December. 1925
INDUSTRIAL AND ENGINEERING CHEMISTRY
volatile coal and much more than the dust from the lignite. That is, the dust from the lignite may contain 1 to 3 per cent combustible, the dust from the high-volatile coal 5 to 10 per cent combustible, whereas the dust from the lowvolatile coal may run 10 to 50 per cent combustible. For this reason the length of flame is deceptive as an indicator of completeness of combustion. For the same reason the analysis of furnace gases is not a good indicator of the completeness of combustion. The analysis may show no combustible gases and still the dust may contain a high percentage of carbon. The analysis of furnace gases gives information only as to the amount of excess air used in combustion. The length of flame is affected by the amount of air mixed with the coal. The ignition is the quickest and the flame shortest when only part of the air needed for complete combustion is mixed with the coal when the latter enters the furnace and the rest is supplied along the path of gases. The amount of air to be introduced with the coal varies with the percentage of volatile matter of coal. With low-volatile coals three to four parts of air mixed with the coal give the quickest ignition and shortest visible flame. With highvolatile coal five parts of air to one part of coal may be required for the best results. The visible flame is shorter when the air and coal are subjected to rapid mixing as they enter the furnace and during the combustion of the volatile matter. Mixing in the furnace while combustion is taking place is much more effective than mixing outside of the furnace previous to combustion. Although the particles of coal are very small they are very large compared with the molecules of oxygen. The volatile matter distilled from the particles of coal has many times the volume of the particles themselves. Without mechanical mixing the volatile matter would form a sphere around the particle of coal and combustion would take place only at the surface of this sphere, a t a rate only as fast as natural diffusion would bring the volatile matter and oxygen together. The visible flame would be long and smoky. Active mixing disperses the volatile matter in the atmosphere of air and causes it to burn quickly with a short, visible flame. Flame Travel
After the volatile matter has been driven off from the coal particles and burned the residue is coke and ash. This coke must be burned down to ash if the combustion of the coal is to be complete. The combustion of the fixed carbon is not accompanied by a visible flame. When one looks into the furnace against a hot brickwork the space seems to be filled with hot, transparent gas. If, however, he looks against a dark object, such as boiler tubes, he may be able to see a swarm of white hot particles of solid matter. These particles do not form a continuous cloud like the particles of carbon formed by the breaking down of hydrocarbons in the volatile matter, but are individually distinct. The rapidity with which the fixed carbon burns depends to a large extent on the mixing and to a less extent on the amount of air supplied for complete combustion. The mixing consists of relative motion between the solid particles and the surrounding gaseous atmosphere. The higher this relative motion the quicker is the combustion. The relative motion brings the particles of solid carbon in contact with the molecules of oxygen and carbon dioxide, and the solid carbon is gradually changed to carbon dioxide and to carbon monoxide, which in turn by combustion with more oxygen is converted to carbon dioxide. The combustion of the carbon monoxide so formed appears to be very quick, because it is difficult to find carbon monoxide, in measurable quantities in the furnace gases, especially if 1 or 2 per cent of oxygen is present. The furnace for burning pulverized fuel must be so designed
1233
that the path of gases through it is of sufficient length so that the fixed carbon is almost completely burned at the point where the gases leave the furnace. This length of the gas travel through the furnace is called the flame travel. It is a property of the furnace design. The length of visible flame is largely a property of the coal. The visible flame ends a t the section of the flame path where the combustion of the volatile matter is completed. The combustion of the fixed carbon continues without visible flame until its combustion is completed, or until the particles of coke have entered among the boiler tubes and combustion ceases because of too low temperatures. I n burning powdered coal the rapidity of combustion depends only slightly on the speed of chemical reaction, but almost entirely on the rapidity of mixing. The question is not how fast oxygen will combine with carbon when the two are brought together, but how fast the oxygen can be brought into contact with the carbon. I n other words, it is the mixing while combustion is taking place that counts. Mixing may be defined as a change of the relative positions of carbon and the molecules of the furnace gases. This change of the relative positions is accompanied by friction, which must be overcome by the expenditure of energy. Therefore, if active mixing is to be obtained, sufficient energy must be supplied to overcome the friction. The energy is usually supplied in the form of air under low pressure. When the pressure is released as the air or gas enters the furnace this form of energy changes into that of velocity, which in turn causes the mixing.
