Thermal Degradation Characteristics of Chloromethane Mixtures

Environ. Sci. Technol. 1988, 22,438-447

Thermal Degradation Characteristics of Chloromethane Mixtures Philip H. Taylor” and Barry Delllnger Environmental Sciences Group, University of Dayton Research Institute, 300 College Park, Dayton, Ohio 45469-000 1

Controlled, high-temperature incineration has been identified as a promising technology for the disposal of hazardous organic wastes. Chlorinated methanes (methyl chloride, methylene chloride, chloroform, and carbon tetrachloride) are frequently incinerated toxic organics and are simple chlorinated organics, the study of which may provide insight into the chemistry of high-temperature incineration. This paper evaluates the thermal degradation characteristics of the chloromethanes as pure compounds and within mixtures of varying chlorine content as a function of waste feed composition and fuel/air equivalence ratio. Results indicate that the thermal stabilities of the chloromethanes as pure compounds and in mixtures do not correlate with simple molecular parameters, thus illustrating the importance of complex decomposition kinetics (including parent compound re-formation) in determining the relative “incinerability” of these compounds. An analysis of the formation of products of incomplete.combustion (PICs) indicates that the nature and yield of PIC9 produced are strongly dependent on several factors, including oxygen concentration, temperature, and elemental composition of the waste as well as the molecular composition of the waste feed. This investigation has demonstrated that complex chlorinated organic compounds (e.g., hexachlorobenzene and octachlorostyrene) can be formed from much less complex initial reactants, and components of the feed material may be “re-formed” after their initial decomposition by reactions involving other feed components. Until a greater mechanistic data base is acquired, predicting emissions of undecomposed feed conlponents and PICs can be best accomplished by laboratory thermal testing of the actual waste stream to be incinerated. W

Introduction The laboratory approach to the study of hazardous waste incineration is based on both theory and comparison with full-scale emissions data. In an incinerator, toxic organic compounds undergo both flame and thermal destruction. Reaction conditions in the flame are very favorable for complete oxidation as destruction efficiencies (DES)greater than 99.99% are readily achieved in a properly operating system (1-5). Thermal destruction occurs outside the flame envelope and is typified by lower temperatures (800-1100 “C) and longer residence times (1-6 s) (6). A two-zone thermal model has been proposed demonstrating that the relative mass emissions of toxic organic compounds are governed by the conditions of the thermal zone (7). This model suggests that laboratory-scale flow reactor measurements of the thermal stability of hazardous organic compounds can be effectively used to understand and predict many of the organic emissions from full-scale incinerators (8-12). Partial verification of this model has recently been documented as principal organic hazardous constituents (POHCs). DES in an oxygen-deficient environment correlate reasonably well with full-scale emission data (13). However, the emissions of toxic organic products of incomplete combustion (PICs) depend on complex reaction mechanisms. The current data base on PIC formation is very limited; consequently it is difficult to predict PIC emissions without experimental data on the thermal 438

Environ. Sci. Technol., Vol. 22, No. 4, 1988

decomposition properties of the waste stream or a simulated test mixture. Ideally, the goal of laboratory thermal decomposition research is to gain sufficient fundamental knowledge to predict a priori the relative yields of POHCs and PICs from an incinerator effluent. This goal is consistent with reducing the complexity of full-scale “trial-burn” testing, since one then has only to monitor the PIC9 and POHCs of greatest yield as predicted by laboratory testing and analysis. None of the previously or currently proposed incinerability hierarchies (7,9,13-19) directly address the issue of PIC emissions as they are only concerned with predicting the thermal stability of the POHCs in the feed material. This has not been previously perceived as a problem because PICs resulting from the incineration of hazardous waste are not currently regulated by the U.S. EPA. However, full-scale and pilot-scale testing programs have shown that PICs are produced and emitted from thermal destruction facilities (20-23). In a recent study ( I I ) , we have shown that a simple five-component mixture of hazardous organic compounds can produce over 150 PICs in measurable yields. Numerous other laboratory studies have confirmed that PICs may be formed from virtually any material under the appropriate conditions ( 1 , 5 , 1 0 , 1 2 , 2 4 , 2 5 ) .Since the PICs produced in the incineration of hazardous wastes may be of equal or greater toxicity than the feed material, it is of key importance to better understand the fundamental processes of PIC formation. This paper presents the results of a laboratory thermal degradation study of chloromethane mixtures under various reaction conditions. Questions to be addressed are as follows: (1)How is the thermal stability of the chloromethanes affected by variation of the chlorine, hydrogen, and oxygen concentration? (2) Can the thermal stability of chloromethane mixtures be correlated with simple molecular or physical parameters? (3) How do PIC yields and stability vary as a function of oxygen, chlorine, and hydrogen concentration? (4) Can fundamental reaction pathways be identified such that PIC identity and yield can be predicted for various oxygen concentrations and waste feed compositions?

