Nitric oxide and composition measurements within diffusion flames around simulated ethanol and ethanolpyridine droplets

COMBUSTION AND F L A M E 25, 107-120 (1975)

107

Nitric Oxide and Composition Measurements Within Diffusion Flames Around Simulated Ethanol and Ethanol-Pyridine Droplets* DENNIS E. LUDWIG, FREDIANO V. BRACCO, and DAVID T. HARRJE Guggenheim Laboratories, Princeton University, Princeton, New Jersey

The composition of major stable species, including nitric oxide, and the temperature were measured within the diffusion flames around simulated ethanol droplets burning in air at atmospheric conditions. Nitric oxide measurements were also made with ethanol seeded with various percentages of a nitrogen containing compound (pyridine). The fuel droplet was simulated by a 1.2 mm porous carbon sphere supported by a fine stainless steel fuel line. Quartz microprobes, quartz coated thermocouples of platinum/platinum-13% rhodium, a gas chromatograph, and a chemiluminescent analyzer were used. The results include documentation of significant oxygen penetration to the simulated droplet surface, and pyrolysis and partial oxidation of ethanol near the surface. The measured nitric oxide concentrations for both pure ethanol and pyridine seeded ethanol were greater than expected in spite of measured flame temperatures considerably lower than predicted.

Introduction The processes of combustion of liquid hydrocarbon fuel sprays are important in relation to both efficiency and emissions of such common chemical energy conversion devices as gas turbines, Diesel engines, direct injection stratified charge engines, and home and industrial furnaces. One major pollutant emitted by these systems is nitric oxide. Nitric oxide is formed from reactions between atmospheric oxygen and nitrogen in the high temperature region where combustion takes place. However, organic nitrogen compounds found in typical fuel oils can also be very significant additional sources of nitric oxide. Few in. vestigations of this problem have been carried out, but the information available suggests that there is a definite relationship between the amount of nitrogen in the fuel and the nitric oxide produced under specific operating conditions [ 1]. However, there does not appear to be a strong relationship between the molecular form of the fuel bound nitrogen and the amount of NO produced [2]. *This work was conducted under EPA Grant No. R-800844, Dr. B. Martin, Program Manager.

This work was undertaken to gather additional information on the structure of droplet diffusion flames, on the formation of nitric oxide within these,flames, and on the relationship between the concentration of fuel bound nitrogen and the concentration of nitric oxide produced. This was accomplished by measuring temperature and concentration profiles of nitric oxide and of the major stable species within diffusion flames around small porous carbon spheres fed steadily with liquid fuels. In this paper, a summary of the techniques used and of the data obtained is given. For more information and details the reader is refered to Ludwig [3]. The experiment was also designed so as to yield data as closely comparable with the theoretical results of Bracco [4] as possible. This theory covers specifically the steady state configuration of the porous sphere experiment. It neglects gravitational effects and efforts were made to minimize these effects in the experiment by careful selection of the sphere size. Even so substantial differences between the theory and the experiment remain as discussed later on in this paper. The theory does not employ the collapsed flame Copyright © 1975 by The Combustion Institute Published by American Elsevier Publishing Company, Inc.

108

D.E. LUDWIG, F. V. BRACCO, and D. T. HARRJE

approximation. It would appear that the only flame structure computations made for finite thickness reaction zones (i.e., without employing the collapsed flame assumption) are those of Lorell, Wise, and Carr [5], of Kassoy and Williams [6], and of Bracco. However only Bracco also calculated nitric oxide concentration profiles, which could be compared with the measured ones. These results were reported first in 1971 [7, 8] and more extensively in 1972 [4]. Although numerous other computations of nitric oxide in droplet diffusion

flames have followed [9-I 1], they relate mostly to unsteady effects, not relevant in this porous sphere experiment, while still adopting the simplifying assumption of collapsed flame which has been shown to be incorrect when nitric oxide concentration profiles are of interest [4]. To add perspective to the results presented in this paper, it is useful to summarize the conclusions reached by Bracco [4], and confirmed by others, about spray combustion and nitric oxide formation:

(1) Diffusion flames around hydrocarbon drops burning in air could be significant sources of NO, particularly at high air temperatures. (2) The effects of nitric oxide diffusion must be included in the calculation for the formation of nitric oxide within the flame. (3) The ratio of the mass production rate of nitric oxide to the mass combustion rate of fuel increases with increasing drop radius. (4) The amount of nitric oxide produced per mass of fuel burned decreases as the spray becomes finer. Finally, for a review of theoretical and experimental studies of the various aspects of the droplet burning problem, the reader is referred to the paper by Williams [12].

