Chloroaromatic formation in incineration processes

The Science of the Total Environment 269 Ž2001. 1᎐24

Review paper

Chloroaromatic formation in incineration processes Philip H. Taylor a,U , Dieter Lenoir b a

En¨ ironmental Sciences and Engineering Group, Uni¨ ersity of Dayton Research Institute, 300 College Park, Dayton, OH 45469-0132, USA b ¨ Institut fur Chemie, GSF-Forschungszentrum fur ¨ Okologische ¨ Umwelt und Gesundheit GmbH, Ingolstadter ¨ Landstraße 1, D-85765 Neuherberg, Germany Received 22 March 2000; accepted 20 September 2000

Abstract The high-temperature reactions of chlorinated hydrocarbons are reviewed with a primary focus on the gas-phase molecular growth chemistry and elementary reaction mechanisms leading to the formation of chlorinated benzenes and chlorinated polycyclic aromatic hydrocarbons. Recent heterogeneous mechanistic studies of the chlorination and condensation of aliphatic hydrocarbons at lower temperatures are also summarized. CopperŽII. valent species play an important role as catalyst and reagent. The main thermal pathways for chlorinated dibenzodioxins and furans have been deduced by these laboratory experiments, which try to model the complex reality of the post-incineration zone of municipal and hazardous waste incinerators. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Chlorinated hydrocarbons; Pyrolysis; Molecular growth; Chlorinated benzenes; Chlorinated polycyclic aromatic hydrocarbons; Polychlorinated dibenzo-p-dioxins; Furan

1. Introduction Chlorine is a leading industrial product, giving rise to the manifold of chloro-organic products in the US and other industrialized countries ŽC & E U

Corresponding author. Tel.: q1-937-229-3604; fax: q1937-229-2503. E-mail address: [email protected] ŽP.H. Taylor.; [email protected] ŽD. Lenoir..

News, 1999.. Chlorocarbons are widely used as solvents in syntheses, as cleaning agents, as starting materials and in polymer, pesticide and other product manufacturing applications. Several chlorocarbons are persistent in the environment. Therefore, chlorocarbons are present in the atmosphere, in both municipal and hazardous material combustion, as well as in destructionr clean-up processes related to the above industrial applications. A better understanding of the pyrol-

0048-9697r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 8 - 9 6 9 7 Ž 0 0 . 0 0 8 2 9 - 9

P.H. Taylor, D. Lenoir r The Science of the Total En¨ ironment 269 (2001) 1᎐24

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ysis and combustion of chlorinated hydrocarbons ŽCHCs. is thus of considerable practical and fundamental importance, since emissions of CHCs can be minimized by primary measures applying the knowledge of the relevant formation pathways. It has been previously shown that very few, if any, organic components of the feed-stock survive direct contact with the flame and, furthermore, only minimal organic by-products are formed ŽOppelt, 1986; Dellinger et al., 1986a,b.. The vast majority of the observed pollutants in the stack effluent of incinerators thus originate from chemistry occurring outside the flame. In fact, most of the pollutants are probably formed in the hightemperature, post-flame zone or at even lower temperatures further downstream in quenched gases or as a result of surface-catalyzed reactions. Table 1 summarizes the conditions and types of reactions Žin order of importance. expected in each of five reaction zones that have been identified for combustors. In the most general sense, the mechanisms of pollutant formation and de-

struction are expected to be relatively consistent within a zone. This ‘Zone Model’ is useful to classify the types of reactions occurring within a given zone and consider their impact on emissions in more detail ŽDellinger et al., 1991.. The reader is referred to Dellinger and Taylor Ž1998. for further details concerning the zone model of hazardous waste combustion. In this manuscript, we review laboratory studies of the molecular growth reactions of CHCs. From an environmental perspective, the molecular growth of CHCs is significant because by-products of these reactions include chlorinated aromatic, chlorinated polycyclic aromatic hydrocarbons ŽClPAH., and polychlorinated dibenzo-p-dioxins and furans ŽPCDDrF.. The potential emission of these products into the environment is dubious due to their potential health effects ŽRamamoorth and Ramamoorth, 1997.. This review is divided into two sections. The first section describes gas-phase studies of aliphatic chlorocarbons. Mechanisms and rate parameters associated with the formation of chlo-

Table 1 Dominant mechanisms of pollutant destruction and formationa Zone

Reaction conditions

Decomposition mechanisms

Formation mechanisms

1

Pre-flame

T s 473᎐1273 K tr < 1 s wO2 x ; 50% excess air

i ii iii

i ii iii

2

Flame

T s 1273᎐2073 K tr F 0.01 s wO2 x ; 50% excess air

iii ii i

ii i iii

3

High-temperature thermal

T s 873᎐1373 K tr s 1᎐10 s wO2 x s 50᎐100% excess air

i ii iii

iii ii i

4

Gas quench

T s 353᎐873 K tr ; 10 s wO2 x s 3᎐9%

i ii

iii

5

Surface catalyst

T s 423᎐773 K tr s 10 s to 10 min wO2 x s 3᎐9%

iv

iv

a

Decomposition mechanism: i, concerted molecular elimination; ii, bond fission; iii, bimolecular radical attack; iv, surface-catalyzed decomposition. Formation mechanism: i, concerted molecular elimination; ii, complex radical-molecule pathways; iii, recombination and association reactions; iv, surface-catalyzed synthesis.

P.H. Taylor, D. Lenoir r The Science of the Total En¨ ironment 269 (2001) 1᎐24

rinated aromatic and ClPAH are emphasized. The second section describes heterogeneous gas᎐solid kinetic studies of the chlorination and condensation of aliphatic hydrocarbons. These studies, conducted at much lower temperatures, provide an alternative route to the formation of chlorinated aromatics, ClPAH, and PCDDrF, which relate to the conditions of the post-combustion zone of municipal waste incinerators.

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ated analytical techniques Žtypically gas chromatography coupled to specific detectors. interfaced directly to special flow reactors, drop tube reactors, and stabilized flames have been used to unravel the real complexity of thermal reactions of chloro-organic compounds under oxidative and pyrolytic conditions. These more recent studies are the focus of this review. 2.1. Chloromethanes

2. Gas-phase molecular growth reactions In 1964, Aubrey and Van Waser Ž1964. showed empirically the existence of an equilibrium between chloroaliphatic and aromatic compounds: 9 C 2 Cl 6 l 12 CCl 4 q C 6 Cl 6 Ž cy .

Ž Rxn 1 .

There were several further investigations on thermal reactivity of chloroaliphatic and aromatic compounds between 1960 and 1980. Most of these early studies were performed using sealed ampoule techniques. For a review, see Choudhry and Hutzinger Ž1983.. Since 1980, modern methods of elucidating reaction mechanisms using hyphen-

The oxidative pyrolysis Žsubstoichiometric . of methylene chloride Ž CH 2 Cl 2 . , chloroform ŽCHCl 3 . and carbon tetrachloride ŽCCl 4 . has been investigated by Tirey et al. 1990a; Taylor et al. 1991 using a micro-bore, fused silica, tubular flow reactor coupled to GC-MS detection. Data were obtained over a temperature range of 573᎐1273 K for the following conditions: chlorohydrocarbonroxygen equivalence ratio of 3.0, chlorohydrocarbon concentrations of 2.7" 0.1= 10y5 mol ly1 , gas-phase residence time of 2.0 s, and reactor pressures of 1.15" 0.05 atm. This study first illustrated the dominance of pyrolytic reaction behavior leading to the formation of several thermally stable reaction by-products and the observation of chlorinated molecular growth reactions result-

Fig. 1. Product distributions from the atmospheric pressure oxidative pyrolysis of CHCl 3 . Fuelroxygen equivalence ratio s 3.0, wCHCl 3 x 0 s 2.7= 10y5 mol ly1. t r s 2.0 s. Ps 1.15 atm. Curves drawn through data based on smoothed interpolated fit.

