JOSEPH GRUMER, MARGARET E. HARRIS, and VALERIA R. ROWE Flame Research Section, Division of Explosives Technology, Bureau of Mines, Pittsburgh, Pa.
Yellow-Tipping of Bunsen Burner Flames and Related Exchangeability of Fuels in Gas Utility Systems A method for predicting the exchangeability of fuel gases in
-
gas-distribution systems useful when gas utilities must replace fuels supplied to homes and industries with gas of different composition from that to which appliances have been adiusted
THE
Bureau of Mines, through cooperative agreement with the American Gas Association, is engaged in research on the combustion characteristics of gases and: to a lesser extent, on gasburner design. These investigations are designed to provide fundamental knowledge and are preliminary to the solution of practical problems. The present paper is one of a series of studies on the flash back, blowoff (7, 70), and yellowtipping (8) properties of gaseous fuels and the influence of gas appliances that utilize them. I t presents theoretical considerations and data summarizing the limiting conditions under which gas-burner flames are yellow. The yellow-tip limit is defined as the fuelair composition of the stream within the port for which yellow is just perceptible anywhere in the flame above the port. Yellow-tipping is not as serious a limitation in gas-burner operation as are flash back and blowoff. One cannot heat satisfactorily with a burner that is in flash back or in blowoff, but one can heat with a burner operating with yellow flames. Many such burners are used and often yellow flames are highly desirable for radiant heating. Where yellow-tipped flames are undesirable, it is usually because they deposit carbonaceous material which fouls heating surfaces above the burner and decreases heating efficiency. Under some circumstances, yellow-tipped flames also may give off aldehydes which are irritating, or carbon monoxide in concentrations exceeding safe limits. Therefore, in designing burners or exchanging gases on existing burners, it is important to avoid yellow flames that yield such objectionable results. Correspondingly, in designing burners and selecting fuels for given heating applications, it is important to understand the fundamental nature of the yellow-tipping phenomenon. The information presented here makes it possible to predict the yellowing or
2052
nonyellowing of flames for most fuels on single-port burners in free air at room temperature and pressure. Purther research is expected to relate the new body of information to the performance of present-day gas appliances. The latter very often contain multiport burners operating in free air or in enclosures, which may further complicate analysis of the over-all performance of the appliance. Experimental and Theoretical Figures 1 to 7 give flash back and blowoff curves, and yellow-tip limits on a variety of ports for methane, natural gas, propane, ethylene, propylene, benzene, and acetylene. Some of these flames are shown in Figure 8. The coordinates in Figures 1 to 7 are the fraction of stoichiometric-i.e., the volumetric gas percentage divided by the percent gas in a stoichiometric mixture of fuel with air-and the critical boundary velocity gradient-i.e., the rate of change of steam velocity at the edge of the stream mixture a t the exit plane of the burner port. For steady laminar flow through a round port this gradient is g = 4V/rR3, where Vis the volumetric flow and R is the port radius. The following observations are made from these figures and photographs.
1. The leanest limit for each fuel (in terms of fuel-air composition of the stream in the port) is independent of flow, burner diameter, and oxygen content of the secondary air. Corroborating evidence has been reported for benzene flames ( 2 )* 2. For a given flow, the limit is richer for narrow flames (small diameters) and becomes independent of diameter for wide flames (large diameters). 3. For a given diameter, the limit is richer for small flames (low flows) and becomes independent of flow for tall flames (high flows). 4. At the limit, yellow does not appear below or as part of the primary combustion zone. For many hydrocarbon
INDUSTRIAL AND ENGINEERING CHEMISTRY
mixtures, particularly of the propane type, the top of the primary cone is open. When a yellow ethylene flame was inverted, the blue-green primary combustion surface was clearly visible under the yellow in the burned gas. The same result was obtained when a yellow toluene-air flame and a yellow acetylene-air flame were inverted. (An inverted flame is one where the apex of the primary cone is the part of the flame nearest the plane of the port.) This trend has been observed with all the fuels listed in Tables I and I1 (8). The chemistry of carbon formation in flames has been the subject of many excellent studies (7, 3, 4, 76-79). In most instances, it has been suggested that the reaction involved is the pyrolysis of the parent hydrocarbon. However it is possible that this does not completely describe the essential mechanism. Yellow-tipped flames of different fuels have different physical appearances, suggesting a different chemistry for each case (see Figure 8). Moreover, the yellow in these flames is a secondary mantle phenomenon, with a blue flame zone appearing upstream of the yellow. The priority of blue combustion suggests that the yellow corresponds to a subsequent reaction of oxyhydrocarbon fragments rather than to the reaction of hydrocarbon fragments. I t would appear pertinent to probe yellow-tipped flames for temperature and chemical profiles before drawing firm conclusions on the chemistry of yellow formation in premixed flames. Corroborating evidence that yellowing of aerated flames is a secondary mantle Table I.
Constant Yellow-Tip
Limits
for Single-Component Fuels Fuel Methane Ethane Propane n-Butane Ethylene Propylene
F c , Exptl. 1.80 1.87 1.61 1.57 1.88 1.44
Fuel F c , Esptl. Isobutylene 1.40 Acetylene 2.10 Benzene 1.18 Toluene 1.34 Natural gas 1.78
Table II. Comparison of Experimental and Calculated Values of F, for Two-Component and Multicomponent Fuels Fuel Composition F,, Exptl. F,, Calcd. Two-Component Fuels 76.0% C2H4, 24.0% Hz 72,5% C2H4, 27.5% CH a 53.1% CzHa, 46.9% C3Hs 74.4% CzH4, 25.6% CaHs 90.0% C2H4, 10.0% CaHs 55.4% CaHs, 44.6% Hz 81.6% CaHs, 17.4% Hz,l.O% C3Hs
1.90
1.88
1.85
1.86
1.72
1.75
1.68
1.81
1.78 1.76
1.85 1.61
1.61
1.61
Multicomponent Fuels 70.1%
CaHs, 15.770 Hz,13.7% CO, 0.5% CQHR 37.4%- CH4, 33.4% CzH4, 15.2% Hz, 14.0% Nz 33.5% CH4, 30.1% CzHa, 13.4% Hz, 12.8% Nz, 10.2%
cog
1.60
1.61
1.90
1.84
1.88
1.84
1.76
1.77
1.90
1.84
29.1% CHI, 26.2% CzH4, 22.1% C3H8, 11.8%
Hz, 0.2%
CaH6, 10.6% Nz 32.1% CHI, 28.4% CzH4, 12.5% Hz, 27.0% Nz 42.6% CH4, 18.1% CzH4, 17.0% Hz, 9.1% co, 2.2% CzHs, 1.9% C3Hsl 0.2% C3HBt 0.2% C4Hin, 0.1% C4H8, 5.2% Coz,3.4% Nz 75.2% CHI, 22.2Yo C3Hs, 2.6% CzHs 62.1% CHI, 35.5'70 CaHs, 2.4% CzHz 74.2% CHI, 13.4% C3H6, 9.6% CaHs, 2.5% CzHs, 0.3%
co9
.
