Environ. Sci. Technol. 2004, 38, 5217-5223
Formation of Dioxins in the Catalytic Combustion of Chlorobenzene and a Micropollutant-like Mixture on Pt/γ-Al2O3 VINCENT DE JONG,† MARIUSZ K. CIEPLIK,‡ AND ROBERT LOUW* Center for Chemistry and the Environment, Leiden Institute of Chemistry, Gorlaeus Laboratories, Leiden University, P.O. Box 9502, NL-2300 RA Leiden, The Netherlands
Catalytic combustion over a 2 wt % Pt/γ-Al2O3 catalyst of chlorobenzene (PhCl) and of a micropollutant-like mixture representative for a primary combustion offgas has been investigated. Typical conditions were 1000-1500 ppm of organics in the inflow, contact times ∼0.3 s, 16% O2 in nitrogen at ∼1 bar, and temperature range 200-550 °C. PhCl reacts considerably slower than when processing Clfree compounds such as heptane. At intermediate temperaturessand incomplete conversionsbyproducts are formed, especially polychlorobenzenes (PhClx). These are accompanied by polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs) at levels of about 10-6 relative to PhClx. Additional HClsmade by co-reacting PhCl with tert-butylchloridesleads to much higher levels of PhClx and PCDD/Fs. Using the micropollutant-like mixture, the total chlorine input is reduced almost 20-fold, but it nevertheless leads to a 30-fold higher PCDD/F output. This is ascribed to reaction of the small amounts of (chloro)phenols in the mixture. The congener/isomer patterns of the PCDD/Fs for the mixture and with PhCl per se are quite comparable with those found in emissions from incinerators. As carbon is not present nor formed on the catalyst surface, de-novo formation therefrom cannot be involved. Rather condensation of phenolic entities or like precursors must have occurred. Consequences and options to ensure safe application are briefly discussed as well.
Introduction Catalytic combustion is an attractive technology for the complete, clean conversion of hydrocarbon fuels into energy and for the purification of waste gases with low contents of organics. Supported noble metals (especially Pt) are wellsuited (1-5), but a number of oxides, mixed oxides, and perovskites are also used (6-11). When halogensespecially chlorinesis present in the feed, several problems can arise, to wit: corrosion due to the formed HCl, and in some cases formation of volatile and reactive (oxy)chlorides. For example, chromium oxychloride (CrO2Cl2) has a boiling point of only 117 °C (12) and when * Corresponding author phone: (+31)71-527-4289; fax: (+31)71-527-4451; e-mail:
[email protected]. † Present address: ALcontrol Laboratories, Steenhouwerstraat 15, NL-3194 AG Hoogvliet, The Netherlands. ‡ Present address: Energy Research Centre of the Netherlands, Wetserduinweg 3, NL 1755 LE Petten, The Netherlands. 10.1021/es034820y CCC: $27.50 Published on Web 08/24/2004
2004 American Chemical Society
a 12 wt % Cr2O3/2 wt % CrO3 catalyst was tested for the destruction of a chlorine-containing mixture, formation of volatile chromium oxychlorides was indeed observed (13). For more recent experience, see the work of Corella and co-workers (14, 15). It appears that supported noble metals, especially Pt, are much better suited for the catalytic combustion of halogen-containing (waste) gases than oxidic catalysts, certainly when halogen levels are substantial. Another feature is the possibility that Deacon-type reactions occur, which can create substantial amounts of Cl2 and hence lead to chlorinated byproducts, such as polychlorobenzenes (PhClx) from chlorobenzene (PhCl) (16) and CCl4 from CH2Cl2 (17-19). These byproducts might not be a problem during proper operation with complete mineralization of organic chlorine and full conversion of all organics into CO2, but if the feed for instance temporarily contains too little organics for the catalyst bed to maintain the desired working temperature, this can result in a drop in temperature with the formation of unwanted byproducts. Of course gradual - deactivation of the catalyst is a threat as well. Earlier studies on the combustion of PhCl over a 2 wt % Pt/γ-Al2O3 catalyst showed the formation of PhClx