Environ. Sci. Technol. 2002, 36, 2760-2765
Amines Compounds as Inhibitors of PCDD/Fs De Novo Formation on Sintering Process Fly Ash C EÄ L I N E X H R O U E T , * CAROLINE NADIN, AND EDWIN DE PAUW University of Lie`ge, Mass Spectrometry Laboratory, B6c Sart Tilman, B-4000 Lie`ge, Belgium
The polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) are highly toxic compounds produced by some natural processes and different human activities. Waste incineration as well as steel and iron industries, in particular, the sintering process, are among the principal sources of these pollutants. In this paper, two inhibitors, triethanolamine (TEA) and monoethanolamine (MEA), are tested relating to their ability to prevent the de novo formation of PCDD/Fs on sinter plant fly ash. The amounts of both PCDDs and PCDFs, formed by thermal treatment of the fly ash, decrease when inhibitors are added. The best results, up to 90% reduction of the PCDD/ Fs formation, are obtained when MEA is mixed with the fly ash at the highest concentration tested (2 wt %). The addition of inhibitors modifies the PCDFs/PCDDs ratios and, under some experimental conditions, the PCDD/Fs homologue distributions, suggesting that more than one pathway for the de novo formation of PCDD/Fs exist. On the other hand, no modification in the PCDD/Fs isomer distributions is observed as a result of the addition of inhibitors, in accordance with the possible thermodynamic control of these distributions. The temperature tested, 325 and 400 °C, does not affect the inhibition activity; however, longer reaction times (4 h instead of 2 h) give better percentages of PCDD/Fs reduction. The results suggest that the two inhibitors and especially MEA can reduce the PCDD/Fs formation on sinter plant fly ash under various conditions of temperature and reaction time, making them suitable for use in the real process. Tests performed in parallel at a real sinter plant are in good agreement with the laboratory experiments and confirm that the use of inhibitors is an appropriate technique for the prevention of PCDD/Fs emissions from sintering processes.
Introduction Since the discovery of polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) in the flue gas and fly ash of municipal waste incinerators (MWI) by Olie and co-workers in 1977 (1), the formation of these toxic compounds has been studied intensively. Recent reviews summarized the most important trends and results (2, 3). Two mechanisms can be found in the literature to describe the formation of PCDD/Fs in the incinerators or other combustion processes: a formation from precursors and the * Corresponding author phone: +32-4-366-34-22; fax: +32-4-36634-13; e-mail:
[email protected]. 2760
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so-called de novo synthesis. The first pathway involves reactions of chemical similar compounds to PCDD/Fs, such as chlorophenols, formed initially as products of incomplete combustion. The de novo synthesis consists of PCDD/Fs formation from macromolecular carbon present in the fly ash. Some constituents of the fly ash such as copper and iron chlorides have been shown to catalyze these heterogeneous reactions between the gas phase and the fly ash. Many authors postulate that this synthesis could take place essentially in the post combustion zone of the incinerators at a temperature around 300 °C (4). The de novo formation of PCDD/Fs is supposed to be strongly correlated with the metal-catalyzed oxidation of carbon in the fly ash (5). The oxidative degradation of the carbon structure gives mainly gaseous products CO2 and CO as well as, in a minor pathway, some small aromatic compounds including PCDD/Fs. The laboratory studies concerning the formation of PCDD/ Fs use only MWI fly ash (6-16) or model mixtures (6, 13, 17, 18). However, beside the waste incinerators, the iron and steel industries are known to be important sources of PCDD/ Fs in different countries (19, 20). Considering the very large gas flow volumes discharged from these industrial processes, the contribution of the dioxin pollution