Environ. Sci. Technol. 2006, 40, 1263-1269
Formation of Dioxins from Combustion Micropollutants over MSWI Fly Ash M A R I U S Z K . C I E P L I K , †,| V I N C E N T D E J O N G , †,⊥ J E L E N A B O Z O V I C ˇ ,‡ PER LILJELIND,§ STELLAN MARKLUND,§ AND R O B E R T L O U W * ,† Leiden Institute of Chemistry, Gorlaeus Laboratories, Leiden University, P.O. Box 9502, 2300 RA Leiden, The Netherlands, Chemistry Department, University of Belgrade, Serbia, and Environmental Chemistry, Umeå University, SE-901 87 Umeå, Sweden
Formation of polychlorinated dibenzofurans and dibenzop-dioxins (PCDD/Fs) from a model mixture of products of incomplete combustion (PICs) representative of municipal solid waste incineration (MSWI) flue gases, over a fixed bed of MSWI fly ash has been investigated. For comparison, a single model compound (chlorobenzene) was also briefly studied. A newly developed lab-scale system enabled the application of (very) low and stable concentrations of organic substancessof 10-6 M or lesssto approach realistic conditions. Samples taken at several time intervals allowed the observation of changes in rates and patterns due to depletion of the carbon in fly ash. The model flue gas continuously produced PCDDs and PCDFs after the de novo reaction had ceased to occur. Dioxin output levels are comparable to those of “old” MSW incinerators. Replacing the PIC trace constituent phenol by its fully 13Clabeled analogue led to, e.g., PCDD with one labeled ring as prominent product, meaning that the formation is about first order in phenol, contrary to earlier assumptions. The meaning of the results for the formation of dioxins in the MSWI boiler is discussed.
Introduction Formation of polychlorinated dibenzofurans and dibenzop-dioxins (PCDD/Fs) in a variety of thermal processes is a well-established fact. Ever since “dioxins” have been detected in effluent gases and fly ash of a Dutch municipal solid waste incineration (MSWI) facility by Olie et al. (1), discussion is ongoing on pathways to, and the formation mechanism of, these obnoxious compounds. Besides high-temperature gasphase reactions of aromatics (2-4), lower-temperature catalytic reactions involving fly ash appear to be at least as important. The latter can be subdivided into two categories. * Corresponding author phone: +31 (0) 71 5274289; fax +31 (0) 71 5274451; e-mail:
[email protected]. † Leiden Institute of Chemistry. ‡ Visiting student from University of Belgrade, Serbia. § Umeå University. | Present address: Energy Research Center of The Netherlands (ECN), Westerduinweg 3, P. O. Box 1, 1755 ZG, Petten, The Netherlands. Phone: +31 (0) 224 564700; e-mail:
[email protected]. ⊥ Present address: PURAC, P.O.Box 21, 4200 AA Gorinchem, The Netherlands. Phone: +31 (0) 183695890; e-mail: vincentdejong@ tiscali.nl. 10.1021/es052225l CCC: $33.50 Published on Web 01/11/2006
2006 American Chemical Society
The first advocates PCDD/F formation from carbonaceous material present in the fly ash. This de novo pathway has been extensively studied by Stieglitz et al. (5) and Olie et al. (6), among others. Laboratory experiments have been performed batch-wise, with real or model ashes, and with “native” carbon or added surrogates, mostly in the range of 250-400 °C. The de novo formation usually takes place over a long time (up to several hours) but decreases in time as the carbon on/in the fly ash gets depleted. The dioxin congener patterns observed in de novo experiments resemble those found in real-life samples, with outputs of PCDDs about an order of magnitude lower than those of PCDFs. While this observation is a stronghold for those advocating this formation pathway, yields of PCDD/Fs in these tests are very low, typically up to 1 µg/g ash in 1-2 h. These levels have to be reconciled with the real time scale, up to several minutes, and the conditions for catalytic reaction in the MSWI postcombustion zone (boiler and air pollution control devices). The second category concerns catalytic reactions on fly ash of semivolatile organic compounds in the MSWI offgas. This “precursor” scenario tallies with a seconds time scale, easily accessible also in (model) laboratory experiments. A key question, however, is whether the very low concentrations of (potential) precursor