O2 mixtures

Chemical Structures of Fuel-Rich Flames of trans-C2H2Cl2/CH4/Ar/O2

Mixtures

MARCO J. CASTALDI and SELIM M. SENKAN* Department of Chemical Engineering, Universityof California, Los Angeles, California 90024

Temperature and species concentrationprofileswere obtained for atmospheric-pressure, premixed,laminar, fiat flamesof trans-C2H 2C12/CH4 mixturesunder fuel-richconditionsat several CI/H ratios. Sampleswere withdrawn from within the flames using a heated micro-probe followedby gas analysisvia on-line capillary gas chromatography/massspectrometry(GC/MS). Mole fraction profileswere determinedfor the reactants and for CO, CO2, HC1,H20 , H2, CH3CI, CH2C12,CHC13,CC14,C2HCi, C2H3C1,C3H5C1,C2H2, C2H4, C2H6, C4H4, C4H2, C6H6, and C~oHs. The role of these species, in view of our current understandingof the mechanismof combustion of trans-C2H2Cl2 and CH4, are discussed. INTRODUCTION Incineration represents a viable method for the safe treatment of hazardous chemical wastes which frequently contain chlorinated hydrocarbons (CHC) [1]. However, our continued use of this important technology will largely depend upon our ability to understand and control the formation and emission of potentially toxic by-products, such as chlorinated aromatics that are precursors for dioxins and furans, in incineration. Consequently, understanding the flame chemistries of chlorinated hydrocarbons and the effects CHCs have on hydrocarbons flames are important. Flame structure studies are also important for the development and verification of fundamental models describing the destruction of hazardous materials and for the development of clean combustion technologies with minimal emission of potentially dangerous by-products. Trans-C2H2Cl2 (TDCE) is a synthetic organic chemical which is used as a low temperature extraction solvent in the manufacture of organic materials such as dyes, perfumes, lacquers, and thermoplastics [2]. Recently, it has also been used in blends of hydrochlorofluorocarbons, in an attempt to replace chlorofluorocarbons under the Montreal Protocol. Although TDCE is not classified as a carcinogen, it is on the EPA's list of priority pollutants [3]. We are not aware of any prior studies in which the combustion of TDCE has been investigated. The only related work appears to

*Corresponding author. COMBUSTION AND FLAME 104:41-50 (1996)

Copyright© 1996by The CombustionInstitute Published by Elsevier ScienceInc.

be the laser-induced reactions of TDCE studies by Guckert and Carr [4], where the major reaction products were determined to be cisC2H2C12 and C2HCI. In related research, the flame structures of a variety of chlorinated hydrocarbons have been established, including those for C2HC13 [5, 6] and 1,3-C2H4C12 [7], representing systems with high and low C 1 / H ratios, respectively. These experimental investigations, in conjunction with detailed chemical kinetic modeling [8, 9] also revealed the importance of the C I / H ratio and the isomeric structures of CHCs in affecting the nature of intermediates that form in the combustion of chlorinated hydrocarbons. Consequently, we began a series of investigations involving the three isomeric structures of C2H2C12, i.e., transC2H2C12 (TDCE), cis-C2HzCl 2 (CDCE), and 1,1-C2HzC12. In this paper we report the results of an experimental study on the flame chemistry of trans-C2H2C12; experiments with other isomers of C2H2C12 are in progress and will be reported in the future.

EXPERIMENTAL The experimental system has been described in detail previously [10]: thus only a brief explanation is provided. The atmosphericpressure, premixed, laminar, flat-flames of trans-C2H2C12/CH 4 were stabilized over a 50mm-diameter porous bronze burner with a concentric Argon shield gas distributor. Species sampling was accomplished by withdrawing gases through a 6 mm o.d. quartz microprobe with a tapered tip having an orifice diameter of 0010-2180/96/$15.00

