J. E. QUON,' 8.
Ten simple gaseous hydrocarbon fuels were burned as laminar diffusion flames in a cylindrical burner open only a t the top and supplied with a constant rate of secondary air. T h e combustion nucleus concentration in the effluent was monitored with a photoelectric nucleus counter as the fuel rate and type was varied. The reciprocal of the volumetric fuel rate a t which prolific nucleus concentration occurs in the effluent was considered to be a measure of the relative air pollution potential.
D. TEBBENS, and J. F. THOMAS
Univ. of California, Sanitary Engineering Research Laboratory, Berkeley, Calif.
Cornbustion Nuclei An Index for Classifying Gaseous Hydrocarbon Fuels
A
METHOD is proposed for the detection of the beginning of particle issuance from diffusion flames. It is then applied to define a relative air pollution potential of simple gaseous hydrocarbon fuels. Calibration for a two-compartment, expansion-type, Wilson cloud chamber, photoelectric nucleus counter is presented. For the conditions of the experiment, there is a maximum nucleus-free fuel rate for each fuel which denotes the beginning of carbon formation. The reciprocal of this fuel rate was defined as the relative air pollution potential and was used to classify the hydrocarbon fuels. Fuels examined and their order of increasing air p d l u t i o n potential are: methane, ethylene, ethane, propane: propylene, 1-butene, 2-butene, butane, butadiene, and acetylene. Propylene has both a lower and upper limit of fuel rate for which the nucleus concentration increases without bound. This was not characteristic of other fuels tested. The lowest nucleus concentration for propylene was found to be considerably higher than for other fuels. Desirability of propylene as a fuel or present in other fuels in significant amounts should be questioned in arcas \vhere air pollution is a problem. .4parameter indicating the number of collisions per second per bond between a fuel molecule and oxygen molecules at the upper limit of flammability shows a qualitative relationship to the relative air pollution potential of a fuel. Combustion effluent particles with radii of 1 mp to 100 mp may be termed combustion nuclei. The size of the particles resulting from nonsooting flames may be estimated from their physical properties. From diffusion losses in cylindrical tubes, the average radius of the particles in nonsooting flames is found to be approximately 4 mp. Parker and Wolfhard ( 8 ) , who examined flocs of carbon particles condensed on a cold probe in various zones of a diffusion flame, found the flocs to be composed of particles of 5 mp in radius in nonsooting hydrocarbon flames and particles of 50 mp in radius in sooting hydrocarbon flames.
There is little doubt that the supply of combustion nuclei in urban areas Present address, The Technological Institute, Northwestern University, Evanston, Ill.
can be attributed largely to the amount and type of activity associated with the combustion of carbon-containing fuels. Nucleus counts in urban areas are lo4 to t05,cc. in order of magnitude, whereas those in areas relatively free from the activities of man are in the order of 10z/cc. ( 4 ) . Nucleus concentration is a result of the dynamic equilibrium between production and loss of nuclei by physical properties such as coagulation, diffusion, and ventilation.
These combustion nuclei may have an important although vaguely understood role in bringing about many of the effects of air pollution. Increased fog persistance (7) and frequency ( 6 ) can be attributed to combustion nuclei. Reduction in visual range is possible through coagulation where there is a continuous supply of combustion nuclei. .4ttempts to correlate the smoking point of a fuel or the dimensions of the flame at the smoking point with the thermodynamic, chemical, and physical properties of the fuel have had varying degrees of success. From an air pollution standpoint, the smoking point of a fuel is not the best index of its air pollution potential, since most combustion processes avoid the production of smoke. Combustion nuclei are present in effluents from all combustion processes involving the use of carbon-containing fuels. A high concentration is indicative of poor combustion. Detection and measurement of these Combustion nuclei is possible, and the concentration of combustion nuclei can logically serve as a n index of air quality for effluents from combustion processes.
Apparatus General. The nucleus counter, combustion chamber with secondary air and fuel system, and mixing chamber with dilution air system are shown schematically on pages 235 and 236. Dilution and secondary air, and fuel, were filtered through cotton and have an initial nucleus concentration of less than 10*/cc. Dilution and secondary air were metered by rotameters with an accuracy The gaseous fuels (Matheson) of zk27,. were C.P. grade with the exception of ethane, which had a purity of 95%. v./v. All flows are reported at 70" F. and 760mm. Hg. Combustion Chamber, The combustion chamber used was a cylindrical glass tube, 18 inches in length and 2.20 inches in diameter. The burner tube used extended 6.5 inches into the combustion chamber, and consisted of 0.020 inch diameter stainless steel tubing supported by a concentric glass tube. Filtered secondary air was distributed by a wire screen and metered by a rotameter calibrated in place. Compressed gas was metered by a capillary orifice calibrated to within 0.1 cc./min. The orifice was calibrated for ethylene by collecting the gas over a 20% solution of Na2S04. Calibrations for other gases were obtained from their
h
I lq U Burner system a.