Experimental Approach Decomposition of the chloromethanes and PIC formation as a function of temperature was measured by the combined analytical capability of the thermal decomposition unit-gas chromatographic (TDU-GC) system and the thermal decomposition analytical system (TDAS). The TDU-GC was employed for the quantitative profile measurements, while the TDAS was used for PIC identifications. A generalized block diagram of these thermal instrumentation systems is presented in Figure 1. Details of these systems have been discussed at length elsewhere (26-28). A brief summary of their salient features is presented in the following paragraph. Both systems utilize a high-temperature fused-silica reactor in which a gas stream exhibiting a segregated flow

0013-936X/88/0922-0438$01.50/0

0 1988 American Chemical Society

TDU-GC SYSTEM

Flgure 1. General block diagram of the thermal decomposition unit-gas chromatographic(TDU-GC) system and the thermal decomposition analytical system (TDAS). The two systems differ only in the in-line analytical instrumentation employed.

pattern is exposed to temperatures as high at 1050 "C for mean residence times of 0.5-5.0 s. Reactor design ensures that each molecule experiences a square-wave thermal pulse with a very narrow residence time distribution. Heated transfer lines connect the insertion chamber to the reactor and the reactor to the effluent analysis system. The major differences in these thermal instrumentation systems arise in their in-line analytical capability. The analytical function of the TDU-GC is performed by a Varian Vista 4600 programmed-temperature gas chromatograph in conjunction with a CDS 401 computer data system. The TDAS employs a Varian 2400 programmedtemperature gas chromatograph coupled to a LKB 2091 mass spectrometer. A minicomputer is used to assist data acquisition, storage, and analysis. For the chloromethane mixture thermal degradation tests, four different compositions were chosen in an attempt to simulate the wide range of oxygen concentration and waste feed composition commonly encountered in the incineration of hazardous waste mixtures. Furthermore, the mixture compositionswere initially chosen to evaluate the effect of elemental composition (Le., C/H/Cl ratios) versus molecular composition (e.g., relative concentration of CCl, to CH3C1) on the yield and identity of PICs. To quantitatively describe the varying reaction conditions from oxygen starved to oxygen rich, the equivalence ratio relation was used:

As defined above, when 4 < 1 an oxygen-rich stoichiometry exists, while for 4 > 1a fuel-rich stoichiometry is present. Specific details of the reaction stoichiometry, Cl/H ratios, and mixture component concentrations are presented in Table I. Samples were prepared by injecting specific amounts of high-purity (>99.9%) carbon tetrachloride (CCl,), chloroform (CHCl,), methylene chloride (CH2C12),and methyl chloride (CH,Cl) into a 1-L bulb, which was purged with either room air or nitrogen. In a similar manner, the pure components were prepared in an oxygen/nitrogen atmosphere such that their gas-phase reactor concentration was identical with the overall gas-phase reactor mixture concentration (2000 ppm). A total of 250-pL aliquots of the samples was injected into the insertion chamber of TDU-GC at a rate of 50 pL/s. Data were acquired over a temperature range of 300-1050 "C at a mean gas-phase residence time of 2.0 s. The thermal degradation data for the excess air sample was gathered in a carrier of flowing dry nitrogen, maintaining the 4 = 0.05 stoichiometry. The near pyrolytic sample was analyzed in an atmosphere of flowing dry nitrogen, with only trace levels of oxygen (estimated to be less than 10 ppm) available for combustion. Regarding