with levels as high as 12% of the total chlorine input at temperatures around 400 °C at conversions of PhCl of around 90% (16, 20). For Pd on several supports, the production of PhClx was found to be even higher (21). Varying the support, lowering the Pt dispersion, and adding H2O during the combustion of PhCl helped to suppress, but could not eliminate, the formation of PhClx (21). A better “therapy” is to add a fair excess of, for example, heptane to PhCl; rates of combustion of PhCl are much larger, and the amount of PhClx is down to trace levels only (1). A key feature appears to be that ‘poisoning’ of the catalyst surface by chlorine is counteracted by its removals due to water or due to the hydrogen of the alkane (1, 22, 23). When PhClx are formed it is possible, if not likely, that also higher-chlorinated polyaromaticsssuch as polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs)sare produced. These “dioxins”, infamous for their toxicity, are highly undesired; therefore, we thought it of prime importance to check the performance of a Pt/γalumina (γ-Al2O3) catalyst also in this context. In practice the aim is of course to achieve “complete” destruction of the target compound(s)sand of possible intermediate or side productssunder acceptable conditions, for instance at temperatures which the catalyst can tolerate. However, to get an insight into the underlying chemistrysand in the consequences of a malfunctioning catalystsreactions must be conducted (also) at incomplete conversions, which we have done. First, PhCl has been studied; as such, and next with tertbutyl chloride (t-BuCl, 1 mol/mol PhCl) as an additional chlorine source. The reason to add t-BuClsfound to be very rapidly combusted even at the lowest temperatures studied - is to obtain a level of HCl common for off-gases of thermal (waste) combustion facilities, and see its effect on the behavior of model compound PhCl. Next, the behavior of a complex, micropollutant-like mixture has been investigated. This is meant to mimic catalytic combustion of PICs, products of incomplete combustion, in said off-gases. The composition of this mixture is based on the analysis of a common municipal solid waste (MSW) facility off-gas (24). By using a micro-dosage pump, the PIC concentrations in the feed gas to the catalyst bed (16% O2 in nitrogen) could be very low, at even (sub) ppm levels, to approach real conditions also in this respect. Sampling on polyurethane foam (PUF) plugs for typically 18 h allowed proper PCDD/F analysis. VOL. 38, NO. 19, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Schematic overview of the on-line setup.
Experimental Section On-Line Measurements. The experiments were performed using an on-line setup with a sampling unitsa polyurethane foam (PUF) plug adsorbersand the feeding of a liquid input mixture (PhCl and/or other organics), as depicted in Figure 1. The latter was done by means of a microinjection pump (CML, Microdialysis 100) connected by a thin capillary (0.28 mm) to a glass mixing chamber of 1 mL capacity, and filled with quartz wool. The injected liquid (typical inflow of 0.225 µL min-1) was evaporated in this heated (T ∼300 °C) glass chamber, and transported to the reactor by the carrier gas, metered by two electronic mass flow controllers (Brooks 5850 TR), one for N2 and one for O2. The reactor was a vertical quartz tube (internal diameter 5 mm) with a quartz filter on which the catalyst bed rested. For each experiment 0.3 g of catalyst was used, with a bed height of ca. 10 mm. The outflowing gases were directed by heated stainless steel tubing (T > 150 °C) to a sampling loop by means of two six-port sampling valves (Valco C6WT-HC) for on-line analysis on a Hewlett-Packard 5890 series II gas chromatograph, utilizing a capillary (CP SIL 5 CB, 50 m, × 0.32 mm, film thickness 0.4 µm) and a packed column (Carbosphere 80100 mesh). The capillary column was at the end split into two streams, which were connected to a flame ionization detector (FID) and an electron capture detector (ECD). The FID was used for on-line quantification of hydrocarbons etc. and