SSD10010-2180(95)00101-B

42 about 0.075-0.10 mm. The microprobe as well as the porous burner was cooled by hot ethylene glycol at 125°C [11]. The sampling probe pressure was maintained at less than 100 torr by means of a mechanical vacuum pump to provide rapid removal of gases from the hot flame zone and to minimize the condensation and/or adsorption of: high molecular weight species on surfaces in the sampling line. Under these conditions the partial pressures of all the species reported here, including C10H 8 [naphthalene, b.p. 218°C, Pv(100°C) = 20 torr], were significantly lower than their vapor pressures, thus the acquisition of accurate concentration data was possible. In addition, the flow through the probe orifice would be nearly choked and result in the extraction of gas samples spatially averaged over 2-3 orifice diameters in the upstream direction. As we shall see below, these conditions were sufficient for the acquisition of spatially resolved species concentration profiles in atmospheric-pressure flames under fuel-rich conditions. TDCE was delivered to the burner using a high-pressure liquid chromatography pump (ISCO Model 2354). The pump was used with a flow restriction valve in the downstream to further minimize pressure oscillations. The liquid fuel was injected into preheated gases (Ar, CH4, and 02) through a syringe needle for rapid and steady evaporation. The steadiness of the liquid flow as well as evaporation was visually verified by monitoring a rotameter that was installed along the liquid delivery line and by observing the luminosity of the flames at the onset of soot formation. The identification and quantification of species were accomplished using a computer-controlled gas chromatograph/mass spectrometer (GC/MS) system (Hewlett-Packard 5890/ 5971A). The GC was equipped both with a capillary column (0.3 mm × 50 m fused silica) which was directly interfaced to a quadrupole mass spectrometer, and two packed columns (6 ft Porapak Q and 6 ft Molecular Sieve 5X) that were connected to a thermal conductivity detector through a multiport valving mechanism. Consequently, the complete analysis of the gas samples, i.e. both high- and low-molecular-weight compounds, was possible in a single experiment. The samples were introduced

M . J . CASTALDI AND S. M. SENKAN into the G C / M S by computer-controlled vane injection. Standard procedures of gas analysis using GC and MS were employed throughout the experiments. Calibration of reactants was made by withdrawing unburned gases directly through the sampling probe. For all gaseous intermediate and product species, certified gas mixtures acquired from Matheson Co. were used. We estimate an accuracy of about +15% for the determination of the mole fractions of major C 1 and C 2 species and _+20% for the remaining species. For species for which calibration standards were not available the relative ionization cross section method was used [12]. This method is expected to be accurate within a factor of 2. Species profiles were generated by moving the quartz probe relative to the stationary burner setup with the aid of a vertical translator having a precision of about 0.01 mm. We estimate that an uncertainty of about _+0.3 mm exists in the absolute positions of the species profiles. However no such uncertainty would exist among the profiles of different species in a given experiment. Temperature profiles were measured by using a 0.075-mm P t - P t / 1 3 % Rh thermocouple with a bead diameter of about 0.15 mm immediately after the concentration measurements. It was freshly coated by silica and glazed before each experiment to minimize catalysis. The thermocouple was positioned parallel to the burner surface to minimize conduction along the wires, and was kept in the flame for as little time as possible to prevent the degradation of the coatings. The temperature profiles reported here correspond to direct thermocouple readings and were not corrected for radiation effects in an attempt to better establish gas temperatures. This was not done because such corrections can introduce additional uncertainties, primarily due to the unknown emissivity of the soot coated thermocouple beads [13]. Nevertheless, such calculations can be made from the information provided here by assuming a plausible emissivity leading to temperature corrections as high as 100 K in the hottest part of the flame. We also estimate an uncertainty of _+0.25 mm in the absolute positions of the temperature profiles.

CHEMICAL STRUCTURES OF FUEL-RICH FLAMES RESULTS AND DISCUSSION

In Table I, the precombustion mixture compositions and other features of the flames studied are presented. The equivalence ratios (~b = stoichiometric 0 2 required/actual O 2 used) were determined by considering HCI as the preferred oxidation product [14]. These particular compositions were determined to be suitable for the intended microprobe studies after different equivalence ratios, argon dilutions, and fuel ratios ( T D C E / C H 4) were explored. At higher equivalence ratios, the formation of large amounts of carbonaceous deposits plugged the probe orifice and prevented the successful undertaking of long experiments. At lower argon dilutions and lower equivalences ratios, flames were positioned so close to the burner surface that the acquisition of quantitative flame structure data was not possible by the microprobe used. At higher argon dilutions, flames became too weak and attached to the sampling probe. We have used CH 4 as an auxiliary fuel because its combustion characteristics are reasonably well understood [15, 16]. As seen in Table I, all the flames were in the fuel rich region with varied amounts of dilution, C I / H ratios and equivalence ratios. Flame A had an equivalence ratio of 2.34 and a C1/H ratio of 0.119. Flame B had a lower equivalence ratio (2.02), but higher C I / H ratio (0.157). Flame C had the highest equivalence ratio (2.40) and the C1/H ratio (0.173); however, it was also the most difficult flame to sample because of excessive soot formation. Before experimental results are discussed, a number of issues concerning the accuracy of

