1,1-Dichloroethane Combustion Chemistry

Comhusr. Sci. ond Tech., 1995, Vol. 106, pp. 69-82 Reprintsnvailable directly from the publisher Photocopying permitted by license only

0 1995 OPA (Overseas Publishers Association) Amsterdam B.V. Published under licensc by Gordon and Breach Science Publishers SA

Printed in Malaysia

Chemical Structures of Fuel-Rich, Premixed, Laminar Flames of 1,1 -C,H4CldCH410dAr

I. A. GARGUREVICH, MARC0 CASTALDI and S . M. SENKAN* ~ e ~ a r t m e n t o f Chemical Engineering, University of California, Los Angeles CA 90024

(Receiued August 8. 1994: infinal Jorm February 6, 1995)

ABSTRACT-Temperature and species mole fraction profiles were determined in atmospheric-pressure, premixed, laminar, flat flames of I, 1-C,H,CI, and CH, under fuel-rich conditions at different CI/H ratios. Samples were withdrawn from within the flame using a heated microprobe followed by gas analysis by on-line capillary gas chromatography-mass spectrometry (GC/MS). The mole fraction profiles were determined for all the reactants and for CO,CO,, HCI, H,O, H,,C,H,,C,H,, C,H,,CH,CI.C,H,CI, CH,CCI,,CH,CHCICH,,CH,CCICHCH,,C,H,,C,H,,C,H, (benzene), and C , H, (napthalene). The roleofthesespecies,in view ofthecurrent ideasofthe mechanismofcombustion of 1.7-DCE isdiscussed and compared to earlier measurements made in I,2-C,H,CI, flames.

Key Words: Flame chemistry, chlorinated hydrocarbons, toxic by-products

INTRODUCTION

Safe destruction anddisposal of hazardous chemical wastes, which frequently contain chlorinated hydrocarbons (CHC), is an important challenge facing the manufacturing industry in the US and abroad, and incineration represents an effective method of treatment(Senkan, 1988). However, thecontinued useofthis important technology will largely depend on our ability to better understand and control the emission of potentially toxic by-products that form in these devices. Consequently, it is of practical interest to investigate the chemical structures of the flames of chlorinated hydrocarbons, as well as the flames of mixtures of CHCs and hydrocarbon fuels, to better establish the nature of potentially toxic intermediates that may form during their combustion. Flame structure studies are also important for the development and validation of detailed chemical kinetic mechanisms describing the combustion of CHCs. Detailed mechanisms, once developed, can then be combined with models describing the transport phenomena to simulate the combustion and emission behav- ior of practical systems from fundamental considerations (Ho and Bozzelli, 1992; Fisher and Koshland, 1992; Won and Bozzelli,1992; Chang and Senkan,1989; Karra et a/., 1988). The development of such fundamental models will be crucial for the design and operation of clean combustion systems that will lead to the emission of lowest possible levels of pollutants.

Over the past years, we have been undertaking systematic studies directed towards the development of a better understanding of the chemical kinetics of combustion of

Author to whom correspondence should be addressed

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70 I. A. GARGUREVICH AND S. M. SENKAN

select CHCs. Previously we reported experimental species mole fractions profiles in fuel-rich flames of CH,CI (Karra and Senkan, 1987), CH2CI2 (Qun and Senkan, 1990), CHCI, (Lee et al., 1993). CCI, (Xieqi et al., 1993), C,HCI, (Chang ef al., 1986 and Changand Senkan, 1988), and I,2-C2H4CI, (I,2-DCE)(Kassem and Senkan, 1991). In parallel, detailed chemical kinetic mechanisms describing the major features of these flames have also been developed and partially tested using available experimental data (Karra et a/., 1988; Chang and Senkan, 1989; Qun and Senkan, 1994). In particular, preliminary modeling studies of I,2-DCE combustion (Kassem, 1990) indicated the need to accurately describe the reactions of chlorinated hydrocarbon isomers to quantitatively account for experimental observations. Consequently, we undertook the studies reported in this paper to establish the similarities and differences that exist between the flame structures of the two isomers of dichloroethane, 1, 2-C2H4C12 and I , I-C2H4C12.

