January 1955
INDUSTRIAL AND ENGINEERING CHEMISTRY
( I , 3 ) a t times with transformed coordinates. The curves were apparently based on experience with rock dusting as a means for controlling coal dust explosions in mines and upon experiments by Taffanel and associates (7-9) w t h coal dust flames in a refractorylined conical tube. The experimental data were not sufficient to establish quantitative relations with proximate analyses. Nevertheless, DeGrey presented curves for coals of various volatile matter and ash contents. The curves showed maximum flame velocities a t dispereions ranging from about 40 to 60 cubic feet of air per pound of coal. Curves from the present investigation lie entirely on the lean side of the maxima shon-n by DeGrey. Furthermore, they do not indicate the high ratio of flame velocities, nearly 3 to 1, for a coal of 30% compared with 15% volatile matter content, that was shown in the earlier curves. The strong influence of volatile matter content, in the earlier estimates of flame velocities, was probably based upon experience with explosions in mine galleries, where concentration of energy in the moving flame front depends more upon rapid combustion of volatile matter than upon sloTver and probably incomplete combustion of fixed carbon. The present investigation indicates that a higher furnace temperature is needed to establish a flame with the coal of lower volatile matter content, but that the flame velocity is not so markedly dependent upon volatile matter content. Limitation of primary air is a commonly practiced means for getting good ignition with coals of lower volatile matter content. According to Traustel’s analysis of the heattransfer system, the improvements result, a t least in part, from the reduced thermal load resulting from the restricted amount of air associated with the coal prior to the point of ignition. It may also result, as is inherently recognized in Traustel’s as well as Nusselt’s analyses, from an increased radiant intensity of the flame. A study of the ignition of individual particles in a pressured atmosphere ( 5 ) showed that convective heat transfer and the “time-dependent” rather than the “steady-state” form of the equations for the heat-transfer system from the coal to the surrounding atmosphere, must be considered. iz hindering effect of pressure was observed which was not readily overcome by oxygen enrichment. It could be explained on the basis of the idea that, within the short time prior to ignition, the thermal or the convective yave advanced only into a limited portion of the at-
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mosphere associated with the coal. If such a limitation exists, the limitation given by Traustel for the heat capacity of the entire amount of associated atmosphere mould seem of secondary importance. Though the analyses of Nusselt and Traustel may be questioned on theoretical grounds, the importance of the effects with which they deal has been substantiated experimentally. The ignition process involves heat transfer to the coal by radiation and heat loss by conduction into the immediately surrounding atmosphere. Anything that changes the rates of these processes will change the rate of flame propagation. Because radiant heat transfer through transparent media depends upon angle factors, but not on distance, the propagation of a pulverized coal flame depends upon the size and geometry of the flame a9 well as upon the fuel distribution and the aerodynamic pattern. The pulverized coal flame differs in this rerpect from gaseous flames. In so far as fuel gases are transparent to thermal radiation, Aame propagation depends upon energy transfer from zones within or immediately adjacent to that portion of the flame front under consideration. While the propagation of a gaseous flame depends upon local conditions, the propagation of a pulverized coal flame depends upon the flow of radiant energy throughout the flame. LITERATURE CITED (1) Craig, O., Trans. Am,. SOC.Mech. Engrs., 61,369-72 (1939). (2) DeGrey, M. A.,Reo. nadt., 19, 645-55 (1922). (3) Kreisinger, H., Proe. Engrs. SOC.West. Penn., 39, 244-9 (1923). (4) Nusselt, W., Z . V e r . d e u f . Ing., 68, 124-8 (1924). (5) Omori, T. T., and Oming, A. A , Trans. A m . Soc. Mech. Enqrs., 72, 591-7 (1950). (G) Orning, A. A., “Combustion of Pulverized Coal.” pp. 1522-87, Chap. 34 of “Chemistry of Coal Utilization,” H. €I. Lowry, ed., New York, John Wiley & Sons, 1045. (7) Taffanel, J., Ann. mines, Mirn., SQrie11, 2, 167-205 (1912). (8) Taffanel, J., and Durr, il., Colliery Guardian, 103,227. (9) Taffanel, J., and Durr, A., Cornit6 central des houillbes de France, Paris, 1911. (10) Traustel, S., Feuerungstech., 29, 1-6, 25--31, 49-60 (1941).
ACCEPTED October 12, 1951. Based on a D.Sc. thesis a t the Carnegie Institute of Technology entitled “A Study of the Conditions Necessary for Producing Small Flames with Pulverized Coal“ by Bimalendu Ghosh, 1951.
RECEIVED for review June 16, 1954.