by these sources is important. In Belgium, in 1995, it was estimated that 34% of the total dioxin emissions came from the industrial sector and among the different industries, the sinter plant was responsible for 24% of the dioxin emissions (21). Despite this fact, PCDD/Fs studies on sinter plants are quite sparse (22-29). PCDD/Fs emissions can, of course, be controlled by means of flue gas cleaning systems which remove the PCDD/Fs present in the gas or by the way of the inhibition technique which tries to avoid or reduce the formation of PCDD/Fs in the process. A recent review presents a comparative evaluation of techniques for controlling the formation and emission of PCDD/Fs in MWI (30). Although PCDD/Fs formation in combustion processes is being studied widely, studies on inhibition are quite sparse, especially on sinter plants. Various inhibitors have been tested both in laboratory and in pilot plants to reduce the PCDD/Fs formation relating to incineration processes. Different studies reveal the inhibition ability of some basic compounds such as NH3 (31, 32), CaO (33), and NaOH and KOH (34). Different sulfur compounds were also investigated. Na2S and Na2S2O3 are able to reduce PCDDs formation on MWI fly ash (35, 36). SO2 was also largely investigated (32, 37-40). The presence of SO2 seems to deplete the Cl2 levels through the gas-phase reaction
Cl2 + SO2 + H2O T 2HCl + SO3 thereby converting a major chlorinating agent Cl2 in a less active form HCl (39). On another hand, the presence of SO2 could also affect the PCDD/Fs biaryl synthesis by poisoning the catalyst (40). Based on the concept of “poisoning” the catalyst, another group of compounds is used as inhibitors for the PCDD/Fs formation: compounds able to form complex with the catalyst at low temperature, which can lead to covalent bonds and therefore poisoning upon heating. Among compounds well-known to complex Cu2+, functionalized amines (ethanolamine and triethanolamine) are very effective inhibitors (34, 41). Concerning the mechanism of inhibition, an interaction between the catalyst and the inhibitor is supposed. Lippert et al. have observed the existence of a Cu-N bond during the reaction of bromobenzene on alumina with Cu as catalyst and ethanolamine as an 10.1021/es015776u CCC: $22.00
2002 American Chemical Society Published on Web 05/02/2002
TABLE 1. Elemental Analysis of the Original Fly Ash fly ash composition (in wt %) Mg
Al
Si
P
S
Cl
K
Ca
1.04
2.17
3.62
0.24
4.07
9.55
9.07
7.83
Cr
Mn
Fe
Cu
Zn
Pb
C
0.04
0.31
49.90
0.17
0.34
5.98
3.34
inhibitor (200 °C) (42). Azides formation takes place already at 200 °C. Such compounds are known to be intermediates in the formation of copper nitrides, stable at higher temperature. Urea has also been examined in this way by several research teams as a potential PCDD/Fs inhibitor (41, 4651). In the present paper, we report on a series of inhibition experiments carried out with sinter plant fly ash. We have showed in a previous study (51, 52) that this fly ash is very active in de novo formation of PCDD/Fs, and we investigate here the possibility to prevent this formation by using inhibitors. Two different inhibitors are examined: triethanolamine (TEA) and monoethanolamine (MEA). These two compounds were chosen for the following reasons. In a preliminary study with sintering process fly ash (51), TEA was shown to give better results than triethylamine and urea. On the other hand, MEA shows similar inhibition activity than TEA on incinerator fly ash and both are shown to be very effective inhibitors in the literature (34, 41). Different parameters such as the amount of inhibitors, the temperature, and the reaction time are investigated. The different experiments are performed at two temperatures (325 and 400 °C): the two optimal temperatures found for the de novo synthesis of PCDD/Fs on this sinter plant fly ash (see ref 52). The homologue and full isomer distributions are also examined to get a better understanding of the inhibition mechanism. Preliminary results of our investigation have been published before (53).
