INDUSTRIAL AND ENGINEERING CHEMISTRY
1238
and E are the slopes and intercepts in Figure 6. The complete equation is therefore, F = K log u B logD E (2) The Coefficients in the complete Equation 2 (functions of the air-gas ratio) are summarized in Table 11.
+
+
-
T a b l e 11-Coefficients for E q u a t i o n (Length of flame in inches K log u f B log D f E) Air-gas ratio K B E 0 17.6 33.9 25.2 0.5 23.3 32.6 18.9 1.0 23.0 43.0 14.0 39.0 8.6 2.0 21.5
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Factors Affecting Utility of Secondary Air in Gaseous Combustion By E. W. Rembert a n d R. T. Haslam MASSACHUSETTS INSTITUTE OF TECHNOLOGY, CAMBRIDGE.MAS%
The application of the general equation to actual furnace design depends on the ability to evaluate the coefficients under actual operating conditions, these coefficients changing with the type of furnace and the kind of fuel gas used. If 1.0,
Vol. 17, No. 12
I
These experiments were carried o u t to obtain s o m e definite ideas regarding t h e mechanism by which oxygen in secondary air is consumed by a combustible gas. T h e results show that t h e total amount of air supplied to a free-burning flame materially affects the fraction thereof that is utilized for combustion. I n all cases investigated, t h e r a t i o of utilized t o supplied air passes through a m a x i m u m with respect to supplied air, t h e actual values depending upon port velocity a n d diameter. T h e fraction of the supplied a i r that is utilized for combustion increases with both port velocity a n d diameter, owing probably t o an increased rate of mixing. Several practical aspects of t h e results are discussed, including t h e fact that, by plotting t h e log of t h e air supplied t o that a m o u n t which is utilized, a flat curve results which can be closely approximated by a straight line, thereby permitting accurate extrapolation of “two-point’’ d a t a o n excess air requirements.
I R introduced into a fire box, above that theoretically required for complete combustion of the fuel, decreases the thermal efficiency of a furnace, because the stack gases can seldom be cooled to the initial temperature of the air used for combustion. The use of a certain amount of excess air in a furnace operating with long-flame combustion is necessary; if this amount is judiciously chosen, the sensible heat carried away in the stack gases is equal to that which would be lost as potential heat were the secondary air reduced. P r a c t i c a l l y , the only data relating directly to the factors affecting the excess AIR air requirement of a furnace are those included in the U. s. B u r e a u of M i n e s Bulletin 135, attention to which has been called in a previous paper. The results of this work are open to two serious objections from a point of view of general appIication. In the first place, stratification of the gases in the flue renders the average values of the gas analyses of doubtful accuracy. Furthermore, t h e experiFigure 1 mental conditions are important in the determination of excess air requirements, because the baffle arrangement, gas velocity, and distribution will-vary in different furnaces. It is possible to obviate the difficulties involved in sampling by thoroughly mixing the total volume of flue gas, when working on a small scale.
A
C Figure 6
it is desired, for example, to extend the fire box of a gasfired furnace, using the same port arrangement and size, so that the furnace will burn twice the quantity of gas that it will handle at present, we have an equation of the form of L = K log 11 c for which the coefficients can be determined in the existing furnace by two simultaneous measurements of flame length and port velocity.
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Bureau of Standards Completes over 173,000 Tests During the fiscal year ended June 30, 1925, the Bureau of Standards completed 173,261 tests. This represents a considerable increase over previous years; 115,729 tests having been completed in 1923, and 135,852 in 1924. The fee value of the work is $547,543.35. This is also an increase over past years; the 1923 and 1924 figures being $419,915.70 and $509,850.87, respectively. All income so received is turned back into the Treasury of the United States, the expenses incurred in the tests being absorbed by the annual congressional appropriations for the bureau’s maintenance. 2 1 great variety of materials and devices were tested, including analytical weights, chemical glassware, thermometers, sugar samples, radium, cement and concrete, leather, and paper. Almost every industry and branch of the Government has been served by the bureau.
INDUSTRIAL AND ENGINEERING CHEMISTRY
December, 1925
5
5
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DWCTCR OF PORT-0.281 INCHES
1239
1
4
3
0 3 4 UTILIZED AR -GAS RATIO
Figure 2
?.
