Shock tube investigation of ignition in methaneoxygennitrogen dioxideargon mixtures

COMBUSTION A N D F L A M E 24, 173-180 (1975)

173

Shock Tube Investigation of Ignition in Methane-Oxygen-Nitrogen Dioxide-Argon Mixtures ERNEST A. DORKO, DAVY M. BASS, and ROBERT W. CROSSLEY Departmeti t of A ero-Mechanical Engineering, Air Force Institute of Technology, Wright-Patterson Air Force Base, Ohio 45433

and KARL SCHELLER Aerospace Research Laboratories, Chemistry Research Laboratory, Wright.Patterson Air Force Base, Ohio 45433

The ignition of CH4-O2-NO2 mixtures highly diluted with Ar was examined in the region behind a reflected shock wave in a single pulse shock-tube. The measurements covered temperatures of 1200 to 1800°K, pressures of 3 to 20 atm, total gas concentrations of 3.3 - 4.5 X I0 "s mol/cc and fueloxidizer mixture equivalence ratios of 0.38-1.60. The induction times ranged from 13 to 580 psec for a maximum dwell time of about 700 psec. When this system was compared to a CH4-O2-Ar system, it was found that NO2 addition shortens the induction time. The ignition temperature was also lowered by about 200°C by the addition of NO 2. The dependence of induction time on concentration for an oxidizer ratio range of 25-75% NO2 is correlated by the relationship: 7" = 6.45 X 10-15 exp(28700/RT) [CH4] "0"32 [O2] "0"20 [NO2] "1"30 [Ar] +1"°° sec A product analysis showed that during preignition methane decreases substantially when NO2 is present and much larger quantities of CO are formed than when no NO2 is involved. It was also found that a mixture of CH4-NO2-Ar produced pressure and temperature oscillations. It is suggested that an important step when NO 2 is present is its decomposition to produce NO and O. The reaction between CH4 and O becomes an important contributor to chain branching. Other aspects of the mechanism are discussed qualitatively.

Introduction In recent work in this laboratory, the ignition characteristics of methane-oxygen [ 1,2,3] and ethane-oxygen [4] mixtures highly diluted with argon have been examined in the region behind a reflected shock wave. These studies have determined the dependence of ignition delay times on reactant concentration and temperature and small amounts of fuel additives. In the course of the ethane-oxygen investigation [4], it was found that the addition of slight quantities of NO2 to the • mixture (02 : NO2 = 35 : 1) reduced the ignition delay times by a factor of approximately 2 over the entire temperature range examined. The present study was undertaken in order to explore in greater detail the effect of nitrogen oxides in combustion promotion in an effort to obtain a • basic understanding of their role. Methane rather

than ethane was selected as the fuel in view of the greater amount of experimental and analytical data available regarding its oxidation. Experimental Section The shock tube used in these experiments was 1 in. i. d. The tube, its accessory equipment, and its operation were described in an earlier publication [3]. One change has been made. The glass driven section has been replaced by an equivalent section of stainless steel tubing which was machine polished to minimize turbulent flow at the wall. Several reaction mixtures were prepared manometrically on a high pressure line and were stored in stainless-steel cylinders at 40 psia. Both the line and the cylinders were seasoned for 24 h with nitrogen dioxide and then pumped down to a few microns before the preparation of the

Copyright © 1975 by The Combustion Institute Published by American Elsevier Publishing Company, Inc.

174

ERNEST A. DORKO, et al.

mixtures. A calculation was made to correct for the equilibrium between NO2 and N204. [5] The composition of the mixtures is shown in Table 1. The first five mixtures served to indicate the change in induction time as a function of % NO2 in the oxidizer and the last four mixtures were used to determine the concentration dependence of induction time. TABLE 1 The Reaction Mixtures Used No.