Experimental Section Materials. The following materials were used: solution of 2,3,7,8-Cl-substituted 13C12-labeled PCDD/Fs (EPA 1613 LCS, Campro Scientific, Veenendaal, The Netherlands); toluene (p.a., Baker); hexane (p.a., Baker); dichloromethane (p.a., Vel); dodecane (Merck); sulfuric acid (95-97%, Baker); sodium chloride (p.a., Merck); potassium hydroxide (p.a., Merck); sodium sulfate anhydrous (Baker); aluminum oxide (activated, neutral, type 507c, Aldrich); glass wool (DMCS treated, Alltech Europe); technical dry air (Air Liquide, Belgium), triethanolamine (97%, Acros), and monoethanolamine (99%, Acros). Fly Ash. Fly ash, described in Table 1, was collected in the electrostatic precipitator of a Belgian sintering plant. This electrofilter consists of three fields and is operating at 120130 °C. The fly ash used in this study comes from field 3 and was stored at ambient temperature prior to lab experiments. Around 72.5 wt % of the fly ash has a size under 40 µm. All experiments were conducted with extracted fly ash in order to minimize potential interferences from adsorbed organic precursors and native PCDD/Fs. Prior to experiments, all fly ashes were Soxhlet extracted with toluene (2 × 24 h), rinsed with hexane, and air-dried at room temperature. This fly ash, cleared of natives PCDD/Fs, is called “extracted fly ash” or “fly ash” in the rest of the paper as opposed to the “original fly ash”, which refers to the fly ash coming directly from the sintering plant without any pretreatment and containing the native PCDD/Fs. Only trace amounts of PCDD/Fs were found in the extracted fly ash.
Inhibitors. Two different inhibitors have been tested in this study: triethanolamine (TEA) and monoethanolamine (MEA). Extracted fly ash was mixed with 0.5, 1, or 2 wt % of inhibitors. The inhibitor was preliminary dissolved in methanol and then mixed with the fly ash, and the methanol evaporated. Experimental Apparatus. Five grams of sample (fly ash or fly ash + inhibitor) was packed into a horizontal glass tube reactor (16 cm long, 3 cm diameter) with glass wool as plugs. The tube was placed in a chromatographic furnace, and the samples were heated under a flow of technical air (100 mL/min). Three kinds of experiments were performed: 325 °C during 2 and 4 h, 400 °C during 2 h. Products evaporating from the fly ash were collected using two coldtraps in series (100 mL of toluene cooled with ice). Each experiment was performed in duplicate or triplicate. Cleanup. The slightly modified EPA-8280 was followed for classical PCDD/Fs analysis. Detailed method has been described before (52). Analysis. All analyses were performed by HRGC/HRMS using Mat95-XL high-resolution mass spectrometer and Hewlett-Packard 6890 Series gas chromatograph. The GC conditions were optimized to separate most of the PCDD/Fs as followed: column, SP2331 capillary column (Supelco, 60 m × 0.25 mm i.d., 0.2 µm film thickness); splitless injection of 2 µL of the extract at 275 °C; initial oven temperature, 150 °C; temperature programming, 150 °C, held for 1 min, then increased at 15 °C/min to 200 °C, then increased at 1.2 °C/ min to 273 °C, and held during 18 min. Helium was used as carrier gas. The mass spectrometer was operated in the electron impact ionization mode using selected ion monitoring. The mass spectrometer was tuned to a minimum resolution of 10 000 (10% valley) and was operated in a mass drift correction mode using FC5311 to provide lock masses. The two most abundant ions in the chlorine clusters of the molecular ion were recorded for each congener of native and labeled PCDD/Fs. The source temperature was set to 270 °C. Identification and Quantification. The TCDD-OCDD and TCDF-HpCDF congeners were analyzed. No analyses of the species without chlorine or less than four chlorines were performed. Native concentration was determined by isotopic dilution using the 2,3,7,8-Cl-substituted labeled PCDD/Fs to quantify all the native isomers within homologues assuming equal response for all isomers within an isomer group and no isomer-selective losses during the cleanup. Since 13COCDF was not present in the solution standard, this congener was not quantified. The different congeners were identified according to ref 54.