the ECD was used for (poly)chlorinated organics (up to PhCl6). CO and CO2 were quantified by injection on a packed column followed by hydrogenation to CH4 in a methanizer; the resulting mixture was then analyzed on a FID. At all relevant temperatures at least a duplicate on-line analysis was performed. When the reactor reached the set temperature the first injection on the GC was after 15 min in order to guarantee a stable situation. Off-line PCDD/Fs measurements were performed after Soxhlet extraction of said PUF plugs with 110 mL of a 10/1 heptane/acetone mixture for approximately 18 h, with 20 turnovers h-1. The resulting extract was subsequently 5218
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concentrated to 0.5 mL - first with a rotary evaporator and next with a Zymark Turbovap II apparatus, and sent for external HRGC/HRMS analysis. Catalyst. A 2 wt % Pt/γ-Al2O3 catalyst was prepared following the homogeneous deposition precipitation method as described by Geus (25). For 10 g material, 9.80 g γ-Al2O3 (Degussa C) was suspended in 250 mL water; 0.308 g H2[Pt(OH)6] (Johnson Matthey, 62.5% Pt) was dissolved in 3 mL hot concentrated nitric acid and added to the suspension under continuous stirring. Subsequently a 10-g portion of urea was added while increasing the temperature to ca. 80 °C and stirring for 15 h. Due to slow decomposition of urea the pH increased from 2 to 8, causing the Pt precursor to precipitate on the γ-Al2O3 support. The suspension was filtered off, washed with demineralized water, and dried overnight at 90 °C. Finally the catalyst was calcined for 3 h in flowing air at 600 °C. After crushing a sieved fraction of 150-300 µm was collected and used. Chemicals. The following chemicals were available and used as such: benzene (Merck Uvasol, >99%), acetone (Baker, >99.5%), naphthalene (Janssen Chimica, >99%) chlorobenzene (Baker Analyzed, >99%), phenol (Aldrich, >99%), phenanthrene (synthesized, >99%), 1,1,1-trichloroethane (Janssen Chimica, >97%), benzaldehyde (Baker Analyzed, >99%), dibenzofuran (Fluka, >95%), monobenzofuran (synthesized, >99%), toluene (Baker Analyzed, >99%), 2-chlorophenol (Fluka, >99%), 2,4,6-trichlorophenol (Fluka, >99%), tetrachloroethylene (Baker Analyzed, >99%), hydrogen (Air Products, 99.995%), methane (Air Products, 99.995%), nitrogen (Air Products 99.995%). The composition of the model mixture of products of incomplete combustion (PIC) is given in Table 1
Results Byproduct Formation during Combustion of PhCl on Pt/ γ-Al2O3. To get an insight in the formation of byproducts during the combustion of PhCl on 2 wt % Pt/γ-Al2O3, a series of experiments was performed between 150 and 550 °C. Figure
TABLE 1. Composition of the PIC-like Feed Mixturea compound
ratiob
concnc [mg m-3]
benzene naphthalene phenanthrene acetone 1,1,1-trichloroethane benzaldehyde dibenzofuran monobenzofuran toluene phenol 2-chlorophenol 2,4,6-trichlorophenol chlorobenzene tetrachloroethylene
(-) 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
7640 4179 1743 568 652 519 329 116 90 92 50 39 44 162
a Inflows: liquid mixture 0.225 µL min-1, gases N + O ) 0.27 mol 2 2 h-1, with 16% O2, total Cl for 18 h ) 2.4 µmol. b Relative ratio to benzene c (mol/mol). At room temperature and 1 bar, with the mixing ratio (see footnote a).
2 depicts conversions of PhCl and total PhClx productions at various temperatures. Consonant with the earlier observations of Van den Brink et al. (16), the catalytic oxidation of PhCl is indeed a sluggish and complex process especially at intermediate conversions. Increasing the reaction temperature results in higher combustion rates for PhCl, but full conversion is reached only at temperatures of 500 °C and above. The selectivity to PhClx strongly depends on the temperature. Already at ca. 250 °C higher-chlorinated products from PhCl could be detected. A highest selectivity of 7% (yield on converted PhCl, sum of (poly)chlorinated benzenes) is observed at around 400 °C, which is comparable to previous results with this catalyst (16, 20). The isomer ratio of p-, m-, and o-PhCl2 was remarkably constant at 4:3:1 at all temperatures. To determine the PCDD/Fs formed and emitted, an experiment was performed at a set temperature of 350 °C. Over a period of 18 h heavier organic byproducts were collected on a PUF-plug. Results on (tetra- and higher chlorinated) PCDD/F isomer groups are collected in Table