% CH4

% 02

% NO2

1 2 3 4 5 6 7 8 9

4.00 4.00 4.00 4.00 4.00 8.00 8.80 3.08 2.85

8.00 6.00 4.00 2.00 8.00 8.80 8.22 2.85

2.00 4.00 6.00 8.00 8.00 2.20 8.22 2.85

The gases were purchased from Matheson Gas Products. Methane, oxygen, and argon were listed as 99.99%, 99.99%, and 99.999% pure, respectively. Nitrogen dioxide was 99.5% pure and helium driver gas was 99.995% pure. All gases were used without further purification. Reflected shock temperatures were computed from the measured incident shock velocities, using the conservation equations and the ideal gas equation of state assuming thermal equilibrium of all degrees of freedom but no reaction before ignition [1 ]. The molar enthalpies of methane, oxygen, nitrogen dioxide and argon were taken from JANAF thermochemical data [6] and were presented as six-term polynomials whose coefficients were evaluated by the leastsquares method. Ignition delay times were monitored by pressure signals originating from a 603A Kistler pressure transducer whose signals were recorded on Polaroid film from a 551 Tektronix dual-beam oscilloscope. The ignition delay times were signified by a sudden change in the slope of the trace. Under these test conditions 7"was reproducible to about 5-10%. Samples of the quenched, shocked gas mixture were withdrawn through a toggle valve in

the end plate of the shock tube. They were withdrawn into pre-evacuated bulbs and analyzed. Hydrocarbon components were analyzed by use of a gas chromatograph (Hewlett-Packard Series 700) with a 6 ft × 1/8 in. column packed with Porapak Q (Waters Associates). The analysis was performed with the column at 50°C. Flame ionization was the detection method. The nonhydrocarbon components of the gas mixture were analyzed with a Consolidated Electrodynamics Corp.-Type 21-103 mass spectrometer. The instruments were calibrated with known mixtures of gases prior to analysis. Methane was analyzed by both methods and was used to correlate the results of the two.

Results Approximately 200 shocks were run with the mixtures shown in Table 1. Data were collected over a temperature range of 1200°K to 1800°K, a pressure range of 3 to 20 atm, and a fuel-oxidizer mixture equivalence range of 0.38-1.60. The total gas concentration ranged from 3.3 to 4.5 × 10"s mols/cc. The effect on induction time of an increasing percentage of NO2 in the oxidizer portion of a stoichiometric mixture of methane-oxidizer is shown in Fig. 1. A plot at three representative

I000

800 1540°K

400

200

%

20

40

60

80

I00

%NO 2 Fig. 1. Ignition delay time (7") vs % NO 2 in the oxidizer portion of the mixture. The curves cover a total gas concentration range of 3.3 to 4.5 X l0 -s mol/cc and are shown for representative temperatures. (See Table 1 for a description of the mixtures used.)

IGNITION IN CH 4- 02 - NO 2 - Ar MIXTURES temperatures of the ignition delay time (r) oxidizer composition is given. It can be seen that the addition of relatively small amounts of NO2 substantially shortens the ignition delay time. As the relative amount of NO2 is increased above 25% to about 75% the effect levels out. When NO2 becomes the main component of the oxidizer mixture, the reaction mechanism appears to change radically, so that when a mixture containing only NO2 as oxidizer is shocked there is no discernible reaction in the temperature range studied. A series of shocks in this temperature range with a mixture containing only NO2 as oxidizer gave no indication of a pressure increase which would signal the onset of ignition. Undoubtedly reaction is occurring [7], but the processes that occur do not lead to the pressure and temperature rises that are used to characterize ignition. However, when a threshold temperature of about 2000°K was reached, a very interesting oscillation appeared on the pressure trace. This oscillation occurred up to the maximum temperature obtained which was 2825°K. In seeking further information regarding the nature of the oscillation, the temperature history during ignition was followed by means of a thin-film