P.H. Taylor, D. Lenoir r The Science of the Total En¨ ironment 269 (2001) 1᎐24

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ing in the formation of hexachlorobenzene, C 6 Cl 6 Žcy., from simple chlorinated methane precursors, CHCl 3 and CCl 4 . The facile formation of C 6 Cl 6 Žcy. from the oxidative pyrolysis of chloroform is illustrated in Fig. 1. Key elementary reactions leading to the formation of C 6 Cl 6 Žcy. are presented and discussed by Taylor and Dellinger Ž1991.. The formation of higher molecular weight compounds from the pyrolysis and combustion of chlorinated hydrocarbons has been observed for the following precursors: CHCl 3 ŽTirey et al., 1990a, 1991., CCl 4 , ŽTirey et al., 1990a,b; Taylor et al. 1991., dichloroacetylene ŽC 2 Cl 2 . ŽTaylor et al., 2000., trichloroethene ŽC 2 HCl 3 ., ŽChang and Senkan, 1988, 1989; Mulholland et al., 1992; Taylor et al., 1993, 1994., tetrachloroethene ŽC 2 Cl 4 . ŽBallschmiter et al., 1986; Tirey et al., 1990b; Taylor et al., 1996a., hexachloropropene ŽC 3 Cl 6 . ŽTaylor et al., 1996b., and 1,3-hexachlorobutadiene ŽC 4 Cl 6 . ŽTaylor et al., 1996c; Baillet et al., 1996.. Tirey et al. Ž1990b. proposed a mechanism including the formation of the following intermediate species: C 2 Cl 2 , trichlorovinyl radical ŽCl C⫽CCl 2 ., C 2 Cl 4 , C 2 HCl 3 , C 4 Cl 4 , 1,3-pentachlorobutadienyl radical ŽCCl 2 s CClCCl s C Cl., and C 4 Cl 6 that required further experimental and theoretical analysis. The mechanism was based on mechanisms proposed to account for observed products from the pyrolysis and combustion of unsaturated aliphatic hydrocarbons ŽCole et al., 1984; Westmoreland et al., 1989. and methyl chloride ŽWeissman and Benson, 1984.. However, based on the absence of production of chlorinated aromatics from methyl chloride and methylene chloride as compared with chloroform and carbon tetrachloride, Dellinger and Taylor Ž1990. proposed that chlorine facilitates condensation reactions resulting in the formation of chlorinated aromatic species. They suggested that the tendency to form high molecular weight species is attributable to the propensity for chlorinated radicals to undergo reversible, additionrelimination Ždisplacement. type molecular growth reactions: 䢇

䢇

C 2 Cl 3 q C 2 Cl 4 l C 4 Cl 7 )TC 4 Cl 6 q Cl

Ž Rxn2.

Chlorine substitution can play a major role in the rate of these molecular growth reactions through inductive destabilization of the initially formed adduct and through rapid elimination of chlorine. Chlorine elimination reduces the probability of the reverse decomposition to reactants. The lower C᎐Cl bond energies in CHCs Žvs. C᎐H bond energies in hydrocarbons. may favor these types of molecular growth reactions. Tirey et al. Ž1990b. and Dellinger and Taylor Ž1990. presented QRRK calculations illustrating that chemically activated displacement of Cl atoms from olefinic and acetylenic species by olefinic radicals is favored for CHCs as compared with the similar displacement of H atoms from hydrocarbons. They proposed that this is in part responsible for the high yields of chlorinated aromatics for the more chlorinated of these species. The following paragraphs summarize recent studies of the pyrolysis and combustion of unsaturated CHCs and the development of core reaction models describing molecular growth leading to the formation of chlorinated aromatic species. 2.2. Trichloroethene The chemical structure of atmospheric pressure, fuel-rich Ž ␾ s 1.36., pre-mixed, laminar flames of C 2 HCl 3 were investigated using a flat-flame burner by Chang and Senkan Ž1988.. Species mole fraction profiles were determined by using micro-probe sampling coupled with on-line mass spectrometry. A number of new intermediates including C 2 HCl 3 O, C 2 Cl 4 O, C 3 Cl 4 O, C 3 HCl 5 , C 3 Cl 4 , C 3 Cl 6 and C 4 Cl 6 were identified and quantified. These measurements provided new insights for the development of detailed chemical kinetic mechanisms describing the combustion of C 2 HCl 3 and other chlorinated hydrocarbons. A detailed mechanism was subsequently published by Chang and Senkan Ž1989.. Although model predictions were generally satisfactory, several discrepancies were observed between model and experiment. In addition to better experimental measurements of reactive species including C 2 Cl 2 and chlorocarbon radicals, the model requires further refinement. Improved rate parameters for reactions of chlorocarbon radicals with

P.H. Taylor, D. Lenoir r The Science of the Total En¨ ironment 269 (2001) 1᎐24

O 2 , elementary reactions of C 2 Cl 2 , and molecular growth processes are specific issues that need to be addressed. Measurements of condensed-phase products of C 2 HCl 3 pyrolysis in a drop-tube reactor obtained over a temperature range of 1100᎐1500 K were reported by Mulholland et al. Ž1992.. A predominance of perchloroaromatics was found, in sharp contrast to the largely unsubstituted polycyclic aromatic hydrocarbons obtained from the pyrolysis of a mixture of C 2 HCl 3 and toluene under similar conditions. Detailed characterization of product tars from the pyrolysis of pure C 2 HCl 3 using GC-MS analysis revealed the following: C 4 Cl 6 , C 6 HCl 5 , C 6 Cl 6 , C 8 H 4 Cl 6 , C 8 Cl 6 , C 7 HCl 7 , C 8 Cl 8 , C 10 HCl 7 , C 10 H 4 Cl 6 , C 9 Cl 8 , C 10 Cl 8 , C 10 HCl 7 , C 12 Cl 8 , C 12 Cl 10 and C 14 Cl 8 . A reaction mechanism beginning with C 2 Cl 2 formation from C 2 HCl 3 via concerted HCl elimination was proposed. Aromatic ring formation and growth were proposed to occur by successive additions of C 2 Cl 2 to C 2 HCl 3 , in a manner similar to that proposed by Dellinger and Taylor Ž1990.. The oxygen-free pyrolysis of C 2 HCl 3 was investigated by Taylor et al. Ž1993, 1994. using a 1-cmi.d., fused silica, tubular flow reactor under lami-

5

nar flow conditions coupled to on-line GC-MS detection. Data were obtained over a temperature range of 573᎐1273 K for the following conditions: C 2 HCl 3 initial concentrations of 5.4" 0.5 = 10y5 mol ly1 , gas-phase residence time of 1.7 s, and reactor pressures of 1.83 " 0.3 atm. Pronounced molecular growth was observed above 1000 K as evidenced by the formation of C 2 Cl 4 , C 4 Cl 4 , and C 6 Cl 6 Žcy. as major Ž) 5 mol.%. products and dichlorodiacetylene ŽC 4 Cl 2 ., C 4 Cl 6 , pentachlorobenzene w C 6 HCl 5 Ž cy .x , hex achloroethynylbenzene w C 8 Cl 6 Ž cy .x , oc tachloroethenylbenzene wC 8 Cl 8 Žcy.x, octachloronaphthalene wC 10 Cl 8 cy.x, and C 12 Cl 8 Žcy. as minor Ž- 5 mol.%. products. A detailed reaction kinetic model was developed describing molecular growth up to the formation of C 8 Cl 6 Žcy. and C 8 Cl 8 Žcy.. Comparison of model predictions with experiment are summarized in Fig. 2. A core pyrolysis model that accounts for the major products up to eight carbon atoms is depicted in Table 2. Sensitivity and production rate analyses indicated that unimolecular C᎐Cl bond fission is the dominant initiation step. The Cl atoms produced then abstract the single H atom

Fig. 2. Comparison of model predictions with major product distributions from the pyrolysis of C 2 HCl 3 . Closed symbols: experimental data. Solid and broken curves: model predictions. wC 2 HCl 3 x0 s 5.4= 10y5 mol ly1 . t r s 1.7 s. P s 1.83 atm.

