I
PHILIP F. KURZ Baitelle Memorial Institute, Columbus 1, Ohio
lntluence of Diborane on Flame Speed of Propane-Air Mixtures
THIS ARTICLE of of
describes the r c s ~ l t s studies the influence ofdiboraue, in additive amounts and aa a s u x d f u y fuil, on the h e specds of propane-air mixtures. This work was part of a krger investigation concerned with a study of combustible additives which affect the burning of hydmearbons. A fuel additive is d e h e d , arbitrarily, as a subs*mce which is added to the primary fuel in an w n t not greater than 5 volume %. A combustible substance added to the hydrocarbon in amounts rangihgfrom 5 to 50 volume % is deaigmted a secondary fuel. The present work was undtrtaken to teat the validity of a working hypothe& ppeviously (4) set up to explain a portion of t k mechanism of hydrocaban combustion. According to this hypothesi, the combustion js promoted by OH radicals. These radicals serve aa energy carriers and are initiated by a partial oxidation of hydrogen atoms first prod u d m low concentration fran the demmpmirion of hydrocarbon molecules
by an extunal inlluencte.g., an activation or ignition source which may be thermal, eiectrical, or aciinic. The OH radicala are regenemted by a chain naction and thereby repeat the cycle. If thin hypothesis is c o m t , the speed of combustion of hydrocarbons might be accekrated by the addition ofa s u b stance which can liberate hydrogen atoms or hydroxyl radicals in the flame. Certain hydrides of clements in Pe&xlic G m p a 111, V, and VI appear+ to be pcsibie sourced of hydragen atoms, and were selected for expcrimenral SNdics.
Two Gmup VI hydrides, hydrogen sulfide, and hydmgen selenide, were found to be strong inhibitors. They
0.8
I00 0.7 85
-I
a6
8
g 8
a
c
0.5
0 0 r
t 0
5:
0.4
0
LL
0.3
0.2 Numbers ot ends of curves show per
0.1
cent of stoldJometric oir in mixture
0.0 Argon in Mixture, volume pbr Cbnl Figure 1. Influence of argon on Rams speeds of propane-air inixmes on a 1.1 1cm. burner Va U, NO. IO
OCrOmR 1956
1863
0
0%
8 n L
2
0.6
c
3 0
g
Propane-Air-Diborane
‘\
0.4
n
(0.23 vol. per cent)-Argon
I
4.2 vol. per cent diborcne in fuel
07
Air in Mixture, percentage of stoichiometric
Figure 2. Flame speeds of propane-air and propane-air-diborane-argon mixtures on a 1.1 1 -cm. burner with a Smithells flame separator
retarded the rate of the combustion reactions of hydrocarbon-air mixtures to a much greater extent than might have been predicted on the assumption that their effects ~7ouldbe proportional to their concentrations in the fuel mixture. The results of this work with Group VI hydrides have been reported (3-5). Ammonia, a Group V hydride, was found to influence hydrocarbon flame speeds essentially as does an inert gas such as nitrogen (6). Diborane influences hydrocarbon combustion in an unique manner. Experimental Procedure
The 1.I I-cm. Bunsen burner apparatus was the same as that used in the work with Group V and VI hydrides and has been described (4). The diborane used was prepared by the Callery Chemical Co., Callery, Pa. Its purity was stated to be 99 mol. ye B ~ H B .T o prevent dissociation, it was stored a t dry-ice temperature pending use. The propane used was Phillips
Table
1.
Composition and
Petroleum Co.’s Research Grade, stated to be 99.9% pure. Compressed air in commercial cylinders was used to ensure close control over the air flow at all times. A modification of the method of Smith and Pickering (7) was used to measure flame speeds. This has been described earlier ( 4 ) .
Experimental Results
Influence of Argon on Flame Speed of Propane. T o inhibit decomposition of the diborane while in temporary storage at room temperature, and to facilitate flow measurements, argon was added to the diborane. hIixtures containing about 10 to 15 volume yo diborane were prepared just before use. The flame speeds of propane-air-argon mixtures were measured on the 1.11-cm. Bunsen burner to provide a reference base. Figure 1 shows the influence of argon alone on the flame speed of propane.
Flame Speeds of Mixtures
Propane-Diborane-Air-Argon
Lean and near-stoichiometric rich mixtures
Air in Mixture, Percentage of 8toichiometric
Fuel, Volume 7% CsHs B2H6
(1)
(2)
118.4 112.0 109.2 106.5 103.7 101.2
100 91.4 87.6 84.1 80.9 78.0
1 864
(3)
...