43

our measurements should be stated. First, the profiles within 0.5 mm from the burner surface must be considered unreliable because of possible sampling-probe burner-surface interactions. Second, because of errors in sampling and/or calibrations, carbon balances generally deviated from the 100% level. However, these deviations were generally within + / - 20%, thus were satisfactory. Third, because of the formation of soot and soot precursors, the amounts of which were not quantified, carbon balances remained poor in post flame region.

Flame A

In Fig. 1, temperature and mole fraction profiles for TDCE, CH4, 02, Ar, CO, and CO 2 are presented. In this and subsequent figures, lines have been drawn through the data points as a visual aid to indicate trends. Experimental carbon atom balances together with the mole fraction profiles of H20, HCI and H 2 are given in Fig. 2. The HCI and H 2 0 mole fraction profiles were obtained from CI and O atom balances, respectively. As seen in Figure 2, the carbon balances deviated slightly from 100%, thus were reasonable considering the presence of diffusional effects. Since the hydrogen calibration was good, an independent determination of HzO mole fractions using H-atom balances was also made. These com-

03

1200

1000 0.2~

TABLE I

°f

Experimental Conditions and Features of the Flames Studied Hame

A

B

C o.1

Composition (%v) Ar CH 4 02 TDCE Cold Gas Velocity (cm/s) Equivalence Ratio C1/H Ratio Fuel Ratio (TDCE/CH 4)

37.6 26.6 28.7 7.17 10.17 2.34 0.119 0.27

49.6 18.4 25.2 6.88 8.88 2.02 0.157 0.373

30.9 27.6 30.5 11.0 8.40 2.53 0.166 0.398

o~ I

2

3

4

Dimnce atx)ve bum~. smface (n~-a)

Fig. 1. Temperature and species mole fraction profiles for TDCE, CH 4, O 2, CO, CO 2, and Ar in flame A.

44

M. J. CASTALDI AND S. M. SENKAN 0.000

no~

~4

CH~CI*IO 0.0C5 0.3 •

~

'

1

0

oo 0,004

l

I

0.2-

0.1 -

•

0,003

HCl 0.001

o,o i

l I i i i i 1 2 3 Distanc=above b u r n ~ sun'ace(ram)

i 4

i

o.ooo

• 1

2

. 3

4

Distance above bttm~ surfa~ (ram)

Fig. 2. Species mole fraction profiles for H2, H20, and HCI, as well as carbon balances alongflameA.

Fig. 3. Speciesmole fractionprofilesfor cn3c1, CH2C12, C2H3C1, C2HCI,and C3H5C1in flameA.

pared favorably with the mole fractions for H 2 0 determined from the O-atom balances. As seen in Fig. 1, TDCE was completely consumed within 2.5 mm from the bumer surface while CH 4 and O 2 penetrated to about 4.5 and 3.5 mm, respectively. The decay of CH 4 and O 2 followed similar patterns, and CO formed as the major product in this fuel-rich flame. Peak flame temperature was 1220°C, and occurred at about 2.5 mm from the burner surface, after the consumption of the halogenated species. The subsequent decrease of the flame temperature was likely to be caused by radiative heat losses from the sooting flame. The decrease in Ar concentration can be attributed to the molar expansion of gases in the flame upon combustion. Figure 2 shows the HCI concentration leveling off at 12% at about 2.5 mm from the burner surface; this coincides with the complete consumption of TDCE. The H E also leveled off at about 35% at 4.0 mm from the burner surface, corresponding to the point of near complete conversion of CH 4. In Fig. 3, the mole fraction profiles for the chlorinated intermediates CHaC1, CH2C12, CEH3C1, C2HCl, and C3HsCI are presented. As evident from Fig. 3, C2HaCI was produced at the highest level and peaked at 1.5 mm from the burner surface at a level of 3000 ppmv (parts per million by volume). The maximum