P.H. Taylor, D. Lenoir r The Science of the Total En¨ ironment 269 (2001) 1᎐24

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Table 2 C 2 HCl 3 core pyrolysis and molecular growth mechanism Reaction C2 HCl3 ª C2 HCl2 q Cl C2 HCl3 q Cl ª C2 Cl3 q HCl C2 Cl3 q Mb ª C2 Cl2 q Cl q M C2 Cl3 q Cl2 ª C2 Cl4 q Cl C2 Cl3 q C2 HCl3 ª C4 HCl6 C4 HCl6 ª C4 HCl5 q Cl C4 HCl5 q Cl ª C4 Cl5 ŽIc . q HCl C4 Cl5 ŽI. ª C4 Cl4 q Cl C2 Cl3 q C2 Cl2 ª C4 Cl5 ŽNd . C4 Cl5 ŽN. ª C4 Cl4 q Cl C4 Cl4 ª C4 Cl3 ŽI. q Cl C4 Cl4 ª C4 Cl3 ŽN. q Cl C4 Cl3 ŽN. q C2 Cl2 ª C6 Cl5 ŽLe . C6 Cl5 ŽL. ª C6 Cl5 Žcy. C6 Cl5 Žcyf . q Cl ª C6 Cl6 Žcy. C4 Cl5 ŽN. q C2 Cl2 ª C6 Cl7 ŽL. C6 Cl7 ŽL. ª C6 Cl6 Žcy. q Cl C6 Cl5 Žcy. q C2 Cl2 ª C8 Cl6 Žcy. q Cl C6 Cl5 Žcy. q C2 Cl4 ª C8 Cl8 Žcy. q Cl C6 Cl5 Žcy. q C2 Cl2 ª C8 Cl8 Žcy. q Cl

log k Žcm3 moly1 sy1 kcaly1 . 15.0᎐91.0r␪a 12.3᎐9.5r␪ 15.0᎐32.8r␪ 12.5᎐4.8r␪ 11.0᎐7.0r␪ 13.0᎐18.0r␪ 12.3᎐5.0r␪ 13.4᎐43.9r␪ 12.0᎐6.0r␪ 13.4᎐34.1r␪ 16.0᎐68.4r␪ 16.0᎐84.0r␪ 11.7᎐5.0r␪ 10.0 12.6q 1.2r␪ 12.5᎐5.0r␪ 10.0 11.7᎐5.0r␪ 11.7᎐5.0r␪ 11.7᎐5.0r␪

a

␪ s 2.303= R = T. M, third body molecule. c I, secondary, resonance-stabilized radical intermediate; see Taylor et al. Ž1994. for chemical structures. d N, primary radical intermediate; see Taylor et al. Ž1994. for chemical structures. e L, linear radical intermediate; see Taylor et al. Ž1994. for chemical structures. f cy, cyclical intermediate; see Taylor et al. Ž1994. for chemical structures. b

in C 2 HCl 3 yielding C 2 Cl 3 and HCl. The major initially formed radical, C 2 Cl 3 , either loses a Cl atom to form C 2 Cl 2 or reacts with Cl 2 to give C 2 Cl 4 . Trichlorovinyl radical also adds to either C 2 HCl 3 , C 2 Cl 2 , or C 2 Cl 4 via Cl displacement to give C 4 HCl 5 Žundetected., C 4 Cl 4 Žmajor., or C 4 Cl 6 Žminor., respectively. Except for the very lowest temperatures, model calculations indicate that the C 2 Cl 3 radical chain is the dominant conversion cycle for C 2 HCl 3 under our experimental conditions. The overall result is that rapid abstraction of H from C 2 HCl 3 by Cl atoms removes most H-containing species from the reaction system before C 4 species can be formed. Any H-containing C 2 or C 4 species that form at trace levels are rapidly destroyed by additional H abstraction by Cl. When a hydrogen-rich fuel is added, there is competition for the available Cl atoms and H-containing CHCs would be expected to survive and result in the formation of a variety of H-containing

molecules. This is in agreement with observations of others working on mixed H and Cl systems ŽMulholland et al., 1992.. Reaction of C 4 Cl 5 ŽN. and C 4 Cl 3 ŽN. radicals with C 2 Cl 2 are the dominant pathways to formation of C 6 Cl 6 Žcy.. Reaction of C 6 Cl 5 Žcy. with C 2 Cl 4 and C 2 Cl 3 with C 6 Cl 6 Žcy. contribute nearly equally to C 8 Cl 8 Žcy. formation while the reaction of C 2 Cl 2 with C 6 Cl 5 Žcy. dominates C 8 Cl 6 Žcy. formation. The conversion of C 2 Cl 3 into C 2 Cl 4 inhibits both the formation of larger molecular weight species and the destruction of C 2 HCl 3 . 2.3. Tetrachloroethene The oxygen-free pyrolysis of C 2 Cl 4 was investigated by Taylor et al. Ž1996a. under the identical conditions described for C 2 HCl 3 . Pronounced molecular growth was observed above 1100 K as evidenced by the formation of C 6 Cl 6 Žcy.. Hexachlorobenzene and heavier chloro-arene forma-

P.H. Taylor, D. Lenoir r The Science of the Total En¨ ironment 269 (2001) 1᎐24

tion have also been reported by Ballschmiter et al. Ž1986. in extended exposure, sealed tube pyrolysis experiments Ž3 h duration. involving both liquid and gaseous phases. Using the C 2 HCl 3 model as the starting point, a detailed reaction kinetic model was developed by Taylor et al. Ž1996a,b,c. describing molecular growth up to the formation of C 12 Cl 8 Žcy.. Comparison of model predictions with experiments are summarized in Fig. 3. A core pyrolysis model that accounts for the major products containing up to six carbon atoms is depicted in Table 3. Sensitivity and production rate analyses indicate that C᎐Cl bond fission is the dominant initiation step. Chlorine atom addition to C 2 Cl 4 followed by dissociation of C 2 Cl 5 into CCl 3 and CCl 2 radicals is insignificant. This result is consistent with the lack of formation of CCl 4 and hexachloroethane ŽC 2 Cl 6 . as stable products. The trichlorovinyl radical, C 2 Cl 3 , either loses a Cl atom to form C 2 Cl 2 or reacts with Cl 2 to reform C 2 Cl 4 . Another sink for C 2 Cl 3 is addition to C 2 Cl 2 or C 2 Cl 4 followed by Cl elimination to give C 4 Cl 4 and C 4 Cl 6 , respectively. Experimental results, supported by model predictions, indicated that these species rapidly convert to resonance-stabilized C 4 Cl 3 ŽI. and C 4 Cl 5 ŽI. radi-

7

cals, respectively, and do not achieve significant concentrations. Although C 4 Cl 5 ŽI. and C 4 Cl 3 ŽI. formation is kinetically and thermodynamically favored over C 4 Cl 5 ŽN. and C 4 Cl 3 ŽN. formation, the model predicts that the former do not significantly contribute to molecular growth. Reaction of C 4 Cl 5 ŽN. and C 4 Cl 3 ŽN. with C 2 Cl 2 are the dominant pathways to C 6 Cl 6 Žcy.. The dominant sources of C 4 Cl 5 ŽN. and C 4 Cl 3 ŽN. are the decomposition of C 4 Cl 6 and C 4 Cl 4 , respectively, followed by isomerization of the respective C 4 Cl 5 ŽI. and C 4 Cl 3 ŽI. intermediates. The dominant pathway to formation of C 6 Cl 6 Žcy. is shown in Fig. 4. This pathway applies to both C 2 HCl 3 and C 2 Cl 4 pyrolysis. Trichlorovinyl radical, the primary radical formed from both reactants, is the precursor species. 2.4. Hexachlorobutadiene The oxygen-free pyrolysis of 1,3-hexachlorobutadiene ŽC 4 Cl 6 . was investigated by Taylor et al. Ž1996c. under the identical conditions described for C 2 HCl 3 and C 2 Cl 4 . Pronounced molecular growth was observed above 1050 K as evidenced

Fig. 3. Comparison of model predictions with major product distributions from the pyrolysis of C 2 Cl 4 . Closed symbols: experimental data. Solid and broken curves: model predictions. wC 2 Cl 4 x 0 s 5.4= 10y5 mol ly1 . t r s 1.7 s. Ps 1.83 atm.