Flame-Stability Studies on an Inverted Bunsen Burner d
PHILIP F. KURZ Battelle Memorial Institute, Columbus 1 , Ohio
F
LAME-stability studies have been carried out on an inverted Bunsen burner. The experimental work dealt with a comparison of the stability limits of upright and inverted flames of single and mixed hydrocarbon fuels and with the influences of nitrogen on the stability limits of hydrocarbon-air mixtures. DESCRIPTION O F APPARATUS
Figure 1 shows a schematic diagram of the inverted Bunsen burner and the auxiliaries for metering gas flows, for quenching flash back, and for cooling the burner tube. The burner tube is of brass, 19.1 mm. in inside diameter and about 122 em. long. The water jacket extends upward about 107 cm. from the plane of the burner port. Cooling water is circulated through the jacket by means of a pump. Effluent water is returned to a sump for recirculation. Flash backs were quenched by introducing nitrogen into the fuel-air mixture a t the inlet to the burner tube.
A Smithells tube to exclude ambient air from the environs of the burner port is essential. Unshielded flames curl upward and move about under the influence of random currents in the ambient air. Fuel and air flows were metered by means of critical flow orifices. Compressed air from commercial cylinders was used in the experimental work in order to ensure close control over the air flow a t all times. DESCRIPTION O F IlriVERTED FLAMES
Figure 2 shows sketches of lean and rich hydrocarbon-air flames a t the last stable composition prior to blowoff on the inverted Bunsen burner. The sketches represent flames produced a t moderate air rates-7 to 9 liters per minute. B t lower air rates the flames are shorter and are more rounded at the tip, and a t high air rates the flames are longer and more pointed. Lean ethylene flames are essentially the same as lean flames of other hydrocarbons. However, because shielded rich ethylene
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flames ale stable a t fuel-air compositions where inost other hydrocarbon8 will not burn stably, near-limit rich ethylene-air flames are very different fiom methane-air and other hydrocarbon-air flames. The primary flame cone of rich, near-limit ethylene-air flames is violet a t lo1-v air rates and blue a t moderate and high air rates. At low and moderate air rates the primary flame cone is enshrouded with a bright orange-yellow mantle which is streaked with black streamers showing the occurrence of soot formation. Rich methane flames near the blowoff point produce only a simple unshrouded blue-green or blue primary flame cone. The effluent from the primary flame cones of rich mixtures burns in disklike diffusion flames a t the end of the Smithells tube, as shown in Figure 2. BEHAVIOR OF IYVEKTED FLAMES
The flammability limits ieported by Coward a,nd Jones ( 1 ) for various hydrocarbon-air mixtures are invariably wider for upward prepagation in a tube than when the flame is propagated downward. The widening of the flammability limits with upward propagation is especially marked in the instance of ethylene-air mixtures a t the rich limit. Table I lists their values for the flammability limits of typical hydrocarbon-air mixtures for upn-ard and downnrard propagation in tubes 150 cm. long. The data of Table I suggest that a marked difference in flamestability limits might also exist between upiight and inverted flames on a Bunsen-type burner.
TABLE I. FLAMMABILITY LIMITSO F HYDROCARBON-AIR MIXTURES I N TUBES 150 CM.LOKGAND CLOSED AT THE FIRISG END
Hydrocarbon Methane Ethane n-Pentane Ethylene Propylene Acetylene Benzene Toluene
Tube Diameter, Cm. 7.5 5.0 5 0 5 0 5.0 5.0
5.0 5.0
Direction of Propagation Upward Downward Fuel at Limit, Lean Rich Lean Rich 14.85 5.95 13.35 5 35 3.32 10.0 3.15 14.8 8.0 1.49 4.56 1.43 3.13 33.3 3.42 15.3 9.6 2.20 7.2 2.21 78 2.80 63.5 2 60 1.48 5.55 L4.3 7.45 1.31 6.75 1.32 4.60 ~
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coincide within the region of laminar flow on burners 11 mm. or less in diameter. This results because the lean and rich blowoff limits are progressively narrowed a8 the burner-jet velocity is increased and will eventually coincide, provided the flow through the burner tube remains laminar. PROCEDURE FOR BIKGLE FUELS. Air flow was set at a low rate of about 3 liters per minute, and fuel a a s admitted a t an increasing rate until a stable, lean flamc was produced The flow of fuel was decreased by convenient increments until the flame blen- off. This was the lean blowoff point. Then the flow of fuel was increased by increments until, after passing through a region of flash back, a stable, rich flame resulted. Further increases in fuel Aon-, by convenient increments until the rich flame blew off, defined the rich limit. Then the air flow was set at a higher ratc and thc procedure outlined above was repeated until a range - of air rates up to about 21 liters per minute had been covered. 1’ R 0 C E U U R E