concentrations of CH3CI, CH2C12, C2HCI and C3H5C1 were about 500, 100, 400, 30 ppmv, respectively, and they all peaked at about 2.0 mm from the burner surface. The rapid decrease in the concentration of chlorinated intermediates beyond 2.0 mm can be attributed to the significant consumption of TDCE at this point (see Figure 1), as their formation is intimately linked to the CHC fuel. The formation of C2HC1 in the flames was consistent with the results of Guckert and Carr [4]. In Fig. 4, the mole fraction profiles for C 2 hydrocarbon intermediates C2H6, C2H4, and CEH 2 are presented. As expected, C2H 2 was most abundantly produced, reaching levels as high as 4.5% at about 2.5 mm from the burner surface. In addition, it penetrated well into the post flame zone. In contrast, both C2H 4 and CEH 6 were produced at significantly lower concentrations, i.e., peak concentrations of 0.45% and 0.035%, respectively, and were rapidly destroyed. These results are totally consistent with our understanding of the combustion kinetics of CH 4 [15, 16]. Similarlythe profiles for C4H2, C4H4, C6H 6 (benzene), and C10Hs (naphthalene) are presented in Fig. 5. The relative ionization method was used for the quantification of C4H2, C4H4, and C10H s. The formation and the relative levels of these higher molecular weight hydro-

CHEMICAL STRUCTURES OF FUEL-RICH FLAMES 0.05

45

0.3

1200

CIH4*IO ~ ,

0.04

1000

j

0.03 -

J

J ~ 0.02-

os

o.1

0.01-

C2H6*I~I ~ ~ o.o

0.[30

°!

o.2

i

~

]

;

I

2

3

4

Disumc¢ above bun~

suzf~¢

(ram)

Fig. 4. Species mole fraction profiles for C2H6, C2H4, and C2H 2 in flame A.

carbon intermediates are also consistent with prior experiments involving the mixtures of CHCs and CH 4 [7, 17, 18]. From Fig. 5, it can also be seen that the levels of C 6 H 6 and c 4 n 4 were about the same, i.e., with peak concentrations of about 175 and 130 ppmv, respectively. The levels of C a l l 2 were about 10 times larger, with peak concentration of about 1700 ppmv. On the other hand the levels of C10H 8 were lower, with a peak value of about 13 ppmv. All

i

~

~

1

2

200 3

4

Fig. 6. Temperature and species mole fraction profiles for TDCE, CH4, O2, CO, CO2, and Ar in flame B.

of these species persisted in the flame well into the post flame zone. Flame B Figure 6 presents the temperature and species profiles for C H 4 , 02, mr, CO2, CO and TDCE for Flame B. Experimental carbon balances together with H 2 0 , H2, and HCI are shown in Fig. 7. The profiles for H 2 0 and HCI mole fractions as well as the carbon balance were obtained by the same method used in Flame A, i.e., by O-, CI-, and C-atom balances, respec-

0.002 0.2

I

.

.

.

.

-

-

.

0.001"

l 0,1

0.000

I

I

I

1

1

2

3

4

0.0

i

i

1

t

i

2

i

i

i

3

4

i

,bo,,, b . m = ~at"~c, ( ~ )

Fig. 5. Species mole fraction profiles for C4H2, C4H4, C6I-I6, and C]0Hs in flame A.

Fig. 7. Species mole fraction profiles for H2, H20 , and HCI, as well as carbon balances along flame B.

46

M. J. CASTALDI AND S. M. SENKAN

tively. Independent H-atom balances also resulted in the determination of H 2 0 mole fractions that were comparable to those obtained by O-atom balances. As seen from the species and temperature profiles presented in Fig. 6, Flame B was positioned closer to the burner surface than Flame A (Fig. 1). This result is consistent with the lower equivalence ratio of Flame B (4) = 2.02) compared to Flame A ((h = 2.34). From Fig. 6 it can be seen that TDCE was consumed within 2.0 mm of the burner surface with 0 2 surviving until about 2.5 mm, while c n 4 penetrated well into the post flame zone. A peak flame temperature of about 1210°C occurred at about 2.0-2.5 mm above the surface. As seen in Fig. 7 the HCI peaked at about 9% of 1.5 mm above the burner, a level similar to Flame A. However, the peak H 2 concentration was only 1 / 2 of the value of Flame A, or 15% in this flame. On the other hand peak H 2 0 levels increased by a factor of about 2 in Flame B, reaching 15%. These results are consistent with the lower equivalence of Flame B. The mole fraction profiles of chlorinated intermediates, i.e. C3H5C1 , C2H5C1, C2HCI, and CH3CI, a r e presented in Fig. 8. Here, C2H3C1 was produced in the highest concentration of about 3500 ppmv and peaked at about 1.0 mm above the burner. The peak