Results and Discussion Two inhibitors (TEA and MEA) were tested to study their effect on the catalytic activity of the sinter plant fly ash toward the prevention of PCDD/Fs formation by de novo synthesis. The inhibition tests were compared to reference experiments, which consist of thermal experiments performed without inhibitors mixed with the fly ash. Percentages of inhibition were calculated from the reference tests. The entirety of the different experiments performed and the results obtained are summarized in Table 2. Figure 1 shows a part of the results (325 °C and 2 h). The margins of errors are generally acceptable except for one test run (325 °C, 4 h, 0.5%TEA). This result has, for all that, been included in the table. However, the reader has to be well aware of the margin of error reported. The results indicate that a clear reduction in both PCDDs and PCDFs concentrations occurred when inhibitors are used. Depending on the temperature and the reaction time investigated, the global inhibition yields are up to 90%. The results obtained are generally better with MEA than with VOL. 36, NO. 12, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 2. Summary of the Experimental Conditions and Results test run
∑PCDDa
∑PCDFa
∑PCDF/ΣPCDDb
325 °C, 2 h, no inhibitor 325 °C, 2 h, TEA 0.5% 325 °C, 2 h, TEA 1% 325 °C, 2 h, TEA 2% 325 °C, 2 h, MEA 0.5% 325 °C, 2 h, MEA 1% 325 °C, 2 h, MEA 2% 325 °C, 4 h, no inhibitor 325 °C, 4 h, TEA 0.5% 325 °C, 4 h, TEA 1% 325 °C, 4 h, MEA 0.5% 325 °C, 4 h, MEA 1% 325 °C, 4 h, MEA 2% 400 °C, 2 h, no inhibitor 400 °C, 2 h, TEA 0.5% 400 °C, 2 h, TEA 1% 400 °C, 2 h, MEA 1%
175 ( 24 95 ( 25 108 ( 18 25 ( 3 154 ( 7 64 ( 11 44 ( 12 320 ( 44 209 ( 105 194 ( 15 78 ( 1 79 ( 7 48 ( 14 130 ( 16 59 ( 11 63 ( 5 18 ( 8
2185 ( 431 1918 ( 119 2413 ( 108 485 ( 68 3406 ( 2 1817 ( 214 775 ( 387 4667 ( 347 2757 ( 2024 3551 ( 184 623 ( 181 481 ( 38 459 ( 4 1552 ( 384 1384 ( 157 1500 ( 39 340 ( 167
12.4 ( 0.4 21.9 ( 3.5 23.3 ( 2.4 19.6 ( 0.4 22.2 ( 0.5 28.8 ( 0.8 16.4 ( 2.2 14.7 ( 0.5 12.2 ( 2.0 18.4 ( 0.8 8.1 ( 1.2 6.2 ( 0.5 10.6 ( 1.6 15.1 ( c 23.4 ( 0.7 23.9 ( 0.7 18.9 ( 0.7
a
Concentrations in ng/g, mean value ( range.
b
% PCDD inhibitiond
% PCDF inhibitiond
45 ( 16 38 ( 13 86 ( 2 12 ( 13 63 ( 8 75 ( 8
12 ( 18 -11 ( 22 78 ( 5 -56 ( 31 17 ( 19 65 ( 19
35 ( 34 40 ( 10 76 ( 3 75 ( 4 85 ( 5
41 ( 44 24 ( 7 87 ( 4 90 ( 1 90 ( 1
43 ( 14 39 ( 11 83 ( 8
11 ( 18 3 ( 16 78 ( 11
Calculated for each experiment; mean value ( range is given. c Less than 0.05.
d
Relative to
the experiments without inhibitor under identical conditions using A ( a/B ( b ) A/B ( x1/B2a2+A2/B4b2, for example: PCDDs 325 °C, 2 h, TEA 2%: (25 ( 3)/(175 ( 24) ) 14 ( 2%, corresponding to an inhibition yield of 86 ( 2%.
FIGURE 2. Inhibition yields for the PCDDs as a function of the temperature, reaction time 2 h.
