ADAPTATION OF THE BUNSEN BURNER TO NATURAL GAS G . Ross ROBERTSON, UNIVERSIN OP CALIFORNIA AT LOSANGELES
The recent advent of uatural gas into American laboratories has emphasized the fact that the classical Bunsen burner i s unsuited to use with parafin hydrocarbons. A large percentage of free hydrogen i s required i n the fuel if a hot, stable flume i s to be obtained. Manufacturers are offering new burners adapted to natural gas. In these the upper tube, or stack, is enlarged and perhaps modified in shape; the gas orifice is reduced or its regulation improved; air-ports are enlarged, and denrices are attached to the mouth, or port, to prevent blowout of a hydrocarbon.tlame. . . . . .
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When the city of Heidelberg in 1855 adopted the new-fangled improvement of illuminating gas, Robert Wilhelm Bunsen was implediately confronted with the problem of using it in the university laboratory. One of his British students' offered the eminent director the awkward Argand lamp which London was already using to heat flaqks and retorts. Bunsen, profoundly disgusted with the performance of this device, proceeded forthwith to bore holes in the side of a metal tube, and then passed the new fuel through the tube in such a manner that air was drawn into the gaseous stream. Result-the Bunsen burner, an invention which has given Bunsen more widespread fame than all the rest of his important researches. In its day the Bunsen burner was rightly classed as an important invention, and it immediately afforded its inventor an opportunity to conduct investigations2 of flame reactions. Nowadays, however, the designer of gas burners realizes that Bunsen had a relati9ely well-behaved fuel to deal with in common coal gas. It was easy to adjust the burner to give a flame of high effi~iency.~ Histoty now repeats itself after a fashion, seventy-five years later, in the recent widespread invasion of natural gas into American laboratories. To be sure, the native fuel has had limited use, both domestic and industrial, for decades; but not until 1930 and 1931 did this rich new gas reach a sufficient number of chemical laboratories to raise a real chorus of complaint, audible in Chicago and Pittsburgh. Manufacturers of burners have now listened, and peculiar-looking devices are being offered to us. Bunsen Burner Inadequate Every chemist whose fuel service has shifted to natural gas has had occasion to regard his Bunsen burner with the same disgust which Bunsen expressed over the Argand lamp. A "soft" blue or even yellow flame replaces the hot hissing flame with the green inner cone. A change to the 'ROSCOE, J. Chem. Soc.. 77, 547 (1900). BUNSEN,Ann.. 138,257 (1866) THORPE, J C h m . SOL 31,627 (1877). 1963
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NOVEMBER, 1932
JOURNAL OF CHEMICAL EDUCATION
more modem Tirrill-type burner, with its wider air-mixing chamber and adjustable needle-valve, affords but slight relief. To be sure, the Tirrilltype burner usually eliminates yellowness of flame. To 'accomplish anything more, one must increase the gas pressure, screw in the needlevalve and open the air-ports wide-whereupon the flame promptly blows itself out. Chemical Composition of Gas The following typical analyses of commercial fuel gases reveal the principal cause of the difficulty: Hydrogen Carbon monoxide Methane Ethane Propane iso-Butane %-Butane iso-Pentane, etc. Ethylene Benzene Carbon dioxide
Nitrogen Oxygen Density (air = 1) B. t. u. value
1
Cool Gar
Cnr6urdrd Walrr Go,
57.9% 6.15
35.5% 32.0
26.97
.. .. .. ..
0.4 0.98 3.15 3.53 0.85 0.38 527
Oil Gar
1
] 12.7 .. ..
51.08% 10.43 . 27.5
.. ..
..
.. ..
..
90 1.7 4.2 4.7 0.2 0.65 550
1.8 0.15 3.75 4.30 0.35 0.43 505
DIY Nofulol Gas
0.00 0.1 a9 7 2 0.15 0.1
..
0.00 0.00 0.2 0.5 0.25 0.63 1100
C~lilindc? Gar
0.00 0.00 0.00
trace 28.7 25.3 46 trace 0.00 0.00 0.00 0.00 0.00 1.86 3037
C
The oil gas described above is the fuel made by cracking petroleum on hot bricks, and was the principal domestic gas of pre-war days in California. At present it is used for little more than a standby reserve in the major centers, the 1100 B. t. u. natural gas having supplanted it generally. The figures given for "dry natural gas" are rounded-offvalues, subject to slight variation, for the standard city fuel dispensed by several companies in the Los Angeles district. I t is the residue of ordinary natural gas from which absorption gasoline and "cylinder gas" have been stripped. The cylinder gas is a liquefied fuel now being dispensed by a leading western utility in small towns distant from the major gas lines. Its boiling range is -17' to -l°C., and its vapor pressure at room temperature of the order of four atmospheres. Free Hydrogen
It is the presence of free hydrogen in large amount in artificial gases which makes the old-fashioned Bunsen burner usable. Hydrogen burns rapidly in a highly restricted space. Its velocity of flame propagation is roughly ten times that of a paraffin. Even the 50'% mixtures show more than three times the velocity of the hydrocarbon flame. Furthermore,
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1965
in the presence of hydrogen the carbon monoxide of water gas seems to burn more rapidly than it does in the pure state. Accordingly there is little danger of spontaneous blowout of a flame if the fuel has a t least 30% free hydrogen. The downward rush of flame toward the port or mouth of the burner is much faster than the upward current of gas and air. As a result of the great speed of combustion of a hydrogen mixture, the flame is concentrated in a small space and thus its temperature is higher. The resulting temperature advantage is of particular interest to the glassworker who is not provided with cylinder oxygen. Beinr of low fuel value uer cubic.foot,.hydrogen reauires but little air for . combustion. Accordingly Bunseu found no difficulty in providing adequate air intake facilities. A couple of small air-ports were s&icient. Actually there was more concern about the strike-back of the flame than of blowout.