175 platinum heat gauge embedded in the end flange, near the pressure transducer. Representative simultaneous pressure and temperature traces are shown in Fig. 2 for an initial reaction temperature of 2277°K. These traces indicate that the periods of the two oscillations are identical (42/lsec), but 180 ° out of phase with each other. Experiments utilizing mixtures 6 through 9 were performed in order to determine the concentration dependence of the ignition delay or induction time in the manner previously reported. [1] Differences in the ratio of densities were taken into consideration in determining the mixture compositions so that at the reflected shock temperature, the concentration ratio was an integer number (concentrations were varied by a factor of either 3 or 4). The experimental conditions utilized are given in Table 2 and experimental results are shown in Fig. 3. The ordinate differences between properly selected pairs o f lines yielded experimental values for the exponents in the concentration dependencies [1 ]. These were used in turn to obtain a single correlation for all the experimental data by plotting log(r/[CH4 ]-0.32 [02 ] -0.20 [NO2 ] -I .:30 [Ar] + 1.oo vs I/T in Fig. 4.

A B

0.2 v/cm

5my/cm TIME ( I00/

sec./cm )

Fig. 2. Representative oscilloscope traces obtained at 2277°K for a mixture of CH4-NO2-Ar. These traces shows the pressure oscillations (trace A) and the temperature oscillations (trace B) at the end plate of the shock tube in the reflected shock. (Time, temperature, and pressure increase to the right and up on the traces.)

176

ERNEST A. DORKO, et al.

I000

~00

,

rS"K,

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,4oo

T 5 °K

1200

80C

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~'. BICH4) 40C -3.1 cd

20¢

1400

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1021

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1,2oo

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0

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- 4.1

m ~k

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t

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.zo

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I031T 5 °K

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The fit of data is quite satisfactory. A least squares program was used to determine the slope and intercept of the line from which the proportionality factor/~ is evaluated. The experimental relationship for the ignition delay time in terms of temperature and composition is then expressed by Eq. (1) as =

6.45 X 10 "Is exp (28,700/RT)

(1) [CH, ] -0.a 2 [02 ] -o.2o [NO2 ] - I ,ao [Ar] + 1.oo sec. It is of interest to note that the concentration dependencies of the induction time are significantly changed by the addition of NO2. In a mixture of CI-I4-O2-Ar [1 ] (shocked under conditions similar to those used in the present experiment) for comparison: r

=

3.62 X l 0 "14 exp(46,500/RT)

(2) [CH4] +0.33 [02 ] - 1 . o 3 [Ar] o

d8

eo

Fig. 3. Ignition delay time (r) vs the reciprocal of the refleeted shock temperature (Ts) for the mixtures described in Table 2.

r

I

oT

Fig. 4. A plot of log/Jvs the reciprocal of the reflected shock temperature (Ts) for all data points. The value for was determined by a least squares fit of the experimental data.

The addition of NO2 decreases the pre-exponential constant and the temperature dependency of the ignition delay. Furthermore, it introduces a dependence upon argon, reduces the oxygen depend- , ency and reverses the sign of the fuel dependency. The fuel is now a promoter of ignition while argon acts as an inhibitor. It must be noted at this point that the relationship determined in Eq. (1) is valid for an NO2/O2 oxidizer ratio range of 25-75% NO2. This range of oxidizer ratios corresponds to the relatively flat region of the curves shown in Fig. 1. In this region the kinetics of the reaction are relatively independent of the specific value of the oxidizer ratio and so the relationship of Eq. (1) is a true indicator of the concentration dependencies of ignition delay time. Evidence that this.is the case is shown graphically in Fig. 1. As the reflected shock temperature increases, causing a corresponding increase in concentrations of the species present, the ignition delay time decreases. At the higher temperature r becomes vanishingly small. How-

IGNITION IN C H 4 - 0 2 - NO 2 - Ar MIXTURES

177

TABLE 2 Summary of the Experimental Conditions for the Concentration Dependence Experiments

Group

Mixture

Approximate initial pressure (torr)

