Measurement of the soot concentration and soot particle sizes in propane oxygen flames

Eighteenth Symposium (International) on Combustion

The Combustion Institute, 1981

MEASUREMENT OF THE SOOT CONCENTRATION AND SOOT PARTICLE SIZES IN PROPANE OXYGEN FLAMES H. BOCKHORN, F. F E T T I N G , U. MEYER, R. RECK, G. W A N N E M A C H E R

Institut fiir Chemische Technologie, Technische Hochschule Darmstadt, 6100 Darmstadt, Bundesrepublik Deutschland

Soot concentrations and particle sizes were measured by light scattering and probe measurements in the burnt gas region of atmospheric pressure propane-oxygen flames and propane-oxygen flames to which hydrogen or ammonia were added. The results show that the soot concentrations in propane-oxygen flames, to which hydrogen is added are lower compared to propane-oxygen flames. The decrease of soot concentration is much stronger when ammonia is added. Associated with the reduction of soot concentration is a reduction of mean particle size of the soot particles and a lower breadth of the particle size distributions. Electron micrographs of soot particles from the probe measurements showed that soot particles from flames with high soot concentrations (propane oxygen flames) are aggregates with chain or cluster structure while the structure of the particles from flames with lower soot concentration (propane oxygen flames with hydrogen or ammonia added) is more compact. Light scattering measurements were evaluated by nonlinear regression analysis of the scattering curves using different scattering models. Light scattering measurements and probe measurements which give high consistency for flames with low soot concentration and a more compact structure of the particles are discussed with respect to particle structure. The soot inhibiting effect of hydrogen and ammonia added to propane oxygen flames and their influence on the soot particle sizes are discussed as a strong deceleration of the reactions which lead to nucleation and a deceleration of the surface growth reactions for the built nuclei. This can be elucidated by simulation of particle growth of soot particles in these flames by a model including nucleation, surface growth and coagulation.

I. Introduction Many investigations deal with the formation of soot during the partial b u r n i n g of hydrocarbons (a review is given bye). In addition to localizing the conditions for soot formation in flames and its dependence upon different parameters, the explanation of the soot formation mechanism and the growth of soot particles were the main considerations. The effect of additives in preventing the emission of soot and polycyclic hydrocarbons is also of considerable interest. Among the additives affecting the soot concentration in hydrocarbon flames and the properties of the soot particles are water, 2 methanol 3 and salts or organic compounds of several metals. 4'5'~ The effect varies according to the chemical properties and the structure of the additives: Water and methanol act indirectly as oxidizing agents by increasing

the concentration of hydroxyl radicals w h i c h react with the soot particles according to Cs + OH CO + 1 / 2 H~ in all stages of particle growth and therefore reduce the amount of soot formed. 2"3Alkaline earth metals act by catalysing the radical reaction H~O ~ H + OH, so that their final effect also consists in an acceleration of soot particle oxidation. 5'6 Alkaline metals induce higher electrical charges on the soot particles. This prevents coagulation; the particles remain smaller and therefore can b u r n faster in an oxidizing atmosphere, ~ This work investigates the effect of two different additives, namely hydrogen and ammonia. For this purpose, the soot concentration and the distribution of particle sizes were determined in fiat propaneoxygen flames and in fiat propane-oxygen flames to which hydrogen or ammonia was added. Probe measurements and light scattering measurements were made at atmospheric pressure.

1137

1138

SOOT

II. Experimental Burner System and Apparatus The burner was a water-cooled porous plate fiat flame burner with 100 mm diameter. The gases used were of technical quality (propane: 99%; oxygen: 99.5%; hydrogen: 99.9%; ammonia: 99.98%; nitrogen: 99.99%). The fiat flames were enclosed in short quartz cylinders placed on the burner plate. They were stabilized by a wire grid which was located 40 to 60 mm above the burner plate. The total flow was 172 ema/sec at STP for all flames. The probe measurements were performed with a critical orifice placed at the tip of a quartz cone. This cone had an angle of 40~ and was attached to a plate by a plane ground joint; the gas was drawn through it by a rotary vane pump. To determine the soot particle dimensions, a small mica plate was briefly placed in the overexpanded jet in the orifice (diameter: 800 ~tm). This plate was placed about 30 mm behind the orifice. Thereafter the sample was exposed to a carbon platinum vapor under a 30~ angle and then inspected under an electron microscope. To determine the soot concentration, a boron silicate filter was placed in the over-expanded jet (orifice diameter: 250 I~m) for a longer time; this retained the solid particles. The gas volume drawn from the flame was measured behind the rotary vane pump. In order to perform the scattered light measurements, a chopped laser beam (633 nm, 5 mW) was passed through the flame gases containing the particulate matter. An image of a certain sample volume-12.0 mm a in the case of a 90~ scattering angle--was produced on the first dynode of a photomultiplier fixed in the observation plane at an angle of 90~ with respect to the incoming beam and of a photomultiplier movable in a range of 180~ in the plane of observation by polarisation filters, interference filters and the corresponding optical parts (for details seer'8). The signals of both photomultipliers were amplified by lock-in-amplifiers, converted into logarithmic expressions and subtracted from each other, so the absolute intensity as well as the intensity of scattered light normalized to that of 90~ could be determined for the vertically and horizontally polarized components of the scattered light. The experimental set-up is similar to that used conventionallyin light scattering measurements (see for exampleZ4).