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Burner Design
I
In the conventional burner design, as illustrated in Figure 1, the usefulness of the "stack," or vertical tube, and the airports is obvious even to the laboratory beginner. Surpris' ingly few students, and by no means all instructors, realize the signscance of the gas or$ce, which directs a small jet of fuel a t high velocity into the air-mixing chamber. Without this orifice the desired blue flame is not readily produced; while the FIGrm LABORAhot blue flame with the green cone, TORY BURNBRDESIGN so much . prized for hi~h-temperature . work, is quite unattainable. This information might well appear in manuals of general chemistry, and would serve to clear up some of the mystery as to "poor burners." The standard Bunsen burner has a relatively large gas orifice, small air-ports, and a small ('/le-inch) stack. All three of these specifications are out of line for natural gas. The supposedly obvious remedies lead to trouble, however. Since the air-gas mixture is too rich, one may reduce the size of the orifice, leaving air-portsopen a t full. Unfortunately, this causes a serious loss in primary air injection. When a fine stream of fuel gas emerges
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JOURNAL OF CHEMICAL EDUCATION
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under pressure from a minute orifice past a ring of air-ports into an airmixing chamber, the proportion of air sucked in is subject to simple calculation. The amount of air captured, per unit of heat value used, varies inversely as the B. t. u. rating of the gas, and directly as the square root of the gas density. For example, let us compare natural gas of 1200 B. t. u. value, density 0.68, and artificial gas, 550 B. t. u. and density 0.42:
This calculation tells us that the natural gas will draw in only 58.2% of the air which would have entkred with the equivalent (in heat value) of artificial gas. Theoretical Remedies Such a result at once leads to three propositions:~(a) increase the gas
pressure in order to encourage a vigorous air draft; (b) reduce the orifice size so as to keep the mixture lean; and (c) enlarge the air-ports. By these modifications one may raise the.percentage air intake to the desired value. When all this is done, the resulting flame would probably be wonderful if it did not promptly blow itself out. The gas current in the stack is too rapid for the downward propagation of the flame. Venturi Tube A partial remedy for the above troubles is found in the classical Venturi tube, represented in Figure 2(A). The U. S. Bureau of Standards4 has 'BERRYam Co-WOXERS,U.S. Bureau of Standards Technologic Paper No. 193 (1921); No. 222 (1922).
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ADAPTATION OF BUNSEN BURNER
1987
shown that a constriction of the stack near its lower end to 43% of the port or mouth diameter, combined with the proper taper of tube, gives maximum efficiency and a concentrated, hot blue flame. Unfortunately, the cost of manufacture of such a stack is too high for the economics of common student burners, although the idea is carried out in a very few models for special purposes. The necessity for multiple openings a t the top of such a burner adds further to its cost. The device is of course to he recommended where the customer can afford its use. Economical Design The simplest and one of the most effective single remedies for Bunsen burner troubles is a mere increase in size of stack, still preserving cylindrical form. Roughly this means a 30 to 125% enlargement.in cross-sectional area, as seen in recent commercial burners. Three valuable rezults ensue-first the gas and air have a much greater space in which to mix: secondly the current is slowed down; and finally the resulting flame is shorter, wider, and therefore more efficient by direct impact. Manufacturers of the new natural gas burners differ slightly in preference of size, using tubing ranging from to inch outside diameter. Prevention of Blowout Even the increase of stack diameter still leaves the natural gas flame somewhat unstable. A sudden draft, or a turn-up of the gas, and the flame is extinguished. To remedy this'nuisance i t is necessary to break up the stream of gas a t the port or place ofractual flame, and to reduce the velocity of a t least a part of the stream a t that place. In the common kitchen stove the system of multiple holes in a wide iron top is satisfactory in view of the fact that all of the vessels to be heated are of large size. Such a scheme is too expensive for the laboratory, and particularly unsuited to the heating of test tubes and crucibles. The Bunsen burner port may then be castellated and slightly constricted, as shown in Figure 2(B). The peripheral gas current is retarded, and there tends to be a circular pilot flame fixed a t the port opening where i t belongs. The "flame intensifier" (C) shows this idea carried out to an extreme. Such a device effectively stops blowout, gives a wide "rosebud effect, and is efEcient in service with wider vessels such as beakers and porcelain dishes. With test tubes and small distilling flasks it meets criticism by reason of its breadth of flame. The well-known Meker-type grid gives a similar retardation of gas current and effectively stops the blowout. It is interesting to observe that the device was designed not for this purpose, hut rather to prevent exactly the opposite difficulty, viz., the downward strike of a coal-gas flamerich in hydrogen.