EXPERIMENTAL

The experimental facilities have been described in detail previously (Chang and Senkan, 1988; Kassem et ul., 1989), thus only a briefexplanation will be provided. The atmospheric-pressure,premixed, laminar, flat-flames of 1, I-C2H4CI,/CH4 were stabil- ized over a 50mm diameter porous bronze burner with a concentric shield gas (Ar in the present experiments) distributor. Reactant gases and Ar were separately metered using mass flow meters (MKS, Burlington, MA), and then were mixed and heated to about 150•‹C. The liquid fuel was separately metered and delivered by a high pressure liquid pump (ISCO 2354, Lincoln, NE), and was injected into the preheated reactant gases to insure rapid and steady evaporation. The steadiness of the liquid flow was established by the use of a rotameter that was placed downstream along the liquid delivery line as well as by observing the luminosity of the flame at the onset of soot formation.

Gas samples were obtained by withdrawing gases from within the flame using a 6 mm OD quartz microprobe. The end of the probe was conically tapered at an angle of about 15" and had a 0.075-0.10mm diameter orifice at its tip (Kassem et ul., 1989). The microprobe as well as the porous burner was heated by ethylene glycol to 125•‹C. The sampling probe pressure was maintained at about 100Torr by means of a mechan- ical vacuum pump to provide rapid removal of gases from the hot flame zone and to minimize the condensation and/or absorption of high molecular weight species on surfaces in the sampling line. Under these conditions, the partial processes of all the gases, including C,,H, [napthalene, b.p. 21S•‹C, P,(10O0C) = 20Torr)], were signifi- cantly lower than their vapor pressures, thus the determination of accurate flame concentration profiles was possible. Gases sampled were then transported to the analysis system through glass-coated lines that were electrically heated to about 200•‹C.

A number of practical difficulties exist in the acquisition of accurate flame structure data from 1 atm pressure flames. These difficulties include probe-induced distortions in the concentration profiles due to flame attachment and spatial averaging, especially in regions of steep concentration gradients, and the possible continuation of reactions in the sampling probe and transfer lines. Since the complete elimination of all of these

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1 , I-C,H,CI,/CH,/O,/Ar FLAMES 71

problems is impossible in intrusive micro-probe sampling, we have expended consider- able time and effort in the past to develop optimal flame sampline, techniaues (Karra . . . - . .

and Senkan. 1987: Kassem et a/., 1989). These investigations led to the conclusion that, by using microprobes and the operating conditionsnoted above, the acquisition of reliable flame structure data in atmospheric pressure should be possible.

Although probe-induced distortions are expected to be less in sub-ambient pressure . flames that exhibit broader reaction zones, 1 atm flame studies are important and vital in developing a better understanding of the nature of combustion intermediates under practical conditions. This is due to the fact that high molecular weight by-products and intermediates, e g , aromatics, polyaromatics and soot, form via pressure- dependent radical-radical recombination and radical addition reactions. Conse- quently, sub-atmospheric pressure flame experiments, although desirable from a spatial resolution point of view, would be of less utility in establishing the relevant flame chemistry of halogenated hydrocarbons in practical incinerators.

The identification and quantification of species were accomplished using a computer-controlled gas chromatograph/mass spectrometer (GC/MS) system (Hewlett- Packard 5890/5971A). The G C was equipped both with a capillary column (0.3 mm x 50m fused silica) which was directly interfaced to a quadrupole mass spectrometer, and 2 packed columns (6ft. Porapak Q and 6ft. Molecular Sieve 5X) which were connected to a thermal conductivity detector (TCD) through a multiport valving mechanism. Consequently, the complete analysis of the gas samples, i.e. both the high and low molecular weight compounds, was possible in a single experiment. The samples were introduced into the GC/MS by computer controlled valve injection. Standard procedures ofgas analysis using G C and MS were employed throughout the experiments.