The Mechanism of Combustion in the Bunsen Cone' By E. W. Rembert and R. T. Haslam MASSACHUSETTS INSTITUTE
OF
TECHNOLOGY, CAMBRIDGE, MASS.
Varying the temperature of the furnace walls surrounding a flame burning as a Bunsen cone between 110" and 1100" C. had no effect upon the rate of combustion in the cone as determined by the analysis of the stack gases. This fact is verified also by direct calculation from existing data on radiation from flames, assuming the radiation to be thermal. The fact that combustion was complete at a short distance above the port with only 5 per cent excess air, indicates that the rate of combustion is governed by the rate at which oxygen is mixed with the combustible gases. Experiments were made to determine the effect of port velocity, port diameter, and the air-gas ratio upon the height of the cone, with the following results: 1-The rate of change of cone area decreases with increasing port velocity, becoming practically constant at high velocities. 2-The rate of change of cone height decreases with port diameter, in some cases passing through a maximum. 3-The rate of combustion in the Bunsen cone increases with increase in the primary air-gas ratio, which result is in accord with thermal considerations. The results are found to coardinate in such a manner as to indicate that heat transfer from the flame surface to the cold gas is the factor controlling combustion of gas with premixed air. 1 This and the three following papers represent abstracted portions of a thesis submitted by E. Wayne Rembert to the Faculty of the Massachusetts Institute of Technology in partial fulfilment of the requirements for the de gree of Doctor of Science in Chemical Engineering.
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INDUSTRIAL A N D ENGINEERING CHEMISTRY
1234
HEMICAL reaction in the gaseous phase is important in the design and operation of furnaces burning any fuel. Even coal may be fired so that the potential heat in combustible gases rising from the fuel bed is more than half the total heat in the coaL2 This heat must be liberated by combustion in secondary air that is admitted above the fuel bed; the rate a t which the oxygen and combustible combine determines the length of combustion chamber required for a given service. This work was undertaken to study the mechanism by which a stream of combustible gas burns under various conditions. It is hoped to eliminate some of the confusion now existing in regard to combustion phenomena, and to bring the design of furnaces a few steps nearer a firm, scientific basis.
C
A large amount of work has been done on radiation from both luminous and nonluminous flamesI3 but the results are somewhat conflicting, owing to differencesin experimental conditions. However, it is safe to say that a nonluminous flame of city gas will radiate not over 15 per cent of its total heat to the surroundings at room temperature. For Cambridge city gas, with a heat of combustion of 540 B. t. u. per cubic foot, the theoretical flame temperature is 2280' C. Then, assuming for the moment the radiation to be thermal, the heat loss is:
where
Preliminary Experiments
If a furnace is constructed so that the flame impinges directly upon some comparatively cold surface, such as boiler tubes, soot formation is excessive and the excess air requirement is greater than that consistent with good furnace practice. I n order to eliminate these difficulties, the grate, or oil burner, is often placed in an extension combustion chamber known as the "Dutch oven," which is usually successful in the elimination of the troubles experienced with the short fire-box. The general explanation for the results obtained through the use of a Dutch oven is that (1) a longer path is provided for the travel of gases and the time of contact is thus increased; and (2) the walls of the oven, as well as the inclosed flame, are practically out of sight of the boiler tubes: the gases are held a t a higher average temperature than before, and an increased rate of combustion results. The flame may lose heat to the walls by radiation and convection; the relative amounts transferred by each mechanism have not been established. Therefore, the following experiments were undertaken in order to determine the effect of radiation losses from the flame upon the rate of combustion, as determined by the analysis of the stack eases. Tge c o m p o s i t i o n of the gases from a flame, burning as a DIAMETER OF PORT-0.360 INCHES Bunsen cone, was determined by sampling a t a point 4.25 inches (10.8 cm.) above the port, diameter 0.360 i n c h (0.915 cm.). T h e average port velocity of the gas mixture was 78 feet per second (23.8 meters per sceond). The flame burned in .. a small furnace built PCRCCNT OF THEORETICAL UR SUPPLED of 1-inch (2.54-cm.) Figure 1 s t a n d a r d DiDe. 12 inches (30.5 cm.) long. The wall temperature was c'orkrolled by water cooling or by varying the thickness of insulation on the walls. The experimental results are shown on Figure 1, in which the amount of carbon monoxide in the air-free flue gas is plotted against the total air supplied. The points cover wall temperatures of 110', 650', and l l O O o C., and show very clearly that the composition of the stack gases is independent of the wall temperature, a result in accord with the physical phenomena involved, as will be shown. The actual thickness of the zone of combustion is only a fraction of an inch, hence the only mechanism by which the flame can lose heat is by radiation.