compounds in the offgas can lead to the PCDD/F levels and compositions observed in practice. The composition of an MSWI offgas is extremely complex. Analyses list hundreds of recognized substances, both inorganic and organic (5, 7), covered by the general term products of incomplete combustion (PICs). Concentrations also differ greatly. Some PICs can be present at a high ppm level (e.g., acetylene), others, such as (chloro)phenols, constitute only ppb levels or even less. All this makes it extremely difficult to mimic the real conditions on a lab scale. Thus far, simplification has been the approach by those attempting research in this field. Usually one or two compounds have been chosen to model the flue gas, often narrowing the choice only to (chloro)phenols (8-10). Indeed, phenols are demonstrated potent dioxin precursors, both in the gas phase (2-4) and in fly ash-mediated reactions. (810). In model gas-phase reactions, “slow combustion” gives PCDFs as the prevailing products, with (chlorinated) phenoxy radicals as key intermediates (3). In catalytic reactions of phenols over fly ash, PCDDs heavily dominate. While these studies prove that formation of PCDD/Fs from gas-bound substances over the surface of fly ash on a seconds time scale can be a prominent reaction, results as yet are likely to be too far from those in reality, because the concentrations of the phenols or other potential precursors used in these tests usually have been orders of magnitude larger than those present in MSWI flue gases. This paper reports on our attempt to bridge the gaps between the real world situations and laboratory approaches. To this end, a cocktail with 14 compounds representative of important categories of PICs in a real MSWI flue gas has been used, and with concentrations of 10-6 M or less to approach those in practice. For comparison, a simple model compound, chlorobenzene, has also been studied. Employing a standard fly ash still containing its “native” carbonswhich allows a proper contribution by de novo reactionssthis approach aims at disclosing the relative importance of the various mechanistic pathways to PCDD/Fs in MSWI post combustion zones. VOL. 40, NO. 4, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Composition of the PIC Model Mixture compound
ratio relative to benzene concentration M [mol/mol] (x × 10-6)a
benzene naphthalene phenanthrene acetone trichloroethane benzaldehyde dibenzofuran benzofuran toluene phenol 2-chlorophenol 2,4,6-trichlorophenol chlorobenzene tetrachloroethylene a
dna 1/3 1/10 1/10 1/20 1/20 1/50 1/100 1/100 1/100 1/250 1/500 1/250 1/100
49 16 4.9 4.9 2.4 2.4 1.0 0.5 0.5 0.5 0.2 0.1 0.2 0.5
At a feed rate of 1.25 µL/min and a 6.6 L/h gas flow (25 °C).
Experimental Section Setup. The experimental setup and the experimental procedure have been described earlier (11). Analyses. Outflowing gases were directed by heated stainless steel tubing (T > 150 °C) to sampling loops by means of two Valco C6WT-HC six-port sampling valves, for online gas chromatographic analysis on a Hewlett-Packard 5890 Series II instrument, provided with a capillary (CP SIL 5 CB, 50 m × 0.32 mm, film thickness 0.4 µm) and a packed column (Alltech, Carbosphere 80-100 mesh). The capillary column end was split into two with connections to a flame ionization detector (FID, to quantify hydrocarbons, etc.) and an electron capture detector (ECD, for (poly)chlorinated organics up to hexachlorobenzene). CO and CO2 were quantified by injection on a packed column (Alltech, Carbosphere 80-100 mesh) followed by hydrogenation/hydrogenolysis to CH4 in a methanizer, and then analyzed on a FID. For off-line PCDD/F analysis the PUF-plugs were Soxhletextracted with 110 mL of a 10:1 heptane/acetone mixture (ca. 10 turnovers/hour). The extract was concentrated to 0.5 mL employing a Zymark TurboVap II apparatus. Dioxin analyses were performed at Umeå University by means of HRGC/HRMS, as described in ref 11. Chemicals. All the used chemicals were at least of 99.5% purity (for more details see ref 11). Throughout, the standardized European MSWI fly ash used was the same as that previously employed by Stieglitz et al. (5). The fly ash was used as received to evaluate possible contributions of both de novo and “precursor” reactions.