FIGURE 1. Total amounts of PCDDs (A) and PCDFs (B) obtained in the different tests (temperature 325 °C, reaction time 2 h). Mean value ( range. TEA and with the biggest amount of inhibitors studied (2 wt %). The inhibition test run at 325 °C, 2 h, with 0.5% MEA, gives in reality an increase in PCDFs concentrations (inhibition yield: -56 ( 31%). The margin of error relating to the PCDFs amount is however very small, only (2 ng/g, corresponding to a relative error of 0.06%. This error is not realistic since 10-20% of error are generally accepted for the only measure of PCDD/Fs in samples. The results indicate that MEA is a more effective inhibitor than TEA. A possible explanation could be the difference between the stability of the MEA and TEA complexes with copper, a well-known active catalyst for the PCDD/Fs formation. The literature data concerning the stability and the thermal behavior of those complexes are quite sparse. Casassas et al. (55) have shown that, in solution, ethanolamines act as bidentate ligands through the amino and hydroxyl groups. MEA forms complexes by addition of two molecules of the ligand to the copper ion while TEA behaves 2762
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differently with, at low ligand concentration, only one ligand molecule added to the metal ion, followed by dimerization of the monomer. The catalyst deactivation is supposed to proceed via the formation of nitrides (42). Since the number of nitrogen atoms available to form such Cu-N bonds is higher in the MEA-Cu complexes as compared to the TEACu complexes, this could lead to a better inhibition. Considering the extreme complexity of the fly ash and the PCDD/Fs mechanisms of formation, this explanation is only tentative and should be supported by more experiments on model systems. Inhibition as a Function of the Temperature. Figure 2 presents the inhibition yields obtained for some experiments as a function of the temperature of reaction. It can be observed that, for the TEA, the temperature of reaction has no effect on the percentage of inhibition obtained. For the MEA, it seems that the inhibition is slightly better at 400 °C than at 325 °C. The same trend was observed for the PCDFs (not shown). These results are quite surprising since we could think that a higher temperature of reaction would induce a partial vaporization or destruction of the inhibitors and thus a decrease in the inhibition. Nevertheless, these results are very encouraging relating to the sinter plant itself. Actually, the sintering process presents a large range of temperature along the strand and in the different wind boxes located below the strand that collect the off-gas. Depending of the wind box position along the belt, the temperature (measured below the belt) varies between 50 and 400-450 °C. The results suggest that the great variation of temperature along the process will not affect the performance of the inhibitors and that the inhibition can remain effective at different temperatures.
FIGURE 3. Inhibition yields for the PCDDs (A) and PCDFs (B) as a function of the reaction time, temperature 325 °C. Inhibition as a Function of the Reaction Time. Figure 3 presents the inhibition yields obtained for some experiments as a function of the reaction time. Except for the PCDDs with TEA as inhibitor for which the reaction time seems to have no influence, longer reaction time (4 h instead of 2 h) gives better inhibition results. The difference between the percentages of inhibition obtained at 2 and 4 h is particularly important when the inhibition at 2 h is low (for example with low amount of inhibitors). The percentage of PCDDs inhibition with MEA goes up from 12% at 2 h to 76% at 4 h for 0.5 wt % of inhibitor, although it goes only from 75% at 2 h to 85% at 4 h for 2 wt % of inhibitor. Longer reaction times do not involve loss of inhibitory activity through evaporation or destruction of the inhibitor, which is an advantage for the use of this technique in the industrial process, where various reaction conditions may exist, notably for the residence time. PCDFs/PCDDs Ratios, Homologues, and Isomers Distributions. To try to get more information concerning the mechanisms of PCDD/Fs formation during de novo synthesis and the way of action of the inhibitors, the PCDFs/PCDDs ratios as well as the homologue and full isomer distributions were investigated for all the experiments performed in this study. Indeed, as pointed out by Olie et al. (35, 36), “if only one route of formation with one catalyst exists