I
* Bur. Mines. Bull. 186.
Vol. 17, No. 12
0
=
rate of heat loss
T , = flame temperature,
O
K.
TT = wall temperature, " K. K
=
constant
which, for a wall temperature of 1100" C., becomes:
9 e
=
K[(25.53)4-(13.73)4]= 385,600 K
and, for a wall temperature of 110" C.:
ea
= K[(25.53)'-(3.83)']
= 420,800
K
The ratio of the heat lost by radiation to the walls a t 110" C. to that a t l l O O o C. is
or an increase of only 1.3 per cent of the total heat of combustion. This change in the heat radiated gives a change in the theoretical flame temperature of approximately 30" C. It can be concluded, therefore, that in ordinary furnace practice the change in the rate of heat transfer by radiation from the flame to the walls is small even if the temperature of the wall changes greatly. The fact that the composition of the stack gases, sampled 4.25 inches (10.8 cm.) above the port a t a velocity of 78 feet (23.8 meters) per second, showed complete combustion with only 5 per cent excess air, as contrasted with the usual excess air requirements of industrial furnaces, demonstrates very clearly that the rate of gaseous combustion in furnaces is controlled by the rate of mixing of secondary air with the combustible gases. Factors Influencing the Rate of Combustion in the Bunsen Cone
Combustion in a Bunsen cone occurs only when the fuel gas and air are premixed. The rate a t which the flame is propagated, as measured by the area of the cone, must be directly proportional to the rate at which the mixture is raised to its ignition temperature. Bunsen4 first determined this normal rate of flame propagation rate by measuring the velocity a t which the flame flashed back through the port. Later Guoyb measured the area of the cone and thus calculated the rate of flame propagation for several pure gases mixed with air. Mallard and Le Chateliere employed this method on several industrial and pure gases. Michelsonl modified the experimental procedure by employing a photographic method for the determination of cone areas. The most complete study of the phenomena of combustion U Helmholtz, Weid. Beibl., 14, 589; Julius, Ibid., 14, 602; David, Phil. Mag., 89, 66, 84 (1920); 42, 86s (1921); Callander, British Assoc. Repts. 1910; Haslam, Lovell, and Hunneman, I n d . Eng. Chem., 17, 272 (1925). 4 "Gasometrische Methoden," 2nd ed., 1877. 6 A n n . Chem. Phys., 18, 6 (1879). 6 A n n . mines, 4, 296 (18S3). 7 Ann. Phys. Chem., 87, 1 (1889).
.
INDUSTRIAL AND EN(XNEERING CHEMISTRY
December, 1925
in the Bunsen cone has been carried out by Ubbelobde* and his co-workers, whose work brings out the following facts: I-Their results do not check those of Michelson and Mallard and LeChatelier. The following table shows maximum flame speeds of various gases in air:
Ubbelobde and Hofsasz Michelson Mallard and LeChatelier
31.5 45.0
..
200 277 430
Discussion of Results
27.5 62:O
2-Some mixtures of carbon monoxide and hydrogen gave a higher r a t e of propagation than did pure hydrogen. 3-Dilution with an inert gas lowers the rate of flame propagation. 4-An i n c r e a s e i n the initial temperature of the gas increases the flame speed. &Smaller diameter b u r n e r s give higher rates of flame propagation.