Evaluation of Probe Measurements To determine particle size distributions, the areas (silhouettes) and the anisometry of the particles were calculated from electron micrographs, using the procedure proposed by9. Because of the irregularities in the particle structure, the volumes of the particles

were calculated as the volumes of ellipsoids with equal semiaxes. The particle size is finally given as the diameter of a sphere of equal volume, in order to enable comparison with the results of the light scattering method. Particle size histograms were based on the evaluation of 200-500 particles each. The boron silicate filter was weighed after drying to determine the soot concentration in the gas sampies. The weight of the soot was related to the corresponding gas volume at STP.

Evaluation of the Light Scattering Measurements The relationship between the scattering intensity I s originating at a scattering center, e.g. a soot particle in a hydrocarbon flame, and particle properties as well as optical parameters is fundamental for the evaluation of the measurements:

Is = f(d,h,m,O,q~,r~, Io)

(1)

In detail, this relationship reads: Is =

lo ~

4~2t'B

[(ill + i~2) s i n ~ + (i~, + i22)cos2q~]

(2)

I,, denotes the intensity of the incoming light of wave length h, r~ is the distance between scattering center and observer, and ~pis the polarisation angle. The i~j are the squares of the amplitude functions of the scattering matrix i , = [S~j[~ which result for every scattering problem from the solutions of Maxwell's equation under the corresponding boundary conditions. For the case of spheres only, the nondiagonal elements of the scattering matrix are equal to zero, therefore the scattering intensity [Eq. (2)] contains only the amplitude functions S tt and $22 which consist of the known Mie coefficients and of scattering angle dependent functions derived from Legendre polynoms (see..... ). Solutions for Maxwell's equations are also known for general spherical geometry ~ and for pairs of spheres, la'14 For these cases, the solutions contain all the amplitude functions of the scattering matrix, because of the rotational dissymmetry. Additionally, the functions depending on the scattering angle also contain properties of the particles. For the case of pairs of spheres, the solutions can only be handled if multipole fields are not considered. Therefore, these solutions are only applicable for values of the particle size parameter, a = ~r 9 d/h, up to one. To clarify the effect of different scattering models, calculated scattering functions for spheres as well as for spherical particles with a semiaxis ratio of

MEASUREMENT OF SOOT CONCENTRATION IN PROPANE OXYGEN FLAMES 2, having the same volume were compared. It appears that there are no substantial differences between these two scattering models for particles of the size of those investigated here (d up to 200 nm). 7 On the contrary, the evaluation of scattering curves for multiple centre scattering yields particle sizes which are too small by up to 50% under the assumption of spherical geometry. Furthermore, the results show that the mutual influence of scattering disappears if the origins have distances of more than three particle diameters. According to Eq. (1) the measured scattering intensities yield the particle size d, the refraction index m ~ n - n ' i , as well as the corresponding parameter for a particle size distribution for polydisperse particle systems. Parameter estimation was performed by nonlinear regression analysis using the Marquardt procedure. 7'8 Spherical geometry or multiple center scattering were used as scattering models, depending on particle size. Based on inspection of the electron micrographs, logarithmic normal distributions were assumed as particle size distributions. In addition to the mean particle size d~ (first moment of the distribution) its standard deviation a~ appears as a further parameter in this assumption which is generally applicable for agglomerating systems ~5 or soot particles. ~'~ The soot concentration can be determined by measuring the extinction if this is caused only by soot particles, because K,~, = n r 9 ~,~,

(3)

nv means the particle number density and ~ the cross-section for extinction averaged over all particle sizes; ~ , can be calculated by the amplitude functions of the scattering matrix, aa In the flames investigated, other components besides the soot particles can contribute to extinction (see e.g.~8); in this case the soot concentration calculated from measured extinction coefficients is too high. The soot concentration may then be determined by measuring the absolute scattering intensity, which is given by TM. U = n T 9 F. ' exp(Kox, 9 L) 9 U ~ / G ~

(4)

with U s and U L as amplified voltages of the photomultiplier for light scattering measurements, respectively for pure power measurements of the laser. G ~ is a parameter combining all optical factors for the devices and has to be determined anew for every set up. ~'s means the averaged total scattering function according to Eq. (2) which has to be calculated from the particle properties resulting from the scattering curves. K ~xtis the extinction coefficient which has to be measured separately. The solutions of the scattering problems are such that in many cases the parameters a~ and a~ cannot

1139

be determined independently, but only as a correlation function which may be approximately determined as a , = [1,59' c%(~r, = 1) - 0,221 9 tr~ 2''6

(5)

when performing nonlinear regression analysis. 7'~ Values for a~ and c r have to be calculated using additional information, namely, the measured extinction coefficient and the absolute scattering intensity.