Experiment, Equipment and Results

Droplet Size Since one of the objectives is to compare the experimentally determined composition and temperature profiles with a theoretical model that neglects gravitational effects, the simulated droplet must be of a size that minimizes these effects• The presence of gravity creates buoyancy forces that induce convection currents and distort the flame front from that of a sphere. As a result, for a given droplet radius, the mass burning rate is increased over that of the gravity free case. Williams [12] references the following semiempirical mass burning rate formula, which corrects for the effect of natural convection:

m• (nat) = rh ° (1 +

Gr'3),

where

rh° = 47rrt ~

In (1 + B), p

Gr=- gp2(2rl)3 ~2

= 1

L

AT , Tavg

+

gY

],

t

Tavg = (Tflame + Too)/2. The gravitational effects are small if (.2/B'44)Gr .3 < < 1. Using ethanol as a fuel (B --- 3) and assuming properties for air at 1150 °K, a T flame = 2000 °K, and a Too = 300 °K, this relation suggests that the droplet diameter must be much less than 1.15 cm. Thus a 1 mm sphere should be a good choice• This size is amenable to construction and is in the upper size range of droplets actually found in certain sprays. The SupportedSphere For this work, the supported porous sphere technique was used. Fuel is continuously supplied to

NO AND COMPOSITION IN DIFFUSION FLAMES

109 reaction rate of a typical rapid reaction mechanism such as

the surface of a sphere of inert material, in this case, a porous carbon. This technique gives a steady state flame and is useful for studying aerodynamic effects, flame structure, and burning rate coefficients. The effects of different sphere diameters may also be studied. In order to deliver a constant flow of fuel to the porous sphere, a synchronous motor is geared to a precision threaded rod, which in turn is mechanically linked to push a syringe plunger at a constant rate. From the syringe, the fuel flows through a teflon line to a .304 mm o.d. stainless steel hypodermic tube, which supports the porous sphere from below (Fig. l(a)). While many inert materials were tried, the only successful material from which the spheres were made was a porous carbon material readily available to the lab. Due to its porosity, the smallest diameter that could be fabricated was about 1.2

k CH2 0 + OH -+ H20 + HCO, at the flame and sampling line conditions. Such a reaction rate drops by at least a factor of about 2000.

/~mm

DIA.P

HIGHTEMR

L

~ QUARTZMICROPROBE

(b)

Fig. 1. Sketches showing construction of the droplet and quartz microprobe.

The quartz probe orifice diameter was selected based on a spacial resolution of 10 diameters, anticipated species gradients through the flame, the requirement that the species mean free path be much smaller than the probe orifice, the requirement that the probe mass flow rate be much smaller than the fuel flow rate, and sampling time considerations. This analysis indicated that the implied problems could be avoided by using a probe whose orifice diameter was about 30/a (Fig.l(b)).

Gas Sampling Considerations

The technique of probing flames with quartz microprobes is documented and widely used [13-17]. The reader is referred to the literature for discussions regarding the usefulness and limitations of such probes. The quartz probe and associated sampling lines (Fig. 2) were connected in series with a gas chromatograph sampling loop. Design considerations limited the sampling line-loop pressure to a minimum of 2 cm Hg. Quenching efficiency of this design was checked by comparing the drop in the

The Gas Chromatograph

A thermal conductivity gas chromatograph (Carle Instruments Model 8004 modified for our pur-

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PRESSURE

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(o)

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=

110

D.E. LUDWIG, F. V. BRACCO, and D. T. HARRJE so that only NO concentrations were measured in this study. For details of the instrument design and data interpretation techniques, the reader is refered to Ludwig [3].

poses) was selected for the purpose of analyzing the basic stable chemical species expected in the flame structure. Figure 3 indicates the chromatograph design. By knowing the species elution times through the various columns, the sample rerouting valves can be switched to effect the following separation:

