December, 1925
INDUSTRIAL AND ENGINEERING CHElMIXTRY
1215
FLAME SYMPOSIUM Papers presented before the Section of Gas and Fuel Chemistry at the 69th Meeting of the American Chemical Society, 13altimore, M d . , April 8 to 10, 1925
Conditions Governing the Efficiency of Gas Burners
perhaps the earliest illustration of a primitive gas burner. 50argument is needed to prove the )Tastefulness from the standpoint of either light or heat from such a burner. The combustion, such as it is, is altogether supported by secondary By S. W. Parr air and that portion of the gas which does not have a fair UNIVERSITY OF ILLINOIS, U R B A N A , ILL. chance to burn floats away in the form of filaments of carbon, producing neither heat nor light, but only smudge. From such a burner it is a simple step to conceive of a This paper points out the inherent inefficiency of the ordinary gas burners where a large proportion of the air flattened pipe giving to the flame a greater area and conserequired for combustion is supplied a s secondary air. quently a better chance for the secondary air; and so we The advantages and difficulties of increasing the primary have the “bat’s wing” burner. After this in logical order air i n gas burners of the Bunsen type are pointed out, and would come the “fish tail,” with two jets so arranged as attention is called to the advantage, from the standpoint still further to increase the superficial area of the flame. of combustion, of raising the pressure of the gas. Then would come the “cock spur,” in which we can easily see the forerunner of the modern acetylene burner, which T IS interesting to follow the development of burners for still further multiplies the flame area. Following the “cock the utilization of manufactured gas as supplied to city spur” burner would naturally come the Argand with its mains. Any one undertaking a study of this sort a t the multiplication of jets, but still utilizing exclusively secondary present time must bear in mind, of course, that the purpose air for combustion. After this would come the Welsbach of a burner has shifted from that of the production of light burner, in which the Bunsen principle is utilized for the to one where the production of heat is the main consideration. production of heat without light, and the incandescent Indeed, the literature relating to burners and their efficiency mantle brought into service for the production of light as to have any specific meaning today would need to have an a secondary effect following the production of heat. We come now to a brief reference to the Bunsen burner, interchange of these words, using “heat” where we formerly would have used “light.” This may be very well illustrated presumably developed by Professor Bunsen some time about by a single reference. I n the British Gas Referee’s report 1850, and consequently in use for the most part by chemists to the Board of Trade for 1871 occurs a statement which, for upwards of seventy-five years. The main thing accomwith a change of words as suggested above, has striking plished in developing this burner was the ability to burn gas without making a smoky flame, but in the production of present-day significance. It would read: heat i t is probably the limiting illustration in the matter of Every improvement in the construction of gas burners is inefficiency. equivalent, in its economical effects, to the discovery of a method An explanation of this somewhat severe reference to the of cheapening the manufacture and supply of gas; for it enables the public to obtain more heat from the gas which they consume Bunsen burner is in order. The average city gas as a t present and pay for. By using good burners instead of bad ones, con- manufactured requires slightly more than 4 parts of air sumers may obtain from 30 t o 50 per cent more heat while their to 1 part of gas for complete combustion. A Bunsen burner gas bills remain the same. may be made to function in a very satisfactory mannerIt is the purpose of this paper to review very briefly the that is, without any luminosity in the flame-with a ratio burner situation as i t exists today and, if possible, to point as low as 1 part of air t o 4 parts of gas. S o w a flame of any out the factors involved, if we are to realize the greatest considerable volume produced by a Bunsen burner with such amount of effective heat for a given volume of gas burned. a ratio must utilize its remaining air in the form of secondary air in some way brought into contact with the surface of Increase of Primary Air the flame. Note that the volume of this secondary air is I n the first place, the basic statement is here proposed that, fifteen times as great as that of the air utilized by the burner in general, the highest efficiency in the combustion of gas is at its minimum ratio for the production of a nonluminous attained in direct proportion as we secure completeness of flame, and this without taking into account the still larger combustion from use of primary rather than secondary air. volume of entrained air which performs no other function It is only necessary, by way of illustration, to recall the various than that of diluting the heat and carrying i t o f f into space. methods whereby manufactured gas has been burned. T o Obviously, this large amount of entrained air is the natural illustrate, first, the opposite condition-namely, that of the result of the greatly extended flame area due to the use of a initial production of heat where only secondary air is em- low air ratio. It is evident, therefore, that if the precise ployed-the first burner was undoubtedly a tube with a volume needed for complete combustion could be sent up round opening. This is a fairly safe statement to make, even through the interior of the tube and mixed with the gas as if there were no other method of arriving at such a conclusion primary air, there would be a corresponding reduction in the than that of deductive reasoning, the theory being that con- superficial area of the flame. Hence, we would have no such duits were made before burners and that round holes in all loss by entrainment of excess air, and by thus centralizing probability preceded either square or narrow ones. A very the combustion within a small area we would be able to utilize old notebook sketch has recently been found which bears the to the highest degree the heat produced. name of Dixon with the date of 1750. Although i t is priProbably the question as to why so low a ratio of air to gas marily of interest as pushing the date for the discovery of the is used in the Bunsen burner is seldom asked in this conmanufacture of coal gas back some fifty years, it also affords nection. The answer is a simple one. At ordinary tempera-
