February, 1930
I X D CSTRIAL A S D ESGIIINEERISG CHEAUISTRY
179
Chemistry and Physics of the Combustion of Gaseous Fuels’ Burner Design and Combustion of Fuels in Industrial Furnaces G. E. Seil, H. A. Heiligman, and C. N. Witherow E. J. L A V I ~ AON D COMPANY, PLYMOUTH MEETING,PA.
The reasons for desiring to control the chemical carbon monoxide and oxygen, HE chemical constituconstituents of the furnace atmosphere are discussed. although not at temperatures ents of the products of The chemical and physical effects of excess carbon which would affect the ware. c o m b u s t i o n play a monoxide, excess oxygen, neutral atmosphere, and Oxygen may enter the stack very important part in most varying atmosphere are enumerated. The atmosphere gases owing to an inward leakhigh-temperature industrial in gas-fired tunnel and periodic kilns is traced throughage of air after the temperaoperations. I n the burning out the burn and compared with coal-fired kilns. ture of the products of comof ceramic and r e f r a c t o r y Control of kiln atmosphere when using solid, liquid, bustion has dropped below the wares the color and quality of and gaseous fuels is discussed. point at which carbon monthe finished product are, t o Various methods of burning gas industrially are oxide in the concentrations an appreciable extent, deterdescribed, with a full discussion of an annular orifice present unites with oxygen. m i n e d b y t h e atmospheric Very o f t e n in coal-fired type burner. This burner provides its own Venturi condition of the kiln during kilns, a furnace atmosphere for inducing air, and can use air which is too hot to be the b u r n . I n muffle kilns, will vary from strongly recompressed. This burner can be used in conjunction where the products of comducing to strongly oxidizing. with a rigid Venturi block. Curves and experimental b u s t i o n d o n o t coine into A f t e r t h e addition of fresh data on producer gas, city gas, and oil burning are direct contact with the ware, f u e l t o a n y fire box, the shown. the furnace structure itself Droducts of combustion leavmay be neatly affected by the- oxidizhg 0; reducing altion of the products of combus- ing that fire box are very stroigly reducing. As the fuel tion. I n fact, mufile kilns are largely used in industries bed diminishes in thickness the amount of carbon monoxide where direct contact with the products of combustion may in the gases entering the kiln at that point decreases, soon be injurious bo the ware, in order that the kiln atmosphere a neutral point is reached, and finally a strongly oxidizing may be so controlled as to give the ware certain desired char- condition exists. This cycle is repeated with every addition acteristics. I n metallurgical operations the effect of kiln at- of coal and, if kiln atmosphere were plotted against time, with oxidizing atmospheres as positive values and reducmosphere is too obvious for discussion. An oxidizing atmosphere a t any particular point in the ing atmospheres as negative values, a saw tooth curve would furnace is due to the presence of oxygen in the atmosphere result. adjacent to and surrounding the ware a t that point, and is It is only necessary to consider very briefly the effects of determined by the amount of free oxygen in the products of oxidizing and reducing conditions on several industrial combustion a t that point. The replacement of the oxygen operations to see that the ability to control these conditions with carbon monoxide normally causes reducing character- is a very vital factor in economical production. istics. The presence of free carbon, or solid combustible Effect of Furnace Atmosphere in Brick Manufacture matter, cannot directly cause a reducing atmosphere, but may be responsible for the formation of carbon monoxide, I n the manufacture of building brick from red burning d i i c h gives the atniosphere reducing characteristics. At clays, the burning temperature and the color of the mare high temperatures it is theoretically impossible to have any are appreciably affected by the atmospheric conditions tar, free carbon, or suspended solid fuel in an atmosphere of of the kiln. In reducing atmospheres the melting points carbon dioxide, because of the following reaction: of clays are normally lower and the burning of clay bricks c + coz = 2co can be finished at a lower temperature. The color of the If the molecular relationship between the total csombustible finished ware is determined by the oxidizing or the reducing matter and the oxygen is such that the excess of oxygen or action of the kiln gases, and the “black headers” obtained carbon monoxide at m y giren point is so small as to have in this industry are the direct result of reducing conditions. no appreciable effect, the atmosphere a t that point is con- It has been demonstrated on a laboratory scale that if i t sidered neutral. were possible to maintain a reducing atmosphere throughout At times it is difficwlt to‘ determine by analysis whether the burn an entire kiln of black brick would be obtained. the furnace atmosphere is oxidizing or reducing. For The quality and color of brick made from buff burning example, in a furnace TThich contains free oxygen and sulfur clays, both solid colored and speckled, are greatly affected dioxide the oxygen tends to determine the atmosphere as by the kiln atmosphere during the burn. The background oxidizing, while the sulfur dioxide tends to give the atmos- color of a brick of definite composition is determined by phere reducing characteristics. The ware burned where this the atmospheric condition of the kiln a t the end of the burn. condition exists approaches in appearance the ware burned A brick without colorants will have a buff color when the in a reducing atmosphere. firing is finished oxidizing, and a gray or blue-gray color The analysis of the stack gases is no indication of the actual when the firing is finished reducing. By changing from kiln atmosphere, for in stack gases it is possible to find both oxidizing to reducing conditions at the finishing temperature, it is possible to change the brick color from buff to gray, Received September 27, 1929. Presented before t h e DLvision of Gas or vice versa, and by regulating conditions a t the end of the and Fuel Chemistry a t t h e 78th Meeting of t h e American Chemical Society, Minneapolis, Minn., September 9 t o 13, 1929. burn any shade betmeen buff and gray can be obtained.