Thermocouple Design and Temperature Measurement

Porapak Q - (C2H2 + C2H4), H20, ethanol, Molecular Sieve - 02, N2, CO, CH4, Porapak T - CO2. By maintaining the Porapak T and molecular sieve 5A at 20 °C and the Porapak Q between 85 °C and 90 °C, and by employing helium carrier at 60 cc/ min, an analysis time of 19 min could be obtained. Detector response was recorded on a Bristol Dynamaster chart recorder, and peak areas were estimated using the standard practice of multiplying peak height times the peak width at one-half the height. The Chemiluscent Analyzer

In order to minimize the conversion of flame produced NO to NO2 during sampling and analysis, it is desirable to analyze the gas samples in an "on-line" fashion at low pressure. At the time of this work, however, commerci.al NO analyzers were not designed to sample gases at the low pressure encountered in our sampling lines. Therefore, a special NO analyzer had to be fabricated. Consultation with AeroChem Research Laboratories yielded the necessary design parameters. The resulting instrument was sensitive only to the photon emitted by the reaction N O + O a ~ N O 2 +02 +hu

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Temperature measurements in the flame were made using standard thermocouple techniques that are well documented [ 17-20]. The thermocouples were constructed from platinum/platinum13% rhodium. These materials withstand the flame temperatures without significant oxidation or deterioration and yield a higher response than other combinations. Since temperature measurements were made in regions where kinetic reactions were occurring, the thermocouples were coated with quartz to minimize catalytic effects. The actual thermocouple design and method of probing the flame is shown in Fig. 4. This choice was based on minimizing aerodynamic disturbances from the legs while permitting a geometry amenable to various temperature correction techniques described below. Practical considerations limited the thermocouple wires to .0015 in.; smaller diameters would be preferable. Since in this experiment it is not possible to align the thermocouple wires along surfaces of constant temperature a method must be devised to calculate the error introduced by heat conducted along the probe wires. This was found to be a difficult task, and in the time frame allowed for this work, no reliable method was established. However, several estimation techniques, whereby the measured temperature profile was used to estimate the temperature gradient along the wire, indicated that the measured temperature between the simulated droplet surface and the flame zone may be as much as 300 °K higher than the actual one, depending on the probe position (see Fig. 7).

~

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NO AND COMPOSITION IN DIFFUSION FLAMES

111

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35-

~~ 33" ~-u~

Burning Rate Data

In order to check if the chosen simulated droplet size was indeed small enough to reduce significantly gravity effects, the variation in the mass burning rate with droplet diameter was studied. Simplified theories for gravity free burning droplets predict a constant value for rh °/r I . Figure 5 shows that, with gravity, this ratio rises rapidly for diameters greater than 3 mm, but tends to a more constant value for smaller diameters. The value for the burning rate measured by Agoston, Wise, and Rosser [21 ] for a 6.5 mm diameter sphere burning ethanol under natural convection is 5.1 X 10 "3 g/sec. The value obtained in this work is 5.2 X 10"3 g/sec. For smaller diameters, the burning rates in this work are about 20% higher than those reported by Wise, Lorell, and Wood [22]. Temperature Data Theoretical calculations [4], which have been used as a basis for comparison are plotted in Fig. 6. Experimental temperature profiles are plotted in Fig. 7. Using the measured value of the mass fraction of ethanol at the simulated droplet surface YF l' the temperature at the surface (which is wetted with ethanol) may be calculated from the Clausius-Clapeyron equation:

YF'I=

WF W

_ _

e

R rt

yielding Tl ~ 331 °K.

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I

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Fig. 5. Steady burning rates vs porous sphere diameter; air at T = 300 °K,p = 1 arm, with natural convection. Theoretically, T t = 325 °K for YF,/= .75 and equal molecular weights. These two are in good agreement, however the value is not as measured with the thermocouple even though the junction was touching the liquid surface. Since the thermocouple is located in a region of steep temperature gradients, the major source of error in this region of the flame is conduction of heat to the junction through the probe wire. The profile obtained in Fig. 7 results from the use of an estimation of the temperature gradients along the probe obtained by applying the heat conduction equation. No great weight should be placed on the validity of this corrected profile curve near the drop surface; it is presented only to indicate that heat conducted along the probe is significant in this region. For details of this estimation technique, the reader is referred to Ludwig [3]. As the probe junction moves toward the region of maximum flame temperature, the gradient along the wire becomes smaller, reducing the effects of error in its estimation. In this case, the correction due to conductive transfer was found to be about 35 °K. Also in this region, error due to radiation was found to be about 80 °K. These corrections