III. Results and Discussion Particle Sizes

The structure of the soot particles is more complex for flames with higher soot concentration, e.g. C3Hs---O~, flames than for those with lower soot concentration, e.g. C3Hs---O~--N2wNH~ flames. The particles of the former exhibit pronounced chain structure, while flames of the latter kind deliver particles with a shape similar to spherical geometry. This can be shown qualitatively by electron micrographs of soot particles from flames with different soot concentrations (Fig. 1). All the samples are taken at a height of 25 mm above the burner plate. Figure 1, left, corresponds to a C 3 H 8 - - O ~ flame with a soot concentration of 3 mg/1, Fig. 1, center, to a C3Hs----Oz--N2--H 2 flame with 1.3 m g / l soot and Fig. i, right, to a C3Hs---q92--N2wNH3 flame with 0.5 m g / l soot. The C / O ratio was 0.7 for all flames. Figure 2 shows particle size distributions as derived from probe measurements and light scattering measurements for the same flames at the same position. First of all, one notices that the observed particle sizes are essentially smaller in the light scattering measurements than in probe measurements in the case of flames with high soot concentration (C,~H 8--flame with 3 mg / 1soot). A mean value for the particle size of about 120 nm is derived from light scattering measurements, whereas a value of 160 nm results from probe measurements. Highly consistent results for both methods can be found for flames with soot concentrations lower than 2 mg/l; these are the C,H~-----O2--N~--H ~ and C 3 H s - - O ~ - - N z~NH~ flames. The actual discrepancy for flames with high soot concentration is even larger than shown in Fig. 2, since the method of evaluation of the electron micrographs is inherently biased by the fact that the dimensions of a particle with cluster or chain structure are expressed as the dimensions of a sphere of equal volume. The actual size of the particle is surely larger than that which results from this simplification. On the other hand, it is impossible to comprehend the loose collection of a cluster or a particle with chain structure as a whole because

1140

SOOT

10Ohm FIG. 1. Electron micrographs of typical soot particles in a a) CaHa-"q32 flame b) C3Hs-O~--N2--H 2 flame and c) C3Hs----O2--N2--NH3 flame. Height above the burner H = 25 ram. Composition of feed in % per volume: a) Calls:32.0; O~:68.0 b) Calls:29.4; O2:63.1; Nz:l.9; H2:5.6 e) C~Hs:29.4; O2:63.1; Nz:4.0; NH,:3.5

the scattering waves do not interfere except for addition of intensities if the single scattering centres have distances more than three particle diameters. Therefore evaluation of the light scattering measurements yields a larger number of smaller particles instead of such particle agglomerate as a whole. This is also reflected by the fact that a multiple center model gives a better approximation for flames with particles having a chain structure (Fig. 1); the corresponding calculation is represented by the dashed line in Fig. 2. Furthermore, when the volume is determined from the electron micrographs the empty spaces within the clusters are eventually included in the volumes; the volume estimated in this way may therefore appear larger than the actual volume. The optical method, on the other hand, only reflects the interference of light with actual matter in those loose aggregates. If one assumes a dense packing of identical spheres for the estimation of this influence, then one obtains a value of 0.7 for the ratio of the sum of all single volumes and the volume of the whole aggregate. The optical method therefore "sees" a particle, the diameter of which is too small by a factor of 0.9 (these systematic errors become more important as the structures of the aggregates become more complex). Some further aspects of the light scattering method have to be discussed in this context. The nonlinear

regression analysis of the measured scattering data gives estimated values for the parameters a,, ~r, n and n'. (The computations clearly show that regression is improved by varying the real and imaginary parts of the refraction index, instead of assuming them to be constant. The following values were obtained for the refraction index 25 mm above the burner: C3Hs"-O 2 flame C3H~--O2--N~--H~ flame C3Hs---O2--N2--NH~ flame

n= 1.1-0.37i n = 1.3 - 0.74 i n= 1.3-0,94i

In 17 n = 1.6 -- 0.6 i is assumed.) The estimated parameter values vary within a confidence interval. Individual confidence intervals for the true contours for the sum of square surface are best suited in the present case of nonlinear regression analysis,z~ According to them the estimated parameter values lie within these intervals, with 99% probability. Large differences for the confidence intervals appeared for the different runs. The mean confidence interval for a~ is 10%, 5% for cr 15% for n and as much as 25% for n'. This means that the particle size distribution can be derived from the scattering data with higher confidence than the refraction index. A physical interpretation of the refraction index obtained from this regression analysis therefore is only meaningful if very large dif-

M E A S U R E M E N T O F SOOT C O N C E N T R A T I O N IN PROPANE OXYGEN F L A M E S

0.01 ~

~

1141

Probe Measurements Light Scattering Measurements,

--

Spheres .....