8.6 12.4 15.9 19.1 22.0
Fuel, EwivaEent % CsHa BzHs (4) 100 94.7 92.2 89.8 87.6 85.5
(5)
... 5.3 7.8 10.2 12.4 14.5
INDUSTRIAL A N D ENGINEERING CHEMISTRY
piame Speed of l$Lri&Te, Ft./Sec.
Contribution to Flame Speed, Ft./Sec. CaHs BzHa
(6)
(7)
0.73 0.74 0.74 0.75 0.82 0.83
0.73 0.74 0.74 0.72 0.71 0.69
(8)
... 0.00 0.00
0.03 0.11
0.14
Argon i n
Mixture, Vol. % (9)
0 1.94 2.87 3.79 4.68 5.56
These data are for unshielded flames burning in ambient air. Preliminary Experiments with Propane-Diborane Mixtures. A brief series of preliminary experiments with propane-air-diborane mixtures containing increasing amounts of diborane ranging from 0.02 to 0.25 volume yo of the fuel-air mixture showed that the flame speed of the mixture was about the same as the flame speed of propane. When the diborane was present in amounts exceeding 0.2 volume yo. corresponding to about 4 to 5 volume yo of the fuel, difficulties were encountered in obtaining photographic images with inner cones sharply defined, for flamespeed measurements, because of the intensity of the bright green shroud which enveloped the primary flame cone. Attempts at Flame-Splitting with a Smithells Tube. A Smithells (8) tube was used in an attempt to delineate the inner cone more sharply. I t was hoped that by excluding ambient air from the environs of the Bunsen flame the intensely bright green shroud which enveloped the primary cone could be displaced to the top of the Smithells tube. This was not successful. A green mantle persisted around all propane-air-diborane flames, and there was practically no green color in any of the separated diffusion flame cones until the rich blowoff limit was reached. The implications of the unseparated green shroud and the pale blue secondary flame are that diborane was probably burning preferentially in or near the primary flame front, thus leaving the propane only partially consumed and requiring it to complete its combustion in a diffusion flame in the ambient air at the outlet of the Smithells tube. This selective combustion has been mentioned briefly (2) and has also been observed in the twin flames by Berl and Dembrow (7). When about 15% or more diborane was present in the fuel there was a visible effusion which appeared to be solid-laden vapors from below, the primary flame cone. This suggests that some preflame thermal dissociation of diborane occurs in the gas stream approaching the flame front. I t is possible that higher hydrides of boron may have been formed in this dissociation process and that these were condensing as visible solid particles. Influence of Diborane in Additive Proportions. Figure 2 compares the flame speeds of propane-air mixtures with those of propane-air-diborane-argon mixtures containing 0.23 volume Yo diborane on the 1.11-cm. burner with a Smithells separator. A diborane-argon mixture (10.3 to 89.7) was used. The propaneair mixture has a slightly higher flame speed than does the quaternary mixture containing added diborane (0.23%) and argon (2.1%).
Influence of Diborane as a Secondary Fuel. A diborane-argon mixture containing 14.0 volume yo BzHe was used in another series of experiments. The objective was to evaluate the effect of stepwise increases in diborane concentration on the flame speeds of lean and rich propane-air mixtures. The experiments were conducted on the 1.1I-cm. burner with the flame separator attached. The lean starting point was at 118% of stoichiometric air. The flows of propane and air were held constant, and the diborane-argon flow was increased in increments of 100 cc. per minute ranging from 200 to 1000 cc. per minute. This resulted in diborane concentrations ranging from 0.32 to 1.45 volume yo of the quaternary mixture (8.6 to 32 volume yo diborane in the fuel). The addition of diborane caused a change in the percentage of stoichiometric air in the mixture from 118.4 (zero diborane) to 