2, entry 1. The total of ∼0.13 nmol compared with the output of PhClx at 350 °C (∼2.5% or 0.11 mmol, cf. Figure 2) comprises a molar ratio of 1.2 × 10-6. The (overall) PCDF/PCDD ratio is close to 2; interestingly, this is not unlike the pattern commonly found for dioxins from (municipal waste) incineration (Table 2, entry 2). Also, the degrees of (poly) chlorination do not differ much. Combustion of a t-BuCl/PhCl Mixture. The additional chlorine source was applied to see if it has an impact on the byproduct formation. An input mixture of 750 ppm PhCl and 750 ppm t-BuCl was used, all other flows were kept the same. Exploratory experiments have shown that t-BuCl gave a quantitative yield of HCl and COx at even mild conditions. The conversion data and the PhClx production at various temperatures are plotted in Figure 3. The lowest applied temperature for this series was 350 °C, focusing on the area with high PhClx outputs as in Figure 2. The output of PhClx is clearly higher than that from PhCl per se (Figure 2). Now the highest selectivity to PhClx is observed at 419 °C and found to be 41%. For PhCl per se, the highest selectivity to PhClx (ca. 7% see Figure 1) was reached at 397 °C. So, the conversion of PhCl may be a little slower in the presence of t-BuCl, but the formation of (oxy)chlorinated byproducts is greatly enhancedsand about equally important as oxidative degradation. Next the influence of t-BuCl on the formation of PCDD/ Fs was investigated. A temperature of 360 °C was used while the byproducts were again collected for an 18 h period. Data are in Table 2, entry 3. With a 25% lower PhCl level in the feed, the total dioxin output is increased from ∼0.13 to ∼0.33 nmol, while the PCDF/PCDD ratio is up from 1.6 to ∼3.0. The average degree of (poly)chlorination is quite the same for the PCDFs but markedly larger for the PCDDssfrom ∼4.9 in entry 1 to ∼6.6 in the present case (entry 3). Altogether, the selectivity to PCDD/F remains relatively low, with an output of PhClx at 360 °C of 1.3 mmol, the 0.33 nmol dioxins are a factor 0.3 × 10-6 less. Micropollutant-like Mixture. To mimic the out-flowing gas of a MSW incinerator, a mixture has been composed based on the data of Jay and Stieglitz (24), see Table 1. For practical reasons, a selection of substances has been made
FIGURE 2. PhClx production during combustion of PhCl on 2 wt % Pt/γ-Al2O3. PhCl conversion (+), total output of PhClx (9), PhCl2 (b), PhCl3 ([), PhCl4 (2), PhCl5-6 (×). Inflows: PhCl 1000 ppm, O2 + N2 0.25 mol h-1; τvoid ∼ 0.33 s at STP; [O2] ) 16%. VOL. 38, NO. 19, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 2. PCDD/F (tetra through octa) Production during Combustion of Various Organics on 2 wt % Pt/γ-Al2O3a PhClb (1)
T4CDF P5CDF H6CDF H7CDF O8CDF T4CDD P5CDD H6CDD H7CDD O8CDD sum PCDF sum PCDD
flue gasc (2)
PhCl/t-BuCld (3)
PIC mixturee (4)
PCDD/F [ng]
PCDD/F [10-9mol]
PCDD/F
PCDD/F [ng]
PCDD/F [10-9mol]
PCDD/F [ng]
PCDD/F [10-9mol]
2.8 (6) 5.3 (11) 9.4 (20) 5.7 (12) 7.2 (15) 8.9 (19) 4.3 (9) 2.0 (4) 1.3 (3) 1.4 (3) 29 (64) 18 (36)
0.009 0.016 0.025 0.014 0.016 0.025 0.012 0.005 0.003 0.003 0.079 0.049
(25) (17) (12) (8) (4) (4) (6) (9) (9) (9) (64) (36)
10.2 (8) 20.0 (16) 20.8 (16) 20.6 (16) 22.5 (18) 1.5 (1) 4.6 (4) 6.9 (5) 9.1 (7) 11.4 (9) 94 (74) 34 (26)
0.033 0.058 0.055 0.050 0.051 0.005 0.013 0.018 0.021 0.025 0.247 0.081
324 (27) 261 (22) 138 (11) 84 (7) 24 (2) 115 (10) 129 (11) 84 (7) 36 (3) 15 (1) 831 (70) 379 (30)
1.1 0.76 0.37 0.20 0.05 0.37 0.38 0.22 0.09 0.03 2.4 1.1
a Total PCDD/F production in 18 h; conditions, see Figure 2; values in parentheses are wt % of total PCDD/Fs. b PhCl (1000 ppm) ) 4.5 mmol in 18 h, at 350 °C. c Data from ref 26. d PhCl (750 ppm)/t-BuCl (750 ppm) ) 6.75 mmol of Cl in 18 h, at 360 °C. e PIC mixture ) 2 µmol of Cl in 18 h, at 350 °C.