84.2% CH4, 7.6'70 CzHz, 5.3% CZH6, 1.6% CaHs, 0.6% C4Hin, 0.3% C&, 0.4% COz 91.6% CH4, 4.0% C7Hs, 3.2% CzHs, 0.7% CaHs, 0 2 % CaHs, 0.3% coz
08
12
16 20 24 28 32 36 40 GAS CONCENTRATION, FRACTION OF STOlCHlOMETRlC
44
48
52
Flame-characteristics diagram for 100% methane
60,000
1.80
1.82
1.76
1.76
1.71
1.73
40,000 30,000
20,000 10,000
8,000 1.66
1.74
7 6,000
23
8 4,000
X 3,000 I--
5 2,000
1.76
1.81
67.6% CH4, 26.8'7' C I H ~ . 2.3% CqHn.
coz
04
Figure 1 .
72.5% CHI, 15.9% CZH4, 7.7% Hz, 2.6% CzHe, 0.4% C3Ha, 02% CaH8, 0.2% C4Hin, 0.5%
con
0
s a
0
1.79
1.82
1,000
3
800
& a
400 300
>
2
600
3
1.77
1.82
s
200
E
loo
s 2
* 8 0
1.74
1.78
60 0
Q ,294 1914 Q 249
20 10 0
413
206
30
phenomenon is found in recent studies (72, 20). In particular, Street and Thomas (20) distinguished between fuels with the yellow obviously in the secondary mantle and those with the yellow directly atop the blue or bluegreen cone. Such differences were observed in this laboratory, but because
315
.
40
0.4
0.8
12
Figure 2.
1.6 20 2.4 2.8 3.2 36 40 GAS CONCENTRATION, FRACTION OF STOICHIOMETRIC
4.4
4.8
Flame-characteristics diagram for natural gas
89.5% CH4, 6.7% CzHe, 2.7% C3Hs, 0.4% C&3,0.4% VOL. 48, NO. 11
CaHio, 0.3% COP NOVEMBER 1956
2053
5.2
I
1
1
1
60,000
I
I
I
1
I I
I
40,000 I
30,000
~
r(
I
I
I
LEGEND
I
Tube diameter, cm. x 1.023 .891
0 0 A
.776
0
* -D
*
.699 ,611 .413 ,354 ,249
,195
Port
Figure 3.
Flame-characteristics diagram for
100,000
100,000 80,000 60,000 40,000 30.000
98.6% C3H8, 1.4% C3Hs
1
I
1
,
I
I
I
20,000
40,000 r(
I
30,000t1 -5
.624 354
10,000
7 8000 in
n 6000
?5
y
4,000
8 d
2,000
2
3000
u
g
9 2 & 8
*
300
zc
200
$ ,249
Diameter, cm. Depth, cm. o 0.952 0.635
CK
*
,776
Port dimensions, cm. 0.654 X 3.18 ,318 x 2.5 ,354x 1.28 ,196x 1.29
400
5 P
Tube diameter, cin.
1,000 800 600
A
100 80
0
4
,796 ,595 ,239
,635 ,635 ,635
60
40 30 20
10
Figure 5. Flame-characteristics diagram for 99.2% Figure 4.
2054
Flame-characteristics diagram for 1 00% ethylene
INDUSTRIAL AND ENGINEERING CHEMISTRY
0.4% C3Hs, 0.4% CzHe
C3H6,
the blue was observed upstream of the yellow on inverted flames, all yellow in flames is judged to be part of the secondary burning. Constant Yellow-Tip Limits. Observations 1 to 3 may be explained in terms of the diagram of an idealized yellow-tip limit flame (Figure 9). The yellow zone is a spot on the axis at a height, h, above the port. The flame is tall enough so that only radial diffusion of secondary air to the yellow spot is significant. The diameter, 2R*, is such that secondary air just fails to reach the axis at the plane of the yellow spot in the time the gas takes to flow from the port to that plane. Thus, by definition, the axial filament of the flame is isolated from all other gases and a t the axial filament in the yellow zone the ratio of atoms of (hydrogen plus carbon) over oxygen is the same a t the unburned and burned faces of the primary flame front and in the yellow zone. This means that we are dealing with known reactants-the feed mixture to the burner port. If the diameter of the port were increased still more, the isolated axial filament would increase in width and the yellow zone would increase in width and height. Should there exist for a given fuel a characteristic fuel-air mixture a t which yellow is just barely produced, this analysis indicates that it can be identified by looking for the yellow-tip limit that is independent of increasing tube diameter and increasing flow. Experimentally such limits, designated as constant yellowtip limits (F,), have been found for a wide variety of fuels and mixtures, as evidenced by Figures 1 to 7 and Tables I and 11. Table I gives the constant yellow-tip limits of 11 yellow-tipping single-component fuels, including those of primary interest to the gas industry. The constant yellow-tip limit of mixtures of these fuels and mixtures of these fuels with inerts, or hydrogen and carbon monoxide is obtained by taking a weighted average of the constant yellowtip limits of the components, as in the following equation:
100,000 80,000 60,000 40,000 30,000
I1
1
100,000 80,000 60,000
40,000 30,000
g
v)
z:
20,000
v)
g 10,000
3
8,000 6,000
I
I
I PI
\ \ I
I
4,000 3,000 2,000
1,000 800 600
Tube diameter, cm.
+ ..
400 300
200
I 100 0
Figure 7.