0.004

concentrations of the C2H5C1, C 2 H C I , and CH3C1 w e r e about 20, 400, and 190 ppmv, respectively. The C2 hydrocarbon intermediates, which consist of C2H6, C 2 H 4 , and C2H2, are shown in Fig. 9. Again C2H 2 was most abundantly produced, reaching levels of about 3% at about 1.5 mm, and penetrated well into the post flame zone. The peak concentrations of C 2 H 6 and C 2 H 4 w e r e 0.019% and 0.25%, respectively. The levels of these C2 intermediates were slightly lower than those observed in Flame A, a result consistent with the equivalence ratios of these two flames. The profiles for the highest molecular weight species detected in Flame B are given in Fig. 10. These species again consisted of C 4 H 2 , C4H4, C6H6 (benzene), and C]0H s (naphthalene). The peak concentrations for C4H2, C4H4, C6H6, and C10H s were 1200, 100, 100, and 5 ppmv, respectively. Flame C

This flame had the highest equivalence ratio (~b = 2.53) and the highest C I / H ratio of 0.166 (Table I). These conditions rendered the sampiing of this flame difficult because of excessive soot formation. Consequently, useful data only up to 3.5 mm from the burner surface were

0.04 C2HCI*IO

0.003 -

o.O3-

O.0O2-CC~H~COI0*I~~ i

o.001 •

0.01 -

CzH~*IO0

0.000

-

Distaace above bura~" ~

(=am)

Fig. 8. Species mole fraction profiles for CH3C1, CH2C12, C2H3Cl , CEHCl , and C3H5C1 in flame B.

O.QO I

i 1

'

r 2

DL~Umc© above ~ ' n ~

t 3

:

t 4

sm~aco ( r a m )

Fig. 9. Species mole fraction profiles for C2H6, C2H4, and CEH 2 in flame B.

CHEMICAL STRUCTURES OF FUEL-RICH FLAMES

47

0.0015 0.4

•

t 120 .~

.

T

•

o.'.,. ~ a o

•

100~

~

0.0010 CiHI*10 I

0.2-

| 0.0005

cao~*lO0 ~Q

0.0 0.0000

I 2

I I

T 3

D i l a t e above bm'ncr~

t 4

I 1

'

l 2

Disu~ce above btmm- ~ r f ~

(ram)

Fig. 10. Species mole fraction profiles for C4H2, C4H4, C6H6, and CloH 8 in flame B.

obtained due to the plugging of the sampling probe orifice. However, some additional species not observed previously in other flames were detected in this flame, thereby providing additional insights to the combustion chemistry of TDCE.

In Fig. 11 the temperature and species profiles for CH4, 02, AF, CO2, CO and TDCE are presented. Experimental carbon balances together with H20, Ha, and HC1 are shown in Fig. 12. Again the profiles for H 2 0 and HC1 mole fractions were obtained by O- and Cl-

O.3

[ 0

3 (~)

Fig. 12. Species mole fraction profiles for H 2, H20 , and HC1, as well as carbon balances along flame C.

atom balances, respectively. As evident from these figures, the reaction zone was slightly broader and the peak flame temperature lower than the other flames, because of the higher equivalence ratio of Flame C. Peak H 2 and HCI concentrations were higher than Flame A, a result that is consistent with the higher equivalence ratio and C1/H ratio of Flame C. As expected, this flame also produced the lowest levels of H 2 0 (Fig. 12). In Fig. 13, the mole fraction profiles for the chlorinated intermediates, that include CH3CI,

1200

0.004

o, 0.2.

c~

4 o

r

1000

co

OI3CJ..10

°j

•

cc11.1co 0.0~0.1

4OO

0.o

L 1

,

,

2

3

200

Dimnct abo,,e b u n ~ sur/~e ( n ~ )

Fig. 11. Temperature and species mole fraction profiles for TDCE, CH4, 02, CO, CO2 and Ar in flame C.