10
5
PORl VCLWTY-f7.
PORT DIAMCER- INWCS
Figure 3
Experimental
The apparatus used for studying long-flame combustion in secondary air is shown diagrammatically in Figure 1. It consists of a cylindrical chamber of sheet metal, bolted to a flat base of the same material. The top of this chamber is tapered to a stack fitted with a blank sliding gate through which a length of pipe is projected. This arrangement permits thorough mixing of the stack gases and consequently eliminates the difficulties of stratification in sampling. A length of sheet metal pipe, 6 inches (15 cm.) in diameter and 3 feet (91 cm.) long, was connected to the base of the combustion chamber, surrounding the burner tube. The former was closed a t the bottom and fitted with a piezometer ring near the base, through which secondary air was admitted. A glass window was provided in the side of the combustion chamber for observation of the flame under various conditions. The flow of air and gas was measured by means of short-tube flowmeters. The experimental procedure consisted in setting the flowmeters for the desired air-gas ratio, all the air as secondary. After a period of 20 minutes, a sample of the gas was taken from the top of the stack pipe and analyzed in a standard Burrell apparatus. Simultaneously with the sampling of the flue gas, a sample of the fuel gas was analyzed in a Williams apparatus. For the purpose of making calculations on the formation of soot, primary air was admitted into the burner after each sample of flue gas had been drawn, in such quantity as to insure a blue flame, and sufficient secondary air admitted to insure complete combustion. A flue gas sample was analyzed in the Burrell for carbon dioxide and oxygen. The results of these experiments are best presented in plots of the type shown in Figure 2 , giving the variation of the air utilized for combustion, with the total air supplied to the flame. The numerical values of these quantities were obtained by simple stoichiometric calculations from the analyses of the flue gas. These curves, for all conditions of port velocity and diameter investigated, have the same general shape. For purposes of discussion, the results have been calculated over to curves of the types shown in Figures 3 and 4, showing the effects of port velocity and diameter, respectively, on the amount of air utilized for combustion, the total amount of air being kept constant. The experiments were conducted with Cambridge city gas; an average analysis of this gas is as follows:
1.0
a5
0
PER Y C .
Figure 4
co2
Illurninants 0 2
Per cent 3.1
co
4.6
HI
0.8
CHI
Per cent 18.1 42.0 24.9
Discussion of Results
If the excess air admitted to a furnace is reduced below a certain value, the stack gases contain a quantity of combustible gas mixed with free oxygen. This condition can be explained only by the fact that heat transfer from the flame has been more rapid than diffusion or mixing of oxygen with the fuel gas, and a t some point in the travel of the gases the temperature of the reacting mixture has fallen below its ignition temperature. Inspection of Figure 2 will show that the extrapolation of the curves to the point of zero on the axis of utilized air falls positive on the axis of supplied air, which means that before any of the fuel gas can be burned enough air must be supplied to the flame to maintain a concentration of oxygen in the region of combustion sufficiently high to overcome the heat loss to the gas.
SUPRILD~RATIO
Figure 5
The difficulty of removing the last traces of combustible from the stack gases can also be seen by inspection of Figure 2. However, the actual numerical results of this work cannot be applied to commercial furnaces; it is necessary to obtain the optimum excess air requirement for each type of furnace from an actual test. If the ratio of utilized air to supplied air is plotted against the total air supplied, the curves pass through a maximum,
INDUSTRIAL A N D ENGINEERING CBEMISTRY
1240
the actual values of which depend on the port velocity and diameter (Figure 5 ) . Upon plotting the logarithm of the air supplied against the air consumed, a flat curve results. This can be approximated by a straight line over a comparatively wide range, independent of port diameter. Such a relation is shown in Figure 6. This obviously offers a convenient method for the
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Vol. 17, No. 12
Relative Rates of Combustion of Constituents of City Gas Burning in Secondary Air By E. W. Rembert and R . T. Haslam MASSACHUSETTS INSTITUTE OF TECHNOLOGY, CAMBRIDGE, M A S S .