T
I N D U S T R I A L A N D ENGINEERING CHEMISTRY
180
The chemistry of this color change is simple. After the organic matter has been oxidized, the color of the brick is determined by the state of oxidation of the iron in the clay. If fired in an oxidizing atmosphere and the iron is present as Fez03 a buff color is obtained, but when the bricks are finished in a reducing atmosphere, the iron is present in a partly reduced state and the lower oxides of iron cause the gray or blue-gray color of the brick. PlRL
COOLIN6
7
CAR
PREHEAT
HOVWWT
Figure 1-Tunnel Kiln
6
--c
Figure 2-Simple
Type of Induction Burner
When granulations of very definitely sized manganese particles are added to clay, as in manufacture of “speckled” face bricks, the kiln atmosphere not only affects the color of the brick itself, but determines the size and nature of the manganese spots. Bricks burned in an oxidizing atmosphere are buff colored, and the spots are small and sharp in outline. When h i s h e d in a reducing atmosphere, the brick will be gray or bluish gray, and the spots will be large and round, because the melting point of the claymanganese mixture is depressed in a reducing atmosphere. I n order to produce a buff brick containing large round spots it is necessary to burn in a reducing atmosphere until the firing is nearly finished, and then to finish in a strongly oxidizing atmosphere. I n the manufacture of bricks and shapes for refractory purposes, it is generally desirable to burn in an oxidizing atmosphere, Strongly reducing conditions a t the end of the burn are injurious to fire clay, silica, magnesite, and chrome refractories, although slightly reducing conditions in the early stages of the firing are unavoidable with certain types of equipment. The conditions of the kiln atmosphere are very largely determined by the type of kiln, the fuel used, and the method of burning. I n comparing fuels for industrial uses, a consideration of the ease with which a fuel permits control of the rate of burning and of the kiln atmosphere is indicative of the value of the fuel. Fuel cost should not be considered on the B. t. u. basis alone, but on the cost per unit of perfect h i s h e d ware. Control of Kiln Atmosphere with Different Fuels
SOLIDFuELs-Sofid fuels, such as coal and coke, are usually the least expensive on a B. t. U. basis. With fuels of this type it is very difficult to control accurately the rate of burning and the composition of the combustion products. If hand-stoked, a t the time of firing the grate contains a thick bed of fuel, and the air passing through the grate is insufficient for complete combustion. The fire box is in effect a gas producer, and the products of combustion are strongly reducing. I n mechanically stoked coal-fired furnaces this difficulty is not eliminated, because the fuel bed varies in thickness and the gases leaving the deepest part of
Val. 22, No. 2
the bed are reducing in nature. Although provision is usually made for the admission of air above the fire box, this difficulty is never completely eliminated when solid fuels are used. I n addition to the injurious effects of reducing gases in the kiln atmosphere, it is possible under certain reducing conditions to deposit finely divided carbon on the ware in certain parts of the kiln. This condition is possible with practically all fuels, but it is most likely to occur when burning bituminous coal, or its fluid derivatives which contain considerable tar, oil, and volatile hydrocarbons. An oxidizing atmosphere will cause the combustion of this deposit in place and subject the ware to the intense local heating effect of the incandescent carbon. The result is unevenly burned, glazed, or cracked ware. LIQUIDFuELs-In burning with liquid fuels, such as oil, the fuel can be comminuted so that the surface contact with the supporter of combustion approaches that pomible with gaseous fuels. Powdered coal is considered a liquid fuel because of the ease with which i t can be handled. The fuel and the air are mixed as intimately as possible at the burner and the quantities of each can be accurately controlled, giving close regulation of the burning conditions. The turbulence with which the mixing is done and to some extent the velocity of the combustible mixture determine the flame temperature and the products of combustion. When it is possible to supply all the necessary oxygen a t the burner as primary air, the flame temperature attainable is determined by the
GA5
-
Figure 3-Induction
Figure 4-Induction
Burner with Venturi Throat