0.001 CflC:l~'100

0.000

'

; t

'

I 2

D/stance above bum~ surface ( m m )

Fig. 13. Species mole fraction profiles for CH3C1, CH2C12, CHC13, CC14, C2H3C1, C2HC1, and C3H5C1 in flame C.

48

M. J. CASTALDI AND S. M. SENKAN

CH2C12, CHCI3, CCI 4, C2HCI, C3H5C1 and C2H3C1 are presented. Both CHCI 3 and CC14 represent new species detected in this flame, which is a consequence of its high C I / H ratio. As can be seen in Fig. 13, there is a poor correlation of the species concentration profiles for several of the CHC intermediates. For example, CC14, CH2C12, CHCI3, and C3H5C1 were formed early on in the flame, reaching their peak mole fractions at about 1.0 mm from the burner surface. In contrast, CH3CI, C2H3C1 , and C2HCI levels peaked significantly later. This behavior of the CHC mole fraction profiles is inconsistent with the measurements obtained in Flames A and B. Consequently, the results for H C H profiles in Flame C should be used with caution. As noted above, there were sampling problems with Flame C because of excessive soot formation and the premature plugging of the probe orifice. The relative ordering of the chlorinated intermediates with respect to their peak concentration levels was C2HC1 (5000), C2H2C12 (2000), C2H3C1 (1250), CH3C1 (450), C3H5C1 (25), CCI 4 (20), CHCI 3 (17), and CH2C12 (10), where the numbers in parentheses represent ppm values. The concentration profiles for C 2 hydrocarbon intermediates, i.e., C2H6, C2H4, and C2H 2 are presented in Fig. 14. These profiles, unlike the CHCs, were consistent with our

0.05

e~n,

other measurements in Flame A and B. Again C2H 4 was most abundantly produced reaching 4.0% peak concentration at 2.6 mm above the surface. The peak levels of C2H 4 and C2H 6 were 0.38% and 0.02%, respectively. These values are slightly higher than those observed in Flame A, a result which is consistent with higher equivalence ratio of this flame. Finally, in Fig. 15 the mole fraction profiles for C4H2, C4H4, c 6 n 6 (benzene) and C10H 8 (naphthalene) are presented. Again these profiles are consistent with the results of Flames A and B. The peak concentrations for these species were 150, 1100, 230, and 20 ppmv for C4H2, C4H4, Crn 6 and C10H8, ppmv, respectively. As expected, these values are also higher than other flames reported here. Based on thermochemical considerations as well as our current knowledge of the kinetics and mechanisms of chlorinated hydrocarbon reactions [19], the following remarks can be made with regard to the major reaction pathways leading to the formation of chlorinated intermediates observed in the experiments. In fuel rich flames, the destruction of fuels and intermediates occur as a result of C1, H, OH, and to a lesser degree O radical attack. The attack of these radicals on TDCE is a complicated process because of the competition between simple abstraction and addition, and the isomerization of the resulting adduct in the latter case. Consider for example the attack of C1 radicals. In the case of simple abstraction, C1 radicals will primarily result in H-atom abstraction,

0.04

CI + t-CHCICHCI ~ CHC1CCI + HCI, [6.05]

(1)

0.03

"~

0.02.

0.01

0.00 Distance above burner surface (ram)

Fig. 14. Species mole fraction profiles for C2H6, C2H4, and C2H 2 in flame C.

because the Cl-atom abstraction reactions are considerably endothermic, e.g. 20-30 kcal/mol [18]. In the above and subsequent reactions, the numbers in parentheses correspond to enthalpies of reaction in kcal/mol units. Alternately, CI can also add to the TDCE, and result in the production of CHCI2CHC1 radical. The energy rich CHCI2CHCI radical can then decompose by the /3-scission of the C-C1 bond, as C - H bond dissociation is more endothermic. However, this process can result in

CHEMICAL STRUCTURES OF FUEL-RICH FLAMES

49

and contribute to the production of CHCICCI radicals:

0.~

(OH, O) + t-CHCICHC1 CHC1CCI + (H20, OH), [ - 10.0, 6.86]

(5)

In addition, its unimolecular decomposition also results in TDCE destruction:

Ji ~

ciellt

t-CHCICHCI + M

0.11111

CzHCI + HCI + M,

0.~ Dist,~c© ~ov© burner surface (ram)

Fig. 15. Species mole fraction profiles for C4H2, C4H4, Call 6, and C10H8 in flameC.