P.H. Taylor, D. Lenoir r The Science of the Total En¨ ironment 269 (2001) 1᎐24

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Table 3 C 2 Cl 4 core pyrolysis and molecular growth mechanism Reaction

log k Žcm3 moly1 sy1 kcaly1 .

C2 Cl4 ª C2 Cl3 q Cl C2 Cl3 q Mb ª C2 Cl2 q Cl q M C2 Cl4 q Cl ª C2 Cl3 q Cl2 C2 Cl3 q C2 Cl4 ª C4 Cl7 ŽNc . C2 Cl3 q C2 Cl2 ª C4 Cl5 ŽN. C4 Cl7 ŽN. ª C4 Cl6 qCl C4 Cl6 ª C4 Cl5 ŽId . q Cl C4 Cl6 q Cl ª C4 Cl5 ŽI. q Cl2 C4 Cl5 ŽI. q Cl ª C4 Cl5 ŽN. q Cl C4 Cl5 ŽN. ª C4 Cl4 q Cl C4 Cl4 ª C4 Cl3 ŽI. q Cl C4 Cl4 q Cl ª C4 Cl3 ŽI. q Cl2 C4 Cl3 ŽI. q Cl ª C4 Cl3 ŽN. q Cl C4 Cl5 ŽN. q C2 Cl2 ª C6 Cl7 ŽLe . C6 Cl7 ŽL. ª C6 Cl6 Žcyf . q Cl C4 Cl3 ŽN. q C2 Cl2 ª C6 Cl5 ŽL. C6 Cl5 ŽL. ª C6 Cl5 Žcy. C6 Cl5 Žcy. q Cl ª C6 Cl6 Žcy.

15.0᎐91.4r␪a 15.0᎐32.8r␪ 14.6᎐34.0r␪ 12.0᎐6.0r␪ 12.0᎐6.0r␪ 12.9᎐20.7r␪ 16.3᎐70.2r␪ 14.8᎐15.2r␪ 10.4᎐18.6r␪ 13.4᎐34.1r␪ 16.0᎐68.4r␪ 14.6᎐13.4r␪ 10.4᎐18.6r␪ 12.5᎐5.0r␪ 10.0 11.7᎐5.0r␪ 10.0 12.6q 1.2r␪

a

␪ s 2.303= R = T. M, third body molecule. c N, primary radical intermediate; see Taylor et al. Ž1994. for chemical structures. d I, secondary, resonance-stabilized radical intermediate; see Taylor et al. Ž1994. for chemical structures. e L, linear radical intermediate; see Taylor et al. Ž1994. for chemical structures. f cy, cyclical intermediate; see Taylor et al. Ž1994. for chemical structures. b

by the formation of C 6 Cl 6 Žcy., C 8 Cl 8 Žcy., and C 12 Cl 8 Žcy.. Thermal degradation studies of 1,3C 4 Cl 6 in air using a tubular flow reactor have also been reported by Baillet et al., 1996.. Detection of aromatic species such as C 6 Cl 6 Žcy. and C 8 Cl 8 Žcy. verified the importance of molecular growth processes, even in the presence of excess oxygen. Using the C 2 Cl 4 model as the baseline model, Taylor et al. Ž1996c. developed a detailed reaction kinetic model was developed describing molecular growth up to the formation of C 12 Cl 8 Žcy.. Comparison of model predictions with experiment are summarized in Fig. 5. A core pyrolysis model that accounts for the major products up to 12 carbon atoms is depicted in Table 4. Sensitivity and production rate analyses indicate that unimolecular C-Cl bond fission to form C 4 Cl 5 ŽI. radicals is the dominant initiation step. The resulting Cl atoms rapidly add to the double bond of C 4 Cl 6 to form C 4 Cl 7 ŽN.. This radical can revert to C 4 Cl 6 and Cl atoms or

dissociate to C 2 Cl 4 and C 2 Cl 3 . The latter process dominates and accounts for C 2 Cl 4 as the major organic product. Although initial formation of C 4 Cl 5 ŽI. radicals is kinetically and thermodynamically favored over C 4 Cl 5 ŽN. radicals, the two species are in equilibrium with each other. Thus, the C 4 Cl 5 ŽI. radicals act as a reservoir for the formation of the more reactive C 4 Cl 5 ŽN. radicals. There are three reaction sinks for the more reactive C 4 Cl 5 ŽN. radicals. One reaction involves ␤ C᎐Cl scission yielding the other major product, C 4 Cl 4 . A second pathway involves the addition of C 4 Cl 5 ŽN. to C 4 Cl 4 resulting in the formation of C 8 Cl 9 ŽN.. This linear radical can then cyclize via an intramolecular Cl abstraction pathway involving a 1,4-Cl shift to form octachloroethenylbenzene wC 8 Cl 8 Žcy.x and Cl atoms. A third sink for C 4 Cl 5 ŽN. involves addition to C 2 Cl 2 yielding the linear C 6 Cl 7 ŽL. radical, which then cyclizes to form C 6 Cl 6 Žcy. and a Cl atom. Primary and secondary C 4 Cl 3 and C 4 Cl 5 radicals, produced from the dissociation of C 4 Cl 4 and

P.H. Taylor, D. Lenoir r The Science of the Total En¨ ironment 269 (2001) 1᎐24

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tion pathways leading to formation of C 8 Cl 8 Žcy. and C 12 Cl 8 Žcy. are shown in Fig. 6. 2.5. Hexachloropropene

Fig. 4. Reaction pathway diagram for molecular growth via C 2 Cl 2 addition to even carbon number chlorinated radicals.

C 4 Cl 6 react rapidly with C 8 Cl 6 Žcy., eventually leading to the formation of C 12 Cl 8 Žcy.. There was no evidence that the reactions of C 2 species with C 10 Cl 8 Žcy. play an important role in C 12 Cl 8 Žcy. formation. The dominant 1,4-Cl isomeriza-

The oxygen-free pyrolysis of hexachloropropene was investigated by Taylor et al. Ž1996b. under the identical conditions described for C 2 HCl 3 , C 2 Cl 4 and 1,3-C 4 Cl 6 . Pronounced molecular growth was observed above 800 K as evidenced by the formation of C 4 Cl 6 , C 6 Cl 6 Žcy., C 6 Cl 8 , C 8 Cl 8 Žcy. and C 12 Cl 8 Žcy.. Using the C 4 Cl 6 model as the baseline, a detailed reaction kinetic model was developed describing molecular growth up to the formation of C 12 Cl 8 Žcy.. The initial thermal degradation of C 3 Cl 6 produces the even-carbon species C 2 Cl 4 , as well as three oddcarbon species, C 3 Cl 5 , C 3 Cl 4 Ža., and C 3 Cl 3 . These species form from a series of C᎐Cl bond fission reactions wC 3 Cl 5 , C 3 Cl 4 Ža. and C 3 Cl 3 x or from C᎐Cl bond fission followed by chemically activated Cl displacement of the CCl 3 group ŽC 2 Cl 4 .. Each of these species can, in principle, undergo reactions that ultimately form C 6 Cl 6 Žcy.

Fig. 5. Comparison of model predictions with major product distributions from the pyrolysis of C 4 Cl 6 . Closed symbols: experimental data. Solid and broken curves: model predictions. wC 4 Cl 6 x 0 s 5.4= 10y5 mol ly1 . t r s 1.7 s. Ps 1.83 atm.