Burner with Two Venturi Throats
turbulence of the combustible mixture. It is also possible to have both carbon monoxide and oxygen in the products of combustion if the proper amount of air is added without the proper turbulence or without control of the velocity of the combustible mixture. I n order to obtain complete combustion of the mixture a t the burner, not only must the burner supply the proper turbulence, but the rate of flame propagation must be equal to or greater than the velocity of the combustible mixture. Where only a portion of the air is admitted as primary air, the burner becomes a prcducer-gas generator and, if one-half of the air required for combustion is admitted as primary air, a true producer gas is formed, causing reducing conditions in the kiln until this gas comes into contact with the secondary air, when complete combustion takes place.
February, 1930
I N D U S T R I A L AIVD ENGINEERING C H E M I S T R Y
I n regulating the burning conditions when liquid fuels are used consideration must be given, not only to the fuelair ratio and to the composition of the products of combustion, but also to the flame temperature and to the velocity and rate of flame propagation of the combustible mixture. These are best controlled for ideal conditions when all the air is admitted a t the burner as primary air, either cold or preheated. With delayed combustion the flame temperature is lower and there is a slower rate of heat transfer, since heat transfer is dependent on a function of the temperature differential. I n slow combustion a portion of the heat is removed from the conibustible mixture by the ware before
Figure 5-Annular
Orifice T pe Induction Burner with Venturi %hroat
combustion is complete, and the removal of this portion of the sensible heat decreases the flame temperature. The total number of heat units developed from the fuel will be the same in either case, provided the velocity of the gases through the kiln is not sufficient to remove part of the combustible material before it has had sufficient time for complete combustion. GASEOUSFuELs-Gaseous fuels are in phase with the supporter of combustion and maximum surface contact is possible. With the proper turbulence a homogeneous combustible mixture is obtained. For this reason gaseous fuels, being the most flexible, approach the ideal for industrial operations. Theoretically it is possible to mix the fuel and the air in such proportions as to obtain perfect conibustionthat is, to bring every particle of combustible material into contact with exactly the proper amount of the supporter of combustion and to have neither excess fuel nor excess air
Figure 6-Diagram
of Forces Acting in Induction Burner
in the products of combustion. However, it is essential that the fuel-air ratio be ideal in every portion of the mixture. If, for example, 1 cubic foot of city gas is mixed with 4.5 cubic feet of air so that any sample of the mixture, no matter how small, contains this ratio, the mixture is ideal. Since it is extremely difficult to obtain and maintain this condition, the process must determine which shall be in slight excess in the combustible mixture. I n some cases, as for instance steam-boiler heating, a slight excess of air is desirable in order to insure complete Combustion, and has no harmful
181
effects. I n the melting and treating of metals, where a slightly reducing atmosphere is desirable in order t o avoid oxidation of the charge as far as possible, the combustible mixture should contain a slight excess of the fuel. I n the burning of ceramic and refractory ware, the operator can adjust the mixture to give oxidizing or reducing conditions as required. The amount of air added should always be as close to the theoretical as possible to give the desired effect, since excess air must be heated and therefore consumes a certain amount of fuel, while a deficiency of air allows some fuel to leave the furnace in an unburned state. Since the excess or deficiency of air should be as small as possible, the volume and weight of the products of combustion are in a certain measure determined by the type of fuel used. In the operation of direct gas-fired tunnel kilns, it is essential that the temperature and weight of the products of combustion be sufficient to carry the heat to the ware in the preheat zone, and that the products of combustion do not precipitate moisture on the ware entering the kiln. Gaseous fuels themselves vary in chemical composition, in calorific value, and in the volume of air required for combustion. Natural