[34.0]

(6)

and lead to the formation of CzHC1, as noted in the experiments. The unimolecular decomposition of CHC1CC1 radicals, produced by Reactions 1, 3, and 5 can also contribute to CzHC1 formation: CHCICCI + M ~ CzHCI + CI + M,

[27.9]

(7) the production of a mixture of trans-C2HzCl e and Cis-CzH2C12: C1 + t-CHCICHCI ~ [CHCI2CHCI]* c-CHC1CHC1 + C1, (2) where []* represents chemically activated adduct. For the case of H radicals, the situation is somewhat different. First, H radicals can abstract both H and CI atoms from TDCE: H + t-CHCICHCI (CHC1CH, CHCICC1) + (HC1,H2), [ - 8.25, 5.01]

(3)

Upon addition to TDCE, the most likely reaction path for the adduct will be the loss of C1 by the /3-scission of the C-CI bond: H + t-CHC1CHCI ~ [CHCICH2CI]* C2H3C1 + CI,

[-17.3]

(4) This reaction also results in the production of C2H3C1, as observed in the experiments. In addition, the following abstraction reactions can also contribute to TDCE destruction: As in the case of CI radicals, the attack of electronegative OH and O radicals would be expected to abstract only H atoms from TDCE,

It should be noted that analogous reactions can also be formulated to describe the destruction of the cis-C2H2Cl 2. The destruction of the c n 4 fuel similarly would occur by the following reactions: (El, H, OH, O) + CH 4 CH 3 + (HCI, H 2 , H 2 0 , OH), [1.67, 0.62, - 14.4, 2.48]

(8)

The chlorination of the CH 3 radicals produced by Reaction 8 can then lead to CH3CI: CH 3 + t-CHC1CHC1 ~- CH3CI + CHC1CH

[11.1]

(9)

Similarly, the formation of CH2CI 2, as seen in the experiments can be accounted for by the preferential formation of CH2CI from CH3C1 by the attack of C1, OH, and O: CH3CI + (C1, OH, O) CH2CI + (HC1, H20, OH), [ - 3.97, - 20.0, - 3.16]

(10)

followed by the subsequent chlorination of the CH2CI: CH2CI + t-CHCICHCI CH2CI z + CHCICH,

[-15.9]

(11)

50

M. J. CASTALDI AND S. M. SENKAN

The formation of CHCI3, as well as CCl 4 detected in chlorine-richer Flame C, can then be accounted for by considering reaction sequences similar to Reactions 9 and 10. The production of C3H5C1 can be explained by considering the following substitution process, which also proceeds via the formation of a chemically activated adduct:

pears to be reasonably in place to qualitatively account for the new species observed in flames.

This research was supported, in part, by funds from the U.S. Environmental Protection Agency Grant No: R819178-01, the National Science Foundation, Grant No: CTS-9311848 and the UCLA Center for Clean Technologies.

CH 3 + t-CHCICHCI ~ [CH3CHC1CHCI]* CH3CHCHC1 + C1 (12) Clearly, Reaction 11 also would contribute to TDCE destruction as well. As evident from the above reaction, the formation of chlorinated intermediates are intimately related to the presence of TDCE in the flames, and therefore their production rates would be expected to decrease as the concentration of TDCE decreases. Indeed, this behavior has been observed in Figs. 3, 8, and 13 where the mole fractions of CHC intermediates peaked early in the flame zone, where TDCE destruction was virtually complete (Fig. 1). Plausible reaction pathways describing the formation and consumption of hydrocarbon intermediates can similarly be proposed. Since CH 4 was the major hydrocarbon fuel used in this flame, its reactions and the reactions of its daughter products must be considered to account for the experimental results. Because these reactions have been described quite extensively in our earlier flame structure studies [7, 17, 18], they will not be repeated here. In summary, the chemical structures of fuel-rich flames of T D C E / C H 4 established by using heated microprobe sampling and G C / M S provide new insights on the nature of intermediates associated with the combustion of TDCE in particular and of chlorinated hydrocarbons in general. These measurements provide new quantitative information that should be considerable utility for the development and verification of detailed chemical kinetic mechanisms. Our current understanding of the thermochemistry and reaction kinetics of hydrocarbons and chlorinated hydrocarbons also ap-

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