Calibration of reactants was made by withdrawing unburned gases directly through the sampling probe. For gaseous intermediate and product species, a certified gas mixture acquired from Matheson Co. (Cucamonga, CA) was used. We estimate an accuracy of about f 15% for the determination of the mole fractions of major C, and C, species and f20% for the remaining ones. For species for which calibration standards were not available the relative ionization cross section method was used (Fitch and Sauter, 1983). This method is expected to be accurate within a factor of 2.

Species profiles were generated by moving the sampling probe relative to the stationary burner with the aid of a vertical translator having a precision of about 0.01 mm. We estimate that an uncertainty of about f 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 Pt-Pt/l3%Rh thermocouple freshly coated and glazed by silica immediately following the composition measurements. The thermocouple used had a bead diameter of about 0.15 mm, and was kept in the flame for as little time as possible to minimize soot accumulation and to prevent the degradation of the coatings. The temperaturesreported here also represent direct thermocouple readings and were not corrected for radiation losses, because such corrections require the knowledge of the emissivity of the soot coated thermocouple bead, which is not known (Bradley and Mattews, 1968). In addition, soot accumulation increases the bead diameter, which in return decreases the heat transfer coefficient, thus

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thermocouple measurements. Nevertheless, gas temperatures can be estimated from the information provided by assuming plausible emissivitiesand bead diameters Tor the soot coated thermocouple. These calculations result in radiation corrections in excess of 100 K in the hottest part of the flame, resulting in nearly flat temperature profiles in the post flame zone. We also estimate an uncertainty of f O.Zmm in the absolute positions of the temperature profiles.

RESULTS AND DISCUSSION

In Table I, the pre-combustion mixture compositions and other features of the flames studied are presented. The equivalence ratios (@ = stoichiometric 0, requiredJactual 0, used) were determined by considering HCI as the preferred oxidation product. These particular compositions were determined to be suitable for the intended microprobe studies after different equivalence ratios, argon dilutions, and fuel ratios ( I , 1-DCEJCH,) were explored. At higher equivalence ratios and fuel ratios the formation of large amounts of carbonaceous deposits plugged the probe orifice and prevented the successful undertaking of longexperiments. At lower argon dilutions and lower equivalence ratios flames were positioned so close to the burner surface that the acquisition of spatially resolved flame structure data was not possible by the microprobe used. At higher argon dilutions flames became too weak and became attached to the sampling probe. We have used CH, and an auxiliary fuel because its combustion characteristics are well understood.

From Table I it can be seen that Flame A had a equivalence ratio of 2.28 and Cl/H ratio of 0.17. This flame also exhibited the broadest reaction zone and provided information on the largest number ofspecies. Flame B had a slightly higher equivalence ratio (cD = 2.36); however, more CH, had to be used to sample this flame t o prevent excessive soot rormation. This, unfortunately resulted in a lower CI/H ratio of 0.10, and led to the detection of a smaller number of chlorinated species. Experiments under fuel-lean were also conducted to explore the flame chemistries of pure 1, I-DCE. However, these flames, in spite of higher Ar dilutions, were extremely thin and positioned close to the burner surface, rendering microprobe studies impractical

TABLE 1

Pre-reaction compositions and other features of the flames sludied

Flame A B C

Composition (%v) Ar 39.3 34.5 49.0 CH, 20.4 27.7 0.0 - 0, 29.6 30.8 42.3 I . I-DCE 10.7 7.0 8.7 Cold Gas Velocitv Icmlsec) 11.5 10.3 10.0 . . , . Equivalence Ratio 2.28 2.36 0.5 CI/H Ratio 0.17 0.10 0.5 I , I-DCE/CH, Ratio 0.52 0.25 -

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I . I-C,H,CI,/CH,/O,/Ar FLAMES 73

because of probe burner surface interactions. Nevertheless, samples were withdrawn and analyzed to identify the nature of intermediates formed.

Before experimental results are discussed, a number of issues concerning the accuracy of 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, concentration profiles obtained represent values that are spatially averaged over 2-3 orifice diameters, or 0.15-0.3 mm, in the upstream direction. However, 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. Finally, because of unavoidable errors in sampling, detection and calibrations, carbon balances generally deviated from 100% level. In this work, atom balances were made by using Argon as the internal standard.