gas has a calorific value of 1100 B. t. u. per cubic foot and requires about ten volumes of air for complete combustion. Therefore the combustible mixture will contain 1100 B. t. u. in 11 cubic feet, or about 100 B. t. u. per cubic foot. The flame intensity is low, owing to the large amount of nitrogen brought in with the air. The products of combustion will contain large amounts of steam, due to the methane, and condensation may become a troublesome factor, especially in the operation of direct-fired kilns. I n burning natural gas it is possible to deposit fine carbon on the ware, which, as has been previously explained, may lead to serious operating difficulties. City gas requires about four and a half times its volume of air for combustion, and has a calorific value of about 550 B. t. u. per cubic foot. In this case a perfect combustible mixture will have a calorific power of 100 B. t. u. per cubic foot, but owing to the smaller amount of inert gases present, the flame intensity is greater. The volume and weight of the products of combustion are smaller than in the case of natural gas, and they contain steam which may condense on the ware. The possibility of condensation in the cooler parts of the kiln is greater because of the smaller volume of
Figure 7-Test
Furnace with Burners Using Preheated Air
the products of combustion, assuming the same temperature in both cases. I n the case of producer gas made from bituminous coal, gas having a calorific power of about 160 B. t. u. is mixed with about one and one-fourth times its volume of air, resulting in a combustible mixture with a calorific power of about 70 B. t. u. per cubic foot. Producer gas made from anthracite coal contains about 150 B. t. u. per cubic foot, and the figures given for bituminous producer gas are approximately true for anthracite producer gas.
182
I N D USTRIAL A N D ENGINEERING CHEMISTRY
VOl. 22,
KO,
2
4000 cubic feet of producer gas and wilI require 5000 cubic feet of air per hour. A small change in the pressure or volume of the producer gas means very little change in the amount of air required, and unless perfect theoretical combustion is taking place the kiln temperature will be very little changed by slight variations in the producer gas or air supplied. Control with Various Types of Burners
Figure 8-Best
Induction Burner Using Preheated Air
Bituminous producer gas is not always a satisfactory fuel because of its non-uniformity. The amount of tar per thousand cubic feet varies from time to time, and the amount of air required for combustion will vary accordingly. The producer itself requires constant attention in the regulation of the depth of the fuel bed and poking to avoid troublesome clinker formation. Anthracite producer gas is more uniform in composition and anthracite coal lends itself more readily to gasification. Producer gas burns with a low flame temperature, but this disadvantage affects only unusually high temperature operations and can be overcome by preheating the producer gas or the air, or both. The great advantage of clean producer gas over other gaseous fuels lies in the ease with which the gas and air mixture can be regulated. One volume of producer gas requires approximately one and one-fourth volumes of air for complete combustion. It is very easy to control the gas and air mixture to give very little variation in the kiln atmosphere, as a slight change in the producer gas would not cause great variations in the amount of air required
Since regulation of the burning of the fuel is so vital a factor in controlling the chemical constituents of the products of combustion, due consideration must be given to the type of burner used for burning liquid and gaseous fuels, with special reference to the relationship between the burner action and the furnace action. I n general there are four types of furnaces: (1) the independent unit, where no effort is made to bring the seneible heat of the waste gases back into the process; (2) a series of related individual units, in which air for combustion is preheated by passing it through a unit in which the firing has just been completed, and in which the products of combustion in the unit being fired are used to preheat the ware in the next unit to be fired; (3) the regenerative furnace, where checkerwork is heated by the hot products of combustion, and where the direction of the flow of gases is then reversed so that the air is preheated by the checkerwork: (4) the continuous furnace, in which production is
\
no050
-
I
OOOtS
1-
00000