10

P.H. Taylor, D. Lenoir r The Science of the Total En¨ ironment 269 (2001) 1᎐24

The C 3 Cl 3 recombination pathway, in conjunction with the low temperature C 3 Cl 5 recombination pathway, produced the best agreement with the C 6 Cl 6 Žcy. profile. The pericyclic self-addition of C 3 Cl 4 was insignificant at all temperatures. The chemically activated C 2 q C 4 radical addition reactions followed by Cl elimination were unimportant below 1100 K. A core reaction pathway diagram for the pyrolysis of C 3 Cl 6 is depicted in Fig. 8. Rate parameters associated with the formation of C 6 Cl 6 Žcy. from C 3 Cl 3 radicals are given in Table 5. A notable observation from this study was the formation of C 8 Cl 8 Žcy. in ; 1 mol.% yields, but the absence of C 10 Cl 8 Žcy.. This observation suggested a strong role of C 4 species as opposed to C 2 species in the molecular growth chemistry. The C 4 pathway accounted for more than 50% of the yield of C 8 Cl 8 Žcy., the remainder being due to reaction of pentachlorophenyl radical wC 6 Cl 5 Žcy.x with C 2 Cl 4 . 2.6. Summary

Fig. 6. Schematic of molecular growth pathways involving resonance-stabilized secondary C 4 radicals.

and larger species. Consequently, four reaction kinetic schemes for the formation of C 6 Cl 6 Žcy. were evaluated: Ž1. C 2 and C 4 radical addition reactions followed by Cl elimination from the chemically activated adduct; Ž2. pericyclic selfaddition of C 3 Cl 4 Ža.; Ž3. self-recombination of C 3 Cl 5 radicals; and Ž4. self-recombination of C 3 Cl 3 radicals. A summary of the model fits to the C 2 Cl 2 , C 4 Cl 6 , C 6 Cl 6 Žcy., C 6 Cl 8 and C 8 Cl 8 Žcy. profiles are shown in Fig. 7.

Chemical kinetic modeling of the pyrolysis of C 2 HCl 3 , C 2 Cl 4 , C 3 Cl 6 , and C 4 Cl 6 has resulted in a self-consistent set of rate parameters for molecular growth reactions involving radical addition followed by Cl elimination. For the general class of perchlorinated radical addition reactions included within these four models, reasonable agreement with the data were obtained using values of log A s 12.0" 0.3 cm3 moly1 sy1 and Ea s 6.0" 1.0 kcal moly1 . For Cl elimination from the stabilized adduct, reasonable fits were obtained using values of log A s 13.2" 0.2 cm3 moly1 sy1 and Ea s ⌬ Hr q Ž1.0" 1.0. kcal moly1 . 1,4-Cl isomerization reactions Žvia intramolecular abstractions . were also found to be important molecular growth reactions for the pyrolysis of C 4 Cl 6 and to a somewhat lesser extent for C 3 Cl 6 . In these two systems, the larger concentrations of C 4 radicals opened C 4-radicalrC 4-molecule addition pathways that required 1,4-Cl isomerization for stabilization of the initially formed adduct. A simple bimolecular abstraction model of this process resulted in estimates of log A and Ea of 10.7 cm3 moly1 sy1 and 23 " 3 kcal moly1 , re-

P.H. Taylor, D. Lenoir r The Science of the Total En¨ ironment 269 (2001) 1᎐24

spectively. However, reasonable agreement with the C 8 Cl 8 Žcy. and C 12 Cl 8 Žcy. profiles from the C 4 Cl 6 experiments required considerably larger activation energies of 31 " 3 kcal moly1 . These C 4 pathways are attractive and potentially important molecular growth channels for CHCs. Clearly, both Cl displacement and isomerization reactions require further study at the elementary reaction level in order to elucidate more fully their role in the combustion of CHCs and formation of potentially hazardous by-products. The implication of the results of the experimental and modeling effort on CHC pyrolysis is the manner in which weaker carbon-chlorine bonds Žvs. carbon-hydrogen bonds. facilitate certain reactions. Chlorine atom displacement, inter-

11

nal chlorine atom abstractions, and chlorine migrations can all facilitate molecular growth. Formation of perchlorinated aromatic species that are resistant to oxidation may actually be more kinetically favored than for the corresponding hydrocarbons. The limitation in further model development is the lack of experimental elementary rate measurements for many key reactions. In addition to chlorine isomerizations, ⫽CCl᎐Cl dissociations, H abstraction by Cl atoms, Cl atom transfer reactions, and radical addition reactions all represent major classes of reactions for which a paucity of rate data exists. Although serious attempts have been made to estimate the rates of these reactions, a larger database of validated CHC reac-

P.H. Taylor, D. Lenoir r The Science of the Total En¨ ironment 269 (2001) 1᎐24

12

Fig. 7. Comparison of model predictions with major product distributions from the pyrolysis of C 3 Cl 6 . Closed symbols: experimental data. Solid and broken curves: model predictions. wC 3 Cl 6 x 0 s 5.4= 10y5 mol ly1. t r s 1.7 s. Ps 1.83 atm.

tion rate and thermochemical data are needed to improve the development of extended reaction mechanisms.

3. Surface-catalyzed molecular growth reactions Combustion processes are one of the major sources of polychlorinated dibenzo-p-dioxins and

polychlorinated dibenzofurans ŽPCDDrF. in the environment ŽLiem and van Zorge, 1995; Thomas and Spiro, 1995.. Their formation in combustion systems has been attributed to several routes, including de novo synthesis from particulate carbon and reactions of chlorinated aromatic precursors ŽVogg and Stieglitz, 1986; Dickson and Karasek, 1987; Stieglitz et al., 1991; Dickson et al., 1992; Milligan and Altwicker, 1993; Addink

Table 5 C 3 Cl 4 degradation mechanism and C 3 Cl 3 reactions to C 6 Cl 6 Žcy. a,b Reaction

Af

nf

Ef

C3 Cl4 Ža. ª C3 C3 q C1 C3 Cl4 Ža. ª C3 Cl4 Žp. C3 Cl4 Ža. q Cl ª C3 Cl4 Žp. q C1 C3 Cl4 Žp. ª C3 Cl3 q C1 C3 Cl4 Žp. q Cl ª C3 Cl3 q C12 C3 Cl4 Ža. q Cl ª C3 Cl3 q C12 2C3 Cl3 ª C6 Cl6 ŽYNE. 2C3 Cl3 ª C6 Cl6 ŽENE. C6 Cl6 ŽYNE. ª C6 Cl6 ŽENE. C6 Cl6 ŽYNE. ª C6 Cl6 Ž4r. C6 Cl6 Ž4r. ª C6 Cl6 ŽFUL. C6 Cl6 ŽFUL. ª C6 Cl6 Žcy. C6 Cl6 Ž4r. ª C6 Cl6 Žcy. C6 Cl6 ŽENE. ª C6 Cl6 Ž4r.

7.0E16 5.0E13 2.6E10 5.0E16 2.5E13 4.0E13 5.0E12 5.0E12 5.0E10 5.0E12 5.0E11 5.0E13 5.0E13 5.0E12

0 0 0 0 0 0 0.5 0.5 0 0 0 0 0 0

66.5 70.0 8.9 61.5 8.1 14.5 0 0 28.0 28.5 52.8 52.8 52.8 24.0

a b

Rate parameters in cm3 mol per s per kcal units. For molecular structures associated with the various chemical abbreviations, consult Taylor et al. Ž1996b..

P.H. Taylor, D. Lenoir r The Science of the Total En¨ ironment 269 (2001) 1᎐24

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Fig. 8. Core reaction pathway diagram for the pyrolysis and molecular growth of C 3 Cl 6 .

and Olie, 1995; Milligan and Altwicker, 1996; Sidhu et al., 1996.. In the de novo pathways, PCDDrF or PCDDrF-like species are expelled directly from the particulate carbon matrix after chlorolysis of this material ŽVogg and Stieglitz, 1986; Stieglitz et al., 1991; Milligan and Altwicker, 1993; Addink and Olie, 1995.. In the original paper by Vogg and Stieglitz, the direct conversion of elementary carbon to PCDDrF was suggested. Later it was shown by Luijk et al. Ž1994. that conversion of carbon matrices to

PCDDrF can occur via chloroaromatic compounds like chlorophenols. However, the term de novo is still restricted to formation of PCDDrF from elementary carbon independent of the specific transformation pathways. In the alternative precursor pathways, chlorinated benzenes and phenols react to form PCDDrF ŽDickson and Karasek, 1987; Dickson et al., 1992; Milligan and Altwicker, 1996; Sidhu et al., 1996.. These precursors may be present in sufficient quantities in the waste feed of some hazardous waste incinerators;