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JOURNAL OF CHEMICAL EDUCATION
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For the many who prefer a simple torch-like Bunsen flame, the sleeve and baffle system (D) or the hexagonal insert (E) solves the blowout problem. In both of these devices, as in the castellated tip, a portion of the gas-air current is retarded until its velocity is insacient to permit the lifting of the flame from the burner port. In the sleeve device the retardation is accomplished by the deflection of a small fraction of the fuel through four holes into the annular space, where it has plenty of time to escape and burns with stability. In the stack equipped with hexagonal insert the outer passageways are so narrow, as in the Meker grid, that the current velocity is reduced to a satisfactory degree and the flame is stable. Both devices operate well, and involve no great manufacturing expense. It should be noted that patent applications have been filed for these devices. Orifice Problem In the old Bunsen burner the gas orifice is a simple aEair. I t is virtually a miniature fishtail outlet, flattened only partially to give the rather coarse opening suited to coal gas. Simple tinkering with a penknife or a hammer is adequate for adjustment of opening. Such crude construction was preferred to the drilled orifice largely because of the fact that adjustment did not require the services of a machine shop. When adapted to natural gas the fishtail orifice is necessarily very narrow, and thus unsatisfactory. The slightest corrosion or entrance of foreign solid matter throws the adjustment out. Accordingly a drilled orifice running from 72 (wire-drill size)'up to 65 or larger is used in the new models. As suggested in Figure 1 such an ofifice should be drilled out from below with a large drill to a point near the tip. The final distance is run through with the fine drill. In this way the pressure of gas at the point of escape is not diminished by friction in a long passageway. Such construction is of particular value in districts where the gas pressure rnns below five inches of water. In certain very recent models the orifice is displaced to the side wall of the stack. This not only permits a convenient air supply from below, but serves as a protection from troublesome foreign matter, such as molten sulfur, which often falls into burners. Irregular Gas Service
All would now be lovely if the gas company would only serve gas of uniform B. t. u. value, summer and winter. The ideal is nearly attained in a city such as Los Angeles with a happy climate and vast petroleum resources. Farther east, where the thermometer is free upon occasion to fall below zero, uniformity is almost impossible. Standby gas plants must be drafted into service, and winter gas becomes different from summer gas. So arises the popularity of the Tirril-type burner, familiar in most laboratories.
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ADAPTATION OF BUNSEN BURNER
1969
In the past some dissatisfaction has arisen from two defects in the Tirrill. Faulty centering of the needle point causes misdirection of the h e gas stream into the stack. The percentage air intake falls, and the flame becomes soft and ineffective. Many old burners now in service have this failing without much hope of remedy consistent with economy of repair. Secondly, faulty threading, which may after all be the cause of the aforementioned difficulty, also permits leakage. To remedy such a situation certain manufacturers favor the use of a packing gland, or nut with soft washer, in the base. Other manufacturers, unwilling to add this expense to the burner, prefer to confine attention to accurate threading and avoid leaks in a simpler manner. It is quite possible that all of the present-day Tirrill-type burners have a much larger needle than is desirable with natural gas. Certamly any of them has a much larger gas capacity than is warranted by any ordinary use of the device. Furthermore, in view of the fact that corrosion of the brass needle and valve seat reduces the effectiveness of the natural gas flame markedly, there should be profitable research in the application of more resistant metals to this situation. While the foregoing account covers the principal patterns of conventional burners, there still remains a variety of novel commercial devices, many not yet well tried out by the trade. This includes burners with stacks widely separated from orifice, and burners with no stack at all-merely a top grid or mantle. Cylinder G'as
*
Public utilities are discovering that it is more economical to distribute propane and butane in small towns than to build and operate local gasmanufacturing plants. Undoubtedly a small demand will arise for laboratory burners in such regions, remote from metropolitan gas lines. The commercial cylinder gas described in the analytical table above is highly unsuited to the common Bunsen burner. It gives but little better satisfaction in the ordinary Tirrill burner, or the fixed-orifice natural gas burner adapted to 1100-B. t. u. gas. The large-stack Tirrill-type with wide air-ports and antiblowout device gives fairly good flames with the 3000-B. t. u. gas up to a flame height of about four inches. If the flame be turned up to higher volume it becomes slightly yellow. It is likely that the ideal student burner of low cost, suited to propane or butane, is yet to be presented commercially. The Venturi- or Meker-type burner, when provided with ample air-ports, will just nicely take care of butane gas. Satisfactory burners of this type are already on the market. It is possible to obtain a single burner which will handle anything from low-grade water gas to straight butane with satisfactory results.