CH3COCH3
where coefficients n,, nu, etc., are decimal percentages of the yellow-tipping components in a fuel mixture and Zn = n, n,. The adequacy of Equation 1 is shown by the data in Table 11, where experimental and calculated values of F, are compared for a wide variety of mixtures. This rule is not expected to apply to mixtures in which the concentration of nonyellow-tipping components exceeds about 50 to 60% of the total components. I t is significant to note that the experimental constant yellow-tip limits for methane, ethane, propane, ethylene,
0.4
0.8
2.0
2.4
Flame-characteristics diagram for
97.3%
1.2
1.6
2.8
GAS CONCENTRATION, FRACTION OF STOICHIOMETRIC
3.2
C2H2, 2.7%
VOL. 48, NO. 1 1
NOVEMBER 1956
2055
Figure 8. A. 6.
Ethylene, round port Natural gas flame, round port
c. D.
and propylene are leaner than the fuelair composition at which free carbon is predicted thermodynamically as one of the combustion products. Experimental and calculated thermodynamic values are compared in Table 111. Street and Thomas (20) report "critical concentrations of air. required to suppress carbon formation," obtained in a Smithell separator for a large number of fuels, including ethylene, propane, propylene. isobutylene, acetylene. benzene, and toluene. The near agreement with data in Table I is noteworthy because of the drastically different experimental conditions ; however. the disagreement is in excess of experimental error. It is possible that a given fuel may have two slightly different characteristic limits. depending upon the presence or absence of secondary flame adjacent to the primary flame. Moreover, in the experiments reported by Street and Thomas, the flames are surrounded by cooling products of primary combustion which can diffuse into the unburned gas. Possible complicating factors in the Smithell mantle are convective currents, temperature gradients. the chemical change in the
20.56
Yellow-tipped aerated flames E. Propylene, slot port, front view F. Propylene, slot port, side view
Propane, round port Propylene, round port
burned gas as it cools. Concentration gradients across the flame front are high, and products of combustion diffusing toward the unburned gas would not all be consumed by the primary flame. Street and Thomas do not report the role of port diameter and flow. Nonconstant Yellow-Tip Limits. The preceding section considered yellow-tip limits for relatively large ports and flows. However the concept of a constant yellow-tip limit characteristic for each fuel can be used to correlate the nonconstant yellow-tip limits observable on smaller ports and for smaller flows as follows. The displacement of a diffusing molecule is given by the equation
where D' is the diffusion coefficient and t is the time available for diffusion. I n Figure 9 distance 2 is the width of the flame a t the plane of the yellow spot. As for large ports, this is roughly equal to the radius, R*,and the time, t , is the quotient of the height, h, of the yellow spot in the flame and the axial velocity, Uaz. We can write
INDUSTRIAL AND ENGINEERING CHEMISTRY
G.
Benzene, round port
R*@IN 2D'h/LTa,
(3)
V a z is related to the product of the radius and the boundary velocity gradient g-e.g., for steady laminar flow, U,, = g R j 2 . Therefore, Equation 3 can be written as: R*(3)
-
k D ' h jg
(4)
where k is a proportionality constant. Equations 3 and 4 show the following trends of the parameters governing the nonconstant )-ellow-tip limit (symbol F,) : 1. If R > R*,the yellowtip Emit must be unaffected by secondary air because, by definition of R*,air cannot reach the axis. As R increases beyond R*, the yellow zone grows from a point to a streak of appreciable width and height. If R < R*,secondary air does reach the axis and more fuel must be added to the burner stream to compensate for this excess air if yellow is to be observed. 2 . k is a function relating the velocity at the axis to the boundary velocity gradient or to the total flow. I t reflects changes in velocity profiles caused by changing port shape, depth, etc. For a given velocity profile, as in steady laminar flow, k is a constant.
3. D' should be approximately the same for large groupings of fuels. The hot gases through which secondary air diffuses to the axis are composed mainly of nitrogen, water, carbon dioxide, and carbon monoxide, with some hydrogen. I t is estimated that the temperatures of these hot gases (see Table 111) do not differ enough for large groups of fuels to affect the diffusion coefficient appreciably.
Incidentally, because Equation 3
according to
D' N V / r h
(5)
where V is the flow velocity in cubic centimeters per second, it should be possible to estimate D' for steady laminar flow from the flow and the height of the yellow spot in the flame a t the constant yellow-tip limit. Table IV contains such flow and height measurements for a fuel consisting of 76.0y0 ethylene and 24.0% hydrogen. Some extraneous flame dimensions are included for possible future applications. All of the reported flames measured 1.00 to 1.05 cm. at the base and 1.5 cm. a t their widest cross section. D' is shown by the data in Table IV to be between 3.3 and 5.2 sq. cm. per second. This experimental range may be compared with calculated diffusion coefficients (77)-namely, 3.1 sq. cm. per second a t 1200' K. and 4.9 sq. cm. per second a t 1600' K. The calculated values assume that the products of combustion of 0.1155 CzHc
+ 0.03650.177 Hz 4+ 0.671 Nz 0 2
are 0.1230 HzO
+ 0.2310 CO + 0.1445 Hz
+ 0.671 Nz,
and that the diffusion coefficient, D', for the products of combustion and oxygen are given by a linear weighted average of the diffusion coefficients of each product and oxygen. This comparison supports the applicability of Equation 2 to the yellow-tip limit and also suggests a new experimental method for a rough measurement of diffusion coefficients in flame gases. 4. h depends on the flow and on the average burning velocity, &, of the primary combustion cone of the yellowtipping flame. For a right circular conical flame,
is weakly dependent on the chemical composition of the fuel, approaching zero for very rich fuel-air mixtures. The fraction h/g of Equation 4 is nearly independent of flow, particularly for parabolic flow, where
This explains the approximate inde-
Table 111.
Comparison of Experimental and Thermodynamic Values of Constant Yellow-Tip Limits" Initial Constant Yellow-T i p Limits, F , Temp.,
' K.
300 298 523 523
exptl. calcd. exptl. calcd.
CHa 1.80 2.7
C2Hs 1.87 2.84
CaHs 1.61 2.87
2.87
2.98
2.99
...
..*
...
CzHa 1.88 2.66
..* ...
Thermodynamic Flame Temperatures, K .