Equivalence ratio (fuel/oxidizer)

Reflected shock temp (°K) 1288-1583

A B

6 7

100 100

1.00 1.60

C D E

8 9 9

100 100 400

0.38 1.00 1.00

ever, at lower temperatures r decreases somewhat as the % NO2 increases, indicating that the NO2 concentration dependence (-1.30) is operating sufficiently to overcome the inhibiting effects of the argon (which of course is present in the highest concentration). In the regions 0-25% NO2 and 75-100% NO2, the concentration dependence expression would be expected to be quite different than Eq. (1) and it would be a function of % NO2. A product analysis using mixture 3 was performed. The results for the carbon-containing species are shown in Fig. 5. A material balance for carbon at the temperature at which ignition begins (1280°K) shows that 65% of the methane has reacted prior to ignition. During this time, 47% of the methane yields CO, 4.5% yields CO2, and 5.5% yields C2 H4 or C2 H6. This result is different from that obtained previously [1 ] for a CH4-02Ar mixture. For this mixture only 15% of the methane has reacted at the onset of ignition (ca. 1480°K). It is also to be noted that the ignition temperature has been lowered about 200 ° by the addition of NO2. The greater rate of disappearance of methane in mixtures containing NO2 is associated with the production of much larger quantities of CO. These facts taken in conjunction indicate that the methane disappears by oxidation rather than by decomposition. Thus, the NO2 produces earlier oxygen attack on the methane at a much lower temperature than prevails in its absence. It is also significant to note that the large quantity of CO formed during the induction period is not consumed upon ignition. It is evidently produced at a greater rate than its oxidation to CO2. At longer times than the dwell time available in this shock tube (ca. 700/~sec) the product gases would presumably attain equilibrium CO2 composition with enough available oxygen.

1323-1599 1359-1685 1491-1847 1310--1637 I00.0

--

50.O

20.O

I0.0 5O i,i

2.0

•

LO

~0.5 0.; • OC'H4 A C2H4

OC2Hs -'C02 • co

0.0.•

0.02 0.01

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14()0

Fig. 5. Production distribution as a function of reflected shock temperature for a mixture containing 4% CH4, 4% NO2, and 4 % 0 2 .

In addition to the carbon-containing species, observations of the changes in oxygen, nitrogen, and nitrogen oxides were attempted. Oxygen decreases monotonically throughout the reaction. At the ignition temperature, the amount of oxygen has decreased to 32% of its original value and thereafter it decreases rapidly to 1% or less of original. This result is as expected considering the changes manifest in the carbon-containing

178

ERNEST A. DORKO, et al.

species. During the analysis of the nitrogen oxides, it was not possible to differentiate between NO2 and NO, the two oxides which could be reasonably postulated for the reaction, since their mass spectral cracking patterns were nearly identical. However, the significant result was that the total amount of the two remains constant throughout the temperature range observed. The conclusion drawn from this observation is that NO2 is transformed to NO under the experimental conditions. Further support for this conclusion is the fact that no detectible amount of N2 forms even after ignition. The amounts of hydrogen and water were not analyzed.

Discussion The phenomenological relationship for the ignition delay for methane-oxygen mixtures containing substantial quantities of NO2 differs very markedly from that obtained in its absence. In the latter instance, Eq. (2), the induction period is highly dependent on the oxygen concentration and much less so on the fuel. Furthermore, the methane itself acts as an inhibitor for its own ignition and the argon concentration has no effect on the ignition delay time. The addition of NO2 reduces the temperature dependence of the induction time, introduces a strong argon and NO2 dependency, reduces the effect of oxygen concentration and reverses the effect of the fuel concentration. The power dependence of the induction time on the fuel is now negative (i.e., it functions as an ignition promoter). It will be useful to attempt to rationalize the effect of NO2 on the combustion of methane in terms of the rather complete kinetic model advanced previously [2] for the methane-oxygen reaction in an argon bath. In that work, it was found necessary to postulate a methane-oxygen initiation step in addition to the obvious methane decomposition in order to explain the experimental results for temperatures below 1750°K [2] viz.,