Table I. Experimental Conditions % by volume

component

Cl/H = 5

CHsCl CH;Clz CHClB CCl,

7 7

40

33 53

20 10

reaction atmosphere temperature range, "C mean gas-phase residence time, s instrumentation

Cl/H

=T

30

m, where 6 = [fuel/airla,t"a~/[fUel/airlato~ch 300-1050

q5

= 0.05 or

2.0

thermal decomposition unit-gas chromotograph (TDU-GC) thermal decomposition analytical system (TDAS)

data precision, day-to-dayuncertainty of the raw data (GC area count for a given POHC and PIC over a wide range of POHC destruction) was found to be less than or equal to 5%. Thermocouple and flow meter calibrations indicated exposure temperature and residence time uncertainties of less than 1% . Chromatographic analyses of the TDU-GC reactor effluent were obtained by using a 15 m x 0.332 mm fusedsilica capillary column with a 5 ym thick DB-5 stationary phase. The GC oven temperature was ramped from -99 to 250 "C at 20 deg/min. Helium was used as the GC carrier gas, and a hydrogen flame ionization detector (HFID) was used for solute detection. For the PIC identification measurements, the TDAS sample introduction parameters were identical with the thermal decomposition experiments. Chromatographic analysis of the effluent was conducted by using a 30 m X 0.332 mm fused-silica capillary column with a 5 ym thick DB-5 stationary phase. The GC oven temperature was ramped from -70 to 250 "C at 22 deg/min. Helium was used as the GC carrier gas. Solute analysis was conducted by using a mass spectrometer with an ion source temperature of 250 "C, elution energy of 70 eV, and an accelerating voltage of 3.5 kV. The scan range of the mass spectrometer was m / e 33-400. PIC identifications were confirmed on the basis of relative retention time and mass spectral data analysis. For the excess air samples, identifications were obtained at a temperature of 600 "C for both Cl/H ratios. The near pyrolytic PIC identifications were acquired for temperatures equal to 600 and 700 "C for Cl/H ratios of 1:l and 800 "C for Cl/H ratios of 51. These temperatures were chosen in an attempt to identify as many of the PICs as possible for a given experimental condition. Excellent results were obtained as all the PICs produced under the excess air conditions were identified, and approximately Environ. Sci. Technol., Vol. 22, No. 4, 1988

439

t

i

0 CHC13 0 CCI4

EXPOSURE TEMPERATURE, C'

-

Figure 2. Pure compound chloromethane thermal degradation profiles for i$ = 0.05 and f, = 2.0 s.

A CH3CI Cn2CI2 0 CHC13

0 CCI4

EXPOSURE TEMPERATURE, 'C

-

Flgure 4. Chloromethane mixture (CVH = 5: 1) thermal degradation profiles for 4 = 0.05 and t = 2.0 s.

mixture in the absence of mixture effects. These profiles also serve as a base line for elucidation of the effect of mixture interaction on the thermal stability of each component. From Figure 2, the decreasing order of stability under oxidative conditions on the basis of the temperature for 99% destruction is CH3C1 > CH2Clz = CC14 > CHC1,


Fuure 3. Pure compound chloromethane thermal degradation profiles for 4 = 03 and 7, = 2.0 s.

90% of the PICs produced under near pyrolytic conditions were identified.

POHC Results The fraction (by weight) of each POHC that survives a high-temperature exposure can be determined by dividing its integrated HFID response at the temperature in question into its integrated HFID response at a nondestructive reference temperature ( 2 1 ) . In this report, the thermal degradaticrn profiles are quantitatively compared in terms of a destruation efficiency (DE). This DE is a measure of the gas-phase thermal stability of a component for a given set of experimental conditions. The relationship between DE (%) and fraction remaining ( Wi) at a given temperature and residence time is DE = (1 - WJ100 The temperature required for 99% DE of a given compound at a mean gas-phase residence time, Er, of 2.0 s [Tg9(2)(C)]has been previously proposed as a ranking index and will be used in this discussion (9).For a given set of experimental conditions, the larger the value of this index, the more stable the component under investigation. Pure Compound Stability. Figures 2 and 3 depict the thermal degradation profiles of the chloromethanes as pure compounds under oxidative and pyrolytic conditions. These plots effectively summarize the overall thermal degradation behavior of the original components of the test 440