... ...
C~HB 1.44 2.75 1.47
...
...
298 970 1030 1050 523 1000 1090 1150 Calculations made by S. R. Brinkley, Jr., R. W. Smith, Jr., and H. E. Edwards, Mathematical and Theoretical Physics Section, Bureau of Mines.
pendence of flow and yellow-tip limits of all tall flames of a fuel (see Figures 1 eo 7). Because D' and Fu are weakly dependent on the chemical composition of the fuel, it is likely that chemical composition can be used as the basis for preliminary grouping of fuels. Three such groups have been tested, one containing mixtures of methane, ethane, propane, propylene, butane, and isobutylene; a second containing mixtures of the last five fuels with ethylene; and a third containing mixtures of methane and ethylene. These mixtures are of practical interest to the gas utilities.
5. If g (or Uaz)is low enough, the flame height above the yellow zone may be of the order of R*. This height is y of Figure 9 and not h of Equation 4. Secondary air can reach the yellow zone as readily from the top of the flame as from the side, and because in this case the amount of secondary air at the yellow is increased, F,/F, < 1. Organization of Yellow-Tip Limits. The basic tendency of the fuel to yellowtip is expressed by its value of F,. The yellow-tip fraction, F,/F, ( F , being the nonconstant yellow-tip limits for small ports and flows), is acceptable for comparison of all yellow-tip limits of all fuels. Within each group, four parameters need be considered-the fuel composition, the ratio of fuel and air that is being supplied to the burner port, the port diameter, and the flow rate. These four parameters may be organized graphically as follows: Yellow-tip limits, such as those for methane and natural gas (Figures 1 and 2) are obtained for a wide range of ports and flows. The points plotted in the two graphs are functions of F,, the yellowtip limit, and g,, the critical boundary velocity gradient. For each port diameter, yellow-tipped flames occur to the right of the respective curve. (This type of yellow-tip limit plot has two disadvantages. I t is difficult to interpolate between port diameters; and there is no apparent means for extending data obtained with one fuel to a new and untested fuel.) Starting with such
.
I
.
graphs, values of g, are read for values of
F, selected so that the yellow-tip fraction F J F , is equal to 1.0, 0.95, 0.90, 0.85, etc. Taking g, as the ordinate and the port diameter as the abscissa, curves of constant F , / F , are plotted for each fuel. From these intermediate curves, which are omitted from this paper, a new set of graphs is prepared (see Figure lo), in which the ordinate is the port diameter and the abscissa is the fuel composition, expressed as ratios of the constituents of the fuel. The flow rate, expressed as the critical boundary velocity gradient, is fixed as a selected value for each graph in a set. Enough graphs are included in a set to cover an adequate range of flows. A family of curves is drawn in each graph. Each curve is the locus of points of constant F J F , , for the fixed selected flow, covering the range
I
Figure 9. Schematic yellow-tip limit flame for critical port radius, R* VOL. 48, NO. 1 1
NOVEMBER 1956
2057
of port diameters and fuel consumption. Figure 10 covers fuels made up of mixtures of methane and the propane group, as well as mixtures of the propane group with ethylene. The propane group is an average of ethane, propane, propylene, butane, and isobutylene. A similar set of graphs for fuels made up of mixtures of methane and ethylene, including oil gases, is shown in Figure 11, This graphical method includes the four variables affecting nonconstant yellow-tip limits-fuel composition, fuelair composition, port diameter, and flow. The resulting graphs are called composite yellow-tip fraction diagrams. The use of these composite yellow-tip fraction diagrams will be illustrated by calculations of yellow-tip limits of a fuel composed of a mixture of natural gas and propane (62.170 methane, 35.57, propane, and 2.4y0 ethane).
Table IV.
Dimensions of Constant Yellow-Tip Limit Flames of 76.0ajC C2H~-24.07~ Hz Fuel Flume Height, Cm. Difusion Flow, V , Fraction of To t o p of To t i p Coeff., D', Cc./Sec. Stoichiometric yellow none, h of blue cone Total Sq. Cm./Sec.
Tube Diam., Cm. 0.776
111.4 89.9 65.3 43.2 111.4 90.1 65.3 43.3 111.4 90.0 65.3 43.3
0.891
1.023
1.90 1.91 1.93 1.95 1.90 1.91 1.93 1.94 1.90 1.91 1.93 1.94
7.40 6.70 4.90 3.33 7.00 5.60 4.54 3.00 7.00 5.60 4.54 9.00
are obtained in the same way for the 0.413- and 0.249-cm. tubes. The constant yellow-tip limit of the fuel, F,,is calculated using Table I and Equation 1 :
Chemical analysis places the yellow-tip limits of this fuel in the class of yellowtipping gases called methane-propane group-ethylene fuels (Figure 10). The fuel is located on each of the composite diagrams through its methane-propane group ratio, or its propane groupmethane ratio (the fuel composition ratio has been adjusted between 0 and 1). In the present case, the fuel composition ratio is propane group-methane = 37.9/62.1 = 0.61. Calculations are made for port diameters of 0.776, 0.413, and 0.249 cm. Figure 10 shows that for an abscissa of 0.61 and an ordinate of 0.776, the value for F,/F, is about 0.75. This reading is noted in column 1 of Table V. The flow for this point is given in the legend of Figure 10 and is found in column 2 of Table V. Values of F J F , and g,
+
1 [(0.621 X 1.8) (0.355 X 1 .o 1.61) (0.024 X 1.87)] = 1.73
-
+-
The experimental value of F, is 1.71, which was used in constructing Table V and Figure 12. The calculated value of 1.73 could have been used equally well. Dividing F, by F,/F, (Table V, column l), the values of F, in column 3 are obtained. Values of g, (column 2) us. F, (column 3) for each port diameter give the curves of Figure 12. These calculated curves are in good agreement with the experimental points of the same figure. A diagram such as Figure 12 may be converted into units of per cent primary air us. B.t.u./hour-sq. inch, as has been explained (5).
31 29 27
25 gq=300. g -Critical boundary velocity
2.3
'-gradient, seconds-] F=Gas concentration, fraction of stoichiometric. F,=Constant vellow.tio limit.
2.1
5
19
d
2 17
i!
15
m 13 11 9
7
5 3 1
.