FLAME A

In Figure 1, the temperature and mole fraction profiles of 1, I-DCE, CH,, 0, and Ar as well as of CO and CO, are presented for Flame A (@ = 2.28,1,1-DCE/CH, = 0.52,C/H = 0.17). In this and subsequent figures lines have been drawn through the data points as a visual aid to indicate the trends. Experimental carbon atom balances, together with the mole fraction profiles of H,O, HCI and H, determined through O-, Cl- and H- atom balances, respectively are shown in Figure 2. Carbon balances - obtained from the ratio (carbon conten; of gas phase species/Ar)/(carbon fed to the reactor/Ar) x 100, were generally within + 10%. thus reasonable. Atom balances similarly were conducted;sing Argon as the reference gas. The experimental temperature profile was also corrected for radiation losses using emissivity of 0.3 in the early

Mance above burner rurfoce (rnm)

FIGURE 1 Temperature and species mole fraction profiles for 1, I-DCE, CH,,O,.CO,CO, and Ar for flame A.

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Olstonce above burner surface (mm)

FIGURE 2 Species mole fraction profiles for H,. H,O, and HCI, as well as carbon balances along flame A.

parts of the flame and 0.7 in the post flame zone, and this is indicated by the dashed line in Figure 1.

As seen in Figure I, the temperature increased from about 400 K near the burner surface to a maximum of 1600 K at 3.5 mm along the flame,a level that is slightly higher than the I,2-DCE flames studied by Kassem and Senkan, 1991. This, however, is an expected consequence of the greater argon dilution present in the 1,2-DCE flame studies. We explored the possibility of using the same argon dilution to better compare our results with the earlier experiments. However, at argon concentrations larger than indicated in Table 1, 1, I-DCE flames became weak and attached to the sampling probe, and this led to a marked hystheresis in species concentration profiles.

From Figure 1 it can be seen that 1, I-DCE was completely consumed within 3.0mm from the burner surface while some CH, penetrated into the post flame zone. The penetration of 0, was less than CH, but greater than 1,l-DCE, and was consumed at about 4 m m from the burner surface. Also evident from this figure, C O forms as the major product in this fuel-rich flame, as opposed to CO,.

In Figure 2, it can be seen that the products H,O, HCI, and H, all formed a t levels comparable to one another in this flame. However, the mole fraction for HCI leveled off, while the levels of H,O decreased and those of H, increased beyond 3.5 mm from the burner surface. The decrease in H,O mole fraction profile mirrors a similar increase in C O as shown in Figure 1.

In Figure 3, the mole fraction profiles for the chlorinated intermediates CH,CI, C2H,C1, C,H,CI,(CH,CCI,), C,H,CI(CH,CHClCH,), and C,H,CI (CH,CCICHCH,) are presented. It should be noted that both CH,CI and C,H,CI were also observed in the fuel-rich flames of I,2-DCEJCH, (Kassem and Senkan, 1991). However, the formation of C,H,CI,,C,H,CI, and C,H,CI appear to be specific to 1 , l -DCE combustion, and as such should be useful for the development and verification detailed

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I . I-C,H,C12/CH,/0,/Ar FLAMES

"' r 1

0 I 2 1 5

Distance above burner surface (rnrn)

FIGURE 3 Species mole fraction profiles To CH,CI.C2H,CI,C,H,CI,.C,Ii,CI and C,H,CI for flame A.

kinetic mechanisms. As evident from Figure 3, the concentrations of all of the chlorinated intermediates peaked early on in the flame and were rapidly destroyed shortly thereafter. As in the case for 1.2-DCE combustion (Kassem and Senkan, 1991), C,H,CI was the most abundant CHC intermediate in I, 1-DCE flames, reaching a peak molar concentration ofabout 1.3% in the present experiments.The order ofabundance of the remaining chlorinated hydrocarbons, with respect to their peak concentrations, were CH,CI (O.O5%), C2H2C12 (0.02%), C,H,CI ( I2 ppmv, parts per million by vol- ume), and C,H,CI (9 ppmv).