Envlron. Sci. Technol., Vol. 22, No. 4, 1988

Close inspection of Figure 2 indicates that, although CH2C12is initially more stable than CCl,, CH2C12becomes much more sensitive to oxidative degradation at higher temperatures, and the curves intersect at 750 "C (99.15% DE). From Figure 3, the decreasing order of stability in the absence of oxygen on the basis of the temperature for 99% destruction is CH3C1 > CCl, > CH,C12 > CHC13 Close inspection of Figure 3 indicates that, although CCll is initially much less stable than CH2C12,the DE curves cross a t a temperature of 800 OC (95.5% DE) with CHzClz more easily degraded a t higher temperatures. With the exception of the higher stability of C C 4under near pyrolytic conditions, the relative stabilities for these components vary little with change in reaction atmosphere. In contrast, the absolute thermal stability for each chloromethane component is significantly greater under pyrolytic conditions. With respect to the individual components, CH3C1and CCl, are most sensitive to the change in oxygen concentration. On the other hand, as the oxygen concentration is reduced, T,(2)(C) for CHC13and CHiC1, increases by less than 100 OC. Mixture POHC Stability. Figures 4-7 present oxidative and near pyrolytic degradation profiles for chloromethane mixtures at Cl/H ratios of 5:l and 1:l. Comparison of the overall appearance of these profiles with those in Figures 2 and 3 demonstrates the complex nature of oxidative and pyrolytic degradation of these mixtures. Close inspection of the mixture profiles in relation to the effect of oxygen concentration and waste feed composition on thermal stability results in the following observations: (1)The absolute thermal stability of the chloromethanes is sensitive to changes in elemental composition of the mixture. (2) Nearly all chloromethane mixture components are less stable in oxygen-rich versus near pyrolytic environments.

Table 11. Pure Compound Thermal Stability of the Chloromethanes

component

M , / g (2% kcal/g

LBDE (3@, kcal/mol

4 = 0.05

3.25 1.70 0.75 0.24

83.5 79.0 77.0 70.4

805 755 595 750

CHSCl CHzClz CHC13

cc1,

7'99(2)(C)

q5 =

m

>loo0

835 645 940

Table 111. Chloromethane Reactive Species Equilibrium Mole Fractionsa ~5

= 0.05

component

C1

c10

OH

4 = m , C1

CH3C1 CHzClz CHC13 CCll

2.33-5 6.63-5 1.23-4 1.6E-6

5.33-7 1.53-6 2.63-6 3.53-6

1.33-7 8.33-8 3.53-8

C1.03-8 2.93-6 6.43-4 9.OE-4

"Calculated at a temperature of 800 "C.

( 5 ) Under oxygen-deficient conditions, re-formation is only observed for CH3C1and CH2C12.

POHC Discussion The U.S. EPA is currently regulating organic emissions

CHC13

cc!,

i

0010

3 w

'

4bo

5bo

I

76,

66,


I

8bo

1

1000

960

-+

Figure 8. Chloromethane mixture (CI/H = 5:i) thermal degradation profiles for 4 = and t = 2.0 s.

of components initially present in the feed material. Because of the complexity of hazardous waste mixtures, it is feasible to monitor only limited numbers of compounds in the effluent during a trial burn for a given incineration facility. As a guide for selection of these POHCs, the U.S. EPA is currently using a scale of incinerability based on the heat of combustion per unit mass (AHJg) (29). Results of laboratory and field studies have indicated that this ranking parameter cannot be used to predict the relative incinerability of all POHCs (11, 13,22). It is apparent from the chloromethane pure compound and mixture thermal degradation profiles that the thermal stabilities of chloromethanes are strongly dependent on oxygen concentration and, to a somewhat lesser extent, composition of the waste feed. Table I1 presents a summary of the thermal stability of the chloromethanes as pure compounds. For comparison, each compound's heat of combustion per unit mass (AHc/g) and lowest bond dissociation energy (LBDE) are also presented (29,30). The data in Table I1 indicate that neither AHJg nor LBDE correlate with the pure compound thermal stability of the chloromethanes regardless of the reaction atmosphere. The first result is in agreement with a previous study (11). The second result demonstrates that simple bond homolysis is not, in general, an important aspect of the degradation process, thereby illustrating the importance of chemical reaction kinetics in determining POHC stability. Analysis of Pure Compound Stability. Detailed mechanistic studies of hydrocarbon combustion indicate that critical initial degradation steps are bimolecular reactions involving simple atom abstraction by a reactive intermediate (31,321. To aid in a reaction kinetic analysis, radical concentrations have been calculated for the experimental conditions investigated utilizing a standard equilibrium code (33). Mole fractions calculated at 800 "C are presented in Table 111. Under oxygen-rich conditions, the data in Table I11 indicate that for CH3C1,CH2C12,and CHCl,, C1 atoms, C10 radicals, and OH radicals are the reactive species in highest concentration. Similar calculations for CC14 indicate the presence of C1 atoms and C10 radicals. The lower thermal stability of CHC13, CH2C12,and CHJl under oxidative