Results and Discussion Model PIC Mixtures. The composition of the model PIC mixture employed in this investigation is presented in Table 1. Compounds were chosen from the list of 220 PICs reported by Jay and Stieglitz (7), with prominence as a main criterion; some compounds served in simplification by lumping, e.g., acetone for aliphatic ketones, or benzaldehyde for aromatic aldehydes. First, the PIC cocktail was used as such, with a feed rate leading to the concentrations presented in the table, being over 2 orders of magnitude higher than in reality. Lowering the feed rate 5-fold in a “time-sliced” experiment (see below)s reaching the minimum reliable flowrate of the feeding system, of 12 µL/hsbrought the concentrations already closer to reality. In a following short series the cocktail was diluted 100 times with an 86:14 (vol) mixture of heptane/tertbutylchloride. The latter compound served also as a chlorine (HCl) source, which was absent in the original mixture. 1264
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Together with the 5-fold lower liquid feed rate this meant 500 times lower concentrations of the model PICs. Experiments with the 100% PIC Cocktail. First, three 18 h runs were conducted at 350 °C to evaluate the performance of the new setup. In the first run with no PUF-plug the whole exit gas stream was led to the gas chromatograph for online analysis. It turned out that the flow of organics and conversion to CO/CO2 (some 20/40% mol on carbon input, respectively) was stable throughout. The product mixture was much more complex than the inflowing cocktail. We have refrained from detailed analysis, but next to unconverted substrates, higher PAHs, such as pyrene derivatives, and some chlorinated aromatics have been noticed. During the next experiment, also at 350 °C, products were trapped in a PUF-plug adsorber. Results on PCDD/Fs are shown in Figures 1-3. After turning off the organic flow another sample was collected on a fresh PUF-plug for some 10 h to see if any memory effectssometimes a disturbing factor in dioxin researchswould occur. This was not the case: only 2 ng of PCDD and 8 ng of PCDF were found, compared with ∼230 000 ng of PCDD/F in the first catch. Next, an experiment was performed at a 5-fold lower feed rate of the undiluted PIC cocktail, at 325 °C. Samples were taken for dioxin analysis after the first 39 min and subsequently after 1, 2, 9.5, and 17.5 h runtime, to obtain insight into the production of PCDD/Fs as the experiment proceeds. From Figure 4 it is immediately clear that during the first 2 h or so, output rates of PCDD/F are firmly time-dependents which may tally with de novo activityswhile later on a steady reactivity persists whereby the dioxins must have been formed from precursors in the PIC cocktail (see below). 1% PIC Cocktail in Heptane/tert-Butylchloride. To achieve realistic low levels of the model PICs, the original mixture was diluted and the feed rate was lowered, as described in the preceding section. First, control experiments with heptane only and with the heptane/t-BuCl mixture were performed, again at 325 °C, keeping the feed rate at the same low level of 12 µL/h. Less than 2% of t-BuCl survived and ∼86% of the chlorine could be found back as HCl in a simple water trap. Translated into concentration terms this equals [HCl] ∼350 ppm, matching with common concentrations found in MSWI flue gases (12). Next, an 18 h experiment with the 1% PIC cocktail was conducted. The respective