Light Scattering Measurements. Muttipte Center Scattering

0,005

O,C2

200

--Particte

Z,O0

I

600

Size d / n m

0.~ I

0.01

0.01

i

b) ,0.00

0

,

,

200 ~.00 --Particle Size d / n m ~

(

H=lOmm

a) I/ ! " ~ ' ~

H 9 20 mm

//'/~\~

~

H:/.0 mm

160

0

200

Particle Size

300

d/nm

0,02l

b) 400

=lOmm

Partcte Size d/nm

0~1

F1G. 2. Particle size distributions of soot particles m a a) C 3 H s - - O 2 flame, b) C3H~-----O2--N~--H ~ flame and c) C 3 H 8 - - O 2 - - N 2 - - N H 3 flame. Height above the burner H = 25 ram. Composition of feed as in Fig. 1. ferences appear. 8 On the other hand, variations of the refraction index scarcely influence the calculated size distributions. A further point influencing the evaluation of the light scattering measurements is the anisotropy of the soot particles as pointed out in 16. This anisotropy causes cross polarization effects in the scattered light. Measurements of the cross polarization in a flame showed that while it is less than 2% of the vertically polarized component while in the range of 90 ~ scattering angle it is of the same magnitude as the horizontally polarized component. When simulating this influence by estimation of the parameters a~, (rg, n and n ' from the scattering curves, where the horizontally polarized components

- -

360

Particle Size d / n m

0,0;

c)

>.

//•-•

H= 10 mm

>,0~1

I

100

0

--Particle Fl(;. 3. Particle size distribution of soot particles for various heights above the burner for a a) C 3 H s - - O 2 flame, b) C 3 H , - - - - O z - - N r ~ flame and c) C 3 H s - - - O ~ - - N 2 - - N H 3 flame. Composition of feed as in Fig. 1. Figure 3a additionally shows the particle size distributions, which are calculated by the solution of Eq. (6).

2oo

16o

0

200 Size d/nm

Light Scattering Measurements. Spheres

Light Scattering Measuraments, Multiple Center Scattering Calculated

300

1142

SOOT

are reduced by one half in the range 8 5 0 -< 0 -< 95 ~, one finds that the real and imaginary parts of the refraction index decrease by about 10%. The value of c~ increases slightly whereas ~r decreases. Altogether, the changes of the different parameters remain within the confidence intervals mentioned above. The strong broadening of the particle size distributions as a function of the flame height above the burner is shown in Fig. 3. The green-blue oxidation zone of the flame stabilizes 2.8 mm to 3.0 mm above the burner plate. Data were taken between 10 mm and 45 mm above the burner. At the same time as the strong broadening of the particle size distributions is observed, the most frequent particle diameter doubles in the range between 10 and 20 mm. The distribution resulting from evaluation according to the multiple center scattering model for the C3Hs---O ~ flame is also shown (dashed line). The mean values of the particle size show a strong increase from 30 nm to 100 nm in the range between 10 and 20 mm above the burner plate. The profiles of the optically measured mean diameters and profiles developed from the probe measurements for the same three flames are shown in Fig. 4. The increase is less for both the flames with lower soot concentrations. The optically determined particle sizes are smaller than those from probe measuremen ts for the C 3H , -432 flames, as discussed above. A better agreement is obtained by using the multiple center model to evaluate the scattering curves. The average values for the particle sizes obtained in this way are also shown in Fig. 4. The distributions according to this model are broader which means that ~r is larger (see also Fig. 2 and 3). Figure 3 additionally shows, the particle size distributions which can be calculated for the C3H8---O 2 flame, by solving Eq. (6) according to the conditions represented by Fig. 7, curve 1. As can be seen from Fig. 3a, reasonable agreement between optically measured particle size distributions and calculated size distributions is achieved, the calculated particle size distributions being more symmetrical than the log-normal distributions assumed for the evaluation of the scattering curves. Generally, mean values and standard deviations are in the usual order of magnitude; e.g.'8 find d~ = i00 nm and ~, = 1.5 for a soot generator with propane as fuel.

i

4. Probe Measurements 9 Light Scattering Measurements, Spheres Light Scattering Measurements,

200

I00

4-

E Iso 4".-.."" N

lOO 4-fl

4-

J o f

~ 1 7 6

l

0

10 --Height

I

20 30 40 above Burner H/mm

~--

F]c. 4. Profiles of mean particle size for a C3H8----O = flame (upper), C3H,-----Oa--N2--H 2 flame (middle) and a C 3 H s - - - O 2 - - N 2 - - N H 3 flame (lower). Composition of feed as in Fig. 1.