100 100 300 400 500 CUBIC FEET ff ClTV GAS PER WOYP AI 6 8 F AND UI*-SPGR a490
v
uu Figure 9-Same
Burner as in Figure 8, but Water-cooled
for combustion, As a concrete example, consider a burner which consumes 1000 cubic feet of city gas per hour. This burner requires approximately 4500 cubic feet of air per hour for perfect combustion. The slightest change in the gas pressure or gas volume means exactly four and onehalf times that change in the volume of air required for combustion. A burner of the same B. t. u. capacity will burn
Figure 10
continuous, and in which preheating of the air and the ware are done in the same kiln. Any of these furnaces may be either of the direct-fired or muffle type. Furnaces of the independent unit type are very inefficient from the viewpoint of heat economy. No attempt is made to utilize the sensible heat in the waste gases, and the heat lost in this may may represent a large part of the sensible heat in the products of combustion. I n the burning of ceramic and refractory wares in periodic kilns the time factor and the poor heat economy are almost prohibitive, because the ware must be set in a cold kiln, burned, and removed after the kiln has cooled sufficiently to permit entrance before the cycle is completed. I n some metallurgical operations the furnace is held close to its operating temperature and both the charged and the discharged materials are close to this temperature, and the time element is not so vital a factor in efficient production. I n this process
I N D CSTRIA L A X D EXGIA-EERISG C H E N I S T RY
February, 1930
the inefficiency lies in the heat removed by the finished product. When the furnace consists of several related units so arranged that they can be set, preheated, fired, cooled, and unloaded in rotation, the thermal efficiency is fairly high. However, regulation of conditions a t all periods of the heat treatment is difficult, because the preheating of the air and the preheating of the mare must be adjusted in accordance with conditions in the unit being fired. The ideal furnace is one in which the entire heat treatment is carried out continuously, and a typical furnace of this kind is the tunnel kiln now being widely used in the
01
I
180
I
200
I
120
CUSiC FEET 07 C I T Y G I 5
I
240
PEQ HOUR
I
160
AI
PBO
bO*f AND I4 4*-
300
320
5P GP 3 4 9 0
Figure 11
ceramic and refractory industries. The general arrangement of this kiln is shown in Figure 1. The ware is carried through the kiln on cars, passing successively through three zones-the preheat zone, the burning zone, and the cooling zone. The economy of this kiln is apparent. Production is continuous; the air passing over the ware in the cooling zone is used as preheated air for the combustion of the fuel, and the hot products of combustion impart their eensible heat to the ware in the preheat zone. A tunnel kiln, properly designed, permits of very close regulation of the time-temperature curve of the ware, and of the atmospheric conditions in all parts of the kiln, prorided the fuel and method of burning are carefully chosen. I n firing a kiln of this type with coal, part of the air for combustion is necessarily admitted with the fuel, and this primary air is a t atmospheric temperature. In order to take advantage of the sensible heat in the hot gases corning from the cooling zone, only a portion of the total air admitted is added as primary air, so that a t the fire box nearest the cooling zone an approximate producer gas is burned with hot air, and regulation of the constituents of the products of combustion is not difficult. However, as the hot air passes through the fire zone the concentration of oxygen is greatly decreased owing to secondary combustion in the successive fire boxes and the dilution of the air by the products of combustion, so that control of atmospheric conditions in the preheat zone is sometimes very difficult. This condition holds for oil-burning installations, although use of this fuel permits of much closer regulation than is possible with solid fuel, since oil can be fed continuously througli a burner in small regulated amounts with regulated amounts of added air. This is also true for gas burning when part of the air is added as primary air and part as secondary air. However, with gaseous fuels it is possible, by the use of induction burners, to take the hot air from the cooling zone, which cannot be compressed or blown because of its heat, and lead it through a flue t o the burners, so that all the air required for combustion enters as primary preheated air.