The relative combustion rates of the components’ 6f city gas, burning in secondary air, can be expressed in terms of the general mass action expressions for reaction rates, providing that over 80 per cent of the gas has been burned. The change of relative percentages of the constituents with port velocity and diameter is small. Methane and ethylene are believed to burn with oxygen according to the equations:
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C2Hi
and where DIAMETER OF PORT- 0.360 I N W U DIAMETER (x PORT-0.610 INCHES DlAMETER OF PORT- O B K ) INCHES
CH4
+
+ 02 0 2
+ 2H2 = CO + HZ + H20 = 2CO
Hydrogen and carbon monoxide, burning simultaneously in the presence of small quantities of methane. and ethylene, follow an expression of the type
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3.0
40
1/H2 = !E! (1/CO) kco
UTILIZEO~RATIO
Figure 6
extrapolation of “two-point” data, and permits the optimum excess air requirement of a furnace to be obtained with greater ease and speed. Referring to Figure 3, it will be observed that, for a given amount of primary air supplied, the amount that is utilized increases rapidly with port velocity for low values of the latter, owing probably to an increase in the rate of mixing with respect to heat transfer. These curves show very clearly the desirability of using a high port velocity in industrial furnaces. The variation of utilized air with port diameter (Figure 4) shows the same general shape as the curves involving port velocity, except that the change in slope is more abrupt.
Consumption of Turpentine and Rosin by Industries The Bureau of Chemistry, United States Department of Agriculture, has compiled the following statistics from reports made by individual users, showing the total quantities of turpentine and rosin used during the years 1922 and 1923 by certain industries in the United States:
Paint and varnish Soap Paper Rosin, oil, pitch, and printing ink Shoe Dolish and leather dressinas Automobiles Sealing wax, fly paper, insulation Oils and greases Linoleum, oil cloth, roofing Iron and steel Ship yards Pharmaceuticals and chemicals Matches and wooden ware
TOTAL
-In 1922---In 1923Turpentine Rosin Turpentine Rosin 50-gal. 500-lb. 50-gal. 500-lb. bbls. bbls. bbls. bbls. 141,691 179,319 105,803 217,317 580 282,700 635 286,755 276 248,856 182 162,353 57,058 189 56,129 1,200 992 16,135 792 19,627 548 7,791 6,823 450 14,255 1,308 1,403 23,941 1,165 13,612 7,751 793 29,239 533 33,663 68 401 14,228 15,749 291 65 377 43 536 1,708 313 2,593 283 43 3.631 26 3,190
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174,376
134,096
754,927
the slope being approximately kar/&o = 4.9. Hydrogen and methane burn according to a similar expression, the slope being approximately
4 3
= 3.0
kH*
and for carbon monoxide
tco
=
14.5
N PREVIOUS papers‘ the writers have shown that the rate of mixing of oxygen with the fuel gas is the factor controlling the rate of combustion of gas stream, burning in secondary air. Kothing has been said, however, of the relative rates of combustion of the several constituents of a composite gas. In an article by one of the writers2 on the simultaneous combustion of carbon monoxide and hydrogen, it was assumed that the general expressions for reaction rates in homogeneous systems held for combustion in secondary air, and, using the data included in U. S. Bureau of Mines Bulletin 135, it was concluded that the two gases burn with oxygen according to the reactions:
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2co + 0 2H2
+
2 0 2
= 2c02 = 2H20
It was found that hydrogen burned about three times as fast as carbon monoxide. However, these results are not directly comparable with those obtained in the following work, since the fuel was a producer gas rising from the fuel bed of a stoker-fired furnace. Also, among other differences, the air was admitted at several points along the flame instead of symmetrically around the gas stream. Experimental
902,010
It is probable that the statistics showing the domestic consumption of turpentine and rosin do not include all of these materials used in industries manufacturing other articles in which the identity of the turpentine or rosin is lost. On the other hand, a small percentage of turpentine and rosin, more especially turpentine, reported as used by an industry-the paint and varnish industry, for example-may be sold again as such to retail trade.
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The results herein presented are derived directly from data obtained from the experimental work described in the previous paper. This work consisted in sampling the flue gas resulting from the combustion of Cambridge city gas in 1
Pages 1233 to 1240, this issue.
* Tnis JOURNAL,16, 679 (1923).