The following work is not intended to furnish actual rates of c o m b u s t i o n in the 3 4 B u n s e n c o n e , but EPATIO rather to explain the Figure 2 divergence of existing results and to establish the physical mechanism by which a gas burns under these conditions. Experimental
City gas and air were metered through flowmeters connected by a tee to the burner tube, 42 inches (1.07 meters) long. It was found necessary to use a burner of this length in order to eliminate fluctuations in the cone due to eddies from the mixing tee. I DIAMETER Of PORT- 0.610 INCHES I 1 S u r r o u n d i n g t h e burner was a-&inch standard iron pipe, through which secondary air was drawn by n a t u r a l d r a f t . The flame was housed in a rectangular box ? 18 inches (45.8 cm.) 2 square by 4 feet (1.22 L meters) high, the top of which was fitted 9w5i with a tapered hood 0 0 of sheet metal. A g l a s s w i n d o w was provided in one side of the combustion chamber; the height of the cone was deterPORT V E L W T Y - E E T P C R YUM mined by s i g h t i n g Figure 3 past two meter sticks spaced 9 inches (22.8 cm.) apart, the tip of the cone forming the third point in the line. The experimental procedure consisted in maintaining Ubbelobde and Hofsasz, J . Gasbcl., 66, 1225, 1253 (1913); Ubbelobde and Dommer, I b i d . , 61, 733,757 (1913). 8
a constant flow of gas and varying the primary air. A typical set of curves is shown in Figure 2. For purposes of discussion, these curves have been calculated over to those of the types as shown by Figures 3, 4, 5, and 6 showing the effect of variation of port velocity, port diameter, and airgas ratio, respectively, other factors remaining constant.
Flame speed, cm. per second co Hz C H4
INVESTIGATOR
1235
A stream of a mixture of combustible gas and air flowing from a port of circular section will possess a certain definite ignition temperature and rate of combustion for any given set of conditions, providing the air-gas ratio is between the explosive limits of the gas under consideration. Assume for the present that the velocity of the gas stream is uniform across the entire area of the port. Then, when a point in the stream is ignited, the flame will propagate in every direction with an absolute velocity V . However, a condition of dynamic equilibrium will be reached, providing the port velocity is not too high, in which the flame surface will remain stationary and will be of such area as is required for the transmission of sufficient heat to raise the gas stream to its ignition temperature. Under this condition of equilibrium, if the flow of heat is to be steady the flame must be propagated normal to the surface with an absolute velocity V , and this velocity must be equal to the component of the port velocity normal to the surface a t the same point. It is obvious that the only shape of flame surface for a circular port is a cone with the port as a base.
16
3
$
z
2 L $
4
0
a
I6
PCUT V L O U T Y
- FEET PER SECOND
24
Figure 4
The problem of defining the area of the cone under any given set of conditions is extremely complicated from a mathematical point of view, because of uneven distribution of velocity through the section of the gas stream, heating of the gas stream by the zone of secondary combustion, and increase in the rate of heat transfer with increase in velocity, due to turbulence. EFFECTOF SECONDARY ConmusTroN-It is experimentally observed that the gas stream is heated in part from the zone of secondary combustion. Table I gives some idea of the magnitude of this effect for a burner 20.8 mm. (0.820 inch) in diameter. of Secondary C o m b u s t i o n on Height of Bunsen C o n e Cone height, inches Port velocity With secondary Without secFt./sec. air ondary air 2.12 2.28 5.9 6.5 2.31 2.52 4.75 5.2
Table I-Effect
Air-gas ratio
EFFECTOF VELocITY-The experimental curves (Figures 3 and 4) show the variation of cone height with velocity. As the velocity is increased turbulence sets in, and as a re-
INDUSTRIAL AND ENGINEERING CHEMISTRY
1236
sult the heat transfer is greater, the cone relatively shorter. For higher velocities the cone height is about constant. It was observed that a definite region of measurable thickness exists in the Bunsen flame, bounded by two superimposed cones. It is probable that the thickness of this section measures the rate a t which chemical combination occurs. This conclusion is borne out by the fact that the thickness increases with increased velocity. Table I1 illustrates the magnitude of this effect. Table 11-Effect of Port Velocity on the Thickness of the Bunsen Cone Port Diameter Height of cone Thickness Air-gas velocity of port in inches of cone ratio Ft./sec. Inches Upper Lower Inches 2.7 2.6 0.1 6.8 5.6 1.2 o.610 1.6 1.6 0.0 4.0 3.5 0 .. 5 . 5 0.469 2.3 2.1 0.2 3.4 2.9 0.5 4.8 3.7 1.1
Vol. 17, No. 12
A higher temperature difference results in a higher rate of heat transfer. Also, the ignition temperature of the mixture probably decrease^.^ 9
Falk, J. A m . Chem. Soc., as, 1517 (1906); 29, 1536 (1907); Dixon, Soc. ( L o n d o n ) , 93, 661 (1938); Taffanel, Compt. rend., lST, 714