BINARY FUEL MIXTURES.When binary hydrocarbon mixtures were PALEstudied, the lean BLUE PRIMARi limit for fuel A CCNE was determined at ilRIGul a chosen air rate by the method o u t l i n e d in t h e foregoing section. SOOT Then fuel B was introduced a t a ORANGEpredetermined PALElow rate of flow SECONDand the flow rate ARY DIFFUSlGN of fuel A was deFLAME creased until the lean limit of the RICH hEAR-LIMIT LEAN RICH NEAR-LII mixture was HYDROCARBON METHANE FLAhlES ETHYLENE F L A M E reached. N e x t FLAMES the flow rate of Figure 2. Near-Limit Lean and f u e l B w a s inRich Hydrocarbon Flames on creased by a conInverted Bunsen Burner venient increment and the flow rate of fuel h was decreased again until lean blowoff was reached. The addition of successive increments of fuel B was continued, each time reducing the requirements of fuel A, until a t length the only combustible present was fuel B. The procedure for observing the rich limits of fuel mixtures was similar t o that for the lean-limit studies. Briefly, the rich limit of fuel A was established at the chosen air rate; then fuel B was introduced in successive increments permitting the observation of rich limits for a series of binary mixtures of fuels A and B; finally, the rich limit of fuel B was established. Usually the behavior of binary mixtures was observed a t two or three different rates of air flow. By means of this procedure it was usually possible to cover the whole range of fuel compositions from 100% A t o 100% B. IWR
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STABILITY EXPERIMENTS WITH SIhGLE FUELS EXPERIMENTAL PROCEDURE FOR UNDILUTED MIXTURES
In burners with Smithells tubes, which exclude ambient air from the environs of the burner port, it is possible, a t any Gxed rate of input of air within the region of laminar flow, t o induce lean-type blowoff by decreasing the flow of fuel to reach the limit, and rich-type blowoff by increasing the flow of fuel. With some slow-burning hydrocarbons it is possible to increase the jet velocity until lean-type and rich-type blowoff limits
Flame-stability experiments were carried out on the shielded inverted 19.1-mm. Bunsen burner with several hydrocarbon fuels. Results for methane-air and ethylene-air flames are presented. METHBNE-AIR FLrlwzs. Figure 3 shows the relationship between the composition of methane-air mixtures a t lean and rich blowoff on the inverted burner and the values of the critical boundary velocity gradient a t blowoff. The agreement of the re-
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
January 1955
sults obtained with a variety of metering orifices was good. Each different combination of metering orifices for fuel and air is represented by a point of different shape on the curves. The narrowing of both limits of stability is apparent as the burner-jet velocity is increased. Methane-air flames a t rich blowoff ETHYLENE-AIRFLAMES. are unshrouded, have no yellow tip, and are nonsmoky-that is, the primary flame cone is similar to the primary cone of lean flames, and is blue-green or blue in appearance. Rich ethylene flames at and near the limit of stability on shielded burners are very different from rich near-limit methane-air flames. Ethylene shows a bright orange-yellow shroud around the primary flame cone, which is smoky a t the lower air rates on shielded burners. As the air rate is increased, the shroud and the sootiness gradually disappear and only the blue or blue-green primary flame cone remains near the burner port. Because of these differences between rich methane and rich ethylene flames and because of the large differences in the rich limits of ethylene shown in Table I for upward and downward propagation, ethylene was chosen for investigation. Figure 4 shows the relation between the composition of ethylene-air mixtures a t the lean and rich blowoff limits on the inverted 19.1-mm. Bunsen burner and the critical velocity gradient a t blowoff. These data and those presented for methane-air flames show that consistent results can be obtained in flame-stability studies with a shielded inverted Bunsen burner.
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blowoff limits for ethylene-air flames over the entire range of air rates from 3.1 t o about 19 liters per minute on the 19.1-mm. burner. This illustrates a significant difference between combustion of quiescent rich mixtures and the combustion of the flowing rich mixtures used in the flame-stability studies. It is concluded from t h e data shown in Figures 5 and 6 and from the results of experiments with other hydrocarbons, t h a t all hydrocarbons, except perhaps acetylene, will have identical stability limits on upright or inverted Bunsen burners shielded to exclude ambient air from the environs on the burner port. BEHAVIOR O F BINARY HYDROCARBON MIXTURES
INVERTED FLAMES OF METHANE-ETHYLENE MIXTURES Methane-ethylene mixtures were studied on inverted 19.I-mm. Bunsen
COMPARISON O F FLAME-STABILITY LIMITS ON UPRIGHT AND INVERTED BURNERS
METHANE-AIRFLAMES. Earlier studies in this laboratory determined the stability limits of methane-air flames on shielded upright Bunsen burners. It has also been found ( 2 ) that, when the critical velocity gradient is used as a basis for comparison, the stability limits of shielded flames are essentially independent of burner diameter. Figure 5 shows t h a t essentially identical results are obtained for the stability limits of methane-air flames burned on upright and inverted Bunsen burners of slightly different diameters. This is not surprising, inasmuch as the limits of flammability (Table I) for methane-air mixtures are not much wider for upward than for downward propagation in tubes. Because of the large difference in ETHYLENE-AIRFLAMES. the rich limits of flammability shown in Table I for ethylene-air mixtures for upward and downward propagation of flames in tubes, a significant difference in the stability limits of upright and inverted rich ethylene-air flames might be expected, but none was found. Figure 6 shows no difference between upright and inverted rich
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Figure 3. Stability Limits of Methane-Air Flames on Shielded Inverted 19.1-Mm. Bunsen Burner
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Stability of Methane-Air Flames
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