92.3 (1.45 volume % diborane). A similar procedure was followed in experiments with rich mixtures. The rich starting point was 92.1% of stoichiometric air (zero diborane). Stepwise addition of the diborane-argon mixture increased the diborane content of the quaternary mixture from 0.31 to 1.43 volume yo (6.8 to 26.8 volume yo diborane in the fuel). The richest mixture contained 75.5y0 of stoichiometric air. Figures 3 and 4 compare the flame speeds of these propane-air diboraneargon mixtures with those of corresponding propane-air-argon mixtures obtained by interpolation of the data given in Figure 1. In establishing the composition of the fuel-air mixture relative to stoichiometric
I
89
13
56
J
I
I
3.8
19
00
Argon in Mixture, volume per cent 32 0 273 22.0
15.9
8.6
Diborane in Fuel, volume per cent
1.2
Air in Mixture, percentage of stoichiometric Figure 3. Flame speeds of lean and near-stoichiometric rich propane and .propane-diborane mixtures on a 1.1 1 -cm. Bunsen burner ~
Table II. Air i n Mixture, Percentage of Stoichiometric
~~
Calculation of Air Available for Propane after Diborane Burns Preferentially in Rich Mixtures Fuel, Air for CsHs Equivalent % Stoichiometric- Air Air Percentage of CaHs BnHa Per 100 mol8 For BeHa For CaHs Available Volumes stoichiometric Rich, Near-Stoichiometric Mixtures
(1) 98.8 96.5 94.4 92.3
(2) 83.5 81.6 79.7 78.0
(3) 16.5 18.4 20.3 22.0
(4) 2381 2381 2381 2381
88.2 86.3 84.6 82.9 81.3 79.8 78.4 77.0 75.5
95.8 93.8 91.9 90.1 88.4 86.7 85.1 83.5 82.0
4.2 6.2 8.1 9.9 11.6 13.3 14.9 16.5 18.0
2381 2381 2381 2381 2381 2381 2381 2381 2981
Wb 393 438 483 524
Wd
1988 1943 1898 1857
(7Y 2352 2298 2248 2198
1959 1860 1765 1674
98.6 95.7 93.0 90.1
2281 2234 2188 2145 2104 2064 2026 1988 1952
2100 2055 2014 1974 1936 1900 1867 1833 1798
2000 1908 1821 1738 1659 1583 1512 1440 1369
87.8 85.4 83.2 81.0 78.8 76.7 74.6 72.5 70.1
(Q)O
Moderately Rich Mixtures
a
100
147 193 236 277 317 355 393 429
(3) X (4)/100. (4)/100. (4) x (1)/100. (7) ( 5 ) . (8) + (7).
* (2) X
-
~~
VOL. 48,
NO. IO
OCTOBER 1956
1865
proportions, diborane was assumed to bum according to the following equation
BJIi
d.8 1 2 5.5 3.8 1.9
g
I
1.
7boronr in Fuel, volume per cont
0
a i i Q
c
0.
f
r
U-
,
. )
:: *
Awn
0.1
E
1
I
e
-
B i 0 1 - b 3H10 (1)
Figure 5 shows that the presence of up to 20 volume % of d i k e in the fuel only inhibits the flame speeds of lean mixtum, and that 32 volume 3'% of diborane in the fuel cauaed an increase of only about 32% in the b e speed of slightly rich mixturrs. However, in moderately rich mixtures the increases in flame speed resulting from increases in diborane concentration are considerably more pronounced. Figure 6 shows the approximate effect on flame speed of changing the propaneair cornpaition from lean or slightly rich to moderately rich a t nine different rates of diborane flow. The lines donnecting, the respective data points are dotted because only two points are available for each diffennt diborane rate. Theae results were surprising inasmuch as accelerating effects had been believed posible with concentrations of diborane within the additive range of less than 5 volume % of the fuel. It is apparent that diborane is effective in increasing the flame speed of propane only when present as a second& fuel.
Argon in Mixtun, volume per cent
0
+ 301
*Propow-Air-Argon 0.4
0.2
Analyair of Data
0.q
Figure 4.
70 80 90 100 Air in Mixture, percrntoga of StOiChiOm&iC
Flame speeds of rich propane and propane-dibarone mixtures on a
1.1 I-cm.Bunsen burner
Table 111.
Air in Mh,& P€Tw?tUae of s?&hMnLtric
~
C o b b t e d Contributions of Propane and Diborane to Flame Speeds of Rich Mixtures
cdcd. bOntributiOntn Fd. Eguimled 76 CIX. BlE,
'mRwl.3 spasd Of
M W l d , Ft./cbc., C,& E&,
96.5
94.4
92.3
92.1 88.2 06.3 84.6 82.9 81.3 79.8 78.4 77.0 75.5
1866
83.5
18.4
0.64
0.67
0.21 0.30
79.7 78.0
20.3 22.0
0.62 0.59
.0.35
16.5
..