112

D.E. LUDWIG, F. V. BRACCO, and D. T. HARRJE

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rI

applied, to our measurement yield a maximum flame temperature of 1830 °K, still 170 °K lower than predicted theoretically. The sensitivity of the correction techniques to the parameters that were estimated and subject to error was then investigated. The results suggested that the corrected

maximum temperature plotted in Fig. 7 is probably correct within 50 °K. It should be pointed out that discrepancies between expected and measured temperatures have been reported by other researchers. Aldred, Patel, and Williams [23] studied the burning of spheres of liquid n-heptane (boiling point of 371 ° K ) a n d reported droplet "surface" temperatures of 750°K. Their maximum flame temperatures were 300 - 400 °K lower than their calculated value. They apparently did not correct for conductive effects, nor did they discuss this aspect. Gas Chromatograph Data

The measured flame composition is presented in Figs. 8-10 in terms of mole percent vs the distance from the center of the simulated droplet divided by its radius. The simulated droplet was burned in air under conditions of natural convection at 1 atm pressure, and an ambient temperature between 300 °K and 310 °K. It is seen that the reproducibility of the measured stable species profiles is very good. The minor species are generally in agree-

NO AND COMPOSITION IN DIFFUSION FLAMES

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r/r,,

Fig. 8. Measurements within the steady porous sphere flame: composition (Run #1); r l = .66ram, ethanol in air, T = 300 °K, p = latm, with natural convection.

Fig. 9. Measurements within the steady porous sphere flame: composition (Run #2); r l = .66mm, ethanol in air, T = 300 °K, p = 1 atm, with natural convection.

ment except that in Runs No. 1 and 3, the data suggest a dip in the concentrations of CO, C2 H6, and C2 H4 + C2 H2 near r / r I = 1.3. Run No. 3 was selected as the basis for comparison with theory since this run exhibited minimum scatter in the data, and because this run agreed qualitatively with Run No. 1. Although water was not directly analyzed, it was deduced as the principal specie not accounted for by the analyzed species. It should be kept in mind, however, that the profile for water in Fig. 10 does include small percentages (probably not more than a total of 3-4%) of compounds not analyzed (such as aldehydes). For calculation purposes, however, this profile was assumed to be H20.

Since the theoretical results are presented in terms of mass fractions, the data of Fig. 10 were converted to mass fractions (%) and are plotted in Fig. 11. Several immediate observations can be made: (1) Even for these small simulated droplet sizes, the flame is still compressed by about a factor of 3 compared with the theoretical prediction. (2) Calculated and experimental values for the mass fractions at the simulated droplet surface are in excellent agreement (compare Fig. 6 and 11). (3) The experimental data indicate significant oxygen penetration to the surface. (4) Pyrolysis and partial oxidation of the ethanol is taking place between the simulated droplet

114

D.E. LUDWIG, F. V. BRACCO, and D. T. HARRJE

surface and the region where primary combustion is occurring. In order to locate these regions, the rate of disappearance of ethanol, and the rate of formation of CO2 were obtained by using the experimental species profiles and by employing the conservation of species equation, Fick's law, global

.or 7O

BLUE LUMINOUS

I

~ ,'

ZONE

I

I

=-..-..--N=

flame structure and burning rate data observed by several researchers are now in order. At the end of this section, these general observations are more specifically applied to the comparison between the theoretical and experimental data mentioned in this paper.

8O

i,_ 6 0 '

Z

~ 50" •

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50

BALANCE, PRIMARILY

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/\

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BALANCE . . . .PRIMARILY ....

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2

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.

.

.

.

/f

°'

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6 7 8910

r/r,

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r/r e 4 CH4 I ~I ,~,3

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3

c=H** f

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2

3

4

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6 78910

r/r~,

2

3 r/rt

Fig. 10. Measurements within the steady porous sphere flame: composition (Run # 3); r I = .66mm, ethanol in air, T = 300 °K, p = 1 atm, with natural convection.

Fig. ] ]. Measurements within the steady porous sphere flame: composition (Run # 3 ) ; r / = .66ram, ethanol in air, T = 300 OK, p = i atm, with natural convection.

continuity, and the measured fuel burning rate. These rates are plotted in Figs. 12 and 13. Apparently, in addition to the thermal decomposition of ethanol, partial oxidation is also occurring in the same region.