Soot Concentrations The soot concentrations increase with increasing fuel concentrations. This can be seen from Fig. 5, where the soot concentration at 30 mm height above the burner plate for C3H,---O 2 flames, C 3 H s - - O ~ - - N 2, C3H8---O 2--N2--H= and C 3 H s - - - O ~ - - N 2 - - N H flames is shown as a func-

tion of the propane concentration. Clearly, the soot concentration increases for all flames with increasing propane concentration, e.g. from 3 mg/1 at C / O = 0.7 to 7 mg/1 at C / O = 0.9 for the C 3 H 8 - - O 2 flame. The soot concentration in the

MEASUREMENT OF SOOT CONCENTRATION IN PROPANE OXYGEN FLAMES C 3 H s - " 4 ) 2 - - N 2 - - H 2 flame with 1.9% N 2 and 5.6% H 2 in its feed is distinctly lower than that in the C a H s - 4 3 2 - - N 2 flame with 7.5% N2. The soot concentration is reduced even more in the C 3 H s - - ~ 2 - - N 2 - - N H a flames, their feed contains C3H 8 and NH3 with a constant ratio of 8.5. No variation of the soot concentration was observed between 15 and 45 mm above the burner for the probe measurements. The error ranges shown in Fig. 5 for the probe measurements are determined experimentally from a series of runs. The soot concentrations determined by light scattering according to Eq. (4) are higher than those determined by probe measurements by a factor of 2 to 5. This stems from the fact that all systematic errors for u and c r discussed above enter into the averaged total scattering functions Fs according to Eq. (4). It can also be traced back to the fact that the soot particle volume is determined by the use of n T, a~ and ~ , assuming a density of p.~ = 1.86 g / c m 3 for the soot. Especially c r has a strong influence; a systematic investigation ~ shows that a 10% variation ofa~ in the range of c r = 1.5 causes a 40 to 50% variation of the soot concentration. Furthermore it has to be mentioned that the logarithmic normal distributions for the particle sizes are only approximations which do not completely consider, for example, the large particles (see Fig.

.~_

/,i

13

0J

0,8 C/0 -Ratio

a9 ~'~

Flc. 5. Soot concentrations for C~Hs---O 2 flames

(o), C a H ~ - - - O 2 - - N 2 flames with 7.5% per volume

N2 (X), C3Hs---432--N2--H2 flames with 1.9% per volume N 2 and 5.6% per volume H 2 (VI) and C 3 H s - - O z - - N ~ - - N H ~ flames with 7.5% per volume (N 2 + NH3) and [ C a l l s ] / [ N H 3 ] = 8.5 (A). Height above the burner H = 30 ram.

1143

2 top). Measurements of the absolute scattering intensity yield soot concentrations that are too high, if the model does not adequately consider large particles because the scattering intensity is dependent on d" as a first approximation. Therefore the light scattering method is not as suitable as probe measurements for the determination of soot concentrations in polydisperse systems with absorption. Simulation of the Particle Growth Particle growth for an agglomerating polydisperse system can be simulated by the solution of the particle balance :1~ i=l

dn k dt

1 ,=k- l | Z 13''n , n , - n k ~ 13,k n, 2 ~=1 t=1

(6)

i--k--i

in which nk, n, or n, mean the particle densities of size classes k, i and j. 13, and 13,kare the collision parameters for the particles of class i with those of class J or k; they have to be determined according to the Knudsen numbers in question. Equation (6) has to be integrated numerically to calculate the particle size distribution for a certain reaction time. For the numerical integration the size distribution is separated into 400 particle classes and the number of collisions for the particles in each class is calculated according to the corresponding collision parameters. The particle numbers for each class are calculated anew for every time interval of the integration, with respect to the particles that form or disappear by collision. The time intervals for the integration were chosen such that the calculated collision numbers were small compared to the particle numbers. Figure 6 shows the profile of the mean particle diameter calculated from the solution of Eq. (6) for the C 3 H , - - O ~ flame with C / O = 0.7. It was assumed that all the measured soot (see Fig. 5) is present as "nuclei" of 1 nm radius at the time t = 0, this time corresponding to the beginning of the soot forming zone. This means that coagulation is the only physical process taking place. Curve 1 shows the mean diameter for free molecular coagulation at 1500 K. A change of the temperature to 1800 K (curve 2) or a correction of the collision parameters according to Stokes-Cunningham ~ (curve 3) has almost no influence on the mean diameters of the particles which lie appreciably below the measured particle sizes (curve 6, evaluation with the multiple center model). Better consistency is only achieved when the structure of the agglomerates is considered by assuming a ratio of 0.7 between the sum of the single particle volumes and the volume which is effective for the collision (curve 4). This assumption considers that the volume