1
__ C3-He GRObP CHI
1
CHa C,H8 GROUP
1
-
C2Ha C3Hg GROUP
~
C3H8 GROUP
Figure 10. Yellow-tip fractions for methane-propane group-ethylene for g, = 300 Propane group i s average of ethane, propane, propylene, n-butane, and isobutylene. diagram
2058
INDUSTRIAL AND ENGINEERING
CHEMISTRY
1
C2H4
fuels Composite
5.21 4.21 3.36 2.40 4.89 4.02 3.35 2.23 4.92 3.77 3.37 2.21
16.4 14.0 10.5 7.7 13.0 11.3 9.5 7.5 13,O 11.3 9.5 7.5
4.8 3.3 4.2 4.1 5.2 5.1 4.6 4.6 4.9 5.1 4.5 4.7
Influence of Burner-Design Parameters on Yellow-Tipping. PORT DEPTH. Virtually identical yellow-tip limits were obtained on round ports of '/c-inch length and on long ports of a t least 60 diameters length. INITIAL TEMPERATURE. The initial temperature of the unburned gas was observed to have a very minor effect on yellow-tip limits, as indicated by tests with propylene, where the constant yellow-tip limit was observed to be invariant with preheat, within experimental error (1.44 to 1.47, between 300' and 523' K.). The nonconstant yellow-tip limits for varying port diameter and flow were found to vary slightly with the initial temperature. The change was attributed to the observed warming of secondary air by th.e burner. It seems permissible to conclude for practical purposes That yellowtip limits are independent of the initial temperature of the burner stream, provided that temperatures are low enough and the flows rapid enough to preclude chemical reaction within the burner. This conclusion is corroborated by the experiments of Street and Thomas (20) for propane, propylene, benzene, and kerosine. They observed that increasing the temperature up to 773' K. slightly reduced the critical air-fuel ratio for the suppression of yellow in flames. Clark ( 2 ) noted an appreciable lowering of air-fuel ratio for yellowing of preheated benzene flames but did not evaluate the ambient air temperature. NONCIRCULAR CIIANXELS.Some comparisons were made between yellow-tip limits on rectangular, square, and triangular channels, and those on circular channels. With respect to the constant yellow-tip limits, results on noncircular channels were almost identical to those on circular ones. The constant yellow-tip limit of the fuel was generally observed if one side of the channel was longer than R". When in addition the short side was much smaller than R*, scant data show that the port behaved as a tube of diameter equal to LTVO to three times the short side.
This was attributed to the relatively limited availability of ambient air. O n a circular channel, the yellow spot in a flame receives diffusing secondary air along radial paths equally from all points on the circumference of the secondary mantle. For the above type of noncircular channel, secondary air can reach the yellow spot only from a small region along the center of the long sides of the channel. MULTIPORT BURNERS.No tests were conducted with such burners. However, it is apparent that, if the multiports are far enough apart, they will behave as single ports. If the ports are closer, the flames will more or less coalesce. The system will tend to behave as a single large port and probably show constant yellow-tip limit of fuel. Predicting Exchangeability of Fuel Gases in Gas Distribution Systems with Reference to Occurrence of Yellow-lipping Gas utilities often must increase or replace their send-out fuels to meet the demands of homes and industries. If the chemical composition of the new send-out gas differs from the composition on which the community gas appliances have been adjusted, it is desirable to predict how the burners will function on the new supply.
The Bureau of Mines has developed a theoretical method of evaluating the possibility of flash back and blowoff difficulties (6, 9, 73) based on burners with upright ports in free air at room temperature and pressure. Research is in progress to ascertain the applicability of the concepts to burners operating under practical conditions. T o date, the method has been successful in every trial and has been tested in several instances against known experience of gas utilities. The present paper extends the method to predict the exchangeability of fuels with respect to yellowtipping and also reports the results and comparisons between predicted performance and the performance reported by gas utilities. Tests of methods of predicting exchangeability of fuels are not easily obtained. There exists no trustworthy scaling procedure for extending results with a dozen or a hundred gas burners, checked in a laboratory under known conditions, to many millions or thousands of burners operating on a gas utility’s lines with unknown adjustment and environment. As a consequence it seems that only comparisons between prediction and actual experience of a gas utility under clearly defined operating conditions are meaningful tests of any method of predicting exchangeability of fuels on utility lines. 20,000,
I
Table
Sample Calculations of Yellow-Tip Curves Fc = 1.71
Tube Diameter, Cm.
FJFv
0,
F,
0.776
0.75 0.89 0.97 0.98 0.98 0.98
300 800 3,000 10,000 20,000 40,000
2.28 1.92 1.76 1.75 1.75 1.75
0.413
0.58 0.79 0.85 0.86 0.86
800 3,000 10,000 20,000 40,000
2.95 2.17 2.01 1.99 1.99
0.249
0.56 0.69 0.69 0.69
3,000 10,000 20,000 40,000
3.05 2.48 2.48 2.48
I n order to predict whether a burner operating with a given fuel gas a t given air-shutter, gas-orifice, and gas-linepressure settings will maintain a stable nonyellow flame, the mixture composition and boundary velocity gradient a t the burner port must be known. These two quantities are determined in an atmospheric gas burner by the amount of air that is entrained. Thus the airentrainment performance of a burner
,
I
V.
I
I
60 50 40
30
20
I
I
LEGEND Tube diameter, cm X 1914 A 0611
1023
.776
* @ Q
,
1
I
I
1
p
0 1
I
I
I * x
I
I ,354 ,249 195
I
0
0
X
12 16 20 24 28 GAS CONCENTRATION, FRACTION OF STOICHIOMETRIC
1 I
32
36
Figure 1 1. Yellow-tip fractions for methane-ethylene Figure 12. Flame-characteristics diagram for 62.1 % CHd, 35.5% fuels for g, = 300 CaHs, 2.4% CzH8 Composite diagram
Comparison of experimental points and calculated curves VOL. 48, NO. 1 1
NOVEMBER 1956
2059
can be represented by a point (or a curve) in a diagram of the boundary velocity gradient us. mixture composition -the same units being used to define flame-stability limits and yellow-tip limits. The performance point is now plotted on the flame-characteristics diagram of the fuel (such as Figures 1 to 6). If it falls in the stable blue-flame region, the burner will maintain a stable nonyellow flame; if not, flash back, blowoff, or yellow-tipping will result, depending on the location of the performance point. Knowing the performance point of a burner with the adjustment fuel (fuel a), we can calculate the performance point of the same burner with a substitute fuel (fuel x ) . Using Equations 8 and 9 :
5Fa
[do s2/1 - (1 - d,)FS].'/2 [do S2/1 - (1 - do)FS]."2
B blOW3ff
for
X.
