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only when gas and air come into contact, obeys all the laws of a heterogeneous reaction. I n this case the velocity of the reaction depends upon the rate of diffusion of the air and gas to the interface and upon the rate of diffusion of the combustion products away from the interface, as well as upon the rate of chemical reaction in the interface. If the gas is emitted with a higher velocity, introducing turbulence and eddy currents into the flow stream, the rate of combustion is increased because the rates of mixing are increased. This principle is recognized and widely used by combustion engineers. A gas-gas heterogeneous reaction may be of importance in gaseous explosions. I n internal combustion engines this type of reaction might be expected in engines using a stratified charge, and in the fuel injection engines such as those working on the Diesel cycle, in which the air used for combustion is compressed to a high pressure and temperature in the engine cylinder and the fuel subsequently injected, vaporized, and burned during the power stroke. A high degree of turbulence, however, causes such intimate mixing that the gasgas heterogeneous reaction rapidly approaches homogeneous reaction as the rates of mixing exceed the rate of a homogeneous chemical reaction. (2) Gas-Liquid Heterogeneous Reaction
A gas-liquid reaction is a reaction occurring a t the interface of gaseous and liquid phases. I n general, the same statement may apply to gas-liquid reactions as was made concerning gas-gas heterogeneous reactions. I n fact, evidence indicates t’hat liquids are vaporized before oxidation takes place in most combustion reactions. Whether or not this is always true is probably of no importance, as in either case the reaction is a surface or heterogeneous reaction and therefore governed by the same laws. It is well-known that oil fuel may be burned more quickly and completely if finely atomized and injected with turbulent flow into the furnace or engine than if introduced as a viscous stream. If the liquid is uniformly distributed throughout the mixture in the form of a fine mist, the explosive reaction takes place in a mixture that may be considered essentially homogeneous and the reaction approaches a homogeneous reaction. This statement is directly supported by the work of Haber and Wolff,ll who found that the properties of an explosion occurring in a fuel-mist air mixture are similar to the properties of a progressive homogeneous gaseous explosion. (3) Gas-Solid Heterogeneous Reaction A gas-solid reaction is a reaction occurring a t the interface of gaseous and solid phases. I n addition to reactions between gases and solids as dust explosions and explosions in a coal calorimeter bomb, the catalytic action of solid surfaces on explosions of gaseous mixtures is extremely important.12 Many reactions between gases are greatly catalyzed by solid surfaces, particularly when the surfaces are heated to high temperatures. ??le socalled “surface combustion” furnace is the extreme application of this principle to industrial furnaces. Many investigators have emphasized the important effect of hot surfaces on the combustion of explosive mixtures in internal combustion engines. I n general, the engine knock in a high-compression, multiple-cylinder engine may be largely stopped by properly cooling the suspected hightemperature areas.13 As the exhaust valve is .probably the 2. angeu. Chem., 36, 373 (1923). Sokal, J . Sac. Chem. I n d . , 43, 2831‘ (1924). 18 Holloway, Huebotter, and Young, J . SOC.Automofive Eng., 12, 111 (1923); Young and Holloway, Ibid., 14, 315 (1924); 16, 255 (1924); Horning, J . Sac. Automobile Eng., 14, 142 (1924). 11 12
Vol. 17. No. 12
most highly heated area within the combustion chamber, proper cooling of the exhaust valve or its complet,e elimination makes more difference than any other surface factor.l% Catalysis
Midgley and Boyd15 have shown that catalysts mixed with the liquid fuel used in internal combustion engines have very important effects on the explosion. These catalysts become intimately mixed with the fuel and uniformly distributed throughout the explosive mixture as vapor or at least as a fine mist. I n this condition as a part of the homogeneous mixture these catalysts influence all homogeneous reactions as well as heterogeneous reactions involving the explosive mixture. Surface catalysis of gaseous reactions makes the reaction heterogeneous. For this reason all gaseous reactions catalyzed by surfaces are included under heterogeneous reactions. As a large number of gaseous reactions are subject to contact catalysis, catalytic heterogeneous gaseous explosions are veryimportant, particularly in internal combustion engines. 14
15
Abell, J . SOL.Automotive Eng., 13, 301 (1923). THISJOURNAL, 14, 894 (1922).
Length of Visible Flame and Length of Flame Travel in Combustion of Powdered Coal By Henry Kreisinger COMBUSTION ENGINEERING CORP., NEWY o = , N. Y.
T
HE length of flame in the combustion of powdered coal’ is generally understood to be the length of the visible flame in the furnace. The visible flame is produced by the combustion of the volatile matter of coal; that is, it is the combustion of the gaseous combustible in the coal. T h e combustion of the gaseous combustible is made visible by the presence of small particles of incandescent carbon formed. by the breaking down of heavy hydrocarbons. Visible Flame
The length of the visible flame depends on the percentage. of the volatile matter in the coal, the composition of thevolatile matter, the amount of air supplied with the coal, and the rapidity of mixing of the coal and the air. Other factors being the same, coal with a high percentage. of volatile matter will give a longer visible flame than coals having a low percentage of volatile matter, and to the ordinary observer it appears that high-volatile coals require larger furnaces and longer time for complete combustion than coals having a low percentage of volatile matter. I n otherwords, high-volatile coals seem to be more difficult to burn in pulverized form than low-volatile coals. Thus, Illinois coal when burned in powdered form will give a long, dense. flame, which may be somewhat smoky. Pocahontas and Kern River coals will give a bright, intensely hot flame, which will appear considerably shorter than the flame from theIllinois coal. Texas and Korth Dakota coals will also give a long flame, which, however, may not be so long as the flame from the Illinois coal. The flame will be bright yellow and ordinarily will not be smoky. If a sample of the fine dust particles carried by the furnace gases is collected a t the outlet of the gases from the furnace,. the dust from the low-volatile matter coal will contain a higher percentage of unburned carbon than the dust from the high-
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I N D U S T R I A L A N D 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
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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 t o 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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