1144

SOOT

'~176

I

J ++_++

0

+

10

20

Height

above

+

30 Burner Hiram

t~0

Ftc. 6. Comparison of profiles of measured and calculated mean particle size for a C a H ~---O z flame. Composition of feed as in Fig. la. 1 free molecular coagulation; 1500 K 2 free molecular coagulation; 1800 K 3 Stokes-Cunningham correction; 1500 K 4 free molecular coagulation; ratio of actual particle volume to collisional volume, 0.7; 1500 K 5 like 4; elliptical shape of particle, ratio of half axes 0.25 6 mean particle size measured by light scattering (multiple center scattering)

and therefore the collisional cross section of a particle is larger when it is regarded as an agglomerate of spheres and not as a single sphere of equal mass. Therefore the faster growth of the mean diameter in this case is due to the larger collision parameter. The additional assumption of an elliptical particle shape also improves the results for flames with high soot concentration (curve 5, ratio of semiaxes = 0.25). The reason for the faster growth of the mean diameter compared to simple spheres is the larger collisional cross section of a particle with lower symmetry compared to a sphere with equal volume. The average collisional cross sections of the ellipsoids are calculated by integrating of the silhouettes over the two solid angles. A more realistic simulation of particle growth assumes that the nuclei appear at the beginning of the soot forming zone with a certain time function and not in a single moment. Such a time function is developed by L9 from the analysis of the particle growth in low pressure sooting flames. The time in which all the measured soot is formed by nucleation and surface growth (H = 10 mm above the burner) is the upper limit for the time in which nuclei can form. The surface growth of the particles is assumed to be proportional to the total surface of the particles and to the amount of gas that is condensing to soot. Figure 7, curve 1 shows the course of the mean particle diameter for the

C3H8---O ~ flame under these assumptions (curve 2 reflects the measured data). The nucleation rate was set in such a way that the total number of nuclei formed equals the number which results from expressing the total amount of soot by the smallest single particle to be found on the electron micrographs. It seems reasonable to assume that these particles are grown as single particles by surface growth and have not been subjected to agglomeration. This results in a slower particle growth in the very first phase, as can be seen from curve 1. The agreement with the measured curve is much better than in Fig. 6, especially in the range 10 mm -< H -< 25 mm. This assumption results in the main amount of the soot (98.5%) deriving from surface growth and only 1.5% from delayed nucleation. The measured particle sizes can only be reproduced approximately by the solution of Eq. (6) for the case of the C3Hs----O2--N2--NH 3 flame with C / O = 0.7 if the nucleation rate is drastically reduced compared to the C ~ H s - - O 2 flame (factor 10 -2, Fig. 7, curve 3). This means that for the C3Hs----O2--N2--NH 3 flames, very much fewer nuclei are present at the beginning of the soot formation and that these nuclei grow to relatively large particles by surface growth before agglomeration begins because of the gradually increasing particle number. This finally leads to larger particle

150

I++,+~+ / = ~S ' ] : ~ I

10

20 H~gh|

above

" + +

.... ++.+~++-+'s--+ --

30 Burner

t*O HIrn rn

FIc. 7. Comparison of profiles of measured and calculated mean particle size for a C3H8---O2 flame and a C 3 H s - - - O 2 - - N 2 - - N H 3 flame. Composition of feed as in Fig. la and lc. 1 C3Hs----492 flame; like 5 in Fig. 6; delayed nucleation and surface growth 2 C3H8---O 2 flame; like curve 6 in Fig. 6 3 C3Hs-----O2--Nz--NH , flame; like 1; delayed nucleation and surface growth 4 C3Hs----O2--N2--NH3 flame, mean particle size measured by light scattering 5 C3Hs---O2--Nz--NH3 flame, mean particle size measured by probe measurements.

MEASUREMENT OF SOOT CONCENTRATION 1N PROPANE OXYGEN FLAMES diameter in the late phase of particle growth, as compared with pure agglomeration. However, at the same time the particles should exhibit a simpler structure (compare Fig. I). In this case only 0.2 960 of the measured soot quantity is formed by nucleation. The agreement with the particle diameter measured by the light scattering method is not as good as for the C3H~--O 2 flame (curve 4). Particle diameters derived from probe measurements (curve 5) lie below those determined optically. The reason for this may be that the underestimation of particle sizes is not as pronounced for evaluation by the light scattering method as from the electron micrographs because of the relatively compact structure of the particles in this flame (Fig. 1). The soot decreasing effect of the additives investigated here seems to be due to the fact that fewer nuclei are formed and not that the soot particles grow more slowly because of an increased oxidation rate. The rate of nucleation in this case is decreased considerably more than the rate of surface growth which is reduced only by a factor corresponding to the ratio of the soot quantities formed. This is in agreement with earlier observations . 20 in which smaller particle number densities were found with decreasing soot concentration in a C 2H 2----02flame. It seems reasonable to assume that "nuclei" are formed by reaction of reactive unsaturated hydrocarbons (e.g. components formed of polyacetylenes by radical reactions 2~ and that these "nuclei" also adhere to the surface of soot particles already being formed. It is known that hydrogen reduces the concentration of those polyacetylenes in C2H2---O2--H z flames. 21 Also, measurements of concentration profiles in low pressure C3Hs----O2--NH3 flames showed that the concentration of polyacetylenes and higher hydrocarbons, e.g. polycyclic aromatic hydrocarbons is lower compared to C 3H~-----O2 flames. 22Therefore the soot diminishing action of hydrogen and ammonia consists in a reduction of the concentration of these intermediates. This reduction can effect a much stronger decrease of the nucleation rate compared to the surface growth rate because the reactions leading to nucleation must be of a higher order than those for surface growth. The reduction of the concentration of the higher hydrocarbon components may be explained in the case of hydrogen by a shift of the overall reactions between the polyacetylenes to lower components,2~ whereas in the case of ammonia, small hydrocarbon radicals may be trapped by ammonia or nitrogen hydrogen radicals, hydrogencyanide being formed at the same time and growth of polyacetylenes and higher components being blocked, z2 The strong reduction of the nucleation rate compared to surface growth finally leads to particles with the more compact structure found in the C3H~----O2--N~--NH3 flames for which the agglomeration rate is slow