183
Types of Induction Burners
From the foregoing discussion it will be seen that the temperature and atmosphere are most easily controlled when gaseous fuels are used. A clean anthracite producer gas is especially adaptable in this respect. A properly designed continuous furnace offers the greatest opportunity for this control, but its operation requires careful supervision. The mechanical details of construction, the refractory materials used in the various zones, and the methods of operation have been so well developed that it is no longer necessary to experiment with these factors. -4t the time the experimental work described in this paper was started, there was no burner which would burG gaseous fuels by inducing primary air a t 1100" C. against a pressure differential. I n designing a gas burner the following factors must be considered: (1) fuel efficiency, ( 2 ) ease of regulation, (3) simplicity of design, and (4) simplicity of installation, to permit of easy removal, cleaning, repair, replacement, etc. In a continuous furnace atmospheric conditions are best controlled by admitting a large proportion of the air necessary for combustion in regulated amounts a t the burner, but in order to secure maximum fuel efficiency this air must bring back to the furnace the sensible heat which was held by the finished ware a s it left the fire zone, and which it releases as it approaches the exit. Preheating the air may be accomplished by the use of expensive checkerwork bringing back to the furnace the sensible heat of the products of combustion, or by the use of burners which induce the air preheated by the ware leaving the fire zone to flow directly to the burners. This latter method can be readily incorporated into standard continuous furnace practice. Induction burners utilize the pressure of the gas through the medium of a jet to inspirate sufficient air for combustion. Several types are illustrated. Figure 2 shows a very simple type which consists of a gas jet and a tube, with means of regulating by orifice both the gas and air flow. This type is used in the Bunsen burner and in the common gas range. Figure 3 shows a burner using a Venturi throat in addition to the jet. The T'enturi throat aids in the proper mixing
Figure 12-Annular-Orifice Type of Oil Burner
and turbulence of the air and gas, decreases the losses due to friction and eddy currents, and builds up a secondary pressure which is utilized in the design of certain types of furnaces. Figure 4 shows a burner which uses two Venturi throats, securing greater efficiency in the induction of the air and in mixing and turbulence of the air and fuel. Figure 5 shows a burner of the annular orifice type, combined with a Venturi throat. This is the most efficient type of induction burner, and a full description of its advantages will be given later. Principle of Operation of Induction Burner
The induction type of gas burner operates on two basic principles:
I N D U S T R I A L A N D ENGINEERING CHEMISTRY
184
(1) The changing of relatively high pressure and low velocity to low pressure and high velocity. The constant mass and increased velocity tend to entrain or inspirate the air for combustion. (2) The utilization of a device, in this case the Venturi throat, to secure proper mixing of air and gas, to cut down losses from eddy currents and friction, and to build UP a secondary pressure which will nullify the harmful effects of furnace back pressure due t o the rapid combustion of the gases, by acting as a guide for the movement i f the c o m b u s t i b l e mixture.
I n order to calculate the forces acting in the induction type of burner, the total energy a t different points of a limited system, as shown in Figure 6, can be considered. F o r t h e sake of simplicity, a simple j e t a n d Venturi are used in this illustration, as the basic principles which apply in this case are also applicable to the annular orifice type of burner. The total energy entering the system is the sum of the potential energy and the kinetic energy of the gas leaving the jet a t position A and is represented by the equation: Figure 13 E, = P C. , " C M V 2 (1) -_ ., where E , = total energy in gas leaving jet a t point A '/q
~~~
P
= potential energy, due to work done compressing
C
= constant depending on density of gas
M V
gas to pressure existing a t this point
= mass of gas
= velocity of gas
At point B the high pressure, P, and the low velocity, V , are changed into a low pressure, Pz, and a high velocity, Yz,and these factors result in a region of diminished pressure just beyond the jet, causing the entrainment of the air required for combustion. The total energy in the gas a t this point is represented by the equation:
Es
= p2 -k '/z
CzMeVz'
(2)
where Ea
= total energy in gas at point B Pe = potential energy in gas a t point B C2 = constant depending on density of gas, frictional
losses, and general efficiency of system
Ma = mass of gas Vz = velocity of gas a t point B
P, must be great enough to overcome any back pressure caused by constriction at throat of Venturi At point C the mixture has passed through the Venturi throat, and the total energy in the mixture is represented by the equation: E. = Pa I/z C3M3Va2 (3)
+
where Eo = total energy in mixture a t point C Pa = potential energy in mixture at point C c8 = constant depending on density of mixture, frictional losses, and general efficiency of system = mass of mixture Vs = velocity of mixture a t point C P8 must be great enough to overcome back pressure of furnace
The function of the Venturi is to guide the expanding gases and to control the direction of the velocity change, thus decreasing the losses due to friction and eddy currents. The high velocity in the throat and the shape of the throat prevent the back pressure of the furnace from baffling and
Vol. 22, No. 2