During the last two decades, innumerable cap culations have been made of droplet burning rate and flame structure by various authors, employing somewhat different assumptions, and innumerable measurements have been taken with a variety of experimental techniques. Yet the vast majority of theoretical and experimental data are mismatched and therefore difficult to compare. No comprehensive theory exists that includes all the factors known to influence the burning

Comments on the Flame Structure and Burning Rate

Some general comments on the reasons for the disagreement between the calculated and the measured

NO AND COMPOSITION IN DIFFUSION FLAMES

115

-8

+11

mE -7

÷10

E-6

÷9

N-° -5.

÷8

-r 0 - 4

+7

°3

'~E

x N

2

r/r/ Fig. 12. Calculated rate of consumption of ethanol vs position in flame based on Run #3.

.3 *2÷1 0

rate and the flame structure of a droplet of a given fuel in a given, infinite, oxidizing atmosphere. They include natural and forced convections, unsteady heating of the liquid, variations of the transport and thermodynamic properties within the flame, and finite chemical kinetic rates (including gas phase pyrolysis and partial oxidation of the often complex fuel molecule). Accordingly, there are no adequate theoretical results for comparison with the burning rate and flame structure data obtained with falling or suspended (often on quartz filaments) drop experiments. There are only qualitative results, which agree with measured burning rates and flame stand off distances to within, perhaps, a factor of 3. However, for practical purposes, semiempirical correlations, with empirically determined coefficients, have been obtained, which allow the experienced user to estimate burning rates to within 20% or 30% of the measured ones [12, 24]. Let us abandon the most complex of the cases and consider the simplest. In the simplest theory, the steady vaporization and combustion of a constant radius, constant temperature, "droplet" are considered, and natural and forced convections neglected. The corresponding experiment is the porous sphere experiment. Not generally appreciated is the fact that even in this case the agreement between experimental and theoretical results is poor mostly because they are still mismatched. First, there are no available steady (obtained with porous spheres) burning rate and flame struc-

m

-I 2

.l,m 3 r/to

Fig. 13. Calculated rate of production of C O 2 tion in flame based on Run//3.

vs

posi-

ture data that are free of natural convection effects. For that the sphere would have to be less than l m m in diameter for standard hydrocarbon fuels and air. The flame around porous spheres of about 1 mm diameter, as used in the work reported in this paper, is egg-shaped with the porous sphere at about the center of the flame indicating small, but not negligible, gravity effects (most researchers have employed 5 to 10 mm spheres or cylinders, which are fully influenced by gravity effects). Second, no one has yet calculated the burning rate and the flame structure including all the effects that are still known to influence the results of this, the simplest of all cases. Commonly made assumptions include:

1. Negligible surface temperature gradient on the liquid side, 2. Equal binary diffusion coefficients for all species, 3. Infinitely fast kinetic rates (collapsed flame) and stoichiometric combustion or, 4. One-step, finite-rate reaction, 5. Negligible radiation,

116 6. Lewis number equal to 1.0, 7. pD = constant, 8. Equal and constant specific heats, 9. Equal molecular weights. Kassoy and Williams [6] avoided (only partially) assumptions 2, 3, 6 and 7 and found a strong dependence of the calculated burning rate, and of such flame structure parameters as flame position and maximum flame temperature, on the assumed transport and thermodynamic properties. Variation in any of the above three quantities by a factor of 2 was found by varying the transport and thermodynamic properties. Sensitivity of the results to the fuel and oxidizer reaction orders of the one-step, finite-rate reaction was also demonstrated. Bracco and Schier (not published) maintained the assumptions of collapsed flame and negligible surface temperature gradient on the liquid side, but removed all other assumptions and confirmed the results of Kassoy and Williams on the influence of the transport and thermodynamic properties by extensive

D.E. LUDWIG, F. V. BRACCO, and D. T. HARRJE numerical computations. The influence of the assumed chemical kinetic parameters, when finiterate reactions are used, had already been demonstrated by Lorell, Wise and Carr [5]. It is to be emphasized that both the calculated burning rate and the calculated flame position vary by similar percentages when the above parameters are varied. The general belief that burning rates can be predicted more accurately than flame geometry is largely incorrect. However, it is true that the size of the flame is apparently overestimated by a factor of 2 or 3 if the various parameters are adjusted to give what are believed to be reasonable burning rate and flame temperature. Conversely, better agreement with what one would think the size of the flame should be can be obtained accepting what are believed to be larger errors in the burning rate and flame temperature. Comparing now the theoretical results of Fig. 6 with the experimental results of Fig. 11, one notices that

