a flame, the latter will flatten out. W hen th e gases from th e fire box strike the stone the sam e thing happens and consequently th e y show a tendency to hug the sides of th e kiln, hence the oval cross section.
T he writer has generally preferred to flare the kiln ou t after leaving th e burning zone, w ith the idea not only th at there would be less chance for th e charge to choke and arch, bu t also th at as th e lim e spread into the enlarged area there would be a tend
ency for the charge to roll over and separate, and thus expose fresh surface to the flame. If drawn ou t through a cylinder of the sam e cross section it would probably m erely sink down in a body.
W here w e use a gas as a fuel w e em ploy practically the same construction of kiln; th a t is, we draw the lining in above the gas ports and open it out again below these for the reason explained above. (See Fig. 2.)
h e i g h t o f s h a f t— The height of the shaft above the fire boxes is im portant. It is p retty generally the practice to follow standard lines in the m atter of the height. A t the sam e time it is probable th at a careful stu dy of conditions would show that in the case of som e stone very much higher kilns could be used w ith advantage than in the case of others. There is no question but that, within reason, the higher the kiln the better the econom y which can be obtained, because a longer tim e is given th e stone to absorb the heat from the gases. The height, however, is lim ited by the draft produced. Where natural draft is employed there is a lim it to th e height which is allowable, and 30 to 35 ft.
above the fire b o s is generally considered ample. It is quite probable, however, th a t by the use of induced draft higher shafts could be employed.
c h a r g in g— The charging arrangement shown on the top of the kiln illustrated in Fig. 2 will give satisfaction for closed-top kilns used w ith induced draft. It is similar to those used on the smaller blast furnaces. I t is quick acting and closes the opening tightly, preventing any cutting down of the draft due to air leakage here. If each kiln is provided w ith its own individual fan much better operation will be secured. T his arrangem ent is particularly desirable where the fans can be directly connected to electric motors.
stonte .b in— Practically all of the newer kilns e m p lo y a stone bin on top of the kiln. T his is really part of the sh aft bu t, as the temperature a t this point is low, the bin is usually left unlined.
There is no question about the advantages of the ston e bin, and, for practical purposes, the larger i t is th e better. T he idea, of course, is alw ays to have in the sh aft of th e kiln a bod y of stone sufficient to absorb the h eat properly. If th e ston e bin is m ade of the general design of the shaft, th e gases probably pass up
through the bin w ith som e degree of uniform ity, and it serves not only to hold the stone for the tim e when the kiln is not charged, but also as part of the shaft.
c o o l i n g c o n e—M o st of the better types of kiln are provided with a cooling cone. T his is m ade of steel plate and is so placed th a t the lime is cooled alm ost entirely by radiation. T he ideal arrangem ent for cooling the lim e would unquestionably be one in which the air necessary for com bustion passed up through th e lim e and cooled the latter, and thus recovered the heat carried off b y the lime. In the case of coal-fired kilns, no practical m ethod has been worked out by which this result can be achieved.
In the case of gas-fired kilns, however, the air for the combustion is allowed to enter a t the bottom of th e cone, passing up through che lim e and abstracting th e heat from the latter.
I t w ould probably be b etter to allow the sh aft to project for 4 or 5 ft. below the fire boxes before th e steel cone is placed on th e latter. T h is would leave a considerable bod y of h ot lime below the burning zone. N o h eat can be transferred from this h ot lim e to that being burned because radiation takes place only from a hotter bod y to a colder one, b u t there is alw ays consider
able air which enters th e kiln where th e lim e is drawn and the h ot bod y of lim e below the arches would serve to h eat this and prevent its cooling off th e lim estone in the burning zone. There is a m echanical objection to th e use of such a form of coolcr, however, in th a t it would n ecessitate raising th e fire boxes about 6 or 7 ft. higher from th e floor.
q u i c k - a c t i n g d r a w—A ttention has been called to the desir
ability of reducing to a m inimum th e excess air entering the kiln.
U nquestionably th e largest therm al loss is in th e excess air which enters th e kiln a t th e tim e of discharging th e lime and
Fi g. 4 — Fo r m sf o r Pe d e s t a l s
during the stoking of th e fire. T h e former m ay be cu t dow n to a m inimum b y the use of a very quickly acting draw on th e cooling cone.