1145

compared to surface growth because of the low particle density numbers.

IV. Conclusions The comparative measurements of soot particle sizes and soot concentrations in C3H~----O2, C3Hs--K)2--N2--H2 and C3Hs--Oz--N2--NH3 flames by the light scattering method and by probe measurements elucidate the following points: The systematic errors that occur when applying the light scattering method to polydisperse particle systems with aggregate structures result in particle sizes which are too small. Application of different scattering models (spheres and multiple center scattering) elucidate these inherent limitations. Evaluation of the scattering curves by nonlinear regression analysis which is the only reasonable method, yields the mean particle sizes, the parameter of the particle size distribution, and the real and imaginary part of the refraction index within certain confidence intervals. These confidence intervals have the property that variations of the scattering curves by changes in the nature of the particles, e.g. by anisotropy, result in variations of the calculated parameters lying within them. This is due to the nature of the solutions of the differential equations of the scattering problem. Measurements of the soot concentrations by direct determination of the mass (probe measurements) are preferable to the light scattering method. The latter gives values which are mostly too high because of the absorption by other components present in the flames and because of systematic errors in the particle size parameters which enter twice into the determination of the soot concentration. The systematic errors are the larger the more complex the structures of the aggregates are. The evaluation of particle sizes from electron micrographs also yields particle sizes which are too small compared with the actual particle sizes if the dimensions of a particle with complex structure are characterized by only one parameter. The soot concentrations decrease noticeably in C.~H 8"--O2 flames if H 2 is added to them. The effect is even stronger if NH3 is added instead of H2. At the same time, the mean particle size decreases together with the breadth of the particle size distribution. The chemical effect of H2 and NH 3 on the soot formation is not the same as that of H20 or alkaline- and alkaline earth metals. These slow down the soot particle growth by increasing the rate of the reaction C s + OH --~ CO + 1/2 H2, whereas H 2 and NH3 interfere with the reactions leading to the formation of "soot nuclei." They cause a decrease in surface growth during the first phase of particle growth, and a drastic reduction of "nucleation." This can clearly be shown by comparing the measured particle sizes with the particle sizes

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SOOT

calculated using a model that includes coagulation, surface growth and nucleation.

Acknowledgement We are greatly indebted to the Deutsche Forschungsgemeinschaft for substantial financial help.

REFERENCES 1. WAGNER, H. G.: Seventeenth Symposium (International) on Combustion P. 3, The Combustion Institute, 1979 2. MOLLER-DETHLEFS, K. AND SCHLADER, A. F.: Combustion and Flame 27,205 (1976) 3. CHAKRABORTY,B. B. AND LONG, R.: Combustion and Flame 12, 168 (1968). 4. SPENGLEB, G. AND HXtJPT, G.: Erd61 u n d Kohle 22, 679 (1969) 5. COTTON, D. H., FRISWELL, N, J. AND JENKINS, D. R.: Combustion and Flame 17, 87 (1971) 6. HAYNES, B. S., JANDER, H. AND WAGNER, H. GG.: Seventeenth Symposium (International) on Combustion P. 1365, The Combustion Institute, 1979 7. BOCKHORN, H., FETT1NG, F., MEYER, U. AND WANNEMACHER, G.: in progress 8. MEYER, U.: Dissertation, Technische Hoehschule Darmstadt, Darmstadt, 1979 9. MEDALIA,A. I.: Journal of Colloid and Interface Science 24, 393 (1967) 10. M1E, G.: Annalen der Physik 11, 25, 377 (1908) 11. VANDE HULST, H. C.: Light Scattering by Small Particles, John Wiley and Sons, New York, 1957