overall dioxin outputs are in Figure 1, with compositions shown in Figures 2 and 3. While the output from “heptane per se” is quite like that from Stieglitz’ standard experiment, i.e., the output obtained when stripping with synthetic air only (Figure 1), the run with t-BuCl gives about twice as muchsan effect which, one way or another, may be ascribed to the HCl formed therefrom. The presence of (only) 1% of the PIC cocktail led to a further increase, to about twice as much PCDDs and ∼50% more PCDFs. This is suggestive of a contribution by components in that PIC cocktail. To obtain a better insight into this, the phenol in the diluted PIC cocktail was replaced by its fully 13C -labeled analogue. The resulting outputs of the 13C6 containing dioxins and furans are depicted in Figures 1 and 5. The total is well-comparable with that of the former experiment and it is immediately clear from Figure 5 that substantial fractions of especially the PCDDs contain one labeled ring, so the other must stem from some component(s) in the PIC cocktail. Much smaller but still detectable proportions of PCDDs are fully derived from the small amount of nonchlorinated phenol in the feed. The degree of isotopic labeling in the PCDFs is decidedly smaller, but not negligible. Chlorobenzene/Heptane. Chlorobenzene has often been nominated as a precursor for dioxins in fly ash-mediated reactions. For comparison, we have therefore reacted a 1:1 (molar) mixture of chlorobenzene/heptane at 350 °C. The output of PCDD/F differed little from that with “heptane per
FIGURE 1. Overview of PCDD/F outputs. Data from Stieglitz et al. (5) are recalculated per 0.30 g of ash. Numbers in parentheses are yields relative to those of the heptane per se experimen. 100% PIC experiment results, designated by #, secondary axis (right). 13C6-phenol experiment feed ) heptane/t-BCl/1%PIC wherein plain phenol was replaced with a 13C6 labeled homologue.
FIGURE 2. Congener distribution chart. se” or with Stieglitz’ standard experiment (see Figure 1); only the small proportion of PCDDs is several times higher. So, this simple benzene derivative gives at best only a little dioxin, in this case, in addition to the PCDD/Fs stemming from native carbon in the ash. Phenols are much more reactive. In a competition experiment at 325 °C with phenol and chlorobenzene at ∼3 × 10-5 M and a contact time of ∼1 s, Born (13) has found that the former compound is at its onset, but phenol reacted already for 20%. At 360 °C, chlorobenzene reacted substantially, but phenol reacted to completion. Neither aliphatic hydrocarbons (here, heptane) nor the HCl
source tert-butylchloride are significant as precursors for dioxins. Comparison of PCDD/F Outputs. Under our conditions, in 18 h 0.12 Nm3 of (synthetic) offgas is passed over 0.30 g of fly ashsor ∼0.4 Nm3/g of ash. In real MSWI facilities, typical ash/gas ratios are around 0.6 Nm3/g (14). Figure 1 shows that the PCDD/F outputs vary greatly and span a range of over 600. The absolute “winner” is the undiluted PIC cocktail, denoted 100% PIC (large bar, left in the figure, values on secondary axis), with over 230 µg of the 2,3,7,8-substituted PCDD/Fs. It is worth mentioning that the total output of VOL. 40, NO. 4, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 3. 2,3,7,8-Substituted congener distribution chart (Hagenmaier plot). For penta- to hepta-homologues, only the extra positions are given in labels, i.e., 1-PeCDF ) 1,2,3,7,8-pentachlorodibenzofuran).