~'ool--TFA r

ft

'

l

'

I

I

'

"

I

"

"

I

10


-+

Flgure 7. Chloromethane mixture (CVH = 1:l) thermal degradation profiles for 4 = and t = 2.0 8.

(3) For all waste feed compositions and oxygen concentrations, CH3C1 was found to be the most thermally stable component, and CHC1, was found to be the most thermally labile component. (4) Under oxygen-rich conditions, POHC re-formation phenomena (as evidenced by a positive inflection in the POHC profile in the mixture versus the pure compound) are observed for CH3C1, CH2C12,and CC4, regardless of Cl/H ratio.


441

Table IV. Thermal Stability of Components of Chloromethane Mixtures

Table V. Chloromethane Mixture Reactive Species Equilibrium Mole Fractionsn

Ta0(2)(C) mixture ratio

C1

4 = 0.05 c10

OH

5:1 1:1

1.3E-4 6.93-5

2.93-6 1.5E-6

2.23-8 8.33-8

~

component CH,Cl CHzClz CHCls CCll

Cl/H = 5 4 = 0.05 4=m 830 820 625 780

985 815 660 930

Cl/H = 1 4 = 0.05 4=m 900 760 645 790

1100 840 680 705

versus pyrolytic conditions in conjunction with thermochemical calculations indicates that OH radical reactivity may be responsible for the observed effects. In a recent study performed in this laboratory (34),the decreasing order of reactivity of OH radicals with chloromethaneswas determined to be CHCl, 3 CH2Cl2> CH,C1 > CC14 With the exception of CCl,, the relative stability of the chloromethanes under oxygen-rich conditions is in good agreement with the relative OH reactivities of the chloromethanes. This result is consistent with OH abstraction of a H atom as a major reaction channel for initial degradation of these compounds. Pure compound studies of CC14 degradation indicate a substantial dependence on oxygen concentration. The only oxygenated radical present in substantial quantities for this reaction mixture is the C10 radical. A survey of the literature indicates a paucity of data regarding C10 reactivity. However, conservative estimates of Arrhenius parameters for C1 atom abstraction by C10 radicals versus unimolecular decomposition indicate the rate constant for the former pathway to be 3 orders of magnitude larger at a temperature of 600 "C. Since the C10 concentration is greatest for CC14 compared to the other chloromethanes, this reaction may be a significant channel for the initial degradation of CClk In the absence of oxygen, equilibrium calculations for CC14, CHCl,, and CH2C12(see Table 111) indicate that C1 atoms are the reactive species in highest concentration. Due to the inherent stability of CH&1 at the temperature where the calculation was performed, similar calculations indicate the absence of any substantial radical concentration for this molecule. A survey of the literature indicates only one prior study of C1 atom reactivity with a chloromethane, CH3C1 (35). Thus, relative C1 atom reactivities toward the chloromethanes are presently unknown. As a result of thermochemical considerations, one may expect the highest C1 atom reactivity with hydrogen-containing chloromethanes (i.e., chlorine abstraction by C1 atoms is endothermic). The higher relative stability of CC14compared with that of CH2C12and CHC13 is consistent with this hypothesis. Furthermore, the extremely high thermal stability of CH3C1as a result of insufficient C1 atom concentrations is also consistent with this radical initiation mechanism. The observed reactivity of CH3C1, CH,C12, and CHC1, also correlates with the strength of the abstractable carbon-hydrogen bond (30). Analysis of Mixture POHC Stability. Table IV presents a summary of the thermal stability of chloromethane mixture components as a function of waste feed concentration and reaction atmosphere. Contrasting relative component stabilities in Table IV with each component's AH,/g and LBDE (see Table 11) indicate that no correlation exists between mixture stability and these simple molecular ranking parameters. The lack of a correlation between LBDE and mixture component stability is also consistent with the pure compound results and further illustrates the importance of complex reaction 442


a

$ , =

-, c1

7.43-4 2.93-6

Calculated at a temperature of 800 "C.