during de novo synthesis, an inhibitor can only reduce the amount of PCDD/ Fs formed but not change the PCDFs/PCDDs ratios, the congener or isomer distributions. Parallel formation pathways catalyzed by various species can be affected by an inhibitor in a different way and consequently such a change in ratios or patterns can occur”. PCDFs/PCDDs Ratios. Figure 4 presents the PCDFs/ PCDDs ratios for the different temperatures and reaction times investigated. These ratios were calculated for the total amounts of PCDD/Fs as well as for the different chlorofamilies. Two different trends can be established. At a relatively short reaction time (2 h, Figure 4A,C), the PCDFs/ PCDDs ratios rise as a result of the use of inhibitors. These results are in perfect agreement with those of Olie et al. (36), who performed inhibition tests on incinerator fly ash during 60 min, as well as with the results of Hell et al. (56). To our knowledge, this research team is the only one who performed inhibition tests on the same substrate as ours (sintering process fly ash, 300 °C, 2 h). These results prove that with a reaction time of 2 h, at 325 or 400 °C, the inhibition is always better for the PCDDs than for the PCDFs. This fact can also be visualized in Table 2. Since the inhibitor acts in a different way on the PCDDs and PCDFs concentrations, these two compound families are certainly formed in the de novo synthesis by different pathways. At a longer reaction
FIGURE 4. PCDFs/PCDDs ratios for the different experiments (A: 325 °C, 2 h; B: 325 °C, 4 h; C: 400 °C, 2 h). Mean value ( range.
FIGURE 5. Homologue distributions obtained for the reference and inhibition tests. Mean value ( range. A: PCDFs, 325 °C, 2 h; B: PCDDs, 325 °C, 4 h. time (4 h, Figure 4B), the differences between the PCDFs/ PCDDs ratios of the reference and the inhibition tests become smaller. With increasing reaction time, the inhibition yields reached for the PCDDs and the PCDFs are in the same range. Homologue Distributions. Some of the homologue distributions obtained for the different inhibition tests are presented in Figure 5. As it can be observed, the homologue distribution of the PCDFs (325 °C, 2 h, Figure 5A) seems not to be affected by the presence of the inhibitors. Actually, the differences found as a result of the addition of inhibitors are small and are of the same order of magnitude as the variations between replicates. The inhibition yields obtained are identical for the different chlorofamilies. The same results were observed for the PCDDs and PCDFs for the different experiments performed during 2 h (not shown). However, in Figure 5B presenting the PCDDs homologue distribution obtained for experiments performed at 325 °C but during a longer reaction time (4 h), the homologue distribution is affected by the presence of the inhibitors. The lower chlorinated species (tetra- and penta-) decrease in comVOL. 36, NO. 12, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 6. Part of the PeCDF isomer distribution obtained for the reference and inhibition tests. Mean value ( range. Temperature 325 °C, reaction time 2 h. parison to the reference test, whereas the most chlorinated (OCDD) rise from 13% in the reference test to values between 25 and 53% for the different inhibition test runs. These results are in contrast with those of Olie et al. (36) who performed PCDD/Fs formation tests on incinerator fly ash in the presence of different inhibitors during 60 min. With some of the inhibitors tested, Olie et al. found a shift toward lower chlorinated homologues as compared with the uninhibited experiment and explained this trend as a result of a decrease of the metal ions available to catalyze chlorination. On the other hand, Ruokoja¨rvi et al., who performed tests with gaseous inhibitors in a pilot plant, observed no statiscally significant change in PCDD/Fs homologue profiles (32). It is however, difficult to compare different studies since the experimental conditions are very different: origin of the fly ash, inhibitors tested, and especially the reaction time, which seems to affect greatly the results. Hell et al. (56) found that TEA does not significantly affect the average level of chlorination in the sinter plant fly ash, but, on the other hand, their inhibition yields are smaller than ours (between 30 and 56%). In our study, the decrease of the lower chlorinated homologues and the rise of the most chlorinated ones, observed for a longer reaction time (4 h), are not easily explained. We can first imagine that the