J. Chem.
(1913); 118,42 (1914).
Factors Influencing Length of a Gas Flame Burning in Secondary Air By E. W. Rembert and R. T. Haslam
t
MASSACHUSETTS INSTITUTE OF TRCHNOLOOY, CAMBRIDGE, MASS.
Experiments were carried out to determine the effects of port velocity, port diameter, and the primary air-gas ratio upon the over-all length of a gas flame burning in E F F E C TO F P O R T free space. It was found that the length of the flame is DIAMETER-w i t h a n determined by an equation of the form: increase in the diamL = K log u B log D E eter of the port, the l e n g t h of t h e p a t h where u is the port velocity, D is the port diameter, and t h r o u g h which heat K,B, and E depend on the primary air-gas ratio. A practical application of the results in connection must be t r a n s f e r r e d from the zone of sec- with the rational design of furnaces is included in the ondary combustion in- paper.
+
creases for any given height of cone, resulting in a decrease in the over-all rate of heat transfer. This decrease in heat transfer must manifest itself in an increased rate of c h a n g e of cone area with the amount of gas WRT MAMCTCR - INWCS burned. Figure 5 A variation in the diameter of the port, for any given velocity and air-gas ratio, also affects the shape of the flame surface. The ratio of the mass of gas in turbulent motion to the total gas flowing increases with increase in port diameter, resulting in a higher over-all rate of heat flow. 14 The data (Figure 5) indicate that the com12 b i n e d e f f e c t s of increased heat transfer and a decrease in the 10 curvature of the cone s u r f a c e actually dei s crease the height of a cone for larger values of diameter, the ve1 6 locity and air-gas ratio remaining constant. EFFECTOF AIR-GAS RATIO-A typical set 2 of curves showing the variation of cone height with the air-gas ratio is 0 2 3 4 shown in Figure 6. As a & - RATIO the air-gas ratio inFigure 6 c r e a s e s the temperature of the flame approaches a maximum corresponding to the theoretical air requirement of the gas under consideration. D
~
Ei
+
K a previous paper' the authors have discussed the
I
mechanism of combustion of a stream of gas premixed with air. It was shown that port velocity, port diameter, and the primary air-gas ratio affect the dimensions of the Bunsen cone in such a way as to indicate that the rate of combustion under these conditions is controlled by the rate a t which he$ is transferred from' the flame to the cold air mixture. In industrial furnace practice, combustion of a gas with premixed air offers no serious problems except in the design of burners. RIost of the combustion problems encountered in the industries arise in the process of burning in secondary air; yet our knowledge of the phenomena involved is meager. The designer is forced to base his work upon a qualitative knowledge of the factors involved and upon empirical data obtained in field tests. Practically the only work that involves a thorough study of the factors influencing the rate of combustion of a gas in secondary air is that contained in the U. S. Bureau of Mines Bulletin 135. By sampling the reacting gases a t various points along the flame, the length of combustion space required for various conditions of furnace operation was determined. The data included in $ this work also show the Y marked tendency of hot gases to stratify according to their density, making sampling of stack gases difficult. The present work includes a study of the effects of port velocity, port diameter,
x
2
1
Page 1233,this issue.
=RAT0 GAS
Figure 1
*
