Smoke Limits of Bunsen Burner Ethylene-Air Flames Carbon deposits from smoke in gas appliances and engines are a serious problem. This article looks into the mechanism of flames giving off smoke JOSEPH GRUMER and MARGARET E. HARRIS Bureau of Mines, U. S. Department of the Interior, Pittsburgh, Pa.
Figure 1.
Smoking flames of ethylene-air Tube diameter 2.47 cm.
A
T h e smoke limit needs to be considered in designing gas-burning appliances, but no adequate treatment of the problem exists. Some yellow flames richer than the yellow-tip limit ( 4 ) burn clean in free air or enclosed in gas appliances; others do not. Spectroscopic studies of luminous hydrocarbonair flames have also been made ( 3 ) , but the chemical nature of the yellow emitters and their precursors has not been identified. I t is not known whether a given species and a specific mechanism of formation are common to all yellow flames or under what conditions the appearance of yellow results in the escape of carbonaceous material. Determination of smoke limits and chemical species composition should lead to basic information on the fuels and fuel-air ratios that can be used in gas appliances without evolving smoke. This study is primarily concerned with gas appliances but the information is applicable to other gaseous and liquid fuel burners.
Experimental Streams of ethylene and air were separately metered: mixed in suitable mixing chambers, and fed to long cylindrical glass tubes which served as burners. The streams were ignited and permitted to burn in free air, and smoke limits were determined visually. Hot gases leaving the flame were viewed in an evenly illuminated room against a black background. The observer looked over a shield which blocked off direct light from the luminous flame. A strong beam of light was directed crosswise to his line of sight. Particulate matter in the nonluminous hot gases leaving the flame was made visible by reflection of this beam into the eyes of the observer. This technique is highly sensitive and reproducible. The smoke limit is the point where smoke is first observed leaving the flame and is expressed in units of flow us. fuel-air composition of the premixed stream. The
570
Fraction af stoichiometric Flow, cm./sec. Type 1 flames, A, are farmed at low velocities and low primary aeration
procedure is similar to that previously employed ( 2 ) and differs from the more usual definition of a smoke limit (51, which is the minimum flow of pure fuel at which smoke is observed against a white background.
Discussion of Results
Smoke Limits. Two types of smoke limits were found. Figure 1,A: shows a Type 1 smoke limit flame, a short flame formed at low flow velocity and low primary aeration (including air), The smoke appears in a steady stream at the top of a yellow flame. These flames are fairly steady, frequently showing nonperiodic changes in height. This particular flame varied in height from 12.7 to about 10 cm. When the flame lengthened, its top broadened and changed from yellow to orange-black. Smoke issued from the top as a constant thin streak. In its shortened state the flame top was sharp and of the same yellow intensity as the body of the flame. h-o smoke could be seen a t such times. B shows a Type 2 smoke limit flame, which flickers greatly and resembles a turbulent diffusion flame. Shifting regions of orange-black tongues emitting smoke appear in the periphery. The over-all flame lengthens and shortens nonrhythmically-this particular flame changed from a height of about 35 cm. to about 28 cm. Smoke appeared from various parts of the flame as puffs which generally became visible some distance downstream from the flame and did not appear to be continuous with it. The height of this type of flame was very sensitive to minor changes in primary aeration. C shows a smoking ethylene-air flame considerably richer than the smoke limit.
INDUSTRIAL AND ENGINEERING CHEMISTRY
OY0
9.96 1.4
Type 2 flames, E, flicker and resemble a turbulent diffusion flame
B 2.44 -
71 .a
C 5.52
12.7
C.