inhibition is better for the lower chlorinated species, but this is not true for a shorter reaction time (2 h). Other explanations are that, after the inhibition, chlorination reactions of the PCDD/Fs take place or that catalytic dechlorination is reduced by inhibition. Indeed, copper compounds as well as Fe2O3 in a minor way are known to catalyze the dechlorination/hydrogenation reaction of PCDD/Fs (57). In our sinter plant fly ash, the Cu content (see Table 1) is similar to what is found in MWI fly ash (between 0.02 and 0.5 wt % (58)); the Fe content is however more considerable. The presence of this huge amount of dechlorination catalyst entrained a dechlorination effect during annealing (experiment without inhibitors), an effect reduced in the inhibition tests in which the inhibitors block the dechlorination catalysts. Nevertheless, the decrease of the lower chlorinated species is environmentally interesting since these species are the most toxic (highest TEF-values). Full Isomers Distributions. Part of the results concerning the full isomers distributions are presented in Figure 6 for the PeCDF at 325 °C, 2 h. All the isomer distributions were 2764
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calculated by setting the sum of each homologue to 100% and calculating the relative contribution of each peak. As it can be observed from Figure 6, the PeCDF isomer distribution obtained at 325 °C and 2 h is not influenced by the addition the inhibitors. The same results were found for all the PCDDs and PCDFs families for the different temperatures and reaction times studied (see Figures S1-S6, Supporting Information). The inhibitors appear not to be able to alter the isomer distribution within homologues. These results are consistent with earlier observations that isomer distributions of PCDD/Fs formed on fly ash by de novo synthesis do not depend on the experimental conditions (temperature and reaction time) (52) and appear to be thermodynamically controlled (11, 14). Nevertheless, it has been shown in the literature that thermodynamic control can only partially explain isomer distributions observed, and thus it is certainly not the only explanations for why inhibitors do not influence isomer distributions. Since the PCDD/Fs pattern is not influenced by the addition of TEA or MEA, inhibitors are able to reduce the global amounts of PCDD/Fs but not to suppress the formation of the toxic 2,3,7,8-substituted isomers selectively. Inhibition Tests Performed at a Real Sintering Process. In parallel to these laboratory experiments, inhibition tests were performed in an industrial sinter plant. The same two inhibitors were tested: triethanolamine (TEA) and monoethanolamine (MEA). The way of introduction of the inhibitors was however totally different: the inhibitors were dissolved in water and introduced in the operating process by the way of spraying nozzles placed in wind boxes located below the strand. Reference tests were performed without inhibitor injection and compared to inhibition experiments in which the inhibitor was injected continuously and measurements carried out after different times. The results obtained at the sinter plant are in good agreement with the laboratory experiments. TEA and MEA were both effective in preventing the PCDD/Fs formation in the industrial process. MEA gives better results with percentages of inhibition up to 90% (calculated on the basis of TEQ). Better results were obtained for longer times between the beginning of the inhibitor injection and the sampling. This latency is not surprising since time is necessary to obtain a good spreading of the inhibitor in the process and especially
on the walls where adsorbed fly ash can produce great amounts of PCDD/Fs by de novo synthesis.
Acknowledgments The authors would like to thank Mr. J. -M. Brouhon from the C.R.M (Centre de Recherches Me´tallurgiques de Lie`ge) for interesting discussion and critical reading of the manuscript. C. Xhrouet was funded as fellow by the F.N.R.S (Fonds National de la Recherche Scientifique Belge).
Supporting Information Available Figures S1-S6 of all the full isomer distributions (obtained for the different inhibitors, temperatures, and reaction times investigated). This material is available free of charge via the Internet at http://pubs.acs.org.
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Received for review November 5, 2001. Revised manuscript received March 26, 2002. Accepted April 11, 2002. ES015776U
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