In Figure4 the mole fraction profiles for hydrocarbon intermediates C,H,,C,H4,C,H6,C4H,,C4H4,C6H, (benzene), and Cloh8 (napthalene) are presented for Flame A. All these species, with the exception of C l o H 8 were also present in I, 2-DCE flames(Kassem and Senkan, 1991). It is likely that even C,,H, was present in I, 2-DCE flames, but was not detected because of sampling probe temperature limita- tions. That is, in previous experiments the sampling probe was kept at 50•‹C by circulating warm water (Kassem and Senkan, 1991). This temperature would be too low to maintain high molecular weight products, such as CloH8, in the gas phase. This situation has improved by the use of ethylene glycol a t 125•‹C in the present study. Addition experiments using 150•‹C probe temperature were also conducted to check the validity of our C l o H 8 measurements. The results of these measurements were virtually identical to those made using 125•‹C probe, suggesting the reliability of our determina- tions for naothalene.

A comparison of the concentration profiles for hydrocarbon intermediates (Fig. 4) to those for chlorinated hvdrocarbons oresented in Figure 3 reveals that the former were - produced and reached their values considerably later in the flame. In fact, some of the acetylenic and aromatic species even penetrated into the post flame zone, where no chlorine containing species were detected (except HCI).

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Distance above burner surface (mm)

FIGURE4 Species mole fraction profiles for C,H,,C,H,,C,H,,C,H,.C,H,,C,H,. and C,,H, for flame A.

As evident from Figure 4, the most abundant hydrocarbon intermediate was C,H; reaching a leak concentration level of about 4%. It was followed by C,H, (0.5% peak concentration), C,H, (0.03%), C4H, (600ppmv), C,H, (lOOppmv),C,H, (55 ppmv) andC,,H, (IOppmv). Both C2H, and C4H2 peaked at about 3.75mm from the burner surface and penetrated well into the post flame zone. These results are consistent with our understanding of the combustion kinetics of CH, (Warnatz, 1984; Westbrook and Dryer, 1984) relative to chlorinated hydrocarbons (Senkan, 1994).

FLAME B

In Figure 5 the mole fraction profiles for 1, I -DCE,CH, ,02 ,C0 ,C0 , and Ar, as well as the temperature profile, both direct thermocouple readings and radiation corrected values, are presented for Flame B (@ = 2.36,CIJH = 0.1). As evident from the profiles shown in this figure, Flame B exhibits a narrower reactionzone and was positioned closer to the burner surface than Flame A. For example, 1 , l -DCE was completely consumed within 2.0mm and the temperature maximum occurred at about 2.75mm from the burner surface. Again C O was the major product, as opposed to CO,, in this fuel- rich flame. In addition. significant methane penetrated well into the post flame zone. Unlike Flame A (Fig: I), the temperature profile in Flame B exhibited a well-defined maxima (see Fig. 5). As noted before, this was likely caused by excessivesoot accumulation on the thermocouples bead which is consistent with the slightly higher equivalence ratio of flame B. Soot accumulation on thermocouples would increase the effective bead size and this, in return, would decrease convective heat transfer relative to radiation. These events manifest themselves as an apparent decrease in thermocouple measurements.

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FIGURES Temperature and species mole fraction profiles for I . I-DCE.CH,.O,,CO.CO, and Ar for flame B.

Carbon balances, as defined earlier, as well as mole fraction profiles for HCI, H z and. H 2 0 , determined through CI-,H- and 0-atom balances, are presented in Figure 6. As seen from this figure, H z and H 2 0 were also produced early in the reaction zone at levels close to one another, and reached levels of about 20% in the post flame zone. In this case, the mole fractions of H,O slightly decreased and those for H z slightly increased in the post flame zone, to a lesser extent than Flame A. The HCI mole fractions leveled offat a value of about 10% which is consistent with the lower chlorine loading ofthis flame. Carbon balances wereagain within f 20%. thus were reasonable.