(8) o
Figure 13.
where do is the fuel-gas specific gravity, p o is the gas-line pressure, and S is the stoichiometric fuel-gas percentage. F has been defined as the fuel-gas concentration in the burner stream. expressed as the fraction of the stoichiometric percentage, and g, as the boundary velocity gradient. Equations 8 and 9, and the flamecharacteristics diagrams of fuels a and x. are all the tools needed. T o predict whether fuel x will burn satisfactorily on a burner that has been using fuel a, we must know the specific gravities and the gas-line pressures of both fuels, and the mixture composition and boundary velocity gradient in the burner when fuel a is used. Then, using Equations 8 and 9, we calculate F, and pz for the same burner using fuel x . The last step consists of locating the F, and g, on the flame-characteristics diagram of fuel x . If the air-entrainment performance point falls in the stable blue-flame region, the burner will perform satisfactorily, and fuels a and x are exchangeable on this particular burner. The method can be extended very simply to evaluate the behavior of a large number of burners on exchanged fuel gases, as in the case of burners on the lines of a gas company. by treating all the burners as a statistical group. For this purpose. use is made of the limits of the stable blue-flame region of fuel a. I t is certain that in a community that has been serviced satisfactorily by fuel a, all burners in use are in adjustment a t the existing line pressure, so that the air-entrainment performance point, or curve of mixture composition DS. boundary velocity gradient for each burner, is contained in the stable blueflame region of the fuel. The equations developed for a single burner can be
2060
CU'W
0.4
0.8
1.2
2.0 24 2a 32 3.6 GAS CONCENTRATION,FRACTION OF STOICHIOMETRIC
16
4.0
44
48
5.2
Predicted burner performance in community changing from fuel gas
a to x used to calculate the shift of the borders of the stable blue-flame region of fuel a to form a new region which contains the air-entrainment performance points of each burner in the community when fuel x is substituted for fuel a. For this purpose we substitute the coordinates of the blowoff, flash back, and yellow-tip curves of fuel a in Equations 8 and 9 and solve for the coordinates of fuel x . New curves, l x , 2x, (3x).,, and a group of 3x curves are obtained. .41though we started out with flame-stability and yellow-tip curves of fuel a. Equations 8 and 9 do not result in like curves of fuel x. Curves I x , 2x, ( 3 x ) F e and 3x are the new limits of the region which contains the air-entrainment performance points of all burners in the community considered. SVe now determine experimentally or calculate the flame-stability diagram (7, I O ) , including yellow-tip limits of fuel x . If the limits of the new region for fuel x are within the stable blue-flame region of fuel x . then fuel x can be substituted for fuel a. If the new borders are oumide the stable blue-flame region of fuel x , it appears generally possible to judge whether the fuels are or are not exchangeable to some extent, and whether the situation can be improved by minor changes in burner adjustments, composition of fuel x, or gas-line pressure. Furthermore, because fuel x can be a mixture of fuel a with a supplementing fuel, the method can be used to predict burner performance with supplemental fuel gases used to meet peak load demands. Examples have been given (6, 9. 73) predicting exchangeability with reference to flash back and blowoff difficulties. Exchangeabilit)., including the
INDUSTRIAL AND ENGINEERING CHEMISTRY
likelihood of yellow-tipping. is evaluated in related manner and will be illustrated by analyzing the exchangeability of the fuels of Table VI ( 7 4 , on burners in a community adjusted to natural gas. The steps involved are: Calculation of Flame-Characteristics Diagrams. Flash back and blowoff curves are obtained for each fuel (7, 70). Constant yellow-tip limits are obtained for each fuel by use of Table I and Equation 1. Nonconstant yellow-tip limits for each fuel are obtained for a representative range of port diameters by use of composite yellow-tip fraction diagramsnamely, Figure 10 and the relation F, = F J i F c ' F A . For the three substitute fuels, these are the solid curves of Figure 13. Calculation of Air-Entrainment Performance Borders of Burners in Community. The curves obtained for the adjustment gas (natural gas) in steps l a to c are the starting point. The coordinates of each curve of the adjustment gas are substituted in Equations 8 and 9. Like coordinates for curves l x (blowoff), 2x (flash back), ( 3 x ) ~ , (constant yellow-tip limit): and 3x (nonconstant yellow-tip limits) are obtained. These are the dashed curves in Figure 13. Comparison of Flame-Characteristics Diagram and Air-Entrainment Performance Diagram. The following is predicted from the coplots of Figure 13. There will be no blowoff troubles and appreciable flash back difficulties; no yellow-tipping on burner ports exceeding approximately 1.6-cm. diameter and appreciable yellow-tipping on smaller ports. These conclusions are in partial diagreement with results obtained
Table VI.