For use on our kilns w e h ave designed a v ery quickly acting draw which is shown in F ig. 3. T h e doors m ay be adjusted by means of lock nu ts to an accurate fit w ith th e bottom of th e cone, and, a s th ey are swung from beam s resting in th e concrete pedestal and n ot from th e cone itself, th e y are n o t affected by th e warping of th e latter. T h e doors are opened and closed by a single push and pull on th e operating levers. T hese latter are located outsid e of the kiln and perm it th e operator to stand aw ay from the d u st and h eat w hen the lim e is being drawn.
Allowing a h o t b o d y of lim e to rem ain a t all tim es betw een the
T H E J O U R N A L OF I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y
cooling cone and th e burning zone will also reduce th e loss of heat due to the drawing of th e lime, as we h ave said.
h a n d s t o k e r s— W e have recently tried hand stokers in place of the ordinary hand firing in order to cut down excess air used to burn the coal. T he qu antity of coal consum ed in a single furnace of a lim ekiln is n ot large enough to w arrant th e in
stallation of m echanical stokers such as are used on boilers and large furnaces. T here are a number of hand stokers which have been successfully used for heating small m etallurgical furnaces, and one of th e m ost successful of these has been adapted to our type of lim ekiln. T he result has been to increase greatly the capacity of the kiln, cu t in half th e labor necessary for stoking, and decrease m aterially th e fuel consum ption. W e also find that with th e kilns on which we have tried this stoker there is practically no core obtained, whereas on the same kilns w ithou t the stoker the core w ill probably run to about as much as 5 per cent of the lime. In view o f the increased kiln outp ut obtained by the use of stokers, this is one of the cheapest w ays of increasing the capacity of a flame kiln 1 ime plant.
p r e v e n t i o n o f h e a t l o s s b y r a d i a t i o n— N e x t to the excess air, the largest item of w aste in lim e burning is unquestionably due to radiation from the w alls of the fire box and the kiln shell.
It is, therefore, better practice to prevent this as much as possible.
A new m aterial which is both cheap and efficient is sil-o-cel.
This can be obtained in blocks, b u t a more satisfactory form for the use in limekilns is the powder. I t is quite the general practice to fill in between th e brick and the shell w ith a thin layer of sand.
An appreciable saving of fuel can be effected b y the use of sil-o- cel in place of this sand backing.
s t r u c t u r e o f k i l n— Referring to th e structural features of the kiln, the m ost popular m aterial for th e shell is steel. T here are some points, however, in favor of reinforced concrete. I t is generally known th a t steel will radiate heat more rapidly than concrete. Some heat is therefore saved b y the use o f a concrete shell. The concrete shell is also more permanent, in th a t it does not require continual painting and is not subject to corrosion from the acids generated from the sulfur in the coal.
In some types of kiln the steel is run below th e firing floor.
T his has seemed rather expensive construction, how ever, and we have built our kilns on concrete pedestals. W ooden forms are used and are m oved from one kiln to the next during con
struction (Fig. 4).
A weak point in the flame kiln has alw ays been th e arches.
The writer has tried out several different kinds o f brick, bu t his preference is now for specially m ade blocks of large dim ensions and of exact k ey to fit the radius of the opening betw een th e kiln and th e firebox. W e use a block approxim ately 17 x 11 x 15 x 18 in. and use one row of these. Fig. 5 show s th is block, which is much stronger than th e standard 9 in. brick. T h ey have a much larger key and wedge in more firmly. Owing to their w eight, these blocks are not easily pushed out of place b y a bar w hen poking down a hang-up. A s th ey are specially m ade, the m anu
facturer will usually see th a t they are better burned and annealed and made of better clay m ixtures than his standard brick.
T he cooling cone'has alw ays been a weak spot in lim ekiln con
struction, and a t some plants the advan tage o f obtaining cool lime has not proved sufficient to com pensate for th e frequency w ith which the cone has to be renewed. W e have found th a t th e
72G T H E J O U R N A L OF I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y Vol. 13, N o. 8 life of the cone can be considerably increased b y proper ventilation
a t the top, so th a t a current of air can pa-s around th e cone a t this point. As generally p’aced, the top of the cone is in a con
fined space and th e air a t this point is prae:ically stagnant. B y placing a few ven ts so th a t the air can pass up betw een the cone and th e side of the kiln, a current of air can be m ade to pass around the cone a t this point and reduce its temperature m ate
rially and increase its life. If desired, th is air m ay be conducted into the firebox below the grate, b u t it is doubtful w hether any appreciable saving in fuel would be effected thereby.