12. ASANO,S. ANDYAMAMATO,G.: Applied Optics 14, 29 (1975) 13. TruNKS,W.: Annalen der Physik V 22, 561 (1935) 14. Lips, G. ANn LEVINE, S.: Journal of Colloid and Interface Science 33, 455 (1970) 15, HIDY, G. M. AND BROCK, J. R.: The Dynamics of Aerocolloidal Systems, Vol. 1, Pergamon Press, Oxford, 1970 16. D'ALESSIO, A., DI LORENZO, A., BORGHESE, A., BEREa'rA, F. ANt)MASl, S.: Sixteenth Symposium (International) on Combustion P. 695, The Combustion Institute, 1977 17. DALZELL, W. H., WILLIAMS, G. C . AND Hoa'rEL, H. C.: Combustion and Flame 14, 161 (1970) 18. CmPPET, S. ANt) GRAY, W. A.: Combustion and Flame 31, 149 (1978) 19. WERSROURG,B. L., HOWARt), J. B. ANn WILLIAMS, G. C.: Fourteenth Symposium (International) on Combustion, P. 929 The Combustion Institute, 1973 20. HOMANN, K. H. AND WAGNER, H. GG.: Eleventh Symposium (International) on Combustion, P. 371 The Combustion Institute, 1967 21. HOMANN, K. H. ANO WAGNER, H. GG.: Berichte der Bunsengesellschaft fiir Physikalische Chemic 69, 20, (1965) 22. BOCKHORN, H.: Dissertation, Technische Hochschule Darmstadt, Darmstadt, 1976 23. HIMMELBLAU, D. M.: Process Analysis by Statistical Methods, John Wiley and Sons, New York, 1970 24. WHITBY, K. T. ANt) WILLEKE, K.: Proc. Aerosol Measurement, Workshop University of Florida, Gainesville, 1977

COMMENTS

Insofar as the observed effects of H~ and NH~ addition on soot formation are concerned, have flame temperature measurements a n d / o r calculations been carried out in connection with your experiments? If so, how do the soot results compare with flame temperature trends?

compared to flames to which pure N z was added. Furthermore, we have shown that temperature variations affect only slightly the results of the simulation of the particle growth. Thus the exact values of the flame temperatures are not of great importance concerning the conclusions drawn from our measurements.

Author's Reply. Temperature measurements in the burnt gas region of the investigated flames were performed only for orientation with thermocouples. We think that the reported results are not affected very much by a variation of the flame temperature, for the soot inhibiting additives (H z and NHa) were added in small amounts and together with N 2. In that way the total added volume fraction was 7.5% in all cases and the soot inhibiting effect could be

D. E. Rosner, Yale University, USA. My question concerns the possibility of preferential deposition of the smaller size range particles on the solid mica plates (targets) in your soot sampling probe--i.e., the particle size distribution (PSD) in your flames may be relatively poorer in small size particles than it appears from your electron microscope (EM)

C. P. Bankston, Jet Propulsion Laboratory, USA.

MEASUREMENT OF SOOT CONCENTRATION IN PROPANE OXYGEN FLAMES results. If so, this systematic correction ("deconvolution") should be made prior to comparison with the corresponding results of light scattering measurements. Considering only particle deposition by Brownian diffusion across a laminar stagnation point gas flow boundary layer, in the absence of thermophoretic effects and over the size range 50-600 nm (cf. Fig. 2), one would expect more than a 20-fold ratio in capture efficiency (small vs. large). Fortunately, however, thermophoresis augments the deposition of large particles more than the small ones,~ 3 and this phenomena would be expected to reduce appreciably the aforementioned correction factor (perhaps to nearer 2-fold, with exact values depending upon the ratio of the mica target temperature to the stagnation temperature of the combustion gas mixture entering the smoke). Alternatively, if your targets were themselves permeable to the gas being sampled, then the capture efficiency would become even less sensitive to particle size--ie, with adequate target "suction," no corrections to E M / P S D data would be required before comparison with the results of the non-intensive light scattering technique. Indeed, it would be interesting to make EM comparisons of the PSDs on porous vs. solid targets at the same temperatures.

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REFERENCES 1. ROSNER, D. E., AND FERNANDEZ DE LA MORA, J., "'Recent Advances in the Theory of Salt/Ash Deposition in Combustion Systems," Proc.

DOE / EPRI Conference on Advanced Materials/Alternate Fuel-Capable Directly Fired Engines, Report CONF 790749, (Dec. 1979), pp 301-330. 2. ROSNER,D. E., "Thermal (Soret) Diffusion Effects on Interracial Mass Transport Rates," 1. Physiochemical Hydrodynamics (Pergamon Press) (in press, 1980), 3. ROSNER, D. E., AND SESHADRI, K., (This Symposium).

Author's Reply. We think that the conditions sampiing the flame gases by the used probe were chosen in a way, that the preferential deposition of smaller sized particles on the solid mica plates could be avoided. However, we agree with you that this aspect should be clarified by comparisons of particle size distributions from electron micrographs of soot particles deposited on solid and porous target under the same conditions.