P
E co sa
P
I
I
I
b
.!
a small change in the fuel rate results in a disproportionate increase in the concentration. A change in the flow rate of ethane from 37 to 40 cc./min. resulted in a corresponding change in the nucleus concentration from 106 to 2 X 106/cc. For ethylene, 1-butene, 2-butene, and butadiene, the nucleus concentration increased with fuel rate to a maximum of about 50,00O/cc. then decreased to a value of about 100. After this, the increase in nucleus concentration with fuel rate is similar to that for the saturated aliphatic hydrocarbons. This maximum in the nucleus concentration was observed only in the four unsaturated aliphatic hydrocarbons mentioned above. Propylene has two values of fuel flow for which the nucleus concentration is 2 200,000, cc. T h e lowest propylene
Relative Maximum Nucleus-Free Fuel Rate Fuel Methane Ethane Propane Butane Ethylene
Propylene
1-Butene Cis-2-But ene Butadiene Acetylene
Q*,
Cc./hIin. 178 37.0 25.5 16.5 50.5 25.5 19.0 19.5 15.5 4.0
1iQm"
0.562 X 2.70 3.92 6.06 1.98 3.92 5.26 5.13 6.45 25.0
1.36 3.73 3.79 4.50 3.18 4.20 4.24 4.13 5.66 48.3
100 65.8 51.7 40.5 57.0 54.1 42.7 42.4 45.3 17.5
. , _, : !
4
I
I20
80
40
I
VOL. 53, NO. 3
MARCH 1961
237
fuel flow for which nucleus concentration measurements were made was 1.5 cc./min. Propylene flow rates below this value did not support combustion. T h e acetylene curve is similar to that of the saturated aliphatic hydrocarbons, but the lowest value of nucleus concentration is in the order of 20,00O/cc. T h e behavior of the unsaturated aliphatic hydrocarbons can be attributed to higher flame temperatures. T h e observed maximum flame temperatures ( 3 ) of the unsaturated aliphatic hydrocarbons were a t least 100” F. higher than those for the saturated series. Ethylene a n d acetylene have even higher flame temperatures. T h e higher temperature can result in cracking of the fuel to yield combustion nuclei. Carbon \vas sometimes noticeable on the burner tube rim with ethylene a n d inevitably was present in large amounts with acetylene flames. T h e nucleus concentration is governed, in part, by consumption of combustion nuclei in the flames. At low fuel rates? the yello\v luminous zones a r e small and the ability of the flame to consume combustion nuclei is correspondingly low. This inability to consume combustion nuclei formed by cracking of the fuel accounts for the difference in behavior between saturated and unsaturated aliphatic hydrocarbons. For the same fuel flow rate, the presence of carbon on the burner rim gives higher values of nucleus concentration t h a n if the burner rim were free of carbon. Xucleus concentrations ivere measured with burner rim free of carbon. As the fuel rate is increased. the yellow luminous region of the flame increases, and ability to consume combustion nuclei increases disproportionately with respect to production, resulting in a decrease in nucleus concentration. -4s the maximum ability of the flame to consume combustion nuclei is reached, the nucleus concentration increases rapidly with fuel rate. Propylene is especially prone to high combustion nuclei yields. T h e minim u m nucleus concentration is about 50,000:’~~.a n d increases very sharply as the fuel rate is varied in either direction. (This has some importance in air pollution control.) T h e results indicate that it is difficult to burn propylene nucleus-free a n d its use as a fuel or impurity in other fuels in significant quantity should be questioned in areas \\here air pollution is a severe problem.