(1) In the experiment, gravity effects were small, but not negligible. (2) The theoretical results were obtained employing assumptions 1, 2, 4-9. (3) Theoretical and experimental data were for two different sphere sizes (exact scaling laws are obtained only from oversimplified theories). (4) The theoretical and experimental burning rates differ by 20% (using the results of Fig. 11 for the experimental burning rate and scaling the theoretical results with the assumption that rh ° cc r, which is derivable only with oversimplified theories), the maximum temperature by 10%, and the flame size by a factor of 3. Moreover, the compositions are fundamentally different even if the two results are arbitrarily rescaled and superimposed. In the theory, the ethanol molecule is assume to maintain its integrity up to the flame region as a consequence of the adopted one-step, finite rate reaction. In the experiment, significant pyrolysis and partial oxidation are found. (5) Better agreement between any one of the above parameters could have been obtained by adopting different values for the transport, thermodynamic and kinetic rate constants, but it is doubtful that much better agreement could have been achieved for all of them simultaneously using realistic values for the constants. (6) It is surmised that satisfactory agreement would be found if gravity effects were eliminated in the experiment and if a complete theory were employed. Such a theory would need the often missing data of the high and low temperature kinetics of pyrolysis, and oxidation of the complex fuel molecule. Ob/,iously, the computation of nitric oxide within the flame are also strongly influenced by the flame structure inaccuracies. At most, one can hope to predict general trends. Accordingly, the good agreement (after arbitrarily rescaling and superimposing the theoretical and the experimental flame structures) between the calculated and the

measured nitric oxide concentrations discussed in the next section is pleasing, but mostly fortuitous. Nitric Oxide Data

Nitric oxide profiles for pure ethanol and for ethanol seeded with various percentages of pyridine are shown in Figs 14-17. Theoretically, the maxi-

NO AND COMPOSITION IN DIFFUSION FLAMES mum NO concentration calculated by Bracco [4] for pure ethanol is about 20 ppm. Experimentally, we measured about 48 ppm. Since the rate constants for the kinetic mechanism used to calculate NO formation are fairly well established the difference between the two numbers is probably due to (1) Approximation of the transport, thermodynamic, and chemical kinetic properties in the flame structure calculations, (2) Mismatch between theory and experiment discussed in the previous section, (3) Invalid assumptions regarding equi, librium oxygen concentrations and steady state N used to predict NO, and (4) Inaccuracies inherent in the present equipment design and data reduction methods. For a fuel-nitrogen content of 1%, the calculated maximum NO concentration increased to 360 ppm. Experimentally, we measured maximum NO concentrations near 800 ppm. The difference is again probably due to the same reasons previously discussed plus an additional one. In arriving at 360 ppm, it was assumed that the nitrogen compound releases N atoms at the rate at which the fuel burns and which contribute to a new steady state N concentration. These assumptions were necessary for lack of any better kinetic data. Recent research [1, 2] indicates, however, that nitrogen is not released as atoms, but reacts via active intermediates and radicals. The results from this work do confirm the suggestion that diffusion flames can be a very strong source of NO from combustion of nitrogen containing fuels. Published data for NO formation in diffusion flames is scarce, but work by A. Williams [25] on a 5.85 mm radius cylinder burning under our conditions originally indicated NO concentrations in the forward stagnation region of the flame zone to be 3 ppm for pure ethanol and the same order of magnitude for ethanol containing.l% wt. pyridine. Between the liquid surface and the flame zone, he measured concentrations an order or magnitude higher for ethanol-pyridine than for pure ethanol. His conclusion was that most of the NO is destroyed in the flame zone. However, more recent results from the same group [26] show a much better agreement with our results both in magnitudes and trends. On the other hand, they now report the presence of a minimum in the nitric oxide concentration between the cylinder

1 17

~o

40

2O

I0

r/r/

Fig. 14. Measurements within the steady porous sphere flame: nitric oxide; r I = .66mm, ethanol in air, T = 300 °K, p = 1 atm, with natural convection. 300

250

200" E o z

150, o I00

0.2% PYRIDINE

-

50"

2

3

4

5

6

7 8910

r/r,,

Fig. 15. Measurements within the steady porous sphere flame: nitric oxide; r l = .648mm, ethanol with 0.1% and 0.2% pyridine in air, T = 300 °K, p = 1 atm, with natural convection.

surface and the flame. Our results do not show the presence of such a minimum. Their results, if confirmed, would indicate production of nitric oxide at, or near, the cold cylinder surface.