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however, they must be formed in other combustion systems known to emit more PCDDrF than hazardous waste incinerators ŽThomas and Spiro, 1995.. In the previous section, it was demonstrated that complex CHCs readily form from aliphatic CHCs at elevated temperatures. However, the purely gas-phase molecular growth mechanisms of CHCs leading to the formation of PCDDrF have been unable to account for the concentrations observed in the flue gas of full-scale combustors ŽShaub and Tsang, 1983; Kilgroe et al., 1991.. Shaub and Tsang Ž1983, 1985. explored this issue in detail through the development of a simple 13-step gas-phase reaction mechanism and a separate heterogeneous catalytic model of dioxin formation and concluded that heterogeneous rates were much larger than homogeneous rates. Furthermore, they concluded that the more rapid heterogeneous rates were necessary to account for full-scale MWI data. Sidhu et al. Ž1996. challenged the kinetic rates used in the gas-phase dioxin model and concluded, based on experimental measurements of PCDDrF formation from 2,4,6-trichlorophenol, that the estimated rate for pre-dioxin formation from trichlorophenol precursors was underestimated by Shaub and Tsang Ž1983.. Grotheer and Louw Ž1996. conducted an experimental study to verify the hypothesis of Sidhu et al. Ž1996. and concluded that the homogeneous rates in the reaction mechanism proposed by Shaub and Tsang were indeed correct and that their experiments and those by Sidhu et al. Ž1996. could be interpreted in terms of a different gas-phase model incorporating phenoxy radical recombination as the key step in dioxin formation. None of the studies conducted to date have considered a detailed kinetic model with incorporation of reversible chemistry and must be considered preliminary. An assessment of the current situation suggests that heterogeneous processes account for at least two-thirds of the PCDDrF emissions from thermal disposal facilities ŽUS-EPA, 1998.. Additional studies are needed to further address this important question. Chlorinated benzenes ŽClBz. and phenols ŽClP. are found in the flue gas of hazardous as well as of municipal waste incinerators and a general

correlation of ClBz and ClP with PCDDrF has been deduced by Kaune et al. Ž1998. and Blumenstock et al. Ž1999.. Chlorinated benzenes and phenols have been determined as ultimate precursors for PCDDrF, but the relevant precursors for these chloroaromatic compounds are open for discussion. In a qualitative study, Scholz et al. Ž1992. detected formation of PCDDrF by heating aliphatic and aromatic precursors on a copper wire. Froese and Hutzinger Ž1993, 1996. demonstrated that simple C 2 hydrocarbons, e.g. acetylene, ethylene and ethane, form chlorinated benzenes, chlorinated phenols, and PCDDrF when exposed to an HCl-air mixture over various SiO 2rmetal oxide surfaces. Although these experiments indicated a prominent role of surface-catalyzed reactions in formation of PCDDrF, product yields were greatest at elevated temperatures Ž873 K. where homogeneous gas-phase reactions also occur. Furthermore, neither a plausible overall mechanism nor a detailed surface model was deduced from these empirical studies. Consequently, key issues such as how these molecules adsorb on the catalyst, when and how chlorination occurs, how molecular growth proceeds, and why the products desorb from the surface remain unresolved. Lenoir et al. Ž1998., Wehrmeier et al. Ž1998a. and Taylor et al. Ž1999. recently reported that a simple hydrocarbon present in all combustion systems, acetylene wC 2 H 2 x, is readily converted to C 6 Cl 6 Žcy. and other CHCs through a catalytic reaction with cupric oxide ŽCuO. and HCl on a borosilicate surface at temperatures ranging from 423 to 773 K. Copper species were selected for the laboratory experiments because this metal has been shown to be present in all kinds of fly ashes of MWIs and HWIs. Model fly ashes of SiO 2 ᎐CuCl 2 mixtures have been previously studied by Dickson et al. Ž1992.. Trace levels of ; 0.05% copper as a constituent of fly ash were sufficient to act as catalyst for PCDDrF formation from chlorophenol precursors. Initially, we investigated FeCl 3 and CuCl 2 under the same experimental conditions. No conversion of acetylene to chloroaliphatic and chloroaromatic compounds was observed within detection limits using FeCl 3 ŽWehrmeier, unpublished data.. However, as discussed in the following pages, copper species

P.H. Taylor, D. Lenoir r The Science of the Total En¨ ironment 269 (2001) 1᎐24

were observed to be efficient catalysts for conversion of acetylene into chloroaliphatic and chloroaromatic compounds including PCDDrF.

4. Experimental results 4.1. Formation of ¨ olatile chlorinated aliphatic and aromatic compounds: a new model for the postcombustion zone of incinerators Experiments were conducted using a specially designed fixed-bed reactor system capable of detailed on-line analysis of volatile and semi-volatile chlorinated species, see Wehrmeier et al. Ž1998a. for details. Reactions of C 2 H 2 in a very fuel-lean oxygen atmosphere on CuCl 2 impregnated borosilicate foams produced a variety of chlorinated organic compounds ŽWehrmeier et al., 1998a; Taylor et al., 1999.. This system was able to chlorinate C 2 H 2 to form chlorinated C 2 compounds. C 2 , C 4 , C 6 and C 8 compounds were the

15

major products observed in the gas-phase. The most striking feature in the distribution of reaction products was the high degree of chlorination. A majority of products were perchlorinated. An interesting observation was the lack of unchlorinated acetylene polymerization products, e.g. vinylacetylene, benzene, ethynylbenzene and naphthalene. Furthermore, cracking products and compounds containing an odd number of carbon atoms were not detected under these reaction conditions. The major reaction products formed in the C 2 H 2-CuCl 2rBS system are illustrated in Fig. 9. Reaction products with a lower degree of chlorination Že.g. pentachlorobutadiene . were predominately formed at higher temperatures. Maximum product yields were typically formed at 573 K. Adding HCl to the C 2 H 2-CuCl 2rBS system increased the total product yield. The same reaction products were formed, but the distribution was shifted towards higher chlorinated compounds, e.g. C 2 HCl 3 was not detected. A few

Fig. 9. Predominant gas-phase products for the reaction of C 2 H 2 on CuCl 2 rBS at 150⬚C, 300⬚C and 400⬚C, respectively. wC 2 H 2 x total s 167 ␮mol, wCuCl 2 x 0 s 350 " 80 ␮mol, t r s 2.0 s, reactor gas: 4% O 2 in He. Error bars represent relative standard deviations in the product measurements.

16

P.H. Taylor, D. Lenoir r The Science of the Total En¨ ironment 269 (2001) 1᎐24

minor reaction products were not quantified and hence, these products are not included in summation of product yields. These products included CCl 4 , C 2 HCl 5 , and mono- through pentachlorobenzene. Approximately 15 mol.% C 2 H 2 was transformed to chlorinated and condensed reaction products in the presence of a CuCl 2 impregnated borosilicate foam at 573 K. In the presence of 1100 ppm HCl, yields rose to just over 17 mol.% of the initial C 2 H 2 concentration. The amounts of CO and CO 2 were not determined. CuO was tested for its catalytic activity towards C 2 H 2 condensation and chlorination in the C 2 H 2-CuOrBS and C 2 H 2rHCl-CuOrBS system, respectively. Reaction products for the C 2 H 2rHCl-CuOrBS system were only found at and above 573 K with maximum yields at 673 K. In contrast to the C 2 H 2-CuCl 2rBS system, significant amounts of reaction products were formed at 773 K. The two systems investigated without adding copper species, C 2 H 2-BS and C 2 H 2rHClBS, did not result in any reaction products at temperatures up to 773 K. Acetylene was unreactive in the gas-phase at temperatures between 473 and 773 K. Time-dependent studies indicated that chlorination and condensation reactions are fast processes, readily occurring at t r s 1.0 s with maximum rates at t r s 1.5᎐2.0 s. Product yields for C 2 and C 4 species generally decreased for residence times greater than 2.0 s. Yields of C 6 species achieved a steady-state or decreased with increasing reaction time. The lack of a shift in molecular weight distribution of reaction products with increasing residence time suggests that condensation reactions are occurring very rapidly compared with the time-scale of the experiments. It is likely that C 2 H 2 is partially oxidized to CO during the experiments, which may reduce CuŽII. species to CuŽI. species. CuŽI. and CuŽII. species were identified from post-experiment XPS analysis of the borosilicate surfaces. The fraction of CuŽI. species increased from 30% at 423 K to 40% at 623 K. CuŽI. and CuŽII. species containing chlorine were identified from XPS analysis of the C 2 H 2rHCl-CuOrBS experiments, lending credibility to the hypothesis that CuClrCuCl 2 is