on the lines of a major utility was being inconvenienced by necessary curtailment of its supply of natural gas during severe winter weather. The natural gas supplied had the composition given in Table VII. The company desired to set up an automatic gas making system to supplement its supply of natural gas from the utility. The chemical manufacturing processes concerned required that the heat output of the burners and the flame length should remain nearly constant during operation. Flames should remain stable and the burners operate at stoichiometric fuel-air composition. What substitute fuel would meet these requirements? A satisfactory substitute fuel was selected in the following manner. 1. Because the flame length had to be the same a t stoichiometric for the natural gas and the substitute fuel, the latter had to be a fuel of roughly the same burning velocity at stoichiometric as natural gas. Any fuel consisting of an unspecified ratio of saturated hydrocarbon with air would roughly meet this requirement. This is known from existing burning velocity data. Propaneair was a favored choice because of its availability. This choice was also indicated by the near equality of the flamestability diagrams of propane and natural gas (9), which means that the same flash back and blowoff limits exist for equal values of the fraction of stoichiometric. 2 . The next logical step was to determine the proper propane-air ratio for the substitute fuel. Equation 8 was employed to identify the required ratio. As the burners had to operate at stoichiometric on fuel a (natural gas) and fuel x (the substitute propane-air fuel), F,/Fa of Equation 8 is unity. If the chemical composition of fuel x is restricted, as in this case, to a definite hydrocarbon mixture, C,H2,+2, or to a mixture of C,H2 L+ 2 with air, then
Composition of Fuel Gases ( I 4) Natural Gas
Hydrogen Methane Ethane Higher paraffins Ethylene Propylene Higher olefins and diolefins Aromatics Inerts Stoichiometric fuel gas, yo Specific gravity Heating value, B.t.u./cu. foot
36.6 36.0 10.8 0.4 10.9 2.5 0.6 0.9 1.3 10.2 0.534 958
using critical burners and appliances as a measure of exchangeability (74). Actual experience in a community under known operating conditions is needed to determine which evaluation of the exchangeability of these fuels for natural gas is correct. The agreement encountered so far between predictions of exchangeability of fuel gases and experience in the field can be illustrated by means of a problem recently handled. A utility had a district of 10,000 customers, or roughly, 50,000 to 60,000 burners, adjusted to burn satisfactorily on PE gas 1 of Table VII. I t was considering substituting PE gases 2, 3, or 4 in place of No. 1. There was the question whether any of the three substitute gases could be used either as a complete substitute or mixed with the adjustment gas PE No. 1. The required analysis and prediction were made as in the preceding examples. The conclusions drawn were that with gases 2 and 3 there would be virtually no flash back and blowoff difficulties but a little yellow-tipping, gas 3 being very slightly less troublesome than 2. I t was clearly apparent that if substitute gases 2 and 3 were mixed with the adjustment gas in progressively increasing amounts, the service department of the utility could easily handle the small number of complaints that would progressively arise. With reference to substitute gas 4, the evaluation indicated
Table VII. Analysis Unsaturates (C2H4) CsHs C4H10 CsHiz
co
Hz
coz Nz
02 Suecific Kravity Sioichiometric-fuel-gas percentage Heating value, B.t.u./cu. foot Gas-line pressure
42.2 36.2 11.3 0.4 6.8 1.1 0.6 0.5 0.9 11.1 0.469 877
39.0 31.4 5.7 0.4 15.3 3.1 1.7 0.9 2.5 10.5 0.537 941
... ... ... ... ... ...
96.6 1.6
1.8 9.5 0.570 993
that there would be no blowoff but some flash back; there would be no new yellow-tipping on large ports (over l/Z-inch diameter) but some new yellowtipping on smaller ports. Substitute gas PE 4 was adjudged to be a borderline substitute for the adjustment gas 1. The exchanges were recently made using substitute gases 2 and 3 with satisfactory results. From other considerations, the utility judged that gas 4 would give more trouble than it could reasonably handle and this gas was not sent into the lines. Since this method was first proposed in 1949 ( 6 ) , there have been six instances in which predictions by it were compared against experience of utilities (7, 9, 73), including the somewhat unique case of choosing a substitute propane-air fuel to be used to adjust burners ahead of the availability of natural gas. Information was on hand a t the time of publication in all of these instances except one (73), permitting comparison between predicted performance of burners on the substitute gas and experience of utilities. Satisfactory results were obtained. The experience of the utility concerned in this exchange has since become known, and was as predicted (75). An interesting and somewhat related problem illustrates the potential versatility of basic understanding of burner performance in contending with problems met in the use of utility-supplied fuel gas. A large chemical company
do = qd,
+ (1 - 4)
(10)
where q = percentage of C,H2,+ 2 in the hydrocarbon plus air fuel, and d, = specific gravity of C,Hz,+z (1.55 for C8Hs).
Composition of Fuel Gases I
P E Gas 1 PE Gas 2 P E Gas 3 P E Gas 4 62.5 75.4 72.9 25.4 17.6 5.2 4.7 4.8 5.4 1.6 1.3 1.1
... ... ...
... ...
...
... ... ...
... ... ... 3.2
2.0 18.5 1.0 7.6 2.1
1.5 9.3 1.3 5.2 1.2
1.4 11.2 1.6 5.5 1.1
34.7 1.2 10.1 2.4
0.556 11.9 868 Same
0.586 10.6 949 Same
0.581 10.8 931 Same
0.576 12.6 783 Same
BC Natural Gas 79.3 5.6
where S, = fuel-gas percentage in a stoichiometric mixture of C,H2, + 2 (4.02 for CaHs). Equation 8 can be solved for q. Substituting numerical values, Equation 8 becomes
...
3.4 1.0 0.1
... ... 0.1 10.5 ... 0.678 9.36 1029
,
[l - (1
- 0.678) (1.0) (0.0936)] 0.678(0.0936)2 [l (0.55 q ) (1.0)(0.0402/q)] (1 0.55 q ) (0.0402/q)2
+
q is found to 59y0 propane cific gravity of of 1525 B.t.u. VOL. 48, NO. 1 1
+
be 0.59, so that fuel x is plus 41% air with a spe1.325 and a heating value per cu. foot. 0
NOVEMBER 1956
2061
3. Having tentatively identified a satisfactory substitute the question of heat output was next considered. As the flow of gas through an orifice is directly proportional to the square root of the pressure and inversely proportional to the square root of the gravity, the flow of propane-air will be (0.678/1 .33)Ij2 of that with natural gas for equal line pressures. The heat throughput (B.t.u. per hour) of propane-air will be (1 525/1029) X (0.678/1.33)1/2 = lO6Y0 of that with natural gas. The throughput could be brought down to its former value by dropping the line pressure by a factor of (1/1.06)1/2or 97%. 4. Having passed this hurdle, it was then possible to consider whether the propane-air flame would be stable. This was done using Equation 9 and The nusolving for the ratio of g,/g,. merical substitutions are g, - [ l - (1 - 0.678) (1.0) (0.0936)]”2 ga (1 - (1 - 1.33) (1.0) (0.0682)]’/2
and gz/ga = 0.974. This meant that the burners would operate with about the same values of the boundary velocity gradient on both fuels. At 100% primary air, the critical blowoff and flash back gradients are 2300 and 420, respectively, for natural gas, and 2650 and 590 for propane (or propane-air). Therefore the burners will operate stably on propane-air unless they have been burning with flows near flash back with natural gas, a very unlikely adjustment for this equipment. The substitute fuel, consisting of 59% propane and 41% air, was accordingly recommended. An automatic system supplying this fuel to compensate for a decrease in the supply of natural gas was built for the chemical plant and has given two winters of service. Operating personnel report that it is impossible to distinguish from the processing of chemicals in the plant when the change from natural gas to mixtures of natural gas and propane-air occurs, in agreement with the calculated predictions. Incidentally, somewhat different ratios of propane to air as the substitute fuel were tried and found less satisfactory.