FIGURE 4. PCDD/F yields in the time-sliced experiment. mono- through octa-substituted dioxin was only about twice (∼490 000 ng) that of the tetra-octa homologues: 233 300 ng, of which ∼14 800 ng was PCDFs. The dioxin output resulting from subsequent stripping with the carrier gas only, for 10 hours (denoted as stripping) is minimal, proving two things: residual reactivity of native carbon and memory effects were negligible. The PCDF/PCDD ratio of Cl5 > Cl6 > Cl7 (Cl8), which can also be reconciled with this, accepting that there is limited capacity for further (oxy)chlorinationsof precursors, or PCDD/Fs, or both. Details on the “time-sliced” experiment are in ref 16; in brief, the distributions for the final 15.5 h are not unlike that of the 100% PIC run. One difference is that the DFs now constitute ∼20%, pointing at less dominance by phenol condensation at the lower concentration. The DF/ DD ratios for the first hourswhen de novo contributessare ∼0.7. Variation within the furans as well as dioxins is rather limited (16). The heptane-only run gave only a small proportion (∼7%) of PCDDs; the PCDFs are dominated by the tetra- and pentacongeners, which is probably due to the lack of gas-phase chlorine in this case. With t-BuCl added, the PCDFs understandably are biased to the Cl7/Cl8 congeners. For the other experiments with a low PIC feed rate (including that with chlorobenzene), the PCDF/DD ratios vary little; with 15-20% PCDD, this is about a factor of 2 larger DF/DD ratio than that representative for MSWI flue gases. The chlorination patterns do differ, but not very much, and are therefore not unlike those commonly observed from MSWIs. Hagenmaier plots (Figure 3) for the 2,3,7,8-substituted congeners show that despite some differences, the patterns for heptane/t-BuCl and for the 1% PIC cocktail experiments agree fairly well with those from MSWI flue gases. Lab-Scale vs MSWI. For a full understanding of the complex chemistry, more experimentsse.g., with other components in the PIC cocktail labeledsmust be conducted, but it is worthwhile to tentatively compare data from our laboratory experiments with real-world MSWI data. In the time-resolved experiment a steady output of ∼400 ng/h of PCDFssby reaction over 0.3 g fly ash with a contact time of 0.1 s, see Figure 4shas been obtained from an input of 12 µL/h (or ca. 10 mg/h) PIC mixture. Note that the PIC level (180 mg in 18 h, 120 L) is equivalent to ∼1500 mg/Nm3. Based on the (∼0.6% w/w) phenol in the mixture this is a yield of ∼0.4%. Levels in real primary combustion offgases strongly depend on the technology and the quality of the combustion; values of 10-20 mg/Nm3 (with C1/C2 hydrocarbons excluded) (17) have been reported, but with better technology, and applying secondary combustion, levels are down to 500 µg/Nm3 (7), with typically, some 5-20 µg/Nm3 of phenols (18, 19). Despite these 2 orders of magnitude differences, the composition of PIC mixtures shows much less variation (17). Anyway, whether our dioxin yield of ∼0.4% can be extrapolated linearly down to much lower PIC levels is uncertain. The run with labeled phenolsimplying a total PIC input level of ∼15 mg/Nm3 with phenol at ∼100 µg/Nm3sis more revealing, in two ways. At least a considerable part of the total output of nearly 1400 ng must have been made from precursors. A reasonable estimate is 800 ng (in 18 h) or ca. 16 ng/h, equivalent with ∼2.2 µg/Nm3. This is nearly 0.7% 1268
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on the phenol in the feed, a value not much different from that of the “time-sliced” experiment. Whether a formation level of 0.4-0.7% PCDD/F, based on phenol, is realistic for catalytic processes in boilers, etc. will be briefly discussed below. Importantly, the direct evidence of formation of a part of the PCDD/F from labeled phenol as a precursor under conditions allowing translation to real incinerator conditionsswhereby comparable outputs of PCDD/F appear to have been formed from exhaustive de novo reactionscalls for a critical (re)evaluation of the possible importance of the latter formation pathway (20). A modern industrial boilersvery efficient in heat exchange, and therefore also in allowing intense gas/wall, solid, interactionscan be considered a very large-scale version of a configured catalyst; the numerous pipes are on the average covered with a cm-thick ash layer, which will be in intimate contact with the combustion offgas (14). With common dimensions and conditions, the temperature region relevant for fly-ash-mediated dioxin formation is passed by the offgas in no more than a few seconds. In our laboratory experiments the contact time between the offgas and the fly ash is only ∼0.1 s. The degree of conversion of the phenol has not been measured, but probably a fair part will have been (oxidatively) degraded (13). Should the contact time have been for example 1 s, linear behavior to get a 10-fold increase in dioxin output is unrealistic in view of the depletion of the phenol. Nevertheless a severalfold increase, to some 5 µg/Nm3, is a reasonable expectation. Such a