the dominant chlorinating and condensation system in these experiments. The system CuCl 2r CuCl has a low redox potential of 0.153 eV, which initiate single electron transfer reactions. A series of experiments with C 2 Cl 2 were conducted to determine if the major reaction products were similar to the C 2 H 2-CuCl 2rBS system. A distinct similarity was observed as illustrated in Fig. 10. Gas-phase experiments with C 2 Cl 2 at 573 K indicated significant dimerization of the starting material Ž; 8 mol.% C 2 Cl 2 conversion. with tetrachlorovinylacetylene identified as the major product. In the presence of the catalyst support, significant changes in reactivity were observed with tetrachloroethylene, tetrachlorovinylacetylene and hexachlorobutadiene the major products Ž; 15 mol.% C 2 Cl 2 conversion.. Addition of CuO to the surface resulted in further changes in reactivity with tetrachloroethylene, tetrachlorovinylacetylene, hexachlorobutadiene, and hexachlorobenzene the major products Ž; 9 mol.% C 2 Cl 2 conversion at 573 K.. The presence of CuO resulted in increased yields of hexachlorobenzene at the expense of the other products. These experiments demonstrate that molecular growth of dichloroacetylene Žbeyond dimerization. does not readily occur in the absence of copper. The acetylene and dichloroacetylene experiments further indicate that chlorination and formation of C 6 and other chlorinated aromatic compounds at these low temperatures requires the presence of copper. 4.2. Formation of chlorophenols and PCDD r F as non-¨ olatile products In the C 2 H 2 experiments, chlorinated benzenes were detected in the gas-phase, while chlorophenols and PCDDrF were only observed bound to the surface of the borosilicate coated with different copper species. Although certainly possible, we cannot state unequivocally that the chlorophenols and PCDDrF are formed in the short contact times Ž1᎐10 s. of our experiments. Experiments by Gullett et al. Ž1994. and Wikstrom and Marklund Ž2000. have shown rapid formation of PCDDrF on surfaces in a fluidized bed and spouted bed reactor with short contact

P.H. Taylor, D. Lenoir r The Science of the Total En¨ ironment 269 (2001) 1᎐24

Fig. 10. Major gas-phase products Žmol.%. for the reaction of C 2 Cl 2 in the gas-phase and on CuOrBS at 300⬚C. wC 2 Cl 2 x total s 167 ␮mol, wCuOx 0 s 300 " 75 ␮mol, t r s 2.0 s, reactor gas: 4% O 2 in He. Error bars represent relative standard deviations in the product measurements.

times Žsec.. However, in our case, we cannot rule out adsorption of the PCDDrF precursors on the surface and subsequent reaction during the 60min duration of our experiment. It is likely that PCDDrF formation occurred over a wide range of reaction times Žseconds to minutes. during the course of our experiments.

17

The PCDDrF homolog profile obtained from C 2 H 2-CuCl 2rBS and C 2 Cl 2-CuOrBS experiments at 573 K is illustrated in Figs. 11 and 12, respectively. This pattern is similar to the pattern of homologs found in fly ash of waste incinerators ŽLenoir et al., 1998; Wehrmeier et al., 1998b.. However, this pattern varies significantly depending on the type of incinerator and the incineration conditions. In general, the maximum is observed for hexachlorinated dibenzo-p-dioxins ŽWehrmeier et al., 1998b.. The high yields of PCDDrF extracted from the surface infers significant levels, i.e. ; 10%, should have also been observed in the gas-phase. This expectation is derived from results of municipal waste incinerators. The ratio of PCDDrF remaining on fly ash and further transferred by the flue gas at the ESP unit at 573 K is approximately 10:1 ŽFroese and Hutzinger, 1993.. The larger concentrations of copper on the BS surface compared to typical copper concentrations in fly ashes Ž; 0.05%. may have been in part responsible for the large surface-bound PCDDrF yields and the lack of detection of PCDDrF in the gas-phase. Furthermore, the BS matrix may have a stronger absorption coefficient compared with a typical fly ash matrix.

Fig. 11. Homolog profile for surface-bound PCDDrF from C 2 H 2 experiments at 300⬚C. wC 2 H 2 x total s 167 ␮mol, wCuCl 2 x0 s 344 " 80 ␮mol, t r s 2.0 s, reactor gas: 4% O 2 in He.

18

P.H. Taylor, D. Lenoir r The Science of the Total En¨ ironment 269 (2001) 1᎐24

Comparison of the C 2 H 2 and C 2 Cl 2-derived PCDDrF homolog profiles indicates substantial differences. Dichloroacetylene preferentially forms higher chlorinated homologs ŽCl 6 ᎐ 8 CDDrF.. Yields of Cl 4 CDF and Cl 5 CDF were very low and yields of Cl 4 CDD and Cl 5 CDD were below detection limits. Yields of Cl 8 CDD were more than a factor of two to three larger for C 2 Cl 2 than C 2 H 2 and C 2 H 2rHCl. However, as the level of chlorination decreased, relative PCDDrF yields become increasingly larger from C 2 H 2 . PCDDrF dechlorination reactions have been offered as an important mechanism in rationalizing PCDDrF homolog profiles from both laboratory experiments using actual fly ashes and full-scale data. The results presented here appear inconsistent with this mechanism. The C 2 Cl 2 data do not support the contention that significant dechlorination of PCDDrF occurs on the surface, based on surface retention times of up to 60 min. The C 2 H 2 and C 2 Cl 2 experiments reported here give higher chlorinated PCDDrF isomers preferentially. Within a group of isomers, the pattern belongs mainly to the minor second principal component of a statistical principal component analyses of a large number of fly ash samples from technical incinerators in Europe

Fig. 12. Homolog profile for surface-bound PCDDrF from C 2 Cl 2 experiments at 300⬚C. wC 2 Cl 2 x total s 167 ␮mol, wCuOx0 s 300 " 75 ␮mol, t r s 2.0 s, reactor gas: 4% O 2 in He.

Fig. 13. Concentrations of tetrachlorodibenzo-p-dioxin isomers from C 2 H 2 experiments at 300⬚C. wC 2 H 2 x total s 1510 ␮mol, wCuCl 2 x 0 s 344 " 80 ␮mol, t r s 2.0 s, reactor gas: 4% O 2 in He.

ŽWehrmeier et al., 1998b.. This is valid not only for the tetrachlorinated PCDD Žsee Fig. 13., but also the other homologs. The second principal component results in formation of the thermodynamic most stable isomers, e.g. 2,3,7,8-TCDD. But this distribution is kinetically less stable yielding from further destruction and dechlorination the first component that is observed in full-scale incinerators ŽWehrmeier et al., 1998b.. This consecutive pathway is not occurring in the borosilicate model results presented here, because this special system models preferentially the formation pathways ŽTaylor et al. 2000.. With proper interpretation, these results demonstrate the relevance of these experiments to large-scale combustion systems. Chlorinated benzenes and chlorinated phenols are the most likely precursors to PCDDrF. These classes of compounds are always found together with PCDDrF in the effluents of incinerators and have been suggested as indicator compounds ŽKaune et al., 1998.. Chlorinated benzene concentrations are typically approximately three orders of magnitude higher than PCDDrF in incinerator effluents. In the 573 K experiment where chlorinated benzene and PCDDrF yields were measured, the ratio of chlorinated benzenes to PCDDrF was 112. Chlorophenol yields derived

P.H. Taylor, D. Lenoir r The Science of the Total En¨ ironment 269 (2001) 1᎐24

from extractionrclean-up of the surface from C 2 H 2-CuCl 2rBS experiments at 523, 573 and 623 K are shown in Fig. 14. The concentrations of chlorophenols vary significantly with temperature, exhibiting a minimum at 573 K. This contrasts inversely with the PCDDrF temperature dependence, also shown in Fig. 14, which exhibits a maximum at 573 K. At 523 and 623 K, chlorophenol concentrations are much larger than PCDDrF, consistent with previous studies ŽKaune et al., 1998.. However, at 573 K, total chlorophenol concentrations are approximately two orders of magnitude lower than PCDDrF. This is an unexpected result and may be an indication of rapid conversion of surface-bound chlorophenols to PCDDrF on the copper-coated borosilicate surface.