CH4 + A r

k3 -~ CH3 + H + A t

(3) k 3 = 4 . 0 0 × 1017 e x p ( - 8 8 , 0 0 0 / R T ) cc tool "l sec "1

k4 and

CH4 + O -'~ CH3 + HO2

(4) k4 = 8.00 X 1013 exp(-56,000/RT)cc mol "1 sec "l In view of the overwhelming influence of N O 2 o n the induction time, we must undoubtedly add as an initiating step its unimolecular decomposition, [8, 9, 10] i.e., ks

NO2 + Ar ~ N O

+ O + Ar

(5) ks = 1.12 × 1016 exp (-65,000/RT) cc mol-I sec "l Comparison of the relative rates of these three reactions indicates that the NO2 decomposition will proceed at least an order of magnitude faster than either of the other two reactions for the temperatures and pressures prevailing in our experiments. Though the concentration of O atoms produced in the decomposition of NO2 is kept to a low level by the reaction with NO2 as shown [11] 0 + NO 2 "+ NO + 0 2

(Sa) ksa = 1.95 X 101 a exp (-1060/RT) cc mol "l sec "l the O atoms will be present in sufficient quantity to initiate chain branching by reaction with methane [2, 12] 0 + CH 4 ~

CH3 + OH

(6) k6 =

2.10 × 1013 exp(-4,550/T) cc mol "1 sec "l

(An order of magnitude calculation assuming a steady state between reactions (5) and (5a) suggests that the O atom concentration will be about 10-s moles per mole of initial NO2 in the temperature range 1500-2000°K.) The change in the dependence of r on the concentration of CH4 from +0.33 in the absence of NO2 to -0.32 in the present case requires an explanation. Hydrocarbon fuels can behave as both ignition promoters and inhibitors (or radical scavengers [3, 4] ). The final concentration de-

IGNITION IN CH 4 - 0 2 - NO 2 - Ar MIXTURES pendence then results from a balance between the two roles for the hydrocarbon fuel in the specific system under consideration. An example of a fuel exerting its full promoting role is found in recent cyanogen-oxygen ignition studies [13] in which the ignition delay time was found to vary as (C2 N 2 ) ' I . Since in this case cyanogen cannot function as a scavenger in a mixture with only 02 and argon, its full promoting influence is exerted. In view of the dominant role of the NO2 decomposition, it is not surprising to find its effect much greater than that of oxygen. However, the fact that its power dependency is appreciably greater than unity deserves comment. In addition to the role played by NO2, its decomposition product, NO, may also act to enhance ignition by replacing the inactive HO2 radical produced in Reaction (4), by more active OH species in the following manner HO2 + NO ~ NO2 + OH, AH = -9.3kcal. (7) This reaction also leads to the regeneration of NO2. The NO2 itself may play another part in the ignition, besides its function as a source of O atoms through decomposition. It may react directly with the methyl radicals produced in Reactions (4) and (6) as follows: CH3 + NO2 ~ CH30 + NO, AH = -17.0kcal;

(8) CH30

+ 02 "-* HCHO + HO2, AH = -26.0 kcal.

(9) (The HCHO is oxidized to CO in subsequent reactions.) This scheme provides chain propagation through the regeneration of OH radical. Since Reactions (7) through (9) are all exothermic, they constitute a plausible mechanism under our experimental conditions. It seems reasonable to assume that the large power dependency found for NO2 reflects its secondary as well as its primary role in the oxidation of methane. Ultimately a computer simulation will be required to ascertain the validity of this argument. The abnormally large inhibiting effect of argon cries out for explanation in view of the lack of