FIGURE 3. Combustion of PhCl with t-BuCl on 2 wt % Pt/γ-Al2O3: PhCl conversion (2), total PhClx (9), PhCl2 (b), PhCl3 ([), PhCl4 (2), PhCl5-6 (×). Inflows: PhCl 750 ppm, t-BuCl 750 ppm, O2 + N2 0.25 mol h-1; τvoid ∼0.1 s at 250 °C. since over 200 different compounds were identified. Those chosen represent all important categories of PICs with the exception of S-containing derivatives. By applying a low inflow rate, concentrations have been obtained that in total are ∼1500 ppm C again. However, these are about 2 orders of magnitude larger than those of the PICs in real combustion off-gases. Due to the complex mixture and the low level of most of the individual compounds, we have refrained from analyzing the conversions of the individual compounds and/ or identifying any intermediate (Cl-free) products. The combustion could however be followed through the total COx production. COx was already detected at 200 °C and reached 90% at 400 °C. By on-line ECD analysis some (poly)chlorinated products could be seen but also due to the low concentrations it was not possible to identify any of these. On the basis of retention times, it appears that both chlorinated aliphatics/olefins and traces of PhClx were present in the effluent gas. To quantify PCDD/Fs formed with this realistic inflow mixture, an 18 h run was performed with the reactor temperature set at 350 °C. This implies a total conversion of 5220
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around 60%, based on COx production. (Semi)volatile organics were collected on a PUF plug kept at -25 °C. Data are collected in Table 2, entry 4. Compared with the results from PhCl per se (entry 1), levels of PCDD/Fs have greatly increased from 0.13 to 3.5 nmol, notwithstanding that the concentrations of key aromatics in the feed are much lower than in the model experiments: benzene ∼220 ppm, and phenol only 2 ppm (∼10-6 M). Note also that the PCDF:PCDD ratio again is close to 2 (Table 1, entry 4). Dioxin and Furan Patterns. Figure 4 depicts relative amounts of the tetra- to octa-isomers together with data representative for incinerator flue gases. It underlines that the PCDD/F ratios (here as “% PCDDs”) are strikingly similar despite the large variation in type and/or concentration of the organics employed. The variations in chlorination patterns, degrees, are limited too despite the large differences in chlorine (input) levels. Further insight into conformity or difference in patterns can be obtained by considering the distributions within homologuessespecially those with a sizable toxic equivalence
FIGURE 4. Homologue distributions, data for flue gas samples from ref 26.
FIGURE 5. Relative abundances of 2,3,7,8-substituted PCDD/Fs; data for MSW incinerator from ref 28. factor (TEF) (27). Data are shown in Figure 5. While part of the differences may be caused by the different levels of chlorination, it is worth noting that some isomers are at a rather constant level/ratio, such as the (relative toxic) 2,3,4,7,8-P5CDF, whereas especially 2,3,7,8-TCDD and 1,2,3,7,8-P5CDD show important variations (see Figure 5) and are rather prominent in the reaction with PhCl per se.
Discussion Catalytic Combustion of Chlorobenzene. The model reactions with PhCl underscore that Pt is suited for the combustion of organochlorine compounds, but with care. The oxidative degradation is relatively slow, and substantial levels of higher chlorinated benzenes PhClx are formed in a parallel process. The isomer distribution of the dichlorobenzenes (with some 40% meta) precludes a common electrophilic substitution mechanism. The chemistry is best explained by combination of surface-bound Cl and an (o-, m-, or p-)chlorophenyl entity (in a reductive elimination reaction);
the latter species result from chemisorption of PhCl by C-H bond fission and are subject to oxidation as the major pathway (16, 20). Chlorine at the surface also acts as a poison for the overall conversion, which can be relieved by coreacting suitable hydrocarbons (1, 22). Dioxins (PCDD/Fs) are also clearly formed; under conditions of partial conversion of the ∼1000 ppm of PhCl at levels much higher than those in flue gases of classical MSW incinerators.This is further discussed below. Added tert-butylchloride is very rapidly converted to give ∼100% HCl; discrete reaction to (sorbed, or even free) isobutene, by catalyzed HCl elimination is likely. Catalytic combustion of isobutene is expectedly very rapid (22). Anyway, organic intermediate products could not be detected. It firmly promotes the formation of PhClx, however, meaning that its hydrogen is of no value for removal of Cl from the surface and, hence, suppression of chlorination. It appears that a hydrocarbon co-fuel must have a reactivity comparable to that of PhCl to be effective (1). Altogether, VOL. 38, NO. 19, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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t-BuCl