1 18

D.E. LUDWIG, F. V. BRACCO, and D. T. HARRJE

1200.

700

II00 " I000-

600

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400.

g d

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600-

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500 .78% PYRIDINE

300

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500 200 •

200

0.99% ~"~\ PYRIDINE ~

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2

3

4

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6

7

Fig. 16. Measurements within the steady porous sphere flame: nitric oxide; r I = .66mm, ethanol with 0.38% and 0.78% pyridine in air, T = 300 °K, p = 1 atm, with natural convection.

7

8 910

In Fig. 18 a plot of peak NO ppm vs molar percent pyridine in ethanol is given. The range is from

2000 I000'

¢;t

Oz I00 -4

•

0.01

6

Fig. 17. Measurements within the steady porous sphere flame: nitric oxide; r / = .66mm, ethanol with 0.99% pyridine in air, and r l = .648mm, ethanol with 1.98% pyridine in air, T = 300 °K, p = 1 atm, with natural convection.

8910

r/r~

I0

5

r/~,

,

,

. . . .

,

,

i

,

•

, , , ,

•

0,1 1.0 MOLAR PERCENT PYRIDINE IN ETHANOL

Io.o

Fig. 18. Peak measured nitric oxide concentration vs pyridine molar percent.

NO AND COMPOSITION IN DIFFUSION FLAMES 190 ppm for .1% fuel N to about 1200 ppm for 4% fuel N. It is difficult to make definitive statements from the plot of Fig. 18, which is based only on peak NO measured, however it suggests (with some physical justification) that as the amount o f N in the fuel increases, its effectiveness in enhancing the production of NO decreases. This would appear to result in a larger fraction of the nitrogen forming N2, and possibly other nitrogen compounds, resulting from an insufficient oxygen

119 concentration in the region between the droplet surface and the flame zone. A decrease in NO yield with increasing concentration of fuel bound nitrogen has been reported by several investigators [1, 27].

Conclusions

The following are the primary conclusions regarding the equipment and experimental technique:

(a) Using ethanol as a fuel, a 1 to 2 mm diameter porous sphere is a good choice for reducing gravitational effects to a minimum while still permitting flame structure studies. (b) The gas chromatograph and the general "on-line" sampling technique reduces data scatter to a minimum and thus reproduces the flame structure very well. (c) The NO analyzer reproduces NO profiles with an accuracy better than 10% in most cases. (d) The thermocouple technique is useful for obtaining temperature profiles. The smallest possible thermocouple wire (.0005 in. or less) should be used if it is desired to minimize conductive effects along the wire. The following primary conclusions were drawn after data reduction: (a) The effect of heat conduction along the thermocouple wire is difficult to compute precisely. However, (b) The maximum flame temperature measured is lower than expected (1800 °K) and the error in the measured temperature is probably no more than 50 °K. (c) Gas composition and the droplet surface temperature are in excellent agreement with theory. (d) Oxygen penetration to the droplet surface is significant. (e) Pyrolysis and partial oxidation of ethanol occur very near the droplet surface at about the same location. (f) Production of nitric oxide is greater than predicted theoretically, both for pure ethanol, and for ethanol containing pyridine. (g) Diffusion flames can be significant sources of NO particularly when the fuel contains nitrogen. This work was conducted under Grant No. R - 8 0 0 8 4 4 from The Environmental Protection Agency, Dr. B. G. Martin, Grant Monitor. The cooperation o f the A eroChem Research Laboratories and o f Drs. A. K. Varma and 1:. Dryer is gratefully acknowledged. References

1. Martin, G. B., and Berkau, E. E., AIChE Symposium Series, Air Pollution and Its Control 68, 45 (1972). 2. Turner, D. W., Andrews, R. L., and Siegmund, C. W.,

AIChE Symposium Series, Air Pollution and Its Control 68, 55 (1972), 3. Ludwig, D. E., Nitric Oxide and Composition Profiles Around Burning Droplets of Ethanol and EthanolPyridine Mixtures, M.S. Thesis, Princeton University, AMS Report No. 1137-T, 1973.