19

Fig. 14. Homolog profile for surface-bound chlorophenols from acetylene experiments at 250, 300, and 350⬚C. wC 2 H 2 x total s 167 ␮mol, wCuCl 2 x 0 s 344 " 80 ␮mol, t r s 2.0 s, reactor gas: 4% O 2 in He. 䢇

5. Mechanistic interpretation

䢇

5.1. Chloroaromatic formation

䢇

In the experiments of Wehrmeier et al. Ž1998a. and Taylor et al. Ž2000., the following experimental observations indicate that acetylene is chlorinated prior to surface-catalyzed condensation:

䢇

䢇

no acetylene polymerization products were found with either catalyst investigated; condensed reaction products were only found with a chlorine source present; yields of reaction products increased with additional chlorine; reaction products were predominantly perchlorinated; and major products from C 2 Cl 2rBSrCuO were similar to those for C 2 H 2rBSrCuCl 2 .

Fig. 15. Acetylene and dichloroacetylene chlorinationrcondensation mechanism.

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P.H. Taylor, D. Lenoir r The Science of the Total En¨ ironment 269 (2001) 1᎐24

Consistent with their experimental results and in analogy with mechanisms found in solution chemistry, Wehrmeier et al. Ž1998a., proposed the following reaction mechanism Žsee Fig. 15. for the gas-surface chlorinationrcondensation reaction of acetylene on copper species. Acetylene is chlorinated by CuCl 2 in a ligand transfer oxidation mechanism to mono- and dichloroacetylene and other less unsaturated chlorinated C 2 compounds. Chlorinated acetylenes then form oligomers in a surface-catalyzed reaction. Stahl et al. Ž1986. has shown that reactive dichloroacetylene is stabilized in metal complexes. These metal-dichloroacetylene complexes react with additional dichloroacetylene to form chlorinated metallacylopentadienes ŽSunkel, ¨ 1990, 1991 ., which are intermediates in dichloroacetylene condensation reactions. According to the Glaser and Cadiot᎐Chodikiewicz reaction, Cuq complexes are possible catalysts ŽChodikiewicz, 1957.. Chlorinated metallacylopentadiene forms chlorinated butadienes in oxidative chlorination reaction with CuCl 2 or reacts with dichloroacetylene in an insertion or Diels᎐Alder type reaction to form chlorinated benzenes ŽSunkel, 1990, 1991.. Another route to ¨ hexachlorobenzene from aliphatic precursors may occur from octachlorohexatriene-1,3,5 ŽC 6 Cl 8 . by thermal cyclization and elimination of chlorine, as

Fig. 16. Thermal cyclization of C 6 Cl 8 Žtrans. to C 6 Cl 6 Žcy.. Two C 6 Cl 8 compounds were detected in the product mixture.

Fig. 17. Catalytic cycle for formation of hexachlorobenzene from dichloroacetylene via copper-stabilized chlorovinyl radicals.

illustrated in Fig. 16. This is an uncatalyzed reaction that occurs at temperatures between 423 and 453 K. An alternative mechanism involving the CuCl 2rCuCl system can be envisioned for formation of hexachlorobenzene from dichloroacetylene, cf. Fig. 17. Interactions between the d orbitals of the Cu and the ␲ electrons of the dichloroacetylene result in the formation of chlorovinyl radical intermediates stabilized by the adjacent copper:

A stable desorbed product, hexachlorobenzene, is formed by ring closure. The formation of hexachlorobenzene as the only C 6 product from dichloroacetylene reactions in the presence of the copper catalyst is experimental support for this mechanism. Trimerization of acetylene derivatives to benzenes has been performed in solution by Reppe et al. Ž1969., using dissolved zero-valent Ni complexes. Mechanistic interpretations could not be developed at this time. Chloroacetylene was not investigated. These results show that dichloro-

P.H. Taylor, D. Lenoir r The Science of the Total En¨ ironment 269 (2001) 1᎐24

acetylene is a key compound in the thermochemistry of chlorinated aliphatic and aromatic compounds. Due to technical difficulties, this compound has not been detected in the effluents of large incinerators. However, related C 2 compounds like trichloroethylene and tetrachloroethylene are often reported in the flue gas of incinerators ŽDellinger et al., 1991..

21

bital calculations has recently been proposed for the copper-catalyzed oxidative coupling of chlorophenols involving dinuclear phenolatebridged CuŽII. species ŽOkamoto and Tomonari, 1999; Tupparainen and Ruuskanen, 1999..

6. Summary

5.2. Chlorophenols and PCDD r F The conversion of chlorinated benzenes to chlorinated phenols are likely to occur on the surface of actual fly ashes or model surfaces, e.g. BSrCuO and BSrCuCl 2 . Special metallic oxides like Al 2 O 3 or CuO have been shown to convert bound bromobenzene to phenol by a nucleophilic substitution reaction at 200⬚C ŽLippert et al., 1991.. This is postulated to occur at the superbasic sites of this catalytic surface. It is likely that the chlorine in surface-bound chlorobenzene will also be converted to an OH group on this type of catalytic surface. The higher chlorinated benzenes are expected to be more reactive towards nucleophilic aromatic substitutions ŽMarch, 1985.. An alternative suggestion ŽDellinger and Taylor, 1998., conversion on SiO 2 , is also a possible mechanism. Surface-bound chlorophenoxy radicals are plausible intermediates to the formation of PCDDrF. This is consistent with the lack of detection of gas-phase chlorophenols in our experiments. Conversion of these surface-bound radicals to PCDD may occur through reaction with adsorbed chlorophenol molecules or via reaction with adjacent surface-bound chlorophenol radicals ŽDickson et al., 1992; Dellinger and Taylor, 1998.. These reactions should be faster than the corresponding gas-phase Cl displacement reaction, which does not occur until temperatures in excess of 500᎐600⬚C. These reactions are described as Ullmann-II reactions, and are known to be catalyzed by copper species. In experiments reported in the literature, various chlorophenols have been converted to PCDD by fly ashes and model fly ashes in yields from 8 to 22% ŽKarasek and Dickson, 1987.. A mechanism based on semi-empirical SMI and ab initio molecular or-

Recent gas᎐solid kinetic experiments indicate that a common form of copper in a combustion system, i.e. cupric oxide, can catalyze the chlorination of a simple hydrocarbon, i.e. acetylene, in the presence of HCl, and also catalyze molecular growth to form higher molecular weight CHCs. These reactions occur at temperatures of 300᎐400⬚C. Under certain conditions, the observation of mostly perchlorinated or highly chlorinated products indicates that chlorination followed by molecular growth of CHCs is faster than molecular growth of hydrocarbons under postcombustion conditions. This is significant in that for the first time a viable pathway for forming chlorinated aromatic precursors to PCDDrF from a hydrocarbon-dominated system has been identified. The experimental results also demonstrate that dichloroacetylene is an important precursor in the surface-catalyzed chlorination and molecular growth of acetylenic compounds. The mechanisms of chlorination and condensation discussed herein provide a tool towards minimization of the formation of chloro-aromatic compounds including PCDDrF in the post-combustion effluents of incinerators. The detailed mechanisms of conversion of chlorinated aromatic compounds to PCDDrF are the subject of continuing investigations.

Acknowledgements This research was partially supported by the National Science Foundation and the US Environmental Protection Agency. The authors gratefully acknowledge the contributions to various

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