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INDUSTRIAL AND ENGINEERING CHEMISTRY
1216
tures such a mixture is not combustible, except by accession of secondary air; it is not even explosive and could not be made to burn if it were in an inclosed space and subjected to an electric spark; hence, by maintaining such a ratio in the Bunsen burner we are free from any tendency on the part of the flame to “strike back.” The first notable departure from the conditions which are present in the Bunsen burner is found in the burners of the MBker type. Here under favorable conditions the primary air may be increased up to substantially the theoretical amount required for complete combustion of the gas passing through the burner tip. Increase i n Rate of Flow
But the high ratio of air thus secured produces an explosive mixture which requires special consideration in order to avoid the tendency to flash back and burn at the tip. The expedient employed as a remedy is the insertion in the gas flow of a grid having a total cross-sectional area for the open spaces sufficiently reduced so that the rate of travel forward of the gas mixture shall be greater than the rate of propagation of flame in the reverse direction. The speed of flame propagation in an explosive mixture has been made the subject of numerous investigations. I n the initial stage of flame propagation there is a period of uniform speed which precedes the explosive stage. This is designated in the literature as the period of “uniform movement.” I n studies by Wheeler,’ and later by Paymaq2 it is shown that the rate of flame propagation in the most highly explosive air-gas mixture is on the average about 1 meter per second. Comparing this value with the rate of flow of the gases in the larger part of a burner of the hlbker type, it is evident that conditions are present which would result in the striking back of the flame. For example, the speed of flow in meters per second may be represented by the formula:
Vol. 17, No. 12
extended application of the principle of full combustion by use of all air as primary air has not yet been attained. Increase in Gas Pressure
It will be a t once evident a t this point that the purpose of this paper must be merely to state the problem. By so doing, it sometimes happens that a service is rendered which results in at least aiding, if it does not directly lead to a solution of the matter. The chief factor involved is the variable and relatively low pressure of gas as supplied to city mains. The jet action or Venturi effect is of such a low order, and so easily obscured or obliterated, that it is exceedingly difficult to utilize in any broad and comprehensive manner. This difficulty is readily solved by industrial plants, where the volume of gas used warrants it, by inserting a booster in the system and bringing the gas to the burner a t a uniform pressure of several pounds per square inch. It is possible that some time, to facilitate a much higher distribution of gas through existing mains, city gas companies will want to carry pounds of pressure where they now carry inches. This method, using local pressure-control valves, would afford conditions where this particular problem could be readily solved. Any adequate appreciation of the fuel field now opening up to the gas manufacturers must lead directly, it would seem, t o a consideration of this matter of pressure as one that must be given a place from now on. Meanwhile, it may well be emphasized that, despite the handicaps, and even though the present number of burner designs is legion, it is an inviting field for the chemist and it may quite properly be said that the game is well worth the candle.
Chemical Equilibrium in Gases Exhausted b y Gasoline Engines
I,
R=
A X 3600 in which R = rate of flow in meters per second V = total volume in cubic meters of combined gases A = cross section of mixing chamber in square meters
With a mixing tube having a diameter of 30 mm. (approximately ll/s inch) the cross-sectional area A is 706.8 sq. mm. Assuming a burner tip delivery 0.3398 cubic meter (12 cubic feet) per hour and using an air-gas ratio of 4:1, the volume V mould be 5 X 0.3398 or 1.699 cubic meters (60 cubic feet). Substituting we have 1.699
R = 0.0007068 X 3600
By W. G. Love11 with T. A. Boyd GENERALMOTORS RESEARCH CORP., DAYTON, OHIO
Aside from the fact that it is complex, too little is known about the actual course of combustion in gasoline engines. The composition of the final combustion products, however, has been established by many investigators, and this information gives some insight into the character of the combustion reaction itself. With the purpose of getting further knowledge of the combustion reaction, which occurs so speedily in gasoline engines, the authors have closely examined a number of exhaust gas analyses, with some interesting results, as outlined i n this paper.
R = 0.667
That is to say, the rate of movement forward would be 0.667 meter (approximately 2.2 feet) per second. Hence the necessity of speeding up the rate of flow by some means in order to prevent the travel of the flame backward. These conditions, so simple to name, are not always easy to maintain. Any condition tending to impede the established rate of flow endangers the proper functioning of the flame. For example, a crucible or beaker placed too close to the area of discharge of the mixed gases may slow down the rate of flow t o such an extent that striking back occurs; or, the heating up of the grid through which the gases flow may act as a damper and thus slow down the rate of flow, while wide changes in gas pressure add other complications. I n fact, the conditions for successful operation are so limited that any J . Chem. SOC.(London), 106, Pt.11, 2606 (1914). s Ibid., 116, 1454 (1919). 1
ONSIDERABLE data on the composition of the gases
C
exhausted by automobile engines are available in the literature. Of these the most comprehensive and consistent analyses have been made by Fieldner, Straub, and Jones,l and by Fieldner and Jones.2 The results obtained by these investigators are especially valuable in that, besides being very complete, they cover a wide range of the usual variables, such as fuel-air ratio, speed, grade, engine design, and operating conditions. On account of these facts, the study reported in this paper is based entirely upon the results of Fieldner and his associates. From a consideration of these data the authors have found, first, that the components of the gas exhausted by gasoline engines are in such proportions that the calculated value of the equilibrium constant, K , of the water-gas reaction 1 8
THISJOURNAL, 13, 51 (1921). Ibid., 14, 594 (1922).