has acted as a “perfect” additional source for HCl. It led to nearly thrice as much PCDD/F compared with PhCl per se under like conditions, notwithstanding a ∼25% lower PhCl input level. Apparently, the population on the catalyst surface alters in a way that there is more Cl (lowering the overall rate of conversion of PhCl) that accelerates chlorination but also leads to more PCDD/F (see below). Micropollutant Mixture. Refraining from a discussion of the fate of individual constituents, we may note that the cocktail contained little aliphatics which could have “helped” in keeping the catalyst clean and highly active. Given that, let us focus at the PCDD/Fs observed. With an input level of the major constituent benzene ∼5 times lower than that of PhCl, the dioxin output nevertheless increased ∼25-fold. This is notwithstanding the much lower chlorine level: in a standard PhCl experiment the total input in a 18 h run was ∼ 4.5 mequiv of Cl and in the runs with the cocktail only about 0.24 mequiv. Not surprising, the chlorination pattern is somewhat biased to the lower chlorinated congeners, but the average degree of chlorination of the PCDFs is still close to 5.0. Pathways to PCDD/F. Without neglecting some substantial differences in the congener/isomer compositions, the outputs from PhCl and the PIC cocktail are quite comparable, also with the PCDD/F profiles common for MSW facilities, where catalysis by fly ash is important. In the latter case, catalysis cannot be due to Ptsor any other noble metals but is based on copper (and maybe iron also) (29). Nevertheless, the similarities suggest common underlying mechanisms. In contrast with fly ash, our catalyst does not contain nor forms any carbon, not so strange under the very “lean” conditions employed. Therefore, de novo formation (30)s common term for making dioxins from macromolecular structures such as carbon and sootsdoes not have to be taken into account. Instead, “precursor” pathways must be involved. Consider the result with the PIC cocktail (Table 1, entry 4). The output of 3.5 nmol means nearly 0.02 mol % based on the total input of dibenzofuran, of ∼20 µmol in 18 h. Repeated chlorination of dibenzofuran may have contributed to at least the PCDF production. More likely candidates are the “trace” amounts of phenols in the cocktail (31-33) expressed on the intake of phenol itself the PCDD/F outputsunder the used conditionssis 0.07%. In the reactions with PhCl per se, chlorinated phenoxy entitiessnot necessarily free chlorinated phenolssare likely intermediates. That PCDD/F patterns (from precursors) are a function of catalyst and conditions, and pertinent mechanistic aspects will be discussed separately (34). Which compounds in a realistic cocktail are really involved in dioxin formation is worth further study (35)snot only for oxidation on well-defined catalysts but also in fly ash mediated processes in combustors. Noteworthy is that the PIC cocktail, in reaction over MSW incinerator fly ash, also leads to a “common” PCDD/F product with a DF:DD ratio of ∼4 (33, 35, 36). Performance and Safety. As mentioned in the Introduction, in practice, a successful catalytic combustion should be complete and durable, the more so if chlorine is involved. When processing “recalcitrant” chlorinated aromatics, cofeeding of a proper alkane (such as heptane with PhCl) is highly beneficial, as it increases rates for chlorobenzene(s) and suppresses (oxy)chlorinationsand almost certainly also dioxin formationsas side reactions. A safe margin in working temperature is of course also advisable. For a safe application of (in this case, supported Pt) in the destruction ofseven, dioxin-containingswaste gases, this approach should be further tested, also including the effect of increased HCl levels, and checked for compliance with regulation. Reactions conducted at incomplete conversion also serve as a model for malfunctioning by deactivation of the catalyst or by temperatures lower than required. Although primarily 5222
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TABLE 3. Toxicity Levels TEQ [ng Nm-3] PhCl PhCl + t-BuCl PIC mixture
93a 31 571
a For PhCl, the main components that contribute to the total TEQ are (with I-TEF factors in parentheses): 2,3,7,8-T4CDD (1)*71.5 ) 71.5 ng Nm-3; 1,2,3,7,8-P5CDD (0.5)*31.6 ) 15.8 ng Nm-3; 1,2,3,4,7,8-H6CDF (0.1)*52 ) 5.2 ng Nm-3; other components give a neglible contribution.
meant for levels in food, etc., it is worthwhile to express the outputs of PCDD/Fsat the conditions mentioned in Table 1sas ng I-TEQ/Nm3 (Table 3). These can be compared with values, common for “raw” MSW incinerator combustion offgases, viz., ∼0.1-0.75 µg m-3 (36), for “full” toxic 17 PCDD/ Fs; when expressed in TEQ, these numbers will be about a factor 50 less or 2-15 ng Nm-3, which is still much higher than an aimed or regulated emission level of