value is compatible with reported emissions from old incinerators, (ca. 100 ng TEQ/Nm3) e.g., those equipped with only an ESP (21, 22). The slow de novo formation of dioxins, by passing (synthetic) air over MSWI fly ash at temperatures around 325 °C is well documented. Added vapors of benzene or chlorobenzene are still hardly reactive, but phenol (and chlorinated phenols) then reacts to a substantial degree. At the hitherto commonly used concentrations, of 10-4 to 10-6 M, phenols produce mostly PCDDssa pattern unlike those observed from incinerators, with PCDF/DD ratios of about 3-4. Our cocktail of 14 PICs to mimic the composition of the trace organics in a primary combustion offgas better approached real world conditions in the laboratory, but with phenols at an input level of ca. 10-6 M the high dioxin output (nearly all is PCDD) unrealistically overshadows any de novo contribution. After bringing the PIC input level down by a factor 500, closer to reality, the “precursor” (phenols) and de novo pathways appear to become of comparable importance; also, the PCDF/DD ratios are quite like those in practice, and do not differ much between the two contributions. The crucial role of phenols is clearly shown: reacting 13Clabeled phenol with other (all 12C) cocktail and fly ash constituents yields labeled dioxins and furans. Remarkably, most of these have only one aromatic ring stemming from the labeled phenol. Hence, the formation of PCDD/F under realistic conditions appears to be first-order in phenol. In sum, “precursor” chemistry fueled by PICs in a primary combustion offgas can explain at least a part of the formation of PCDD/Fs downstream, to begin with, in the boiler. As mentioned above, for a more detailed understanding, more experiments will be needed, e.g., by labeling of other components in the PIC mixture, or changing its composition. This will also hold for the answer to the question inasmuch as de novo pathways are, or can be, important (19, 20)swith the presence of native carbon in the ash as a prerequisite. It will be important too to design and conduct lab-scale experiments which approach real incinerator conditions as closely as possible. While our 18 h type fixed-bed experiments have been designed to pass the requisite volume of primary combustion offgas (∼120 L) over the chosen amount of fly ash (0.3 g), most of the time (∼16 h) this ash was deprived
of its native carbon with its associated chemistry and physics. Nevertheless, through sampling at various time intervals and through labeling (hitherto, only of the phenol in the PIC mixture) novel insights have been obtained, including that (real) fly ash can steadily form PCDDs and PCDFs from a realistic mixture of trace organics, albeit, as yet, at concentrations above those in practice. Finally, it is of interest to consider the result with the 100% PIC cocktailsaltogether with an input level of 1500 ppm as C6 ringsfrom an engineer’s or a regulator’s viewpoint. The ∼240 000 ng of PCDD/F from the 18 h run (in altogether nearly 120 L of air, see Table 1) means no less than 2 million ng/Nm3. And even taking 2 zeros off to approach a more common PIC level for MSWI primary flue gases, the remaining >104 ng/Nm3 is still impressive in view of regulated emission limits of 1-0.1 ng TEQ/Nm3. This clearly demonstrates the potency and activity of MSWI flyashessnotwithstanding their capacity as an oxidation catalyst (10)sto create dioxins, etc. under practical circumstances. A more general message is that catalytic oxidation processes involving chlorinated organics in which conversions are only partialsfor example with poor, or deactivating, catalysts, with a boost in input, or due to low temperatures (11)scan create high levels of TEQ rather than remove dioxins, etc..
Acknowledgments Financial support of the European Union through project ENV4-CT97-0587 (MINIDIP) is gratefully acknowledged.
Literature Cited (1) Olie, K.; Vermeulen, K. P.; Hutzinger, O. Chlorodibenzo-p-dioxins and Chlorodibenzofurans are Trace Components of Fly Ash and Flue Gas of some Municipal Incinerators in the Netherlands. Chemosphere 1977, 8, 455-459. (2) Schaub, W. M.; Tsang, W. Dioxin Formation in Incinerators. Environ. Sci. Technol. 1983, 17, 721-730. (3) Wiater, I.; Born, J. G. P.; Louw, R. Products, Rates and Mechanism in the Gas-phase Condensation of Phenoxy Radicals. Eur. J. Org. Chem. 2000, 6, 921-928. (4) Sidhu, S. S.; Maqsud, L.; Dellinger, B. The Homogeneous, Gasphase Formation of Chlorinated and Brominated Dibenzo-pdioxin from 2,4,6-trichloro- and 2,4,6-tribromophenols. Combust. Flame 1995, 100, 11-20. (5) Stieglitz, L.; Bautz, H.; Roth, W.; Zwick, G. Investigation of Precursor Reactions in the de-novo Synthesis of PCDD/PCDF on Fly Ash. Chemosphere 1997, 34, 1083-1090. (6) Addink, R.; Olie, K. Mechanisms of Formation and Destruction of Polychlorinated Dibenzo-p-dioxins and Dibenzofurans in Heterogeneous Systems. Environ. Sci. Technol. 1995, 29, 14251435. (7) Jay, K.; Stieglitz, L. Identification and Quantification of Volatile Organic Components in Emissions of Waste Incineration Plants.