kinetics in determining component stability within the mixture. To facilitate reaction kinetic analysis of mixture component stability, radical concentrations have been estimated for the experimental conditions investigated (33). Mole fractions calculated at 800 O C are presented in Table V. Under oxygen-rich conditions, these calculations indicate that for all chloromethane components C1 atoms, C10 radicals, and OH radicals are the reactive species in highest concentration. The same calculation under oxygen-deficient conditions indicates the presence of only C1 atoms, regardless of Cl/H ratio. Comparison of thermal degradation profiles of the mixture components with those of the pure compounds yields four major findings that are inherently related to reaction kinetics. First, nearly all chloromethane components are less stable in oxygen-rich versue near pyrolytic conditions. This result is consistent with the presence of OH radicals and their high reactivity with chloromethanes as discussed in the analysis of pure compound stability. The major exception to this result is the decrease in thermal stability of CC14under low-chlorine, near pyrolytic conditions. For cc14, inspection of Table I11 (near pyrolytic) versus Table V (near pyrolytic, Cl/H = 1:l)indicates that the radical population decreases in the mixture. This implies that the decrease in CCll stability in the mixture cannot be attributed to radical reactivity. Thus, more complex reaction kinetics must be considered to explain the decrease in stability of CC14 under near pyrolytic, low-chlorine conditions. A second finding from the analysis of Figures 4-7 is that, for all waste feed compositions and oxygen concentrations, CH3C1was found to be the most stable component and CHC1, was found to be the most thermally labile component. Under oxygen-rich conditions, this finding is consistent with the relative OH reactivities of the chloromethanes. Furthermore, the increase in stability of CH,Cl under near pyrolytic conditions as one decreases the chlorine concentration is consistent with a chlorine atom abstraction degradation channel (see Table V). The low stability of CHC1, under oxygen-deficient conditions is not consistent with a chlorine atom abstraction mechanism, as one would expect CHC13degradation behavior to parallel that of CH3Cl. Previous tracer studies have indicated that concerted HC1 elimination is an alternative low-energy pathway for near pyrolytic CHC13 degradation (36, 37). A third observation is that the initiation of the degradation of the components in the mixtures occurs at lower temperatures than the components as pure compounds (with the exception of chloroform) for each corresponding oxygen concentration. This is particularly striking for CH3C1and CH2C12for Cl/H = 5 under oxidative conditions, their decomposition initially paralleling that of CHC1,. It appears that the degradation of CHC13 initiates the formation of a radical pool resulting in degradation of the other components at a temperature lower than possible as a pure compound. This increase in the radical pool is observed for kinetic and equilibrium calculations that we

1

IO6: I

z

t

n

-

-

'

0

CzCh CqC14

A

CzHCI3

A

1

'

1

'

1

'

1

'

1

'

-

1 400

300

500 600 700 800 EXPOSURE TEMPERATURE 'C+

1000

900

Flgure 8. Thermal reaction product profiles for a chloromethane mixture (CVH = 1:l)at f, = 2.0 s under oxidative (6= 0.05) conditions.

10'1 300

'

I

400

-

' ' 1 ' I ' I 500 600 700 800 EXPOSURE TEMPERATURE, OC

'

900

'

'

1000

Flgure 9. Thermal reaction product profiles for a chloromethane mixture (CVH = 5 1 ) at f, = 2.0 s under oxidative (q5 = 0.5)Conditions.

have performed at 600-700 "C. The last major finding is that, under oxygen-rich conditions, all mixture component stabilities are ultimately greater than pure compound stabilities, regardless of Cl/H ratio. This is in spite of the observed decrease in temperature for the onset of oxidative degradation for the mixtures. Comparison of pure compound versus mixture total reactive species concentration under oxidative conditions indicates that the mixture reactive species concentration levels are always greater, regardless of waste feed composition. The significance of this result is that the increase in mixture component stability must now be ascribed to POHC re-formation reactions. A prime example of POHC re-formation under oxygen-rich conditions is the large concentration increase (factor of 8 at 700 "C) of C C 4 under low-chlorine conditions.