179 such an effect in hydrocarbon systems lacking NO2. Detailed kinetic calculations for methane [2] have shown that the acceleration of the initiating Reaction (3) by argon is completely counterbalanced by its buffering effect in reducing the temperature rise due to subsequent reaction, resulting in a virtually nil effect on induction time. In the present instance, the endothermicity of initiation steps (3) and (4) is supplemented by the predominantly important decomposition (5). The summed effect of these reactions is to increase the importance of the bath effect of argon in reducing temperature rise with respect to the promotional role. The retarding effect of argon found in the cyanogen study [13] and attributed to the greater endothermicity of its initiating step buttresses this line of reasoning. Two important observations remain to be discussed. No ignition was observed in CH4-NO2 mixtures in the absence of oxygen at temperatures below 2000°K, while mixtures of equivalent stoichiometry with oxygen ignited at 1280°K. By lack of ignition, it is meant that no temperature or pressure rise was observed to indicate the occurrence of a net energy-releasing phase of reaction. Coupled with this observation is the oscillatory nature of the temperature and pressure traces for the reaction from 2000 to 2300°K. These observations can be explained in kinetic terms if an oscillatory reaction is postulated. Above 2000°K the oscillations are observable. Below this temperature they are damped sufficiently so that no change in the temperature or pressure trace is obtained under the experimental conditions. An excellent discussion, of oscillatory kinetic phenomena which have been reported to occur in hydrocarbon flames, has been given by Yang and Gray [14]. The observation of this phenomenon has not previously been reported in shock tubes. A note of caution is in order, however. The oscillatory pattern of the pressure and temperature traces could also be due to the occurrence of transverse instabilities in the shock wave which is passing through the gas mixture and initiating an exothermic reaction [ 15]. Work is being continued in an attempt to elucidate the kinetic scheme of the reaction between CH4 and NO2. This work includes a com-

180 puter simulation o f the kinetic steps o f the reactions and further experiments to decide if oscillatory kinetics are occurring for this system.

This work was supported in part by the Air Force Systems Command through the AFSC/AFIT Research Support Fund. The authors wouM like to thank Dr. Assa Lifshitz for helpful discussions concerning the inhibitory role o f argon. References 1. A. Lifshitz, K. Scheller, A. Bureat and G. B. Skinner, Combust. Flame 16, 311 (1971). 2. G. B. Skinner, A. Lifshitz, K. Scheller, and A. Burcat, & Chem. Phys. 56, 3853 (1972). 3. IL W. Crossley, E. A. Dorko, K. Scheller, and A. Burcat, Combust. Flame 19, 373 (1972). 4. A. Burcat, R. W. Crossley, K. Seheller, and G. B. Skinner, Combust. Flame 18, 115 (1972).

ERNEST A. DORKO, et al. 5. F. H, Verhoek and F. Daniels, J. Amer. Chem. Soc. 53, 1250 (1931). 6. Joint Army, Navy, and Air Force Thermochemical Panel, Thermochemical Data, The Dow Chemical Co., Midland, Michigan. 7. P. G. Ashmore and K. F. Preston, Combust. Flame 11,125 (1967), 8. R. E, Huffman and N. Davidson, J. Am. Chem. Soc. 81, 2311 (1959). 9. H. Hiraoka and R. Hardwiek, 3". Chem. Phys. 39, 2361 (1963). 10. J. Troe, Ber. BunsengesellschaftPhys. Chem. 73, 144 (1969). 11. F. S. Klein and J. T. Herron, J. Chem. Phys. 41 1285 (1964). 12. J. T. Herron,lnt. J. Chem. Kinetics 1,527 (1969j. 13. A. Lifshflz, K. Scheller, and D. Bass, J. Chem. Phys., 60, 3678 (1974). 14. C. H. Yang and B. F. Gray, J. Phys. Chem_ 73, 3395 (1969). 15. R. A. Strehlow, Phy. Fluids, 121, 96 (1969).

Received July 22, 1974; revised October 21, 1974