Summary Since 1950 the Bureau of Mines, in cooperation with the American Gas Association, has been engaged in studies on the fundamental combustion characteristics of fuel gases. It is planned to extend these studies to the fundamentals of burner and appliance design. The theoretical and experimental work on gas burner fuels clearly shows that the problem of the over-all performance of gas burners can be profitably approached in terms of factors that are inherent in the fuel gas, factors that are inherent in the design of the burner,
2062
and factors that are inherent in the design of the appliance housing the burner. These factors are interrelated and frequently modify one another in a complex manner. Fundamental flash back and blowoff characteristics of all fuel-gas mixtures likely to be of interest to the gas industry have been reported in earlier publications. The present paper deals with fundamental yellow tipping characteristics of fuel gases. Constant yellowtip limits have been obtained as a limiting value for increasing port diameter and flow. These are the foundation of a graphical method of correlating yellowtipping over a range of practical port diameters. The influence of port shapes, depths, and temperatures has been explored briefly. I n addition, the Bureau of Mines method of predicting the exchangeability of fuel gases has been extended to include the limitation imposed by yellow-tipping. Satisfactory agreement has been obtained between predictions of the likelihood of flash back, blowoff, and yellow-tipping with exchanged fuel gases and the actual experience of gas utilities.
Y
= nonconstant yellow-tip
ax
= = =
C
u, w , etc.
at axis of port constant yellow-tip limit components of a fuel
Superscripts - = =
*
average or root mean square value minimum for constant yellow-tip limit
-
Nomenclature fraction of stoichiometric nonconstant yellow-tip limit constant yellow-tip limit yellow-tip fraction boundary velocity gradient, seconds-’ port radius, centimeters volumetric rate of flow, cc. per second linear rate of flow, cm. per second diffusion coefficient, sq. cm. per second time, second height, centimeters (per cent/lOO) of C,H,+2, in fuel 2x
3% (3x)x8.J
S,
po
d,, do
X S
S,
n
curves for borders of air entrainment performance of burners
=
= burning velocity, cm. per second = gas-line pressure, cm. of water = specific gravity of C,H2, + 2 = specific gravity of fuel = displacement of molecule by diffusion, cm. = stoichiometric volumetric per cent of fuel gas/100 = fuel percentage in a stoichiometric mixture of C,H2, + 2 = fractional concentration of a component in a fuel
Subscripts a
= fuel gas used for adjusting
X
= substitute fuel gas = blowoff
burners
B F
INDUSTRIAL AND ENGINEERING CHEMISTRY
=
flash back
Literature Cited (1) Behrens, H., “Fourth
Symposium (International) on Combustion,” p. 538, Williams & Wilkins, Baltimore, 1953. ( 2 ) Clark, T. P., Natl. Advisory Comm. Aeronaut. Research Mem. RME52624 (1 952). (3) Comerford, F. M., Fuel 32, 67 (1953). ( 4 ) Gaydon, A. G., Wolfhard, H. G., “Flames, Their Structure, Radiation and Temperature,” Chapman & Hall. London. 1953. ( 5 ) Grumer, ‘J., Gas 30, 47 (1954). ( 6 ) Grumer, J., IND.ENG. CHEM.41, 2756 (1949). ( 7 ) Grumer, J., Harris, M. E., Ibid., 44, 1547 (1952). ( 8 ) Grumer, J., Harris, M. E., Rowe, V. R.. “Studv of Yellow-TiaDine Characteristick of Fuel G&es,’ Interim Report 2, Project PDC-3GU, American Gas Association, July 1954. (9) Grumer, J., Harris, M. E., SchuItz, H., IND.ENG. CHEM, 44, 1554 (1952). (10) Ibid., 47, 1760 (6955). (11) Kreiger, F. J., Rand Corp., Research Memo. 649 (1951 ). (12) Kurz, P. F., IND.ENG. CHEM.47, 621 (1955). (13) Lewis, B., Grumer, J., Gas Age 25, 25-8, 72-80 (May 11, 1950). (14) Linden, H. R., Guyer, J. J., Pettyjohn, E. S., “Production of Natural Gas Substitutes by Pressure-Hydrogasification of Oils,” Institute of Gas Technology, Paper CEP-54-7, AGA Chemical, Engineering and Manufactured Gas Production Conference, May 24-26, 1954, Pittsburgh, Pa. (15) Mong, P. E., Pacific Gas and Electric GO., private correspondence. ( 1 6 ) Porter, G., Advisory Group for Aeronautical Research and Development, AG13/M9 (May 3-7, 1954). (17) Robertson, W. W.,Humphrey, N. B., Anderson, R. C., J . Chem. P h y . 21, 2093 (1953). (18) Schalla, R. L., McDonald, G. E., IND.ENC.CHEM.45, 1497 (1953). (19) Smith, R. S., Gordon, A. S. J . Chem. Phys. 22, 1150 (1934). (20) Street, J. C., Thomas, A., Fuel 34, 4 (1955). RECEIVED for review September 6, 1955 ACCEPTEDMay 23, 1956 Division of Gas and Fuel Chemistry, 127th Meeting, ACS, Cincinnati, Ohio, April 1955. Research supported by the American Gas Association, Project PDC-3-GU. Material supplementary t o this article has been deposited as Document No. 4987 with the AD1 Auxiliary Publications Project, Photoduplication Service, Library of Congress, Washington 25, D. C . A copy may be secured by citing the document number and by remitting $1.25 for photoprints or $1.25 for 35-mm. microfilm. Advance payment is required. Make checks or money orders payable to Chief, Photoduplication Service, Library of Congress.