On th e last kilns which we have b u ilt, w e have em ployed a water jack et around th e upper part of th e cone, th e idea being not only to cool th e lime but a t the same tim e to protect th e cone.
We have m ade no effort to w ater-jacket th e whole cone, bu t only a space of about a foot, as near the top of th e cone as th e jacket can be placed.
W e have alw ays m ade th e platform s and walkw ays on top of our kilns of steel. For the floor plates we usually use perforated steel plates or a better and more rigid material— Irving’s subway grating.
Th e Ro t a r y Ki l n
Turning our atten tion next to the rotary kiln, th e losses here are practically th e same as in th e flame kiln. T he losses due to radiation and to h eat carried oil by th e products of com bustion arc greater than w ith sh aft kilns, while the losses due to incom plete com bustion and h eat carried off b y the lim e are less. A com plete general discussion of th e rotary kiln lim e plan and a comparison w ith flame and gas fired kilns will be found in Rock Products of Decem ber 4, 1920.
p u l v e r i z e d c o a l— The principles of heat transference are
th e sam e in rotary kilns a s in sh aft kilns. I t is desirable to h a v e a high initial flame temperature and to cut dow n to a minimum the excess air used in burning. Pulverized coal is much easier to regulate th an producer gas and gives a higher flame tem pera
ture. C onsequently it is to be preferred to producer gas when good coa! low in ash can be secured. W ith pulverized coal, we can cu t down th e excess air used in burning to alm ost the exact am ount required by theory, and we can also burn our fuel com pletely.
Pulverized coal is unquestionably a more efficient fuel for heating the rotary kiln, than producer gas, and where com plaints as to the fuel econom y of the rotary kiln have com e to our atten tion, we have usually found th a t the gas producer was at fault and th a t th e heating value of th e g as supplied the kiln w as low. I t m ust be remembered th a t in gasifying coal some of its heating value is lost, so th a t the gas which is m ade from a ton of coal will not have the sam e number of heat un its as this am ount of coa! itself. These losses in even the best gas producers are a t least 10 per cen t and often w ill run as high as 25 per cent, w ith perhaps 15 to 20 per cen t as an average loss for m echanically stirred producers. In pulverizing coal, none of its thermal value is lost, so th a t we get the ful' heating value of the coal.
In addition to this, pulverized coal can be burned com pletely, and th e qu antity of excess air necessary to do th's is probably sm aller than with a n y other fuel.
W e m ust also remember th a t w ith pulverized coal we are feed in g into th e kiln a uniform stream of fuel of th e sam e heating valu e a t all tim es, whereas with producer gas the qu ality of fu el fluctuates w ith the condition of th e producer.
B y th e use of pulverized coal, we introduce into th e products
•of com bustion in the kiln itself practically all of th e heat un its
■of the coal. T he com bustion is alm ost perfect, and there is no loss of heat due to carbon in the ash or to radiation from the producer w alls and gas piping.
PR O D U C ER GAS— The only producers which will give satisfaction
and good fuel econom y w ith either vertical or rotary limekilns arc the autom atically stoked, and preferably autom atically fed,
producers, as th ey can be m ade to furnish a constan t supply of gas of uniform quality.
W here producers are used in order to improve the quality of th e lim e th ey should be set close to the kiln in order to reduce to a m inim um the loss b y radiation from the gas mains.
f a c t o r s i n f l u e n c i n g E c o n o m y— In a general w ay, it m ay be said th a t better results will be obtained by a large body of mate
rial m oving slowdy through the kiln th an from a small bod y of stone m oving rapidly. T his m eans th e kiln m ust not be pitched a t too great an inclination.
If th e diam eter of th e k'In is too large compared w ith the length, the fuel requirements will be high. Other things being equal, the longer the kiln the better fuel econom y.
T h e size of ston e burned influences the econom y to some extent. As a general rule, better results can be obtained by screening out the du st from the coarse rock before burning. This
T h e size of ston e burned influences the econom y to some extent. As a general rule, better results can be obtained by screening out the du st from the coarse rock before burning. This