PIC Results Figure 8 presents thermal reaction product profiles for chloromethane mixtures under oxidative, low-chlorine loadings. PICs detectable (area count >200) were 1,lC2HzC12, 1,2-CzH2Cl2(both geometric isomers), C2HC13, C2C14,and C2H2C14,(molecular weights 1x W0) were C1 atoms. These calculations also indicated that the C1 atom concentration was a strong function of Cl/H ratio, with concentrations decreasing as the Cl/H ratio was diminished. For low Cl/H ratios, examination of Table VI indicates that C2C12,C2HC13,and C2C14are the high-yield PICs produced. The low C1 atom concentrations are thus indirectly responsible for the slow destruction rates of these molecules (see Figure 10) because the only other reactions possible under these conditions are high-energy unimolecular bond homolysis reactions and concerted HC1 elimination: CZClZ

B

PIC

CzHC13

FORMATION

C&14

CCI, CCI,

B

PIC

on +

CI

4

'CC13 CCI)

. +

ClW

cl,

FORMATION

CI CI

'

CI C I ClI CII H CI

HI CIl

proportionation and addition reactions with subsequent C1 and/or HCl elimination indicates that many of the same PICs should be generated under oxygen-deficient versus oxygen-rich conditions as one varies the Cl/H ratio. Examination of Table VI indicates agreement with the proposed reaction scheme with C2H4Cl2, 1,2-C2HzCl2, CZHCl3,and C2C14among the PICs observed under these conditions. Comparison of PIC destruction for these

-

+ 'C1 'C2HClZ + 'C1 'C&1

+

'C2C13 'C1

(1) (2)

(3)

A simple kinetic scheme has been presented for the formation of chloroethane and chloroethylene compounds. The lack of OH radicals and the small quantities of C1 atoms present under these conditions imply some unimolecular reaction pathway for the formation C2Cl2. Research in this laboratory analyzing PICs formed in the near pyrolytic thermal degradation of CZHCl3indicates that CZClzis the predominant PIC at temperatures of 500-600 "C. In previous modeling studies of CzHC13/airpremixed flames, it was suggested that eq 4 represented an additional low-energy unimolecular decompositionpathway for C2HC1, degradation (42): CzHC13

-

C&12

+

HC1

(4)

The reaction scheme presented in the preceding paragraph is consistent with the types of PICs produced and their destruction rates observed under low-chlorine conditions. For high-chlorine loadings, a radical chain mechanism analogous to the preferred route to benzene formation in aliphatic, premixed flames can be constructed that is consistent with the observed PIC production (43). C12dissociation/recombination represents a pathway for the formation and destruction of C1 atoms: Clz

+M

-

'C1

+ 'C1+

M

(5)

Chain propagation with C1 atom abstraction of an H atom from C2HC13follows: C2HC13 + 'C1-

'CzC13 + HC1


(6) 445

Reactions of 'C2C13radicals with C2Clzpropagate the chain resulting in the formation of C,C1, radicals:

-

*CzC13+ C2C12

'C4Cl,

(7)

Formation of hexachlorobenzene results from a third chain propagation reaction: *C4C15+ C2C1,

ring

closure

c&l6

+ 'C1

This mechanism results in the following net reaction: C2HC13 + 2C2C12

-+

C&l,

+ HC1

(9)

C8H8may be formed by a similar reaction scheme involving C4C14instead of C2C12:

Examination of this mechanism indicates that eq 5 and 6 are the most critical reactions, as it is the presence of C1 atoms under high-chlorine loadings that results in the formation of higher molecular weight perchlorinated compounds. Two other elements worth noting are the formation of hexachlorobenzene, being quadratically dependent on the dichloroacetylene concentration, and the competing nature of trichloroethylene degradation by unimolecular decomposition, eq 4, and C1 atom attack, eq 6. Further examination of Table VI indicates that additional perchlorinated species produced in smaller yields include c&14, C3C16,and c4c14. c4c14 formation can be envisioned with the following disproportionation reaction:

-

'C2C13 + C2C12

C4C14+ 'C1

(11)

C3C14and C3C16formation may result from reactions of 'CC13 radicals with C2Clzand C2Cl.4 'cc13

+ C2clz

'CCl,

+ C2C14

-

c3c14 + 'c1

(12)

C3C16+ 'C1

(13)

These reactions are evidenced by the higher yields of C3C14 and C3C16under high-chlorine loadings (where 'CC1, radicals are in highest concentration). In closing, the swift radical chain reactions are such that higher molecular weight compounds can easily form from simple chemical species under the proper conditions. The destruction of these high molecular weight PICs will not occur until extremely high temperatures where they become unstable and fragmentation reactions ultimately resulting in HCl and C12 combustion end products predominate.

chloromethanes are "apparently" more stable under mixture conditions due to re-formation. Conclusions from a statistical comparison of hazardous waste field-scale and laboratory-scale results (13) indicate that measured POHC DES of all but the most stable compounds are influenced by their re-formation as PICs from reaction with other components of the waste stream. This experimental study confirms this hypothesis and also demonstrates the exceedingly complex nature of predicting POHC emissions for a given hazardous waste stream. Analysis of PIC formation/destruction processes for chloromethane mixtures indicates that as the oxygen concentration decreases and chlorine concentration increases, PICs become more numerous, complex, and difficult to destroy. Under oxidative conditions, a radicalmolecular reaction scheme involving H and/or C1 atom abstraction by OH followed by radical-molecule disproportionate reactions is consistent with the nature and yield of PICs produced. Under oxygen-deficient conditions, a similar reaction mechanism is consistent with the type and yields of PICs produced under low-chlorine loadings. However, a complex radical chain mechanism is more consistent with the large yields of higher molecular weight PICs formed under high-chlorine loadings. Overall, analysis of PIC formation for this hazardous waste mixture indicates that the nature and yield of PICs produced are strongly dependent on several factors including oxygen concentration, temperature, and molecular structure of the waste feed components. Prediction of the nature and yields of PIC emissions is thus extremely difficult without knowledge of thermal degradation mechanisms for the given hazardous waste stream. Until a greater mechanistic data base is acquired, predicting POHC and PIC emissions can be best accomplished by laboratory thermal testing of the actual waste stream to be incinerated.

Acknowledgments We acknowledge stimulating discussions with Douglas L. Hall, John L. Graham, and Sueann L. Mazer. We thank Debra Tirey and Sueann L. Mazer for assistance in PIC identifications. Registry No. CC14,56-23-5;CHC13,67-66-3;CH2C12,75-09-2; CH&1,74-87-3; CZC12, 7572-29-4; 1,1-CzH&12,75-35-4;1,2-CzH&lz, 540-59-0; C2HC13,79-01-6; CzCl4, 127-18-4;C2C16,67-72-1; C6Cl6, 118-74-1; C&&, 24648-09-7; C2HIC12, 1300-21-6.

Literature Cited Bose, D.; Senkan, S. M. Combust. Sei. Technol. 1983, 35, 187-202.

Summary Chloromethane pure compound and mixture thermal stabilities have been evaluated as a function of waste feed composition and reaction atmosphere. Results indicate that simple, single parameter incinerability ranking approaches do not correlate with the data, illustrating the importance of complex degradation kinetics in determining thermal stability. Specifically, pure compound results indicate that a reduction in oxygen concentration enhances the thermal stability of all components. Relative stabilities under oxygen-rich conditions indicate that OH abstraction reactions are a significant initial degradation channel for CHC13, CH2C12,and CH3C1. Mixture results also indicate that nearly all chloromethanes are less stable in oxygenrich versus oxygen-deficient conditions. Evaluation of absolute thermal stability indicates that a majority of the 446


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Received for review October 27, 1986. Revised manuscript received August 3,1987. Accepted October 21,1987. This research was partially supported by the U S . E P A under Cooperative Agreements CR-810783 and CR-811777.


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Thermal Degradation Characteristics of Chloromethane Mixtures

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