9.3.8
4.2 6.2
91.P
8.1
0.85 0.77. 0.72 0.66
16.5 18.0
0.51 0.46 0.38 0.33 0.27
95.8 90. I
88.4 86.7 8S.l 83.5 82.0
9.9 11.6 13.3 14.9
dliaUrs, . Ft./SsC.
0.-
IHDUIIIIAL AHD EHOlNElRlNQ CHEMISTRY
remaining oxwn. Table I presents a summary of thr pertinent data for lean propane-diborane mixtuds. The fuel mixture (columns 4 and 5) is described in terms of the equivalent percentages of fuels, based on relative oxygtn demand. Inasmuch as the molar stoichiometricoxygen requirement of propane 5 0 9 , and of diborane 3 0 e ,the equivalent percentages of propane and diborane are as follows:
4
81.6
100.
EW.
Flomsped
Mixh,r.r
Rih, N.or-Stoihbm.hic
98.8
'
At least two plausible assumptions may be maye in interpreting these data-in lean mixtures. the Dromne in propanediborane mixtures i; contributing th6 flame speed .of the mixture. to the fullest extent of its concentration in the fuel; and in rich mixtures, the diborane burns selectively' and completely and the propane is rhnsurned only to the extent of reacting with the
0.88 0.94 0.87 0 .99
0.40
..
0.03
0.I2 0.19 0.$7 0.32 0.38 0.45
-
Equivalent percentage C a s = Vo-
0.85 0.80
0.84 0.85
6.86 0.85 0.84 0.83
0.H)
0.83
0.62
0.89
'
+vcc% 0.6V-
Equivalent percentage BJIe
-
0.6VVc+, 0.6V&x,
+
x
100
x
100
*
where , V and Vm are the volumetric percentages of pmpane and diborane, respectively, in the fuel mixture. Column 7 shows the calculated contribution of propane to thoflame speed
.,., .,
,.,
.
. *
ADDITIVES #N CUlLS of the mixture. These calculated values are the product of the flame speed of a propane-air-argon mixture, containing the game amount of argon and having the same fuel-air stoichiometry as the propanediborane mixNre (from Figure 3), and the equivalent percentage of propane in the mixture (wlumn 4). For example, for the fvst mixture, the contribution of propane is calculated as follows: (0 78x0 947) = 0.74
The values in column 8 are the differences between wlumn 7 and the ex-ental values for the flame speed of the m i x t w a (wlumn 6). Table I1 shows the methcd of establishing the richneas of the residual pmpane& &Nres when it is m u l e d that diborane burns preferentially, uSing its stoichiometric requirement of air. Table 111 show the calculated contributions of propane and diborane to the flame s@ of rich propanediborane mixtures. The data given in the last column M Table I1 and Figure 1 were used to establish the flame speeds of propane-won baac mixtures. These values were then used to calculate the contribution of propane to the flame sped of the quaternary mixture. For example, for the moderately rich mixture containing 83.5 equivalent % propane, the next-to-last line in Table II shows that the propane is burning with 72.5% ofitn stoichiometric air requirement (the diborane is asaumcd to acquire its ftlll requirement of air for comhustion to B a r and €I& This) mixture . wntains 8.0% atgon. Then P i i r e 1 shows that a propane-argon-air mixof that composition will have a flame speed of 0.4 foot per second. The wntribution of the propane to the p r o p a n e d i h n e mixture is then the product of the equivalent pe'centa@ of propane in the mixture by the value of the flame sped of propane under the conditions stated. Thus
Dibarone in Fuel, volume par c n t Figure 5. Relationship between change in flame speed of propane-diborane mixtures and percentage of dibarone in the fuel
0.835 X 0.4 = 0 33
The contribution of the diborane is then the difference between the cakulated contribution ofthe propane and the experimental valw of the flame s p e d of the mixture, as fallows: 0 83
- 0.33 = 0.50
Inasmuch an it appears to be warranted to calcuIate the contribution of the propane to the flame speed of the ' mixture in the manner sh above, it may be equall) justi6ahle o use the contribution of the diborane as establishcd by diffurnoe to estimate h e speeds for dibwazu+rgon mixhued.