The table shows that unsaturation does not necessarily mean a higher air pollution potential, as smoking point studies on hydrocarbon fuels would indicate. t\s fuel rate is increased beyond the maximum nucleus-free rate, the nucleus concentration increases without bound, and a fuel rate is reached where visible smoke begins to issue from the flame. T h e increase in the maxim u m nucleus-free fuel rate necessary to
238
reach the smoking point is different for different fuels. For butadiene? the maximum nucleus-free rate is the same as the smoking point. For butane, the maximum nucleus-free rate is 16.5 cc.,’ min. a n d the smoking point is 59 cc. min. This shows that if the weight of carbon us. fuel floiv from the inception of carbon nuclei production to the smoking point could he obtained, one Lvould find the slopes of the curves to he different for different fuels. T h e classification of fuels with respect to their air pollution potential as determined by the inception of carbon production (combustion nuclei) and by the smoking p3int is different. Schalla, Clark, and I l c D o n a l d (9) give a n example of relative smoking tendencies of hydrocarbon fuels. T h e percentage of stoichiometric air a t Lvhich maximum nucleus-free fuel rate occurs is in the neighborhood of 400Yc except for methane and acetylene. This suggests that availability of oxygen is important in influencing the processes of carbon formation and hence the air pollution potential of a fuel. .4 collision parameter was calculated in a n attempt to correlate qualitatively the relative air pollution potential of a fuel tvith the availability of oxygen. This parameter indicates the numbcr of collisions per second per bond bet\veen a fuel molecule a n d oxygen molecules a t the upper flammabi1it)- limit. From elementary kinetic theory (5). I
and
Z’
CY
*v2cT:(c;
+
6;)’ 2
(10)
Subscripts e a n d g = ethylene a n d any other gas R = microammeter reading nT = relative nucleus concentration a,-4, 6 , B = constants k = coagulation constant for combustion nuclei d = diffusion coefficient for the nuclei-flask system Q = volumetric rate of flushing air V = volume of the flask no = concentration at time zero t = time Z,l o = light transmission ratio r = radius of water droplets n = concentration of water droplets 1 = length of light path K = scattering-area coefficient for water droplets in air for visible light TC’ = xveight of water released by adiabatic expansion of the counter with allo~vancemade [or expansion of the primary chamber 7 = viscosity of the gas p = density of the gas c = average speed of qas molecules u = diameter of gas mglecules .\- = concentration of gas molecules 7’ = number of collisions per second betLveen a a , molecule and u ? molecules .\-! = concentration of a? ~ n ~ ) l r c u l e s ua = l ’ ? ( U i CT?) T = absolute temperature .lf = molecular iveight I; = constant such that Z for methane equal 100 L = upper flammable limit of fuel v. c . .If I = molecular \\eight of furl .if? = molecular IveiSht of oxygen f = number of bonds in a fuel molecule
+
T h e average speed of a molerules. c.
Cited
(11) From Equations 9. 10, and 11. the collision parameter. Z , is:
Literature
T h e value ua \vas calculated from available viscosity data a t 20’ C. (2). p \vas taken to be proportional to M, and T \vas the maximum observed flame temperature (,?). T h e calculation of Z assumes: 7 a P 2 : p CY 44: .I-? cy (1 - L ) ; and the relative u does not change \vith temperature. T h e collision parameter describes only the initial process a t the instant tvhen the mixture of fuel and oxygen reaches the upper flammable limit. T h e error in Z due to uncertainty in the flame temperature to be used is largely minimized because 2 a T I 2 . T h e lack of adequate data to eliminate the last assumption can lead to gross errors in the value of Z. However. the collision parameter does enable a qualitative correlation of the relative air pollution potential lvith the availability of oxygen (table and Figure 2).
869 (1931). (4) Landsberg, H., Er,oeb. Kosrn. Phys. 3, 155 (1938). (5) Loeb, L. B.; “ T h e Kinetic Theory of Gases,” 2nd ed., McGraw-Hill. New York, 1934. (6) Mikhailovskaia, A. N..J . Gcophys.: MOSCOW 6, No. 4. 307 (1929). (7) Neuberger, H., in “Proceedings of the United States Technical Conference on Air Pollution” (McCabe, L.. editor) I McGraw-Hill, New York. 1952. (8) Parker, LV. G., W‘olfhard. H . G., J . Chem. SOC.1950, p. 2038. (9) Schalla, R . L.. Clark, T . P.! McDonald, G. E., 9.ii. C. ,4., .inn. Repts.. Rept. 1186 (1954). (10) Stratton, J . X., Houshton. H . G.; Phys. Ret.. 38, 159 (1931).
r
CY
(T/,i4)12
Nomenclature
INDUSTRIAL AND ENGINEERING CHEMISTRY
Q = the volumetric flow rate = the molecular weight
(1) Houghton. H. G.. Chalker. \V. K.. J . Ofit. SOC.Aw. 39, No. 11. 955 (1949). (2) ”International Critical Tables.’‘ Val. V, 1, McGraw-Hill: New York, 1929. (3) Jones, G. \V., Lewis. B., Friauf. J. B., Perrot. G. St. J.: J . .4m. Chem. Sod. 53,
RECEIVED for review October 1-, 1960 ACCEPTED December 7,1960 Division of Water and Waste Chemistry, Symposium on Air Pollution. 138th Meeting, ACS: New York. N. Y., September 1960. Work is supported by Research Grant RG-4281 National Institutes of Health, Public Health Service. and is in cooperative effort within t h e University of California of the School of Public Health and Department of Engineering.