In Figure 7 the mole fraction profiles for the chlorinated intermediates, CH,CI,C,H,CI,and C2HiC12 (CHCICHCI) are presented. As evident from the comparison of these results to those presented in Flame A (Fig. 3), the spectrum of chlorinated hydrocarbon intermediates produced in Flame B were narrower. This result, however, is not surprising because of the lower CI/H ratio of Flame B. The lower CI/H ratio results in a greater fraction of CI to be tied up as HCI, thereby lowering the probability of formation of chlorinated hydrocarbons. As seen in Figure 7, all the CHCs were also produced early on in the flame and were destroyed within 2.0 mm from the burner surface; this corresponds to the point of complete destruction of 1, I-DCE (Figure 5). The order of abundance of these chlorinated species, based on their peak concentrations that occurred at about 2.5 mm from the burner surface, were C,H2C12 (0.029%), C2H3CI (2.3%), and CH,CI (0.05%). The higher peak mole fractions of C2H3CI seen in this flames can be attributed to the earlier decomposition of 1.1-DCE, caused by the earlier increase in the temperature along Flame B (see Fig. 5).

In Figure 8, the mole fraction profiles for C2H2, C2H,, C,H,,C,H,,C,H,(benzene), and C,,H, (napthalene) are presented for Flame B. As evident from this figure, these hydrocarbons were produced later in the flame compared to chlorinated intermediates.

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I . A. GARGUREVICH AND S. M . SENKAN

0.3 S . . . , . 3; ;

Carbon --. 6 K

0.2 - e e YO

HCI 0.1 - . - D D O "

1 0.0

' 0 1 2 3 4 5 6

Distance above burner surface (rnm)

FIGURE 6 Species mole fraction profiles for Hz, H,O, and HCI. as well as carbon balances along flame B.

Distance above burner surface (rnrn)

FIGURE 7 Species mole fraction profiles for CH,CI.C,H,CI and C,H,CI, for flame B.

As in the case for Flame A, several acetylenic species penetrated into the post flame zone. A comparison of these results with those presented in Figure 4 (Flame A) also reveals the earlier production of all the hydrocarbon intermediates in the flame zone, consistent with earlier fuel consumption and narrower reaction zone of Flame B. The order of abundance of the hydrocarbon intermediates, again based on peak concentrations were, C 2 H 2 (3.8%),C2H, (0.75%),C2H6 (700ppmv1, C,H2 (650ppmv), C6H6

. (80ppmv), C,H, (25ppmv),andC,,H, (1 ppmv). These levels, with the exception of C,,H,, were close to those measured in Flame A.

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FIGURE8 Species mole fraction profiles for C,H,,C,H,,C,H6,C,H,,C,H,,C6H6, and C,,H, for flame B.

As noted above, experiments with fuel-lean Flame C were also conducted to establish, albeit qualitatively, the nature of intermediates, produced in these flames. These studies revealed the presence of highly chlorinated species, such as CCI,, CHCI,, C,CI,, C,HC13, CH3CC13,CH3CHCI,, as well as oxygenated chlorinated species, such as COCI, and CH,CICOCI. These results are consistent with our earlier fuel-lean C H C flame studies (Chang er dl., 1986).

As evident from the foregoing discussion, our understanding of the similarities and differences in the flame chemistries of I, I - and I, 2-DCE has improved substantially as a result of this and prior work. Based on thermochemical considerations as well as our current knowledge of the kinetics and mechanisms of chlorinated hydrocarbon reactions (Senkan, 1994), the following remarks can be made with regard to the major reaction pathways associated with the formation of intermediates observed in these experiments.