4. Braceo, F. V., Fourteenth Symposium {International) on Combustion, The Combustion Institute, Pittsburgh, Pa., 1973, p. 831. 5. Lorell, J., Wise, H., and Cart, R. E., 3. Chem. Phys. 25,325 (1956). 6. Kassoy, D. R., and Williams, F. A., Liquid Droplet Combustion with Finite Rate Chemistry, 6th Aerospace Sciences Meeting (Paper No. 68-181), N.Y., Jan. 1968. 7. Braeco, F. V., A Theoretical Model for Diesel Combustion, Combustion Institute Central States Section Spring Meeting, Ann Arbor, MI, March 1971. 8. Bracco, F. V., NO Formation in Droplet Diffusion Flames, (Paper No. WSCI 71-29), Combustion Institute Western States Section Fall Meeting, lrvine, CA, Oct,,bcr 1971. 9. Sanders, C. F., Teixeira, D. P., and de Volo, N. B.,

The Effect of Droplet Combustion on Nitric Oxide

120

10. 11. 12. 13. 14. 15. 16. 17. 18.

19.

20.

21.

22.

23. 24. 25.

Emissions by Oil Flames, (Paper No. 72-7), Combustion Institute Western States Spring Meeting, Seattle, Wash., April 1972. Altenkirch, R. A., Shahed, S. M., and Sawyer, R. F., Combust. Sci. Technol. 5,147 (1972). Kesten, A. S., Combust. Sci. TechnoL 6, 115 (1972). Williams, A., Combust. Flame 21, 1 (1973). Fristrom, R. M., Prescott, R., and Grunfelder, C., Combust. Flame 1,102 (1957). Smith, S. R., and Gordon, A. S.,J. Chem. Phys. 22, 1150 (1954). Westenberg, A. A., Raezer, S. D., and Fristrom, R.M., Combust. Flame 1,467 (1957). Prescott, R., Hudson, R., Fornet, S., and Avery, W., J. Chem. Phys. 22, 145 (1954). Fristrom, R. M., and Westenburg, A. M., Flame Structure, McGraw-Hill, N.Y., 1965. Kaskan, W. E., Sixth Symposium (International) on Combustion, The Standing Committee on Combustion Symposia, 1954, p. 134. Bascombe, K. N., Tenth Symposium (International) on Combustion, The Combustion Institute, 1965, p. 55. Friedman, R., Fourth Symposium (International) on Combustion, The Standing Committee on Combustion Symposia, 1952, p. 259. Agoston, G. A., Wise, H., and Rosser, W. A., Sixth Symposium (International} on Combustion, The Standing Committee on Combustion Symposia, 1954, p. 708. Wise, H., Lorell, J., and Wood, B., Fifth Symposium (International) on Combustion, The Standing Committee on Combustion Symposia, University of Pittsburgh, 1954, p. 132. Aldred, J. W., Patel, J. C., and Williams, A., Combust. Flame 17, 139 (1971). Braeeo, F. V.,AIAA J. 12, 1534 (1974). Williams, A., Fourteenth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, Pa., 1973, p. 841.

D.E. LUDWIG, F. V. BRACCO, and D. T. HARRJE 26. Hart, R., Nasralla, M., Williams, A., Combust. Sci. Technol., to be published. 27. Fenimore, C. P., Combust. Flame 19, 289 (1972). Received 8 November 19 74; revised 31 January 19 75

Nomenclature transfer number, defined in text B average specific heat at constant pressure, P cal/g°K Gr Grashof Number, defined in text i stoichiometric ratio, ( y o / Y F ) stoich heat of vaporization, cal/g L molecular weight, g/mole W W average molecular weight, g/mole mO mass burning rate, gravity free case, g/sec m°(nat) mass burning rate under the influence of natural convection, g/sec R universal gas constant, 1.987 cal/mole °K r distance from the droplet center, cm T temperature, °K mass fraction Y average thermal conductivity, cal/sec cm°K gas density, g/cm 3 p /a dynamic viscosity, g/sec cm rate of formation of a species, g/cm 3 sec Subscripts boiling condition b F fuel flame within the flame zone l at the droplet surface oxidizer O forr ~oo ~o