Flame richer than smoke limit
Two flames appeared to the eye-a central one reaching from the port of the burner to the top of the flame, and a second hanging like an umbrella from the top of the flame and extending about halfway upstream toward the port. I t seemed that combustion !vas stopped at the borders of the central flame, perhaps secondary air was drafted into the combustion products, and the resultant mixture ignited. The over-all flame changed shape frequently and irregularly and was about 26 cm. tall. This flame illustrates the possibility of combustions ending and beginning under suitable circumstances. These two types of smoke limits were measured, using a range of flame port diameters from 0.25 to 2.47 cm. The maximum height of smoke limit flames was also measured. As indicated by the arrows, smoking flames occur to the right of each curve and yellow nonsmoking flames to the left (Figures 2 and 3). There is no e\,idence of a smoke limit characteristic of the fuel gas, as was observed in studies of yellow tipping ( 4 ) . As burner tube diameter and flow are increased, smoke limits tend to approach the constant yellow-tip limit, indicating a gradual transition from yellow tipping to smoking. and that the smoking tendency of a fuel can be represented as a fuel-air ratio that is fuel-richer than the constant yellow-tip limit of the fuel, shown in previous studies to be characteristic of the fuel and independent of the burner on Lvhich the fuel is being burned. I t is the leanest fuel-air mixture at which yellow can be observed anywhere in the flame burning in free air. The value for ethylene is F, = 1.88. The curves of flame height and of flow us. fuel-air composition at the smoke
50
4 Figure 2. Average flow velocity of smoke limit flames o f ethylene-air
I I
1
I
46
42
As burner tube diameter and flow increase, there i s a gradual transition from yellow tipping to smoking
38
34
y1
30 Y
I c
b
5
u, 26 c
13
a
Figure 3. Heights smoke limit flames o f ethyleneair
6
Type 1 limit flames show no change in height and nearly constant flow far a wide range of fuel-air ratios
limit show t\vo regions. The Type 1 limit flames, which include the pure diffusion flame of 1 0 0 ~ ofuel, show no change of flame height and nearly constant flow rate for a wide range of fuelair ratios. Large changes in the concentration of primary air and the presence of unlimited secondary air do not influence these limits. As the primary mixture is further aerated, t!iere is a transition to another type of limit flame where the limit is strongly dependent on the primary oxygen present. The transition point depends on the tube diameter and is at leaner mixtures for the larger tubes. This indicates that secondary air is being consumed by these flames. I n Figure 4. the time for a gas element a r the axis of the flame to flow from the port to the end of the flame is plotted as a function of composition. The slopes of the curves decrease lvith decreasing primary air. This indicates that the height of a Type 1 flame is not such that enough secondary air can diffuse into the flame for stoichiometric combustion. :"-
22
of
4
.-
~
--
~
8
_
18
14
10
6
1
1
Figure 5 bears more evidence that the availability of secondary air does not make for complete combustion in Type 1 flames. As the smoke limit is approached from the yellow flame side. the residence time of an axial element in the flame increases, indicating that time is being taken to gain secondary air to prevent smoking. However, in the region of smoking flames. time spent in the flame decreases as more smoke is produced. It appears that reaction is being stopped between secondary air that could be obtained by- flame lengthening. and combustibles. Chemical Composition of Burning Gases in Smoking Flames. Figure 6 shows flame structure obtained on a 2.47-cm. burner at conditions where the flash back and smoking flame regions intersect. A streak of smoke was observed issuing from the top of the flame,
I _
4
_
Figure 4. Residence time in smoke limit flames o f ethylene-air
3
5 7 9 GAS CONCENTRATION fRhCT!OY
I1
13
I 14
17
OF 5T3ICHlOMLTRlC
The flame in Figure 6>.4! was about 10 cm. long when it smoked and 7.5 cm. long Lvhen it did not. 'The entire flame was in free space above the mouth of the glass tube. A slight decrease in fuel flow caused an inner core of flame to separate and move upstream into the burner, as shown in B. For such flames, the upper section was on top of the tube mouth and the lower section was inside the tube. A4sthis separation progressed, the remaining flame on top of the burner in free air steadied and ceased smoking. The center core of the fire inside the tube showed on its upstream face a horizontal blue burning zone preceding the yellow. Soot was observed on the inside walls of the glass tube around the yellow zone. This indicates that the initial stages of smoking occur very close in space and time to the blue burning zone. l'he blue zone is thin, as are the primary cones of usual blue flames. Reaction times must be correspondingly short. The yellow zones are much more extended, showing slower reaction time. Furthermore, this sequence of the zones on a time scale indicates that the yellow
Enough secondary air cannot diffuse into flame for stoichiometric combustion
b Figure 5. Residence times for yellow and smoking ethylene-air flames
2 GAS CONCEhTRATICY
FRACTION OF STOICe1IOYETR
14
c
16
Availability of secondary air does not give complete combustion in Type 1 flames
3
I
1
1
I
5
7
9
11
0,CENTIMETERS PER SECOND VOL. 51, NO. 4
APRIL 1959
571
Figure 6. Smoking flames of ethyleneair at flash-back limit Tube diameter 2.47 cm. Fraction of stoichiometric Flow, cm./sec.