As in the case of I, 2-DCE, the destruction of I, I -DCE in flames should occur via the attack of small radicals CI, H, OH, and 0 in an approximate order of importance in fuel-rich flames. For the case of electronegative radicals CI,OH and 0 , the primary mechanism of reaction will be the abstraction of H atoms, because C1 atom abstrac- tions are. far too endothermic (about 20 kcal/mole). In addition, the abstraction of .*-hydrogens will be favored because of bond dissociation and activation energy considerations:

.* - H abstraction

I, I-DCE + (Cl,OH,O) F! CH3CC12 + (HCI, H 2 0 , 0 H ) [-8.65, -24.7, -7.81 (1)

P-H abstraction

1, I-DCE + (CI, OH, 0 ) G CHC12CH2 + (HCI, H,O,OH) C1.24, - 14.8,2.08] (2)

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80 I . A. GARGUREVICH AND S. M. SENKAN

where the numbers in square parentheses represent the heats of reaction in kcal/mole units. For the case of H radical attack, the abstraction of both H and CI atoms from 1,l-DCE is possible:

I, 1-DCE + H *CH3CHCI + HCI [- 23.81 (3)

I, I-DCE + He(CH,CC12,CHCI,CH,) + H, [-9.7,0.2] (4)

Similarly, the destruction of CH, would occur via the following reactions:

CH4 + (CI,OH, H,0)*CH3 + (HCI, H20 , H,,OH) C1.67, - 14.4,0.62,2.48] (5)

Clearly then the subsequent reactions of CH,CCI,,CHCI,CH,,CH,CHCI and CH, radicals will determine the nature of chlorinated intermediates produced in the flames of 1, I-DCE. For the CH,CC12 radicals, the following unimolecular decomposition:

and the H abstraction reaction:

result in the formation of CH,CCI, as noted in the experimental flames. Similarly, the reaction:

CHC12CH2 + 0, *CH2CC12 + HO, [- 16.91 (8)

would also contribute to CH2CCI, production. The other likely reaction of the CHC12CH, radical would be:

CHC12CH, + M aC,H,CI + CI + M [13.5] (9)

Reaction (9) as well as the following reactions of the CH,CHCI radical:

result in the production of C,H,CI as observed in the experiments. As noted above, the levelsofC,H,CI present in 1,l-DCE flames were similar to those observed in I, 2-DCE flames (Kassem and Senkan, 1991). This result is surprising because the C1 chain present in I,2-DCE combustion mechanism should have been less important in the case of I , I-DCE, because of the expected dominance of a-H abstraction reaction (1). Apparently, at the high temperatures associated in flames, the B-H abstraction reaction (2) must also be important, and together with reaction (9) constitutes a closed chain process leading to the rapid production of C,H,CI. In addition, reaction (I) together with reaction (I 1) would also be expected to contribute to C,H,CI formation.

The formation of CH,CI is likely to proceed via the chlorination of the CH, radicals in the flame:

The production of higher molecular weight CHCs noted in the experimental flames can then be accounted for by the following,chemically activated processes. For the case of C,H,CI, its formation can be explained by the following radical recombination

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process:

where [I* represents the energy rich adduct, which upon collisions with other species, M, produces the ground state C3H7C1. It should be noted that CH,CHCl was formed via reaction (3). The production of C,H,CL can similarly be accounted for by the following addition/decomposition process:

Reaction pathways responsible for the procluctiorl of hydrocarbon intermediates can also be formulated to account for experimental results. These intermediates are closely linked to the combustion chemistry of CH, (Warnatz, 1984), and plausible reaction mechanisms has been discussed extensively in prior publications, thus will not be repeated again (see for example ]Lee et al., 1993).

In summary, the chemical structures of fuel-rich flames of 1, 1-C2H4C12/CH4 established by using heated microprobe sampling and GC/MS provide new insights on the nature of intermediates associated with the cornbustion of 1,l-DCE, particularly with regard to the differences in the flame structures of 1,2-DCE. These measurements provide useful information for the development and verification of detailed chemical kinetic mechanisms for the combustion of 1, 1-, and 1,2- isomers of DCE. Our current understanding of the thermochemistry and reaction kinetics of hydrocarbons and chlorinated hydrocarbons also appears to he realsonably in place to qualitatively account for the new species observed in flames.

This research was supported, in part, by funds from the: US Environmental Protection Agency Grant No: R819178-01, the National Science Foundation, Grant No: CTS-9319170 and the UCLA Center for Clean Technologies.

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1,1-Dichloroethane Combustion Chemistry

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