,
"4
Es-
Speeds of Mixture. The values given in Tablca I and 111 for the
of Diborane-&Argcm
plrunc
AIR IN MIXTURE, PERCENTAGE OF Sl0lCHK)METRIC Figure 6. R e l a t i d i p between changer in flame speed of propane-diborane mirtures and in mixture composition a t nine levels of constant diborane Aaw VQL. 48. NO. 10
ocrolcl 1956
1867
on the rich side of the stoichiometric point (Figure 2, propane-air mixtures). Discussion
0
z
8 W v)
a w n I-
W W
IL
aW W
a. v)
W
I
4
LL
AIR IN MIXTURE, PERCENTAGE OF STOICHIOMETRIC Figure 7. Estimated flame speeds of diborane-air-argon mixtures (argon: diborane = 6.03:l.OO)
contribution of diborane to the flame speed of the propane-diborane mixtures may be used to estimate the flame speeds of diborane-air-argon mixtures containing argon in the ratio
- A_ _ _-
BzHs
6.03 1.00
This is done by dividing the contribution of the diborane to the flame speed of propane-diborane mixtures by the equivalent percentage of diborane in the mixture and multiplying by 100. Diborane flame speeds calculated in this manner are plotted in Figure 7. Also in Figure 7, the symbols, x ; are values of flame speeds of lean and rich propane flames. The circles represent values of flame speeds estimated for diborane from the data for lean and near-stoichiometric rich mixtures. The triangles represent similar estimates for moderately rich mixtures. For rich mixtures a reasonably smooth continuous curve can be drawn through the upper seven triangles and the upper three circles as shown by the solid line. Thus, it appears that, when the diborane to propane ratio exceeds a certain amount-which is different for lean and
1 868
for rich mixtures-the estimated flame speeds of diborane assume reasonable values that follow a smooth curve, as would be expected. However, in lean mixtures, when there is less than about 17 equivalent 7 ' diborane in the propanediborane mixture the propane exerts such a strong inhibiting influence on the diborane that it is not possible to estimate reasonable values for the flame speed of diborane alone (lower four circles, Figure 7). Similarly, in rich mixtures, if the diborane to propane equivalence ratio is less than 1 to 12, the strong inhibiting influence of propane makes estimation of the flame speed of diborane unreliable (lower two triangles, Figure 7). The solid curve (Figure 7) suggests that diborane has a high flame speed when burned with air in rich mixtures. The maximum flame speed is not covered in the range of mixtures studied. It may occur in the region of air-fuel composition near 60% of stoichiometry where hydrogen-air mixtures also reach their maximum. This is significantly different from the behavior of hydrocarbon-air mixtures, which usually achieve a maximum flame speed slightly-
INDUSTRIAL AND ENGINEERING CHEMISTRY
It has been shown that when diborane is present in small amounts in mixtures with propane, the flame speed of the mixture is less than that of propane. This is true with lean mixtures containing up to about 20 volume % diborane in the fuel and with rich mixtures containing up to about 6 volume 7' diborane. Under these conditions it appears that propane inhibits the combustion of diborane or that there is mutual inhibition. This suggests that any increase in hydrogen atom concentration caused by the presence of diborane does not accelerate the combustion processes of hydrocarbons in the lean or rich flames in question. In rich flames of propane-diborane containing more than about 6 volume 7 0 diborane, the flame speed of the mixture exceeds that of propane. However, there is evidence that the diborane is burning selectively. One of the results of this selective combustion is that the combustion of propane in rich mixtures is retarded a t least to the extent to which diborane pre-empts its stoichiometric oxygen requirements and thereby causes the propane to burn in a richer mixture and a t a slower rate. Accordingly, in these rich mixtures the diborane may be considered to inhibit the combustion of propane. Such a process masks any effect of the increased hydrogen atom concentration due to thc presence of diborane in the flame. Acknowledgment
The author wishes to express his thanks to R. S. Litton, who assisted ably and faithfully in carrying out the experimental work, and to J. F. Foster, Division of Fuels and Physical Chemistry at Battelle for his helpful interest. literature Cited (1) Berl. W. G.. Dernbrow., D.., ATature 170, 367 (1952). Kurz, P. F., Fuel 33, 250 (1954). Zbid., 34, 463 (1955). Kur7, P. F., IND.ESG. CHEM.45, 2361 (1955). Ibid., 47, 297 (1953). Kurz, P. F., unpublished results. Smith, F. A , , Pickering, S. F., J . Research Natl. Bur. Standards 17, No. l ( 1 9 3 6 ) . ( 8 ) Smithells, -4,, Jngle, H., Trans. Chem. SOC.,London 61, 204 (1892).
RECEIVED for review October 27, 1955 ACCEPTED March 17, 1956 Work done under sponsorship of Chemistry Research Branch, Aeronautical Research Laboratory, Wright Air Development Center, Wright-Patterson Air Force Base. Ohio.