wasting the useful energy of the jet in eddy currents and high frictional losses. The discussion indicates briefly the principles on whieh induction burners operate, but since there are many variables which are difficult to determine and control, mathematical calculations are unsatisfactory in this type of equipment. Although induction burners using cold air have been successfully used in heating oDerations, there are few induction burners which can &dice sufficient hot air for the combustion of the fuel. Using preheated air places a greater burden on the burner, since the entrainment of a much larger volume of air is necessary. The volume of a constant weight of gas varies directly with the absolute temperature, other conditions being constant, and a given weight of air will have approximately four times the volume a t 1100" C. that it has a t ordinary temperatures. This larger volume means increased velocity, increased losses due to friction and eddy currents, and therefore considerably more energy is required to inspirate the same amount of air a t 1100' C. than a t ordinary temperatures. Tests of I n d u c t i o n Burners
I n order to test induction burners, using preheated air a t various temperatures, a test furnace, shown diagrammatically in Figure 7, was built. This furnace was so constructed that all the air used for combustion entered the furnace through Carborundum tubes, B, and was taken to the burner, D, through a flue, C. The Carborundum tubes and the flue were filled with broken brick to secure sufficient heating surface to preheat the air, and the volume blower, A , was used to supply the necessary pressure. Pressures were measured by means of inclined gages in the hot-air flue, in the chamber before the Vent& throat, and in the furnace chamber. Samples for gas analysis were taken from the stack a t point E. Each burner was placed in position and burning tests were run a t various flue pressures, f u r n a c e p r e s s u r e s , and temperat u r e s of p r e h e a t e d air. T h e efficiency of the burner as a device for ins p i r a t i n g air was determ i n e d , a t different gas pressures, by analysis of the stack gases, which indicated excess or deficiency of air. Annular-Orifice Type of Burner I
I
I
I
I
2 GALL 4 Q 0 1 6L PER LYOUR I The burner which met all conditions most sucFigure 14 cessfully is shown in Figure 8. Gas under pressure is forced through the annular orifice and, as has been previously explained, the hollow cylinder of rapidly moving gas inspirates the air necessary for combustion. By changing dimensions this burner can be made to pass any desired number of cubic feet of gas per hour a t any desired pressure. The amount of air inspirated can be controlled by an air regulator, so arranged that it can be placed in any desired position in the air opening. Figure 9 shows the same type of burner, watercooled, for use in furnaces where the burners are subjected to unusually high temperatures. The annular-orifice type of induction burner has the following advantages over the jet type:
(1) The surface of high-velocity gas exposed for the entrainment of air in a jet type of burner is the surface of a thin cylinder of gas, the diameter of which is the size of the jet opening. In the annular-orifice type the surface exposed is thr: inside of a hollow cylinder the size of the air opening, and this surface is much greater than that exposed by a jet passing the same amount of gas a t the same pressure. Further, when the burner is used in combination with a Venturi throat, as shown in Figure 5, both inside and outside surfaces of the gas cylinder are used t o entrain air. ( 2 ) The air is drawn into a thin-walled cylinder of gas, and the gas and air are more intimately mixed than in the jet type of burner. (q) There is almost no danger of clogging due to dust and foreign particles in the gas. Should a particle of dust become wedged in the annular orifice, only a very small part of the effective area is lost, and it is practically impossible to close the annular orifice completely. On the other hand, one particle may practically close the effective area of a round orifice. (4) The annular orifice is really a center Venturi, which varies with the gas pressure and adapts itself to give maximum efficiency regardless of the gas pressure. The jet-type burner and F'enturi throat must be in definite relationship in order to obtain maximum efficiency. ( 5 ) The use of a regulating device on the air opcning of the annular-orifice type permits accurate control of the air and gas mixture and of the atmospheric conditions in the furnace. The gas-air ratio may :tlso be controlled by the pressure differential due to the absolute fire-box pressure. ( 6 ) Air a t 1100" C. which cannot be compressed or blown can be induced by a burner of this type.
(1) The annular orifice must be large enough to pass the required amount of gas a t the given pressure, and the orifice and the pressure must be such as to inspirate the required volume of air against the furnace back pressure. The area of the orifice and the gas pressure necessary under different conditions can be determined by a study of the curves in Figures 10 and 11, and reference to the standard handbooks. ( 2 ) The area of the center orifice must be sufficiently large t o pass the required volume of air a t a reasonable velocity. (3) The gas pressure required depends on the back pressure of the furnace, the temperature of the induced air, and the B. t. u. value of the gas used. (4) The induction burner works oil a weight, not a volume, basis. If the frictional losses and the general efficiency of the system were constant a t all temperatures, the mass of air inspirated would be constant. At higher temperatures there is a large increase in the frictional losses duc to increased volume and to increased velocity, and the decrease in the amount of the gas inspirated is not directly proportional to the increase in temperature, but varies with the density and chemical nature of the gas.