Chemosphere 1995, 30, 1249. (8) Kanters, M. J. Thesis, Leiden University, Leiden, The Netherlands, 1996 (in English). (9) Addink, R.; Drijver, D. J.; Olie, K. Formation of Polychlorinated Dibenzo-p-dioxins/Dibenzofurans in the Carbon/Fly Ash System. Chemosphere 1991, 23, 1205-1211. (10) Born, J. G. P. Thesis, Leiden University, Leiden, The Netherlands, 1994 (in English). (11) De Jong, V.; Cieplik, M. K.; Louw, R. Formation of Dioxins in the Catalytic Combustion of Chlorobenzene and a Micropollutant-like Mixture on Pt/γ-Al2O3. Environ. Sci. Technol. 2004, 38, 5217-5224. (12) Wikstro¨m, E. Thesis, Umeå University, Umeå, Sweden, 1999 (in English). (13) Born, J. G. P.; Mulder, P.; Louw, R. Fly ash mediated reactions of phenol and monochlorophenols: oxychlorination, deep oxidation, and condensation. Environ. Sci. Technol. 1993, 27, 1849-1863. (14) Private communication, Dr. J. G. P. Born, N. V. Huisvuilcentrale (MSWI) - Noord Holland, Alkmaar, The Netherlands. (15) Ryan, S.; Wikstro¨m, E.; Gullett, B. K.; Touati, A. Investigation of the Pathways to PCDDs/Fs from an Ethylene Diffusion Flame: Formation from Soot and Aromatics. Organohalogen Compd. 2004, 66, 1119-1125. (16) Cieplik, M. K. Thesis, Leiden University, Leiden, The Netherlands, 2003 (in English). (17) Eduljee, G. H. In Issues in Environmental Science and Technology, Vol 2: Waste Incineration and the Environment; Hester, R. E., Harrison, R. M., Eds.; Royal Society of Chemistry, Turpin Distribution Services Ltd.: Cambridge, 1994; pp 71-95. (18) Zimmermann, R.; Blumenstock, M.; Heger, H. J.; Schramm, K.W.; Kettrup, A. Emission of Nonchlorinated and Chlorinated Aromatics in the Flue Gas of Incineration Plants during and after Transient Disturbances of Combustion Conditions: Delayed Emission Effects. Environ. Sci Technol. 2001, 35, 10191030. (19) Zhang, X. J. Emissions of Volatile Organic Compounds from Large-scale Incineration Plants. J. Environ. Sci. Health 1998, A33, 279-306. (20) Huang, H.; Buekens, A. Chemical Kinetic Modeling of de novo Synthesis of PCDD/F in Municipal Waste Incinerators. Chemosphere 2001, 44, 1505-1510. (21) Johnke, B.; Stelzner, E. Evaluation of the German Dioxin Measuring Program at Refuse Incineration Plants. Waste Manage. Res. 1992, 10, 345-355. (22) Rappe, C. In Review of Dioxin Emissions in Hong-Kong; Environmental Resources Management on behalf of Environmental Protection Department, Government of the Hong-Kong Special Administrative Region of People’s Republic of China; report CB(2)1845/99-00(03); 1999, p 18.
Received for review November 7, 2005. Accepted December 6, 2005. ES052225L
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