A 2.69 5.0
B 2.5 5.7
Initial stages of smoking occur very close to blue burning zone
zone reactions are fed by the products of the blue zone combustion. This is not the sequence of events in a yellow 100% fuel flame, where, along a stream tube, yellow combustion precedes blue combustion. Table I contains mass spectrometric analyses of samples withdrawn from regions within and adjacent to the enclosed flame, a t the axis or near the wall and at the plane of the blue flame base and at the yellow zone. Sampling was done by fine quartz capillaries introduced through ground-glass joint side arms on the tube. Oxyhydrocarbon compounds were not detected. Therefore these smoking flames have not been shown to involve reactions and products characteristic of cool flames. The major products of burning detected
are simple-hydrogen, carbon monoxide, carbon dioxide, and water. (Water was evaluated by striking a hydrogen and oxygen balance.) Acetylene and methane appear in concentrations up to roughly 2%. Other hydrocarbons are present in only trace amounts. There is no sign of a progressive build-up of high molecular weight materials. T h e species found are about the same as in samples taken from diffusion flames of methane (7) and propane (7, 6), but carbon dioxide-carbon monoxide ratios are less than unity in the enclosed premixed flames listed in Table I and larger than unity in diffusion flames (7, 6, 7). Oxygen availability as measured by the carbon dioxide-carbon monoxide ratio does not change the types of compounds in very rich hydrocarbon-air flames. The submergence within the tube of the blue and yellow zones excludes secondary air from these two regions. Therefore the loss of gas phase carbon and presumably the formation of solid carbonaceous material in the flame can be followed by the carbon-nitrogen ratio in the flaming gases. A decrease of about 270 in the ratio was observed with the downstream distance in the sequence A-C, indicating some smoke formation. These analyses cannot be used to identify the precursors of solid carbon, beyond noting that the precursors must be single carbon compounds. I n the first three columns of Table I, the carbonnitrogen ratio and the per cent ethylene in air remaining in the products are higher in the flame gases than in the gases fed to the flame. Such discrepancies have also been observed with rich propane-air flames ( 7 ) . T h e first and last columns indicate a radial transfer of gaseous carbon compounds. These differences can be explained on the basis
of diffusion across the flame zone, along concentration gradients. For example, ethylene from the unburned gas will diffuse toward the burned gas because its concentration is lower there, and carbon dioxide and monoxide will diffuse toward the unburned gas. The nitrogen content of both flame and unburned gas is high; so there is no high concentration gradient for nitrogen. An exchange of this sort will enrich the carbon content of the flame gases, as ethylene is a twocarbon molecule and carbon dioxide is the slowest diffusing of the four molecules mentioned.
Conclusions Smoke limits characteristic of the fuel gas and independent of burner diameter and flow rate do not exist. No smoke limits have been found which are leaner than the constant yellow-tip limit for the fuel. The latter is characteristic of the fuel oxidant system only. There are two types of smoke limit flames: the richer primarily a function of flow rate, the leaner primarily a function of fuel-air ratio. The availability of unlimited ambient secondary air does not prevent escape of smoke from flames. Gases within smoking ethylene-air flames do not contain significant quantities of oxygenated or high molecular weight hydrocarbons.
Acknowledgment The authors are indebted to Joseph M. Singer for help with the chemical sampling studies.
Nomenclature
F, Table 1.
h
Compositions of Ethylene-Air Flame
D
Failure to detect oxyhydrocarbon compounds indicates no relation to cool flames
(Enclosed in 2.5-cm. i.d. tube; 15.9% ethylene) 0.48 Cm. from Wall rldjacent t o Yellow, 1 cm. Top of yellow, Blue above blue 3 cm. above blue Argon Nitrogen Oxygen Hydrogen Carbon monoxide Carbon dioxide Methane Ethylene Acetylene Methylacetylene Benzene Diacetylene Ethane or formaldehyde
0.7 61.5 11.1 5.7 7.4 0.90 0.8 10.7 1.0 0.04 0.0 0.01 0.1
0.7 60.9 0.65 11.0 18.5 2.5
coz/co
0.12 0.119 0.015 0.092 0.478 0.530 16.8
0.135 0.304 0.041 0.181 0.478 0.508 35.4
CO/Nz C02/Nz HdN2 (C/Ndorig. (C/Ndgaaeou produots (% CZH4 in air)produota
572
1.7 1.7 2.2 0.04 0.02 0.03 0.04
INDUSTRIAL AND ENGINEERING CHEMISTRY
0.7 60.3 0.4 11.8 18.9 2.5 1.9 1.1 2.3 0.04 0.02 0.02 0.04 0.132 0.314 0.042 0.196 0.478 0.506 36.5
Probe a t Axis in Blue 0.8 61.9 10.1 6.6 8.5 1.0 0.8 8.8 1.2 0.05
Trace 0.02 0.1 0 . 118 0 . 136 0 . 016 0 . 106 0 . 478 0 . 493 15. 4
= fuel-gas concentration
for constant yellow-tip limit, fraction of stoichiometric = height,cm. = average flow velocity, cm. per second
literature Cited (1) Bur. Mines, Flame Research Labora-
tory, unpublished work. (2) Clark, T. P., Natl. Advisory Comm.
Aeronaut., Research Mem. RME52G24 (1952). (3) Gaydon, A. G., Wolfhard, H. G., “Flames, Their Structure, Radiation and Temperature,” Chapman & Hall, London,*l953. . (4) Grumer, J., Harris, M. E., Rowe, V. R.. Bur. Mines Rept. Invest. 5225 (1956). (5) Schalla, R. L., McDonald, G. E., TND. ENG.CHEM.45, 1497 (1953). (6) Smith, S. R., private correspondence. ( 7 ) Smith, S. R., Gordon, A. S., J . Phys. Chem. 60, 759 (1956). RECEIVED for review June 12, 1958 ACCEPTED December 8, 1958 Research supported by the American Gas Association, Project PDC-3-GU.