A discussion of the atinular-orifice type of burner would lie incomplete without a brief description of its application as an oil burlier (Figure 12). Air under pressure of 25 pounds per square iiicli is forced through the aiiiiular orifice, drawing the oil through tlie ceiiter orifice. The oil is very finely comminuted in bliis process. The air pressure aud the oil pressure may be regulated to give a combustible mixture of any desired B. t. u. value. The curves iu Figures 13 and 14 show the relationship between air pressure, oil level, and Burners of the annular-orifice type can be designed to oil consumption. The fuel-air ratio may be controlled meet any requirements, and a properly designed and care- by three methods: (1) variation in the size of tlie oil fully nianufactured burlier will give satisfactory awvice 11-it11 orifice, (2) variation in the air pressiut:, atid (3) variat>ioti very little care. In dwigning and operating a hwner of this i i i the oil level with reference to t.hc Iiuriier. The last type several factors must be horne in mind: method affords the best regulation.
-
Possibilities of Production of Radium and Vanadium from Carnotite' H. A. Doerner
Depletion of t h e high-grade ores, increased operating costs, a n d Belgian competition have discouraged domestic production of r a d i u m . A t present price levels t h e r e is little to justify t h e exploitation of c a l n o t i t e for r a d i u m . W h e n an ore which is treated for i t s v a n a d i u m cont e n t c o n t a i n s as m u c h as 1 per cent U 3 0 ~a, u r a n i u m conc e n t r a t e should be recovered as a by-product a n d saved for possible f u t u r e t r e a t m e n t f o r r a d i u m . Analysis of production costs leads t o t h e conclusion t h a t mechanical concentration of low-grade ore m i g h t be an i m p o r t a n t factor in reducing the cost of r a d i u m . As applied t o a shipping ore, no proved m e t h o d of t r e a t m e n t s t a n d s o u t above t h e o t h e r s for cheapness of operat i o n o r efficiency, a n d t h e r e is s m a l l probability that any
new m e t h o d will greatly change t h e cost of extracting r a d i u m from an ore. With t h e possible exception of t h e fluoride m e t h o d of Fleck a n d Haldane, only direct leaching m e t h o d s (with or without a preliminary t r e a t m e n t ) a r e applicable t o a d u s t concentrate from a low-grade ore. As compared t o an ore, t h e processing of s u c h concentrates requires more reagents a n d involves m o r e difficult filtrations. Preliminary roasting of t h e ore results i n i m p o r t a n t benefits, in regard to b o t h c o n c e n t r a t i o n and chemical t r e a t m e n t , which have not been sufficiently realized. These benefits apply particularly t o t h e carbonaceous ores a n d t o t h e modified nitric acid m e t h o d of extracting radium.
.. . . . . . . .. . . .
T
HE carnotite ores of Colorado and Utali were the
chief source of radium for many years. Originally these ores were worked oiily for their vaiiadiiini COP te it3arid were temporarily ahandoned when the 1.ich vai1adiuni deposits iu Peru were discovered. Later, about 1912, the presence and value of radium in those ores became appreciated, and carilotit,rr was treated priricipally for its radium content, the vanadiu~ii and uranium being conaidered a s by-products. 'Received November 20, I Y 2 Y . Published by permission (Not subject to copyright..i
Director, U. S . Bureau of Mines.
01
the
In tlie fa,ll of 1922 news of the very ricli deposits of radiutii ore discovered ill the 13elgiaii Congo caused aliiiost complete wssation uf domestic production. The Congo ore uotitaitis pitchblende aiid a number of alteration products, iiicluding tlie new niiiierals bequerelite: curite, kasolite, stasit,e, a i d dewindite. Reports iiidicate that a considerable amount of ore coiitaiiiitig over 50 per cent L 3 0 8 has been produced. It seemed certain that radiuiii c ( ~ ~be l d extracted from the Coiigo ore a t a iiiuch lower cost tliari from t,lie relatively low-grade caruotite; uiid irl order t u preserve their elaburlttci iuarket,ingorgwiiis:itioris, tlie Iargrr ;211ieric~an producers made
